Morphology‐Conserved Transformations of Metal‐Based ...download.xuebalib.com/xuebalib.com.29936.pdfcarbonate-hydroxides, and metal–organic frameworks (MOFs). Then, we briefly

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Morphology-Conserved Transformations of Metal-Based Precursors to Hierarchically Porous Micro-/Nanostructures for Electrochemical Energy Conversion and Storage

Min Chen, Yueguang Zhang, Lidan Xing, Youhao Liao, Yongcai Qiu,* Shihe Yang,* and Weishan Li*

DOI: 10.1002/adma.201607015

1. Introduction

Hierarchically porous structured materials generally present multiple-level porosity structures in which the pore length scales range from micropores (<2 nm) to mesopores (2–50 nm) and macropores (>50 nm).[1,2] More specifically, these materials always consist of assembled molecular units or their aggregates that are embedded in or intertwined with other units or aggregates, which may, in turn, be similarly organized at increasing size levels.[3–6] These aesthetic architectures enable the structured materials to obtain unique properties and functionalities.[7–11] The assembly comprises numerous hier-archical organizations, each of which exhibits exceptional functionality that ema-nates from structures over a large range of length scales.[12–15] Biological structural hierarchy and integration strongly affect

the performance of a scaffold in a tissue. Likewise, in many cases, the properties and functionalities of materials depend greatly on their structural features, such as size, shape, density, and porosity.[16–25]

Over the past decades, several methodologies have been developed for the synthesis of hierarchical micro-/nano-structures with controlled size, shape, porosity, and compo-sition, and mainly involve replication induced by physical/chemical vapor deposition (CVD);[26] sol–gel methods;[27–29] templates, including hard and soft techniques;[30–43] chemical bath deposition;[44] direct combustion;[45,46] electrochemical deposition;[47] hydrothermal/solvothermal synthesis;[48,49] and lithographic techniques.[50,51] Various emerging applications exist for these hierarchical micro-/nanostructures, such as catalysts,[52–56] drug- and gene-delivery systems,[57,58] hydrogen generation and storage,[59–61] rechargeable batteries,[62–65] and photo voltaics.[66–68] For instance, 3D hierarchically micro- or sub-microsized architectures are always self-assembled from nanometer-sized building units via interactions such as van der Waals forces, hydrogen bonds, and ionic and covalent bonds. These units consist of 0D structures, such as spherical nanopar-ticles; 1D structures, such as nanowires, nanobelts, nanorods, and nanotubes; and 2D structures, such as nanosheets.[34,69–76] These hierarchical architectures have been used to enhance

To meet future market demand, developing new structured materials for electrochemical energy conversion and storage systems is essential. Hierarchically porous micro-/nanostructures are favorable for designing such high-performance materials because of their unique features, including: i) the prevention of nanosized particle agglomeration and minimization of interfacial contact resistance, ii) more active sites and shorter ionic diffusion lengths because of their size compared with their large-size counterparts, iii) convenient electrolyte ingress and accom-modation of large volume changes, and iv) enhanced light-scattering capability. Here, hierarchically porous micro-/nanostructures produced by morphology-conserved transformations of metal-based precursors are summarized, and their applications as electrodes and/or catalysts in rechargeable batteries, supercapacitors, and solar cells are discussed. Finally, research and development challenges relating to hierarchically porous micro-/nanostructures that must be overcome to increase their utilization in renewable energy applications are outlined.

Electrochemistry

M. Chen, Prof. Y. Zhang, Dr. L. Xing, Dr. Y. Liao, Prof. W. LiSchool of Chemistry and EnvironmentSouth China Normal UniversityGuangzhou 510631, ChinaE-mail: [email protected]. Y. Zhang, Dr. L. Xing, Dr. Y. Liao, Prof. W. LiEngineering Research Center of MTEES (Ministry of Education)Research Center of BMET (Guangdong Province)Engineering Lab. of OFMHEB (Guangdong Province)Key Lab. of ETESPG (GHEI)and Innovative Platform for ITBMD (Guangzhou Municipality)South China Normal UniversityGuangzhou 510006, ChinaProf. Y. ZhangSuzhou Institute of Nano-Tech and Nano-BionicsChinese Academy of SciencesSuzhou 215123, ChinaDr. Y. Liao, Prof. Y. Qiu, Prof. S. YangDepartment of ChemistryThe Hong Kong University of Science and TechnologyClear Water Bay, Kowloon, Hong Kong, ChinaE-mail: [email protected], [email protected]; [email protected]. Y. QiuCollege of Environment and EnergyGuangzhou 510006, China

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materials’ magnetic properties for magnetic resonance imaging and magnetic energy storage, increase the surface area for catalysts and photovoltaics, and accommodate volume changes during cycling for lithium-ion batteries (LIBs).[77–79]

Many of the methodologies mentioned above have been uti-lized to produce hierarchical micro-/nanostructures; however, these structures typically have no or low porosity, leading to low active surface areas and poor wettability for electrolytes. The design and construction of hollow micro-/nanostructures can help to address these issues to a certain extent, and sev-eral excellent reviews on recent progress in the synthesis and applications of hollow micro-/nanostructures have been pub-lished.[80–84] The template technique is one of the most useful methods for preparing hollow or porous materials and generally involves the formation of a template (e.g., mesoporous silicas or mesoporous carbon as hard templates, or organized polymers as soft templates), subsequent deposition of materials, and finally, removal of the templates. However, these complicated synthetic procedures are expensive, result in low yields, and are time con-suming, hindering their industrial application. A morphology-conserved transformation approach to hierarchically porous micro-/nanostructures typically involves the transformation of a precursor through calcination or some other process.[85–90] Gen-erally, this approach consists of two main steps: the construc-tion of precursors from metal-based intermediate compounds, and the conversion of the precursors to hierarchically porous structures. Because of its generality, simplicity, and cost effec-tiveness, the morphology-conserved transformation approach has become the most attractive method for preparing hierar-chically porous micro-/nanostructures with intriguing architec-tures and unique properties and functionalities.

Here, we summarize the hierarchically porous micro-/nano-structures constructed via morphology-conserved transforma-tion and discuss their electrochemical/photoelectrochemical applications. In the first section, we systematically survey a variety of precursors for hierarchically porous micro-/nano-structures, including metal hydroxides, metal carbonates, metal carbonate-hydroxides, and metal–organic frameworks (MOFs). Then, we briefly review the key applications of hierarchically porous micro-/nanostructures in rechargeable batteries, including LIBs, lithium–sulfur (Li–S) batteries, lithium–air (Li–O2) batteries, supercapacitors, and dye-sensitized solar cells (DSSCs). Here, we cover the major work in each area, pre-senting the relevant work in each subarea as brief overviews.

2. Various Precursors to Hierarchically Porous Micro-/Nanostructures

Hierarchically porous structured materials have many excep-tional properties, such as ultrahigh surface areas, controlled pore sizes and shapes, and nanoscale effects, making them the most promising candidates for energy conversion and storage device applications. However, how to control the structures, sizes, and shapes of the materials remains chal-lenging in current material syntheses. Fortunately, the conser-vation of morphology from metal-based precursors to porous metal oxides is an effective method compared with intricate template approaches.[91–98] Electrospinning is a universal

morphology-conserving transformation method for producing hierarchically porous micro-/nanofibers in different forms, such as core–shell hollow porous micro-/nanofibers.[99–101] Given the broad scope of the existing review articles addressing electrospinning methods, the following Sections here will focus on work utilizing other interesting precursors.[102,103]

2.1. Metal Hydroxide Precursors

The most common metal-based precursors are metal hydroxides, such as Mn(OH)2,[104] Co(OH)2,[105] Ni(OH)2,

[106] Sn(OH)2,[107]

Min Chen received her M.S. (2013) in Physical Chemistry from South China Normal University. She is now pursuing her Ph.D. in Prof. Li’s Group in the School of Chemistry and Environment, South China Normal University. Her current research focuses on nano materials for high-energy-density lithium-ion batteries.

Yongcai Qiu received his Ph.D. (2012) in the Nano Science and Technology program at Hong Kong University of Science and Technology (HKUST). He then did his postdoctoral research at HKUST in 2013–2014 and joint postdoctoral research at Chinese Academy of Sciences and Stanford University in

2015–2016. His current research interests cover the preparation, understanding, and applications of functional nanomaterials in energy storage and conversion devices

Weishan Li received his Ph.D. in 1996 from South China University of Technology. He is now a professor in the School of Chemistry and Environment, South China Normal University, and the director of the Engineering Research Center of Technology for Electrochemical Energy Storage, Minister of

Education, China. His current research focuses on new materials for electrochemical energy conversion and storage.

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Ce(OH)2,[108] La(OH)3,[109] In(OH)3,[110,111] Y(OH)3,[112] and Ni–Co bimetallic hydroxides.[113] In the synthesis of metal hydroxides, sodium hydroxide,[114–116] ammonium hydroxide,[117] and hydrazine hydrate[118] are typically used as the hydroxide source. Among them, sodium hydroxide and ammonium hydroxide are the most commonly used sources of insoluble metal hydroxides because their reaction principles are simple. For example, Wang et al.[119] prepared nanocrystalline-assembled bundle-like CuO particles through the calcination of an as-pre-pared bundle-like Cu(OH)2 precursor. In the typical formation of bundle-like Cu(OH)2, NaOH was directly added to a mixed aqueous solution of CuCl2 and C6H8O7. In addition, core–ring structured NiCo2O4 nanoplatelets were obtained by annealing a mixed precursor of β-Co(OH)2 and Ni(OH)2 at 200 °C. The coprecipitation of Co(OH)2 and Ni(OH)2 was also achieved using sodium hydroxide as the precipitant.[90] Needle-like Co3O4 nano-tubes were successfully converted from a β-Co(OH)2-nanorod precursor, which was produced by reacting Co(NO3)2 with ammonia solution.[117]

In addition to NaOH and NH3·H2O, alkaline conditions can also be generated via the thermal decomposition of urea in water, or the hydrolysis of weak acidic salts. Urea has been shown to play a very critical role in the hydrothermal synthesis method. Li et al.[120] prepared a mesoporous ultrathin NiO nanowire network through the thermal decomposition of an α-Ni(OH)2 precursor at 300 °C for 2 h in air. The α-Ni(OH)2 precursor was obtained by hydrothermal treatment of NiCl2 in the presence of urea in aqueous solution. Similarly, urchin- and flower-like hierarchical NiO microspheres were successfully synthesized by Pan et al.[121] via calcining α-Ni(OH)2 precur-sors. The morphologies of the precursors were controlled by simply tuning various sources of nickel salts in the nickel-salt–urea–H2O ternary system. Anions were observed to interca-late in the crystal lattice of α-Ni(OH)2 and strongly influence the self-assembly process, thereby determining the resulting morphology and structure.

The mechanism by which urea tailored the resulting mor-phology was proposed as follows: At the appropriate tempera-ture (normally above 80 °C), urea gradually hydrolyzes and releases NH3 and CO2, creating weak alkaline conditions for the generation of metal hydroxide or metal carbonate hydroxide nuclei. The growth of nuclei can then be controlled by the release rates of NH3 and CO2, affording sufficient time for the spontaneous self-assembly of nanoblocks to minimize the mutual interaction energy. The related reaction formulas could be written as follows:[122,123]

For the thermal decomposition of urea in water:

CO NH 2H O 2NH CO2 2 2 3 2( ) + → + (1)

M NH H O M OH NH1 3 2 1 4x x xxx( )+ + → ++ +

(2)

where M1 refers to metal elements and M1(OH)x is insoluble. For the growth of hierarchically structured precursors, except for the decomposition of urea, the application of appropriate weak acid salts (e.g., sodium acetate) is another feasible method to obtain alkaline conditions for precipitating metal ions.

For the hydrolysis of a weak acidic salt using acetate (Ac) salt as an example:

M Ac H O HAc M OH2 2 2y y yy

y( ) + → + ++ − (3)

M OH M OH2 2yyy( )+ →+ −

(4)

where M2 refers to metal elements and M2(OH)y is insoluble. Hydrolysis of the acetate group resulted in the formation of OH− (Equation (3)), then, the OH− combined with metal ions to produce metal hydroxides. For instance, nanoporous NiO structures with an actinia shape[124] were fabricated by calcining an α-Ni(OH)2 precursor. In a typical experiment, the α-Ni(OH)2 precursor was obtained hydrothermally by reacting NiCl2 and NaAc in a mixed solution of ethylene glycol (EG) and water. NaAc offered a weakly basic environment for the formation of sheet-like α-Ni(OH)2. In addition, hierarchical NiCo2O4 tetrag-onal microtubes were fabricated via the thermal transformation of a layered nickel–cobalt-hydroxide precursor, prepared from Ni(Ac)2 and Co(Ac)2 in a solution containing 1,3-propanediol and isopropyl alcohol.[125]

Surfactants, such as poly(ethylene glycol) (PEG),[126] poly(vinylpyrrolidone) (PVP),[127] and sodium dodecyl sulfate (SDS),[122] have been demonstrated to play important roles in the formation of more complicated micro-/nanomaterials because of their amphiphilic features. For instance, 3D micro-flowery In(OH)3 structures assembled from 2D nanoflakes were prepared via a hydrothermal approach with the assistance of SDS. Additionally, In2O3 retaining its morphology was pro-duced by annealing the In(OH)3 precursor.[122]

2.2. Metal Carbonate Precursors

Metal carbonates, such as manganese carbonate,[128] cobalt carbonate,[129] and Co–Mn bimetal carbonate, are another type of precursor that can be used to prepare hierarchically porous structures.[130] When generating metal carbonates, inorganic salts or the decomposition of certain organics is typically used as the source of the carbonate ion. NH4HCO3 and Na2CO3 are the most commonly used inorganic salts to provide carbonate ions for the synthesis of various metal carbonates.[20,131–134] For example, LiNi0.5Mn1.5O4 hollow microspheres were obtained through morphology-conserved synthesis following a three-step route.[135] First, spherical MnCO3 was produced by a simple precipitation method with MnSO4 as the manganese source and NH4HCO3 as the precipitant. Then, the as-formed MnCO3 microspheres were thermally decomposed into porous MnO2, which inherited the microsphere morphology of MnCO3 after the annealing process. Finally, LiOH and Ni(NO3)2 were impregnated into the as-synthesized mesoporous MnO2 micro-spheres to form LiNi0.5Mn1.5O4 hollow microspheres. This formation process was analogous to the Kirkendall effect, in which the outward diffusion of Mn and Ni atoms was quicker than the inward diffusion of O atoms. A similar three-step method was applied to prepare LiMn2O4 microspheres.[131] Additionally, a hierarchically porous LiNi1/3Co1/3Mn1/3O2 struc-ture was synthesized by a two-step process: The carbonate precursor (Ni1/3Co1/3Mn1/3)CO3 was prepared using Na2CO3 and NH3HCO3 as precipitants and then mixed with a lithium source to produce the final product.[136]

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Various organics can be used to synthesize metal carbonate precursors, but they share certain common features and con-tain –CH2OH, –CHO or –COOH groups. Under a given set of reaction conditions (e.g., hydrothermal), these compounds decompose and then generate CO3

2− anions, which combine with metal cations to form metal carbonate nuclei and subse-quently grow to produce the metal carbonate precursor.[137–141] Typically, fructose and β-cyclodextrin have been used as multi-functional polyol reagents to prepare different MnCO3 micro-structures. Hierarchically porous Mn2O3 has been generated via calcining the as-synthesized MnCO3.[142] The proposed mechanism is described in Equation (5–9). The redox reactions between permanganates and poly-based organic molecules (e.g., fructose and β-CD, where CD = cyclodextrin (C6H10O5)7) would generate MnCO3 and MnOOH precipitates, which would nucleate during pyrolysis of the polyol-based organic molecules.

− + → + − + +− −4R CH OH MnO MnOOH 4R CHO OH H O2 4 2 (5)

R CHO MnO MnOOH R COO H O4 2− + → + − +− − (6)

5R COO 2MnO 6H 2Mn 5CO 8H O42

2 2− + + → + +− − + + (7)

CO H O 2H CO2 2 32+ → ++ − (8)

Mn CO MnCO232

3+ →+ (9)

Strong oxidants are required for the redox reaction. KMnO4 is among the most commonly used oxidants for the synthesis of manganese-based precursors. Similar syntheses have been developed by other researchers. For instance, by changing the raw mass ratio of D-maltose to KMnO4, a series of manga-nese-based precursors (amorphous MnO2 flower, γ-MnOOH nanorod, MnCO3 cube, polyhedron, spindle, and fusiform) were synthesized and sintered at 600 °C to obtain various α-Mn2O3 morphologies.[143]

In contrast to the contribution of KMnO4 mentioned above, KMnO4 has also been used to oxidize low-valence manganese compounds, such as the following:

3MnCO 2KMnO 5MnO K CO 2CO3 4 2 2 3 2+ → + + (10)

Using this reaction, hierarchically hollow microspheres and microcubes of MnO2

[86] were prepared by retaining the mor-phologies of the MnCO3 precursor (Figure 1). The well-defined shape of MnCO3 could be adjusted by changing the synthesis conditions. The pathway to hierarchically hollow MnO2 struc-tures is shown in Figure 1G. Briefly, various morphologies of MnCO3 precursors were prepared in advance via a hydro-thermal method. Then, KMnO4 reacted with MnCO3 to form core–shell MnCO3@MnO2, in which MnO2 was the shell and MnCO3 the core. After removing the MnCO3 core by HCl etching, the MnO2 shell retained the original framework, and hollow structures were produced. In addition, by prolonging

the reaction time of KMnO4 with MnCO3, the thickness of MnO2 could be increased. Similarly, Cao et al.[134] synthesized various morphologies of Mn2O3, including hollow-structured spheres, cubes, ellipsoids, and dumbbells, based on morpholog-ically controllable MnCO3 precursors. Other similar approaches have been proposed to produce LiMn2O4.[85,144,145] For instance, through the hydrothermal process, β-MnO2 was obtained by oxidation of Mn(CH3COO)2·4H2O in the presence of peroxy-sulfate ((NH4)2S2O8). Then, spinel LiMn2O4 was prepared by reacting LiOH with the as-synthesized β-MnO2 nanorods.[145]

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Figure 1. Transmission electron microscopy (TEM) images of a MnCO3 microsphere (A), MnCO3@MnO2 (B) after chemical reaction (Equation (10)), MnCO3@MnO2 (C) after partial removal of MnCO3, and hollow MnO2 (D) with complete removal of MnCO3. Scanning elec-tron microscopy (SEM) images of MnCO3 microcubes (E) and MnO2 microcubes (F). Schematic illustration (G) of the formation process of hierarchically hollow MnO2 nanostructures: i) MnCO3 precursors with different morphologies, ii) MnO2@MnCO3, and iii) hierarchically hollow MnO2 nanostructures. Reproduced with permission.[86] Copyright 2008, Wiley-VCH.



Surfactants were also applied to con-struct metal carbonate precursors, which underwent processes similar to those of metal hydroxides.[146] Typically, three types of highly uniform Co3O4 products (peanut-like, capsule-like, and rhombus topography) were synthesized from various CoCO3 pre-cursors by using Co(CH3COO)2·4H2O, PVP, diethylene glycol (DEG), and urea as reaction materials.[129] A precipitation–dissolution–renucleation–growth–aggregation mecha-nism was proposed to explain the formation of the precursors. Initial primary precipitates were produced instantaneously when super-saturation was reached. Unstable precipitates underwent dissolution, renucleation, and crystallite growth. The CoCO3 crystallization process can be described simply as fol-lows: When heated at approximately 90 °C, the urea decomposes into CO2 and OH− (Equation (11)). Under sealed conditions, the dissolved CO2 is largely converted to car-bonate ions (Equation (12)). Then, Co2+ com-bines with the generated carbonate ions to form the CoCO3 precipitate (Equation (13)), and finally, CoCO3 is annealed in air to obtain the target Co3O4 (Equation (14)). The formation process of CoCO3 is heavily dependent on dissolution of the carbonate ion in Equation (12).

CO NH 3H O 2NH CO 2OH2 2 24

2( ) + → + ++ − (11)

2NH CO 2OH 2NH CO H O42

432

2+ + → + ++ − + − (12)

Co CO CoCO232

3+ →+ − (13)

6CoCO O 2Co O 6CO3 2 3 4 2+ → + (14)

Analogously, CoCO3 nanostructures were successfully fabri-cated via a solvothermal route using PVP as a capping reagent. Intriguing anisotropic porous Co3O4 nanocapsules were then formed by heat treatment of CoCO3 in air.[147]

In addition to the surfactant, the geometry and strength of the chelating agent play crucial roles in controlling the shape.[148] For example, 3D hierarchically-assembled lotus-shaped porous MnO2

[148] was synthesized by the calcination of a MnCO3 precursor. The growth of lotus-like MnCO3 depended on the chelating agent (citric acid) present in a simple aqueous solution. Indeed, rods, spheres, and nanoaggregates of MnCO3 could also be synthesized by varying the chelating agent (citric acid, tartaric acid, and oxalic acid, respectively).

Interestingly, some intriguing structures were also pre-pared by controlling the thermal decomposition tempera-ture.[149] For example, triple-shelled, cubic-like porous Mn2O3 was fabricated by controlling the thermal decomposition tem-perature of the MnCO3 precursor (Figure 2).[149] The MnCO3 precursor was sintered at 300 °C for 1 h with a ramping rate of 1 °C min−1, followed by heating at 600 °C for 1 h with

2 °C min−1. At the initial stage of calcination, a large tem-perature gradient (ΔT1) existed along the radical direction, resulting in a Mn2O3 shell at the surface of the MnCO3 core. The hierarchically porous structure was produced by two forces from opposite directions: the contraction force (Fc) from the decomposition of MnCO3, which promoted the inward shrinkage of the MnCO3 core, and the adhesion force (Fa) from the relatively rigid Mn2O3 shell, which prevented its inward contraction.[149] With a large ΔT1, Fc exceeded Fa during the early stage. Thus, the inner core shrank inward and became isolated from the outer shell. Similar to the first heat treatment (ΔT1), triple-shelled Mn2O3 was produced by the second heat treatment (ΔT2). With prolonged heating, Fc decreased rapidly. When Fa surpassed Fc, the direction of material movement was reversed and, as a result, the inner core contracted outward, leaving a hollow cavity in the center. Similarly, double-shelled CoMn2O4 hollow microcubes were prepared by heating Co0.33Mn0.67CO3 precursor at 600 °C for 5 h, with a ramping rate of 2 °C min−1 in air.[150]

Additionally, by precisely controlling the thermal treatment, Wang et al.[151] synthesized hollow MnO2 via the partial thermal decomposition of MnCO3. Through a partial calcination pro-cess, the MnCO3 on the surface was converted to an oxida-tion layer of MnO2, leaving the MnCO3 inner cores, and thus forming a “core@shell” (MnCO3@MnO2) structure. Acid treat-ment was applied to remove the inner cores and form porous hollow MnO2. This approach was also applied to prepare other hollow MOx (M: Fe, Co, Ni, etc.).[151] The pore sizes could also be changed by controlling the annealing temperatures. For example, Chang et al.[152] successfully synthesized Mn2O3

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Figure 2. SEM images of a MnCO3 nanocube (A), SEM (B), and TEM (C) images of a triple-shelled Mn2O3 hollow nanocube, and a schematic illustration (D) of the formation of a triple-shelled Mn2O3 hollow nanocube. Reproduced with permission.[149] Copyright 2014, Royal Society of Chemistry.



microspheres with different pore sizes through the morphology-conserved transformation of MnCO3 at different annealing temperatures. The nanoparticles within the microspheres became obviously larger, and the diameter of the microspheres decreased as the annealing temperature increased. This result might be attributable to the aggregation and regrowth of the nanoparticles, accompanied by the pores becoming larger and increasingly inhomogeneous, until disappearing completely.

2.3. Metal Carbonate Hydroxide Precursors

Metal carbonate hydroxides are also promising precursors for the preparation of hierarchically porous metal oxides. To synthesize a metal carbonate hydroxide, OH− and CO3

2− must exist or be generated in the reaction solution. The metal ion might form metal hydroxide, metal carbonate, or metal car-bonate hydroxide, depending on its properties (e.g., the solubility of the corresponding metal salts).[153–158] Therefore,

not every metal ion can form a metal carbonate hydroxide. Gen-erally, Zn-based carbonate hydroxide,[159] Co-based carbonate hydroxide,[160,161] and Ni-based carbonate hydroxide[162] are uni-versally used compounds.

Zn-based carbonate hydroxide is the most representative example, and is thus used to introduce the metal carbonate hydroxide precursor. Various methods, such as the chemical-bath deposition method,[163] reflux method,[155] hydrothermal method,[159] and solvothermal method,[164] have been devel-oped to synthesize zinc-based carbonate hydroxide precur-sors, including Zn5(CO3)2(OH)6

[155,164] (Figure 3A–E) and Zn4CO3(OH)6

[159,163,165] (Figure 3F–J). The reaction processes can be expressed as follows:

5Zn 6OH 2CO Zn CO OH232

5 3 2 6( ) ( )+ + →+ − − (15)

)()( )(+ + +

→ ⋅

+ − −4Zn 6OH CO H OZn CO OH H O may be 0 or 1

232

2

4 3 6 2

x

x x (16)

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Figure 3. SEM image of the Zn5(CO3)2(OH)6 precursor (A), which is composed of plate-like nanostructures with edge thicknesses of approximately 19 nm, and porous ZnO nanoplates (B). Typical TEM image of an individual porous ZnO nanoplate (C). Selected-area electron diffraction (SAED) patterns (D) and high-resolution TEM (HRTEM) image (E) of porous ZnO nanoplates. Reproduced with permission.[155] Copyright 2008, Wiley-VCH. SEM images of the Zn4CO3(OH)6 precursor (F) and 3D porous architecture of ZnO (G). Magnified TEM image of the ZnO nanosheets (H) and cor-responding SAED pattern of H (I). Schematic illustration of the formation of porous structures (J). Reproduced with permission.[159] Copyright 2010, The American Chemical Society.



For example, in Jing’s work,[155] a plate-like Zn5(CO3)2(OH)6 precursor with edge thicknesses of approximately 19 nm was prepared in the presence of urea, and then annealed at 400 °C for 2 h to generate porous ZnO nanoplates. Addi-tionally, a 3D porous ZnO architecture consisting of inter-connected nanosheets was fabricated by the conversion of a layered Zn4(CO3)(OH)6·H2O precursor.[159] In the aforemen-tioned preparation, urea played an important role in deter-mining the final morphology. Similarly, assisted by urea, Lei et al.[165] fabricated porous ZnO microspheres by calcining Zn4(CO3)(OH)6 microspheres. During the annealing pro-cess, a porous structure was probably formed because of the release of H2O and CO2 in the thermal decomposition of the precursors. The thermal decomposition could be described as follows:

Zn CO OH H O ZnO CO 3 H O3 6 2 2 2z x y zx y( ) ( ) ( )⋅ → + ↑+ + ↑ (17)

The Co-based intermediate compound is another impor-tant candidate precursor for the synthesis of Co3O4 or CoO. For example, Xiong et al.[166] synthesized chrysanthemum-like Co(CO3)0.5(OH)·0.11H2O through a hydrothermal route with the assistance of urea and sodium chloride. The precursor was then transformed into mesoporous Co3O4 under normal annealing conditions. Cubic CoO nanonets were successfully prepared by annealing a monoclinic Co2(OH)2CO3-nanosheet precursor in an Ar atmosphere.[158] Similarly, Co3O4 nanorods, nanobelts, nanosheets, and cubic/octahedral nanoparticles exposing the high-energy (110) crystal plane were success-fully synthesized by annealing the Co(CO3)0.5(OH)·0.11H2O precursor at 250 °C.[160] Moreover, cobalt–nickel bimetallic hydroxide carbonate[154,167,168] and cobalt–manganese bime-tallic hydroxide carbonate[53] with hierarchical structures were synthesized as precursors to obtain the final hierarchically porous products. Specifically, a sea-urchin-like NiCo2O4 spinel structure was prepared through a calcination process with morphology conserved from a bimetallic (Ni, Co) carbonate hydroxide. The bimetallic carbonate hydroxide precursor was formed via a sequential crystallization process. Specifically, monometallic nickel carbonate hydroxide first nucleated, then evolved into flower-like microspheres, grew into bime-tallic hydroxide carbonate nanorods via localized dissolution-recrystallization, and finally formed a sea-urchin-like structure. Additionally, nickel–cobalt-oxide microspheres consisting of mesoporous thorn arrays were obtained by the calcination of a well-designed NiCo(OH)2CO3.[154]

Weak acid salts combined with (NH4)2CO3 have also been used to provide CO3

2− and OH−. For instance, 3D nest-like porous ZnO was synthesized by Wang et al. by annealing the Zn5(CO3)2(OH)6 precursor. The precursor was obtained through a hydrothermal process in which OH− and CO3

2− were provided by the hydrolysis of zinc acetate and the dissociation of (NH4)2CO3, respectively.[169]

Hexamethylenetetramine is also useful for the preparation of metal carbonate hydroxide precursors. When the reaction temperature exceeds 120 °C, hexamethylenetetramine decom-poses, releasing OH− and CO3

2 in water.[105,170] For example, Ni2(OH)2CO3 microspheres assembled from nanosheets were obtained through a hydrothermal route in the presence of

hexamethylenetetramine. Hierarchically porous NiO micro-spheres were fabricated by further heat treatment at 300 °C for 2 h.[156]

As mentioned above, employing surfactants, such as SDS,[156] Pluronic F127,[171] and cetyltrimethylammonium bromide (CTAB),[172] is an effective approach to achieving metal car-bonate hydroxide precursors with various morphologies.[171,173] For instance, urchin-like hollow Co3O4 spheres were success-fully synthesized by Chen et al.[153] via the thermal decomposi-tion of a Co(CO3)0.5(OH)·0.11H2O precursor obtained with the assistance of CTAB.

2.4. MOF Precursors

MOFs are also among the most promising candidate precur-sors for hierarchically porous materials.[174–177] As the linkers of MOFs, ligands are of great importance because they often contain functional groups (e.g., –OH, –COOH, and –NH2) that provide coordination sites. Based on the different styles of ligands, we divide the precursors of MOFs into several classes: metal carboxylate complexes, metal hydroxide acetate com-plexes, metal alkoxide complexes, metal glycolate complexes, Prussian blue (PB) coordination complexes, and metal imida-zolyl/pyridyl complexes.

2.4.1. Metal Carboxylate Complexes

Carboxylate linkers are among the most important and widely used ligands for constructing MOFs. In forming metal car-boxylate complexes, certain carboxylic acids, including mono(carboxylic acid), di(carboxylic acid), and poly(carboxylic acid), should be added to coordinate with metal ions. We will introduce several types of carboxylic acid ligands in the fol-lowing section.

Formic acid and acetic acid are the most convenient mono(carboxylic acid)s for preparing metal carboxylate com-plexes. For instance, a porous Mn2O3 polyhedral structure was produced from Mn-based MOFs generated by reacting HCOOH and CH3COOH with manganese salts.[178] Hierar-chically porous V2O5 microspheres were developed by Zhang et al.[179] via an additive-free solvothermal method and subse-quent calcination process. V2O5 microspheres were produced by the reaction of CH3COOH and vanadium (V) oxytriisoprop-oxide (VO(OiPr)3.

Oxalic acid is one of the most readily available dicarb-oxylate ligands for forming metal coordination compounds. Notably, the morphologies of oxalate-based MOFs can be regu-lated by surfactants and solvents. For example, nanospheres, nanocubes, and nanorods of nickel oxalate coordination poly-mers have been modulated using various surfactants (CTAB, TX-100, and Tergitol) and solvents (isooctane, n-hexane, and cyclohexane).[180] Li’s group prepared a hierarchically porous, layered, lithium-rich oxide by optimizing the molecular size of PEG.[181] Among these reported surfactants, CTAB is the most widely used for preparing 1D oxalate-based MOF precur-sors. A variety of porous nanorod-like manganese oxide com-pounds (MnO, Mn2O3, and Mn3O4) have been successfully

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prepared by annealing nanorod-like manganese-oxalate precur-sors under given reaction conditions.[182] Additionally, aligned spinel porous CoFe2O2 nanorods were successfully synthesized by roasting a rod-like CoFe2(C2O4)3 precursor.[183] A porous Li[Li0.19Mn0.32Co0.49]O2 nanorod structure was synthesized by Chen et al. from a Mn–Co-oxalate nanorod precursor. Specifi-cally, oxalate ligands were used to coordinate manganese ions and nickel ions to form a 1D bimetal nanorod structure with the help of CTAB.[180,184–186] Then, the 1D Mn–Co-oxalate precursor was converted to porous Mn–Co oxide under heat treatment. Finally, LiOH was impregnated into the as-obtained porous Mn–Co oxides, yielding 1D porous Li[Li0.19Mn0.32Co0.49]O2 by a subsequent annealing process.

The hydrolysis of diethyl oxalate can also be used to produce metal-oxalate precursors. For example, Fatemeh et al.[187] syn-thesized nickel oxalate nanostructures via reaction of nickel salts and diethyl oxalate in ethanol solution. By controlling the heat treatment and reaction conditions, the NiC2O4 pre-cursor could be converted into dandelion-like and rod-like NiO nanostructures.

1,4-Benzenedicarboxylic acid (H2BDC) is also a general ligand for the formation of controllable structured MOFs.[188] For example, spindle-like porous α-Fe2O3

[189] was obtained from an iron-based MOF (MIL-88-Fe) (Figure 4A,B). This MOF was synthesized by a modified solvothermal method using FeCl3·6H2O and H2BDC as raw materials. α-Fe2O3 nanoassem-bled spindles[190] were successfully prepared by the simple calcination of the Fe-MOF precursor. Additionally, porous Fe2O3@TiO2

[191] could also be derived from the Fe-based MOF,

which was synthesized by the reaction of H2BDC and FeCl3 in dimethylformamide (DMF). In addition to monometal MOFs, bimetal MOFs have also been reported. For example, bimetal ZnO/ZnCo2O4 nanosheets were derived from a Zn–Co-MOF precursor, which was first fabricated by refluxing a mixture of terephthalic acid, zinc nitrate, cobalt acetylacetonate, and sur-factant PVP in a mixed solvent (DMF:ethanol = 5:3) at room temperature.[192] Furthermore, a hollow ZnO/ZnFe2O4/C octahedral structure was derived from a Fe-based MOF, in which the octahedral Fe(III)-MOF-5 was synthesized through the reaction of terephthalic acid, Fe(acac)3, Zn(NO3)2, and PVP in a mixed solvent (DMF:ethanol = 5:3).[193] Mesoporous NixCo3−xO4 nanorods inherited the morphology of their Co–Ni-bimetallic-MOF precursor, which was fabricated by the reaction of 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC), Co(NO3)2, and Ni(NO3)2 in a mixed solvent of DMF–ethanol–water (1:1:1 [v/v/v]).[194]

The in situ formation of a coordination polymer template is another useful approach for constructing porous metal oxides. For example, Liu et al.[195] synthesized an amorphous hexagonal-ring precursor via the reaction of organic bridging ligands, H2BDC, and zinc acetate in DMF solvent, followed by sintering at 550 °C to form polycrystalline ZnO rings. A tem-plate-directed growth mechanism has been demonstrated to be feasible for morphological control; this mechanism involved a concurrent two-step process of growing secondary coordina-tion polymers and dissolving the initial supporting template. Jung et al.[196] used the same ligands to prepare unusual ZnO hexagonal tubes. The initially formed metal-coordination-

polymer particles acted as templates for the growth of the final hexagonal tubes. Other researchers have successfully synthesized Ti-coordination-polymer and Sn-coordi-nation-polymer precursors to obtain final porous structures.[197]

Some natural amino acids that contain dicarboxylate groups can be regarded as func-tional ligands to obtain various MOFs.[198] For instance, cobalt-aspartate MOF has been used as the precursor in the synthesis of porous tubular Co3O4. Similarly, L-glutamic acid was employed as an assisting ligand to prepare a 3D zinc-glutamate MOF.[199]

Poly(carboxylic acid)s are also used to con-struct precursors with porous structures.[200] For example, Ji et al.[201] synthesized porous Mn2O3 nanowires through calcining wire-like Mn-BTC (H3BTC = 1,3,5-benzenetricarbox-ylic acid) MOFs. In addition, well-dispersed and size-controlled CuO nanostructures were successfully synthesized by pyrolyzing a Cu-based MOF (Cu–BTC) precursor.[202]

2.4.2. Metal Hydroxide Acetate Complexes

Metal hydroxide acetate complexes belonging to a family of layered compounds are extensively used as precursors to obtain

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Figure 4. SEM image of the as-prepared MIL-88-Fe (A) and TEM images of the final spindle-like α-Fe2O3 (B). Reproduced with permission.[189] Copyright 2012, American Chemical Society. SEM images of the Ni–Co precursor (C) and TEM images of the as-obtained mesoporous Ni0.37Co oxide prisms (D). Reproduced with permission.[204] Copyright 2015, Wiley-VCH.



hierarchically porous structures.[174] Two synthetic methods (refluxing and hydrothermal) are often adopted to prepare such layered compounds; this process involves the hydrolysis of acetate salts in alcohol/polyol medium.[203] For instance, Lou’s group[204] obtained yolk–shell Ni/Co bimetallic oxide nano-prisms (Figure 4C,D) by heating an Ni/Co hydroxide acetate precursor at 350 °C in air, at a rate of 2 °C min−1. The precursor was prepared by refluxing metal acetate and PVP in ethanol solution at 85 °C. The reaction by which the Ni–Co precursor is formed can be described by Equation (18):

) ))

( (() )( (

+ − +→ ⋅ +Co CH COO (5 )Ni CH COO 4H O

Ni,Co OH CH COO 2H O 2CH COOH3 2 3 2 2

5 2 3 8 2 3

x x (18)

Similarly, hollow Co3O4 boxes were well designed through annealing a precursor of 1D cobalt hydroxide acetate (Co5(OH)2(CH3COO)8·2H2O) prisms. The precursor was obtained in ethanol solution.[87] Laurence et al.[205] fabricated a series of metal hydroxide acetates (metal = Zn, Co, and Ni) by reacting Zn(CH3COO)2/Co(CH3COO)2/Ni(CH3COO)2 and polyols (DEG or 1,2-propanediol) in ethanol. Then, the final products ZnO, Co3O4, and NiO were obtained from the decom-position of the metal hydroxide acetate precursors.

In addition, metal hydroxide acetate complexes could be generated from the reaction of metal acetate and aqueous ammonia. However, such reactions are usually performed at a relatively low temperature (i.e., lower than 100 °C). For instance, Song et al.[206] synthesized basic zinc hydroxide ace-tate nanobelts via the reaction of zinc acetate and ammonia at low temperature. Then, ZnO nanobelts consisting of nano-particles were obtained by the calcination of the precursor at 500 °C for 2 h.

2.4.3. Metal Alkoxide Complexes

Metal alkoxides (M(OR)n) are well-known and widely studied compounds. They can be formed by the reaction of various metal ions (e.g., Co, Zn, Fe, Mn, Ti, and Ni) and polyols (e.g., EG, DEG, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, and glycerol) under alkaline conditions.[205,207] For the precipitation of these alkoxides, the polyol medium is the primary consideration. Among these polyols, EG is the most effective ligand for the precipitation of metal alkoxide compounds with divalent metal ions, such as Co(II) and Mn(II).[208] The reaction mechanism by which metal alkoxides are produced from EG can be described as follows:[205]

HOC H OH 2OH OC H O 2H O2 4 2 42

2+ ↔ +− − (19)

M2

OC H O M(OC H O)2 42

2 42

xxx+ → ↓+ −

(20)

As shown by Reaction Equation (19) and (20), an alkaline environment is clearly vital for the precipitation of metal alkoxides. Alkaline conditions can be obtained by the hydrol-ysis of an anionic group, such as acetate or totally/partially

deprotonated alcohols produced by acid–base equilibrium reac-tions. For instance, in the presence of sodium acetate, which provides weak alkaline conditions, hollow cobalt alkoxide microspheres consisting of nanosheets were synthesized via the reaction of Co(II) and EG under solvothermal conditions. Finally, the precursor could be converted into porous products, conserving the desired structure.[209] Similarly, Mn2O3 hierar-chical microspheres assembled from porous nanosheets were successfully synthesized by a two-step method.[210] The first step was the formation of the Mn(C2H4O2)2 microsphere pre-cursor assembled with nanosheets, obtained by the reaction of manganese acetate and EG. The second step was the calcina-tion of the precursor. Hierarchically mesoporous CoMn2O4 microspheres assembled with porous nanosheets were fabri-cated by the reaction of Mn(CH3COO)2 and Co(CH3COO)2 in EG solution. The bimetal alkoxide powder was formed by the coordination of Co(II) and Mn(II) cations with EG, and subse-quent transformation to CoMn2O4 by thermal decomposition treatment.[176]

In addition to anion hydrolysis, some weak bases, such as organic amines/urea, can also be used to produce an alkaline environment. For example, Zhong et al.[211] prepared novel 3D flower-like iron oxide nanostructures by an EG-intermediary self-assembly process. Typically, FeCl3, urea, and tetrabu-tylammonium bromide (TBAB) were dissolved in EG. Then, the mixture was refluxed at 195 °C for 30 min. After natural cooling, a green iron alkoxide precipitate was collected as the precursor. Finally, α-Fe2O3, γ-Fe2O3, and Fe3O4 were synthe-sized without morphological alteration after sintering the pre-cursor. A 3D flower-like cerium precursor was also prepared by mixing cerium salt with a solution of urea and TBAB dis-solved in EG at 180 °C. Qiu et al.[122] used ZnCl2, CoCl2, and tert-butylamine as raw materials to generate zinc–cobalt hybrid alkoxides (HO or Cl)M-OCH2CH2O-M(Cl or OH) as hexagonal nanodisks, in a mixed solution of EG and water. After heat treatment, porous ZnCo2O4 hexagonal nanodisks consisting of nanoparticles were obtained.

As a reaction medium, EG cannot fully control all morpholo-gies, although it is undoubtedly an effective reagent for the syn-thesis of various structures. Polyols other than EG might play similar roles in the reaction medium and create metal alkox-ides with more varied structures. For instance, CuO hollow cubes, spheres, and urchin-like particles could be converted from Cu2O precursors. The Cu2O nanocubes were synthesized by dissolving copper(II) acetylacetonate in 1,5-pentanediol.[203] Additionally, porous Co3O4 was derived from a crystallized Co-alkoxide (Co(C3H6O2)) precursor obtained by the reaction of cobalt acetate and 1,2-propanediol at a constant temperature of 160 °C.[207]

2.4.4. Metal Glycolate Complexes

Metal glycolate complexes are also used as precursors to pre-pare hierarchically porous structures. EG as the reaction medium reacts with metal ions and subsequently oligomerizes. The mechanism by which metal glycolates are produced is dis-tinct from that by which metal alkoxides are generated, and can be simply described as by Reaction (21):[212,213]

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OH

OH

O

O OHOH

OO

MO

*O* n

OH OH

MX+

HOC2H4OH

at certain temperature

HOC2H4OH

HOC2H4OH

OC2H4OH

HOC2H4O

OC2H4OH

HOC2H4O

M

M M M

M

(21)

In this mechanism, the numbers of M–O covalent bonds and M→OH coordination bonds are controlled by the valence and coordination numbers of the metal ion.[212] Here, for the example of M2+ (e.g., Sn2+, Mn2+ and Pb2+), the coordination number is two.[213,214]

Based on the above reaction principle, Xia’s group[212] expected that EG might play a significant role in the wire-like morphology, because they discovered that EG could coordinate with Ti(IV), Sn(II), In(III), and Pb(II) cations to form chain-like coordination complexes (Figure 5) under reflux. When the chain-like complexes were long enough, they aggregated into bundles and then precipitated from the reaction medium as uniform nanowires. Ultimately, metal oxides were converted by further calcining the nanowire precursors in air at elevated temperatures.

EG also plays a key role in the establishment of hierarchi-cally porous structures. For instance, 3D hierarchical flower-like MgO spheres were successfully prepared through a solvothermal route, followed by an annealing process. The magnesium glycolate precursor was obtained in the presence of EG.[215] Vanadium glycolate with a microspherical struc-ture was readily obtained in EG medium.[216] A nanosheet-assembled hierarchically hollow V2O5 microsphere was suc-cessfully prepared by calcining the V-glycolate precursor generated by the reaction of VOC2O4 and EG.[217] Further-more, EG was found to completely replace the acetylacetonate ligand of vanadium(III) acetylacetonate to form vanadium glycolate.[218,219] Hierarchically porous V2O5 micro-/nanostruc-tures[219] were synthesized via the initial formation of a cor-responding precursor, followed by calcination. In this case, the proposed growth process can be described as a fast nucleation of primary particles, followed by aggregation and, finally, crys-tallization of the primary particles.

Additionally, the morphology of the metal glycolates was determined to be strongly related to the concentrations of metal cations, EG, and surfactants. For instance, Cao et al.[220] used a

wet chemical method to synthesize a series of Cu-glycolate pre-cursors, which were then converted to hierarchically nanostruc-tured CuO. By changing the initial Cu(CH3COO)2 concentration, they acquired various shaped Cu-glycolates, including micro-plate, doughnut-like, and multilayered structures. Qu et al.[221] prepared hierarchically porous anatase TiO2 nanopillars via the thermal treatment of a Ti-glycolate precursor. They also found that the lengths of 1D Ti-glycolate nanopillars were strongly dependent on the volume ratio of EG/tetrabutyl titanate. Indeed, when the volume ratio was changed from 20:1 to 40:1, the pre-cursor length increased from approximately 5 µm to approxi-mately 20 µm. When the ratio was increased further to 60:1, the length of the precursors reached approximately 50 µm.

PVP has been shown to be crucial for the formation of uni-form hollow microspheres[222,223] because of its strong coordi-nation with metal ions through functional groups (–N and/or CO) in EG solution. For example, Lou’s group[224] prepared ball-in-ball hollow ZnMn2O4 microspheres via a two-step method. In the first step, the hollow-structured ZnMn-glycolate precursor was obtained by the reaction of Zn(CH3COO)2 and Mn(CH3COO)2 in EG/PVP solution. Then, the metal glycolate hollow spheres were calcined in air at 500 °C for 4 h, to gen-erate the unique ZnMn2O4 ball-in-ball hollow microspheres.

Generally, hierarchically porous structures can be obtained from the simple calcination of metal glycolate precursors in air, accompanied by the release of CO2 and H2O. Unfortunately, the structures of some precursors may collapse, agglomerate, and transform into other morphologies or generate new crystal phases,[206,217] especially at high calcination temperatures (≥700 °C). To obtain high-crystallinity anatase TiO2, Qu et al.[221] developed a post-treatment method in which ethylenediamine (EN) is used to prevent the growth and accumulation of TiO2 nanoparticles, which leads to pore collapse. This method can be divided into three steps: First, the titanium glycolate nan-opillar precursor is formed and calcined at 300 °C for 2 h to achieve stable amorphous TiO2 nanopillars. Second, the stable amorphous TiO2 nanopillars are refluxed in aqueous EN solu-tion at pH 10–11. Third, the products are calcined at 700 °C to form hierarchically porous anatase TiO2 nanopillars. According to Qu’s experiments, without EN post-treatment, the sample consisted of large TiO2 particles with few pores after calcination at 700 °C. In contrast, after EN treatment, the TiO2 nanopillars consisted of many tiny compact TiO2 nanoparticles (approxi-mately 10 nm) with porous structures. These results implied that EN treatment restrained the growth and aggregation of

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Figure 5. TEM images of SnO2 nanowires (A) obtained by calcining Sn-glycolate, TEM images of In2O3 nanowires (B) generated by calcining In-glycolate, and TEM images of PbO nanowires (C) obtained by calcining Pb-glycolate. Reproduced with permission.[212] Copyright 2004, The American Chemical Society.



TiO2 nanoparticles and the phase transformation from anatase to rutile, thereby preserving the porous structures.

Under certain other conditions, EG could serve as the carbon precursor in the pyrolytic process. For example, a ZnO/carbon composite[225] was converted by annealing a zinc glycol ate precursor in an Ar atmosphere, whereas mesoporous ZnO nanotubes were obtained in air. Here, the initial zinc glycolate precursor fibers were co-assembled by mixing Zn(CH3COO)2 with EG at 150 °C, and the zinc ion and zinc glycolate separately acted as the structural constructor and the build-in template, respectively, during the subsequent carboni-zation process.

2.4.5. PB Coordination Complexes

PB coordination complexes (Fe4[Fe(CN)6]3·xH2O), in which iron ions are bridged by CN groups (–(Fe–CN–Fe)–), are consid-ered as potential precursors for nanoporous iron oxides.[226,227] For instance, crystalline α-Fe2O3 and γ-Fe2O3 with different hollow cavities could be selectively synthesized by calcining various PB precursors.[226] Additionally, Fe2O3 microboxes with various shell structures have been prepared by controlling the annealing temperature of PB microcubes.[228]

PB analogs (PBAs) with the chemical formula M3II[MIII

(CN)6]2·nH2O (M = transition metals) are constructed from octahedral MIII(CN)6

3− complexes bridged into a simple cubic lattice by M2+ ions.[229,230] PBAs, as a class of crystalline MOFs, have been recently fabricated into various morphologies and diverse compositions with uniform sizes.[231] For example, Co3O4 microframes were synthesized by etching Co–Co PBA(Co3[Co(CN)6]2·xH2O) microcubes in ammonia solu-tion, followed by annealing treatment.[232] Additionally, hollow porous CoFe2O4 nanocubes were successively obtained after roasting the Co[Fe(CN)6]0.667 precursor.[183]

2.4.6. Metal Imidazolyl/Pyridyl Complexes

Zeolitic imidazolate frameworks (ZIFs) have attracted substan-tial attention regarding the preparation of porous structures because of their potential technological impact and combined characteristics of pore size regimes.[233] The bonding of imid-azole with metal ions can form some of the most stable com-plexes via heterocyclic-N ligands.

ZIF-67 is one of the most representative ZIFs.[234,235] The original Co/Zn-ZIF-67 precursors were typically produced in a mixed solution of cobalt and zinc sources, 2-methylimid-azole, and various solvents. For example, using Ni foam as the substrate, porous nanosheet-structured ZnO and Co3O4 were obtained from the reaction of cobalt nitrate and zinc nitrate with 2-methylimidazole in deionized water.[174] Porous Co3O4 concave nanocubes with extremely high specific surface areas were fabricated by annealing Co-based ZIF-67.[235] Li et al.[194] successfully prepared Co0.4Zn0.19S @N and S co-doped carbon dodecahedra derived from Co/Zn-ZIF-67. The synthetic route involved three steps: First, Co/Zn-ZIF-67 was designed as the precursor. Then, the intermediate product, Co/Co3ZnC dodeca-hedra with an N-doped carbon overlayer was obtained by

calcining the precursor. Finally, the product, Co-Zn-S@N and S co-doped carbon dodecahedra, was formed through a sulfuriza-tion process.

Co-MOFs can also be constructed by the coordination of the Co(II) ion with an N-rich pyrimidine ligand. For instance, porous Co3O4/N-C particles with a fish-scale structure were successfully synthesized by sintering a well-designed Co-MOF at 500 °C in a nitrogen atmosphere, where the Co-MOF was constructed by the coordination of the Co(II) ion with an N-rich bidentate ligand (4,6-di(1H-imidazol-1-yl)pyrimidine) and 5-nitroisophthalic acid.[236]

To summarize, scaling the design of MOFs from the molec-ular level to the macroscopic scale is innovative and valuable. To create well-defined materials, the rigidity, aspect ratio, and geometric features of the organic blocks and the coordination of the metal ions must be controlled. The mechanism by which such hierarchical structures are formed may involve the aggre-gation of building blocks into complicated 3D structures and their subsequent reaction, dissolution, and re-deposition in hydro/solvothermal reactions.

2.5. Other Precursors

In addition to the precursors mentioned above, some metal inorganic salts, such as metal hydroxyoxides, hydrotalcite-like salts, and nitrates, are occasionally used as solid precursors to synthesize their nanostructured metal oxides after chemical/thermal conversion.[237] For instance, Li et al.[238] used a simple ionic liquid-assisted hydrothermal method to fabricate a hier-archical α-GaOOH precursor, and then obtained hierarchically porous α-Ga2O3 structures. Song et al.[239] synthesized porous α-Fe2O3 microspheres by calcining bundle-like β-FeOOH precursors at 700 °C. Ultrathin transition-metal hydroxides, including δ-FeOOH, α-Co(OH)2, and α-Ni(OH)2 nanosheets, were synthesized by mixing metal salt with freshly prepared NaBH4 solution in the presence of CTAB, followed by conver-sion to the corresponding metal oxides.[240] As reported by Chen et al.,[89] hollow Co3O4 spheres were attained via annealing the CoOOH precursor. Additionally, the hollow Cu2(OH)3NO3 microspheres fabricated by Zhang’s group[241] were reduced by glutamic acid to obtain the final porous multishell Cu2O struc-ture. Flower-like ZnO nanosheets were converted from a sim-ilar morphology of Zn5(OH)8(NO3)2·2H2O by a simple thermal treatment. The Zn5(OH)8(NO3)2·2H2O precursor was obtained by an electrodeposition method.[171] Porous spinel Co3O4 was converted at 150–600 °C from two monometal hydrotalcite-like compounds (CoII

0.74CoIII0.26(OH)2.01(NO3)0.21(CO3)0.02·0.6H2O

and CoII0.74CoIII

0.26(OH)1.99(CO3)0.13(NO3)0.01·0.7H2O) that were prepared via combining precipitation and anion-exchange methods.[242] Hierarchical star-like Co3O4 was successfully derived from a Co(OH)F precursor via a convenient hydro-thermal method and subsequent annealing treatment in air.

Another interesting case is the morphology-conserving trans-formation of porous micro-/nanostructures from the amor-phous phase to the crystalline phase.[68,243] Representatively, the two-step synthesis of mesoporous TiO2 microspheres consists of precursor preparation and final hydrothermal fixing.[68,244,245] Chen et al.[68] proposed a self-assembly mechanism for micro-sphere formation. By adjusting the long-chain alkylamines, they

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prepared microspheres with different porosities, suggesting that the long-chain alkylamine acted as the structural directing agent in mediating the microsphere formation. A similar syn-thesis of mesoporous TiO2 microspheres was also reported.[246] The proposed mechanism for the formation of the porosity and crystallization of the microspheres was the elimination of –OCH2CH3 (–OEt) groups and long-chain alkylamines in the subsequent hydrothermal Oswald ripening process.

In summary, this approach is useful and effective for applying various precursors in the fabrication of hierarchically porous structures. The mechanisms by which different types of products were obtained were systematically studied, and the overall formation process was determined to involve sev-eral main steps: the nucleation, regrowth, and self-assembly of nanocrystals into hierarchical structures, and the final con-version to a porous structure via thermal treatment or other methods. In the preparation process, the influencing factors, such as the concentration of OH−, alkaline substance, selective ligands, the time consumed by the reaction, the ratio of various solvents, and the additive surfactants, should be considered in terms of the synthesis.[113,240,247–250] To obtain the final hierar-chical porous structures, calcining the precursors in air or inert atmosphere at a certain temperature and for a certain time is necessary. To prevent the products from cracking, shrinking, and collapsing during calcination, the calcination temperature and time, heating/cooling rate, and ambient atmosphere must also be carefully considered.

3. Applications of Hierarchically Porous Micro-/Nanostructures

Hierarchically porous micro-/nanostructure materials have been recognized as the most promising materials for applica-tions in the electrochemical energy conversion and storage fields.[251–254] In this section, we focus on the recent progress involving metal-based hierarchically porous micro-/nanostruc-tures applied in LIBs, Li–S batteries, Li–O2 batteries, superca-pacitors, and DSSCs.

3.1. LIBs

Over the past decades, LIBs have been most commonly used in portable electronic devices, such as smart phones and laptop computers, because of their high energy density, wide tem-perature window, and lack of a memory effect.[60,63,65] How-ever, increasing the energy/power density, cycling stability, and safety of selective electrode materials remains a challenge that must be overcome in the development of LIBs for next-genera-tion electric vehicles.[255]

Fortunately, hierarchically porous micro-/nanostructure materials can enhance the electrochemical performance (e.g., rate capability and cycling stability) of LIBs.[12,58,62,251,252] The advantages of these materials can be explained as follows. First, the transport of both electrons and lithium ions is greatly affected by the sizes of the nanomaterials. The ion diffusion in solid-state electrode materials is directly related to the transport path, as expressed by Equation (22):[88]

L /D2τ = (22)

where L, D, and τ represent the diffusion length, ion-diffusion coefficient, and time consumption of the diffusion process, respectively. Therefore, the diffusion length of the lithium ion in nanocrystalline grains within hierarchically porous structured electrode materials can be reduced, resulting in improved rate capabilities. Second, hierarchically porous micro-/nanostruc-tured electrode materials possess relatively high specific surface areas, which could increase the contact area between the elec-trode and the electrolyte, and thereby enhance utilization of the active materials and increase the gravimetric capacity. Third, the unique structure could offer enough space to accommodate the volume change during the charge/discharge process, giving rise to better cyclic stability. The applications of these special structure materials in LIBs generally fall into two broad catego-ries: anode materials and cathode materials.

3.1.1. Anode Materials

Transition-metal-based oxides, such as monometal oxides (e.g., cobalt oxide, nickel oxide, iron oxide, and manganese oxide) and mixed-metal oxides (AxB3−xO4; A, B = Co, Ni, Zn, Mn, Fe, etc.), have been employed as anode materials because of their relatively high theoretical capacities (e.g., 890 mA h g−1 for Co3O4,[256] 718 mA h g−1 for NiO,[257] 1007 mA h g−1 for Fe2O3,[258] and 1019 mA h g−1 for Mn2O3

[259]) compared to com-mercialized graphite (≈372 mA h g−1). These high capacities are attributable to the conversion reaction between lithium and metal oxides in the electrolyte, as follows (Equation (23)):[1,88]

M O 2 Li 2 e M 2 Li O2y y x yx y + + → ++ − (23)

Despite the high capacity, the practical use of metal oxides as anode materials remains limited by the high irreversible capacity losses that occur during the first charge/discharge cycle and deterioration of the active materials during long cycling.[65,84] The former problem is often ascribed to the innate characteristics of most anode materials, whereas the latter results from large volume changes (e.g., ≈100% volume expan-sion for Co3O4

[207] and Fe3O4[260]) during the lithium insertion/

extraction process.[65]

Cobalt oxides with hierarchically porous structures can be easily formed via a morphology-conserved transforma-tion method,[65,261] and exhibit improved lithium-ion storage performances. There are two main types of cobalt oxides: CoO and Co3O4. Their theoretical capacities are 715 and 890 mA h g−1, respectively. Voltage plateaus for the CoO elec-trodes are observed at 0.7 and 2.1 V during the initial discharge and charge, respectively.[261] In the case of the Co3O4 elec-trodes, the voltage plateaus occur at ≈0.8–1.3 V during initial discharge, and 2.2 V during the charge process.[256] Substan-tial attention has been focused on Co3O4 materials because of their higher capacities and easy preparation. For instance, hierarchically hollow Co3O4 nanoneedles originating from β-Co(OH)2 nanoneedles were well designed to improve the electrochemical performance of LIBs.[117] The results of elec-trochemical testing revealed that the first discharge capacity of

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the as-synthesized sample was approximately 1290 mA h g−1 at 150 mA g−1 between 3 V and 10 mV and that a reversible capacity of 1079 mA h g−1 was maintained after 50 cycles. This good electrochemical performance was attributed to the short transport distance of the lithium ions and the enhanced inter-connection among individual nanoparticles. A porous Co3O4 nanobelt array fabricated from a uniform and well-aligned Co(CO3)0.5(OH)0.11H2O nanobelt array precursor was found to exhibit good electrochemical performance.[169] Charge/dis-charge testing revealed that the Co3O4 nanobelt array retained a specific capacity of 770 mA h g−1 over 25 cycles at 177 mA g−1. Even at high current densities of 1670 and 3350 mA g−1, spe-cific capacities of 510 and 330 mA h g−1, respectively, were achieved after 30 cycles. Single- (S-Co), double- (D-Co), and triple-shelled (T-Co) hollow spheres (Figure 6) assembled from Co3O4 nanosheets were successfully synthesized by calcining the cobalt glycolate precursor.[262] Based on initial charge/dis-charge testing, the discharge capacities of S-Co, D-Co, and T-Co were approximately 1199.3, 1013.1, and 1528.9 mA h g−1, respectively. According to the cycling performance testing results, S-Co, D-Co, and T-Co maintained capacities as high as

680, 866, and 611 mA h g−1, respectively, after 50 cycles at a rate of C/5; all of these values were better than that of a commer-cial sample (C-Co). Furthermore, even at a high current rate of 2 C, D-Co was able to deliver a capacity of 500.8 mA h g−1, indicating that D-Co has good rate capability.

Manganese-based oxides have also been extensively researched because of their low cost, environmental friendli-ness, high capacity, and low reaction voltage (0.2–0.5 V during initial discharge).[59,65] A variety of morphologies and crystallo-graphic structures have been obtained from the conversion of different types of precursors. The crystallographic structures of manganese oxide mainly include cubic rock salt (MnO, 756 mA h g−1),[263] inverse spinel (Mn3O4, 937 mA h g−1),[264] hexagonal corundum (Mn2O3, 1019 mA h g−1),[259] and a man-ganese dioxide structure (MnO2, 1223 mA h g−1).[263,265,266] In these reports, manganese-based oxides with hierarchically porous structures were found to exhibit enhanced lithium storage capacities. Mn2O3, which is a well-known functional transition-metal oxide with structural flexibility, has attracted extensive attention because of its distinctive physico-chemical properties.[142] For example, porous Mn2O3 nanomaterials were

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Figure 6. SEM (A,C,E) and TEM (B,D,F) images of the three samples: A,B) S-Co, C,D) D-Co, and E,F) T-Co. G) The first cycle discharge/charge curves of S-Co, D-Co, and T-Co. H) Cycling performance of the three as-prepared samples and commercial Co3O4 product (C-Co) at a current rate of C/5 (178 mA g−1). I) Charge/discharge curves of D-Co at different current densities. Reproduced with permission.[262] Copyright 2010, Wiley.



fabricated by Qian’s group[104] via the conversion of a simple Mn(OH)2 precursor. The as-prepared Mn2O3 exhibited a high and stable reversible capacity. Specifically, the porous Mn2O3 nanoflowers could maintain a capacity of ≈521 mA h g−1 after 100 cycles at a current density of 300 mA g−1, indicating that the porous structure was crucial for the enhanced elec-trochemical performance of Mn2O3. Triple-shelled Mn2O3 hollow nanocubes[149] derived from MnCO3 also exhibited excellent electrochemical performance. When evaluated as an anode material for LIBs, this material could deliver revers-ible capacities of 606 and 350 mA h g−1 at current densities of 500 and 2000 mA g−1, respectively.

Iron oxides have also been investigated intensively as promi sing anode materials for LIBs because of their high theoretical capacity, low cost, environmental friendliness, and high resistance to corrosion. Two typical iron oxide phases, hematite (α-Fe2O3, 1007 mA h g−1)[76,190,228,258,267,268] and mag-netite (Fe3O4, 924 mA h g−1),[255] could be obtained via the morphology-conserved transformation method. The discharge plateau for iron oxides lies at approximately 0.8 V.[258,260] Fe2O3 nanostructures with various features have been evaluated to improve the resulting electrochemical properties.[269] For instance, Fe2O3 microboxes with a well-defined hollow struc-ture and hierarchical shell originating from a PB precursor were found to display a high specific capacity and excellent cycling performance, with the highest reversible capacity of 945 mA h g−1 in the 30th cycle at 200 mA g−1.[228] As described above, spindle-like porous α-Fe2O3 has shown enormously enhanced lithium storage performance.[189] The capacity of the porous α-Fe2O3 was 911 mA h g−1 after 50 cycles at a rate of 0.2 C. Even when cycled at a high rate (10 C), a reversible capacity of 424 mA h g−1 could be obtained.

Copper oxides Cu2O (375 mA h g−1) and CuO (674 mA h g−1) have also been utilized as anode materials because they are non-toxic and naturally abundant, and can be prepared by a simple method.[65,190,203,270] For example, the as-prepared bundle-like CuO mentioned above exhibited excellent electrochemical per-formance, with a high rate capability.[119] The initial discharge capacity of CuO was 1179 mA h g−1 at a rate of 0.3 C, and a capacity of 666 mA h g−1 was retained after 50 cycles. Even at a high rate (6 C), a capacity of 361 mA h g−1 was maintained.

Binary transition-metal oxides have also been widely studied as anode materials for LIBs because of their higher theoret-ical specific capacity, superior rate performance, and better cycling stability compared to mono-transition-metal oxides.[204] Obtaining these mixed metal oxides via the thermal treatment of diverse precursors is simple. The electrical/ionic conduc-tivity of Ni–Co-based oxides was greatly improved, leading to enhanced electrochemical properties, especially the rate capability.[204,271] For instance, mesoporous Ni0.37Co oxide nanoprisms with a yolk–shell structure derived from Ni–Co-based hydroxide acetate could deliver an initial discharge capacity of 1394.4 mA h g−1 and maintain a reversible capacity of ≈1028.5 mA h g−1, after 30 cycles at a current density of 200 mA g−1. Among these Ni–Co-based oxides, NiCo2O4, with a spinel structure (i.e., in which nickel occupies the octahedral sites, and cobalt is distributed over both octahedral and tetra-hedral sites), has attracted the most attention because of its significantly enhanced electrochemical performance.[272,273] For

instance, mesoporous NiCo2O4 microspheres[274] originating from Ni0.33Co0.67CO3 were shown to retain a reversible capacity of 1198 mA h g−1 after 30 cycles, at a current density of 200 mA g−1. Even at a current density of 800 mA g−1, a capacity of 705 mA h g−1 was maintained after 500 cycles. Other binary metal oxides, such as CoMn2O4, have higher electronic conduc-tivities than MnOx.[176] Double-shelled hollow CoMn2O4 micro-cubes derived from Co0.33Mn0.67CO3 cubes[130] were observed to have a discharge capacity of ≈830 mA h g−1 at a current density of 200 mA g−1, and maintained a capacity of 624 mA h g−1 after 50 cycles. Foam-like porous spinel MnxCo3−xO4 was obtained from Mn3[Co(CN)6]2·nH2O nanocubes.[229] Electrochem-ical testing revealed that the initial discharge capacity was 1395 mA h g−1 at a current density of 200 mAg−1, and that a high charge capacity of 733 mA h g−1 was retained after 30 cycles.

Anode materials with lower charge voltages could deliver higher energy densities.[275,276] ZnMn2O4 has attracted exten-sive attention because its operating voltage is much lower than those of Co- or Fe-based oxides, and because it is low cost and environmentally friendly.[150,224,277] For example, ZnMn2O4 ball-in-ball hollow microspheres obtained from ZnMn-glycol ate (Figure 7)[224] were found to deliver an initial discharge/charge capacity of ≈945/662 mA h g−1 at a current density of 400 mA g−1. During the cycling performance testing, the discharge capacity gradually decreased to 490 mA h g−1 after approximately 50 cycles, maintained a steady state for dozens of cycles and then began to increase, becoming as high as 750 mA h g−1 after 120 cycles. Correspondingly, the Cou-lombic efficiency was approximately 70% for the first cycle, quickly increased to 98% after several cycles, and then stabi-lized at nearly 100% for the remaining cycles. After cycling for 120 cycles at a current density of 400 mA g−1, the same cell was subjected to rate capability testing. This cell was able to deliver specific capacities of 683, 618, 480, and 396 mA h g−1 at current densities of 600, 800, 1000, and 1200 mA g−1, respectively. The enhanced electrochemical performance of the ZnMn2O4 ball-in-ball hollow microspheres is attributable to the following: The small average size of the primary nanoparticles could reduce the diffusion distance of Li+, and the void space in the porous structure could serve as an electrolyte reservoir, leading to good rate capability. More importantly, the unique ball-in-ball hollow morphology of ZnMn2O4 could significantly improve the struc-tural integrity, by alleviating the mechanical strain induced by volume expansion during repeating cycling to some extent, resulting in excellent cycling stability.

Metal vanadates can also be used as anode materials. Indeed, the design and synthesis of metal vanadates with various morphologies and structures have attracted tremen-dous interest.[278,279] For instance, porous Co2V2O7 hexag-onal nanoplatelets were obtained via thermal treatment of a Co2V2O7·nH2O precursor,[279] and exhibited good cycling stability and rate capability. Specifically, porous Co2V2O7 maintained a reversible capacity of 866 mA h g−1 with almost 100% capacity retention after 150 cycles at a current density of 0.5 A g−1, and delivered specific capacities of 813 mA h g−1, 666 mA h g−1, 594 mA h g−1, 518 mA h g−1, and 344 mA h g−1 at 0.2 A g−1, 0.5 A g−1, 1 A g−1, 2 A g−1, and 5 A g−1, respectively.

Enhanced lithium storage performance has also been reported for many other metal oxides, such as NiO,[156,280] MnCo2O4,[281]

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Mn1.5Co1.5O4,[282] CoFe2O4,[183] ZnCo2O4,[46,48,122,168,192] and CaSnO3,[28,29] which can also be produced by the morphology-conserved transformation method.

3.1.2. Cathode Materials

The currently available cathode materials include layered LiCoO2 (≈140 mA h g−1),[283] spinel LiMn2O4 (≈120 mA h g−1),[76] and olivine LiFePO4 (≈170 mA h g−1).[284] To meet the demands for higher energy and power densities in LIBs, new structured cathode materials with higher specific capacities and high voltages must be developed. Among the metal-based cathode materials reported to date, lithium-rich layered oxide materials (i.e., xLi2MnO3·(1−x)LiMO2 [M = Mn, Ni, Co, Fe, Cr, etc.]) have attracted much attention because of their high capacities (i.e., > 250 mA h g−1) and high operation voltages (i.e., > 4.6 V at room temperature).[127,181,250] For instance, Wu’s group[250] produced hierarchical nanostructured Li1.2Ni0.2Mn0.6O2 with exposed (010) planes (HSLR), which was converted from a quasi-spherical Ni0.2Mn0.6(OH)1.6 precursor (Figure 8). High specific discharge capacities of 230.8, 216.5, 188.2, 163.2, and 141.7 mA h g−1 were obtained at rates of 1 C, 2 C, 5 C, 10 C, and 20 C at 2–4.8 V, respectively. This outstanding high rate capability could be ascribed to two key factors: the unique hier-archical structure, including the special directional alignment of the nanoplates that facilitated the rapid diffusion of Li+, and the hierarchical structure that provided efficient 3D electron transport networks.

LiNi1/3Co1/3Mn1/3O2 is another typical layered cathode mate-rial in which the valences of the nickel, cobalt, and manganese

ions are +2, +3, and +4, respectively. This material exhibits an enhanced electrochemical performance relative to LiCoO2, LiNiO2, and LiMnO2.[133,285] LiNi1/3Co1/3Mn1/3O2 is typically obtained through first synthesizing metal carbonate precur-sors and then reacting them with lithium sources at high tem-perature. Hierarchically porous structured LiNi1/3Co1/3Mn1/3O2 can achieve remarkable performances in terms of high rate capabilities and long cycling stabilities. For example, LiNi1/3Co1/3Mn1/3O2 hollow microspheres exhibited a high dis-charge capacity of 157.3 mA h g−1 at 0.2 C after 100 cycles, and 120.5 mA h g−1 at 0.5 C after 200 cycles. Even at a high rate (5 C), a high capacity of 114.2 mA h g−1 was maintained.[133]

Spinel LiMn2O4 and LiNi0.5Mn1.5O4 possess 3D lithium-ion channels, and thus exhibit good rate capabilities. For instance, porous LiMn2O4 spheres exhibited stable cycling ability and high rate capability.[131] The capacity retention was 94% after 100 cycles at 1 C. Additionally, the discharge capacity remained at ≈83 mA h g−1 at 20 C. The spinel LiMn2O4 nanorods synthesized by Cui’s group delivered high charge storage capacities at high power rates. Cycling stability tests revealed that capacity reten-tion of more than 85% was maintained for over 100 cycles.[145] Porous cubic LiMn2O4 (Figure 9) derived from a MnCO3 pre-cursor exhibited an excellent high rate capability and a long-term cycle life. At a high rate of 30 C, this material could still deliver a reversible capacity of 108 mA h g−1 and, more impor-tantly, its capacity retention was 80% at 10 C after 4000 cycles.[76] Additionally, LiNi0.5Mn1.5O4 hollow microspheres/microcubes consisting of nanosized particles showed high capacity, excel-lent cycling stability, and exceptional rate capability.[135] These structures could deliver a discharge capacity of approximately 120 mA h g−1 at 0.1 C. Even at 20 C, the retained capacity was still

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Figure 7. Typical field-emission SEM (FESEM) (A,C) and TEM (B,D) images of the ZnMn–glycolate precursor (A,B) and ZnMn2O4 ball-in-ball hollow microspheres (C,D). The first and second charge/discharge profiles (E) obtained at a current density of 400 mA g−1. Cycling performance and the corresponding Coulombic efficiency (F) at 400 mA g−1. Rate capabilities (G) at various current densities. Reproduced with permission.[224] Copyright 2014, Wiley-VCH.



104 mA h g−1. Furthermore, the capacity retention could reach 96.6% at 2 C after 200 cycles. LiNi0.5Mn1.5O4 porous nanorods with an ordered P4332 phase derived from a MnC2O4 precursor could deliver reversible capacities of 140 and 109 mA h g−1 at rates of 1 and 20 C, respectively.[286] Impressively, a capacity retention of 91% was maintained after 500 cycles at 5 C. This remarkable performance was attributed to the porous 1D nano-structures, which could accommodate strain relaxation during cycling and provide short lithium-ion-diffusion distances along the confined dimension.

Other cathode materials, such as V2O5,[179,219,257] derived from various precursors have also been investigated. For instance, Lou et al.[179] reported 3D porous V2O5 hierarchical microspheres with good cycling ability and excellent rate capability. Indeed, a stable capacity of 130 mA h g−1 at 0.5 C was maintained after 100 cycles. Even at 30 C, a capacity of 105 mA h g−1 was achieved.

Notably, several suitable strategies, including coating with carbonaceous materials[255,260] and TiO2,[259] doping a spinel phase on the surface of porous materials,[287] or using functional additives,[288–293] are often applied to generate hierarchically porous micro-/nanomaterials with improved electrochemical performance. For example, Wang et al.[294] synthesized carbon-coated α-Fe2O3 hollow nanohorns onto carbon nanotube (CNT) backbones to enhance electron transport and prevent agglomer-ation. This unique hybrid structure exhibited a stable capacity of 800 mA h g−1 over 100 cycles at a current density of 500 mA g−1. Even at high current densities of 1000–3000 mA g−1, capaci-ties of 420–500 mA h g−1 were maintained. Cubic Mn2O3@TiO2 established on Mn2O3 porous nanocubes displayed supe-rior charge/discharge performance.[259] A reversible capacity of 263 mA h g−1 at 6000 mA g−1 was achieved for Mn2O3@TiO2, whereas unmodified Mn2O3 exhibited a value of only 9.7 mA h g−1. Moreover, using additives in electrolytes is one

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Figure 8. FESEM images (A,B) and TEM images (C,D) of HSLR. HRTEM image (E) of the region marked in (D). The inset is a magnified image of the frame shown in (E). F) Corresponding SAED pattern of D. G) Charge/discharge curves of HSLR at various rates. H) Cycling performance of HSLR at different rates. I) Cycling performance of HSLR materials at C/10; the inset shows the corresponding voltage profile and dQ/dV plots. Reproduced with permission.[250] Copyright 2014, Wiley-VCH.



of the most effective approaches to enhance the electrochemical performance of electrode materials because of the in situ for-mation of a uniform interphase film that isolates direct contact between the electrolyte and electrode and prevents metal ion dissolution.[288–293] Therefore, a hybrid approach must be devel-oped to further improve the electrochemical performances of these hierarchically porous micro-/nanostructure materials.

Table 1 summarizes the electrochemical performance of hierarchically porous structures with applications in LIBs that were produced from the various precursors mentioned above.

3.2. Li–S Batteries

Li–S batteries have been considered as promising candidates for next-generation electrochemical energy-storage devices because of their overwhelming advantages in energy density (theo-retical capacity of 1672 mA h g−1; theoretical energy density of 2500 W h kg−1).[298–306] In addition, sulfur is abundant and environmentally benign. During discharge, Li+ ions originate from the lithium-metal anode and move to the sulfur cathode through the electrolyte, forming Li2S as the final discharge product at the cathode; during charge, Li+ ions are extracted from Li2S and move back to the lithium-metal anode.[306–310] This reaction is reversible. Unlike the insertion-compound cathodes in LIBs, sulfur undergoes more complicated compositional and structural changes in this reversible reaction, resulting in soluble polysulfide intermediates. Unfortunately,

the dissolved polysulfide species shuttle between the cathode and anode, which could cause loss of the active material and result in low Coulombic efficiency.[311]

Various strategies have been adopted to strongly suppress the shuttle effect and mainly include: i) constructing core–shell structured materials[234] and ii) forming metal oxides or chal-cogenide/carbon/S composites.[312,313] The former is compli-cated and unsuitable for large-scale production. In contrast, the latter is simple. Metal oxides or chalcogenides can form strong bonds with polysulfides, and thereby substantially improve the Coulombic efficiency and cycling stability. Hierarchi-cally porous structures with unique architectures have shown great potential for entrapping polysulfides. A typical case was reported by Nazar’s group.[312] They used hierarchically hollow V2O5 spheres, synthesized by the conversion of vanadyl oxa-late precursor, to entrap polysulfides. A discharge capacity of approximately 1400 mA h g−1 was obtained in the first cycle at C/50 between 1.8 and 2.5 V. Additionally, a stable specific capacity of 1000 mA h g−1 was obtained at C/5, and a capacity of 820 mA h g−1 was obtained after 300 cycles.

3.3. Li–O2 Batteries

Li–O2 batteries can provide the highest theoretical energy density (3500 W h kg−1) among energy-storage devices.[314–318] However, challenges relating to the application of Li–O2 bat-teries are more severe than for Li–S batteries. The poor oxygen

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Figure 9. SEM image (A) and TEM image (B) of porous LiMn2O4. Rate performance of the as-prepared porous LiMn2O4 (C). Deep cycle performance at a rate of 10 C for porous LiMn2O4 (D). Reproduced with permission.[76] Copyright 2014, Royal Society of Chemistry.



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Table 1. Electrochemical performances of various hierarchically porous metal oxides.

Precursors Product composition and morphology

Precursor preparation methods

Conversion methods

Electrochemical performance

Current density [mA g−1]

Cycle number

Reversible capacity

[mA h g−1]

Metal hydroxides Urchin- and flower-like hierarchical NiO

microspheres[121]

Solvothermal Calcine in air at 500 °C for 2 h 0.2 C 50 163

Single-crystal Co3O4 nanoneedles[249] Refluxing Anneal at 300 °C 150 50 1079

Nanocrystalline-assembled bundle-like CuO[119] Solution route Heat at 400 °C for 6 h 201 50 666

Porous Mn2O3 with flower-like structure[104] Hydrothermal Heat to 600 °C with a heating

rate of 10 °C min−1

300 100 521

Porous NixCo3−xO4 nanosheets[271] Hydrothermal Calcine at 450 °C for 2 h with a

temperature ramp of 1 °C min−1 in air

100 50 1330

Flower-like CaSnO3[295] Hydrothermal Anneal at 1000 °C 60 50 547

Sheet-like Co3O4[105] Refluxing Anneal at 300 °C in air for 2 h 200 100 1200

Hierarchical Li1.2Ni0.2Mn0.6O2 nanoplates with

exposed (010) planes[250]

Co-precipitation Mix with Li2CO3 and calcine

at 900 °C for 12 h

250 40 219.3

0.5Li2MnO3·0.5LiNi0.5Mn0.5O2 (10-µm-sized

secondary particles consisting of sub-microm-

eter-scale, flake-shaped primary particles)[296]

Co-precipitation Mix with LiOH and calcine

at 900 °C for 15 h

0.2 C 130 233

Metal carbonates Anisotropic column-shaped porous Co3O4

nanocapsules[147]

Solvothermal Heat at 400–600 °C for 2 h in air 130 22 ≈1000

Hierarchical mesoporous nanostructures

of Mn2O3[142]

Hydrothermal Heat in air at 600 °C for 2 h 200 150 ≈380

Mn2O3 microspheres[152] Hydrothermal Calcine in air at 500 °C for 3 h with a

ramping rate of 2 °C min−1

200 200 524

Triple-shelled Mn2O3 hollow nanocubes[149] Precipitation Sinter at 300 °C for 1 h with a ramping rate

of 1 °C min−1 and then at 600 °C for 1 h

with a ramping rate of 2 °C min−1 in air

500 100 533

Double-shelled CoMn2O4 hollow microcubes[130] Precipitation Anneal at 600 °C with a ramping rate

of 2 °C min−1

200 50 624

Mesoporous single-crystalline NiCo2O4

nanoribbons[267]

Hydrothermal Calcine at 300 °C for 5 h with a ramping


200 60 1198

MnCo2O4 spinel quasi-hollow spheres[274] Hydrothermal Heat to 600 °C with a ramping rate

of 4 °C min−1 for 10 h

200 25 755

ZnMn2O4 hollow microspheres[150] Precipitation Calcine at 600 °C 400 100 607

Mn1.5Co1.5O4 core–shell microspheres[275] Hydrothermal Heat at 600 °C with a ramping rate

of 2 °C min−1 for 10 h

400 300 618

Porous cubic LiMn2O4[76] Precipitation Mix with CH3COOLi and sinter

at 700 °C for 10 h.

30 C 4000 108

LiNi0.5Mn1.5O4 hollow microcubes[135] Precipitation Mix with LiOH and Ni(NO3), then

calcine at 800 °C for 20 h in air

2 C 200 121

Porous LiMn2O4 spheres[131] Precipitation Mix with LiOH and then sinter

at 750 °C for 10 h

1 C 100 117

Uniform LiNi1/3Co1/3Mn1/3O2 hollow

microspheres[133]

Precipitation Mix with LiOH/LiNO3 and heat

at 900 °C for 10 h

0.2 C 100 157.3

Metal hydroxide

carbonates

Nickel–cobalt-oxide mesoporous

thorn microspheres[154]

Hydrothermal Anneal at 400 °C for 2 h 900 50 545.5

Co3O4 nanobelt[169] Hydrothermal Heat at 350 °C for 4 h with a heating

ramp of 1 °C min−1

1.5 C 25 788.7

Mesoporous Co3O4[166] Hydrothermal Heat at 300 °C in air with a heating


100 30 1240

Cubic CoO nanonets rich in pores[158] Hydrothermal Heat at 500 °C in Ar atmosphere with a

ramping rate of 5 °C min−1

1000 100 637



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Precursors Product composition and morphology

Precursor preparation methods

Conversion methods

Electrochemical performance

Current density [mA g−1]

Cycle number

Reversible capacity

[mA h g−1]

MOFs Polyhedron porous Mn2O3[178] Hydrothermal Heat at 750 °C with a temperate ramp

of 5 °C min−1 for 4 h in air

1000 1200 819.8

CuO nanostructures[202] Precipitation Calcine at 550 °C for 2 h in air 0.1 C 50 320

3D hierarchical porous ZnO/ZnCo2O4

nanosheets[192]

Refluxing Anneal at 500 °C in air with a heating


1000 250 1016

Spindle-like mesoporous α-Fe2O3[192] Solvothermal Calcine at 380 °C in air 0.2 C 50 911

3D porous V2O5 hierarchical microspheres[179] Hydrothermal Anneal at 350 °C in air 0.5 C 100 130

Ni0.3Co2.7O4 nanorod[235] Microwave-irradiation Calcine at 450 °C in air 100 200 1410

α-Fe2O3 nanospindles[190] Solvothermal Pyrolyze at 550 °C for 2 h with a heating


100 40 ≈921

Porous LiNi0.5Mn1.5O4 nanorods[286] Precipitation Calcine at 700 °C for 6 h in air 5 C 500 ≈116

Hierarchically porous

0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2[181]

Precipitation Calcine at 800 °C in air for 12 h 4 C 200 112

Porous layered lithium-rich oxide nanorods[184] Precipitation Calcine at 700 °C for 10 h and then

at 750 °C for 5 h

2C 100 142.8

Yolk–shelled Ni–Co mixed oxide[204] Refluxing Calcine in air at 350 °C for 2 h with a

heating rate of 2 °C min−1

200 30 1028.5

Hierarchically mesoporous CoMn2O4[176] Solvothermal Calcine at 600 °C for 3 h 100 65 942

Porous ZnCo2O4 nanoflakes[122] Hydrothermal Heat to 400 °C and maintain for 5 h in air. 80 25 ≈810

ZnMn2O4 ball-in-ball hollow microspheres[224] Refluxing Anneal at 500 °C in air for 4 h with

a ramping rate of 1 ° C min−1

400 40 490

Hierarchically hollow Mn2O3 microspheres[214] Solvothermal Anneal at 750 °C for 10 h in air with

a heating rate of 5 °C min−1

500 140 580

Hierarchically hollow V2O5 microspheres[217] Hydrothermal Heat in air at 350 °C for 2 h with a heating


300 50 128

Porous ZnFe2O4/α-Fe2O3 micro-octahedrons[223] Solvothermal Heat in air at 350 °C for 2 h with a heating


200 75 1752

1D crystallized Co3O4 nanofibers[174] Hydrothermal Calcine at 500 °C in air 1000 150 720

Porous NiO microtubes[280] Precipitation Heat at 500 °C 100 100 583

Fe2O3 microboxes[228] Precipitation Anneal to 550 °C 200 30 945

Porous spinel MnxCo3−xO4 nanocubes[229] Precipitation Calcine 400 °C for 1 h 200 30 733

Porous CoFe2O4 nanocubes[183] Precipitation Calcine at 350 °C for 4 h with a ramping


1C 200 1115

Porous ZnO/ZnFe2O4/C octahedra[193] Refluxing Heat to 500 °C at a heating rate

of 1 °C min−1 in N2

2000 100 988

Porous N-doped carbon-coated Co3O4

fish-scale structures[236]

Hydrothermal Calcine at 500 °C in N2 for 2 h at a heating

rate of 0.5 °C min−1

1000 500 612

Co3O4 hollow spheres[262]

Single-shelled

Double-shelled

Triple-shelled

Hydrothermal Calcine at 400–600 °C for 1.5–5 h 178 50 680

866

611

Others MnOx hollow nanospheres[297]

MnO2

Mn3O4

MnO

Hydrothermal Heat in air at 300 °C for 10 h (MnO2)

Heat at 280 °C for 3 h in H2/Ar (Mn3O4)

Heat at 400 °C for 6 h in H2/Ar (MnO)

100 60 840

1165

1515

LiMn2O4 nanorods[145] Hydrothermal Mix with LiOH and then calcine at

700 °C for 10 h

148 100 100

Loaf-like ZnMn2O4[277] Hydrothermal Calcine at 700 °C for 2 h in air 500 100 517

Table 1. Continued.



reduction reaction (ORR) (i.e., because of the generation of Li2O2 particles during discharge) and oxygen evolution reac-tion (OER) (i.e., because of the sluggish Li2O2 back reaction to the original reagents, Li and O2) activities during discharge and charge result in low energy efficiency, a short lifetime, and poor charging rates. Therefore, the development of effective cathode catalysts that dramatically improve the electrochemical perfor-mance of Li–O2 batteries is required. Transition-metal oxides exhibited comparative advantages, such as low cost, high chem-ical stability, and high catalytic activity, as cathode catalysts for Li–O2 batteries. Various porous metal oxides have been widely applied in Li–O2 batteries. In particular, metal oxides (e.g., MnO2,[314,318,319] Co3O4,[102,320] and MnCo2O4

[321]) originating from various precursors have frequently been documented to exhibit desirable catalytic activities, because of their large sur-face areas, uniform pore structures, and hierarchical pore sizes.[229,231,270,322]

MnO2, which is a low-cost and plentiful source material, has been widely used in Li–O2 battery catalysts because of its relatively high electrochemical reactivity.[314,323] For example, Zhang and co-workers[314] manufactured hierarchically porous δ-MnO2 nanoboxes by the conversion of a PBA precursor, and found that they exhibited high catalytic activities during the ORR/OER process. The resulting battery showed a relatively low overpotential of 270 mV and enhanced discharge capacities of 4368 and 3324 mA h g−1 at 0.08 and 0.16 mA cm−2, respec-tively. Moreover, excellent cyclic stability (1000 mA h g−1 after 112 cycles, and 500 mA h g−1 after 248 cycles at 0.16 mA cm−2) was achieved. Thackeray’s group[319] synthesized needle-like α-MnO2/ramsdellite-MnO2 from an Li2MnO3 precursor. As an electrode/electrocatalyst for Li–O2 batteries, these oxides provided extremely high reversible capacities and relatively low polarization voltages during the early cycles. Additionally, multiporous MnCo2O4 microspheres originating from a car-bonate precursor were fabricated as an efficient bifunctional catalyst for nonaqueous Li–O2 batteries.[321] A stable reversible capacity of 1000 mA h g−1 was achieved, at a current density of 250 mA g−1 over 50 cycles.

3.4. Supercapacitors

Supercapacitors, which are another type of electrochemical energy-storage device, have attracted tremendous attention in recent years because they can exhibit higher power density and better cyclability than LIBs.[324–330] There are two main types of supercapacitors: electrochemical double-layer capacitors and pseudocapacitors. In a typical electrochemical double-layer capacitor, the energy is physically stored on the surface of accu-mulated electrodes, in which the electrode materials, such as diverse carbon materials, are not electrochemically active.[331,332] Thus, the capacitance depends directly on the contact surface area between the conductive electrodes and the electrolyte, and thus porous carbon materials with high surface areas are required.[328] By contrast, the electrical energy of a pseudocapac-itor is stored by a series of Faradaic reactions. These Faradaic electrochemical processes have been demonstrated to improve both the working voltage and the specific capacitance, leading to a far greater energy density.[324,331] Over the past decade,

monometal oxides (e.g., NiO,[44] MnO2,[333] and CoOx[160]) and

mixed metal oxides (e.g., NixCo3−xO4 (0 < x ≤ 1), ZnCo2O4,[140] and CuFe2O4

[334]) have frequently been used as electrode mate-rials in pseudocapacitors.

Among these metal oxides, nickel oxides have been inten-sively studied for supercapacitor applications because of their large surface areas, high pseudocapacitive behaviors, and high chemical/thermal stabilities.[335] As mentioned above, the spe-cific surface area is a critical factor contributing to the electro-chemical performance of a supercapacitor. The morphology of NiO has been found to be closely related to the specific sur-face area and specific capacitance. To date, many hierarchical structures with high surface areas have been derived from designed precursors and well utilized to enhance the charge storage properties of supercapacitors. For instance, NiO nano-flakes prepared via the conversion of an Ni(OH)2 precursor exhibited high power densities, at high rates and excellent cycle lives.[335] For example, the as-synthesized NiO microstructure manifested specific capacitances of 942, 804, 696, and 613 F g−1 at 5, 10, 20, and 30 mA, respectively, and only approximately 1.5% of the capacitance was lost after 1000 cycles, at a discharge current of 5 mA. Zhang et al.[336] prepared a hierarchically porous NiO film on a nickel foam substrate by the conversion of β-Ni(OH)2. The as-prepared NiO film displayed superior capacitive performance, including a high discharge capacitance, excellent rate capability, and good cycling stability. Capacitances of 232, 229, 213, and 200 F g−1 were obtained at 2, 4, 10, and 20 A g−1, respectively, and a specific capacitance of 348 F g−1 could be delivered at 2 A g−1 after 4000 cycles.

Co-based oxides (mostly Co3O4) with various morphologies have been prepared by the morphology-conserved-transfor-mation method and applied in supercapacitors. For instance, the hollow Co3O4 boxes mentioned above exhibited specific capacitance values of 278, 216, 198, and 176 F g−1 at discharge current densities of 0.5, 1, 2, and 5 A g−1, respectively. After 500 cycles at 2 A g−1, the specific capacitance still exceeded 250 F g−1, indicating good cycling stability. Porous Co3O4 with a rhombic dodecahedral structure, which was derived from the con-version of ZIF-67, displayed outstanding capacitive performance and stability.[337] Specifically, it delivered a high specific capaci-tance of 1100 F g−1 at a current density of 1.25 A g−1, and even achieved 437 F g−1 at 12.5 A g−1. The obtained electrode showed almost no capacitance decay after 2000 cycles and had a capaci-tance retention of more than 95.1% after 6000 cycles at 6.25 A g−1.

MnO2, which possesses the advantages of high theoretical specific capacitance, low cost, low toxicity, and natural abun-dance, is also a promising electrode material.[331,333] Sev-eral types of crystallized MnO2 exist, including α-, β-, γ-, and δ-MnO2. Among these crystallized forms of MnO2, α-MnO2 shows the highest theoretical specific capacitance (1380 F g−1) and has attracted substantial attention. For example, α-MnO2 nanoflowers have been used as an electrode material,[338] and their capacitance was 211.5 F g−1 at a current density of 1 A g−1. Even at a current density of 10 A g−1, the capacitance value was 128 F g−1.

In addition to monometal oxides, spinel-type bimetal oxides, such as nickel–cobalt oxides (NixCo3−xO4, 0 < x < 1), are the most commonly used electrode materials for superca-pacitors because of their high electronic conductivity values

Adv. Mater. 2017, 1607015



(10−1–10 S cm−1), which benefit their rate capabilities.[157,331] For example, porous nickel–cobalt oxide nanowires were used as binder-free electrodes for the assembly of a NixCo3−xO4 nanowire/activated carbon (AC) asymmetrical supercapac-itor.[157] The resulting device exhibited high specific capaci-tances of 1479 F g−1 at 1 A g−1, and 792 F g−1 at 30 A g−1. Yolk–shell Ni–Co mixed oxide delivered a very high capacitance of over 1000 F g−1 at a current density of 10 A g−1, and did not exhibit any capacitance decay (i.e., 98% capacitance retention) after 15 000 cycles.[204] Hierarchical NiCo2O4 tetragonal micro-tubes derived from nickel–cobalt double-layered hydroxide microtubes have also been used as a supercapacitor electrode material.[125] The as-prepared NiCo2O4 microtube electrode delivered high specific capacitances of 1387.9, 1286.0, 1180.1, 1009.8, and 862.7 F g−1 at current densities of 2, 5, 10, 20, and 30 A g−1, respectively. A specific capacitance of 1055.4 F g−1 was retained after 12 000 cycles, at a current density of 10 A g−1.

Another binary metal oxide with a spinel structure, ZnCo2O4, also exhibited good electronic conductivity and was investigated for supercapacitor applications.[73,337] For example, uniform ZnCo2O4 nanowire arrays grown directly on nickel foam via the morphology-conserved method delivered high specific capaci-tances of 1625, 1536, 1444, 1182, and 962 F g−1 at current densi-ties of 5, 10, 20, 40, and 80 A g−1, respectively.[339] A capacitance retention of 94% was obtained after 5000 cycles at a high cur-rent density of 20 A g−1.

Combining different metal oxides is also an effective approach for improving the charge storage properties. For instance, meso/macroporous ZnCo2O4/MnO2 hierarchical core–shell nanocone forests on a 3D nickel-foam substrate exhibited excellent electrochemical performance in terms of their specific capacity, cyclic stability, and rate performance.[140] These structures displayed exceptional specific capacitance values of 2339 and 1526 F g−1 at current densities of 1 and 10 A g−1, respectively. Furthermore, the long-term capacity retention was almost 95.9% after 3000 cycles at 2 A g−1, and 94.5% after 8000 cycles at 10 A g−1. This striking electrochem-ical performance was attributed as follows: First, the highly conductive 3D Ni-foam substrate totally eliminated binders and conductive additives, and enhanced the electron transport. Second, the mesoporous MnO2 shell, which had a huge surface area, could increase the contact area between the electrode and electrolyte, thereby accelerating ion diffusion.

3.5. DSSCs

Hierarchically porous micro-/nanostructures are also regarded as promising photoelectrodes for electrochemical solar-energy-related applications, especially in DSSCs.[243,268,340–345] A com-plete DSSC system contains a photoanode, consisting of a mesoporous oxide layer (typically, TiO2,[66,243,346,347] ZnO,[67,268] SnO2,[1] or WO3

[348,349]) deposited on a transparent conductive glass substrate; a monolayer of dye sensitizer covalently bonded to the surface of the mesoporous oxide layer, to harvest light and generate photon-excited electrons; an electrolyte containing a redox couple, to collect electrons at the counter electrode and effect dye regeneration; and a counter electrode made of a plat-inum-coated conductive glass substrate, or via the mesoporous

substitution of platinum.[341] The mesoporous semiconducting oxide is an essential part of the whole system, since the con-version efficiency depends directly on the amount of dye mol-ecules absorbed on the internal surface. However, the light absorption capability is the main obstacle to improving the con-version efficiency of DSSCs.

TiO2 (anatase) is commonly utilized as a photoanode because of its superior chemical inertness and electronic configuration compared with other metal oxides (ZnO[67,268] and SnO2

[1]). Therefore, plentiful research efforts have been devoted to the design and optimization of mesoporous TiO2 electrodes, to enhance the power conversion efficiency of DSSCs.[68,243,346,347] For instance, Caruso’s group[68] synthesized mesoporous TiO2 beads with surface areas of up to 108.0 m2 g−1 and tunable pore sizes as photoanodes. They found that mesoporous TiO2 beads with sub-micrometer-sized particle diameters and high specific surface areas could enhance light harvesting without sacrificing the accessible surface for dye loading, resulting in improved power conversion efficiency (7.20%). Kim and co-workers[343] fabricated mesoporous spherical TiO2 structures with diameters of approximately 250 nm, which were used as photoanodes for DSSCs and exhibited a power conversion efficiency of 8.44%. This remarkable performance was ascribed to the hierarchically porous structure with a large pore diameter, high surface area, and fairly good scattering effect.

4. Conclusion

In conclusion, hierarchically porous micro-/nanostructured materials have great potential applications in many fields, espe-cially LIBs, Li–S batteries, Li–O2 batteries, supercapacitors, and DSSCs. Several representative precursors, such as metal hydroxides, metal carbonates, metal hydroxide carbonates, and MOFs, have been used to create hierarchically porous micro-/nanostructured materials via the morphology-conserved trans-formation approach. Figure 10 describes the hierarchically porous micro-/nanostructured materials derived from various precursors and their applications in LIBs, Li–S batteries, Li–O2 batteries, supercapacitors, and DSSCs. Notably, metal-based precursors are most likely to be developed in the future, for applications in electrochemical energy conversion and storage devices.

The general precursors and formation mechanisms were systematically introduced. i) Suitable precursors should be carefully chosen because the morphologies and structures of the final materials are closely related to the precursor spe-cies. The morphologies of various precursors were affected by, for example, ions, ligands, solvents, surfactants, and pH. ii) The fabrication of hierarchically porous micro-/nanostruc-tures involved the formation of a desirable precursor through a nucleation-crystal-growth self-assembly process, followed by a morphology-conserved transformation involving a thermal treatment, selective etching, or a hydrothermal method. Methods with compositional controllability and structural tailoring will undoubtedly enhance the performance of these advanced energy conversion and storage materials.

The properties and applications of some representative hier-archically porous micro-/nanostructures were briefly described.

Adv. Mater. 2017, 1607015



Hierarchically porous micro-/nanostructured materials have the unique feature of micro- or sub-microsized architectures consisting of nanosized units, and are endowed with the advan-tages of both hierarchical micro-/nanostructures and porous structures, such as high specific surface areas, porosity, and short ion-/electron-diffusion pathways. When utilized in LIBs, these materials can avoid agglomeration during electrochemical cycling and minimize interfacial contact resistance. Nanosized units can enhance Li diffusion and alleviate inner stress, thereby improving the electrode capacity and rate capability. Further-more, the porosity of the materials can enhance the contact sur-face areas between the electrode materials and electrolytes, and thus facilitate Li+ diffusion. Furthermore, porosity can also alle-viate the strain induced by volume changes during lithium inser-tion/extraction, leading to good long-term cycling performance. When used in Li–S batteries, this unique structure can not only support loading a high amount of S or Li2S and achieving high sulfur utilization, but also restrict polysulfide diffusion and stabi-lize the dissolved polysulfides within the cathode. When applied in Li–O2 batteries and supercapacitors, the hierarchically porous structure avoids serious particle aggregation, creates convenient channels for electron transport and electrolyte ingress, increases

the electroactive surface area, and enhances Faradaic reactions. When used in DSSCs, the hierarchical porous structure presents the following additional functions: The porosity ensures a large surface area for the adsorption of dye molecules, and the spheri-cally shaped architecture with sub-micrometer diameters can generate efficient light scattering in the sunlight spectrum.

We conclude by highlighting the fabrication of inorganic hier-archically porous micro-/nanostructures with designed sizes, shapes, compositions, and functionalities. Although consider-able work has been performed to develop suitable synthetic methods, the morphology-conserved transformation approach from precursors to hierarchically porous micro-/nanostructures is thought to be the most promising candidate method because of its generality, simplicity, and cost effectiveness. Although this method usually consists of a two- or even three-step process, the porosity structure can be better tuned by precisely control-ling the experimental parameters in next-step processes (e.g., the annealing temperature and ramping rate). Furthermore, new functionalities can be introduced by an extra-step process. For example, when reacting with NaOH, the precursors can form hierarchically porous cathode materials for application in sodium-ion batteries. Much work remains to be done in the

Adv. Mater. 2017, 1607015

Figure 10. Schematic illustration of the syntheses of hierarchical porous micro-/nanostructure materials from various precursors, and their applications in LIBs, Li–S batteries, Li–O2 batteries, supercapacitors, and DSSCs. Mx(OH)y: metal hydroxide precursors; Mx(CO3)y: metal carbonate precursors; Mx(OH)y(CO3)z: metal carbonate hydroxide precursors; and MOFs: metal–organic-framework precursors.



near future to achieve the commercial application of hierarchi-cally porous structured materials. Notably, the integrated par-ticle size, tap density, and interfacial properties between the electrode material and electrolyte are also crucial for electro-chemical energy conversion and storage devices. Many useful strategies to control and optimize the morphologies of electrode materials remain to be systematically established. Addition-ally, the favorable interface at the operating voltage should be further investigated. Researchers from different fields must work together to achieve the commercial application of hierar-chically porous micro-/nanostructure materials with designed functionalities. Based on the research achievements in this area, we expect that the synthetic strategy of morphology-conserved transformation from precursors to hierarchically porous micro-/nanostructures will bring about a more prosperous future.

Acknowledgements

This work is financially supported by the National Natural Science Foundation (Grant Nos. 51471073 and 21403287), the key project of Science and Technology in Guangdong Province (Grant Nos. 2012A010702003 and 2013B090800013), the Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant No. 201509030005), and the scientific research project of the Department of Education of Guangdong Province (Grant No. 2013CXZDA013).

Keywordshierarchically porous micro-/nanostructures, morphology-conserved transformation, rechargeable batteries, solar cells, supercapacitors

Received: December 29, 2016Revised: February 17, 2017

Published online:

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