Emerging applications of graphene and its derivatives in carbon capture and conversion: Current status and future prospects.pdf

Emerging applications of graphene and its derivatives in carboncapture and conversion: Current status and future prospects

Amin Taheri Najafabadi n

Department of Chemical and Biological Engineering, Clean Energy Research Center, The University of British Columbia, 2360 East Mall, Vancouver, BC,Canada V6T 1Z3

a r t i c l e i n f o

Article history:Received 1 March 2014Received in revised form9 August 2014Accepted 17 September 2014Available online 1 November 2014

Keywords:CO2Carbon capture and conversionGrapheneCatalysisElectrocatalysisPhotoelectrocatalysis

a b s t r a c t

Alarming carbon dioxide emissions and its detrimental environmental impacts (e.g. climate change andglobal warming) are the major consequences of the undue reliance of the modern civilization on fossilfuels. Long-term solutions to address these issues are based on developing sustainable alternatives forthe human energy thirst. However, the versatilities offered by the carbonaceous fuels have stillpreserved their popularity as the main source of energy for a wide variety of applications. After decadesof practicing conventional carbon capture and storage, researchers believe the ultimate solution ofrealistically facing with CO2 sequestration problem is the chemical conversion of carbon dioxide tovaluable products. However, substantial development of state-of-the-art materials remains the majorbottleneck of such technologies. Graphene, as the rising star of the materials world in 21st century, offersgame-changing prospects towards a more sustainable future for fossil-fuel-based economies. This two-dimensional planar sheet of sp2-bonded carbon atoms is the most widely studied nanomaterial since itsdiscovery in 2004. Here we aim to highlight various aspects of graphene research in carbon dioxidecapture and conversion from materials viewpoint. After presenting an overview of the most commonand effective synthesis and doping/functionalization methods, the application of graphene and itsderivatives in CO2 capture and conversion is discussed in detail. Catalytic, electrocatalytic andphotoelectrocatalytic use of graphene-based compounds could potentially revolutionize some of thecurrent techniques for CO2 transformation to valuable chemical commodities. CO2 to grapheneconversion pathways are also covered extensively in this review paper as another intriguing relationof graphene with CO2.

& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15162. Graphene synthesis overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517

2.1. Scalable top-down production techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15182.2. Chemical doping/functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1520

2.2.1. Doping (condensed matter). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15202.2.2. Functionalization (chemical synthesis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522

3. CO2-assisted graphene production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15254. Graphene-assisted CO2 capture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1527

4.1. CO2 capture background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15274.2. Graphene-based sorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1527

5. Graphene-assisted CO2 conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15315.1. CO2 conversion background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15315.2. Catalytic reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1532

5.2.1. Hydrogen-assisted CO2 reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15325.2.2. Methane-assisted CO2 reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533

Contents lists available at ScienceDirect

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Renewable and Sustainable Energy Reviews

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5.3. Electrocatalytic reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15345.4. Photoelectrocatalytic reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1535

6. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1537References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1537

1. Introduction

Energy and environmental issues are two of the major chal-lenges facing modern civilization by the mid-century [1]. From thisperspective, fossil fuels are double-edged swords that require adelicate balance between their benets and drawbacks. On onehand, they are unquestionably versatile energy supplies; where oiland natural gas supply close to 90% of our current energy needs,making so many industrial activities economically feasible [2]. Theadaptabilities offered by the carbonaceous fuels have still pre-served their popularity as the main source of energy for a widevariety of applications. Firstly, they are available in a wide range ofphysical formats (gas, liquid and solid) spreading almost all overthe world. Secondly, myriad of technological advances are madewith respect to their implementation for various applications indifferent scales. Thirdly, owing to high energy density and port-ability, their superior properties as fuels in transportation industrycannot be overstated [3]. On the other hand, the increasinglyalarming climate change issues have occurred due to notableamounts of carbon dioxide emissions from undue consumptionof fossil fuels [4]. Burning each gram of carbon in fossil fuelsreleases more than 3.5 g of carbon dioxide (CO2), accumulation ofwhich is now approaching 1 Tt in the atmosphere [5]. In orderto limit the consequent global mean temperature increase by2.02.4 1C, the world-wide CO2 emission must be reduced5080% by 2050 versus that of 2000 [6,7].

Long-term solutions to address the aforementioned problemsare based on the development of sustainable alternatives forquenching the ever-increasing human energy thirst. Meanwhile,in the current fashion of runaway fossil fuel intakes, CO2 reductionstrategies particularly from large-scale energy consumers (e.g.power stations and cement works) rely mainly on three proposed

generic solutions for CO2 capture and storage (CCS): pre- and post-combustion capture, and oxy fuel combustion [8,9]. Afterwards,the puried CO2 is sent for physical storage options such asdeep ocean sequestration [10], geological storage [11], limitedindustrial uses (e.g. mineral processing and soda companies)[12], etc. However, researchers believe that storing CO2 some-where other than atmosphere is not the permanent solution to theproblem [13,14]. In fact, the long term ecological and environ-mental impacts of using the earth as a gigantic reservoir for carbondioxide are not well-understood [10,15]. The potential hazards ofCO2 leakage to the earth surface remarkably increases the risk ofthis approach [16,17]. It also imposes signicant nancial burdenson the industrial rms from expensive CO2 capture equipment tohefty sequestration costs ultimately translated into higher pricesfor the end-users [18,19].

Based on the facts mentioned, chemical transformation of CO2can be the realistic solution to the CO2 sequestration concerns;which could ultimately lead to the utopia for fossil-fuel-basedeconomies with taking full advantage of the carbonaceous fuelswhile minimizing their negative environmental impacts. Thisapproach of recovering CO2 in order to synthesize useful productsis capable of sustainable reduction of carbon emissions, and isknown as carbon capture and conversion (CCC) [20]. The economicadvantages of producing valuable chemicals from CO2 providefurther incentives for major emitters to move towards thisdirection.

The author has recently published a critical review paper on thestate-of-the-art CCC technologies, addressing more efcient mate-rial development as the main bottleneck in this eld of research[21]. Graphene, as the rising star of the materials world in 21stcentury, offers game-changing prospects towards a more sustain-able future for fossil-fuel-based economies. This two-dimensional

Fig. 1. Graphene, the mother of all carbon dimensions with myriad of applications (right, Reprinted with permission from Ref. [22], Copyright 2011, American PhysicalSociety).

A. Taheri Najafabadi / Renewable and Sustainable Energy Reviews 41 (2015) 151515451516

planar sheet of sp2-bonded carbon atoms is the most widely studiednanomaterial since its discovery in 2004 by Dr. Andre Geim andDr. Konstantin Novoselov [22,23]. Considering its numerous uniqueproperties, graphene can be implemented promisingly within manyareas of energy and environmental research [24,25]. It has a largetheoretical specic surface area (2630 m2 g1) [26], high intrinsiccharge mobility (200,000 cm2 v1 s1) [27], strong Young's mod-ulus (1.0 TPa) [28], and excellent thermal conductivity(5000Wm1 K1) [29], while having signicant optical trans-mittance (97.7%) for application as transparent conductive elec-trodes [30,31].

Graphene has been used for many key applications, particu-larly: (i) in energy-related areas, modied graphene materialshave been used in solar cells [32,33], while metallic/metaloxides combined with graphene are utilized in lithium ionbatteries [34,35], supercapacitors [36,37], andfuel cells [38,39];(ii) in the environmental pollution remediation area, grapheneand magnetic graphene nanomaterials have been employed asadsorbents for heavy metal ions and organic pollutants [40],while several transition-metal oxide graphene hybrids are studiedfor the degradation of toxic organic pollutants [41]. Graphene-based materials have been also examined as the pollutantsensors [42] including CO2 sensing applications [43,44]. Impor-tantly, chemical treatment of graphene remarkably broadensits applications in cross-disciplinary areas with structuraldoping and surface functionalization [45,46]. Fig. 1 demonstratesgraphene as the mother of all carbon dimensions with myriad ofapplications.

This paper aims to highlight various aspects of grapheneresearch in carbon dioxide capture and conversion from materialsperspective. After presenting an overview of the most commonand effective synthesis and doping/functionalization methods, theapplication of graphene and its derivatives in CO2 capture andconversion is discussed in detail. Catalytic, electrocatalytic andphotoelectrocatalytic use of graphene-based compounds couldpotentially revolutionize some of the current techniques for CO2transformation to valuable chemical commodities. CO2 to gra-phene conversion pathways are also covered extensively here asanother intriguing relation of graphene with CO2. Since this reviewpaper is multi-disciplinary in nature, the audience across theassociated areas might not be necessarily familiar with all theaspects. Therefore, the author has provided a concise backgroundfor each section, and depending on their expertise, readers cancertainly skip some of the introductory sections. Such approachmaintains the comprehensiveness of the paper without requiringto reach other references for some of the crucial terminologies andbackground.

2. Graphene synthesis overview

There are two fundamentally different approaches to preparegraphene sheets (GNs) as single-(or a few) layers of atoms, namelytop-down and bottom-up [47,48]. The top-down approach startswith macroscopic structures, breaking them down into smallerones. In fact, Geim and Novoselov developed a top-down approach

Nomenclature

NMP 1-methyl-2-pyrrolidoneNPD 4-nitrophenyl diazoniumARPES angle-resolved photoemission spectroscopyAPCVD atmospheric pressure chemical vapor depositionEg band-gap energyCCC carbon capture and conversionCCS carbon capture and storageCMS carbon molecular sieveCNT carbon nanotubeCCM catalytic combustion of methaneCVD chemical vapor depositionDRM CO2 reforming of methaneCB conduction bandDFT density functional theoryDOS density of statesDMF dimethylformamideERC electrochemical reduction of carbon dioxideEF Fermi energyFLG few-layer grapheneFWHM full-width at half-maximumf-HEG functionalized hydrogen exfoliated grapheneGCMC grand canonical Monte CarloGO graphene oxideGN graphene sheetsG-silica graphene-based mesoporous silicaGMNO graphene-Mn3O4 hybridGIC graphite intercalation compoundHOMO highest occupied molecular orbitalHRTEM high-resolution transmission electron microscopyHEG hydrogen exfoliated grapheneIGCC integrated gasication combined cycle

LDH layered double hydroxideLUMO lowest unoccupied molecular orbitalMOF metal organic frameworkOFET organic eld effect transistorsORR oxygen reduction reactionPOM partial oxidation of methanePGNB periodic graphene nanobudsPES photoemission spectroscopyPECVD plasma-enhanced chemical vapor depositionPANI polyanilinePEI polyethyleneiminePPy polypyrrolePS polystyrenePT polythiophenePC propylene carbonaterGO reduced graphene oxideRWGS reverse water-gas shiftRTIL room temperature ionic liquidSWCNT single wall carbon nanotubeSEG solvent exfoliated graphenesccm standard cubic centimeters per minuteH0 standard enthalpy changeS0 standard enthropy changeG0 standard Gibbs free energy changeSMR steam methane reformingg structural parameter for C60 fullereneSOCl2 thionyl chlorideTiNS TiO2 nanosheetsTEM transmission electron microscopyVB valence bandWE working electrodeXPS X-ray photoelectron spectroscopy

A. Taheri Najafabadi / Renewable and Sustainable Energy Reviews 41 (2015) 15151545 1517

so-called micromechanical cleavage to extract single sheets ofatoms from three dimensional graphitic crystals using scotch tapeexfoliation [23]. Graphite, as an earth-abundant starting materialfor the top-down preparation of graphene, offers a cost-efcientand environmentally-friendly alternative to bottom-up nano-car-bon based synthesis. However, the key challenge here is tosurmount the strong cohesive energy of the -stacked layers ingraphite (5.9 kJ mol1 carbon) [49,50]. Other top-down methodsinclude wet chemical or electrochemical synthesis from graphiteintercalation compounds (GICs) [5153], direct liquid phase exfo-liation [54], and solution-based chemical reduction of grapheneoxide (GO) [55,56]. GNs produced by the top-down approach areusually mixtures of monolayers, bilayers and multilayers (typicallycomposed of three to ten monolayers), in the form of irregularlystructured akes (at or folded sheets) [57]. Such morphologiesstill suit the majority of GNs applications in the energy sector, bothfor energy storage and conversion [24,58].

On the other hand, production of GNs with only a few layersand defect-free is certainly looked-for. Furthermore, employing astraightforward top-down pathway is difcult to produce largepieces of GNs (in the size of mm) that would be needed forfabricating chips or other electronic devices. Therefore, bottom-uptechniques have been intensely investigated for technologicalapplications, in which single-layers of graphene are grown ina step-wise manner from carbon atoms. This is mainly achievedvia epitaxial growth of GNs on a substrate by chemical vapordeposition (CVD) [59,60], plasma-enhanced CVD (PECVD) [61,62],solvothermal synthesis [63,64], pyrolysis [65,66], and thermaldecomposition of silicon carbide (SiC) wafer under ultrahighvacuum conditions [67,68].

All the above-mentioned processes have been studied in detail,and described in several review articles [6971]. Additionally, ahigh-throughput approach to determine the number of atomicplanes in GN samples has been recently introduced via opticalmicroscopy and subsequent image processing [72,73]. Here, wemostly focus on scalable top-down GNs production techniques

considering the demanding industrial applications of grapheneespecially in carbon capture and conversion. The complementarydoping/functionalization aspects, which greatly broaden grapheneapplications, are also covered to provide the reader with sufcientbackground for the subsequent sections.

2.1. Scalable top-down production techniques

At present, chemical oxidation of graphite to graphite oxidefollowed by exfoliation to graphene oxide (GO), and then reducingGO to graphene usually by chemical or thermal reduction hasemerged as a promising method due to its low-cost and massproduction potential [7478]. Graphite oxide is usually synthe-sized by oxidizing graphite via concentrated sulfuric acid, nitricacid and potassium permanganate based on the Hummers method[79,80]. It is important to note that although graphite oxide andGO share similar chemical properties (i.e. surface functionalgroups), their structures are different. GO is a monolayer materialproduced by the exfoliation of graphite oxide [69]. Fig. 2 depictsthe graphene production pathway via effective reduction of themono-layered graphene oxide sheets.

Several reducing agents are proposed for chemical reduction ofGO sheets including hydrazine [8285], and sodium borohydrate[86,87]. Hydrazine (N2H4), unlike other strong reductants, isnonreactive with water and is suggested to be the most effectivein synthesizing ultrathin and ne graphene sheets [69]. Through-out the reduction process, the brownish GO dispersion in waterturns black, and the reduced sheets begin to agglomerate andprecipitate in the reaction vessel [77,88]. Such trends in colorchanges and dispersibility of the reduced GO (rGO) in water arerendered by the removal of oxygen atoms from the GO sheets,making the nal product more blackish and less hydrophilic [69].The restoration mechanism of the conjugated GN network throughhydrazine-assisted GO reduction has been proposed by Stankovichet al. in Scheme 1 [77]. First, hydrazine partakes in a ring-openingreaction with epoxides and transforms the epoxides into

Fig. 2. Graphene production pathway via effective reduction of the mono-layered graphene oxide sheets. Reprinted with permission from Ref. [81], Copyright 2011, Wiley-VCH.


hydrazino-alcohols [89]. This derivative then reacts further, con-verting to an amino-aziridine moiety which goes through thermalelimination of di-imide to form a double bond [77]. Stable aqueoussuspensions of rGOs were obtained via pH adjustmentsusing ammonia solutions during reduction with hydrazine [90].However, without surfactant-assisted stabilization, rGOs tend toagglomerate in organic solvents due to their hydrophobicity[77,88].

Sodium borohydride (NaBH4), as another promising reductiveagent, is reported to be more effective in removal of GO's oxygen-containing groups compared to hydrazine [86]; although borohy-dride is prone to hydrolysis by water [69]. The resulting rGOs showlower sheet resistances of 59 k/m2 (comparing with 780 k/m2for a hydrazine-reduced sample under similar circumstances), andhigher C:O ratios of 13.4:1 (compared to 6.2:1 for hydrazine) [86].Other GO chemical reduction pathways include the use of hydro-quinone [91], gaseous hydrogen (after thermal expansion) [92],strong alkaline solutions [93], and solvothermal methods [94].While hydrogen-assisted GO reduction was proved to be promis-ing (C:O ratio of 10.814.9:1), hydroquinone and alkaline solutionsdid not emerge as effective as hydrazine and sodium borohydridebased on semi-quantitative results [69].

Nonetheless, most of the reported chemical production tech-niques use harsh oxidizers (e.g. H2SO4/KMnO4), and an excess of

organic solvents (e.g. dimethylformamide/tetrahydrofuran), whichare not environmentally benign [70,95]. Besides, the successivereduction of GO sheets to graphene typically requires a strongchemical reducer (e.g. hydrazine/sodium borohydride), and hightemperature heating in order to recover the graphitic structure bylowering the lattice defects [56,96,97]. The severe poisonous andexplosive characteristics of hydrazine or sodium borohydridederivatives dictate safety measures when large quantities areused, making the process challenging in real conditions [98].A handful of scalable environmentally friendly processes areavailable to reduce GO to graphene either by chemical or electro-chemical pathways [93,99]; however, developing more integratedgreen approaches to the synthesis of graphene is still of greatinterest.

To address the above-described issues, researchers have uti-lized electrochemical methods as a part of the GN fabricationprocess [100106]. In principle, GN electrosynthesis employs anionically conductive solution (electrolyte) and a power sourceto drive the structural changes through the graphite precursor(e.g. rod, plate, or wire) placed as the working electrode (WE). Thisoffers a number of potential advantages including ease of opera-tion and control over the entire synthesis process, being more eco-friendly with elimination of harsh oxidizers/reducers, relativelyfast fabrication rates, and high mass production potential atambient pressure/temperature. Presumably, direct exfoliation ofthe graphene sheets from graphite would overcome the lowelectronic conductivity of graphene lms chemically reduced fromGO derivatives [77,105]. As shown in Fig. 3, GNs electrosynthesisparticularly in the aqueous media is typically perceived to involvethe partial oxidation (or reduction) of the host graphite (WE) that

Fig. 3. Electrochemical approaches of (a) oxidation, intercalation and exfoliation (negative ions are shown in red color), and (b) reduction, intercalation and exfoliation toproduce single and multilayer GN akes (positive ions are shown in deep blue color). Reprinted with permission from Ref. [52], Copyright 2013, Elsevier. (a) Positive currentto produce "oxidised" GN akes and (b) Negative current to produce "non-oxidised" GN akes. (For interpretation of the references to color in this gure legend, the reader isreferred to the web version of this article.)

Scheme 1. Proposed reaction pathway for epoxide reduction with hydrazine.Reprinted with permission from Ref. [77], Copyright 2007, Elsevier.


leads to the intercalation of anions (or cations) from the electro-lyte. The intercalation process prompts structural expansion in thegraphite matrix which was reported as blisters in early investiga-tions [107]. This has been suggested to yield ultrathin GNs whenhigh voltages are applied to the graphitic electrodes in sulfuricacid [101], and KOH solutions [108] at the anode and cathode,respectively. Nonetheless, the exact exfoliation mechanism is stilldebated, and more exhaustive experiments are required to verifythe described pathways illustrated in Fig. 3.

Due to limitations imposed by water electrolysis in aqueouselectrolytes, non-aqueous solvents or mixtures of aqueous withnon-aqueous components are generally employed to provide awider electrochemical window [103105]. Accordingly, amongstanodic electro-exfoliations, large anions of PF6

, and/or BF4 in

ionic liquids have shown remarkable tendency to intercalategraphite electrodes, yielding gram-scale quantities of carbonnanostructures especially GN akes [105,106]. Air- and moisture-stable room temperature ionic liquids (RTILs), salts of large organiccations with relatively bulky inorganic counter-ions, are moltensalts with melting points close to room temperature [109,110].Owing to low vapor pressure, high chemical and thermal stability,solvating capability, non-ammability with potential recyclability,RTILs have received much interest as green solvents in organicsynthetic processes to replace classic, toxic and volatile molecularsolvents [111,112]. Consequently, ILs are extensively used as areaction medium for the fabrication of conducting polymers andnanoparticles [113116]. Additionally, their tunable physicochem-ical properties enable in-situ functionalization of GNs which is themost important approach for expanding the graphene applicationsthroughout many key research themes [117,118].

Among the cationic electro-exfoliation reports, lithiumco-intercalation with propylene carbonate (PC) has been proposedfor high-yield production of few-layer graphene (FLG) (470%)[103]. The process resembles the destructive effects of PC as amolecular solvent on the graphitic electrodes in lithium-ionbatteries [119,120]. Intercalation of tetraalkylammonium cationsfrom 1-methyl-2-pyrrolidone (NMP) solutions is also reportedwith signicant exfoliation rates as a viable choice for GN synth-esis with low energy consumption and ease of operation [121].In any case, a subsequent sonication treatment is suggested tohave complementary role in improving the quality of the electro-synthesis products particularly with boosting the exfoliation of theexpanded akes into the ultrathin sheets [103,106,121].

2.2. Chemical doping/functionalization

Any effort toward tailoring graphene characteristics by insert-ing external atoms or molecules to its sp2 network falls intochemical doping/functionalization category. Physicists and che-mists look at this subject from quite different perspectives, whichwe have aimed to combine them here. Physicists are mostlyinclined to view this from electronic point of view, where anyexternal impurities can affect the band structure of graphene,therefore focusing on the doping concept [122,123]. This is alsoparaphrased as the adatom phenomenon for creating new bodystates that do not appear in pure graphene [124,125]. On the otherhand, chemists are more interested in such modications forfunctionalization and improving the processability of graphene;hence they tend to discuss the concept from chemical synthesis/activation approach [117,118]. This has often caused confusionwith misplacing the related categorizations which we tried toaddress here.

In the most general classication, any impurity introduced tographene can interact in two major states. First, the impurity candisrupt the sp2 network and cause sp3 defect regions via bondingwith graphene. This is something which is typically associated

with substitutional doping or covalent functionalization. Secondgroup consists of adatoms that tend to interact with graphene lessrigorously therefore do not cause sp3 defects in the graphenelattice. Such modications, so-called surface transfer doping ornoncovalent functionalization, are typically reversible under espe-cial circumstances. Importantly, substrate, chemical remnants andambient gases can render involuntary electronic effects in gra-phene [126128]. Here, the author briey overviews the conceptfrom condensed mater to chemical synthesis aspects.

2.2.1. Doping (condensed matter)From condensed matter standpoint, graphene is constructed

via hybridization of s, px and py atomic orbitals forming sp2-bonded carbon atoms via three strong bonds with three adjacentatoms. The remaining pz orbital on each carbon center overlapswith those from the neighboring atoms, establishing a lled bandof orbitals (valence band), and an empty band of n orbitals(conduction band). Since the valence and conduction bands touchat the Brillouin zone corners, graphene emulates a zero-band-gapsemiconductor [60]. Fig. 4A depicts the described energy distribu-tion in graphene, and the corresponding inset for zero-band-gap atone of the Dirac points (i.e. the intersection of the valence andconduction bands) [129].

From vast amount of literature, doping of graphene can beclassied as (i) electrical doping (via changing gate voltage)[132,133], (ii) substrate-induced doping (via interactions withthe support) [131], and (iii) chemical doping (via insertion ofexternal chemical species) [130,134]. Fig. 4B illustrates the strongambipolar eld effect in pristine graphene (i.e. mismatch betweenFermi level and Dirac point), resulted from electrical doping.Fig. 4C shows the transference of the Dirac point relative to theFermi level prompted by chemical doping and draws a simulta-neous comparison with the substrate-induced doping. We mainlyfocus on the graphene chemical doping concept here, consideringits paramount importance to the context of this review article.

Before putting graphene to the test, chemical doping wasrecognized to extend carbon nanotubes (CNTs) application invariety of research areas [135139]. As previously described, twochemical categories for ne-tuning of the graphene electronicproperties via doping include (i) adsorption of gas [42], metal[140], or organic molecules to the graphene surface [141] (i.e.surface transfer doping [142,143]), and (ii) substitutional doping,which introduces heteroatoms, such as nitrogen [144], and boron[145], into the conjugated graphene lattice [146,147]. Conse-quently, graphene band gap tuning via doping is employed tofabricate high performance electronic devices [148,149].

The mechanism of chemical doping in graphene resembles thatof carbon nanotubes, however the latter is still debated [150153].Surface transfer doping occurs through charge transfer from theadsorbed dopant (or graphene) to graphene (or dopant) [131,142].Charge transfer direction is determined by the relative position ofdensity of states (DOS) at the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO) of thedopant compared to the Fermi level of graphene. If the dopant'sHOMO is above the graphene's Fermi level, charge ows fromdopant to graphene, and the dopant is considered donor; con-versely, if the LUMO is below the Fermi level of graphene, chargetransfers from graphene layer to dopant, and the dopant acts as anacceptor [129,154]. Consequently, graphene can be p-type or n-type doped according to the described electron exchange patterns[154,155]. p-type doping lifts the graphene's Dirac points abovethe Fermi level, while n-type doping pushes the Dirac pointsbelow the Fermi level. In general, surface-adsorbed moleculeswith electron-withdrawing groups (electronegativity) inducep-type doping in electronic structure of graphene, and molecules


with donating groups (electropositivity) lead to n-type doping.In contrast, the substitutional doping mechanism in pristinegraphene still remains uncertain. For this category, p-type dopingmostly takes place by adding atoms with fewer valence electronsthan carbon (e.g. boron), while n-type doping is induced by addingatoms with more valence electrons than carbon (e.g. nitrogen).

Three main characterization tools are generally employedto monitor the chemical doping including: (i) photoemissionspectroscopy (PES), especially X-ray photoelectron spectroscopy(XPS), and angle-resolved photoemission spectroscopy (ARPES),(ii) Raman spectroscopy, and (iii) charge mobility measurements.From XPS spectrum, the dopant presence can be conrmed by thecharacteristic peak of the representative dopant's element, and thecorresponding peak area characterizes the doping level (i.e. surveyscan analysis). Furthermore, the chemical and electronic states ofthe dopant can be obtained by analyzing the high-resolutionspectra of the element (i.e. narrow scan analysis). ARPES iscommonly employed as a strong probe to explore the bandstructure of graphene at Fermi level near the Dirac point in theBrillouin zone [156158]. This is achieved by relating the energydistribution of electrons to their momentum along the high-

symmetry directions measured by ARPES. Comparing the Fermilevel position (at zero bonding energy) with the Dirac point (near,below or above the Fermi level) determines whether the grapheneis pristine or doped.

Raman spectroscopy is another useful technique to analyze thenumber of layers, disorder and doping in graphene [159163]. Dueto the relatively facile recognition of single- and few-layer gra-phene akes by optical microscopy on silicon substrates withtypically 300 nm of SiO2 deposits, most of Raman studies arecarried out on a similar substrate [130]. For electrically dopedgraphene, it is well-known that the G band, the main characteristicband in graphene, sharpens and upshifts in both electron and holedoping [133,164]. The full-width at half-maximum (FWHM) of Gband also decreases for both kinds of doping.

The corresponding carbonaceous G peak in graphene is alsoinuenced by chemical doping. In the case of surface transferdoping, there is a handy empirical correlation to identify thedoping type: n-type doping downshifts and stiffens the G peak,while p-type doping upshifts and softens the representative Gpeak (Fig. 5) [165167]. Adsorption of aromatic groups is alsoreported to induce an asymmetry or splitting in the G band for

Fig. 4. (A) Left: the electronic structure of the sp2 bonded graphene. Right: zoom-in of the Dirac point where valance and conduction bands intersect. Reprinted withpermission from Ref. [129], Copyright 2009, American Physical Society. (B) Ambipolar electric eld effect in single-layer graphene (the position of the Dirac point and theFermi energy (EF) are shown in the insets as a function of gate voltage). Reprinted with permission from Ref. [130], Copyright 2007, Nature Publishing Group. (C) A schematicdiagram for the Dirac point and the Fermi level positions as a function of doping; where the upper panel is n-type doped, pristine and p-type doped free-standing graphene(ac), and the lower section illustrates n-type doped, pristine and p-type doped epitaxial graphene grown on silicon carbide (SiC) (df). Reprinted with permission from Ref.[131], Copyright 2008, American Chemical Society.


graphene-containing samples (Fig. 5) [168]. Nonetheless, the trendfor substitutional doping is entirely different, where both nitro-gen- and boron-doped graphene show upshifted G bands [169].

Electrical conductivity (or resistance) of any graphene-containing substrate is measurable by eld effect devices. Theconductivity (or resistivity) relation with gate voltage of pristinegraphene is a characteristic V-shaped curve (Fig. 4). Typically, theminimum conductivity (or maximum resistivity) point (i.e. theDirac point of pristine graphene) is observed at the zero gatevoltage; while p-type (n-type) doping of graphene shifts the pointtoward positive (negative) gate voltages. In the case of band-gapopening, graphene-based transistors will exhibit higher on/offcurrent ratios, where theoretical/experimental studies have sug-gested many novel properties for such pn junctions often shapedby local top gate control [170174]. Promising pn junctions havebeen generated via selective chemical doping, and their propertieshave been investigated extensively [175].

As mentioned earlier, graphene is highly sensitive to surfacetransfer doping, and most of such doping processes are reversible.This two-dimensional material with its ultrahigh conductivity,which alters swiftly upon atomic/molecular absorbance, is thereforean excellent candidate for high-precision sensors. A linear conduc-tivity response to various NO2 concentrations is reported (Fig. 6a),which greatly facilitates the application of graphene-based sensorsto detect even individual NO2 molecules [42]. This has become astrong incentive to develop high performance sensors based ongraphene chemical manipulations [176180]. Fig. 6 presents otherencouraging cases of gas sensing and the related electronic effectson GNs. Graphene-based biosensors for detecting bacteria [181],glucose [182], pH and proteins [183,184] have also been successfullyfabricated. Additionally, DNA detection via GO is reported [185,186],while graphene is suggested to be an ideal material for DNAsequencing [187192].

It is important to achieve controllable, air-stable and high-performance n-type, p-type or even ambipolar eld effectthroughout the doping process. Pristine graphene and rGO oftenshare similar p-type characteristics at ambient conditions due tothe involuntary doping originated from chemical residues oroxygen molecules in the air, which is not fully explored yet.However, graphene embedded in insulating polymer matrices,like polystyrene (PS), is noticeably less affected by such ambientdoping [193]. Consequently, graphene as a buffer layer betweenthe electrodes and the organic semiconductors of organic eld

effect transistors (OFETs) has exhibited competitive work func-tions similar to those for Ag and Cu [194,195]. Moreover, theanalogous molecular structure of graphene and organic semicon-ductors like pentacene affords strong interactions amongstthem, lowering the carrier injection barriers, thus improving thedevice overall performance (Fig. 6d).

Other applications have also been reported including use ofnitrogen-doped graphene as an efcient metal-free electrocatalystfor oxygen reduction reaction (ORR) in fuel cells (Fig. 7) [196,197],and graphene doping with thionyl chloride (SOCl2) for achieving highelectrical conductivities [198]. More practical applications are antici-pated to emerge soon due to the rapid progress on grapheneresearch. This is further elaborated in the upcoming sections relatingdoping to graphene competence for CO2 capture and conversion.

2.2.2. Functionalization (chemical synthesis)From chemistry viewpoint, functionalization and dispersion of

graphene sheets are of crucial importance for their end applica-tions. This enables graphene to be processed by solvent-assistedtechniques, such as layer-by-layer assembly, spin-coating, andltration [117,199]. It also avoids agglomeration of graphenemonolayers throughout GO reduction process, and preservesgraphene's intrinsic properties. Hence, surface modied graphenehas been utilized for the fabrication of polymer nanocomposites[200], super-capacitor devices [201], drug delivery systems [202],solar cells [203], memory devices [204], transistor devices [205],biosensors [206], etc. Moreover, it is indicated in the next sectionsthat functionalization has a remarkable role in enhancing gra-phene's CO2 capture capacities along with introducing new cata-lytic properties for CO2 conversion.

GO is widely used as a precursor for the synthesis of proces-sable graphene, owing to its highly oxygenated surface withabundant hydroxyl, epoxide, diol, ketone, and carboxyl functionalgroups [207,208]. As shown in Fig. 8, such oxygen-containingfunctionalities can result in a broad range of dispersibility in waterand various organic solvents via alteration of the van der Waalsinteractions [209213]. Edge-positioned carbonyl and carboxylgroups afford strong hydrophilicity for the GO sheets, makingthem readily dispersed in water [214,215]. As illustrated in Fig. 9,according to various functionality scenarios, different modelstructures have been proposed for GO [213,216].

As discussed in Section 2.1, chemical reduction of GO lead to arapid irreversible precipitation due to the agglomeration of GNs,unless a stabilizer is used. Therefore, to address this issue, GOsurface is modied prior to reduction, which is usually performedeither by noncovalent or covalent functionalization [199,217].Serving an example, reduction of alkylamine-modied GO resultsin stable dispersion of functionalized graphene sheets in organicsolvents. Impregnation of carboxylic or sulfonate groups on thebasal planes of graphene is also reported to afford producingwater-dispersible graphene sheets [218220]. On the other hand, anumber of papers have discussed synthesis of functionalizedgraphene sheets directly from natural graphite [221223]. Forinstance, ionic-liquid-assisted graphene electrosynthesis fromgraphitic anodes remains functional groups on the ultrathinexfoliates [104,106]. Nonetheless, obtaining high-yield dispersionsof non-functionalized GNs has still sustained a wave of research[106,222].

Discussing the aforementioned classications, noncovalentgraphene functionalization mainly involves physical surfaceadsorption of molecules/atoms via hydrophobic, van der Waals,and electrostatic forces (equivalent to surface transfer doping).Successful cases include adsorption of surfactants or small aro-matic molecules, polymer wrapping, and biochemical interactionswith DNA and peptides [224227]. Similar to doping, noncovalent

Fig. 5. Raman shift trends for the graphene's G band upon interaction with (a) 1 Msolutions of monosubstituted benzenes, and (b) with various concentrations ofaniline and nitrobenzene. Reprinted with permission from Ref. [165], Copyright2008, Royal Society of Chemistry.


functionalization is a well-practiced concept for the surfacemodication of carbon-based materials, and it has been frequentlyemployed to alter the physicochemical properties of the sp2

networks in CNTs [228233]. Consequently, similar techniquescan be borrowed for further surface modication of graphene viavarious types of organic compounds.

Fig. 7. (a) Metal-free nitrogen-doped graphene as an efcient electrocatalyst for oxygen reduction reaction. Reprinted with permission from Ref. [144], Copyright 2013, RoyalSociety of Chemistry. (b) Schematic illustration of various types of nitrogen-doped graphene (gray for the carbon, blue for the nitrogen, and white for the hydrogen atom).A possible defect structure is shown in the middle of the ball-stick model. Reprinted with permission from Ref. [182], Copyright 2010, American Chemical Society. (Forinterpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) Linear dependency of chemically induced charge transport, n, to various concentration, C, of NO2. Lower inset: graphene characterization by the electric eldeffect. (b) Changes in resistivity, , at zero-band-gap point of graphene induced by exposure to various diluted gases (1 ppm). The positive (negative) sign of changes indicateelectron (hole) doping. (c) Constant mobility of charge carriers in graphene with increasing chemical doping. The parallel shift is caused by a negligible scattering effect of thecharged impurities induced by chemical doping. Reprinted with permission from Ref. [42], Copyright 2007, Nature Publishing Group. (d) Schematic of charge transfer at theF4-TCNQ/graphene interface. Reprinted with permission from Ref. [141], Copyright 2007, American Chemical Society.


In contrast with noncovalent interactions, covalent functiona-lization involves rehybridization of sp2 carbon atoms through thegraphene network into the sp3 conguration, associating with theloss of electronic conjugation (equivalent to substitutional doping)[234]. This is mainly due to the rigorous interactions of the dopantwith the honeycomb graphene, and it is often irreversible. Suchstrong surface modications can be achieved in a variety of waysincluding nucleophilic substitution, electrophilic addition, con-densation, and combinatory addition; which most of them useGO as the starting material [235237].

Briey, the epoxy functionalities of GO are the target groups forthe nucleophilic substitution reaction. It involves attacking amine(single bond NH2) groups of the organic modiers, bearing a lonepair of electrons, to the GO's epoxy groups. Comparing with othertechniques, nucleophilic substitution is more facile and takes placeat room temperature in an aqueous medium. Thus, it has emergedas a promising scalable production method for GNs functionalizedwith all types of aliphatic and aromatic amines, amino acids,amine-terminated biomolecules, ionic liquids, low molecularweight polymers, and silane compounds [235246].

Electrophilic substitution occurs when an electrophile agentdisplaces a hydrogen atom. Impulsive insertion of aryl diazoniumvia reduction of 4-nitrophenyl diazonium (NPD) tetrauoroborateis a prominent example for such electrophilic reactions withgraphene surface [247]. Stable dispersions of functionalized GNsin organic solvents have also been obtained via electrophilicsubstitution of aryl diazonium salt on the surface of surfactant-wrapped graphene [248,249].

In condensation reaction, two molecules (or functional groups)combine to form one single molecule with a loss of entropy.It occurs when isocyanate, di-isocyanate, and amine compoundsform amides and carbamate ester linkages with GO body.

Stankovich et al. used a series of isocyanate compounds for thesurface modication of GO [51,250]. The reaction took place indimethylformamide (DMF) under nitrogen atmosphere. In the caseof using solid isocyanates, both the isocyanate and GO were loadedinto a ask prior to the DMF addition. The resulting functionalizedGO was readily dispersed in DMF therefore useful in the prepara-tion of polymer nanocomposites [51]. Similar to the organicisocyanate, organic di-isocyanate is also emerged successfully inGO functionalization and cross-linking [251].

In organic addition reactions, two or more molecules merge toform a larger molecule. A promising case is put forward recentlywith 1,3-dipolar cycloaddition of azomethine ylide on the surfaceof graphene. Thus, a similar concept has been employed for thefunctionalization of epitaxial graphene by cycloaddition of azido-trimethylsilane [252]. After removing N2, nitrene reacts withgraphene via an electrophilic cycloaddition reaction or a biradicalpathway to form functionalized graphene. The GNs obtained fromdirect liquid-phase exfoliation of graphite in pyridine and NMPhave been functionalized using a similar procedure [253,254]. Theresulting graphene sheets are easily dispersible, making themmore compatible with the processes that involve mixing, blending,or dispersion.

As mentioned earlier, it has become more evident that bothcovalent and noncovalent modications signicantly improve theprocessability of graphene [255257]. However, this adverselyaffects the electrical conductivity of surface modied GNs, and isobserved to decrease the electron mobility noticeably comparingto that of pristine graphene [208,251]. In addition, both functio-nalization techniques severely decrease the surface area of themodied GNs due to the structural damages induced by graphitechemical oxidation, subsequent sonication, functionalization andchemical reduction [117,199]. To address the aforementioned

Fig. 8. As-prepared graphite oxide dispersions in water and 13 organic solvents by bath ultrasonication for 1 h. Top: Dispersions immediately after sonication. Bottom:Dispersions three weeks after sonication. The yellow color of the o-xylene sample is due to the solvent itself. Reprinted with permission from Ref. [211], Copyright 2008,American Chemical Society. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)

Fig. 9. Variations in the proposed model structures for the GO sheets regarding the presence (left, Reprinted with permission from Ref. [212], Copyright 1998, AmericanChemical Society) or absence (right, Reprinted with permission from Ref. [213], Copyright 2009, Nature Publishing Group) of carboxylic acids on basal plane edges.


issues, several studies have examined preparing functiona-lized GNs directly from graphite in a one-step process using,for instance, ionic liquids and surfactants via direct liquid-and electro-exfoliation methods [102,106]. Nonetheless, thebottom line is that all the related studies have appreciated theeffectiveness of graphene functionalization in preventing GNsagglomeration in various solvents, and affording more physico-chemical properties for a broad range of applications.

3. CO2-assisted graphene production

Following the discussions in Section 2.1, there is a great interesttowards sustainable production of graphene for preserving thegreen image of this material. Consequently, new methodologiesare pursued upon using CO2 as the graphene precursor to hit twobirds with one stone. This provides a unique opportunity withlarge-scale chemical conversion of CO2 to graphene and itsderivatives considering their increasing global demand. The pro-ducts could be even employed afterwards as excellent CO2sorbents (see Section 4.2). Due to the paramount importance ofthis newly born research concept, we have particularized all thecritical synthesis parameters as they could spark more out-of-the-box ideas in this eld.

Recently, it was found that burning magnesium ribbons in aCO2-rich environment produces ultrathin carbonaceous sheets viathe reaction presented in Scheme 2 [258]. Although the metalCO2propulsion system for Mars missions had been explored in early90s [259], thorough characterization of the resulting carbonproducts remained untouched until 2011 [258]. Indeed, formationof few-layer graphene from combustion of magnesium metal incarbon dioxide was unprecedented, and prompted further incen-tives to explore more pathways from CO2 to carbon nanostructures[260]. It was also a signicant development over the previousreport which had suggested graphene synthesis by CO reduction(rather than CO2) using Al2S3 [261].

In the initial studies, 3 g of Mg ribbon was ignited inside a dryice vessel covered with another dry ice block. After the completionof Mg combustion in CO2, the black deposits were collected andtreated with 1 M HCl solution to remove the MgO and anyremaining Mg metal. Both Mg and MgO react with hydrochloricacid to form MgCl2 which is water-soluble. Thus, pure carbonmaterial was obtained as the product by ltration and subsequentvacuum drying with the yield of 680 mg (92%). Fig. 10a and bshows transmission electron microscopy (TEM) images of the FLGprepared by the described technique, in which graphene sheetswith varying lengths between 50 nm and 300 nm have beenisolated [258]. Fig. 10c clearly shows the large crystalline graphenestructures as the combustion products, while high-resolution TEM(HRTEM) (Fig. 10d) suggests the number of layers ranging from37. The measured lattice spacing of this material was about 3.5 ,which is in good agreement with the thickness of a monolayergraphene (3.4 ) [28]. The inset image in Fig. 10d corresponds tothe diffraction pattern of FLG, which is an indication of crystal-lization. Although the exact graphene formation mechanism is stillunder investigation, the high temperature generated during mag-nesium combustion undoubtedly plays a crucial role. It is hypothe-sized that magnesium ignition in gaseous CO2 renders quickescape of the solid products from the reaction center. Thus, thesp2 bonded carbon atoms have very low retention times todevelop their 3D multi-layer structure (graphite), and only FLGsynthesis is kinetically favored. Catalyst morphology effect on

monolayer graphene formation with higher yields using similartechniques is still subject to more studies.

Apart from the above-mentioned report, it is still challenging todirectly break the CQO bonds due to the thermodynamicalstability of CO2, limiting the practicality of CO2 to grapheneconversion. Moreover, nding reliable ways to precisely rule overthe GNs layer number, morphology, crystallinity, size, edge struc-ture, and even spatial orientation of the products is of greatinterest. Delicate control of the aforementioned characteristicsduring graphene synthesis is crucial to ne-tuning its electricproperties and device performance. Yet, the discussed chemicaltransformation of CO2 into graphene has not completely fullledthese aspects. In this regard, a more controllable epitaxial growthof single-layer graphene domains was recently put forward viacatalytic activation of CO2 [262]. Graphene domains, with a shapeevolution from hexagonal to more rounded, were fabricated oncopper foils in an atmospheric pressure chemical vapor deposition(APCVD) chamber by chemical activation of CO2 [262]. Unlike CH4,decomposition of CO2 under CVD conditions is more difcult dueto the signicant energy barriers for CO2 conversion. Therefore,the corresponding CVD apparatus for CO2-assisted graphenesynthesis was modied with a catalytic activation module togenerate more reactive intermediates such as methanol, formicacid, and methane as indirect carbon precursors [263]. The utilizedcatalyst for CO2 activation in the recent study was a Ni-basedcatalyst (Ni/Al2O3), which is among the most successfully exam-ined materials for CO2 conversion to synthetic natural gas [263266]. In fact, CO2 reduction to form CC and CH bonds is ofintense interest in the context of viewing CO2 as a feedstock toproduce valuable chemical commodities via organic synthesis (seeSection 5) [264266].

Fig. 11a shows the modied CVD apparatus diagram based onthe described scheme for graphene synthesis from CO2. Comparingwith the previous CVD growth routes, the Ni-based catalyst for

Scheme 2. Burning magnesium metal in a CO2-rich environment to producegraphene heterostructures.

Fig. 10. TEM images of few-layer graphene obtained from magnesium combustionwith CO2. (a) Graphene nanosheets with an average length of 50100 nm.(b) Larger size graphene sheets with average lengths of 300 nm. (c) CrystallineGNs with an average length of 200 nm. (d) High-resolution TEM image of few-layergraphene with the number of layers ranging from 37. Inset: the electrondiffraction pattern of GNs. Reprinted with permission from Ref. [258], Copyright2011, Royal Society of Chemistry.


CO2 activation is loaded at the upstream of the copper foil whichwas placed at the center of the quartz tube. The Cu foils were rstannealed at 1000 1C at a 200 sccm H2 ow rate for 30 min.Subsequently, the alumina-supported nickel catalyst was heatedwith a heating tape while maintaining the 200 sccm H2 ow, andthen a mixture of 5 sccm CO2 and 200 sccm H2 was introduced tothe chamber for graphene growth during 30 min. Afterwards, CO2injection was stopped and 150 sccm Ar was put through until theH2 ow reduced to 50 sccm. Finally, the substrates were slowlyquenched from 1000 1C to 800 1C, and the furnace was swiftlycooled down to room temperature by opening the joints.

Beside the efforts to produce carbon nanostructures directlyfrom CO2, there has been another line of research looking intoutilizing other sources of CO2 [267]. Since it is not much con-venient to carry out magnesium combustion using dry ice, thenew study reported GNs production by the co-calcination ofcalcium carbonate (CaCO3) with magnesium powder (Mg) [267].This offers a facile and scalable approach to synthesize few-layergraphene by calcining the mixture of magnesium metal andcalcium carbonate. The number of layers are reported to vary from4 to 10 by a simple calcination at 850 1C, where the reactiontemperature was observed to be vital for graphene formation[267]. GNs were only detected for calcinations at 850 1C, whileother temperatures (ranging from 700 to 900 1C) led to theformation of amorphous carbon materials. In addition to graphenesheets, CaO and MgO were also collected after the reaction. It isnoteworthy that CaO is currently used as a recyclable sorbent forcarbon capture in uidized bed chemical looping congurations,

where it efciently adsorbs CO2 and converts to CaCO3 [268,269].The described graphene synthesis method using calcium carbo-nate opens up the possibility of large-scale in-situ production ofcarbon nanostructures in a continuous CaO/Mg regeneration modelinked with the main CO2 capture process.

The melting point of magnesium is 649 1C with boiling point of1090 1C while calcium carbonate decomposes at 825 1C. Therefore,in the reaction at 850 1C magnesium remains liquid with very lowvapor pressure. It is observed that magnesium droplets wrapcalcium carbonate powder tightly, where the CaCO3 contents werefar greater than that of Mg. Thus, the molten magnesiumsurrounding calcium carbonate surface forms a nano-stretchedlm. Also, magnesium combustion in pure CO2 is suggested tooccur at two reaction zones [270]: (i) an outer zone (shell) wheremagnesium initially reduces carbon dioxide to CO and coverts tomagnesium oxide, and (ii) an inner zone, at the particle surface,where CO is further reduced by the remaining liquid magnesiumand results in producing solid carbon and solid magnesium oxide.Accordingly, calcium carbonate decomposition into carbon dioxideand then carbon dioxide reduction by magnesium into graphenecan be presumed to follow the Scheme 3. In this case, since thereducing agent (Mg) was effectively wrapped and stretchedaround the CaCO3 particles to form a nanoscale layer, the ultrathincarbon sheets were produced as the result.

Other attractive options for CO2 conversion to carbon allotropesinclude reducing CO2 to diamonds [271,272], and nanotubes[273,274], either through direct CO2 splitting or via reaction withmetals at pressures higher than 70 MPa. Recently, porous carbonsynthesis from high-pressure supercritical CO2 was reported usingalkali metals for CO2 reduction [275]. As a notable follow up, CO2reaction with sodium borohydride enabled producing boron-doped porous carbon at more moderate conditions [276]. Theresults introduced a more energy efcient pathway for CO2reduction to porous carbon under atmospheric pressure at tem-peratures below 500 1C; while XPS analysis conrmed doping ofthe products with boron throughout the synthesis process [276].

Nevertheless, complete CO2 reduction to carbon nanomaterialswithout using metal-containing co-reactants and applyingextreme conditions remains unmapped. Just recently, a novelmetal-free pathway for CO2 conversion to graphene oxide wasput forward using ammonia borane (NH3BH3) [277]; which isknown to have a signicant hydrogen storage capability [278]. Thereported CO2-to-GO pathway comprises two consecutive steps ofcarbon xation and then graphenization. The carbon xation is theCO2 reaction with NH3BH3 under mild temperature and pressure(To100 1C and Po3 MPa) to produce a solid compound contain-ing methoxy (OCH3), formate (COOH), and aliphatic groups. After-wards, the graphenization phase takes place with the pyrolysis ofthe obtained solid at high temperatures (T4600 1C), and atmo-spheric pressure of nitrogen to produce graphene oxide-boronoxide nanocomposites. Fig. 12 shows the reaction pathway forCO2-to-GO using ammonia borane with representative TEMmicro-graph. After all, more extensive studies are required for advancingthis concept, and turning it to a feasible process for cost-effectiveCO2 remediation in large scales.

Fig. 11. Graphene CVD growth on copper foils via catalytic activation of CO2.(a) Schematic diagram. (bf) SEM images of graphene domains development oncopper foils with 200 sccm H2 and different ow rates of CO2 (330 sccm) after30 min of growth time. (g) The characteristic length of the domains as the functionof CO2 ow rates. Reprinted with permission from Ref. [262], Copyright 2013, RoyalSociety of Chemistry.

Scheme 3. Underlying reactions of calcium carbonate conversion to graphenenanosheets via magnesium reduction.


4. Graphene-assisted CO2 capture

4.1. CO2 capture background

Concerning negative impacts of CO2 as a greenhouse gas onglobal warming and climate change [279,280], it is crucial tond inexpensive, effective, and robust solutions for substantialCO2 reduction from industries that heavily rely on fossil fuels[281,282]. For instance, in pulverized-coal plants, post-combustionCO2 capture is a common practice where CO2 removal is carriedout from the typically dilute (o15 % by volume) ue gas (i.e.exhaust gas). Aside from the current infrastructures, severalalternative power plant designs have been proposed that mayallow for more efcient CO2 capture [283285]. In pre-combustioncapture, the fuel is gasied with steam, air, or O2 to producesynthesis gas (H2/CO mixtures). CO is typically further oxidized toCO2 by H2O in a shift reactor to produce additional hydrogen.The gas mixture (now mostly H2 and CO2) is then separated into aH2-rich fuel and CO2 for storage/conversion. In contrast to typicalpost-combustion capture conditions, the target gas stream forseparation in this case is typically pressurized, containing appreci-able CO2 concentrations (415 %). Pre-combustion capture is thusmostly related to the coal-red plants with integrated gasicationcombined cycle (IGCC) [286,287]. Oxy-combustion is anotheralternative power plant design that could lead to more efcientCO2 capturing. In this case, a hydrocarbon fuel is combusted withpure O2, rather than air, producing an exhaust of CO2 and H2Ovapor. This leads to a ue gas with an easily separable component,H2O, remaining an essentially pure CO2 stream for storage or otherdispositions. Importantly, such designs require an air separationunit, which adds to the monetary burdens of this technology[288]. After CO2 separation by any of the described methods, thepuried CO2 can be considered whether for (semi-)permanentstorage [289291] or conversion into useful products [14,292,293].

Considering the above-mentioned criteria, three types of gasmixtures are targeted for CO2 capture and separation technologies:the components of ue gases (mainly CO2/N2), pre-combustion gasmixtures (H2/CO2), and natural gas streams containing CO2(mainly CH4/CO2). However, aside from the major components inue gases, natural gases, and hydrogen, other gases/compoundscan be found in these mixtures. For instance, trace amounts ofwater are inevitable in most systems which generally add morecomplexities to the separation process; while under certaincircumstances it may positively contribute to it [294,295]. Still,only a handful of computational investigations have includedwater to study its possible effects more extensively [280,296].Consequently, different adsorbents may better suit one applicationor another, on the basis of the operating temperature, pressure,CO2 mole fraction, presence of impurities, and other factors.According to various adsorption behaviors of graphene and its

derivatives, their suitability for each of the three described majorCO2 mixtures is conceivable throughout this paper.

In general, employing high capacity nanostructured materialspotentially reduces the costs associated with carbon capture (orseparation). For instance, membranes or particle-based adsorp-tion-driven processes can be used for CO2 separation from N2-richue gases [297,298]. CO2 is currently removed from ue gasstreams via absorption by amine-based aqueous solutions likesimple alkanolamines or liqueed NH3 [299,300]. CO2 selectivity isa key factor for the cost-effectiveness of the adsorption-mediatedseparation of CO2 from ue gases [301,302]. To address this issue,numerous adsorbent materials have been put to test recentlyincluding zeolites [303,304], metal organic frameworks (MOFs)[305,306], and highly alkaline adsorbents [294,307]; where alka-line systems have shown a high selectivity for CO2 adsorption overN2 [308,309]. Moreover, carbon [310,311] and silica [312,313]materials have also received a great attention in this regard.Complementarily, chemical looping combustion congurations areproposed to continuously circulate the sorbent via regenerativecycles [314,315]. Among materials for solution-based separation,ionic liquids have also exhibited promising uptakes with highselectivity towards CO2 [316,317].

Quality (or utility) of ideal CO2 adsorbents is characterized byseveral parameters [318,319], which generally includes fast adsorp-tion/desorption kinetics, large adsorption capacity, high regener-ability/stability, and a wide yet exible operability range.Nevertheless, no ideal adsorbent exists in reality, and the practicalityof each adsorbent must be decided over its strengths and weak-nesses in the context of a particular process design. Ultimately, thoseadsorbents that perform reasonably well within a specic CO2separation process scenario would emerge as viable candidates.

4.2. Graphene-based sorbents

Carbon-based adsorbents are among the most promisingmaterials for CO2 capture due to their chemical inertness, highsurface area, and low cost [320]. Porous carbons have revealed asignicant advantage over zeolites in terms of hydrophobicity forCO2 capture. However, the competitive adsorption of water onother hydrophilic carbonaceous surfaces still reduces their CO2uptake capacities [280,321]. Several types of carbon materials havebeen studied for CO2 capture including activated carbons[322,323], carbon molecular sieves (CMS) [324,325], carbon nano-tubes (CNT) [326,327], and graphene heterostructures as the mostrecent constructs [328,329]. Among those, graphene has thelargest surface area (2630 m2 g1) [26], and is thus presumed tooutperform the others [330,331].

As pointed out in Section 2.1, scalable top-down graphenesyntheses, mainly via GO reduction, do not usually result in apristine monolayer of graphite, rather producing a few-layergraphene with lattice defects [332,333]. Therefore, CO2 capturebehaviors of GNs prepared by such methods have been unex-pected to some degree [328,333,334]. Recently, relatively highuptakes of hydrogen and carbon dioxide by rGO sheets with awide range of surface areas is reported [335]. A reasonablehydrogen uptake of 1.7 wt% was observed at 1 atm and 77 K,where the values were linearly correlated with the surface area.Thus, upon extrapolation, the hydrogen uptake by single-layergraphene was projected to exceed 3 wt%. The H2 adsorption at100 atm and 298 K was found to surpass 3 wt%, which suggestsmuch higher uptakes by the monolayer graphene via linearextrapolation [335]. On the other side, rGOs showed remarkablyhigher uptakes for CO2, approaching 35 wt% at 1 atm and 195 K.This was further backed by another study on rGOs obtained fromvarious thermal reduction conditions showing high CO2 capturecapacities (248 wt% under 298 K and 30 bar) for the samples

Fig. 12. Formation of graphene oxide nanocomposites from carbon dioxide usingammonia borane with representative TEM micrograph showing curled and stackedakes. Reprinted with permission from Ref. [277], Copyright 2012, AmericanChemical Society.


reduced at 300 1C [336]. Such uptakes were signicantly higherthan those for commercial zeolites (7-fold) and activated car-bons (3.5-fold) under similar experimental conditions [336].

Fig. 13 compares the typical sorption isotherms for H2 and CO2on a graphene supercell, and depicts various adsorption orienta-tion scenarios. The rst-principles calculations show parallel andperpendicular orientation for hydrogengraphene interactions[154,328], suggesting up to 7.7 wt% of theoretical hydrogenadsorption on monolayer graphene. On the other hand, CO2molecules sit parallel to the honeycomb network, resulting in amaximum uptake of 37.9 wt% for single-layer graphene [335].

It is found that stacking the graphene layers in the samplesdecreases the H2 uptake, while the edge sites signicantly con-tribute to CO2 adsorption on GNs [337]. Since the related uptakecalculations have only taken basal planes into account, edge effectscan be the source of discrepancy between theoretical estimationsand experimental values of adsorption onto GNs [338]. Theaforementioned discrimination between the edge sites and basalplanes in graphitic carbons is also observed using molecularorbital calculations, based on SO2 and CO2 adsorptions [339].Moreover, strong interactions of CO2 and H2O with graphenenanoribbons at room temperature have been reported experimen-tally with less reversible adsorption; comparing to N2 moleculesadsorbed less rigorously with perfectly reversible cycles at 77 K[340]. Therefore, the meaningful pattern of edge effects in gra-phene nanoribbons became more evident; while basal-plane-dominant GNs had exhibited reversible adsorption for CO2 andH2O.

A few theoretical works have explored the determining role ofedges on the physicochemical properties of nanographenes as theycontain numerous edge sites with various types of defects [341344]. The edges' contribution to the molecular adsorption by GNswas isolated from that for basal planes using grand canonicalMonte Carlo (GCMC) simulations [329]. The edge sites of GNsshow relatively strong Coulombic interactions due to the partialcharging at the vertices, while basal planes hardly afford suchinteractions. The modeling results revealed that the edge sites aremore inclined toward CO2 adsorption, while N2 mostly sits on thebasal planes. This leads to an extremely high selectivity for CO2adsorption over N2 on the edge sites, which the number exceeds30 in pressures below 0.02 MPa [329]. Therefore, ne-tuning ofthe edge sites versus basal planes in GNs can suit their applicationsfor selective adsorptions, reactions, and separations. Fig. 14 illus-trates the discriminating effect of the nanographene edges forselective CO2 adsorption with related comparisons.

From surface chemistry standpoint, basic adsorbents showmore afnity towards CO2 capture due to the slightly acidic natureof CO2 [345]. Consequently, adsorbents functionalized with amineshave shown high CO2 adsorption uptakes [346348], whichimproves further when nitrogen is effectively incorporated withinthe support's framework [349351]. Thus, researchers have aimedto improve the CO2 capture characteristics of amine-based carbonmaterials by increasing the surface density of the amine groups onthe support as well as strengthening the amine-support immobi-lization [352354]. Along this approach, polyaniline (PANI), as arich source of nitrogen-containing groups, has been impregnated

Fig. 13. (a) Hydrogen sorption isotherms at 1 atm and 77 K. (b) Carbon dioxide sorption isotherms. (c) Binding energies of a single molecule on a graphene supercell as afunction of distance with various adsorption orientations (shown in insets) for hydrogen (upper panel) and carbon dioxide (lower panel). Reprinted with permission fromRef. [328], Copyright 2008, American Chemical Society.


on various supports and membranes with promising results [355358]. The suitable supports for such applications should meetseveral criteria including strong attraction for the amine-containing molecules, high surface area with proper pore sizedistribution, reasonable mechanical strength and hydrothermalstability. Graphene matches very well with the stated require-ments, and therefore polyanilinegraphene nanocomposites haveshown encouraging CO2 capture performance [356,358].

In the rst study, GNs were prepared via hydrogen-inducedthermal exfoliation of GO, named as hydrogen exfoliated graphene(HEG) [359]. HEGs alone reached the adsorption capacities of 21.6,18 and 12 mmole g1 at temperatures of 25, 50 and 100 1C,respectively (pressure was xed at 11 bar) [360]. These valueswere found to outperform other carbon nanostructures (e.g.activated carbons and carbon nanotubes) as well as zeolites undersimilar operating conditions [360]. HEG was then functionalizedwith oxygen-containing functional groups to form functionalizedgraphene (f-HEG). This initial functionalization created anchoringsites for the subsequent addition of PANI molecules to the f-HEG,providing more uniform accommodation for nitrogen-rich groupsover the f-HEG surface. The f-HEG was further coated withnanostructured PANI, yielding a PANI-f-HEG nanocomposite via achemical impregnation process [361]. CO2 adsorption capacities atthe pressure of 11 bar and temperatures of 25, 50 and 100 1C wereobserved as 75, 47 and 31 mmol g1, respectively. Comparing withthe above-mentioned CO2 uptake values for non-functionalizedHEG, this is a far greater performance accompanied by a higherdegree of recyclability [356].

In the other study, PANI-rGO composites were synthesizedusing chemical activation starting from pretreatment with 7 MKOH solutions followed by activation at various temperaturesbetween 400 and 800 1C under nitrogen ow inside a tube furnace[358]. This was inspired by the recent synthesis of high surfacearea porous carbons (3100 m2 g1) via chemical activation ofgraphene for application in supercapacitors with high-capacityenergy storage [362]. KOH partially reacts with graphene at hightemperatures and remains a signicant pore volume within thenal product after being washed away by HCl solutions [363]. PANIwas grafted to the surface of GO sheets prior to the reduction withhydrazine (a typical protocol for GN functionalization, see Section2.2.2) [358]. A highly reversible CO2 uptake of 2.7 mmol g1 at298 K and 1 atm (5.8 mmol g1 at 273 K and 1 atm) was observedover the samples activated at 700 1C. The N-doped graphenemaintained a practical stability in the course of recycling, showingonly an initial decrease of 10% (32.7 mmol g1) in CO2 storagecapacity before reaching the cycling equilibrium. The synthesized

materials also displayed selectivity towards CO2 adsorption com-pared to H2, N2, Ar or CH4. Fig. 15 represents the describedrecyclability and gas selectivity for the NG7 samples (979.6 m2 g1

surface area), which showed the highest CO2 uptakes (NG7 standsfor chemical activation at 700 1C).

N-doped porous carbons are also synthesized via chemicalactivation of polypyrrolegraphene composites using KOH solu-tions [364]. Polypyrrole (PPy) is an environmentally benign basicCO2 sorbent with excellent thermal stability and can be easilyproduced in large scales [365]. However, the low surface areaalong with costly nature of PPy have constrained its application forCO2 capture at the industrial level. In this regard, interfacing PPywith graphene substantially increases its surface area, and theresulting composites have also exhibited a high adsorption capa-city for aqueous mercury [366]. After chemical activation, nitrogenwas found to be doped within the porous carbon rather than thegraphene matrix. The PPyGN nanomaterials showed a selectiveand reversible adsorption for CO2 (4.3 mmol g1) over N2(0.27 mmol g1) at 298 K and 1 bar [364]. The scalability of thesynthesis method and excellent recyclability of the N-dopedporous carbons make them highly competitive for practicalapplications.

As the latest follow up to the abovementioned line of research,S-doped microporous carbon materials, obtained by the chemicalactivation of polythiophene (PT) grafted rGOs, were examined forCO2 adsorption [367]. With a signicant CO2 uptake of4.5 mmol g1 at 298 K and 1 atm, the S-doped samples showedremarkable selectivity over N2, CH4 and H2 with a stable recyclingadsorption capacity of 4.0 mmol g1 [367]. The microporosity(0.6 nm pore size), surface area (1567 m2 g1), and oxidized Scontent of the porous carbon (6.5 wt%) were found to be thedetermining factors for the remarkable CO2 adsorption resultedfrom chemical activation at 700 1C. Noticeably, S-doped chemicallyactivated graphene reached higher CO2 adsorption values thanthose for N-doped materials prepared and examined under similarconditions [367].

Successful synthesis of graphene-based mesoporous silica(G-silica) sheets is reported recently with sandwich-like structureand high surface area [368]. Conning GNs within individual poroussilica sheets enables the end product to have broader applicationsparticularly in ultrafast energy storage [368,369]. It was found thatG-silica sheets can serve as an effective host for immobilizingpolyethyleneimine (PEI), as another amine-rich compound (denotedas PEI-G-silica) [370]. The resulting PEI-G-silica sheets not onlyhad an ultrathin structure with high PEI surface density, butalso exhibited superior thermal conductivities originated from GNs

Fig. 14. (a) CO2 and N2 adsorption isotherms at 273 K on nanographenes ( for CO2, for N2), edge sites (solid curve), and basal planes (dashed curve), and (b) snapshots.Reprinted with permission from Ref. [329], Copyright 2012, American Chemical Society.


presence. Such features afforded the synthesized nanocomposite anefcient CO2 diffusivity/adsorption as well as rapid thermal transferfor faster regeneration kinetics. Accordingly, high CO2 uptake of19 wt% with reasonable regenerability at 75 1C and atmosphericpressure was achieved by PEI-G-silica sheets [370].

GrapheneMn3O4 (GMNO) hybrid porous materials have alsoshown promising carbon dioxide adsorption capacities [371].Metal oxides that offer various basic sites are extensively studiedas alkaline CO2 adsorbents [372,373]. Obviously, the surface area isanother determining factor, and specic strategies are followed tobetter benet from basicity on higher accessible areas. Thisincludes synthesis of porous metal oxide materials [374], andimpregnating metal oxide nanoparticles on high surface areamaterials [375]. Particularly, in the synthesis of GMNO hybrids(second strategy), MnO(OH)2 colloid, obtained by the hydrolysis ofMn2 in basic solution, was used as the Mn3O4 precursor. Afterhydrothermal reaction of GO and MnO(OH)2, they were simulta-neously reduced to graphene and Mn3O4, respectively, with anenhanced crystallization. Finally, as-prepared GMNO hybrid mate-rials showed a high specic surface area ranging from 140 to680 m2 g1. The CO2 uptakes reached up to 11 wt% for the samplesprepared with initial Mn2/GO mass ratios of 4 (541 m2 g1

surface area) under sorption isotherm conditions [371]. This wassignicantly higher than CO2 adsorption capacity of pristinegraphene (7.7 wt%) and Mn3O4 (0.6 wt%) in similar experimentalsettings [371].

In a separate study, graphene oxide enhanced the CO2 adsorp-tion capacity of the layered double hydroxides (LDHs) [376]. LDHs,also known as hydrotalcite-like compounds, are a large class ofinorganic materials with some unique basic properties useful fordiverse applications [377,378]. They are composed of synthetic 2Dnanostructures with basic anionic clays conned by the positivelycharged brucite-like Mg(OH)2 layers. A fraction of divalent cationsin LDHs, octahedrally bonded with hydroxyl groups, is typicallysubstituted by trivalent cations. This results in an excess of positivecharge which is then balanced by the intercalating anions. Theremaining interlayer space is usually lled by the water moleculeswhich weakly interact with the material [379,380]. LDHs, aspromising CO2 sorbents, require lower regeneration energies withbetter long-term stability than other alkali metal oxides (e.g.calcium oxides) [296]. In addition, they exhibit fast adsorption/desorption kinetics, particularly in the water presence, which notonly makes them applicable to pre-combustion CO2 capture, butalso enables them for sorption-enhanced hydrogen production[381]. Despite the described advantages, LDHs have shown rela-tively low CO2 adsorption capacities [382]. The recent studydemonstrated that GO assembly with LDHs provides a lightweight

charge-harmonizing 2D material that boosts the CO2 uptakecapacity, and multicycle stability of the end product. Consequently,this increased the CO2 storage capacity of LDH by 62% withonly 7 wt% of GO incorporation [376]. The experimental procedurefor producing GOLDH composites was based on the directprecipitation of the LDH nanoparticles onto GO using wetimpregnation technique. Unsupported LDHs were synthesized byco-precipitation with Mg/Al ratio of 2, which is reported to beoptimal for CO2 capture [383]. Fig. 16 presents the schematic forthe LDHGO assemblies with related CO2 sorption data as afunction of GO loading at 573 K and P(CO2) of 0.2 bar.

Interest in the inorganic graphene analogs containing boron,carbon, and nitrogen, BxCyNz, has arisen by a recent study on agraphene-like borocarbonitride (BCN) sample with high surfacearea and noticeable CO2 uptakes at low temperatures [384];similar to the behavior of hexagonal (graphene-like) boron nitride(g-BN) based on computational modeling [385]. Therefore, moreexperimental research is conducted on the adsorption of CO2 andCH4 by graphene-like BxCyNz compounds with compositions closeto BC2N [386]. The synthesized materials showed large surfaceareas (15001990 m2 g1), and a very high uptake for both CO2(up to 64 wt%) and CH4 (up to 15 wt%) at 298 K and 5 MPa; whichwere increasing exponentially with the surface area of the adsor-bent. The CH4 uptakes changed proportionally to those for CO2,and they were both considerably higher than the values reportedfor their adsorption on nanographenes [329].

On the theoretical track, grand canonical Monte Carlo (GCMC)simulations are performed to design new constructs via evaluatingfullerene-intercalated graphene nanocontainers (NanoBuds) foradsorption of CH4 and CO2 [387]. It was found that the fullerenesof type C180 other than C60, as intercalation reagents for construct-ing the model structure, could improve the storage capacity [387].Afterwards, further in-depth GCMC studies were conducted onadsorption of CH4 and CO2 gases and CO2 purication from CH4/CO2 and N2/CO2 binary mixtures by the C60 intercalated graphite(Fig. 17) [388]. First, it was perceived that the pristine material isnot suitable for gas storage, mainly due to its low porosity (0.45)and large crystal density (1.57 g/cm3). Accordingly, the maximumuptakes of 4.04 mmol/g for CH4 and 4.96 mmol/g for CO2 werecalculated for room temperature adsorption. However, the mate-rial exhibited a remarkable selectivity for CO2 at ambient condi-tions, reaching up to 8 and 50 for the CH4/CO2 and N2/CO2mixtures, respectively. Prior to that nding, a novel construct so-called periodic graphene nanobud (PGNB) was reported using therst-principles method, where the C60 fullerenes were covalentlybonded to the graphene monolayer [389]. Therefore, it opened thepossibility of controlling the distance between the C60 fullerenes

Fig. 15. (a) Recycling of the NG7 composite material, and (b) its selectivity towards CO2 adsorption over Ar, N2, H2 and CH4. NG7 stands for chemical activation at 700 1C.Reprinted with permission from Ref. [358], Copyright 2013, IOP Science.


for further modication of the material. This affords up to 200%increase in CO2 adsorption, approaching 12 mmol/g at 6 MPa forboth mixtures without degrading the selectivity to CO2 [388]. Suchimprovements are mainly achieved via decreasing crystal density,thus increasing porosity originated from enlargement of g(structural parameter for C60). Ultimately, it implies that C60intercalated graphite could be still an excellent material for CO2purication, especially for N2/CO2 systems at room temperatureunder aforementioned circumstances. Fig. 17(a and b) demon-strates the effect of structural parameter enlargement on theabsolute CO2 uptakes in C60 intercalated graphite at 298 K withequimolar mixtures of CH4/CO2, and N2/CO2, respectively.

5. Graphene-assisted CO2 conversion

5.1. CO2 conversion background

Carbon dioxide, as the most oxidized state of carbon, is anonpolar linear molecule which is highly stable from thermody-namics angle. Both terms of enthalpy and entropy changes (H0

and S0, respectively) of the Gibbs free energy change G0 arenot favorable for converting CO2 to other useful chemical com-modities [390]. Therefore, reactions involving CO2 reductionrequire signicant energy input to overcome the costs associatedwith thermodynamical barriers. It also necessitates proper cata-lysis to boost the conversion kinetics, and enhance the productselectivity [391].

Undoubtedly, energy supplies for CO2 conversion should beprovided from sustainable low-carbon sources to effectively con-trol atmospheric CO2 levels. As the most scientically soundscenario, energies from intermittent resources (e.g. electricityand hydrogen produced by solar, wind, wave, geothermal, biofuelor nuclear) must be employed to chemically transform CO2.In association with hydrogen (or methane), CO2 can be convertedinto high-density fuels perfectly compatible with the currentenergy infrastructures. Additionally, a wide range of useful pro-ducts can be synthesized from CO2, which are currently producedby unsustainable carbonaceous reagents. This intriguing approachoffers the prospect of decarbonizing fossil-fuel-based economieswithout any revolutionary change in the platform required bysubstituting other alternatives such as electricity and hydrogen.

Fig. 16. (a) Schematic of the LDH-GO hybrids for CO2 capture. (b) Average CO2 uptakes as a function of GO content at 573 K and P(CO2)0.2 bar. Reprinted with permissionfrom Ref. [376], Copyright 2012, American Chemical Society.

Fig. 17. Absolute CO2 uptakes of C60 intercalated graphite versus g at 298 K with equimolar compositions of: (a) CH4/CO2 mixture; (b) N2/CO2 mixture. (c) Snapshots of puregas adsorption in C60 intercalated graphite at 298 K and 6 MPa (top for top view, and bottom for side view). Reprinted with permission from Ref. [388], Copyright 2010,American Chemical Society.


In fact, there are numerous theoretical pathways proposed forCO2 chemical transformation which are studied extensively byseveral researchers in the past few years [392401]. The majorityof them mainly rely upon use of CO2 as a co-reactant with otherchemicals owning higher Gibbs free energies (e.g. hydrogen,methane, unsaturated compounds, small-membered ring com-pounds, organometallics, etc.) for facilitating chemical reductionof carbon dioxide. Such energetic chemicals meaningfully enhanceCO2 reducibility with releasing high chemical energies in thecourse of reaction [392]. Fig. 18 exemplies the appropriateselection of reactions that can lead to a negative Gibbs free energychange for the overall CO2 reduction route.

Among the suggested solutions for CO2 chemical conversion,graphene and its derivatives can serve remarkably in three majorcategories of catalytic, electrocatalytic, and photoelectroncatalyticCO2 reduction to primary fuels and chemical commodities (i.e.methanol and formic acid), using methane (Scheme 4) andhydrogen (Scheme 5) as the main co-reactants. Each of thementioned categories will be discussed in detail throughout thissection with more emphasis on the future prospects by elaboratingthe theoretical aspects. Fig. 19 portrays the critical role of advancedcatalytic materials, in this case graphene, to achieve carbon-

neutral cycles for carbonaceous fuels by efcient

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Emerging applications of graphene and its derivatives in carbon capture and conversion: Current status and future prospects.pdf