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Novel solid-state glycine-nitrate combustion for controllable
synthesis of hierarchically porous Ni monolith
Qin Guo, Ying Zhao, Jiatu Liu, Cheng Ma, Hangyu Zhou, Boyun Huang and Weifeng Wei *
State Key Lab for Powder Metallurgy, Central South University,
Changsha, Hunan, P.R.China 410083.
Abstract:We demonstrate a novel solid-state glycine-nitrate route for not only the scalable
combustion synthesis of hierarchically porous Ni monolith, but also control over impurities,
microstructure topography and size. The as-synthesized porous Ni monolith may find instant
applications as electrode current collectors, catalyst and catalyst substrates or sensors.
Key words: porous metal foam; combustion synthesis; hierarchical pore; supercapacitor; energy density
Porous metal foam, due to its excellent electronic
conductivity, high permeability, low density, and high
specific surface area, has been widely employed as
electrode backbone, catalysts and catalyst substrates,
and sensors1-7. Recently, its important role has been
highlighted by the surge of binder-free high-
performance supercapacitors and lithium ion batteries
(LIBs) electrodes: direct deposit of active material on
three-dimensional conductive current collector for both
enhanced electronic conductivity and simplified
electrode preparation procedure4, 5, 8-10. Although direct
deposit has been easily achieved, the supporting
backbone was obtained by elaborate process11 even
with highly toxic precursors in the case of Incofoam®
Ni foam12; still, the large pore size (200μm to 2mm)
and consequent low specific surface area of the inactive
backbone may counteract the specific capacity and
energy density of the electrode as a whole6, 13. Thus,
alternative porous Ni foam with optimized pore
structure by facile method would be most enticing not
only for higher energy density for free-standing
advanced electrodes but also its low-cost application in
fields as energy storage, catalyst and sensors.
Glycine-nitrate combustion (GNC), a novel
illustration of sustainable redox reaction and propellant
chemistry, has proved to be a simple but cost-effective
method for scalable synthesis of porous and fine
advanced ceramics, catalysts and nanomaterials14, 15.
Typically, an oxidizer (O) and a fuel (F) are first mixed
in solution. When heated, the solution turns to vicious
gel and begins to foam after gelation. Then the redox
reaction initiates at critical ignition temperature (Tig)
and sustains due to intensive heat release. Meanwhile,
solid-state products are released, sintered into different
microstructure and made porous by the gaseous ones14-
17. Solution GNC received intensive attention when
Avarma etal18 obtained transition metal/alloy/cermet
rather than oxides by tuning F/O ratio, of which the
mechanism has been revealed through detailed studies
by Manukyans etal19, 20.
* Email: weifengwei@csu.edu.cn
Recently, modified combustion or redox method
has been employed to first obtain nanostructured
porous metal powder, followed by post-shaping into
monolith which began to serve as current collector of
supercapacitor and LIB13, 21. However, the as-prepared
metal backbone may suffer from friable problem and
increased contact resistance compared with its
continuous counterparts. Till now, continuous porous
metal monolith with considerable strength has not yet
been realized by this simple and cost-effective
combustion method due to the following reasons: (1)
foaming problem during gelation makes it difficult to
achieve uniform density of the products; (2) much gas
released within such a short time gives the combustion
spraying nature; (3) the instant and spraying nature
makes the sintering of products insufficient to achieve
reliable strength.
Based on previous pioneering studies, we report
in this communication the successful synthesis of
continuous hierarchically-porous Ni monolith by a
novel solid-state GNC: before heated to Tig, the viscous
gel with high F/O ratio was sufficiently dried to solid-
state which was then shaped and ignited in solid state
under mechanical confinement. By integrating solution
GNC into solid-state combustion, we achieved such
merits as follows: (1) molecular-level blending of
simple chemical reagents as in solution combustion
was inherited rather than expensive micro-scale metal
or non-metal powders mixed by tedious ball milling in
solid-state combustion; (2) foaming problem in
solution combustion was annulled by subsequent
shaping process which not only promises uniform
density of products but also steady self-propagating
combustion behaviour as in conventional solid-state
combustion; (3) less gas especially water would burst
out and breach the architecture after intensified drying.
To obtain continuous monolith, we highlighted the
premised role of released gas compared with
conventional gasless solid-state combustion; (4) up-
scale of GNC was well weaved into mature procedure
as shaping and post-processing could be omitted.
Impressively, a tuneable parameter as vacuum drying
temperature (Td) was then proposed for effective
control over propagating velocity (v) and maximum
temperature (Tmax) of combustion wave, and thus the
control over impurities, microstructure topography and
size. Theoretically, the solid-state GNC may represent
an illustration which blurs the boundary between
solution combustion and solid-state combustion. This
method can be modified for the controllable synthesis
of other porous metal/alloy/cermet monolith. The as-
synthesized porous Ni monolith may find instant
applications as electrode current collectors, catalyst
and catalyst substrates.
Figure 1 schematic illustration of solid-state glycine-nitrate
combustion synthesis of porous Ni monolith
The typical solid-state GNC started with nickel
nitrate hexahydrate (N) and glycine (G) solution
followed by a two-step drying, grinding and net
shaping, ended with confined solid-state combustion
(Figure 1) and detailed fabrication procedures were
presented in ESI. The process is simple and cost-
effective with small energy consumption and little
demand in equipment; the shape and size of the
metallic monoliths can be easily tailored, suggesting
industrial scaling-up flexibility; the as-synthesized Ni
monolith is competent for instant application as
conductive supporting backbone. It is worthy to note
the “nickel cycle” enclosed by nitrate acid dissolving,
which gives nickel nitrate to cycle in GNC.
The bonding state of precursors after two-step drying
was characterized by Infrared Spectroscopy (IR). As
depicted in Figure 2, blue shift of asymmetric
stretching peaks of COO- at 1610 cm-1 indicated the
coordination between Ni2+ and COO-, which was
further verified by the vibration band of M-O at about
553 cm-1. The shifts of bending vibration and stretching
vibration of N-H at 1514 cm1 and 3180 cm-1
respectively, and the absence of NH3+ vibration at
2131cm-1 confirmed the amino group coordination
with metal ions 22, 23. Such results validated the
molecular-level blending of GN precursors even in
solid state. The other noticeable result of two-step
drying was dehydration. As the increasing weight loss
and thermal gravimetric−differential scanning
calorimetry (TG-DSC) results indicated (ESI Tab. S 1
and Fig. S 1 ), more water as tied water from nitrate
salts and inter-molecular dehydration between glycine
was removed when precursor gel was dried at higher Td
20 . After sufficient drying, less gas especially water
would burst out and breach the architecture.
Figure 2 Infrared spectroscopy of GN precursors dried at different Td
The solid-state GNC process was fairly steady as
shown in ESI video, and propagating parameters as
Tmax and v were carefully measured. As shown in
Figure 3, Tmax presented a growing tendency with the
increasing Td, which may due to less heat consumption
to remove water from precursors, as suggested by
Manukyans etal’s calculation in iron-nitrate-glycine
system24. On the other hand, the propagating velocity
also increased from 0.6 mm/s to 6.7 mm/s with the
increase of Td, which may due to higher Tmax and the
consequent larger pre-combustion zone heated to Tig.
In this solid-state GNC, Tmax and v falls into similar
range with that of Manukyans etal’s20 and Varma’s
study18 in solution combustion (ESI Tab. S 2), which
implies solid-state GNC still inherited major
characteristics of solution combustion in term of Tmax
and v. It is noteworthy that low Tmax is more conducive
than high Tmax (> 2000K) in conventional solid-state
combustion in terms of the formation of nano-scale
microstructures with high surface area14, 15.
Nevertheless, solid-state combustion pathway is of
great practical value for not only annulling foaming
problems in solution combustion, achieving steady
self-propagating combustion behaviour and uniform
density of products, also post-processing is omitted.
Noticeably, the mechanical confinement transmitted by
two opposing quartz plates not only restrained the
spraying in axial direction, but also provided friction or
adhesion force in radical direction, which is a simple
but effective method to continuous monolith
compared to high pressure atmosphere25.
Figure 3 T-t profile and velocity of propagating combustion waves with precursors dried at different Td
Figure 4 Scanning electronic microscopy of porous Ni monolith with precursors dried at different Td (a) 70 ℃(b) 100 ℃ (c) 160
℃ (d) 200 ℃ and (e)(f) Td=160℃ confined by 7 MPa N2
pressure (scale bar a~d 4 μm, e 100μm, f 10μm)
The composition and microstructure of combustion
products were further characterized by Energy
Dispersive Spectroscopy (EDS) and Scanning
Electronic Microscopy (SEM). Less impurities as O
and C remained in final products when the precursors
were dried at higher Td (ESI ESI Tab. S 3) and pure
metal was obtained at Td =200 ℃. This may due to
more sufficient pyrolysis of nitrate and glycine and
subsequent redox reaction under higher Tmax,which can
avoid unwanted C, O impurities and post-heat
treatment for metallic foam15, 25, or on the other hand,
control element doping for functional oxide materials.
The microstructure was interconnected curved
sheets or branches with hierarchical nano-micro-scale
pores (ESI Fig. S 2). The porous nickel sheets mainly
composed of single layer of sintered grains, which
grew from about 300 nm to 4 μm as Td increased from
70℃ to 200℃ (Figure 4 (a) to (d)), which can be
ascribed to enhanced sintering by higher Tmax and less
gas release. Consequently, the pore structure, the void
space between sintered structures, varied. It is
interesting to notice the microstructure of hollow
porous microsphere when combustion was confined by
N2 pressure over 4 MPa (Figure 4 (e) and (f), ESI Fig
S 3). The formation of the unique microstructure
resulted from interplay between internal released gas
and external high pressure N2 on the released liquid Ni
droplet. High-pressure atmosphere, though demanding
and complicated, provided unique control over the
sintered microstructure.
The specific surface area of the pure metal sample
(Td =200 ℃) was 5.66 m2/g, density 0.2~0.4 g/cm3 and
the pore was in the range of 2nm~400 μm concentrated
at ~10 μm and 140 μm, as revealed by Mercury
Intrusion Porosimeter (ESI Fig. S 4). Compared with
Incofoam® Ni foam (450~3200μm)6, 12, the optimized
hierarchical pore structure may promise higher areal
specific capacitance and energy density for free-
standing advanced supercapacitors and LIBs electrodes.
Conclusions
In summary, a novel solid-state glycine-nitrate
process, where desired merits of solution
combustion and solid state combustion met, was
demonstrated for the scalable synthesis of
hierarchically porous Ni monolith. Also, tuneable
parameter as vacuum drying temperature was
proposed for the effective control over propagating
velocity and maximum temperature of combustion
wave, and thus the control over impurities,
microstructure topography and size. This method
may be modified for the controllable synthesis of
other porous metal/alloy/cermet monolith. The as-
synthesized porous Ni monolith may find instant
applications as electrode current collectors,
catalyst and catalyst substrates or sensors.
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Electronic Supplementary Information
Novel solid-state glycine-nitrate combustion for controllable
synthesis of hierarchically porous Ni monolith
Qin Guo, Ying Zhao, Jiatu Liu, Cheng Ma, Hangyu Zhou, Boyun Huang and Weifeng Wei
State Key Lab for Powder Metallurgy, Central South University,
Changsha, Hunan, P.R.China 410083.
Email: weifengwei@csu.edu.cn
Experimental Section
Materials Synthesis. Ni(NO3)2•6H2O (AR) was employed as oxidizer, glycine (AR) as fuel. In
a typical case, 50mM Ni(NO3)2•6H2O (O) and 75mM glycine (F) with F/O=1.5 were first resolved
in 30ml DI water to form uniform solution. Then, the solution was heated at 100℃for 30min to
vaporize water until the solution turned to viscous transparent gel. Afterwards, the gel was dried at
200℃ for 12h in vacuum oven and solidified into irregular foam. It is noteworthy that the precursor
is highly hygroscopic that foam was immediately transferred into Ar-filled glove box After being
artificially grinded into powders, precursors of 4g were weighed and compacted into a pellet of
16mm in diameter under pressure of 10MPa. Finally, the precursor pellet placed between two
opposing quartz plates confined by 10N mechanical pressure was locally ignited by Ni-Cr resistance
wire with 700mA current in air. For high pressure atmosphere confinement, the precursor pellet was
placed in a 10L stainless steel reactor filled with 4MPa N2 before locally ignited by Ni-Cr resistance
wire with 700mA current.
Process Monitor. Maximum temperature (Tmax) of combustion wave was carefully measured by
a 0.1 mm K type thermal couple. Data were collected by Angilent 34980A with 34921A module at
250 Hz frequency. The combustion processes in air were recorded by camera, based on which the
propagating velocities were estimated.
Materials Characterization. The pyrolysis behavior of gel precursors was analyzed by thermal
gravimetric−differential scanning calorimetry (TG–DSC, Netzsch STA449C Jupiter) in air at
heating rate of 10℃/min. The bonding state of solid state precursors was characterized by Fourier
Transformation Infrared Spectroscopy (Thermo Scientific Nicolet 6700) in the range of 4000-400
cm-1 as KBr discs. Scanning electron microscope (SEM) images and energy dispersive X-ray
spectroscopy (EDX) spectra were acquired on a FEI Qanta FEG 250 microscope operated at 10 kV.
The specific surface area and pore size distribution were characterized by mercury intrusion
porosimeter (Micromeritics AutoPore IV 9500 ).
Results section
Tab. S 1 weight loss ratio for precursors dried at different Td
Td
(℃)
Δm
(g)
Weight loss ratio
(%)
Theoretical weight loss ratio
(%)
70 3.9243 26.97 12.4
100 4.7362 32.54
160 5.4443 37.41 24.7
200 6.4259 44.16
Note:
1) F/O=1.5 with Ni(NO3)2•6H2O 50mM (14.55g) and glycine 75mM (5.63g).
2) Weight loss ratio was calculated based on the mass of Ni(NO3)2•6H2O
3) theoretical weight loss ratio is calculated based on the following equations1:
Ni(NO3)2 • 6H2O = Ni(NO3)2 • 4H2O + 2H2O T = 70℃
Ni(NO3)2 • 4H2O = Ni(NO3)2 • 2H2O + 2H2O T = 160℃
3Ni(NO3)2 • 2H2O = Ni3(NO3)2(OH)4 + 4HNO3 + 2H2O T = 250℃
Fig. S 1 Thermal gravimetric−differential scanning calorimetry (TG–DSC) of gel precursors with
F/O=1.5
Tab. S 2 Tmax and velocity compared with reference (F/O=1.5)
Combustion type Td
(℃)
Tmax
(℃)
Velocity
(mm/s)
Solid-state GNP 70 786 0.65
100 948 0.85
160 1120 6.27
200 1147 6.69
Gel combustion1 95 750~1150 0.8~1.1
Solution combustion2 Preheating >200 930~970 4~12
Note: data from reference were estimated value under same F/O ratio.
Tab. S 3 Energy dispersive spectroscopy of as-synthesized porous Ni monolith with precursors dried
at different Td (At%)
Td (℃) 70 100 160 200
Ni 85.3 84.4 96.8 100
O 8.9 3.7 3.2
C 5.8 11.9
Fig. S 2 Low magnification SEM of as-fabricated Ni with precursors dried at different Td (a)
70 ℃(b) 100 ℃ (c) 160 ℃ (d) 200 ℃ (scale bar 20 μm )
(b) (a)
(c) (d)
Fig. S 3 SEM of Ni (Td=160℃) confined by different N2 pressure (a) 1atm (b) 1MPa (c) 2MPa (d)
3MPa (e) 4MPa (f) 7MPa (scale bar 100 μm)
(e)
(f)
(b) (a)
(c) (d)
Fig. S 4 Mercury Intrusion porosimeter of Ni monolith (Td=200℃ by mechanical confinement) in
different range
Reference
1. K. V. Manukyan, A. Cross, S. Roslyakov, S. Rouvimov, A. S. Rogachev, E. E. Wolf and A. S. Mukasyan, Journal of
Physical Chemistry C, 2013, 117, 24417-24427.
2. P. Erri, J. Nader and A. Varma, Advanced Materials, 2008, 20, 1243-+.
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