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.

Notes and references 1. H. Wang, G. Wang, Y. Ling, F. Qian, Y. Song, X. Lu,

S. Chen, Y. Tong and Y. Li, Nanoscale, 2013, 5,

10283-10290.

2. B. C. Tappan, S. A. Steiner, 3rd and E. P. Luther,

Angewandte Chemie, 2010, 49, 4544-4565.

3. D. R. Rolison, J. W. Long, J. C. Lytle, A. E. Fischer,

C. P. Rhodes, T. M. McEvoy, M. E. Bourga and A. M.

Lubers, Chemical Society Reviews, 2009, 38, 226-252.

4. G. Zhu, Z. He, J. Chen, J. Zhao, X. Feng, Y. Ma, Q.

Fan, L. Wang and W. Huang, Nanoscale, 2014, 6,

1079-1085.

5. Z. Sun, S. Firdoz, E. Y.-X. Yap, L. Li and X. Lu,

Nanoscale, 2013, 5, 4379-4387.

6. W. Ni, H. B. Wu, B. Wang, R. Xu and X. W. Lou,

Small, 2012, 8, 3432-3437.

7. W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi and X.

Sun, Analyst, 2013, 138, 417-420.

8. G. Q. Zhang, H. B. Wu, H. E. Hoster, M. B. Chan-Park

and X. W. Lou, Energy & Environmental Science,

2012, 5, 9453-9456.

9. J. Yan, Q. Wang, T. Wei and Z. J. Fan, Advanced

Energy Materials, 2014, 4.

10. G. Zhang and X. W. Lou, Advanced Materials, 2013,

25, 976-979.

11. B. Bugnet and D. Doniat, Google Patents, 1989.

12. V. Paserin, S. Marcuson, J. Shu and D. S. Wilkinson,

Advanced Engineering Materials, 2004, 6, 454-459.

13. G. Gao, H. B. Wu, S. Ding, L.-M. Liu and X. W. Lou,

Small, 2014, n/a-n/a.

14. S. T. Aruna and A. S. Mukasyan, Curr Opin Solid St

M, 2008, 12, 44-50.

15. M. Lackner, Combustion synthesis: Novel routes to

novel materials, Bentham Science Publishers, 2010,

Chapter 16.

16. K. C. Patil, S. T. Aruna and T. Mimani, Curr Opin

Solid St M, 2002, 6, 507-512.

17. K. C. Patil, S. T. Aruna and S. Ekambaram, Current

opinion in solid state and materials science, 1997, 2,

158-165.

18. P. Erri, J. Nader and A. Varma, Advanced Materials,

2008, 20, 1243-+.

(b)

(c)

(a)

(b)

(d)

(f)

(e)

19. A. Kumar, E. E. Wolf and A. S. Mukasyan, Aiche

Journal, 2011, 57, 2207-2214.

20. 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.

21. Y. J. Fu, Z. B. Yang, X. W. Li, X. H. Wang, D. Q. Liu,

D. K. Hu, L. Qiao and D. Y. He, Journal of Materials

Chemistry A, 2013, 1, 10002-10007.

22. G. Socrates, Infrared and Raman characteristic group

frequencies: tables and charts, 3rd edition, John Wiley

& Sons, 2004.

23. F. H. A. Al-Jeboori, T. A. M. Al-Shimiesawi and O. M.

N. Jassim, Journal of Chemical & Pharmaceutical

Research, 2013, 5.

24. A. S. Mukasyan, P. Epstein and P. Dinka, Proceedings

of the Combustion Institute, 2007, 31, 1789-1795.

25. B. C. Tappan, M. H. Huynh, M. A. Hiskey, D. E.

Chavez, E. P. Luther, J. T. Mang and S. F. Son, J Am

Chem Soc, 2006, 128, 6589-6594.

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-+.