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Effect of cobalt precursors on Co3O4 anodic catalyst for a membrane-free direct borohydride fuel cell
Journal of Alloys and Compounds 724 (2017) 474e480
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of cobalt precursors on Co3O4 anodic catalyst for a membranefree direct borohydride fuel cell Jinfu Ma*, Xiangyu Gao, Dewei Wang, Tong Xue, Shaolin Yang School of Materials Science and Engineering, North Minzu University, Yinchuan, 750021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2017 Received in revised form 3 July 2017 Accepted 5 July 2017 Available online 6 July 2017
The fuel discharging efficiency of direct borohydride fuel cell (DBFC) depends on both the anodic catalyst activity and borohydride hydrolysis. In this paper, Co3O4 anodic catalysts prepared from different cobalt sources used in DBFC have been investigated. It showed that the catalytic performance of Co3O4 was associated with its microstructure that relied on the precursor. The maximum power densities obtained was 40, 62 and 28 mW cm2 of DBFC with Co3O4 catalyzed anode from CoCl2 (marked DBFC-A), Co(NO3)2 (marked DBFC-B) and Co(CH3COO)2 (marked DBFC-C), respectively. The Co3O4-A has the highest number of electron transfer (n ¼ 7.06) which is close to the theoretical value (n ¼ 8), and DBFC-A showed good voltage stability with specific capacity of 720 mAh.g1. A rapid voltage attenuation occurred in DBFC-B with the specific capacity of 330 mAh.g1 because Co3O4 from Co(NO3)2 has the higher catalytic activity for both borohydride oxidation reaction (BOR) and hydrolysis which lead to the lower fuel utilization efficiency. © 2017 Elsevier B.V. All rights reserved.
Keywords: Direct borohydride fuel cell Co3O4 Cobalt sources Discharge capacity Anodic catalyst
1. Introduction
O2 þ 2H2 O þ 4e /4OH
Many problems, such as the fossil energy shortage, haze weather, increasingly rapid growth of electricity demand of mobile devices, force people to look for new alternative energy and to explore the effective way of energy conversion. Fuel cell is one of the most promising devices for environmental friendly power generation by converting chemical energy directly into electrical energy without limitation of the Carnot efficiency. Borohydride is one of energetic materials, it can be catalyzed to produce hydrogen as fuel in acidic and neutral solution, and it can be oxidized to generate electrons directly in alkaline solution. Based on the alkaline mechanism, direct borohydride fuel cell (DBFC) [1e10], a liquid fuel cell with low operating temperature, appeared, and its working mechanism is as follows: Anode reaction:
BH4 þ 8OH /BO 2 þ 6H2 O þ 8e
Ea0 ¼ 1:24Vvs:SHE
Cathode reaction:
* Corresponding author. E-mail address: [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jallcom.2017.07.041 0925-8388/© 2017 Elsevier B.V. All rights reserved.
(1)
Ec0 ¼ 0:4Vvs:SHE
(2)
E0 ¼ 1:64Vvs:SHE
(3)
Overall reaction:
BH4 þ 2O2 /BO 2 þ 2H2 O
Hydrolysis reaction(side reaction):
BH4 þ 2H2 O/4H2 þ BO 2
(4)
The advantages of DBFC, such as high theoretical open-circuit voltage (OCV), high H-capacity and environmental safety, are considerable. To be honest, like the other fuel cells, there is still a great distance away from the actual application. Through a great deal of efforts, the problem of high cost and borohydride crossover has been solved with non-noble catalysts development and membrane-free cell structure [6,11,12]utilization. However, how to improve the fuel conversion efficiency and power density become the key problems at present. In order to solve problems mentioned above, anodic behavior of the borohydride oxidation (as reaction (1)) and borohydride hydrolysis (as reaction (4)) must be investigated. Ultimately, these decide the properties of anodic catalyst. As reported, 8e transfer of reaction (1) could be achieved by Au catalyst only with 100% fuel conversion efficiency but a very low power density. Recently, hydrogen storage alloys [13,14], noble metals and their alloys [4,5,15e24], and metal-oxide composite
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materials [1,6,25], and Co-based catalysts [6,9,12,26e28] have been studied due to their high activity for borohydride oxidation reaction (BOR). Li's [27] and yang's [12] researches showed that the CoO could be used as anode catalyst for DBFC, and the amorphous Co-B can improve the kinetics performance effectively and the Au@Co-B improved about 80% compared with the DBFC which employed pure Au as anode catalyst [6]. Here, Co3O4 was firstly applied to BOR and the effect of cobalt precursors on Co3O4 anodic catalyst has been investigated. 2. Experimental details 2.1. Preparation of Co3O4 All the chemicals of AR-grade were purchased from Aladdin reagent co., LTD. The Co3O4 samples were prepared by the homogeneous precipitation method. Added NH3$H2O drop-wise to 1 M CoCl2$6H2O solution under vigorous stirring at 30 C, the pH value of the suspension after precipitation reaction was monitored to 8.5e9, until the precipitate color become green. After a continuous stirring for 30 min, the obtained precipitate was filtered and washed with deionized water and alcohol to remove the unreacted chemicals. After drying at 80 C for 24 h, the precipitate was heated at 275 C with a ramp of 5 C$min1 for 2 h. The object product of Co3O4 was obtained which was mark Co3O4-A sample. The procedures of Co3O4 preparation from cobalt sources of Co(NO3)2$6H2O and Co(CH3COO)2$4H2O were as same as Co3O4-A sample with different calcination temperature, noted as 220 C and 300 C, respectively. The obtained products were marked Co3O4-B and Co3O4-C.
discharging performances of the DBFCs were measured by using a battery testing system (from Neware Technology Limited, Shenzhen, China). In order to evaluate the performances of the electrodes, the VeI characteristic curves for the cathode were generated using a conventional three-electrode electrochemical system consisting of the gas diffusion electrode as working, the Pt wire as counter and the Hg/HgO as reference electrodes. All the tests were operated at ambient temperature.
2.2. Characterization techniques
3. Results and discussion
Phase compositions of the prepared Co3O4 samples were characterized by an X-ray diffractometer (XRD-6000,Shimadzu, Japan) with CuKa radiation in the 2q range of 10e80 . XPS spectra were obtained from a Thermo Fisher Scientific XPS instrument (K-Alpha, America). Morphology of the prepared Co3O4 samples were tested by field emission scanning electron microscope (FESEM, Zeiss supra55,Germany) operating at 5e10 kV.
3.1. Analysis of the precursor of Co3O4
2.3. Preparation of electrodes and the DBFC
Fig. 1. TGA profiles of the precursor of Co3O4.
Thermogravimetric analysis (TGA) was carried out to investigate the thermal behavior of the different cobalt precursors which are CoCl2$6H2O, Co(NO3)2$6H2O and Co(CH3COO)2$4H2O. The TGA curves of the precursors are shown in Fig. 1. It can be seen that there are three distinct weight loss stages. In curve (a), the first weight loss of about 7.2% occurs at 30e175 C, which was ascribed to the evaporation of water. A weight loss was about 8% in the second stage corresponds to the thermal decomposition of Co(OH)2
The LaNiO3 catalyzed cathode was prepared by method of our previous work [11]. To prepare the anode, Co3O4 was mixed together with acetylene black (5 wt.%) and polyvinyl alcohol as adhesive, then the mixture was smeared onto a Ni-foam. After drying at 80 C in vacuum for 2 h, the anode was compacted by 5 MPa pressure. In order to activate the anode, it was pretreated in the 6 M KOH and 0.8 M KBH4 aqueous solutions for 24 h. The loading mass of Co3O4 is 100 mg cm2. In this study, we made a DBFC in which the fuel was stored in an attached fuel tank (18 mL) and the cathode worked by air-breathing without any auxiliary facilities. The electrodes were placed with a distance of 1 mm to prevent short-circuits, without using any Nafion membrane or polymer electrolyte membrane. 2.4. Test equipments and methods As testing, the fuel solution consisted of KOH and KBH4 was added to the fuel tank, and the source of oxygen was obtained from air through the gas diffusion layer. The line sweep voltammetry (LSV) and chronoamperometry tests were employed by using a computer controlled CHI920D electrochemistry workstation (CH Instrument, Inc., USA). The
Fig. 2. XRD patterns of as-prepared Co3O4 samples.
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(reaction (5)). From 275 C, an endothermic peak occurs which can be attribute to the thermal decomposition of the precursors into Co3O4 (reaction (6)). There was no weight loss after 425 C. A similar analysis can be conducted shown in curve (b) and curve (c), only the initial temperature of thermal decomposition of Co(OH)2 is different. It appears at 220 C and 300 C, respectively.
Co2þ þ 2NH3 $H2 O/CoðOHÞ2 þ 2NH4þ
(5)
6CoðOHÞ2 þ O2 /2Co3 O4 þ 6H2 O
(6)
3.2. The structure and morphology characterization As shown in Fig. 2, XRD patterns of Co3O4 produced from different cobalt precursors exhibit the typical cubic system and polycrystalline nature, which are in good accordance with the JCPDS42-1467. The diffraction peaks at 19.02 , 31.24 , 36.70 , 38.62 , 44.74 , 55.70 , 59.36 and 65.26 can be respectively denoted as (111), (220), (311), (222), (400), (422), (511) and (440) reflections. The sharp diffraction peaks indicated a higher crystallinity could be observed, it is in agreement with Deng et al. reported [29]. In order to detect the compositions of Co3O4 products, X-ray photo-electron spectroscopy (XPS) measurements were performed, as shown in Fig. 3a. The Co 2p spectra of the sample A exhibits two peaks at 780.8 eV and 796.1 eV, which are corresponding to the Co2p3/2 and Co2p1/2 spin orbit peaks of Co3O4, respectively. The energy difference between the Co2p3/2 peak and Co2p1/2 peak is 15.3eV, Co 2p3/2 and Co2p1/2 peaks could be satisfactorily assigned two peaks, which corresponded to Co3þand Co2þ, respectively (Fig. 3b) confirming the distinctive trait of the Co3O4 phase [29e31]. The satellite structure found in 788eV and 804eV further confirmed the co-existence of Co(II) and Co(III) on the surface of Co3O4 [32]. The morphology of as-synthesized Co3O4 from different cobalt precursors were characterized by SEM. It can be observed that Co3O4-A was consisted of regular cubic shape particles with length of ~30 nm (shown in Fig. 4a). Rod-like Co3O4-B (Fig. 4b) and bamboo-shaped Co3O4-C (Fig. 4c) have been identified. At the low magnification SEM images, Co3O4-A (Fig. 4a’) and Co3O4-B(Fig. 4b’) have sponge-like structure while Co3O4-C has obviously agglomeration (Fig. 4c’). 3.3. Kinetic properties of
BH4
electro-oxidation
According the Randles-Sevcik equations described in equations (7) and (8), the number of electrons of BH4 electro-oxidation can be obtained.
Ip ¼ 2:99 105 a1=2 n3=2 ACD1=2 v1=2
(7)
1:857RT na ¼ F Ep Ep=2
(8)
where Ip is the oxidation peak current(A), n is the total number of exchanged electrons, A is the geometric electrode area(cm2), v is the scan rate(V.s1), C is the bulk concentration of BH4 (mol.cm3), and D is the diffusion coefficient. Because in this paper the KBH4/KOH solution was used which was different from other reported research on DBFC used NaBH4þNaOH solution. And the diffusion coefficient(D) of BH4 in KBH4þKOH solution has not been reported, so the D was tested by
Fig. 3. (a) XPS spectrum of as-prepared Co3O4 samples and (b) fitting XPS spectrum of Co2p.
Bard'method [33]. In this test, a 25 mm diameter Pt micro-disk was used. According equation (9), the plot of id(t)/id,ss ~ t1/2 was carried out to evaluate the D.
. id ðtÞ id;ss ¼ 0:7854 þ p1=2 4 aðDtÞ1=2
(9)
where a is the radius of the micro-disk (mm), id,ss is the steady state value of the tip current(A) from chronoamperometry curve(Fig. 5a). In Fig. 5b, the pffiffi experimental points show that id(t)/id,ss is a linear function of 1= t ,thus agreeing with the theoretical prediction. A linear regression was used to compute the slope S, then D was evaluated to be 4.41 106 cm2/s by equation (9). The peak current (oxidation peaks of LSV in Fig. 6a) increases linearly with the square root of scan rate as shown in Fig. 6b. Using the Randles-Sevcik equations, the number of electron transfer (n) of Co3O4-A, Co3O4-B and Co3O4-C were calculated to be 7.06, 4.81 and 5.15, respectively. The n(¼7.06) of Co3O4-A is close to the theoretical value n(¼8), and the highest n of Co3O4-A means highest fuel utilization efficiency.
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Fig. 4. SEM images of Co3O4. high magnification (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C; low magnification (a0 ) Co3O4-A,(b0 ) Co3O4-B,(c0 ) Co3O4-C.
3.4. The polarization characteristics of different anodes Fig. 7 shows the anodic polarization characteristics of DBFCs with anodic catalysts from different cobalt sources. By using a 0.8 M NaBH4/6 M KOH (optimized in our previous researches [34,35])
electrolyte, at a scan rate of 50 mV s1, polarization degree (Dj) increased with potential shifts from 1.20 V to 0.8 V. At the lower potential(<-1.10 V), the polarization potentials are very close. The Dj presented obvious difference at higher potential (>-1.10 V), and increased by order of Co3O4-B < Co3O4-A < Co3O4-C. It means a
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Fig. 6. (a) The LSVs and (b) the scan rate dependence of peak current obtained on the Co3O4 catalysts (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. (the Co3O4 modified Glassy carbon electrode) Electrolyte: 1 M KOH/20 mM KBH4. Fig. 5. (a) Chronoamperometry curve at a micro-disk electrode and (b) Plot of the experimental ratio id(t)/id,ss against the inverse square root of time according Bard's method [32].(20 mM KBH4 in 1 M KOH with a 25 mm Pt micro-disk).
higher current discharge ability of DBFC is favored when use Co3O4B as anodic catalyst.
3.5. Cell performance The performance of DBFCs employing Co3O4-A, Co3O4-B and Co3O4-C are presented in Fig. 8. The OCVs of DBFC-A, DBFC-B and DBFC-C are 1.05 V, 0.93 V and 0.91 V, respectively. With the increasing discharge current density, the polarization degree is with DBFC-B < DBFC-A < DBFC-C sequence. The maximum power densities were obtained as 40, 62 and 28 mW cm2 of DBFC-A, DBFC-B and DBFC-C, respectively. By contrast, a membrane-free DBFC used Ag-Mn3O4/C as cathode catalyst and commercial Pt/C as anode catalyst, a maximum power density of 17 mW cm2 was obtained [36]. Fig. 9 shows the discharge curves of DBFCs using different anodic catalysts. Under the constant discharge current density of 50 mA cm2 and fixed fuel volume of 18 mL, the cell voltage stays around 0.72 V(DBFC-A), 0.75 V(DBFC-B) and 0.55 V(DBFC-C) at
Fig. 7. Polarization curves of Co3O4 catalyzed anodes (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. Electrolyte: 6 M KOH/0.8 M KBH4, Scan rate:50 mV s1.
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Fig. 8. Polarization curves of Co3O4 catalyzed anodes (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. Electrolyte: 6 M KOH/0.8 M KBH4.
Fig. 9. Specific capacity of Co3O4 anodic catalyzed DBFCs (a) DBFC-A,(b) DBFC-B,(c) DBFC-C. Electrolyte(fuel): 6 M KOH/0.8 M KBH4, Fuel volume: 18 mL.
479
initial stage, respectively. These voltages are corresponding to values in DBFC V-I curves(Fig. 8). With the increase of the discharge time, DBFC with different anodic catalysts showed different attenuation trend, in the discharge time of about 10 h, DBFC-A showed good voltage stability and obtains a specific capacity of 720 mAh.g1. However, a rapid voltage attenuation happened in DBFC-B, the specific capacity just reached 330 mAh.g1. DBFC-C displayed the lowest discharge voltage platform and obtained a moderate specific capacity of 475 mAh.g1. Meanwhile, drastic voltage fluctuations, due to a large amount of H2 produced by side effect (reaction (4)), occurred in the discharge process. It indicated the obvious catalytic competition from reactions (2) and (4) occurs in Co3O4-C.As a conclusion, DBFC-A showed the best specific capacity. This relative order of specific capacity is consistent with that of the electron transfer number. Stability of DBFC is one of key factors which is related to its practical applications. Fig. 10 showed the stability of DBFCs at a constant discharge current density of 50 mA cm2 with a continuous fuel supply (5 mL min1). As it can be seen from Fig. 9, the initial cell voltages of DBFC-A, DBFC-B, DBFC-C is 0.73 V, 0.75 V, 0.55 V, respectively. In the 500 h discharge test, the cell voltage of DBFC-A keep an ideal stability with just 0.3% decay, that of DBFC-B decrease from 0.75 V to 0.63 V (16% decay). The cell voltage of DBFC-C has a big attenuation from 0.55 V to 0.14 V. From the results, it shows that the DBFC-A has an ideal practical stability which is helpful to satisfy practical applications. The results of the present study show that the rod-shaped Co3O4 crystal has excellent catalytic ability but has lower specific capacity and stability. On the contrary, cubic-like Co3O4 crystal has lower catalytic ability but has excellent specific capacity and stability. By analysis, in the alkaline borohydride solution, Co3O4 can catalyze both BOR and borohydride hydrolysis [37,38], research has show that the higher surface area of Co3O4, the higher catalytic activity for borohydride hydrolysis [39]. In this study, rod-shaped (Co3O4-B) provided higher surface area than cubic-like Co3O4 (Co3O4-A) and bamboo-shaped Co3O4 (Co3O4-C), so it has excellent catalytic ability both for BOR and borohydride hydrolysis, which led to lower fuel utilization efficiency from BOR. The cubic-like Co3O4 has appropriate surface area thus the lower hydrogen generation rate from borohydride hydrolysis, which made Co3O4-A has higher fuel utilization efficiency and reasonable stability.
4. Conclusions
Fig. 10. Stability test of Co3O4 anodic catalyzed DBFCs (a) DBFC-A,(b) DBFC-B,(c) DBFCC. Electrolyte(fuel): 6 M KOH/0.8 M KBH4, Constant discharge current density: 50 mA cm2, Fuel supply way: continuously supply with a peristaltic multi-channel pump at rate of 5 mL min1.
In this study, Co3O4 was used as anodic catalyst for DBFC, and its catalytic properties depend on the cobalt sources. The Co3O4 crystals from different cobalt sources show various micromorphology. Co3O4-B (from Co(NO3)2) has the lowest anodic polarization and the highest cell power density, and the Co3O4-C(from Co(CH2COO)2) has the highest anodic polarization and the lowest cell power density. The specific capacity and stability tests show that DBFC-A possesses the best practice performance, for instance, i) its capacity (720 mAh.g1) is beyond 51.6% and 118% to that of DBFC-A and DBFC-C, respectively. It indicates Co3O4-A has the best restrain ability for BH 4 hydrolysis. ii) Co3O4-A also has the best long-term discharge stability. From results, the rod-shaped crystal is helpful to improve the catalytic ability for both BH 4 oxidation and hydrolysis, and the cubic-like crystal could improve the fuel utilization efficiency and the cell stability. However, these results are not fully explained by the widely investigation, the crystallinity, main parameters in preparation which impact catalytic activity for Co3O4 has not shown in this work. Specifically, the catalytic mechanism of BOR catalyzed by Co3O4 should be investigated in future research.
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Acknowledgments This work was supported by the National Natural Science Foundation (NSFC) of China (61440051, 21303257), Natural Science Foundation of Ningxia, China (No. NZ14097), Chinese Academy of Sciences (CAS) “Light of West China” Program. Key research project of North Minzu University in 2015 (2015KJ18, 2015KJ28). References [1] D.H. Duan, H.H. Liu, X. You, H.K. Wei, S.B. Liu, Anodic behavior of carbon supported Cu@Ag core-shell nanocatalysts in direct borohydride fuel cells, J. Power Sources 293 (2015) 292e300. [2] M. Martins, B. Sljukic, C.A.C. Sequeira, O. Metin, M. Erdem, T. Sener, D.M.F. Santos, Biobased carbon-supported palladium electrocatalysts for borohydride fuel cells, Int. J. Hydrogen Energy 41 (2016) 10914e10922. [3] M. Zhiani, I. Mohammadi, N. Salehi, Carbon supported Fe-Co nanoparticles with enhanced activity and BH4(-) tolerance used as a cathode in a passive air breathing anion exchange membrane direct borohydride fuel cell, Rsc Adv. 5 (2015) 23635e23645. [4] P.Y. He, X.Y. Wang, Y.J. Liu, X. Liu, L.H. Yi, Comparison of electrocatalytic activity of carbon-supported Au-M (M ¼ Fe, Co, Ni, Cu and Zn) bimetallic nanoparticles for direct borohydride fuel cells, Int. J. Hydrogen Energy 37 (2012) 11984e11993. [5] B. Sljukic, J. Milikic, D.M.F. Santos, C.A.C. Sequeira, D. Maccio, A. Saccone, Electrocatalytic performance of Pt-Dy alloys for direct borohydride fuel cells, J. Power Sources 272 (2014) 335e343. [6] S. Li, L.N. Wang, J. Chu, H.Y. Zhu, Y.Z. Chen, Y.N. Liu, Investigation of Au@Co-B nanoparticles as anode catalyst for direct borohydride fuel cells, Int. J. Hydrogen Energy 41 (2016) 8583e8588. [7] C.I. Karadag, G. Behmenyar, F.G.B. San, T. Sener, Investigation of carbon supported nanostructured PtAu alloy as electrocatalyst for direct borohydride fuel cell, Fuel Cells 15 (2015) 262e269. [8] D.M.F. Santos, B. Sljukic, L. Amaral, J. Milikic, C.A.C. Sequeira, D. Maccio, A. Saccone, Nickel-rare earth electrodes for sodium borohydride electrooxidation, Electrochim. Acta 190 (2016) 1050e1056. [9] K. Ye, X.K. Ma, X.M. Huang, D.M. Zhang, K. Cheng, G.L. Wang, D.X. Cao, The optimal design of Co catalyst morphology on a three-dimensional carbon sponge with low cost, inducing better sodium borohydride electrooxidation activity, Rsc Adv. 6 (2016) 41608e41617. [10] G.R. Li, Q.Q. Wang, B.H. Liu, Z.P. Li, Porous carbon as anode catalyst support to improve borohydride utilization in a direct borohydride fuel cell, Fuel Cells 15 (2015) 270e277. [11] J. Ma, Y. Liu, Y. Liu, Y. Yan, P. Zhang, A membraneless direct borohydride fuel cell using LaNiO3-catalysed cathode, Fuel Cells 8 (2008) 394e398. [12] X.D. Yang, Y.N. Liu, S. Li, X.Z. Wei, L. Wang, Y.Z. Chen, A direct borohydride fuel cell with a polymer fiber membrane and non-noble metal catalysts, Sci. Rep. 2 (2012). [13] G. Lota, A. Sierczynska, I. Acznik, K. Lota, AB(5)-type hydrogen storage alloy modified with carbon used as anodic materials in borohydride fuel cells, Int. J. Electrochem. Sci. 9 (2014) 659e669. [14] D.M. Zhang, G.L. Wang, K. Cheng, J.C. Huang, P. Yan, D.X. Cao, Enhancement of electrocatalytic performance of hydrogen storage alloys by multi-walled carbon nanotubes for sodium borohydride oxidation, J. Power Sources 245 (2014) 482e486. [15] D.H. Duan, X. You, J.W. Liang, S.B. Liu, Y.F. Wang, Carbon supported Cu-Pd nanoparticles as anode catalyst for direct borohydride-hydrogen peroxide fuel cells, Electrochim. Acta 176 (2015) 1126e1135. [16] B. Sljukic, J. Milikic, D.M.F. Santos, C.A.C. Sequeira, Carbon-supported Pt0.75M0.25 (M ¼ Ni or Co) electrocatalysts for borohydride oxidation, Electrochim. Acta 107 (2013) 577e583. [17] L.H. Yi, L. Liu, X. Liu, X.Y. Wang, W. Yi, P.Y. He, X.Y. Wang, Carbon-supported Pt-Co nanoparticles as anode catalyst for direct borohydride-hydrogen peroxide fuel cell: electrocatalysis and fuel cell performance, Int. J. Hydrogen Energy 37 (2012) 12650e12658.
[18] L.H. Yi, W. Wei, C.X. Zhao, L. Tian, J. Liu, X.Y. Wang, Enhanced activity of Au-Fe/ C anodic electrocatalyst for direct borohydride-hydrogen peroxide fuel cell, J. Power Sources 285 (2015) 325e333. [19] B. Choi, W.H. Nam, D.Y. Chung, I.S. Park, S.J. Yoo, J.C. Song, Y.E. Sung, Enhanced methanol tolerance of highly Pd rich Pd-Pt cathode electrocatalysts in direct methanol fuel cells, Electrochim. Acta 164 (2015) 235e242. [20] X. Liu, L.H. Yi, X.Y. Wang, J.C. Su, Y.F. Song, J. Liu, Graphene supported platinum nanoparticles as anode electrocatalyst for direct borohydride fuel cell, Int. J. Hydrogen Energy 37 (2012) 17984e17991. [21] H.Y. Qin, K.J. Chen, C. Zhu, J.B. Liu, J. Wang, Y. He, H.Z. Chi, H.L. Ni, Z.G. Ji, High electrocatalytic activity for borohydride oxidation on palladium nanocubes enclosed by {200} facets, J. Power Sources 299 (2015) 241e245. [22] I. Merino-Jimenez, M.J. Janik, C.P. de Leon, F.C. Walsh, Pd-Ir alloy as an anode material for borohydride oxidation, J. Power Sources 269 (2014) 498e508. [23] J. Liu, Q.L. Zhao, C. Wu, Y. Wang, W. Wei, X.Y. Wang, L.H. Yi, Performance improvement of activated nanoporous carbon supported gold catalyst as an anode for direct borohydride-hydrogen peroxide fuel cells, Rsc Adv. 4 (2014) 17129e17135. [24] F.G.B. San, O. Okur, C.I. Karadag, I. Isik-Gulsac, E. Okumus, y Evaluation of operating conditions on DBFC (direct borohydride fuel cell) performance with PtRu anode catalyst by response surface method, Energy 71 (2014) 160e169. [25] D.H. Duan, J.W. Liang, H.H. Liu, X. You, H.K. Wei, G.Q. Wei, S.B. Liu, The effective carbon supported core-shell structure of Ni@Au catalysts for electrooxidation of borohydride, Int. J. Hydrogen Energy 40 (2015) 488e500. [26] L. An, T.S. Zhao, X.H. Yan, X.L. Zhou, P. Tan, The dual role of hydrogen peroxide in fuel cells, Sci. Bull. 60 (2015) 55e64. [27] S. Li, Y.N. Liu, Y. Liu, Y.Z. Chen, Study of CoO as an anode catalyst for a membraneless direct borohydride fuel cell, J. Power Sources 195 (2010) 7202e7206. [28] S. Li, X.D. Yang, H.Y. Zhu, X.Z. Wei, Y.N. Liu, Ultrafine amorphous Co-W-B alloy as the anode catalyst for a direct borohydride fuel cell, Int. J. Hydrogen Energy 38 (2013) 2884e2888. [29] S. Deng, X. Liu, N. Chen, D. Deng, X. Xiao, Y. Wang, A highly sensitive VOC gas sensor using p-type mesoporous Co3O4 nanosheets prepared by a facile chemical coprecipitation method, Sens. Actuators B Chem. 233 (2016) 615e623. [30] M. Pal, R. Rakshit, A.K. Singh, K. Mandal, Ultra high supercapacitance of ultra small Co3O4 nanocubes, Energy 103 (2016) 481e486. [31] V.R. Mate, A. Jha, U.D. Joshi, K.R. Patil, M. Shirai, C.V. Rode, Effect of preparation parameters on characterization and activity of Co3O4 catalyst in liquid phase oxidation of lignin model substrates, Appl. Catal. A General 487 (2014) 130e138. [32] Y. Feng, L. Li, S. Niu, Y. Qu, Q. Zhang, Y. Li, W. Zhao, H. Li, J. Shi, Controlled synthesis of highly active mesoporous Co3O4 polycrystals for low temperature CO oxidation, Appl. Catal. B-Environ. 111 (2012) 461e466. [33] G. Denuault, M.V. Mirkin, A.J. Bard, Direct determination of diffusion coefficients by chronoamperometry at microdisk electrodes, J. Electroanal. Chem. Interfacial Electrochem. 308 (1991) 27e38. [34] J. Ma, Y. Liu, P. Zhang, J. Wang, A simple direct borohydride fuel cell with a cobalt phthalocyanine catalyzed cathode, Electrochem. Commun. 10 (2008) 100e102. [35] J. Ma, J. Wang, Y. Liu, Iron phthalocyanine as a cathode catalyst for a direct borohydride fuel cell, J. Power Sources 172 (2007) 220e224. [36] C. Grimmer, R. Zacharias, M. Grandi, B. Pichler, I. Kaltenboeck, F. Gebetsroither, J. Wagner, B. Cermenek, S. Weinberger, A. Schenk, V. Hacker, A membrane-free and practical mixed electrolyte direct borohydride fuel cell, J. Electrochem Soc. 163 (2016) F278eF283. [37] O.V. Netskina, O.V. Komova, S.A. Mukha, V.I. Simagina, Aqueous-alkaline NaBH4 solutions: the influence of hydride decomposition on catalytic properties of Co3O4, Catal. Commun. 85 (2016) 9e12. [38] G.R.M. Tomboc, A.H. Tamboli, H. Kim, Synthesis of Co3O4 macrocubes catalyst using novel chitosan/urea template for hydrogen generation from sodium borohydride, Energy 121 (2017) 238e245. [39] T.L. Pfeil, T.L. Pourpoint, L.J. Groven, Effects of crystallinity and morphology of solution combustion synthesized Co3O4 as a catalyst precursor in hydrolysis of sodium borohydride, Int. J. Hydrogen Energy 39 (2014) 2149e2159.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of cobalt precursors on Co3O4 anodic catalyst for a membranefree direct borohydride fuel cell Jinfu Ma*, Xiangyu Gao, Dewei Wang, Tong Xue, Shaolin Yang School of Materials Science and Engineering, North Minzu University, Yinchuan, 750021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2017 Received in revised form 3 July 2017 Accepted 5 July 2017 Available online 6 July 2017
The fuel discharging efficiency of direct borohydride fuel cell (DBFC) depends on both the anodic catalyst activity and borohydride hydrolysis. In this paper, Co3O4 anodic catalysts prepared from different cobalt sources used in DBFC have been investigated. It showed that the catalytic performance of Co3O4 was associated with its microstructure that relied on the precursor. The maximum power densities obtained was 40, 62 and 28 mW cm2 of DBFC with Co3O4 catalyzed anode from CoCl2 (marked DBFC-A), Co(NO3)2 (marked DBFC-B) and Co(CH3COO)2 (marked DBFC-C), respectively. The Co3O4-A has the highest number of electron transfer (n ¼ 7.06) which is close to the theoretical value (n ¼ 8), and DBFC-A showed good voltage stability with specific capacity of 720 mAh.g1. A rapid voltage attenuation occurred in DBFC-B with the specific capacity of 330 mAh.g1 because Co3O4 from Co(NO3)2 has the higher catalytic activity for both borohydride oxidation reaction (BOR) and hydrolysis which lead to the lower fuel utilization efficiency. © 2017 Elsevier B.V. All rights reserved.
Keywords: Direct borohydride fuel cell Co3O4 Cobalt sources Discharge capacity Anodic catalyst
1. Introduction
O2 þ 2H2 O þ 4e /4OH
Many problems, such as the fossil energy shortage, haze weather, increasingly rapid growth of electricity demand of mobile devices, force people to look for new alternative energy and to explore the effective way of energy conversion. Fuel cell is one of the most promising devices for environmental friendly power generation by converting chemical energy directly into electrical energy without limitation of the Carnot efficiency. Borohydride is one of energetic materials, it can be catalyzed to produce hydrogen as fuel in acidic and neutral solution, and it can be oxidized to generate electrons directly in alkaline solution. Based on the alkaline mechanism, direct borohydride fuel cell (DBFC) [1e10], a liquid fuel cell with low operating temperature, appeared, and its working mechanism is as follows: Anode reaction:
BH4 þ 8OH /BO 2 þ 6H2 O þ 8e
Ea0 ¼ 1:24Vvs:SHE
Cathode reaction:
* Corresponding author. E-mail address: [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jallcom.2017.07.041 0925-8388/© 2017 Elsevier B.V. All rights reserved.
(1)
Ec0 ¼ 0:4Vvs:SHE
(2)
E0 ¼ 1:64Vvs:SHE
(3)
Overall reaction:
BH4 þ 2O2 /BO 2 þ 2H2 O
Hydrolysis reaction(side reaction):
BH4 þ 2H2 O/4H2 þ BO 2
(4)
The advantages of DBFC, such as high theoretical open-circuit voltage (OCV), high H-capacity and environmental safety, are considerable. To be honest, like the other fuel cells, there is still a great distance away from the actual application. Through a great deal of efforts, the problem of high cost and borohydride crossover has been solved with non-noble catalysts development and membrane-free cell structure [6,11,12]utilization. However, how to improve the fuel conversion efficiency and power density become the key problems at present. In order to solve problems mentioned above, anodic behavior of the borohydride oxidation (as reaction (1)) and borohydride hydrolysis (as reaction (4)) must be investigated. Ultimately, these decide the properties of anodic catalyst. As reported, 8e transfer of reaction (1) could be achieved by Au catalyst only with 100% fuel conversion efficiency but a very low power density. Recently, hydrogen storage alloys [13,14], noble metals and their alloys [4,5,15e24], and metal-oxide composite
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materials [1,6,25], and Co-based catalysts [6,9,12,26e28] have been studied due to their high activity for borohydride oxidation reaction (BOR). Li's [27] and yang's [12] researches showed that the CoO could be used as anode catalyst for DBFC, and the amorphous Co-B can improve the kinetics performance effectively and the Au@Co-B improved about 80% compared with the DBFC which employed pure Au as anode catalyst [6]. Here, Co3O4 was firstly applied to BOR and the effect of cobalt precursors on Co3O4 anodic catalyst has been investigated. 2. Experimental details 2.1. Preparation of Co3O4 All the chemicals of AR-grade were purchased from Aladdin reagent co., LTD. The Co3O4 samples were prepared by the homogeneous precipitation method. Added NH3$H2O drop-wise to 1 M CoCl2$6H2O solution under vigorous stirring at 30 C, the pH value of the suspension after precipitation reaction was monitored to 8.5e9, until the precipitate color become green. After a continuous stirring for 30 min, the obtained precipitate was filtered and washed with deionized water and alcohol to remove the unreacted chemicals. After drying at 80 C for 24 h, the precipitate was heated at 275 C with a ramp of 5 C$min1 for 2 h. The object product of Co3O4 was obtained which was mark Co3O4-A sample. The procedures of Co3O4 preparation from cobalt sources of Co(NO3)2$6H2O and Co(CH3COO)2$4H2O were as same as Co3O4-A sample with different calcination temperature, noted as 220 C and 300 C, respectively. The obtained products were marked Co3O4-B and Co3O4-C.
discharging performances of the DBFCs were measured by using a battery testing system (from Neware Technology Limited, Shenzhen, China). In order to evaluate the performances of the electrodes, the VeI characteristic curves for the cathode were generated using a conventional three-electrode electrochemical system consisting of the gas diffusion electrode as working, the Pt wire as counter and the Hg/HgO as reference electrodes. All the tests were operated at ambient temperature.
2.2. Characterization techniques
3. Results and discussion
Phase compositions of the prepared Co3O4 samples were characterized by an X-ray diffractometer (XRD-6000,Shimadzu, Japan) with CuKa radiation in the 2q range of 10e80 . XPS spectra were obtained from a Thermo Fisher Scientific XPS instrument (K-Alpha, America). Morphology of the prepared Co3O4 samples were tested by field emission scanning electron microscope (FESEM, Zeiss supra55,Germany) operating at 5e10 kV.
3.1. Analysis of the precursor of Co3O4
2.3. Preparation of electrodes and the DBFC
Fig. 1. TGA profiles of the precursor of Co3O4.
Thermogravimetric analysis (TGA) was carried out to investigate the thermal behavior of the different cobalt precursors which are CoCl2$6H2O, Co(NO3)2$6H2O and Co(CH3COO)2$4H2O. The TGA curves of the precursors are shown in Fig. 1. It can be seen that there are three distinct weight loss stages. In curve (a), the first weight loss of about 7.2% occurs at 30e175 C, which was ascribed to the evaporation of water. A weight loss was about 8% in the second stage corresponds to the thermal decomposition of Co(OH)2
The LaNiO3 catalyzed cathode was prepared by method of our previous work [11]. To prepare the anode, Co3O4 was mixed together with acetylene black (5 wt.%) and polyvinyl alcohol as adhesive, then the mixture was smeared onto a Ni-foam. After drying at 80 C in vacuum for 2 h, the anode was compacted by 5 MPa pressure. In order to activate the anode, it was pretreated in the 6 M KOH and 0.8 M KBH4 aqueous solutions for 24 h. The loading mass of Co3O4 is 100 mg cm2. In this study, we made a DBFC in which the fuel was stored in an attached fuel tank (18 mL) and the cathode worked by air-breathing without any auxiliary facilities. The electrodes were placed with a distance of 1 mm to prevent short-circuits, without using any Nafion membrane or polymer electrolyte membrane. 2.4. Test equipments and methods As testing, the fuel solution consisted of KOH and KBH4 was added to the fuel tank, and the source of oxygen was obtained from air through the gas diffusion layer. The line sweep voltammetry (LSV) and chronoamperometry tests were employed by using a computer controlled CHI920D electrochemistry workstation (CH Instrument, Inc., USA). The
Fig. 2. XRD patterns of as-prepared Co3O4 samples.
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(reaction (5)). From 275 C, an endothermic peak occurs which can be attribute to the thermal decomposition of the precursors into Co3O4 (reaction (6)). There was no weight loss after 425 C. A similar analysis can be conducted shown in curve (b) and curve (c), only the initial temperature of thermal decomposition of Co(OH)2 is different. It appears at 220 C and 300 C, respectively.
Co2þ þ 2NH3 $H2 O/CoðOHÞ2 þ 2NH4þ
(5)
6CoðOHÞ2 þ O2 /2Co3 O4 þ 6H2 O
(6)
3.2. The structure and morphology characterization As shown in Fig. 2, XRD patterns of Co3O4 produced from different cobalt precursors exhibit the typical cubic system and polycrystalline nature, which are in good accordance with the JCPDS42-1467. The diffraction peaks at 19.02 , 31.24 , 36.70 , 38.62 , 44.74 , 55.70 , 59.36 and 65.26 can be respectively denoted as (111), (220), (311), (222), (400), (422), (511) and (440) reflections. The sharp diffraction peaks indicated a higher crystallinity could be observed, it is in agreement with Deng et al. reported [29]. In order to detect the compositions of Co3O4 products, X-ray photo-electron spectroscopy (XPS) measurements were performed, as shown in Fig. 3a. The Co 2p spectra of the sample A exhibits two peaks at 780.8 eV and 796.1 eV, which are corresponding to the Co2p3/2 and Co2p1/2 spin orbit peaks of Co3O4, respectively. The energy difference between the Co2p3/2 peak and Co2p1/2 peak is 15.3eV, Co 2p3/2 and Co2p1/2 peaks could be satisfactorily assigned two peaks, which corresponded to Co3þand Co2þ, respectively (Fig. 3b) confirming the distinctive trait of the Co3O4 phase [29e31]. The satellite structure found in 788eV and 804eV further confirmed the co-existence of Co(II) and Co(III) on the surface of Co3O4 [32]. The morphology of as-synthesized Co3O4 from different cobalt precursors were characterized by SEM. It can be observed that Co3O4-A was consisted of regular cubic shape particles with length of ~30 nm (shown in Fig. 4a). Rod-like Co3O4-B (Fig. 4b) and bamboo-shaped Co3O4-C (Fig. 4c) have been identified. At the low magnification SEM images, Co3O4-A (Fig. 4a’) and Co3O4-B(Fig. 4b’) have sponge-like structure while Co3O4-C has obviously agglomeration (Fig. 4c’). 3.3. Kinetic properties of
BH4
electro-oxidation
According the Randles-Sevcik equations described in equations (7) and (8), the number of electrons of BH4 electro-oxidation can be obtained.
Ip ¼ 2:99 105 a1=2 n3=2 ACD1=2 v1=2
(7)
1:857RT na ¼ F Ep Ep=2
(8)
where Ip is the oxidation peak current(A), n is the total number of exchanged electrons, A is the geometric electrode area(cm2), v is the scan rate(V.s1), C is the bulk concentration of BH4 (mol.cm3), and D is the diffusion coefficient. Because in this paper the KBH4/KOH solution was used which was different from other reported research on DBFC used NaBH4þNaOH solution. And the diffusion coefficient(D) of BH4 in KBH4þKOH solution has not been reported, so the D was tested by
Fig. 3. (a) XPS spectrum of as-prepared Co3O4 samples and (b) fitting XPS spectrum of Co2p.
Bard'method [33]. In this test, a 25 mm diameter Pt micro-disk was used. According equation (9), the plot of id(t)/id,ss ~ t1/2 was carried out to evaluate the D.
. id ðtÞ id;ss ¼ 0:7854 þ p1=2 4 aðDtÞ1=2
(9)
where a is the radius of the micro-disk (mm), id,ss is the steady state value of the tip current(A) from chronoamperometry curve(Fig. 5a). In Fig. 5b, the pffiffi experimental points show that id(t)/id,ss is a linear function of 1= t ,thus agreeing with the theoretical prediction. A linear regression was used to compute the slope S, then D was evaluated to be 4.41 106 cm2/s by equation (9). The peak current (oxidation peaks of LSV in Fig. 6a) increases linearly with the square root of scan rate as shown in Fig. 6b. Using the Randles-Sevcik equations, the number of electron transfer (n) of Co3O4-A, Co3O4-B and Co3O4-C were calculated to be 7.06, 4.81 and 5.15, respectively. The n(¼7.06) of Co3O4-A is close to the theoretical value n(¼8), and the highest n of Co3O4-A means highest fuel utilization efficiency.
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Fig. 4. SEM images of Co3O4. high magnification (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C; low magnification (a0 ) Co3O4-A,(b0 ) Co3O4-B,(c0 ) Co3O4-C.
3.4. The polarization characteristics of different anodes Fig. 7 shows the anodic polarization characteristics of DBFCs with anodic catalysts from different cobalt sources. By using a 0.8 M NaBH4/6 M KOH (optimized in our previous researches [34,35])
electrolyte, at a scan rate of 50 mV s1, polarization degree (Dj) increased with potential shifts from 1.20 V to 0.8 V. At the lower potential(<-1.10 V), the polarization potentials are very close. The Dj presented obvious difference at higher potential (>-1.10 V), and increased by order of Co3O4-B < Co3O4-A < Co3O4-C. It means a
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Fig. 6. (a) The LSVs and (b) the scan rate dependence of peak current obtained on the Co3O4 catalysts (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. (the Co3O4 modified Glassy carbon electrode) Electrolyte: 1 M KOH/20 mM KBH4. Fig. 5. (a) Chronoamperometry curve at a micro-disk electrode and (b) Plot of the experimental ratio id(t)/id,ss against the inverse square root of time according Bard's method [32].(20 mM KBH4 in 1 M KOH with a 25 mm Pt micro-disk).
higher current discharge ability of DBFC is favored when use Co3O4B as anodic catalyst.
3.5. Cell performance The performance of DBFCs employing Co3O4-A, Co3O4-B and Co3O4-C are presented in Fig. 8. The OCVs of DBFC-A, DBFC-B and DBFC-C are 1.05 V, 0.93 V and 0.91 V, respectively. With the increasing discharge current density, the polarization degree is with DBFC-B < DBFC-A < DBFC-C sequence. The maximum power densities were obtained as 40, 62 and 28 mW cm2 of DBFC-A, DBFC-B and DBFC-C, respectively. By contrast, a membrane-free DBFC used Ag-Mn3O4/C as cathode catalyst and commercial Pt/C as anode catalyst, a maximum power density of 17 mW cm2 was obtained [36]. Fig. 9 shows the discharge curves of DBFCs using different anodic catalysts. Under the constant discharge current density of 50 mA cm2 and fixed fuel volume of 18 mL, the cell voltage stays around 0.72 V(DBFC-A), 0.75 V(DBFC-B) and 0.55 V(DBFC-C) at
Fig. 7. Polarization curves of Co3O4 catalyzed anodes (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. Electrolyte: 6 M KOH/0.8 M KBH4, Scan rate:50 mV s1.
J. Ma et al. / Journal of Alloys and Compounds 724 (2017) 474e480
Fig. 8. Polarization curves of Co3O4 catalyzed anodes (a) Co3O4-A,(b) Co3O4-B,(c) Co3O4-C. Electrolyte: 6 M KOH/0.8 M KBH4.
Fig. 9. Specific capacity of Co3O4 anodic catalyzed DBFCs (a) DBFC-A,(b) DBFC-B,(c) DBFC-C. Electrolyte(fuel): 6 M KOH/0.8 M KBH4, Fuel volume: 18 mL.
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initial stage, respectively. These voltages are corresponding to values in DBFC V-I curves(Fig. 8). With the increase of the discharge time, DBFC with different anodic catalysts showed different attenuation trend, in the discharge time of about 10 h, DBFC-A showed good voltage stability and obtains a specific capacity of 720 mAh.g1. However, a rapid voltage attenuation happened in DBFC-B, the specific capacity just reached 330 mAh.g1. DBFC-C displayed the lowest discharge voltage platform and obtained a moderate specific capacity of 475 mAh.g1. Meanwhile, drastic voltage fluctuations, due to a large amount of H2 produced by side effect (reaction (4)), occurred in the discharge process. It indicated the obvious catalytic competition from reactions (2) and (4) occurs in Co3O4-C.As a conclusion, DBFC-A showed the best specific capacity. This relative order of specific capacity is consistent with that of the electron transfer number. Stability of DBFC is one of key factors which is related to its practical applications. Fig. 10 showed the stability of DBFCs at a constant discharge current density of 50 mA cm2 with a continuous fuel supply (5 mL min1). As it can be seen from Fig. 9, the initial cell voltages of DBFC-A, DBFC-B, DBFC-C is 0.73 V, 0.75 V, 0.55 V, respectively. In the 500 h discharge test, the cell voltage of DBFC-A keep an ideal stability with just 0.3% decay, that of DBFC-B decrease from 0.75 V to 0.63 V (16% decay). The cell voltage of DBFC-C has a big attenuation from 0.55 V to 0.14 V. From the results, it shows that the DBFC-A has an ideal practical stability which is helpful to satisfy practical applications. The results of the present study show that the rod-shaped Co3O4 crystal has excellent catalytic ability but has lower specific capacity and stability. On the contrary, cubic-like Co3O4 crystal has lower catalytic ability but has excellent specific capacity and stability. By analysis, in the alkaline borohydride solution, Co3O4 can catalyze both BOR and borohydride hydrolysis [37,38], research has show that the higher surface area of Co3O4, the higher catalytic activity for borohydride hydrolysis [39]. In this study, rod-shaped (Co3O4-B) provided higher surface area than cubic-like Co3O4 (Co3O4-A) and bamboo-shaped Co3O4 (Co3O4-C), so it has excellent catalytic ability both for BOR and borohydride hydrolysis, which led to lower fuel utilization efficiency from BOR. The cubic-like Co3O4 has appropriate surface area thus the lower hydrogen generation rate from borohydride hydrolysis, which made Co3O4-A has higher fuel utilization efficiency and reasonable stability.
4. Conclusions
Fig. 10. Stability test of Co3O4 anodic catalyzed DBFCs (a) DBFC-A,(b) DBFC-B,(c) DBFCC. Electrolyte(fuel): 6 M KOH/0.8 M KBH4, Constant discharge current density: 50 mA cm2, Fuel supply way: continuously supply with a peristaltic multi-channel pump at rate of 5 mL min1.
In this study, Co3O4 was used as anodic catalyst for DBFC, and its catalytic properties depend on the cobalt sources. The Co3O4 crystals from different cobalt sources show various micromorphology. Co3O4-B (from Co(NO3)2) has the lowest anodic polarization and the highest cell power density, and the Co3O4-C(from Co(CH2COO)2) has the highest anodic polarization and the lowest cell power density. The specific capacity and stability tests show that DBFC-A possesses the best practice performance, for instance, i) its capacity (720 mAh.g1) is beyond 51.6% and 118% to that of DBFC-A and DBFC-C, respectively. It indicates Co3O4-A has the best restrain ability for BH 4 hydrolysis. ii) Co3O4-A also has the best long-term discharge stability. From results, the rod-shaped crystal is helpful to improve the catalytic ability for both BH 4 oxidation and hydrolysis, and the cubic-like crystal could improve the fuel utilization efficiency and the cell stability. However, these results are not fully explained by the widely investigation, the crystallinity, main parameters in preparation which impact catalytic activity for Co3O4 has not shown in this work. Specifically, the catalytic mechanism of BOR catalyzed by Co3O4 should be investigated in future research.
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