C as a high performance cathode catalyst in direct borohydride fuel cell

Journal of Alloys and Compounds xxx (xxxx) xxx

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CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell Junkang Jia a, Xingxing Li a, Haiying Qin a, *, Yan He b, c, Hualiang Ni a, Hongzhong Chi a a

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR China Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, PR China c Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201210, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 July 2019 Received in revised form 15 November 2019 Accepted 16 November 2019 Available online xxx

Development of non-noble metal cathode catalysts was a continuous hotspots of fuel cell research. In this work, the CoO nanorods/C with different mass content of CoO were prepared by hydrothermal method and were investigated as cathode catalysts in direct borohydride fuel cell. The CoO nanorods exhibited an average length about 1e2 mm and an average diameter about 15 nm. The 10 wt% CoO nanorods/C achieved the best oxygen reduction reaction (ORR) activity compared to the 5 wt% CoO nanorods/C and 20 wt% CoO nanorods/C. The onset reduction reaction potential of the 10 wt% CoO nanorods/C catalyst toward ORR was 0.821 V (vs. reversible hydrogen electrode), and the number of electron transfer was about 4.01. The ORR on the catalyst was mainly based on an appropriate four-electron reaction pathway. The half-wave potential difference of the 10 wt% CoO nanorods/C after 5000 cycles was 26 mV, suggesting a good catalytic stability. The DBFC using the cathode catalyst achieved a maximum power density of 410 mW cm
2 at 60  C. The experimental results confirmed that the CoO nanorods/C had excellent catalytic performance in DBFCs. © 2019 Elsevier B.V. All rights reserved.

Keywords: CoO nanorods Electrocatalyst Oxygen reduction reaction Direct borohydride fuel cell

1. Introduction As a promising power generation technology, fuel cell has high efficiency, environmental friendliness and energy conservation [1e4]. Direct borohydride fuel cell (DBFC) has received widespread attention since its high theoretical power density of 5.67 Ah$g
1, high theoretical open circuit potential of 1.64 V [5e7], and the ability to use non-noble metals as cathode catalysts [8e11]. The ORR determines the overall efficiency of DBFC since the kinetics of ORR is much slower than that of the borohydride oxidation reaction. Due to fuel crossover with alkaline electrolyte, NaOH was usually formed at the cathode [10]. The non-noble metal cathode catalyst with high catalytic activity and good tolerance toward borohydride is a research hotspot of DBFC in recent years. Transition metals-N/C (Me/N/C, Me ¼ Co, Fe, Ni, etc.) are considered to be a promising candidate thanks to their good catalytic activity for ORR in alkaline environments [12e15]. Ma et al. [14,15] studied a DBFC using activated carbon-supported iron phthalocyanine as cathode catalyst. The cathode catalyst had good

* Corresponding author. E-mail address: [email protected] (H. Qin).

electrocatalytic activity and reached a performance of 92 mW cm
2. A DBFC using a highly active Fe-AAPyr as cathode catalyst could achieve a performance of 137 mW cm
2 [16]. Qin et al. [17] reported that a DBFCs with platelet-like Co(OH)2-PPy-C exhibited a power capability of 83 mW cm
2 under ambient conditions. He et al. [18] synthesized a nanobundles CoOOH-PPy-C as the cathode catalyst and got a high power density (101 mW cm
2) for the DBFC. The cell voltage was only reduced by 4% after 80 h of operation. Besides transition metal, the carbon support was considered to be another key factor determined the catalytic activities. The DBFC with the Co/N-macroporous carbon showed an excellent capability of 215 mW cm
2 [11,19]. Among these studies, most publications dealing with cathode catalyst focus on transition metal or carbon support and the best reported durability test maintains 80 h so far. It has been reported that one-dimensional nanostructured electrocatalysts have demonstrated unique and unusual physical and chemical properties endowed by dimensional confinement and anisotropy [20]. In this work, one-dimensional CoO nanorods/C with different mass fraction of CoO were prepared. The influences of morphologies and content of CoO on the catalytic activity towards ORR were investigated and discussed. Finally, a high catalytic

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Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065

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activity and good durability ORR catalyst (10 wt% CoO nanorods/C) is presented after optimization. 2. Experimental 2.1. Synthesis of CoO Nanorods/C First, 1.06 g spherical graphite (BP2000) and 60 mL deionized (DI) water were placed in a Teflon-lined and were stirred for 5 min. Then, 0.6 g CO(NH2)2, 0.15 g NH4F and different mass fractions of Co(NO3)2$6H2O (the mass ratio of CoO: C was 1:19, 1:9 and 1:4) was mixed in 15 mL DI water. Then the mixture was stirred by a magnet rotor for 5 min to prepare a uniform solution [21]. Pure CoO nanorods was prepared by similar method and investigated for comparison. The homogeneous solution was then transferred to the mixture of spherical graphite (BP2000) and deionized water and stirred for 30 min. The Teflon-lined was sealed in the stainless steel autoclave. Then the reaction vessel was held at 120  C for 6 h. The product was washed, dried and then annealed in Ar at 500  C for 2 h. 2.2. Characterization The phase analysis of the prepared samples were identified by X-ray diffractometer (XRD, the model was Rigaku D/max 2550 PC, Cu Ka). The chemical valence of Co in the prepared sample was identified by X-ray photoelectron spectroscopy measurements (XPS, the model was Thermo scientific ESCALAB 250 Xi). The XPS Peak 4.1 software was used to fit the raw data. Scanning electron microscopy (SEM, the model was FEI Apreo S Hivac) and transmission electron microscopy (TEM, the model was JEM-2100) were employed to analyze the microstructure. 2.3. Electrochemical measurements A three-electrode electrochemical system was employed to evaluate the electrochemical properties of the catalysts. The catalyst loading on the working electrode was 100 mg cm
2. A 0.1 M KOH solution was used as the electrolyte. All measurements were evaluated by using CHI 733e Electrochemical Station with a rotating instrument (The model was Gamry 710). All potentials mentioned later were transferred to the RHE scale by the equation as follows:

Eðvs: RHEÞ ¼ Eðvs: SCEÞ þ 0:0591  pH þ 0:244

(1)

Cyclic voltammetry (CV), rotating disk electrode (RDE), accelerated durability test (ADT) and rotating ring disk electrode (RRDE) tests were adopted to evaluate the catalytic properties and the experimental details could be found elsewhere [22e26]. The catalyst loading of CV, RDE and RRDE was 100 mg cm
2. The catalyst loading of ADT was 150 mg cm
2. 2.4. Cell performance measurements DBFC tests were performed at 30 and 60  C. The test area of the cells is 6 cm2. The Co(OH)2-PPy-BP was used as the anode catalyst and the prepared catalysts were used as the cathode catalyst. The loadings of cathode and anode catalyst were 5 mg cm
2. During assembling the fuel cell, the anode and cathode was just compressed on either side of Nafion 212 membrane together to form a mechanical contact. There was no need to make membrane electrode assembly by hot-pressing like proton exchange membrane fuel cell. The fuel was the solution contained a solution of 5 wt% NaBH4 and 10 wt% NaOH was used as fuel and wet O2 as oxidant.

3. Results and discussion XRD results of the synthesized CoO nanorods and 10 wt% CoO nanorods/C are shown in Fig. 1. The standard PDF cards of carbon (#75e1621) and CoO (#43e1004) are also given. The XRD results of the prepared CoO nanorods and CoO nanorods/C both display five main sharp peaks located at 2q ¼ 36.5 , 42.4 , 61.5 , 73.9 and 77.5 corresponding to (111), (200), (220), (311) and (222) of CoO respectively, which are consistent with the data of CoO standard PDF card. It is confirmed that the expected CoO could be synthesized. High resolution XPS spectra of the Co2p core-level in 10 wt% CoO nanorod/C is shown in Fig. (b). Four peaks corresponding to the Co2p core-level are extracted from the fits of the Co2p XPS spectra. Two peaks Co 2p3/2 and Co 2p1/2 locate at 780.6 and 796.1 eV, respectively. Two satellite peaks Co 2p3/2 satellite and Co 2p1/2 satellite locate at 804.1 and 786.3 eV, respectively. According to the spectrum of pure CoO, the binding energies of CoO 2p3/2 and CoO 2p1/2 are located at 781.1 and 796.3 eV respectively [27]. The difference of the binding energy of Co 2p3/2 and Co 2p1/2 in the asprepared 10 wt% CoO nanorod/C is 15.5 eV and the spectrum of the as-prepared 10 wt% CoO nanorods/C is similar to the spectra of pure CoO, which indicates the presence of Co2þ in the catalyst and the cobalt oxide is a monoxide. This result agrees well with the XRD result. Nanorod-shaped CoO was successfully prepared as shown in Fig. 2(a). SEM observation shows that the CoO nanorods mixed with carbon particles together in the10 wt% CoO nanorods/C (Fig. 2(b)). TEM images of the obtained CoO nanorods are given in Fig. 3(a), in which a batch of nanorods with diameters of 10e15 nm and lengths of 1e2 mm are observed. The diffraction patterns corresponding to the CoO nanorod indicate the good crystallinity of the CoO nanorod (Fig. 3(a)), which is in agreement with the XRD results. The crystallographic nature of the individual CoO nanorod was investigated by using HRTEM observations. Fig. 3 (b) is a section of a CoO nanorod, showing the growth direction as <311>. These {111} side planes are also observed in the CoO nanorod. In other words, the CoO nanorod mainly grows along the <311>direction and preferentially exposes the {111} planes. Fig. 3(c) and (d) show TEM images of the prepared 10 wt% CoO nanorods/C. A batch of nanorods with diameters of about 15 nm and lengths of about 1 mm are mixed with carbon particulars. The CV curves of CoO nanorods and the10 wt% CoO nanorods/C are shown in Fig. 4(a). The CoO nanorods and the 10 wt% CoO nanorods/C exhibit no obvious peak in N2-saturated alkaline electrolyte. In contrast, there is a sharp reduction peak of the two catalysts in O2-saturated alkaline electrolyte, which confirms electrocatalytic activity of CoO nanorods and the 10 wt% CoO nanorods/C towards ORR. However, the 10 wt% CoO nanorods/C composite catalyst shows much more positive onset potentials and higher cathodic currents towards ORR than those of pure CoO nanorods, suggesting synergistic ORR activity of the hybrid of CoO nanorods and carbon nanoparticles. Fig. 4 (b) shows the linear scan voltammograms of the CoO nanorods, 5 wt% CoO nanorods/C, 10 wt % CoO nanorods/C and 20 wt% CoO nanorods/C tested on RDE under 1600 rpm. The 20 wt% CoO nanorods/C shows a half-wave potential (E1/2) of 0.705 V and an onset potential (Eonset) of 0.906 V, comparable to those of the Pt/C catalyst (E1/2 ¼ 0.821 V, Eonset ¼ 0.93 V) and superior to those of CoO nanorods, 5 wt% CoO nanorods/C and 10 wt% CoO nanorods/C (Fig. 4(b) and Table 1). By contrast, CoO nanorods show the worst ORR activity (E1/2 ¼ 0.601 V, Eonset ¼ 0.703 V) among these catalysts. After mixing with carbon nanoparticles, considerable improvement in activity could be visualized for 5 wt% CoO nanorods/C (E1/2 ¼ 0.645 V, Eonset ¼ 0.772 V) and 10 wt% CoO nanorods/C (E1/2 ¼ 0.696 V, Eonset ¼ 0.821V) due to the CoeC synergetic effect.

Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065

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Fig. 1. (a) XRD spectra of the prepared CoO nanorods and the 10 wt% CoO nanorods/C, the standard PDF card of CoO (#43e1004) and carbon (#75e1621) are also supplied; (b) Co 2p XPS spectra of the 10 wt% CoO nanorods/C.

Fig. 2. SEM image of (a) CoO nanorods and (b) 10 wt% CoO nanorods/C.

Fig. 3. (a) TEM image and (b) HRTEM of CoO nanorods, inset in (a, b) is the select area diffraction pattern corresponding to the CoO nanorod; (c) TEM image and (d) HRTEM of the 10 wt% CoO nanorods/C.

Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065

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Fig. 4. (a) The CV curves of the CoO nanorods and 10 wt% CoO nanorods/C in oxygen (solid) or nitrogen (dash) saturated 0.1 M KOH, all the data plotted in CV curves are IR-free; (b) RDE curves of the CoO nanorods, 5 wt% CoO nanorods/C, 10 wt% CoO nanorods/C and 20 wt% CoO nanorods/C in O2-saturated 0.1 M KOH at 1600 rpm; (c) K-L plots and (d) Tafel plots derived from the RDE results of the CoO nanorods, 5 wt% CoO nanorods/C, 10 wt% CoO nanorods/C and 20 wt% CoO nanorods/C.

Table 1 ORR performances of the as-synthesized catalysts. Catalyst

Onset potential (V vs. RHE) Half-wave potential (V vs. RHE)

Electron transfer numbers

Tafel slope (mV$dec
1)

Current density at 0.8 V (mA cm
2)

CoO nanorods 5 wt% CoO nanorods/C 10 wt% CoO nanorods/ C 20 wt% CoO nanorods/ C

0.703 0.772 0.821

0.601 0.645 0.696

4.0 3.79 4.01

87.7 117.1 75.9

0.99 1.24 3.13

0.906

0.705

3.84

85.7

4.19

According to the K-L equation, LSV curves were tested upon various rotating speeds to obtain the electron transfer number (n) of different catalysts towards ORR. The n of CoO nanorods, 5 wt% CoO nanorods/C, 10 wt% CoO nanorods/C and 20 wt% CoO nanorods/C are calculated to be about 4.0, 3.79, 4.01 and 3.84 at 0.40 (vs. RHE), respectively (Fig. 4 (c)). It is indicated that the ORR mainly proceeds via a 4 electron transfer pathway on the CoO nanorods and the CoO nanorods/C. Fig. 4(d) shows the slope of Tafel plots in low overpotential range are 87.7, 117.1, 75.9 and 85.7 mV$dec
1 for the CoO nanorods, 5 wt% CoO nanorods/C, 10 wt% CoO nanorods/C and 20 wt% CoO nanorods/C. It could be seen that the 10 wt% CoO nanorods/C shows the best ORR kinetics performance (n ¼ 4.01, Tafel slope ¼ 75.9 mV$dec
1) among these catalysts, implying a good CoeC synergetic effect needs a reasonable mass ratio of Co and C to realize. For convenience, the CoO nanorods/C mentioned in the later of this manuscript is 10 wt% CoO nanorods/C. The RRDE test of the 10 wt% CoO nanorods/C was carried out to further understand the electron transfer pathway during the ORR reaction. Fig. 5 shows the n and the yield of H2O2 at a rotating ring electrode speed of 1600 rpm. Fig. 5(a) shows that the n is close to 4 and the yield of the H2O2 is below 15%. It could be concluded that

the 10 wt% CoO nanorods/C is mainly through a 4-electron process rather than a 2-electron process towards ORR based on the RDE and RRDE results. The durability of the 10 wt% CoO nanorods/C and conventional Pt/C catalyst in the alkaline electrolyte was investigated by ADT (Fig. 6). The E1/2 shift of the 10 wt% CoO nanorods/C is 26 mV while Pt/C declines 42 mV after 5000 cycles of LSV. The result indicates that the durability of the 10 wt% CoO nanorods/C should be slightly prior to the Pt/C in alkaline electrolyte. The outstanding durability of the 10 wt% CoO nanorods/C might be ascribed to the nanorods structure of CoO to prevent them from aggregation. Many studies have proven that it is a promising way to enhance the properties of the catalyst by changing their morphology, such as surface area and symmetry [28e31]. The comparison of electrocatalytic activity of the Co-based catalyst with different morphologies towards ORR in the alkaline medium is shown in Table 1. The Eonset and E1/2 shift of the CoO nanorods/C towards ORR is higher than that of CoOeN catalyst with nanoparticle (Table 2) [32], showing that the electrochemical activity of CoO nanorod/C towards ORR is higher than that of CoO nanoparticle/C. Moreover, the durability and electron transfer number towards ORR of the CoO

Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065

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Fig. 5. (a) The transfer electron number from RRDE, (b) the yield of H2O2 of the 10 wt% CoO nanorods/C.

Fig. 6. The ADT evaluation of (a) the 10 wt% CoO nanorods/C and (b) the Pt/C by 5000 cycles at 50 mV s
1 in O2-saturated 0.1 M KOH solution at 25  C.

Table 2 Comparison of ORR activities of reported catalysts in alkaline medium. Samples(a)

Heteroatom

Morphology

Eonset (V vs RHE)

DE(b) onset (mV)

Tafel slope (mV$dec
1)

DE(c) 1/2 (mV)

n

Ref.

CoO/C Co@CoeNeC Co/CoO@CoeNeC NeCoO

Co, Co, Co, Co,

Nanorod Nanoparticle Nanoparticle Nanoparticle

0.821 1.020 0.915 0.775


109 þ90
15
155

75.9 59 e e


26/5000 cycles
17/2000 cycles e e

4.0 e 3.8 3.9

This work [35] [25] [32]

C N, C N, C N

(a) All the samples were tested in 0.1 M KOH solution; (b) DEonset was Esample compared with EPt/C (0.93V); (c) DE1/2 after ADT in O2-saturated 0.1 M KOH solution.

nanorod/C is superior to the other Co-based ORR catalysts reported to date (Table 2). It has been found that cobalt oxide nanoparticles with different surface structure exhibited catalytic activity, which is attributed to the surface of Co2þ ions located at tetrahedral sites with the presence of {111} lattice planes of the nanoparticles was the catalytic active sites towards ORR [33]. It might also be the case in the prepared CoO nanorods/C, in which the nanorods structure results in highly exposed {111} planes (Fig. 3). Furthermore, the highly stressed surface configuration of nanowires could enhance the ORR activity as confirmed by reactive molecular dynamics simulations [34]. Li reported that one-dimension geometry of jagged Pt nanowires is available for remaining the stability of ultrafine nanostructures and benefiting the charge transport, leading to better performance along with enhanced stability, which is in line with the presented results [33]. The cell performances of DBFC using the prepared CoO nanorods and CoO nanorods/C cathode are shown in Fig. 7. The DBFC using pure

CoO nanorods has an open cell voltage (OCV) of 1.12 V at 30  C, which is comparable to those of the DBFC using CoO nanorods/C cathode, operated at 30  C (1.11 V). However, the DBFC using pure CoO nanorods exhibit a maximum power density (Pmax) of only 35 mW cm
2. At the same time, the DBFCs using the 5 wt% CoO nanorods/C cathode, 10 wt% CoO nanorods/C cathode and 20 wt% CoO nanorods/C cathode have Pmax of 199, 248 and 173 mW cm
2. Although the CoO nanorods exhibited catalytic properties towards ORR through a 4-electron process, a good conductivity is necessary to ensure its feasibility in DBFC as the cathode catalyst. The DBFC using the 10 wt% CoO nanorods/C cathode achieved a Pmax of 410 mW cm
2 at 60  C, much higher than that operated at 30  C (248 mW cm
2). The Pmax of various materials as cathode catalysts for DBFC are summarized in Fig. 8. The CoO nanorods/C in this work exhibits excellent cell performance as a DBFC cathode catalyst. It is suggesting that the feasibility of the CoO nanorods/C catalyst in the application for DBFC.

Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065

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Fig. 7. The cell performance of the DBFC using the CoO nanorods, 5 wt% CoO nanorods/C, 10 wt% CoO nanorods/C and 20 wt% CoO nanorods/C cathode operated at (a) 30  C and (b) 60  C. Anode: Co(OH)2-PPy-BP, the loading of catalyst: 5 mg cm
2, fuel: 5 wt% NaBH4-10 wt% NaOH, electrolyte: Nafion NRE-212 membrane, wet O2 at 100 mL min
1 under 0.2 MPa.

effects of CoeC. This work clearly shows that CoO nanorods/C is a high-performance catalyst in DBFC. Author Contributions Section Junkang Jia: Data curation, Methodology, Formal analysis, Writing-Original draft; Xingxing Li: Investigation, Visualization, Software; Haiying Qin: Conceptualization, Supervision, WritingReviewing and Editing, Funding acquisition; Yan He: Visualization, Funding acquisition; Hualiang Ni: Writing- Reviewing and Editing; Hongzhong Chi: Writing- Reviewing and Editing Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 8. Cell performance comparison of various cathode catalysts in DBFC. Membraneless DBFC with the cathode catalysts of CoPc [14](,), FePc/Ac [15]( ), MnO2 [36](), Eu2O3 [37](△), LaNiO3/C [38](▽), La0.8Sr0.2CoO3 [39](9), LaNi0.8Co0.2O3 [40](8), LaNi0.9Ru0.1O3 [41]( ), Sm2O3 [42](*), CeO2 [42](☉) and LaCoO3 [43]( ); Hydrophilic polypropylene diaphragm(-) DBFC with Fe-AAPyr [16] as cathode catalysts; Nafion NRE-212(C) DBFC with the cathode catalysts of NCX_WH [44], Pt/C [45] and CoO nanorods/C(this work); Nafion N112(:) DBFC with the cathode catalysts of Co-IAA/BP [46](pyrolyzed), Pt/C [47], Co/N-MPC [11] and Co(OH)2-PPY-C [48]; Anion exchange membrane( ) DBFC with Hypermec™K14 [49] as cathode catalysts; Nafion N117(+) DBFC with the cathode catalysts of FeTMPP [13], Pt/C [50], RuO2 [51], Co(OH)2-PPY-C [17,48], Co-PPY-C [52], CoOOH-PPY-C [18] and FeS-PPy-BP [26]; Nafion NRE-211( ) with the cathode catalysts of RuO2 [51] and Co (OH) 2-PPY-C [48].

⋄

4. Conclusions In summary, we synthesized CoO nanorods and CoO nanorods/C by hydrothermal method. Microstructure characterizations indicated the successful preparation of the CoO nanorods with an average length about 1e2 mm and an average diameter about 15 nm. The ORR on the Co nanorods/C catalyst is mainly a fourelectron pathway. After 5000 cycles, the E1/2 of the CoO nanorods/C is 26 mV, which is better than the E1/2 of the Pt/C (42 mV). Both the n and the E1/2 suggest the good catalytic performance of the prepared CoO nanorods/C. The DBFC assembled with CoO nanorods/C realized a Pmax of 410 mW cm
2 at 60  C, which is nearly the best performance of the DBFC tested at comparable conditions. The remarkable electrocatalytic performances are attributed to its unique microstructure, having mainly exposed {111} lattice planes on the surface and the significant synergetic

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Please cite this article as: J. Jia et al., CoO nanorods/C as a high performance cathode catalyst in direct borohydride fuel cell, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153065