| Direct borohydride fuel cell（DBFC）is a type of direct liquid fuel cells,which has the advantages of high energy conversion efficiency,environmental friendliness,free of noise during electricity generation,and good stability.In addition,it has a high energy density(9.3 k Wh kg-1)and a high theoretical open circuit voltage（1.64 V）compared to other types of fuel cells,so it has received widespread attention in recent years.One of the main bottlenecks hindered the commercialization of DBFC is the sluggish kinetic of the borohydride oxidation reaction（BOR）for the anode.Although precious metal catalysts such as Pd and Pt,have good catalytic activity and stability for BOR,the expensive price and rareness of the precious metals limits their application in DBFCs.Compared with the precious metal catalysts,it is found that some non-noble metals,such as Ni metal has a higher catalytic activity than that of the noble metal catalysts,but it possesses an insufficient stability during BOR process due to the Ni oxidation at high potentials.To solve this problem,this study constructed highly efficient and stable Ni based bimetallic BOR electrodes（Pd@Ni and Ni-Co@Ni）based on the electrodepostion method.To reveal the origin of the enhanced stability and performance of the two electrode,various characterization,such as scanning electron microscope,energy dispersive spectroscopy,X-ray diffraction,and electrochemical methods,were performed.Based on the results from the two electrodes,an electrode（Ni-Co@Ni F）with a reduced mass transfer resistance was also proposed in this study.Convential electrodes for BOR are usually prepared by spraying method.To increase the integrity of the electrode,polymer binders are usually needed.However,the binder addition not only led to the increased ohmic resistance of the catalyst layer,but also resulted in the blockage of the passway of the reactants,both detrimental to the electrode performance.In this study,we used electrochemical deposition method to obtain the Pd@Ni electrode with an efficient catalytic activity.It was found that due to the complete exposure nickel active sites and the synergistic effect of Ni and Pd,the electrode exhibit an excellent catalytic activity and prolonged stability.Electrochemical characterization showed that the electrode showed the best catalytic activity at a Pd loading of 0.3 mg cm-2.The current density of Pd@Ni can reach 138 m A cm-2 at a potential of-0.7 V vs.Hg/Hg O.The DBFC with Pd@Ni electrode exhibit the maximum power density of 63 m W cm-2 at room temperature.To avoid the utilization of precious metal while achieve the similar stability of Pd@Ni,a Ni-Co@Ni electrode were also prepared by co-electrodeposition of Ni and Co on Ni mesh in this study.It was found that the surface of the non-precious metal electrode is a leaf-like structure and Ni is partly covered by Co.The electrochemical tests revealed that the introduction of Co protected the Ni metal from electrochemical oxidation during the operation,and the synergistic effect between Ni and Co was responsible to the enhanced catalytic activity of the electrode.When the electrode potential was-0.7 V vs.Hg/Hg O,the Pd@Ni-0.3 electrode had the highest current density of 138 m A cm-2.The DBFC with the Pd@Ni-0.3 electrode achieved the maximum power density of 67 m W cm-2.This value was comparable to that of the Pd@Ni electrode.To further improve the DBFC performance,nickel foam was used as the substrate for the electrodeposition of Ni and Co due to its well-developed porous structure.It was found that the open structure of Ni foam is conducive to the exposure of the active sites of the catalysts and the reduction of the mass transfer resistance.It was also shown that the Ni-Co@Ni F-0.3 electrode exhibited a current density of 530 m A cm-2 at-0.88 V vs.Hg/Hg O.The DBFC with the Ni-Co@Ni F-0.3 electrode showed a maximum power density of 75 m W cm-2 at room temperatures,which was 12%higher than that of ni-Co@Ni electrodes. |
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