| The direct borohydride fuel cell(DBFC) has attracted much attention as a potential candidate of high power supplier due to its high capacity of 5.67 Ah g-1,high electromotive force(EMF) of 1.64 V and high theoretic energy conversion rate(91%).However,DBFC system for portable and mobile applications suffers from its high cost caused by the use of platinum as cathode catalyst.The development of non-platinum catalysts to replace the conventional platinum-based catalysts is one of the critical issues in the research of DBFC.In the present work,a transition metal macrocycles composite,polypyrrole modified carbon-supported cobalt hydroxide(Co(OH)2-PPY-C) has been fabricated and its electrochemical performances as cathode and anode catalysts for DBFC are evaluated.Co(OH)2-PPY-C was prepared by chemical method using Co(NO3)2·6H2O,pyrrole and carbon as the raw materials.The surface composition,microstructure were characterized by typical material identification methods such as powder X-ray diffraction,X-ray photoemission spectroscopy,field emission scanning electron microscopy,transmission electron microscopy.Its electrochemical performances were systematically studied by electrochemical analysis methods including the galvanostatic discharge,cyclic voltammetry, electrochemical impedance spectroscopy and rotation disk electrode cyclic voltammetry etc. Based on the obtained results,some main factors to influence catalyst activity are evaluated and discussed.The catalytic mechanisms of oxygen reduction reaction(ORR) and borohydride oxidation reaction(BOR) on Co(OH)2-PPY-C were suggested.The prepared Co(OH)2-PPY-C isβ-Co(OH)2 with a shape of platelet dispersed on carbon particles.As a cathode catalyst,Co(OH)2-PPY-C exhibits fairly good electro-catalytic activity and durability comparable to Pt/C for electrochemical oxygen reduction in the DBFC.At ambient condition,a maximal power density of 65 mW cm-2 has been obtained by the DBFC using hydrogen storage alloy composite as the anode catalyst,Co(OH)2-PPY-C as the cathode catalyst,Super P as the catalyst support material and Nafion 117 as the electrolyte. Co(OH)2-PPY-C is an applicable cathode catalyst for the DBFC and the number of electrons involved in the ORR is 2.9.It has been found that the Co(OH)2 platelet would be transformed into CoHO2 with a shape of needle during operation.It is considered CoHO2 is also a good ORR catalyst like Co(OH)2.The DBFC using Co(OH)2-PPY-C as the anode catalyst exhibits better cell performances compared to the DBFC using hydrogen storage alloy as anode catalyst.A maximal power density of 83 mW cm-2 has been obtained at ambient condition. The Co(OH)2 platelet would be also transformed into CoHO2 in the shape of needle during Co(OH)2-PPY-C being used as anode catalyst in DBFC,and the apparent coulombic number of BOR on Co(OH)2-PPY-C is 3.0.The arrangement pattern of cobalt atoms on the surface of catalyst is a key factor deciding the apparent coulombic number of BOR.When the Co(OH)2-PPY-C is used as ORR catalyst in alkaline medium,oxygen is electroreduced with a 4-electron-reaction on the cobalt sites and a 2-electron-reaction on the PPY modified carbon substrate.The catalysis of the Co(OH)2-PPY-C is strongly dependent upon the catalyst support material as well as the loading of PPY and Co(OH)2.The electron number of ORR and BOR on the catalyst could reach up to 3.7 and 4 respectively when using BP 2000 instead of Super P as the catalyst support material.The catalytic activity of catalyst could be improved with the increase of loading of PPY on the catalyst,but the number of reaction electron keeps unchanged.There is a optimum loading of Co(OH)2 on the PPY modified carbon.When the atomic proportion of Co to N is close to 1:2,the catalyst demonstrates a better ORR catalytic activity.The influences of fuel composition,operation temperature and applied electrolyte membrane on the performance of the DBFC using Co(OH)2-PPY-BP as cathode and anode catalyst are investigated.Maximal power density of 207 mW cm-2 at room temperature and 324 mW cm-2 at 60℃have been achieved respectively when the DBFC uses N 211 as the electrolyte,5 wt.%NaBH4-10 wt.%NaOH solution as the fuel.When Co(OH)2-PPY-C is annealed at temperatures ranging from 400 to 800℃,the diffraction peak of Co(OH)2 disappears and the diffraction peaks of Co3O4,CoO and Co appears.Meanwhile,the platelet grows up and turns to particles.The ORR and BOR catalyst activity of catalyst is improved with increasing the annealing temperature but decreased when annealing temperature reaches up to 800℃.Co(OH)2-PPY-C annealed at 600℃exhibits a better catalytic activity.From above obtained results,it can be concluded that the Co(OH)2-PPY-C catalyst has demonstrated a rather promising possibility to substitute commercialized platinum catalyst.It is believed that these achievements in this research have given a certain contribution to the development of DBFC technology.
|
PDF Full Text Request