| Direct NaBH4-H2O2 fuel cell is a novel type of fuel cell using liquid fuel and oxidizer.NaBH4 is dissolved in an alkaline electrolyte (usually NaOH) and directly utilized as fuel at the anode.NaBH4,is considered to be an attractive fuel for fuel cells due to its high reactive activity and high energy density. Besides, it is safe,non-toxic,chemically stable and easy to be transported in its dry state. However,the hydrogen evolution usually occurred during anodic oxidation of NaBH4,which not only decreased the fuel utilization and lowered the cell performance, but also increased complication of cell structure. Hydrogen evolution has been identified as the main problem hindering the development and application of DBFC. Therefore, it is necessary to study the anodic oxidation of NaBH4 and find a reasonable solution to solve the hydrogen evolution problem.Electrooxidation of BH4- on Pt-Ru nanoparticles (nPt:nRu=l:l in NaOH solution was investigated by cyclic voltammetry.An effective solution for depression of hydrogen evolution was studied and achieved by a quasi-8e-reation of BH4-.Direct NaBH4-H2O2 fuel cells using Pt-Ru alloy as anode catalyst were constructed and their performances were evaluated.Results revealed that the borohydride oxidation in 2 mol/L NaOH solution showed an eight electron process when the concentration of BH4- is lower than 0.03 mol/L.The eight electron oxidation was believed to proceed mainly via two consecutive steps: the formation of adsorbed hydrogen on Ru by catalytic hydrolysis of BH4-, and the subsequent electrooxidation of hydrogen on Pt.The main electrooxidation reaction of BH4- on Pt-Ru changed as BH4- concentration increased. A strong influence of OH- and BH4- concentration on the electrocatalytic oxidation of BH4- was found. The effect of BH4- concentration was more significant than the concentration of OH-. A peak power density of 345 mW/cm2 at a cell current density of 629 mA/cm2 was obtained for the optimized DBFC operating at 55℃.The optimized concentration of anode fuel and cathode oxidizer was 0.2 mol/L NaBH4+3 mol/L NaOH,0.6 mol/L H2O2+3 mol/L NaOH, respectively.The optimized atomic ratio of Pt to Ru in the anode catalyst was 3:1.Constant current discharge was carried at 250 mA/cm2 for 8000 s,results indicated that cell voltage decreased from 0.7 V to 0.4 V,and the power density decreased by 43%.Constant voltage discharge measurement at 0.7 V for 8000 s demonstrated that the current density decreased from 260 mA/cm2 to 119 mA/cm2,and the power density decreased by 54%.The decline of cell performance is mainly caused by the decrease of fuel and oxidizer concentration during the operation. |
PDF Full Text Request