Investigation On The Anode Catalyst And Mea For Direct Sodium Borohydride Fuel Cell

In recent years, direct sodium borohydride fuel cell (DSBFC) has been receiving increasing attention because of its advantages, such as high voltage (1.64 V), high energy density (9.3 kWh·kg-1), and using non-noble metal as anode catalyst. The preparation of anode catalyst, preparation technology of membrane electrode assembly (MEA) and effect of operating conditions on the DSBFC performance have been investigated in this dissertation.PtNi/C, AuNi/C and AuPt/C catalysts with various molar ratios were prepared by using impregnation method. EDAX results showed that the molar ratio and metal loading of each catalyst were near to the expect values. Cyclic voltammetry (CV) and steady-state polarization measurement results indicated that 7:3 was the optimal molar ratio for the three binary alloy catalysts, and PtNi(7:3)/C showed the best performance. The absence of characteristic peaks of Ni in XRD pattern of PtNi(7:3)/C catalysts, positive shift of 2θfor characteristic peaks of Pt, and the decrease of the d value indicated that Ni had entered into Pt lattice and formed alloy with Pt. The same results were obtained in AuNi(7:3)/C catalyst. Heat treatment can result in the higher alloying between binary metals, and the catalyst activity can be further improved. XPS patterns of PtNi(7:3)/C shows that the electronic state of Pt was changed, i.e. increase of Pt0 content, due to the electron effect of Ni. This may be one of the reasons to the performance improvement.The MEA with PtNi(7:3)/C as the anode catalyst has the best performance. The increase of catalyst loading can improve the cell performance, but exorbitant catalyst loading can make the electrode too thick and inhibit the diffusion of the fuel. The optimized catalyst loading is 1 mg·cm-2. Nafion can bond and fix the catalyst meanwhile it can also decrease the catalyst active sites. Therefore the best content of Nafion is 10 mass%. The proportion of hydrophobic pore to hydrophilic pore in the anode diffusion layer can be regulated by PTFE content. The experimental results indicated that the MEA with 5% PTFE content had the best performance. The carbon powder in the diffusion layer can provide safe working place for the catalysts, but exorbitant carbon powder loading would make the diffusion layer too coarse and generate cracks, on the other hand, the carbon powder with high thickness can inhibit the diffusion of the fuel. The best carbon powder loading is 1 mg·cm-2. The performance of the MEA with Nafion 117 membrane was higher than that of the MEA with AEM membrane. Na+ pre-treatment on the electrolyte membrane can shorten the MEA activation time and improve the discharge stability. Under the optimized preparation technologies, the maximum power density of the MEA was 25.2 mW·cm-2 and 54.8 mW·cm-2 at 25℃and 60℃, respectively.The DSBFC performance was significantly improved with the increase of operating temperature. And, the concentration of NaBH4 also had effects on the performance of DSBFC, where the optimal concentration is 1.0 mol·L-1. The possible reason is that the high concentration of NaBH4 will increase the cathode polarization and reduce the fuel utilization. In addition, the electrolyte, NaOH is another factor, where the optimal concentration of NaOH is 6.0 mol·L-1. And the cathode working conditions also play an important role on the performance of DBFC. It has been found that the pure and humid oxygen used, and the removal of NaOH from the cathode will improve the long-term performance of DSBFC.To overcome the low utilization of fuel in DSBFC, a novel composite membrane electrode assembly (MEA) is developed, in which the anode zone includes two sections. The utilization of fuel can be increased to 100 % when the area ratio of the two sections is 3:1.