| Borohydride as fuel for fuel cell has attracted much attention in recent years due to its high electrooxidation activity and high specific energy.Theoretically, one BH4- can generate eight electrons. Pratically, one BH4- usually generates less than 8e because of the incomplete electrooxidation of BH4- and its hydrolysis, which lead to the reduction of borohydrides utilization. Therefore, inhibition of NaBH4 hydrolysis to H2 and improvement of NaBH4 utilization are important research topic for the development of direct borohydride fuel cells.In this thesis, surface modification of AB5-type hydrogen storage alloys by Pd was studied to improve the performance of hydrogen storage alloy electrocatalysts. The surface morphology of the AB5-type hydrogen storage alloys was characterized by SEM. The distribution of Pd on the hydrogen storage alloy surface was examined by EDS. The mass percent of Pd in hydrogen storage alloys was measured by ICP-Ms. The electrochemical oxidation of BH4- on Pd/AB5 electrode and the utilization of NaBH4 were studied by cyclic voltammetry and chronoamperometry methods.The results of hydrogen storage alloys surface chemical reduction treatment and Pd surface modification show that the catalytica performance of hydrogen storage alloy was significantly enhanced after being immersered in 6 mol·dm-3 NaOH +0.1 mol·dm-3 NaBH4 solution for four days and then reacting with 8 mL of 10 mmol·dm-3 Pd solution for 5min at room temperature. After modifications, the specific surface area was increased due to Ni-enrichment and Pd-deposition on the surface, which lead ot the improvement of NaBH4 electro-oxidation activity. The oxidation peak current density increased from about 18 mA·cm-2 at -0.65V to 90 mA·cm-2 at-0.15V after Pd-doping treatment in 0.1 mol·dm-3 NaBH4+1 mol·dm-3 NaOH solution. The stablelized current density on Pd/AB5 electrode reached around 36 mA·cm-2 at -0.7 V, which is around 22 mA-cm-2 on AB5 electrode.For the Pd/AB5 electrode, in 1 mol·dm-3 NaOH solution, NaBH4 oxidation current density increased with the increase of the concentratation of NaBH4. In 0.1 mol·dm-3 NaBH4 solution, with the increase of NaOH concentratation, NaBH4 oxidation current density increased first and then decreased.1 mol·dm-3 NaOH was found to be the optimum electrolyte concentration. NaBH4 oxidation is largely influenced by its concentration polarization at constant OH- concentration. The study of scan rate on the NaBH4 electro-oxidation shows that, the NaBH4 oxidation reaction mainly controlled by diffusion and the number of electrons transfer for NaBH4 oxidation reaction is around 7.NaBH4 utilization efficiency decreased with the increase of oxidation potential on the AB5 electrode. However, the NaBH4 utilization efficiency increased with the increase of oxidation potential on the Pd/AB5 electrode. Therefore, Pd/AB5 electrode shows higher utilization efficiency of NaBH4 than the AB5 electrode at high-current. The electro-oxidation rate of NaBH4 is higher than its hydrolysis rate on Pd/AB5 electrode. In 6 mol·dm-3 NaOH+0.1 mol·dm-3 NaBH4 solution, the utilization efficiency of NaBH4 at -0.3 V is 29.4% and 46.1% for AB5 and Pd/AB5 electrode, respectively. With the increase of NaBH4 and NaOH concentration, the utilization efficiency of the NaBH4 decreased on Pd/AB5 electrode. It is 42.4% in 0.1 mol·dm-3 NaBH4 and 1 mol·dm-3 NaOH solution at -0.7 V, and 32.7% in 1 mol·dm-3 NaBH4 and 6 mol·dm-3 NaOH solution. |
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