Studies On Anode Catalysts For Direct Borohydride Fuel Cell

Direct borohydride fuel cell (DBFC) is promising direct liquid fuel cell due to itshigh theoretical cell voltage and high energy density. However, the anodic oxidationof borohydride in alkaline medium is found to be accompanied with hydrogenevolution that reduces fuel efficiency. Therefore, how to depress hydrogen evolutionwithout impairing polarization properties becomes the target of research. In this thesis,the anodic oxidation of borohydride on several electrocatalysts was investigated usingthe electrochemical methods, XRD and TEM techniques.The main results obtainedare as follows:1. The Pd catalyst supported on multiwalled carbon nanotubes (Pd/MWCNT)was prepared through the liquid phase reduction method and its electrocatalyticactivity for borohydride oxidation was investigated.Compared with othercarbon-supported Pd catalysts, the Pd/MWCNT catalyst demonstrated improvedpolarization properties due to its smaller Pd particles and better dispersion of catalystparticles. The hydrogen evolution behavior on the Pd/activated carbon sample wasfound to be dependent on the concentrations of NaOH and NaBH₄. On the other hand,borohydride oxidation on Pd/carbon black and Pd/MWCNT displayed near 4ereaction mechanisms.2. The electro-oxidation of borohydride on carbon-supported Au and Ag wasstudied. The results suggest that the fuel efficiency of borohydride electro-oxidation ishighly dependent on the concentration ratio of [OH^-]/[BH₄^-]. At [OH^-]/[BH₄^-]=1, only3e reaction stoichiometry was obtained. When [OH^-]/[BH₄^-] was increased to 2, thereaction was more than 4e. If [OH^-]/[BH₄^-] became 5, the reaction was found to benear 8e. Detailed cyclic voltammetry (CV) studies reveal that BH₃OH^- is involved inthe 3e reaction when [0H^-]/[BH₄^-]=1. The non-8e reaction stoichiometry occurring onAu and Ag, both of which are inactive for the chemical hydrolysis reaction, suggeststhat there exists an inherent competition between OH^- and H₂O during borohydrideelectro-oxidation. If OH^- ions are not sufficient for each BH₄^- to accomplish the 8eelectro-oxidation, part of BH₄^- will react simultaneously with H₂O to generate hydrogen.3. The hydrogen evolution behaviors on Ni,Pd/C and Pt/C were examined. Itwas found that the anodic reaction on Ni was a 4e process. On the contrary, thehydrogen evolution on Pd/C and Pt/C(C: activated carbon )anodes could be depressedby increasing the concentration ratio of [OH^-]/[BH₄^-]. The Ni-Pd/C and Ni-Pt/Ccomposite anodes were prepared by adding Pd/C and Pt/C into the Ni anoderespectively. It was found that hydrogen evolution was depressed by adding Pd/C andPt/C in the Ni anode and the reaction stoichiometry was more than 4. The highconcentration ratio of [OH^-]/[BH₄^-] is beneficial to the depression of hydrogenevolution. In a 6M NaOH-1M NaBH₄ solution, the hydrogen evolution on Ni-Pd/Cand Ni-Pt/C composite anodes almost ceased and near-8e reaction occurred at largercurrents. In addition, the polarizations of the composite anodes became large with theincrease of [OH^-]/[BH₄^-] concentration ratios and the Ni-Pd/C anode revealed smalleroverpotentials than the Ni-Pt/C anode.