Studies On The Anode Catalyst For Direct Borohydride Fuel Cells And Bionic Metal Polar Plate Design

Fuel cell（FC） is an electrochemical conversion device which converts chemically energy from fuel and oxidant into electricity directly.The direct borohydride fuel cell （DBFC） takes borohydride solution as fuel.It has advantages such as higher theoretical voltage and power density than fuel cells with hydrogen gas as the fuel.At the same time,borohydride could be storaged and transported in a solid state or liquid state,it is much more convienient than the hydrogen.We want to explore the possibility using rare-earth hydrogen storage alloy as anodic catalysts for DBFC,and the reaction mechanism of the interface between electrode/electrolyte is also discussed in detail.The hydrogen storage alloy used as catalyst shows excellent catalytic activity to the oxidation and hydrolysis of BH₄^-,it also can adsorb and storage the hydrogen released from BH₄^-hydrolysis and then generates electricity further,which enhance the fuel utilization.In this work,LaNi_4.5Al_0.5 alloy is used as anodic catalyst for DBFC.The effect of electrolyte,reaction,temperature and discharge current on the electrochemical behaviours and electrode performance is investigated.Self-reduction, mechanical ball milling and heat treatment methods are applied to modify LaNi_4.5Al_0.5 alloy with Au,Ag,and Si elements.After modification,the side reaction,the hydrolysis of BH₄^- is suppressed and fuel utilization is enhanced to 65%,the largest power density reaches to 200 mW cm^-2.SEM-EDS,XRD,CV,EIS and polarization curve have been carded out to reveal the relationship between material structure,composition,particle size and the electrochemical performance of hydrogen storage alloys used as the catalysts in DBFC.It found that the direct 8e conversion was quite difficult,however,4e conversion can be realized along with generating electricity further from the hydrogen released form BH₄^- hydrolysis,thus fuel utilization was enhanced.The reaction mechanism in discharge process has been observed by in-situ IR.Quantum chemistry calculation has been used to investigate the energy change for BH₄^- dehydrogenation. It is indicated that the H^-in BH₄^- could be replaced by OH^- step by step,along with the process for hydrogen evolution,or H⁺ and electron generation.The evolution of hydrogen is unavoidable in DBFC because of the hydrolysis of borohydride,which will result in a gas-liquid biphase flow in the channel of bipolar plate and reduce the touch between the catalyst and the fuel,which reduces the discharge power density.In order to lessen the effect of hydrogen released from BH₄^-, a novel bionic leaf venation structure polar plate has been designed.It found that the novel polar plate not only exhibits the minimum flow resistance and better liquid distribution,but also eliminates the gas generated from the reaction process in the most shortest path,which enhances the fuel utilization and output power.