The Use of Heteropoly Acids in Proton Exchange Membrane Fuel Cells

摘要

The heteropoly acids,HPA,are a large and diverse class of inorganic oxides with very high room temperature proton conductivities and interesting electrochemistry.Over 100 reports on the improvement demonstrated by HPA in proton exchange membrane,PEM,fuel cells have appeared in recent years.Most of these reports concern only the few commercially available HPA based on the Keggin structure,but there are many more HPA that can be readily synthesized in which charge and shape can be varied.We have undertaken a systematic study of proton conduction/structure activity relationships in the HPA and have shown that these materials can have high proton diffusion coefficients,over a range of temperatures from low to elevated,and relative humidity's from dry to fully humidified,all of which are of interest to the development of PEM fuel cells with greater versatility.Previously we have demonstrated very high current densities for a single fuel cell in which the only proton conducting component is the HPA using dry inlet gases at room temperature.Doping of perfluorosulfonic acid ionomers with HPA generally changes the protonic conduction of the material and can lead to increases in the observed diffusion coefficients,as well as an increase in the temperature at which the maximum diffusion coefficient is observed.In a fuel cell,when membrane components are doped with HPA the changes in proton conduction do not necessarily agree with the changes in performance observed in ex-situ testing.The effects in conditioned membrane electrode assemblies in a fuel cell can be dominated by a dramatic lowering of the electrode membrane interfacial resistance,as the HPA are mobile in these systems and may have migrated towards the electrode.Similar results are obtained when HPA are deliberately adsorbed on to a PEM fuel cell carbon supported anode demonstrating that the HPA can be used as the ionomer in a fuel cell electrode.Large enhancements to the proton conductivity of the membrane in a working fuel cell will only be realized when HPA are immobilized,whilst a number of immobilization strategies for HPA are available these will only be realized if nanoparticles of HPA particles can be synthesized in which the binding chemistry does not degrade the proton conducting ability of the HPA.