The anode in a PEFC reformate system

Abstract: Polymer electrolyte fuel cells (PEFC) constitute one ofseveral types of fuel cells that can be used to converthydrogen to electricity and heat. PEFC co-generation systemsbased on fossil fuels are on the edge of commercialisation.Biogas may be a future fuel in these kinds of systems. Thecommon denominator for these fuels is the carbonmonoxide-containing product gas from the fuel processor,poisoning the catalyst of the fuel cell.A model for the carbon monoxide (CO) oxidation mechanism onPtRu/C in Nafion® was validated by steady-state andopen-circuit potential decay experiments in a porous electrode.Furthermore, a natural gas fuelled system with an autothermalreformer and PEFC has been built from purchased components. Theconstruction was lead by ABB AB with support from KTH,especially in designing the control system. The differentcomponents of the system were evaluated at KTH.It is shown that the oxidation of CO can be described with amodel that takes into consideration the adsorption of CO on Ptand the adsorption of H2O on Ru. The adsorbed CO can be eitherlinearly or bridge-bonded to the surface. The two forms cannotbe distinguished from each other by the method used in thiswork. It is also shown that the oxidation of CO cannot bedescribed by a model describing the surface as homogeneous,where all the catalyst sites are treated equally. Therate-determining step is the oxidation step, with the otherreactions in pseudo-equilibrium up to about 0.5 V vs. RHE,where the adsorption of CO is not in equilibrium but still notrate-determining. However, at the lowest potentials, theadsorption of water might be the rate-determining step. Theopen circuit decay method confirmed the accuracy of thesteady-state measurements. This technique does not suffer fromnoise or the placement of the reference electrode since thepotential gradient in the electrolyte disappears as soon as thecurrent is interrupted. Furthermore, the model presentedpredicts the coverage of COads on the catalyst surface. Thus,the model can be used together with a model for H2 oxidation.The experimental set-up of the single cell is a useful platformfor future work on CO kinetics with different catalysts and atdifferent temperatures. It is possible to study the hydrogenoxidation in presence of CO with existing equipment but purehydrogen oxidation will still be challenging.The PEFC system is based on a fuel processor for 2-10 kWeland a fuel cell stack of 2 kWel maximum. Furthermore, itcontains a DC/AC inverter, a control system, a gas analysissystem and extensive measurement equipment, such asthermocouples and pressure meters. The research PEFC systemwith infrastructure is a useful platform for future researchalthough various components may need to be replaced.