Performance of the High Temperature PEM Membrane Electrode Assembly

High temperature polymer electrolyte membrane (HT-PEM) fuel cells provide an attractive alternative to low temperature (LT) PEM fuel cells owing to several advantages that include a tolerance to carbon monoxide so that reformate may be used, simplified cooling systems and balance of plant, higher value waste heat and no need for humidification of reactants. PEMs derived from phosphoric acid-doped polybenzimidazoles (PBI) have so far generated the most interest for high temperature PEM fuel cell operation. However, in order to realize a commercially competitive product there are several challenges that remain regarding the performance, durability and cost – particularly the cost associated with the platinum-based electrodes.

This communication will provide details on some of the HT-PEM MEA performance targets most recently achieved by Danish Power Systems, namely;

• MEA performances of >0.7 V cm^-2 at 0.2 A cm^-2 and >0.5 V at 0.8 A cm^-2 using dry H₂/Air.
• MEA lifetime of 12000 h at 0.23 A cm^-2 using dry H₂/Air with a degradation rate of <10 µV h^-1.
• An integrated 5 kW stack/reformer system operating for 3000 h using methanol reformate as fuel.

The task of optimizing MEA performance is carried out using rotating disc electrode voltammetry and CO stripping/electrochemical surface area studies in order to improve electrode properties (porosity and phosphoric acid management). Investigations into MEA durability are conducted using post mortem analyses on MEAs tested under operating conditions. These studies are comprised of cross-sectional SEM imaging and X-ray diffraction to monitor catalyst layer/membrane degradation and catalyst particle growth, respectively. Studies using reformate have also led to promising results with an MEA performance of 0.58 V cm^-2 at 0.385 A cm^-2 with a degradation rate of 30 µV h^-1 using wet H₂ (30 mol%)/Air.

In addition to reaching performance benchmarks, a reduction in the standard deviation for MEA performance to <5% has been achieved through efforts aimed at improving the uniformity of the membrane and catalyst layer thicknesses.