The physical and chemical state of phosphoric acid fuel cell assemblies after long term operation: surface and near-surface analysis

Abstract

An attempt was made to correlate performance losses with materials compatibility and compositional changes in phosphoric acid fuel cell (PAFC) electrode assemblies as a function of operation time. Westinghouse PAFC stacks were run under a constant operating regime and portions of some of the single cells were analyzed after stack operation for 5000 and 16 000 h along with appropriate reference samples. The PAFC assemblies were disassembled, sectioned where appropriate and analyzed using scanning and transmission electron microscopy (SEM and TEM), Rutherford backscattering spectroscopy (RBS), electron microprobe analysis (EMP), and X-ray photoelectron spectroscopy (XPS). The profiles of the Pt catalyst in both the anode and the cathode layer did not show any preferential loss or peaking. The most pronounced change in cell composition detected following stack operation for 5000 and 16 000 h was the increase in Pt/C ratio, that was related to loss of carbon from the cathode electrocatalyst. In contrast, the anode catalyst layer maintains the same ratio of Pt:C following 16 000 h of operation. The loss of carbon is thought to occur by an electrochemical mechanism and is enhanced at the higher potentials experienced by the air cathode in the fuel cell. In addition, TEM results clearly demonstrate the well recognized phenomenon of Pt particle agglomeration in the cathode catalyst layer, which is seen to be quite substantial after 5000 h of stack operation. This behavior was not observed at the anode electrocatalyst layer. The mechanical integrity of the assemblies was found to be quite satisfactory after 5000 h, but much less so after 16 000 h. Questions regarding carbon and Pt corrosion, Pt migration, and the chemical and physical integrity of the PAFC structures are addressed and are all postulated to be contributing to the observed cell performance losses.