INVESTIGATION ON DEGRADATION PHENOMENA OF PEM ELECTROLYZER OPERATING AT HIGH CURRENT DENSITIES

Kurzfassung

On the path to an emission free energy economy PEM water electrolysis (PEMWE) is a promising technology for the sustainable production of hydrogen with high current densities and thus high production rates of hydrogen. Long lifetime and platinum group metal loadings are major challenges for a widespread implementation of PEMWE. This work presents the result of 3000 h stack operation at high current densities of 4 A cm-2 (Figure 1 a) and the accompanied electrochemical in-situ characterization of the 8-cell stack. Half of the cells in the stack had standard noble metal loading, the other half reduced metal loading. After operation physical ex-situ analysis of selected cells were performed using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The general findings were similar in all cells. Electrochemical impedance spectroscopy (EIS) analysis at different time steps indicates mainly a decrease in ohmic resistance over time (Figure 1b). A significant loss of ionomer at anode cross-sections was detected by conductive AFM, whereas a loss of noble metal was observed at the cathodes. The loss of noble metals from the electrodes could be confirmed using EDX. XPS analysis revealed a small concentration of Ir and a small, but widely distributed amount of Pt in the membrane. After operation, a significant heterogeneous enrichment of Ti was measured on the cathode side but within the detection limits of EDX no Ti was observed in the membrane itself (Figure 1c). It is concluded that the coverage of Ti is originated from the cathode-side Ti bipolar plates. A significant thinning of the membranes after operation was detected using nitrogen cleaved samples for SEM, confirmed by the detected fluorine release during operation. This is in good agreement with the electrochemical results. The associated change of permeability of the membrane leads to an increased crossover of gases and water which affects the system behaviour negatively due to lower gas quality. Small particles and large degraded ionomer areas were observed by AFM-based surface potential measurements in the membrane after operation.