Electrochemical Characterization for Improvement of PEM Electrolysers

Abstract

These years the interest in electrical energy from renewable energy sources such as e.g. wind power is increasing. However, the demand of society for electrical energy does not always correspond to the production rate, and a method to store the electrical energy is needed. Electrolysis of water into hydrogen is one such method. It converts electrical energy into chemical energy bound in hydrogen molecules, which can then be stored. Knowing of performance limiting processes in operating PEMECs should enable engineering of improved electrolysis cells with a better performance allowing for economic storage of a bigger fraction of the renewable electrical energy. In this thesis polymer electrolyte membrane electrolysis cells (PEMECs) are investigated electrochemically in order to gain knowledge of the processes limiting the performance of PEMECs by contributing to the differential cell resistance. Three different PEMEC types from EWII A/S and Paul Scherrer Institute (PSI) were investigated with cyclic voltammetry (CV) combined with current density - voltage (i,V) curves and electrochemical impedance spectroscopy (EIS) measured at three different temperatures (approximately 54, 61 and 69 °C, but the exact temperature varied a bit between the cells) and at four to six different current densities (0.07, 0.35, 0.69, 1.00, 2.00, 3.00 A cm-2) for the EIS measurements. The influence of platinum coating of the titanium current collector was also investigated. The electrochemical results were supplemented with some scanning electron microscopy. Based on the experimental results a new hypothesis for the internal resistance of PEMECs was suggested. It was found that the performance of PEMECs are limited by proton conduction resistance of Nafion in the electrolyte and in the anode and by constrictions of the current path at the electrolyte/electrode interfaces. An optimized structure of the PEMECs has been suggested, which includes a thinner Nafion membrane that still prevents H2 cross over. It also includes IrOx anode catalyst particles as small as possible, but still being able to form triple phase boundaries with the Nafion binder and the water and produced gas in the electrode. This in combination with a titanium current collector with smaller pores than the titanium felt applied in the experiments in this thesis, but still very porous to prevent mass transport limitations,  will decrease the current constrictions in the PEMECs. By applying smaller IrOx particles the thickness of the anode catalyst layer can be decreased, though still obtaining the same proton conduction from the same amount of Nafion binder in the anode catalyst layer, and this will further decrease the differential cell resistance and increase the performance of the PEMECs. The presented hypothesis is new and discovered from the combination of EIS, CV and iVcurves. Only limited amounts of EIS on PEMECs is reported in literature, probably due to frequent observations of noise in the EIS measurements. This phenomenon has been investigated in this thesis. Based on EIS measurements a hypothesis has been presented, which suggests that the noise originates from the unstable gas bubble release at the electrodes caused by the AC perturbation and that the noise is dependent on the microstructure of the particular PEMEC.