Cesium dihydrogen phosphate as electrolyte for intermediate temperature proton exchange membrane water electrolysis (IT-PEMWE)

Abstract

In this work the potential application of CsH2PO4 as intermediate temperature electrolyte for Proton Exchange Membrane Water Electrolysis (PEMWE) was studied. This material, from the phosphate-based solid acid family, was previously reported as a promising electrolyte for intermediate temperature PEM fuel cells although no study as electrolyte in a PEMWE system had been carried out before. The physico-chemical properties of phosphate-based solid acids in terms of structure and morphology were investigated and their thermal stability evaluated. Proton conductivity at the intermediate temperature range (150 – 300 °C) was measured and the influence of humidity on the stability of CsH2PO4 in terms of dehydration and water solubility determined. Different approaches for the fabrication of CsH2PO4-based membranes are proposed in order to improve the mechanical properties and reduce the thickness and ohmic resistance of the electrolyte. Membrane fabrication techniques including casting of polymer/CsH2PO4 composites, glass-fibre reinforcement, polymer doping or electrospinning were developed and the resulting membranes characterised in terms of structure, proton conductivity and mechanical stability. The compatibility of CsH2PO4 with IrO2 was evaluated and compared to standard acid electrolyte solutions in a three-electrode half-cell in the low temperature range (40 – 80 °C). The performance of IrO2 towards oxygen evolution reaction (OER) in a CsH2PO4 concentrated solution exhibited poor activity, which was attributed to a high kinetic activation caused by the high pH and high phosphate concentration in solution. Finally the performance of CsH2PO4 as solid-state electrolyte in the electrolysis cell was evaluated at intermediate temperatures (235 – 265 °C). Electrodes were optimised in terms of catalyst and ionomer loading for an intimate catalyst/electrolyte contact and characterised by cyclic voltammetry. The electrolysis system was characterised by quasi-steady polarisation curves and electrochemical impedance spectroscopy. The maximum performance obtained by a Pt/CsH2PO4/IrO2 MEA system at 265 °C was 20 mA cm-2 at 1.90 V. This low activity, in good agreement with the results obtained in the half-cell, was mainly attributed to kinetic losses generated in the CsH2PO4/IrO2 interface. The low acidity of the electrolyte is considered to affect the active oxidation state of iridium, Abstract ii creating a non-hydrated oxide layer, which influenced negatively to the performance of the electrolyser. It is therefore concluded that despite the promising results reported for CsH2PO4 as electrolyte in intermediate temperature fuel cells, this material, and presumably the rest phosphate-based solid acids, are not to be considered as potential electrolytes for PEM water electrolysers.