Solid polymer electrolyte water electrolysis

Abstract

Studies have been made on electrocatalysts and their plating methods to solid polymer electrolyte (SPE). Perfluorosulfonic acid polymer membranes (Dupont manufactured Nafion) were used as SPE. Noble metals and their alloys were directly attached to both sides of the membrane without a binder by special metal plating methods developed by the present authors. The methods, utilizing reactions of a metal salt solution with a reducing agent on the membrane surface, made it possible to increase the adhesive strength of electrocatalysts to the membrane and also to eliminate, almost perfectly, the resistance of electrocatalyst/SPE interface. Pretreatments of the membrane were also investigated in order to improve the adhesive property. It was found that hydrothermal treatment and gas plasma treatment were more effective. The constituents of cell voltage, i.e. anodic and cathodic overvoltages and ohmic drop, were measured for five noble metals and their alloys. The anodic overvoltages were a major constituent of voltage losses and depended distinctly on the kinds of electrocatalysts used. The anodic overvoltage increased in the following order: Ir < Rh < Rh/Pt < Pt/Ru < Pt < Pd. Pure Ru had high initial activity. However, it was corroded during oxygen evolution. The electrodes based on Ir-Ir alloys were the best electrocatalysts for oxygen evolution and had a Tafel slope of 40–60 mV decade −1. The effects of the operating temperature on the cell performance were also studied. The cathode and anode were a thin layer of Pt and similar layer of Ir alloy, respectively. The cell voltage decreased with an increase in temperature. At a current density of 500 mA cm −2 and at 90°C, the cell voltage was 1.56–1.59 V corresponding to a thermal efficiency (based on ΔH) of 93–95%.