Electrocatalytic and thermal activation of anodic oxygen- and cathodic hydrogen-evolution in alkaline water electrolysis

In micro-electrolysis cells (4 cm2 electrode area) which possess a sandwich-configuration as used in advanced water electrolysis[1] different anodic and cathodic electrocatalysts, which did not contain noble metals were investigated over the current density range from 10−4 to 1.0 A cm−2 and a temperature range from 30 to 130°C (electrolyte: 50 wt% caustic potash). Mechanically activated nickel electrodes, RuO2 doped anodes and Pt-black covered cathodes served as comparison standards for H2 and O2 evolution. IR-drop connections were not applied. Nonetheless the measuring-method used allowed to keep IR-induced mistakes in voltage-reading in single-electrode voltages below 40 mV even at the highest current densities of 1.0 A cm−2. Plots of voltage vs log current densities for anodic oxygen evolution possess slopes between 40 and 70 mV (100°C) dec−1 of current density which in some cases—especially at low temperatures—increased up to a value of 2 RT/F at higher current densities. By increasing the temperature these steeper parts of the anodic current—voltage curves very often disappear. The current-voltage curves of anodic oxygen evolution for different temperatures unexpectedly run nearly parallel to each other ie with a slope which is nearly independent of temperature and thus cannot be described according to the Butler—Volmer equation with a constant value of the formal charge-transfer coefficient βi. The effective activation energies obtained from dln io/d(1/T) range from 70 to 100 kJ mole−1. O2 overpotentials at current densities around 1 A cm−2 are most efficiently decreased by (i) application of mixed oxides containing cobalt in at least two different valency states (CoII/CoIII or CoIII/CoIV) and (ii) by use of higher working temperatures; roughened surfaces, however, are only of limited value in this respect. Voltage vs log current curves for cathodic hydrogen evolution show a pattern which is in agreement with the Butler—Volmer equation. The effective charge-transfer coefficient is close to 0.5 and increases slightly above this value for Raney-metal activated cathodes. The effective activation energies lie between the limits of 40 and 55 kJ mol−1. Hydrogen evolution overpotentials are most efficiently decreased by (i) preparation of cathode surfaces with high roughness factors, (ii) using Ni, Co or Fe as cathode material and (iii) by increasing the working temperatures.