Spark-plasma-sintered porous electrodes for efficient oxygen evolution in alkaline water electrolysis

Abstract

Porous electrodes for alkaline water electrolysis were prepared by spark plasma sintering, a short-time sintering technique, in combination with a space holder method. After removal of the space holders, highly porous layers of polycrystalline Ni and of a nanocrystalline Ni-Fe alloy were obtained on a metallic substrate. Both porosity and thickness of the electrocatalytic layers can be controlled by the space holder volume content and the sintering process conditions, for example, the applied pressure and temperature. The active surface of the electrode can be increased significantly by a roughness factor of up to 1,120 determined by double layer charging. The porous layers are efficient towards oxygen evolution reaction (OER), whereas activity is greatly influenced by the chemical composition. The porous Ni-Fe electrodes exhibit an extremely low OER-overpotential of 230 mV at 0.3 A cm−2 in highly concentrated KOH (29.9 wt.-%) at 333 K. Beside the high surface area, the efficiency of the porous Ni-Fe layer is characterized by a high intrinsic activity resulting in a low Tafel slope of around 23 mV dec−1 at low and 50 mV dec−1 at high current densities as well as a high turnover frequency (TOF) of approximately 3.4 s−1 at 0.3 V. The porous Ni electrodes have a lower intrinsic activity with higher Tafel slopes and lower TOF. Moreover, an excellent stability and activity under realistic operating conditions of intermittent electrolysis (up to 1 A cm−2) for 100 h was proven for the porous Ni-Fe electrode.