High-Performance Cathodes for Alkaline Water Electrolysis in a Zero-Gap Setting: Ni–Sn/Ni Foam Prepared by Galvanostatic Electrodeposition

Abstract

In an attempt to obtain fully functional cathode materials for zero-gap alkaline water electrolysis, Ni foam substrates with various pore diameters were modified through galvanostatic electrodeposition of Ni–Sn alloys as an easily scalable procedure. To optimize the production process for each substrate, Ni–Sn alloys were electrodeposited at five different constant current densities. The obtained cathodes were primarily subjected to hydrogen evolution in 1 M KOH to evaluate their activity, while the best-performing samples were further investigated in 30 wt % KOH at 70 °C in a three- and two-electrode arrangement. Detailed electrochemical impedance spectroscopy analysis of hydrogen evolution reaction (HER) conducted with a three electrode arrangement indicated two semicircles on the Nyquist plots that confirmed that the adsorption of intermediate (H_ads) is potential dependent. Relevant HER parameters such as exchange current density and relaxation time showed exceptional performance of optimized electrodes. During zero-gap single cell tests with bare Ni foam used as the anode, onset voltages for Ni–Sn cathodes were around 1.64 V (for bare foams, 1.99 V), with cell voltage at 1 A cm^–2 being as low as 2.03 V (for bare foams, 2.57 V). The cathodes were also subjected to a long-term stability test, showing excellent activity preservation. Great stability, low cell voltage, and low production cost confirm their suitability for industrial applications. Top-view as well as cross-section electron microscopy analysis have shown that the entire foam surface was evenly covered with Ni–Sn coating. The composition of the investigated coatings was within the range of Ni_(1+x)Sn (0 < x < 0.5) metastable phase and practically independent of deposition current density. Aberration-corrected scanning transmission electron microscopy revealed that the so-called metastable phase is in fact the Ni₃Sn₂ phase, which is shown for the first time for electrodeposited Ni–Sn alloys.