Hydrogen Production from Liquid Ammonia Electrolysis Using a High Entropy Alloy Catalyst Composed of IrRhRuCoNi

Abstract

Liquid ammonia electrolysis is a promising route to generate hydrogen. It is performed at room temperature to obtain highly purified hydrogen isolated at the cathode. Theoretically, hydrogen can be extracted by electrolyzing liquid ammonia at a voltage of 0.077 V vs H₂/NH₃ at 25 °C. However, the onset voltage exceeds the theoretical value in practice owing to the high overpotential associated with the anodic reaction in the amide ion oxidation process. In this study, we prepared high entropy alloy IrRhRuCoNi powder to serve as an anodic catalyst. This powder exhibited a high specific surface area (66.2 m²/g) and was synthesized by reducing the oxide precursor in molten LiCl–CaH₂ at 600 °C. A comprehensive structural analysis involving X-ray diffraction and scanning/transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed that the synthesized powder consisted primarily of a face-centered cubic structure with a well-dispersed mixture of Ir, Rh, Ru, Co, and Ni at the nanoscale. The IrRhRuCoNi powder was then supported on carbon nanotubes and used as an anodic catalyst in liquid ammonia electrolysis. Chronoamperometry measurements demonstrated that the anodic catalyst exhibited current densities comparable to a commercial ruthenium black catalyst, reaching 0.93/4.20 mA/cm² at 0.3/0.5 V vs H₂/NH₃ after 300 s. Furthermore, the catalytic performance remained stable over a 50 h period. Notably, the alloyed catalyst maintained high current densities of 1.03/3.50 mA/cm² at 0.3/0.5 V vs H₂/NH₃ even during cyclic voltammetry measurements at an exceptionally slow scan rate of 2 mV/min from 0 to 0.5 V vs H₂/NH₃. In contrast, the commercial ruthenium black catalyst exhibited substantial decreases in current densities under these conditions. These results demonstrate the superior performance of the synthesized high entropy alloy IrRhRuCoNi catalyst for liquid ammonia electrolysis compared to the single Ru catalyst, which is likely attributed to the synergistic effects arising from the multielemental composition.