The role of nickel hydroxide phases in wastewater electrolysis for sustainable green hydrogen production

Abstract

The energy sector plays a major role in driving climate change. Sustainable green hydrogen production through wastewater electrolysis is a promising alternative to fossil fuel consumption, without putting at risk precious resources, such as freshwater, at periods of high energy demand. This study investigates how interfacial engineering can be implemented in order to enhance the electrocatlytic activity of earth abundant nanostructures, such as nickel hydroxide, in water/wastewater electrolysis. It was demonstrated that the Ni(OH)₂ sample containing a mix of α- and β-phases phases (αβ1-Ni(OH)₂) exhibited the best oxygen evolution reaction (OER) performance due to the synergistic interface between the two phases, which enhanced its catalytic activity and reaction kinetics compared to single-phase samples. It showed a low Tafel slope of 18.8 mV dec⁻¹ and potentials of 1.42, 1.63, and 1.82 V vs. RHE at current densities of 10, 100, and 500 mA cm⁻², respectively. This αβ1-Ni(OH)₂ sample was also tested for urea oxidation reaction (UOR). While UOR kinetics were slower (Tafel slope of 31.5 mV dec⁻¹), the potentials at 10 and 100 mA cm⁻² (1.40 and 1.60 V vs. RHE) were slightly lower than those for OER, indicating higher energy efficiency at lower current densities. However, at 500 mA cm⁻², the UOR potential increased significantly to 2.20 V vs. RHE, compared to 1.82 V vs. RHE for OER, likely due to competition between UOR and OER at high current densities. Post-stability testing revealed surface degradation (cracking, sintering, oxidation, and oxygen defect formation) in OER conditions, while the same sample remained morphologically and chemically stable during UOR testing. This new knowledge provides valuable information into the design and interfacial engineering of Ni(OH)₂ nanostructures for wastewater electrolysis.