Critical insights into eutectic molten hydroxide electrolysis for sustainable green hydrogen production

Abstract

In addressing global energy demands, the focus on hydrogen gas production from renewable sources intensifies. This research review investigates hydrogen production via steam splitting using eutectic molten hydroxide (NaOH–KOH%) electrolysis, a promising solution for escalating energy needs. A pivotal aspect involves developing a novel reference electrode for eutectic molten hydroxide, enveloping Ni/Ni(OH)₂ with an alumina or mullite tube ionic membrane. The mullite-covered electrode proves stable and reusable from 225 to 300 °C, showcasing a novel advancement in electrochemical stability. Compared to silver and platinum quasi-reference electrodes, the designed reference electrode demonstrates superior stability and efficacy in controlling the platinum working electrode, marking a significant innovation. Moreover, an intriguing cyclic voltammetry study examines different working electrodes, including Ni, Pt, Ag, Mo, and stainless steel (SS) in eutectic molten hydroxide at different temperature conditions. The observed reduction potential for hydrogen evolution follows the order: Ni > Pt > Ag > SS > Mo, corroborated by chronoamperometry, underscoring the reliability of the findings. In the pursuit of high-temperature eutectic molten hydroxide electrolysis to split steam into hydrogen fuel, cathodes of nickel, platinum, and stainless steel are deployed alongside stainless steel and graphite anodes. Operating within the temperature range of 225 to 300 °C and applying voltages ranging from 1.5 to 2.5 V, stainless steel as an anode yields impressive current efficiencies at 300 °C: 90.5, 80 and 68.6% for nickel, stainless steel, and platinum cathodes, respectively. This study positions steam splitting via molten hydroxides as a promising alternative for hydrogen production, poised for integration with renewable energy sources, marking a transformative step in sustainable energy practices.