Investigating direct alkaline seawater electrolysis and its impact on the oxygen evolution reaction

As the global energy landscape changes to address climate change, hydrogen has become a vital link between renewable electricity and difficult-to-electrify industrial sectors. Nonetheless, producing hydrogen through electrolysis depends on highly purified freshwater, a resource that is often limited in areas with the greatest renewable energy potential. As a result, an investigation into the feasibility of producing hydrogen directly from alkaline seawater, bypassing the need for energy-intensive desalination, has been explored in this thesis. The primary scientific challenge in direct seawater electrolysis is the competition at the anode between the oxygen evolution reaction (OER) and the corrosive chlorine oxidation reaction (ClOR). An evaluation of earth-abundant OER catalysts and their interactions with seawater using advanced electrochemical and materials characterisation techniques is presented. NiFe(OH)2 exhibits high OER activity in alkaline simulated seawater, requiring 327 and 393 mV to achieve 500 and 1000 mA cm-2 respectively at 298 K during initial testing. However, accelerated degradation and post-testing characterisation reveal chloride-induced corrosion pathways that compromise long term stability. To address this limitation, two established corrosion mitigation strategies were applied: electrostatic repulsion via sulphide modification (NiFe-S) and ion-selective modification using chromium incorporation (NiFe-Cr variants). The chromium based ion selective strategies more effectively suppressed hypochlorite formation, but NiFe-S demonstrated superior overall stability, achieving a degradation rate of 0.78 mV h-1 at 100 mA cm-2 over 50 hours, compared to 1.6 mV h-1 for unmodified NiFe(OH)2 in 1 M KOH & AbsOcean. Rotating ring-disc electrode (RRDE) analysis enabled estimated quantification of hypochlorite generation, leading us to conclude that both modification strategies reduced parasitic chlorine chemistry, with NiFe-S maintaining low ClOR Faradaic efficiencies (0.061-0.043% at 100-200 mA cm-2). Integrating optimised catalysts into an AEM electrolyser demonstrated that long-term performance in seawater is governed not only by catalyst chemistry but also by system level factors, including membrane stability, electrolyte management, and current-collector corrosion. These findings highlight that successful seawater electrolysis requires co-optimisation of catalyst selectivity, corrosion resistance, and device architecture. Overall, this work proposes design principles for achieving selective and durable oxygen evolution in alkaline seawater, advancing the feasibility of practical direct seawater electrolysis.