Currently, fossil energy accounts for more than 80% of the global energy supply, and its excessive use has led to serious environmental pollution and carbon emission problems. Developing green hydrogen as a clean energy carrier is crucial for achieving carbon neutrality. Using electricity from renewable energy sources to electrolyze water for hydrogen production is an ideal approach. However, traditional electrolysis relies on high-purity fresh water, which exacerbates water shortages. China is rich in marine resources, and the coupling of renewable energy sources such as offshore wind power with seawater electrolysis can efficiently and locally absorb fluctuating electrical energy, having both economic and environmental advantages. Nevertheless, the complex components in seawater severely limit the electrolysis efficiency and the lifespan of the electrolyzer. The catalyst is the core challenge for technological breakthroughs. Transition metal-based materials have become the focus of research due to their abundant reserves, low cost, and corrosion resistance. Therefore, this review elaborately discusses the research progress of transition metal catalysts for seawater electrolysis.

Firstly, this review discusses the principles and challenges of direct seawater electrolysis. Seawater electrolysis is similar to pure water electrolysis, involving two half-reactions, namely the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. However, compared with pure water electrolysis, the catalysts for seawater electrolysis face more challenges. The composition of seawater is very complex and varies by region. It has been measured that the total concentration of all ions in seawater is, on average, 3.5%(in mass fraction), among which the ions with relatively high contents include Cl^-, Br^-, Na⁺, Mg²⁺, Ca²⁺, K⁺, etc. In addition to the above ions, there are also numerous microorganisms and plankton in seawater, which may lead to side reactions at the cathode and anode, corrosion by chloride ions, deposition of microorganisms and metal ions, and poisoning of the catalyst, reducing the efficiency of water electrolysis and the lifespan of the equipment.

In response to the above challenges, this review summarizes the strategies for the development of seawater electrolysis catalysts. For OER catalysts, the primary consideration is the suppression of chlorine-related chemical reactions. Developing catalysts that can achieve high OER selectivity and high activity at a lower overpotential and designing catalysts for reactions under alkaline conditions are ideal solutions to avoid the oxidation reaction of Cl^-. Secondly, anode catalysts need to have good corrosion resistance, and resistance to Cl^- corrosion can be achieved by designing a Cl^- barrier layer. For HER catalysts, developing highly active HER catalysts can promote the HER kinetic process and improve the efficiency of seawater electrolysis. At the same time, the cathode also faces the problems of Cl^- corrosion and the deposition of metal ion impurities (such as Mg²⁺ and Ca²⁺) that can block the active catalytic sites. Designing metal oxide, hydroxide, and anion barrier layers on the surface of the catalyst can effectively solve the above problems.

In recent years, in order to reduce the overpotential of the electrolysis process and improve the electrolysis efficiency, selectivity, and the lifespan of the catalyst, researchers have made great efforts in the field of developing seawater electrolysis catalysts and designed various catalysts, including noble metal electrode materials such as Ir and Pt, as well as non-noble metal electrode materials such as Ni and Co. Noble metal catalysts have excellent performance but have low reserves and high costs, making large-scale application difficult. Transition metal-based materials have become a research hotspot due to their abundant reserves, low cost, and corrosion resistance. This review elaborately discusses the research progress of transition metal catalysts for seawater electrolysis, such as those based on Ni, Co, Mo, Mn, and Cr. This article also compares and analyzes various hydrogen production equipment used for electrolyzing seawater.