Challenges and Opportunities for Seawater Electrolysis: A Mini-Review on Advanced Materials in Chlorine-Involved Electrochemistry

Abstract

Hydrogen (H2) is an important component in the framework of carbonneutral energy, and the scalable production of H2 from seawater electrolysis offers a feasible route to address global energy challenges. With abundant seawater reserves, seawater electrolysis, especially when powered by renewable electricity sources, has great prospects. However, chloride ions (Cl − ) in seawater can participate in the anodic reaction and accelerate the corrosion of electrode materials during electrolysis. Although the oxygen evolution reaction (OER) is thermodynamically favorable, the chlorine evolution reaction is highly competitive because fewer electrons are involved (2e − ). These two problems are compounded by the dearth of corrosion-resistant electrode materials, which hinders the practical applications of seawater electrolysis. Therefore, intensive research efforts have been devoted to optimizing electrode materials using fundamental theories for practical applications. This review summarizes the recent progress in advanced electrode materials with an emphasis on their selectivity and anti-corrosivity. Practical materials with improved selectivity for oxygen generation, such as mixed metal oxides, Ni/Fe/Co-based composites, and manganese oxide (MnOx)-coated heterostructures, are reviewed in detail. Theoretically, alkaline environments (pH > 7.5) are preferred for OER as a constant potential gap (480 mV) exists in the high pH region. Nevertheless, corrosion of both the cathode and anode from ubiquitous Clis inevitable. Only a few materials with good corrosion resistance are capable of sustained operation in seawater systems; these include metal titanium and carbon-based materials. The corrosion process is usually accompanied by the formation of a passivated layer on the surface, but the aggressive penetration of Clcan damage the whole electrode. Therefore, the selective inhibition of Cl − transport in the presence of a robust layer is critical to prevent continuous corrosion. Advances in anti-corrosion engineering, which encompasses inherently anti-corrosive materials, extrinsically protective coating, and in situ generated resistive species, are systematically discussed. Rational design can impart the material with good catalytic activity, stability, and corrosion resistance. Finally, we propose the following opportunities for future research: 1) screening of selective and anti-corrosive materials; 2) mechanism of competitive reactions and corrosion; 3) evaluation of anti-corrosive materials; 4) industrial-scale electrolysis with high current density; 5) optimization of experimental conditions; and 6) development of integrated electrolyzer devices. This review provides insights for the development of strategies aimed at tackling chlorine-related issues in seawater electrolysis.

Nickel–Cobalt Hydrogen Phosphate on Nickel Nitride Supported on Nickel Foam for Alkaline Seawater Electrolysis

“…The presence of ions (such as Mg 2+ , Ca 2+ , Cl – , Br – , etc.) and bacteria/microbes in seawater may poison electrodes/catalyst materials, which leads to their instability for a long-term operation and even catalyst deactivation, so the above issue puts more demands for exchange membranes in seawater electrolyzers. , Furthermore, the other critical issue is the competition between chloride chemistry reaction and OER on the anode. , According to the Pourbaix diagram, the standard electrode potentials difference between OER and chloride chemistry keeps increasing with the increasing pH and remains a maximum value of 480 mV when the pH is above 7.5. Therefore, developing high-performance alkaline electrocatalysts with overpotential below 480 mV at a desired current density provides the possibility to boost effective seawater splitting without chlorine chemistry side-reactions.…”

“…and bacteria/microbes in seawater may poison electrodes/catalyst materials, which leads to their instability for a long-term operation and even catalyst deactivation, so the above issue puts more demands for exchange membranes in seawater electrolyzers. 14,15 Furthermore, the other critical issue is the competition between chloride chemistry reaction and OER on the anode. 16,17 According to the Pourbaix diagram, 18 the standard electrode potentials difference between OER and chloride chemistry keeps increasing with the increasing pH and remains a maximum value of 480 mV when the pH is above 7.5.…”

Advanced Two-Dimensional Materials for Green Hydrogen Generation: Strategies toward Corrosion Resistance Seawater Electrolysis─Review and Future Perspectives

“…The electrolyzer design can help in the improvement of reaction efficiency; however, these are major research areas to be investigated thoroughly and deep exploration of catalyst materials is necessary for saline environments. 334 Seawater electrolysis is mainly used in chloro-alkali industries for producing chlorine gas; however, the importance of hydrogen fuel in the growing market across the world exceeds the chlorine demand. The researchers have developed different kinds of electrodes to deal with numerous problems during seawater electrolysis for the production of H 2 or Cl 2 .…”

“…The dependence of local cations, pH, ionic conductivity, and magnetic fields on the catalytic process may offer new insights for optimized catalytic performances. The electrolyzer design can help in the improvement of reaction efficiency; however, these are major research areas to be investigated thoroughly and deep exploration of catalyst materials is necessary for saline environments . Seawater electrolysis is mainly used in chloro-alkali industries for producing chlorine gas; however, the importance of hydrogen fuel in the growing market across the world exceeds the chlorine demand.…”

Heterointerface Engineering of Ni2P–Co2P Nanoframes for Efficient Water Splitting

“…Hydrogen (H 2 ) is a promising energy carrier to replace traditional fossil fuels. − Electrochemical water splitting, driven by a renewable source (such as water-, wind-, solar-derived electricity), is a clean and efficient pathway for H 2 production. − To decrease cost, highly efficient electrocatalysts are required to reduce the overpotentials of the two half-reactions for water splitting, including cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). − Although Pt and RuO 2 /IrO 2 exhibit the state-of-the-art catalytic activities for HER and OER, respectively, their industrial applications are limited by the high costs of noble metals. , The latter has led to intensive efforts devoted to developing efficient noble-metal-free electrocatalysts.…”