Recent Advances in Self-Supported Transition-Metal-Based Electrocatalysts for Seawater Oxidation

Seawater electrolysis is a promising and sustainable technology for green hydrogen production. However, some disadvantages include sluggish kinetics, competitive chlorine evolution reaction at the anode, chloride ion corrosion, and surface poisoning, which has led to a decline in activity and durability and low oxygen evolution reaction (OER) selectivity of the anodic electrodes. Benefiting from the lower interface resistance, larger active surface, and superior stability, the self-supported nanoarrays have emerged as advanced catalysts compared to conventional powder catalysts. Self-supported catalysts have more advantages than powder catalysts, particularly in practical large-scale hydrogen production applications requiring high current density. During electrolysis, due to the influx of bubbles generated on the electrode surface, the powdered nanomaterial is peeled off easily, resulting in reduced catalytic activity and even frequent replacement of the catalyst. In contrast, self-supported nanoarray possessing strong adhesion between the active species and the substrates ensures good electronic conductivity and high mechanical stability, which is conducive to long-term use and recycling. This minireview summarizes the recent progress of self-supported transition-metal-based catalysts for seawater oxidation, including (oxy)hydroxides, nitrides, phosphides, and chalcogenides, emphasizing the strategies in response to the corrosion and competitive reactions to ensure high activity and selectivity in OER processes. In general, constructing three-dimensional porous nanostructures with high porosity and roughness can enlarge the surface areas to expose more active sites for oxygen evolution, which is an efficient strategy for improving mass transfer and catalytic efficiency. Furthermore, the Cl- barrier layer on the surface of catalyst, particularly that with both catalytic activity and protection, can effectively inhibit the competitive oxidation and corrosion of Cl-, thereby delivering enhanced catalytic activity, selectivity, and stability of the catalysts. Moreover, developing super hydrophilic and hydrophobic surfaces is a promising strategy to increase the permeability of electrolytes and avoid the accumulation of large amounts of bubbles on the surface of the self-supported electrodes, thus promoting the effective utilization of active sites. Finally, perspectives and suggestions for future research in OER catalysts for seawater electrolysis are provided. In particular, the medium for seawater electrolysis should be transferred from simulated saline water to natural seawater. Considering the challenges faced in natural seawater splitting, in addition to designing and synthesizing self-supported catalysts with high activities, selectivity, and stability, developing simple and low-cost natural seawater pretreatment technologies to minimize corrosion and poisoning issues is also an important topic for the future development of seawater electrolysis. More importantly, a standardized, feasible evaluation system for self-supported electrocatalysts should be established. In addition, factors such as the intrinsic activity, density of accessible active sites, size, mass loading, substrate effects, and test conditions of the catalyst should be fully considered.