Anion modification for enhanced anode catalyst performance in seawater electrolysis

Abstract

Direct seawater electrolysis presents a promising pathway for sustainable “green hydrogen” production. However, the complex composition of seawater, particularly the presence of chloride ions (Cl−), poses significant challenges to the structural stability and electrocatalytic performance of oxygen evolution reaction (OER) catalysts. Although recent studies have demonstrated that anion modification can improve the stability and activity of catalysts, the extent of these improvements varies considerably across different anions, and the underlying mechanisms remain poorly understood. This review examines the electrochemical behavior of anions related to their physicochemical properties and provides a comprehensive overview of recent advances and remaining challenges in anion-oriented strategies for seawater electrolysis. First, we propose a novel framework for determining anion properties based on adsorption energy, ionic potential, and acid-base character, which evaluates the physicochemical properties of anions from three dimensions and serves as a guideline for selecting modification materials for catalysts. Second, we critically discuss the underlying mechanisms by which anion modification enhances OER stability and activity in seawater, with a focus on chlorine chemistry and oxygen evolution dynamics. Classical approaches for stability improvement, such as the introduction of external anions and the regulation of Cl− and hydroxide ions (OH−), are discussed. We also summarize mechanisms for activity enhancement, including electronic structure modulation, active species engineering, and mass transfer optimization. Finally, we outline future research directions for anion modification strategies and highlight persistent challenges.

1. Introduction

Amid growing global energy security concerns, the pursuit of sustainable energy development has become an international priority [1]. Energy transition plays a central role in this effort. Hydrogen, as an environmentally benign, abundant, and high-energy-density carrier (with a high heating value of 141.8 MJ/kg at 298 K), serves dual roles in energy storage and CO2 mitigation, positioning itself as a highly promising alternative to conventional fossil fuels [2–6]. Among existing hydrogen production technologies, water electrolysis powered by renewable energy (such as wind, solar, and tidal energy) provides a carbon-free approach for generating clean and sustainable “green hydrogen” [7–12]. However, conventional water electrolysis relies on high-purity water feedstocks, limiting its application in water-scarce regions. In contrast, seawater constitutes approximately 96.5% of global water resources and exhibits higher conductivity (~5 S·m−1) than pure water (conductivity ~5.5 × 10−6 S·m−1), making it an attractive electrolyte [12–17]. Moreover, seawater electrolysis demonstrates inherent geographical synergies with emerging marine-based renewable energy technologies, such as offshore wind and tidal power, thereby offering opportunities to optimize the spatial layout of hydrogen supply chains [18].

Current seawater electrolysis strategies are broadly categorized into indirect and direct approaches [19]. The indirect method utilizes desalinated seawater for electrolysis, primarily through systems integrating reverse osmosis and electrolysis units, and is commonly referred to as “two-step” seawater electrolysis. This approach avoids interference from complex impurities in seawater and has gained widespread adoption [20–23]. However, significant limitations are to be considered, including the large footprints of desalination infrastructure, high capital expenditure, and substantial maintenance costs [24–25]. What’s more, large-scale seawater desalination considerably raises hydrogen production costs (~12.1 $·kg−1), while the disposal of concentrated brine byproducts poses serious threats to marine ecosystems [26–28]. Therefore, the development of direct seawater electrolysis technology emerges as a more cost-effective and environmentally sustainable alternative.

Seawater exhibits a complex ionic composition, dominated by approximately 0.55 M Cl−, 0.48 M Na+, 0.05 M Mg2+, and 0.01 M Ca2+. Among these ions, the aggressive Cl− induce severe corrosion during electrolysis, posing a major threat to long-term system stability.