The use of renewable energy for hydrogen production through water electrolysis is a critical pathway for green hydrogen generation. Compared to pure water electrolysis, direct electrolysis of seawater offers several advantages, such as raw material availability and application diversity. However, the complex composition of seawater presents significant technical challenges, particularly the competitive chloride oxidation reaction (ClOR) at the anode, which leads to equipment corrosion [
1]. Although iron-nickel-based catalysts have been identified as promising electrode materials for alkaline seawater electrolyzers (ASWE) [
2,
3], issues such as limited potential windows for anodic oxygen evolution reaction (OER), insufficient current density, and reduced operational stability due to corrosive oxidation remain unresolved [
4–
6]. Additionally, the incomplete understanding of mechanisms of ClOR hinders the structural design of high-performance electrodes. Therefore, identifying the key factors governing corrosion processes and optimizing electrode structures to mitigate ClOR while enhancing OER efficiency are critical for advancing seawater electrolysis technologies.
In a recent publication in the
Journal of American Chemical Society, Shen et al
. [
7] proposed an innovative anti-corrosion strategy by designing a rare-earth metal protection layer on transition metal electrodes. The oxygen-affinitive Eu
2O
3 layer created a local OH
−-enriched microenvironment, effectively limiting Cl
− adsorption and oxidation on the electrode surface, thus significantly extending the lifespan of electrodes. The developed Eu
2O
3/FeNi
2S
4 electrodes exhibited stable operation for over 100 h in a kilowatt-scale ASWE system and demonstrated promising economic feasibility based on technical economic analysis (TEA), offering a potential solution to the corrosion challenges in seawater electrolysis.
The real-time corrosion behavior of a traditional Ni mesh anode in an ASWE was investigated using electrochemical quartz crystal microbalance (EQCM) and differential electrochemical mass spectrometry (DEMS) to analyze electrolytic products (Figs. 1(a)‒1(c)). The results showed that the oxidation product HClO significantly contributed to anodic metal corrosion. Competitive adsorption studies confirmed that negatively charged ions (e.g., OH−, ) in the electrolyte suppressed Cl− accumulation at the interface, mitigating corrosion and maintaining catalyst activity. To address these challenges, the researchers electrodeposited element europium (Eu), a rare metal, onto the FeNi2S4 layer. Through calcination under argon protection, Eu2O3/FeNi2S4 composite electrodes were formed, leveraging the difference in oxygen affinity between rare earth metals and transition metals. X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the Eu2O3 layer enhanced charge transfer and electron delocalization between Fe and Ni via oxygen affinity, interfacial strain, and dielectric screening, creating a dynamic redox environment essential for OER catalysis.
To further understand the reaction process at the rare-earth oxide interface, a series of in situ spectroscopic tests were conducted. In situ fluorescence testing using N-ethoxycarbonylmethyl-6-methoxyquinolinium bromide (MQAE) labeling revealed reduced Cl‒ adsorption and oxidation on Eu2O3-modified surfaces (Figs. 1(d) and 1(e)). Additional in situ infrared spectroscopy, rotating ring-disk electrode (RRDE) measurements (Fig. 1(f)), and isotopic kinetic analyses conjointly demonstrated Eu2O3/FeNi2S4 superior OH− adsorption capacity compared to FeNi2S4, thanks to the exceptional oxygen affinity of the Eu2O3 layer. This OH−-enriched local microenvironment effectively stabilized interfacial pH at high potentials (> 1.7 V), inhibiting Cl− adsorption. Density functional theory (DFT) calculations indicated that rare-earth incorporation reduced OH− adsorption energy while increased Cl− adsorption energy at Ni active sites, promoting OER selectivity (Fig. 1(g)). During OER, Ni2+ was oxidized to high-valence active species, which were stabilized by Eu2O3 due to its strong oxygen affinity and electron-capturing capability, ensuring sustained electrode activity and stability.
The anodic catalytic performance of Eu2O3/FeNi2S4 was evaluated in an alkaline seawater electrolyte, demonstrating enhanced OER performance in the potential range of 1.0–2.0 V compared to FeNi2S4 (Fig. 1(h)). For instance, at a potential of 1.79 V, the Eu-modified electrode achieved a current density of 100 mA cm-2, a twofold improvement over FeNi2S4. Long-term stability tests showed that Eu2O3/FeNi2S4 remained stable for 1000 h at 500 mA/cm2, confirming the effectiveness of the Eu2O3 protective strategy. Time-resolved in situ UV-visible absorption spectroscopy tracked the evolution of high-valence metal active species, confirming that Eu2O3 did not participate directly in the electrochemical reaction. Fourier transform cyclic voltammetry (FtaCV) analysis further revealed that Eu2O3 stabilized high-valence active Ni sites in the reconstructed layer, concurrently improving OER activity and durability (Fig. 1(i)). An ASWE system constructed with Eu2O3/FeNi2S4 as the anode and NiS2 as the cathode, with a total electrode area of 1081.5 cm2 demonstrated stable operation for 100 h at 1.94 V and 500 mA/cm2 in 30% KOH electrolyte at 80 °C (Fig. 1(j)). TEA confirmed that the system met profitability thresholds for commercial hydrogen production (Fig. 1(k)), validating the practical viability of the rare-earth protection strategy for large-scale seawater electrolysis applications.
In summary, Shen et al. [
7] presented an innovative oxygen-affinitive rare-earth Eu
2O
3 modification strategy that stabilizes Fe/Ni active sites by creating OH
−-enriched interfaces, blocking chlorine corrosion pathways and offering a promising solution for efficient ASWE systems. Notably, a combination of
in situ electrochemical characterization techniques was used to validate the interfacial evolution of key intermediate species, supporting the proposed reaction mechanisms and offering methodological insights for future water/seawater electrolysis research. Similar protection strategies were also explored by Sha et al. [
8], who demonstrated a phosphate passivation layer on NiCoP-Cr
2O
3 electrodes, achieving 10000 h of stable seawater electrolysis at 0.5 A cm
-2. However, the effectiveness of rare-earth protection strategies in industrial-scale ASWE devices under high current densities (> 1 A/cm
2) and long-term operation (> 10000 h) still requires further verification. Additionally, batch preparation is also a necessary condition for electrode applications. While the current preparation process is available for industrial scale-up, its multi-step nature increases production costs, and streamlining synthesis would significantly improve cost-effectiveness and facilitate mass production. Compared to market-validated alkaline water electrolysis (AWE, > 80000-h lifespan) and proton exchange membrane electrolysis (PEM, > 3 A/cm
2 current density) technologies, ASWE devices still fall short of application benchmarks, whose electrodes requires further optimization and verification. Protection layer engineering strategies offer a promising pathway to address these challenges, potentially accelerating the engineering application of seawater-to-hydrogen technology.
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