The increasing demand for hydrogen energy calls for more economical and green hydrogen production pathways.[1] Compared to conventional water electrolysis, which relies on scarce high-purity freshwater, seawater electrolysis has attracted increasing attention due to its use of natural seawater.[2] However, the complex composition of seawater introduces significant challenges. Because catalysts are highly sensitive to electrolyte compositions,[3] directly using seawater can obscure fundamental mechanisms and hinder rational catalyst design. Therefore, focusing on the effect of individual ion is essential for establishing more solid scientific basis for future seawater electrolysis development.

Chloride ion (Cl^-), which is difficult to remove even through water purification,[4] is widely regarded as the primary cause of corrosion and competitive side reactions.[5] To address it, catalyst design generally falls into two categories: enhancing intrinsic OER activity or introducing a protective outer layer. The former is limited by the scaling relationship between OER and CER,[6] while the latter has been reported to reduce catalytic activity by blocking exposed active sites.[7,8] Among the protective layer materials, δ-MnO₂ is widely used due to its high oxygen evolution reaction (OER) selectivity and corrosion resistance. However, because of its low intrinsic OER activity when intercalated with proton or alkali metal cations, δ-MnO₂ has been primarily regarded as a protective layer rather than an active phase, limiting its potential to achieve both high selectivity and OER activity.

In our previous study, we demonstrated that transition metal (TM) cations (Fe³⁺, Ni²⁺) intercalated into interlayers of δ-MnO₂ act as active sites and significantly enhance OER activity. Based on this, this work further focuses on the influence of Cl^- on δ-MnO₂ intercalated with Fe³⁺ and Ni²⁺. Linear sweep voltammetry (LSV) curves and chronoamperometry (CA) reveal an enhanced activity of Fe³⁺/MnO₂ in the presence of Cl^-, while Ni²⁺/MnO₂ shows decreased activity. X-ray photoelectron spectroscopy (XPS) spectra of TM sites, Mn, and residual Cl indicate that this contrasting behavior follows the Hard and Soft Acid and Base (HSAB) theory that Cl^- coordinates with weaker Lewis acids (Ni²⁺), suppressing active sites, but binds to lattice defects when stronger Lewis acids (Fe³⁺) are present. Ex/in situ Raman, XRD, and operando FTIR further reveal that Cl^- facilitates the phase transformation of MnO₂ and the formation of a water-rich interlayer environment. Overall, this work uncovers the critical role of interaction between electrolyte ions and catalysts, highlights the potential role of Cl^- in enhancing OER activity for future seawater electrolysis.