Ni-Xides (B, S, and P) for Alkaline OER: Shedding Light on Reconstruction Processes and Interplay with Incidental Fe...

Ni-Xides (X = B, P, or S) exhibit intriguing properties that have endeared them for electrocatalytic water splitting. However, the role of B, P, and S, among others, in tailoring the catalytic performance of the Ni-Xides remains vaguely understood, especially if they are studied in unpurified KOH (Un-KOH) because of the renowned impact of incidental Fe impurities. Therefore, decoupling the effect induced by Fe impurities from inherent material reconstruction processes necessitates investigation of the materials in purified KOH solutions (P-KOH). Herein, studies of the OER on Ni₂B, Ni₂P, and Ni₃S₂ in P-KOH and Un-KOH coupled with in situ Raman spectroscopy, ex situ post-electrocatalysis, and online dissolution studies by ICP-OES are used to unveil the distinctive role of Ni-Xide reconstruction and the role of Fe impurities and their interplay on the electrocatalytic behavior of the three Ni-Xide precatalysts during the OER. There was essentially no difference in the OER activity and the electrochemical Ni²⁺/Ni³⁺ redox activation fingerprints of the three precatalysts via cyclic voltammetry in P-KOH, whereas their OER activity was considerably higher in Un-KOH with marked differences in the intrinsic activity and evolution of the Ni²⁺/Ni³⁺ fingerprint redox peaks. Thus, in the absence of Fe in the electrolyte (P-KOH), neither the nature of the guest element (B, P, and S) nor the underlying reconstruction processes are decisive activity drivers. This underscores the crucial role played by incidental Fe impurities on the OER activity of Ni-Xide precatalysts, which until now has been overlooked. In situ Raman spectroscopy revealed that the nickel hydroxide derived from Ni₂B exhibits higher disorder than in the case of Ni₂P and Ni₃S₂, both exhibiting a similar degree of disorder. The guest elements thus influence the degree of disorder of the formed nickel oxyhydroxides, which through their synergistic interaction with incidental Fe impurities concertedly realize high OER performance.