Transition metal chalcogenides (TMCs) are efficient oxygen evolution reaction (OER) pre‐electrocatalysts, and will in situ transform into metal (oxy)hydroxides under OER condition. However, the role of chalcogen is not fully elucidated after oxidation and severe leaching. Here we present the vital promotion of surface‐adsorbed chalcogenates on the OER activity. Taking NiSe2 as an example, in situ Raman spectroscopy revealed the oxidation of Se‐Se to selenites (SeO32−) then to selenates (SeO42−). Combining the severe Se leaching and the strong signal of selenates, it is assumed that the selenates are rich on the surface and play significant roles. As expected, adding selenites to the electrolyte of Ni(OH)2 dramatically enhance its OER activity. And sulfates also exhibit the similar effect, suggesting the promotion of surface‐adsorbed chalcogenates on OER is universal. Our findings offer unique insight into the transformation mechanism of materials during electrolysis.

NiMo alloys are efficient electrocatalysts in alkaline water electrolyzer for the hydrogen evolution reaction (HER). Metals are usually considered to be stable during the cathodic process. However, the actual behaviors of Mo in the NiMo alloys are unexplored. Here, we present the instability of Mo in the Ni 4 Mo alloy as a highly efficient HER electrocatalyst in an alkaline medium. Mo in Ni 4 Mo is oxidized and dissolved in the form of MoO 4 2À first. The dissolved MoO 4 2À will re-adsorb on the electrode surface and polymerize. Theoretical calculations indicate that the adsorption of the dimer Mo 2 O 7 2À can promote the HER activity of metal Ni. The addition of MoO 4 2À to the electrolyte can not only repair the durability of Ni 4 Mo alloy, but also facilitate the HER activity of pure metal of Ni, Fe, and Co. Our findings provide insight into the structural transformation mechanism and performance-enhanced origin of cathodic materials under the reaction conditions.

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To further our understanding of the role of solution chemistry in directing nucleation processes new experimental and computational data are presented on the solution and crystallisation chemistry of tolfenamic acid (TA), a benchmark polymorphic compound.