Mn-Based Polyoxometalates with Organic–Inorganic Large-Spaced Layers as the Highly Efficient Electrocatalysts for Acidic and Alkaline Hydrogen Evolution Reaction

Polyoxometalates are promising options for electrocatalysts due to electronic sponges properties. Organic-inorganic hybridized polyoxometalates are the focus of research in the field of electrocatalytic hydrogen evolution through substitution by transition metal ions or modification by organic ligands. Using a one-step solution method, two new Mn-based polyoxometalates were successfully created, Na₄(C₄H₇N₂)[{Mn₃WO₃}[BiW₉O₃₃]₂(H₃O)₈]·7H₂O (1), Na₇(C₃H₃N₂)[{MnWO₃}{BiW₉O₃₃}]·14H₂O (2). Both compounds, 1 and 2 are made up of two {BiW₉O₃₃}^9- polyoxyanions encapsulated in 10-nuclear high-nuclear clusters containing Mn ions and further connected to organic ligands through hydrogen bonding. It is worth noting that compound 1 forms large-spaced organic-inorganic layered three-dimensional structures with two {BiW₉O₃₃}^9- polyoxyanions encapsulating 10-nuclear high-nuclear clusters containing Mn ions as the inorganic layer and 2-methylimidazole ligand as the organic layer eventually. However, compound 2 forms a three-dimensional structure with an imidazole ligand organic layer wrapped in an inorganic layer. The two compounds were used in the electrocatalytic hydrogen evolution reaction (HER) in acidic solutions (0.5 M H₂SO₄) and alkaline solutions (1 M KOH). Compound 1 has exceptional catalytic activity, with overpotential values of 30 and 140 mV, respectively, which were significantly lower than compound 2 (74 mV/328 mV), Na₁₂[(Na(H₂O)₂)₆(BiW₉O₃₃)₂]·27H₂O (abbreviated as {BiW₉}) (369 mV/534 mV), [{NaO₂(H₂O)₂}₂{(Mn(H₂O)₂)₂(WO₂)₂ (BiW₉O₃₃)₂}]·16H₃O⁺·XH₂O (abbreviated as {BiW₉-Mn})(285 mV/435 mV) at 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH electrolyte solution. Meanwhile, compound 1 showed excellent durability (1000 cycles) and stability (24 h) under both conditions. This study provides a new and convenient way to design organic-inorganic layered polyoxometalates for electrocatalytic hydrogen evolution reactions.