POM-Based Water Splitting Catalyst Under Acid Conditions Driven by Its Assembly on Carbon Nanotubes

Development of efficient and stable bifunctional electrocatalysts for water electrolysis under acidic conditions is essential for sustainable hydrogen production. A novel vanadium polyoxometalate (POM)-based material, Na₄(H₂O)₁₂[(CH₂OH)₃CNH₃]₂[V₁₀O₂₈]·4H₂O (1) is presented, incorporating non-innocent cations and whose electrocatalytic activity can be switched from the production of oxygen to hydrogen through its assembly on carbon nanotubes (CNT). A physical mixture (1/CNT) shows remarkable oxygen evolution reaction (OER) activity, with an overpotential of 0.34 V at 10 mA cm^{- 2}, outperforming commercial IrO₂ (0.45 V) and approaching Ir/C (0.31 V), with 80% Faradaic efficiency. In contrast, directed assembly (1@CNT) unlocks TRIS ⁺= [(CH₂OH)₃CNH₃]⁺ groups functionality, enabling high hydrogen evolution reaction (HER) efficiency, with an onset potential of -0.07 V, close to Pt/C, and 94% Faradaic efficiency. Mechanistic studies, strongly supported by in-operando confocal microscopy and theoretical calculations, reveal that the modulation of crystal interactions and the local microenvironment is key to orchestrating the OER/HER tuning. OER is proposed to proceed via an alcohol oxidation reaction (AOR), while HER benefits from TRIS⁺ moieties acting as a "proton sponge". This work provides a compelling approach for rational design of bifunctional molecular electrocatalysts based on earth-abundant elements and controlled nanoassembly, with clear relevance for advancing green hydrogen production technologies.