Three-dimensional 14-core manganese-sodium-based polyoxometalate with unique channels as bifunctional electrocatalysts for overall water splitting†

Polyoxometalates (POMs) with excellent redox capacity, high negative charge properties, good structural stability and polyatomic active sites are considered excellent candidates for water splitting. In this work, the synthesis process of compounds was explored by adjusting the pH, solvent volume and reaction temperature to obtain the most suitable conditions for crystal precipitation by a one-step solution method. Single crystal X-ray diffraction tests showed that two POM compounds modified by Mn group and organic ligand were successfully synthesized, which were [{Na₁₂(H₂O)₁₁Mn₂(OH)₈}{W₁₂O₄₀}]·8H₂O (1), (Hdmap)₆[H₂W₁₂O₄₀]·3H₂O (2) (dmap = 4-dimethylaminopyridine). CCDC 2306403 and CCDC 2327916 contain the supplementary crystallographic data for this paper. Compound 1 showed a layered three-dimensional structure wrapped by 14-core sodium ions and manganese ions with a unique channel (11.73 Å × 24.71 Å). Compound 2 possessed a layered three-dimensional structure of dmap-{W₁₂}-dmap. For HER. Compound 1 exhibited the best overpotential value of 199 mV for HER at a current density of 10 mA cm⁻² and a slope of 63.95 mV dec⁻¹, which were lower than those of Na₁₀[H₂W₁₂O₄₂]·27H₂O (440 mV, 495.76 mV dec⁻¹), compound 2 (322 mV, 73.86 mV dec⁻¹) and blank carbon cloth (552 mV, 750.85 mV dec⁻¹). Compared to Na₁₀[H₂W₁₂O₄₂]·27H₂O and compound 2, compound 1 also exhibited a lower overpotential value of 398 mV and the Tafel slope of 46.24 mV dec⁻¹ for OER. Meanwhile, compound 1 offered excellent stability after 24 hours and durability after 2500 cycles for both HER and OER. Surprisingly, compound 1/CC||compound 1/NF required only 1.48 V to drive water electrolysis, which was close to the value of Pt/CC||RuO₂/NF (1.43 V). Moreover, the CP curve of compound 1/CC||compound 1/NF was controlled for 20 h, reflecting that compound 1 has considerable durability.