Pure reduced polyoxometalate materials as electroactive materials for assembling proton energy storage devices

Abstract

The integration of efficient proton transport and reversible redox activity in a single material is highly desirable for advanced electrochemical devices, yet remains challenging. Herein, two novel crystalline pyridine-decorated polyoxometalates, namely H₁₀{Cu^II_0.5[Mo^V₆O₁₂(OH)₃(HPO₄)₄]₂}₂·8HPy·24H₂O (1) and H₈Cd^II[Mo^V₆O₁₂(OH)₃(HPO₄)₄]₂·2Cl·2HPy·2Me₂NH (2), are designed and synthesized via a hydrothermal route. The pyridine molecules endow the materials with remarkable proton conductivity, reaching 9.63 × 10^–3 S cm⁻¹ (1) and 2.21 × 10^–3 S cm⁻¹ (2) at 85 °C and 95% RH. The excellent proton conduction stems from the ordered hydrogen-bonding networks facilitated by both the pyridine N sites and the terminal/surface oxygen atoms of the {P₄Mo^V₆O₃₁} anions. Furthermore, the title complexes as electrochemically active materials are loaded onto the surface of carbon paper to assemble solid-state proton energy storage devices; the 1-CP@PANI-SC devices can achieve an outstanding specific capacitance of 330.12 F g⁻¹ and cycling stability of 94.2% after 1000 cycles. Crucially, electrochemical analysis coupled with proton conduction studies indicates that the pre-established proton-conducting pathways significantly facilitate the transport of charge-compensating protons (H⁺) during the rapid redox reactions of molybdenum centers, thereby enhancing the pseudo-capacitive kinetics and overall electrochemical efficiency. This work not only presents high-performance multifunctional electroactive materials but also establishes a material design principle that links proton conduction with charge storage dynamics for next-generation energy storage systems.