Proton-insertion-pseudocapacitance of tungsten bronze tunnel structure enhanced by transition metal ion anchoring

Abstract

The one-dimensional channel array of hexagonal tungsten bronze (WO₃) offers an electron transfer matrix, but its overwhelming H⁺ adsorption hinders it from being a good supercapacitor electrode material. Inspired by the Volcano plot on the relation between transition-metal and free energy of H-adsorption, we propose a new strategy to anchor transition metal ions (Zn²⁺, Cu²⁺, Ni²⁺, Ag⁺, Au³⁺ and Ir³⁺) into the WO₃ lattice to improve proton-insertion based pseudocapacitance. Among the variety of transition metals, Zn²⁺ exhibits the optimal O 2p band center, which matches well with the best experimental capacitive behavior. The molar ratio of Zn/WO₃ ranges from 0.2 to 0.6. The specific capacitance for Zn²⁺-anchored WO₃ (390 F g⁻¹) reaches 202% of that of WO₃ (193 F g⁻¹) at 0.5 A g⁻¹ with robust stability (259 F g⁻¹ at 3 A g⁻¹ for 3000 cycles). Density functional theory confirms that O 2p is shifted down by the d-filling cations, which corresponds to alleviated O–H interaction and facilitated H⁺ desorption. The band tuning by transition-metal-ion incorporation would break new ground on developing high-capacitance metal oxide supercapacitors.