Interface Storage Mechanism in Aqueous Ammonium-Ion Supercapacitors with Keggin-Type Polyoxometalates-Modified Ag-BTC

Ammonium-ion supercapacitors (AISCs) offer considerable potential for future development owing to their low cost, high safety, environmental sustainability, and efficient electrochemical energy storage capabilities. The rapid and efficient charge-transfer process at the AISC can endow them with high capacitive and cycling stabilities. However, the prolonged intercalation/deintercalation of NH₄ ⁺ in layered and framework materials often results in the cleavage of the active sites and the deconstruction of the framework, which makes it difficult to achieve long-term stable energy storage while maintaining high capacitance in the electrode materials. Herein, highly redox-active polyoxometalates (POMs) modified [Ag₃(µ-Hbtc)(µ-H₂btc)]_n (Ag-BTC) is used as electrode materials. POMs effectively promote the pseudocapacitance storage of NH₄ ⁺ through a similar interface storage mechanism. At a current density of 1 A g^-1, {PMo₁₂}@Ag-BTC exhibited a specific capacitance of 619.4 mAh g^-1 and retained 100% of its capacitance after 20,000 charge-discharge cycles. An asymmetrical battery with {PMo₁₂}@Ag-BTC and {PW₁₂}@Ag-BTC as positive and negative electrode materials, respectively, achieved an energy density of 125.3 Wh kg^-1. The interface-capacitance process enables the full utilization of metal-O_x (x = b, c, t) sites within the POMs, significantly enhancing charge storage. This study emphasizes the considerable potential of POM-based electrode materials for NH₄ ⁺ intercalation/deintercalation energy storage.