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

Abstract 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.