Functional Modification of Dawson-Type Arsenomolybdate for Enhanced Ultracapacitor Performance and Nitrate-to-Ammonia Production

Abstract

Polyoxometalates (POMs) are promising electrocatalysts and pseudo-capacitive materials due to their reversible multi-electron redox properties. In this study, Dawson-type mono-arsenic-capped arsenomolybdate are anchored into channels of {Cu(trz)₂}₇ metal–organic network yielding a solution-stable host-guest structure, [{Cu^I(trz)₂}₇{As^IIIAs^V₂Mo^V₄Mo^VI₁₄O₆₂}]₂·3H₂O (2), which exhibits higher conductivity and specific capacity, excellent rate performance and cycle stability than (biz)₉(Hbiz)₃{As^III_1.5As^V₂Mo₁₈O₆₂}₂·2H₂O (1) and most reported POMs, ascribing to the excellent Faraday properties of POMs, metal–organic conductive network, and the advantages of host-guest structure in surface area and stability. The AC//2-CPE device demonstrates energy density and power density of 25.45 Wh kg⁻¹ and 1991.53 W kg⁻¹, and 92.4% capacity retention after 10 000 cycles. Moreover, compound 2 as nitrate reduction reaction (NO₃RR) electrocatalyst achieves a current density of 150 mA cm⁻² at −0.5 V, ammonia production rate of 15.28 mg h⁻¹ cm⁻², and Faradaic efficiency of up to 90%. Density functional theory is employed to thoroughly investigate the adsorption active sites and the detailed energetic steps corresponding to the overall reaction pathway of NO₃RR regulated by compound 2. This study reveals that encapsulating POMs clusters into a metal–organic network can increase the redox active sites, improve stability, and conductivity, thereby enhancing the energy storage and catalytic activity of POMs at the molecular level.