Potential‐Controlled Ammonium‐Assisted Reversible Electrodeposition of Polyoxometalates for Hybrid Redox Flow Batteries

Counter-cations play a key role in modulating the redox chemistry and phase behavior of polyoxometalates (POMs). In this study, we report a potential-controlled, ammonium-assisted electrodeposition of POMs from nonaqueous electrolytes. Owing to its tetrahedral geometry and hydrogen-bonding capability, NH$_4$$^+$ uniquely stabilizes reduced POM clusters at the electrode interface, enabling reversible solid-to-solution transitions. Using the mixed-valence vanadium cluster [V$_{18}$O$_{46}$(NO$_{3}$)]$^{5-}$ and Keggin-type POM cluster, [PMo$_{12}$O$_{40}$]$^{3−}$ as model systems, we demonstrate controlled reversible charge-transfer dynamics, including effective mass deposition under reductive potentials and complete dissolution upon oxidation, as confirmed by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) measurements. Microscopic and spectroscopic investigations reveal that electrodeposition at low potentials involves partial reduction of redox-active metal centers within the POM framework, accompanied by NH$_4$$^+$ incorporation via hydrogen-bonded networks, forming NH$_4$$^+$-coupled POM-derived deposits, which exhibit a significantly different pathway from the classic metal electrodeposition (metal plating). ... mehrLeveraging this reversible behavior, we propose the design of POM electrodeposition-based hybrid redox flow battery (RFB), where NH$_4$$^+$-driven POM electrodeposition/dissolution in the negative half-cell enables electron storage/release during charge/discharge processes, and validate the design through proof-of-concept battery demonstrations. These findings provide important initial insights into NH4+-driven reversible POM electrodeposition and highlight its potential in advancing electrochemical energy storage technologies.