Thermodynamics of transition metal electrodeposition in concentrated aqueous electrolytes

Electrodeposition of transition metals (TMs) is important for energy storage and sustainable metal production (such as ironmaking), but the reactivity of the M2+/M redox pair in concentrated aqueous electrolytes has never been examined, despite its critical role in regulating TM electrodeposition faradaic efficiency, a critical performance metrics for metal batteries and electrowinning. For the first time, we systematically examine how concentrated electrolyte affects the thermodynamics of TM electrodeposition by combining experimental, theoretical, and computational methods. Our study revealed that the electrodeposition potential (Eeq) of a wide range of TMs (Fe, Cr, Co, Ni, Zn) is strongly dependent on electrolyte concentration. The classical thermodynamic model, the Nernst equation, cannot quantify such concentration dependence due to its neglect of metal-anion complexation, a unique structural feature of concentrated aqueous electrolytes of TM ions due to their strong cation-anion interaction. By examining the energy landscape of the electrodeposition reaction and the complex formation equilibria, we derived a chemistry-agonistic physical model that successfully predicts Eeq in a wide range of concentrated electrolytes and for representative TMs. A unified thermodynamic framework of metal deposition is proposed by generalizing our model, which reduces to previously proposed models under limiting conditions and covers electrodeposition from dilute electrolyte to molten salt electrolyte. The fundamental and practical implications of the results are discussed, shedding light on future electrolyte engineering for a wide range of electrode reactions and engineering applications.