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). Although M2+/M redoxes, including Fe2+/Fe, have been investigated in concentrated aqueous electrolytes, thermodynamic measurements and modeling of the equilibrium M2+/M potential considering metal–chloride complexation in highly concentrated electrolytes remain elusive. For the first time, we systematically examine how a concentrated electrolyte affects the thermodynamics of TM electrodeposition by combining experimental, theoretical, and computational methods. Our study revealed that the electrodeposition potentials (Eeq) of a wide range of TMs (Fe, Cr, Co, Ni, Zn) are strongly dependent on the 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 together with the metal–ligand complexation equilibria, we develop a thermodynamic model that predicts the equilibrium electrodeposition potential in concentrated electrolytes. The model is general in structure and can be applied to other aqueous Mn+/M systems when speciation and activity data are available. The model is used for predicting the electrodeposition potential of selected TMs in both supported and unsupported electrolytes, and the prediction agrees very well with experiments. A unified thermodynamic framework for metal deposition is proposed by generalizing our model, which reduces to previously proposed models under limiting conditions and covers electrodeposition from a dilute electrolyte to a 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.