Electrochemical Crystallization in Aqueous Electrolytes: Unlocking the Complex Chemistry of a Seemingly Simple Molecule

Compared to other synthetic routes, electrochemical synthesis offers unique control over reaction kinetics, allowing precise modulation of the driving force and reaction rate, dictated by the electrode potential E and current density J, respectively. Most electrochemical systems rely on a liquid electrolyte to conduct electrical current through the concerted motion of ions. Ideally, this liquid phase remains chemically stable under the applied potential. Among all solvents, water (H2O), unsurprisingly, was the first used for electrochemical systems due to its non-toxicity, abundance, and exceptional ability to dissolve and dissociate ionic salts. Despite its early dominance, research shifted toward nonaqueous electrolytes—such as organic solvents and ionic liquids—because of their wider electrochemical stability windows, enabling the deposition of a broader range of materials. However, interest in electrochemical crystallization from aqueous electrolytes has resurged; this is largely driven by two factors. First, water’s environmental friendliness makes it attractive for large-scale and sustainable applications. Second, water’s inherent reactivity—primarily due to spontaneous proton production (H_2 O ↔ H^+ + 〖OH〗^-)—presents both challenges and, counterintuitively, unique opportunities. In particular, recent research into emerging aqueous battery chemistries based on in-situ electrochemical crystallization of metals such as Zn, Cu, and Sn at the anode, and compounds like MnO2 at the cathode has offered exciting new insights into the associated complex interfacial phenomena. Drawing in part on advances from the battery field, this Review aims to offer perspectives on tailoring aqueous systems for the electrochemical synthesis of crystalline matter. In contrast to nonaqueous systems, the proton-induced reactivity in aqueous electrolytes introduces additional complexities. Continuous gas generation and pH-induced precipitation can lead to porous, passivating interphases, posing significant challenges to precise control. However, this same proton activity also enables the formation of a diverse library of materials—such as oxides and hydroxides—that are often inaccessible through nonaqueous electrochemical paths. These aqueous crystallization processes offer a cost-effective and highly controllable approach for synthesizing critical materials for energy and sustainability applications such as electrochemical energy storage. In this Review, we first explore the key physicochemical processes involved in aqueous electro-crystallization of metals, a simpler group of materials with well-defined lattice structures and stoichiometries. We then turn our attention to critical considerations for growing more complex compounds, characterized by intricate lattice symmetries and stoichiometries. We argue that achieving precision control in aqueous electrochemical crystallization requires a holistic understanding of proton activity, its influence on crystallization, and the ability to regulate interfacial chemical kinetics and transport phenomena across multiple length scales. With such control, electrochemical crystallization in aqueous systems offers a sustainable and versatile platform for the precision synthesis of materials essential to energy technologies and sustainability.