Complex Displacement Deposition: A Self-Templating Mechanism for Forming High-Surface-Area Catalysts.

Controlling the structure and composition of metal deposits is central to advancing electrocatalysis and functional materials. Here, we introduce Complex Displacement Deposition (CDD), a general electrodeposition framework in which an electrochemically inert displacing agent competes with a target metal for a shared complexing agent, decoupling precipitation from metal reduction and granting access to architectures unattainable by conventional electrodeposition. We demonstrate CDD using the Cu-citrate-Ca system, in which interfacial alkalinization drives Ca2+ to displace Cu2+ from its citrate complex, producing a previously unreported ternary Ca-Cu-citrate-hydroxide phase. The reversible formation and electrochemical reduction of this phase establish a dynamic self-templating cycle that generates hierarchical, high-surface-area copper networks. Evaluated for the electrochemical nitrate reduction reaction, these electrodes achieve partial current densities of 302 mA cm-2 and ammonia yield rates of 1.41 mmol h-1 cm-2, exceeding comparable metallic copper and many other copper-based catalysts. Based on these results, we identify within CDD two thermodynamically distinct displacement regimes. An indirect regime, demonstrated here, proceeds through a ternary intermediate. A direct regime, predicted thermodynamically but not demonstrated here, yields a pure metal hydroxide. CDD thus establishes a modular strategy for electrodepositing high-surface-area metals, metal-oxide composites, and compositionally complex architectures.