Unraveling the Secrets of Ordered Pd₂Ge Intermetallic Catalysts for Multifunctional Electrochemical Performance

ConspectusThe term "intermetallic" refers to a new metallic structure generated by the formation of intermetallic bonds of two or more different constituting metals. In solid-state synthesis, a very high temperature and a long duration are required to address the challenge of breaking the stable homometallic bonds and generating heterometallic bonds with the periodic diffusion of all metals forming the new compound. In nanoparticle synthesis via solution-phase methods, different atoms experience competition between the rate of reduction and diffusion, which majorly controls the formation of ordered and disordered compounds between two different metals. Intermetallic compounds (IMs) provide a unique combination of thermodynamic stability, long-range atomic ordering, heteroatomic surfaces, and electronically tunable frameworks, making them highly active and versatile for electrocatalysis. In this context, Pd₂Ge stands out as a chemically intriguing intermetallic template for generating highly stable and efficient electrocatalysts. In this Account, we summarize the multiyear research of our group establishing Pd₂Ge as a model platform for understanding how elemental diffusion, site-specific substitution, active interface generation, and electronic structure tuning can transform a single ordered intermetallic into a multifunctional electrocatalyst family. We showed that the solution-phase synthesis of Pd₂Ge nanoparticles is achievable through careful control of the reduction kinetics and diffusion pathways, despite the significant reduction potential mismatch between Pd²⁺ and Ge⁴⁺. Our approach achieves the simultaneous coreduction of Pd²⁺ and Ge⁴⁺ precursors, effectively suppressing GeO₂ formation and enabling the clean evolution of the Pd₂Ge phase. After overcoming the challenge of binary intermetallic synthesis, a major challenge in intermetallic chemistry is the controlled incorporation of a third metal without disrupting the long-range order; this is governed by the reduction potential, atomic-size matching, orbital-overlap, site-preference energetics, and diffusion barriers. We demonstrate that the Pd sites in Pd₂Ge can accommodate Ni, Co, Pt, and Cu through element-specific diffusion and reduction kinetics, enabling substitution up to a variable diffusion limit while preserving the ordered framework. This provides an atomic-level example for experimentally probing multimetal diffusion and lattice accommodation in a stable intermetallic matrix. Site-selective substitution has been proven by powder X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). The exact charge transfer mechanism after different element substitution has been evidenced via XPS and XAS. Our group has explored the electrochemical properties of different metal-substituted Pd₂Ge in the ethanol oxidation reaction (EOR), oxygen reduction reaction (ORR), formaldehyde oxidation reaction (FAOR), and oxygen evolution reaction (OER). The exact reaction mechanism and the active site determination of this intermetallic for these reactions have been exhaustively determined via different operando spectroscopic and analytical techniques and DFT calculations. This Account gives a broad overview and a guideline about the intermetallic generation and full exploration of a stable intermetallic and how fine-tuning of the intermetallic gives rise to different electrochemical superiorities. Collectively, this Account presents the first integrated exploration of Pd₂Ge as a robust and electronically programmable intermetallic, showing how kinetic control, thermodynamic driving forces, and site-selective multimetal diffusion can be leveraged to design high-performance electrocatalysts across diverse reaction environments.