The development of earth‐abundant catalysts for selective electrochemical CO₂ conversion is a central challenge. CuSn bimetallic catalysts can yield selective CO₂ reduction toward either CO or formate. This study presents oxide‐derived CuSn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO₂ reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X‐ray absorption spectroscopy for in situ study, and quasi in situ X‐ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu⁰ under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO‐selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate‐selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn⁰ and Sn oxide species. Density functional theory simulations suggest that Sn^δ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn⁰ sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts.

Current time-resolved in situ approaches limit the scope of mechanochemical investigations possible. Here we develop a new, general approach to simultaneously follow the evolution of bulk atomic and electronic structure during a mechanochemical synthesis. This is achieved by coupling two complementary synchrotron-based X-ray methods: X-ray absorption spectroscopy (XAS) and X-ray diffraction. We apply this method to investigate the bottom-up mechanosynthesis of technologically important Au micro and nanoparticles in the presence of three different reducing agents, hydroquinone, sodium citrate, and NaBH₄. Moreover, we show how XAS offers new insight into the early stage generation of growth species (e.g. monomers and clusters), which lead to the subsequent formation of nanoparticles. These processes are beyond the detection capabilities of diffraction methods. This combined X-ray approach paves the way to new directions in mechanochemical research of advanced electronic materials.

The integration of multiple elements in a high‐entropy state is crucial in the design of high‐performance, durable electrocatalysts. High‐entropy metal hydroxide organic frameworks (HE‐MHOFs) are synthesized under mild solvothermal conditions. This novel crystalline metal–organic framework (MOF) features a random, homogeneous distribution of cations within high‐entropy hydroxide layers. HE‐MHOF exhibits excellent electrocatalytic performance for the oxygen evolution reaction (OER), reaching a current density of 100 mA cm⁻² at ≈1.64 V_RHE, and demonstrates remarkable durability, maintaining a current density of 10 mA cm⁻² for over 100 h. Notably, HE‐MHOF outperforms precious metal‐based electrocatalysts despite containing only ≈60% OER active metals. Ab initio calculations and operando X‐ray absorption spectroscopy (XAS) demonstrate that the high‐entropy catalyst contains active sites that facilitate a multifaceted OER mechanism. This study highlights the benefits of high‐entropy MOFs in developing noble metal‐free electrocatalysts, reducing reliance on precious metals, lowering metal loading (especially for Ni, Co, and Mn), and ultimately reducing costs for sustainable water electrolysis technologies.