Cross-Scale Supply–Demand Matching of Reactive Species in Electrocatalytic Reduction Reactions

Conspectus Electrocatalytic reduction reactions, such as the hydrogen evolution reaction (HER) and the carbon dioxide, carbon monoxide, and nitrogen electroreduction reactions (CO2RR, CORR, NRR), provide promising and sustainable routes for energy conversion, carbon utilization, and low-emission chemical manufacturing. Despite significant advances in catalyst development, the practical implementation of these reactions, particularly under high-current-density conditions, remains severely constrained by inefficiencies in activity, selectivity, and stability. The electrocatalyst performance has traditionally been interpreted and optimized largely in terms of intrinsic activity descriptors, such as adsorption free energies of reaction intermediates, electronic-state characteristics of active sites, and related electronic-structure parameters, including the d-band center and density of states. These descriptors are typically evaluated in modeling under the implicit assumption that reactive species are continuously and sufficiently supplied to catalytic active sites. Under practical operating conditions, especially at high reaction fluxes, however, this assumption is often no longer valid. A critical limitation arises from the mismatch between the supply capacity of reactive species (e.g., *H, lattice O, and CO2) and the demand strength imposed by active sites. At high current densities, electrochemical interfaces operate far from equilibrium, such that reaction performance is no longer governed solely by the intrinsic catalytic activity of a single active site but also by the rates of generation, replenishment, transfer, and utilization of elementary reaction species. Such supply–demand imbalances can reduce overall reaction rates and alter reaction pathways. However, the role of species supply–demand matching across different scales has rarely been systematically incorporated into reaction mechanisms or catalyst design strategies. In this Account, we propose a reactive species cross-scale supply–demand matching framework in electroreduction reactions across atomic, nanoscale, and macroscale regimes. We demonstrate that catalytic performance is governed not only by intrinsic activity but also by whether the availability of reactive species becomes insufficient, excessive, or appropriately matched to the demand of active sites. At the nanoscale, we uncover hydrogen spillover as an interfacial mechanism that couples *H generation and consumption sites, enabling *H supply–demand balance in alkaline HER. At the atomic scale, extending this concept to proton-coupled electron transfer reactions reveals that efficient hydrogen transfer is governed by spatial proximity, requiring angstrom-scale site separation. Furthermore, we identify lattice oxygen as an alternative atomic-scale supply pathway in CO2RR and provide direct experimental evidence for lattice oxygen participation. At the macroscale, we develop quantitative CO2 supply–demand matching models that correlate mass transport conditions, electrode structures, and reaction flux limitations. By integrating these examples across different scales, this Account reframes the understanding of electroreduction reactions from an intrinsic activity-centered view toward a supply–demand matching perspective, where the balance between reactive species availability and catalytic demand becomes a key determinant of performance under practical conditions, providing new guidance for designing electrocatalysts, electrodes, and reactors.