How Far Does an Active Site Reach? Defining Interaction Radii in Covalent Organic Framework Electrocatalysts for Nitrate Reduction

Abstract Electrocatalytic nitrate reduction to ammonia has emerged as a compelling strategy for simultaneously closing the anthropogenic nitrogen cycle and enabling decentralized ammonia production. Despite substantial advances in catalyst development, the origin of high selectivity in porous electrocatalysts remains incompletely understood because current interpretations overwhelmingly regard active sites as localized catalytic points. In this Perspective, we argue that this description neglects a fundamental length scale governing multistep nitrate electroreduction. We introduce the concept of an interaction radius, defined as the spatial domain over which an active site measurably perturbs the energetics, dynamics, and transport of reactants, intermediates, and transition states. Within covalent organic frameworks (COFs), this interaction radius emerges from the coupled action of electronic delocalization, electrostatic fields, hydrogen-bonding networks, steric confinement, and solvent organization, transforming the active site from a discrete coordination center into a spatially distributed catalytic entity. We discuss how interaction radii evolve under electrochemical bias, overlap to produce cooperative reaction domains, and ultimately determine intermediate retention, pathway bifurcation, and ammonia selectivity. We further propose experimental and computational strategies for quantifying interaction radii and establish design principles that relate framework chemistry, pore architecture, site spacing, and local solvation to the reaction length scales of nitrate reduction. By reframing COF electrocatalysis through a spatially resolved perspective, this work establishes interaction radius as a quantitative organizing framework with the potential to evolve into a predictive descriptor for rational catalyst design, while providing a transferable conceptual basis for understanding complex electrocatalytic reactions beyond nitrate reduction.