Cation concentration-dependent reaction kinetics in single-atom catalysts for electrochemical CO₂ reduction

Electrolyte cations are widely recognized as critical promoters in electrochemical CO₂ reduction reaction (CO₂RR), yet the field has largely focused on cation identity while overlooking a more fundamental and practical parameter, namely cation concentration. Whether continuously increasing cation concentration enhances catalytic activity remains an open and consequential question, and the lack of mechanistic understanding has limited the rational design of electrolyte environments. Here, we demonstrate that cation concentration is not merely a secondary parameter, but a decisive kinetic regulator that fundamentally governs CO₂RR performance. By integrating constant-potential ab initio molecular dynamics simulations with experiments, we reveal a previously unrecognized nonmonotonic ("volcano-type") dependence of catalytic activity on K⁺ concentration over Ni-N-C single-atom catalysts. At moderate concentrations, K⁺ promotes CO₂ activation by restructuring the interfacial hydrogen-bond network and stabilizing key intermediates. Strikingly, further increasing K⁺ concentration leads to over-stabilization of *CO, impeding its desorption and suppressing overall reaction rates. This "double-edged" effect establishes an intrinsic trade-off between intermediate activation and product release. Experimental measurements directly validate this prediction, exhibiting a pronounced rise-and-fall trend in CO partial current density.