Deciphering cation-coupled mechanisms in electrochemical CO2 reduction via electrokinetic analysis

The electrochemical reduction of CO₂ to formic acid or formate represents one of the most economically promising route for CO₂ utilization. While substantial advances in catalyst design and electrolyzer engineering have been achieved in recent years, critical uncertainties remain regarding the reaction pathway and the often-debated role of alkali metal cations. Resolving these discrepancies requires precise kinetic analysis under well-defined conditions.

In this study, the researchers systematically investigate the kinetics of CO₂ reduction to formic acid or formate across a wide pH range, enabled by two key developments: the identification of BiPO₄ as a stable precatalyst under acidic conditions through comprehensive screening, and the implementation of sensitive ion chromatography for accurate product quantification, even at low current density where conventional methods struggle. The electrokinetic data suggest that the reaction proceeds via sequential electron and proton transfers rather than proton-coupled electron transfer as proposed by many computational simulations. Notably, the rate-determining step (RDS) transitions from the proton transfer step at low overpotential to the first electron transfer step at high overpotential, with the proton source dependent on electrolyte pH. Furthermore, through K⁺ reaction order analysis and crown ether chelation experiments, the researchers demonstrate that the alkali cations are not merely spectators but actively participate in the reaction, likely by stabilizing negatively charged intermediates via electrostatic interactions.

Importantly, building on all the experimental results, the researchers propose a comprehensive mechanistic framework for CO₂RR to formic acid or formate with cation involvement in this study. CO₂ is first adsorbed onto the catalyst to form *CO₂. The reduction proceeds via sequential electron and proton transfers, with the electron transfer to form *CO₂^- as the RDS at high overpotential and the protonation of *CO₂^- to *OCHO as the RDS at low potential. In the chemical step, the proton donor is identified to be free H⁺ in electrolytes with pH < 4.3, and it exhibits a mixed nature at pH > 4.3. Although cations do not directly participate in the reaction, they likely stabilize *CO₂^- anion via dipole-field interaction or short-range Coulombic interaction, as the adsorption of negative charged species alone is energetically unfavorable under highly cathodic potentials. Moreover, the hydration shell of cations may act as a preferred proton donor due to its close proximity and competitive pK_a. Following the formation of *OCHO, the intermediate accepts a second electron and subsequently desorbs as formate at pH > 3.75 (pK_a of formic acid) or undergoes further protonation to yield formic acid at pH < 3.75.

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