Insights Into CO₂ Loss, pH Effects, and Tafel Kinetics in Ni Single Atom-Driven Bicarbonate Electroreduction

Electrochemical reduction of bicarbonate offers an attractive pathway for converting captured CO₂ into valuable chemicals under mild conditions, thereby bypassing prior CO₂ release. Herein, we examine the catalytic performance and mechanism of a Ni single-atom catalyst in bicarbonate electrolysis to CO and H₂, emphasizing the impact of CO₂ (carbon species) escaping from the electrolyte, a critical and often-overlooked factor in the literature that affects selectivity and efficiency. We investigate three cell configurations: (1) closed cell with no CO₂ escape, preserving reactive carbon species; (2) open cell with moderate escape, causing gradual depletion; and (3) Ar-purged cell with significant escape, accelerating degassing and losses. In an H-cell, however, CO selectivity declines over time in both open and Ar-purged setups due to changes in the electrolyte. Infrared spectroscopy, pH monitoring, and quantitative carbonate-speciation analysis indicate that loss of CO selectivity stems from the depletion of reactive carbon species (dissolved CO₂ from bicarbonate dissociation) and buffer shifts, rather than catalyst deactivation. Selectivity is restored by pH adjustment. Kinetic analyses, including Tafel slopes (∼118 mV dec^-1) and electrochemical impedance spectroscopy, reveal a rate-determining step in which a pre-equilibrium chemical reaction is coupled to electron transfer to adsorbed CO₂ intermediates.