Insights Into CO ₂ Loss, pH Effects, and Tafel Kinetics in Ni Single Atom‐Driven Bicarbonate Electroreduction

ABSTRACT Electrochemical reduction of bicarbonate offers an attractive pathway for converting captured CO 2 into valuable chemicals under mild conditions, thereby bypassing prior CO 2 release. Herein, we examine the catalytic performance and mechanism of a Ni single‐atom catalyst in bicarbonate electrolysis to CO and H 2 , emphasizing the impact of CO 2 (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 CO 2 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 CO 2 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 CO 2 intermediates.