Controlling hydrocarbon chain growth and degree of branching in CO2 electroreduction on fluorine-doped nickel catalysts

Nickel-based materials can facilitate the electrocatalytic CO₂ reduction (CO₂R) reaction to generate hydrocarbons up to C₆. Here we show that fluorine doping alters the nature of the Ni active sites, which proves instrumental in tuning the selectivity of the CO₂R. We interrogate the CO₂R reaction mechanism using intermediate surrogates, including aldehydes, alkyl iodides and acetylene. Aldehydes are electroreduced to alcohols and deoxygenated intermediates. Among the latter, unsaturated hydrocarbon intermediates (RCH_2−x*, where the asterisk represents surface-bound species and x = 1 or 2) reacting with *CO dictate chain propagation, modulated by competitive C–C coupling and C–H hydrogenation reactions. Compound branching in the hydrocarbons initiates from *CO coupling with two *CH₂ species, and the branch-to-linear hydrocarbon ratio can be doubled using a pulsed potential strategy. An inverse H/D kinetic isotope effect promotes deuterated hydrocarbon formation with a Faradaic efficiency of 22.2%. This work reveals mechanisms and strategies for the conversion of CO₂ into linear and branched hydrocarbons, thus advancing electrosynthetic fuel development.