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

Abstract

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 (RCH2−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.