Abstract
Taking advantage of renewable energy sources to convert CO2 to fuels and chemicals
electrochemically has the potential to sustainably produce currently fossil-derived molecules
artificially, using systems that mimic photosynthesis. The ultimate goal is solar fuels or “electrofuels”,
and by developing the technology into an efficient and scalable one, a circular economy can be formed
around CO2-neutral production and subsequent use of fuels, Fig. 1. In addition, it presents an avenue
for efficient utilization of intermittent, renewable energy along with the vast, pre-existing
infrastructure for storing and transporting these molecules. Nonetheless, considerable progress must
still be made to achieve industrially relevant targets for the technology to be considered for large-
scale implementation, as featured in a recent technoeconomic and carbon emissions analysis [1].
At the heart of this development is the electrochemical CO2 reduction reaction (CO2RR), which
takes place on the surface of an electrocatalyst. Thus, it is vital to gain a fundamental understanding
of how different parameters influence the selectivity and efficiency of the CO2RR. Over the past few
years, considerable work has been done to gain mechanistic insight into various aspects of the
electrocatalysis, such as the initial activation of CO2, the importance of stabilizing key intermediates,
and their relation to the observed overpotentials [2]. The effect of the molecular nature or catalyst
morphology (particle size, shape, roughness, defect density and composition) on CO2RR selectivity has
also been examined [3], [4] in detail. More recently, attention has been given to (local) electrolyte
effects and the role of mass transport [5] in relation to product selectivity. Finally, potential pulses
leading to a change in the catalyst morphology, oxidation state and local pH have been featured as an
additional avenue to tune the product selectivity [6]. Progress in understanding these fundamental
aspects of the CO2RR will be crucial in developing both solar-driven and electricity-driven systems.
Currently, the best Faradaic efficiencies (FEs) have been reported for the 2-electron products
carbon monoxide (CO) and formic acid (HCOOH) as well as 12-electron product ethylene (C2H4).
Electrochemical engineering efforts are progressing these best candidates towards commercial
relevancy through innovations in gas diffusion electrodes, ion-exchange membranes, flow-cell
architectures and process optimization. In order to enhance this progress and expand the range of
favorable CO2RR products, further advancements must be made in understanding the electrocatalysis.
