Electrochemical energy conversion and storage processes, powered using renewable electricity, provide a sustainable approach to tackling global energy and climate concerns. These processes usually consist of multiple and branched elementary steps involving diverse intermediates and electron/proton transfers. A typical such process is CO₂ reduction reaction (CO₂RR).

In CO₂RR, the judicious stabilization of key intermediates in order to favor a desired pathway is central to the improvement of reaction selectivity. Traditional strategies focus on the control of electrocatalyst materials morphology, grain boundaries, facets, and dopants.

However, the selectivity towards a specific product, especially economically-desirable C₂₊ products, is still low in neutral media – the best catalyst reported so far shows a Faradaic efficiency (FE) of 60% for ethylene at a partial current density of 7 mA cm^-2. This low performance results in a low energy efficiency.

We reported in 2018 that high concentrations of hydroxide ions can mediate the selective production of ethylene, with a record FE of 70% for a Cu catalyst in 10 M KOH. However, large amounts of CO₂ are consumed by the alkaline electrolytes to form carbonate – i.e. much CO₂ is wasted under these highly alkali conditions.

Figure 1. The device that operates CO₂-to-ethylene conversion.

More recently, we adopted the approach that – if we could work in neutral media, yet discover how to sustain high selectivity (>70%), and do all this at commercially-relevant current densities (e.g., 300 mA cm^-2) – offer a further advance toward the sustainable production of fuels and chemical feedstocks via CO₂RR.

Collaborating with the groups of Professor Jonas Peters and Professor Theo Agapie at Caltech, the joint team developed a molecular tuning strategy – the functionalization of the surface of electrocatalysts with organic molecules – that stabilizes intermediates for enhanced CO₂RR to ethylene. We investigated a library of molecules, derived via electro-dimerization of arylpyridiniums, with the aid of organic synthesis, operando spectroscopies and theoretical calculations. We found that the adhered molecules stabilize a CO-bound intermediate in an atop bound configuration, favoring reduction to ethylene instead of evolution as gaseous CO.

As a result of this strategy, we achieved CO₂RR, in neutral electrolyte, to ethylene, having record performance for this medium: FE of 72% at a partial current density of 230 mA cm^-2. The system maintains the high selectivity towards ethylene while operating at high current for 190 hours, delivering a full-cell energy efficiency of 20%. Due to the use of neutral electrolyte, the CO₂ is not consumed by the electrolyte: the CO₂ utilization efficiency herein is approximately 100%, where in prior energy-efficiency-record-setting reports in alkali media, it was much lower.

The full paper can be found here: https://www.nature.com/articles/s41586-019-1782-2.