CO2 Electroreduction on Modified Cu Catalysts: Using Subsurface Dopants to Enhance Catalytic Performance

The electrochemical reduction of CO₂ (CO₂RR) over Cu to a single class of target hydrocarbons – ethylene represents one prominent example of intense recent interest – is attracting considerable attention. It provides an avenue to the synthesis of high energy density and high economic value feedstocks and fuels using renewable electricity. However, before CO₂RR can be widely applied, at least two significant issues need to be addressed: (1) Previous research has shown that modified copper is especially selective in CO₂RR; however, diverse C1 and C2 species are generated simultaneously. It is of interest to enhance the selectivity to a single high-value hydrocarbon product at high activity; (2) High Faradaic efficiencies for C2 products have been achieved by inducing surface Cu^δ+ sites in Cu via oxygen species. Since such Cu^δ+ sites are expected to be readily reduced to Cu⁰ under CO₂RR, it is of interest to increase the stability of the Cu^δ+ sites in Cu.

We took the view that the electronegativity of new dopants could increase Cu^δ+ sites and offer a subsurface doping strategy that keeps the Cu^δ+ sites stable under the reducing potentials used in CO₂RR. In DFT calculations (Figure 1), we found that boron and carbon elements under negative applied potential can be stable at the subsurface of a Cu(111) slab, and that they greatly alter the Cu oxidation state to be positive, and as a consequence enhance the selectivity of ethylene products from CO₂RR. Our corresponding experiments also achieve a high Faradaic efficiency for ethylene of ~52% for boron-doped Cu and ~78% for carbon-doped Cu. Importantly, boron-doped and carbon-doped copper showed ~40 h and ~170 h stability for CO₂RR to ethylene. This theoretical and experimental combined work indicates a role for subsurface chemistry in enhancing the selectivity and stability of the Cu based CO₂RR system.