CO2-to-synfuels manufacture based on renewable electricity offers pathways to low-carbon chemical feedstocks and fuels. To achieve this ideal, we have pursued the realization of higher-selectivity and higher-productivity catalysts to turn CO2 into hydrocarbons. Indeed there remains room to increase selectivity, activity, and overall energy efficiency.
Cu as so far been the most efficient in converting CO2 to the more valuable products - multi-carbon hydrocarbons such as ethylene. Early reports of its promise were from Hori. Y. (Chem. Lett. 14, 1695-1698 (1985)). The effect of facets on the product distribution was investigated in the 1990s, with Cu(100) facets enabling the highest C2+ hydrocarbon selectivity among the low-index Cu facets. This later served as a design principle for improving Cu catalysts. One route to Cu(100)-rich catalysts is the colloidal syntheses of nanocubes, during which capping agents adsorb on the Cu surface to lower the surface energy of Cu(100) facets and stabilize its cubic morphology.
We posited that CO2 reduction intermediates could also adsorb on Cu surface and help stabilize Cu(100) facets.
We therefore investigated the effect that adsorbed CO2RR intermediates had on the energetics of Cu facets. We began with density functional theory calculations and found that increasing the coverage of CO2RR intermediates lowers the surface energy of Cu(100) facets; for instance, a CO2RR intermediate coverage of 0.33 ML results in Cu(100) surface energy of 1 J cm-2, which is 0.26 J cm-2 more stable than that of Cu(111) (Figure 1a). The equilibrium shapes of the resultant Cu crystals predicted by Wulff construction (Figure 1b) exhibit an increase in Cu(100) exposure compared to Cu without intermediates and with H* adsorption only (labeled Cu-HER).
Figure 1. Wulff construction clusters of Cu showing the coverage of CO2RR and HER intermediates. a Calculated intermediate coverage and corresponding surface energies of Cu(111), Cu(100) and Cu(211) facets. b Wulff construction clusters of Cu without and with adsorption of CO2RR and HER intermediates.
We then proceeded to synthesize Cu catalysts under CO2 reduction conditions (labeled Cu-CO2) via an electrodeposition approach. Using the grazing-incidence wide-angle X-ray scattering and hard X-ray absorption spectroscopy beamlines at the Advanced Photon Source at Argonne National Laboratory, we observed that Cu growth was decelerated in the presence of CO2 by ~60% compared to synthesis under the usual H2 evolution conditions (Figure 2a). A CO2-rich environment increased the Cu(100):Cu(111) ratio to 1.4:1, as verified by both OH- electroabsorption and Pb underpotential deposition. This represented a 70% increase in the ratio of Cu(100) facets to total facet area (Figure 2b and c). This is in agreement with the picture presented in theoretical studies wherein the energetics of Cu facets under CO2 reduction conditions alter the Cu surface.
Figure 2. Analysis of the catalyst formation and the surface structures. a The ratio of metallic Cu to Cu precursor over the course of catalyst formation. b, c The surface area and ratio of Cu(100) and Cu(111) facets.
Finally, we equipped alkaline flow cells and a membrane electrode assembly systems with Cu catalysts fabricated under CO2 reduction conditions. We achieved a 90% Faradaic efficiency toward C2+ products at current densities of 580 mA cm-2. A 37% full-cell energy efficiency at 300 mA cm-2 was achieved in alkaline flow cells. Consistent high-selectivity-at-high-current-density performance was demonstrated in 65-hour operation using the membrane electrode assembly systems.
For more information, please find our paper here.