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The electrochemical and photoelectrochemical reduction of CO₂ presents an attractive route towards fuel formation from renewable energy. However, the dearth of selective, active and inexpensive catalyst materials remains a key challenge on the way to renewable production of hydrocarbon fuels. Here, we address these challenges through the design of novel devices and selective earth-abundant catalysts for the electrochemical^[1–3] and solar-driven^{[1,4–6]} reduction of CO₂. In addition, we provide insight into the origin of selectivity on electrocatalysts based on copper, paving the way towards more rational design of electrochemical fuel forming devices.

We pioneer the application of Atomic Layer Deposition (ALD) to direct the catalytic selectivity of copper catalysts. Copper-based catalysts derived from the reduction of CuO are known to reduce CO₂ but simultaneously exhibit poor selectivity. ALD coating of CuO with thin layers of SnO₂ allows for formation of CO with high selectivity even at low overpotentials, comparing favorably to conventional gold catalysts, known to selectively convert CO₂ to CO. We use kinetic studies, high resolution transmission electron microscopy and gas chemisorption analysis to explain the observed selectivity changes on the basis of decreased binding strengths for both H and CO intermediates on the Sn-containing catalyst surface.

We subsequently combine our new catalyst with an earth abundant oxide anode to effect overall splitting of CO₂ to CO and O₂. Driven by a 3-junction solar cell, this device achieved long-term solar splitting of CO₂ into CO and O₂ at an efficiency of 13.4 %, constituting the current world record.^[1]

Going beyond the production of CO, we investigate the further reduction of CO to hydrocarbons. Insights into the factors controlling the branching between different hydrocarbon species and hydrogen as a side product are scarce, precluding any rational design of selective catalysts. To remedy these shortcomings, we devise a platform allowing for the systematic study of the relevant reaction parameters. Exploiting non-aqueous electrolytes, we isolate activation-controlled kinetic data in electrochemical driving force, proton availability, solvent dielectric and reagent concentration. Our observations provide unique insight into the mechanism of hydrocarbon production on copper electrodes, demonstrating that the competition between H and CO for surface sites plays an important role in governing the product selectivity between methane, ethylene and hydrogen. These results establish a new paradigm in controlling hydrocarbon product selectivity in electrochemical fuel formation.^[2]

References

[1] M. Schreier, F. Héroguel, L. Steier, S. Ahmad, J. S. Luterbacher, M. T. Mayer, J. Luo, M. Grätzel, Nat. Energy 2017, 2, 17087.

[2] M. Schreier, Y. Yoon, M. N. Jackson, Y. Surendranath, Angew. Chem. Int. Ed. 2018, DOI 10.1002/anie.201806051.

[3] G. P. Lau, M. Schreier, D. Vasilyev, R. Scopelliti, M. Grätzel, P. J. Dyson, J. Am. Chem. Soc. 2016, 138, 7820–7823.

[4] M. Schreier, P. Gao, M. T. Mayer, J. Luo, T. Moehl, M. K. Nazeeruddin, S. D. Tilley, M. Grätzel, Energy Env. Sci 2015, 8, 855–861.

[5] M. Schreier, L. Curvat, F. Giordano, L. Steier, A. Abate, S. M. Zakeeruddin, J. Luo, M. T. Mayer, M. Grätzel, Nat. Commun. 2015, 6, 7326.

[6] M. Schreier, J. Luo, P. Gao, T. Moehl, M. T. Mayer, M. Grätzel, J. Am. Chem. Soc. 2016, 138, 1938–1946.