Electrocatalysts for Direct CO₂ to C2+ Chemical Conversions with High Selectivity and Energy Efficiency

Carbon dioxide emission from fossil fuel combustion has generated an on-going debate on its impact to the global environmental and ecological systems. New carbon capture, sequestration and conversion technologies are widely pursued as potential solutions to mitigate the concerns. The electrochemical CO2 reduction reaction (CO2RR) to hydrocarbon fuels and chemicals using renewable electricity offers an attractive “carbon-neutral” or even “carbon-negative” mitigation strategy. CO2 is an inexpensive carbon source and can be used as a feedstock for producing high value chemicals. Key challenges facing the current CO2RR electrocatalysis include improving energy efficiency by reducing the overpotentials; increasing the process selectivity by enhancing a single-product Faradaic efficiency (FE); and lowering the system operating cost by prolonging the catalyst stability. The U.S. produced 5.1 billion metric tons of CO2 in 2018 from industrial processes, posing a major concern on its impact to the climate.[1] The CO2 can be captured and used as raw material to produce to value-added chemicals. According to DOE’s “Bandwidth Study on Energy Use and Potential Energy Saving Opportunities in U. S. Chemical Manufacturing”,[2] C2+ chemicals such as ethanol (C2H5OH), acetic acid (CH3CO2H) and acetone (C3H6O) are among top 74 major chemicals produced in the US. Making these chemicals using sequestrated CO2 represents a huge opportunity for US manufacturing while improving the environment. Techno-economic analysis (TEA) of low-temperature electrochemical conversion of CO2 to C2+ chemical, particularly ethanol, has been conducted by several studies. All analyses indicate that profitable manufacturing of C2+ chemicals by electrochemical CO2 reduction reaction (CO2RR) depend strongly on conversion selectivity (Faradaic efficiency, or FE) and the difference between the actual and theoretical operating potentials (overpotential). Argonne National Laboratory, through collaboration with Northern Illinois University, recently prepared a series of highly active and durable electrocatalysts for CO2RR over commercial carbon support using a novel amalgamated lithium metal (ALM) synthesis technique.[3] The new electrocatalysts offer the several advantages in direct conversion to C2+ chemicals, including a) high FEs (all with peak FE > 90%) with one-step conversion to C2 (C2H5OH and CH3COOH) and C3 (C3H6O) chemicals; b) low voltage with the onset potential as low as -0.4 V and approaching to the theoretic limit; c) low cost catalysts made of earth-abundant transition metal; d) low operating temperature (< 80 °C) and pressure (one bar). including > 90% single-product FE, low onset potential (-0.4 V RHE for ethanol conversion) and excellent durability. In this presentation, we will report discuss the new electrocatalyst performance in terms of FE, onset potential and durability. The structure-function relationship with particular emphasis on CO2 to ethanol conversion and the mechanistic insight gained from operando X-ray absorption spectroscopy and computational modeling will also be reported. Acknowledgement: This work was supported by Argonne National Laboratory LDRD office. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials, an U.S. Department of Energy Office of Science User Facility, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357. [1] https://www.eia.gov/environment/emissions/carbon/pdf/2019_co2analysis.pdf [2] https://www.energy.gov/sites/prod/files/2015/08/f26/chemical_bandwidth_report.pdf [3] Xu, H. P. et al.; Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically-dispersed copper, Nature Energy, 2020, 5, 623-632.