(Energy Technology Division Research Award Address) Electrochemical CO2 Reduction: Path Towards a Carbon Neutral Chemical Industry?

Abstract

The chemical and fuel industry today relies on fossil fuels as its major feedstock and applies a variety of energy-intense thermal / catalytic processes to convert this feed into different intermediates or bulk chemicals and fuels. Many of these processes are responsible for a sizeable fraction of the anthropogenic CO2 emissions that are contributing to global warming and associated issues such as climate change, rising sea levels, and more erratic weather patterns. In contrast, the use of CO2 as the feedstock for the production of bulk chemicals such as CO, ethylene, and ethanol via electrochemical CO2 reduction not only utilizes some of the CO2 that otherwise would be emitted in the atmosphere, it also avoids the sizeable CO2 emissions associated with many of the aforementioned energy-intense processes that use fossil fuels as the feed [1].Over the past decade, a range of active electrocatalysts for the selective reduction of CO2 to different product have been identified. For CO production selectivity easily exceeds 95%, and current densities exceeding 500 mA/cm2 can be achieved at overall energy efficiencies of 45-60% [2,3]. Also, ever more active and selective catalysts for ethylene / ethanol production are being developed. Recently we reported an electrodeposited copper-silver alloy catalyst able to produce ethylene and ethanol at a combined selectivity exceeding 80% (3:1 ethylene to ethanol) at a rate of 170 mA/cm2 [4]. Undoubtedly, research will continue to yield ever more active and selective catalysts for different products. A number of techno-economic analyses have indicated that stability / durability of catalysts and electrodes over thousands of hours will be crucial to achieve economic feasibility (see for example [5]).This presentation will summarize state-of-the-art electrocatalysts for the reduction of CO2 to CO, to ethylene / ethanol, and other products, and how a number of factors, such as electrolyte composition, pH, and electrolysis cell design help optimize electrocatalytic performance. Subsequently, it will review some of the challenges related to enhancing the stability/durability of catalysts and especially of gas diffusion electrodes typically used in electrolysis cells. The presentation will also further explore the techno-economic and life-cycle prospects of CO2 electroreduction technology: which product can be produced in an economically viable and close to carbon neutral fashion using energy from the grid, which only in part originates from renewable sources [6]. A key finding is that reducing the energy requirement of the anode side (by replacing the energy-intense oxygen evolution reaction with a different electrochemical conversion) can be hugely beneficial to achieving economic feasibility and carbon neutrality. For example, co-electrolysis that involves reduction of CO2 on the cathode paired with oxidation of, for example, glycerol (a waste product of biofuel production) at the anode, reduces overall the overall energy requirement of the process by 40-50%. Indeed, techno-economic and life-cycle analyses indicate that co-electrolysis approaches that involve oxidation of organic substrates (biomass, waste streams from industry) on the anode drastically enhance the prospects of CO2 electroreduction technology to be key to a future carbon neutral chemical industry.(1) P.J.A. Kenis, A. Dibenedetto, T.R. Zhang, ChemSusChem, 2017, 18(22), 3091-3093.(2) S. Verma, X. Lu, S. Ma, R.I. Masel, P.J.A. Kenis, PhysChemChemPhys, 2016, 18 (10), 7075-7084.(3) S. Verma, Y. Hamasaki, C. Kim, W. Huang, S. Lu, H.R.M. Jhong, A.A. Gewirth, T. Fujigaya, N. Nakashima, P.J.A. Kenis, ACS Energy Lett., 2018, 3, 193-198.(4) T.T.H. Hoang, S. Verma, S. Ma, T.T. Fister, J. Timoshenko, A.I. Frenkel, P.J.A. Kenis, A.A. Gewirth, J. Am. Chem. Soc., 2018, 140, 5791-5797.(5) S. Verma, B. Kim, H.R. Jhong, S. Ma, P.J.A. Kenis, ChemSusChem, 2016, 9 (15), 1972-1979.(6) S. Verma, S. Lu, P.J.A. Kenis, Nature Energy, 2019, 4, 466–474.