Impact of Catalyst Performance on the Life-Cycle CO2 Emissions of Methanol Production By Direct Electrocatalytic Reduction of CO2

Direct electrocatalytic reduction of carbon dioxide (eCO₂RR) is one technology being developed to convert solar photovoltaic power into energy-dense, easily dispatchable liquid fuels.  In addition to providing a stable and energy-dense storage medium for solar energy, fuels synthesized using solar energy and CO₂ would provide a non-fossil, low-emissions route to liquid fuels for a carbon-constrained future.  Although extensive basic science research is focused on eCO₂RR catalyst development, the full life-cycle emissions of this technology platform have not been quantified.  In particular, the relative importance of catalyst performance improvements, compared to other system parameters, for the emissions profile of a large-scale eCO₂RR process have not been analyzed in the literature.

This work uses a hybrid life cycle assessment (LCA) approach to estimate how the choice of catalyst material and the performance of the catalyst influence the life cycle CO₂ emissions associated with producing methanol by the eCO₂RR process.  Methanol is a potential drop-in transportation fuel, useful in both combustion engines and fuel cells.  In this analysis, the CO₂ reduction electrocatalyst and its carbonaceous support are explicitly modeled using process-based LCA.  For the balance of the CO₂ reduction cell system, a water electrolyzer is used as a surrogate technology, and incorporated using input-output LCA.

The CO₂ emissions intensity of methanol produced by a prospective direct CO₂ electrocatalytic reduction process are dominated by the emissions intensity of the electrical power used to carry out the electrocatalytic reduction.  The emissions intensity is strongly influenced by the electrocatalyst's faradaic efficiency for the desired methanol product, but is only weakly sensitive to catalyst overpotential and current density.  Ancillary processes such as CO₂ capture and product methanol purification generate only a small fraction of the total emissions.  

In a preliminary analysis, the total system CO₂ emissions are 1.24 g CO₂ emitted / g methanol, and 97% of these emissions are from generating the electricity used to drive the electrocatalytic reduction.  (In contrast, methanol production from conventional natural gas reforming has an emissions intensity of 0.50–0.67 g CO₂ emitted / g methanol.)  The CO₂ capture process, the product purification process, and the embodied emissions in the process equipment each contribute 2% or less to the CO₂ intensity of the methanol product.  The CO₂ intensity of the product is therefore influenced by those technical parameters relevant to the generation and use of the electric power, especially the CO₂ intensity of the electricity and the faradaic efficiency of the cathode catalyst for the desired methanol product.  Doubling the faradaic efficiency from 25% to 50% would reduce the CO₂ intensity by one-third (from 1.24 to 0.79 g CO₂ emitted / g methanol).  The emissions intensity is only moderately sensitive to catalyst overpotential: decreasing the cathode potential by about one third, from –0.8 V to –0.5 V, would decrease the emissions intensity by about one tenth (from 1.24 to 1.12 g CO₂ emitted / g methanol).  If the feedstock CO₂ is provided from direct air capture instead of flue gas, the total emissions intensity of the methanol would be higher: at least 1.74 g CO₂ emitted / g methanol, if the direct air capture process is powered by renewable energy sources, and higher still if powered by fossil sources.

 The reference case eCO₂RR system in this analysis contains a nanoparticulate copper catalyst (100 nm particle diameter) that reduces CO₂ to methanol with 25% faradaic efficiency, a current density of 0.5 A cm^-2, and 0.8 V of overpotential.  The anode reaction is water oxidation, carried out with 0.3 V of overpotential.  The feedstock CO₂ provided to the reduction cell is captured from coal-fired power plant flue gas, with an emissions intensity of 97 g CO₂ emitted / g CO₂ captured.  The emissions intensity of the electricity that powers the electrocatalytic reduction is 20 g/kWh, typical of photovoltaic generation.