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Catholyte-free flow cell enables high efficiency electroreduction of CO2 to C2 fuels

Periodic Reporting for period 1 - CF-CO2R (Catholyte-free flow cell enables high efficiency electroreduction of CO2 to C2 fuels)

Okres sprawozdawczy: 2019-06-15 do 2021-06-14

The electrochemical CO2 reduction (CO2R) provides a promising means to utilize and upgrade CO2 towards valuable two-carbon (C2) products (i.e. ethylene and ethanol), yet the current CO2R requires high energy costs that pose major challenging barries to compete with C2 productions from conventional petrochemical routes. Two main hurdles have been identified: (1) significant ohmic loss and the surface reconstruction of catalysts result from the use of liquid electrolyte, and (2) catalysts with heterogeneous active sites yield a variety of products, including undesired one-carbon (C1) products (e.g. CO) and parasitic H2 production from competing hydrogen evolution reactions. Coupling efficient catalysts and systems design to achieve near-unity selectivity towards a single C2 product remains a grand challenge.

These topics are important for society because designing energy-efficient CO2R catalysts and systems accelerates the decarbonization of the oil and gas industry, and promises a carbon-neutral future. To this regard, this MSC Action titled “Catholyte-free flow cell enables high efficiency electroreduction of CO2 to C2 fuels” aims at: (1) design of catholyte-free membrane electrode assembly (CF-MEA) flow cell to enable CO2R with low-energy costs, (2) synthesis of efficient electrocatalysts to produce value-added ethylene and ethanol products at high selectivities, and (3) characterization of the catalyst structure-reactivity relationship using operando spectroscopy techniques, with the goal of accelerating ethylene- or ethanol-selective catalysts that work readily at industrial-relevant system conditions.
The zero-gap CF-MEA is an appealing device for CO2R to reduce ohmic losses, but the C2-selective catalysts developed under alkaline flow cells show decreased C2 selectivities in MEA systems. One of the reasons is due to excess surface coverage of CO2 at catalysts surface enabled by MEA that blocks active sites for carbon-carbon (C-C) coupling reactions. To this end we searched for optimal CO2-to-ethylene conversion conditions in MEA devices via innovative materials design and tunings of local environment, and studied how they could be coupled together to activate CO2 and accelerate CO2R in the most energy-efficient ways. Particularly, silica nanoclusters show enhanced CO2 adsorption and the copper-silica interface delivers a 65% ethylene selectivity from CO2R (Nature Communications 2021). Later, a series of metal phthalocyanine molecules were immobilized at the Cu surfaces to confine CO and C2 intermediates towards a record 73% ethylene selectivity. Two manuscripts under development are related to these research findings.

We then developed new methods to steer the ethylene and ethanol selectivities from CO2R. While the CO2-to-ethylene conversion on Cu-based catalysts is thermodynamically favourable, promoting ethanol production by supressing ethylene formation pathways has mainly been explored using density functional theory (DFT) calculations. Guided by DFT analysis, we unveiled the ethylene-ethanol switching mechanism and then developed efficient strategies to improve ethanol electrosynthesis from CO2R. Our results suggest that tunings of the hydroxyl and CO coverages at the Cu surface offer a great opportunity to promote ethanol and suppress ethylene from CO2R. To do so we employed a new pulsed electrolysis coupled with the design of bimetallic Cu alloys to achieve the ideal combinations of hydroxyl and CO adsorptions. As a result, we achieved 50% ethanol selectivity from CO2R and the related results are currently under manuscript drafting for publication.

We disseminated the project results through diverse means: (1) 8 research talks to world-class universities such as ETH Zurich (Switzerland), Princeton University (US), University of British Columbia (Canada), Shanghai Jiao Tong University (China), etc.; (2) one live educational session to general audience in the “Celebration: EPFL turns 50!” event before the Covid; (3) social media platforms including Twitter, Facebook and LinkedIn since the pandemic.
The project fully achieved the initial objectives, in particular contributed for the development of energy-efficient CF-MEA electrolysis systems and C2-selective catalytic materials. Record-high ethylene (73%) and ethanol (50%) selectivities from CO2R have been achieved. The overall progress of this MSCA project will shine light on CO2R reaction mechanisms and accelerate the research on CO2-to-C2 conversion towards practical applications. The research findings of this CF-CO2R project will help accelerate the reduction of carbon footprint and achieve the 2050 net-zero emission goal set by the European Union. The project has delivered knowledge transfer via outstanding training and the subsequent careers of highly qualified personnel, including the design of CF-CO2R-relevant research projects for PhDs and postdocs, and assistance in writing award-winning MSCA applications. The project also allowed initiating collaborations with other research institutions outside of Europe. For example, the collaborations among the École polytechnique fédérale de Lausanne and Swiss Light Source in Switzerland, University of Toronto in Canada and Advanced Photon Source in United States initiated in the framework of CF-CO2R project contributed to develop lasting relations that can contribute to increase the research communications and knowledge transfer among the participant institutions.
CF-CO2R with Cu:molecule catalyst for selective ethylene/ethanol productions