Periodic Reporting for period 1 - STRATCAT-CO2 (Surface functionalization with thiols: a novel strategy in catalyst design for the efficient reduction of CO2 to C2 products)
Periodo di rendicontazione: 2019-08-01 al 2021-07-31
Since its birth more than 250 years ago, the chemical industry has become a key sector in the European economy: today, it generates 1.1% of the European Union’s gross domestic product and ca. 1.15 million direct jobs. It is also the EU’s largest industrial energy consumer and third largest industrial emitter of carbon dioxide. In its climate action roadmap, the European Commission targets an 80% reduction of greenhouse gas emissions by 2050 (compared to 1990 levels) as well as a nearly exclusive use of low-carbon energy sources in power generation. The chemical industry is a crucial stakeholder for achieving these goals by (1) replacing fossil feedstocks, (2) electrifying its production processes to make use of renewable energy, and (2) providing large-scale solutions for energy storage from intermittent renewable sources. In this context, the Climate Action roadmap sets the stage for nothing less than a fundamental transformation of the chemical industry.
Combining the electrochemical CO2 reduction reaction (eCO2RR) with increasingly affordable renewable energy sources provides groundbreaking opportunities to use CO2 as a medium for energy storage and as a raw material in chemical production. In particular, ethanol and ethylene are high-value products that can be obtained from the eCO2RR, with wide applications as a fuel and as a building block in the chemical industry, respectively.
Practical implementation requires the development of CO2 reduction catalysts with high selectivity toward the target products at low overpotentials. Despite recent progress in achieving these requirements for the simplest eCO2RR products (i.e. carbon monoxide and formate), reaching high performance in the reduction of CO2 to ethylene and ethanol remains a major challenge. Moving the field forward to unlock the vast potential of the eCO2RR requires investigating new concepts in catalyst design while gaining fundamental insights into their mechanistic effect.
In this context, the STRAT-CO2 project sought to investigate a novel strategy for the design of eCO2RR catalysts—based on surface functionalization with organic ligands—targeting the reduction of CO2 to multicarbon products with high efficiency. In particular, the project the following two objectives:
1. To gain a fundamental understanding of how electronic and geometric effects from adsorbed thiols modulate selectivity and activity toward C2 products over surface-functionalized electrodes.
2. Building on this understanding, to develop a catalyst that shows high efficiency in reducing CO2 to ethylene or ethanol at a practically relevant current density of 200 mA cm-2.
Combining the electrochemical CO2 reduction reaction (eCO2RR) with increasingly affordable renewable energy sources provides groundbreaking opportunities to use CO2 as a medium for energy storage and as a raw material in chemical production. In particular, ethanol and ethylene are high-value products that can be obtained from the eCO2RR, with wide applications as a fuel and as a building block in the chemical industry, respectively.
Practical implementation requires the development of CO2 reduction catalysts with high selectivity toward the target products at low overpotentials. Despite recent progress in achieving these requirements for the simplest eCO2RR products (i.e. carbon monoxide and formate), reaching high performance in the reduction of CO2 to ethylene and ethanol remains a major challenge. Moving the field forward to unlock the vast potential of the eCO2RR requires investigating new concepts in catalyst design while gaining fundamental insights into their mechanistic effect.
In this context, the STRAT-CO2 project sought to investigate a novel strategy for the design of eCO2RR catalysts—based on surface functionalization with organic ligands—targeting the reduction of CO2 to multicarbon products with high efficiency. In particular, the project the following two objectives:
1. To gain a fundamental understanding of how electronic and geometric effects from adsorbed thiols modulate selectivity and activity toward C2 products over surface-functionalized electrodes.
2. Building on this understanding, to develop a catalyst that shows high efficiency in reducing CO2 to ethylene or ethanol at a practically relevant current density of 200 mA cm-2.
Five thiols—different in the length of the side chain and the hydrogen-binding abilities of the non-thiol terminal group—were selected for a systematic evaluation of their effect on CO2 reduction. Regarding the preparation of self-assembled monolayers (SAMs) of the thiols on copper surfaces, a pre-adsorption etching step with glacial acetic acid (i.e. to remove surface oxides) greatly enhanced the quality and reproducibility of the obtained SAMs. The functionalized surfaces were thoroughly characterized prior to the reaction and following electrolysis at different potentials by x-ray photoelectron spectroscopy (XPS). The measurements indicated that the stability of the adsorbed thiol SAMs depended on the electrolysis potential and on the length of the thiol side chain. Nevertheless, the adsorbed thiols were found to typically suppress the total current density without influencing much the product selectivity or, in the case of –OH and –NH2-terminated ligands, to promote the HER, particularly in the case of the latter. Despite this interesting result, attempts to exploit this effect under conditions with decreased proton availability (e.g. CO reduction in 0.1 M KOH) did not result in an enhancement of multicarbon product formation.
Initial work in a zero-gap membrane electrode assembly (MEA)-type electrolyzer revealed that the neutralization of CO2 by OH− ions generated in situ by the eCO2RR leads to CO2 crossover as carbonate through the anion-exchange membrane (AEM) and to a non-stoichiometric decrease of the outlet flow from the reactor. If not accounted for, this effect can lead to large overestimations of the Faradaic efficiencies of the gas eCO2RR products and to an incorrect assessment of catalytic performance at elevated current densities. This problem was found to affect a large fraction of published studies, in which figures of merit—particularly for CO2 reduction to ethylene production under alkaline conditions—can be vastly overstated. Therefore, significant work was aimed at correcting these experimental shortcomings and developing robust protocols for the accurate quantification of catalytic performance at industrially relevant current densities. Based on these findings, awareness of this issue among the eCO2RR community is rapidly growing, leading to the adoption of methodological improvements and better data collection practices.
The zero-gap configuration of the electrolyzer (i.e. in which the catalyst layers are directly in contact with the membrane, with no liquid electrolyte in between) enabled the reduction of ohmic losses but complicated the recovery of liquid products. In light of this, ethylene was selected as the target product for the second part of the project. To this end, we evaluated nanostructured copper catalysts with abundant crystallographic facets over which CO2 reduction to ethylene is favored. Compared to benchmark catalysts, this strategy was successful in achieving higher production rates of ethylene, although selectivity over the parasitic hydrogen evolution reaction (HER) still needs to be improved. These results point toward the need for a better understanding of the factors that promote the HER at high current densities, such as water transport across AEMs, electrode flooding and the occurrence of mass transfer limitations. Nevertheless, this work points toward a viable strategy for enhancing the ethylene productivity of the eCO2RR at industrially relevant reaction rates.
Initial work in a zero-gap membrane electrode assembly (MEA)-type electrolyzer revealed that the neutralization of CO2 by OH− ions generated in situ by the eCO2RR leads to CO2 crossover as carbonate through the anion-exchange membrane (AEM) and to a non-stoichiometric decrease of the outlet flow from the reactor. If not accounted for, this effect can lead to large overestimations of the Faradaic efficiencies of the gas eCO2RR products and to an incorrect assessment of catalytic performance at elevated current densities. This problem was found to affect a large fraction of published studies, in which figures of merit—particularly for CO2 reduction to ethylene production under alkaline conditions—can be vastly overstated. Therefore, significant work was aimed at correcting these experimental shortcomings and developing robust protocols for the accurate quantification of catalytic performance at industrially relevant current densities. Based on these findings, awareness of this issue among the eCO2RR community is rapidly growing, leading to the adoption of methodological improvements and better data collection practices.
The zero-gap configuration of the electrolyzer (i.e. in which the catalyst layers are directly in contact with the membrane, with no liquid electrolyte in between) enabled the reduction of ohmic losses but complicated the recovery of liquid products. In light of this, ethylene was selected as the target product for the second part of the project. To this end, we evaluated nanostructured copper catalysts with abundant crystallographic facets over which CO2 reduction to ethylene is favored. Compared to benchmark catalysts, this strategy was successful in achieving higher production rates of ethylene, although selectivity over the parasitic hydrogen evolution reaction (HER) still needs to be improved. These results point toward the need for a better understanding of the factors that promote the HER at high current densities, such as water transport across AEMs, electrode flooding and the occurrence of mass transfer limitations. Nevertheless, this work points toward a viable strategy for enhancing the ethylene productivity of the eCO2RR at industrially relevant reaction rates.
The findings of STRATCAT-CO2 provide a bridge between basic and application-oriented research in the context of CO2 reduction and inform the development of novel catalyst and device design strategies to promote the formation of valuable multicarbon products at industrially relevant current densities. Moreover, the results of this project have pointed out important pitfalls in current experimental approaches and provided a sound methodological basis for future work. Overall, STRATCAT-CO2 has provided valuable stepping-stones for electrochemical CO2 reduction on its march toward technological relevance within a future circular economy based solely on renewable energy.