Cu-Based Bimetallic Electrocatalysts for CO2 Electrochemical Conversion to Valuable Chemicals: Study of Product Selective Distribution

Abstract

The carbon dioxide emissions contribute largely to the global climate change, and this may lead to serious consequences, such as ice melting at the Earth’s poles and the fast-rising of the sea level [1]. The conversion of CO2 to molecules with higher energy is a possible strategy for fuel and/or chemical reactant production or even energy storage. In this context, different metal electrocatalysts are under investigation, but only copper seems to be effective for generating hydrocarbons, mainly methane (CH4) and ethylene (C2H4) [2]. However, further advances are still necessary to increase its product selectivity and stability. Considering this scenario, in this study, the selectivity and stability of copper-based electrocatalysts for the CO2 electrochemical reduction, in KHCO3 aqueous electrolyte, was investigated by on-line Differential Electrochemical Mass Spectrometry (DEMS), under different reaction conditions. Polycrystalline bulk copper electrodes (Cu rod of a rotating electrode) produced only CH4 as the reaction product in the first scan from -0.5 to -2.4 V vs. Ag/AgCl. However, in the subsequent scans, the signal for C2H4 became evident and its magnitude increased, while that for CH4 decreased. During long-term potentiostatic polarization at -2.0 V, after ca. 1 h, CH4 was suppressed and only C2H4 was observed, this being stable for more than 7 h. After this period of polarization, XRD analysis revealed the formation of copper hydroxide. When the reaction was conducted on copper oxide, only C2H4 was observed. Although the Cu2O phase suffer reduction to metallic copper, as revealed by EXAFS [3], some superficial (and metastable) hydroxides may remain or be sustained by the OH- species that are produced during the CO2 reduction in the aqueous media. The formation of OH- may also be responsible for the observed copper hydroxide phase on the surface of the bulk copper electrode, after 7 h of polarization. Interestingly, when the concentration of KHCO3 was reduced from 0.5 to 0.1 molL-1 (reduced buffer capacity) the DEMS signal for C2H4 was increased for all cases, as observed before [4]. These results point out to an important role of the superficial oxides/hydroxides (or high local pH) on the selectivity for C2H4. Importantly, in addition to the increase in the selectivity, it is noted an increase in the stability of the electrocatalyst when the reaction is totally deviated to the formation of C2H4 [5]. The necessary presence of hydroxides on the copper surface for the C2H4 formation is further confirmed when palladium was co-deposited or alloyed with copper. As palladium induces the hydrogen spillover, copper is already present in the metallic state in the as-prepared material, and remains in the same reduced state even after the CO2 reduction, as revealed by XRD analyses. This copper-palladium electrocatalyst produced only CH4 and H2 (to a higher extent), with no measurable C2H4 signal. The adsorbed hydroxide species (or high local pH) may favor the electron transfer with non-simultaneous proton transfer, which favor pathways of CO – CO coupling species, producing ethylene as the final product [4]. Acknowledgements The authors gratefully acknowledge financial support from FAPESP (2014/26699-3, 2016/13323-0 and 2013/16930-7), CAPES and CNPq (306469/20162). [1] H. Khatib, Energy Policy, 2012, 48, 737–743. [2] Hirunsit, P., Soodsawang, W., Limtrakul, J., J. Phys. Chemistry C, 2015, 119, 8238–8249. [3] Camilo, M. R., Silva, W. O., Lima, F. H. B. J. Braz. Chem. Soc., 2017, 28, 1803–1815. [4] Kas, R., Kortlever, R., Yılmaz, H., Koper, M. T. M., Mul, G., ChemElectroChem, 2015, 2, 354 – 358. [5] Dinh, C., Burdyny, T., Kibria, MD G., Seifitokaldani, A., Gabardo, C. M., de Arquer, F. P. G., Kiani, A., Edwards, J. P., de Luna, P., Bushuyev, O. S., Zou, C., Quintero-Bermudez, R., Pang, Y., Sinton, D., Sargent, E. H., Science, 2018, 360 (6390), 783–787.