(Invited) Local Environments on Cu Electrode Surface for Selective CO2 Electrolysis
Authors: Yamauchi Miho 
Published: December 2024

Electrochemical CO2 reduction (eCO2R) attracts much attention as a technology for carbon circulation on the earth and the selective eCO2R for the production of high value-added chemicals is increasingly demanded. Hydroxide-derived copper (OH/Cu) electrodes exhibit excellent performance for eCO2R. However, the role of OH remains controversial and therefore the origin for the selectivity enhancement emerging on HO/Cu has not been fully understood. We the quantitatively evaluated surface OH by electroadsorption for the first time and established a direct correlation between the OH amount and selectivity for the production of CH4 and C2+ on OH/Cu with the help of computational investigations concerning work functions of the surface. Based on these findings, we demonstrated valuable selectivities using OH/Cu electrodes having a controlled OH amount; three OH/Cu electrodes realized their distinct selectivity such as Faradaic efficiency (FE) for the production of CH4 of 78%, FE of 71% for C2+, and the ratio of C2+-to-CH4 >355.[1] The proposed simple strategy for the selectivity control would contribute to further quantity synthesis of value added chemicals using eCO2R.

The eCO2R in acidic electrolytes would have various advantages due to the suppression of carbonate formation. However, its reaction rate is severely limited by the slow CO2 diffusion due to the absence of OH that facilitates the CO2 diffusion in an acidic environment. Here, we designed an optimal architecture of a gas diffusion electrode (GDE) employing a copper-based ultrathin superhydrophobic macroporous layer, in which the CO2 diffusion is highly enhanced. This GDE retains its applicability even under mechanical deformation conditions. The eCO2R in acidic electrolytes exhibits a Faradaic efficiency of 87% for C2+ products.

Direct air capture (DAC) and utilization of the captured CO2 (DAC-U) is a promising carbon neutral strategy to achieve an efficient carbon circulation on earth. DAC using membrane separation (m-DAC) is attracting much attention due to its lower energy consumption and its scalability. However, a higher energy input is still required to produce pure CO2. Given the preferred scalability of m-DAC, its combination with eCO2R may be suitable for on-site applications. Currently, high concentration and high purity CO2 gas is used in eCO2R studies, but, if low concentration and impure CO2 mixed gas becomes available, the cost of DAC-U using m-DAC can be significantly reduced. To date, only a few studies have evaluated eCO2R of CO2 feeds containing O2. Therefore, the development of systems that enable efficient eCO2R in the presence of O2, is critical to the establishment of DAC-U technology. Recently, we achieved eCO2R by direct introduction of 60% air containing CO2 using a porous Cu network formed on a hydrophobic gas diffusion layer. Even in the presence of 12% O2, Faradaic efficiency for the C2+ production reached 85% at a partial current density of 132 mA cm-2. Reference [1] M. Sun, A. Staykov, M. Yamauchi, ACS Catal.,12, 14856-14863 (2022). [2] M. Sun, J. Cheng, M. Yamauchi, Nat. Comm., 15, 491 (2024). [3] A. Anzai, M. Higashi, M. Yamauchi, Chem. Comm. , 59, 11188-11191 (2023).