Electrochemical CO2 Reduction Reactions to Multicarbon Products with a Current Density Exceeding 1.8 a cm-2
Authors: Nakasone Sora , Inoue Asato , Yoshida Ryotaro , Harada Takashi , Nakanishi Shuji , Kamiya Kazuhide 
Published: December 2024

The electrochemical CO2 reduction reaction (CO2RR) using electricity derived from renewable energy sources has attracted intensive attention as a technology for carbon neutral society. Social implementation of CO2RR requires highly selective and high current density production of high value-added C2+ products (C2H4, C2H5OH, n-C3H7OH, CH3COOH, etc.). For this reason, there has been many studies into the direct electrolysis of gaseous CO2 as a substrate by using gas diffusion electrodes (GDEs), which can overcome the problem of limited mass transport due to the low solubility of CO2, to achieve high current density for the reaction. We have recently increased the partial current density for gaseous CO2RR to C2+ products over Cu nanoparticles (CuNPs) on GDEs in alkaline electrolytes to a record value of 1.8 A/cm2.1 This has been achieved by maximizing the area of the three-phase interface through optimum catalyst loading. However, most of the other factors which are sensitive to realize high-current-density CO2RR have been still uncovered. In particular, identifying the requirements for catalysts is crucial because we need to synthesize new catalysts to alter selectivity. In other words, when we aim to achieve both high selectivity and high current density, it is essential to clearly define the catalyst requirements for high-current density CO2RR. Therefore, in this work, we will evaluate the performance of different Cu-based electrocatalysts in CO2RR and analyze how the formation of C2+ products depends on the current density. In addition, we investigated the CO reduction reaction to identify the molecular mechanism of C-C bond formation in high current density electrolysis systems.

Fig. a shows the representative faradaic efficiency for C2+ products (FEC
2+) of the best electrocatalyst in 1 M KOH solutions. The FEC
2+ increased as total current density increased in the range of total current density = 0.2-2.5 A/cm2. The value of FEC2+ reached a maximum of 80 % at 2.5 A/cm2. In this case, the FE of ethylene, ethanol, n-propanol and acetic acid was 47 %, 26 %, 2.9 % and 4.9 %, respectively. The partial current density of C2+ reached up to 2.0 A/cm2(Fig .b). We then explore the requirements for CO2 electrolysis under high current density by investigating the relationship between the obtained electrochemical activities and the physicochemical properties of the electrocatalysts. Thus, we conducted CO2 electrolysis using commercial CuNPs with varying properties, such as oxidation state and surface area. We found that the porosity size within the catalyst layer strongly correlated with the peak of j
C The detailed mechanism of this correlation will be discussed in our presentation.

We next investigated the activity of CO reduction reactions with high current density because CO2 electrolysis with copper-based catalysts is known to produce C2+ products through dimerization reactions of CO intermediates. As a result, the FE for acetic acid production was higher in the high-current density CO reduction reaction compared to the CO2RR. We will discuss the molecular mechanism behind C-C bond formation during high-current density electrolysis in the presentation.

Figure :

Figure.(a) The representative FEC
2+ and (b) partial current density for C2+ products of the best electrocatalyst in 1 M KOH solutions.

References :

[1] Inoue, A.; Harada, T.; Nakanishi, S.; Kamiya, K. Ultra-High-Rate CO2 Reduction Reactions to Multicarbon Products with a Current Density of 1.7 A cm–2 in Neutral Electrolytes. EES Catal.
2023, 1 (1), 9–16, DOI: 10.1039/D2EY00035K

Figure 1