CO2 Electroreduction Reaction on Pt-Cu Catalyst and Its in-Situ Analysis By Infrared Spectroscopy
Authors: Takahashi Hiroki , Ohkubo Keisuke , Taguchi Masami 
Published: December 2020

The concentration of CO2 in the atmosphere continues to increase. As a result, global warming is considered to be progressing due to the greenhouse effect. Therefore, the goal of "achieving a balance between human-made emissions of greenhouse gases and removals by sinks in the second half of this century" is set and decreasing the CO2 emissions has become an urgent issue. Recently, it was found that the CO2 electroreduction in an acidic solution on a Pt-based electrode catalyst, such as Pt or PtRu, proceeds at around 0.05 V vs. SHE, which means that CO2 reduction occur with extremely low overvoltage [1]. Our research group has previously reported that a Pt oxide thin film exhibits a higher CO2 electroreduction activity than a Pt one, and the factors affecting the superior CO2 electroreduction activity was considered by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) [2,3]. In this study, CO2 electroreduction activity of Pt and Pt-Cu electrodes were evaluated and its difference was investigated by in-situ SEIRAS.
The specimen was a Pt thin film or Pt oxide thin film, which were prepared on a Ti substrate by reactive sputtering. The detailed procedure was described in a previous report. The rotating disk electrode was fabricated from a Ti substrate coated with the Pt thin film or the Pt oxide thin film. It was attached to the electrode rotation system as the working electrode, and the CO2 electroreduction activity was evaluated. The counter electrode and the reference electrode were a Pt sheet and an Ag/AgCl in 3.30 mol dm-3 KCl, respectively. The electrolytic solution was a 0.1 kmol dm-3 HClO4 aqueous solution. For in-situ SEIRAS, the electrocatalysts were supported in Au thin film which was prepared by electroless deposition on a semi-cylindrical Si prism. The resolution of the infrared spectrometer was 4.0 cm-1, and the detector was a liquid nitrogen-cooled MCT detector. The IR spectrum measured at 1.2 V vs. RHE was used as a reference.
Fig. 1 shows the cycic voltammograms of the Pt thin film. In Ar-deaerated atmosphere, typical characteristics of Pt was observed. However, the oxidation current was recognized at around 0.6 V vs. SHE in CO2-saturated solution. This anodic peak reflects the CO2 electroreduction activity [2]. Fig. 2 shows the relationship between CO2 electroreduction activities per active surface area and Cu content of the prepared thin films. The CO2 electroreduction activities of the Pt-Cu thin films were higher than that of the Pt thin film. Thus, we used SEIRAS to perform in-situ analysis of the surface adsorption species during the electroreduction to investigate the factors involved in the high electroreduction activity of the Pt Cu electrodes. Fig. 3 shows the Relationship between the intensity of adsorbed linear-CO, HCOO-, and methanol during the CO2 electroreduction reaction on Pt-65.2 at.% Cu/C with time. The potential was maintained at 0.05 V vs. RHE for the CO2 electroreduction to occur. The peaks of the linear-CO and methanol increase with an increase in time. This indicates the amount of linear-CO and methanol adsorbed on the surface was increasing. On the other hand, for HCOO-, although peaks were observed, the absorbance was obviously lower than that for CO. These behaviors were the same as for Pt [3]. Therefore, the difference of CO2 electroreduction activity between Pt and Pt-Cu is not related to the intermediate product and adsorption species. Fig. 4 shows the CO coverage of Pt/C and Pt-65.2at.% Cu/C estimated by CO-stripping voltammetry. Although almost all the surface of Pt was covered by CO, about half of the surface of Pt-Cu maintained clean condition. Therefore, we can say that the better CO2 electroreduction activities of Pt-Cu electrodes are brought by a weak adsorption characteristic of CO.
[1] S. Shironita, K. Sato, K. Yoshitake, and M. Umeda, Electrochim. Acta, 206, 254 (2016).
[2] K. Ohkubo, H. Takahashi, and M. Taguchi, J. MMIJ, 135(2), 8 (2019).
[3] K. Ohkubo, H Takahashi, E.P.J. Watters, and M Taguchi, Electrochemistry, 88(3), 210 (2020).

Figure 1