Effect of CO2 and HClO4 Concentrations on CO2 Reduction Reaction Intermediates at Pt Polycrystalline Electrode
Authors: Kobayashi Shun 
Published: December 2024

Introduction

To achieve carbon neutrality, it is necessary to apply practical CO2 reduction and effective utilization technologies. Recently, there has been an expectation for the electrochemical reduction of CO2 (CO2 Reduction Reaction: CO2RR) using electricity derived from renewable energy. CO2RR with low overvoltage using a polymer electrolyte membrane is an actively researched next-generation technology. It is important to improve Faraday efficiency and analyze CO2RR mechanisms. Cu electrode catalysts are commonly used for CO2RR due to their high Faraday efficiency. However, a challenge remains in reducing the overvoltage of CO2RR to the level of hydrogen generation in normal PEM water electrolysis. Although Pt electrode catalysts have a significantly lower Faraday efficiency for CO2RR than Cu electrode catalysts, they have been reported to have low overvoltage1. Recent research has shown that the presence of water in the supplied CO2 and the alloy composition enhances Faraday efficiency2,3. To reduce CO2RR overvoltage, this study utilized a Pt electrode. Successful CO2RR requires the electrode catalyst surface to have coexisting surface-adsorbed hydrogen (Had) and CO2RR intermediate (COad). This study investigates the effect of electrolyte pH (perchloric acid concentration) and CO2 concentration on the COad and Had, utilizing a Pt polycrystalline electrode.

Experimental

Electrochemical measurements were carried out using a three-electrode cell. The counter electrode was a Pt mesh, and the reference electrode was a reversible hydrogen electrode (RHE). The working electrode was a φ3 mm Pt polycrystalline disk electrode that was mirror-polished with a 0.05 μm alumina suspension. Cyclic voltammetry (CV) measurements were performed with a sweep range of 0.05-1.0 V vs. RHE and a sweep rate of 50 mV/s or 10 mV/s. The results of the CV measurements are presented after IR correction. To evaluate CO2RR, the concentration of HClO4 was adjusted to 0.01-0.5 M as the electrolyte. The CO2 concentration was varied from 0-100 vol% and bubbled for 30 minutes before the CV measurements. For the electrochemical evaluation of CO2RR, the initial potential was kept at 0.05-0.4 V for 5 minutes before the CV measurements, and then the CV measurement was performed.

Results and Discussion

1. CO2RR potential in HClO4 electrolyte saturated with 100% CO2

Figure 2 shows the results of CV measurements performed after holding the initial potentials at 0.05-0.4 V for 5 minutes before starting the CV measurements. The bubbling CO2 concentration during the measurements was 100%, and the HClO4 concentration was 0.1 M. In comparison to the CV peaks observed in the N2 atmosphere, peaks where the CO2RR reduction intermediate (COad) are reoxidized are observed at 0.6-0.7 V in the 100% CO2 atmosphere. The charge of these COad peaks remain constant at low initial holding potentials. However, it decreases significantly as the initial holding potential approaches 0.4 V. Under the measurement conditions of 100% CO2 atmosphere and 0.1 M HClO4 concentration, it can be seen that CO2 is reduced on the Pt electrode at a potential positive of 0 V vs. RHE because the COad peaks are seen between 0.05-0.3 V of the initial holding potentials.

2. Dependence of COad peak on bubbling CO2 concentration

CV measurements were performed by changing the CO2 concentration bubbled in 0.1 M HClO4. Figure 3 shows the results of CV measurements at an initial potential hold of 0.05 V, where the CO2 concentration was changed from 0% to 100%. In the Hupd region, the amount of charge decreases in the CO2 atmosphere because CO2 is already surface-adsorbed as COad at 0.05 V, and the decrease is larger as the CO2 concentration increases. It is evident that the peaks of COad increase as the concentration of CO2 increases. Additionally, when the CO2 concentration is low, the peaks of COad are divided into at least two peaks, indicating the generation of COad with different adsorption structures during CO2RR. As the CO2 concentration increases, the peaks of COad appearing on the low potential side also increase. Additionally, the ratio of COad with different adsorption structures might change depending on the CO2 concentration bubbled.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number JP23K13835 and TEPCO Memorial Foundation.

References

1. M. Umeda, Y. Niitsuma, T. Horikawa, S. Matsuda, and M. Osawa, ACS Appl. Energy Mater.
3, 1119 (2020).

2. Y. Niitsuma, K. Sato, S. Matsuda, S. Shironita, and M. Umeda, J. Electrochem. Soc.
166, F208 (2019).

3. S. Matsuda, S. Yamanaka, and M. Umeda, ACS Appl. Mater. Interfaces
15, 42676 (2023).

Figure 1