Dynamic Oxygen Collection Efficiency: Unraveling the Effects of Perovskites on Carbon Corrosion As Composite Catalysts for Oxygen Electrode
Authors: Ni Jiayuan , Miyahara Yuto , Miyazaki Kohei , Lee Changhee , Abe Takeshi 
Published: August 2024

Introduction

The composite catalyst comprising perovskite and carbon has been extensively studied as a catalyst for the oxygen evolution reaction (OER) at the oxygen electrode in alkaline media. However, the competitive corrosion of carbon under harsh OER potential necessitates additional caution when determining OER activity from the overall oxidation current. The use of a rotating ring-disk electrode (RRDE) is acknowledged for its ability to quantitatively determine the faradaic efficiency of the OER. [1] However, for practical applications, the generation of oxygen bubbles can cause unpredictable disturbance, leading to the erroneous collection efficiency. In this study, we aim to establish a dynamic current-response collection efficiency during the generation of oxygen bubbles. This was achieved by modelling with an Au/Au RRDE, well-known for its electrochemical stability under harsh conditions. In addition, using this calibrated collection efficiency, we investigate the influence of perovskite catalysts on the degradation behaviors of carbon materials at OER potentials in alkaline media.

Materials and electrochemical measurements

Measurements of oxygen gas collection efficiency were conducted using a modeled RRDE comprising an Au disk electrode and an Au ring electrode. The RRDE served as the working electrode, with a Pt wire as the counter electrode, a reversible hydrogen electrode (RHE) as the reference electrode, and a solution of Ar-saturated 1 mol dm-3 KOH as the electrolyte. The disk electrode underwent a 5-minute chronopotentiometry with varied oxidation currents, while the ring electrode underwent chronoamperometry at 0.3 V (vs. RHE). The collection efficiency was calculated from the ring current attributed to the oxygen 2-electron reduction reaction.
Two kinds of perovskite catalysts, namely Sr2Co0.8Fe0.2O3Cl[2] (hereafter SCFOC) and LaCoO3 (hereafter LCO), and two carbonaceous materials, namely graphitized carbon black (TOKABLACK#3845, hereafter TB) and ketjen black (EC-600JD, hereafter KB), were used to evaluate the influence of perovskite on the carbon degradation behavior. Four composite catalysts, each comprising perovskite and carbon with a weight ratio of 5:1, were loaded on a disk electrode of the glassy carbon disk/Au ring RRDE. A cell analogous to the aforementioned design was assembled. Electrochemical measurements were conducted through chronoamperometry, employing various potentials at the disk electrode, and maintaining a potential of 0.3 V vs. RHE at the ring electrode, for a duration of 5 minutes.

Results and discussion

Figure 1 illustrates a three-dimensional representation of the oxygen gas collection efficiency of the Au/Au RRDE as two functions of disk current density and measurement time. It was evident that the oxygen gas collection efficiency varied significantly from the theoretical value (25.6 %) and was dependent on the disk current density and measurement time. The constrained solubility of oxygen in aqueous solution and the hydrophobic nature of the PTFE between the ring and the disk might cause a decrease in the collection efficiency due to the partial retention of oxygen bubbles on the electrode as the oxygen generation current increased and the measurement time elapsed. This suggests that, unlike typical solution reactions, the dynamic determination of the oxygen gas collection efficiency is essential, considering current density and measurement time.
Figure 2 illustrates the computation of faradaic oxygen efficiency, utilizing dynamic oxygen collection efficiency. The composite catalyst composed of TB and SCFOC was taken as an illustrative example. The variation in collection efficiency, synchronized with the shape of disk current density, was employed in determining the faradaic oxygen efficiency. This dynamic collection efficiency yielded a comparatively smooth faradaic oxygen efficiency.
Utilizing the calibrated faradaic oxygen efficiency, we proceeded to assess the capacity of carbon degradation by integrating the current density of the carbon oxidation reaction (COR) calculated by subtracting the OER current from the overall current. As depicted in Figure 3, the COR capacity as a function of the potential suggested that the components of the composite catalysts had distinct impacts on carbon degradation. Specifically, SCFOC appears to enhance degradation, whereas LCO exhibits a comparatively less pronounced effect.
A more comprehensive discussion of these findings will be presented at the upcoming meeting.
[1] I.S. Filimonenkov et al., Electrochim. Acta
321, 134657 (2019).
[2] Y. Miyahara et al., Chem. Mater.
32, 8195 (2020).

Figure 1