Thin-Film Study of Delithiated LiCoO2 Catalyst for Oxygen Evolution Reaction in Alkaline Media
Authors: Asano Kenshi , Miyahara Yuto , Tsujimoto Shota , Sakamoto Ryo , Lee Changhee , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Renewable energy is indispensable for realizing the sustainable society, and the use of hydrogen energy as an energy carrier is recently gathering attention. It is therefore strongly needed to develop energy conversion devices with the production of hydrogen, and anion-exchange membrane water electrolysis is extensively studied due to its cost effectiveness and relative safety[1]. In these systems, however, sluggish kinetics of oxygen evolution reaction (OER) require large overpotential, which motivates us to explore efficient OER electrocatalysts in alkaline media.

Objective

Among OER catalysts, LiCoO2 is known as a highly active catalyst in alkaline media. In addition, it has been reported that the catalytic activity of LiCoO2 powder can be improved by delithiation[2]. In order to discuss the electrochemical properties of the delithiated LiCoO2 more in detail, the use of thin-film electrodes is effective because it contains no binder or conductive addictive. Pulsed laser deposition (PLD) method is a favorable method to fabricate thin film electrodes because dense and pinhole-free films can be grown by this method. In this study, we prepared LiCoO2 thin-film electrodes by the PLD method, followed by the electrochemical delithiation of them. Then, their OER activities were evaluated to compare their electrocatalytic behaviors.

Experimental

LiCoO2 thin films were prepared by PLD. LiCoO2 was deposited on Pt substrate. After preparing the LiCoO2 thin film, it was characterized by X-ray diffraction (XRD) measurement.
Electrochemical delithiation of LiCoO2 thin films was conducted by using a three-electrode cell. LiCoO2 thin film grown on Pt substrate was used as a working electrode, and Li metal was used as counter and reference electrodes. A solution of 1 mol dm-
3 LiPF6 in propylene carbonate was used as the electrolyte. The delitiation process was performed by charging the cell at 4.30 V vs. Li+/Li after two charge-discharge cycles between 3.5 V and 4.2 V (vs. Li+/Li). After this process, the resultant delithiated LiCoO2 thin films (referred to as De-LiCoO2 thin-films) were characterized by XRD.
Regarding the electrochemical evaluation of the OER activities for De-LiCoO2 thin-film, a three-electrode cell was used. De-LiCoO2 thin film was used as a working electrode, Pt mesh was used as a counter electrode, and Hg/HgO was used as a reference electrode. A solution of 0.1 mol dm-
3 KOH (O2 gas was bubbled for 90 seconds) was used as an electrolyte. Cyclic voltammetry was carried out at a sweep range from 0.2 to 0.8 V (vs. Hg/HgO). The electrode was cycled for 10 times at a scan rate of 10 mV s-
1.

Results and Disccusion

Figure 1 shows XRD patterns of LiCoO2 and De-LiCoO2. Before delithiation, the XRD pattern was in good agreement with the simulated patterns generated from ICSD database (Number 51182). After the delithiation, the XRD peaks at 2θ ≈ 19° were lowered with an appearance of a new peak at 2θ ≈ 18.4°, suggesting that the LiCoO2 thin film was successfully delithiated by the delithiation process employed in the present study.
Comparison of cyclic voltammograms between the LiCoO2 and the De-LiCoO2 thin films is shown in Fig. 2. The LiCoO2 thin film showed larger Co3/4+ peak at around 1.5 V, which is ascribed to both lithium removal into electrolyte and Li ordering within the surface/bulk[3]. On the other hand, the De-LiCoO2 thin film did not show such peaks, implying that lithium content of the as-prepared De-LiCoO2 was much lower than that of the pristine LiCoO2. The De-LiCoO2 showed a different OER activity from that of pristine LiCoO2, implying that the OER activity was dependent upon lithium content of the thin films. The effect of lithium content on the OER activity and its quantitative discussion is under investigation, and details will be discussed in the conference.

Reference

[1] Stefania Marani, Paolo Salvi, Paolo Nelli, Rachele Pesenti, Marco Villa, Mario Berrettoni, Giovanni Zangari, and Yohannes Kiros., Electrochimica Acta
82, 384-391 (2012).
[2] Zhiyi Lu, Haotian Wang, Desheng Kong, Kai Yan, Po-Chun Hsu, Guangyuan Zheng, Hongbin Yao, Zheng Liang, Xiaoming Sun, and Yi Cui., Nature Communications 5, 4345 (2014).
[3] Graeme Gardner, Jafer Al-Sharab, Nemanja Danilovic, Yong Bok Go, Katherine Ayers, Martha Greenblatt, and G. Charles Dismukes., Energy & Environmental Science
9, 184 (2016).

Figure 1