Introduction

Cobalt oxide (CoO) has emerged as a prominent catalyst in electrochemical water splitting (EWS) applications, demonstrating significant efficacy in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to its impressive charge transfer capabilities and large surface area1, 2, 3, 4, 5. Recently, Liu . synthesized CoO quantum dots and combined them with TiO materials to improve the efficiency of charge transfer between the two materials to increase the water-splitting activity of the material6. Similarly, Yuan et al. have developed a composite material combining CoO with nitrogen-doped carbon, supported on Ni foam, to facilitate comprehensive water-splitting for both HER and OER reactions. Despite CoO's efficiency, the use of Ni foam substrates has been noted to pose stability issues in acidic environments, suggesting a preference for basic conditions for optimal water splitting reactions7.

Recently, La developed CoO/CC for overall water splitting and demonstrated that this material has an efficient catalytic performance8, 9. Furthermore, CoO/CC also expressed a high ESW performance at a NaSO electrolyte that does not react or interfere with most of the target electrode or electrochemical reactions, respectively10. Moreover, NaSO solution is a stable electrolyte over electrodes based on SnO-glass 11.

Various methods, including hydrothermal synthesis, sol-gel, electrochemical deposition, and sputtering, have been explored for synthesizing CoO on substrates12, 13, 14, 15, 16, 17. Among these, electrochemical deposition stands out as a straightforward, eco-friendly approach that minimizes chemical use and production time. This method is especially beneficial for fabricating electrodes for EWS reactions, which necessitate the integration of catalysts with conductive substrates to ensure efficient electron transfer. The selection of an appropriate conductive electrode—be it graphite, carbon-based materials like carbon nanotubes (CNTs) or carbon cloth (CC), metal foams or meshes, conductive polymers, or transparent conductive oxides such as FTO and ITO—is crucial for optimizing EWS efficiency and paving the way for future practical applications18, 19.

This study aims to synthesize CoO on various conductive substrates (FTO, ITO, and CC) using electrochemical deposition under low potential and temperature conditions. Characterization of the CoO coatings was conducted via X-ray diffraction (XRD) patterns and SEM observations. Furthermore, a NaSO solution was used as an electrolyte for the EWS evaluation over catalytic electrodes.

Methods

Results

X-ray Diffraction Analysis

The XRD patterns of CoO decorated on various substrates are depicted in Figure 1. The patterns reveal peaks corresponding to the underlying substrates of CC, FTO, and ITO. Additionally, distinct peaks at 2θ values of 19°, 31.2°, 36.8°, 44.8°, 59.3°, and 65.2° are observed, which correlate with the (111), (220), (311), (400), (511), and (440) lattice planes of CoO, respectively [JCPDS #80-1532]. Notably, the (440) and (311) planes of CoO are prominently featured when deposited onto FTO and ITO substrates, respectively. Conversely, the crystalline intensity of CoO on the CC substrate appears to be weaker.

Scanning Electron Microscopy Observations

SEM analysis was conducted to examine the morphology of CoO on the substrates, as illustrated in Figure 2, Figure 3, Figure 4. Figure 2 presents CoO/ITO with a porous structure distinctly different from the smooth structure of the underlying ITO. Figure 3 highlights the morphology of CoO/FTO, indicating slight variations with non-uniform CoO particle sizes. In contrast, Figure 4 shows CoO/CC densely covered with a uniform layer of CoO, approximately 4-5 µm thick, exhibiting a consistent porous structure across the CC substrate.

Linear Sweep Voltammetry Analysis

LSV measurements were utilized to evaluate the EWS activity of CoO on various substrates, as depicted in Figure 5. The HER performance, showcased in Figure 5(a), reveals CoO/CC as the most active electrode, achieving the lowest onset potential of -382 mV at a current density of 10 mA cm. Comparatively, CoO/FTO and CoO/ITO require significantly higher overpotentials of -975 mV and -1610 mV, respectively, to reach -10 mA cm, underscoring the superior HER efficiency of CoO/CC.

OER activities, shown in Figure 5(b), indicate that none of the electrodes—FTO, ITO, or CC—exhibit effective EWS performance for OER, as measured in 1.0 M NaSO at a scan rate of 10 mV s. This suggests that while CoO/CC demonstrates promising HER capabilities, improvements are needed to enhance OER performance across all evaluated substrates.

Discussion

XRD patterns depicted in Figure 1 reveal distinctive crystal structures among the conductive substrates used. Notably, both FTO and ITO exhibit high crystallinity, with predominant orientations at the (110) and (222) planes, respectively. This variance in diffraction peaks can be attributed to the unique crystalline characteristics of each substrate, which in turn influence the nucleation and growth of CoO during the electrochemical deposition process. Specifically, the pronounced crystalline growth of CoO along the (440) and (311) planes on FTO and ITO substrates, respectively, contrasts with the weaker crystal intensity observed on the CC substrate. However, the presence of both (440) and (311) planes of CoO on the CC substrate suggests a nuanced texture development that directly impacts CoO's morphology21.

The observed disparities in CoO crystal growth across different substrates might be elucidated by oriented aggregation phenomena, where crystals tend to cluster around emerging seed sites. This process results in a non-uniform size distribution of CoO, as evidenced by the SEM images in Figure 4. These images further demonstrate that the CC substrate facilitates a more uniform distribution of CoO compared to FTO and ITO substrates, enhancing the HER performance of CoO.

Moreover, the electrochemical measurements highlighted in Figure 5 and Table 1 indicated that CoO/CC and CoO/FTO electrodes require relatively low cell voltages of 1.13 V and 1.34 V, respectively, to achieve a current density of 10 mA cm. This contrasts with the difficulty in determining the necessary voltage for the CoO/FTO electrode, underscoring the effectiveness of the electrochemical deposition method in preparing CoO on various substrates.

The comparative analysis of overpotentials for HER and OER across CoO-decorated substrates reveals significant variations, possibly due to differences in electrolyte environments. While prior studies predominantly employed 1.0 M KOH, this research utilized 1.0 M NaSO, showcasing the CoO/CC electrode's superior activity and lower overpotential in comparison to other electrodes. This suggests that the choice of electrolyte can markedly influence the EWS performance of CoO electrode systems, with the CoO/CC configuration exhibiting enhanced activity at reduced overpotentials.

Conclusion

In summary, we has successfully synthesized CoO on various substrates, including FTO, ITO, and CC, through an electrochemical deposition, as confirmed by XRD and SEM analyses. The findings reveal that CoO exhibits distinct (440) and (311) planes when deposited on the CC substrate, which significantly influences its morphology. Notably, the CoO/CC electrode demonstrates superior EWS performance in both HER and OER. Specifically, the CoO/CC electrode achieved the lowest observed overpotentials of -382 mV for HER and 1130 mV for OER at a consistent current density of 10 mA cm, utilizing a 1.0 M NaSO electrolyte. These results underscore the potential of CoO/CC as a highly effective catalyst for EWS applications, highlighting its promising capacity for energy conversion processes. The study paves the way for further exploration into the optimization of CoO-based electrodes for sustainable hydrogen and oxygen production.

ACKNOWLDEDMENTS

This research is funded by Vietnam National University Ho Chi Minh City (VNU-HCM) under Grant B2022-18-03.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.