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Article

In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting

by
Wei Duan
,
Shixing Han
,
Zhonghai Fang
,
Zhaohui Xiao
* and
Shiwei Lin
State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering, Hainan University, No. 58 Renmin Road, Haikou 570228, China
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(10), 4134; https://doi.org/10.3390/molecules28104134
Submission received: 20 April 2023 / Revised: 5 May 2023 / Accepted: 14 May 2023 / Published: 16 May 2023
(This article belongs to the Collection Green Energy and Environmental Materials)
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Abstract

:
Electrocatalytic water splitting is a crucial area in sustainable energy development, and the development of highly efficient bifunctional catalysts that exhibit activity toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of paramount importance. Co3O4 is a promising candidate catalyst, owing to the variable valence of Co, which can be exploited to enhance the bifunctional catalytic activity of HER and OER through rational adjustments of the electronic structure of Co atoms. In this study, we employed a plasma-etching strategy in combination with an in situ filling of heteroatoms to etch the surface of Co3O4, creating abundant oxygen vacancies, while simultaneously filling them with nitrogen and sulfur heteroatoms. The resulting N/S-VO-Co3O4 exhibited favorable bifunctional activity for alkaline electrocatalytic water splitting, with significantly enhanced HER and OER catalytic activity compared to pristine Co3O4. In an alkaline overall water-splitting simulated electrolytic cell, N/S-VO-Co3O4 || N/S-VO-Co3O4 showed excellent overall water splitting catalytic activity, comparable to noble metal benchmark catalysts Pt/C || IrO2, and demonstrated superior long-term catalytic stability. Additionally, the combination of in situ Raman spectroscopy with other ex situ characterizations provided further insight into the reasons behind the enhanced catalyst performance achieved through the in situ incorporation of N and S heteroatoms. This study presents a facile strategy for fabricating highly efficient cobalt-based spinel electrocatalysts incorporated with double heteroatoms for alkaline electrocatalytic monolithic water splitting.

1. Introduction

Hydrogen is receiving increasing attention, due to its low carbon footprint and high energy density. The replacement of traditional fossil fuel sources with hydrogen is considered an effective strategy for efficiently solving today’s increasingly serious environmental and energy problems [1]. For large-scale hydrogen production, electrocatalytic water splitting is an efficient and simple method. It is a pollution-free process that does not emit greenhouse gases, unlike other methods such as fossil fuel reforming hydrogen production or methanol/ammonia splitting hydrogen production [2,3]. Hydrogen production from electrocatalytic water can be achieved through various methods, such as photoelectrochemical water oxidation [4,5], alkaline electrocatalytic water splitting [6], electrocatalytic water splitting using proton exchange membranes [7], and solid oxide electrolysis of water for hydrogen production [8]. Alkaline electrocatalytic water splitting is the most mature and widely applicable of these methods [9]. The OER at the anode and the HER at the cathode were the major half-reactions involved in electrocatalytic water splitting. The electrocatalytic splitting of water into H2 and O2 requires a thermodynamic potential of 1.23 V. However, in practical situations, higher driving voltages are often required to overcome the energy barriers associated with charge transfer and mass transport in the multi-step catalytic process of HER and OER [10,11,12]. The noble metal Pt [13] and Ir [14], etc., demonstrate excellent catalytic performance in electrocatalytic water splitting owing to their appropriate adsorption/desorption free energies for reaction intermediates [15,16]. In spite of this, their high price and rarity prevent them from being widely used as electrocatalysts in HER and OER applications. Therefore, alternative catalysts based on non-noble metals are urgently needed [17,18].
Transition metal compounds’ catalytic materials have the potential to replace noble metals in commercial applications, which can be attributed to their cost-effectiveness and high activity, and the outermost atomic layer electronic structure was controllable, rendering them highly appealing for diverse applications [19]. Ni-, Co-, and Fe-based catalysts, in particular, have demonstrated higher electrocatalytic activity for HER [20,21,22] and OER [23,24,25], making them the focus of interest for many researchers. Bifunctional electrocatalysts can catalyze both OER and HER reactions for water splitting, so the development of electrocatalysts with bifunctional catalytic activity is more valuable for commercial applications. Co3O4 is a promising candidate catalyst, due to the fact Co3O4 consists of Co2+ and Co3+ ions, with three unpaired d electrons on Co2+ and all d electrons of Co3+ being paired. The bond between cobalt and oxygen exhibits a covalent character in the ion bond dominated Co3O4 [26]. With its attractive electronic properties, Co3O4 can serve as a reliable and efficient catalyst in a wide range of electrochemical catalytic reactions [27,28,29]. Due to its rich and controllable surface electronic structure, the electrocatalyst Co3O4 was also used for water splitting [30,31]. While Co3O4 exhibits satisfactory OER catalytic activity, it has yet to achieve commercial viability. Additionally, its poor HER activity poses limitations on its overall water splitting performance. Thus, it is imperative to explore practical and feasible approaches to enhance its bifunctional electrocatalytic activity for water decomposition.
The electrocatalytic reaction of water splitting occurs on the surface of a catalyst, and therefore, its surface structure is closely related to its catalytic activity. Various methods exist to modulate the surface properties of a catalyst, such as morphology modulation [32], electronic structure modulation [33], heterojunction engineering [34], and defect engineering [35], etc. Defect engineering is a useful strategy for regulating catalytic performance, since it can modulate the electronic structure of catalyst surfaces and adjust the adsorption/desorption process of reactants and products on the catalyst surface by redistributing charges [36]. Previous studies have demonstrated that as an OER catalyst, surface modifications can alter the catalyst’s electronic structure, reduce reaction intermediates’ (OH*, O*, OOH*) energy barriers and enhance intrinsic catalytic activity [37]. As a HER catalyst, cobalt-based compounds are converted to complexes of cobalt metal and Co2+ during the HER process [38]. Cobalt, serving as the active center, contributes significantly to both the adsorption of surface water molecules and protons during HER, and the desorption of HER surface intermediates [39]. Moreover, the replacement of certain anions in cobalt-based compounds has the potential to adjust the electronic configuration of cobalt, thereby lowering the free-energy barrier for H* adsorption and promoting the release of H2 from the active site, thus accelerating the HER. Elements such as C [40], N [41], P [42], and S [43] have been demonstrated to be effective dopants for Co-based catalysts in HER. Introducing cation/anion vacancies onto the surface of a catalyst has been affirmed as an effective strategy for enhancing catalytic activity. Recent studies suggest that dual doping enhances the catalytic activity of catalysts more effectively than single doping [44], and in situ filling of vacancies in the catalyst can lead to significant improvements in catalytic activity, and these improvements are more pronounced than those achieved with a single vacant site [45].
In this study, we attempt to employ a combination of plasma etching and in situ heteroatom filling strategies to etch the surface of Co3O4 to create abundant oxygen vacancies and fill the oxygen vacancies with N and S heteroatoms simultaneously. Ar plasma etching reduced the size of the Co3O4 nanosheets, creating a substantial number of defects and exposing more active sites, while the introduction of N and S optimized the surface and structure of the catalyst. The obtained N/S-VO-Co3O4 showed a significant enhancement in the electrocatalytic decomposition activity of water in 1.0 M KOH, especially the HER catalytic activity, compared with the pristine Co3O4, and N/S-VO-Co3O4 exhibited significant long-term catalytic stability for both HER and OER. Furthermore, the double electrode composed of N/S-VO-Co3O4 exhibited catalytic activity comparable to that of Pt/C and IrO2 in an alkaline overall water-splitting simulated electrolytic cell and showed excellent long-term catalytic stability.

2. Results and Discussion

The synthesis method for N/S-VO-Co3O4 is illustrated in Scheme 1 (details are given in the Supplementary Materials). Briefly, during the preparation of N/S-VO-Co3O4, oxygen vacancies were etched on the surface of Co3O4 by using Ar plasma. Simultaneously, NH3 and H2S generated via the thermal decomposition of thiourea were used as N and S sources to fill the oxygen vacancies in VO-Co3O4.
Initially, we employed the HER and OER characteristics of the catalyst as indicators in a 1.0 M KOH solution. By adjusting the plasma treatment time, we established the optimal preparation conditions. The experimental results showed that the use of Ar plasma assistance for the introduction of N and S heteroatoms could significantly improve the HER and OER properties of the catalyst, as shown in Figure S1. Additionally, with Ar plasma assistance, the catalysts exhibited the highest catalytic activity compared to direct introduction, and surpassed the performance of catalysts with single heteroatoms under equivalent conditions, as depicted in Figure S2. The optimal parameters for the synthesis of N/S-VO-Co3O4 have been determined. Co3O4, VO-Co3O4, and N/S-VO-Co3O4 were used as the study objects, and the source of catalytic activity of N/S-VO-Co3O4 was explored in the following experiments by combining various characterization and electrochemical measurements.
The catalysts’ morphologies were examined using scanning electron microscopy (SEM); SEM revealed that the morphology of Co3O4 was observed as nanoclusters that were formed by closely stacked nanosheets, which grew vertically to the substrate and were uniformly distributed, and the surfaces of the pristine Co3O4 nanosheets were observed to be clean, smooth, and dense, as shown in Figure 1a and Figure S3a. After the plasma-etching process, the surface of the VO-Co3O4 nanosheets became porous, discontinuous, and loosely planar, and the larger nanosheet structure was etched into a smaller nanosheet structure, while bulk phase defects appeared on the nanosheets, as shown in Figure 1b and Figure S3b. Similar to the microscopic morphology of VO-Co3O4, the nanosheets of N/S-VO-Co3O4 became porous, discontinuous, and loosely planar, as shown in Figure 1c and Figure S3c. To further analyze the elemental species and their distribution on the surface of the catalyst, energy-dispersive X-ray spectroscopy (EDS) was used to probe the elemental distribution (mapping) on the surface of the N/S-VO-Co3O4, as seen in Figure S4. The EDS mapping results showed that the elements Co, O, S, and N were evenly distributed on the surface of N/S-VO-Co3O4. Notably, the concentrations of N and S heteroatoms were relatively low, indicating that their presence in the Co3O4 nanosheets was limited.
To gain further insight into the microscopic morphology and structure of the catalysts, we used transmission electron microscopy (TEM) to analyze their morphology, structure, and elemental distribution. The TEM images revealed that the morphology of Co3O4 is a nanosheet structure with smooth edges, while the morphology of N/S-VO-Co3O4 bears resemblance to that of VO-Co3O4 following Ar plasma etching. The thickness of the Co3O4 nanosheet was reduced and the edges of the nanosheet appeared to be chipped, consistent with the SEM results, as shown in Figure S5. High-resolution transmission electron microscopy (HRTEM) was subsequently employed to observe the obtained range of catalysts, as shown in Figure 1d–f. There two diffraction stripes with different lattice spacings were found on Co3O4, where the lattice stripe with a distance of 0.46 nm corresponds to the (111) crystal plane of Co3O4, while the lattice stripe of 0.24 nm corresponds to the (311) crystal plane of Co3O4 (JCPDS 43-1003) [46]. The lattice diffraction stripes on the VO-Co3O4 were the same as those on the Co3O4, indicating that the Ar plasma etching did not cause a significant structural change for the Co3O4. Similarly, the lattice diffraction stripes observed on N/S-VO-Co3O4 were the same as those observed on Co3O4, indicating that the introduction of N and S heteroatoms did not cause significant phase transformation of Co3O4. The elemental compositions of Co3O4 and N/S-VO-Co3O4 were obtained via EDS in the High-Angle Annular Dark-Field (HADDF) mode for further analysis of the elemental species and distribution in the nanoclusters, as shown in Figures S6 and S7. The characteristic peaks of N and S elements were observed in the EDS spectrum of N/S-VO-Co3O4; this indicates the successful introduction of N and S. The amount of each element in Co3O4 and N/S-VO-Co3O4 were shown in Tables S1 and S2, respectively. In N/S-VO-Co3O4, the loading content of N and S elements were 4.31% and 13.29%, respectively, demonstrating that the amount of N and S elements introduced in N/S-VO-Co3O4 was relatively small. Figure 1g shows the EDS mapping, which confirms the homogeneous distribution of the elements Co, O, S, and N on the surface of N/S-VO-Co3O4, compared with Figure S8. It can also be observed that the amounts of N and S elements were relatively small, consistent with the SEM mapping.
The catalysts’ crystal structures were analyzed using X-ray diffraction (XRD). As depicted in Figure 2a, diffraction peaks at 2θ = 31.27°, 36.85°, 44.81°, 59.35°, and 65.23° were observed for the obtained range of catalysts, corresponding to the (220), (311), (400), (511), and (440) crystal planes of Co3O4. (JCPDS 43-1003). This observation suggests that the lattice structure of the catalysts remained largely unchanged despite undergoing Ar plasma etching or introducing N and S heteroatoms. Raman was also utilized to investigate the structural properties of the catalysts, and the Raman spectra of Co3O4, VO-Co3O4, and N/S-VO-Co3O4 are shown in Figure 2b. There were three obvious peaks observed in the Raman spectra at 475 cm−1, 521 cm−1, and 683 cm−1, corresponding to the F 2 g , F 2 g 2 , and A 1 g phonon modes of the Co3O4 crystals, respectively [47]. There were no other peaks belonging to cobalt sulfate or cobalt nitrate in the Raman spectrum, which indicated that the crystal structure of Co3O4 remained unchanged after the introduction of N and S heteroatoms. Additionally, a small shift at A1g could be observed, which could be attributed to the different electronegativity [48] of the introduced N and S heteroatoms.
X-ray photoelectron spectroscopy (XPS) was utilized to examine the elemental composition and metal valence states of Co3O4, VO-Co3O4, and N/S-VO-Co3O4. As shown in the XPS survey spectra of Co3O4, VO-Co3O4, and N/S-VO-Co3O4, Figure S9. The characteristic peaks of N 1s and S 2p were found only in the survey spectrum of N/S-VO-Co3O4 indicating the successful introducing of N and S heteroatoms. For the Co 2p spectra in Figure 2c, it can be observed that the Co 2p spectra of all catalysts can be decomposed into eight fitting peaks, indicating the presence of different valence states of Co in Co3O4. The fitting peaks at 780.1 eV and 795.1 eV were attributed to Co3+, and the peaks at 782.2 eV and 797.2 eV were attributed to Co2+. The remaining four fitting peaks were considered to be satellite peaks of Co 2p [49]. By comparing the areas of the peaks of Co3+ and Co2+ in the Co 2p spectra of each catalyst, the valence change of Co during the Ar plasma treatment and introduction of N and S heteroatoms can be analyzed. The XPS analysis revealed that the content of Co2+ increased following Ar plasma etching. This indicates that the reduction in the valence state of Co was caused by the oxygen vacancies on the catalyst surface induced by Ar plasma etching [50]. Conversely, the Co2+: Co3+ ratio decreased after introduction of oxygen vacancies with N and S heteroatoms, indicating that the introducing of N and S heteroatoms caused a slight increase in the valence of Co, possibly due to the electronegativity difference compared to O atoms, as seen in Table S3. Figure 2d shows the decomposed O 1s spectra, which exhibits four characteristic peaks (OI, OII, OIII, and OIV) at the binding energies of 529.9, 530.7, 531.4, and 532.4 eV, respectively. These peaks correspond to the lattice oxygen, oxygen vacancies, surface-adsorbed hydroxyl groups, and surface-adsorbed oxygen-containing species of Co3O4, respectively [51,52,53]. The oxygen vacancy concentration in the surface of the catalyst can be established by comparing the ratio of OII:OI in the O 1s spectrum [54]. Following plasma treatment, the ratio of OII:OI in VO-Co3O4 increased, as seen in Table S4, indicating that plasma treatment increased the concentration of the oxygen vacancies of the catalyst. After introduction of N and S heteroatoms, the oxygen vacancy concentration of N/S-VO-Co3O4 decreased; nevertheless, it remained higher than that of the pristine Co3O4, indicating that N and S heteroatoms maybe in situ filled in the oxygen vacancies, as we reported previously. In the S 2p spectrum of the N/S-VO-Co3O4 in Figure 2e, the S 2p can be decomposed into four fitted peaks and the peaks at 161.5, 162.7, 164.1, and 167.5 eV, which were ascribed to 2P3/2, 2P1/2, Co-S, and Co-O, respectively [55], indicating the successful introduction of the S atoms and the S atoms bonding with the Co and O atoms on the surface of Co3O4. In Figure 2f, the N 1s spectrum was ascribed to the bonding of N with metal elements, indicating the successful introduction of N elements [56].
By combining the morphological and structural characterizations of SEM, TEM, XRD, Raman, and XPS, as well as the previous reported experimental design and the characterization results, it is reasonable to speculate that plasma constructs surface oxygen defects while being able to fill the introduced N and S heteroatoms in situ in the oxygen vacancies [56,57,58]. We determined the in situ filling of the oxygen vacancies of Co3O4 by N and S heteroatoms. Subsequently, we investigated the impact of these modifications on the catalytic performance of the catalyst in the alkaline water electrolysis by means of thorough electrochemical tests.
The electrochemically active specific surface area (ECSA) was utilized to evaluate the variations in intrinsic activity of the range of catalysts via cyclic voltammetry (CV). The CV tests were carried out on Co3O4, VO-Co3O4, and N/S-VO-Co3O4 with different scan rates (v). The linear relationship diagrams, as shown in Figure S10, were drawn based on their respective current density (ΔJ/2) and v. The Cdl of Co3O4, VO-Co3O4, and N/S-VO-Co3O4 were determined to be 0.43, 0.65, and 1.13 mF cm−2, respectively, based on the slopes of the fitted line, and the ECSA value of N/S-VO-Co3O4 was further calculated to be 28.3 cm2, which was much larger than that of VO-Co3O4 (16.3 cm2) and Co3O4 (10.8 cm2). These outcomes suggest that the introduction of oxygen vacancies effectively improves the ECSA, resulting in a significant increase in the quantity of catalytic reaction sites. Additionally, the number of catalytic sites was further increased by filling the N and S heteroatoms.
The HER catalytic activities of Co3O4, VO-Co3O4, and N/S-VO-Co3O4 were evaluated in a 1.0 M KOH solution. As depicted in Figure 3a,b, the HER catalytic activity of N/S-VO-Co3O4 was greatly improved after Ar plasma etching and in situ filling of N and S heteroatoms; a mere overpotential of 181 mV was required to achieve −10 mA cm−2, which is much lower than the overpotentials required for VO-Co3O4 (318 mV) and Co3O4 (430 mV), respectively. The Tafel slope of each catalyst was shown in Figure 3c, the slope of N/S-VO-Co3O4 was 79 mV dec−1, which was much lower than VO-Co3O4 (118 mV dec−1) and Co3O4 (162 mV dec−1), while the smaller Tafel slope reflects the faster reaction kinetics of the catalyst during the HER reaction, which is better than most of the Co-based catalysts reported currently, as shown in Table S7 [58,59,60,61,62,63,64,65,66,67]. To further analyze the kinetic processes involved in the HER process, electrochemical impedance spectroscopy (EIS) tests were conducted at −1.3 V vs. SCE. The Charge transfer resistance (Rct) of each catalyst during the HER reaction was obtained from EIS fitting, with results shown in Table S5. It is generally believed that the smaller the impedance value, the higher the charge transfer efficiency of the corresponding catalyst. Among them, N/S-VO-Co3O4 possesses the smallest electrochemical impedance (Rct = 4.21 Ω), which is much smaller than that of VO-Co3O4 (Rct = 35.83 Ω) and Co3O4 (Rct = 105.64 Ω), as shown in Figure 3d. The enhancement of the charge transfer rate could be attributed to the incorporation of oxygen vacancies and in situ filling of N and S heteroatoms. The catalytic stability of N/S-VO-Co3O4 for HER was evaluated through 1000 cycles of CV within a range of −0.3 V to 0.1 V vs. RHE at a scan rate of 100 mV s−1. The comparison of polarization curves before and after the 1000 cycles CV almost overlap, as shown in Figure S11a. Furthermore, N/S-VO-Co3O4 was subjected to a 12-h durability test at −10 mA cm−2, as depicted in Figure S11b. Notably, the potential of the catalyst remained relatively steady throughout the test. Collectively, these results indicate that N/S-VO-Co3O4 demonstrates good catalytic stability during HER.
The OER catalytic activity of Co3O4, VO-Co3O4, and N/S-VO-Co3O4 was also evaluated and compared. As depicted in Figure 4a,b, the OER activity of N/S-VO-Co3O4 required an overpotential of 294 mV to reach 10 mA cm−2, which was lower than that of VO-Co3O4 (328 mV) and Co3O4 (405 mV). Additionally, the Tafel slope of each catalyst was shown in Figure 4c; the Tafel slope of N/S-VO-Co3O4 (71 mV dec−1) was lower than that of VO-Co3O4 (124 mV dec−1) and Co3O4 (170 mV dec−1), reflecting the faster OER reaction kinetics of the N/S-VO-Co3O4 catalyst due to the smaller Tafel slope, indicating it is better than most of the Co-based catalysts reported currently, as seen in Table S8 [56,60,68,69,70,71,72,73,74,75,76]. EIS was also performed to further analyze the kinetic processes in the OER process, at 0.55 V vs. SCE. As shown in Figure 4d, N/S-VO-Co3O4 has the smallest electrochemical impedance (Rct = 2.12 Ω), which is smaller than that of VO-Co3O4 (Rct = 4.68 Ω) and Co3O4 (Rct = 6.10 Ω); the EIS fitting results are shown in Table S6. This indicates that the OER charge transfer rate and kinetics of N/S-VO-Co3O4 were also enhanced.
Finally, the OER stability of N/S-VO-Co3O4 was also tested using 1000 cycles of CV in the range of 1.2 to 1.6 V vs. SCE. The OER polarization curves before and after 1000 cycles of CV are shown in Figure S12a, and the LSV curves before and after 1000 cycles of CV almost overlap. Additionally, N/S-VO-Co3O4 was subjected to a 12-h durability test at 10 mA cm−2, as depicted in Figure S12b; the potential of the catalyst remained relatively steady throughout the test. In summary, N/S-VO-Co3O4 also demonstrated good OER catalytic durability.
The prepared catalysts were compared with benchmark noble metal-based catalysts Pt/C and IrO2, as illustrated in Figure S13. Despite requiring a higher overpotential than Pt/C to achieve −10 mA cm−2 in HER, N/S-VO-Co3O4 exhibited a smaller Tafel slope, indicating faster HER kinetics than Pt/C. Regarding OER, N/S-VO-Co3O4 exhibited both a lower overpotential required at 10 mA cm−2 and a smaller Tafel slope than IrO2, suggesting faster OER kinetics for N/S-VO-Co3O4 than for IrO2.
Benefiting from the gratifying bifunctional activity of N/S-VO-Co3O4 in alkaline electrolytes, N/S-VO-Co3O4 exhibits catalytic activity comparable to that of Pt/C || IrO2 in an alkaline overall water-splitting simulated electrolytic cell. As shown in Figure 5a,b, the required voltage of the N/S-VO-Co3O4 || N/S-VO-Co3O4 dual-electrode at the current density of 10 mA cm−2 was only 1.715 V, which is much lower than that of the dual-electrode electrolytic cell consisting of VO-Co3O4 || VO-Co3O4 (1.834 V) and Co3O4 || Co3O4 (1.872 V), and comparable to Pt/C || IrO2 (1.681 V). Moreover, the N/S-VO-Co3O4 || N/S-VO-Co3O4 dual-electrode exhibited favorable electrocatalytic stability in 1.0 M KOH, as shown in Figure 5c, with no significant voltage increase at a constant current density of 10 mA cm−2 for 60 h, showing that it outperformed that of Pt/C||IrO2.
To gain further insight into the mechanism underlying the bifunctional catalytic activity of N/S-VO-Co3O4 in electrolytic water splitting, we conducted in situ Raman analysis in 1.0 M KOH to dynamically monitor the HER and OER processes of N/S-VO-Co3O4. For comparison, Co3O4 was also measured under the same conditions. As illustrated in Figure 6a,b, the Raman spectra of Co3O4 and N/S-VO-Co3O4 did not exhibit significant changes after the applied potential during HER, consistent with the ex situ Raman results in air presented in Figure 2b. This observation indicates that the lattice structure of Co3O4 did not undergo significant changes during HER. Therefore, it is reasonable to hypothesize that the in situ filled N and S heteroatoms served as moderators on the adsorption/desorption of the reaction intermediates during HER, which enhanced the HER activity of N/S-VO-Co3O4 [41,43]. In contrast, during OER, a new characteristic peak becomes observable in the Raman spectra of N/S-VO-Co3O4, ranging from 550 to 620 cm−1, once the applied potential reached 1.424 V vs. RHE, as illustrated in Figure 6d. This peak is indicative of the production of CoOOH [77], while this phenomenon was not observed in Co3O4, as shown in Figure 6c. Previous reports suggest that the high-valent Co4+ in CoOOH is the truly active phase of Co-based catalysts during OER in alkaline electrolytes; it can be inferred that the transition from Co3O4 to CoOOH is facilitated by the in situ filling of oxygen vacancies with N and S heteroatoms, leading to a lower overpotential required for OER.
Ex situ characterization was also performed on N/S-VO-Co3O4 to analyze its structural changes after the catalytic reaction. Figure S14a shows the XRD results, which indicate that the structure of N/S-VO-Co3O4 did not undergo significant changes before and after the electrocatalytic water splitting. The spinel structure of Co3O4 was still maintained. On the other hand, a slight shift can be observed in the Co 2p spectrum of N/S-VO-Co3O4 after OER, as shown in Figure S14b, indicating the production of more high-valent Co. This shift was not observed after HER. Figure S14c illustrates the change in the S 2p spectrum of N/S-VO-Co3O4 after the catalytic reaction. When compared with the initial N/S-VO-Co3O4, it was observed that the S species did not undergo significant changes after HER. However, after OER, all S species could be observed to shift to the S–O bonded form. This shift is related to the in situ reconfiguration of N/S-VO-Co3O4 during the OER process. Figure S14d demonstrated that the elemental species of N composition did not undergo significant changes before and after HER and OER. The N species exists in the form of N bonded to metal elements. In summary, it can be concluded that the lattice structure of N/S-VO-Co3O4 remains stable during the HER reaction, and the elemental species of N and S do not undergo significant changes. However, during the OER process, N/S-VO-Co3O4 undergoes a structural transformation, leading to the production of more CoOOH. Furthermore, the S element is further oxidized to produce S O 4 2 − during the structural transformation phase.

3. Conclusions

In summary, we successfully etched the spinel-structured Co3O4 nanosheets via Ar plasma to create oxygen vacancy-rich nanosheets (VO-Co3O4) and subsequently filled the oxygen vacancies with N and S heteroatoms to form N/S-VO-Co3O4. SEM and TEM characterization revealed that the Ar plasma etching generated numerous porous defects on the surface of the Co3O4 nanosheet, which increased the specific surface area of the catalyst and promoted adsorption and desorption of reactants and products in electrocatalytic water splitting. Furthermore, XRD and Raman analysis demonstrated that the in situ filling of N and S atoms did not significantly alter the crystal structure of Co3O4, indicating the preservation of the crystal structure of Co3O4 in the obtained catalysts. However, XPS analysis confirmed that the surface electronic structure of Co and O in the catalysts underwent a transformation. This transformation resulted in a significant improvement in the electrocatalytic activity of N/S-VO-Co3O4 for water splitting in a 1.0 M KOH electrolyte. The HER and OER performances of N/S-VO-Co3O4 were greatly enhanced compared to pristine Co3O4. It only required 181 mV (for HER) and 294 mV (for OER) overpotentials to reach a current density of 10 mA cm−2. These values were much lower than the overpotentials of 430 mV (for HER) and 405 mV (for OER) required for pristine Co3O4. Additionally, N/S-VO-Co3O4 exhibits good catalytic stability. Benefiting from the favorable bifunctional HER and OER electrocatalytic activity, the dual-electrode N/S-VO-Co3O4 || N/S-VO-Co3O4 required 1.715 V to reach the current density of 10 mA cm−2, in an alkaline overall water-splitting simulated electrolytic cell, which was comparable to a dual-electrode consisting of Pt/C || IrO2. Moreover, N/S-VO-Co3O4 || N/S-VO-Co3O4 exhibited good catalytic stability in long-term overall water splitting compared to Pt/C || IrO2. The in situ Raman analysis further revealed that the in situ filled N and S heteroatoms played a moderating role in the adsorption/desorption of reaction intermediates during HER and facilitated the transition from Co3O4 to CoOOH during OER. This study offers a new approach for the development of effective dual-heteroatom-introduced electrocatalysts for alkaline electrocatalytic overall water splitting.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28104134/s1, Preparation, measurement and characterization details, Figure S1–S14, Table S1–S8. Figure S1. (a) HER and (b) OER polarization curves of N/S-VO-Co3O4 with different plasma treatment times in 1.0 M KOH. Figure S2. (a, c) HER and (b, d) OER polarization curves of catalysis compared with different treatment in 1.0 M KOH. Figure S3. SEM images of (a) Co3O4, (b) VO-Co3O4, (c) N/S-VO-Co3O4. Figure S4. SEM image and corresponding EDS mappings of N/S-VO-Co3O4. Figure S5. TEM images of (a) Co3O4, (b) VO-Co3O4, (c) N/S-VO-Co3O4. Figure S6. EDS Spectrum of Co3O4. Figure S7. EDS Spectrum of N/S-VO-Co3O4. Figure S8. HADDF-TEM image and corresponding EDS mappings of Co3O4. Figure S9. XPS survey of Co3O4, VO-Co3O4 and N/S-VO-Co3O4. Figure S10. Cyclic voltammogram curves of (a) Co3O4, (b) VO-Co3O4, (c) N/S-VO-Co3O4. (d) Corresponding Cdl value of Co3O4, VO-Co3O4 and N/S-VO-Co3O4, respectively. Figure S11. (a) HER polarization curves of N/S-VO-Co3O4 initial and after 1000 cycles CV. (b) Chronopotentiometry curve of N/S-VO-Co3O4 at −10 mA cm−2. Figure S12. (a) OER polarization curves of N/S-VO-Co3O4 initial and after 1000 cycles CV. (b) Chronopotentiometry curve of N/S-VO-Co3O4 at 10 mA cm−2. Figure S13. The (a) HER and (b) OER polarization curves of different catalysts in 1.0 M KOH. (c) Tafel slope of N/S-VO-Co3O4 compared with Pt/C, during HER. (d) Tafel slope of N/S-VO-Co3O4 compared with IrO2, during OER. Figure S14. (a) XRD, (b) Co 2p, (c) S 2p and (d) N 1s spectra of N/S-VO-Co3O4 before and after HER/OER. Table S1. Elemental ratios of Co3O4 from EDS spectrum. Table S2. Elemental ratios of N/S-VO-Co3O4 from EDS spectrum. Table S3. The relative ratio of Co2+ and Co3+ in N/S-VO-Co3O4, VO-Co3O4 and Co3O4 by Co 2p spectra fitting. Table S4. The relative ratio of oxygen vacancies in N/S-VO-Co3O4, VO-Co3O4 and Co3O4 by Co 2p spectra fitting. Table S5. Electrochemical impedance spectroscopy fitting results for N/S-VO-Co3O4, VO-Co3O4, Co3O4 during HER at −1.30 V vs. SCE. Table S6. Electrochemical impedance spectroscopy fitting results for N/S-VO-Co3O4, VO-Co3O4, Co3O4 during OER at 0.55 V vs. SCE. Table S7. Comparison of HER activity for N/S-VO-Co3O4 and recently reported Co-based catalysts. Table S8. Comparison of OER activity for N/S-VO-Co3O4 and recently reported Co-based catalysts.

Author Contributions

Conceptualization, W.D. and Z.X.; experiment, W.D.; writing—original draft preparation, W.D.; writing—review and editing, W.D., S.H., Z.F. and Z.X.; data curation, W.D. and Z.X.; supervision, Z.X.; funding acquisition, Z.X. and S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Hainan Provincial Natural Science Foundation of China (Grant No. 223QN186), Scientific Research Starting Foundation of Hainan University (Grant No. KYQD(ZR)-22018), and the specific research fund of the Innovation Platform for Academicians of Hainan Province (YSPTZX202123).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We gratefully acknowledge the financial support provided by Hainan Provincial Natural Science Foundation of China (Grant No. 223QN186), Scientific Research Starting Foundation of Hainan University (Grant No. KYQD(ZR)-22018), and the specific research fund of the Innovation Platform for Academicians of Hainan Province (YSPTZX202123).

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

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Molecules 28 04134 sch001 550
Scheme 1. Schematic of Ar plasma-assisted in situ filling of N and S heteroatoms.
Scheme 1. Schematic of Ar plasma-assisted in situ filling of N and S heteroatoms.
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Figure 1. (a–c) SEM and (d–f) HRTEM images of Co3O4, VO-Co3O4, and N/S-VO-Co3O4, respectively. (g) HADDF-TEM image and corresponding EDS mappings of N/S-VO-Co3O4.
Figure 1. (a–c) SEM and (d–f) HRTEM images of Co3O4, VO-Co3O4, and N/S-VO-Co3O4, respectively. (g) HADDF-TEM image and corresponding EDS mappings of N/S-VO-Co3O4.
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Figure 2. (a) XRD spectra. (b) Raman spectra. (c) Co 2p spectra. (d) O 1s spectra of Co3O4, VO-Co3O4 and N/S-VO-Co3O4. (e) S 2p and (f) N 1s spectrum of N/S-VO-Co3O4.
Figure 2. (a) XRD spectra. (b) Raman spectra. (c) Co 2p spectra. (d) O 1s spectra of Co3O4, VO-Co3O4 and N/S-VO-Co3O4. (e) S 2p and (f) N 1s spectrum of N/S-VO-Co3O4.
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Figure 3. (a) HER polarization curves (with 95% iR-compensation), (b) corresponding overpotentials at −10 mA cm−2, (c) Tafel slope, (d) Nyquist plots at −1.30 V vs. SCE of the N/S-VO-Co3O4, VO-Co3O4 and Co3O4.
Figure 3. (a) HER polarization curves (with 95% iR-compensation), (b) corresponding overpotentials at −10 mA cm−2, (c) Tafel slope, (d) Nyquist plots at −1.30 V vs. SCE of the N/S-VO-Co3O4, VO-Co3O4 and Co3O4.
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Figure 4. (a) OER polarization curves (with 95% iR-compensation), (b) Corresponding overpotentials at different current density, (c) Tafel slope, (d) Nyquist plots at 0.55 V vs. SCE of the N/S-VO-Co3O4, VO-Co3O4 and Co3O4.
Figure 4. (a) OER polarization curves (with 95% iR-compensation), (b) Corresponding overpotentials at different current density, (c) Tafel slope, (d) Nyquist plots at 0.55 V vs. SCE of the N/S-VO-Co3O4, VO-Co3O4 and Co3O4.
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Figure 5. (a) Polarization curves and (b) corresponding voltages at 10 mA cm−2 of the N/S-VO-Co3O4 || N/S-VO-Co3O4, VO-Co3O4 || VO-Co3O4, Co3O4 || Co3O4 and Pt/C || IrO2. (c) Chronopotentiometry curves of the N/S-VO-Co3O4 || N/S-VO-Co3O4 and Pt/C || IrO2 at 10 mA cm−2. All the stability date without iR compensation.
Figure 5. (a) Polarization curves and (b) corresponding voltages at 10 mA cm−2 of the N/S-VO-Co3O4 || N/S-VO-Co3O4, VO-Co3O4 || VO-Co3O4, Co3O4 || Co3O4 and Pt/C || IrO2. (c) Chronopotentiometry curves of the N/S-VO-Co3O4 || N/S-VO-Co3O4 and Pt/C || IrO2 at 10 mA cm−2. All the stability date without iR compensation.
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Figure 6. In situ Raman spectra of Co3O4 and N/S-VO-Co3O4 during HER and OER. (a) Co3O4 in HER. (b) N/S-VO-Co3O4 in HER. (c) Co3O4 in OER. (d) N/S-VO-Co3O4 in OER.
Figure 6. In situ Raman spectra of Co3O4 and N/S-VO-Co3O4 during HER and OER. (a) Co3O4 in HER. (b) N/S-VO-Co3O4 in HER. (c) Co3O4 in OER. (d) N/S-VO-Co3O4 in OER.
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MDPI and ACS Style

Duan, W.; Han, S.; Fang, Z.; Xiao, Z.; Lin, S. In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting. Molecules 2023, 28, 4134. https://doi.org/10.3390/molecules28104134

AMA Style

Duan W, Han S, Fang Z, Xiao Z, Lin S. In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting. Molecules. 2023; 28(10):4134. https://doi.org/10.3390/molecules28104134

Chicago/Turabian Style

Duan, Wei, Shixing Han, Zhonghai Fang, Zhaohui Xiao, and Shiwei Lin. 2023. "In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting" Molecules 28, no. 10: 4134. https://doi.org/10.3390/molecules28104134

APA Style

Duan, W., Han, S., Fang, Z., Xiao, Z., & Lin, S. (2023). In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting. Molecules, 28(10), 4134. https://doi.org/10.3390/molecules28104134

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