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

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.


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 H 2 and O 2 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. Co 3 O 4 is a promising candidate catalyst, due to the fact Co 3 O 4 consists of Co 2+ and Co 3+ ions, with three unpaired d electrons on Co 2+ and all d electrons of Co 3+ being paired. The bond between cobalt and oxygen exhibits a covalent character in the ion bond dominated Co 3 O 4 [26]. With its attractive electronic properties, Co 3 O 4 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 Co 3 O 4 was also used for water splitting [30,31]. While Co 3 O 4 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 Co 2+ 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 Co 3 O 4 to create abundant oxygen vacancies and fill the oxygen vacancies with N and S heteroatoms simultaneously. Ar plasma etching reduced the size of the Co 3 O 4 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-V O -Co 3 O 4 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 Co 3 O 4 , and N/S-V O -Co 3 O 4 exhibited significant long-term catalytic stability for both HER and OER. Furthermore, the double electrode composed of N/S-V O -Co 3 O 4 exhibited catalytic activity comparable to that of Pt/C and IrO 2 in an alkaline overall water-splitting simulated electrolytic cell and showed excellent long-term catalytic stability.

Results and Discussion
The synthesis method for N/S-V O -Co 3 O 4 is illustrated in Scheme 1 (details are given in the Supplementary Materials). Briefly, during the preparation of N/S-V O -Co 3 O 4 , oxygen vacancies were etched on the surface of Co 3 O 4 by using Ar plasma. Simultaneously, NH 3 and H 2 S generated via the thermal decomposition of thiourea were used as N and S sources to fill the oxygen vacancies in V O -Co 3 O 4 . 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.

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 Figures 1a and S3a. After the plasmaetching 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 Figures 1b 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-V O -Co 3  The catalysts' morphologies were examined using scanning electron microscopy (SEM); SEM revealed that the morphology of Co 3 O 4 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 Co 3 O 4 nanosheets were observed to be clean, smooth, and dense, as shown in Figure 1a and Figure S3a. After the plasmaetching process, the surface of the V O -Co 3 O 4 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 V O -Co 3 O 4 , the nanosheets of N/S-V O -Co 3 O 4 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-V O -Co 3 O 4 , 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-V O -Co 3 O 4 . Notably, the concentrations of N and S heteroatoms were relatively low, indicating that their presence in the Co 3 O 4 nanosheets was limited. 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 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 Co 3 O 4 is a nanosheet structure with smooth edges, while the morphology of N/S-V O -Co 3 O 4 bears resemblance to that of V O -Co 3 O 4 following Ar plasma etching. The thickness of the Co 3 O 4 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 Co 3 O 4 , where the lattice stripe with a distance of 0.46 nm corresponds to the (111) crystal plane of Co 3 O 4 , while the lattice stripe of 0.24 nm corresponds to the (311) crystal plane of Co 3 O 4 (JCPDS 43-1003) [46]. The lattice diffraction stripes on the V O -Co 3 O 4 were the same as those on the Co 3 O 4 , indicating that the Ar plasma etching did not cause a significant structural change for the Co 3 O 4 . Similarly, the lattice diffraction stripes observed on N/S-V O -Co 3 O 4 were the same as those observed on Co 3 O 4 , indicating that the introduction of N and S heteroatoms did not cause significant phase transformation of  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 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 2g , F 2 2g , and A 1g phonon modes of the Co 3 O 4 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 Co 3 O 4 remained unchanged after the introduction of N and S heteroatoms. Additionally, a small shift at A 1g 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 Co 3 Figure S9. The characteristic peaks of N 1s and S 2p were found only in the survey spectrum of N/S-V O -Co 3 O 4 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 Co 3 O 4 . The fitting peaks at 780.1 eV and 795.1 eV were attributed to Co 3+ , and the peaks at 782.2 eV and 797.2 eV were attributed to Co 2+ . The remaining four fitting peaks were considered to be satellite peaks of Co 2p [49]. By comparing the areas of the peaks of Co 3+ and Co 2+ 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 Co 2+ 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 Co 2+ : Co 3+ 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  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 2P 3/2 , 2P 1/2 , 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 Co 3 O 4 . 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]. X-ray photoelectron spectroscopy (XPS) was utilized to examine the elementa position and metal valence states of Co3O4, VO-Co3O4, and N/S-VO-Co3O4. As shown XPS survey spectra of Co3O4, VO-Co3O4, and N/S-VO-Co3O4, Figure S9. The charac peaks of N 1s and S 2p were found only in the survey spectrum of N/S-VO-Co3O4 in ing the successful introducing of N and S heteroatoms. For the Co 2p spectra in Fig   Figure 2. 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 Co 3 O 4 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  Figure 3a,b, the HER catalytic activity of N/S-V O -Co 3 O 4 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 V O -Co 3 O 4 (318 mV) and Co 3 O 4 (430 mV), respectively. The Tafel slope of each catalyst was shown in Figure 3c, the slope of N/S-V O -Co 3 O 4 was 79 mV dec −1 , which was much lower than V O -Co 3 O 4 (118 mV dec −1 ) and Co 3 O 4 (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 Cobased 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 (R ct ) 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-V O -Co 3 O 4 possesses the smallest electrochemical impedance (R ct = 4.21 Ω), which is much smaller than that of V O -Co 3 O 4 (R ct = 35.83 Ω) and Co 3 O 4 (R ct = 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-V O -Co 3 O 4 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-V O -Co 3 O 4 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-V O -Co 3 O 4 demonstrates good catalytic stability during HER.
The OER catalytic activity of Co 3 O 4 , V O -Co 3 O 4 , and N/S-V O -Co 3 O 4 was also evaluated and compared. As depicted in Figure 4a,b, the OER activity of N/S-V O -Co 3 O 4 required an overpotential of 294 mV to reach 10 mA cm −2 , which was lower than that of V O -Co 3 O 4 (328 mV) and Co 3 O 4 (405 mV). Additionally, the Tafel slope of each catalyst was shown in Figure 4c; the Tafel slope of N/S-V O -Co 3 O 4 (71 mV dec −1 ) was lower than that of V O -Co 3 O 4 (124 mV dec −1 ) and Co 3 O 4 (170 mV dec −1 ), reflecting the faster OER reaction kinetics of the N/S-V O -Co 3 O 4 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 12 Ω), which is smaller than that of V O -Co 3 O 4 (R ct = 4.68 Ω) and Co 3 O 4 (R ct = 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-V O -Co 3 O 4 were also enhanced.
Molecules 2023, 28,4134 transfer rate could be attributed to the incorporation of oxygen vacancies and in si of N and S heteroatoms. The catalytic stability of N/S-VO-Co3O4 for HER was e through 1000 cycles of CV within a range of −0.3 V to 0.1 V vs. RHE at a scan ra mV s −1 . The comparison of polarization curves before and after the 1000 cycles CV overlap, as shown in Figure S11a. Furthermore, N/S-VO-Co3O4 was subjected to a rability test at −10 mA cm −2 , as depicted in Figure S11b. Notably, the potential of lyst remained relatively steady throughout the test. Collectively, these results indi 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 e and compared. As depicted in Figure 4a,b, the OER activity of N/S-VO-Co3O4 req overpotential of 294 mV to reach 10 mA cm −2 , which was lower than that of VO-Co mV) and Co3O4 (405 mV). Additionally, the Tafel slope of each catalyst was show ure 4c; the Tafel slope of N/S-VO-Co3O4 (71 mV dec −1 ) was lower than that of VO-Co mV dec −1 ) and Co3O4 (170 mV dec −1 ), reflecting the faster OER reaction kinetics of VO-Co3O4 catalyst due to the smaller Tafel slope, indicating it is better than most o based catalysts reported currently, as seen in Table S8 [56,60,[68][69][70][71][72][73][74][75][76]. EIS was a formed to further analyze the kinetic processes in the OER process, at 0.55 V vs. shown in Figure 4d, N/S-VO-Co3O4 has the smallest electrochemical impedance (R Ω), which is smaller than that of VO-Co3O4 (Rct = 4.68 Ω) and Co3O4 (Rct = 6.10 Ω) fitting results are shown in Table S6. This indicates that the OER charge transfer kinetics of N/S-VO-Co3O4 were also enhanced.
Finally, the OER stability of N/S-VO-Co3O4 was also tested using 1000 cycles the range of 1.2 to 1.6 V vs. SCE. The OER polarization curves before and after 100 of CV are shown in Figure S12a, and the LSV curves before and after 1000 cycl Finally, the OER stability of N/S-V O -Co 3 O 4 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-V O -Co 3 O 4 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-V O -Co 3 O 4 also demonstrated good OER catalytic durability.
The prepared catalysts were compared with benchmark noble metal-based catalysts Pt/C and IrO 2 , as illustrated in Figure S13. Despite requiring a higher overpotential than Pt/C to achieve −10 mA cm −2 in HER, N/S-V O -Co 3 O 4 exhibited a smaller Tafel slope, indicating faster HER kinetics than Pt/C. Regarding OER, N/S-V O -Co 3 O 4 exhibited both a lower overpotential required at 10 mA cm −2 and a smaller Tafel slope than IrO 2 , suggesting faster OER kinetics for N/S-V O -Co 3 O 4 than for IrO 2 .
Benefiting from the gratifying bifunctional activity of N/S-V O -Co 3 O 4 in alkaline electrolytes, N/S-V O -Co 3 O 4 exhibits catalytic activity comparable to that of Pt/C || IrO 2 in an alkaline overall water-splitting simulated electrolytic cell. As shown in Figure 5a Molecules 2023, 28,4134 almost overlap. Additionally, N/S-VO-Co3O4 was subjected to a 12-h durability t mA cm −2 , as depicted in Figure S12b; the potential of the catalyst remained r steady throughout the test. In summary, N/S-VO-Co3O4 also demonstrated good O alytic durability. The prepared catalysts were compared with benchmark noble metal-based Pt/C and IrO2, as illustrated in Figure S13. Despite requiring a higher overpoten Pt/C to achieve −10 mA cm −2 in HER, N/S-VO-Co3O4 exhibited a smaller Tafel slo cating faster HER kinetics than Pt/C. Regarding OER, N/S-VO-Co3O4 exhibited lower overpotential required at 10 mA cm −2 and a smaller Tafel slope than IrO2, su faster OER kinetics for N/S-VO-Co3O4 than for IrO2.
Benefiting from the gratifying bifunctional activity of N/S-VO-Co3O4 in alkal trolytes, N/S-VO-Co3O4 exhibits catalytic activity comparable to that of Pt/C || Ir alkaline overall water-splitting simulated electrolytic cell. As shown in Figure  required   To gain further insight into the mechanism underlying the bifunctional catalytic activity of N/S-V O -Co 3 O 4 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-V O -Co 3 O 4 . For comparison, Co 3 O 4 was also measured under the same conditions. As illustrated in Figure 6a,b, the Raman spectra of Co 3 O 4 and N/S-V O -Co 3 O 4 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 Co 3 O 4 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-V O -Co 3 O 4 [41,43]. In contrast, during OER, a new characteristic peak becomes observable in the Raman spectra of N/S-V O -Co 3 O 4 , 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 Co 3 O 4 , as shown in Figure 6c. Previous reports suggest that the high-valent Co 4+ in CoOOH is the truly active phase of Co-based catalysts during OER in alkaline electrolytes; it can be inferred that the transition from Co 3 O 4 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. To gain further insight into the mechanism underlying the bifunctional catalytic tivity of N/S-VO-Co3O4 in electrolytic water splitting, we conducted in situ Raman anal in 1.0 M KOH to dynamically monitor the HER and OER processes of N/S-VO-Co3O4. comparison, Co3O4 was also measured under the same conditions. As illustrated in Fig  6a,b, the Raman spectra of Co3O4 and N/S-VO-Co3O4 did not exhibit significant chan after the applied potential during HER, consistent with the ex situ Raman results in presented in Figure 2b. This observation indicates that the lattice structure of Co3O4 not undergo significant changes during HER. Therefore, it is reasonable to hypothe that the in situ filled N and S heteroatoms served as moderators on the adsorption sorption of the reaction intermediates during HER, which enhanced the HER activit N/S-VO-Co3O4 [41,43]. In contrast, during OER, a new characteristic peak becomes obs able in the Raman spectra of N/S-VO-Co3O4, ranging from 550 to 620 cm −1 , once the app potential reached 1.424 V vs. RHE, as illustrated in Figure 6d. This peak is indicativ the production of CoOOH [77], while this phenomenon was not observed in Co3O4 shown in Figure 6c. Previous reports suggest that the high-valent Co 4+ in CoOOH is truly active phase of Co-based catalysts during OER in alkaline electrolytes; it can be ferred that the transition from Co3O4 to CoOOH is facilitated by the in situ filling of oxy vacancies with N and S heteroatoms, leading to a lower overpotential required for OE Ex situ characterization was also performed on N/S-V O -Co 3 O 4 to analyze its structural changes after the catalytic reaction. Figure S14a shows the XRD results, which indicate that the structure of N/S-V O -Co 3 O 4 did not undergo significant changes before and after the electrocatalytic water splitting. The spinel structure of Co 3 O 4 was still maintained. On the other hand, a slight shift can be observed in the Co 2p spectrum of N/S-V O -Co 3 O 4 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-V O -Co 3 O 4 after the catalytic reaction. When compared with the initial N/S-V O -Co 3 O 4 , 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-V O -Co 3 O 4 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-V O -Co 3 O 4 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-V O -Co 3 O 4 undergoes a structural transformation, leading to the production of more CoOOH. Furthermore, the S element is further oxidized to produce SO 2− 4 during the structural transformation phase.
Molecules 2023, 28, 4134 11 of Ex situ characterization was also performed on N/S-VO-Co3O4 to analyze its structur changes after the catalytic reaction. Figure S14a shows the XRD results, which indica that the structure of N/S-VO-Co3O4 did not undergo significant changes before and aft the electrocatalytic water splitting. The spinel structure of Co3O4 was still maintained. O the other hand, a slight shift can be observed in the Co 2p spectrum of N/S-VO-Co3O4 aft OER, as shown in Figure S14b, indicating the production of more high-valent Co. Th shift was not observed after HER. Figure S14c illustrates the change in the S 2p spectru of N/S-VO-Co3O4 after the catalytic reaction. When compared with the initial N/S-V Co3O4, it was observed that the S species did not undergo significant changes after HE However, after OER, all S species could be observed to shift to the S-O bonded form. Th shift is related to the in situ reconfiguration of N/S-VO-Co3O4 during the OER proces Figure S14d demonstrated that the elemental species of N composition did not under significant changes before and after HER and OER. The N species exists in the form of bonded to metal elements. In summary, it can be concluded that the lattice structure N/S-VO-Co3O4 remains stable during the HER reaction, and the elemental species of N an S do not undergo significant changes. However, during the OER process, N/S-VO-Co3 undergoes a structural transformation, leading to the production of more CoOOH. Fu thermore, the S element is further oxidized to produce SO 2-4 during the structural transfo mation phase.

Conclusions
In summary, we successfully etched the spinel-structured Co3O4 nanosheets via A plasma to create oxygen vacancy-rich nanosheets (VO-Co3O4) and subsequently filled t oxygen vacancies with N and S heteroatoms to form N/S-VO-Co3O4. SEM and TEM cha acterization revealed that the Ar plasma etching generated numerous porous defects o the surface of the Co3O4 nanosheet, which increased the specific surface area of the cataly and promoted adsorption and desorption of reactants and products in electrocatalytic w ter splitting. Furthermore, XRD and Raman analysis demonstrated that the in situ fillin

Conclusions
In summary, we successfully etched the spinel-structured Co 3 O 4 nanosheets via Ar plasma to create oxygen vacancy-rich nanosheets (V O -Co 3 O 4 ) and subsequently filled the oxygen vacancies with N and S heteroatoms to form N/S-V O -Co 3 O 4 . SEM and TEM characterization revealed that the Ar plasma etching generated numerous porous defects on the surface of the Co 3 O 4 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 Co 3  good catalytic stability in long-term overall water splitting compared to Pt/C || IrO 2 . 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 Co 3 O 4 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.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.