Next Article in Journal
Electron Beam Irradiation Modified UiO-66 Supported Pt Catalysts for Low-Temperature Ethyl Acetate Catalytic Degradation
Next Article in Special Issue
Cation Vacancies Anchored Transition Metal Dopants Based on a Few-Layer Ti3C2Tx Catalyst for Enhanced Hydrogen Evolution
Previous Article in Journal
Red- and Black-Phosphorus-Based Materials for Photocatalytic Water Disinfection
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 3
10.3390/catal15030219
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Defect-Rich Co(OH)2 Induced by Carbon Dots for Oxygen Evolution Reaction

by
Xuan Han
1,
Chao Guo
2,*,
Hui Wang
1,
Weijuan Xu
1,
Qinlian Liu
1,
Qingshan Zhao
1,* and
Mingbo Wu
1
1
State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China
2
Department of Information Engineering, Shandong Foreign Trade Vocational College, Qingdao 266100, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(3), 219; https://doi.org/10.3390/catal15030219
Submission received: 26 January 2025 / Revised: 22 February 2025 / Accepted: 24 February 2025 / Published: 26 February 2025
(This article belongs to the Special Issue Novel Electrocatalysts for Boosting Oxygen/Hydrogen Evolution Reactions)
Browse Figures
Versions Notes

Abstract

Hydrogen production from water electrolysis offers a highly promising and sustainable route to solve the energy crisis. However, it is severely limited by the sluggish kinetics of the oxygen evolution reaction (OER) occurring on the anode. Herein, employing carbon dots functionalized with benzene sulfonate groups (BS-CDs) as a distinctive inductor, a Co(OH)2 catalyst featuring abundant defects was synthesized for an enhanced OER. The highly hydrophilic nature of BS-CDs exerts a significant interfacial induction effect on the growth dynamics of Co(OH)2, fostering the formation of elevated crystal defects and a substantial quantity of oxygen vacancies. The resulting BS-CDs/Co(OH)2 catalyst requires an overpotential of only 340 mV to achieve a current density of 10 mA cm−2 in alkaline media, demonstrating markedly improved OER activity compared to pristine Co(OH)2 and N-CDs/Co(OH)2 induced by amine-modified CDs. Furthermore, the structural integrity of the catalyst is maintained, with a current retention rate of 92% observed following a 20 h stability assessment. This work provides a novel approach for developing cost-effective transition metal catalysts that exhibit exceptional catalytic efficiency and excellent stability for the OER.

Graphical Abstract

1. Introduction

The escalating depletion of non-renewable resources like fossil fuels has emerged as a pressing global challenge [1]. The pursuit of clean and sustainable energy sources is pivotal in mitigating future environmental degradation and energy scarcity. Hydrogen generation via water electrolysis has received significant attention for its eco-friendly and renewable nature [2,3]. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are integral to this process [4]. Especially, the high overpotential and complex four-electron-coupled proton transfer mechanism of the OER present major obstacles to the widespread commercialization of water electrolysis [5,6]. Noble metal-based oxides, such as IrO2 and RuO2, have demonstrated effectiveness in enhancing OER efficiency [7,8]. Nonetheless, their scarcity and exorbitant costs severely impede large-scale implementation [9]. Therefore, there is an urgent need to develop cost-effective, durable, and efficient catalysts capable of replacing precious metals.
Non-noble transition metal (TM, e.g., Mn, Fe, Co, and Ni)-based hydroxides and their derivatives show excellent OER catalytic performance in practical applications [10,11,12]. Among them, cobalt hydroxide (Co(OH)2) has the advantages of low cost, simple preparation, easy morphology control, and excellent chemical performance, which makes it one of the most promising OER catalysts to replace noble metals [13,14]. Cobalt-based hydroxides are powdery and generally synthesized by the co-precipitation of cobalt salts and bases in aqueous solution [15,16]. The hydrothermal method is the most commonly used approach for the preparation of Co(OH)2. For example, Lv et al. prepared N-doped Co(OH)F nanowire arrays on conductive carbon fiber paper via the hydrothermal method, and the potential could reach 1.540 V at 10 mA cm−2 with a Tafel slope of 69.75 mV dec−1 in 1 M KOH solution [17]. Liu et al. synthesized three-dimensional α-Co(OH)2 hollow nanoflowers (α-Co(OH)2/HNF), assembled using ultrathin nanosheets at room temperature, using Cu2O nanoballs as templates [18]. Merely 310 mV of overpotential and a 68.9 mV dec−1 Tafel slope are needed to achieve a current density of 10 mA cm−2 in 1 M KOH. However, the powdery electrocatalyst is prone to aggregation and detachment during the reaction process, causing magnetic electron attraction and subsequent electron loss [19,20,21]. Co(OH)2 still faces problems such as poor conductivity and insufficient active sites, underscoring the importance of developing highly efficient Co(OH)2-based catalysts [22,23].
The graphite frameworks of carbon dots (CDs) have unique photoelectrochemical properties that render them exceptional electron donors or electron acceptors [24,25,26,27]. The electron transport behavior and effective surface area of composite materials based on CDs could be effectively regulated and ultimately improve catalytic performance [28,29,30,31]. For instance, Tian et al. successfully combined CDs with MnO2 nanoflowers, which effectively enhanced electrocatalytic activity in an alkaline medium [32]. In view of this fact, it can be envisioned that the charge transfer ability and the exposure of active sites are expected to be concurrently enhanced upon coupling CDs with transition metal hydroxides [33,34,35]. However, rare reports focus on the integration of CDs with Co(OH)2 and the mechanism by which the varying surface properties of CDs influence electrochemical performance. Consequently, the use of CDs to regulate and improve the electrocatalytic performance of Co(OH)2 holds significant importance and promising applications for the advancement of efficient OER catalysts.
Herein, a defect-rich Co(OH)2 electrocatalyst was fabricated as a distinctive inductor by employing carbon dots functionalized with benzene sulfonate groups (BS-CDs) to enhance the OER through a facile in situ hydrothermal strategy. To elucidate the regulatory mechanisms, a comparison analysis was conducted between the BS-CDs/Co(OH)2 catalyst induced by BS-CDs with strong hydrophilicity and N-CDs/Co(OH)2 induced by amine-modified CDs (N-CDs). The influence of the hydrophilicity of CDs on Co(OH)2 morphology and the electrocatalytic performance of the OER were investigated. The highly hydrophilic characteristics of BS-CDs facilitate a strong interaction with the Co2+ precursor, promoting the formation of an amorphous structure, elevating the defect density, and generating a substantial quantity of oxygen vacancies. Consequently, the incorporation of BS-CDs significantly improves the electrocatalytic performance, achieving an overpotential of 340 mV at 10 mA cm−2, superior to the pristine Co(OH)2 and N-CDs/Co(OH)2. Moreover, the favorable combination between BS-CDs and Co(OH)2 contributes to excellent stability during the OER process, with a retention rate of 92% observed after a 20 h testing period.

2. Results and Discussion

As depicted in Figure 1, the synthesis of BS-CDs functionalized with benzene sulfonate groups was accomplished via an electrochemical exfoliation methodology. In this procedure, low-cost petroleum coke was designated as the electrode material. BS-CDs were generated under an applied voltage of 40 V during the electrolysis process. Subsequently, suction filtration followed by dialysis was sequentially implemented to purify the resultant products. Notably, sodium sulfanilate was chosen as the electrolyte, which possesses the ability to chemically react with the newly formed carboxyl groups on the surfaces of the BS-CDs. This reaction results in the formation of an amido linkage, facilitating the in situ covalent modification of a substantial number of benzene sulfonate groups on the surfaces of the CDs. In the context of the control group experiments, aqueous ammonia was employed as the electrolyte for the preparation of amine-modified CDs (N-CDs).
The morphology and particle size of the CDs were characterized by TEM and AFM. As evidenced by the TEM images (Figure 2a,d), both types of CDs exhibit a spherical shape, with BS-CDs exhibiting a more uniform dispersion. The average particle size distribution (Figure 2a,d, inset) for N-CDs and BS-CDs are measured at 2.61 ± 0.09 nm and 2.54 ± 0.04 nm. The high-resolution TEM (HRTEM) images (Figure 2b,e) clearly reveal the lattice fringes of the CD samples, with a lattice spacing of 0.21 nm corresponding to the (100) crystal plane of graphite. These observations are further corroborated by the AFM images presented in Figures S1 and S2. Comparative analyses of the CDs and raw petroleum coke using XRD (Figure S3) and Raman spectra (Figure S4) provide additional validation for the successful synthesis of N-CDs and BS-CDs. To elucidate the surface functional groups present in N-CDs and BS-CDs, XPS characterization (Figure S5) and FTIR analysis (Figure S6) were carried out, revealing the presence of amine groups and benzene sulfonate groups on the surfaces of the N-CDs and BS-CDs, respectively. It is worth mentioning that a variety of oxygen-containing functional groups can also be observed, attributed to the oxidation effect of the electrochemical exfoliation process. The contact angles of the N-CDs and BS-CDs (Figure 2c,f) are determined to be 49° and 16°, respectively, indicating the high hydrophilicity of BS-CDs, which can be ascribed to the presence of hydrophilic benzene sulfonate groups.
The structure and morphology of the catalysts were characterized by TEM and HRTEM analysis. The pristine Co(OH)2 reveals a nanowire-like morphology in the absence of CDs (Figure 3a). Upon the incorporation of N-CDs, a block-like morphology with a close aggregation of nanowires is observed (Figure 3b). After the addition of BS-CDs, BS-CDs/Co(OH)2 nanoparticles with heterogeneous sizes emerge (Figure 3c). The morphological characteristics observed in the TEM images align with the SEM results presented in Figure S7. The HRTEM images distinctly illustrate the successful integration of Co(OH)2 with the CDs. According to the standard card for Co(OH)2 (JCPDS: 38-0547), the lattice fringe of 0.53 nm corresponds to the XRD diffraction peak observed at 17.1° (Figure 3d). In the HRTEM images of N-CDs/Co(OH)2 (Figure 3e) and BS-CDs/Co(OH)2 (Figure 3f), lattice spacings of 0.245, 0.246, 0.251, 0.270, and 0.264 nm correspond to diffraction angles of 36.1°, 35.9°, 35.0°, 33.5°, and 33.4°. In addition, the inter-planar distance of 0.21 nm is ascribed to the (100) plane of N-CDs and BS-CDs [36]. The existence of diffuse reflection rings in the Fast Fourier Transform (FFT) images (Figure 3e,f, inset) indicates that specific regions within the material lack long-range order. This observation suggests that the composite material exhibits a shorter-range ordered structure and reduced grain size, ultimately leading to a decrease in crystallinity. Elemental distribution analysis via the energy-dispersive X-ray (EDX) mapping of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 (Figure 3g–i) reveals an even distribution of elements throughout the samples.
A comprehensive series of characterization measurements were undertaken to thoroughly investigate the interactions between Co(OH)2 and BS-CDs. As depicted in Figure 4a, Raman tests reveal the presence of the typical D band and G band in the spectra of N-CDs/Co(OH)2 and BS-CDs/Co(OH)2, indicating the existence of graphite carbon and further confirming the successful introduction of CDs. As can be seen in the XRD patterns (Figure 4b), Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 show respective diffraction peaks corresponding to the standard card (JCPDS: 38-0547) of Co(OH)2 [37]. It can be seen that the peak for Co(OH)2 is sharp, indicating its good crystallinity. In contrast, the diffraction peak intensities of N-CDs/Co(OH)2 and BS-CDs/Co(OH)2 are significantly weakened, indicating that N-CDs and BS-CDs have an inhibitory effect on the growth of Co(OH)2 crystal, in accordance with the TEM analysis results. The FT-IR spectra of the samples are displayed in Figure 4c. The FT-IR curves for N-CDs/Co(OH)2 and BS-CDs/Co(OH)2 closely resemble that of Co(OH)2, with the emergence of two new peaks at 1603 and 1187 cm−1, which are assigned to the vibration of N-H and S=O, respectively. The N-H peak in N-CDs/Co(OH)2 primarily originates from the amino groups present on the surface of N-CDs, while the N-H and S=O peaks in BS-CDs/Co(OH)2 are primarily derived from the sodium sulfonate groups inherent in BS-CDs. Moreover, the relative concentration of oxygen vacancies was assessed using electron paramagnetic resonance (EPR) spectroscopy. As illustrated in Figure 4d, compared to pristine Co(OH)2 and N-CDs/Co(OH)2, the BS-CDs/Co(OH)2 sample exhibits the most pronounced signal at g = 2.004, indicative of trapped electrons associated with oxygen vacancies [38]. Along with the TEM and XRD analyses, these results provide sufficient evidence for the defective characteristics of BS-CDs/Co(OH)2. The rich defects should be attributed to the intercalation of nanosized BS-CDs, inducing the increased density of crystal defects and a substantial quantity of oxygen vacancies, ultimately enhancing catalytic performance.
XPS was performed to further investigate the chemical environment of the catalysts, as well as the coupling effect between the CDs and Co(OH)2. As displayed in Figure 5a, Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 predominantly consist of four elements: C, N, O, and Co. The N content in N-CDs/Co(OH)2 and BS-CDs/Co(OH)2 is significantly higher than that in Co(OH)2, primarily due to the introduction of N in amino-modified N-CDs and benzene sulfonate-modified BS-CDs. Figure 5b shows the peak of C in N-CDs/Co(OH)2 and BS-CDs/Co(OH)2. The predominant forms of C in both samples include C-C/C=C (284.8 eV), C-N (286.4 eV), C=O (288.3 eV), and N-C=O (289.4 eV). The two peaks at 780.8 eV and 796.7 eV in the Co 2p spectrum (Figure 5c) correspond to Co2+ 2p3/2 and Co2+ 2p1/2 of Co2+ in Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2. The energy separation of 16 eV between the Co 2p3/2 and Co 2p1/2 peaks further confirms the presence of Co2+ [39]. In comparison to pure Co(OH)2, the Co peaks in BS-CDs/Co(OH)2 and N-CDs/Co(OH)2 exhibit shifts, indicating enhanced interactions between BS-CDs/N-CDs and Co(OH)2. As shown in the high-resolution O 1s XPS spectra (Figure 5d), oxygen in the three samples exists in the forms of Co-O (531.0 eV), O-H (531.8 eV), and C=O (533.2 eV), primarily originating from Co(OH)2 and various oxygen-containing functional groups on the surface of the CDs. When BS-CDs were added, the Co-O peaks of BS-CDs/Co(OH)2 weakened compared to N-CDs/Co(OH)2 and Co(OH)2. This phenomenon indicates that the lattice oxygen in BS-CDs/Co(OH)2 has decreased, leading to the formation of more oxygen vacancies, which is consistent with the EPR test results.
To explore the induction and regulation effects of CDs on the catalytic efficiency of Co(OH)2, the OER performance test was carried out on the materials. Figure 6a shows the OER polarization curves of the samples. The linear sweep voltammetry (LSV) curves suggest that the obtained BS-CDs/Co(OH)2 catalyst displays superior OER activity compared to both N-CDs/Co(OH)2 and pure Co(OH)2. At a current density of 10 mA cm−2, the overpotential of BS-CDs/Co(OH)2 is 340 mV, which is comparable to that of RuO2. This value is significantly lower than the overpotentials observed for N-CDs/Co(OH)2 (370 mV), Co(OH)2 (570 mV), N-CDs (630 mV), and BS-CDs (730 mV). The Tafel plot depicted in Figure 6b is obtained by processing the LSV data of the materials. Notably, the Tafel slopes for BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2 are measured at 113, 123, and 216 mV dec−1, respectively, among which BS-CDs/Co(OH)2 exhibits the lowest Tafel slope, revealing the fastest OER reaction kinetics of the BS-CDs/Co(OH)2 catalyst. In comparison to Co(OH)2, the Tafel slopes for both BS-CDs/Co(OH)2 and N-CDs/Co(OH)2 demonstrate significant reductions, suggesting that the incorporation of BS-CDs and N-CDs can efficiently enhance the OER reaction rate. To facilitate a clearer comparison of the performance among BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2, the overpotentials and Tafel slopes for each were plotted in a consolidated bar chart in Figure 6c. It is evident that BS-CDs/Co(OH)2 possesses the lowest overpotential and Tafel slope, exhibiting the most favorable OER catalytic performance. Compared to various reported Co-based electrocatalysts, BS-CDs/Co(OH)2 demonstrates exceptional catalytic performance with a relatively low overpotential (Table S1).
To elucidate the role of CDs in enhancing the conductivity of the materials, electrochemical impedance spectroscopy (EIS) tests were conducted on BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2, and the test results are shown in Figure 6d (impedance diagram after fitting). The EIS parameters of all the electrocatalysts can be found in Table S2. Remarkably, the electrochemical impedances of BS-CDs/Co(OH)2 and N-CDs/Co(OH)2 are significantly lower than that of Co(OH)2, indicating that the incorporation of CDs into Co(OH)2 substantially improves the charge transfer capability of the materials. Among the samples, BS-CDs/Co(OH)2 exhibited significantly reduced charge transfer impedance and the highest electron conductivity. According to the previous analysis, BS-CDs possess a higher degree of graphitization than N-CDs, facilitating faster charge transfer. Moreover, the interaction between BS-CDs and Co(OH)2 is primarily governed by electrostatic forces, which leads to stronger electron interactions compared to the coordination-based interactions observed between N-CDs and Co(OH)2. Consequently, the enhancement of conductivity in Co(OH)2 due to the incorporation of BS-CDs is more pronounced.
Figure 7 illustrates the cyclic voltammetry (CV) curves of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 scanned at different scanning rates (20, 40, 60, 80, 100, and 120 mV s−1) within the non-Faraday potential range (0.9–1.0 V vs. RHE). The charge–discharge current density differences of each sample at 0.95 V voltage under these scanning rates were calculated, and a linear fitting of half the current density difference against the scanning rate was performed, resulting in the linear relationship depicted in Figure 7d. The slope of the line corresponds to the double-layer capacitance (Cdl) value. As can be seen, the electrochemical surface area (ECSA) and the double-layer capacitance (Cdl) are linear, and a larger Cdl value signifies a greater ECSA [40]. Accordingly, the Cdl values of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 are determined to be 9.22, 23.34, and 28.85 mF cm−2, respectively. BS-CDs/Co(OH)2 exhibits the highest Cdl, thereby indicating that it possesses the largest electrochemical active surface area.
The performance of catalysts not only depends on their catalytic activity, but also on their long-term stability. Therefore, in order to explore the long-term stability of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2, both the timing current method test and cyclic CV test were conducted. Figure 8a–c illustrates the variations in current density over time for Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 at a constant voltage of 1.75 V vs. RHE. After 20 h of testing, the current retention rate of BS-CDs/Co(OH)2 reaches an impressive 92%, while N-CDs/Co(OH)2 exhibits a retention rate of 68%, and the pristine Co(OH)2 shows only a 34% retention rate. This indicates that the current density reduction for BS-CDs/Co(OH)2 is negligible after prolonged durability testing, signifying its excellent long-term stability. The lattice oxygen species on the catalyst surface may participate in the OER process through diverse oxygen release pathways, leading to performance degradation. The attenuated participation of lattice oxygen has been identified as a critical determinant of operational stability in OER catalysts. As evidenced by EPR and XRD analyses, the BS-CDs/Co(OH)2 composite exhibits the highest oxygen vacancy concentration and compromised crystallinity, effectively suppressing the involvement of lattice oxygen during OER operation. This unique structural characteristic consequently endows BS-CDs/Co(OH)2 with optimal stability compared to the other specimens. These findings were further corroborated by cyclic CV tests. Figure 8d–f presents the LSV curves of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2 before and after 3000 CV cycles. The corresponding overpotential increases at a current density of 10 mA cm−2 were found to be 173, 67, and 22 mV, respectively. It is evident that BS-CDs/Co(OH)2 demonstrates the smallest overpotential change, indicating the most stable performance among the tested samples. The inherent stability of pure Co(OH)2 is poor, highlighting the significant role that the incorporation of CDs plays in enhancing the stability of the material.
The aforementioned results unequivocally demonstrate that the incorporation of BS-CDs significantly improves the electrocatalytic OER performance of Co(OH)2, with effects markedly superior to those observed with N-CDs. Considering the diverse physical and chemical properties demonstrated, this improvement can be attributed to the highly hydrophilic nature and pronounced negative charge of BS-CDs, which facilitate electrostatic attraction with Co2+ ions, thereby fostering a strong association between the two entities. This interaction effectively occupies the positions of OH− ions, generating additional crystal defects and oxygen vacancies. While N-CDs also possess a certain degree of negative charge, the presence of -NH2 groups allows Co2+ ions to coordinate with these functional groups during the hydrothermal process. However, this coordination does not interfere with the binding of Co2+ to OH− ions, implying that the introduction of N-CDs does not alter the existing oxygen vacancies. This distinction elucidates the differences between the two types of CDs in influencing material morphology. To sum up, the highly hydrophilic nature and strong negative charge of BS-CDs empower them to replace OH− ions during the hydrothermal synthesis process, leading to an increased density of defects and the generation of a substantial quantity of oxygen vacancies, ultimately enhancing the catalytic performance of the OER.

3. Materials and Methods

3.1. Chemicals and Reagents

Calcined petroleum coke and petroleum asphalt were obtained from China National Offshore Oil Corporation. Ammonium hydroxide (25.0–28.0 wt%), urea (99%, AR), potassium hydroxide (99%, AR), and anhydrous ethanol (99%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium p-aminobenzene sulfonate (97 wt%), dichloromethane (99.5%, AR), and cobalt chloride hexahydrate (99%, AR) were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). Highly purified water was generated using a Millipore system (UPD-I-10T, Chengdu, China).

3.2. Preparation of Catalysts

3.2.1. Synthesis of BS-CDs and N-CDs

An amount of 12.0 g of petroleum coke was added to a mixture of 50 mL of dichloromethane and 8.0 g of petroleum pitch to form a paste. The paste was pressed into a discoid precursor via hot-compaction treatment after evaporating most of the dichloromethane. A disk electrode was obtained after calcinating the sample in a tube furnace under N2 at 800 °C for 2 h. Then, two electrodes were inserted into 100 mL of 2 wt% sodium sulfanilate solution and used as a counter electrode and a working electrode, respectively. A static potential of 40 V was applied to the system using direct current power. After 3 h of electrochemical oxidation, the anode disk electrode corroded completely. The mixture was subjected to membrane filtration to eliminate the larger fractions. Finally, the BS-CD powder could be collected through freeze-drying. The N-CD congener was prepared through the same procedure, replacing sodium sulfanilate with 5 wt% ammonia solution.

3.2.2. Synthesis of BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2

Amounts of 3 mmol of CoCl2 6H2O, 3 mmol of urea, and 100 mg of BS-CDs were dissolved in 20 mL of deionized water (DI) under continuous vigorous stirring to form a solution. Subsequently, the above solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 100 °C for 12 h. Finally, the obtained samples were washed three times each with water and ethanol and dried at 60 °C for 12 h to obtain BS-CDs/Co(OH)2. Similarly, BS-CDs were substituted with N-CDs to prepare N-CDs/Co(OH)2. Additionally, the aforementioned process was conducted without the inclusion of CDs to produce Co(OH)2.

3.3. Material Characterization

Scanning electron microscope (SEM) images were taken with a MERLIN scanning electron microscope (ZEISS, Oberkochen, Germany) at 20 kV, and transmission electron microscope (TEM) images were recorded with a JEM2100F scanning transmission electron microscope (JEOL, Tokyo, Japan) at 200 kV. The morphology and particle size of the CDs were measured by a Multimode8 SPM atomic force microscopy (AFM, Bruker, Billerica, MA, USA). The X-ray diffraction (XRD) patterns of the resultant materials were acquired on a X’Pert3 X-ray Diffractometer (PANalytical, Almelo, The Netherlands). Raman spectra were recorded with a Renishaw Raman system model 2000 spectrometer (Renishaw, London, UK). X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250XI X-ray photoelectron spectrometer (Thermo Fisher, Waltham, MA, USA). All self-standing electrodes were characterized using the resultant materials directly.

3.4. Electrochemical Measurements

All electrochemical tests were conducted using a Shanghai Chenhua CHI 760E electrochemical workstation (Shanghai, China) and a three-electrode system. The electrolyte comprised 1.0 M of KOH solution, the counter electrode was a Pt electrode, and the reference electrode was a Ag/AgCl electrode. An amount of 2 mg of the sample was weighed and dispersed in 5 μL of Nafion and 800 μL of ethanol. Then, 5 μL of the suspension was dropped on a glassy carbon electrode 3 times. After drying, this was used as the working electrode. All current densities were normalized by dividing the measured current by the geometric area of the glassy carbon electrode.
The linear sweep voltammetry test was conducted at a scan rate of 5 mV s−1 in a saturated O2 environment, and the voltage conversion relative to the standard hydrogen electrode (RHE) was ascertained using the Nernst equation:
E(RHE) = E(Ag/AgCl) + 0.197 V + 0.059 × pH
To determine the electrochemical double-layer capacitance (Cdl), the cyclic voltammetry (CV) curve within the non-Faraday potential window (1.02–1.22 V vs. RHE) was measured at different scan rates of 20–120 mV s−1. After data processing, the linear relationship between the charge–discharge current density difference at 1.12 V vs. RHE and the scan rate was obtained, and the linear slope was Cdl. The cyclic stability was tested using a high-sweep CV method at a scan rate of 100 mV s−1, with a total of 3000 cycles conducted. The timing current method and the fixed voltage value test, considering current in relation to time (20 h) curve and the current retention rate, were also used to judge the long-term stability of the samples.

4. Conclusions

In summary, by employing benzene sulfonate group-functionalized BS-CDs as a distinctive inductor, we successfully prepared a defect-rich BS-CDs/Co(OH)2 catalyst through a straightforward in situ hydrothermal strategy. The BS-CDs exhibit a highly hydrophilic character and a pronounced negative charge, which significantly influences the interfacial dynamics of Co(OH)2 growth, promoting the emergence of enhanced crystal defects and oxygen vacancies. As a result, the synthesized BS-CDs/Co(OH)2 catalyst exhibits remarkable OER activity, requiring only a 340 mV overpotential to achieve 10 mA cm−2 under alkaline conditions, significantly outperforming both pristine Co(OH)2 and amine-modified N-CDs/Co(OH)2. Furthermore, the catalyst’s structural stability is evidenced by a retention rate of 92% over a 20 h period, underscoring its durability for practical applications. This work not only underscores the mechanism and potential of CDs in catalyst design, but also contributes to the development of cost-effective and efficient transition metal catalysts for sustainable energy solutions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15030219/s1, Figure S1: (a) AFM 2D and (b) 3D images of N-CDs. (c) The width and height profiles along the line in the 2D image. Figure S2: (a) AFM 2D and (b) 3D images of BS-CDs. (c) The width and height profiles along the line in the 2D image. Figure S3: XRD patterns of petroleum coke, N-CDs, and BS-CDs. Figure S4: Raman spectra of petroleum coke, N-CDs, and BS-CDs. Figure S5: (a) XPS survey spectra of N-CDs and BS-CDs, and high-resolution XPS spectra of (b) C 1s, (c) O 1s, and (d) N 1s. Figure S6: FTIR spectra of N-CDs and BS-CDs. Figure S7: SEM images of (a) Co(OH)2, (b) N-CDs/Co(OH)2 and (c) BS-CDs/Co(OH)2. Table S1: Comparison of the electrocatalytic OER performance of BS-CDs/Co(OH)2 with other Co-based catalysts reported in alkaline solutions. Table S2: EIS parameters of the samples. Refs. [41,42,43,44,45,46,47,48].

Author Contributions

X.H.: formal analysis, data curation, writing—original draft preparation, visualization. C.G.: conceptualization, resources, supervision. H.W.: methodology, validation, formal analysis, data curation. W.X.: data curation, writing—original draft preparation. Q.L.: investigation, writing—original draft preparation, visualization. Q.Z.: resources, writing—original draft preparation, visualization, supervision, project administration. M.W.: supervision, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 22208375), China; the National Key R&D Program of China (No. 2019YFA0708700), China; the Fundamental Research Funds for the Central Universities (24CX02025A), China; the Key Technology Research and Industrialization Demonstration Projects in Qingdao City (24-1-4-xxgg-6-gx), China.

Data Availability Statement

The original contributions presented in this study are included in the article and Supplementary Materials; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Waidelich, P.; Batibeniz, F.; Rising, J.; Kikstra, J.S.; Seneviratne, S.I. Climate damage projections beyond annual temperature. Nat. Clim. Change 2024, 14, 592. [Google Scholar] [CrossRef] [PubMed]
  2. Dresp, S.; Dionigi, F.; Klingenhof, M.; Strasser, P. Direct Electrolytic Splitting of Seawater: Opportunities and Challenges. ACS Energy Lett. 2019, 4, 933–942. [Google Scholar] [CrossRef]
  3. Tang, J.; Guo, K.; Guan, D.; Hao, Y.; Shao, Z. A semi-vapor electrolysis technology for hydrogen generation from wide water resources. Energy Environ. Sci. 2024, 17, 7394–7402. [Google Scholar] [CrossRef]
  4. Wang, P.; Zheng, J.; Xu, X.; Zhang, Y.-Q.; Shi, Q.-F.; Wan, Y.; Ramakrishna, S.; Zhang, J.; Zhu, L.; Yokoshima, T.; et al. Unlocking Efficient Hydrogen Production: Nucleophilic Oxidation Reactions Coupled with Water Splitting. Adv. Mater. 2024, 36, 2404806. [Google Scholar] [CrossRef] [PubMed]
  5. Ren, X.; Zhai, Y.; Yang, N.; Wang, B.; Liu, S. Lattice Oxygen Redox Mechanisms in the Alkaline Oxygen Evolution Reaction. Adv. Funct. Mater. 2024, 34, 2401610. [Google Scholar] [CrossRef]
  6. Wang, X.; Zhong, H.; Xi, S.; Lee, W.S.V.; Xue, J. Understanding of Oxygen Redox in the Oxygen Evolution Reaction. Adv. Mater. 2022, 34, 2107956. [Google Scholar] [CrossRef]
  7. Huang, H.; Kim, H.; Lee, A.; Kim, S.; Lim, W.-G.; Park, C.-Y.; Kim, S.; Kim, S.-K.; Lee, J. Structure engineering defective and mass transfer-enhanced RuO2 nanosheets for proton exchange membrane water electrolyzer. Nano Energy 2021, 88, 106276. [Google Scholar] [CrossRef]
  8. Tang, J.; Zhong, Y.; Su, C.; Shao, Z. Silver Compositing Boosts Water Electrolysis Activity and Durability of RuO2 in a Proton-Exchange-Membrane Water Electrolyzer. Small Sci. 2023, 3, 2300055. [Google Scholar] [CrossRef]
  9. Gao, G.; Sun, Z.; Chen, X.; Zhu, G.; Sun, B.; Liu, S.; Yamauchi, Y. Recent advances in Ru/Ir-based electrocatalysts for acidic oxygen evolution reaction. Appl. Catal. B Environ. Energy 2023, 343, 123584. [Google Scholar] [CrossRef]
  10. Wang, Y.; Li, L.; Shi, J.; Xie, M.-Y.; Nie, J.; Huang, G.-F.; Li, B.; Hu, W.; Pan, A.; Huang, W.-Q. Oxygen Defect Engineering Promotes Synergy Between Adsorbate Evolution and Single Lattice Oxygen Mechanisms of OER in Transition Metal-Based (oxy)Hydroxide. Adv. Sci. 2023, 10, 2303321. [Google Scholar] [CrossRef]
  11. Zhang, L.; Fan, F.; Song, X.; Cai, W.; Ren, J.; Yang, H.; Bao, N. A novel septenary high-entropy (oxy)hydroxide electrocatalyst for boosted oxygen evolution reaction. J. Mater. 2023, 10, 348–354. [Google Scholar] [CrossRef]
  12. Zuo, S.; Wu, Z.; Zhang, G.; Chen, C.; Ren, Y.; Liu, Q.; Zheng, L.; Zhang, J.; Han, Y.; Zhang, H. Correlating Structural Disorder in Metal (Oxy)hydroxides and Catalytic Activity in Electrocatalytic Oxygen Evolution. Angew. Chem. Int. Ed. 2023, 63, e202316762. [Google Scholar] [CrossRef] [PubMed]
  13. Tong, Y.; Chen, P.; Zhou, T.; Xu, K.; Chu, W.; Wu, C.; Xie, Y. A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,N-Decorated Graphene. Angew. Chem. Int. Ed. 2017, 56, 7121–7125. [Google Scholar] [CrossRef] [PubMed]
  14. Xiao, Z.; Wang, Y.; Huang, Y.-C.; Wei, Z.; Dong, C.-L.; Ma, J.; Shen, S.; Li, Y.; Wang, S. Filling the oxygen vacancies in Co3O4 with phosphorus: An ultra-efficient electrocatalyst for overall water splitting. Energy Environ. Sci. 2017, 10, 2563–2569. [Google Scholar] [CrossRef]
  15. An, J.; Choi, H.; Lee, K.; Kwon, K.-Y. Cobalt-Doped Iron Phosphate Crystal on Stainless Steel Mesh for Corrosion-Resistant Oxygen Evolution Catalyst. Catalysts 2022, 12, 1521. [Google Scholar] [CrossRef]
  16. Yan, Y.; Chen, Y.; Shao, M.; Chen, X.; Yang, Z.; Wang, J.; Chen, H.; Ni, L.; Diao, G.; Chen, M. Construction of Ultrathin Cobalt-Based Nanosheet Arrays on Titanium Mesh as Asymmetric Electrodes for Overall Water Splitting. ACS Sustain. Chem. Eng. 2023, 11, 2499–2510. [Google Scholar] [CrossRef]
  17. Lv, J.; Yang, X.; Zang, H.-Y.; Wang, Y.-H.; Li, Y.-G. Ultralong needle-like N-doped Co(OH)F on carbon fiber paper with abun-dant oxygen vacancies as an efficient oxygen evolution reaction catalyst. Mater. Chem. Front. 2018, 2, 2045–2053. [Google Scholar] [CrossRef]
  18. Liu, H.; Guo, D.; Zhang, W.; Cao, R. Co(OH)2 hollow nanoflowers as highly efficient electrocatalysts for oxygen evolution reaction. J. Mater. Res. 2017, 33, 568–580. [Google Scholar] [CrossRef]
  19. Gong, C.; Zhao, L.; Li, D.; He, X.; Chen, H.; Du, X.; Wang, D.; Fang, W.; Zeng, X.; Li, W. In-situ interfacial engineering of Co(OH)2/Fe7Se8 nanosheets to boost electrocatalytic water splitting. Chem. Eng. J. 2023, 466, 143124. [Google Scholar] [CrossRef]
  20. Zhi, Y.; Li, Z.; Tang, Y.; Peng, J.; Zhang, Z.; Li, C.; Bao, R.; Chen, G.; Yi, J.; Chen, J.; et al. “Electron-reservoir” CeO2 layer on S-Co(OH)2 to stabilize lattice oxygen for boosting oxygen evolution reaction at large current density. Nano Energy 2024, 134, 110565. [Google Scholar] [CrossRef]
  21. Yu, X.; Chen, Y.; Wu, Y.; Chu, X.; Liu, B.; Hu, B.; Che, G.; Jiang, W.; Liu, C. Self-reconstructed ZIF-L/Co(OH)2 heterointerface modulates electronic structure of Ru and boosts electrocatalytic hydrogen evolution. Chem. Eng. J. 2024, 490, 151926. [Google Scholar] [CrossRef]
  22. Kim, D.-H.; Lee, Y.-K. Understanding highly active and durable Fe-decorated Co(OH)2 catalysts in alkaline oxygen evolution reaction by in situ XANES studies. Chem. Eng. J. 2024, 490, 151701. [Google Scholar] [CrossRef]
  23. Pei, M.-J.; Shuai, Y.-K.; Gao, X.; Chen, J.-C.; Liu, Y.; Yan, W.; Zhang, J. Ni and Co Active Site Transition and Competition in Fluorine-Doped NiCo(OH)2 LDH Electrocatalysts for Oxygen Evolution Reaction. Small 2024, 20, 2400139. [Google Scholar] [CrossRef] [PubMed]
  24. Hong, Q.; Wang, Y.; Wang, R.; Chen, Z.; Yang, H.; Yu, K.; Liu, Y.; Huang, H.; Kang, Z.; Menezes, P.W. In Situ Coupling of Carbon Dots with Co-ZIF Nanoarrays Enabling Highly Efficient Oxygen Evolution Electrocatalysis. Small 2023, 19, 2206723. [Google Scholar] [CrossRef] [PubMed]
  25. Jiang, Y.; Yu, J.; Song, H.; Du, L.; Sun, W.; Cui, Y.; Su, Y.; Sun, M.; Yin, G.; Lu, S. Enhanced water-splitting performance: Interface-engineered tri-metal phosphides with carbon dots modification. Carbon Energy 2024, 6, e631. [Google Scholar] [CrossRef]
  26. Wu, L.; Qin, H.; Ji, Z.; Zhou, H.; Shen, X.; Zhu, G.; Yuan, A. Nitrogen-Doped Carbon Dots Modified Fe–Co Sulfide Nanosheets as High-Efficiency Electrocatalysts toward Oxygen Evolution Reaction. Small 2023, 20, 2305965. [Google Scholar] [CrossRef]
  27. Xia, C.; Zhu, S.; Feng, T.; Yang, M.; Yang, B. Evolution and Synthesis of Carbon Dots: From Carbon Dots to Carbonized Polymer Dots. Adv. Sci. 2019, 6, 1901316. [Google Scholar] [CrossRef]
  28. Fani Rahayu Hidayah, R.; Didik, A.; Lazar, B.; Arturo, S.-A.; Francisco, R.-Z.; Ferry Anggoro Ardy, N.; Vivi, F. The Role of Solvent in Carbon Quantum Dot Synthesis on the Performance of MoS2 Nanosheet/Carbon Quantum Dot Heterostructures as Electrocatalysts for the Hydrogen Evolution Reaction. ACS Appl. Nano Mater. 2025, 8, 1479–1489. [Google Scholar] [CrossRef]
  29. Ma, M.; Zhu, W.; Liao, F.; Yin, K.; Huang, H.; Feng, K.; Gao, D.; Chen, J.; Li, Z.; Zhong, J.; et al. Sulfonated carbon dots modified IrO2 nanosheet as durable and high-efficient electrocatalyst for boosting acidic oxygen evolution reaction. Nano Res. 2024, 17, 8017–8024. [Google Scholar] [CrossRef]
  30. Han, M.; Zhu, S.; Lu, S.; Song, Y.; Feng, T.; Tao, S.; Liu, J.; Yang, B. Recent progress on the photocatalysis of carbon dots: Classification, mechanism and applications. Nano Today 2018, 19, 201–218. [Google Scholar] [CrossRef]
  31. Zulfajri, M.; Abdelhamid, H.N.; Sudewi, S.; Dayalan, S.; Rasool, A.; Habib, A.; Huang, G.G. Plant Part-Derived Carbon Dots for Biosensing. Biosensors 2020, 10, 68. [Google Scholar] [CrossRef] [PubMed]
  32. Tian, L.; Wang, J.; Wang, K.; Wo, H.; Wang, X.; Zhuang, W.; Li, T.; Du, X. Carbon-quantum-dots-embedded MnO2 nanoflower as an efficient electrocatalyst for oxygen evolution in alkaline media. Carbon 2019, 143, 457–466. [Google Scholar] [CrossRef]
  33. Jin, B.; Wang, Q.; Sainio, J.; Saveleva, V.A.; Jiang, H.; Shi, J.; Ali, B.; Kallio, A.-J.; Huotari, S.; Sundholm, D.; et al. Amorphous carbon modulated-quantum dots NiO for efficient oxygen evolution in anion exchange membrane water electrolyzer. Appl. Catal. B Environ. Energy 2024, 358, 124437. [Google Scholar] [CrossRef]
  34. Ni, Q.; Zhang, S.; Wang, K.; Guo, H.; Zhang, J.; Wu, M.; Wang, L. Carbon quantum dot-mediated binary metal–organic framework nanosheets for efficient oxygen evolution at ampere-level current densities in proton exchange mem-brane electrolyzers. J. Mater. Chem. A 2024, 12, 31253–31261. [Google Scholar] [CrossRef]
  35. Wang, B.; Song, H.; Qu, X.; Chang, J.; Yang, B.; Lu, S. Carbon dots as a new class of nanomedicines: Opportunities and challenges. Coord. Chem. Rev. 2021, 442, 214010. [Google Scholar] [CrossRef]
  36. Feng, X.; Shi, Y.; Niu, W.; Zhang, N.; Wang, C.; Lei, Y.; Yang, H.; Wang, H. Sea urchin-like cobalt selenide/N-doped carbon dots as efficient bifunctional electrocatalysts for water electrolysis. J. Ind. Eng. Chem. 2023, 133, 162–171. [Google Scholar] [CrossRef]
  37. Fang, Y.; Xue, Y.; Hui, L.; Yu, H.; Liu, Y.; Xing, C.; Lu, F.; He, F.; Liu, H.; Li, Y. In situ growth of graphdiyne based heterostructure: Toward efficient overall water splitting. Nano Energy 2019, 59, 591–597. [Google Scholar] [CrossRef]
  38. Wang, X.-R.; Liu, J.-Y.; Liu, Z.-W.; Wang, W.-C.; Luo, J.; Han, X.-P.; Du, X.-W.; Qiao, S.-Z.; Yang, J. Identifying the Key Role of Pyridinic-N–Co Bonding in Synergistic Electrocatalysis for Reversible ORR/OER. Adv. Mater. 2018, 30, 1800005. [Google Scholar] [CrossRef]
  39. Chen, Z.; Zheng, W.; Zhang, X.; Zheng, Y. Coupling of chrysanthemum-shaped cobalt hydroxide and nitrogen-doped carbon dots for high-performance hybrid supercapacitors. J. Electroanal. Chem. 2022, 925, 116666. [Google Scholar] [CrossRef]
  40. Zhang, Y.; Foster, C.W.; Banks, C.E.; Shao, L.; Hou, H.; Zou, G.; Chen, J.; Huang, Z.; Ji, X. Graphene-Rich Wrapped Petal-Like Rutile TiO2 tuned by Carbon Dots for High-Performance Sodium Storage. Adv. Mater. 2016, 28, 9391–9399. [Google Scholar] [CrossRef]
  41. Xu, Z.; Li, W.; Yan, Y.; Wang, H.; Zhu, H.; Zhao, M.; Yan, S.; Zou, Z. In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution. ACS Appl. Mater. Interfaces 2018, 10, 22102–22109. [Google Scholar] [CrossRef] [PubMed]
  42. Peng, H.; Zhang, W.; Song, Y.; Yin, F.; Zhang, C.; Zhang, L. In situ construction of Co/Co3O4 with N-doped porous carbon as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Catal. Today 2019, 355, 286–294. [Google Scholar] [CrossRef]
  43. Aljabour, A.; Awada, H.; Song, L.; Sun, H.; Offenthaler, S.; Yari, F.; Bechmann, M.; Scharber, M.C.; Schöfberger, W. A Bifunctional Electrocatalyst for OER and ORR based on a Cobalt(II) Triazole Pyridine Bis-[Cobalt(III) Corrole] Complex. Angew. Chem. Int. Ed. 2023, 62, e202302208. [Google Scholar] [CrossRef] [PubMed]
  44. Wang, K.; Guo, Y.; Chen, Z.; Wu, D.; Zhang, S.; Yang, B.; Zhang, J. Regulating electronic structure of two-dimensional porous Ni/Ni3N nanosheets architecture by Co atomic incorporation boosts alkaline water splitting. InfoMat 2021, 4, e12251. [Google Scholar] [CrossRef]
  45. Kumar, M.; Nagaiah, T.C. A multifunctional cobalt iron sulfide electrocatalyst for high performance Zn–air batteries and overall water splitting. J. Mater. Chem. A 2022, 10, 4720–4730. [Google Scholar] [CrossRef]
  46. Zhao, Y.; Dongfang, N.; Triana, C.A.; Huang, C.; Erni, R.; Wan, W.; Li, J.; Stoian, D.; Pan, L.; Zhang, P.; et al. Dynamics and control of active sites in hierarchically nanostructured cobalt phosphide/chalcogenide-based electrocatalysts for water splitting. Energy Environ. Sci. 2022, 15, 727–739. [Google Scholar] [CrossRef]
  47. Ma, R.; Hao, R.; Zhou, W.; Cao, Y.; Chai, H. Hollow CoP microspheres as superior bifunctional electrocatalysts for hydrogen evolution in a broad pH range and oxygen evolution reactions. J. Solid State Chem. 2022, 316, 123499. [Google Scholar] [CrossRef]
  48. Yang, L.; Zhang, B.; Fang, B.; Feng, L. A comparative study of NiCo2O4 catalyst supported on Ni foam and from solution residuals fabricated by a hydrothermal approach for electrochemical oxygen evolution reaction. Chem. Commun. 2018, 54, 13151–13154. [Google Scholar] [CrossRef]
Catalysts 15 00219 g001
Figure 1. Formation illustration of N-CDs and BS-CDs.
Figure 1. Formation illustration of N-CDs and BS-CDs.
Catalysts 15 00219 g001
Catalysts 15 00219 g002
Figure 2. (a,b) TEM images and (c) contact angle of N-CDs. (d,e) TEM images and (f) contact angle of BS-CDs.
Figure 2. (a,b) TEM images and (c) contact angle of N-CDs. (d,e) TEM images and (f) contact angle of BS-CDs.
Catalysts 15 00219 g002
Catalysts 15 00219 g003
Figure 3. (a–c) TEM images, (d–f) HRTEM images, and (g–i) element mapping images of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2.
Figure 3. (a–c) TEM images, (d–f) HRTEM images, and (g–i) element mapping images of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2.
Catalysts 15 00219 g003
Catalysts 15 00219 g004
Figure 4. (a) Raman spectra, (b) XRD patterns, (c) FTIR spectra, and (d) EPR of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2.
Figure 4. (a) Raman spectra, (b) XRD patterns, (c) FTIR spectra, and (d) EPR of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2.
Catalysts 15 00219 g004
Catalysts 15 00219 g005
Figure 5. (a) XPS survey spectra of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2, and high-resolution XPS spectra of (b) C 1s, (c) Co 2p, and (d) O 1s.
Figure 5. (a) XPS survey spectra of Co(OH)2, N-CDs/Co(OH)2, and BS-CDs/Co(OH)2, and high-resolution XPS spectra of (b) C 1s, (c) Co 2p, and (d) O 1s.
Catalysts 15 00219 g005
Catalysts 15 00219 g006
Figure 6. (a) OER polarization curves, (b) Tafel plots, (c) OER performance parameters, and (d) EIS Nyquist plots of BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2.
Figure 6. (a) OER polarization curves, (b) Tafel plots, (c) OER performance parameters, and (d) EIS Nyquist plots of BS-CDs/Co(OH)2, N-CDs/Co(OH)2, and Co(OH)2.
Catalysts 15 00219 g006
Catalysts 15 00219 g007
Figure 7. Cyclic voltammograms of (a) BS-CDs/Co(OH)2, (b) N-CDs/Co(OH)2, and (c) Co(OH)2 with different scan rates (20, 40, 60, 80, 100, and 120 mV s−1). (d) Capacitive currents against scan rate and corresponding Cdl value of catalysts at 0.95 V.
Figure 7. Cyclic voltammograms of (a) BS-CDs/Co(OH)2, (b) N-CDs/Co(OH)2, and (c) Co(OH)2 with different scan rates (20, 40, 60, 80, 100, and 120 mV s−1). (d) Capacitive currents against scan rate and corresponding Cdl value of catalysts at 0.95 V.
Catalysts 15 00219 g007
Catalysts 15 00219 g008
Figure 8. Long-term stability tests of (a) Co(OH)2, (b) N-CDs/Co(OH)2, and (c) BS-CDs/Co(OH)2 at a constant voltage for 20 h. LSV curves of (d) Co(OH)2, (e) N-CDs/Co(OH)2, and (f) BS-CDs/Co(OH)2 before and after 3000 CV scans.
Figure 8. Long-term stability tests of (a) Co(OH)2, (b) N-CDs/Co(OH)2, and (c) BS-CDs/Co(OH)2 at a constant voltage for 20 h. LSV curves of (d) Co(OH)2, (e) N-CDs/Co(OH)2, and (f) BS-CDs/Co(OH)2 before and after 3000 CV scans.
Catalysts 15 00219 g008
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Han, X.; Guo, C.; Wang, H.; Xu, W.; Liu, Q.; Zhao, Q.; Wu, M. Defect-Rich Co(OH)2 Induced by Carbon Dots for Oxygen Evolution Reaction. Catalysts 2025, 15, 219. https://doi.org/10.3390/catal15030219

AMA Style

Han X, Guo C, Wang H, Xu W, Liu Q, Zhao Q, Wu M. Defect-Rich Co(OH)2 Induced by Carbon Dots for Oxygen Evolution Reaction. Catalysts. 2025; 15(3):219. https://doi.org/10.3390/catal15030219

Chicago/Turabian Style

Han, Xuan, Chao Guo, Hui Wang, Weijuan Xu, Qinlian Liu, Qingshan Zhao, and Mingbo Wu. 2025. "Defect-Rich Co(OH)2 Induced by Carbon Dots for Oxygen Evolution Reaction" Catalysts 15, no. 3: 219. https://doi.org/10.3390/catal15030219

APA Style

Han, X., Guo, C., Wang, H., Xu, W., Liu, Q., Zhao, Q., & Wu, M. (2025). Defect-Rich Co(OH)2 Induced by Carbon Dots for Oxygen Evolution Reaction. Catalysts, 15(3), 219. https://doi.org/10.3390/catal15030219

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop