Co doping modulates the electronic structure of nickel diselenide and promotes the simultaneous occurrence of glycerol oxidation and hydrogen evolution reactions

Abstract

Using the glycerol oxidation reaction as an alternative to the traditional water oxidation reaction to reduce the required potential and accelerate the electrocatalytic process is an effective strategy for lowering energy consumption. In this study, a simple one-step hydrothermal method was employed to prepare a 10% Co–NiSe₂ bifunctional catalyst that is applicable to both the hydrogen evolution reaction and glycerol oxidation reaction. In the hydrogen evolution reaction, 10% Co-doped NiSe₂ requires 153 mV less overpotential than undoped NiSe₂ to achieve a current density of 20 mA cm⁻². The introduction of cobalt (Co) reduced the overpotential of the catalyst by 153 mV in the glycerol oxidation reaction, achieving selective conversion of glycerol to formate. The modified electrode exhibited higher catalytic activity, higher current density, and improved cycling stability, which can be attributed to the increased active sites on the material's surface and the optimization of its electronic structure due to cobalt doping. These characteristics make the 10% Co–NiSe₂ self-supported electrode a promising non-precious metal-based electrocatalyst suitable for efficient and energy-saving electrocatalytic systems, particularly in the fields of water splitting for hydrogen production and the selective oxidation of organic compounds.

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1. Introduction

In light of the escalating energy crisis and the challenges posed by global warming due to fossil fuel consumption, the development of clean and efficient renewable energy sources has become increasingly urgent. As global energy demand continues to rise, advancing sustainable technologies is of paramount importance. In this context, hydrogen energy, known for its high energy density, conversion efficiency, and environmental benefits, has emerged as a key focus among various renewable energy options.^1–6 Among the various methods for producing hydrogen energy, water electrolysis stands out as a key technique. It plays a crucial role in facilitating the transition to clean energy, particularly by enhancing the flexibility and sustainability of the overall energy system when integrated with renewable energy sources. With ongoing technological advancements and decreasing costs, water electrolysis is poised to occupy a more prominent position in the future energy landscape.^7–14 Despite the numerous advantages of the hydrogen evolution reaction, it still encounters several challenges. The high overpotential associated with the oxygen evolution reaction restricts the efficient advancement of water electrolysis for hydrogen production. Additionally, there is a risk of explosion due to the potential cross-reaction between the generated oxygen and hydrogen.^15–20

Glycerol, a highly functionalized molecule, is both inexpensive and readily available. It is non-toxic and biodegradable, offering high product yields and excellent selectivity in various chemical reactions. These advantageous properties make glycerol an ideal green solvent, well-suited for promoting green chemistry and sustainable development in reaction processes.^21–26 In recent years, the strategy of combining glycerol electrooxidation with the hydrogen evolution reaction has garnered significant attention as an innovative approach to generate high-value-added products and reduce the costs associated with water electrolysis. Under electrocatalytic conditions, glycerol can be transformed into various high-value-added chemicals, including 2-hydroxyacetone, glyceric acid, and glycolic acid.²⁷ Compared to traditional water oxidation reactions, glycerol electrooxidation has a lower theoretical oxidation potential, enabling it to occur at a reduced voltage. This not only decreases energy consumption but also enhances overall energy conversion efficiency.^21,28,29 An important application of glycerol electrooxidation is the direct production of formate, which is particularly significant for the preparation of formic acid fuel. Formic acid, as a liquid hydrogen carrier, demonstrates great potential in hydrogen energy storage and transportation. It can release hydrogen gas through a dehydrogenation reaction under mild conditions, providing fuel for devices such as fuel cells. Additionally, formic acid is a stable liquid at room temperature and pressure, making it easy to store and transport. It is also safer and more convenient than pure hydrogen gas. Therefore, combining glycerol oxidation reactions with hydrogen evolution reactions not only generates high-value-added chemicals but also effectively reduces energy consumption. This strategy holds significant promise in the field of energy storage and conversion.^30–39

Although precious metals and their oxides, such as Pt, RuO₂ and IrO₂, demonstrate exceptional catalytic performance in the hydrogen evolution reaction (HER) and glycerol oxidation reaction (GOR), their high cost, limited natural availability, and inadequate stability in certain situations hinder their practicality for large-scale commercial applications. Consequently, there is an urgent need to develop efficient electrocatalysts that are abundant in resources, cost-effective, and exhibit outstanding stability.^19,40–44 Transition metal chalcogenides (such as NiSe₂, CoSe₂, and FeSe₂) have emerged as highly attractive alternatives to precious metal catalysts (including platinum, ruthenium, and iridium) in electrochemical processes such as the HER and GOR. Their appeal stems from their ease of synthesis, low cost, excellent catalytic performance, and good stability.^45–47 For instance, Zhou et al.⁴⁸ identified a new, highly effective, and stable porous NiSe₂ electrocatalyst through a straightforward acid treatment and direct selenization. This electrocatalyst exhibits commercial performance that is nearly comparable to that of the precious metal Pt catalyst, but at a significantly lower cost. Qin et al.⁴⁹ demonstrated exceptional performance in both the OER and the HER in alkaline electrolytes by incorporating a small amount of Ru into selenides. This approach not only reduces costs but also enhances electrochemical activity, achieving an OER overpotential (η₁₀) of 210 mV and an HER overpotential (η₁₀) of 59 mV. Additionally, they reported an outstanding overall water-splitting voltage of 1.537 V at a current density of 10 mA cm⁻², along with a faradaic efficiency of 100%. Wang et al.⁵⁰ developed a two-dimensional nanopore Ni–Co selenide nanorod array (Ni_XCo_1−XSe) as a bifunctional electrode, suitable for hydrazine oxidation reactions in alkaline electrolytes and HER in acidic media. The optimized Ni_0.5Co_0.5Se₂ functioned effectively as both cathode and anode catalysts, achieving a power density of 13.3 mW cm⁻² at a current density of 54.7 mA cm⁻² in an alkaline-acidic hydrazine fuel cell, demonstrating excellent long-term stability and faradaic efficiency. The experimental data provide compelling evidence for the use of transition metal selenides as bifunctional catalysts in the electrochemical field.

In this paper, a 10% cobalt-doped NiSe₂ catalyst was successfully synthesized using a one-step hydrothermal method. This catalyst demonstrates exceptional bifunctional electrocatalytic performance under alkaline conditions, making it suitable for both the HER and the GOR. By varying the amount of cobalt doping, it was determined that the catalyst's performance peaked at a 10% doping level. Compared to pure NiSe₂, the 10% cobalt-doped NiSe₂ exhibited significantly enhanced catalytic activity and stability in both the HER and GOR. This achievement offers a novel strategy for developing efficient, low-cost, non-precious metal-based bifunctional electrocatalysts. It particularly highlights promising applications in the fields of water splitting for hydrogen production and the selective oxidation of organic compounds.

2. Results and discussion

By utilizing selenium powder as the selenium source, along with nickel nitrate and cobalt nitrate as the sources of nickel and cobalt, respectively, nickel selenide (NiSe₂) nanoparticles with varying levels of cobalt doping were successfully synthesized using a one-step hydrothermal method. Fig. 1 illustrates the flowchart of the synthesis process. X-ray diffraction analysis (XRD) results indicate that all Co–NiSe₂ samples, regardless of cobalt doping levels, exhibit characteristic diffraction peaks of pure hexagonal NiSe₂, consistent with JCPDS card number 65-1843, with no other impurity phases detected (Fig. 2). Notably, as cobalt is introduced, the intensity of the characteristic diffraction peaks of NiSe₂ in the XRD patterns shows varying degrees of attenuation, suggesting that cobalt doping results in a decrease in the material's crystallinity. Furthermore, no diffraction peaks for cobalt selenide were observed in the XRD patterns, indicating that cobalt exists within the NiSe₂ structure in a doped form rather than forming a separate selenide phase.

Fig. 1 The synthesis process of Co–NiSe₂. (a) Se and NaOH, (b) Ni(NO₃)₂⋅6H₂O and Co(NO₃)₂⋅6H₂O.

Fig. 2 XRD patterns of nickel selenide doped with cobalt at different concentrations.

The results of scanning electron microscopy (SEM) characterization indicate that the surface of the 10% Co–NiSe₂ catalyst exhibits a uniform and consistent nanosheet morphology, arranged in a stacked formation (Fig. 3a). Transmission electron microscopy (TEM) further confirmed that the material possesses a hexagonal nanosheet structure (Fig. 3b). In the high-resolution transmission electron microscopy (HRTEM) image of 10% Co–NiSe, lattice fringes with a spacing of 0.314 nm are clearly observable, corresponding to the (200) crystal plane of NiSe₂ (Fig. 3c). Fig. 3d presents the selected area electron diffraction (SAED) pattern of 10% Co–NiSe, revealing the material's crystal structure characteristics. Additionally, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and corresponding elemental mapping analyses confirm the uniform distribution of Se, Co, and Ni elements across the nanosheets. Compared to undoped NiSe₂ (Fig. S1a and b), the 10% Co–NiSe₂ maintains a similar hexagonal nanosheet morphology. Notably, the nanosheets in the 10% Co–NiSe₂ sample exhibit a tendency to thicken. This suggests that the introduction of Co did not alter the fundamental shape of the material.

Fig. 3 Morphological characterization images of 10% Co–NiSe₂: (a) SEM image. (b) TEM image. (c) HRTEM image. (d) SEAD image. (e–h) HAADF-STEM images and corresponding elemental mapping results.

To further investigate the chemical state of surface elements in the 10% Co–NiSe₂ catalyst, X-ray photoelectron spectroscopy (XPS) was employed for analysis. The full XPS spectrum confirmed the presence of Se and Ni elements. However, due to the relatively low doping concentration of cobalt, the signal peak intensity in the XPS full spectrum was weak and challenging to identify directly (Fig. 4a). In the high-resolution Se 3d spectrum of 10% Co–NiSe₂, a broad peak was observed at a binding energy of approximately 59 eV, which is attributed to the formation of Se–O bonds on the surface as a result of oxidation. Two main peaks were identified at 54.8 eV and 55.6 eV, corresponding to Se 3d_5/2 and Se 3d_3/2, respectively. In the Ni 2p spectrum, the core peaks for Ni 2p_3/2 and Ni 2p_1/2 were observed at 853.3 eV and 871.2 eV, confirming the presence of Ni²⁺. Notably, the introduction of Co resulted in a positive shift in the Ni 2p_1/2 peak position, specifically an increase in binding energy of 0.56 eV, and the Ni 2p_3/2 shifted towards higher binding energy by 0.1 eV indicating a decrease in electron cloud density around the Ni atoms. In the Co 2p spectrum, the spectrum corresponds to the binding energy peaks of Co 2p_1/2 (795.7 eV) and Co 2p_3/2 (777.9 eV) for Co²⁺ ions. The results indicate that cobalt has been successfully introduced into the NiSe₂ catalyst without altering the valence states of the elements. However, the Ni 2p peak exhibits a shift towards higher binding energy, suggesting that Co doping has altered the local charge distribution around the nickel atoms, thereby impacting the performance of the catalyst.

Fig. 4 XPS measurement spectra and high-resolution XPS spectra of 10% Co–NiSe₂ and NiSe₂. (a) Full spectra, (b) Se 3d, (c) Ni 2p, (d) Co 2p.

To assess the catalytic activity of Co–NiSe₂, electrochemical tests were performed using a conventional three-electrode system in a 1 M KOH electrolyte solution at a scan rate of 5 mV s⁻¹. Under identical experimental conditions, linear sweep voltammetry tests were conducted on Co–NiSe₂ catalysts with varying cobalt doping levels. As illustrated in Fig. 5a, the electrocatalytic activity of the material initially increased and then decreased with rising cobalt doping ratios, peaking at a 10% cobalt doping level. Specifically, when comparing 10% Co–NiSe₂ to undoped NiSe₂, the overpotential required to achieve a current density of 20 mA cm⁻² was reduced by 153 mV for 10% Co–NiSe₂ under the same conditions (see Fig. 3b). This finding indicates that an optimal amount of cobalt doping significantly enhances the electrocatalytic activity of NiSe₂, while excessive doping may result in diminished catalytic performance. To evaluate the performance differences among various catalysts, the Tafel slopes of NiSe₂ materials with different cobalt doping levels were analyzed. The results revealed that 10% Co–NiSe₂ exhibited the smallest Tafel slope (Fig. 3c), suggesting that this catalyst exhibits higher intrinsic activity and a more efficient charge transfer process. Additionally, electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the reaction kinetics of each catalyst. As shown in Fig. 3d, 10% Co–NiSe₂ displayed the lowest charge transfer resistance, further confirming its significant advantage in electron transfer rates during electrocatalysis. The capacitance (C_dl) obtained from CV at various scan rates in the non-Faraday range is utilized to determine the ECSA of the catalysts (Fig. S2). The C_dl values for NiSe₂, 5% Co–NiSe₂, 10% Co–NiSe₂, 15% Co–NiSe₂, and 20% Co–NiSe₂ are 25, 32, 33, 31, and 32 mF cm⁻², respectively. The addition of cobalt enhances the electrochemically active area of the catalysts at various doping levels, particularly at the 10% cobalt doping level, where the effect is most pronounced. Therefore, by precisely controlling the cobalt doping ratio, effective optimization of the NiSe₂ catalyst's performance can be achieved. To comprehensively evaluate the catalytic performance of 10% Co–NiSe₂, it is essential to assess the stability of the catalyst during long-term operation, in addition to considering ECSA and electrocatalytic activity. Fig. 3f illustrates the stability test of 10% Co–NiSe₂ at a current density of 10 mA cm⁻² for 10 h, during which the overpotential remains stable without significant fluctuations. The results indicate that 10% Co–NiSe₂ not only exhibits excellent electrocatalytic activity but also demonstrates good stability during prolonged operation. This further confirms the reliability and durability of this catalyst in practical applications, thereby establishing its potential value in the field of electrocatalysis.

Fig. 5 Electrochemical hydrogen evolution reaction (HER) performance in 1.0 M KOH (pH = 14). (a) Linear sweep voltammograms of different catalysts. (b) Comparison of linear sweep voltammograms with 10%Co–NiSe₂ and NiSe₂. (c) Tafel slope curves. (d) Nyquist plots of the catalysts. (e) C_dl plots. (f) The V–t test of 10%Co–NiSe₂ at a current density of 10 mA cm⁻².

The glycerol oxidation performance of the catalyst was evaluated in a mixed solution of 1 M KOH and 0.1 M glycerol. Initially, the electrochemical performance of nickel selenide materials with varying Co doping ratios was assessed using LSV (Fig. 6a). The experimental results indicated that the most significant enhancement in electrocatalytic activity occurred in the NiSe₂ sample doped with 10% Co. This finding aligns with the hydrogen evolution reaction, suggesting that an optimal level of Co doping can effectively enhance the electrocatalytic activity of NiSe₂ materials. Furthermore, a notable difference in current response was observed between the solution containing 0.1 M glycerol and that without glycerol. Under identical current density conditions, the overpotential was successfully reduced by 153 mV following the addition of 0.1 M glycerol (Fig. 6b). This phenomenon demonstrates that the inclusion of glycerol significantly enhances the electrocatalytic activity of the material. As shown in Fig. 6c, the Tafel slope of 10% Co–NiSe₂ is 49.6 mV dec⁻¹, which is considerably lower than that of undoped NiSe₂ (85 mV dec⁻¹). This indicates that, compared to other catalysts, 10% Co–NiSe₂ exhibits faster reaction kinetics. Electrochemical impedance testing indicates that 10% Co–NiSe₂ exhibits a lower charge transfer resistance (Fig. 6d). This finding further confirms its enhanced electron transfer rate, facilitating more efficient electron transport during the electrocatalytic process. Additionally, the electrochemically active surface area is a crucial factor influencing electrocatalytic activity. The magnitude of the C_dl value is directly proportional to the size of the electrochemically active area. Compared to other catalysts, 10% Co–NiSe₂ has the highest C_dl value, with an ECSA of 425 cm², which is 1.3 times greater than that of undoped Co–NiSe₂ (325 cm²) (Fig. 6e). CV curves of NiSe₂ at different scan rates during the UOR are shown in Fig. S3. This indicates that 10% Co–NiSe₂ possesses a larger ECSA, providing more potential active sites and further enhancing its electrocatalytic activity. Similarly, consistent with the results of the HER, the addition of Co results in varying degrees of enhancements in the active area of the catalysts with different doping levels. This phenomenon confirms that Co doping not only optimizes the electronic structure of the material but also significantly increases the ECSA, thereby synergistically improving its overall electrocatalytic performance. To evaluate the stability of the 10% Co–NiSe₂ electrode in the GOR, the catalyst electrode underwent 1000 cycles of continuous CV scanning and long-term chronopotentiometry testing (Fig. 6f). The results indicated that, after 1000 cycles of CV scanning, the electrocatalytic activity of the 10% Co–NiSe₂ electrode experienced a slight decrease; however, it remained at a high level, demonstrating the electrode's excellent long-term stability and durability. The inset illustrates the stability test conducted over a 10-hour period at a current density of 50 mA cm⁻². During this time, the potential of the electrode exhibited minimal change, further confirming its stability under prolonged operating conditions. Qualitative and quantitative analyses of the electrolyte post-electrolysis were performed using nuclear magnetic resonance (NMR) spectroscopy. In the ¹H NMR spectrum, in addition to the characteristic peaks of glycerol and water, distinct peaks corresponding to formate were also detected in the electrolyte after electrolysis (Fig. 6g). The emergence of new peaks indicates the production of formate during the electrolysis process, further confirming the occurrence of the electrochemical reaction. Characteristic peaks of formate were also observed in the ¹³C NMR spectrum (Fig. 6h). These results align with the observations in the ¹H NMR spectrum, further validating the formation of formate. To evaluate the faradaic efficiency of the electrolysis process, we analyzed samples taken after 10 h of electrolysis, with the results presented in Fig. 6i. The findings indicate that the faradaic efficiency of the 10% Co–NiSe₂ electrode is approximately 60% after 10 h of electrolysis at a current density of 100 mA cm⁻². This faradaic efficiency suggests that while the majority of the input electrons contributed to the formation of target products (such as formate), a portion of the electrons was not effectively utilized. Possible reasons for this inefficiency include the occurrence of side reactions, limitations in ion transport within the electrolyte, or insufficient active sites on the electrode surface. Nevertheless, a faradaic efficiency of 60% still demonstrates the high catalytic activity and selectivity of the 10% Co–NiSe₂ electrode during the electrolysis process.

Fig. 6 Performance of the glycerol oxidation reaction in a 1.0 M KOH + 0.1 M glycerol mixed solution. (a) Linear sweep voltammograms for various catalysts. (b) Comparison of linear voltammetric scans for glycerol oxidation and water oxidation. (c) Tafel slope curves. (d) Nyquist plots for the catalysts. (e) C_dl plots. (f) Polarization curves of 10% Co–NiSe₂ before and after 1000 cyclic voltammetry (CV) cycles. The inset displays the voltage–time (V–t) test of 10% Co–NiSe₂ at a current density of 50 mA cm⁻². (g) ¹H NMR. (h) ¹³C NMR. (i) Faradaic efficiency of glycerol oxidation to formate.

3. Conclusion

In summary, 10% Co–NiSe₂ was successfully synthesized using nickel nitrate, cobalt nitrate, and selenium powder through a one-step hydrothermal method. Electrochemical tests indicate that this electrode exhibits excellent electrocatalytic activity and stability in both the HER and GOR. Compared to the pure NiSe₂ electrode, the 10% Co–NiSe₂ electrode demonstrates a lower overpotential, higher current density, and improved cycling stability. These enhancements are primarily attributed to the increase in active sites on the material's surface and the optimization of its electronic structure resulting from cobalt doping. Therefore, the 10% Co–NiSe₂ catalyst holds significant potential as a promising non-precious metal-based electrocatalyst in renewable energy conversion technologies and the electrochemical conversion of organic substances.

Conflicts of interest

There are no conflicts to declare.

Data availability

The XRD patterns, SEM images, LSV and CV curves, and TOF values supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03279b.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (52362036). The authors extend their gratitude to Scientific Compass (https://www.shiyanjia.com) for providing invaluable assistance with the XPS analysis.

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