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Article

Enhanced Adsorption Ability of CoS-Doped CuS for Promoting Electrochemical Oxidation of HMF

by
Peng Cao
,
Yunliang Liu
,
Ruihua Yang
,
Yaxi Li
,
Yuanyuan Cheng
,
Jingwen Yu
,
Xinyue Zhang
,
Peter Phiri
,
Xinya Yuan
,
Yi Yang
,
Naiyun Liu
*,
Yixian Liu
and
Haitao Li
*
Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(5), 422; https://doi.org/10.3390/catal15050422
Submission received: 19 March 2025 / Revised: 15 April 2025 / Accepted: 23 April 2025 / Published: 24 April 2025
(This article belongs to the Section Electrocatalysis)
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Versions Notes

Abstract

:
In the face of the intensifying energy and environmental challenges, the exploration of clean and sustainable approaches to energy conversion and utilization holds paramount significance. 5-Hydroxymethylfurfural (HMF), as a biomass platform compound with great potential, has drawn extensive attention for its oxidation to prepare 2,5-Furandicarboxylic acid (FDCA). In this study, a CoS-doped CuS composite catalyst (CoS–CuS) was synthesized via a one-step microwave–hydrothermal method for the electrocatalytic oxidation of HMF. The catalyst was comprehensively analyzed by means of multiple characterization techniques and electrochemical testing methods. The results demonstrate that the doping of CoS optimizes the surface electronic structure of the catalyst, enhancing its adsorption capabilities for HMF and OH−. Compared with the CuS catalyst, CoS–CuS in the 5-hydroxymethylfurfural oxidation reaction (HMFOR) shows a lower onset potential decreasing from 1.32 VRHE to 1.29 VRHE. At a potential of 1.4 VRHE, the current density of CoS–CuS attains a value 2.02-fold that of CuS. Significantly, CoS–CuS demonstrates a substantially higher Faraday efficiency in the generation of FDCA, reaching nearly 89.1%. This study provides a promising approach for the construction of other efficient copper-based electrocatalysts.

1. Introduction

In the process of continuous social progress and development, fossil fuels such as coal, oil, and natural gas have been widely used [1,2,3]. However, simultaneously, a series of severe problems such as energy crises and environmental pollution have emerged [4,5,6]. As a result, the exploration of clean and renewable alternative energy sources has become a central issue that captures the attention of the entire world [7,8,9,10]. Biomass energy, as a green and sustainable energy source, holds great development potential and is regarded as one of the key options to replace fossil fuels [11,12,13,14]. Within the diverse spectrum of biomass platform compounds, 5-hydroxymethylfurfural (HMF) has garnered significant interest owing to its remarkably wide-ranging application potential, as it is amenable to conversion into a multitude of high-value-added chemicals [15,16,17]. Among these derivatives, 2,5-Furandicarboxylic acid (FDCA) serves as a crucial starting material in the synthesis of polyethylene furanoate (PEF) [18,19]. Considering the structural resemblance between FDCA and terephthalic acid, polyethylene furanoate (PEF) is anticipated to emerge as a viable substitute for petroleum-based polyethylene terephthalate (PET) [20,21,22,23]. However, traditional catalytic oxidation methods for HMF generally require high-temperature and high-pressure environments and rely on noble metal catalysts [22,23]. These factors have severely restricted its large-scale promotion and application [24].
In contrast, electrocatalytic technology, with its remarkable advantages of mild reaction conditions, low catalyst costs, and high reaction efficiency, shows extremely broad application prospects [25,26,27]. In terms of the environment, electrocatalysis powered by renewable electricity can reduce CO2 emissions by 72% compared to thermal catalysis, so electrocatalysis has certain environmental advantages [28]. Regarding process costs and industrial-scale applications, according to techno-economic analyses, current process costs (electrolytes, membranes, energy input) account for approximately 60% of the total expenses; therefore, the process costs cannot be underestimated [29]. Pilot-scale flow reactors have demonstrated the ability to stably produce FDCA for over 1000 h with an activity decay of less than 5%, meeting industrial durability standards [30]. The above indicates that electrocatalysis has the potential for industrial applications in the production of value-added compounds but it also faces numerous challenges.
Within this context of electrocatalytic technology, the choice of materials plays a crucial role. Different materials exhibit distinct characteristics that can significantly influence the performance of the electrocatalytic process. Copper (Cu) has attracted extensive attention because of its weak activity in the oxygen evolution reaction (OER) side reaction and hydrogen evolution reaction (HER) side reaction, resulting in high selectivity during the electrocatalytic process [31,32,33]. For example, Li [34] et al. constructed Pt nanoparticle-modified CuO nanowires (Pt/CuO@CF) for the selective electrochemical oxidation of HMF to FDCA under alkaline conditions. Pt/CuO@CF exhibited excellent catalytic performance at different HMF concentrations (10–100 mM). The Faraday efficiency of FDCA was higher than 95%, and it had excellent long-term stability (15 cycles). Yu [35] et al. prepared a bimetallic Cu–Co oxide/hydroxide electrocatalyst (CuOCoOOH) through co-electrodeposition and electrochemical activation methods. The introduction of Co enhanced substrate adsorption, promoted defect construction, and aided the generation of high-valent active species. In the reaction of electrochemically oxidizing HMF to FDCA, both the yield and Faraday efficiency reached 98%.
The adsorption of HMF is crucial as the initial step of its catalytic conversion [36,37]. In this process, the adsorbed OH− on the electrode surface extracts the proton H from the HMF molecule, generating H2O and reaction intermediates [38,39]. Especially during the electrocatalytic conversion of HMF in an alkaline system, the appropriate adsorption ability of the electrocatalyst for organic molecules (such as HMF) and OH− species is directly related to the smooth progress of the reaction [40]. Copper sulfide (CuS) is a promising electrocatalyst for the HMF oxidation reaction (HMFOR). Compared with other transition metal sulfides, the competitive reaction on the CuS electrode, the oxygen evolution reaction (OER), is not significant [41,42]. Even at relatively high potentials it can still maintain a high Faraday efficiency. However, CuS has rather weak capabilities in adsorbing and activating OH− species as well as HMF molecules. As a consequence, the intrinsic activity of CuS in the HMFOR reaction remains unable to achieve the expected level. Therefore, exploring appropriate strategies to optimize the adsorption and conversion processes of electrocatalysts for HMF molecules and OH− species is of vital significance for improving the performance of CuS in HMFOR.
Cobalt atoms have a special electronic structure. The electron distribution in their 3d and 4s orbitals enables them to have various interactions with organic molecules and hydroxyl groups [42]. For hydroxyl groups, cobalt can form chemical bonds with the oxygen atoms in the hydroxyl groups. The lone electron pairs of the oxygen atoms form coordination bonds with the empty orbitals of cobalt. This chemical bond interaction endows cobalt with a strong adsorption ability for hydroxyl groups. Li [43] et al. successfully synthesized a 3% CoS2–7% CuS heterostructure catalyst using an electrodeposition–hydrothermal method, where CoS2 nanoparticles (NPs) were decorated on CuS nanosheets (NSs) supported on carbon cloth (CC) for highly efficient HER. The optimized 3% CoS2–7% CuS catalyst demonstrated exceptional electrocatalytic performance with outstanding cycling stability across all pH conditions. Systematic characterization revealed that the CoS2 NPs significantly increased the density of surface catalytic sites, while the synergistic effect between Co2+/Co3+ and Cu+/Cu2+ multivalent states substantially enhanced charge transfer efficiency.
In this study, we successfully synthesized CoS–CuS catalysts via a microwave-assisted hydrothermal method. The microwave–hydrothermal method was selected as the synthetic approach due to its distinct advantages, including: (1) rapid reaction kinetics (enabling energy-efficient processing) [44], (2) precise control over nanoparticle morphology and size distribution [45], (3) high crystallographic phase purity [46], and (4) environmental benignity [47]. Through a series of in situ characterization techniques and electrochemical testing methods, the results clearly show that the introduction of a small amount of CoS significantly enhances the adsorption ability and conversion activity of CoS–CuS for the organic molecule HMF and OH− species. Especially, in the reaction process from FFCA to FDCA, it greatly promotes the dehydrogenation reaction and the electron transfer process. Benefiting from the doping of CoS, the onset potential is significantly reduced (decreased from 1.32 VRHE to 1.29 VRHE). At a potential of 1.4 VRHE, the current density is 2.02 times higher than that of pure CuS. This study deeply analyzes the internal mechanism of enhanced adsorption of CoS–CuS, providing strong theoretical guidance for the design of efficient electrocatalysts with heterogeneous interfaces.

2. Results

As illustrated in Figure 1, CoS–CuS nanoparticles were synthesized via a one-step microwave–hydrothermal method. Nickel foam was selected as the substrate owing to its remarkable advantages in large-scale synthesis and electrochemical applications. It serves as a highly cost-efficient conductive current collector and has been verified by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to have no interference with the catalyst layer. In the synthesis procedure, a mixed solution of copper nitrate, cobalt nitrate, and thioacetamide (TAA) was prepared in deionized water. Then, pre-cleaned nickel foam was added to the solution, followed by a microwave–hydrothermal treatment to obtain the CoS–CuS composite. The nanoparticle morphology was characterized using scanning electron microscopy (SEM). Figure 2a presents the low-magnification transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping of CoS–CuS. The morphology of CoS–CuS nanoparticles is clearly discernible, and the elements Cu, Co, and S are homogeneously distributed on the catalyst, with an exceedingly low Co content.
The crystalline structure was further investigated by XRD. As shown in Figure 2b, the characteristic diffraction peaks at 29.2°, 32.7°, and 47.8° correspond to the (102), (006), and (107) planes of hexagonal CuS (PDF#78-0876), respectively. Simultaneously, distinct peaks at 35.2° and 54.2° were indexed to the (101) and (110) planes of CoS (PDF#75-0605) [48]. Notably, the coexistence of CuS and CoS diffraction signatures in the composite pattern, without detectable peak shifts or impurity phases, verifies the successful formation of a heterostructured CoS–CuS system with effective phase dispersion.
To precisely define the surface valence states and chemical environments of the CuS and CoS–CuS, XPS measurements were performed. In marked contrast to the pure phases, significant variations in the elemental valence states of the CoS–CuS catalyst were discerned. Based on the XPS spectrum of the copper 2p orbital (Figure 2c), the peaks at 932.28 eV and 952.28 eV are assigned to Cu⁺ in the Cu 2p3/2 and Cu 2p1/2 states, respectively. The peaks at 934.58 eV and 954.78 eV correspond to Cu2+ in the Cu 2p3/2 and Cu 2p1/2 states, respectively [49]. Notably, the composite catalyst exhibited a 0.1 eV positive binding energy shift compared to pure CuS (932.18 eV and 952.18 eV for Cu+; 934.68 eV and 954.88 eV for Cu2+) [50], accompanied by attenuated satellite peaks—both observations indicating significant electronic interaction between the CoS and CuS phases. The presence of Cu⁺ species can be attributed to the mild reducing environment created by thioacetamide during synthesis, consistent with previous reports of surface reduction in CuS systems [51]. More importantly, the Co doping-induced modifications in the Cu coordination environment were found to promote Cu⁺ formation while facilitating interfacial electron transfer from CuS to CoS, as evidenced by the binding energy shifts. This electronic redistribution may enhance the catalyst’s dual adsorption capacity for both HMF and OH− species, ultimately boosting its electrocatalytic performance in alkaline media [52]. XPS spectrum of the S 2p region of CoS–CuS (Figure S1, Supporting Information) demonstrates the existence of photoelectron peaks at binding energies of 162.28 eV and 163.48 eV [53]. These peaks can be, respectively, attributed to S 2p3/2 and S 2p1/2, which are characteristic of S2− species. High-resolution transmission electron microscopy (HRTEM) was utilized to characterize the micro-morphology and structure of the catalyst. The fabricated catalyst consists of nanoparticles, and the CuS (102), (006), (101), CoS (101) and Cu2S (024) crystal planes were observed in the HRTEM images (Figure 2d). The aforementioned results confirm the successful preparation of the CoS–CuS catalyst.
The electrocatalytic performance of the CoS–CuS composite was systematically evaluated and compared with monometallic CuS and CoS catalysts to elucidate the synergistic effects of cobalt doping. Using a standard three-electrode configuration, linear sweep voltammetry (LSV) analysis in 1 M KOH with 10 mM HMF (Figure 3a) revealed that the CoS–CuS composite exhibited a lower HMFOR onset potential (1.29 VRHE) than pure CuS (1.32 VRHE), though still higher than monometallic CoS (1.22 VRHE). Notably, while CoS showed the most favorable onset potential, its current density was substantially lower than that of the CoS–CuS composite, highlighting the limitations of pure CoS. To optimize the Co doping ratio, a series of CuS catalysts with varying theoretical CoS concentrations (0.5× to 3× relative to the standard Co precursor) were tested (Figure S2, Supporting Information) The composite with the standard doping ratio (0.04 mmol Co(NO3)2·6H2O precursor) demonstrated the highest current density for HMFOR, achieving 134 mA cm−2 at 1.4 VRHE, which is 2.02 times that of CuS, confirming the promoting effect of CoS doping (Figure 3b). The Tafel slope value is an important parameter for evaluating the reaction rate and mechanism of an electrocatalytic reaction. As shown in Figure 3c, the Tafel slope of the CoS–CuS catalyst is 32.8 mV dec−1, while that of CuS is 40.6 mV dec−1. This indicates that the CoS–CuS catalyst has a better catalytic reaction response for HMFOR.
In the electrochemical impedance spectroscopy (EIS) analysis (Figure S3, Supporting Information), compared with CuS, a reduction in the charge transfer resistance of CoS–CuS in the low-frequency region of the 1.0 M KOH and 50 mM HMF electrolyte was detected. This indicates that CoS–CuS has a lower charge transfer resistance than CuS. Double-layer capacitance (Cdl) measurements (Figure S4, Supporting Information) further confirmed that CoS–CuS possessed the highest electrochemical active surface area (ECSA, 0.03978 mF cm−2) among the catalysts (CuS: 0.02896 mF cm−2; CoS: 0.02597 mF cm−2), consistent with its larger specific surface area (12.4008 m2 g−1, vs. 7.8087 m2 g−1 for CuS and 1.4442 m2 g−1 for CoS, Figure S5, Supporting Information). This enhanced surface area of CoS–CuS provides more accessible active sites and facilitates greater electrolyte–catalyst interfacial contact during the HMFOR, thereby substantially improving the reaction efficiency. Additionally, the open circuit potential (OCP) was measured to detect changes in the content of organic adsorbents in the inner Helmholtz layer [54]. When 50 mM HMF was injected, the OCP change of CoS–CuS (0.421 VRHE) was more significant than that of CuS (0.199 VRHE), indicating that more HMF molecules were adsorbed in the inner Helmholtz layer (Figure 3d). This also means that the introduction of CoS is beneficial to the adsorption of HMF, which explains why CoS–CuS has a better response in LSV. It is speculated that the active sites may have a stronger capture ability for HMF, thus accelerating the reaction.
Given the coexistence of aldehyde and hydroxyl functional groups in the HMF molecule, the HMFOR exhibits two distinct reaction pathways [55]. One pathway generates a 2,5-diformylfuran (DFF) intermediate, while the other generates a 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) intermediate. Subsequently, both intermediates are oxidized to 5-formyl-2-furandicarboxylic acid (FFCA), and FFCA is further oxidized to finally produce FDCA. Conducting real-time qualitative and quantitative monitoring of this complex reaction pathway of the HMF oxidation reaction can help reveal the specific reaction path and catalytic efficiency of the HMF oxidation reaction. (Figure 4a). In this study, high-performance liquid chromatography (HPLC) was used to analyze standard samples and products. Calibration curves for five substances involved in the HMFOR were prepared (Figures S6 and S7, Supporting Information) to illustrate the changes in the concentrations of reactants and different products with the amount of charge transfer. Considering the convenience of sampling and calculation and to avoid the irreversible decomposition of HMF at high concentrations due to the Cannizzaro reaction, low-concentration conditions were adopted to accurately evaluate the activity of the catalyst. In an electrolyte of 1 M KOH and 10 mM HMF, a constant potential of 1.4 VRHE was applied to the CoS–CuS electrode.
As can be seen from Figure 4b, during potentiostat electrolysis, the current density continuously decreased due to the continuous consumption of the substrate in the reaction system. The variation in charge (C) in Figure 4b reflects the dynamic characteristics of the electrocatalytic oxidation process. As the current density continuously decreases over time, the rate of charge accumulation also gradually slows until reaching a plateau at 57.8 C, which is attributed to the progressive consumption of the substrate (HMF) in the reaction system. This trend aligns with the depletion of HMF and the accumulation of FDCA shown in Figure 4c,d. Notably, the reaction pathway was unambiguously determined by the distinct distribution of intermediates. DFF was observed as the major intermediate, while HMFCA was detected only in trace amounts. This result conclusively establishes that Pathway 2 (HMF → DFF → FFCA → FDCA) dominates under the catalytic influence of CoS–CuS. Furthermore, the stable current and charge accumulation curves demonstrate the robustness of the CoS–CuS catalyst, as no abrupt changes are observed. This behavior highlights the catalyst’s ability to maintain consistent performance throughout the reaction.
Subsequently, potentiostatic tests were carried out on CoS–CuS and CuS between 1.4 VRHE and 1.6 VRHE (Figure 5a). The products were detected and the HMF conversion rate, FDCA yield, and Faraday efficiency were calculated. From 1.4 VRHE to 1.6 VRHE, the Faraday efficiencies of CoS–CuS for catalyzing FDCA were 89.1%, 88.5%, 85.1%, 84.7%, and 82.4%, respectively. At 1.4 VRHE, the conversion rate of CoS–CuS for catalyzing HMF reached 97.1%, the yield of FDCA was 95.8%, and the Faraday efficiency was 89.1%. The Faraday efficiencies of CuS in the same potential range were 83.2%, 82.5%, 81.9%, 80.8%, and 80.3%, while CoS exhibited significantly lower efficiencies of 80.0%, 76.7%, 63.4%, 42.3%, and 28.6% at corresponding potentials (Figure 5b,c). These results demonstrate the superior HMFOR catalytic performance of the CoS–CuS composite. To enable direct comparison of FDCA production rates among CoS–CuS, CuS, and CoS catalysts, we systematically evaluated their performance within the potential range of 1.4–1.6 VRHE (Figure S8, Supporting Information). To study the stability of the CoS–CuS catalyst, six independent cycles were carried out on the same electrode, and the products of each test were analyzed. The stability of CoS–CuS was rigorously evaluated through six consecutive electrolysis cycles at 1.4 VRHE. The catalyst maintained consistently high FDCA yields between 94.9 and 95.9%, with minimal fluctuations in conversion rate and Faraday efficiency (Figure 5d). Post-reaction characterization by XRD revealed no detectable phase transformations or structural changes in the CoS–CuS heterostructure (Figure S9, Supporting Information), demonstrating its exceptional durability. This outstanding stability, combined with the consistently high performance across multiple cycles, positions CoS–CuS as a robust and reliable catalyst for HMFOR applications.
In situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was utilized to explore the adsorption behavior of electrocatalysts towards organic molecules and OH− species. In a solution consisting of 1 M KOH and 50 mM HMF, in situ FT-IR experiments were carried out on CoS–CuS and CuS samples at 1.3 VRHE for a duration of 20 min. As presented in Figure 6a,c, the in situ ATR-FTIR spectra demonstrated that, under a potential of 1.3 VRHE, as the testing time progressed, the intensity of the O–H vibration peak within the range of 3100–3400 cm−1 during the CoS–CuS test increased continuously and reached its maximum at 20 min. This phenomenon indicates an enrichment of a greater quantity of OH− species in the vicinity of the CoS–CuS material surface. In contrast, the peak intensity of CuS at the corresponding position remained nearly unchanged, suggesting that CoS–CuS exhibits a stronger adsorption capacity for OH− species [56,57,58,59,60].
Simultaneously, the in situ infrared tests in Figure 6b,d revealed that, for the CoS–CuS catalyst, the intensity of the C–O signal peak at 1216 cm−1 gradually increased with the extension of the testing time. However, for the CuS catalyst, only a minor change in the peak intensity at 1216 cm−1 was observed. Given that the potential of 1.3 VRHE is lower than the onset potential of HMFOR and that only HMF in the electrolyte system contains C–O bonds, it can be deduced that the peak intensity at 1216 cm−1 is attributed to the stretching vibration of the C–O bonds in the carboxyl group of HMF [42,60,61]. This indicates that CoS–CuS has a stronger adsorption ability for HMF compared to CuS, which is consistent with the results of OCP test.
The above-mentioned results suggest that the doping of CoS is conducive to the co-adsorption of OH− and HMF by the CoS–CuS catalyst. From the perspective of the reaction process, the electrocatalytic oxidation of HMF in an alkaline environment involves the participation of organic molecules and OH− species. The adsorption of HMF represents the initial step in its catalytic conversion. Subsequently, the surface-adsorbed OH− abstracts the proton H from HMF, forming H2O and reaction intermediates. Therefore, a stronger adsorption capacity for OH− species and HMF enables the catalyst to accumulate OH− species and HMF at a lower potential, thereby accelerating the reaction process [37]. Based on these experimental results it can be reasonably deduced that the introduction of CoS can significantly enhance the adsorption of organic molecules and OH− species, and simultaneously promote electron transfer, ultimately accelerating the reaction process. As shown in Figure 6e,f, by plotting the signal values from in situ infrared spectroscopy against the testing time, the difference in the adsorption amounts of OH− and HMF between the two catalysts can be more intuitively obtained. In light of the aforementioned experimental outcomes, schematic representations of the initial reaction within the inner Helmholtz layer are presented (Figure 7a,b).

3. Materials and Methods

3.1. Materials and Characterizations

All reagents and materials were used without further purification, including cobalt nitrate hexahydrate, copper nitrate hexahydrate, ammonium persulfate, nickel foam, absolute ethanol, potassium hydroxide, 5-hydroxymethylfurfural, hydrochloric acid, and acetone, were analytical-grade reagents and purchased from Sigma Aldrich (St. Louis, MO, USA) and Sinopharm (Beijing, China). All reagents and chemicals were used as received without further purification.
Morphology and microstructure analysis: Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7800F, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Talos F200s, Thermo Fisher Scientific, Waltham, MA, USA) coupled with energy-dispersive X-ray spectroscopy (EDS). Crystallographic structure determination: X-ray diffraction (XRD, Bruker D2 Phaser, Bremen, Germany) with Cu Kα radiation (λ = 1.5418 Å), operated at 30 kV and 10 mA. X-ray photoelectron spectroscopy (XPS) analysis was collected on a Thermo Scientific (Waltham, MA, USA) K-Alpha+ X-ray photoelectron spectrometer using Al Kα radiation.

3.2. Synthesis of CoS–CuS and Control Catalysts

The CoS–CuS was synthesized by a one-step microwave hydrothermal method. Specifically, a 1 cm × 2 cm piece of nickel foam was taken as the substrate. The nickel foam was ultrasonically cleaned with 1 M hydrochloric acid and acetone successively and then dried for later use. Amounts of 6 mmol of thioacetamide, 2.6 mmol of copper nitrate hexahydrate, 0.04 mmol of cobalt nitrate hexahydrate, and 10 mmol of urea were accurately weighed and uniformly mixed in 70 mL of distilled water to form a brown suspension. Then, the nickel foam with the suspension was subjected to microwave hydrothermal treatment at a power of 500 W for 4 min. After cooling to room temperature, the nickel foam was taken out, washed with distilled water and anhydrous ethanol successively, and vacuum-dried at 60 °C to obtain CoS–CuS. Similarly, CuS was obtained by not adding cobalt nitrate hexahydrate to the mixed solution. The synthesis method of CoS is the same as that of CuS, except that 2.6 mmol of copper nitrate hexahydrate is replaced with 2.6 mmol of cobalt nitrate hexahydrate. The synthesis of 0.5CoS–CuS and 3CoS–CuS follows the same method as that of CoS–CuS, except the amount of cobalt nitrate is adjusted to 0.02 mmol and 0.12 mmol, respectively.

3.3. In Situ FT-IR Testing

A 100 nm Au film was thermally evaporated onto a polished Si prism (PuDi vacuum PD-400, Wuhan Pudi Vacuum Technology Co., Ltd., Wuhan, China) after the substrate underwent sequential polishing with 0.05 μm Al2O3 suspension and ultrasonic cleaning in acetone/deionized water. The working electrode was fabricated by airbrushing catalyst ink onto the Au-coated prism, with in situ FT-IR measurements performed in a three-electrode spectro-electrochemical cell (working electrode: catalyst/Au film; counter electrode: Pt wire; reference electrode: Ag/AgCl) using an MCT-equipped FT-IR spectrometer (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA). Prior to measurement, electrode activation was achieved through cyclic voltammetry (0–1.8 VRHE, 0.05 V s−1) until stabilization, followed by background spectrum collection at open-circuit potential. Spectra were then acquired at 1.3 VRHE (2 min intervals over 20 min) with 4 cm−1 resolution and presented as background-subtracted transmission spectra.

3.4. Electrochemical Measurement

All electrochemical experiments were carried out at room temperature using a CHI-660 electrochemical workstation (CHI Instrument, Shanghai, China) in a typical three-electrode cell configuration. The prepared self-supported catalyst, a platinum electrode, and an Hg/HgO electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The working electrolytes for the OER process and HMFOR process were 1 M KOH (pH = 14) and 1 M KOH + 10 mM HMF (pH = 14), respectively. According to the Nernst equation, the measured potentials (vs. Hg/HgO) were expressed on the reversible hydrogen electrode (RHE) scale: E (vs. RHE) = E (vs. Hg/HgO) + 0.098 + 0.0591 × pH. The reference electrode and counter electrode were commercial Hg/HgO and platinum sheet electrodes, respectively. Before starting the electrochemical test, cyclic voltammetry (CV) scanning was performed at a sweep rate of 50 mV s−1 to fully activate the working electrode until the curves became stable. Linear sweep voltammetry (LSV) curves without iR compensation were collected at a scan rate of 5 mV s−1.

3.5. HPLC Analysis

High-performance liquid chromatography (HPLC, Agilent 1260 Infinity Series, Santa Clara, CA, USA) equipped with an ultraviolet–visible (UV–Vis) detector and an Agilent Zorbax C18 (250 mm × 4.6 mm, 5 μm) column was used to identify and quantify the substrate (HMF), intermediates (HMFCA, FFCA, and DFF), and the final oxidation product (FDCA). In a typical experiment, 20 μL of the electrolyte was collected during potentiostatic electrolysis, diluted to 2 mL with deionized water, and analyzed by HPLC. Regarding the analysis conditions, the UV–Vis detector had a wavelength of 265 nm. The mobile phases A and B were chromatographic-grade methanol and 5 mM ammonium formate aqueous solution with a volume ratio of 3:7. The flow rate was fixed at 0.6 mL min−1. The column temperature was maintained at a constant 30 °C, and each separation lasted 10 min. The external standard method was used. Standard solutions of known concentrations of commercially available pure reactants, intermediates, and final products were applied to calibration curves to identify and quantify the products. The theoretical charge of the electrochemical oxidation reaction was calculated as follows: 6 × 10 mM × 10 mL × 96,485 C mol−1 = 57.8 C. Equations (1), (2) and (3) were used to calculate the HMF conversion rate, FDCA selectivity, and Faraday efficiency, respectively.
HMF conversion rate (%) = [n (consumed HMF)/n (initial HMF)] × 100%
Product yield (%) = [n (generated specific product)/n (initial HMF)] × 100%
Faraday efficiency (%) = [n (generated FDCA)/(charge amount/(6 × F))] × 100%
where F represents the Faraday constant (96,485 C mol−1) and n represents the molar mass of the identified product.

4. Conclusions

Overall, in this study, a CoS-doped CuS catalyst, namely CoS–CuS, was successfully synthesized. The unique heterogeneous structure of CoS–CuS enables precise control over the adsorption properties of HMF and OH−, significantly enhancing the selective conversion efficiency of HMF to FDCA. Electrochemical test data indicate that compared with the single CuS catalyst, CoS–CuS exhibits a lower onset potential. At a potential of 1.40 VRHE, its current density is more than twice that of CuS, and the Faraday efficiency for generating FDCA (FE FDCA) is higher, reaching 89.1%. The cyclic performance test further confirms that CoS–CuS demonstrates excellent cyclic stability during HMFOR. Analysis combined with the results of in situ infrared spectroscopy reveals that the incorporation of CoS effectively optimizes the electronic structure and reaction microenvironment of CoS–CuS. This optimization promotes more rational adsorption behaviors of HMF and surface-adsorbed OH (OH ads) on the catalyst surface. Therefore, the introduction of CoS into CuS powerfully drives the conversion process of HMF. These findings fully demonstrate that CoS–CuS, as a highly efficient and stable catalyst with excellent performance, holds great potential for applications in the field of value-added utilization of biomass resources.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050422/s1, Figure S1: XPS S 2p spectra of the CuS and CoS–CuS samples; Figure S2: The LSV curves of 0.5 CoS–CuS, CoS–CuS, and 3 CoS–CuS for HMFOR and OER; Figure S3: EIS Nyquist plots in 1.0 m KOH with 50 mm HMF; Figure S4: CV curves and Cdl values of the CoS–CuS, CuS and CoS catalyst obtained at different scan rates in 1 M KOH and 10 mM HMF; Figure S5: N2 adsorption/desorption isotherms of CoS–CuS, CuS and CoS; Figure S6: HPLC standard curves for HMF, FDCA, FFCA, HMFCA and DFF; Figure S7: HPLC calibration curves for HMF, FDCA, FFCA, HMFCA and DFF; Figure S8: Comparative analysis of FDCA production rates over CoS–CuS, CuS, and CoS catalysts at various applied potentials; Figure S9: XRD patterns of CoS–CuS, CuS, and CoS after HMFOR testing.

Author Contributions

Conceptualization, P.C. and H.L.; Data curation, N.L.; Formal analysis, R.Y., X.Z. and X.Y.; Funding acquisition, N.L. and H.L.; Investigation, Y.Y. and Y.C.; Methodology, Y.L. (Yunliang Liu); Project administration, H.L.; Resources, H.L.; Software, Y.L. (Yixian Liu); Supervision, H.L.; Validation, Y.L. (Yaxi Li), R.Y., Y.C. and X.Z.; Visualization, H.L.; Writing—original draft, J.Y. and P.C.; Writing—review and editing, N.L. and P.P. 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 (Grants 52072152, 51802126), the Jiangsu University Jinshan Professor Fund, the Jiangsu Specially-Appointed Professor Fund, the Open Fund from Guangxi Key Laboratory of Electrochemical Energy Materials, Zhenjiang “Jinshan Talents” Project 2021, China PostDoctoral Science Foundation (2022M721372), the “Doctor of Entrepreneurship and Innovation” in Jiangsu Province (JSSCBS20221197), the Natural Science Foundation for Colleges and Universities in Jiangsu Province (No. 24KJB480003), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (Nos. KYCX22_3645 and KYCX24_3964), and the Student Research Project of Jiangsu University (Nos. 23A586 and 23B324).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of the synthesis process of CoS–CuS.
Figure 1. Schematic illustration of the synthesis process of CoS–CuS.
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Figure 2. (a) TEM and elemental mapping images of CoS–CuS, (b) XRD patterns of CoS–CuS, CuS and CoS (the black heart symbols represent CuS, while the blue diamond symbols denote CoS), (c) XPS Cu 2p spectra of the CuS and CoS–CuS samples, (d) HRTEM of CoS–CuS.
Figure 2. (a) TEM and elemental mapping images of CoS–CuS, (b) XRD patterns of CoS–CuS, CuS and CoS (the black heart symbols represent CuS, while the blue diamond symbols denote CoS), (c) XPS Cu 2p spectra of the CuS and CoS–CuS samples, (d) HRTEM of CoS–CuS.
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Figure 3. (a) LSV curves of CoS–CuS, CuS and CoS in 1.0 m KOH electrolyte with and without HMF, respectively, (b) the HMFOR current densities of CoS–CuS and CuS at 1.4 VRHE, (c) Tafel slopes of CoS–CuS and CuS in 1.0 M KOH and 10 mM HMF electrolyte, (d) OCP changes of CoS–CuS and CuS in 1.0 m KOH aqueous solution after HMF.
Figure 3. (a) LSV curves of CoS–CuS, CuS and CoS in 1.0 m KOH electrolyte with and without HMF, respectively, (b) the HMFOR current densities of CoS–CuS and CuS at 1.4 VRHE, (c) Tafel slopes of CoS–CuS and CuS in 1.0 M KOH and 10 mM HMF electrolyte, (d) OCP changes of CoS–CuS and CuS in 1.0 m KOH aqueous solution after HMF.
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Figure 4. (a) Two possible reaction pathways for HMF oxidation into FDCA, (b) current density and passed charge versus electrooxidation time for HMFOR, (c) the HPLC signals for anodic products, (d) the concentration of HMF, intermediates, and FDCA after 57.8 C charge was passed through the anode at different charges.
Figure 4. (a) Two possible reaction pathways for HMF oxidation into FDCA, (b) current density and passed charge versus electrooxidation time for HMFOR, (c) the HPLC signals for anodic products, (d) the concentration of HMF, intermediates, and FDCA after 57.8 C charge was passed through the anode at different charges.
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Figure 5. HMF conversion, FDCA yield, and FE obtained by: (a) CoS–CuS, (b) CuS, and (c) CoS for different potential test of HMFOR, (d) the stability of HMFOR on CoS–CuS.
Figure 5. HMF conversion, FDCA yield, and FE obtained by: (a) CoS–CuS, (b) CuS, and (c) CoS for different potential test of HMFOR, (d) the stability of HMFOR on CoS–CuS.
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Figure 6. In situ ATR-FTIR spectra of (a,b) CuS and (c,d) CoS–CuS for HMFOR. The graphs illustrating the time-dependent adsorption amounts of (e) OH− and (f) HMF by CuS and CoS–CuS.
Figure 6. In situ ATR-FTIR spectra of (a,b) CuS and (c,d) CoS–CuS for HMFOR. The graphs illustrating the time-dependent adsorption amounts of (e) OH− and (f) HMF by CuS and CoS–CuS.
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Figure 7. Schematic diagram of initial reaction in the inner Helmholtz layer of CuS (a) and CoS–CuS (b).
Figure 7. Schematic diagram of initial reaction in the inner Helmholtz layer of CuS (a) and CoS–CuS (b).
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Cao, P.; Liu, Y.; Yang, R.; Li, Y.; Cheng, Y.; Yu, J.; Zhang, X.; Phiri, P.; Yuan, X.; Yang, Y.; et al. Enhanced Adsorption Ability of CoS-Doped CuS for Promoting Electrochemical Oxidation of HMF. Catalysts 2025, 15, 422. https://doi.org/10.3390/catal15050422

AMA Style

Cao P, Liu Y, Yang R, Li Y, Cheng Y, Yu J, Zhang X, Phiri P, Yuan X, Yang Y, et al. Enhanced Adsorption Ability of CoS-Doped CuS for Promoting Electrochemical Oxidation of HMF. Catalysts. 2025; 15(5):422. https://doi.org/10.3390/catal15050422

Chicago/Turabian Style

Cao, Peng, Yunliang Liu, Ruihua Yang, Yaxi Li, Yuanyuan Cheng, Jingwen Yu, Xinyue Zhang, Peter Phiri, Xinya Yuan, Yi Yang, and et al. 2025. "Enhanced Adsorption Ability of CoS-Doped CuS for Promoting Electrochemical Oxidation of HMF" Catalysts 15, no. 5: 422. https://doi.org/10.3390/catal15050422

APA Style

Cao, P., Liu, Y., Yang, R., Li, Y., Cheng, Y., Yu, J., Zhang, X., Phiri, P., Yuan, X., Yang, Y., Liu, N., Liu, Y., & Li, H. (2025). Enhanced Adsorption Ability of CoS-Doped CuS for Promoting Electrochemical Oxidation of HMF. Catalysts, 15(5), 422. https://doi.org/10.3390/catal15050422

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