Electrocatalytic Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Bis(hydroxymethyl)furan Using CuIr Bimetallic Nanowires

Abstract

Electrocatalytic hydrogenation (ECH) represents an environmentally friendly pathway for converting 5-hydroxymethylfurfural (HMF) into the high-value chemical 2,5-bis(hydroxymethyl)furan (BHMF). However, its selectivity and Faradaic efficiency are often constrained by competitive hydrogen evolution at the cathode and insufficient supply of active hydrogen at the surface. To address this challenge, this study developed an Ir-decorated copper oxide nanowire catalyst (denoted as CuIr) featuring a hydrogen-rich adsorption (H_ads) surface. The incorporation of Ir significantly enhances the catalyst’s water dissociation capacity, creating abundant H_ads sources that selectively accelerate HMF hydrogenation while suppressing side reactions. Under a mild applied potential of −0.45 V vs. RHE and a current density of approximately −20 mA cm⁻², the optimal CuIr₄₀ catalyst achieved near-complete conversion of HMF (99%), a BHMF yield of 99%, and a high Faradaic efficiency of 97% within 120 min of electrolysis. Mechanistic studies reveal that this catalytic leap stems from the synergistic functional interaction between Cu and Ir sites in substrate activation and hydrogen supply. This work presents a novel strategy for designing efficient electrocatalysts for biomass hydrogenation by regulating surface H_ads concentration.

1. Introduction

With the escalating environmental and resource challenges associated with fossil fuel consumption, the conversion of renewable biomass into high-value-added chemicals has emerged as a critical pathway for achieving sustainable development [1,2,3,4]. The catalytic conversion of biomass not only offers a forward-looking solution for producing high-value chemicals but also helps alleviate the depletion of finite fossil fuel resources [5,6,7,8]. Among numerous biomass derivatives, furan-based aldehydes—particularly 5-hydroxymethylfurfural (HMF)—serve as pivotal platform molecules bridging biomass resources and the chemical industry due to their unique chemical structure [9,10,11]. HMF can be synthesized from biomass carbohydrates and demonstrates exceptional application potential among numerous platform compounds [12,13]. Its hydrogenation product, 2,5-bis(hydroxymethyl)furan (BHMF), serves as a crucial precursor for synthesizing bio-based polymers, fuels, and pharmaceutical intermediates, offering broad application prospects [14,15,16,17,18]. Currently, the industrial conversion of HMF to BHMF typically requires high-temperature and high-pressure conditions, relying on H₂ as the hydrogen source [19]. Consequently, developing a catalytic system capable of efficiently converting HMF under mild conditions holds significant importance [5,20,21].

In recent years, electrocatalytic hydrogenation (ECH) technology has garnered significant attention as a green and highly efficient alternative solution [22]. Due to its ability to achieve highly efficient selective reactions under mild conditions, coupled with the advantages of energy-tunable and green sustainable properties, it is regarded as a highly promising catalytic technology [23]. In ECH systems, selective regulation of reaction products can be achieved by adjusting parameters such as current density and voltage [24]. Copper-based, nickel-based, and other non-precious metal catalysts have been widely utilized in selective hydrogenation reactions, allowing for the targeted synthesis of desired products through the modulation of metal electronic structures, surface acid–base properties, and support synergistic effects [25,26]. Among numerous electrode materials, copper(Cu)-based catalysts demonstrate application potential in the ECH of biomass-derived aldehydes due to their low cost and moderate adsorption strength towards carbonyl groups [27,28,29]. The rich crystal surface structure provides a structural design basis for regulating reaction pathways [30].

In this context, the use of Cu-based catalysts for the hydrogenation of HMF to BHMF has emerged as a research focus. It has been demonstrated that Cu can effectively adsorb and activate the C=O bond in HMF molecules, which is crucial for its hydrogenation conversion [31]. For example, Yang et al. systematically reviewed studies on the electrocatalytic hydrogenation of furanic aldehydes, detailing reaction parameters and mechanisms involving HMF [31]. Elsayed et al. achieved efficient catalytic reduction of HMF to BHMF under hydrogen-free conditions using a copper–iron oxide bimetallic nanocatalyst [32]. To further enhance performance, researchers often combine Cu with other metals to construct bimetallic catalysts. For instance, Liu et al. prepared CuM (M = Pt, Pd, Au, Ag) bimetallic catalysts via a simple replacement method, and these catalysts exhibited enhanced activity in the electrocatalytic conversion of furanic aldehydes [33]. Studies have also indicated that combining Cu with noble metals (e.g., Pt, Pd) or non-noble metals can effectively modulate the electronic structure of Cu, strengthen its adsorption and activation toward HMF, and simultaneously improve the catalyst’s ability to generate adsorbed hydrogen (H_ads), thereby enhancing the selectivity and reaction kinetics for BHMF [33,34]. However, single Cu-based catalysts frequently encounter challenges of low Faraday efficiency (FE) and BHMF selectivity in the hydrogenation of HMF. This is primarily attributable to competition from the hydrogen evolution reaction (HER) and insufficient supply of active H_ads on the catalyst surface, leading to side reactions [27,35]. Research indicates that the generation and utilization efficiency of H_ads is one of the key factors determining the selectivity of furan–aldehyde compounds toward ECH [36,37]. Its formation depends on the characteristics of the electrode surface and involves direct interactions between the reactants and the electrode surface [38].

To overcome the aforementioned limitations, constructing bimetallic catalysts to harness intermetallic synergistic effects represents an effective strategy for enhancing the performance of copper-based catalysts [39,40]. By combining two metals, not only can their respective advantages be leveraged, but the overall catalytic performance, selectivity, and stability can also be optimized through adjustments to the electronic structure [41]. Common precious metals such as platinum (Pt) and palladium (Pd) are widely used due to their high reactivity [42], but they come with a high cost. In contrast, iridium (Ir) is regarded as an ideal component for promoting H_ads generation due to its exceptional water dissociation capability and moderate adsorption strength towards hydrogen species [43,44]. Introducing Ir into the Cu matrix is expected to significantly enhance the H_ads enrichment capacity on the catalyst surface by constructing bimetallic active sites [45]. Accordingly, this study designed and prepared a novel nanowire catalyst featuring iridium species decorated on a copper oxide substrate. Through an electrolytically induced displacement method, Ir was etched onto the surface of copper nanowires, successfully constructing this composite architecture. Electron spin resonance (ESR) analysis reveals that this catalyst possesses highly active H_ads, effectively promoting water dissociation and establishing a H_ads-rich interfacial environment. Kinetic analyses and spectroscopic characterizations demonstrate that Ir species catalyze the formation of interfacial H_ads, thereby enhancing HMF hydrogenation selectivity and effectively suppressing undesirable dimerization side reactions. This research offers novel insights for the rational design of interfacial aquatic environments and provides a viable approach for the efficient catalytic conversion of biomass-derived molecules.

2. Results and Discussion

2.1. Characterization of the Physicochemical Properties of Catalysts

The synthesis process for the Ir-decorated copper oxide nanowire catalyst (denoted as CuIr_x) is exhibited in Figure 1. Employing the galvanic replacement method, copper nanowires were immersed in an IrCl₃ solution for surface etching and deposition, successfully yielding a CuIr bimetallic nanowire catalyst loaded with iridium species. The iridium content of the samples was quantified by ICP, as shown in Table S1.

Figure 1. The schematic illustration for CuIr_x preparation.

As shown in Figure S2, the SEM image of the Cu nanowire clearly demonstrates its nanowire structure. With increasing electrostatic displacement time, the CuIr₁₀, CuIr₂₀, CuIr₃₀, CuIr₄₀, and CuIr₅₀ samples retained their distinct nanowire structure without exhibiting any signs of damage or deformation. Figure 2 presents a comparative analysis of CuIr₄₀ and Cu nanowires. Figure 2a–d shows SEM and TEM images of Cu nanowires before and after galvanic replacement, respectively. It can be observed that the surface of the etched nanowires becomes rougher, which will enhance contact between the catalyst surface and HMF. Figure 2e–h presents the EDX elemental distribution maps for CuIr₄₀, revealing uniform dispersion of Ir across the surface. This confirms successful galvanic replacement, wherein Ir has etched away portions of the Cu atoms. Furthermore, Figure 2i,j displays the SAED patterns for CuIr₄₀ and Cu NW. The results indicate no new diffraction rings emerged prior to or following the etching process. Further HRTEM characterization (Figure 2k,l) revealed, through measurement of lattice spacings, that the (111), (200) and (220) crystal planes of Cu exhibit spacings of 0.2325 nm, 0.1834 nm and 0.1243 nm, respectively, whereas the Ir (111) crystal plane spacing is 0.2325 nm. (200), and (220) crystal planes were determined to be 0.2325 nm, 0.1834 nm, and 0.1243 nm, respectively, whilst the Ir (111) plane measured 0.2467 nm. Based on the distribution of surface lattice planes, this indicates that the Ir etching did not remain confined to the surface but penetrated into the nanowire structure.

Figure 2. The SEM images of samples Cu NW and CuIr₄₀ (a,b), TEM images of Cu NW and CuIr₄₀ (c,d), EDX images of CuIr₄₀ (e–h), SAED images of CuIr₄₀ and Cu NW (i,j), and HRTEM images of Cu NW and CuIr₄₀ (k,l).

To investigate the influence of varying metal deposition amounts on the specific surface area of the catalyst, N₂ adsorption–desorption experiments were performed. The results are presented in Figure 3, with corresponding data summarized in Table 1. The presence of hysteresis loops in the isotherms (Figure 3) confirm the mesoporous nature of all catalysts. Compared to the pristine Cu NW, the introduction of Ir generally resulted in an increase in specific surface area and a shift in the average pore size toward a smaller range. This structural evolution toward more defined mesoporosity likely enhances the substrate adsorption capacity. The observed non-linear variation in surface area with etching time is attributable to the complex galvanic replacement process, which can lead to non-uniform etching and deposition. The increased specific surface area provides a greater number of active sites and improves metal dispersion, strengthening the interaction between Ir and the nanowire support. This contributes to enhanced catalyst stability and water dissociation capability, thereby facilitating the supply of H_ads for the protonation step in the hydrogenation reaction.

Figure 3. The N₂ adsorption-desorption isotherms and pore size distribution of samples Cu NW (a,a1), CuIr₁₀ (b,b1), CuIr₂₀ (c,c1), CuIr₃₀ (d,d1), CuIr₄₀ (e,e1) and CuIr₅₀ (f,f1).

Table 1. The S_BET value and average pore diameter of catalysts.

Catalysts	SBET (m2 g−1)	Average Pore Size (nm)
Cu NW	4.9365	12.848
CuIr10	5.4038	10.852
CuIr20	5.2135	10.271
CuIr30	5.3174	10.735
CuIr40	5.1735	10.116
CuIr50	5.2671	10.363

As shown in Figure 4, X-ray diffraction (XRD) was used to analyze the crystal structures of Cu NW and CuIr catalysts. Three characteristic diffraction peaks were observed at 43.7°, 50.9°, and 74.3°, corresponding to the Cu (111), Cu (200), and Cu (220) planes, respectively. These characteristic peaks confirm the presence of a well-defined Cu crystalline structure in the samples. No distinct peaks corresponding to Ir were detected, and no significant peak shifts in the Cu diffraction pattern were observed after Ir etching, which could be attributed to the low Ir content [46].

Figure 4. XRD pattern of prepared catalysts.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the electronic structure changes in surface elements (Ir, Cu, and O) in the catalysts, as shown in Figure 5. The full XPS spectra of CuIr₄₀ and Cu NW are displayed in Figure 5a. Figure 5b shows the Ir element peaks in the CuIr₄₀ catalyst, indicating the presence of Ir primarily in the metallic state with a small portion in the oxidized state. A detailed analysis of the O 1s region for CuIr₄₀ and Cu NW is shown in Figure 5c. The results reveal two primary oxidation states of oxygen species on the surface, corresponding to Cu-O and Cu-O-Cu. The former arises from slight oxidation of the electrode surface and adsorption of oxygen atoms, while the latter likely results from the partial conversion of Cu₂O during the calcination process [47]. Figure 5d illustrates the change in the valence state of Cu on the surfaces of CuIr₄₀ and Cu NW. Both catalysts exhibit strong peaks associated with Cu⁰/Cu⁺ species, with only minor satellite peaks of Cu²⁺, indicating that most of the surface copper species have been reduced to lower oxidation states or metallic copper [48]. Notably, a positive shift of 0.43 eV in the Cu peak was observed on the surface of CuIr₄₀, which was absent in Cu NW, suggesting the formation of a Cu-Ir bimetallic interaction at the catalyst surface.

Figure 5. The XPS spectra of CuIr₄₀ and Cu NW (a) high-resolution spectra of Ir 4f (b), O 1s (c), and Cu 2p (d).

2.2. Catalytic Performance

As shown in Figure 6, to evaluate the electrocatalytic performance of the prepared catalysts, linear sweep voltammetry (LSV), cyclic voltammetry (CV), and Tafel slope measurements were carried out for CuIr_x nanowire catalysts with different immersion durations. Figure 6a,b show that the CuIr₄₀ catalyst exhibited the highest current density over a wide potential range both before and after the addition of HMF substrate, significantly outperforming the other samples. Therefore, CuIr₄₀ was selected as the optimal catalyst for further investigation. Figure 6c presents the CV curves recorded in 0.1 M KOH. All catalysts displayed pronounced peaks around 0.4–0.45 V, which can be attributed to the desorption of H_ads species, indicating that H_ads desorption occurred at these potentials. Notably, the desorption peak observed on CuIr₄₀ was significantly more intense, implying a higher generation of H_ads on its surface. This observation was further confirmed by ESR analysis, which highlighted the crucial role of Ir in enhancing H_ads formation. Figure 6d presents the Tafel plots of the different catalysts. In line with the LSV results, CuIr₄₀ exhibited the lowest Tafel slope, indicating superior reaction kinetics and the lowest overpotential barrier. This also suggests that the proton–electron coupling process on its surface is well-regulated, and the overall hydrogenation reaction is primarily governed by the adsorption and desorption behavior of H_ads.

Figure 6. LSV curves of different catalysts measured in 0.1 M KOH without (a) and with (b) 20 mM HMF, CV curves of different catalysts (c), and Tafel plots of different catalysts (d).

As shown in Figure 7, to further investigate the charge transfer capability of the catalysts during the electrocatalytic process, electrochemical impedance spectroscopy (EIS) was conducted on different catalyst samples. The results reveal that the pristine Cu NW sample, without any metal doping, exhibits relatively high charge transfer resistance. In contrast, upon the incorporation of Ir, the CuIr_x series catalysts show significantly reduced impedance values, indicating improved electronic conductivity. This suggests that the introduction of Ir effectively modifies the surface electronic structure of the catalyst, enhances charge migration efficiency between active sites, and lowers the energy barrier for interfacial charge transfer. Meanwhile, the proton/intermediate diffusion efficiency on the catalyst surface is also improved, indicating that the CuIr_x catalysts possess higher intrinsic activity and faster reaction kinetics in the ECH of HMF. These findings further confirm the significant role of bimetallic synergy in enhancing catalytic performance. The observed non-monotonic variation in charge transfer resistance (Rct) with increasing Ir content can be attributed to the evolution of surface electronic structure and active site distribution. Initially, the introduction of Ir enhances the overall conductivity and facilitates charge transfer, leading to a decrease in Rct. However, beyond an optimal loading (CuIr₄₀), further Ir incorporation may slightly disrupt the optimal electronic coupling between Cu and Ir sites, as inferred from the positive shift in the Cu 2p XPS peak (Figure 5d). This could modestly increase the interfacial charge transfer barrier. Consequently, the minimum Rct attained for CuIr₄₀ corresponds to the most favorable electronic environment for efficient charge transfer, which aligns well with its highest activity and selectivity in HMF hydrogenation. This optimal charge transfer property of CuIr₄₀ is consistent with its superior catalytic activity and selectivity observed in Figure 6 and Figure 8. This structure–activity relationship underscores the importance of optimizing bimetallic composition to achieve synergistic effects in electrocatalytic performance.

Figure 7. EIS spectra of different catalysts.

Figure 8. Catalytic performance of different samples (a,b), reaction kinetics of the best sample (c), performance at different substrate concentrations (d), performance at different voltages (e) and regeneration performance (f).

To investigate the effect of Ir introduction on the hydrogenation performance of HMF, electrochemical measurements and product analyses were conducted on samples subjected to different etching times. As shown in Figure 8a,b, which present the hydrogenation capabilities of different catalysts, the obtained results are consistent with the data from electrochemical tests. The introduction of Ir significantly enhances the selectivity toward BHMF and improves the FE. This demonstrates that increasing the generation of H_ads indeed exerts a positive influence on the hydrogenation reaction. Compared with the Cu NW catalyst, the CuIr_x series not only improved the conversion of HMF but also enhanced the yield of BHMF. The FE for BHMF formation exhibited a volcano-shaped trend across the series, initially increasing and then declining with higher Ir content, with CuIr₄₀ showing the maximum value. These results indicate that CuIr₄₀ is the optimal catalyst. Figure 8c illustrates the hydrogenation kinetics of CuIr₄₀. Both yield and conversion increase synchronously during the reaction, but a fast-to-slow trend is observed over time due to the gradual depletion of HMF substrate and the reduced adsorption efficiency. After 120 min of reaction, a conversion rate and product yield of 99% were achieved, with almost no detectable byproducts. Figure 8d presents the hydrogenation performance of CuIr₄₀ under different substrate concentrations. When the substrate concentration was increased to 2.5 and 5 times the original level, both the FE and selectivity decreased, which may be attributed to insufficient H_ads generation to match the protonation pathway. This result also indicates that the optimal concentration of HMF is 20 mM. Figure 8e shows the hydrogenation performance of CuIr₄₀ at different applied voltages. As the voltage increased, the conversion rate remained relatively stable, but both FE and selectivity declined. This phenomenon could result from the occurrence of HER and excessive hydrogenation involving 4 H_ads under high H_ads conditions. Thus, −0.45 V (vs. RHE) is identified as the optimal applied potential. Figure 8f presents the regeneration performance of CuIr₄₀. After five reaction cycles, the catalyst maintained its performance without significant degradation, indicating excellent reusability.

2.3. Mechanistic Insights into the HMF-to-BHMF Hydrogenation Pathway over CuIr_x Catalysts

As shown in Figure 9, the adsorption and reaction process of HMF on the surface of the CuIr₄₀ electrode was analyzed by electrochemical in situ infrared (ECIR). During the initial adsorption stage, a distinct C=O stretching vibration peak appeared at 1650 cm⁻¹ in the spectrum, and its intensity gradually increased with prolonged reaction time, indicating a stable and continuous adsorption behavior of the aldehyde group from HMF on the catalyst surface. Concurrently, a broad peak associated with the hydroxymethyl (-CH₂OH) group was observed around 3300 cm⁻¹, suggesting that the hydroxymethyl moiety of the HMF molecule also effectively interacted with the catalytic surface. Furthermore, characteristic vibration peaks of the furan ring skeleton were clearly visible, further demonstrating the favorable overall adsorption capability of the CuIr₄₀ catalyst for HMF. This multi-site adsorption behavior facilitates the stable formation of reaction intermediates and promotes subsequent reaction steps [49]. As the electrocatalytic reaction progressed, new infrared bands associated with radical intermediates began to emerge and showed increasing intensity over time. This indicates that the aldehyde group underwent electron transfer on the electrode surface, leading to the formation of radicals such as C·-OH and CH-O·. The generation of these radical intermediates requires the involvement of adsorbed hydrogen species, implying that the CuIr₄₀ surface provides a sufficient quantity and diversity of H_ads species to support the stepwise hydrogenation of the aldehyde group. Considering that Ir sites on the catalyst surface possess high water dissociation activity, it is inferred that a H_ads-rich environment is formed under the synergistic effect of the Cu-Ir bimetallic system. This environment significantly accelerates the conversion of radical intermediates into the target product BHMF. This finding not only confirms the excellent HMF adsorption and catalytic properties of CuIr₄₀ but also reveals the evolution pathway of key intermediates and highlights the mechanistic role of H_ads in the electrocatalytic hydrogenation process.

Figure 9. ECIR spectra of HMF adsorption and reaction on CuIr₄₀.

The generation of radical species was analyzed by ESR spectroscopy, as presented in Figure 10. The introduction of Ir notably enhanced the formation of both DMPO-H* and DMPO-C* adducts (DMPO: 5,5-dimethyl-1-pyrroline N-oxide, a spin-trapping agent). Quantitative integration of the ESR spectra revealed that the signal intensities corresponding to DMPO-H* and DMPO-C* increased by factors of 1.51 and 2.39, respectively, for CuIr40 relative to Cu NW (Supplementary Figure S5). This differential enhancement indicates that Ir not only promotes the formation of active hydrogen species (H*) but also, more markedly, facilitates the generation of carbon-centered radicals (C*) derived from HMF activation. The pronounced increase in DMPO-C* signals suggests that Ir effectively assists in the cleavage and electron transfer of the carbonyl (C=O) group in HMF. The concurrent rise in both radical types supports a synergistic mechanism in which the abundant H* species—supplied through Ir-enhanced water dissociation—efficiently couple with the readily formed C* intermediates on the catalyst surface. This cooperative radical environment directly accelerates the hydrogenation kinetics and improves the overall catalytic activity. Furthermore, the H_ads-rich surface effectively suppresses undesirable dimerization side reactions. Together, these results demonstrate that Ir significantly boosts catalytic performance by co-promoting the formation and coupling of key radical species essential for the selective hydrogenation pathway [50].

Figure 10. ESR spectra of DMPO-trapped radical adducts for different catalysts. DMPO-C* adduct for Cu NW (a), DMPO-H* adduct for Cu NW (b), DMPO-C* adduct for CuIr₄₀ (c) and DMPO-H* adduct for CuIr₄₀ (d).

In summary, as shown in Figure 11, the electrocatalytic hydrogenation mechanism of HMF involves electron transfer from the carbonyl group, the formation of a radical intermediate, and subsequent stepwise hydrogenation. In the CuIr bimetallic catalyst system, Cu primarily facilitates the adsorption and activation of HMF molecules, particularly through directional adsorption at the C=O carbonyl group and initial electron transfer. The introduction of Ir significantly alters the catalyst’s surface reaction environment, exhibiting notably higher activity in water molecule dissociation behavior. Ir exhibits excellent water dissociation capability, effectively decomposing water molecules into protons and hydroxyl groups. The generated H⁺ ions then bind electrons at the electrode surface to form H_ads. This process significantly increases the concentration of H_ads on the catalyst surface, thereby creating a H_ads-rich environment. This enriched H_ads environment exerts a significant influence on the hydrogenation process: First, it enables rapid coupling reactions with free radical intermediates adsorbed on the catalyst surface (e.g., C·-OH, CH-O·), promoting the conversion of intermediates into the target product BHMF. Second, the high H_ads concentration helps lower the reaction energy barrier and increase the reaction rate, thereby achieving higher Faraday efficiency and yield. Additionally, a certain degree of electronic interaction or alloying behavior may exist between Cu and Ir, thereby regulating the electronic structure of Cu and further optimizing its adsorption behavior and reaction selectivity. This synergistic effect plays a crucial role in promoting the directional adsorption of HMF molecules, stabilizing intermediate structures, and enhancing overall catalytic performance. Therefore, the introduction of Ir not only provides active sites for water dissociation but also plays a central role in promoting HMF-to-BHMF conversion by establishing a H_ads-rich catalytic environment.

Figure 11. Mechanism diagram of electrocatalytic hydrogenation of HMF to form BHMF in H_ads-rich environment.

3. Experimental Section

3.1. Reagents and Materials

Copper foam (CF, thickness 1.0 mm) was purchased from Jiangsu Province. Ammonium persulfate (AR), sodium sulfate (AR), sodium hydroxide (AR), ammonium sulfate (AR), potassium hydroxide (AR), hydrochloric acid (AR), ethanol (AR) and iridium (III) chloride tetrahydrate (IrCl₃·4H₂O, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acetonitrile (HPLC grade) and methanol (HPLC grade) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). 5-Hydroxymethylfurfural (HMF, AR) and 2,5-bis(hydroxymethyl)furan (BHMF, AR) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Deionized water was produced by a Milli-Q water purification system. All chemicals were used as received without further purification.

3.2. Synthesis of Catalyst

The pretreated clean foam copper (1 cm × 2 cm) was immersed into an 80 mL aqueous solution containing 6.4 g NaOH and 2.0 g (NH₄)₂S₂O₈, then reacted at room temperature for 30 min. The resulting sample was washed with water and dried with nitrogen gas to obtain the Cu(OH)₂ nanowire precursor. The precursor was heated at 5 °C/min to 200 °C under air atmosphere and calcined for 2 h to convert it into CuO nanowires. Using the CuO nanowires as the working electrode, constant-current reduction was performed at a current density of −40 mA cm⁻² in 0.5 M Na₂SO₄ electrolyte for 30 min, yielding Cu nanowires (Cu NW). Then, the prepared Cu NW were immersed in a 0.3 mM IrCl₃·4H₂O solution for different durations (10 min, 20 min, 30 min, 40 min, and 50 min) to perform galvanic replacement. This process yielded Ir-decorated copper oxide nanowires with varying Ir contents. The obtained samples were designated as CuIr₁₀, CuIr₂₀, CuIr₃₀, CuIr₄₀, and CuIr₅₀, respectively, where the “CuIr” notation refers to this Ir-decorated copper oxide structure. After the galvanic replacement reaction, the samples were thoroughly washed with deionized water to remove residual solution and then dried under a nitrogen flow.

3.3. Electrochemical Testing and Product Analysis

Electrochemical testing was conducted in a 40 mL H-type electrolytic cell equipped with a cation-exchange membrane. In the experiment, the prepared catalyst, electrolytic platinum electrode, and mercury oxide electrode served as the working electrode, counter electrode, and reference electrode, respectively. The working electrode had a geometric surface area of 1 cm². The electrolyte consisted of a 0.1 M KOH solution containing 20 mM HMF (pH = 13). All potentials were converted to reversible hydrogen electrode (RHE) potentials. Constant-potential electrolysis was performed using the chronoamperometric method. The reaction solution was filtered through a 0.22 μm filter head and analyzed quantitatively for HMF and BHMF concentrations using high-performance liquid chromatography (HPLC). In addition, The Faradaic efficiency of HMF was calculated using Equation (1). The HMF yield (%) was calculated by Equation (2), and the HMF conversion (%) was calculated using Equation (3).

$$\mathrm{F}\mathrm{E}\left(\%\right)={\displaystyle \frac{\mathrm{n} \times \mathrm{F} \times \mathrm{Q}}{\mathrm{I} \times \mathrm{t}}}$$

(1)

Here, n represents the molar quantity of HMF after the reaction (mol), F denotes Faraday’s constant (96,485 C mol⁻¹), Q indicates the number of electrons transferred per product molecule, I is the current density (A cm⁻²), and t is the reaction time measured via the electrochemical workstation (s).

$$\mathrm{Conversion}\left(\%\right)={\displaystyle \frac{{\mathrm{n}}_{\mathrm{HMF},0} - {\mathrm{n}}_{\mathrm{HMF}}}{{\mathrm{n}}_{\mathrm{HMF},0}}}$$

(2)

$$\mathrm{Yield}\left(\%\right)={\displaystyle \frac{{\mathrm{n}}_{\mathrm{BHMF}}}{{\mathrm{n}}_{\mathrm{HMF},0}}}$$

(3)

Here, n_HMF,0 denotes the molar quantity of HMF prior to the reaction (mol), n_HMF represents the molar quantity of HMF after the reaction (mol), and n_BHMF indicates the molar quantity of BHMF measured via HPLC after the reaction (mol).

3.4. Cycling Tests

Recyclability tests were performed to evaluate the stability of the catalyst. Under identical electrochemical conditions, the catalyst with the best performance was subjected to repeated HMF hydrogenation cycles. After each reaction, the catalyst was thoroughly washed with deionized water and anhydrous ethanol, then dried under a nitrogen stream. The treated catalyst was reused in the next cycle, and a total of four cycles were carried out. The composition of the reaction solution was analyzed using HPLC.

4. Conclusions

In this study, a series of Ir-decorated copper oxide nanowire electrocatalysts were successfully synthesized for the electrocatalytic hydrogenation of HMF to BHMF. By enhancing the water dissociation capability and constructing a H_ads-rich environment, the catalysts exhibited outstanding catalytic performance. Through the rational control of the number of water dissociation active sites, the optimized CuIr catalyst achieved a remarkable HMF conversion of 99% and BHMF yield of 99% at a current density of −20 mA cm⁻², significantly outperforming the pristine Cu nanowire catalyst. Comprehensive characterizations revealed that Cu plays a key role in the adsorption and activation of the carbonyl (C=O) group, serving as the main active site for HMF binding. Meanwhile, the incorporation of Ir introduces highly active sites for water dissociation, markedly enhancing the generation of H_ads on the catalyst surface. The synergistic effect between Cu and Ir effectively promotes the protonation pathway of the hydrogenation reaction, facilitating the coupling between HMF-derived intermediates and H_ads, while suppressing the formation of undesired dimers. In addition, the nanowire morphology strengthens the interaction between HMF molecules and the catalyst surface, contributing to the high catalytic efficiency. The catalyst also demonstrated excellent stability in repeated cycling experiments, with no significant loss in activity. These findings highlight the crucial role of optimized hydrogen activation in the hydrogenation of furanic aldehydes, offering new insights into the design of electrocatalysts with enriched H_ads environments. This work provides valuable guidance for the development of high-performance Cu-based electrocatalysts for the efficient hydrogenation of furfural derivatives into furan alcohols.