Co–Fe-modified nitrogen-doped multi-walled carbon nanotubes for alkaline hydrogen evolution: performance and structure–performance correlations

Abstract

Developing earth-abundant hydrogen evolution reaction (HER) electrocatalysts that combine high activity with robust stability is critical for practical alkaline water electrolysis. Herein, nitrogen-doped multi-walled carbon nanotubes were employed as conductive tubular scaffolds to construct Fe-NCNT, Co-NCNT, as well as CoFe-NCNT through pyrolysis of metal–organic coordination precursors. Their morphology, crystalline structure, surface chemistry, defect features, and textural properties were systematically examined through SEM, TEM, XRD, XPS, Raman spectroscopy, alongside N₂ adsorption–desorption measurements. Meanwhile, 1 M KOH was used to evaluate their HER performance. CoFe-NCNT preserves a well-defined hollow nanotubular architecture while exhibiting a higher defect density and larger specific surface area, which favor the formation and exposure of Co–N_x/Fe–N_x dual active sites. As a result, for CoFe-NCNT, the overpotential needed to achieve 10 mA cm^–2 is as low as 30.2 mV, and the Tafel slope is 68.4 mV dec⁻¹, outperforming Fe-NCNT, Co-NCNT, and commercial Pt/C, together with good catalytic stability. From the above findings, coupling Co and Fe sites into the N-doped carbon nanotube framework can effectively enhance alkaline HER catalysis.

1 Introduction

Against the backdrop of the global shift into low-carbon energy systems, green hydrogen has been widely regarded as an important energy carrier for renewable energy storage, industrial decarbonization, and the replacement of conventional fossil fuels. Compared with hydrogen production from fossil resources, water electrolysis can be directly coupled with renewable electricity, like wind and solar power, and is therefore considered an appropriate way to produce high-purity hydrogen (Kumar and Lim, 2022). Hydrogen evolution reaction (HER), as a central cathodic half-reaction in water electrolysis, largely determines the overall energy consumption of hydrogen production (Qadeer et al., 2024). Developing electrocatalysts that combine high activity, persistent durability, and low overpotential is therefore an essential issue for improving alkaline water electrolysis efficiency. Recently, transition-metal-based catalysts are widely investigated for alkaline water splitting because of their tunable valence states, rich electronic structures, and relatively low cost (Lakhan et al., 2024). However, under practical high-current-density operation, these catalysts still suffer from insufficient utilization of active sites, hindered bubble release, limited mass transport, and structural degradation (Zang et al., 2024).

Pt-based materials remain an important benchmark for evaluating HER performance; however, they are expensive and scarce, which continue to limit industrial deployment. To increase precious-metal utilization, coupling Pt single atoms with NiO/Ni heterostructures has been shown to modulate the adsorption of OH* and H*, thereby accelerating alkaline HER kinetics (Zhou et al., 2021). Further studies on axial ligand regulation of Pt single atoms have demonstrated that the local coordination environment around the active center plays a decisive role in alkaline hydrogen evolution (Zhang et al., 2022). In single-atom catalysts, the metal oxidation state shows a volcano-type relationship with HER activity, suggesting that the effective charge state of metal center is important for governing H* adsorption strength (Cao et al., 2022). In addition, pyridinic-N-strengthened d–π interactions promote water dissociation at Pt sites (Yu et al., 2022), while the synergistic contribution of Pt, Ni, and P in PtNiP nanowires has also been confirmed to promote the adsorption and dissociation of water (Xiao et al., 2025). For high-current-density operation, Ni single atoms can be connected with Ru nanoparticles through carbon bridges, which can improve the stability of alkaline HER catalysis (Yao et al., 2024; Li et al., 2020). At the same time, non-precious-metal single-atom catalysts arouse wide attention because of their tunable coordination structures and great atom utilization (Ren et al., 2023; Sun J. et al., 2024; Alvarado-Leal et al., 2024). For example, anchoring Co₄N nanoparticles and single-atom Co sites within N-doped carbon nanotube arrays can enhance HER activity, which is achieved through the adjustment of d-band center (Cao et al., 2021). Co single atoms confined within N-doped carbon nanoboxes can also form stable Co–N coordination sites and promote overall water splitting (Li et al., 2023). Recent studies on N-doped carbon nanotubes bearing bimetallic sites further suggest that intermetallic electronic communication can markedly improve alkaline HER kinetics (Gao et al., 2024).

Although single-metal M–N–C catalysts have made considerable progress, an isolated metal center is often insufficient to optimize the multiple elementary steps involved in alkaline HER, including water dissociation, H* adsorption, and H₂ desorption. Dual-atom or bimetallic single-atom catalysts can offer higher metal loading and more diverse active configurations, while regulating intermediate adsorption through electronic coupling between adjacent metal centers (Lu and Wu, 2024). Recent studies on bimetallic single-atom catalysts used to split water have shown that dual-metal centers can generate both metal–metal and metal–support synergistic effects. Wu et al. (2022) employed an interlayer-confinement strategy to construct Fe–Co dual-single-atom sites for oxygen electrocatalysis, demonstrating that confinement can suppress metal migration during pyrolysis. Feng et al., 2024 subsequently prepared abundant FeCo dual-single atoms on N-doped carbon nanocages and highlighted the importance of atomic-site exposure and coordination regulation. More recently, Li et al. (2024) combined AC-HAADF-STEM, Fe/Co K-edge XAFS, and theoretical calculations to identify asymmetric Fe–N–Co configurations and relate their electronic structures to bifunctional oxygen electrocatalysis. Of more direct relevance to HER, Tao et al. (2025) reported phosphorus-enhanced Fe–Co bimetallic single-atom catalysts in which the local bridging environment regulates hydrogen-intermediate adsorption. Even so, their genuine active configurations and structure–activity relationships still require further experimental clarification (Deshmukh et al., 2025). Fe/Co-based bimetallic systems are particularly attractive because of their strong electronic complementarity. For instance, P-bridged FeCo–NC catalysts have been shown to optimize H* adsorption in HER by using neighboring Co atoms to tune the electronic environment of Fe sites (Tao et al., 2025). However, most existing studies on FeCo–N–C materials have focused on ORR/OER catalysis or zinc–air batteries. Representative examples include Fe–Co dual-metal sites that enhance oxygen electrocatalysis (He et al., 2022), Fe–Co dual-single-atom structures that improve cathodic reactions in zinc–air batteries (Wu et al., 2022), and ZIF-derived Fe–Co dual-atom sites that promote ORR kinetics (Zhang et al., 2023). Other related studies, including FeCo dual single atoms supported onto N-doped carbon nanocages (Cheng et al., 2023), FeCo alloys inside N-doped carbon (Kim et al., 2022), neighboring Co single atoms regulating Fe site electronic distribution (Pei et al., 2024), and real active site identification within multimetallic N–C catalysts (Sun B. et al., 2024), all highlight the importance of bimetallic synergy. Nevertheless, systematic studies that Co/Fe-containing species associated with the N/O-functionalized carbon framework and directly correlate their structure with HER performance remain limited.

In this work, carboxylated multi-walled carbon nanotubes were used as conductive scaffolds, while 2-methylimidazole served as both the nitrogen source and coordination ligand. Fe-NCNT, Co-NCNT, and CoFe-NCNT were prepared through pyrolyzing metal–organic coordination precursors. The sample morphology, crystalline structure, elemental valence states, defect features, and specific surface areas were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, alongside N₂ adsorption–desorption measurements. The 1 M KOH was adopted for evaluating their HER performance with linear sweep voltammetry (LSV), Tafel analysis, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), electrochemically active surface area (ECSA) analysis, and durability tests. This study focuses on clarifying how Co/Fe bimetallic co-doping synergistically enhances the exposure of active sites, electronic-structure modulation, charge transfer, and electrolyte diffusion in N-doped carbon nanotubes. The findings provide experimental evidence for rationally designing low-cost, high-efficiency electrocatalysts with high stability for alkaline HER.

2 Materials and methods

2.1 Chemicals

MWCNTs(>98%, length 10–30 μm, outer diameter 10–20 nm, carboxylated) used in the present work were obtained in Shanghai Yansu Technology Co., Ltd (Shanghai, China; mall.shiyanjia.com). 2-methylimidazole, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, anhydrous ethanol, KOH were acquired in Shanghai Macklin Biochemical Co., Ltd, while 5 wt% Nafion solution in Sigma Aldrich. The chemicals were utilized as received.

2.2 Material preparation

Co- and Fe-containing nitrogen-doped carbon nanotube materials were prepared through the pyrolysis of metal–organic coordination precursors. Briefly, multi-walled carbon nanotubes (MWCNTs) were first refluxed in a concentrated H₂SO₄/HNO₃ mixed solution at the 3:1 (v/v) ratio for a 6-h duration. This acid treatment introduced oxygen-containing functional groups, including hydroxyl (–OH) and carboxyl (–COOH), thereby improving hydrophilicity and coordination ability of the nanotubes. After washing and drying, oxidized MWCNTs were obtained. Subsequently, using 2-methylimidazole as both the nitrogen source and coordination ligand, the oxidized MWCNTs, Co(NO₃)₂·6H₂O, as well as Fe(NO₃)₃·9H₂O were dispersed into anhydrous ethanol at the designed ratio, followed by ultrasonic treatment and stirring for 24 h. The metal ions coordinated with the organic ligand and were simultaneously adsorbed onto the surface of the carbon nanotubes, forming a Co–Fe–organic ligand–MWCNT precursor. The Co-to-Fe molar ratio was controlled at 1:1. The obtained precursor was then put into the tube furnace for heating at the 5 °C min^–1 ramping rate to 900 °C at an Ar atmosphere, before additional 3 h of calcination under 900 °C. During pyrolysis, organic ligand was converted into pyrrolic N, pyridinic N, alongside graphitic N species, enabling nitrogen doping of the carbon nanotubes. Meanwhile, the Co and Fe precursors were converted into metal-containing species associated with the nitrogen- and oxygen-functionalized carbon framework. Although these functionalities may suppress extensive metal migration during pyrolysis, the present characterization does not determine the nuclearity or exact local coordination structures of the resulting Co and Fe species.

For comparison, we prepared corresponding monometallic reference catalysts, Co-NCNT and Fe-NCNT, as previously described.

2.3 Materials characterization

Sample morphology was examined through SEM (ZEISS Sigma 300, Germany). Their internal structure and dimensions were observed using TEM (Thermo Scientific Talos F200S, America). Their crystalline structures were analyzed through XRD (Rigaku SmartLab SE, Japan). Using a fully automated surface area and porosity analyzer (BET, Micromeritics ASAP 2460, America), their specific surface areas were determined by collecting N₂ adsorption–desorption isotherms. Their elemental composition and valence states were examined by XPS (Thermo Scientific K-Alpha, America). In addition, Raman spectra (HORIBA LabRAM HR Evolution, Japan) were recorded for all samples to evaluate the defect features of the carbon materials. The content of Co and Fe in the samples was determined by ICP-OES (Thermo Fisher iCAP PRO, America).

2.4 Electrocatalytic performance testing for HER

The preparation of working electrode was shown below. Briefly, the catalyst powder (5 mg) was transferred into the 2-mL centrifuge tube, followed by addition of anhydrous ethanol (750 μL) and 5 wt% Nafion solution (250 μL) and 30 min of ultrasonication, yielding the uniform and stable black ink containing a catalyst at 5 mg mL^–1. Before catalyst loading, 0.3- and 0.05-μm alumina powders were added to sequentially polish the glassy carbon electrode to a mirror-like finish on a chamois leather pad. After polishing, the electrode was ultrasonically cleaned using ultrapure water and anhydrous ethanol (5 min each) before drying under a nitrogen stream. Subsequently, catalyst ink (5.6 μL) was pipetted on the pretreated glassy carbon electrode and evenly drop-cast over the electrode surface, followed by 2 h of drying within the vacuum oven under 60 °C.

A three-electrode configuration was employed for evaluating HER catalytic activity within 1 M KOH as the alkaline electrolyte, and the CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., China) was employed for collecting electrochemical data. The glassy carbon electrode modified with our transition-metal-modified nitrogen-doped carbon nanotube catalyst was used to be the working electrode at the catalyst concentration of 0.28 mg cm^-2. A Pt plate served as counter electrode, whereas the 1 M KOH-filled Hg/HgO electrode was the reference electrode.

LSV in the alkaline electrolyte was performed over a potential between −1.2 and 0.2 V vs. Hg/HgO at the 5 mV s⁻¹ scan rate. The potentials documented were transformed into the reversible hydrogen electrode (RHE) scale in 1 M KOH by Equation 1.

Typically, we obtained the overpotential at 10 mA cm⁻² (η₁₀) based on LSV curves. According to corrected LSV data, linear fitting was conducted to obtain the Tafel plots by Tafel equation, as expressed in Equation 2. The resulting Tafel slope was used to assess the HER reaction kinetics, with a smaller slope indicating faster reaction kinetics.

EIS was carried out at the potential corresponding to η₁₀, with the frequency of 100 kHz to 0.01 Hz, whereas the AC amplitude being 5 mV. An equivalent circuit containing Rs was utilized to fit Nyquist plots, from which solution resistance (R_s) and charge-transfer resistance (R_ct) were extracted.

CV was conducted between −1.2 and 0.2 V vs. Hg/HgO at the 50 mV s⁻¹ scan rate. The electrode was cycled for 100 consecutive scans until the CV curves overlapped, ensuring that the catalyst surface reached a stable state.

To evaluate the catalytically accessible active site density, double-layer capacitance (C_dl) was measured. CV curves were recorded in a non-Faradaic potential region at the 20, 40, 60, 80, 100, and 120 mV s⁻¹ scan rates within 1 M KOH. The ECSA was calculated according to Equation 3.

C_dl refers to double layer capacitance whereas C_s indicates the atomically smooth surface capacitance.

3 Results and discussion

3.1 Materials characterization

Surface morphologies of the transition-metal-modified nitrogen-doped carbon nanotubes, including Co-NCNT, Fe-NCNT, and CoFe-NCNT, were examined through SEM (Figure 1). Pristine multi-walled carbon nanotubes display a typical one-dimensional tubular morphology, with smooth tube walls and well-preserved structural integrity, providing an effective conductive framework and mechanical support for the resulting composites. After single-metal incorporation, both Fe-NCNT and Co-NCNT largely retain the intact tubular architecture of the carbon nanotubes, without obvious structural collapse. Metal-derived nitrogen–carbon particles are uniformly anchored on the tube walls, leading to a noticeably rougher surface. Only a small number of nanoparticles can be observed, and these are well dispersed without apparent aggregation, indicating that the single-metal precursors were successfully immobilized on the carbon nanotube framework after high-temperature pyrolysis. Compared with the monometallic samples, the CoFe bimetallic co-doped CoFe-NCNT exhibits a more optimized morphology. The nanoparticles attached to the tube walls are smaller and more uniformly distributed, while particle aggregation is effectively suppressed. This suggests that the bimetallic effect helps refine the microstructure of the material. Such a rough and porous tubular architecture is expected to expose more available active sites and to promote charge transport and electrolyte diffusion, thereby providing a favorable structural basis for improved electrochemical and electrocatalytic performance.

FIGURE 1

TEM was employed to examine whether the pre-existing tubular morphology of the MWCNT support was retained after precursor incorporation and pyrolysis (Figure 2). All three samples preserve the characteristic hollow structure of the starting MWCNTs, without obvious large-scale fracture or collapse. These observations demonstrate the structural stability of the nanotube scaffold during synthesis but do not indicate that Co or Fe participates in the formation or reconstruction of the nanotubes. Differences in external surface roughness and the apparent aggregation of the supported species are observed among the samples; however, the precise origin of these differences cannot be determined solely from the present TEM images. To further verify the successful anchoring of Co and Fe elements on the carbon nanotube framework and analyze the elemental composition of the material, TEM-EDS characterization was also performed (Figure 2). The TEM-EDS images clearly show the distribution of C, N, Co, and Fe elements on the surface of carbon nanotubes, which is consistent with the expected elemental composition of Co-NCNT, Fe-NCNT, and CoFe-NCNT catalysts. Among them, the Fe content in Fe-NCNT is 4.27%, and the Co content in Co-NCNT is 4.08 wt%. The Fe and Co contents in CoFe-NCNT are similar, at 2.24 wt% and 2.30 wt%, respectively. In addition, the content of Fe and Co in the samples was determined using ICP-OES, as shown in Table 1. The results of ICP-OES further support the successful loading of Fe and Co metal elements onto MWCNT.

FIGURE 2

XRD was employed to examine the crystalline structures of Fe-NCNT, Co-NCNT, and CoFe-NCNT (Figure 3). All three samples exhibit a broad diffraction peak at approximately 26.0°, which is assigned to the (002) plane of graphitic carbon according to Graphite PDF#75–1621. This result indicates that the graphitic framework of the multi-walled carbon nanotubes is retained after precursor incorporation and pyrolysis. Weak diffraction features are also observed in the 42°–45° region, where the (101) reflections of graphitic carbon may overlap. In particular, the signal at approximately 44.4° cannot be assigned unambiguously to a specific Co-, Fe-, or FeCo-containing crystalline phase because several possible phases have reflections in this region. In the absence of other clearly resolved phase-specific reflections, this peak is not regarded as evidence for metallic nanoparticles, an FeCo alloy, atomic dispersion, or an ordered distribution of metal active sites. Therefore, the XRD results primarily demonstrate preservation of the graphitic carbon framework, while the precise structural forms of the Co and Fe species require complementary microscopic and spectroscopic characterization.

FIGURE 3

XPS was carried out for investigating elemental composition and valence states of the three prepared catalysts (Figures 4–6). In the survey spectrum of CoFe-NCNT (Figure 4a), the characteristic signals for C, N, O, Fe, and Co are observed, while no obvious impurity-related peaks can be observed, suggesting that the catalyst is of great surface purity. The surface element content and proportion (Table 2) are generally consistent with the designed feeding composition, supporting the measurement results of ICP-OES. C 1s, N 1s, O 1s, Co 2p, and Fe 2p signals can be observed, which further verifies that metal species are successfully introduced and targeted surface chemical composition is formed. The high-resolution C 1s spectrum is deconvoluted to a dominant C–C/C=C component, together with C–O, C=O, and π–π* satellite features. This result suggests the formation of a partially graphitized conductive carbon network after pyrolysis, while oxygen-containing functional groups at an appropriate amount are retained. These polar groups can improve surface wettability and may also serve as auxiliary anchoring sites for stabilizing metal species. As observed from the N 1s spectrum, pyrrolic N, pyridinic N, graphitic N, and a small amount of oxidized N co-exist. Of these, pyridinic N and pyrrolic N have been generally considered effective coordination sites for transition-metal single atoms, enabling the formation of M–N₄- or M–N_x-like structures and improving atomic-level dispersion stability. Graphitic N, by contrast, mainly modulates the electronic structure of the carbon framework, promotes charge transport, and influences adsorption energetics. The Co 2p and Fe 2p spectra confirm the presence of surface Co and Fe species with mixed oxidation states. These features are consistent with interactions between the metal species and N/O-containing functionalities of the carbon support. However, XPS does not directly determine metal nuclearity and cannot independently distinguish isolated metal sites from ionic clusters, oxide species, or surface-oxidized nanoparticles. Therefore, Co–N_x and Fe–N_x environments are regarded as possible coordination motifs rather than experimentally identified single-atom structures. Here, the Fe 2p spectrum shows discernible Fe²⁺-related features, suggesting that Fe is primarily coordinated with surrounding N/O atoms in a divalent state. Through the comparison of XPS spectra for CoFe-NCNT, Co-NCNT, and Fe-NCNT, the Co 2p and Fe 2p peaks in the bimetallic sample appear broader, with a more pronounced coexistence of different valence components. To quantitatively evaluate the electronic interaction between Co and Fe species, the binding energies of Co 2p_3/2 and Fe 2p_3/2 were calibrated and compared (Table 3). Notably, the Co 2p_3/2 peak of CoFe-NCNT exhibits a positive shift of approximately +0.3 eV relative to that of Co-NCNT, while the Fe 2p_3/2 peak shows a negative shift of approximately −0.4 eV relative to that of Fe-NCNT. The opposite shift directions provide direct evidence for charge transfer from Co to Fe (or to the surrounding N/O ligands) upon bimetallic incorporation. This electron redistribution modifies the d-band occupation of the metal centers, which is expected to optimize the adsorption free energies of H* and OH* intermediates during alkaline HER. Therefore, the binding energy shifts observed in CoFe-NCNT are fully consistent with its enhanced intrinsic activity and faster reaction kinetics.

FIGURE 4

FIGURE 5

FIGURE 6

In addition, shifts in binding energy and changes in peak-area ratios relative to the corresponding monometallic samples can be observed. These changes may arise from electron transfer and electronic re-equilibration between Co and Fe species located in different but neighboring coordination environments, such as conjugated, shared, or adjacent N-coordination sites within the N-doped carbon matrix (Zhao et al., 2022). Such electronic redistribution may further affect d-band occupation and effective charge density of metal centers. These electronic-structure changes are usually associated with more favorable adsorption energies of reaction intermediates, which represents one of the key pathways by which bimetallic co-doping enhances catalytic performance. In O 1s spectra, both C–O and C=O components confirm that surface oxygen species are present. The appearance of metal–oxygen-related components further suggests that some metal sites may be involved in M–O–C or mixed M–O/N coordination environments. Such coordination motifs are not uncommon in single-atom catalysts and are probably associated with real active configurations under reaction conditions, such as M–N_xO_γ species (Feng et al., 2024). Overall, the XPS results demonstrate that Co and Fe are successfully incorporated into the N-doped carbon framework and mainly exist in ligand-coordinated forms, as evidenced by the elemental composition, carbon-framework features, nitrogen species, mixed valence states of Co/Fe, and the corresponding satellite signals. Compared with the monometallic counterparts, the bimetallic CoFe-NCNT sample exhibits stronger electronic interactions and a more tunable local coordination structure, which may contribute to its improved catalytic behavior.

Through Raman spectroscopy, carbon defects and graphitic structures of Fe-NCNT, Co-NCNT, and CoFe-NCNT were analyzed, as shown in Figure 7. All samples show two representative bands at around 1350 and 1564 cm⁻¹, associated with D and G bands, separately. Typically, the D band is linked to defects and chaotic architectures within carbon materials, whereas the G band is caused by in-plane vibration of sp²-hybridized graphitic carbon. The D-to-G band intensity ratio (I_D/I_G) has been commonly adopted for evaluating the defect degree and graphitization level of carbon materials. The values of the monometallic Fe-NCNT and Co-NCNT samples are relatively close, at 1.09 and 1.04, respectively, indicating that both samples contain a certain amount of carbon vacancies and doping-induced defects compared with pristine carbon nanotubes. These defects can offer more active sites to facilitate electrocatalytic reactions. Notably, the bimetallic CoFe-NCNT sample exhibits a significantly increasing I_D/I_G ratio to 1.27, suggesting that Fe/Co co-doping synergistically induces more lattice defects, edge defects, and heteroatom-doping-related defects, thereby increasing the overall defect density of the material. An appropriate density of defect sites efficiently modulates the electronic architecture of carbon matrix and improves adsorption energies of reaction intermediates, while the retained graphitic structure helps maintain good electrical conductivity. Therefore, the abundant defect-associated active sites constructed through bimetallic synergy represent an important structural factor responsible for the superior HER electrocatalytic performance of CoFe-NCNT compared with the monometallic samples.

FIGURE 7

The porous structures and specific surface areas of three prepared catalysts were investigated by N₂ adsorption–desorption measurements (Figure 8). The N₂ adsorption–desorption isotherms of the three samples exhibit hysteresis behavior, indicating the presence of accessible mesoporous textural voids in the assembled catalyst powders. The BET specific surface areas of Fe-NCNT, Co-NCNT, and CoFe-NCNT are 184.6, 191.5, and 246.3 m² g⁻¹, respectively. Thus, CoFe-NCNT has a larger accessible surface area than the two monometallic samples, which facilitates electrolyte contact and mass transport. It should be noted that these adsorption-derived textural properties may originate from several contributions, including intertube and interparticle voids, nanotube bundling, precursor-derived carbonaceous deposits, and supported metal-containing domains. The present data do not demonstrate that Co and Fe directly create pores within the walls of the MWCNTs. In the absence of a metal-free NCNT control prepared under identical conditions, the higher surface area of CoFe-NCNT is therefore treated as an observed textural characteristic rather than direct evidence for a Co/Fe-induced pore-formation mechanism.

FIGURE 8

3.2 HER catalytic activities of samples

HER electrocatalytic activities of Fe-NCNT, Co-NCNT, CoFe-NCNT, and commercial Pt/C were comparatively evaluated within 1 M KOH using LSV, Tafel analysis, EIS, and chronopotentiometric V–t stability tests (Figure 9). As shown by LSV polarization curves in Figures 9a,b, at the same current density, Fe-NCNT, Co-NCNT, and CoFe-NCNT require overpotentials of 38.1, 46.1, and 30.2 mV, separately; these values decrease relative to commercial Pt/C catalyst (61.9 mV). Compared with the monometallic samples, the polarization curve of the CoFe-NCNT bimetallic composite shifts markedly toward lower potentials, and its overpotential is much lower than those of Fe-NCNT and Co-NCNT. This result indicates that the synergy of Fe with Co can effectively optimize the interfacial electronic architecture while enhancing HER electrocatalytic activity. Meanwhile, the mass activities of Fe-NCNT, Co-NCNT, and CoFe-NCNT are markedly enhanced relative to commercial Pt/C, with CoFe-NCNT delivering the greatest mass activity among these tested samples (Figure 9c). This further suggests that the cooperative effect between Co and Fe promotes the HER catalytic activity of the nitrogen-doped carbon nanotube framework. Kinetic differences were examined by Tafel plots-derived Tafel slopes, and the possible reaction pathway was inferred. From Figure 9d, Tafel slopes of Fe-NCNT, Co-NCNT, CoFe-NCNT, and commercial Pt/C are 87.2, 89.6, 68.4, and 95.3 mV dec⁻¹, respectively, suggesting that HER over these catalysts follows the Volmer–Heyrovsky pathway (Dong et al., 2022; Chen et al., 2022). The significantly decreased Tafel slope of CoFe-NCNT, 68.4 mV dec⁻¹, indicates the requirement of a lower overpotential to reach the same current density, confirming its faster HER kinetics and enhanced catalytic performance, conforming to LSV results in Figure 9a. In contrast, the larger Tafel slopes of Fe-NCNT and Co-NCNT suggest greater kinetic resistance during interfacial charge transfer and hydrogen-intermediate conversion. The markedly reduced Tafel slope of CoFe-NCNT suggests that bimetallic co-doping effectively accelerates interfacial charge transfer and hydrogen desorption during alkaline HER. The EIS results in Figure 9e show the greatly decreased charge-transfer resistance of CoFe-NCNT compared with Fe-NCNT and Co-NCNT, indicating substantially improved interfacial conductivity and charge-transfer efficiency. This improvement is associated with the joint contribution of uniformly distributed bimetallic active sites and highly conductive carbon nanotube network. Long-term V–t stability testing at 10 mA cm⁻² and 100 mA cm⁻² (Figure 9f) shows that commercial Pt/C undergoes performance decay during prolonged electrolysis, which may be associated with the loss or deactivation of active components. By contrast, all three carbon-based composites exhibit excellent operational stability. Among them, CoFe-NCNT shows the smallest potential fluctuation and the most stable long-term catalytic performance, suggesting that it can partially overcome the limitations of commercial noble-metal catalysts, including poor durability under extended operation and high cost. To quantitatively evaluate the catalytic durability, the potential decay rates of all four catalysts were calculated from the chronopotentiometric V-t curves (Equation 4). As summarized in Table 4, CoFe-NCNT exhibits the lowest decay rate of approximately 0.78 mV h⁻¹ at 10 mA cm⁻² and 2.15 mV h⁻¹ at 100 mA cm⁻² in 1 M KOH, which is significantly lower than those of Fe-NCNT (1.04 and 2.93 mV h⁻¹), Co-NCNT (1.12 and 3.68 mV h⁻¹), and commercial Pt/C (1.21 and 4.88 mV h⁻¹). The decay rate of CoFe-NCNT is reduced by 25.0%–26.6% compared with Fe-NCNT and 30.4%–41.6% compared with Co-NCNT, highlighting the crucial role of Co/Fe bimetallic synergy in enhancing structural and catalytic stability. These quantitative results unequivocally demonstrate that the bimetallic CoFe-NCNT catalyst not only delivers superior activity but also exhibits excellent long-term durability, making it a promising candidate for practical alkaline water electrolysis. is the potential at the beginning of stability testing (mV vs. RHE); is the potential at the end of stability testing (mV vs. RHE); is the total duration of the test (h).

FIGURE 9

CV, double-layer capacitance (C_dl) calculation, and ECSA normalization were performed to investigate the intrinsic HER catalytic activities of the four catalysts, as shown in Figure 10. Figure 10a displays the CV curves for CoFe-NCNT. ECSA values derived from the CV measurements offer data regarding the electrochemically accessible active site quantity. Figure 10b shows ECSA-normalized LSV curves of Fe-NCNT, Co-NCNT, and CoFe-NCNT, obtained based on C_dl measurement method. After ECSA normalization, CoFe-NCNT still exhibits superior catalytic performance compared with Fe-NCNT and Co-NCNT, suggesting that the combined influence of Co and Fe incorporation on the surface chemistry and electrochemical properties of the catalyst effectively modulates the active site electronic architecture and improves hydrogen-intermediate conversion efficiency during HER. From Figure 10c, CoFe-NCNT possesses a larger ECSA than Fe-NCNT, Co-NCNT, and commercial Pt/C, indicating that Co/Fe bimetallic modification of nitrogen-doped carbon nanotubes offers additional accessible active sites to facilitate the reaction, thus enhancing the overall catalytic activity. Although the comparison with the monometallic samples demonstrates a beneficial effect of Co/Fe co-incorporation, the present results do not directly establish electronic coupling within an atomically defined Co–Fe pair.

FIGURE 10

Compared with the monometallic Fe-NCNT and Co-NCNT samples, CoFe-NCNT exhibits a larger double-layer capacitance, corresponding to a higher electrochemically active surface area. From this finding, Fe/Co bimetallic co-doping efficiently optimizes the porous interfacial architecture of carbon nanotubes, further provides catalytically accessible active sites, and improves electrolyte wetting and ion adsorption at the electrode surface. In contrast, the monometallic samples show relatively lower C_dl and ECSA values, which may be attributed to their less diverse active-site configurations and higher interfacial charge-transfer resistance. As a result, the utilization efficiency of the effective catalytic interface is comparatively limited.

3.3 Enhancement mechanism analysis

Based on the electrochemical results discussed above, CoFe-NCNT exhibits superior HER catalytic activity compared with Fe-NCNT, Co-NCNT, and commercial Pt/C. This improvement can be mainly attributed to a multiple enhancement mechanism involving structural exposure, electronic modulation, and accelerated mass transport under bimetallic synergistic regulation. In terms of morphology, the SEM images (Figure 1) show that CoFe-NCNT preserves an intact one-dimensional carbon nanotube network compared with the monometallic samples. Meanwhile, the particles anchored on the tube walls are smaller and more uniformly distributed, with markedly suppressed aggregation. This produces a rough, porous, and open interface to further expose active sites accessible to the electrolyte. The TEM images (Figure 2) further confirm that CoFe-NCNT retains a continuous hollow tubular structure. The intact multi-walled carbon nanotube framework can serve as a fast electron-transport pathway, while the rough tube walls and interfacial defects shorten ion-diffusion distances and improve the utilization of the reaction interface. The XRD results (Figure 3) show that all samples retain the characteristic graphitic carbon diffraction peak of carbon nanotubes, indicating that the conductive carbon framework is not destroyed during pyrolysis. In addition, the diffraction-peak shift and enhanced metal-related signal observed for CoFe-NCNT suggest that Co/Fe co-doping modifies the carbon-layer lattice environment and local electronic structure, which may help construct a conductive network and active-site distribution more favorable for HER. The XPS spectra (Figure 4) reveal that C, N, O, Co, and Fe co-exist in CoFe-NCNT. In the N 1s spectrum, pyrrolic N, pyridinic N, and graphitic N can contribute to electronic regulation, metal anchoring, and conductivity enhancement, respectively. The mixed valence states and binding-energy shifts of Co 2p and Fe 2p suggest electronic coupling and charge redistribution between Co and Fe species, which may alter the surface interactions with water-, OH-, and H-containing intermediates; however, the corresponding adsorption energies and water-dissociation barriers were not directly determined. The Raman results (Figure 7) show that the I_D/I_G value of CoFe-NCNT increases to 1.27, higher than those of Fe-NCNT and Co-NCNT. This indicates that bimetallic co-doping induces more lattice defects, edge defects, and heteroatom-related defects. Such defects provide more reaction sites and regulate the local electron density of the carbon matrix. BET measurements further show that the specific surface area of CoFe-NCNT is the highest (246.3 m² g⁻¹), exceeding those of Fe-NCNT and Co-NCNT (184.6 and 191.5 m² g⁻¹ separately). The abundant mesoporous structure can facilitate KOH electrolyte penetration, bubble release, and reactant transport. Therefore, the excellent HER performance of CoFe-NCNT does not arise from a single factor. Rather, it results from the combined effects of bimetallic electronic synergy, N-coordinated active sites, defect engineering, high specific surface area, and the conductive carbon nanotube framework.

Alkaline HER involves not only H* adsorption and desorption but also the initial dissociation of water. The Volmer step produces adsorbed hydrogen through the cleavage of an H–OH bond, whereas the subsequent Heyrovsky or Tafel step generates molecular hydrogen. Consequently, a catalyst must balance water activation, H* binding, OH-related intermediate removal, and hydrogen desorption. CoFe-NCNT exhibits a Tafel slope of 68.4 mV dec⁻¹, compared with 87.2 and 89.6 mV dec⁻¹ for Fe-NCNT and Co-NCNT, respectively. This difference is consistent with faster overall Volmer–Heyrovsky kinetics but does not independently identify the rate-determining step or quantify the water-dissociation barrier. The lower charge-transfer resistance of CoFe-NCNT similarly demonstrates improved interfacial electron-transfer kinetics rather than direct evidence of accelerated H–OH bond cleavage. A plausible, but presently unverified, interpretation is that Co- and Fe-containing surface environments provide complementary interactions with alkaline HER intermediates. More oxophilic metal environments may facilitate water/OH interaction, whereas adjacent metal- or N-doped carbon regions may provide H*-binding environments. The higher accessible surface area and ECSA may further increase the number of interfaces at which these processes occur. Nevertheless, theoretical activation-energy calculations or direct spectroscopic and isotope-effect measurements would be required to confirm whether Co/Fe incorporation intrinsically lowers the water-dissociation barrier.

4 Conclusion

In this work, we prepared Fe-NCNT, Co-NCNT, and CoFe-NCNT catalysts through the pyrolysis of metal–organic coordination precursors, and comprehensively analyzed their catalytic performance toward alkaline HER. SEM, TEM, XRD, XPS, Raman, and BET results show that Co/Fe bimetallic co-doping preserves the one-dimensional hollow conductive framework of multi-walled carbon nanotubes, while promoting the uniform dispersion of metal active sites, inducing more defect structures, forming Co–N_x/Fe–N_x coordination active centers, and markedly increasing the specific surface area and mesoporous proportion. Electrochemical measurements demonstrate that the required overpotential of CoFe-NCNT is as low as 30.2 mV at 10 mA cm⁻², which decreases compared with Fe-NCNT, Co-NCNT, and commercial Pt/C. Apart from a Tafel slope of 68.4 mV dec⁻¹, it also delivers reduced charge-transfer resistance, a larger ECSA, and good long-term stability. From the above observations, the superior HER activity of CoFe-NCNT is associated with the combined contributions of bimetallic electronic coupling, N-coordinated active sites, defect engineering, high specific surface area, and the conductive carbon nanotube framework. This study provides a feasible strategy for developing low-cost, high-efficiency non-noble-metal catalysts with high stability for alkaline HER.

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