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Article

Production of Self-Supporting Hollow Carbon Nanofiber Membranes with Co/Co2P Heterojunctions via Continuous Coaxial Co-Spinning for Efficient Overall Water Splitting

by
Ruidan Duan
1,†,
Jianhang Ding
1,†,
Jiawei Fan
1 and
Linzhou Zhuang
1,2,*
1
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
2
Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2025, 15(7), 772; https://doi.org/10.3390/coatings15070772
Submission received: 9 May 2025 / Revised: 15 June 2025 / Accepted: 23 June 2025 / Published: 30 June 2025
(This article belongs to the Special Issue Coatings as Key Materials in Catalytic Applications)
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Abstract

To address mass transport limitations in carbon nanofiber membrane electrodes for overall water splitting, a self-supporting nitrogen-doped hollow carbon nanofiber membrane embedded with Co/Co2P heterojunctions (Co/Co2P-NCNFs-H) was fabricated via continuous coaxial electrospinning. The architecture features uniform hollow channels (200–250 nm diameter, 30–50 nm wall thickness) and a high specific surface area (254 m2 g−1), as confirmed by SEM, TEM, and BET analysis. The Co/Co2P heterojunction was uniformly dispersed on nitrogen-doped hollow carbon nanofibers through electrospinning, leverages interfacial electronic synergy to accelerate charge transfer and optimize the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Electrochemical tests demonstrated exceptional catalytic activity, achieving current densities of 100 mA cm−2 at ultralow overpotentials of 405.6 mV (OER) and 247.9 mV (HER) in 1.0 M KOH—surpassing most reported transition metal catalysts for both half-reactions. Moreover, the electrode exhibited robust long-term stability, maintaining performance for nearly 20 h at 0.6 V (vs. Ag/AgCl) (OER) and over 250 h at −1.5 V (vs. Ag/AgCl) (HER), attributed to the mechanical integrity of the hollow architecture and strong metal–carbon interactions. This work demonstrates that integrating hollow nanostructures (enhanced mass transport) and heterojunction engineering (optimized electronic configurations) creates a scalable strategy for designing efficient bifunctional catalysts, offering significant promise for sustainable hydrogen production via water electrolysis.

1. Introduction

Limited reserves of fossil fuels and their severe environmental impacts have prompted an urgent search for clean and sustainable renewable energy alternatives [1,2]. Hydrogen energy has emerged as one of the cleanest energy carriers, attracting increasing attention due to its environmental friendliness, high efficiency, and zero-emission characteristics [3,4,5]. Among various hydrogen production technologies, water electrolysis stands out as the cleanest approach [5,6], offering high-purity hydrogen with zero carbon emissions and excellent compatibility with renewable energy sources such as solar and wind power. However, the two half-reactions involved in overall water electrolysis––the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER)—suffer from sluggish kinetics, necessitating highly efficient catalysts to enhance the reaction rates and overall efficiency of water electrolysis systems [7,8]. This sluggishness translates to significant overpotentials—the extra voltage beyond the thermodynamic requirement (1.23 V vs. RHE for overall water splitting)—needed to drive the reactions at practical current densities. High overpotentials directly reduce the overall energy efficiency of the system [9]. For instance, many current systems require overpotentials exceeding 300 mV for the OER and 100 mV for the HER to achieve even a modest current density of 10 mA cm−2, a common benchmark for initial activity assessment [8,10].
In recent years, significant efforts have been devoted to developing highly active catalysts for HER and OER. Noble-metal-based electrocatalysts exhibit excellent activity and durability [11]; for example, Pt-based materials remain the most effective HER catalysts, often achieving current densities of 10 mA cm−2 at overpotentials below 50 mV [12], and Ir/Ru oxides exhibit optimal OER performance, typically requiring overpotentials of 250–350 mV to deliver 10 mA cm−2 [13,14]. While these materials demonstrate high efficiency and good stability, their large-scale application is severely constrained by their high cost and limited natural abundance [15]. This has necessitated the development of non-noble metal catalysts, particularly cost-effective, durable bifunctional catalysts. The goal is to identify materials that not only drive both the HER and the OER efficiently, but also achieve this at industrially relevant current densities (e.g., >100 mA cm−2 or even >500 mA cm−2) while minimizing overpotential. Furthermore, long-term operational stability, often defined as maintaining performance for hundreds or thousands of hours without significant degradation, is a critical metric for practical viability. Such catalysts could significantly simplify device fabrication and operation while reducing hydrogen production costs, offering substantial commercial potential [16,17].
Recent advances have yielded numerous non-noble metal and even metal-free electrocatalysts, including transition metal (Fe, Co, Ni, Mn, Mo, etc.) oxides/hydroxides [18,19], carbides [20], nitrides [21], phosphides [22], and sulfides [23]. Among these, transition metal phosphides (e.g., NiP [24], FeP [25], MoP [26], and CoP [27]), especially CoP and Co2P, demonstrate exceptional HER/OER activity and good electrical conductivity [28]. In addition, rational design of heterointerfaces has emerged as a powerful strategy for electrocatalyst optimization. The strong electronic coupling interactions between different phases in these heterogeneous structures can effectively modulate electronic configurations, reduce reaction energy barriers, and optimize interactions with reaction intermediates, thereby promoting chemisorption of reactive species on the catalyst surface and significantly boosting electrocatalytic activity [29,30,31]. Zhong et al. [31] exemplified this approach with Co/CoP@HOMC, where interfacial charge redistribution and hierarchical porosity synergistically achieve ultralow overpotentials (η10 = 120 mV for HER/260 mV for OER) in alkaline media. While promising, these materials often face challenges in simultaneously achieving low overpotentials for both reactions, high current densities, and prolonged stability. For example, many phosphides might show good HER activity but require higher OER overpotentials, or vice versa, and stability can be compromised under harsh oxidative conditions for OER or reductive conditions for HER over extended periods [32,33].
Furthermore, the catalytic performance of transition metal phosphides can be further enhanced through synergistic interactions with self-supporting graphitic carbon materials, which improve both electronic conductivity and structural stability [34,35]. Carbon nanofibers have emerged as particularly promising supports due to their rapid electron transport and excellent electrochemical stability [36]. Heteroatom doping, particularly nitrogen incorporation, plays a crucial role in modifying the electronic structure of carbon supports, creating intrinsic active sites, and improving overall electrocatalytic performance [37]. Meanwhile, nitrogen doping enhances the three-dimensional hydrophilicity of catalyst surfaces, facilitating electrolyte–electrode contact and boosting catalytic efficiency. Wei et al. [38] successfully constructed a heterostructure interface between Fe/Ni phosphides and graphitic carbon nitride (C3N4). The nitrogen species in C3N4 effectively modulates the electronic structure of Fe/Ni sites and optimizes their adsorption strength with oxygen-containing intermediates. The resulting FeNi-C3N4-P catalyst demonstrates exceptional OER performance, achieving a current density of 100 mA cm−2 at an ultralow overpotential of merely 235 mV. Beyond composition, nanostructural design critically determines catalytic performance [39]. Hollow porous carbon nanofibers, with their well-defined internal channels, high surface area, optimal aspect ratio, and superior conductivity, provide abundant active sites while minimizing electron/mass transport distances, accelerating electrolyte diffusion and gas bubble release [40,41]. These features are critical for achieving high current densities by ensuring efficient reactant supply and product removal and can contribute to improved stability by preventing electrode fouling or active site blockage [42]. Core-shell nanostructures with excellent stability, functionality, and dispersion have proven effective for enhancing electrocatalysis [43,44]. Electrospun core-shell nanofibers offer particularly precise control over active site regulation and charge transfer resistance reduction. Li et al. [45] developed a hybrid CoP catalyst anchored on nitrogen-doped hollow carbon spheres supported by carbon nanofibers (CoP/NCF-200) through controlled electrospinning followed by pyrolysis and phosphidation. This catalyst exhibited remarkable activity, favorable kinetics, and excellent stability, attributable to both the synergistic effects between CoP and N-doped carbon and the unique hollow spherical architecture. Therefore, transition metal co-doped carbon catalysts are widely regarded as promising alternatives to noble-metal-based catalysts for water electrolysis.
Several methods, including hydrothermal growth [46], templating with Metal-Organic Frameworks (MOFs) [47], and vacuum filtration [48], have been developed to fabricate self-supporting electrocatalysts. However, electrospinning stands out as a particularly versatile and scalable technique for creating continuous one-dimensional (1D) nanofibrous membranes. A key advantage of this method is the direct production of binder-free, interwoven 3D networks that possess both high mechanical strength and excellent electrical conductivity pathways throughout the material [49]. Furthermore, the use of coaxial electrospinning provides precise control over the fabrication of complex core-shell nanostructures. This capability is essential for rationally designing advanced architectures, such as the hollow fibers presented in this work, where a sacrificial core material can be selectively removed during post-processing to create internal channels that enhance mass transport [50]. While the process requires careful optimization of multiple parameters to achieve a desired morphology, its ability to uniformly embed catalyst precursors within the polymer matrix ensures homogeneous distribution of active sites, making it a superior strategy for synthesizing high-performance, self-supporting electrodes compared to methods that may suffer from particle aggregation or non-uniform growth on a substrate [49].
To address the aforementioned challenges of achieving low overpotentials, high current densities, and robust long-term stability in bifunctional catalysts, particularly concerning mass transport limitations and active site optimization, here, a self-supporting nitrogen-doped hollow carbon nanofiber membrane embedded with Co/Co2P heterojunctions (Co/Co2P-NCNFs-H) was fabricated via continuous coaxial co-spinning. The inner polymethyl methacrylate (PMMA) layer was pyrolyzed during high-temperature carbonization, creating hollow channels that promote gas diffusion and expose abundant active sites. The outer layer, composed of cobalt precursors and polyacrylonitrile (PAN), formed a conductive carbon matrix after carbonization. The Co/Co2P heterojunction leverages interfacial electronic synergy to accelerate charge transfer and enhance bifunctional catalytic activity. Electrochemical tests in 1.0 M KOH revealed exceptional performance, achieving a high current density of 100 mA cm−2 at competitive overpotentials of 405.6 mV (OER) and 247.9 mV (HER) in 1.0 M KOH, alongside robust stability with continuous operation for over 250 h. These metrics address key challenges in the field. This work highlights the critical role of hollow nanostructures in optimizing mass transport and heterojunction engineering in designing high-efficiency electrocatalysts, offering a scalable strategy for advancing sustainable hydrogen production technologies.

2. Experimental Section

2.1. Materials

Polyacrylonitrile (PAN, Mw = 150,000 g mol−1, AR grade) and polymethyl methacrylate (PMMA, AR grade) were purchased from Aladdin Reagent Inc., Shanghai, China. Cobalt(II) acetate tetrahydrate (Co(AC)2⋅4H2O, 99.9%), phosphoric acid (H3PO4, 99.9%), zinc acetate (Zn(AC)2⋅2H2O, 99.9%) and N, N-dimethylformamide (DMF, AR grade) were purchased from Shanghai Titan Scientific Co., Ltd., Shanghai, China.

2.2. Preparation of Self-Supporting Co/Co2P-NCNFs-H Membranes

Synthesis of Co/Co2P-NCNFs-H was performed via coaxial electrospinning followed by a multi-step heat treatment, as illustrated in Figure S1. The procedure is detailed below.

2.2.1. Preparation of Precursor Solutions

Outer Shell Solution (Solution A): First, 0.1 g of Co(AC)2·4H2O (0.4 mmol) was dis-solved in 15 mL of N,N-dimethylformamide (DMF) and sonicated to form a homogeneous solution. Subsequently, 0.2 g of H3PO4 was added, followed by 1.5 g of polyacrylonitrile (PAN). The mixture was stirred overnight to yield a uniform, pink spinning solution.
Inner Core Solution (Solution B): In a separate vessel, 0.5 g of Zn(AC)2·2H2O and 1.0 g of polymethyl methacrylate (PMMA) were dissolved in 15 mL of DMF by stirring in a 50 °C water bath. After a homogeneous solution was formed, 1.0 g of PAN was added, and the mixture was stirred overnight at room temperature to obtain the inner core solution.

2.2.2. Coaxial Electrospinning

The two precursor solutions were placed into separate 10 mL syringes and loaded onto two syringe pumps. Electrospinning was performed under the following conditions:
Setup: A coaxial spinneret consisting of a 17-gauge outer needle (for Solution A) and a 21-gauge inner needle (for Solution B).
Flow Rate (Outer Solution A): 1.0 mL h−1.
Flow Rate (Inner Solution B): 0.5 mL h−1.
Applied Voltage: 19 kV.
Collector Distance: 15 cm. The process was run for 6 h to produce a uniform nanofiber membrane, denoted as Co/H3PO4-PAN.

2.2.3. Post-Treatment: Pre-Oxidation, Carbonization, and Activation

Pre-oxidation: The as-spun Co/H3PO4-PAN membrane was first dried in a vacuum oven at 60 °C to remove residual DMF. It was then transferred to a muffle furnace and heated to 280 °C at a ramp rate of 1 °C min−1 and held for 1 h to stabilize the fiber structure. This yielded the pre-oxidized sample, Co/H3PO4-PAN-O.
Carbonization: The stabilized membrane was placed in a tube furnace and carbonized under an argon atmosphere. The furnace was heated to 900 °C at a ramp rate of 5 °C min−1 and held for 3 h. During this step, the inner PMMA and zinc components were pyrolyzed, resulting in the formation of porous hollow carbon nanofibers.
Activation: The final carbonized sample was immersed in 0.5 M H2SO4 for 2 h for activation, then washed thoroughly with deionized water and dried. The final product was labeled Co/Co2P-NCNFs-H.

2.3. Preparation of Control Samples

Co/Co2P-NCNFs (Solid Fibers): This control sample was fabricated using single channel electrospinning. Only the outer shell solution (Solution A) was used, with an injection rate of 1.0 mL h−1. All subsequent pre-oxidation and carbonization steps were identical to those described in Section 2.2.3.
Co-NCNFs-H (Hollow, P-free Fibers): This control was prepared to investigate the effect of phosphorus. Fabrication followed the coaxial electrospinning procedure described in Section 2.2.2, using the same inner core solution (Solution B). However, the outer shell solution was prepared without the addition of H3PO4. All other processing parameters and post-treatments remained constant.
Co-NCNFs (Solid, P-free Fibers): This solid control sample was prepared by single-channel electrospinning using the P-free outer solution described above at an injection rate of 1.0 mL h−1. The post-treatment conditions were identical to those for the other samples.

2.4. Catalytic Activity Measurements

Electrochemical measurements were conducted using a CHI 660E electrochemical workstation (CHI Instruments, Shanghai, China) in a three-electrode setup with a 1.0 M KOH electrolyte solution at room temperature and ambient pressure. As-prepared Co/Co2P-NCNFs-H was employed as a self-supported working electrode with an exposed area of 1 cm2. An Ag/AgCl electrode and a glassy carbon electrode served as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) was performed in the 0~1.1 V range at a scan rate of 5 mV s−1 to evaluate water oxidation (OER). The linear sweep voltammetry (LSV) method was applied in the −1~−2 V range to obtain the hydrogen evolution reaction (HER) performance with a scan rate of 5 mV s−1. The stability of the catalysts was assessed using the chronoamperometry technique. All potential values were converted to the reversible hydrogen electrode according to Equation (1), and the overpotential (η) was calculated according to Equation (2):
E (RHE) = E (Ag/AgCl) + 0.059 × pH + 0.197
η = E(RHE) − 1.23
All electrochemical data were i-R corrected (0.8 iR).
ECSA evaluation was conducted using the CV method in the non-faradaic current region at scan rates of 20, 40, 60, 80, and 100 mV s−1. The current density difference at selected potentials was plotted versus scan rate. The resulting linear slope equaled Cdl. ECSA was calculated according to Equation (3), using the specific capacitance (Cs) of a standard electrode with a geometric surface area of 1 cm2. The Cs typically ranges from 20 to 60 μF cm−2. Herein, an average value of 40 μF cm−2 was used to calculate the ECSA.
ECSA = Cdl/Cs
Turnover frequency (TOF) was calculated according to Equation (4), where I is the current density at a given overpotential (e.g., η = 200 mV), F is the Faraday constant (96,485 C), and n is the molar quantity of electrochemically active sites.
TOF = I/(4F × n)
Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100 kHz to 0.01 Hz, and OER/HER was tested at 0.6 V/−1.2 V vs. Ag/AgCl, respectively. The relevant Rct values were fitted using ZView2 software. A smaller Rct value suggested that the catalyst has a higher charge transfer rate.

3. Results and Discussion

3.1. Morphology Characterization

The morphology and microstructure of the synthesized samples were systematically characterized. Scanning electron microscopy (SEM) was employed to analyze the fibrous structure of the as-prepared Co/Co2P-NCNFs-H, as illustrated in Figure 1a. The SEM images revealed a tightly interwoven three-dimensional network of nanofibers, ensuring continuous conductive pathways. The fibers exhibited uniform diameters ranging from 200 to 250 nm, with submicron-scale dimensions contributing to their exceptional mechanical strength [51]. Cross-sectional SEM images (Figure 1b,c) confirmed the hollow architecture, attributed to the thermal decomposition of the polymethyl methacrylate (PMMA) core and the phase separation between PMMA and polyacrylonitrile (PAN). The removal of PMMA during carbonization exposed abundant active sites, thereby enhancing catalytic performance. The dual presence of PAN in both inner and outer layers facilitated robust integration of active sites with the carbon shell, effectively preventing metal particle detachment and ensuring structural stability. Notably, the fibers displayed no observable agglomeration, with wall thicknesses uniformly maintained at 30–50 nm. The self-supporting, three-dimensional network of nanofibers provides continuous pathways for electron transport and ensures robust mechanical integrity. Critically, the hollow interior of each nanofiber is intentionally designed to create channels that facilitate rapid electrolyte diffusion to the active sites and efficient release of generated gas bubbles (O2 and H2). This structure increases the overall specific surface area, thereby maximizing the exposure of catalytically active sites [41].
The fine microstructure of the Co/Co2P-NCNFs-H sample was characterized by transmission electron microscopy (TEM). The TEM images clearly reveal the hollow structure of the carbon nanofibers, with no observable metal agglomeration (Figure 1d,e), demonstrating that the N,P-doped carbon matrix effectively promotes uniform dispersion of Co species. High-resolution TEM (HRTEM) analysis showed lattice fringes of 0.206 nm, which belong to the (211) plane of Co2P (Figure S2) [52], but no obvious Co and C lattice is observed, indicating the predominantly amorphous nature of the C and Co components (Figure 1f). This may be due to the catalyst particles being too small and evenly distributed. Furthermore, energy-dispersive X-ray spectroscopy (EDX) elemental mapping confirmed the homogeneous distribution of multiple elements (C, N, P, O, and Co) throughout the Co/Co2P-NCNFs-H sample (Figure 1g). The close proximity of Co and P atoms is expected to promote formation of Co/Co2P heterojunctions, which are anticipated to lower reaction energy barriers and accelerate charge transfer due to synergistic interfacial electronic effects [28]. Furthermore, uniform doping of nitrogen into the carbon framework is expected to modulate the electronic structure, improve surface hydrophilicity for better electrolyte contact, and securely anchor active species to enhance long-term stability.
SEM cross-sectional and TEM images of the control sample Co/Co2P-NCNFs are shown in Figure S3. Unlike the Co/Co2P-NCNFs-H sample, the SEM cross-sectional image of Co/Co2P-NCNFs can be observed as a solid carbon nanofiber structure. From the TEM images, it can be observed that there are carbon shell-encapsulated nanoparticles loaded onto the carbon nanofibers, and the diameter of the particles is between 10~15 nm (Figure S3b,c). The EDS patterns show that the elements C, N, P, O, and Co are distributed in the nanofibers in the Co/Co2P-NCNFs, but the elements Co and P are more concentrated in the nanoparticles, and the nanoparticles are mainly Co/Co2P heterojunctions coated by the outer carbon shell (Figure S4). Through characterization of the microscopic morphology of Co/Co2P-NCNFs-H and control samples, it can be proved that reasonable design of coaxial co-spinning conditions can achieve preparation of hollow carbon nanofibers doped with N and P elements, and that a reasonable flow rate of the inner and outer layers of the solution can lead to more homogeneous dispersion of the Co element during the electrospinning process.

3.2. Composition Study of the Samples

XRD was used to characterize the degree of graphitization and crystal structure of the catalysts. A broad peak located at 26°, representing the amorphous structure of carbon, was detected in all four samples (Figure 2a). Three distinct sharp peaks were detected in both the Co-NCNFs-H and Co-NCNFs samples at 44.2°, 51.5°, and 75.8°, which corresponded to the Co(111), Co(200), and Co(220) crystallographic planes, respectively (JCPDS No. 15-0806) [53]. There are two distinct Co2P peaks in Co/Co2P-NCNFs-H and Co/Co2P-NCNFs samples located at 43.29° and 40.72°, corresponding to the Co2P (211) and Co2P (121) crystal faces (JCPDS No. 32-0306) [54]. Also, weak peaks of Co (111), Co (200), and Co (220) were present in the XRD pattern of the sample, representing the coexistence of metal Co and Co2P within the carbon nanofiber matrix. The small crystallite sizes and lattice strain likely contribute to enhanced catalytic activity via increased density of grain boundaries and interfacial defects, which can serve as additional active sites [55].
Meanwhile, Raman spectroscopy was employed to probe the structural characteristics and defect states of the carbon nanofiber framework (Figure 2b). All samples exhibit two prominent peaks: the D band at ~1350 cm−1, attributed to disordered carbon or defects, and the G band at ~1587 cm−1, corresponding to graphitized carbon atoms [56]. The ratio of the D band to the G band (ID/IG) is used to determine the degree of graphitization and defectiveness of carbon materials. For Co/Co2P-NCNFs-H, the ID/IG ratio was found to be 1.052, which is significantly higher than that of Co-NCNFs (0.96), Co/Co2P-NCNFs (0.978), and Co-NCNFs-H (0.982). This increase reflects N and P doping and hollow structure lead to a large number of defects in the carbon nanofibers. These defects play a crucial role in enhancing electrocatalytic activity by increasing the number of active sites and facilitating electron transfer across the carbon network [57]. Moreover, the higher ID/IG ratio is consistent with the XPS evidence of increased Co–N bonding and oxygenated functional groups (Figure 3b), suggesting synergistic modulation of the electronic structure through heteroatom doping and interface engineering.
The elemental content in the Co/Co2P-NCNFs-H sample was determined according to ICP-OES, in which the percentages of the Co and P elements were 4.51 wt% and 0.8 wt%, respectively, as shown in Table S1. Calculated from the thermogravimetric analysis results, it can be concluded that the Co content in Co/Co2P-NCNFs-H is about 4.46 wt% (Figure 2c), which is basically consistent with the ICP-OES results. The unit loading of elemental Co in this catalytic material was calculated to be about 4.35 mg g−1. The water contact angles of Co/Co2P-NCNFs-H and Co/Co2P-NCNFs samples were also tested to evaluate the effect of the hollow carbon nanofiber structure on hydrophilicity and hydrophobicity; the results are shown in Figures S5 and S6. The comparison shows that the Co/Co2P-NCNFs-H samples with hollow carbon nanofiber structure have better hydrophilicity and aerophobicity, which proves that rational regulation of fiber structure can effectively enhance three-phase interfacial mass transfer in the process of water electrolysis, promote contact between the active sites and the electrolyte solution, and accelerate the escape of bubbles to enhance the catalytic rate of the electrolysis of water.
As obtained by BET nitrogen adsorption-desorption isotherm analysis, the specific surface areas of Co/Co2P-NCNFs-H (254 m2 g−1) and Co-NCNFs-H (233 m2 g−1) were significantly larger than Co-NCNFs (126 m2 g−1) and Co/Co2P-NCNFs (144 m2 g−1), which demonstrated that the hollow structure could significantly enhance the specific surface area of carbon nanofibers to expose more active sites (Figure S7). The specific surface area of Co/Co2P-NCNFs-H is slightly larger than that of Co-NCNFs-H due to the introduction of defects by P doping in the carbon material. Moreover, the pore sizes of Co-NCNFs and Co/Co2P-NCNFs are mostly around 1 nm, whereas the hollow carbon nanofiber material Co/Co2P-NCNFs-H and Co-NCNFs-H samples also have a larger pore distribution, at 2~3 nm (Figure S8), which may be due to the microporous structures left in the inner wall of the hollow fibers after PMMA decomposition. The hollow and porous structures of Co/Co2P-NCNFs-H and Co-NCNFs-H help to promote rapid mass transfer on the electrode surface and push the reaction forward.
XPS analysis was performed to investigate the surface elemental composition, oxidation states, and chemical structure of the prepared Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H. As shown in the full-scan XPS spectrum (Figure S9d), the Co/Co2P-NCNFs-H sample contains Co, C, N, P, and O elements. The corresponding spectra of control samples are presented in Figure S9: all samples exhibit distinct peaks for C 1s, O 1s, and N 1s, while the signals for the Co and P elements appear relatively weak in the full-scan spectra due to their low concentrations. The Co 2p spectrum (Figure 3a) of Co/Co2P-NCNFs-H reveals a significantly attenuated Co0 2p3/2 peak at 778.2 eV compared to other control samples, indicating partial oxidation of metallic cobalt. Simultaneously, the intensity of peaks at 780.6 eV (Co3+ 2p3/2) and 782.4 eV (Co2+ 2p3/2) increases, accompanied by characteristic satellite features at 786.2 and 792.0 eV, consistent with the formation of Co–O and Co(OH)2 species [53]. These observations are further supported by the O 1s spectrum (Figure 3c), where the dominant peak at 530.1 eV corresponds to metal–oxygen bonds. The N 1s spectrum (Figure 3b) shows four deconvoluted peaks located at 398.4 eV (pyridinic N), 399.8 eV (pyrrolic N), 400.3 eV (Co–N coordination), and 401.1 eV (graphitic N) [58]. Among these, the Co–N component displays a significantly higher intensity in Co/Co2P-NCNFs-H, indicating that nitrogen atoms are actively involved in coordination with Co species. This suggests a strong electronic interaction between N-doped carbon and cobalt, which may help stabilize active sites and modulate local charge density. The P 2p spectrum (Figure 3d) displays two main peaks at 129.4 eV (2p3/2) and 130.2 eV (2p1/2), which are attributed to Co–P bonding, corresponding to the Co2P diffraction peaks observed in XRD patterns. A broader peak at 133.5 eV corresponds to surface-oxidized phosphorus (P–O) [28]. Notably, the intensity of the Co–P signal is significantly higher in Co/Co2P-NCNFs-H than in Co/Co2P-NCNFs, suggesting a higher degree of heterojunction exposure, likely enabled by the hollow and porous architecture. This structure enlarges the interfacial contact among Co, P, and N atoms, reinforcing the both Co-N and Co-P coordination bonds and promoting the stability of active sites [45].

3.3. OER Activity Evaluation of Co/Co2P-NCNFs-H

The OER catalytic activity and stability of the self-supporting electrodes Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H were tested in an alkaline electrolyte solution using a three-electrode system. First, the CV curves of the catalysts during the OER process were measured in 1.0 M KOH solution at a scan rate of 5 mV s−1, as shown in Figure 4a. Compared to the control samples, Co/Co2P-NCNFs-H exhibits the highest OER activity, required an overpotential of only 405.6 mV to achieve a high current density of 100 mA cm−2, which is much lower than Co-NCNFs (586 mV), Co/Co2P-NCNFs (464.9 mV), and Co-NCNFs-H (488.3 mV) (Figure 4b). The Tafel slopes for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H were 145.7 mV dec−1, 91.6 mV dec−1, 110.4 mV dec−1, and 76.2 mV dec−1, respectively (Figure 4c), indicating that Co/Co2P-NCNFs-H shows the best intrinsic OER activity and reaction kinetics in an alkaline electrolyte solution. The comprehensive performance of Co/Co2P-NCNFs-H outperformed some of the recently reported transition metal-based OER catalysts (Table 1).
To compare the ECSA of the prepared samples, CV curves were measured at different scan rates (20, 40, 60, 80, and 100 mV s−1), and the Cdl values of the catalysts were calculated (Figure 5a–e). Among them, the Cdl values of Co/Co2P-NCNFs-H and Co-NCNFs-H are similar, at 1.52 mF cm−2 and 1.48 mF/cm−2, respectively, which are significantly higher than those of Co/Co2P-NCNFs (1.01 mF cm−2) and Co-NCNFs (0.9 mF cm−2).
The ECSA was calculated by converting the specific capacitance of a standard electrode with an actual surface area of 1 cm2 into ECSA, and the results are shown in Figure 5f. The ECSA values of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H are 22.5, 25, 37, and 38 cm2ECSA, respectively. This demonstrates that the hollow carbon nanofiber structure can significantly increase the electrochemical active surface area of the catalyst, allowing the active sites to better interact with the electrolyte and improving the utilization of active sites. Additionally, the TOF value for Co/Co2P-NCNFs-H at an overpotential of 450 mV for OER is 27.0 s−1 (Figure 4e,f), which is significantly higher than those of the other control samples, further confirming its highest intrinsic OER activity.

3.4. HER Activity of Co/Co2P-NCNFs-H

Similarly, in a 1.0 M KOH solution, the HER LSV curves of the catalysts were measured at a scan rate of 5 mV/s, as shown in Figure 6a. Co/Co2P-NCNFs-H exhibited the best HER activity. Compared to Co-NCNFs (383.8 mV), Co/Co2P-NCNFs (291.7 mV), and Co-NCNFs-H (314.5 mV), Co/Co2P-NCNFs-H required the lowest overpotential of only 247.9 mV to reach a current density of 100 mA/cm2 (Figure 6b). Additionally, the intrinsic HER kinetics of the prepared catalysts were evaluated using the Tafel slope, as shown in Figure 6c. Compared to Co-NCNFs (187.4 mV dec−1), Co/Co2P-NCNFs (95.4 mV dec−1), and Co-NCNFs-H (117.3 mV dec−1), Co/Co2P-NCNFs-H exhibited the best intrinsic kinetics, with a Tafel slope of 67.7 mV dec−1. From the electrochemical tests, it can be observed that, compared to Co sites, the Co/Co2P heterojunction significantly enhances the catalyst’s dual-function intrinsic OER and HER activity. Furthermore, the hollow carbon nanofibers can increase the utilization of catalytic active centers by exposing more active sites and provide channels for bubble transfer generated during high-current operation. Therefore, the Co/Co2P-NCNFs-H and Co-NCNFs-H samples show better electrocatalytic performance compared to Co/Co2P-NCNFs and Co-NCNFs. Moreover, the TOF value for Co/Co2P-NCNFs-H at an overpotential of 400 mV for HER is 57.4 s−1 (Figure 6e,f), which is also significantly higher than those of the other control samples, further confirming its highest intrinsic HER activity. Many transition metal-based catalysts with excellent performance have been reported in recent years. In comparison, the comprehensive performance of Co/Co2P-NCNFs-H has surpassed some of them (Table 2).
Figure 4g,h and Figure 6g,h show the polarization curves of the samples after ECSA normalization, used to verify the catalytic kinetics and intrinsic active sites of the catalysts during the OER and HER processes. By excluding the influence of surface area on the catalyst, the performance curves of Co/Co2P-NCNFs-H and Co/Co2P-NCNFs after ECSA normalization are nearly identical in both HER and OER and significantly better than those of Co-NCNFs and Co-NCNFs-H. The results confirm that formation of the Co/Co2P heterojunction plays an important role in optimizing the intrinsic kinetics of the catalytic electrode, significantly enhancing the intrinsic activity of the catalytic processes for both OER and HER. Among them, Co/Co2P-NCNFs-H shows the best dual-function intrinsic activity, proving that the hollow structure also contributes to the formation and dispersion of the Co/Co2P heterojunction.
Furthermore, to compare the resistance of the Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H samples during the HER and OER processes, EIS measurements were conducted in a 1.0 M KOH solution over a frequency range of 100,000 Hz to 0.1 Hz (Figure 4d and Figure 6d). From the Nyquist plot, the electronic conductivity of Co/Co2P-NCNFs-H could be enhanced with lower charge-transfer resistance (Rct) of 650 mΩ than Co-NCNFs (3.55 Ω), Co/Co2P-NCNFs (850 mΩ), and Co-NCNFs-H (3.42 Ω) in the OER. Similarly, in the HER process, Co/Co2P-NCNFs-H also exhibits the smallest Rct of 270 mΩ compared to Co-NCNFs (520 mΩ), Co/Co2P-NCNFs (300 mΩ), and Co-NCNFs-H (342 mΩ). This is likely attributed to the activation effect of nitrogen and phosphorus doping in the carbon material and the promotion of charge transfer between interfaces by the Co/Co2P heterojunction.
The electrochemical stability of Co/Co2P-NCNFs-H for the OER and the HER was also tested. The self-supporting Co/Co2P-NCNFs-H electrode exhibited excellent electrochemical stability for both the OER and the HER in a 1.0 M KOH solution. It was able to stably operate for nearly 20 h at 0.6 V (vs. Ag/AgCl) (Figure 4i) and for nearly 250 h at −1.5 V (vs. Ag/AgCl) without significant performance degradation (Figure 6i). This demonstrates that doping of the N and P elements in the carbon material successfully anchors the Co/Co2P heterojunction active sites, effectively preventing aggregation and detachment of metal elements during long-term electrochemical testing, thus improving stability. The excellent mechanical strength and stable three-dimensional structure of the hollow carbon nanofiber structure also ensure long-term stable operation of the electrode’s active sites.

3.5. Characterization of Co/Co2P-NCNFs-H After OER and HER Stability Tests

The microstructure of the Co/Co2P-NCNFs-H samples after stability testing, denoted as Co/Co2P-NCNFs-H-After (OER) and Co/Co2P-NCNFs-H-After (HER), was characterized by SEM and TEM. After a long electrochemical oxidation process (Figure 7a and Figure S10), the three-dimensional structure and hollow channels of the carbon nanofibers were well preserved, with no significant change in fiber diameter. However, partial aggregation of the metal particles encapsulated in the carbon shell was observed on the fibers, which may be the cause of the catalyst’s performance degradation. After more than 200 h of HER testing (Figure 7b and Figure S11), the uniformity and continuity of the Co/Co2P-NCNFs-H-After (HER) fibers, as well as the fiber thickness, were well maintained, and no significant aggregation of metal elements was found on the fibers. These results collectively demonstrate that the Co/Co2P-NCNFs-H sample has good microstructural stability and mechanical strength during electrochemical testing.
Raman analysis was also performed on the post-reaction samples, as shown in Figure 7c. After the HER and OER reactions, the ID/IG ratio of the samples showed no significant changes, indicating excellent stability of the carbon material during the electrocatalytic process. Figure 7d–g presents the XPS characterization of the Co/Co2P-NCNFs-H-After (OER) and Co/Co2P-NCNFs-H-After (HER) samples. The Co 2p spectrum of Co/Co2P-NCNFs-H-After (OER) shows that the Co0 peak disappears and is oxidized to a higher Co2+/Co3+ oxidation state, with a significant increase in the peak area of the M-O coordination in the O 1s spectrum. Additionally, only the P-O coordination feature peak remains in the P 2p spectrum, while the characteristic peaks of P bonding with Co in the 2p1/2 and 2p3/2 orbitals vanish. This could be due to the strong oxidation process, which causes the Co element to partially leach out, weakening the coordination strength between Co and P. For the Co/Co2P-NCNFs-H-After (HER) sample, the Co 2p spectrum shows an increased proportion of the Co0 peak, and the Co-O coordination peak in the O 1s spectrum disappears, confirming that the metal elements are reduced to a lower oxidation state. The P 2p spectrum shows a decrease in the intensity of the P-O coordination feature peak, indicating a decrease in the overall oxidation state of the sample after the HER.
Future work should focus on enhancing the catalyst’s stability, particularly for the oxygen evolution reaction (OER). While the material demonstrated excellent durability for the hydrogen evolution reaction (HER) for over 250 h, its OER stability was shorter, due to metal particle aggregation. Strategies like surface passivation or doping with oxophilic elements could mitigate this oxidative degradation. A crucial next step is to evaluate the electrode in a practical two-electrode electrolyzer to assess its performance under industrially relevant conditions.
To deepen mechanistic understanding, in situ and operando spectroscopy combined with density functional theory (DFT) calculations should be employed. This approach can elucidate the precise nature of the active sites at the Co/Co2P heterojunction and clarify the reaction pathways for both the HER and the OER. Finally, the versatility of the continuous coaxial co-spinning method should be leveraged to synthesize other catalysts, such as different transition metal phosphides or sulfides (e.g., Ni, Fe, Mo), to further tune catalytic activity. A thorough techno-economic analysis is also warranted to confirm the commercial viability of this scalable synthesis strategy for sustainable hydrogen production.

4. Conclusions

In conclusion, this work successfully demonstrates the importance and effectiveness of synergistically combining structural and electronic design principles to create a highly efficient and durable bifunctional electrocatalyst for overall water splitting. A self-supporting N-doped hollow carbon nanofiber electrode anchored with Co/Co2P heterojunctions (Co/Co2P-NCNFs-H) was synthesized via continuous coaxial electrospinning. The significance of this catalyst is rooted in its unique architecture, where the hollow and porous structure provides abundant active sites and facilitates mass/gas transport, while the Co/Co2P heterojunction interface optimizes the electronic configuration to boost the intrinsic catalytic activity for both the HER and the OER. The material demonstrated exceptional bifunctional electrocatalytic activity for overall water splitting in 1.0 M KOH, achieving a current density of 100 mA cm−2 at low overpotentials of 405.6 mV for the OER and 247.9 mV for the HER, alongside outstanding long-term stability (maintaining operation for 20 h at 0.6 V vs. Ag/AgCl for the OER and 250 h at −1.5 V vs. Ag/AgCl for the HER without significant degradation). Structural analyses revealed a uniform hollow nanofiber architecture with robust mechanical stability, while BET measurements confirmed a high specific surface area (254 m2 g−1) and porous structure, facilitating abundant active site exposure. Hydrophilic/superaerophobic properties validated by contact angle tests enhanced electrolyte infiltration and gas dissipation at the triple-phase interface. XRD and XPS confirmed the coexistence of metallic Co and Co2P, forming heterojunction interfaces that optimized electron transfer kinetics. Comparative studies with control samples (Co-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs) highlighted the synergistic interplay between the hollow structure and heterojunction in boosting catalytic performance. This work provides new ideas for the rational design and preparation of hollow carbon nanofiber catalytic materials with large specific surface area, fast mass transfer channels, and uniform dispersion of active sites.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15070772/s1, Figure S1. Schematic illustration of the overall synthesis of the Co/Co2P-NCNFs-H. Figure S2. HRTEM image of nanoparticle on Co/Co2P-NCNFs-H. Figure S3. (a) SEM images, (b) TEM image and (c) HRTEM image of nanoparticles on Co/Co2P-NCNFs. Figure S4. HAADF-STEM image and EDS elemental maps of the Co/Co2P-NCNFs. Figure S5. Droplet (a) and bubble (b,c) contact angles (CA) of Co/Co2P-NCNFs-H. Figure S6. Droplet (a) and bubble (b,c) contact angles (CA) of Co/Co2P-NCNFs. Figure S7. N2 adsorption-desorption isotherms of Co/Co2P-NCNFs-H. Figure S8. Pore size distributions of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H and Co/Co2P-NCNFs-H. Figure S9. Full scan XPS spectra of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H and Co/Co2P-NCNFs-H. Figure S10. SEM image of Co/Co2P-NCNFs-H-After (OER). Figure S11. SEM image of Co/Co2P-NCNFs-H-After (HER). Figure S12. The diagram of overall water splitting on Co/Co2P-NCNFs-H [2,73]. Table S1. The ICP analysis results of Co/Co2P-NCNFs-H. Table S2. The FHWM and binding energy used during XPS deconvolution.

Author Contributions

Conceptualization, R.D.; Formal analysis, R.D., J.D. and J.F.; Investigation, R.D., J.D. and J.F.; Writing—original draft, R.D. and J.D.; Writing—review & editing, L.Z.; Supervision, L.Z.; Project administration, L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by research funding provided by National Natural Science Foundation of China (Grant Nos. 22378119 and 22208092).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 00772 g001
Figure 1. Morphology characterization of Co/Co2P-NCNFs-H. (a–c) SEM images of Co/Co2P-NCNFs-H. (d,e) TEM images of Co/Co2P-NCNFs-H. (f) HRTEM images of Co/Co2P-NCNFs-H. (g) HAADF-STEM image and EDS elemental mapping of Co/Co2P-NCNFs-H.
Figure 1. Morphology characterization of Co/Co2P-NCNFs-H. (a–c) SEM images of Co/Co2P-NCNFs-H. (d,e) TEM images of Co/Co2P-NCNFs-H. (f) HRTEM images of Co/Co2P-NCNFs-H. (g) HAADF-STEM image and EDS elemental mapping of Co/Co2P-NCNFs-H.
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Figure 2. Composition study of the samples. (a) XRD patterns and (b) Raman spectra of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H. (c) TGA analysis of Co/Co2P-NCNFs-H.
Figure 2. Composition study of the samples. (a) XRD patterns and (b) Raman spectra of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H. (c) TGA analysis of Co/Co2P-NCNFs-H.
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Figure 3. XPS characterization of the samples. (a) Co 2p3/2, (b) N 1s, (c) O 1s, and (d) P 2p for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H.
Figure 3. XPS characterization of the samples. (a) Co 2p3/2, (b) N 1s, (c) O 1s, and (d) P 2p for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H.
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Figure 4. OER activity of the samples. (a) OER CV curves, (b) OER overpotential at 100 mA cm−2, (c) OER Tafel slopes, (d) Nyquist plots of OER in 1.0 M KOH, (e) TOF curves, (f) TOF values at an overpotential of 450 mV, (g) OER jECSA curves, (h) jECSA at an overpotential of 450 mV for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H in the OER test in 1.0 M KOH. (i) Chronoamperometry curve of Co/Co2P-NCNFs-H tested at a constant potential of 0.6 V vs. Ag/AgCl in 1.0 M KOH.
Figure 4. OER activity of the samples. (a) OER CV curves, (b) OER overpotential at 100 mA cm−2, (c) OER Tafel slopes, (d) Nyquist plots of OER in 1.0 M KOH, (e) TOF curves, (f) TOF values at an overpotential of 450 mV, (g) OER jECSA curves, (h) jECSA at an overpotential of 450 mV for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H in the OER test in 1.0 M KOH. (i) Chronoamperometry curve of Co/Co2P-NCNFs-H tested at a constant potential of 0.6 V vs. Ag/AgCl in 1.0 M KOH.
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Figure 5. ECSA tests of the samples. CV curves of (a) Co-NCNFs, (b) Co/Co2P-NCNFs, (c) Co-NCNFs-H, and (d) Co/Co2P-NCNFs-H at different scan rates of 20, 40, 60, 80, and 100 mV s−1; (e) double-layer capacitance measurements and (f) ECSA of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H.
Figure 5. ECSA tests of the samples. CV curves of (a) Co-NCNFs, (b) Co/Co2P-NCNFs, (c) Co-NCNFs-H, and (d) Co/Co2P-NCNFs-H at different scan rates of 20, 40, 60, 80, and 100 mV s−1; (e) double-layer capacitance measurements and (f) ECSA of Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H.
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Figure 6. HER activity of the samples. (a) HER CV curves, (b) HER overpotential at 100 mA cm−2, (c) HER Tafel slopes, (d) Nyquist plots of HER in 1.0 M KOH, (e) TOF curves, (f) TOF values at an overpotential of 400 mV, (g) HER jECSA curves, (h) jECSA at an overpotential of 400 mV for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H in the HER test in 1.0 M KOH. (i) Chronoamperometry curve of Co/Co2P-NCNFs-H tested at a constant potential of −1.5 V (vs. Ag/AgCl).
Figure 6. HER activity of the samples. (a) HER CV curves, (b) HER overpotential at 100 mA cm−2, (c) HER Tafel slopes, (d) Nyquist plots of HER in 1.0 M KOH, (e) TOF curves, (f) TOF values at an overpotential of 400 mV, (g) HER jECSA curves, (h) jECSA at an overpotential of 400 mV for Co-NCNFs, Co/Co2P-NCNFs, Co-NCNFs-H, and Co/Co2P-NCNFs-H in the HER test in 1.0 M KOH. (i) Chronoamperometry curve of Co/Co2P-NCNFs-H tested at a constant potential of −1.5 V (vs. Ag/AgCl).
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Figure 7. Structural stability during electrocatalysis. (a) TEM images of Co/Co2P-NCNFs-H-After (OER). (b) TEM images of Co/Co2P-NCNFs-H-After (HER). (c) Raman, (d) Co 2p XPS, (e) N 1s XPS, (f) O 1s XPS, and (g) P 2p XPS spectra of Co/Co2P-NCNFs-H-After (OER) and Co/Co2P-NCNFs-H-After (HER).
Figure 7. Structural stability during electrocatalysis. (a) TEM images of Co/Co2P-NCNFs-H-After (OER). (b) TEM images of Co/Co2P-NCNFs-H-After (HER). (c) Raman, (d) Co 2p XPS, (e) N 1s XPS, (f) O 1s XPS, and (g) P 2p XPS spectra of Co/Co2P-NCNFs-H-After (OER) and Co/Co2P-NCNFs-H-After (HER).
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Table 1. OER performance in 1.0 M KOH compared with other transition metal-based catalysts.
Table 1. OER performance in 1.0 M KOH compared with other transition metal-based catalysts.
CatalystsOverpotential at
100 mA cm−2 (mV)		Tafel Slope (mV dec−1)Ref.
Co/Co2P-NCNFs-H405.676.2This work
p-Co9S8/NC/CF39875[59]
Ni@N-HCGHF47063[60]
Fe3C-Co/NC450-[61]
Ni2P-VP2/NF39856[62]
Ni/NiFe2O436586[63]
Co-ZIF/CDs/CC401147[64]
NiFe LDH/NiS2/VS238499[65]
Table 2. HER performance in 1.0 M KOH compared with other transition metal-based catalysts.
Table 2. HER performance in 1.0 M KOH compared with other transition metal-based catalysts.
CatalystOverpotential at
100 mA cm−2 (mV)		Tafel Slope (mV dec−1)Ref.
Co/Co2P-NCNFs-H247.967.7This work
Co(OH)2@NiFe/NF31174[66]
Ni3S2/Ni(OH)2-5h 36080.8[67]
Ni(OH)2/NiCo2O4 189 (η10)41[68]
NiFeCrSx/NF23667.4[69]
Co2P-Ni3S2/NF110114.2[70]
ac-NiCo(OH)2/NF32090[71]
F-Co2P/Fe2P/IF151.8115.01[72]
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Duan, R.; Ding, J.; Fan, J.; Zhuang, L. Production of Self-Supporting Hollow Carbon Nanofiber Membranes with Co/Co2P Heterojunctions via Continuous Coaxial Co-Spinning for Efficient Overall Water Splitting. Coatings 2025, 15, 772. https://doi.org/10.3390/coatings15070772

AMA Style

Duan R, Ding J, Fan J, Zhuang L. Production of Self-Supporting Hollow Carbon Nanofiber Membranes with Co/Co2P Heterojunctions via Continuous Coaxial Co-Spinning for Efficient Overall Water Splitting. Coatings. 2025; 15(7):772. https://doi.org/10.3390/coatings15070772

Chicago/Turabian Style

Duan, Ruidan, Jianhang Ding, Jiawei Fan, and Linzhou Zhuang. 2025. "Production of Self-Supporting Hollow Carbon Nanofiber Membranes with Co/Co2P Heterojunctions via Continuous Coaxial Co-Spinning for Efficient Overall Water Splitting" Coatings 15, no. 7: 772. https://doi.org/10.3390/coatings15070772

APA Style

Duan, R., Ding, J., Fan, J., & Zhuang, L. (2025). Production of Self-Supporting Hollow Carbon Nanofiber Membranes with Co/Co2P Heterojunctions via Continuous Coaxial Co-Spinning for Efficient Overall Water Splitting. Coatings, 15(7), 772. https://doi.org/10.3390/coatings15070772

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