Synergistic PtCo bimetallic nanocrystals on hollow carbon nanofibers for high-performance alkaline direct methanol fuel cells

Abstract

The high catalytic activity of low Pt bimetallic catalysts in an alkaline medium opens the way for commercializing alkaline direct methanol fuel cells (ADMFCs) to reduce costs and increase efficiency. A highly dispersed PtCo bimetallic catalyst supported on hollow carbon nanofibers (PtCo-HCNFs) was synthesized. This catalyst demonstrated exceptional methanol oxidation performance, robust CO tolerance, and remarkable electrochemical stability, attributed to the synergistic electron transfer between the Pt and Co active components. Porous electrospun polyacrylonitrile (PAN) nanofibers and the hollow carbon structure provide fast charge transfer channels and load space, ensuring that the electrocatalyst has abundant active sites and efficient electron transfer. The optimized PtCo-HCNFs were tested as an anode catalyst for an ADMFC single cell and exhibited a mass power density of 109.8 mW mg_Pt⁻¹. Due to the rapid charge transfer and mass transfer caused by their one-dimensional hollow porous structure and the synergistic effect of the PtCo bimetal, PtCo-HCNFs are expected to be an economical, efficient, and Co-resistant electrocatalyst for the methanol oxidation reaction (MOR). Concurrently, the merits of the low cost and high mass power density of PtCo-HCNFs in ADMFCs will prove to be of significant benefit in the promotion of the commercial application of ADMFCs.

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1. Introduction

In recent years, advancements in vehicle electrification and portable electronics have driven a rapidly growing demand for next-generation power generation devices characterized by high energy density, enhanced portability, and improved safety.¹ Among various fuel cell technologies, alkaline direct methanol fuel cells (ADMFCs) emerge as a promising alternative for next-generation power generation systems, owing to their superior methanol oxidation kinetics, reduced methanol crossover, and exceptional operational durability.^2,3 Crucially, the methanol fuel employed in ADMFCs presents multifunctional benefits including an optimal energy-to-carbon ratio, cost efficiency, environmental compatibility through biodegradation pathways, and enhanced electrochemical responsiveness.^4–11 Despite their technological promise, alkaline direct methanol fuel cells (ADMFCs) are hindered by two critical challenges:¹² sluggish six-electron transfer kinetics during the anodic methanol oxidation reaction (MOR) and progressive electrolyte carbonation. These issues collectively degrade energy conversion efficiency. While Pd- and Pt-based catalysts have dominated MOR research owing to their favorable surface electronic configurations, critical limitations persist.¹³ These include severe CO intermediate poisoning effects and prohibitive material costs, resulting in suboptimal cell performance that fails to meet commercial requirements. This technological impasse has driven extensive investigations into structural/compositional engineering of MOR electrocatalysts, particularly in developing transition metal-incorporated low-Pt systems (Pt–Co,^14,15 Pt–Fe,^16–18 and Pt–Ni (ref. 19)).

Notably, the strategy of Pt-doped transition metal demonstrates dual benefits: mitigating CO adsorption through ligand effects while simultaneously reducing noble metal loading. Advanced characterization reveals that such hybrid catalysts achieve electronic structure modulation via precious metal–additional element synergy, enabling performance metrics comparable to and sometimes surpassing commercial Pt/C benchmarks.^20,21 PtCo bimetallic doping demonstrates enhanced electrocatalytic performance through two interconnected mechanisms: atomic-scale proximity effects and synergistic electronic modulation.^15,22–26 The contracted Pt–Co interatomic distance facilitates dynamic interfacial interactions, wherein hydroxyl species (OH_ads) generated at cobalt sites under low-potential conditions undergo rapid recombination with adsorbed carbon monoxide intermediates (CO_ads) on adjacent platinum active centers.²⁷ This cooperative mechanism drives the complete oxidation of CO_ads to carbon dioxide while simultaneously regenerating catalytic sites, thereby substantially improving both MOR activity and long-term stability. In a recent investigation, Georgiana and colleagues fabricated CoPt nanowires with a diameter of 200 nm and a length of 3 μm via pulsed electrodeposition for methanol oxidation reaction (MOR) applications.¹⁵ The synthesis was conducted in stable hexachloroplatinic acid electrolytes at pH values of 2.5 and 5.5 under multiple controlled potentials. Electrochemical characterization revealed that the MOR current density increased to a maximum of 85 mA cm⁻² as the Pt loading was elevated, which was primarily attributed to the electronic modulation interactions between Pt and Co atoms. Despite these promising electrochemical performance enhancements, two critical limitations persist: the high Pt loading and intricate fabrication procedure still pose challenges for effective cost reduction of the catalyst. Furthermore, the electrodeposition-based synthesis protocols and resultant material architecture lead to the catalyst exhibiting a relatively low specific surface area and spatially confined active sites, which significantly undermines the efficiency of electrochemical reactions.

The morphological and structural characteristics of catalysts play a critical role in determining their methanol oxidation performance.^28–30 One-dimensional (1D) nanofibers have emerged as promising durable supports for fuel cell applications, owing to their exceptional chemical stability, enhanced specific surface area, and superior electrical conductivity.^31,32 Hollow porous carbon nanofibers were strategically fabricated through a coaxial electrospinning technique to optimize mass transfer efficiency and maximize electrochemically active surface area in 1D architectures.^24,33–40 This engineered configuration establishes a robust catalytic system where the unique combination of hollow porosity and fibrous morphology enables high-density dispersion of active components. Furthermore, the three-dimensional interconnected pore network facilitates reactant diffusion while providing abundant anchoring sites for metal catalytic centers, significantly improving catalytic utilization efficiency.

In this work, we develop a precisely controlled synthesis protocol to regulate the spatial distribution of PtCo bimetallic nanoparticles on hollow carbon nanofibers (HCNFs) through a stepwise doping strategy. The optimized PtCo-HCNF catalyst, fabricated through high-temperature calcination (900 °C) followed by chemical reduction, demonstrates exceptional methanol oxidation reaction (MOR) performance in alkaline media. Electrochemical measurements reveal a remarkable mass activity of 735.9 mA mg_Pt⁻¹ at 0.8 V vs. RHE, surpassing the commercial Pt/C catalyst by 3.2-fold. When evaluated as an anode catalyst in alkaline direct methanol fuel cells (ADMFCs), the PtCo-HCNFs exhibit a superior mass power density of 109.8 mW mg_Pt⁻¹ at 80 °C, exceeding commercial Pt/C counterparts by 1.4-fold. The enhanced CO-tolerance originates from the synergistic electronic modulation between PtCo alloys and the support of conductive HCNFs, which simultaneously facilitates efficient electron transfer and stabilizes metallic nanoparticles. This architecture reduces precious metal loading by 376% compared to conventional catalysts and maintains exceptional durability (>50 000 s operation), demonstrating significant potential for practical implementation in next-generation fuel cell systems. The ratio of the mass difference of platinum contained in the Pt/C catalyst and platinum–cobalt–nitrogen-doped carbon nanostructures (PtCo-HCNFs) to the mass of platinum contained in the Pt/C catalyst is 376%. This is achieved under the condition that the mass of the catalyst is constant.

2. Experimental section

2.1. Materials

All reagents were purchased from commercial sources. Cobalt acetylacetonate (C₁₀H₁₆CoO₄), N,N-dimethylformamide (DMF), acrylonitrile–styrene copolymer (SAN, M.W. ∼ 165 000), polyacrylonitrile (PAN, M.W. ∼ 150 000), chloroplatinic acid (H₂PtCl₆·6H₂O), sodium borohydride (NaBH₄), methanol and ethanol were of analytical grade.

2.2. Synthesis of the Co²⁺@PAN nanofiber membrane

The Co²⁺@PAN nanofiber membrane was fabricated via coaxial electrospinning. Specifically, 0.5 g of polyacrylonitrile (PAN) and 0.6 g of cobalt acetylacetonate (C₁₀H₁₆CoO₄) were dissolved in 5 mL of N,N-dimethylformamide (shell solution). 1.25 g of acrylonitrile–styrene copolymer (SAN) was dissolved in another 5 mL of N,N-dimethylformamide (core solution). The Co²⁺@PAN/SAN hybrid matrix films were synthesized under an operating voltage, a coaxial electrospinning shell/core solution flow ratio, and an electrospinning distance of 16 kV, 3 : 1, and 12 cm, respectively.

2.3. Synthesis of PtCo-HCNFs, Co-HCNFs and Pt-HCNFs

A two-step pyrolysis process was used to obtain the final catalyst. The mixed matrix film was pre-oxidized in air at 250 °C for 1 h and then carbonized at 900 °C for 2 h under an N₂ atmosphere at a heating rate of 2 °C min⁻¹. Finally, the Co hollow carbon nanofibers (Co-HCNFs) were synthesized. Following the grinding of Co-HCNFs, 20 mg of the resultant material was added to 15 mL of deionized water. Then, 125 μL of chloroplatinic acid solution was added and stirred for 2 hours. Subsequently, a NaBH₄ solution was added and stirred for 6 hours. Following centrifugation, water washing and drying, PtCo-HCNFs were obtained. The Pt-HCNFs were synthesized without the addition of C₁₀H₁₆CoO₄.

2.4. Materials characterization

The microscopic morphology of the as-synthesized samples was characterized by field emission scanning electron microscopy (FE-SEM, SUPRA 55) and transmission electron microscopy (TEM, JEM-2200FS). A Cu K X-ray source (λ = 1.5405 Å) was used with a Bruker TTR3 diffractometer to capture the X-ray diffraction (XRD) patterns. The valence states of the produced materials were examined using XPS (Thermo Scientific K-Alpha). The specific surface area and pore size distribution were measured by N₂ adsorption–desorption measurements (Quantachrome Autosorb). The chemical compositions of the products were characterized via a PerkinElmer Optima 7000DV inductively coupled plasma optical emission spectrometer (ICP-OES).

2.5. Electrochemical measurements

All electrochemical measurements were carried out on an AUTOLAB workstation using a standard three-electrode system. A glassy carbon electrode (area 0.196 cm²) coated with a catalyst, an Ag/AgCl electrode and a Pt electrode were used as the working, reference, and counter electrodes, respectively. The catalyst ink was synthesised through a complex multistep process involving the amalgamation of the catalyst (5 mg), a 5% Nafion solution (50 μL), isopropyl alcohol (500 μL) and deionized water (500 μL) through the utilisation of ultrasonic waves. A quantity of 15 microlitres of the ink was dispensed onto the surface of a rotating disc electrode and allowed to dry at ambient temperature. The electrolyte solution was a 1.0 M KOH solution with and without 1.0 M CH₃OH. Cyclic voltammetry (CV) curves were measured at a scan rate of 50 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) tests were performed in the frequency range of 0.01–100 000 Hz at open circuit voltage. Anti-CO poisoning studies were conducted in 1.0 M KOH and 1.0 M CH₃OH by simply saturating the solution with CO gas.

2.6. Preparation of the membrane electrode assembly (MEA) and ADMFC performance tests

A spraying method was used to prepare the MEA with an active area of 4 cm². An FAA3 anion exchange membrane (Fumatech, area 3 cm × 3 cm, thickness 50 μm) was impregnated in 0.5 M KOH solution for 12 h before coating to facilitate ion exchange. The synthesized anode catalyst was combined with isopropanol and Nafion (5 wt%) solution (1 mg/150 μL/5 μL) to create the electrocatalyst coating. After two hours of uniform ultrasonic dispersion, the catalyst coating was sprayed onto one side of the carbon fabric gas diffusion layer (GDL, 2 cm × 2 cm). The mass loading of the electrocatalyst was 4 mg cm⁻². Meanwhile, the cathode electrode was similarly coated with the catalyst containing 20% Pt/C. MEAs were created by fixing them with silicone rubber sealing gaskets. The prepared MEAs were mounted in a single-cell test fixture containing a 4 cm² serpentine flow field and physically assembled by applying a certain amount of pressure through screw twisting. Activation was performed at a cell temperature of 65 °C during all single-cell tests. Anode methanol solution (3 M methanol + 4 M KOH, 5 mL min⁻¹) and O₂ (300 mL min⁻¹) were supplied to the anode and cathode, respectively. Until there was no change in the open circuit voltage (OCV) and the cell was stabilized, the polarization curves of the ADMFC single cell were collected through the fuel cell test system using the continuous current polarization mode.

3. Results and discussion

3.1. Synthesis of PtCo-HCNFs

The PtCo-HCNFs were synthesized by coaxial electrospinning, pyrolysis and chemical reduction. Fig. 1 exhibits the preparation process of the composites, where PAN is used as an electrospinning carrier to distribute Co²⁺ uniformly on its surface. After pre-oxidation and high-temperature carbonisation, the material within the fibre is evaporated, and the shell material is fully carbonised to form the structure of hollow porous carbon nanofibres. Cobalt ions are reduced to cobalt at elevated temperatures and distributed uniformly in the hollow structure. Subsequently, a chemical reduction process causes Pt to be uniformly loaded on HCNFs. The hollow porous design has been revealed to reduce the agglomeration of active metals and to maximise the exposure of the catalyst's active specific surface area.

Fig. 1 Schematic illustration of the synthesis process for PtCo-HCNFs.

3.2. PtCo atomic ratio affected catalytic activity

The atomic ratio is an important influencing factor for synthesizing nanocrystal materials. Therefore, the structure and properties of PtCo nanocrystal catalysts were systematically characterized and evaluated at PtCo atomic ratios of 1 : 2, 1 : 2.5, and 1 : 3 to determine the optimal catalytic configuration. First, XRD patterns were used to investigate the crystal structure of the catalysts, and the results are shown in Fig. 2a. The XRD diffraction patterns of the catalysts collected during various stages of the synthesis process were systematically compared to track structural evolution. A broad peak near 2θ = 26° corresponds to the characteristic peak of the graphitized C (002) crystal plane. In addition, the XRD analysis of Co-HCNFs exhibits three distinct diffraction peaks at 44.2°, 51.5°, and 75.8°, corresponding to the (111), (200), and (220) crystallographic planes of metallic cobalt (JCPDS 15-0806), respectively. In contrast, Pt-HCNFs demonstrate a characteristic (111) Pt peak at 39.7° (JCPDS 04-0802). The sequential emergence of these metal-specific diffraction features in PtCo-HCNFs confirms the controlled stepwise deposition of cobalt and platinum nanoparticles on the hollow carbon nanofibers.

Fig. 2 Structural characterization and performance optimization. (a) XRD patterns; (b) CV curves of the PtCo-HCNF electrodes with different PtCo atomic ratios in 1 M KOH solution; (c) CV curves of the PtCo-HCNF electrodes with different PtCo atomic ratios in 1 M KOH solution and 1 M methanol; (d) EIS diagrams of PTCO-HCNF electrodes under different PtCo atomic ratios and partial magnified images of them.

Furthermore, electrochemical tests were performed on samples obtained to ensure that the MOR activity was optimized for the optimal PtCo atomic ratio. Therefore, the catalytic activities of PtCo-HCNFs with PtCo atomic ratios of 1 : 2, 1 : 2.5, and 1 : 3 were compared, respectively. Fig. 2b demonstrates the cyclic voltammetry (CV) for the three electrocatalysts in the 1 M KOH electrolyte at a scan rate of 50 mV s⁻¹. Among them, the PtCo-HCNF catalyst with a PtCo atomic ratio of 1 : 2.5 exhibited the largest enclosed CV curve area and the highest peak current density, suggesting the highest density of accessible active sites in the alkaline medium. The PtCo-HCNF catalyst with an atomic ratio of 1 : 2.5 also demonstrated the highest MOR activity. As shown in Fig. 2c, all the catalysts were subjected to CV tests in methanol solution. The PtCo-HCNF catalyst with a PtCo atomic ratio of 1 : 2.5 exhibits a current density of 11.2 mA cm⁻² at 0.8 V (vs. RHE), which is significantly higher than the oxidation performance of the 1 : 2 (6.4 mA cm⁻²) and 1 : 3 (4.4 mA cm⁻²) atomic ratio counterparts. In addition, the overall MOR kinetics of each catalyst was studied using electrochemical impedance spectroscopy (EIS) measurements to compare the conductivity and charge transport/diffusion ability of the electrode surfaces. The intersection of the high-frequency curve with the X-axis indicates the contact resistance, while the semicircle is an essential feature of the charge transfer process. In Fig. 2d, all the electrodes exhibit the same contact resistance (about 6.4 Ω). Additionally, the semicircle's smaller diameter than that of the other electrocatalysts indicates that the PtCo-HCNF catalyst surface with a PtCo atomic ratio of 1 : 2.5 has the maximum electrical conductivity and lowest electron transfer resistance, which is responsible for its better MOR activity. Therefore, the PtCo-HCNF catalyst with a PtCo atomic ratio of 1 : 2.5 was used for subsequent testing.

3.3. Structure and morphology analysis of PtCo-HCNFs

Scanning electron microscopy (SEM) observed the composites' morphology and structure. Fig. 3a provides a visual representation of the network structure of carbon nanofibres interconnected by PtCo-HCNFs. Fig. 3b reveals that Co-HCNFs retained structural integrity after 900 °C annealing while exhibiting a well-defined hollow architecture. Metallic nanoparticles observed at fractured regions confirm that Co active species are uniformly distributed throughout the fiber's exterior and interior surfaces, effectively suppressing nanoparticle aggregation. PtCo nanoparticles are uniformly loaded within and on the surface of HCNFs (Fig. 3a and d), further demonstrating strong interfacial adhesion between metal active sites and the carbon matrix. Remarkably, this structural configuration maximizes active site accessibility and enhances metal loading capacity, fulfilling the design objectives of hollow carbon nanofibers. The nitrogen adsorption–desorption isothermal line is demonstrated in Fig. S1, with a specific surface area and a total pore volume of 271.6 m² g⁻¹ and 0.253 cm³ g⁻¹, respectively. In addition, the average aperture is between 2 and 10 nm, and the rich middle aperture structure is more conducive to improved mass transfer and electronic transport. TEM was used to characterize the microstructure of PtCo-HCNFs in detail. As shown in Fig. 3e, Co and Pt nanoparticles with different average particle sizes from tens of nanometers to a single-digit nanometer scale were immobilized on the surface of carbon nanofibers, and the metal nanoparticles were uniformly dispersed without obvious aggregation. The high-resolution TEM image (Fig. 3f) exhibits three clear lattice stripes with interfacial crystal distances of 0.204 nm, 0.226 nm and 0.341 nm corresponding to the (111) plane of the Co, the (111) plane of the Pt and the (002) plane of the graphitic carbon layer, respectively, which is consistent with the XRD analysis. The analysis of the elemental mapping images (Fig. 3g, j and k) indicates that the Pt and Co nanoparticles are uniformly immobilized in the hollow carbon nanofibers.

Fig. 3 SEM images of (a and d) PtCo-HCNFs, (b) Co-HCNFs, and (c) Pt-HCNFs, TEM image of (e) PtCo-HCNFs, HRTEM image of (f) PtCo-HCNFs, and (g–k) elemental mapping images of PtCo-HCNFs.

The surface chemistry and valence of PtCo-HCNFs were determined by X-ray photoelectron spectroscopy (XPS), and the survey spectra (Fig. 4a) indicate the presence of Pt, Co, C, N and O elements. In Fig. 4b, the C 1s spectra can be deconvoluted into four independent peaks at 284.6, 285.2, 286.7, and 288.8 eV corresponding to C=C, C–C, C–O, and O–C=O groups, respectively. As illustrated in Fig. 4c, two distinct peaks of the nitrogen type can be discerned in the N 1s spectrum. These peaks correspond to pyridine-N (398.4 eV) and graphite-N (401.1 eV), respectively. It has been demonstrated that graphite N has a beneficial effect on the electronic conductivity of the material, thereby increasing the ultimate current density. In the Pt 4f spectrum (Fig. 4d), the Pt 4f_7/2 peak at 71.58 eV and the Pt 4f_5/2 peak at 74.98 eV are considered metallic Pt. The two Pt peaks at 72.68 and 76.48 eV are attributed to Pt 4f_7/2 and Pt 4f_5/2, respectively. Moreover, the electron binding energy corresponding to Pt⁰ in PtCo-HCNFs (71.58 eV) is positively shifted by about 0.2 eV compared to Pt-HCNFs (71.38 eV), and the electron binding energy corresponding to Co⁰ in PtCo-HCNFs (780.38 eV) is negatively shifted by about 0.2 eV compared to Co-HCNFs (780.58 eV), quite probably owing to the electron transfer between Co and Pt.²⁶ As shown in Fig. 4e, the Co 2p_3/2 peak at 796.9 eV and the Co 2p_1/2 peak at 780.38 eV are indexed to Co⁰, while the peak at 781.9 eV is a characteristic peak of Co²⁺, accompanied by two broad satellite peaks, indicating the presence of CoO.¹² Notably, the absence of Co²⁺ characteristic peaks in Co-HCNFs (Fig. 4e) suggests potential electron transfer pathways between PtCo species. This electronic redistribution likely modulates the catalyst's electronic configuration and surface chemistry, optimizing the reactant/product adsorption–desorption kinetics.^41,42 Binding energy shifts observed for both Pt and Co (XPS analysis) reveal pronounced electronic coupling between the two metallic components, directly evidencing their synergistic interaction that enhances catalytic functionality.

Fig. 4 XPS spectra of the PtCo-HCNFs. (a) Survey spectrum; (b) C 1s spectra; (c) N 1s spectra; (d) Pt 4f spectra of PtCo-HCNFs and Pt-HCNFs; (e) Co 2p spectra of PtCo-HCNFs and Co-HCNFs.

3.4. MOR performance of PtCo-HCNFs

The MOR activity of PtCo-HCNFs was studied using various electrochemical methods and compared with commercial Pt/C. Mass activity (MA) was calculated to benchmark the specific activity per milligram of platinum to quantify the catalytic efficiency of PtCo-HCNFs. ICP-OES quantification confirms a Pt loading of 4.2 wt% in PtCo-HCNFs (Table S1), lower than that in commercial Pt/C (20 wt%). Fig. 5a compares the CV curves of the PtCo-HCNF and Pt/C electrodes without methanol (1 M) in KOH solution. The integrated cyclic voltammogram (CV) area of PtCo-HCNFs exceeds that of commercial Pt/C, demonstrating superior electrochemical activity. This enhancement correlates with increased mass activity and optimized charge transfer kinetics, as evidenced by the reduced peak potential separation. After the addition of methanol, the current response is evident. It demonstrates a high mass activity of 735.9 mA mg_Pt⁻¹ at 0.8 V (vs. RHE), which is significantly higher than the mass activity of commercial Pt/C (301.5 mA mg_Pt⁻¹, Fig. 5b). It also exhibits excellent catalytic ability compared to previously reported Pt-based catalysts (Table S2). This inverse correlation between the Pt content and activity highlights the synergistic advantages of the bimetallic doping strategy, where Co integration modulates Pt electronic states (XPS analysis, Fig. 4c and d) and stabilizes undercoordinated active sites. In addition, the electrocatalytic durability was determined by chronoamperometry at peak potential (Fig. 5c). There is a significant current decay in both the initial phases, which is a combined effect of capacitive discharge and the poisoning of the active site by the intermediate product. Then the electrode tends towards a pseudo-steady state. Furthermore, the stability was evaluated with a continuous CV scan at 50 mV s⁻¹. As shown in Fig. 5d, the current density of the PtCo-HCNF electrode loses 13.4% of its initial value after 1000 cycles. This proves the oxidation stability of methanol brought about by the structural design of the catalyst. However, as shown in Fig. 5f, the performance of the Pt/C electrode was severely degraded after multiple cycles. After replacing the new test solution, the peak potential shifted significantly to the right and still had 40.3% current decay. The Pt/C electrode was severely affected by the by-product poisoning on the electronic and chemical properties of Pt nanoparticles after the test, resulting in an elevated oxidation potential and a significant decrease in durability. The CO-poisoning resistance mechanism of PtCo-HCNFs is schematically illustrated in Fig. 5e. During methanol oxidation, the stepwise dehydrogenation process generates various carbon-containing intermediates, among which CO species tend to strongly adsorb onto Pt active sites, potentially blocking catalytic surfaces. Concurrently, hydrolysis reactions facilitated by Co active sites and the hydroxide-rich electrolyte environment promote the generation of reactive hydroxyl species (OH*) near the Pt–Co interface.^43,44 These activated hydroxyl groups effectively oxidize the adsorbed CO intermediates through Langmuir–Hinshelwood-type surface reactions, enabling efficient CO desorption. This synergistic mechanism between Pt and Co components ensures continuous regeneration of active sites, thereby maintaining the catalyst's electrochemical activity and durability.

Fig. 5 Electrochemical performance of PtCo-HCNFs for the MOR. (a and b) CV comparison of PtCo-HCNFs and Pt/C; (c) 8 h CA stability test of PtCo-HCNFs and Pt/C; (d and f) experimental comparisons of the durability; (e) mechanism analysis of anti-CO poisoning.

Moreover, to validate the necessity for cobalt doping in enhancing catalytic activity and elucidate the synergistic mechanism between PtCo active centers, we performed comparative electrochemical activity assessments of Pt-HCNFs and Co-HCNFs. As demonstrated in Fig. 6a, the Co-HCNF sample exhibited no discernible redox peaks. At the same time, Pt-HCNFs displayed a comparable peak morphology but significantly lower peak current density and reduced CV curve area relative to PtCo-HCNFs, thereby confirming the essential role of Co doping. Fig. 6b reveals that Co-HCNFs solely manifested a hydrogen evolution reaction peak at approximately 1.1 V, indicating their inherent inability for methanol oxidation. The methanol oxidation peak current density of Pt-HCNFs reached 4.2 mA cm⁻² at 0.73 V, markedly inferior to the PtCo-HCNF counterpart with equivalent mass loading (11.2 mA cm⁻² at 0.77 V). The slight positive shift in methanol oxidation potential observed for PtCo-HCNFs could be attributed to metal-to-metal oxide conversion processes.¹² Complementary EIS analysis in Fig. 6c demonstrates that PtCo co-doping substantially reduces interfacial charge-transfer resistance and mass transport limitations. These collective findings substantiate that the synergistic electronic effects between Pt and Co atoms enable PtCo-HCNFs to achieve superior electrochemical performance compared to monometallic catalysts.

Fig. 6 (a) CV curves of the PtCo-HCNF, Pt-HCNF and Co-HCNF electrodes in 1 M KOH solution; (b) CV curves of the PtCo-HCNF, Pt-HCNF and Co-HCNF electrodes in 1 M KOH solution and 1 M methanol; (c) EIS diagrams of PtCo-HCNF, Pt-HCNF and Co-HCNF electrodes and some of their magnified images.

During the methanol oxidation process, the catalyst will unavoidably become poisoned by the reaction by-product CO.⁴⁵ As a result, the resistance of electrocatalysts to CO must be investigated. As shown in Fig. 7a, the PtCo-HCNFs were continuously fed with CO in the methanol electrolyte to keep the gas saturated, and the mass activity decayed by only 16.8% at 0.67 V (vs. RHE) during the oxidation of methanol, which proved that the PtCo-HCNFs still had a high catalytic activity. In contrast, the comparative Pt/C was less resistant to CO poisoning. The oxidation potential in the methanol solution saturated with or without CO increased significantly, and the peak mass activity decayed by 44.8% (Fig. 7b). This is consistent with the long-term stability test, indicating that the Pt/C electrode is more affected by by-products and the PtCo-HCNFs have better stability and resistance to poisoning. This can be attributed to the strong electronic effect between Pt and Co binary atoms and the bifunctional mechanism, where the charge transfer from Pt to Co changes the electronic structure of the bimetallic cluster and reduces the ability of active Pt to adsorb CO by modulating its d-band center. Additionally, it can generate more OH_ads of oxygen-containing species at low potentials to accelerate the oxidative removal of adsorbed CO, thus improving the MOR progress.

Fig. 7 The CO-resistance experiment. CV curves of PtCo-HCNFs and Pt/C in methanol solutions (a) with or (b) without CO saturation.

The ECSA is closely associated with the number of available active sites. Electrochemical active surface area (ECSA), the number of active sites per gram of the material, was determined by the following equation: ECSA (cm² g⁻¹) = Q_CO × ([Pt] × 0.21)⁻¹, where Q_CO is the CO desorption (mC cm⁻²) charge, [Pt] is the loading of Pt, mass per square meter, on the electrode, and 0.21 (mC cm⁻²) is the charge required for oxidation of a hydrogen monolayer on a Pt surface.^46,47 The ECSA of the PtCo-HCNF electrode was calculated to be about 0.7 cm² g⁻¹, which is much higher than that of Pt/C (0.0013 cm² g⁻¹). Large ECSA will enhance the catalytic performance, which is very important for the electrochemical activity of the catalyst.⁴⁸

3.5. ADMFC performance

To evaluate the MOR activity of PtCo-HCNF electrocatalysts in ADMFC devices, single-cell devices based on FAA3 anion-exchange membranes with 20% Pt/C electrocatalysts as cathode electrocatalysts were synthesized (Fig. 8a). Firstly, the effect of different chemical components brought about by the variation of methanol flow on the performance of ADMFC devices was investigated under sufficient oxygen at the cathode. The polarization curves were obtained at anodic flow rates of 5, 10, and 15 mL min⁻¹ and a cathodic flow rate of 300 mL min⁻¹, respectively, as shown in Fig. 8b. The results reveal that the maximum power density of 19 mW cm⁻² and the highest open-circuit voltage (OCV) were obtained at a flow rate of 10 mL min⁻¹ of methanol solution.

Fig. 8 (a) Schematic illustration of the ADMFC; (b) cell voltage and power density polarization plots for the PtCo-HCNF electrode with different anode flow rates; (c and d) cell voltage and mass power density polarization plots for PtCo-HCNFs and 20% Pt/C at different operating temperatures.

At a low flow rate of 5 mL min⁻¹, the ADMFC is in a state of electrical instability due to the loss of concentration caused by insufficient methanol supply. At this time, the performance of the ADMFC cannot be fully reflected. In contrast, at a high flow rate of 15 mL min⁻¹, incomplete methanol consumption due to low catalyst efficiency leads to methanol crossover through the exchange membrane. This phenomenon generates a mixed potential, thereby reducing the maximum power density. A methanol flow rate of 10 mL min⁻¹ was considered the optimal reaction conditions.

In addition, the operating temperature plays a vital role in activating methanol fuel cell electrodes and suppressing the cross-effect of methanol. Therefore, cell assemblies were formed using PtCo-HCNFs and for comparison 20% Pt/C catalysts served as anodes for methanol fuel cell performance tests at different operating temperatures, respectively. Based on the above anode flow rate study, the anode flow rate for all cell performance tests was 10 mL min⁻¹. As shown in Fig. 8c and d, the power density and OCV of the ADMFC devices assembled with both catalysts increased with the increase in operating temperature. This indicates that the higher operating temperature is conducive to the thermal stability of ADMFC devices, the diffusion rate of methanol is higher, and the mass transport of methanol is better. At this time, the kinetic enhancement of the MOR results in better performance. On the other hand, the high temperature also reduces the polarization and the mixing potential due to the water flooding and cross-effect of methanol, which improves the reaction rate while alleviating the performance degradation caused by the gas–liquid two-phase blockage. In contrast, the performance is poorer at low temperatures due to kinetic limitations. The maximum power densities achieved by PtCo-HCNFs at different temperatures are 7.8 mW mg_Pt⁻¹, 11.2 mW mg_Pt⁻¹, 35.8 mW mg_Pt⁻¹, and 109.8 mW mg_Pt⁻¹ (room temperature, 50 °C, 65 °C, and 80 °C). Compared with the 20% Pt/C anode (36.3 mW mg_Pt⁻¹ in 80 °C), the power density was improved at higher operating temperatures, indicating that PtCo-HCNFs have better thermal stability and are more favorable for fuel cell operation at high temperatures.

4. Conclusion

In summary, a highly dispersed hollow carbon nanofiber supported PtCo bimetallic catalyst was synthesized by coaxial electrospinning, two-step pyrolysis and chemical reduction. The optimized PtCo-HCNFs exhibit exceptional methanol oxidation reaction (MOR) activity with a mass activity of 735.9 mA mg_Pt⁻¹ at 0.8 V vs. RHE, representing a 3.2-fold enhancement over commercial Pt/C. In addition, as an anode catalyst for ADMFC single cells, it exhibited a mass power density of 109.8 mW mg_Pt⁻¹, outperforming Pt/C by 41.7%. The MOR performance of PtCo-HCNFs is attributable to the structural effect. Specifically, the one-dimensional hollow porous carbon nanofibres synthesized by coaxial electrospinning provide PtCo-HCNFs with a substantial active metal loading capacity, resulting in an ECSA that significantly exceeds that of Pt/C. Concurrently, many mesoporous pores on the catalyst surface are advantageous for both methanol adsorption and electrolyte diffusion. The strategy of PtCo bimetallic doping has been demonstrated to effectively modify the charge distribution of the atomic configuration, thereby accelerating the electron transfer rate and promoting the adsorption/desorption of reactants and products. The present study proposes a novel strategy for enhancing mass and electron transport in membrane fuel cells by integrating platinum–cobalt bimetallic nanocrystals with hollow carbon nanofiber architectures. The demonstrated performance enhancements underscore its viability for practical implementation in next-generation energy systems. Furthermore, the material cost is exceptionally low, thus enabling the complete replacement of traditional Pt/C, and thereby promoting the wide application of DMFCs in the future market.

Conflicts of interest

There are no conflicts to declare.

Data availability

The primary research results and new data in the article are available. Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cy00831j.

Acknowledgements

This work is supported by the Beijing Municipal Natural Science Foundation (No. 2252043), the National Natural Science Foundation of China (No. 52170019), the Fundamental Research Funds for the Central Universities (No. 06500100), and the “Ten Thousand Plan”-National High-level Personnel of Special Support Program.

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