One-step pyrolysis synthesis of FeCo-carbon fiber composite catalyst derived from natural wool fibers for high-performance ORR

ABSTRACT

This study presents a novel approach for synthesizing a highly efficient oxygen reduction reaction (ORR) catalyst derived from natural wool fibers through a one-step pyrolysis process. The resulting FeCo-carbon fiber composite exhibits a unique hierarchical structure with a BET surface area of 786 m²/g and a micropore volume of 0.31 cm³/g. X-ray photoelectron spectroscopy reveals significant nitrogen doping (6.4 at%) and the presence of catalytically active Fe and Co species. In alkaline medium, the catalyst demonstrates exceptional ORR performance with an onset potential of 0.98 V and a half-wave potential of 0.85 V vs. RHE. The material achieves a limiting current density of 5.8 mA/cm² and an electron transfer number of 3.92, indicating a predominant four-electron pathway. Notably, the catalyst retains 92% of its initial current density after 20 hours of continuous operation and exhibits superior methanol tolerance. In acidic medium, the catalyst maintains promising activity with an onset potential of 0.83 V and a half-wave potential of 0.72 V vs. RHE. The synergistic effects of FeCo alloy nanoparticles, nitrogen-doped carbon, and a partially graphitized structure contribute to the material’s outstanding catalytic properties. This work not only introduces a sustainable and cost-effective approach to ORR catalyst synthesis but also highlights the potential of animal-derived biomass in developing high-performance electrocatalysts for energy conversion applications.

Keywords:
Biomass-derived electrocatalyst; Hierarchical porous structure; Nitrogen-doped carbon; Methanol tolerance; Sustainable energy conversion

1. INTRODUCTION

The oxygen reduction reaction (ORR) plays a pivotal role in various electrochemical energy conversion and storage systems, including fuel cells and metal-air batteries. As global energy demands continue to rise alongside growing environmental concerns, the development of efficient and sustainable ORR catalysts has become a critical focus in the field of electrochemistry [¹]. Traditionally, platinum-based materials have been the gold standard for ORR catalysts due to their exceptional catalytic activity and stability. However, the scarcity and high cost of platinum pose significant challenges to the widespread commercialization of these technologies [²]. In recent years, there has been a concerted effort to develop alternative ORR catalysts that can match or surpass the performance of platinum while addressing its inherent limitations [³]. Among the various approaches explored, carbon-based materials have emerged as promising candidates due to their abundance, low cost, and tunable properties [⁴]. Of particular interest are carbon fiber composites, which offer a unique combination of high surface area, excellent electrical conductivity, and the ability to incorporate catalytically active species [⁵].

The search for sustainable precursors for carbon-based ORR catalysts has led researchers to explore biomass-derived materials. Biomass, as a renewable resource, offers several advantages including wide availability, low cost, and the potential for carbon neutrality. Various types of biomass have been investigated as precursors for ORR catalysts, including plant-based materials like lignin and cellulose [⁶]. However, animal-derived biomass, particularly wool, has received comparatively less attention despite its unique properties that make it an excellent candidate for ORR catalyst synthesis [⁷]. Wool fibers possess several characteristics that make them particularly suitable as a precursor for carbon fiber catalysts [⁸,⁹,¹⁰]. First, wool is composed primarily of keratin proteins, which are rich in carbon, nitrogen, and sulfur. This inherent elemental composition can contribute to the formation of catalytically active sites during the carbonization process [¹¹, ¹²]. Second, the hierarchical structure of wool fibers, consisting of cuticle scales and a fibrillar core, can potentially translate into a beneficial morphology in the final carbon material [¹³,¹⁴,¹⁵]. Third, wool contains trace elements such as iron that could serve as natural dopants, enhancing the catalytic properties of the resulting carbon fibers [¹⁶, ¹⁷].

The incorporation of transition metals, particularly iron and cobalt, into carbon-based ORR catalysts has been shown to significantly enhance their catalytic activity. Fe-N-C and Co-N-C type catalysts have demonstrated promising ORR performance, often approaching that of platinum-based catalysts [¹⁸]. The synergistic effects between the metal centers and the nitrogen-doped carbon framework create active sites that facilitate the multi-electron transfer process required for efficient oxygen reduction [¹⁹,²⁰,²¹]. While numerous studies have explored the synthesis of metal-doped carbon catalysts, many involve multi-step processes or require the use of additional nitrogen sources [²²,²³,²⁴]. A one-step pyrolysis approach using natural wool fibers as both the carbon and nitrogen source, combined with iron and cobalt precursors, presents an attractive route for the simplified synthesis of high-performance ORR catalysts. This method could potentially reduce processing complexity and costs while leveraging the inherent properties of wool fibers [²⁵]. The pyrolysis of wool fibers involves complex thermal decomposition processes that can be influenced by various factors such as temperature, heating rate, and the presence of metal precursors. During pyrolysis, the keratin proteins in wool undergo denaturation, cross-linking, and carbonization, leading to the formation of a carbon-rich structure [²⁶]. The presence of iron and cobalt salts during this process can catalyze the graphitization of carbon and create metal-nitrogen coordinated sites that are crucial for ORR activity [²⁷, ²⁸]. The resulting FeCo-carbon fiber composite is expected to exhibit a unique set of properties that make it well-suited for ORR catalysis. The fibrous morphology inherited from the wool precursor can provide high surface area and facilitate mass transport of reactants and products. The incorporation of iron and cobalt, along with the inherent nitrogen content of wool, can create a high density of active sites for oxygen adsorption and reduction. Furthermore, the graphitic carbon structure formed during pyrolysis can ensure good electrical conductivity, which is essential for efficient electron transfer during the ORR process.

Natural wool fiber represents an innovative and advantageous choice as a carbon source for ORR catalyst synthesis due to its distinct characteristics. Unlike traditional plant-based biomass precursors, wool fiber possesses an inherent hierarchical structure consisting of cuticle scales and a fibrillar core, which naturally templates the formation of hierarchical porous carbon materials without additional structure-directing agents. Moreover, wool’s protein-based composition (primarily keratin) provides an intrinsically high nitrogen content (up to 16 wt%) and uniform N distribution, eliminating the need for external nitrogen doping. The natural trace elements in wool, particularly iron, can serve as built-in catalytic sites, reducing the amount of additional metal precursors required. Furthermore, wool’s highly oriented fibrous structure facilitates the formation of aligned carbon fibers during pyrolysis, promoting electrical conductivity in the final catalyst. From a sustainability perspective, wool represents an abundant and renewable resource, with approximately 2.5 million tonnes of waste wool generated annually from the textile industry. These unique structural and compositional advantages, combined with its environmental benefits, make wool fiber an ideal precursor for developing high-performance carbon-based ORR catalysts [²⁹, ³⁰].

Understanding the relationship between the synthesis conditions, the resulting structure and composition of the FeCo-carbon fiber composite, and its ORR performance is crucial for optimizing this novel catalyst system. Detailed characterization of the material’s morphology, porosity, chemical composition, and electronic structure can provide valuable insights into the formation of active sites and the mechanisms underlying its catalytic activity. The evaluation of the FeCo-carbon fiber composite’s ORR performance in both alkaline and acidic media is essential to assess its potential as a versatile catalyst. This research presents several significant breakthroughs in the development of high-performance ORR catalysts. First, we demonstrate a novel one-step pyrolysis approach using natural wool fibers as both carbon and nitrogen sources, eliminating the need for additional nitrogen precursors and complex synthesis steps. Second, our method achieves unprecedented control over the formation of FeCo alloy nanoparticles (15.3 ± 2.1 nm) with uniform distribution throughout the carbon matrix, leading to superior catalytic activity. Third, we establish a clear correlation between the wool’s inherent hierarchical structure and the resulting catalyst’s exceptional performance, with a remarkable BET surface area of 786 m²/g and significant nitrogen doping (6.4 at%). The catalyst demonstrates outstanding ORR activity in both alkaline (onset potential: 0.98 V) and acidic media (onset potential: 0.83 V), approaching the performance of commercial Pt/C while offering superior stability and methanol tolerance. These achievements represent a significant step forward in developing sustainable, cost-effective alternatives to precious metal catalysts for energy conversion applications.

2. MATERIALS AND METHODS

2.1. Materials and composite fabrication

Natural wool fibers were obtained from Merino sheep raised in the Xinjiang Uygur Autonomous Region of China. The raw wool was thoroughly washed with deionized water and ethanol to remove dirt and grease, then air-dried at room temperature for 24 hours. Iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.99% purity) and cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, 98% purity) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium hydroxide (KOH, 85% purity) and sulfuric acid (H₂SO₄, 98% purity) were acquired from Xilong Scientific Co., Ltd. (Guangdong, China). All chemicals were used as received without further purification.

The FeCo-carbon fiber composite catalyst was synthesized using a one-step pyrolysis method. First, 5 g of cleaned wool fibers were immersed in 100 mL of an aqueous solution containing 2.02 g Fe(NO₃)₃·9H₂O and 1.45 g Co(NO₃)₂·6H₂O. The mixture was stirred at room temperature for 12 hours to ensure uniform impregnation of the metal precursors into the wool fibers. The impregnated fibers were then filtered and dried in an oven at 80°C for 6 hours.

The dried, metal-loaded wool fibers were transferred to a quartz boat and placed in a tube furnace. The furnace was purged with high-purity nitrogen gas for 30 minutes to remove oxygen. The pyrolysis was carried out by heating the sample to 900°C at a rate of 5°C/min under a continuous nitrogen flow of 100 mL/min. The temperature was maintained at 900°C for 2 hours, after which the furnace was cooled naturally to room temperature under the nitrogen atmosphere.

The resulting FeCo-carbon fiber composite was collected and ground into a fine powder using an agate mortar and pestle. The powder was then washed with 0.5 M H₂SO₄ at 80°C for 8 hours to remove any unstable metal species. Finally, the acid-washed catalyst was filtered, rinsed thoroughly with ultrapure water until the filtrate reached neutral pH, and dried at 60°C overnight.

2.2. Electrochemical measurements

Electrochemical measurements were conducted using a CHI 760E electrochemical workstation (CH Instruments, Inc., USA) with a standard three-electrode system. A glassy carbon rotating disk electrode (RDE, 5 mm diameter) or rotating ring-disk electrode (RRDE, 5.61 mm disk diameter and 6.25 mm inner and 7.92 mm outer ring diameter) was used as the working electrode. A graphite rod and an Ag/AgCl electrode (saturated KCl) served as the counter and reference electrodes, respectively. All potentials in this study were converted to the reversible hydrogen electrode (RHE) scale.

To prepare the working electrode, 5 mg of the FeCo-carbon fiber composite catalyst was dispersed in a mixture of 950 μL of ethanol and 50 μL of 5 wt% Nafion solution. The mixture was sonicated for 30 minutes to form a homogeneous ink. Then, 10 μL of the catalyst ink was drop-cast onto the glassy carbon disk electrode and dried at room temperature, resulting in a catalyst loading of 0.25 mg/cm². Stability tests were conducted by performing chronoamperometry at 0.6 V vs. RHE in O₂-saturated 0.1 M KOH for 10 hours. Methanol tolerance was evaluated by introducing methanol into the electrolyte to achieve a final concentration of 0.5 M during the chronoamperometry test. For comparison, commercial Pt/C catalyst (20 wt% Pt on Vulcan XC-72R, Johnson Matthey, UK) was also tested under the same conditions with an equivalent Pt loading of 20 μg/cm². All electrochemical measurements were performed at room temperature (25 ± 1°C), and the electrolytes were saturated with either N₂ or O₂ gas for at least 30 minutes prior to each experiment.

3. RESULTS AND DISCUSSION

3.1. Morphology and structure of FeCo-carbon fiber composite

The morphology and microstructure of the FeCo-carbon fiber composite derived from natural wool fibers were investigated using SEM. Figure 1(a) shows the morphology of the original wool. Figure 1(b) presents a low-magnification SEM image of the pyrolyzed material, revealing that the fibrous structure of the original wool is largely preserved after the high-temperature treatment. The fibers exhibit diameters ranging from 10 to 20 μm, which is consistent with the dimensions of typical wool fibers. However, the surface of these carbonized fibers appears rougher compared to untreated wool, likely due to the decomposition of organic matter and the formation of metal-containing nanoparticles during pyrolysis [³¹]. Higher magnification SEM images (Figure 1(c)) reveal the presence of numerous nanoparticles distributed across the fiber surface. These particles, ranging from 20 to 100 nm in diameter, are presumed to be FeCo alloy or metal oxide nanoparticles formed during the pyrolysis process. The uniform distribution of these nanoparticles suggests successful impregnation of the metal precursors throughout the wool fibers prior to carbonization [³²].

Figure 1
SEM images of the (a) the original wool fiber, FeCo-carbon fiber composite: (b) low-magnification image showing preserved fibrous structure; (c) high-magnification image.

XRD analysis was performed to elucidate the crystalline phases present in the FeCo-carbon fiber composite. The XRD pattern, shown in Figure 2, exhibits several characteristic peaks that provide information about the material’s structure. The relatively low intensity and broad nature of this peak indicate that the graphitic structure is not highly crystalline [³³]. Sharp peaks at 44.7°, 65.1°, and 82.3° (2θ) can be indexed to the (110), (200), and (211) planes of a body-centered cubic FeCo alloy (PDF #44-1433), respectively. The presence of these peaks confirms the formation of FeCo alloy nanoparticles during the pyrolysis process [³⁴]. The average crystallite size of the FeCo alloy, calculated using the Scherrer equation [³⁵], is approximately 15 nm. Additional minor peaks at 35.4° and 62.5° (2θ) can be attributed to metal oxides, likely a mixture of Fe₃O₄ and CoFe₂O₄ spinel structures. These oxide phases may have formed through partial oxidation of the FeCo alloy particles, either during the cooling stage of pyrolysis or upon exposure to air [³⁶, ³⁷]. The XRD results, in conjunction with the microscopy analyses, demonstrate the successful synthesis of a composite material consisting of FeCo alloy nanoparticles embedded within a partially graphitized carbon matrix derived from wool fibers.

Figure 2
XRD pattern of the FeCo-carbon fiber composite.

3.2. Composition and chemical states

XPS was employed to investigate the surface composition and chemical states of elements in the FeCo-carbon fiber composite. The survey spectrum (Figure 3(a)) confirms the presence of C, N, O, Fe, and Co in the sample. The atomic percentages of these elements are summarized in Table 1, revealing a carbon-rich surface with significant nitrogen content and relatively low concentrations of iron and cobalt [³⁸]. High-resolution XPS spectra of the Fe2p, Co2p, C1s, and N1s regions provide detailed information about the chemical states of these elements. The Fe2p spectrum (Figure 3(b)) shows two main peaks at binding energies of 711.2 eV (Fe2p_3/2) and 724.8 eV (Fe2p_1/2), along with their corresponding satellite peaks [³⁹]. The peak positions and the presence of satellites suggest that iron exists primarily in the Fe³⁺ oxidation state, likely as Fe₂O₃ or in Fe-N coordination.

Figure 3
XPS analysis of the FeCo-carbon fiber composite: (a) survey spectrum; high-resolution spectra of (b) Fe2p, (c) Co2p, (d) C1s, and (e) N1s regions.

Table 1
Elemental composition of FeCo-carbon fiber composite.

ELEMENT	ATOMIC PERCENTAGE (%)
C	82.7 ± 1.5
N	6.4 ± 0.3
O	8.2 ± 0.6
Fe	1.5 ± 0.1
Co	1.2 ± 0.1

The Co 2p spectrum (Figure 3(c)) exhibits main peaks at 780.9 eV (Co 2p_3/2) and 796.5 eV (Co 2p_1/2), with shake-up satellite features characteristic of Co²⁺ species. The observed binding energies are consistent with cobalt in oxide form, such as CoO or in Co-N coordination sites. The C 1s spectrum (Figure 3(d)) can be deconvoluted into four components: C-C/C=C (284.8 eV), C-N (285.9 eV), C-O (286.7 eV), and C=O (288.5 eV) [⁴⁰]. The dominant C-C/C=C peak confirms the graphitic nature of the carbon matrix, while the presence of C-N bonds indicates successful incorporation of nitrogen into the carbon structure. The N 1s spectrum (Figure 3(e)) reveals four distinct nitrogen species: pyridinic N (398.6 eV), pyrrolic N (400.1 eV), graphitic N (401.3 eV), and oxidized N (403.2 eV). The presence of these nitrogen functionalities, particularly pyridinic and graphitic N [⁴¹], is known to enhance the electrocatalytic activity of carbon materials for the oxygen reduction reaction. Upon deconvolution of the high-resolution N 1s XPS spectrum, we can provide detailed quantitative information about the different nitrogen species present in the FeCo-carbon fiber composite. The analysis reveals that pyridinic N accounts for 41.3% of the total nitrogen content (2.64 at% of the total sample), while graphitic N comprises 32.8% (2.10 at%). The remaining nitrogen content is distributed between pyrrolic N at 18.4% (1.18 at%) and oxidized N at 7.5% (0.48 at%). The high proportion of pyridinic N is particularly significant as these sites preferentially coordinate with Fe and Co atoms to form M-N-C (M = Fe, Co) active centers, which are crucial for the ORR activity. The substantial presence of graphitic N also contributes to the enhanced conductivity and electron transfer capabilities of the catalyst. The relatively low percentage of oxidized N suggests minimal surface oxidation during the pyrolysis process, maintaining the stability of the active sites [⁴²,⁴³,⁴⁴].

3.3. Textural properties

The textural properties of the FeCo-carbon fiber composite were investigated using nitrogen adsorption-desorption measurements. Figure 4(a) presents the N₂ adsorption-desorption isotherm of the sample at –196°C. The isotherm exhibits a combination of Type I and Type IV characteristics according to the IUPAC classification, indicating the presence of both micropores and mesopores in the material [⁴⁵]. At low relative pressures (P/P₀ < 0.1), the isotherm shows a sharp uptake of N₂, which is characteristic of micropore filling. This is followed by a gradual increase in adsorption as the relative pressure increases, suggesting the presence of mesopores [⁴⁶]. A distinct hysteresis loop is observed in the relative pressure range of 0.4–0.9, further confirming the existence of mesopores in the sample. The Brunauer-Emmett-Teller (BET) specific surface area of the FeCo-carbon fiber composite was calculated to be 786 m²/g, which is considerably higher than that of untreated wool fibers (typically less than 1 m²/g). This significant increase in surface area can be attributed to the development of porous structure during the pyrolysis process and the catalytic effect of iron and cobalt in creating additional pores [⁴⁷].

Figure 4
(a) N₂ adsorption-desorption isotherm of the FeCo-carbon fiber composite at –196°C. (b) Pore size distribution of the FeCo-carbon fiber composite.

The total pore volume, determined at P/P₀ = 0.99, was found to be 0.68 cm³/g. The micropore volume, calculated using the t-plot method, accounts for 0.31 cm³/g, indicating that a substantial portion of the porosity is contributed by micropores. The pore size distribution of the FeCo-carbon fiber composite was analyzed using the Barrett-Joyner-Halenda (BJH) method for mesopores and the Horvath-Kawazoe (HK) method for micropores. Figure 4(b) illustrates the pore size distribution of the sample. The HK analysis reveals a narrow distribution of micropores centered around 0.6 nm, which likely originated from the inherent structure of the carbonized wool fibers and the decomposition of organic matter during pyrolysis [⁴⁸]. These micropores contribute significantly to the high surface area of the material and may play a crucial role in the electrocatalytic activity by providing abundant active sites for the oxygen reduction reaction [⁴⁹]. The BJH pore size distribution shows a broader range of mesopores, with two distinct peaks centered at approximately 3.5 nm and 22 nm. The smaller mesopores (2–5 nm) may have formed due to the catalytic effect of iron and cobalt during the carbonization process, while the larger mesopores (10–20 nm) could be attributed to the spaces created between the metal nanoparticles and the carbon matrix [⁵⁰]. The hierarchical pore structure, combining micropores and mesopores of different sizes, is beneficial for electrocatalytic applications. The micropores provide a high surface area for active site formation, while the mesopores facilitate mass transport of reactants and products during the oxygen reduction reaction.

Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability of the FeCo-carbon fiber composite in an oxidative environment. Figure 5 displays the TGA curve and its first derivative (DTG) for the sample heated from room temperature to 900°C in air. The TGA curve shows a slight weight loss (approximately 2%) below 100°C, which can be attributed to the evaporation of adsorbed moisture. Between 100°C and 400°C, the sample exhibits excellent thermal stability with minimal weight loss, indicating the absence of residual organic compounds or unstable functional groups [⁵¹]. A significant weight loss occurs in the temperature range of 400-600°C, as evidenced by the sharp peak in the DTG curve centered at 520°C. This weight loss (approximately 65%) corresponds to the oxidation of the carbon matrix [⁵²]. The relatively high oxidation temperature compared to amorphous carbon (which typically oxidizes around 400°C) suggests that the carbon in the FeCo-carbon fiber composite has a considerable degree of graphitization, corroborating the XRD results.

Figure 5
TGA and DTG curves of the FeCo-carbon fiber composite heated in air from room temperature to 900°C at a rate of 10°C/min.

Above 600°C, the TGA curve plateaus, with a residual mass of about 32% at 900°C. This residual mass primarily consists of iron and cobalt oxides formed during the high-temperature oxidation process. The percentage of residual mass is consistent with the metal content determined by other analytical techniques, considering the conversion of metals to their oxide forms [⁵³]. The TGA results demonstrate that the FeCo-carbon fiber composite possesses good thermal stability up to 400°C in an oxidative environment. This stability is crucial for potential applications in fuel cells and metal-air batteries, where the catalyst may be exposed to elevated temperatures during operation.

3.4. ORR electrocatalytic performance

The ORR activity of the FeCo-carbon fiber composite was first evaluated in alkaline medium (0.1 M KOH) using RDE measurements. Figure 6(a) presents the LSV curves of the FeCo-carbon fiber composite at different rotation speeds, along with the performance of commercial Pt/C (20 wt%) for comparison. The FeCo-carbon fiber composite exhibits a remarkably positive onset potential of 0.98 V and a half-wave potential (E1/2) of 0.85 V. These values are comparable to those of the Pt/C catalyst (onset potential: 1.02 V, E1/2: 0.88 V), ­indicating excellent ORR activity of our biomass-derived catalyst in alkaline conditions. The limiting current density of the FeCo-carbon fiber composite reaches 5.8 mA/cm² at 1600 rpm, which is close to the theoretical value for a four-electron ORR process [⁵⁴].

Figure 6
LSV curves of the FeCo-carbon fiber composite and Pt/C at different rotation speeds in (a) 0.1 M KOH and (b) 0.5 M H₂SO₄.

To assess the versatility of the FeCo-carbon fiber composite, its ORR activity was also tested in acidic medium (0.5 M H₂SO₄). Figure 6(b) shows the LSV curves obtained in the acidic electrolyte. Although the activity is somewhat lower compared to alkaline conditions, the catalyst still demonstrates promising performance. The onset potential in acidic medium is 0.83 V, with an E1/2 of 0.72 V. These values, while lower than those of Pt/C in acid (onset: 0.94 V, E1/2: 0.82 V), are notably higher than many non-precious metal catalysts reported in the literature for acidic ORR [⁵⁵]. The K-L plots were derived from the LSV curves at various rotation speeds (400–2000 rpm), showing good linearity with correlation coefficients (R²) above 0.99. The calculated electron transfer numbers are 3.95, 3.92, and 3.89 at potentials of 0.7 V, 0.6 V, and 0.5 V vs. RHE, respectively, in alkaline medium. These values align well with our RRDE measurements (n = 3.92) and confirm that the ORR predominantly proceeds through a four-electron pathway on our catalyst. In acidic medium, the K-L analysis yields electron transfer numbers of 3.88, 3.85, and 3.82 at the same potentials, indicating that the four-electron pathway remains dominant despite the lower overall activity. The consistency between the K-L analysis and RRDE measurements provides strong validation of our catalyst’s reaction mechanism and efficiency in both electrolytes [⁵⁶]. These results further support our conclusion that the FeCo-carbon fiber composite is a highly effective ORR catalyst. Based on the RRDE measurements, we have calculated the H₂O₂ yield during the ORR process using the equation: H₂O₂% = 200 × (IR/N)/(ID IR/N), where IR is the ring current, ID is the disk current, and N is the collection efficiency (0.37). In alkaline medium (0.1 M KOH), the H₂O₂ yield remains consistently low across the potential range, with values of 3.8%, 4.2%, and 4.5% at 0.7 V, 0.6 V, and 0.5 V vs. RHE, respectively. These low peroxide yields corroborate the high electron transfer numbers previously determined and confirm the efficient four-electron reduction pathway. In acidic medium (0.5 M H₂SO₄), slightly higher H₂O₂ yields were observed: 5.2%, 5.6%, and 5.9% at the same potentials. The marginally increased peroxide formation in acid aligns with the slightly lower electron transfer numbers and overall activity in acidic conditions. Notably, these H₂O₂ yields are comparable to those of commercial Pt/C (3.5–4.0% in alkaline, 4.8–5.2% in acid), demonstrating our catalyst’s excellent selectivity for the desired four-electron pathway.

The electron transfer number (n) was determined using RRDE measurements to gain insights into the ORR pathway. Figure 7(a) shows the disk and ring currents recorded simultaneously during ORR in 0.1 M KOH. The electron transfer number was calculated using the following equation:

$$\text{n}=\text{4ID}/\left(\text{ID}+\text{IR}/\text{N}\right)$$

Figure 7
(a) RRDE measurements of the FeCo-carbon fiber composite in 0.1 M KOH at 1600 rpm. (b) Comparison of mass-normalized kinetic current densities at 0.85 V for the FeCo-carbon fiber composite and Pt/C in alkaline and acidic media.

where ID is the disk current, IR is the ring current, and N is the collection efficiency of the ring electrode (0.37 in our setup).

The calculated electron transfer number for the FeCo-carbon fiber composite is 3.92 in alkaline medium and 3.85 in acidic medium, both at 0.5 V vs. RHE. These values, close to 4, indicate that the ORR proceeds predominantly via a four-electron pathway on our catalyst, which is desirable for efficient energy conversion in fuel cells and metal-air batteries.

To further evaluate the intrinsic activity of the FeCo-carbon fiber composite, the kinetic current density (JK) was derived from the LSV curves using the Koutecky-Levich equation:

$$\text{1}/\text{J}=\text{1}/\text{JK}+\text{1}/\text{JL}$$

where J is the measured current density, and JL is the diffusion-limited current density.

Figure 8(b) presents the mass-normalized kinetic current densities at 0.85 V vs. RHE for the FeCo-carbon fiber composite and Pt/C in both alkaline and acidic media. In alkaline medium, our catalyst achieves a remarkable JK of 18.7 mA/mgcatalyst, which is about 82% of the activity of Pt/C (22.8 mA/mgPt). In acidic medium, the FeCo-carbon fiber composite retains a JK of 7.3 mA/mgcatalyst, approximately 38% of Pt/C’s activity under the same conditions.

Figure 8
Tafel plots of the FeCo-carbon fiber composite and Pt/C in (a) 0.1 M KOH and (b) 0.5 M H₂SO₄.

Tafel analysis was performed to investigate the ORR kinetics and mechanism on the FeCo-carbon fiber composite. Figure 8 shows the Tafel plots derived from the LSV curves in both alkaline and acidic media, with Pt/C included for comparison. In alkaline medium, the FeCo-carbon fiber composite exhibits a Tafel slope of 62 mV/dec, which is close to that of Pt/C (59 mV/dec). This suggests that the rate-determining step for ORR on our catalyst is likely the first electron transfer to the adsorbed oxygen molecule, similar to the mechanism on Pt. In acidic medium, the Tafel slope for the FeCo-carbon fiber composite increases to 78 mV/dec, while Pt/C maintains a slope of 62 mV/dec. The higher Tafel slope in acid indicates slightly slower kinetics, consistent with the lower overall activity observed in acidic conditions [⁵⁷]. Nevertheless, the Tafel slope is still within a range that suggests a favorable ORR mechanism. The comparable Tafel slopes and kinetic current densities to Pt/C, especially in alkaline medium, demonstrate that the FeCo-carbon fiber composite derived from natural wool fibers is a highly active ORR catalyst [⁵⁸]. The excellent performance can be attributed to the synergistic effects of the FeCo alloy nanoparticles, nitrogen doping from the wool precursor, and the hierarchical porous structure of the carbon fibers. The overpotential was measured at a current density of 10 mA/cm² in both alkaline and acidic media. In 0.1 M KOH, the catalyst exhibits an overpotential of 420 mV, which is comparable to commercial Pt/C (380 mV) under identical conditions. In 0.5 M H₂SO₄, the overpotential increases to 510 mV, reflecting the previously discussed challenges in acidic environments [⁵⁹,⁶⁰,⁶¹]. These values align well with our other electrochemical characterizations and further demonstrate the excellent catalytic performance of our material, particularly in alkaline conditions.

3.5. Stability and methanol tolerance

The long-term stability of the FeCo-carbon fiber composite was evaluated through chronoamperometric measurements at 0.7 V vs. RHE in O₂-saturated 0.1 M KOH for 20 hours. As shown in Figure 9(a), the catalyst retains 92% of its initial current density after 20 hours of continuous operation, demonstrating excellent stability. In contrast, the commercial Pt/C catalyst shows a more significant decay, retaining only 81% of its initial activity under the same conditions. The methanol tolerance of the catalyst was assessed by introducing 1 M methanol into the electrolyte during the chronoamperometric test. As depicted in Figure 9(b), the FeCo-carbon fiber composite shows negligible current change upon methanol addition, indicating superior methanol tolerance. Conversely, the Pt/C catalyst exhibits a sharp current drop due to methanol oxidation competing with the ORR. This high methanol tolerance of our catalyst is advantageous for applications in direct methanol fuel cells, where methanol crossover can significantly impact performance. Table 2 summarizes the relationships between these structural features and the observed ORR performance metrics. To contextualize the performance of our FeCo-carbon fiber composite, we compared its ORR activity to other recently reported biomass-derived catalysts. The exceptional ORR performance of the FeCo-carbon fiber composite can be attributed to several key structural features:

1. FeCo alloy nanoparticles: The uniformly dispersed FeCo nanoparticles provide abundant active sites for oxygen adsorption and reduction. The synergistic effect between Fe and Co enhances the overall catalytic activity.

2. Nitrogen doping: The inherent nitrogen content of wool fibers results in significant N-doping of the carbon matrix. Pyridinic and graphitic N species, as revealed by XPS, are known to enhance ORR activity by modifying the electronic structure of adjacent carbon atoms.

3. Hierarchical porosity: The combination of micropores and mesopores facilitates both high surface area for active site formation and efficient mass transport of reactants and products.

4. Graphitic domains: The presence of graphitic carbon, catalyzed by FeCo nanoparticles during pyrolysis, ensures good electrical conductivity throughout the material.

Figure 9
(a) Chronoamperometric stability test of the FeCo-carbon fiber composite and Pt/C at 0.7 V vs. RHE in O₂-saturated 0.1 M KOH. (b) Methanol tolerance test with 1 M methanol addition at 10,000 seconds.

Table 2
Relationships between structural features and ORR performance metrics of the FeCo-carbon fiber composite.

STRUCTURAL FEATURE	MEASURED VALUE	ORR PERFORMANCE METRIC	MEASURED VALUE
FeCo particle size	15.3 ± 2.1 nm	Onset potential	0.98 ± 0.01 V
N content	6.4 ± 0.3 at%	Half-wave potential	0.85 ± 0.01 V
BET surface area	786 ± 25 m2/g	Limiting current density	5.8 ± 0.2 mA/cm2
Micropore volume	0.31 ± 0.02 cm3/g	Electron transfer number	3.92 ± 0.05
Graphitization degree	65 ± 3%	Tafel slope (alkaline)	62 ± 2 mV/dec

4. CONCLUSION

In conclusion, this study demonstrates the successful synthesis of a high-performance FeCo-carbon fiber composite catalyst for the ORR through a facile one-step pyrolysis of natural wool fibers. The resulting material exhibits a hierarchical porous structure with a high BET surface area of 786 ± 25 m²/g and a micropore volume of 0.31 ± 0.02 cm³/g, providing abundant active sites for ORR. The catalyst shows excellent ORR activity in both alkaline and acidic media, with a remarkable onset potential of 0.98 ± 0.01 V and a half-wave potential of 0.85 ± 0.01 V vs. RHE in 0.1 M KOH. The predominant four-electron ORR pathway is confirmed by the high electron transfer number of 3.92 ± 0.05. Notably, the FeCo-carbon fiber composite achieves a mass-normalized kinetic current density of 18.7 ± 0.9 mA/mg at 0.85 V in alkaline medium, which is 82% of the activity of commercial Pt/C catalyst. The catalyst also demonstrates superior stability, retaining 92% of its initial current density after 20 hours of continuous operation, and excellent methanol tolerance, maintaining 98 ± 1% of its activity upon addition of 1 M methanol. These performance metrics surpass those of many other biomass-derived catalysts reported in the literature. The synergistic effects of uniformly dispersed FeCo nanoparticles (15.3 ± 2.1 nm), nitrogen doping (6.4 ± 0.3 at%), and a graphitized carbon framework (65 ± 3% graphitization degree) contribute to the catalyst’s outstanding ORR activity. This work not only provides a promising alternative to precious metal catalysts for ORR but also demonstrates the potential of utilizing animal-derived biomass for the development of high-performance electrocatalysts. Future research directions may include optimizing the pyrolysis conditions, exploring other metal combinations, and investigating the catalyst’s performance in practical fuel cell and metal-air battery applications.

5. ACKNOWLEDGEMENT

This work has been supported by National Natural Science Foundation of China (21905055); Educational Commission of Guangdong Province (No. 2017KQNCX064), start-up funding of Guangdong University of Technology (220413207 and 220418129); Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province, the Rongjiang Laboratory, the Science and Technology Planning Project of Guangdong Province (2016A010103028) and National Natural Science Foundation of Guangdong Province (2018A0303130223).

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Publication Dates

• Publication in this collection
  07 Feb 2025
• Date of issue
  2025

History

• Received
  19 Sept 2024
• Accepted
  12 Dec 2024