Red Blood Cells-Derived Iron Self–Doped 3D Porous Carbon Networks for Efficient Oxygen Reduction

Abstract

In addition to C, H and O, some biomass is also rich in mineral elements. The recovery and utilization of special mineral elements is of great significance to prepare functional materials and alleviate the current energy shortage. Herein, we describe a facile strategy for making full use of the chemical composition (C, Fe) and special structure of red blood cells (RBCs) from waste pig blood to fabricate a dual metal (Fe, Co)-nitrogen (N)-doped porous carbon catalyst by pyrolysis of a mixture of RBCs biomass, cobaltous acetate, and melamine. The porous catalyst displays a comparable activity for oxygen reduction reaction (ORR) to that of commercial Pt/C catalyst, with a half-wave potential of 0.821 V_vsRHE in alkaline media and 0.672 V_vsRHE in acid electrolyte. Especially, the as-prepared catalyst shows excellent methanol tolerance and stability in both acidic and alkaline electrolytes, which is superior to commercial Pt/C catalysts. The excellent ORR activity of FeCo-N/C_(RBC) can be ascribed to the porous morphology and the cooperation between metal and nitrogen species. This work provides a novel idea of exploiting the composition of renewable biomass to modulate the activity and stability of carbon-based ORR catalysts.

1. Introduction

With the depletion of traditional energy and the environmental crisis, the demand for renewable and green energy is becoming more and more urgent [1,2,3,4]. Eco-friendly energy storage and energy conversion devices, such as rechargeable metal-air batteries and fuel cells with high energy-density and potential, have shown great potential in renewable energy technology [5,6,7,8]. As the cathode catalytic reaction of metal–air batteries and fuel cells, ORR is an important process since it determines the efficiency of the relevant electrochemical energy conversion devices [9]. Until now, the most effective electrocatalysts are Pt-based materials [10,11]. However, the Pt-based catalysts scarcity and high cost, as well as its susceptibility to CH₃OH poisoning have severely limited their general application of related technologies [12,13]. Therefore, exploring novel and effective non-precious ORR electrocatalysts as substitutes for noble metal is of great value [14,15].

Carbon-based catalysts are the most promising ORR non-precious electrode materials because of their superior thermal, mechanical and electronic properties. In this regard, various kinds of carbon materials, such as graphene, activated carbon, biochar, have been extensively explored [16,17]. However, pristine carbon materials with limited ORR activity are inert and have the following disadvantages: (i) low degree of dopant atoms, such as pyridinic N, resulting in a lack of catalytically active sites; (ii) underdeveloped porous structure, leading to low effective specific surface area, which affects the exposure of active sites and the transport of substances; (iii) poor stability. Heteroatom doped carbon-based materials with low cost, easy handleability, earth abundance and long-term stability have been proposed to be an effective way to address these issues [18,19,20,21]. Among the studied doped carbon materials, codoped metal-nitrogen carbon materials have aroused great interest because of their unique electronic structures and promising ORR activities both in alkaline and acidic media [22,23,24]. However, conventional carbon sources for the preparation of carbon materials mainly focus on non-renewable materials, such as petroleum and coal products, which do not meet the requirements of sustainable development [25,26,27]. Therefore, considering the economic benefits and sustainability, biomass should be the best promising carbon source for the synthesis of carbon-based materials.

Biomass is a sustainable and renewable carbon-rich material, which has been widely used as a green carbon source to synthesize various carbon-based products. For example, Wang et al. used grains as precursors to prepare a hollow three-dimensional (3D) porous carbon framework [28]. Sevilla et al. fabricated graphitic carbon nanostructures (GCNs) from pinewood sawdust and investigated the application of GCNs as a support to prepare Pt electrocatalysts [29]. In fact, some biomass not only contains abundant C, but also N, S, P, etc., and Fe, Cu, Mn, Mo, Zn. Therefore, in addition to exploiting the carbon element in biomass, the mineral elements in biomass can also be used to synthesize heteroatom-doped carbon materials to reduce energy waste [30,31,32,33,34]. Thus, more attention should be focused on the utilization of special mineral elements.

Red blood cells (RBCs), are double-concave disc-shaped, contain abundant C, N, Fe and other elements, and could biologically catalyze ORR with Fe–N centers in hemoglobin. Such unique characteristics could be utilized in the preparation of heteroatom doped carbon materials [35,36,37]. Furthermore, RBCs from animals are easily obtained from slaughterhouses. Herein, we describe a facile strategy, in which we can utilize the chemical composition and special structure of red blood cells from waste pig blood to prepare a dual metal (Fe, Co)-nitrogen-doped porous carbon catalyst by pyrolysis of a mixture of RBCs, cobalt acetate, and melamine. The porous FeCo-N/C_(RBC) catalyst with remarkable stability and high tolerance to methanol poisoning exhibits excellent ORR catalytic ability in both acidic and alkaline medium, which is highly comparable to commercial Pt/C catalyst.

2. Results and Discussion

The fabrication process of the RBC-derived iron self–doped 3D porous carbon catalyst (FeCo-N/C_(RBC)) is shown in Figure 1. The synthesis process involves the impregnation of discarded RBCs with Co(Ac)₂, melamine and pyrolysis under high temperature in an N₂ atmosphere. The FeCo-N/C_(RBC) displays a double concave-disk shape with a porous structure.

The morphology and structure of the catalysts were analyzed by FESEM and TEM. The FeCo-N/C_(RBC) displays a porous RBC-shaped structure with a lateral size of about 2 μm (Figure 2a). The SEM image at higher magnification demonstrates that the FeCo-N/C_(RBC) possesses large amounts of pores (Figure 2b). The porous structure increases the specific surface area of the catalyst, which is conducive to exposing more internal active sites and increasing the contact area with the substrates. Furthermore, we also find cobalt acetate and melamine play a critical role in the formation of the porous RBC-shaped structure. Pure RBCs after calcination (namely, Fe-C_(RBC)) have no porous structure and only a small fraction maintain the original morphology of the double concave-disk shape (Figure S1a). With the addition of melamine, the amount of the double concave-disk shape increases (Figure S1b). Therefore, the doping of melamine strengthens the thermal stability of the precursor to a certain extent [22,38,39], and the double concave disk shape cannot be sintered and broken after high-temperature calcination. Moreover, with the addition of cobalt acetate, the catalyst exhibits a porous structure, but cannot maintain the original morphology of RBC (Figure S1c). That may be because cobalt acetate not only provides the cobalt source doping catalyst [40] but also assumes the role of internal pore generation. Then, the samples were further analyzed by transmission electron microscope image (TEM). A TEM image (Figure 2) reveals a hole-like structure inside FeCo-N/C_(RBC), further demonstrating the porous structure of the catalyst. Figure 2d is the high-resolution transmission electron microscope (HRTEM) image of the FeCo-N/C_(RBC), which indicates that there are two different spacing lattice fringes, 2.953 nm and 2.460 nm, which corresponds to the (220) diffraction surface of Fe₂O₃ and (111) diffraction surface of CoO, respectively. Moreover, the HRTEM image and the corresponding element mappings show the uniform distribution of C, N, O, Fe and Co in FeCo-N/C_(RBC) (Figure 2e), indicating the co-existence of N, O, Fe and Co species within the carbon network. Moreover, the Fe, N and C species may be derived from proteins in RBCs. Besides, the content of Fe and Co elements in Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC), determined by inductively coupled plasma mass spectrometry (ICP-MS), are 2.6 wt% and 0.0 wt% (equaling to 0.59 atomic% and 0.00 atomic%), 2.5 wt% and 0.0 wt% (0.57 atomic% and 0.00 atomic%), 2.4 wt% and 4.3 wt% (0.56 atomic% and 0.97 atomic%), 2.3 wt% and 4.3 wt% (0.54 atomic% and 0.96 atomic%), respectively. The ICP-MS results further verify that Fe and Co species are derived from proteins in RBCs and pore-forming agent cobalt acetate, respectively. Therefore, the porous double concave disk shaped FeCo-N/C_(RBC) is successfully fabricated.

The phase composition of Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC) was characterized by XRD (Figure 3a). All samples show two obvious broad diffraction peaks at 23° and 44°, which can be assigned to the stacking of (002) carbon layers and (101) ordered hexagonal carbon layers, respectively. Moreover, the (002) diffraction peak of Fe-N/C_(RBC), FeCo-C_(RBC) and the FeCo-N/C_(RBC) are significantly shifted to a lower angle compared with that of Fe-C_(RBC), indicating that doping can lead to the expansion of the carbon layer spacing and distortion of carbon material structure. However, the (002) and (101) peak intensities of FeCo-N/C_(RBC) are much lower than those of Fe-C_(RBC), Fe-N/C_(RBC) and FeCo-C_(RBC), revealing that FeCo-N/C_(RBC) has a lower degree of graphitization than Fe-C_(RBC), Fe-N/C_(RBC) and FeCo-C_(RBC). This indicates that the addition of Co and N affects the pyrolysis or reduces the graphitization of the catalyst. In addition, the peaks appearing at 36.5°,42.4° and 61.5°can be ascribed to the (111), (200) and (220) lattice planes of CoO (JCPDS No.48-1719), Other peaks at 30.2°, 35.6°, 43.2°, 57.2° and 62.9° can be ascribed to the (220), (311), (400), (511) and (440) planes of Fe₂O₃ (JCPDS No.39-1346). This reveals that most Co or Fe species exist in FeCo-N/C_(RBC) catalyst as CoO or Fe₂O₃. The Raman spectra (Figure 3b) of all four samples show two obvious peaks at ~1350 cm⁻¹ (D band) and ~1580 cm⁻¹ (G band). The D band can reflect the defect and disorder of carbon material caused by vacancy and heteroatomic doping. The G band is assigned to the vibrational phonon E_2g of the sp²-hybridized bond in the carbon atom, revealing the degree of graphitization of carbon material, including C−C and C−N bonds, respectively. The I_D/I_G peak intensity ratios (area) of FeCo-N/C_(RBC), Fe-C_(RBC), Fe-N/C_(RBC) and FeCo-C_(RBC) are 1.27, 1.09, 1.17 and 1.06, respectively. The results demonstrate that the introduction of Co and N increases the concentration of catalyst defect sites, but reduces the degree of graphitization, which may be caused by the distortion of carbon lattice induced by doping. The above results demonstrate that the doping of Co and N improves the defect sites concentration of biomass-derived FeCo-N/C_(RBC) catalyst, confirming the XRD results.

The specific surface area and porosity of the four samples were measured by nitrogen adsorption/desorption techniques. The specific surface area of FeCo-N/C_(RBC), Fe-C_(RBC), Fe-N/C_(RBC) and FeCo-C_(RBC) are 505.2 m²/g, 85.1 m²/g, 292.4 m²/g and 381.4 m²/g (Figure 3c and Figure S2), respectively. Obviously, the doping of Co and N in the precursor significantly improves the specific surface area of the samples and forms a 3D porous network. Moreover, the pore size distributions of the four catalysts were also assessed using the Brunauer–Emmett–Teller (BET) method. The pore-size distribution curves demonstrate that Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC) all possess a typical mesoporous structure (Figure 3d and Figure S2). The pore diameter of FeCo-C_(RBC) and FeCo-N/C_(RBC) is nearly the same, indicating the Co dopant plays an important role in the formation of the mesoporous structure. However, compared with FeCo-C_(RBC) (Figure S2), the specific surface area of FeCo-N/C_(RBC) is higher, which may be due to the doping of melamine strengthening the thermal stability of the precursor, maintaining the pore structure (Figure 2b and Figure S1c). The mesoporous structure and high specific surface area can expand the electrochemically active surface area of the catalyst, and accelerate the transfer rate of reactants and products, obtaining higher electrochemical activity [41,42].

The surface chemical compositions of FeCo-N/C_(RBC) were detected by X-ray photoelectron spectrometry (XPS). The full XPS spectrum of FeCo-N/C_(RBC) (Figure 4a) proves the existence of Co 2p, Fe 2p, O 1s, N 1s and C 1s in the catalyst. Moreover, the atomic contents of C 1s, Fe 2p, N 1s, O 1s and Co 2p in all samples (Table S1) indicate that the Fe and C species are derived from RBCs, and the N species is partly from RBCs. The high-resolution XPS spectra of C 1s indicates that FeCo-N/C_(RBC) contains sp²-hybridized carbon, nitrogen-carbon and oxygen-containing moieties (Figure 4b). Figure 4c shows the Co 2p spectra of the FeCo-N/C_(RBC) catalyst, which can be resolved into four peaks, centered at 780.9, 796.5, 802.9 and 786.2 eV. More specifically, the peaks at 780.9 and 796.5 eV are assigned to Co 2p_3/2 and Co 2p_1/2, respectively, which is similar to the binding energies of CoO 2p_3/2 (781.1 eV) and CoO 2p_1/2 (796.3 eV) [43], indicating the presence of Co²⁺ existing in the form of CoO in FeCo-N/C_(RBC) catalyst. The two peaks at 802.9 and 786.2 eV correspond to Co 2p_3/2 and Co 2p_1/2 satellites, respectively. The Fe 2p spectrum (Figure 4d) can be deconvoluted into four peaks located at 725.8, 712.9, 732.9 and 718.9 eV. The presence of the peaks at 725.8 and 712.9 eV specifically correspond to the binding energies of 2p_1/2 and 2p_3/2 of Fe³⁺ in FeCo-N/C_(RBC) [44], respectively. These results agree well with the XRD result. The high-resolution N1s spectra (Figure 4e and Figure S3) of the four samples can be deconvoluted into four peaks at 398.6, 399.9, 401.1, and 403.8 eV, corresponding to pyridinic N, pyrrolic N, graphitic N, and oxide N, respectively. Generally, pyridinic N is widely recognized as an efficient active site for ORR. The pyridinic N content of Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC) are 20.78%, 15.17%, 20.97%, 25.21%, respectively. Moreover, the high-resolution XPS O 1s spectra of the as-prepared FeCo-N/C_(RBC) catalyst (Figure 4f) are also deconvoluted into metal-O (531.1 eV), metal-OH (532.8 eV) and C-O (536.6 eV) [44]. These oxygen-containing moieties indicate that there are chemically bonded oxygen groups in FeCo-N/C_(RBC), which can physically adsorb oxygen and enhance the ORR. Based on the above results, the FeCo-N/C_(RBC) should have a high catalytic activity towards ORR [33].

The ORR activity of the obtained catalysts was investigated in two kinds of electrolytes including KOH (0.1 M) and HClO₄ (0.1 M) via cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on rotating disk electrode (RDE). Figure 5a shows the LSV curves of Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC), FeCo-N/C_(RBC) and commercial Pt/C in O₂-saturated 0.1 M KOH solution at 1600 rotating speed. The ORR activities of the four catalysts increased in the order of Fe-C_(RBC) < Fe-N/C_(RBC) < FeCo-C_(RBC) < FeCo-N/C_(RBC), and their half-wave potential are 0.68 V_vsRHE, 0.69 V_vsRHE, 0.74 V_vsRHE and 0.83 V_vsRHE, respectively (Figure S4). Compared with Fe-C_(RBC), the addition of Co or N induces a positive shift in the half-wave potential of Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC). The onset potential, half-wave potential and limiting current density of FeCo-N/C_(RBC) is 0.958 V_vsRHE, 0.831 V_vsRHE and 5.547 mA cm⁻² (Figure S4), respectively, and the half-wave potential is only 44 mV lower than that of 20 wt% Pt/C, indicating that FeCo-N/C_(RBC) has excellent ORR activity. This high ORR catalytic activity can be attributed to its unique porous structure constructed by the acetate volatilization and increased conductivity constructed by nitrogen doping, which causes a large specific surface area, exposing more active sites. To verify our hypothesis, the electrochemically active surface areas (ECSAs) of four samples were measured. Moreover, the ECSAs can be assessed by the double-capacitance layer (C_dl). The C_dl is obtained from the linear slope by plotting Δj = (|j_charge − j_{off charge}|) in a faradaic silence potential range against the scan rates (Figure S5). In Figure 5b, the C_dl of Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC) and FeCo-N/C_(RBC) is 3.9 μF cm⁻², 5.4 μF cm⁻², 5.6 μF cm⁻², and 17.9 μF cm⁻², respectively. The C_dl of FeCo-N/C_(RBC) is significantly larger than that of other catalysts, indicating FeCo-N/C_(RBC) has more ECSA, which is in accordance with the results of the nitrogen adsorption/ desorption test. Therefore, the higher ORR activity of FeCo-N/C_(RBC) could be partially attributed to its more electrochemical active sites. The kinetics ORR activity of FeCo-N/C_(RBC) was further evaluated at different rotating rates from 400 to 2025 (Figure S6a). The limiting current density increases with the increase of rotation speed, which is due to the decreased diffusion distance at a higher rotating speed. The corresponding K-L plots of FeCo-N/C_(RBC) at different potentials show good linearity, indicating first order reaction kinetics with respect to the dissolved oxygen concentration and similar electron transfer numbers per oxygen molecule in the ORR process (Figure 5c). Besides, the electron transfer number of FeCo-N/C_(RBC) calculated from the K-L equation is 4.38, indicating a four-electron transfer pathway for the ORR process. Moreover, the Tafel slope is performed to evaluate the kinetic process. The Tafel slope of FeCo-N/C_(RBC) at a low overpotential is 91.14 mV dec⁻¹, which is much lower than that of other catalysts, confirming its superior ORR activities. The low Tafel slope of FeCo-N/C_(RBC) is owing to the abundant active sites and the synergic effect between metal and nitrogen species, which further highlight the structural advantages of FeCo-N/C_(RBC).

Stability is another important parameter to evaluate a catalyst. The current density of the FeCo-N/C_(RBC) catalyst retains 95.3% of the initial value after 12 h long-term continuous ORR test in the alkaline electrolyte under 0.78 V potential (Figure 5e). In contrast, 20 wt% commercial Pt/C under the same conditions retains only 73% activity. In addition to superior ORR activity and stability, methanol tolerance is another important aspect for potential application in fuel cells. When 3 M methanol is added to the electrolyte, only a slight current change could be observed on the FeCo-N/C_(RBC) loaded electrode, and it returns to the previous state with time increased (Figure 5f). In comparison, the current density of commercial Pt/C goes down instantaneously, only 36.76% current density could be retained after the signal is steady. These results demonstrate that FeCo-N/C_(RBC) is the best ORR catalyst among the four as-prepared catalysts, which is contributed to by its more accessible catalytic active sites and the cooperation between metal and nitrogen species. Furthermore, the electrocatalytic ORR performances of the FeCo-N/C_(RBC) are also competitive with previous reported biomass-derived catalysts, especially when considering the recycling of elements and the electrocatalytic property in acidic electrolytes (Table S2).

Encouraged by the excellent performance of FeCo-N/C_(RBC) in alkaline electrolytes, we further explored the catalytic property in acid electrolytes. Figure 6a shows the LSV curves of Fe-C_(RBC), Fe-N/C_(RBC), FeCo-C_(RBC), FeCo-N/C_(RBC) and 20 wt% commercial Pt/C in O₂-saturated 0.1 M HClO₄ solution. Obviously, the FeCo-N/C_(RBC) exhibits the best ORR activity among the four synthesized samples in acidic medium, and its half-wave potential and limiting current density are 0.672 V_vsRHE and 5.503 mA cm⁻² (Figure S7), respectively, approaching those of Pt/C (0.798 V_vsRHE, 6.063 mA cm⁻²). The FeCo-N/C_(RBC) catalyst also shows a smaller Tafel slope at low overpotential (Figure 6b), showing good ORR kinetic performance. Although the half-wave potential and limiting current density of FeCo-N/C_(RBC) catalyst in 0.1 M HClO₄ solution are lower than those of 20 wt% commercial Pt/C, it displays better stability and methanol tolerance. Alternatively, LSV performed at various rotation speeds indicates the shortened diffusion distance at higher rotating speeds (Figure S6b). As shown in Figure 6c, a continuous 12 h ORR test on the FeCo-N/C_(RBC) coated electrode only resulted in 25.9% loss of the initial current density value, whereas that of commercial 20 wt% Pt/C decreases by 32.3%. In the methanol tolerance test, the current density of FeCo-N/C_(RBC) changes slightly with the addition of 3 M methanol, and it restores to 91.2% of the initial value with increased time (Figure 6d). In comparison, the current density of 20 wt% commercial Pt/C plummets instantaneously, only 41.4% current density could be retained after the signal is steady. Note, that FeCo-N/C_(RBC) demonstrates superb ORR activity and stability both in the alkaline and acid medium among the four synthesized catalysts. The excellent ORR property of FeCo-N/C_(RBC) can be ascribed to the 3D morphological and compositional advantages, which are closely related to the rich defects.

3. Experimental Section

3.1. Materials

Cobaltous acetate (Co(CH₃CO₂)₂·4H₂O), Melamine (C₃H₆N₆) and sodium chloride (NaCl) were manufactured by Aladdin Chemical Co., Ltd. (Shanghai, China). Anhydrous ethanol was purchased from Tianjin Deen Chemical Reagent Co., Ltd. (Tianjin, China). Potassium hydroxide (KOH) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. (Tianjin, China). Nafion (5 wt%) was obtained from DuPont Corporation. Commercial 20 wt% Pt/C catalysts were purchased from Shanghai Hesen Electric Co., Ltd. (Shanghai, China). Ultrapure water was used in all the experiments. All chemicals used in this study were analytical grade and not further purification.

3.2. Preparation of Red Blood Cell (RBCs)

The pig blood was purchased from Guangzhou Hongyang Biotechnology Co., Ltd. (Guangzhou, China), and stored in the fridge at 4 °C. RBCs were isolated from the whole blood, centrifuged at 2000 rpm for 5 min, and washed with normal saline 0.9% NaCl solution three times. Then the purified red blood cells were placed in 4% formaldehyde phosphate buffer solution, fixed at room temperature for 4 h, and rinsed with ultrapure water three times. Next, the fixed RBCs were dried at 60 °C for 24 h.

3.3. Preparation of Catalysts

The doped catalyst preparation process was as follows. First, 25 mg melamine and 12.5 mg of cobaltous acetate were added into 80 mL ultrapure water and continually stirred at 70 °C for 5 h to obtain the solution. Then, 20 mg of dried RBCs was added into this solution and stirred for 12 h until a homogeneous mixture was formed. Next, the resulting mixture was filtered and dried at 80 °C overnight. Finally, the dried mixture was heat-treated at 900 °C for 1 h in a nitrogen atmosphere. The obtained black powders were denoted as FeCo-N/C_(RBC). As a control, Fe-C_(RBC), Fe-N/C_(RBC), or FeCo-C_(RBC) represented the catalysts that were synthesized with RBC, or melamine and RBC, or cobalt acetate and RBC as raw materials, respectively. The synthesis procedure was the same as FeCo-N/C_(RBC).

3.4. Physical Characterization

The morphology and size of the prepared catalysts were observed by field emission scanning electron microscopy (FESEM, Hitachi, SU8010, Tokyo, Japan) operated at 15 kV and transmission electron microscopy (TEM, Hitachi JEM-2100) at an acceleration voltage of 200 kV. After the catalyst was digested with mixed acid, the content of the element in the samples was determined by an inductively coupled plasma-mass spectrometer (ICP-MS, ELAN DRC-e, Perkin–Elmer Sciex, Norwalk, CT, USA). The phases and composition of the catalysts were analyzed by X-ray diffraction (XRD, Bruker D8 ADVANCE, Berlin German) with graphite monochromatized Cu Kα (λ = 0.15406 nm). Raman spectra were recorded using a LabRAM HR Evolution (Horiba, Palaiseau, France) with a 532 nm Ar-ion laser. The chemical structure was characterized by an X-ray photoelectron spectroscopy spectrometer (XPS, Thermo Scientific Escalab 220i-XL, Thermo Scientific, Waltham, MA, USA). The pore structure and specific surface area were analyzed by N₂ adsorption-desorption measurements.

3.5. Electrochemical Measurements

The electrochemical measurements were carried out using a CHI 760E electrochemical workstation using a standard three-electrode system in 0.1 M KOH or HClO₄ solution at room temperature. The electrocatalytic performance of the oxygen reduction reaction was measured with a rotating disk electrode (RDE). The catalyst-coated glassy carbon electrode (GC, diameter 5 mm) was used as the working electrode, the saturated calomel electrode (SCE) and graphite rod were used as the reference electrode and the counter electrode, respectively. In this paper, all potentials of the SCE reference electrode were calibrated versus a reversible hydrogen electrode (RHE). Before every measurement, the GC electrode was polished with an Al₂O₃ slurry (50 nm) to a mirror, then cleaned with ultrapure water and dried under an infrared lamp. Then, 2 mg of catalyst and 0.5 mg of carbon black (Vulcan XC-72, Cabot, Shanghai, China) were suspended in 500 μL of isopropanol/nafion solution (485 μL, isopropanol and 15 μL 5 wt% Nafion), followed by ultrasonication for 30 min to form a homogeneous ink. Then, 25 μL of the catalyst ink was dropped onto the upper surface of the GC electrode with a micropipette. The loading amount of catalyst was approximately 0.5 mg cm⁻².

4. Conclusions

In summary, RBCs-derived dual metal (Fe, Co)-nitrogen-doped carbon (FeCo-N/C_(RBC)) with a 3D porous network and high surface area was successfully synthesized by pyrolysis of the mixture of RBCs, cobalt acetate and melamine. The porous FeCo-N/C_(RBC) displays a comparable ORR activity to the commercial Pt/C catalyst, with a half-wave potential of 0.821 V_vsRHE in alkaline media and 0.672 V_vsRHE in acid electrolyte. Moreover, FeCo-N/C_(RBC) also exhibits excellent methanol tolerance and stability in both acidic and alkaline media, which is superior to the commercial Pt/C catalyst. The porous structure enables FeCo-N/C_(RBC) catalyst to have a higher surface area and more active site exposure, resulting in excellent ORR activity. This work provides a novel way for exploiting green, natural and sustainable resources to prepare high performance, durable composite catalysts.