A cobalt-nitrogen co-doped porous carbon material and its preparation and application

Abstract

This invention discloses a cobalt-nitrogen co-doped porous carbon material and its preparation and application. The preparation involves: 1) grinding guanine, potassium chloride, and zinc chloride thoroughly in a mortar, followed by high-temperature pyrolysis to obtain a black solid containing salts; 2) washing the black solid multiple times with deionized water, and drying to obtain a nitrogen-doped porous carbon material; 3) adding the nitrogen-doped porous carbon material, citric acid, and cobalt acetylacetonate to a mixed solvent containing ethanol, ultrasonicating to homogenize, stirring, filtering, drying, and then high-temperature pyrolysis to obtain the cobalt-nitrogen co-doped porous carbon material. The method of this invention is simple, easy to scale up, and the obtained cobalt-nitrogen co-doped porous carbon material has a large specific surface area, high nitrogen content, and abundant reactive sites, exhibiting oxygen reduction reaction catalytic activity far superior to commercial noble metal catalysts. The cobalt-nitrogen co-doped porous carbon material of this invention is used as an electrocatalyst for the oxygen reduction reaction.

Description

Technical Field

[0001] This invention relates to the technical field of electrocatalysts for redox reactions, and more particularly to a cobalt-nitrogen co-doped porous carbon material and its preparation and application. Background Technology

[0002] Fuel cells are a new type of clean energy conversion device that can convert the chemical energy of fuel into electrical energy, and are one of the effective solutions to the current environmental pollution and energy crisis.

[0003] However, fuel cells suffer from a critical problem of low energy conversion efficiency. The kinetic rate of the oxygen reduction reaction at the cathode is a key factor determining this efficiency. While traditional Pt-based catalysts can achieve high current output, Pt is prone to agglomeration and deformation during operation, leading to deactivation and poor durability, which severely hinders the large-scale application of various fuel cells. Therefore, developing low-cost and high-performance non-precious metal electrocatalysts has significant application value.

[0004] Currently, non-precious metal and nitrogen-co-doped porous carbon materials have emerged as a highly efficient alternative to traditional precious metal catalysts. Compared to precious metal catalysts, these catalysts exhibit superior oxygen reduction catalytic activity, excellent stability, and resistance to methanol toxicity. Furthermore, they possess advantages such as high natural abundance, low cost, simple preparation, and easy recovery. In particular, atomically dispersed metal catalysts have demonstrated outstanding catalytic performance and are considered excellent replacements for Pt-based catalysts. Therefore, developing a simple, readily available, and low-cost method for single-atom non-precious metal-nitrogen co-doped carbon-based catalysts will contribute to the industrial maturation of fuel cells. Summary of the Invention

[0005] The purpose of this invention is to provide a cobalt-nitrogen co-doped porous carbon material and its preparation and application. The raw materials used in this invention are non-toxic and harmless to the human body, the process is simple and easy to implement, and it is beneficial to increase the reaction scale to realize the industrialization of fuel cells.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a cobalt-nitrogen co-doped porous carbon material includes the following steps:

[0008] Step 1: Mix potassium chloride and zinc chloride evenly, add guanine, and grind the three together evenly in a mortar to obtain a white powder;

[0009] Step 2: The white powder sample is pyrolyzed under a protective atmosphere and then washed with water to obtain nitrogen-doped porous carbon material;

[0010] Step 3: In a mixed solvent, the nitrogen-doped porous carbon material, cobalt salt, and citric acid monohydrate obtained in Step 2 are added simultaneously. After being mixed evenly by ultrasonication and stirring at room temperature, the mixture is filtered, dried, and pyrolyzed to obtain the cobalt-nitrogen co-doped porous carbon material.

[0011] The cobalt salt is cobalt acetylacetonate.

[0012] The mixed solvent refers to water and anhydrous ethanol in a volume ratio of (1.0 to 2.0): 3.

[0013] The zinc chloride and potassium chloride are anhydrous zinc chloride (ZnCl2) and potassium chloride (KCl).

[0014] The mass ratio of zinc chloride (ZnCl2) and potassium chloride (KCl) to guanine is 5:5:(0.5-1.5).

[0015] The mass ratio of nitrogen-doped porous carbon, citric acid monohydrate and cobalt acetylacetonate is 3:(0-5):(0-5);

[0016] The mass-to-volume ratio of the nitrogen-doped porous carbon to the mixed solvent is (0.1–0.25) g / 100 mL;

[0017] The mass-to-volume ratio of the monohydrated citric acid to the mixed solvent is (0–0.35) g / 100 mL;

[0018] The mass-to-volume ratio of cobalt acetylacetone to the mixed solvent is (0–0.25) g / 100 mL.

[0019] In step one, the mixing of guanine, potassium chloride, and zinc chloride is carried out at 25-30°C; specifically, the three are placed in a mortar and ground repeatedly for 30 minutes.

[0020] In step two, the white powder is subjected to pyrolysis under a protective atmosphere. Specifically, the heating rate during pyrolysis is 2–5 °C / min; after heating to 850–950 °C, it is calcined for 1–3 hours; the pyrolysis is carried out under a protective atmosphere; deionized water is used for washing, with a volume of 50–100 mL used each time, and the number of washes is 3–5. The washing method involves placing the beaker in an ultrasonic instrument for 5–10 minutes.

[0021] In step three, the ultrasonication and room temperature stirring are carried out at room temperature (25–30°C) for 12–14 hours; filtration is performed to separate the solid sample from the solution; drying is carried out at 60–80°C for 12–14 hours; pyrolysis is performed by pyrolyzing the dried solid sample under a protective atmosphere at a temperature of 750–850°C and a heating rate of 2–5°C / min, and calcining is continued for 1–1.5 hours after the temperature reaches 800–900°C.

[0022] The pore volume of the single-atom cobalt-nitrogen co-doped carbon material of this invention is 0.96–1.37 cm³. 3 / g, pore size 3.2–3.56 nm, specific surface area 1203–1682 m² 2 / g.

[0023] The cobalt-nitrogen co-doped porous carbon material of this invention is applied to the electrocatalytic oxygen reduction reaction as an electrocatalyst for the oxygen reduction reaction, i.e., a catalyst in the cathode of a fuel cell.

[0024] Compared with the prior art, the present invention has the following advantages and effects:

[0025] (1) This invention provides a method for preparing single-atom cobalt-nitrogen co-doped carbon material catalysts. The method is simple, easy to operate, and versatile. It can also scale up the experimental scale to a certain extent, thereby increasing the yield while maintaining the properties of the material and realizing industrial production.

[0026] (2) The single-atom cobalt-nitrogen co-doped carbon material prepared by the present invention has atomically dispersed cobalt species, which are doped into the carbon framework in the form of single atoms. The structure has a high specific surface area and its rich pore structure can effectively promote the material exchange efficiency in the solid-liquid interface, fully expose the active sites of the material, and greatly improve the oxygen reduction reaction activity.

[0027] (3) This invention mainly uses guanine as both a carbon and nitrogen source, which is inexpensive, readily available, environmentally friendly, and does not pose a threat to human health. Guanine is rich in nitrogen, which increases the nitrogen content of the material and thus increases the number of effective reaction sites on the catalyst. Molten salt (KCl / ZnCl2) is used as a template, and during the high-temperature calcination process, K... + Amorphous carbon can be transformed into a graphitic carbon structure, thereby improving the conductivity of the catalyst. Zn and Cl - The evaporation and etching processes can enrich the pore structure and specific surface area of ​​the catalyst, thereby improving the accessibility of active sites and enhancing the mass transfer efficiency of reactants, thus improving oxygen reduction performance. This material can be used as a high-performance cathode catalyst in fuel cells to replace noble metal catalysts. Attached Figure Description

[0028] Figure 1 The nitrogen adsorption-desorption isotherms are for the products obtained in Examples 1, 2, 4, and 5.

[0029] Figure 2 The figures show the pore size distribution curves of the products obtained in Examples 1, 2, 4, and 5.

[0030] Figure 3 The linear cyclic voltammetric scan curves are for the products obtained in Examples 1-7. Detailed Implementation

[0031] The present invention will now be described in further detail with reference to specific embodiments.

[0032] Example 1

[0033] A method for preparing a nitrogen-doped carbon material (hereinafter referred to as NC) specifically includes the following steps:

[0034] Weigh 0.4 g of guanine into a mortar, add 2.0 g of KCl and 2.0 g of ZnCl2, and grind in the mortar for 30 min to ensure thorough mixing. Transfer the mixed white powder to a porcelain boat and place it in a tube furnace under a nitrogen atmosphere, heating to 900 °C at a rate of 5 °C / min and calcining for 2 h to obtain a black carbon material mixed with salt. Transfer the obtained solid to a beaker, wash thoroughly five times with 50 mL of deionized water, then immerse the solid in 1.0 mol / L dilute hydrochloric acid for 1 h, followed by washing with a large amount of deionized water. Finally, transfer the obtained black solid to an oven at 80 °C and dry for 12 h to obtain a black nitrogen-doped carbon material, denoted as NC.

[0035] Example 2

[0036] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 20CoO) without the addition of citric acid specifically includes the following steps:

[0037] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1 and 20 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 20CoO.

[0038] Example 3

[0039] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 20Co10) specifically includes the following steps:

[0040] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1, 10 mg of citric acid monohydrate, and 20 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 20Co10.

[0041] Example 4

[0042] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 20Co30) specifically includes the following steps:

[0043] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1, 30 mg of citric acid monohydrate, and 20 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 20Co30.

[0044] Example 5

[0045] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 20Co50) specifically includes the following steps:

[0046] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1, 50 mg of citric acid monohydrate, and 20 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 20Co50.

[0047] Example 6

[0048] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 10Co30) specifically includes the following steps:

[0049] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1, 30 mg of citric acid monohydrate, and 10 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 10Co30.

[0050] Example 7

[0051] A method for preparing a cobalt-nitrogen co-doped porous carbon material (denoted as 30Co30) specifically includes the following steps:

[0052] In a mixed solvent of 5 mL deionized water and 10 mL anhydrous ethanol, 30 mg of NC obtained in Example 1, 30 mg of citric acid monohydrate, and 30 mg of cobalt acetylacetonate were added. After sonication for 5 min, the mixture was stirred at room temperature for 12 h. The filter cake was obtained by suction filtration and dried at 60 °C for 12 h. The dried powder was placed in a porcelain boat and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 1 h to obtain a black powder, denoted as 30Co30.

[0053] The products obtained in Examples 1, 2, 4 and 5 were subjected to N2 physical adsorption tests using a TriStar II 3020 fully automated surface area and pore size analyzer from Micromeritics, USA.

[0054] from Figure 1 As can be seen from this, all catalysts at lower pressures (P / P) 0 =0~0.011) N2 adsorption capacity increases sharply under higher pressure (P / P 0 =0.4~0.9) all exhibited obvious hysteresis loops, mainly due to capillary condensation in the mesopores, showing typical type IV isotherms and type H3 hysteresis loops, indicating the presence of numerous micropores and mesopores in the catalyst. Both types of pores play important roles in the oxygen reduction reaction; micropores allow oxygen molecules in the electrolyte to approach the active site, while mesopores enhance oxygen transport efficiency. Comparison of the specific surface area, pore volume, and pore size data of Examples 1, 2, 4, and 5 (Table 1) shows that the introduction of metals and citric acid significantly affects the pore volume and specific surface area of ​​the material. The specific surface area of ​​the 20CoO catalyst (Example 2) was reduced by approximately 8% compared to the NC catalyst (Example 1), as evidenced by the pore size distribution results ( Figure 2 It can be seen from the data that the introduced cobalt salt occupies a small portion of the micropores (<1 nm), and the mesopore distribution is similar to that of the NC catalyst. This may be because the cobalt occupies part of the micropores, resulting in a decrease in the specific surface area of ​​the catalyst. The specific surface area of ​​20Co30 (Example 4) after the introduction of citric acid is reduced by approximately 22% compared to 20CoO (Example 2), and by 28% compared to NC (Example 1). Notably, 20Co30 (Example 4) has the largest external specific surface area ratio, reaching 93.3%. Figure 2 It is not difficult to observe that with the introduction of citric acid, more micropores (<1nm and 1-2nm) are occupied, resulting in a decrease in specific surface area and an increase in average pore size. This is attributed to the complexing effect of citric acid on metals, allowing more cobalt and nitrogen species to occupy the micropores, while the increase in the proportion of mesopores can improve the oxygen mass transfer effect and the accessibility of active sites. Furthermore, when the amount of citric acid added is further increased, the specific surface area of ​​20Co50 (Example 5) is comparable to that of 20Co30 (Example 4), as can be seen from its pore size distribution diagram (…). Figure 2 As can be seen from the results, with the further addition of citric acid, some large micropores (1-2 nm) appear, resulting in a decrease in the proportion of mesopores. In summary, combining the results of Examples 1, 2, 4, and 5, it can be found that the introduction of cobalt salt alone has little effect on the specific surface area of ​​the material, while the introduction of an appropriate amount of citric acid has a significant impact on the micropore content. The mesopores are mainly 2-10 nm in size, indicating that cobalt atoms are successfully embedded into the NC material under the complexation effect of citric acid. Therefore, this material has excellent ORR potential as an oxygen reduction catalyst.

[0055] The products obtained in Examples 1, 2, 4, and 5 were characterized by X-ray photoelectron spectroscopy to analyze the surface elemental distribution. The instrument used was a K-Alpha X-ray photoelectron spectrometer from Thermo Fisher Scientific, USA. The actual Co loading in Examples 2, 4, and 5 was further determined using an Optima 8300 inductively coupled plasma optical emission spectrometer (ICP-OES) (Table 2).

[0056] Table 1: Specific surface area (S) of the products obtained in Examples 1, 2, 4 and 5 BET ), microporous specific surface area (S) BET ), external specific surface area (S) ext ), the proportion of external specific surface area to total specific surface area (S) ext / S BET ), average pore size and pore volume

[0057]

[0058] Table 2: Surface element atomic percentage (at%) and Co content (wt%) of the products obtained in Examples 1, 2, 4 and 5

[0059]

[0060] Table 2 shows that the nitrogen atom content in 20Co30 and 20Co50 is higher than that in NC (9.58%) and 20Co0 (6.34%), indicating that the introduction of citric acid can significantly increase the nitrogen atom content in the material. This is attributed to the synergistic effect of cobalt salt and citric acid. Nitrogen atoms have greater electronegativity than carbon atoms and contain lone pairs of electrons, which can affect the charge distribution of adjacent carbon atoms, thereby creating sites conducive to oxygen adsorption and accelerating the oxygen reduction reaction. Furthermore, it can be found that the surface cobalt content is highest in the 20Co30 material obtained in Example 4, indicating that an appropriate amount of citric acid can increase the cobalt loading, which is consistent with the ICP-OES results. Cobalt doping can form CoNx active sites, resulting in abundant active sites in the material, thereby accelerating the oxygen reduction reaction.

[0061] The catalysts obtained in Examples 1-7 were tested using linear sweep voltammetry (LSV) with an IGS-6030 electrochemical workstation from Guangzhou Yingsi Sensor Technology Co., Ltd. The results are as follows: Figure 3 As shown in Table 3, the horizontal axis represents voltage with reference to the reversible hydrogen electrode (RHE), and the vertical axis represents current density. The test medium was an oxygen-saturated 0.1 mol / L potassium hydroxide solution, the scan rate was 10 mV / s, and the rotation speed of the rotating disk electrode was 1600 rpm. The half-wave potential, onset potential, and diffusion control current density of the curve were selected as indicators to evaluate oxygen reduction performance.

[0062] Table 3: Half-wave potential, onset potential, and diffusion control current density of samples obtained in Examples 1-7 in 0.1 mol / L potassium hydroxide solution

[0063]

[0064] Depend on Figure 3 The results in Table 3 show that 20Co30 (Example 4) has the highest half-wave potential and onset potential, as well as a comparable diffusion current density. 20Co10 (Example 3) has a higher half-wave potential and onset potential than 20Co0 (Example 2), indicating that the introduction of citric acid can effectively improve the ORR activity of the catalyst. Further increasing the amount of citric acid added resulted in a significant increase in both the half-wave potential and onset potential of 20Co30 (Example 4). However, when excessive citric acid was added, the half-wave potential and onset potential of 20Co50 (Example 5) began to decrease, indicating that catalysts with a high mesoporous ratio and high cobalt loading can exhibit superior ORR activity. This result is mainly attributed to the fact that an appropriate amount of citric acid helps improve the dispersion and loading of cobalt. Furthermore, controlling the amount of cobalt acetylacetone added also has a positive effect. 20Co30 (Example 4) has a higher half-wave potential and onset potential than 10Co30 (Example 6) and 30Co30 (Example 7), indicating that adding a certain amount of cobalt acetylacetone is beneficial to improving the ORR activity of the catalyst. This is attributed to the synergistic effect of cobalt acetylacetonate and citric acid, which results in a high dispersion of cobalt atoms on the NC material, forming abundant active sites. Furthermore, the abundant micropores and large specific surface area of ​​this series of materials expose even more active sites, enhancing reactant accessibility and oxygen mass transfer efficiency. Among all the results, Example 4 exhibits an excellent half-wave potential, primarily due to the catalyst's rich content of CoNx sites and nitrogen species, which work together to catalyze the oxygen reduction reaction.

[0065] In non-noble metal nitrogen co-doped porous carbon materials, cobalt-nitrogen co-doped porous carbon materials are widely used in various fields of electrocatalysis. Cobalt-nitrogen co-doping into carbon materials not only regulates the charge distribution of active carbon sites, but also allows cobalt atoms to combine with nitrogen atoms to form CoNx active sites. During synthesis, increasing the loading of cobalt atoms can easily lead to metal agglomeration, thus reducing the electrochemical performance of the material. Using nitrogen-rich guanine as the carbon-nitrogen source, guanine easily forms highly graphitized nanosheet structures after high-temperature calcination. Simultaneously, to construct materials with abundant pore structure and large specific surface area, molten salts (KCl, ZnCl2) are used as templates and calcined with guanine at high temperatures, which is beneficial for forming nitrogen-doped porous carbon materials with high specific surface area. Furthermore, citric acid and cobalt acetylacetonate are added. Citric acid, with its abundant carboxyl groups, can improve the dispersion of cobalt atoms, thereby inhibiting cobalt atom aggregation. By controlling the amount of citric acid and cobalt salt added, the amounts of citric acid and cobalt acetylacetonate in Example 4 were determined to maximize the dispersion of cobalt atoms. In summary, the cobalt-nitrogen co-doped porous carbon material of the present invention has abundant pore structure, large specific surface area and high nitrogen content, which is conducive to the formation of abundant active sites and has excellent catalytic activity, and is expected to replace commercial noble metal catalysts.

[0066] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt-nitrogen co-doped porous carbon material, characterized in that... Includes the following steps: Step 1: Mix potassium chloride and zinc chloride evenly, add guanine, and grind the three together evenly in a mortar to obtain a white powder; Step 2: The white powder sample is pyrolyzed under a protective atmosphere and then washed with water to obtain nitrogen-doped porous carbon material; Step 3: In the mixed solvent, the nitrogen-doped porous carbon material, cobalt salt, and citric acid monohydrate obtained in Step 2 are added simultaneously. After being mixed evenly by ultrasonication and stirring at room temperature, the mixture is filtered, dried, and pyrolyzed to obtain the cobalt-nitrogen co-doped porous carbon material. The mixed solvent refers to water and anhydrous ethanol in a volume ratio of (1.0–2.0):3; The mass ratio of nitrogen-doped porous carbon, citric acid monohydrate and cobalt acetylacetonate is 3:(0-5):(0-5); The mass-to-volume ratio of the nitrogen-doped porous carbon to the mixed solvent is (0.1–0.25) g / 100 mL; The mass-to-volume ratio of the monohydrated citric acid to the mixed solvent is (0–0.35) g / 100 mL; The mass-to-volume ratio of cobalt acetylacetone to the mixed solvent is (0–0.25) g / 100 mL; In step two, the pyrolysis treatment of the white powder under a protective atmosphere specifically refers to a heating rate of 2-5°C / min during pyrolysis; calcination for 1-3 hours after heating to 850-950°C; and the pyrolysis treatment is carried out under a protective atmosphere. In step three, the ultrasonication and room temperature stirring are carried out at room temperature (25–30°C) for 12–14 hours; filtration is performed to separate the solid sample from the solution; drying is carried out at 60–80°C for 12–14 hours; pyrolysis is performed by pyrolyzing the dried solid sample under a protective atmosphere at a temperature of 750–850°C and a heating rate of 2–5°C / min, followed by calcination for 1–1.5 hours after the temperature reaches 800–900°C.

2. The method for preparing cobalt-nitrogen co-doped porous carbon material according to claim 1, characterized in that, The zinc chloride and potassium chloride are anhydrous zinc chloride (ZnCl2) and potassium chloride (KCl).

3. The method for preparing cobalt-nitrogen co-doped porous carbon material according to claim 2, characterized in that, The mass ratio of zinc chloride (ZnCl2) and potassium chloride (KCl) to guanine is 5:5:(0.5-1.5).

4. The method for preparing cobalt-nitrogen co-doped porous carbon material according to claim 1, characterized in that, In step one, the mixing of guanine, potassium chloride, and zinc chloride is carried out at 25-30°C; specifically, the three are placed in a mortar and ground repeatedly for 30 minutes.

5. A cobalt-nitrogen co-doped porous carbon material, characterized in that... Obtained by the preparation method described in any one of claims 1 to 4.

6. The cobalt-nitrogen co-doped porous carbon material of claim 5 is applied to the electrocatalytic oxygen reduction reaction as an electrocatalyst for the oxygen reduction reaction, i.e., a catalyst in the cathode of a fuel cell.

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