Preparation and performance of the non-platinum catalyst for oxygen reduction reaction in fuel cells

The oxygen reduction reaction (ORR) of the fuel cells is a core of energy conversion, whereas its kinetics is slow due to the limitation of the activation energy barrier, which dramatically limits the commercial development of fuel cells. At present, the main electrocatalyst for the cathode oxygen reduction reaction of fuel cells is the platinum (Pt)-based catalyst. Still, as a precious metal, Pt has a high price, low reserves, and poor stability. These shortcomings seriously affect the mass production and commercialization of fuel cells. Therefore, there is an urgent need for the development of fuel cells to study a low price, sufficient production, high stability, and pollution-free non-Pt catalyst. The main research content of this paper is to use biomass sycamore ball as a carbon source, mixed with ferric chloride (III) hexahydrate and melamine, and prepare the carbon, nitrogen, and iron co-doped ORR catalyst by high-temperature calcination under a nitrogen atmosphere. The physical characterization and electrochemical testing of the obtained samples were conducted at room temperature. The results showed that the pretreatment of sycamore ball powder to iron ratio of 1:3 had the best catalytic performance.


Introduction
As a green renewable energy conversion device, fuel cells (FCs) have become one of the leading sustainable and high-quality energy sources because of their high energy conversion efficiency, low reaction noise, and no pollutants [1].In the continuous development of FC technology, many issues need to be addressed.The reaction rate of FCs is confined by the pace of the cathode oxygen reduction reaction (ORR).The widely used oxygen reduction catalyst is platinum (Pt) -based materials, which have higher catalytic efficiency than other non-Pt catalysts.However, Pt is a precious metal with a high price and limited reserves.When used as a catalyst, it has poor stability and is prone to poisoning.These drawbacks have hindered the commercialization of Pt-based catalysts to a certain extent.Therefore, the key focus of the FCs is to research and design stable, low-cost, abundant, and non-polluting non-Pt materials that can replace Pt-based catalysts.
Research on non-Pt catalysts has made significant progress abroad [2].Currently, the most common non-Pt catalysts are metal or non-metal composite materials, such as metal oxides, nitrogen-doped carbons, metal nitrides, and graphene oxide composites.These composite materials have advantages such as high catalytic activity and stability, demonstrating excellent performance in the cathode ORR.In addition, some new non-Pt catalysts have been proposed, such as single-atom catalysts and IOP Publishing doi:10.1088/1742-6596/2771/1/012014 2 metal/organic framework materials.These catalysts have the advantage of solid controllability and also exhibit excellent performance in the cathode oxygen reduction reaction [3].
In China, research on non-Pt catalysts is also advancing.Domestic researchers are mainly exploring the preparation of new non-Pt catalysts, such as inorganic/organic hybrid materials and metal/carbon composites [4].At the same time, the catalytic mechanism and structure-performance relationship of non-Pt catalysts are being studied to further enhance their performance [5].Exploration of the application conditions of non-Pt materials, such as the ORR under high temperature and high-pressure conditions, and the ORR with aqueous properties, provides essential support for developing fuel cells in China.
Based on this, this paper mainly focuses on the design and development of a method to prepare efficient and stable non-Pt catalysts.Carbon-nitrogen-iron (C-N-Fe) doped fuel cell cathode ORR non-Pt catalysts were prepared using pre-treated biomass "oroxylum indicum" as the carbon base, FeCl3•6H2O as the iron source, and melamine as the nitrogen source.The performance of the catalyst as ORR catalyst of the FCs was evaluated through analysis of the physical characterization and electrochemical testing of the samples.

Experimental
(1) Pre-treatment: The collected Platanus orientalis balls are rinsed with deionized water to remove dust.The cleaned Platanus orientalis balls are placed in an oven and dried at 80ႏ for 12 hours.They are cut with scissors, ground in a grinder and sieved (120 mesh).A suitable amount of Platanus orientalis ball powder and unground Platanus orientalis balls are placed in a crucible.They are then heated in a tubular furnace under a high-purity nitrogen atmosphere, with the temperature ramped to 300ႏ at a rate of 5ႏ/min and held for 2 hours before natural cooling to room temperature, resulting in black pre-treated Platanus orientalis ball powder and unground black Platanus orientalis ball powder.
(2) Pyrolysis treatment: After the samples have naturally cooled, 1 g of black pre-treated Platanus orientalis ball powder is taken and divided into three portions, named N-C-Fe-3, N-C-Fe-6, and N-C.Additionally, 1 g of unground black Platanus orientalis ball powder is taken and named N-C-Fe-3 (untreated).FeCl3•6H2O is mixed with the samples in quantities of 3 g, 6 g, and none, corresponding to N-C-Fe-3, N-C-Fe-6, and N-C, respectively.The isopropanol solution (20 mL) is added to each of the four samples, which then reacts at ambient temperature for 12 hours using magnetic mixing.The specimen is then volatilized using a rotary evaporator at 60ႏ.Subsequently, 4 g of melamine is added to each sample and ground in an agate mortar.The ground samples are then placed in a tubular furnace under a high-purity nitrogen atmosphere, with the temperature ramped to 900ႏ at a rate of 5ႏ/min and held for 2 hours before natural cooling to room temperature, resulting in black powder, which serves as the carbon nitrogen iron co-doped fuel cell cathode oxygen reduction catalyst.Electrode preparation and ORR tests were conducted based on literature reports [6].

Results and discussion
Figure 1 shows the XRD diagram of samples N-C and N-C-Fe-6.It can be observed that both prepared materials exhibit two characteristic diffraction peaks at 25.6 o , corresponding to the (002) crystal plane of graphitic carbon, indicating the formation of a graphitic carbon structure in the samples.Compared to the N-C sample, N-C-Fe-6 displays three distinct peaks at 43.57 o , 44.65 o , and 50.79 o , corresponding to (220), (031), and (210) reflections, which are attributed to the presence of added Fe elements.The XRD analysis confirms that the graphitization degree of the samples increases after high-temperature calcination, and the Fe elements are successfully doped into the synthetic materials.
The SEM images of all samples are shown in Figure 2. It can be observed that the surface of the material becomes porous after high-temperature calcination, with pore sizes smaller than 1 ȝm.A comparison reveals that the non-doped examples appear as small granules after calcination, while the doped samples undergo a significant morphological transformation, exhibiting a "spider-web-like" porous structure.Additionally, different amounts of Fe affect the configuration of the surface.The surface of N-C-Fe-3 shows a finer and more uniform fibrous structure (Figure 2a), whereas N-C-Fe-6 IOP Publishing doi:10.1088/1742-6596/2771/1/0120143 displays larger particle sizes while maintaining a porous structure (Figure 2b).The untreated N-C-Fe-3 exhibits a spider-web-like structure between the N-C-Fe-6 and the N-C-Fe-3 structures (Figure 2c).The formation of this structure may be related to melamine, which serves as both an N-doping agent and a pore-forming agent for etching materials.This aids in forming a porous structure under high-temperature calcination conditions, thereby accelerating the transfer of reactants and improving the electrocatalytic performance of the samples.Figure 3 shows the analysis of the surface elemental content of the samples using an Energy Dispersive Spectrometer (EDS).The mass percentages of each element in Figure 3 indicate that in sample N-C-Fe-3, C, N, and Fe percentages are 72.7%,24.5%, and 0.1%.C, N, and Fe percentages in sample N-C-Fe-6 are 60.5%, 38.3%, and 0.5%.The mass percentages of C and N in N-C are 90.0%and 2.8%, respectively.C, N, and Fe mass percentages in N-C-Fe-3 (untreated) are 78.3%, 7.2%, and 0.1%.Among the four samples, the contents of the Fe element in the three samples are different.The addition of a moderate amount of Fe element contributes to improving the catalytic performance of the catalyst.Cyclic voltammetry (CV) is an essential method for evaluating the catalytic performance of catalyst electrode materials.Figure 4 shows the test results obtained using the cyclic voltammetry method.Upon observing Figures 4(a-d), it is evident that under oxygen saturation and 0.1 mol/L KOH conditions, both N-C-Fe-6 and N-C-Fe-3 exhibit significant reduction peaks at a voltage of 0.8 V, indicating that the pretreated Fe-doped samples have electrocatalytic activity for ORR.
where N represents the collection coefficient, which is 0.37.

Conclusion
Herein, we took advantage of the pre-treated biomass "Platanus orientalis balls" as the carbon source, FeCl3•6H2O as the Fe transition metal source, and melamine as the nitrogen source to prepare a carbonnitrogen-iron doped non-Pt catalyst for the ORR in automotive fuel cells.Scanning electron microscopy revealed a porous surface structure, facilitating mass transfer and enhancing reaction rates.Elemental analysis indicated the presence of C, N, and Fe in the N-C-Fe-3 sample, with a Fe content of 0.1%.Compared to the non-doped Fe sample N-C, the catalytic performance was improved with a slight Fe doping, demonstrating its beneficial effect.The CV testing showed a prominent reduction peak at 0.8 V for the N-C-Fe-3 sample, indicating excellent electrocatalytic activity.The LSV testing revealed good catalytic activity across the entire voltage range.In stability testing, the initial current density of the N-C-Fe-3 sample was high, and after 10800 s of operation, it exhibited the highest retention rate among all the samples, highlighting its superior stability.This research provides new insights and directions for the research of non-Pt catalysts.

Figure 5 (
c) displays that the initial electric current density of sample N-C-Fe-3 is relatively high, and after 10800s of operation, the retention rates of N-C-Fe-3, N-C-Fe-6, N-C, and untreated N-C-Fe-3 are 68.78%,14.43%, 18.61%, and 9.64% respectively.Therefore, the retention rate of sample N-C-Fe-3 is the highest, indicating its superior stability among the four samples.

Figure 5 (
d) suggests that within the voltage range of 0.16V-0.8V,the electron transfer numbers (n) of the four samples are close to 4, with N-C-Fe-3 and N-C-Fe-3 (untreated) exhibiting relatively stable electron transfer numbers.The electron transfer number can be calculated based on the disk current (ID) and ring current (IR) using the following formula: n=4*ID/(ID+IR/N)