Optimizing Oxygen Reduction Reaction through Enhanced Mesoscopic Mass Transport in Ordered Mesoporous Carbon Nanofibers

Energy Chemistry Chemistry Materials Science Inorganic Non-Metallic Materials Engineering Catalysis Energy Engineering Materials Chemistry
oxygen reduction reaction mass transfer carbon nanofiber mesoporosity electrocatalysis zinc-air battery non-precious metal catalysts

Academic Background

As the global demand for green energy continues to grow, fuel cells and metal-air batteries are considered potential solutions for energy conversion and storage due to their high energy density. However, the commercialization of these technologies is hindered by the slow kinetics of the oxygen reduction reaction (ORR) at the cathode. Currently, platinum (Pt) and its alloys are regarded as the most effective ORR electrocatalysts due to their efficient four-electron process and superior catalytic performance. However, the scarcity and high cost of platinum have driven researchers to seek non-precious metal or even metal-free electrocatalysts as alternatives to Pt-based materials.

Carbon-based materials, known for their high conductivity, low cost, and corrosion resistance, are considered potential alternatives. However, compared to Pt-based catalysts, carbon-based materials typically require higher loadings to achieve similar performance. High loadings can lead to increased mass transport resistance, negatively affecting the overall performance of the devices. Therefore, developing carbon materials that can maintain efficient reactant transport under high loading conditions has become an important research direction.

Source of the Paper

This paper was co-authored by Chuyi Zhao, Lei Tan, Jingsan Xu, Xiaotong Wu, Yuanyuan Cui, Chao Lin, Xiaopeng Li, Teng Long, and Wei Luo, from institutions such as Donghua University, Queensland University of Technology, Ningbo University of Technology, and Shanghai Jiao Tong University. The paper was published on November 19, 2024, in the journal Advanced Fiber Materials.

Research Process

1. Synthesis of Ordered Mesoporous Carbon Nanofibers (OMCNFs)

The research team prepared ordered mesoporous carbon nanofibers using electrospinning technology. First, a precursor solution was prepared using polystyrene (PS) and poly(ethylene oxide)-block-polystyrene (PEO-b-PS) block copolymers as soft templates and resol as the carbon source. During electrospinning, rapid solvent evaporation caused micelles to aggregate and self-assemble into an ordered structure. Subsequently, the template was removed through pyrolysis to obtain ordered mesoporous carbon nanofibers (OMCNFs).

2. Preparation of Nitrogen and Sulfur Co-Doped Ordered Mesoporous Carbon Nanofibers (NS-OMCNFs)

To further enhance catalytic performance, the research team doped nitrogen (N) and sulfur (S) elements into OMCNFs by pyrolyzing thiourea. The specific steps included mixing OMCNFs with thiourea and pyrolyzing them under a nitrogen atmosphere, ultimately yielding nitrogen and sulfur co-doped ordered mesoporous carbon nanofibers (NS-OMCNFs).

3. Electrochemical Performance Testing

The research team conducted oxygen reduction reaction (ORR) performance tests on NS-OMCNFs and compared them with disordered mesoporous carbon nanofibers (NS-NMCNFs) and commercial Pt/C catalysts. Tests included linear sweep voltammetry (LSV), rotating ring-disk electrode (RRDE) testing, and electrochemical impedance spectroscopy (EIS) analysis.

4. Zinc-Air Battery Performance Evaluation

To validate the practical application potential of NS-OMCNFs, the research team assembled zinc-air batteries (ZABs) using NS-OMCNFs as the air electrode and tested their discharge performance, power density, and cycling stability.

Main Results

1. Structural Characterization

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that NS-OMCNFs have a continuous long fiber structure with a smooth surface and uniform ordered mesopores. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that nitrogen and sulfur co-doping successfully introduced defect sites, enhancing the electrocatalytic activity of the material.

2. Electrochemical Performance

ORR tests showed that NS-OMCNFs exhibited excellent catalytic activity in alkaline media, with a half-wave potential (E1/2) of 0.85 V, comparable to that of commercial Pt/C. When the catalyst loading was increased, the performance of NS-OMCNFs improved further, while Pt/C performance declined due to increased mass transport resistance. Electrochemical impedance spectroscopy (EIS) analysis revealed that the hydroxide ion diffusion coefficient (DOH-) of NS-OMCNFs was 26 times that of NS-NMCNFs and 206 times that of Pt/C.

3. Zinc-Air Battery Performance

In the assembled aqueous electrolyte zinc-air battery, NS-OMCNFs achieved a maximum power density of 122.4 mW/cm², significantly higher than that of Pt/C. In the solid-state electrolyte zinc-air battery, NS-OMCNFs also demonstrated excellent cycling stability, maintaining stable performance after 40 hours of charge-discharge cycling.

Conclusion

This study successfully synthesized nitrogen and sulfur co-doped ordered mesoporous carbon nanofibers (NS-OMCNFs), which exhibited superior performance in the oxygen reduction reaction compared to traditional Pt-based catalysts and disordered mesoporous carbon fibers. The ordered mesoporous structure significantly improved mass transport efficiency, enabling excellent electrocatalytic performance under high loading conditions. Additionally, the application of NS-OMCNFs in zinc-air batteries demonstrated their great potential in practical energy devices.

Research Highlights

  1. Ordered Mesoporous Structure: Ordered mesoporous carbon nanofibers were successfully prepared using electrospinning and self-assembly techniques, significantly improving mass transport efficiency.
  2. Nitrogen and Sulfur Co-Doping: Nitrogen and sulfur elements were successfully doped into the carbon fibers by pyrolyzing thiourea, introducing numerous active sites and enhancing catalytic performance.
  3. High Loading Performance: NS-OMCNFs maintained excellent electrocatalytic performance under high catalyst loadings, overcoming the performance degradation of traditional catalysts under high loading conditions.
  4. Practical Application Potential: In zinc-air batteries, NS-OMCNFs demonstrated high power density and excellent cycling stability, showcasing their application value in practical energy devices.

Additional Valuable Information

This research was supported by the National Natural Science Foundation of China (52225204, 52173233, 52402231) and the Innovation Program of Shanghai Municipal Education Commission (2021-01-07-00-03-E00109), among other projects.