(Fe, N)-Doped Carbon Black Catalysts for Oxygen Reduction Reaction Applied to Gas-Diffusion Electrodes
Authors: Wang Guanyu , Ikezawa Atsunori , Waki Keiko , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Introduction

Addressing the sluggish kinetics of the oxygen reduction reaction (ORR) in metal-air secondary batteries is crucial for practical applications. (Fe, N)-doped carbon-based catalysts (FeNC) have shown promise in enhancing the ORR activity. However, their conventional production process involves high temperatures and prolonged annealing, which leads to significant weight loss of the carbon matrix, defect healing, and active site embedding, thus reducing their efficacy [1,2]. This study employs a rapid thermal annealing (RTA) method to synthesize FeNC catalysts, overcoming these challenges and demonstrating improved ORR activity. Furthermore, this research applied these catalysts to gas diffusion electrodes (GDEs) in the alkaline solutions by integrating binders and employing thermal pressing, achieving highly active ORR activity and durability.

Experimental

The FeNC were synthesized via the RTA method in an NH3/Ar atmosphere using acid-treated carbon black and FeCl3 powder. The ORR activity was evaluated by the linear sweep voltammetry with a rotating ring-disk electrode (RRDE). GDEs were prepared by cold-pressing hydrophobic carbon paper with a mixed catalyst layer of FeNC and PTFE under 20 kg/cm2 pressure, then hot-pressed at 340°C under 40 kg/cm2 pressure. Electrochemical measurements were performed using a three-electrode cell in an 8 mol dm–3 KOH solution, consisting of a gas chamber, reference electrode chamber, and counter electrode chamber. The X-ray photoelectron spectroscopy (XPS) was utilized to analyze the element concentration of the carbon black-based catalyst surface. The laser Raman spectrometer was used to examine the structure defectiveness.

Results
and Discussion

In this study, we explored the temperature dependence of the (Fe, N)-doped carbon black catalyst. This was achieved by directly heating the catalysts loaded on the surface of electrodes, resulting in highly active ORR activity. Subsequently, to apply this catalyst to a gas diffusion electrode, the method of directly heating an amount of powder was adopted to scale up the production. The obtained FeNC catalyst exhibited significantly high ORR activity in alkaline solutions, with an onset potential of 1.1 V vs. RHE in KOH. RRDE tests indicated that the FeNC catalyst facilitates a four-electron process, directly reducing oxygen to hydroxide ions. The intensity of disordered carbon relative to that of graphitic carbon (I
d/I
g ratio) obtained from Raman spectroscopy, representing the degree of defects on the surface of carbon materials, was 2.35 for the FeNC powder heated at 900°C, which is higher than that of the original carbon black (2.03). The XPS results showed that the N/C% of the sample annealed at 900°C was 4.9%, and the Fe/C% was 1.5%, which exhibited a higher doping level than the traditional long-time annealing sample [3]. These characteristics make them promising for use in metal-air batteries. In the GDE preparation process, polytetrafluoroethylene (PTFE) integration with hot pressing is generally employed to control the hydrophobicity to effectively prevent carbon loss in KOH solutions. In this study, we examined the effects of PTFE content and hot-pressing duration on the performance of ORR catalysts. We found that higher concentrations of PTFE led to the encapsulation of catalytic sites, while lower concentrations decreased hydrophobicity and failed to adequately bind the ORR catalysts. Similarly, prolonged hot-pressing times resulted in fragmentation of the catalyst films, while insufficient time led to uneven melting of PTFE. As a result, the mixture of 83wt% FeNC powder annealed at 900°C and 17wt% PTFE, subjected to a 10-minute hot press (called FeNC-10hp-17%) achieved a balance. The performances of samples prepared under other conditions will be shown in the meeting. Figure 1 displays the durability of the FeNC catalyst, revealing a marginal decline in activity at the low current density region and a slight increase at the high current density region after 25 cycles (5000 minutes), indicating the overall stability of the catalyst. Ongoing stability assessments suggest that the FeNC catalyst may exhibit further potential for enhanced durability. These findings offer a promising solution to expedite the practical utilization of metal-air secondary batteries.

References

[1] G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 332.6028 (2011): 443-447
[2] L. Zhang, K. Waki, Carbon, 189 (2022): 369-376.
[3] F. Charreteur, F. Jaouen, S. Ruggeri, J.-P. Dodelet, Electrochimica Acta, 53.6 (2008): 2925–2938.

Acknowledgment

This work was partially supported by JRP-LEAD with DFG (JPJSJRP20221602)