Fe-N-C Catalysts Surpassing Pt Catalysts in ORR for Fuel Cells
Authors: Miwa Kohei , Tanaka Hirohisa 
Published: December 2024

As countries ramp up efforts towards achieving carbon neutrality, fuel cells are garnering attention. Conventional fuel cells, utilizing precious metals like Pt, face challenges due to resource and cost constraints. Addressing this issue is seen as a key step towards further proliferation of fuel cells. Therefore, we are conducting research on Fe-N-C catalysts, which utilize abundant and affordable metals like iron, aiming to overcome these challenges. Cathode-side oxygen reduction reaction (ORR) occurring at a higher potential and smooth 4-electron reduction are key factors leading to the improvement of fuel cell performance. In this study, we synthesized Fe-N-C catalysts with modifications made by us to enable synthesis in university labs, based on the method reported by Prof. Plamen Atanassov (standard catalyst)1. Subsequently, we aimed for further performance enhancement by improving the catalyst synthesis process.

We mixed the materials for the catalyst (water, glucose, 2-methylimidazole, zinc nitrate hexahydrate, iron (Ⅲ) nitrate nonahydrate), and conducted a 24-hour hydrothermal synthesis at 200 ℃. Afterwards, we conducted heat treatment in an H2 gas atmosphere, followed by acid washing using nitric acid to remove residual metallic iron in the solution. Subsequently, the catalyst was synthesized through heat treatment in an NH3 gas atmosphere. Furthermore, we prepared the catalyst, ionomer (Nafion), and solvent to create catalyst ink, and conducted cyclic voltammetry (CV) measurements using a rotating ring-disk electrode. Firstly, preventing to generate the metallic iron particles that interferes with the catalytic reaction, we reduced the amount of iron nitrate nonahydrate used and extended the acid washing time. Furthermore, we extended the heat treatment time and changed the gas from H2 to NH3. Additionally, for improved catalyst, according to minimizing the iron source, we also reduced the amount of ionomer to achieve an ideal triple phase boundary (TPB).

In cyclic voltammetry, the improved catalyst started the reduction reaction at a potential 0.02 V higher than the platinum catalyst and 0.1 V higher than the standard catalyst. (Fig.1) The Fe-N coordination structure of the improved catalyst was stabilized by changing the heat treatment conditions. In addition, the development in the crystallinity of the carbon is considered to have improved the conductivity. We speculated that an excess of ionomer leads to prevent oxygen from the gas phase against reaching the active sites. Therefore, the optimal ratio of the amount of iron source to the amount of ionomer was considered to be responsible for the oxygen reduction reaction with low overpotential. Next, we analyze the crystal structures of both improved and standard catalysts using XRD (between 10° and 100° 2θ using Cu target). Residual FCC-Fe was observed on the standard catalyst (2θ : 43°, 52°, 74°), while no iron metal or iron oxide particles were observed on the improved catalyst1. (Fig.2) Furthermore, SEM-EDX analysis showed a reduction in residual metallic iron. This suggests that the improved catalyst succeeded in significantly removing metallic iron, which may interfere with the catalytic reaction.

We will continue to utilize XAFS and other techniques to deepen our understanding of the catalyst structure and elucidate the ORR mechanism. Furthermore, by developing non-platinum catalysts that exhibit seamless four-electron reduction reactions, we aim to suppress the generation of hydrogen peroxide and improve the durability of FC systems for further widespread use.

References

1 R. Gokhale, L-K. Tsui, K. Roach, Y. Chen, M. M. Hossen, K. Artyushkova, F. Garzon, P. Atanassov, “Hydrothermal synthesis of platinum-Group-Metal-Free Catalysts: Structural Elucidation and Oxygen Reduction Catalysis”, ChemElectroChem, 5, 14, (2017), 1848-1853

Acknowledgements

The authors would like to express their sincere appreciation and celebration to Professor Dr. Plamen Atanassov of University of California, Irvine for his instruction to the catalyst synthesis. This work was performed under JSPS KAKENHI Grant Number JP22H02188. This work was also supported by the Research Program for Next Generation Young Scientists of “Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials”(235032).

Figure 1