Platinum-based catalysts play an important role in electrocatalysis of the generally sluggish oxygen reduction reaction (ORR) at the polymer electrolyte fuel cell (PEFC) cathode. However, the expensive Pt-based catalysts contribute to more than 40% of the cost of the PEFC stack for automotive applications, constraining the commercialization of fuel cell power systems.¹ To lower the fuel cell catalyst cost, non-platinum group (non-PGM) catalysts have drawn attention as a promising low-cost replacement for Pt-based ORR catalysts.²The development of non-PGM catalysts have been the focus of significant research effort for years, aimed at obtaining active and durable materials, typically via the heat treatment of N-containing organic compounds and transition metal salts in inert atmosphere. One of the keys to the ultimate success of those efforts is the selection of suitable nitrogen precursors for the synthesis. In this presentation, we will demonstrate a rational design of polyaniline-type (PANI-type) polymers, as the precursors for active and durable non-PGM catalysts for oxygen reduction.

Recently, we have developed well-performing non-PGM catalysts by heat-treating PANI, Fe salts, and carbon in N₂ atmosphere.³ The studies of this and similar non-PGM catalysts suggest that Fe-N defects in a carbon matrix may be vital for the ORR activity of such catalysts.⁴ If so, it is essential to promote the formation of Fe-N bonds during the catalyst synthesis. However, current approaches are still quite rudimentary, largely limited to screening of N-rich precursors and optimizing the Fe salt loading. The rationale behind selecting N-rich precursors is still lacking. In many cases, the N-containing groups in polymers, such as PANI, have poor affinity to Fe salts limiting Fe-N bond formation in the polymer precursor before the heat treatment. An approach to increasing the population of such bonds will be given in this presentation.

The subject of this study are PANI-type polymers with N-containing side groups having high affinity to Fe. By this approach, we allow Fe-N bonds to form evenly in the precursor polymers before the subsequent heat treatment. A higher Fe-N bond content is observed compared to previously used PANI-Fe precursors. Following the precursor synthesis, the polymers are converted into a carbon-based non-PGM catalyst via a heat treatment under N₂. The catalyst undergoes further characterization by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, followed by electrochemical characterization and fuel cell testing. The efficiency of the Fe uptake is also determined. This new approach paves the way to rational synthesis of non-PGM ORR catalysts via a rational design of polymer precursors with strong Fe-N interaction, ultimately resulting in a higher ORR active sites.

Acknowledgement

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

References

1. Spendelow, J.; Marcinkoski, J., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2014, (2014).
2. Wu, G.; Zelenay, P., Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 46, 1878-1889 (2013).
3. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 332, 443-447 (2011).
4. Holby, E. F.; Wu, G.; Zelenay, P.; Taylor, C. D., Structure of Fe–Nx–C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling. J. Phys. Chem. C 118, 14388-14393 (2014).