Ionic-liquid-modified non-precious metal catalysts for oxygen reduction reaction -- temperature effect

Fuel cells are electrochemical devices that can convert chemical energy into electricity and are highly efficient and environment-friendly. Due to their potential applications in the zero-emission vehicle sector, low-temperature fuel cells (LTFCs), such as proton exchange membrane fuel cells, have received intensive research attention. One of the main challenges in the development of LTFC technology is the slow kinetics of cathodic oxygen reduction reactions (ORR). LTFC cathodes use a large amount of Pt catalyst, and the high cost and low stability of Pt-based catalysts have severely limited the use of LTFCs. Recently, non-precious metal catalysts (NPMCs), such as Fe-N-C, have emerged as active catalysts for ORR and are expected to reduce the cost of LTFC systems by replacing expensive Pt-based catalysts. However, their activity and stability still need to be significantly improved in order to meet the requirements of practical applications.  

Numerous attempts have been made to improve the NPMC catalytic performance by tuning their geometric/electronic structures via systematic structural engineering. Most recently, modifying electrocatalysts with small amounts of hydrophobic ionic liquids (ILs), which subscribes to the  solid catalyst with an ionic liquid layer (SCILL) concept, has been demonstrated to be an effective approach to improving the ORR performance of precious metals (e.g., Pt, PtNi, and PtNiMo) and NPMCs (e.g., Fe-N-C, ZIF-derived carbon, and N-doped carbon). However, these studies were usually conducted at room temperature, which is lower than the typical operating temperature of LTFCs (60–80 °C). Therefore, before the SCILL concept can be applied to LTFCs, it is necessary to determine whether the activity boosting effect of IL modification can be maintained at elevated temperatures. 

Recently, Prof. Bastian Etzold and his coworkers at the Technical University of Darmstadt investigated the filling behavior of ionic liquids ([BMMIM][NTf₂]) in Fe-N-C catalysts using high-resolution Ar and water-vapor sorption techniques; they systematically studied the IL-associated boosting effect on the ORR performance of Fe-N-C (with/without S-doping) from 20–70 °C. This study is the first to demonstrate that the IL boosting effect is sustainable and becomes more pronounced at elevated temperatures, which is of great significance for the practical application of the SCILL concept in LTFCs.

This work presents a systematic study of the filling behavior of ILs within the pores of NPMCs and the effect of temperature on their ORR performance. The results show that the IL tends to preferentially fill the micropores (especially ultra-micropores), IL modification can significantly increase the ORR activity of Fe-N-C catalysts, and the IL boosting effect is sustained and becomes even more pronounced at elevated temperatures. The beneficial effects of elevated temperature may be due to faster ORR kinetics and improved IL physicochemical properties (e.g., higher O₂ diffusion coefficient). This work reconfirms that the IL-modification strategy is a generic approach to improving ORR catalysts, and the results presented herein are highly inspired by the practical applications of the SCILL concept in LTFCs.

The paper entitled “The effect of temperature on ionic liquid modified Fe-N-C catalysts for alkaline oxygen reduction reaction” was recently published in the Journal of Energy Chemistry. Prof. Bastian Etzold and Dr. Gui-Rong Zhang (now a professor at Tiangong University) are the corresponding authors of this paper.

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 681719).  UIK and IM would like to thank the German research foundation for financial support via GSC1070.

About the journal

The Journal of Energy Chemistry is a publication that mainly reports on creative researches and innovativeapplications of chemical conversions of fossil energy, carbon dioxide, electrochemical energy and hydrogen energy,as well as the conversions of biomass and solar energy related with chemical issues to promote academicexchanges in the field of energy chemistry and to accelerate the exploration, research and development of energyscience and technologies.

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