(Energy Technology Division Research Award Address) Transition Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction Reaction:Surface Chemistry, Morphology and Reactivity

Abstract

One of the main obstacles for global introduction of carbon-neutral hydrogen-based energy technology is the lack of efficient, inexpensive and durable cathode catalysts for the oxygen reduction reaction (ORR). The most effective catalytic materials contain Pt or other metals from Pt group. These platinum group metals (PGM) contribute substantially to the cost of most electrochemical energy technologies (fuel cells, electrolyzers, etc.) due to their scarcity and limited global availability. The search for less expensive materials that use of Earth-abundant elements is crucial and such pursuit has been very active for many years. While the earlier work used molecular catalysts, such as transition metal macrocyclic complexes, especially metal phthalocyanines and metal porphyrins, substantial practical progress has been achieved by introducing materials that are obtained by pyrolysis at high temperatures from organic or organo-metallic precursors that contain nitrogen. Over the last decade, a widespread opinion emerged that such materials can be viewed as graphene derivatives with nitrogen and transition metal moieties displayed in in them, including close structural analogs of MN4centers (resembling those of transition metal chelates), claimed responsible for the ORR activity.1While these pyrolyzed materials exhibit higher activity and promising stability, the nature of the active sites is still a field of debate. This paper will present an opinion on reactivity descriptors for transition metal-nitrogen-carbon (M-N-C) PGM-free catalysts obtained through pyrolysis process. It will discuss the attempts and limitation for de-novo, total chemical synthesis of M-N-C catalysts.2The paper will compare activity of metal-less (metal free) nitrogen doped carbon catalysts3and will point out critical differences between these two classes of PGM-free catalysts for ORR.4It will also introduce the role of surface Lewis basicity of N-containing groups across a range of pH5and will attempt on linking those activity descriptors to both chemistry of the catalyst surface and nano-scale morphology of the catalysts electrolyte interface.6This paper will display a hypothesis on the mechanism of ORR on M-N-C catalysts that is congruent with the findings from pH-dependence, inhibition and kinetic isotope effect studies.7The discussion will lead towards setting a set of realistic, non-formal, structure-based activity loss (durability) descriptors, so lacking in this field that emerges from absolute empiric advance towards a chemical interpretation of the key effects. The paper will discuss PGM-free catalysts interactions with ionomers8 and will provide directions for successful integrating M-N-C materials into membrane-electrode assemblies, as evidenced by nano- and micro-X-ray CT. 9,10 Kishi, et al., Nanomaterials,(2018) DOI: 10.3390/nano8120965Gokhale, et al., ACS Applied Energy Materials, (2018) DOI: 10.1002/celc.201800578Kabir, et al., ACS Applied Materials & Interfaces, 14 (2018) 11623-11632Matanović,et al., Current Opinion Electrochem., (2018) DOI: 10.1016/j.coelec.2018.03.009Rojas-Carbonell et al., ACS Catalysis, 8 (2018) 3041-3053Chen, et al., ACS Applied Energy Materials, 1 (2018) 1942-1949Chen, Iet al., ACS Applied Energy Materials, (2018) DOI: 10.1021/acsaem.8b00959Artyushkova, et al., ACS Applied Energy Materials, 1(2018)68-77Serov, et al., Appl. Catalysis B,237 (2018) 1139-1147J. Normile, et al., Materials Today - Energy, 9 (2018) 187-197