(Invited) Platinum Group Metal-Free Oxygen Reduction Electrocatalysts: Structure-to-Property Relationships and Design Directions

Abstract

Platinum group metal-free (PGM-free) electrocatalysts draw much attention these days as low-temperature (polymer electrolyte) fuel cell technology is approaching its broad market introduction. Among several competing chemistries, catalysts based on metal-nitrogen-carbon (M-N-C) materials system are by far the most popular choice. UNM have developed and commercialized catalysts from this class obtained by pyrolysis of N-containing organic precursor (N and C source) and transition metal salt (Fe, Co, Mn or combination of more than one metal). The pyrolysis is carried after integration of the precursors with a sacrificial support (usually mono-dispersed silica powder) through ball-milling. After the pyrolysis, the silica support is removed by leaching with HF. The catalysts are usually re-pyrolyzed and a second acid wash is carried out to remove remaining of metallic, oxide or carbide particles from the nano-composite. The resulting catalysts show high graphitic content (exceeding 80%) and atomically dispersed transition metal (usually less than 1%). Both nitrogen and transition metal moieties are integrated into the carbonaceous structure as defects in graphene. Design for performance of this type of catalysts depends on an understanding the nature and of these transition metal and nitrogen sites and the ways they participate in the oxygen reduction reaction (ORR). The structure-to-property relationships in these materials will be discussed here through correlating of theirs observed electrocatalytic activity in Rotating Ring-Disk Electrode (RRDE) and performance in single Membrane Electrode Assembly (MEA) polymer electrolyte fuel cells with results of X-ray photoelectron (XPS), X-ray absorption (XAS) and Mössbauer spectroscopy charaterization. We have introduced Density Functional Theory (DFT) calculations of core level binding energy shifts as a powerful tool for interpretation of XPS spectra of such catalysts.1-3 A combination of XPS and first principles calculations of nitrogen-containing model electrocatalysts4-5 was further used to elucidate the chemical nature of the nitrogen defects. We will discuss here how hydrogenation and protonation of different nitrogen-containing defects play a major role in ORR activity of the PGM-free catalysts across different pH ranges. This paper will place a particular emphasis on elucidating the role of nitrogen- and Fe-N-containing in-plane and edge defects on the acid-base steps in ORR catalysis and thus regulation PGM-free catalysts reactivity and selectivity. The nature of Fe-N-containing in-plane and edge defects pre-determines their association with the micro-pores (edge defects) or with the extended graphene surface (in-plane defects). This sets the stage for a “first order” dependence of chemistry of the active site and the morphology of the carbonaceous material, which is the catalysts’ backbone (see the Figure for conceptual illustration). We have shown that these correlations are more pronounced when MEA performance data are being used. In this case, we observe a strong anti-correlation of the overall performance with the “stacking number” of the graphitic domains in these catalysts, which lead us to conclude that in-plane defects are the predominant contributors to catalytic activity in fuel cells.6 Further discussion will bring the importance of these two classes of active sites in different modes of MEA performance loss and will serve as a guide for both active and durable PGM-free Polymer Electrolyte Fuel Cell (PEMFC) cathode catalysts and catalyst layer design. K. Artyushkova, et al., Chem. Commun., 49 (2013) 2539-2541S. Kabir, et al., Phys. Chem. Chem. Phys., 17 (2015) 17785-17789S. Kabir, et al., Surf. Interface Anal., 48 (2016) 293-300I. Matanovic, et al., J. Phys. Chem. C, 120 (2016) 29225-29232K. Artyushkova, et al., J. Phys. Chem. C, 121 (2017) 2836–2843M. Workman, et al., ACS Energy Letters, 2 (2017) 1489-1493 Figure 1