State-of-the-art polymer electrolyte fuel cells (PEFCs) utilize platinum (Pt) or Pt-alloy nanoparticles supported on high surface-area carbon as catalysts for oxygen reduction reaction (ORR) at the PEFC cathode.¹ Platinum group metal-free (PGM-free) catalysts have the potential to reduce the high PEFC cost by replacing Pt with far less expensive materials. Over the past two decades, the growing interest in PGM-free catalysts has led to significant improvements in their performance, which is now approaching that of Pt-based catalysts.^2–5 Several insights into the critical factors, such as the role of transition metal, catalyst precursors and pore-formers have been studied.^3–5However, for further development of PGM-free catalyst understanding the nature of active site(s) in PGM-free catalysts, as well as the determination of the active site density and turnover frequency (TOF) are vital.

PGM-free catalysts have little affinity to “standard” molecular probes, such as carbon monoxide and hydrogen sulfide, that effectively poison ORR active sites in PGM catalysts.^6,7 Only recently, an ex situ and in situ method for reversibly probing the active sites and estimating the active site density has been proposed.^8,9 Malko et al. showed that nitrogen-based molecular probes can poison the active site and then be reductively stripped off in a voltammetric scan.⁹

In this work, we have studied the effect of the transition metal and synthesis approach (type of precursor and catalyst fabrication method) on the active site. Active site density in catalysts derived from different transition metals was estimated using sodium nitrite (NaNO­₂) as probe.⁹Figure 1a shows the reductive stripping curves for Los Alamos-developed Fe- and Co-derived CM-PANI-C catalysts, with Co exhibiting much higher affinity to the probe than Fe. The effect of different synthesis methodology and precursor with same transition metal was also studied. Reductive stripping of the probe from CM-PANI-Fe-C catalysts with and without Zn added as a pore former occurs at the same peak potentials suggesting the presence of identical active site in both cases (though occurring at a higher concentration in the catalyst synthesized using Zn). On the other hand, a catalyst with atomically dispersed Fe appears to bind the probe more strongly than CM-PANI-Fe-C catalysts (Figure 2b). Active site density estimation provides a quantitative assessment for the effect of the transition metal and synthesis methodology for further development of PGM-free catalyst.

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat).

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