Electrochemical Evidence for Two Sub‐families of FeN_xC_y Moieties with Concentration‐Dependent Cyanide Poisoning

Abstract

Iron-nitrogen-carbon (FeÀNÀC) catalysts are one of the most promising family of materials for the successful replacement of noble Pt in low-temperature fuel cells. Despite recent advances in the synthesis, activity, and site-structure of FeÀNÀC catalysts, further improved understanding of the nature of the Fe-based active sites is still needed. In the present study, the existence of two sub-families of FeN x C y moieties is revealed by the concentration dependence of cyanide poisoning on a FeÀNÀC catalyst only comprising of atomically dispersed iron. The analysis of the activity decrease upon contact with various cyanide concentrations and activity recovery following rinsing with water reveals the presence of two sub-groups of ORRactive FeN x C y species. They are discriminated by the reversibility or irreversibility of the cyanide poisoning. From a Mö ssbauer spectroscopy study and poisoning test on a model compound, Fe(II) phthalocyanine (FePc), we hypothesize that the reversibility/irreversibility of cyanide poisoning is governed by the competitive adsorption of oxygen molecule and cyanide anion. It is proposed that FeN x C y moieties with Fe in low spin state reversibly adsorb cyanide, whereas those with a medium spin state irreversibly adsorb cyanide.

Identifying the Types and Characterization of the Active Sites on M−X−C Single‐Atom Catalysts

“…Choi et al distinguished two similar moieties of FeÀ NÀ C via the delicate concentration-dependent cyanide poisoning experiment. [55] As the Figure 10a shows, the catalytically active sites on FeÀ NÀ C will exhibit irreversible loss in efficiency after adding the CN À ions, while the other moieties show the reversible loss. They indicated this reversible poisoning behavior was attributed to the competitive adsorbed O 2 and CN À , which is demonstrated by the Langmuir adsorption model.…”

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

“…77 Probe molecules such as CO, 78 NO, 79 CN À (ref. 80) or SCN À have been explored as surface probes for Fe-N-C materials under electrochemicalconditions, however none was successfully applied for a quantitative evaluation of surface sites. 81 For example, both CN À and tris(hydroxymethyl)aminomethane (Tris) have been successfully employed in partial poisoning studies of Fe-N x surface sites of Fe-N-C catalysts.…”

“…81 For example, both CN À and tris(hydroxymethyl)aminomethane (Tris) have been successfully employed in partial poisoning studies of Fe-N x surface sites of Fe-N-C catalysts. 80,82 This implies that counting the number of Tris molecules or cyanide ions irreversibly adsorbed on Fe-N-C (after washing the surface with electrolyte free of such probe species) would underestimate the total number of surface-located Fe-based sites, due to too weak adsorption on some sites. Recently, new complementary adsorption/desorption techniques were specifically developed for Fe-N-C materials and validated, one based on low-temperature CO adsorption 83 and the other on ambient-temperature NO 2 À /NO adsorption.…”

Synthesis and Active Site Identification of Fe−N−C Single‐Atom Catalysts for the Oxygen Reduction Reaction

“…The chemical probe is a relatively simple characterization for the active moieties and can be applied in most laboratories . Choi and co‐workers designed a delicate concentration‐dependent cyanide poisoning experiment to reveal the co‐existence of two sub‐families of FeN x C y moieties in a Fe−N−C SAC . As shown in Figure A, some ORR‐active FeN x C y moieties were irreversibly poisoned by CN − anion while others were only reversibly poisoned.…”