Synthesis of Cobalt‐Iron Chalcogenide Clusters as Precursors for Catalysts of Oxygen Electroreduction in Alkali Media

Abstract

New cobalt‐iron chalcogenide clusters C5HMe4Co(CO)E2Fe2(CO)6 (E = Se, Te) were prepared and characterized by IR and NMR spectroscopy; their structures were determined by single‐crystal XRD. Catalysts consisting of nanoparticles of bimetallic chalcogenides of cobalt and iron deposited on highly dispersed carbon black Vulcan XC‐72 were made from the synthesized clusters. Structural characteristics of these catalysts were assessed using X‐ray phase analysis (XRD), microprobe analysis (EDX), and transmission electron microscopy (TEM). It was shown that on the obtained catalysts oxygen electroreduction reaction in alkaline medium proceeds according to a four‐electron mechanism, and the catalysts are tolerant to methanol presence.

Promotion of Catalytic Oxygen Reduction Reactions: The Utility of Proton Management Substituents on Cobalt Porphyrins

“…In 0.1 M KOH, the interfacial charge transfer paths involve the outer-sphere electron transfer mechanism undergoing around 2-electron reaction as well as the direct oxygen adsorption on central metal ions followed by reaction with 3–4 electrons from the external circuit to form HO 2 – or H 2 O. The carbon black substrate can be active for ORR due to such an outer-sphere electron transfer process involving a solvated cluster (O 2 (H 2 O) n ) in the electrolyte rather than the active site in the electrocatalyst. , In 0.5 M H 2 SO 4 , only the inner-sphere electron transfer mechanism occurs, where O 2 is chemisorbed onto the cobalt ion, and then reacts with 3–4 electrons to form H 2 O 2 or H 2 O. In both conditions, the −COOH substituents can dissociate protons and form hydrogen bonds with ions (OH – or H 3 O + ) in solution, which may also attract the adsorbed O 2 .…”

“…The carbon black substrate can be active for ORR due to such an outer-sphere electron transfer process involving a solvated cluster (O 2 (H 2 O) n ) in the electrolyte rather than the active site in the electrocatalyst. 51,52 In 0.5 M H 2 SO 4 , only the inner-sphere electron transfer mechanism occurs, where O 2 is chemisorbed onto the cobalt ion, and then reacts with 3−4 electrons to form H 2 O 2 or H 2 O. In both conditions, the −COOH substituents can dissociate protons and form hydrogen bonds with ions (OH − or H 3 O + ) in solution, which may also attract the adsorbed O 2 .…”

Meso‐substituted Metalloporphyrin‐based Composites for Electrocatalytic Oxygen Reduction Reactions

“…[49,52] Caused by the OSET and thereby the substrate reactivity on oxygen reduction under alkaline condition, the given composite catalyst can reach a much higher response current than that in acid. [53] For instance, three ABAB-type cobalt porphyrins with different proton management groups at their peripheral exhibited the limiting current densities 1.5 ~2 times of the magnitude in 0.1 M KOH than in 0.5 M H 2 SO 4 , while their electron transfer numbers showed slightly reduced values in base than in acid. [54] The selectivity towards 4-electron reduction of O 2 , however, often shows poorer in alkali than in acid, which possibly relates to the interfacial charge states, the decreased water activity as well as the restricted proton transfer from water in alkali solution.…”

“…The Co(II)‐HO 2 − anion can react with another H 2 O to produce two OH − anions, followed by two more e − reacting with the intermediates, on which the charge of the hydroxide ligand delocalizes (or isomerization) and the valence state of the transient cobalt ion changes (Figure 3b). [49,52] Caused by the OSET and thereby the substrate reactivity on oxygen reduction under alkaline condition, the given composite catalyst can reach a much higher response current than that in acid [53] . For instance, three ABAB‐type cobalt porphyrins with different proton management groups at their peripheral exhibited the limiting current densities 1.5∼2 times of the magnitude in 0.1 M KOH than in 0.5 M H 2 SO 4 , while their electron transfer numbers showed slightly reduced values in base than in acid [54] .…”