Synchrotron radiation in situ X-ray absorption fine structure and in situ X-ray diffraction analysis of a high-performance cobalt catalyst towards the oxygen reduction reaction

Abstract

Transition metal-based composites are one of the most important electrocatalysts because of their rich redox chemistry. The reaction kinetics of a redox couple is dependent on the chemical valence and is a key issue in electrocatalytic performance. In this study, a metallic Co catalyst was synthesized by pyrolyzing Co(OH). The effect of the chemical valence of Co on the oxygen reduction reaction (ORR) was investigated by comparing the electrocatalytic properties of three Co-based catalysts containing Co, Co, and Co. The electrocatalytic properties were evaluated mainly by linear scan voltammetry (LSV) and a direct borohydride fuel cell (DBFC) where the Co-based catalysts were used as cathodes. The LSV results show that the ORR peak current density increases with a decrease in chemical valence. The DBFC with the Co cathode exhibits highest power density and good durability. In situ X-ray diffraction combined with in situ X-ray absorption fine structure tests was carried out to reveal the dynamic microstructure evolution of the Co cathode during ORR. The in situ results clearly demonstrate the evolution of metallic Co to Co(OH) and then to CoOOH during the ORR.

Robust Electronic Structure: Uncovering the Origins of Fast Oxygen Reduction Kinetics of the NiCo Nanoalloys@N-Doped Carbon

“…Accompanied by atomic rearrangement, the inner metallic Co 0 is exposed to the alkaline electrolyte and oxidized to Co 2+ in Co(OH) 2 , which is another typical pathway to form the CoOOH species. 46 The reconstruction behavior of Ni is remarkably similar to that of Co species but to a lesser extent since the reconstruction products NiOOH and Ni(OH) 2 were detected merely by XPS, not Raman spectra. The reconstruction processes are visibly delivered in eqs 1−7.…”

“…The Co 2+ in surface oxide shells is first partially oxidized to Co 3+ in Co 3 O 4 and then further converts to Co 3+ in CoOOH. Accompanied by atomic rearrangement, the inner metallic Co 0 is exposed to the alkaline electrolyte and oxidized to Co 2+ in Co­(OH) 2 , which is another typical pathway to form the CoOOH species . The reconstruction behavior of Ni is remarkably similar to that of Co species but to a lesser extent since the reconstruction products NiOOH and Ni­(OH) 2 were detected merely by XPS, not Raman spectra.…”

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

“…The obtained information can reveal that different catalysts have different optimal valence states contributing to higher catalytic activity. As summarized above, the low valence state makes more contribution in M-N-C like catalysts [114,116], while high valence is better for applying in transition metal oxides [84]. Moreover, the synergistic effect often existed in dual transition metal systems, with the electron transfer between them [115].…”

“…For the typical single transition metal-based catalyst, it often undergoes valence changes during its catalytic process. Such as Qin et al [114] explored the effect of the valence of Co in a polypyrrole (PPy)-revised carbonloaded metal Co catalyst (Co-PPy-BP) towards ORR in DBFC by in-situ XAFS and XRD. Firstly, in-situ XANES of Co-PPy-BP evolved into standard Co (OH) 2 , and then into CoOOH, which implied that the valence of Co underwent Co 0 → Co 2+ → Co 3+ during ORR.…”