(282g) Active Sites in Nitrogen-Doped Carbon Nanostructures for Oxygen Reduction and Oxygen Evolution Reactions
Electrocatalytic activity measurements using rotating ring disk electrode (RRDE) show significant ORR and OER activity of CNx compared to state-of-the-art Pt and Ir-based catalysts. Analysis of bifunctional electrocatalytic activity demonstrates much better bifunctional characteristics of CNx compared to Ir/C and Pt/C .
Use of poisoning probes is a useful methodology to systematically examine the nature of active sites in catalytic materials. However, due to the absence of a metal centered active site, CNx materials are not susceptible to poisoning by CO, H2S or CN- [3, 4]. We have recently identified phosphate anions (H2PO4-) as probe molecules for poisoning CNx catalysts in an acidic ORR environment . In-situ RRDE measurements show a linear decrease in ORR activity with increasing H2PO4- anion concentration. Transmission IR spectroscopy and Raman spectroscopy reveal the presence of H2PO4- species on H3PO4âsoaked CNx. A linear decrease in ORR kinetic current with a decrease in pyridinic nitrogen content obtained using X-ray photoelectron spectroscopy (XPS) is observed. This poisoning phenomenon is consistent with two possible active site models: (i) pyridinic N as the active site which is rendered inactive by protonation [6, 7], and (ii) carbon sites adjacent to pyridinic N as the active site (i.e. pyridinic N as a marker for the active site) wherein poisoning is caused by a site-blocking effect due to adsorption of H2PO4- species on carbon. OER activity has also been found to increase with increasing pyridinic nitrogen site density in CNx samples.
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