Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CN_x) in Acidic Media Using Phosphate Anion

Abstract

To probe the active sites of nitrogen-doped carbon nanostructures (CN x ), the effect of dihydrogen phosphate (H2PO4 –) anion on their oxygen reduction reaction (ORR) performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CN x was strong and the corresponding ORR activity was not recovered when the catalyst was reintroduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 to the half-cell were observed when CN x catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CN x exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as X-ray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied by an increase of equal magnitude in the surface fraction of quaternary-N species, which would include the pyridinic-NH sites. A linear correlation was observed between the loss in pyridinic-N site density and that in ORR activity. The observed poisoning phenomenon is consistent with the two possible active site models, i.e., pyridinic-N sites, which would be rendered inactive by protonation, or the C sites neighboring pyridinic-N species. These latter species would be poisoned by a site blocking effect if they strongly adsorb the phosphate ions. Strong adsorption of negatively charged phosphate ions on neighboring C atoms would also stabilize the pyridinic-NH sites. By identifying a poison that can be used as a probe, this study provides a first step toward identification and quantification of active sites in CN x catalysts.

Synergistically Interactive Pyridinic‐N–MoP Sites: Identified Active Centers for Enhanced Hydrogen Evolution in Alkaline Solution

“…Further, as shown in Figure c, the nitrogen K‐edge XANES profile of the MoP@NCHSs‐900 electrocatalyst after it had been soaked in 1 m H 3 PO 4 exhibited suppressed peaks for pyridinic N and pyridinic‐N Mo, and increased peaks for pyrrolic N, graphitic N, N‐O, and σ* (C‐N) peaks. The decrease of the π* peaks could be attributed to coordination of nitrogen to H 3 PO 4 . The broadening and enhancement of the σ* (C‐N) peak suggest stronger C−N bonding, which may result from covalent NCHS and H 2 PO 4 − coupling .…”

“…However, for pyrrolic N, because the lone pair of electrons occupies the vertical p z orbital and contributes to the aromatic sextet, it exhibits a much lower basicity than the pyridinic N. Meanwhile, oxide‐type (N‐O) and graphitic N are even less basic than pyrrolic N. Therefore, none of these N configurations can be poisoned by H 3 PO 4 . Furthermore, density functional theory (DFT) calculations show that compared with pyrrolic N and graphitic N, pyridinic N exhibits the lowest adsorption energy (Figure S14), indicating the highest affinity to H 3 PO 4 for pyridinic N. As confirmed by TEM and XRD (Figures S15 and S16), this process would not alter the morphology and crystal structure of MoP@NCHSs‐900 electrocatalysts . Figure a,b shows a monotonic decrease in HER activity with an increase of H 3 PO 4 concentration.…”

Electrochemical Reduction of CO2 to CO by N,S Dual‐Doped Carbon Nanoweb Catalysts

“…Only the current density versus content of pyridinic N exhibited a linear dependence, whereas no correlations was found for the other N species, indicating that the pyridinic N should be responsible for the electrocatalytic CO 2 conversion to CO. We also performed subsequent experiments using phosphoric acid (H 3 PO 4 ) to block the pyridinic N sites, which could in turn change the activity toward CO 2 RR (Figure S7 in the Supporting Information). The pyridinic N has a lone pair of electrons, which is not delocalized, and can be protonated to form a pyridinium structure (pyridinic N−H) in the H 3 PO 4 aqueous solution . The XPS N 1s spectra of NCNW before and after it was soaked in 0.5 m H 3 PO 4 aqueous solution showed that the relative content of pyridinic N significantly decreased, whereas the content of other N species corresponding to pyrrolic N, graphitic N, and oxidized N increased (Figure S7 a in the Supporting Information), suggesting that H 3 PO 4 could preferentially block the pyridinic N sites.…”

“…The HRTEM images of an individual NSCNW-3 nanowire indicated that the lattice spacingw as approximately0 . 35 nm, corresponding to the (2 00)p lane of graphitic carbon (inset of Figure 1c). [23] Electron energy loss spectroscopy( EELS) was also conducted to explore the dopings tates of Na nd Sa toms.…”

“…The pyridinic Nh as al one pair of electrons,w hich is not delocalized, and can be protonated to form ap yridinium structure (pyridinic NÀH) in the H 3 PO 4 aqueous solution. [35,36] The XPS N1ss pectra of NCNW before and after it was soakedi n0 .5 m H 3 PO 4 aqueous solution showed that the relative content of pyridinic Ns ignificantly decreased, whereas the content of other Ns peciesc orresponding to pyrrolic N, graphitic N, and oxidized Ni ncreased ( Figure S7 ai n the Supporting Information), suggesting that H 3 PO 4 could preferentially block the pyridinic Ns ites. Am onotonic decrease in CO 2 conversion activity with an increasei nH 3 PO 4 concentration waso bserved( Figure S7 bi nt he SupportingI nformation).…”