Role of Nitrogen Moieties in N-Doped 3D-Graphene Nanosheets for Oxygen Electroreduction in Acidic and Alkaline Media

Abstract

This study elucidates the synthesis-structure-property correlations of nitrogen moieties present in nitrogen-functionalized graphene nanomaterials toward oxygen reduction reactions (ORRs) and their electrochemical pathways in acidic and alkaline electrolytes. Porous three-dimensional nitrogen-doped graphene nanosheets (N/3D-GNSs) were fabricated using the sacrificial support method and doped with nitrogen using 10 atom % NH under thermal pyrolysis at T = 650, 850, and 1050 °C for evaluating the nitrogen species formed under different temperatures. The abundances of the various nitrogen species formed under pyrolytic conditions were evaluated with X-ray photoelectron spectroscopy. Using rotating ring-disk electrode, we analyzed the role played by the nitrogen moieties influencing the electrochemical activity of the N/3D-GNS supports for oxygen reduction reactions (ORRs) in both acidic and alkaline media. It was demonstrated that the concentrations of the nitrogen moieties: graphitic-N, quaternary, hydrogenated-N (hydrogenated nitrogen combined pyrrolic nitrogen and hydrogenated pyridine) and pyridinic-N varied considerably with pyrolysis temperatures. A decrease in graphitic-N content and an increase in the ratio of hydrogenated-N/pyridinic-N significantly improved the activity of the material. The half-wave and onset potentials as well as the current densities and hydrogen peroxide (HO)/(HO) yields of the N/3D-GNS materials also varied between acidic and alkaline electrolytes but followed the general trend in terms of pyrolysis temperatures and abundance of the nitrogen moieties. Among the synthesized materials, the 3D-graphene nanosheets that were doped with nitrogen at 850 °C, optimized to have the highest hydrogenated-N and lowest pyridinic-N as well as better catalyst-ionomer integration, showed the highest ORR performance. This strategy for the tunable synthesis of nitrogen-doped graphene materials with controlled nitrogen functionalization offers a platform for developing active supports or catalytic nanomaterials for fuel cell applications.

Progress in Nonmetal‐Doped Graphene Electrocatalysts for the Oxygen Reduction Reaction

“…[44] Sun et al developedanew methodt op repare highly nitrogen doped platelet carbon nanofibers (plCNF). [57] The optimal doping temperature is 850 8C, which gave good ORR activity ( Figure 6). Electrochemical experiments showedt hat the carbon atoms near pyridinic Na toms can be used as catalytic centers for ORR, and the reaction is dominatedb yt he four-electron process.…”

“…The chaotic edges of plCNFs are clipped by high-frequency etchingt echnology,w hich produces many exposed edges and patterned edge points [45] where the pyridinicNatoms are preferentially located. The content of nitrogen components such as graphitic N, quaternary N, nitrogen hydride,a nd pyridinic Nv aries greatly with the pyrolysis temperature.T he decrease of graphitic-N content and the increase of hydrogenation N/pyridinic Nr atio would increase the ORR activity.T he ORR mechanistic steps on graphitic, hydrogenation, andp yridinic nitrogen are proposed in Figure 6c and d. [57] Lu et al prepared3 D N-doped graphene (3D-NG@SiO 2 ). The active sites on N-doped graphene can be determined from the changes in chemical composition before and after ORR.…”

“…[57] The optimal doping temperature is 850 8C, which gave good ORR activity ( Figure 6). The content of nitrogen components such as graphitic N, quaternary N, nitrogen hydride,a nd pyridinic Nv aries greatly with the pyrolysis temperature.T he decrease of graphitic-N content and the increase of hydrogenation N/pyridinic Nr atio would increase the ORR activity.T he ORR mechanistic steps on graphitic, hydrogenation, andp yridinic nitrogen are proposed in Figure 6c and d. [57] Lu et al prepared3 D N-doped graphene (3D-NG@SiO 2 ). [58] It was suggested that nitrogen mainly exists in the form of pyridinic No np yrolysis of ap oly(o-phenylenediamine)-containing nitrogen precursor and silica gel.…”

“…Reprinted with permission from Ref. [57].Copyright:2 018 American Chemical Society. ChemSusChem 2019ChemSusChem , 12,2133ChemSusChem -2146 www.chemsuschem.org and electronc loud distribution of adjacent carbon atoms and thus inducet he formation of "active centers", which can directly participatei nO RR.…”

Three-Dimensional Heteroatom-Doped Nanocarbon for Metal-Free Oxygen Reduction Electrocatalysis: A Review

“…3D graphene structures can effectively restrain the restacking between graphene sheets, and therefore expose more active sites, heteroatom-doped 3D graphene materials are expected to show much better electrocatalytic performance for ORR than the 2D ones [84,85]. By using a hydrothermal self-assembly approach followed by high-temperature treatment, as shown in Figure 6a, Qiu et al fabricated 3D porous N-doped graphene aerogels (NPGAs, Figure 6b), which exhibited good electrocatalytic activity and long-term stability in a Li-O 2 cell system [86].…”