Nitrogen-doped ordered mesoporous carbon:Effect of carbon precursor on oxygen reduction reactions

Aniline,pyrrole and phenanthroline,which have different nitrogen compositions,are used as carbon precursors to synthesize nitrogen-doped ordered mesoporous carbons（NOMCs） by the nanocasting method.The effect of the precursor on the resultant NOMC is extensively investigated by nitrogen adsorption-desorption measurements,scanning electron microscopy,X-ray photoelectron spectroscopy（XPS）,cyclic voltammetry and rotating ring-disk electrode measurements.Salient findings are as follows.First,the precursor has a significant influence on the specific surface area and textural properties.The NOMC materials derived from pyrrole（C-PY-900：765 m^2/） and phenanthroline（C-Phen-900：746 m^2/） exhibit higher specific surface areas than the aniline analog（C-PA-900：569 m^2/）.Second,the XPS results indicate that the total nitrogen content（ca.3.1–3.3 at%） is similar for the three carbon sources,except for a slight difference in the nitrogen configuration.Furthermore,the content of the nitrogen-activated carbon atoms is found to closely depend on the precursor,which is the highest for the phenanthroline-derived carbon.Third,the electrochemical results reveal that the electrocatalytic activity follows in the order C-PA-900 C-PY-900 C-Phen-900,confirming that the nitrogen-activated carbon atoms are the active sites for the oxygen reduction reaction（ORR）.In summary,the precursor has considerable influence on the composition and textural properties of the NOMC materials,of which the ORR electrocatalytic activity can be enhanced through optimization of the NOMCs.

An effective poly（p-phenylenevinylene） polymer adhesion route toward three-dimensional nitrogen-doped carbon nanotube/reduced graphene oxide composite for direct electrocatalytic oxygen reduction

Heteroatom-doped nanocarbons have excellent potential for use in the oxygen reduction reaction （ORR）. However, construction of three-dimensional （3D） N-doped carbon materials with good electrocatalytic performance remains a challenge. Herein, a poly（p-phenylenevinylene） （PPV）-precursor adhesion route was developed for construction of 3D N-doped reduced graphene oxide-PPV calcined-carbon nanotubes （N-RGO-PPV（c）-CNTs）. In the synthesis, the PPV- precursor plays the role of a ＂glue＂ for strong adhesion of the RGO and CNTs. At high temperature, PPV can undergo transformation from the glassy state to a viscous state. Thus, the N-RGO-PPV（c）-CNT composite with multi-porous structure and ridge-like folded graphene flakes could be formed during nitridation at high temperature, which was favorable for production of more active sites for the ORR. As an ORR catalyst, the N-RGO-PPV（c）-CNT composites exhibited superior catalytic activity in alkaline electrolyte. The obtained onset potential （Eonset） of 0.92 V and catalytic current density of 5.7 mA·cm^-2 at 0.6 V （vs. RHE） are comparable to those of the 20% Pt/C composite （0.98 V and 5.2 mA·cm^-2）. The electron transfer number for the N-RGO-PPV（c）-CNT catalyst was about 3.99, which is close to that of the 20% Pt/C （4.01） catalyst. Notably, the optimal N-RGO-PPV（c）-CNT catalyst shows better durability and methanol tolerance than commercial 20% Pt/C. The good performance of the N-RGO-PPV（c）-CNT catalyst for the ORR may be attributed to the synergistic effects of the unique 3D structure for effective mass-transfer, the effective N-doping for production of more active sites, and the good contact between the RGO and CNTs for easy charge-transfer.

Identifying the key N species for electrocatalytic oxygen reduction reaction on N-doped graphene

The state of nitrogen in nitrogen-doped graphene(NG)promoting the conversion of molecular oxygen to hydrogen peroxide was investigated.The oxygen reduction reaction(ORR)reactivity of graphitic-N,pyrrolic-N,pyridinic-N in NG was predicted by density functional theory(DFT).A series of NG samples with different contents of these doped nitrogen types were prepared by the low-temperature thermal reduction method and used for the ORR evaluation.The H_(2)O_(2)yield,2e−ORR current efficiency,H_(2)O_(2)selectivity,electron transfer number(n)were systematically studied.The 2e−ORR selectivity was positively correlated with the N content,approaching 100%with increasing N content(0.40 V vs.reversible hydrogen electrode(RHE)),whereas the comparative energy efficiency showed a volcano-type trend related to N content,reaching a maximum of 94%.In addition,N species validation experiments proved the key role of pyrrolic-N in the synthesis of H_(2)O_(2).Compared with a pure graphene catalyst,further contaminant degradation studies on NG electrodes with different pyrrolic-N contents revealed that the lower pyrrolic-N the higher removal of p-nitrophenol(PNP).This work provides insight into the mechanism of ORR on metal-free catalysts and a facile approach to optimize this important environmental catalytic strategy.

Doped-nitrogen enhanced the performance of Nb_(2)CT_(x)on the electrocatalytic synthesis of H_(2)O_(2)

Electrochemical oxygen reduction reaction(ORR)for preparing hydrogen peroxide(H_(2)O_(2))is a promising way to replace the anthraquinone method.The key to H_(2)O_(2)production is the development of catalysts to regulate the oxygen reduction reaction pathway.Here,nitrogen-doped Nb_(2)CT_(x)was prepared by NH3 annealing method.Compared with precursor Nb_(2)AlC(67.01%)and pure Nb_(2)CT_(x)(75.70%),nitrogen-doped Nb_(2)CT_(x)exhibited excellent performance for 2e−ORR with>90%H_(2)O_(2)selectivity(at 0.4 V vs.reversible hydrogen electrode(RHE)).Faradic efficiency of nitrogen-doped Nb_(2)CT_(x)reached 80.75%,whereas those for Nb2AlC and Nb_(2)CT_(x)were 60.35%and 39.27%,respectively.A desirable catalytic stability for 50 h was observed.Density functional theory(DFT)calculations indicated excellent activity of the nitrogen-doped Nb_(2)CT_(x)was attributed to the introduction of N.This excellent activity was conducive to the adsorption of oxygen and promoted the formation of the OOH intermediate.This work can serve as an important reference for regulating the electronic structure of MXene to expand the application area in the electrochemical field.

N-doped carbon nanotubes supported CoSe_(2) nanoparticles:A highly efficient and stable catalyst for H_(2)O_(2) electrosynthesis in acidic media

Electrocatalytic oxygen reduction reaction(ORR)provides an attractive alternative to anthraquinone process for H_(2)O_(2) synthesis.Rational design of earth-abundant electrocatalysts for H_(2)O_(2) synthesis via a two-electron ORR process in acids is attractive but still:very challenging.In this work,we report that nitrogen-doped carbon nanotubes as a multi-functional support for CoSe2 nanoparticles not only keep CoSe_(2) nanoparticles well dispersed but alter the crystal structure,which in turn improves the overall catalYtic behaviors and thereby renders high O_(2)-to-H_(2)O_(2) conversion efficiency.In 0.1 M HClO_(4),such CoSe_(2)@NCNTs hybrid delivers a high H_(2)O_(2) selectivity of 93.2% and a large H_(2)0_(2) yield rate of 172 ppm·h^(-1) with excellent durability up to 24 h.Moreover,CoSe_(2)@NCNTs performs effectively for organic dye degradation via electro-Fenton process.