Efficient Oxygen Reduction Electrocatalyst Based on Edge-Nitrogen-Rich Graphene Nanoplatelets: Toward a Large-Scale Synthesis

The large-scale synthesis of nitrogen doped graphene (N-graphene) with high oxygen reduction reaction (ORR) performance has received a lot of attention recently. In this work, we have developed a facile and economical procedure for mass production of edge-nitrogen-rich graphene nanoplatelets (ENR-GNPs) by a combined process of ball milling of graphite powder (GP) in the presence of melamine and subsequent heat treatment. It is found that the ball milling process can not only crack and exfoliate pristine GP into edge-expanded nanoplatelets but also mechanically activate GP to generate appropriate locations for N-doping. Analysis results indicate that the doped N atoms mainly locate on the edge of the graphitic matrix, which contains ca. 3.1 at.% nitrogen content and can be well-dispersed in aqueous to form multilayer nanoplatelets. The as-prepared ENR-GNPs electrocatalyst exhibits highly electrocatalytic activity for ORR due to the synergetic effects of edge-N-doping and nanosized platelets. Besides, the stability and methanol tolerance of ENR-GNPs are superior to that of the commercial Pt/C catalyst, which makes the nanoplatelets a promising candidate for fuel cell cathode catalysts. The present approach opens up the possibility for simple and mass production of N-graphene based electrocatalysts in practice

Identifying the Active Site in Nitrogen-Doped Graphene for the VO²⁺/VO₂⁺ Redox Reaction

Nitrogen-doped graphene sheets (NGS), synthesized by annealing graphite oxide (GO) with urea at 700–1050 °C, were studied as positive electrodes in a vanadium redox flow battery. The NGS, in particular annealed at 900 °C, exhibited excellent catalytic performance in terms of electron transfer (ET) resistance (4.74 ± 0.51 and 7.27 ± 0.42 Ω for the anodic process and cathodic process, respectively) and reversibility (Δ<i><i>E</i></i> = 100 mV, <i>I</i>_pa/<i>I</i>_pc = 1.38 at a scan rate of 50 mV s^–1). Detailed research confirms that not the nitrogen doping level but the nitrogen type in the graphene sheets determines the catalytic activity. Among four types of nitrogen species doped into the graphene lattice including pyridinic-N, pyrrolic-N, quaternary nitrogen, and oxidic-N, quaternary nitrogen is verified as a catalytic active center for the [VO]²⁺/[VO₂]⁺ couple reaction. A mechanism is proposed to explain the electrocatalytic performance of NGS for the [VO]²⁺/[VO₂]⁺ couple reaction. The possible formation of a N–V transitional bonding state, which facilitates the ET between the outer electrode and reactant ions, is a key step for its high catalytic activity

Co–N Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid

Nitrogen-functionalized graphene materials have been demonstrated as promising electrocatalyst for the oxygen reduction reaction (ORR), owning to their respectable activity and excellent stability in alkaline electrolyte. However, they exhibit unacceptable catalytic activity in acid medium. Here, a hierarchically porous Co–N functionalized graphene aerogel is prepared as an efficient catalyst for the ORR in acid electrolyte. In the preparation process, polyaniline (PANI) is introduced as a pore-forming agent to aid in the self-assembly of graphene species into a porous aerogel networks, and a nitrogen precursor to induce in situ nitrogen doping. Therefore, a Co–N decorated graphene aerogel framework with a large surface area (485 m² g^–1) and an abundance of meso/macropores is effectively formed after heat treatment. Such highly desired structures can not only expose sufficient active sites for the ORR but also guarantee the fast mass transfer in the catalytic process, which provides significant catalytic activity with positive onset and half wave potentials, low hydrogen peroxide yield, high resistance to methanol crossover, and remarkable stability that is comparable to commercial Pt/C in acid medium

Phosphorus and Nitrogen Centers in Doped Graphene and Carbon Nanotubes Analyzed through Solid-State NMR

Graphene and carbon nanotubes (CNTs) have been investigated closely in recent years because of their apparent positive effect on the electrochemical performance of new fuel cell and battery systems as catalyst stabilizers, supports, or metal-free catalysts. This is particularly true for doped graphene and CNTs, where only a small amount of doping with nitrogen and/or phosphorus can have a remarkable effect on the material performance. A direct link between structure and function in these materials is, as of yet, unclear. Doped graphene and CNTs were synthesized using varied chemical vapor deposition-based methods, and ssNMR was used to unambiguously identify dopant atom sites, revealing that these particular synthesis methods result in highly homogeneous populations of installed phosphorus and nitrogen atoms. We present the first experimental ¹⁵N spectrum for graphitic nitrogen in N-doped graphene. ¹⁵N-labeled nitrogen-doped graphene synthesized as reported here produces mainly graphitic nitrogen sites located on the edges of sheets and around defect sites. ¹H–¹H and ¹H–¹⁵N correlations were also used to probe dopant nitrogen sites in labeled and unlabeled N-doped graphene. A nearly homogeneous population of phosphorus in P-doped graphene is found, with an overwhelming majority of graphitic phosphorus and a small amount of phosphate oligomer. Similar findings are noted for the phosphorus sites in phosphorus and nitrogen codoped CNTs with a minor change in chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs vs graphene sheets) with significantly different electronic structures