Rate-limiting oxygen reduction reactions (ORR) and high-cost platinum at fuel cell cathodes are substantial barriers to commercial fuel cell viability. Carbon-based, metal-free ORR catalysts have high catalytic stability and can potentially dramatically reduce cost and increase efficiency of fuel cells. For increased ORR efficiency, nitrogen doping of graphene sp²-hybridized carbon lattice is utilized, to induce a net positive charge on adjacent C atoms to alter the O₂ chemisorption mode and attract electrons from the _­electrode. We have recently reported synthesis of a novel graphene-based hybrid-nanomaterial composed of three-dimensional (3D) out-of-plane fuzzy graphene (3DFG) on 3D Si nanowire mesh template (NT-3DFG). Leveraging graphene’s outstanding surface-to-volume ratio and electrical conductivity, we attained exceptional electrochemically active surface area of ca. 1017 m² g^-1 (with high density of exposed, active graphene edges) and electrical conductivity of ca. 2400 S m^-1. Here we report fine control over 3DFG flake size and edge density by varying the partial pressure of CH₄ precursor as well as the plasma enhanced chemical vapor deposition (PECVD) process time and temperature (Figure 1.A-B). XPS analysis of C1s confirms the presence of graphite via the sp² hybridized C bonds in 3DFG structures (Figure 1.C.I and III). We also show both in-situ and post synthesis N-doping of NT-3DFG through either incorporation of N₂ during the PECVD process or post-synthesis N₂-plasma treatment. XPS analysis of N1s confirms the presence of pyridinic-, pyrrolic-, and graphitic-N doping in the NT-3DFG structure (Figure 1.C.II and IV). In-situ N-doping of NT-3DFG resulted in ca. 87% pyridinic-N doping compared to ca. 52% through post-synthesis N₂-plasma treatment. This pyridinic-N stabilizes the active site responsible for the chemical disproportionation of peroxide ions during ORR. Through cyclic voltammetry under acidic conditions, we demonstrate the functionality and properties of N-doped NT-3DFG as an ORR catalyst. With pyridinic-N doping and a high density of graphene edges, N-doped NT-3DFG demonstrates greater ORR efficiency than existing graphene-based hybrid nanomaterials.

Figure 1. Characterization of N-doped NT3DFG. (A) SEM image of NT-3DFG synthesized with increasing partial pressure of CH₄ and increasing PECVD process time. (B) Effect of increasing PECVD process temperature on NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 10 min. (C) X-ray photoelectron (XPS) spectra of (I) C1s and (II) N1s of in-situ N-doped NT-3DFG synthesized under 25 mTorr partial pressure of CH4 and 400 mTorr partial pressure of N₂ for 30 min at 800˚C; and (III) C1s and (IV) N1s of NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 30 min at 800˚C doped with N post-synthesis under 300 mTorr partial pressure of N₂ for 5 min at 300˚C. Cyan, purple, pink, red, green and blue represent C sp², C sp³, C=O, Pyridinic N, Pyrrolic N and Graphitic N XPS spectra, respectively.