Nanowire-Templated Three-Dimensional Out-of-Plane Fuzzy Graphene as an Oxygen Reduction Reaction Catalyst
Authors: San Roman Daniel , Garg Raghav , Lamprinakos Nicholas , Cohen-Karni Tzahi 
Published: February 2020

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.

Figure 1