Synergetic Effect of Nitrogen and Boron Co-Doped Highly Porous Graphene Network Towards Oxygen Reduction Reaction
Authors: Ghosh Arpita , Ramaprabhu Sundara 
Published: February 2020

Fuel cells are one of the promising energy conversion devices providing high efficiency and low emission. The performance of this device limits by the sluggish oxygen reduction reaction (ORR) kinetics, which plays the crucial role to confine device performance in terms of power density as well as longevity. To overcome the sluggishness of the ORR, supported or unsupported noble metals have been playing the crucial role as cathode catalyst so far. But reducing the platinum usage and increasing its durability are the key hindrance towards the commercialization of fuel cells. Herein we report a novel material, synthesized in single step consists of nitrogen (N) and boron (B) co-doped highly porous graphene structure (BNG), that can be used as catalyst as well as catalyst support material. The synthesis procedure has been optimized in order to increase the yield of B–C and N–C bonds over B–N species during in-situ boron and nitrogen co-doping which is crucial to determine the synergistic effect of both B and N dopants on ORR.
This material has been used as a metal free efficient ORR catalyst along with incorporation of abundant transition metal (Fe) in acidic medium for proton exchange membrane fuel cell (PEMFC). High concentration of dopant atoms makes it an ideal replacement of metal catalysts. Besides the porous network significantly increases the number of triple phase boundary (TPB) leads to superior fuel and catalyst utilization. Owing to its high surface area (~ 1000 m2 g-1) and pore size the faster diffusion of fuel leads to reduction in mass transfer losses as well as enhanced power density. Catalytic activity of both the catalysts BGN and Fe/BGN has been investigated and compared with commercial Pt/C along with singly doped (nitrogen and boron) porous graphene in acid medium. Unlike the boron counterpart, it has been studied widely that introducing nitrogen atom in the sp2 carbon framework tunes the band gap as well as the electrical properties of graphene. Presence of graphitic and pyridinic nitrogen species coordinated with transition metal predominantly enhances the ORR activity.[1] Similarly, being an electron deficient element, boron has the ability to induce electron density variation on the sp2carbon lattice and accordingly favor ORR. The main contrast between the two dopants is, oxygen adsorption takes place on the carbon bonded with nitrogen whereas, in case of boron oxygen adsorbs directly on the boron sites.[2] Effects of mass transfer on kinetics of ORR were investigated with rotating disc electrode (RDE). The hydrodynamic voltammograms were investigated to determine the kinetic parameters using the Koutecky–Levich equation. Polarization study has been carried out with the single cell measurement in order to estimate the durability, ohmic loss and efficiency of the fuel cell and discussed in conjunction with the half-cell results.

References:

[1] T. Ikeda, M. Boero, S.F. Huang, K. Terakura, M. Oshima, J. Ozaki, Carbon alloy catalysts: Active sites for oxygen reduction reaction, J. Phys. Chem. C. 112 (2008) 14706–14709. doi:10.1021/jp806084d.
[2] Yang L, Jiang S, Zhao Y, Zhu L, Chen S, Wang X, et al. Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction. Angew Chemie - Int Ed 2011;50:7132–5. doi:10.1002/anie.201101287.