  Through direct nanoparticle nucleation and growth on nitrogen doped, reduced
graphene oxide sheets and cation substitution of spinel Co3O4 nanoparticles, a
manganese-cobalt spinel MnCo2O4/graphene hybrid was developed as a highly
efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline
conditions. Electrochemical and X-ray near edge structure (XANES)
investigations revealed that the nucleation and growth method for forming
inorganic-nanocarbon hybrid results in covalent coupling between spinel oxide
nanoparticles and N-doped reduced graphene oxide (N-rmGO) sheets. Carbon K-edge
and nitrogen K-edge XANES showed strongly perturbed C-O and C-N bonding in the
N-rmGO sheet, suggesting the formation of C-O-metal and C-N-metal bonds between
N-doped graphene oxide and spinel oxide nanoparticles. Co L-edge and Mn L-edge
XANES suggested substitu-tion of Co3+ sites by Mn3+, which increased the
activity of the catalytic sites in the hybrid materials, further boosting the
ORR activity compared to the pure cobalt oxide hybrid. The covalently bonded
hybrid afforded much greater activity and durability than the physi-cal mixture
of nanoparticles and carbon materials including N-rmGO. At the same mass
loading, the MnCo2O4/N-graphene hybrid can outperform Pt/C in ORR current
density at medium overpotentials with superior stability to Pt/C in alkaline
solutions.
