Graphite and Graphene-Oxide based PGM-free model catalysts for the Oxygen Reduction Reaction (2017)

The world currently relies heavily on fossil fuels such as coal, oil, and natural gas for its energy. Fossil fuels are non-renewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. One alternative source of energy are fuel cells, electrochemical devices that convert chemical energy to cleanly and efficiently produce electricity. They can be used in a wide range of applications, including transportation, stationary, portable and emergency power sources. Their development has been slowed by the high cost of PGM electrocatalysts needed at both electrodes as well as sluggish oxygen reduction reaction (ORR) occurring at the cathode. To replace the costly PGM-based materials, a new generation of PGM-free ORR catalysts has emerged, composed by earth abundant elements such as carbon, nitrogen and transition metals. Current heterogeneous PGM-free catalysts are exceedingly difficult to study using standard analytical techniques. In the following studies, we use well- defined model systems based on graphitic systems such as graphite and graphene oxide. These extensively characterized materials are relatively simple to model, making them ideal platforms for understanding catalytic active sites. In the first study, we investigate a green, solvent-free and sustainable synthesis route to synthesize large amounts of active ORR electrocatalysts based on graphite. We show that the simple ball milling of expanded graphite in presence of metal and nitrogen precursors followed by a pyrolysis step can create active and selective catalysts towards the ORR. We report an improved activity of graphitebased electrocatalysts in alkaline medium with an onset potential (Eonset) up to ~0.89 V and a half-wave potential (E1/2) up to 0.72 V. In the second study, we demonstrate that removal of intercalated water using simple solvent treatments causes significant structural reorganization substantially impacting the ORR activity and stability of nitrogen-doped graphitic systems (NrGO). Contrasting reports describing ORR activity of NrGO-based catalysts in alkaline electrolytes, we demonstrate superior activity in acidic electrolyte with Eonset of ~0.9 V, E½ of 0.71 V, and selectivity for four-electron reduction >95%. Further, durability testing showed E½ retention >95% in N2- and O2-saturated solutions after 2000 cycles demonstrating highest ORR activity and stability reported to date for NrGO-based electrocatalysts in acidic media. In the third study, we report that the activity and selectivity (4e-) of NrGO catalysts for ORR is enhanced using simple solvent and electrochemical treatments. The solvents, which were chosen based on Hansen’s solubility parameters, drive a substantial change in the morphology of the functionalized graphene materials either by i) forming microporous holes in the graphitic sheets that lead to edge defects as well as enhanced oxygen transport, or ii) inducing 3D structure in the graphitic sheets that promote ORR. Additionally, the cycling of these catalysts has highlighted the multiplicity of the active sites, with different durability, leading to a more selective catalyst over time with little to no loss in performance. We demonstrate excellent ORR activity in an alkaline electrolyte with an Eonset up to ~1.08 V and a E1/2 up to 0.84 V. Further, durability testing showed E1/2 loss 2- and O2-saturated solutions after 10,000 cycles, demonstrating a high ORR activity and stability while improving the selectivity towards the 4-electron reduction. The results described in this study will allow for the synthesis of better performing graphitic ORR electrocatalyst with controlled activity and could lead to a better understanding of the active site formation in PGM-free electrocatalysts.