“Metal-free” electrocatalysis: Quaternary-doped graphene and the alkaline oxygen reduction reaction
Graphene is increasingly ﬁnding application within catalysis due to its intrinsic properties [1–۳], and in particular its catalytic activity towards the oxygen reduction reaction (ORR) has generated signiﬁcant interest [4,5].
The ORR is a persistent challenge for researchers of energy applications (e.g. fuel cells, metal-air batteries) due to its slow kinetics [6,7]. Pristine graphene has demonstrated a limited ORR performance due to its zero band gap, which weakens the catalytic activity [8,9].
A promising strategy to make graphene more catalytically active towards the ORR is doping the graphene structure with p-block heteroatoms, such as nitrogen [10–۱۶], boron [17–۲۱], phosphorus [22–۲۴] or sulfur [25–۲۸], since the inclusion of dopant atoms in the 2D graphene structure can modify the electroneutrality of the sp2 carbon lattice and create sites for O2 adsorption .
It has been suggested that the formation of carbon active sites induced by the heteroatoms alters the binding energy of the molecule of O2, favouring the dissociation of the O]O bond . Several methods have been proposed for the doping of graphene with heteroatoms, including chemical vapour deposition (CVD), ball milling, bottom-up synthesis, thermal annealing, wet chemical method, plasma treatment with heteroatom atmosphere or arc-discharge . Among these methods, CVD is one of the most widespread, nevertheless it is a relatively complex process whose application entails the use of hazardous precursors and gases, contaminationbytransitionmetalreagents,highcostoftheprocessand