Synergistic Catalyst–Support Interactions in a Graphene–Mn₃O₄ Electrocatalyst for Vanadium Redox Flow Batteries

The development of vanadium redox flow batteries (VRFBs) is partly limited by the sluggishness of the electrochemical reactions at conventional carbon-based electrodes. The VO²⁺/VO₂⁺ redox reaction is particularly sluggish, and improvements in battery performance require the development of new electrocatalysts for this reaction. In this study, synergistic catalyst–support interactions in a nitrogen-doped, reduced-graphene oxide/Mn₃O₄ (N-rGO-Mn₃O₄) composite electrocatalyst for VO²⁺/VO₂⁺ electrochemistry are described. X-ray photoelectron spectroscopy (XPS) confirms incorporation of nitrogen into the graphene framework during co-reduction of graphene oxide (GO), KMnO₄, and NH₃ to form the electrocatalyst, while transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm the presence of ca. 30 nm Mn₃O₄ nanoparticles on the N-rGO support. XPS analysis shows that the composite contains 27% pyridinic N, 42% pyrrolic N, 23% graphitic N, and 8% oxidic N. Electrochemical analysis shows that the electrocatalytic activity of the composite material is significantly higher than those of the individual components due to synergism between the Mn₃O₄ nanoparticles and the carbonaceous support material. The electrocatalytic activity is highest when the Mn₃O₄ loading is ∼24% but decreases at lower and higher loadings. Furthermore, electrocatalysis of the redox reaction is most effective when nitrogen is present within the support framework, demonstrating that the metal–nitrogen–carbon coupling is key to the performance of this electrocatalytic composite for VO²⁺/VO₂⁺ electrochemistry

Single Stage Simultaneous Electrochemical Exfoliation and Functionalization of Graphene

Development of applications for graphene are currently hampered by its poor dispersion in common, low boiling point solvents. Covalent functionalization is considered as one method for addressing this challenge. To date, approaches have tended to focus upon producing the graphene and functionalizing subsequently. Herein, we describe simultaneous electrochemical exfoliation and functionalization of graphite using diazonium salts at a single applied potential for the first time. Such an approach is advantageous, compared to postfunctionalization of premade graphene, as both functionalization and exfoliation occur at the same time, meaning that monolayer or few-layer graphene can be functionalized and stabilized <i>in situ</i> before they aggregate. Furthermore, the N₂ generated during <i>in situ</i> diazonium reduction is found to aid the separation of functionalized graphene sheets. The degree of graphene functionalization was controlled by varying the concentration of the diazonium species in the exfoliation solution. The formation of functionalized graphene was confirmed using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The functionalized graphene was soluble in aqueous systems, and its solubility was 2 orders of magnitude higher than the nonfunctionalized electrochemically exfoliated graphene sheets. Moreover, the functionalization enhanced the charge storage capacity when used as an electrode in supercapacitor devices with the specific capacitance being highly dependent on the degree of graphene functionalization. This simple method of <i>in situ</i> simultaneous exfoliation and functionaliztion may aid the processing of graphene for various applications

Hydrogen Oxidation and Oxygen Reduction at Platinum in Protic Ionic Liquids

H₂ oxidation and O₂ reduction have been studied as a function of temperature at Pt electrodes in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Hydrodynamic voltammetry showed that the H₂ oxidation reaction (HOR) became hindered at positive potentials (>1.0 V). Electrochemical analysis and X-ray photoelectron spectroscopy revealed that this drop in HOR activity was due to the formation of an adsorbed blocking oxide layer, which formed on the Pt surface due to trace H₂O oxidation at positive potentials. Electrochemical analysis also revealed that the O₂ reduction reaction (ORR) occurred at an appreciable rate only when pre-existing surface oxides were reduced. As the temperature increased, the potential at which the surface oxides were reduced shifted to more positive potentials and the reduction peak narrowed. The net result was significantly higher rates of the ORR at positive potentials at higher temperatures. Finally, even when Pt surfaces were not initially covered with an oxide adlayer, the rate of the ORR increased significantly upon increasing the temperature and some possible reasons for this temperature dependence are discussed