Redox flow batteries (RFBs) are promising energy storage candidates for grid deployment of intermittent renewable energy sources such as wind power and solar energy. Various new redox-active materials have been introduced to develop cost-effective and high-power-density next-generation RFBs. Electrochemical kinetics play critical roles in influencing RFB performance, notably the overpotential and cell power density. Thus, determining the kinetic parameters for the employed redox-active species is essential. In this Perspective, we provide the background, guidelines, and limitations for a proposed electrochemical protocol to define the kinetics of redox-active species in RFBs.

Free-floating, and copper-supported, few-layer graphene sheets were spontaneously modified from an aqueous solution containing nitrobenzenediazonium ions. The infrared spectra of the chemically modified (copper etched) free-floating graphene were measured in transmission mode by manipulating the sheets onto a KBr disc. The major advantage to this method is the ability to release the graphene sheets off the disc to refloat on a water bath, allowing the graphene to be further modified or deposited onto a new substrate suitable for other analysis. In this study, graphene sheets were then mounted onto highly ordered pyrolytic graphite (HOPG) for atomic force microscopy imaging and electrochemical measurements. The results show there are at least two reaction pathways for spontaneous film grafting to graphene: the commonly accepted aryl radical leading to films containing −C−C− linkages and a direct reaction of the diazonium cation with graphene to give films containing −NN− linkages. The ability to manipulate modified graphene sheets onto electrodes with two orientations, with the film exposed to electrolyte solution or sandwiched between graphene and HOPG, leads to different estimates of the surface concentration of electroactive groups. When the film is sandwiched between graphene and HOPG, two electroreduction signals for the nitro group are seen and much larger surface concentrations are measured. This is the first account of such a signal and is tentatively attributed to different peak potentials for reduction of nitro groups at graphene and HOPG. The solution permeability through the graphene sheet and attached films has important electrochemical consequences for systems of this type employed in supercapacitor applications.

The quantum capacitance from large area, 6- to 7-layered graphene, and chemically modified graphene, was determined using electrochemical impedance spectroscopy measured at 100 Hz in a three-electrode electrochemical cell and aqueous acidic solution. Aryldiazonium chemistry was used to modify one side of the few-layered graphene sheets with methoxy- and iodo-phenyl groups. The graphene sheets were then mounted via an aqueous transfer method onto an epoxy substrate. It was found that both sides of the graphene sheets could be accessed by the electrolyte and thus the sheets were in a pseudo free-floating arrangement. The results show a complex quantum capacitance behavior with applied electrode potential, and that behavior was not stable with potential cycling. Chemically modified samples have similar quantum capacitance minimum to unmodified graphene, and all samples have a shallow U-shaped relationship with respect to applied electrode potential. The results show chemically modifying one side of few-layer graphene sheets using diazonium chemistry was not detrimental to the measured quantum capacitance minimum at the potential of zero charge, but the capacitance remained low over a large potential window which is not desirable for supercapacitor applications.