In-Situ Electrochemical Functionalization of Reduced Graphene Oxide: Positive Lead Acid Case

Graphene based nano-sheets have gained prominence in batteries and super capacitors owing to their  high aspect ratio, exceptional transport properties, ionic functionalities, and surface properties.  Variants like pristine graphene, graphene oxide, or chemically/thermally reduced graphene are not equal in ionic and transport properties owing to the presence or grafting of functional groups in the honey comb carbon structure. To perform as an electro-catalyst, functionalization of graphene is done by ex-situ grafting desired ions and molecules, usually covalently and non-covalently. Graphene which has zero band gap, could be treated to create semi-conductor-type functionalities. Functionalization makes its surface more amenable to further processes like deposition and nucleation of guest molecules or ions.

 This work demonstrates the cathodic electro-functionalization of reduced graphene sheets in the presence of lead dioxide and sulphuric acid electrolyte. High resolution SEM and TEM morphology reveals the surface of graphene sheets have been altered with the atoms of Lead, Oxygen and Sulphur. The deconvoluted C 1s spectra of electrochemically functionalized graphene shows carbon types with chemical states appearing at 284.3 eV (graphite, C—C/C═C), 286.3 eV (C—OH), 287.4 eV (C═O), and 288.3 eV (O—C═O), and 290.5 which was ∏-∏ interaction. Compared to reduced graphene, the intensity of C-C peak reduced very much, and was sacrificed for the re-appearance of oxygen functional groups, especially the (O—C═O). Further investigation were done using Raman spectroscopy and FTIR. There was a marked decrease in I(D)/I(G) ratio with presence of a bond peak at 2642 and 2940 cm^-1. However, FTIR reveals that the peaks for most functional groups were still minimized like the reduced graphene, while band showing vibrations of oxides and sulphates appear between 550-1100. There were significant differences in characteristic –OH vibrations between 3000-3700 cm^-1, due to water of crystallization associated with gel-crystal interface.

 This work also determined that the hydroxyl functional groups and carboxyl functional groups were substituted electrochemically by Pb(OH)^- on charging and HSO₃^- ions during discharge in the hydrated gel interface of the positive lead dioxide plate. There were evidences of increased mobility of hydrogen/hydroxyl ions which is desirable, and more efficient for conduction of SO₄^2- back into the electrolyte during discharge. Oxygen containing groups with its inherent functional properties aided the hydrolysis, combination of OH^- and H⁺ and SO4^2- , which made up the hydrated zone which encapsulates the graphene nano-sheets. This optimized ionic conductivity was marked by ~27% increase in discharge capacity after few cycles, and ~38.5% increase respectively after 25 cycles. Progressive functionalization of the reduced sheets enabled the Pb(OH)⁺, HSO₃^- ions decorations on reduced graphene, by substitution of OH^- and other functional groups, and ∏-∏ bond interaction as  prominent mechanisms. Increase in ionic activity aided the nucleation of intermediate active materials, and increase in active sites on the large surface area of the graphene nano-sheets. This resulted in increase in capacity and charge acceptance at 0.25C rate.

 In-situ electrochemical functionalization of reduced graphene nano-sheets in controlled ionic and conductive system will be a cutting edge approach for controlling it’s behavior in energy materials as an electro-catalyst, in a scalable and cost effective way.