Nanostructured materials plasma functionalised for electrochemical energy storage applications

Abstract

The plasma treatment of few-layer graphene (FLG) was investigated for the effect on the performance in supercapacitors and microsupercapacitors. Modifications to the FLG surfaces were proven by comprehensive studies using characterisation techniques including elemental microanalysis, X-ray photoelectron spectroscopy, potentiometric titration, zeta-potential measurements and dispersion stability analysis. A thermal pre-treatment to yield dried FLG was shown to increase the FLG surface charge and density due to the removal of adsorbed water and incorporation of carboxyl and phenolic functional groups. The thermal treatment was used before all characterisation methods were applied. An Ar gas plasma treatment on dried FLG was shown to introduce carboxyl and phenolic surface functional groups and reduce material variability. With increasing treatment time of Ar plasma, the FLG oxygen content increased by 1 at% due to the presence of a larger number of carboxyl functional groups. The introduction of H2 gas at 3 wt% in Ar gas plasma produced a different functionalised FLG with a smaller quantity of carboxyl groups. Compared to the Ar gas plasma treated material, the H2/Ar gas treated material had a lower surface charge and density. NH3 gas plasma increased the nitrogen content of the FLG starting material two-fold according to XPS and elemental microanalysis. A multi-step treatment consisting of H2/Ar mixed gas plasma followed by NH3 gas plasma gave a further surface increase in nitrogen content by five times relative to the starting material. Electrode films were manufactured using polytetrafluoroethylene (PTFE) and styrene-butadiene rubber (SBR) as non-conductive binders and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) as a conductive binder. The electrode films were constructed into supercapacitors and microsupercapacitors. External testing with PTFE binder showed promise for the H2/Ar mixed gas plasma treated material with a specific capacitance of 195 F g-1 at 1 mA cm-2 in sulfuric acid electrolyte, a 50% increase relative to untreated FLG devices. The other treated materials did not match this performance because they did not contain low concentrations of oxygen surface functional groups and had large quantities of sp3 hybridised carbon atoms. The supercapacitor devices were studied with sulfuric acid and potassium hydroxide aqueous electrolytes. The devices with potassium hydroxide electrolyte did not match the performance of the materials in sulfuric acid electrolyte due to materials’ incompatibilities. Supercapacitor testing was additionally carried out with SBR and carboxymethyl cellulose (CMC) in sandwiched devices alongside potassium hydroxide and sulfuric acid electrolytes. The manufacture of the electrode films required 20 wt% of SBR-CMC binder. The electrochemical results were indistinguishable and had large resistances. An extensive investigation into the manufacture of flexible electrode films with FLG, multi-walled carbon nanotubes (MWCNT) and PEDOT/PSS in composite electrode materials was carried out. The electrode films were laser-etched into interdigitated patterns for planar micro-supercapacitor devices with the application of polyvinylalcohol-phosphoric acid gel electrolyte. These devices performed best with a mass ratio of 1:3:1 (PEDOT/PSS:FLG:MWCNT), and with NH3 functionalised FLG and acid functionalised MWCNT. Gravimetric capacitances of 120 F g-1 at 5 mV s-1 and volumetric capacitances of 20 F cm-3 at 5 mV s-1 were obtained for the NH3:MWCNT(Acid) combination during long cycling tests (10,000 cycles) and showed capacity retentions > 80%. In-situ Raman microscopy analysis suggested that the PEDOT/PSS component underwent pseudo-capacitive, reversible changes during cycling tests but the dominant electric-double layer capacitive-like response was due to the FLG and MWCNT materials, which were highly stable.