Differentiation of the non-faradaic and pseudocapacitive electrochemical response of graphite felt/CuFeS2 composite electrodes

Chalcopyrite (CuFeS2) is a widely available copper mineral and like other transition metal sulfides it may be a suitable material for energy storage applications. In this preliminary study, a composite electrode was built from synthetic CuFeS2 sandwiched between graphite. Scanning Electron Microscopy (SEM), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analyses revealed that the post-synthesis CuFeS2 consisted of microspheres with sulfide (S-2 , S-2(2) and S-n(2) ) surface species. The nature of the charge distribution within this composite electrode was determined and compared with a graphite felt (CF) electrode in 0.2M H2SO4 solution at 25 degrees C. The total current response was partitioned into non-faradaic and faradaic components by charging (under-potential deposition) and discharging through cyclic voltammetric analysis. A large amount of the specific current response (93.5%) of the composite electrode was faradaic in nature. The pseudocapacitive under potential adsorption/desorption of H+ ions was reversible and a relatively higher specific capacitance of 1.265 F g(-1) was achieved at 0.1 A g(-1) (as compared to the 0.588 F g(-1) obtained from the CF electrode). Potentiostatic impedance spectroscopy (0 >= eta >= -0.9 V) within the charging regime and cathodic potentiodynamic polarization scans revealed that the graphite fibers exhibited catalytic behavior. The large polarization, higher pseudocapacitance (phi(p)) and faradaic parallel resistance (R-p) within the potential region (E-OCP > E > E degrees(+)(H) / (H2)) confirmed the generation of an intermediate (H-o) species in the CF electrode. The 'internal charge mediator' character of the graphite felt in the composite electrode, therefore, provided a large faradaic current response. Finally, based on the electrochemical analysis, the charge/discharge mechanism of the composite electrode is proposed. The addition of CuFeS2 in CF increased the specific capacitance and maintained the columbic efficiency of similar to 98%. However, confirmation of its potential use as an active electrode material in energy storage devices requires further research. (C) 2016 Elsevier Ltd. All rights reserved.