VALIAÇÃO DE SUPERCAPACITORES DE EM MATRIZ DE PAPEL CAMPINAS 2019 -BASED CARBONO JOÃO PAULO FERREIRA BERTACCHI PREPARATION AND EVALUATION OF ALL-CARBON PAPER-BASED SUPERCAPACITORS

Supercapacitors, fuel cells and batteries, are currently widely used in energy storage. The former are suited for high power density applications. Interest in these devices is due to the demand for clean, low-cost, efficient energy storage, compatible with intermittent energy sources (such as solar and wind) as well as with the emerging field of flexible electronics. This work verifies the applicability of a scalable platform used to produce non-metallic conductors [88,89] to the development of all-carbon, paper-based supercapacitors. Supercapacitor electrodes were prepared by coating filter paper with dispersions of activated carbon and/or graphite in an alkaline cellulose solution. The resulting films were washed, dried and their masses, thicknesses and surface electrical resistances were measured. Calendered and non-calendered electrodes were assembled as symmetric devices and their electrochemical properties were evaluated by galvanostatic charge/discharge cycling and cyclic voltammetry. The results were compared to the best all-carbon paper supercapacitor devices in the literature, showing excellent areal capacitance and internal resistance (350 mF cm and 1.4 Ω, against 103.5 mF cm [69] and 8 Ω ), but low gravimetric capacitance (67.2 F g against 252 F g ). The capacitance of composite electrodes increased with bench aging of the dispersions used to prepare them. Morphological changes on the surface of activated carbon particles were observed, showing an increase in the amount of mesopores. Maximum areal capacitance of these composite electrodes is achieved when the assembled devices are operated up to 1.6 V in cyclic voltammetry experiments. The assembled devices have a 11% increase in capacitance and a 28% reduction in ESR over 1000 cycles of operation. Figure index Figure 1: Summary of support matrices used in the fabrication of high performance flexible supercapacitors. Extracted from reference .................................................15 Figure 2: Most common route to produce graphene; Chemical exfoliation. Extracted from reference ......................................................................................................18 Figure 3: Some materials applied in supercapacitor electrodes and their specific capacitance. Extracted from reference ..................................................................20 Figure 4: How electrolytes properties affect each parameter of a supercapacitor device. Extracted from reference ..........................................................................21 Figure 5: Reaction pathway for cellobiose acid hydrolysis. Extracted from reference ...............................................................................................................25 Figure 6: Materials commonly used in ultracapacitors (A-D) and response of the electrodes in cyclic voltammetry experiments (E, F) and constant current discharge (G, H). A carbon particles. B porous carbon. C RuO2 or MnO2. D lithium ions intercalated in graphene or carbon nanotubes. E purely capacitive behavior of carbon materials in cyclic voltammetry and G constant current discharge experiments. F“battery-like” behavior of pseudocapacitive materials in cyclic voltammetry and H constant current discharge experiments. Extracted from reference ...............................................................................................................26 Figure 7: Ragone plot comparing the typical specific energy and specific power ratings of different electric energy storage/generation technologies. Extracted from reference ...............................................................................................................27 Figure 8: Representation of the potential drop (solid black curve) as a function of the distance to the electrode surface. From the potential value at the electrode surface (ψM), to its value in the Stern layer (ψS) and the bulk solution (ψB). The total double-layer capacitance (C) is a series sum of its stern component (CS) and its diffuse component (CD)...............................................................................................37 Figure 9: Representation of a negatively charged electrode and a random distribution of ions across the diffuse layer, the solution volume is represented by the grid in the background. As parts of a polymer chain adsorb to the electrode surface, part of the volume initially available to the ions is now occupied by the polymer chain. A decrease in degrees of freedom reflects in a decrease in the ions entropy and consequently an increase in their free energy..........................................................39 Figure 10: Steps followed in this work for supercapacitor manufacture and test......41 Figure 11: Steps of application of thick dispersions through paste spreading...........46 Figure 12: Apparatus used in the measurement of the sheet resistance (Ω/ロ) of the films. It consists of a multimeter (resistance measurement mode) with each of its terminals connected to symmetrically spaced copper straps folded around an acrylic square bar. In between the copper straps a square of 2.5 x 2.5 cm allows the measurement of the sheet resistance of a flat film, as one presses the acrylic bar against the film...........................................................................................................47 Figure 13: Acrylic case used to seal the supercapacitor from the ambient and allow electrical contact between the current collectors and the casing exteriors..............49 Figure 14: Representation of the circuit and components of the GCD apparatus....50 Figure 15: Representation of the circuit and components of the CV experimental apparatus. W Working electrode; C Counter electrode; R Reference electrode.51 Figure 16: Simplified equivalent circuit of a supercapacitor, with an equivalent series resistance (ESR) and a leakage resistance (representing the self-discharge process)......................................................................................................................52 Figure 17: Results of a GCD experiment with a 2C2G10A-Non device, showing the charge/discharge currents, the fitted discharge curves in red, their inclination ‘a’, and the ohmic drop ‘Ωdrop’ at the first 10ms of discharge.........................................57 Figure 18: Gravimetric, areal and volumetric capacitances of the symmetric supercapacitors as a function of the applied current density, obtained from GCD experiments. Stars represent non-calendered films while squares represents their calendered counterparts. x:y:z C:G:A stands for the estimated volumetric proportions (in the electrode) of Cellulose:Graphite:Activated carbon..........................................58 Figure 19: Cyclic voltammograms with increasing sweep rates obtained from supercapacitors assembled from A/C. 2C2G10A-Non, as-prepared suspensions and B/D. 2C2G10A-Non-Aged, six months old suspensions. Counter electrode and reference electrode were short-circuited with one of the supercapacitor poles and working electrode was connected to the other pole...................................................61 Figure 20: Areal capacitance of the 2C2G10A-Non and 2C2G10A-Non-Aged symmetric supercapacitors calculated from the cyclic voltammetry integrals as a function of the scan rate.............................................................................................62 Figure 21: Cyclic voltammograms at a sweep rate of 50 mV.s with increasing maximum voltages obtained from supercapacitors assembled from A/C. 2C2G10A-Non, as-prepared suspensions and B/D. 2C2G10A-Non-Aged, six months old suspensions. Counter electrode and reference electrode were short-circuited with one of the supercapacitor poles and working electrode was connected to the other pole.............................................................................................................................63 Figure 22: Areal capacitance of the 2C2G10A-Non symmetric supercapacitors calculated from the cyclic voltammetry integrals as a function of the voltage cut-off.64 Figure 23: Macroscopic view of a 2C2G10-Non film, with visible activated carbon and graphite particles.................................................................................................66 Figure 24: Image of an activated carbon particle of a 2C2G10A-Non film with an exfoliated graphite particle in the lower right corner...................................................67 Figure 25: Close-up image of the surface of the carbon particle of figure 24 showing relatively low roughness and low mesoporosity.........................................................67 Figure 26: Macroscopic view of a 2C2G10-Non-Aged film, with visible activated carbon and graphite particles..................................................................................68 Figure 27: Image of an activated carbon particle of a 2C2G10A-Non-Aged film.......68 Figure 28: Close-up image of the surface of the carbon particle of figure 27 evidencing the appearance of mesoporosity.............................................................69 Figure 29: Behavior of capacitance and ESR of 2C2G10A-Non and 2C2G10A-Aged devices over 1000 cycles of galvanostatic charge and discharge at 5.56 mA cm....70 Figure 30: Diagram of an electrode in contact with the current collector................79