Electrochemical Behavior of Symmetric Electrical Double-Layer Capacitors and Pseudocapacitors and Identification of Transport Anomalies in the Interconnected Ionic and Electronic Phases Using the Impedance Technique.

A double-channel transmission line impedance model was applied to the study of supercapacitors to investigate the charge transport characteristics in the ionic and electronic conductors forming the electrode/solution interface. The macro homogeneous description of two closely mixed phases (Paasch-Micka-Gersdorf model) was applied to study the influence of disordered materials on the charge transport anomalies during the interfacial charge-discharge process. Different ex situ techniques were used to characterize the electrode materials used in electrical double-layer (EDLC) and pseudocapacitor (PC) devices. Two time constants in the impedance model were adequate to represent the charge transport in the different phases. The interfacial impedance considering frequency dispersion and blocked charge transfer conditions adequately described the charge storage at the interface. Deviations from the normal (Fickian) transport involving the ionic and electronic charge carriers were identified by the dispersive parameters (e.g., n and s exponents) used in the impedance model. The ionic and electronic transports were affected when the carbon-based electrical double-layer capacitor was converted into a composite with strong pseudocapacitive characteristics after the decoration process using NiO. The overall capacitance increased from 2.62 F g-1 to 536 F g-1 after the decoration. For the first time, the charge transport anomalies were unequivocally identified in porous materials used in supercapacitors with the impedance technique.

Charge-storage mechanism of highly defective NiO nanostructures on carbon nanofibers in electrochemical supercapacitors.

An electrode composed of highly defective nickel oxide (NiO) nanostructures supported on carbon nanofibers (CNFs) and immersed in an Li+-based aqueous electrolyte is studied using Raman spectroscopy under dynamic polarization conditions to address the charge-storage phenomenon. By this operando technique, the formation of Li2SO4·H2O during the discharge process is verified. At the same time, we observed the phase transformation of NiO to NiOOH. The Ni(OH)2/NiOOH redox couple is responsible for the pseudocapacitive behavior with intercalation of cationic species in the different Ni structures. A 'substitutive solid-state redox reaction' is proposed to represent the amphoteric nature of the oxide, resulting in proton intercalation, while the insertion of Li+ occurs to a less extent. The electrode material exhibits outstanding stability with 98% coulombic efficiency after 10 000 charge-discharge cycles. The excellent electrode properties can be ascribed to a synergism between CNFs and NiO, where the carbon nanostructures ensured rapid electron transport from the hydrated nickel nanoparticles. The NiO@CNF composite material is a promising candidate for future applications in aqueous-based supercapacitors. DFT simulation elucidates that compressive stress and Ni-site displacement lead to a decrease up-to 3.5-fold on the electron density map located onto the Ni-atom, which promotes NiO/Ni(OH)2/NiOOH transition.

A rational experimental approach to identify correctly the working voltage window of aqueous-based supercapacitors.

It is common to find in the literature different values for the working voltage window (WVW) range for aqueous-based supercapacitors. In many cases, even with the best intentions of the widening the operating voltage window, the measured current using the cyclic voltammetry (CV) technique includes a significant contribution from the irreversible Faradaic reactions involved in the water-splitting process, masked by fast scan rates. Sometimes even using low scan rates is hard to determine precisely the correct WVW of the aqueous-based electrochemical capacitor. In this sense, we discuss here the best practices to determine the WVW for capacitive current in an absence of water splitting using complementary techniques such as CV, chronoamperometry (CA), and the electrochemical impedance spectroscopy (EIS). To accomplish this end, we prepare and present a model system composed of multiwalled carbon nanotubes buckypaper electrodes housed in the symmetric coin cell and soaked with an aqueous-based electrolyte. The system electrochemical characteristics are carefully evaluated during the progressive enlargement of the cell voltage window. The presence of residual Faradaic current is verified in the transients from the CA study, as well as the impedance changes revealed by EIS as a function of the applied voltage, is discussed. We verify that an apparent voltage window of 2.0 V determined using the CV technique is drastically decreased to 1.2 V after a close inspection of the CA findings used to discriminate the presence of a parasitic Faradaic process. Some orientations are presented to instigate the establishment in the literature of some good scientific practices concerned with the reliable characterization of supercapacitors.

Application of electrochemically generated ozone to the discoloration and degradation of solutions containing the dye Reactive Orange 122.

Aqueous solutions containing the commercial azo dye Reactive Orange 122 (RO122) were ozonated in acid and alkaline conditions. Ozone was electrochemically generated using a laboratory-made electrochemical reactor and applied using semi-batch conditions and a column bubble reactor. A constant ozone application rate of 0.25gh(-1) was used throughout. Color removal and degradation efficiency were evaluated as function of ozonation time, pH and initial dye concentration by means of discoloration kinetics and COD-TOC removal. Experimental findings revealed that pH affects both discoloration kinetics and COD-TOC removal. A single pseudo-first-order kinetic rate constant, k(obs), for discoloration was found for ozonation carried out in alkaline solutions, contrary to acidic solutions where k(obs) depends on ozonation time. COD-TOC removal supports degradation of RO122 is more pronounced for alkaline conditions. Evaluation of the oxidation feasibility by means of the COD/TOC ratio indicates that the ozonation process in both acid and alkaline conditions leads to a reduction in recalcitrance of the soluble organic matter.