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This paper reports on the electrochemical performance of symmetric electrochemical capacitors operating in an aqueous electrolyte (1 mol L−1 Li2SO4 solution) whose viscosity is modified by various amounts of carboxymethyl cellulose additive.

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The operando monitoring of pH during the charging and discharging of an electrochemical capacitor in an aqueous neutral salt solution is presented. Proper knowledge of transient and limiting pH values allows for a better understanding of the phenomena that take place during capacitor operation. It also enables the proper assignment of the reaction potentials responsible for water decomposition. It is shown that the pH inside the capacitor is strongly potential-dependent and different for individual electrodes; therefore, the values of the evolution potentials of hydrogen and oxygen cannot be precisely calculated based only on the initial pH of the electrolyte. The operando measurements indicate that the pH at the positive electrode reaches 4, while at the negative electrode, it is 8.5, which in theory could shift the theoretical operating voltage well beyond 1.23 V. On the other hand, high voltage cannot be easily maintained since the electrolyte of both electrode vicinities is subjected to mixing. Operando gas monitoring measurements show that the evolution of electrolysis byproducts occurs even below the theoretical decomposition voltage. These reactions are important in maintaining a voltage-advantaged pH difference within the cell. At the same time, the electrochemical quartz crystal microbalance (EQCM) measurements indicated that the ions governing the pH (OH–) that initially accumulated in the vicinity of the positive electrode enter the carbon porosity, losing their pH-governing abilities. pH fluctuations in the cell are important and play a vital role in the description of its performance during the cyclability at a given voltage. This is especially noticeable in cell floating at 1.3 V, where the pH difference between electrodes is the highest (6 units). The increase of the electrode separation distance acts similarly to the introduction of a semipermeable membrane toward the increase of the capacitor cycle life. During floating at 1.6 V, where the pH difference is not as high anymore (4 units), the influence of separation in terms of electrode stability, although present, is less notable.

This paper introduces the method which allows determining the accurate electrode contributions during cyclic voltammetry (CV) scan of electrochemical capacitor. As a result of theoretical considerations, a calculation method which reveals voltammetry response of both electrodes during CV of two-electrode cell with reference is developed. The technique is based on the preservation of charge neutrality where the accurate potential sweep rate of individual electrode is dynamically assigned based on its total contribution to the total two-electrode cell voltage ramp. This practice should be used in the research with CV scans of energy storage devices in order to improve their precision. The technique is not an alternative to real three-electrode measurements, where constant sweep rate of working electrode is applied and an oversized auxiliary electrode is used, but it is rather a supplement, which allows observing the true electrode behavior during operation of the capacitor. The paper provides comparison of CV scans obtained with fixed scan rates of both electrodes with dynamic CV scan for electrochemical capacitors operating in aqueous media of 1 mol L −1 Li 2 SO 4 and 7 mol L −1 KSCN. For the first time, the simple procedure is proposed to visualize the real qualitative electrode responses.

The described research is focused on the hybrid, bifunctional composite materials dedicated to the electrochemical capacitor electrodes. The novel material exhibits both luminescent and capacitive properties. The fabrication process of semiproducts and the final composite is described. The structure and homogeneity of luminophors are confirmed with the XRD analysis. The morphology of materials is also determined by TEM and SEM images. The detailed spectroscopic characterization includes excitation and emission spectra, luminescence decay curves, emission lifetimes, CIE chromaticity indexes. The electrochemical studies of composite electrodes carried out by cyclic voltammetry and impedance spectroscopy exhibit good charge propagation. For the first time, inorganic luminophors containing doped LaF 3 and GdVO 4 have been successfully used for electrochemical capacitor. It is the primary stage to design a new generation of light emitting capacitors utilizing more stable inorganic luminophors than organicbased ones.