To investigate the effect of anions on the electrochemical properties of polyaniline (PANI) for supercapacitors, electrochemical performance tests of PANI with different dopant anions were carried out in the corresponding acid solutions by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) methods. In particular, ionic fluxes and solvent molecules involved in redox processes can be analyzed by the electrochemical quartz crystal microbalance (EQCM) technique and discriminated by simultaneously recording cyclic voltammograms and mass changes during redox switching. The emeraldine base (EB) form of PANI prepared in a protonic acid with bigger anions can be easily doped by a protonic acid with smaller anions, and conversely, PANI-EB is hard to be doped. The anodic reversal potential of potentiodynamic cycling heavily influences the electrochemical stability of PANI. High anodic potentials result in PANI degradation. Its supercapacitive properties including specific capacitance, power density and cycling stability are strongly dependent upon the type of dopant anion. PANI with the dopant anions of oxalic acid has the highest specific capacitance and the best cycling stability among the used acids. The diffusion coefficient of anions plays a key role in determining power density. PANI films with organic dopant anions exhibit better cycling stability than their inorganic counterparts. It is believed that the hydrolysis of PANI facilitated by the additional water molecules accompanied by dopant anions into and out of the PANI matrix is a key factor responsible for the cycling instability.
Due to certain limitations of traditional models, the growth mechanism of porous anodic TiO 2 nanotubes has not been well determined currently. Herein, for the first time, a mathematical model of voltage-time transient curves under constant current conditions is derived theoretically, based on the conception of ionic and electronic currents and Ohm's law. The simulation results show high fidelity to the experimental curves, and illustrate the linear correlation between nanotube length and ionic current. Further, based on this model, the avalanche breakdown can be explained, which shows advantage over the former derivations on compact films. And this model indicates that the discrepancy between compact and porous oxide films lies on the magnitude of electronic current during anodization. Moreover, the proportion of the ionic and electronic current is then calculated during constant current anodization. It can be concluded that the ionic current contributes to the oxide growth while the electronic current gives rise to the oxygen bubble evolution which acts as the growth mould of the oxide. The present results promote the understanding of the growth kinetics of porous anodic oxides from qualitative interpretation to quantitative analyses.
Previous studies suggested that high-field anodization of aluminum can be realized given that aluminum is anodized at a high voltage just below the breakdown value. However, increasing the applied voltage cannot guarantee the enhancement in electric field strength across the barrier layer of porous anodic alumina (PAA) due to a concurrent increase of the barrier-layer thickness. Here, we report comparative studies of aluminum anodization in a highly concentrated (0.75 M) and a dilute (0.1 M) oxalic acid solution. Special attention is given to the field strength during anodization. To calculate the field strengths, the barrier-layer thickness of PAA was determined by a re-anodizing technique. Electrochemical impedance spectra (EIS) were also used to measure the barrier-layer thickness to improve the reliability of the field strength measurements. Both routes can yield the same conclusion: the barrier-layer thickness increases linearly with the applied voltage. For 0.75 M oxalic acid electrolyte, the resulting regression line has a positive intercept. In this instance the field strengths can be enhanced by increasing the applied voltage. Conversely, for 0.1 M oxalic acid solution, the regression line has a negative intercept and the field strengths decrease with the applied voltage. There are different linear dependences between the barrier-layer thickness and the applied voltage that determine the change in the field strength during anodization. In the highly concentrated oxalic acid electrolyte, the high-field anodization of aluminum can be realized by enhancing the applied voltage, but cannot in the dilute acid solution. Moreover, the ordering qualities of PAA films increase with increasing field strength instead of the applied voltage. The present results may provide a decisive step towards a thorough understanding of the PAA film.
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