This review describes recent advances in electrochemical impedance spectroscopy (EIS) with an emphasis on its novel applications to various electrochemistry-related problems. Section 1 discusses the development of new EIS techniques to reduce measurement time. For this purpose, various forms of multisine EIS techniques were first developed via a noise signal synthesized by mixing ac waves of various frequencies, followed by fast Fourier transform of the signal and the resulting current. Subsequently, an entirely new concept was introduced in which true white noise was used as an excitation source, followed by Fourier transform of both excitation and response signals. Section 2 describes novel applications of the newly developed techniques to time-resolved impedance measurements as well as to impedance imaging. Section 3 is devoted to recent applications of EIS techniques, specifically traditional measurements in various fields with a special emphasis on biosensor detections.

Details of the growth properties of polyaniline (PA) have been studied employing electrochemical techniques (potential cycling method) and the results are reported. The electrochemical growth may be characterized as autocatalytic, and the growth rate is first order in aniline concentration. The growth rate is also dependent on the amount of PA film on the electrode as well as the number of potential cycles. The highest oxidized PA, attacked by an aniline molecule, grows to a longer chain, and a net reduction of the PA film upon aniline addition would render the film conducting. This mechanism explains why the film continues to grow at potentials where only passivating behavior would be predicted. In addition, the growth was divided into regions of well‐defined and poorly defined (amorphous) phases; at longer oxidation times during growth, side reactions seem to play more important roles compared to the earlier phase of the growth process.

Ultrahigh density arrays of conducting polypyrrole (PPy) nanorods are fabricated directly on the indium-tin oxide coated glass by an electropolymerization within a porous diblock copolymer template. The nanorods are shown to have conductivity much higher than thin PPy films, due to the high degree of chain orientation, even though the separation distance for two neighboring PPy main chains is as small as 0.37 nm. The ultrahigh density arrays of conducting polymer nanorods have potential applications as sensor materials, nanoactuators, and organic photovoltaic devices.

A model semiconductor-sensitizer layer of CdSe with under- or overlayers of CdS or ZnS by pre- or postadsorption was prepared on the surface of mesoporous TiO2 films by a series of successive ionic layer adsorption and reaction (SILAR) processes in solutions containing corresponding cations and anions. The growth of each semiconductor layer was monitored by taking UV−visible absorption spectra and high-resolution transmission electron microscopy (TEM) images. The all SILAR-prepared multicomponent sensitizer consisting of CdS/CdSe/ZnS layers was evaluated in a polysulfide electrolyte solution as a redox mediator in regenerative photoelectrochemical cells. The CdS and ZnS layers with the CdSe layer sandwiched in between were found to significantly enhance photocurrents. The best photovoltaic performance was obtained from the CdS/CdSe/ZnS-sensitizer with the ZnS layer on the top, yielding an overall power conversion efficiency of 3.44% with a mask around the active film and 3.90% with no mask. The effect of the mask on short-circuit current (J sc) and overall efficiency (η) measurements was shown to be increasingly critical in semiconductor-sensitized solar cells as they exhibit high photocurrents. The polysulfide electrolyte, which acted as an effective electron transfer mediator for CdS and/or CdSe sensitizers, was not as effective for PbS-based sensitizers prepared by the same SILAR process.

Real-time in situ spectroelectrochemical studies have been carried out in N,NЈ-dimethyl formamide containing lithium trifluoromethane sulfonate as an electrolyte and the results are reported. The results indicate that the primary reduction product of the cyclic form of sulfur, S 8c 2Ϫ , undergoes an equilibrium reaction to its linear chain counterpart, S 8l 2Ϫ , which then dissociates into various products. These two dianions and S 3 Ϫ• were produced along with a minor product, S 4 2Ϫ , at the potential corresponding to the first electron transfer. These products were further reduced or dissociated to species including S 7 2Ϫ , S 6 2Ϫ , S 5 2Ϫ , S 4 2Ϫ , S 3 2Ϫ , S 2 2Ϫ , and S 2Ϫ at the second electron-transfer step as evidenced by the spectral shifts observed during electrolysis. The reduction reactions are generally chemically reversible, making it possible to use sulfur reduction as a cathode reaction for Li/S batteries.

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The electrochemical reduction of sulfur has been studied employing in situ spectroelectrochemical techniques. Previously unreported absorption bands in the UV region observed during the electrochemical reduction are assigned to the reduced sulfur species with high electron-to-atom ratios. The spectroelectrochemical studies indicate that reduction products generated at the first reduction step are essentially identical to those formed at the second with S~-and S~-as ultimate reduction products. This suggests that chemical reactions following the first electron transfer to produce S~-are important. The derivative cyclic voltabsorptometric (DCVA) results indicate that all the reduced sulfur species, i.e., S~-, S~-, S~-, S~-, and S~" are generated at the first cathodic wave and three of these, i.e., S~-, S~-and S~" are reduced at the second reduction wave, while other species show a delayed generation. A mechanism consistent with these observations is proposed.

Effects of counterions and growth methods on polyaniline growth have been studied using the electrochemical quartz crystal microbalance and other related techniques. The results indicate that the polymer growth rates and the morphology of polymer surfaces are very different depending on the electrolytes and growth methods used. During the first cycle of the potentiodynamic growth, the weight increase is observed with a low coulombic efficiency. However, the number of electrons required for the deposition of one aniline unit quickly approaches 2.0 beginning from the second cycle, and the current efficiency becomes higher as the number of cycles increases. As the film grows, the anion insertion and deinsertion become increasingly important. For a thick polymer film in the H 2 SO 4 solution, the doping/dedoping process is not reversible, although it is reversible in HClO 4 solutions. During the potentiostatic and galvanostatic polymerization, relatively low charge efficiencies were observed due to the degradation reactions. These results were consistent with the scanning electron microscope images.