Ac electrogravimetry

For electroactive thin films, the electrochemical impedance technique alone is not able to discriminate among the various models which have been proposed in the literature. So the measurement of an electrogravimetric transfer function, Am/AE, by using a fast QCM, in parallel to measurement of the impedance has been proposed to test these various models and to obtain a complete description of the ionic and solvent exchanges between the film and the solution. So far, two groups, one in Korea and the other one in France, have used this technique. 

Kwak et al. have considered that only two types of species may be involved in the charge compensation process cations and anions, which can be solvated or not. They considered the electrochemical capacitance, Aq/AE, and the electrogravimetric transfer function (which they called electrogravimetric capacitance ), which can be separated into two parts  

They considered equivalent circuits of the electrochemical impedance of the electrode/polymer film/electrolyte solution system (Fig. 6). If only one kind of ion transport takes part in the charge 

when only one ion is involved in the ion transport, the electrogravimetric transfer function is 

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C. Gabrielli, J.J. Garcia-Jareno, M. Keddam, H. Perrot, and F. Vicente, Ac-electrogravimetry study of electroactive thin films. II. Application to polypyrrole. J. Phys. Chem. B, 106, 3192... 

EQCM has also been combined with other techniques, such as probe beam deflection [193], scanning tunneling microscopy [233], scanning electrochemical microscopy [234, 235], and UV-VIS spectroelectrochemistry [236, 237], EQCM was also used under an alternative regime, ac electrogravimetry, which allows the fluxes of different ions taking part in the charge compensation process to be separated [43,98,99,107,108,184,205,238-242]. 

In fact, ac electrogravimetry is the combination of electrochemical impedance spectroscopy with a fast quartz crystal microbalance. The fluxes of all mobile species are considered, and the usual conditions and treatments of EIS are applied. Beside the electrochemical impedance, an electrogravimetric transfer function, Aw/A ((o), can be derived which contains the dependences of the fluxes of anions, cations and solvent molecules, respectively, on the small potential perturbation. The complex plane plot representations of electrogravimetric transfer functions for PANI are shown in Figs. 3.17 and 3.18. 

C. Gabrielli, M. Keddam, F. Minonflet, and H. Perrot, ac electrogravimetry contribution to investigation of the anodic behaviour of iron in strlfuric meditrm, Electrochim. Acta 41 2 1 996). 

To Thi Kim L, Debiemme-Chouvy C, Gabrielli C, Perrot H (2012) Redox switching of heteropolyanions entrapped in polypyrrole films investigated by ac electrogravimetry. Langmuir 28 13746-13757... 

Finally, application of other methods of analysis can be recommended. Many of the previous electrochemical studies devoted to conducting polymers were carried out in combination with radiotracer technique, AC electrogravimetry, quartz-crystal microbalance, surface plasmon resonance, and even ellipsometry, and atomic-force microscopy. In this context application of emerging experimental techniques such as local EIS and nonlinear impedance analysis may also be recommended. 

AC electrogravimetry consists in the simultaneous measurements of the electrochemical impedance and the mass/potential, or electro-gravimetric, transfer function. Impedance measurements are already well documented in the literature hence, in this text, only the electrogravimetric transfer function measurement will be described. It uses a fast QCM whose general concepts are first given below. 

The principle of AC-electrogravimetry measurements is given in Fig. 14. The modified QCM working electrode was polarized at a chosen potential V and a sinusoidal small-amplitude potential... 

Figure 14. The AC-electrogravimetry and electrochemical impedance set-up. From Gabrielli et al. ...

AVi/AV co) is the raw experimental transfer function obtained during the AC-electrogravimetry measurements. 

Some examples of the applications of AC electrogravimetry to the study of the redox switching of inorganic and organic films are now reported. First, details about Prussian blue, where mainly cation movement can be detected, will be given. Then, the behaviom of PPY, where cations, anions and solvent participate in the charge compensation process, will be described. Finally, a system of sandwiched polymers will be analysed. 

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