Encyclopedia of Membranes

2016 Edition
| Editors: Enrico Drioli, Lidietta Giorno

Impedance Spectroscopy, Membrane Characterization by

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DOI: https://doi.org/10.1007/978-3-662-44324-8_863
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Synonyms

 Electrochemical impedance spectroscopy;  Impedance spectroscopy

The impedance spectroscopy (IS), or electrochemical impedance spectroscopy (EIS), is a powerful technique for characterizing electrical behavior of systems in which coupled electrical processes occur at different rates (Barsoukov and Macdonald 2005). The IS technique is able to measure quantitatively the electrical resistance in the bulk and interfacial regions of solid and liquid electrolyte materials, including membranes.

IS technique is widely used in the in situ nondestructive characterization of membranes, as well as to investigate concentration polarization and fouling phenomena (Fontananova et al. 2012; Antony et al. 2013).

In EIS experiments, a sinusoidal electrical stimulus (voltage or current) is applied over a frequency range to a pair of electrodes, and the response of the system under investigation is observed by the same or different electrodes. In the first case, the configuration is indicated as a two-probe (or two electrodes) type (Fig. 1a). If two additional electrodes are used to collect the response of the system, the configuration is indicated as four-probe (or four electrodes) type (Fig. 1b). Another possible configuration uses three electrodes, but it is usually employed to characterize only one half of an electrochemical cell, or phenomena occurring on an electrode and it will not be discussed here. The two-probe configuration is usually applied when the membrane is pressed between two solid conductive electrodes, like in the case of the membrane electrode assembly (MEA) for fuel cells (Fontananova et al. 2012).
Impedance Spectroscopy, Membrane Characterization by, Fig. 1

Schematic view of the experimental setup used for membrane characterization by impedance spectroscopy using two-probes (a) and four-probes configuration (b)

On the contrary, if the membrane is in contact with a liquid electrolyte, the four-probe configuration is the most convenient and commonly used (Antony et al. 2013). This second configuration has the advantage to eliminate the contribution of the electrode injecting stimulus/electrolyte charge transfer resistance, from the impedance spectra, focusing the probing on the membrane and its interfaces.

The sinusoidal electrical stimulus is injected through two planar electrodes (working and counter electrode), and the response of the system to the sine wave perturbation is measured by two reference electrodes (indicated as sense and reference) using an impedance analyzer (usually a potentiostat/galvanostat combined with a frequency response analyzer) which measures voltage, current, and phase shift (Fig. 1).

In analogy to Ohm’s law, the impedance is defined as (Barsoukov and Macdonald 2005):
$$ {Z}_{\left(\omega \right)}=\frac{U_{\left(\omega \right)}}{I_{\left(\omega \right)}} $$
(1)
where Z(ω) [Ω] is the impedance, U(ω) [V] is the voltage drop, I(ω) [A] is the current, and they depend on the circular velocity or circular frequency ω [rad/sec] as follows:
$$ {U}_{\left(\omega \right)}={U}_o \sin \omega t $$
(2)
$$ {I}_{\left(\omega \right)}={I}_o \sin\ \left(\omega t+\varphi \right) $$
(3)
where ω = 2πυ and υ [sec−1] is the frequency, t [sec] is the time, φ [°] is the phase shift between voltage and current, and the subscript ° refers to the amplitude of voltage and current in phase; j is the imaginary number \( j=\sqrt{-1} \).
The impedance can be also rearranged as follows:
$$ {Z}_{\left(\omega \right)}=\left|Z\right| \cos \varphi +j\left|Z\right| \sin \varphi $$
(4)
The Eq. 4 indicates that the impedance is composed of two parts, i.e., the real part:
$$ {Z}^{\prime }=\left|Z\right| \cos \varphi $$
(5)
and the imaginary part:
$$ {Z}^{{\prime\prime} }=\left|Z\right| \sin \varphi $$
(6)
The real part of the impedance is the resistance (Z′); the imaginary part is called reactance (Z″).

On the contrary of the measurement in direct current (DC), using an alternate current (AC) over a frequency range, it is possible to distinguish phenomena proceeding at different rates, like bipolar concentration polarization of an ion exchange membrane (IEM) in contact with an electrolyte solution, which induces the formation of an electrical double layer (EDL) and a diffusion boundary layer (DBL) (Sang et al. 2008).

As a consequence, the total electrical resistance is the sum of the contribution for the transport through the membrane and the interfaces.

The bipolar concentration polarization during an AC cycle, caused by the buildup and depletion of ions at the interfaces, is time dependent. The contribution of the interfacial ionic charge transfer through the interfaces layers at low frequencies is greater than at high frequencies, because at high frequencies there is insufficient time for their formation.

The experimental impedance data can be fitted with equivalent electrical circuit models able to represent the physical system and phenomena under investigation (Zhang and Spichiger 2000).

The typical shape of the impedance spectra (Z″ vs. Z′) in the case of an IEM pressed between two solid electrodes exhibits a distinct arc (Fig. 2a). In the case of an IEM separating two liquid electrolyte solutions, additional semicircles appear in the spectra (Fig. 2b). The corresponding equivalent circuits are also shown in Fig. 2.
Impedance Spectroscopy, Membrane Characterization by, Fig. 2

Impedance spectra reported as imaginary (Z″) versus real part of the impedance (Z′) for an ion exchange membrane: pressed between two solid electrodes (a) and separating two electrolyte solutions (b). The spectra are registered respectively with the two- and four-probe configuration, and the corresponding equivalent circuit models are also shown. The resistor is indicated as R, the capacitor as C, and the constant phase element (a nonideal capacitor) as CPE. The subscript “m” is referring to the membrane, “ct” to the charge transfer between electrode and membrane, “m+s” to membrane plus solution, “edl” to the electrical double layer, and “dbl” to the diffusion boundary layer

At high frequencies (ω→∞) the intercept on the real axis gives the membrane (Rm) or membrane plus solution resistance (Rm+s). Of course, the solution resistance can be obtained by blank experiments (without the membrane) and its contribution can be subtracted to obtain the pure membrane resistance.

At low frequencies (ω→0), the intercept on the real axis gives the sum of membrane (plus solution, if present) and the interface resistances.

References

  1. Antony A, Chilcott T, Coster H, Leslie G (2013) In situ structural and functional characterization of reverse osmosis membranes using impedance spectroscopy. J Membr Sci 425–426:89–97CrossRefGoogle Scholar
  2. Barsoukov E, Macdonald JR (2005) Impedance spectroscopy. Theory, experiment, and applications, 2nd edn. Wiley, New JerseyCrossRefGoogle Scholar
  3. Fontananova E, Cucunato V, Curcio E, Trotta F, Biasizzo M, Drioli E, Barbieri G (2012) Influence of the preparation conditions on the properties of polymeric and hybrid cation exchange membranes. Electrochim Acta 66:164–172CrossRefGoogle Scholar
  4. Sang S, Wu Q, Huang K (2008) A discussion on ion conductivity at cation exchange membrane/solution interface. Colloids Surf A 320:43–48CrossRefGoogle Scholar
  5. Zhang Z, Spichiger UE (2000) An impedance study on Mg2+ selective membrane. Electrochim Acta 45:2259–2266CrossRefGoogle Scholar

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© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Institute on Membrane Technology, National Research Council of Italy, ITM-CNRRendeItaly