Impedance Spectroscopy Becomes Limited

Cases in which Impedance Spectroscopy Becomes Limited. One might say that if one understands an interface well, the results of Z-to measurements can be readily understood. Of course, the interest is in the other direction, in using Z-to plots when one does not understand the interlace. Then the task is to find an interfacial structure and mechanism (and its resulting equivalent circuit) that provides a Z that is consistent in its dependence on to with the experimental results of the impedance measurement. This requires finding reasonable parameters to fit the value of the C s and R s as a function of to for the individual elements in the various equivalent circuits. If the shape of the calculated Z-to plot can only be made to match experiment by using C s and R s that are physically unreasonable, the proposed structure and the equivalent circuit to match it are not acceptable and another must be tried. 

Finite-space diffusion takes place during the charging of insertion electrodes at moderate frequencies, transforming into mainly capacitive behavior within the limit of very low frequencies, in contrast to the semi-infinite diffusion for solution redox-species (except for thin-layer solution electrochemistry) electrochemical impedance spectroscopy becomes a very useful diagnostic tool for the characterization of insertion mechanisms ... 

Whatever the limiting mechanism, ultimately the current becomes limited by concentration polarization, i.e., by the transport of redox species from the bulk electrolyte to the semiconductor surface. The situation in this regard is no different from that at metal electrode-electrolyte interfaces. As in the latter case, hydro-dynamic (specifically RDE, Table 2) voltammetry is best suited to study mass transport. AC impedance spectroscopy can be another useful tool in this regard [82]. 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

A plethora of literature exists that discusses how to relate the structure of the imjredance spectrum to the corrosion mechanism and corrosion rate. In the limit of zero frequency, the impedance becomes equal to the polarization resistance discussed above. Situations arise in which this value is not inversely related to the corrosion rate [5]. The reader is referred to two recent symposia on electrochemical impedance spectroscopy that provide excellent "snapshots of the state of the art in determining corrosion rates and mechanisms from the impedance spectra and provide information on the additional types of data needed to make the connection [6,7]. In addition. Ref 3 provides an overview of practical applications of this technology, as well as those mentioned previously [3]. 

---