Electrochemical Analysis for the Identification of Electrode Reaction in Ni-YSZ Anode as for Solid Oxide Fuel Cell

The most widely studied anode for oxidation of hydrogen in solid oxide fuel cells (SOFCs) is the Ni/YSZ cermet. A detailed understanding of the rate-limiting steps in Ni-YSZ anodes is highly desirable from an optimization point of view. One of the most promising way of complex system is to define the reaction by impedance spectroscopy.

In principal, impedance measurement is comprised of electrolyte response, electrode response, and transport limitations for supply of reactants and removal of products. The electrolyte response is readily separated out as a series resistance, in simple geometries it can even be used to indicate the magnitude of the microscopic electrode contact area. For high temperature impedance model, transport restriction are believed to be negligible in typical setups. But anode with low resistance transport restrictions may contributes significantly.

The electrode reactions like charge-transfer reaction are suggested to occur in the vicinity of the active triple-phase boundary (TPB) line. The active TPB is defined as the line where the Ni network, the YSZ network, and open pores meet to form percolation paths for transport of electrons, oxide ions, and gas respectively. The reaction resistance related to any process occurring across the active TPB is coupled in parallel between Ni and YSZ.

This paper (i) brings forth a method of rationalizing impedance data and (ii) presents relevant data for the oxidation of hydrogen on Ni/YSZ cermet anodes produced by tape casting. In the analysis of the obtained impedance spectra, the distribution of relaxation times (DRT) was calculated in order to perform a pre-identification of the physico-chemical processes occurring in the cell. The impedance data are rationalized by fitting to a representative equivalent circuit, thus allowing the dependency of three processes on typical parameters to be derived.