**Electrochemical Impedance Spectroscopy Study of the Mass Transfer in an Anode-Supported Microtubular Solid Oxide Fuel Cell**

Hironori Nakajima *Kyushu University Japan*

#### **1. Introduction**

284 Mass Transfer - Advanced Aspects

Willit J. (2005). 7th International Symposium on Molten Salts Chemistry & Technology,

Yamana, H., Fujii, T. & Shirai, O. (2003). UV/Vis Adsorption Spectrophotometry of some

*Honor of Marcelle Gaune-Escard,* Carry le Rouet, France, June 2003.

*Laboratory,* Toulouse, France, August 2005.

*Proceeding of Overview and Status of Pyroprocessing Development at Argonne National* 

f-elements in Chloride Melt, *Proceeding of International Symposium on Ionic Liquids on* 

Solid oxide fuel cell (SOFC) has advantages including high efficiency power generation by operation at 500-1000 ◦C, which results in a low environmental load. Moreover, SOFCs provide high quality waste heat, and the use of hydrocarbon fuels such as city gas, liquefied petroleum gas, and alcohol is relatively easy. However, for practical use, optimization of the electrode and electrolyte materials and the structure of the cell are required to improve the performance. In addition, optimization of the operation conditions and improvement of cell durability need to be addressed.

Mass transfers of the fuel and oxygen at the anode and cathode, respectively, greatly affect the cell performance by giving rise to the concentration overpotentials which result in the voltage loss of the cell. The concentration overpotentials including the Nernst loss by the fuel and oxygen depletions in the cell (Li, 2007; Morita et al., 2002) also affect the cell durability since they cause current distribution which leads to temperature distribution in the cell and anode oxidation. The current distribution also prevents effective use of whole electrode areas in a cell geometry.

This chapter describes the concentration overpotentials and current distribution in an intermediate temperature anode-supported microtubular SOFC which can be operated in the temperature range of 500-800 ◦C (IT-SOFC) with an analysis by electrochemical impedance spectroscopy (EIS) and cell surface temperature measurements.

EIS has been widely employed for the analysis of fuel cells. In particular, the author's group has developed diagnosis methods of operating status of the polymer electrolyte fuel cell (PEFC) by analyzing the variation of resistances and capacitances of equivalent circuit models of the PEFC (Konomi & Saho, 2006; Nakajima et al., 2008).

For the SOFC, a number of EIS analyses have been reported (Barsoukov & Macdonald, 2005; Esquirol et al., 2004; Horita et al., 2001; Huang et al., 2007; Ishihara et al., 2000; Jiang, 2002; Leonide et al., 2010; McIntosh et al., 2003). Although many of those reports focused on the characterization of developed materials, there were very few reports (Barfod et al., 2007) that analyze each impedance of the anode and cathode in the full cell impedance of a practical cell simultaneously and separately by applying EIS under operation.

O2 + N2

*R*hf

*C*hf

Fig. 1. Experimental set-up of the microtubular SOFC.

and equivalent circuit.

the axial direction of the cell.

Fig. 2. Equivalent circuit of an SOFC.

Thermocouple

of the Mass Transfer in an Anode-Supported Microtubular Solid Oxide Fuel Cell

Quartz tube

Fuel (H2

<sup>287</sup> Electrochemical Impedance Spectroscopy Study

 + N2 )

Voltage

present chapter, the equivalent circuit is not separated into the charge and mass transfer processes since the complex plane plots exhibited only two arcs, whose behavior should be analyzed with simple one R-C branch prior to appropriate separations of overlapping arcs

The Nernst loss by the partial pressure gradient of hydrogen and oxygen ascribed to their consumption leads to current distribution in the axial direction. Ohmic resistance in the anode and cathode electrodes is also attributed to this current distribution. However, the author use the above equivalent circuit for uniform current distribution to obtain average behavior over

As a result, the variations in these circuit parameters are obtained in accordance with current densities, and anode and cathode gas-feed conditions. In addition, the impedance of the mass transfer process can be analyzed as that of the finite length diffusion (Nakajima et al., 2010).

*R*Ohm

Current

Potentio/

galvanostat

Frequency response analyzer

Electric Furnace

*R*lf

*C*lf

EIS with two-electrode set-up on the practical microtubular IT-SOFC was thus carried out. To evaluate the impedance variation of each part of the cell under operation, gas feeding conditions for the anode and cathode were varied.

In addition, very few experimental studies on current distributions in a cell that lead to temperature distributions have been reported to date, although a number of computational analyses have been reported (Campanari & Iora, 2004; Costamagna & Honegger, 1998; Kanamura & Takehara, 1993; Nishino et al., 2006; Suzuki et al., 2008). Thus the current distributions are estimated by using the overpotentials evaluated with EIS.
