Limiting Factors for a Planar Solid Oxide Fuel Cell Under Different Flow and Temperature Conditions P. Leone 1 *, T. Matencio 2 , M. E. Garciä 2 , Z. R. Domigues 2 , A. Lanzini 1 , M. Santarelli 1 1 Department of Energy, Politecnico di Torino, C.so Duca Degli Abruzzi 24, Torino 10129, Italy 2 Universidade Federal de Minas Gerais e UFMG, Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte, MG, CEP 13565-905, Brazil Received September 18, 2012; accepted April 04, 2013; published online June 21, 2013 1 Introduction Research on solid oxide fuel cells is focusing on the reduc- tion of operating temperature [1, 2], improvement of materi- als and structures aiming at increasing the cell robustness towards cycling [3], proving fuel flexibility and tolerance towards contaminants [4, 5]. To this end, it is necessary to provide proper testing and modeling tools to identify perfor- mance limiting factors of an in-operating fuel cell. Electro- chemical impedance spectroscopy (EIS) [6] is recognized as a powerful experimental method to analyze the operation of fuel cells for design, characterization and diagnostics [6–15]. In particular, several models have been proposed to deconvo- lute the spectra of a solid oxide fuel cell and identify the con- tribution of main cell functional layers to different perfor- mance losses [16–20]. The EIS of a fuel cell is sometimes carried out under transi- ents near equilibrium conditions [9, 21], i.e. the fuel cell open circuit voltage is measured and then held on the cell while the impedance test progresses. However, further information can be obtained by impedance analysis under load condi- tions, with the cell’s polarization being an independent vari- able [21]. The polarization (or overpotential) of a fuel cell is the potential difference between the thermodynamic open cir- cuit potential and the operational potential [22]. This voltage loss is a function of current density and depends on the pro- cesses that occur during the fuel cell operation; as these pro- cesses are many, it is generally difficult to interpret the impe- dance diagrams obtained during the fuel cell electrochemical characterization. However, three main potential losses [21] are recognized and related to (i) materials and interfaces ohmic resistances due to the transport of ions and electrons through the materials; (ii) concentration polarizations that are caused by the resistance to fuel and oxidant mass transport through the electrodes; (iii) activation polarizations related to the electrolyte/electrodes electrochemical reactions. In this way, impedance diagrams can at least provide a breakdown of the total loss into an ohmic resistance, R s , measured at high frequency, and a polarization resistance, R p , measured at me- dium and low frequency, reflecting losses due to chemical, electrochemical, and transport processes. Abstract The work investigates the performance of an anode sup- ported solid oxide fuel cell under relevant conditions at dif- ferent flow and temperature settings with the aim to identify performance limiting factors through impedance spectro- scopy. Impedance spectroscopy is used to deconvolute impedance spectra of an in-operating SOFC and identify limiting overpotentials. Those measurements are made under a wide range of flow and temperature conditions. In particular, oxidant flow rate is varied yielding fuel cell operation with 20, 40, 60, 80% oxidant utilization; fuel is instead changed in terms of flow and composition: fuel utili- zation factors in the range of 20–80% are investigated as well as the dilution with nitrogen. The operating temperature is varied in the range between 650 and 800 °C with steps of 50 °C. Results show that charge and mass transport can lead to a performance limitation according to the selected operat- ing range for the investigated cell design. For the investi- gated anode-supported design, a major improvement of per- formance could arise by reducing ohmic resistances (i.e. employing a thin electrolyte) and by an improvement of the anode geometry aiming at enhancing mass transport. In par- ticular, at low temperature and high fuel utilization, fuel oxi- dation seems to be a relevant performance limiting factor. Keywords: Electrochemical Impedance Spectroscopy, Experimental Results, Performance, SOFC – [ * ] Corresponding author, pierluigi.leone@polito.it FUEL CELLS 13, 2013, No. 5, 733–742 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 733 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.201200154