18th European Symposium on Computer Aided Process Engineering – ESCAPE 18 Bertrand Braunschweig and Xavier Joulia (Editors) © 2008 Elsevier B.V./Ltd. All rights reserved.
Modeling comparison of high temperature fuel cell performance: electrochemical behaviours of SOFC and PCFC Jean-Marie Klein and Jonathan Deseure LEPMI, UMR 5631 CNRS-INPG-UJF, ENSEEG, BP 75, 38402 Saint Martin d’Hères, France
Abstract
Fuel cells are highly efficient in terms of energy production, due to their high power efficiency. Solid Oxide Fuel Cells or Protonic Ceramic Fuel Cell are often proposed. The present paper aims to report on the electrochemical performance comparison between both systems by a Computational Fluid Dynamics approach. The developed model consists of mass, energy balances, and an electrochemical model that relates the fuel, air gas composition and temperature to voltage, current density, and other relevant fuel cell parameters. The electrochemical performances of SOFCs and PCFCs are analysed for several flow configurations. The simulations show that, the flow management should be an essential key during the design optimization. In the PCFC operating conditions, steam is produced at the cathodic side and an excessive steam can involve clogging of PCFC cathode. As a result, electrochemical performance of PCFCs decreases more than SOFCs executions. Keywords: Modeling, CFD, Fuel Cell, SOFC,PCFC .
1. Introduction
In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research. Fuel cells are highly efficient in terms of energy production, emit little noise and are non-polluting. The development of solid oxide fuel cells (SOFCs) has reached its new stage with intermediate temperature SOFCs (IT-SOFC). Unfortunately, poor performance can be observed due to the low ionic conductivity of electrolyte at these temperatures. Thus a new class of fuel cells is developed, based on ceramic electrolyte materials that exhibit high protonic conductivity at intermediate temperatures. The protonic ceramic fuel cell (PCFC) is fundamentally different because 2 J-M. Klein et al.
it relies on proton conduction through the electrolyte and not oxygen ions like SOFCs. Few experimental results on PCFC are reported in literature [1]. However, both SOFC and PCFC operations must be taken into account to clarify the fuel flow management of these devices. It can be added that meaningful flux descriptions are required in order to compare these technologies and to determine the limiting process. The aim of this study is to simulate both PCFC and SOFC operations to assess current, temperature and concentration distributions. We have used a CFD RC commercial package to help decision making on some important flow configurations. It should be noted that this study is theoretical and is based on literature data [2-3].
2. Description of the planar fuel cells SOFC
A planar high temperature fuel cell (700°C for PCFC and 800°C for SOFC) is an assembly of an electrolyte sandwiched between a porous anode and a porous cathode. The oxidant gas and fuel are respectively introduced at the cathodic and anodic side: at the same border (co-flow configuration), at the opposite borders (counter flow) and at the orthogonal borders (cross flow). The cell configuration for modeling is a 15 cm square and the thicknesses of anode, electrolyte, cathode and gas channel are respectively set to 30 m, 50 m, 500 m and 500 m. In the present model, a finite volume method using a computational grid [4] allows solving mass, charge, energy, momentum balances including transport through porous media, chemical and electrochemical reactions within porous electrodes in a gas diffusion electrode model. This model deals with three-dimension geometry in steady state conditions for PCFCs and SOFCs. The O2/N2/H2O and H2/H2O mixtures are respectively supplied at the air cathode and at the anode gas channel. In the gas phase, the mass conservation equation is described as follows [4]. It can be noted that all parameters definitions are presented in the work of Klein et al. [4]. . v 0 t (1)
By neglecting compressibility, turbulence effects, the conservation equations for the transport of Nth species, in a porous media, can be presented in the following vector form: Modeling comparison of high temperature fuel cell performance: electrochemical behaviours of SOFC and PCFC 3 2 v v . v.v -p . (2) t
Gas transport within the porous electrode is described by using the Stefan-Maxwell diffusion with convective transport wherein the electrochemical reaction occurs at the triple phase boundary i.e. at the interface between electronic and ionic conductor and gas phase. Current density (related to charge transports) is thus the sum of two
-1 indissociable but different contributions: ionic and electronic conductivities as, aM (
-1 m ) and potentials as and aM (V). For PCFC or SOFC the hydrogen is electrochemically converted into electricity within the anode. Nevertheless, two kinds of electrochemical conversion could be distinguished. The expression of hydrogen oxidation is given by:
1 H 1 O2 1 H O e- 1 H H e- for SOFC 2 2 2 2 2 and for PCFC 2 2 (3-4)
The corresponding kinetics of electrochemical reactions (Eq. (3) and Eq. (4)) within the porous electrode is described by a Butler-Volmer equation at the triple phase boundary :
SOFCand PCFC SOFC SOFC or PCFC F [H ] SOFC or PCFC F [H O] j jSOFC or PCFC exp a 2 exp c 2 at a0 RT a [H ] RT a [H O] 2 0 2 0 (5)
Here the exchange current density is expressed in A m-2. The overpotential (Eq. (5)) is defined as the difference of electronic and ionic potential as:
a aMasEa0 (6)
Ea0 is the potential difference between the electrolyte and the nickel in equilibrium, i.e., when no current is produced. If we consider an electrochemical reaction occurring at anode, charge conservation may thus be expressed from the Ohm’s law as Eq. (7) and mass balances for each gas phase species i is given at steady-state by Eq. (8): S . as as . aM aM jt (7) V eff
'' ' S jt . v w i . J i Mi (i i ) (8) V eff F
On the Eq. (8) the first member corresponds to convection and second term of Eq. (9) 4 J-M. Klein et al.
standing for diffusion of species i uses the effective mass diffusion coefficient D i,eff within the porous medium to be deduced from the free stream diffusion coefficient Di by the so-called Bruggeman model [5]:
J i Di,eff w i w i D j,eff w j Di,eff Di (9-10) j
Oxygen reduction reaction occurs at cathode from electrochemical reactions as:
- 2 - for SOFC 1 4 O 2 e 1 2 O and for PCFC 1 4 O2 e H 1 2 H 2 O (11-12)
The exchange current jct at cathode is then obtained from a Butler-Volmer equation similar to Eq. (5). The electrolyte material, which is a suitable ionic conductor at high temperature, is completely impermeable to electrons circulation. The electrolyte potential is thus expressed by a classical Ohm’s law without any charge creation or consumption within the electrolyte.
3. Result and discussion
Figures exhibit current density at the anode current collector surface for all flow configurations for a cell potential set to 0.3 V. In the case of co-flow configuration (Fig. 1), a linear gradient of current density of 3 mA cm-2 per cell centimeter have been observed for PCFC and SOFC. Nevertheless, the average current density of SOFC is upper than PCFC operation, the order magnitude of this difference is equal to 0.1 A cm-2 .This gradient is due to fuel and oxidant consumption along the cell. Moreover, at the cathodic side of PCFC steam is produced and the kinetics of oxygen reduction is slower than the kinetics of hydrogen oxidation. Therefore, two limiting processes occur at PCFC cathode: oxygen reduction and oxygen access to the active area. For SOFC, water production appears at anodic side and the hydrogen conserves a better access to the catalytic area than oxygen in air steam mixture of PCFC. Indeed SOFC have a high kinetics of oxygen reduction and a good access of oxygen to the active sites. Finally we can observe (figure 2 and figure 3) that for counter flow and cross flow configurations, gradients are higher, and the maximum current density increases. Accordingly Larrain et al. [6], flow configurations influence the cell performance in case of SOFC. The SOFC mode shows a largest homogenous area of current density than PCFC mode. Modeling comparison of high temperature fuel cell performance: electrochemical behaviours of SOFC and PCFC 5
a) b)
Figure 1. Current density for the co-flow configuration a) PCFC b) SOFC
a) b)
Figure 2. Current density for the counter flow configuration a) PCFC b) SOFC
a) b)
Figure 3. Current density for the cross flow configuration a) PCFC b) SOFC 6 J-M. Klein et al.
4. Conclusion
These first results show for similar gradients of current density in SOFC mode and PCFC mode, that average current densities of SOFC are higher than PCFC. These gradient are due to the reactant access, in the case of a PCFC the current density is lower than SOFC case, but the gas access have the same limiting value. The steam can clog the PCFC cathode, and the simulations have emphasized that it is relevant to operate in cross flow configuration for PCFC. The development of high temperature fuel cell depends on operating temperature and under this condition PCFCs are a promising attainment (700°C for PCFC versus 800°C for SOFC).
+References
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