# SOFC fuelled by methane without coking: optimization of electrochemical performance

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## Abstract

In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research as fuel cells produce electric energy with high efficiency, emit little noise, and are non-polluting. Solid oxide fuel cells (SOFCs) are particularly important for stationary applications due to their high operating temperature (1,073–1,273 K). Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. Unfortunately, internal steam reforming in SOFC leads to inhomogeneous temperature distributions which can result in mechanical failure of the cermet anode. Moreover this concept requires a large amount of steam in the fed gas. To avoid these problems, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained by the hydrogen oxidation. However, with GIR, Boudouard and cracking reactions can involve a risk of carbon formation. To cope with carbon formation a new cell configuration of SOFC electrolyte support was studied. This configuration combined a catalyst layer (0.1%Ir–CeO_{2}) with a classical anode, allowing GIR without coking. In order to optimise the process a SOFC model has been developed, using the CFD-Ace+ software package, and including a thin electrolyte. The impact of a thin electrolyte on previous conclusions has been assessed. As predicted, electrochemical performances are higher and carbon formation is always avoided. However a sharp decrease in the electrochemical performances appears at high current densities due to steam clogging.

## Keywords

SOFC Gradual internal reforming Simulation CFD-Ace+## List of symbols

*D*_{i}Diffusion coefficient of the

*i*-th species (m^{2}s^{−1})*D*_{i,eff}Effective diffusion coefficient of the

*i*-th species (m^{2}s^{−1})*E*_{0}Voltage between the electrolyte and the nickel at equilibrium (V)

*F*Faraday constant (96500) (C mol

^{−1})*J*_{i}Diffusion flux of the

*i*-th species (mol m^{−2}s^{−1})*K*_{E1}Equilibrium constant of steam methane reforming reaction (Pa

^{2})*K*_{E2}Equilibrium constant of water gas shift reaction (–)

*K*_{B}Equilibrium constant of Boudouard reaction (Pa

^{−1})*K*_{C}Equilibrium constant of Cracking reaction (Pa)

*M*Molecular weight of the mixture of gases (kg kmol

^{−1})*P*_{i}Partial pressure of the

*i*-th species (Pa)*R*Universal gas constant (8.314) (J mol

^{−1}K^{−1})- (
*S*/*V*)_{eff} Effective surface-to-volume ratio (m

^{2}m^{−3})*T*Temperature (K)

*a*_{c}Carbon activity (–)

*d*_{pore}Pore diameter (M)

*H*Gas mixture enthalpy (J kg

^{−1})*h*_{B}Solid-phase enthalpy (J kg

^{−1})*i*Current density (A m

^{−2})- [
*i*] Molar concentration of the

*i*-th species (kmol m^{−3})- [
*i*]_{0} Reference concentration of the

*i*-th species (kmol m^{−3})*j*_{a0}Exchange current density of the anode (A m

^{−2})*j*_{at}Faradaic current due to anodic reaction (A m

^{−2})*j*_{ct}Faradaic current due to cathodic reaction (A m

^{−2})*P*Total pressure (Pa)

*Q*Heat flux (W m

^{−2})*r*_{k}Reaction rate of the

*k*-th reaction (Kmol m^{−3}s^{−1})*U*_{cell}Cell potential (V)

*v*Fluid velocity (m s

^{−1})*w*_{i}Mass fraction of the

*i*-th species (–)*x*_{i}Molar fraction of the

*i*-th species (–)

## Greeks

*α*Boudouard coefficient (–)

*α*_{a}Anodic Tafel constant (–)

*α*_{c}Cathodic Tafel constant (–)

*β*Cracking coefficient (–)

*δ*Shear stress tensor (–)

*ε*Porosity (–)

*ϕ*_{as}Ionic phase potential at anode (V)

*ϕ*_{aM}Electronic phase potential at anode (V)

*ϕ*_{es}Ionic phase potential of the electrolyte (V)

*γ*Boudouard + Cracking coefficient (–)

*λ*_{B}Thermal conductivity of the solid phase (W m

^{−1}K^{−1})*η*_{a}Anode overpotential (V)

*κ*Permeability (m

^{2})*μ*Dynamic viscosity of the gas mixture (Pa s)

*μ*_{i}Dynamic viscosity of the

*i*-th species (Pa s)*ρ*Mass density of the gas mixture (kg m

^{−3})*σ*_{as}Ionic phase conductivity at anode (Ω

^{−1}m^{−1})*σ*_{aM}Electronic phase conductivity at anode (Ω

^{−1}m^{−1})*σ*_{es}Ionic phase conductivity of the electrolyte (Ω

^{−1}m^{−1})*τ*Tortuosity of pores (m m

^{−1})

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