Energy, Resources and Environmental Technology
Thermodynamic Analysis of Methane-fueled Solid Oxide Fuel Cells Considering CO Electrochemical Oxidation

https://doi.org/10.1016/j.cjche.2014.06.018Get rights and content

Abstract

Thermodynamic analyses in the literature have shown that solid oxide fuel cells (SOFCs) with proton conducting electrolyte (H-SOFC) exhibited higher performance than SOFC with oxygen ion conducting electrolyte (O-SOFC). However, these studies only consider H2 electrochemical oxidation and totally neglect the contribution of CO electrochemical oxidation in O-SOFC. In this short communication, a thermodynamic model is developed to compare the theoretically maximum efficiencies of H-SOFC and O-SOFC, considering the electrochemical oxidation of CO in O-SOFC anode. It is found that O-SOFC exhibits a higher maximum efficiency than H-SOFC due to the contribution from CO electrochemical oxidation, which is contrary to the common understanding of electrolyte effect on SOFC performance. The effects of operating temperature and fuel utilization factor on the theoretical efficiency of SOFC are also analyzed and discussed.

Introduction

Solid oxide fuel cells (SOFCs) have been identified as efficient and environmental-friendly electrochemical devices for power generation. Compared with low temperature fuel cells (such as proton exchange membrane fuel cells), one attraction of SOFCs is their fuel flexibility. At typical working temperatures (i.e. 1073 K), hydrogen fuel or hydrocarbon fuels, such as methane and ethanol, can be utilized in SOFC for power generation as internal reforming of hydrocarbon fuels can occur in the porous anode of SOFC [1], [2], [3], [4]. Electrolyte is a key component of SOFC and can be built with oxygen ion conducting ceramics (i.e. yttria-stabilized zirconia, YSZ) or proton conducting materials, such as BaCeO3 doped with Gd or Nd. The use of different electrolytes not only yields different ohmic losses, but also influences the mass transfer in porous electrodes, as steam is produced in the cathode of H-SOFC, which in turn impedes the diffusion of oxygen [5], [6].

In order to clarify the differences between H-SOFC and O-SOFC and to identify suitable electrolyte materials for SOFC operation, several thermodynamic analyses have been performed for both H2 and hydrocarbon fueled SOFCs. Demin and Tsiakaras were the first who thermodynamically compared the maximum efficiencies of H-SOFC and O-SOFC fed with hydrogen fuel [7]. It was found that the hydrogen fed H-SOFC had an essential advantage as compared to O-SOFC, due to a higher hydrogen concentration in the anode of H-SOFC [7]. In a subsequent study, the model was extended to investigate the methane fed H-SOFC and O-SOFC [8]. It was found that the maximum efficiency of methane fed H-SOFC was evidently higher than that of O-SOFC [8], [9]. In other thermodynamic analyses of SOFCs fed with ethanol or ammonia fuels, it was also observed that H-SOFC performed better than O-SOFC in terms of maximum efficiency due to a higher hydrogen concentration in the anode of H-SOFC [10], [11], [12].

However, in the abovementioned thermodynamic analyses on methane or ethanol fed SOFCs, only H2 electrochemical oxidation was considered while the contribution of CO electrochemical oxidation to O-SOFC power generation was totally neglected. From the experiments, it has been confirmed that CO electrochemical oxidation could occur in the anode of O-SOFC, although its reaction kinetics was slower than that of H2 electrochemical oxidation [13]. Since CO electrochemical oxidation in H-SOFC would not occur, it is still unclear whether the methane fed H-SOFC is superior to O-SOFC in terms of maximum efficiency if H2 and CO electrochemical oxidation reactions are considered for O-SOFC. In this short communication, a simple thermodynamic model is developed to compare the maximum efficiencies of methane fed H-SOFC and O-SOFC, considering the CO electrochemical oxidation in the anode of O-SOFC. Due to a lack of experimental data, comparison of the present simulation results with literature data is not provided. However, comparison can be easily made once the relevant data are available.

Section snippets

The Model

The present thermodynamic analyses are based on the assumption of chemical and electrochemical reactions. The working principles of the methane fed H-SOFC and O-SOFC are shown in Fig. 1(a) and (b), respectively. In operation, H2O–CH4 mixture at a molar ratio of 2:1 is supplied to the anode channel of SOFC while air is supplied to the cathode. In both H-SOFC and O-SOFC, direct internal reforming (DIR) and water gas shift reaction (WGSR) take place in the porous anode, as shown by Reactions (1),

Results and Discussion

In this section, parametric simulations are performed to compare the maximum efficiencies of H-SOFC and O-SOFC running on methane fuel. In this study, the operating temperature (T) and the value of r are varied to examine their effects on the theoretically maximum efficiency of SOFC running on CH4.

Conclusions

A simple thermodynamic model is developed to compare the maximum efficiencies of methane fed H-SOFC and O-SOFC with consideration of CO electrochemical oxidation in O-SOFC anode. It is found that H-SOFC shows a higher maximum efficiency than O-SOFC when CO electrochemical oxidation is excluded, which is consistent with the results in the literature. When CO electrochemical oxidation in O-SOFC is considered, the maximum efficiency of O-SOFC is obviously higher than that of H-SOFC, and this

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Supported by Hong Kong Research Grant Council (PolyU 5238/11E).

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