Hydrogen sulfide-resilient anodes for molten carbonate fuel cells
Highlights
► Systematical study of Fe introduction into conventional Ni–Al anode. ► Improvement of creep resistance and tolerance to H2S. ► High long term stability of cell with low voltage loss of ca. 5 mV. ► Significant recovery of the modified anodes from H2S poisoning effect.
Introduction
Ni–Cr is a state-of-the-art anode material for modern MCFC systems [1]. Ni–Cr also reportedly, however, 'consumes' electrolytes when lithiation occurs in the system. Therefore, Ni–Al has become the preferred choice in anode material selection and has received widespread attention due to its high durability and effectiveness in resisting the creep of stack loadings [2], [3], [4], [5], [6], [7].
Because MCFCs have high operating temperatures (923 K), natural gas, coal gas and bio-fuels are some of the fuel options for anodes. These fuels contain a variety of contaminants and impurities, such as sulfur compounds (H2S, COS, etc.), halides, tars, dust, ammonia, and soloxanes. It is known that H2S is the most harmful impurity for cell performance. The total sulfur content of pipeline natural gas is on the order of 10–80 ppm, with 4 ppm H2S and 4 ppm mercaptans [8]. The hydrogen sulfide concentration in bio-fuels such as landfill gas or anaerobic digestion product gas can be as much as 200 ppm [9], [10]. The product gases from coal gasification typically contain 0.1–1% or more H2S [11]. Before they are used in MCFCs, first stage cleanup processes are applied, during which the tolerance limit of H2S concentration for MCFCs varies from 0.01 ppm to less than 10 ppm [12], [13], [14]. The tolerance limit of H2S, however, depends on the technology, operating conditions and hydrogen gas concentration. If the cleanup processes are not properly completed, higher amounts of hydrogen sulfide gas (>10 ppm) can severely poison the anode material.
Due to a very low dissociation barrier of Ni with H2S, the rapidly strong absorption of H2S on an anode surface has been found [15], [16] to block the active sites, cover the pore network, increase cell polarization, change the wettability of the electrolyte, and thus degrade the cell performance [17], [18], [19], [20]. In the presence of H2S, the reactions that occur in the electrolyte and anode are as follows:
With electrolytewith anode
Under MCFC working conditions, Al in the Ni-based matrix diffuses out from the anode surface to react with water vapor and form an oxide layer. This alumina layer functions as a protective layer and inhibits impurity invasions from materials such as oxygen, carbon or sulfur [21], [22], [23]. It is not sufficient, however, to stop the effects of H2S poisoning. It is well-known that Fe2O3 is an excellent desulfurization reagent for H2S removal from waste gas or natural gas at high temperatures [24]. The effect of Fe2O3 when water is present during H2S regeneration has been discussed [25], [26], and studying the effects of Fe incorporated into conventional MCFC anodes on the resistance to H2S poisoning is meaningful.
In this work, a Ni–Al–Fe alloy anode is prepared. Fe is introduced not only because of its excellent desulfurization capability, but also because of its ability to facilitate hydrogen diffusion [27], [28], [29], [30]. In order for the newly fabricated Ni–Al–Fe alloy anode to function properly, it must improve creep resistance, tolerate H2S poisoning and improve overall electro-chemical performance. All of these factors are evaluated using a single cell test.
Section snippets
Anode fabrication
The raw material for anode tape casting was prepared by mixing Ni-5 wt% Al powder (Twin Energy, >99%, 3–5 μm) with various contents (10–30 wt.%) of Fe powder (>99%, 3–5 μm) and Ni powder (Inco type 255, >99%, 3–5 μm). The green anode sheet prepared by tape casting was sintered at 1223 K under an H2/N2 (10/90) atmosphere for 0.5 h to prepare the alloy. The anode without the addition of Fe was designated as 0Fe sample; the anodes with 10 wt.% and 30 wt.% Fe content were designated as 10Fe and
Results and discussion
Porosity and pore diameter are important factors that significantly affect mass transfer and cell performance [1]. The porosity and mean pore diameter of the sintered anodes with varying iron contents are shown in Table 1. The microstructural properties of the modified anodes are similar to those of the conventional anode (0Fe). It was expected that the mass transfer process would not change after modification with Fe. The XRD patterns for the samples at wide (Fig. 1a) and narrow (Fig. 1b)
Conclusions
Despite being susceptible to the poisoning effect of H2S, anodes containing Fe are found to be more resilient than the anodes that did not contain Fe. The Fe anodes recover more quickly and deposit less sulfur over the cell test operation period. The presence of Fe in the anode samples significantly improves the creep resistance, leading to an overall improvement in cell test performance. Therefore, the Ni–Al–Fe anodes proposed in this study are recommended for MCFCs that are fueled by hydrogen
Acknowledgments
This research was supported by the Renewable Energy R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Republic of Korea (No. 20113030030040).
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