Electrochemical impedance study of the poisoning behaviour of Ni-based anodes at low concentrations of H2S in an MCFC

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Abstract

The effect was investigated of low H2S concentrations, simulating biogas impurity, on the poisoning behaviour of a Ni-based, molten carbonate fuel cell anode. A conventional Ni–Cr anode was coated with ceria using dip coating to form a rare earth metal oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere (H2, CO2, H2O and N2 as balance) with 2, 6, 12, and 24 ppm of H2S by means of electrochemical impedance spectroscopy.

The recorded data showed that the poisoning resistance was enhanced at low coating percentages of ceria; effects depend on H2S concentration and the applied load. These results were confirmed by electrochemical impedance tests where the cerium oxide addition appears through stable polarization behaviour up to 6 ppm of H2S, particularly in the mass transfer region. The protection is explained by the depression of poisoned Ni active sites and formation of a protective layer to the metal surface. The ceria coating layer is a potential solution to reduce H2S poisoning of MCFCs fuelled with biogas.

Highlights

► Low H2S concentrations was used to simulate biogas impurity. ► Dip coating method was carried out to form a cerium oxide thin layer on the surface of the anode. ► Electrochemical studies was performed in symmetric and button cells under anode atmosphere. ► The cerium oxide addition forms a protective layer to the metal surface. ► The ceria coating layer is a potential solution to reduce H2S poisoning of MCFCs fuelled with biogas.

Introduction

One of the ways to maintain the sustainability of energy production is by utilizing renewable energy. This approach has additional advantage to reduce CO2 emissions according to Kyoto protocol, particularly when the energy efficiency can be increased. A fuel cell, an electrochemical device that can convert chemical energy directly into electricity, can be one of the solutions, particularly when coupled with biomass as alternative energy source. The Molten carbonate fuel cell (MCFC) is a kind of fuel cell that operates at high temperature (650 °C) with specific use in stationary power generation [1], [2], [3], [4].

Since MCFC operates at relatively high temperature, it can be fuelled with various sources of energy such as hydrogen, methane, methanol, gasoline, diesel fuel, biogas, natural gas, coal gas, etc. Biogas from anaerobic digestion of organic wastes is a promising candidate during transition from fossil to renewable energy sources. Even though MCFC is able to utilize them, it is also easily poisoned by some compounds inherent to these fuels. The most notable poison in biogas are sulphur-related compounds. In this paper, the scope is limited only to H2S since its presence can decrease the performance and limit the lifetime of the fuel cell system [5]. In other words, despite development of highly efficient clean-up system with low cost, research on modified anodes with high tolerance towards H2S is very important [6], [7], [8], [9]. The poisoning effect of low H2S concentrations, simulating biogas impurity levels, on the behaviour of Ni–Cr and ceria-modified anodes was investigated, based on the evidence that the latter, show better resistance compared to conventional ones [10], [11], [12].

Section snippets

Pre-experiment and symmetric cell experiment

A conventional Ni–Cr anode was coated with 1.5 wt% ceria using dip coating to form a rare earth metal (cerium) oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere with 48 ppm of H2S by means of electrochemical impedance spectroscopy. In order to investigate the effect of poisoning, our approaches are to set up protocols for accurate measurement and diagnosis of poisoning effects using so-called

Pre-experiment analysis

Fig. 2 shows Nyquist plots of a conventional Ni–Cr anode in symmetric cell configuration, as a function of time. The cell was exposed to so-called CC1 gas composition, which consists of H2, CO2, and H2O with composition 25%, 4%, and 40%, respectively, and with N2 as balance, and which represents the real gas coming from an anaerobic digestion system,. Scattered data were observed in the low frequency area, suggesting the diffusion region was not smoothly measured. In addition, the ohmic

Conclusions

EIS analysis on symmetric cells seems to be a reliable method to screen candidates of modified anodes that show resistance to H2S poisoning. Higher water content accelerated the effect of corrosion as well as produced scattered data during EIS measurements. Optimized gas composition gave less fluctuation in EIS spectra. The recorded data showed that the poisoning resistance was enhanced at low coating percentages of ceria; effects depend on H2S concentration and the applied load. The ceria

Acknowledgement

Authors would like to thank Roberto Lo Presti for his help throughout, Massimo de Francesco for XRF analysis, and Masci Amedeo for SEM analysis.

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