Combined Cr and S poisoning in solid oxide fuel cell cathodes

This work aims to compare the effect of combined chromium and sulfur contaminating conditions to the Cr contamination alone on the Cr poisoning mechanisms in (La,Sr)MnO3-(Zr,Y)O-2 solid oxide fuel cell (SOFC) cathodes. Whereas Cr2O3 and (Cr,Mn)(3)O-4 are found at active triple phase boundaries under the Cr-poisoning condition, the formation of SrCrO4 is promoted under combined Cr and S contaminating conditions, where Cr accumulations act as getters incorporating sulfur, to form Sr(Cr,S)O-4 compounds. The identification of this phase is validated on the synthesized and simulated species by scanning/transmission electron microscopy (SEM/TEM) techniques; its possible formation is predicted by thermodynamic analysis of the stability of perovskite compounds in the presence of combined Cr and S polluting conditions. In contrast, sulfur alone is not found to poison active sites in these composite cathodes. These findings suggest that the Cr poisoning degradation mechanism is altered when (La,Sr)MnO3 is exposed to Cr vapors in the presence of sulfur contamination; the access to electrochemical active sites may be hindered by the formation of Sr(Cr,S)O-4 in a similar manner to a Cr-getter effect in (La,Sr)(Co,Fe)O-3 cathodes. (C) 2011 Elsevier B.V. All rights reserved.

Experimental study of single triple-phase-boundary and platinum-yttria stabilized zirconia composite as cathodes for micro-solid oxide fuel cells

Janine Conde, Paul Muralt, Silviu Cosmin Sandu, Yan Yan

Micro-solid oxide fuel cell (mu SOFC) structures with annular Pt electrodes have been fabricated by means of silicon micromachining. The annular cathodes contained a defined triple phase boundary (TPB) length, which allowed the derivation of the ionic current per length of TPB as 1.24 mA m(-1), measured at 450 degrees C at maximal power. Considering literature values for oxygen diffusion activation on Pt, the active zone supplying atomic oxygen to the triple phase boundary was calculated to be 22 nm wide at most. The anode function was provided by a CGO layer, known to be electrically conductive at reducing conditions, with annular Pt current collector. The TPB length was increased by adding a Pt-YSZ composite cathode layer covering the complete YSZ/CGO membrane. The peak power density was increased by a factor of 5 to reach 5 mW cm(-2) with an open circuit voltage of 0.68V at 450 degrees C. It was observed that the Pt grains re-crystallized to large grains, leading to a loss of electrical connectivity in the composite layer. The composite cathode layer was thus inadequate to contact the complete membrane area, leading to a too large area specific resistance (ASR) in the interior of the cell. (C) 2012 Elsevier B.V. All rights reserved.

2012

Optical and structural analysis of sol-gel derived Cu-Co-Mn-Si oxides for black selective solar nanocomposite multilayered coatings

Yannik Antonetti, Olivia Valérie Charlotte Bouvard, Thomas Gascou, Marina Aymara González Lazo, Aïcha Hessler-Wyser, Martin Joly, Pierre Loesch, Martin Python, Andreas Schueler

In this paper, we report the preparation of the precursor solutions for the deposition of Cu-Co-Mn-Si oxides by a sot-gel method in order to produce black selective coatings for CSP receiver tubes. Their optical properties were investigated by means of spectroscopic ellipsometry. High Resolution Transmission Electron Microscopy (HRTEM) was used to visualize the microstructure. Time-of-Flight-Secondary-Ion-Mass-Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) were employed to determine the profile of atomic distribution and their chemical environment in a triple layered coating on a stainless steel (SS) substrate. The studied material shows high potential for the use in multi-layered graded index coatings for solar energy conversion. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier2015

Investigations into the Effects of Biofuel Contaminants on Solid Oxide Fuel Cells

Hossein Madi

Solid Oxide Fuel Cells (SOFCs) have gathered considerable attention as a clean, highly efficient conversion device for the production of both electricity and heat from fuels. Due to their high operating temperatures, SOFCs do not require pure hydrogen as fuel, but exhibit a high fuel flexibility, a major advantage concerning the high cost of hydrogen production. Nonetheless, certain requirements apply in terms of fuel purity. The presence of contaminants such as sulfur compounds, siloxanes, halogens, volatile organic compounds and tars within the fuel stream, even at trace levels can significantly reduce the lifetime of key components such as the fuel cell stack and reformer. Cleaning technologies can remove harmful impurities from the biofuels so as to meet the cleanliness requirements of SOFC stacks, though these come at higher cost. The purpose of this study is to provide guidelines regarding the maximum impurity concentrations in the biofuels which SOFCs can tolerate. This work has two principal aims: (i) to identify any threshold tolerance limits of different contaminants, if they exist and (ii) to investigate degradation mechanisms caused by contaminants. Siloxane D4, hydrogen chloride and sulfur were selected as biogas impurities. In addition, the effect of toluene (a common impurity in wood gasifier product gas) was also investigated on the performance of Ni-YSZ SOFCs. Short tests lasting 100-500 h were carried out to study the influence of the selected impurities on state-of-the-art Ni anode-supported SOFCs. Experiments were performed on single cells provided by Topsoe Fuel Cell (TOFC) and SOLIDpower and in some cases on 11-cell short stacks, provided by TOFC. Contamination tests were performed on simple hydrogen feed and more complex feeds such as simulated biogas or reformed biogas. Furthermore, in the cases of contamination with siloxane D4 and hydrogen chloride, single cell results were compared with those of short stacks. In-situ (electrochemical impedance spectroscopy- EIS) and ex-situ (electron microscopy and chemical analysis) characterization methods were used to identify the prevailing degradation mechanisms. Suggestions were made, which Ni-YSZ anode processes are affected by each contaminant. Siloxane deposit as SiO2 and block electrode pores and TPB (triple-phase-boundary) electrochemical reaction zone; HCl adsorbs on Ni particles to hinder the electrochemical reaction; sulfur, both from H2S and thiophene, adsorbs reversibly on Ni and leads to an immediate sharp cell voltage drop, which remains small at low S-concentrations. It was found that the complete removal of siloxanes is necessary, that S-compounds are acceptable to a level of 0.5 ppm and that there is no need to remove HCl and tar compounds from the biogas, for the concentration levels in which they are expected to be found in anaerobic digester gas.

EPFL2016

Investigations into the Effects of Biofuel Contaminants on Solid Oxide Fuel Cells

Hossein Madi

EPFL2016