Micro-solid oxide fuel cells running on reformed hydrocarbon fuels

Micro‐solid oxide fuel cell (micro‐SOFC) systems are predicted to have a high energy density and specific energy and are potential power sources for portable electronic devices. A micro‐SOFC system is under development in the frame of the ONEBAT project [1‐3]. In this presentation, we report on the fabrication and characterization of a sub‐system assembly consisting of a startup heater and a micro‐reformer bonded to a Si chip with electrochemically‐active micro‐SOFC membranes. A functional carrier including fluidic channels for gas feed and integrated heaters was bonded to a microreformer with an overall size of 12.7 mm x 12.7 mm x 1.9 mm [4‐7]. As a catalyst, a foam‐like material made of ceria‐zirconia nanoparticles doped with rhodium was used to fill the 58.5 mm3 reformer cavity. This micro‐reformer allows for high methane and butane conversion of > 90 % with a hydrogen selectivity of > 80 % at 550 °C in the reformer [7, 8]. A silicon chip with 30 free‐standing micro‐SOFC membranes (390 μm x 390 μm) with a thickness of less than 500 nm was bonded to the carrier‐reformer assembly described above. The micro‐SOFC membrane consisted of an yttria‐ stabilized zirconia thin film electrolyte. Both Pt‐based and ceramic‐based electrode materials were tested regarding the thermal stability and carbon poisoning at temperatures below 600 °C. The functional‐carrier mirco‐reformer micro‐SOFC assembly was electrochemically tested with hydrocarbon fuel between 300 °C and 600 °C. The fuel cell performance and the microstructural evolution of the anode are discussed as well.

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb

EPFL2002

Development of a low temperature co-fired ceramic fuel processor for the micro-scale solid oxide fuel cell system

Bo Jiang, Thomas Maeder, Paul Muralt, Dimos Poulikakos

The miniaturized solid oxide fuel cells (µ-SOFCs) has become an intensively studied device for portable power generation technology due to its wide choice of hydrocarbon fuels, its high energy density and its great operation efficiency. It is being considered as a battery replacement [1]. The µ-SOFC system, which aims to provide electrical energy (≤ 10 W) at an operating temperature of ca. 550°C, consists of a fuel cell unit for the electrochemical conversion [2-3]; a fuel processing unit for the thermal start-up, fuel reforming and total oxidation of exhausts [4-5]; a system packaging that insulates the fuel cell unit from the operating temperature to the ambient as well as provides fluidic and electronic connections [6-8]; and an electronics module for regulating the power output. As a core module of the entire µ-SOFC system, various fuel processing units have been proposed and developed. Most of those modules have been based on microelectromechanical systems (MEMS), which however shows several critical limitations with regard to electrical and fluidic connections and system integration [9-10]. Here we propose a ceramic based meso-scale gas processer combined with thick film and low-temperature co-fired ceramic technology (LTCC). With an overall size of 12 × 30 × 10 mm3, the ceramic processor, made of Heraeus HeroLock 2000 LTCC materials, mainly functions as a meso-scale hotplate that has a cantilever shape to effectively decouple the heat at the hot zone produced by the start-up heater and/or exothermic fuel processing reactions from the cold zone, in which the temperature is near ambient and thus compatible with normal electrical and fluidic connections (Figure 1). Embedded cavities were integrated into the processor during the fabrication process by using a progressive lamination technique. A thick-film crack-free catalyst paste, containing rhodium-doped ceria-zirconia nanoparticles, was dispensed into the reaction chambers as packed catalytic beds for the processing reactions. An integrated thick-film platinum (Heraeus CL11-6109) heater provides the start-up energy for the exothermic reforming reaction of butane or methane as well as total oxidation reactions. Such a meso-scale monolithic ceramic reactor can carry out the gas processing in a thermally self-sustaining manner, rendering itself to be a functional packaging of the entire µ-SOFC system in the future. In this work, the fabrication process of the gas processor will be discussed in detail, and the results of the fuel processing reactions such as reforming, total oxidation and thermal start-up will be presented as well. References: [1] Bieberle-Hütter, A., Beckel, D., et al. Journal of Power Sources, 177(1), 123–130, (2008) [2] Rey-Mermet, S. and Muralt, P. Solid State Ionics, 179(27–32), 1497-1500, (2008) [3] Evans, A., Bieberle-Hütter, A., et al. Monatshefte für Chemie - Chemical Monthly, 140(9), 975–983, (2009) [4] Shao, Z., Haile, S. M., et al. Nature, 435(7043), 795–798, (2005) [5] Santis-Alvarez, A. J., Nabavi, M., et al. Energy & Environmental Science, 4(8), 3041, (2011) [6] Jiang, B., Maeder, T., Muralt, P. Proceedings, Power MEMS 2010, Leuven (BE), 2010 [7] Jiang, B., Muralt, P., et al. Sensors and Actuators B: Chemical, 175, 218–224, (2012) [8] Maeder, T., Jiang, B., et al. Proceedings, 7th International Conference on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT), San Diego (USA), 2011 [9] Hotz, N., Osterwalder, N., et al. Chemical Engineering Science, 63(21), 5193–5201, (2008) [10] Vaccaro, S., Malangone, L., Ciambelli, P. Industrial & Engineering Chemistry Research, 49(21), 10924–10933, (2010)

2013

Développement d'un réacteur microstructuré basé sur des filaments métalliques catalytiques

Chrystèle Horny

The aim of this work is to develop a microstructured reactor based on filamentous catalysts for the Oxidative Steam-Reforming of Methanol (OSRM), to produce hydrogen as feed for a fuel cell, in an autothermal way. Hydrogen is produced by the methanol Steam-Reforming (SR) reaction. This endothermic reaction requires an external heat source which is, in our case, generated by methanol oxidation. The coupling of these two reactions – SR and oxidation, called oxidative steam-reforming of methanol – is performed in a single reactor. As the oxidation is much faster than SR, it occurs in the first part of the reactor, the SR takes place in the second part. If a conventional fixed bed reactor is used, pronounced axial temperature profiles are developed: a hot–spot due to the exothermicity of the oxidation is generated at the entrance followed by a cold–spot due to the SR. The high temperature may damage the catalyst and the low temperature diminishes the rate of reforming reaction leading to poor reactor performance. Thus the temperature control is crucial. Consequently, a microstructured reactor is used. This kind of reactor has multiple parallel channels with a diameter ranging from ten to several hundreds micrometers. These submillimetric dimensions lead to a high surface to volume ratio and a much higher heat transfer coefficient than in the traditional heat exchangers. These characteristics allow to increase heat exchange between reactions and to avoid hot–spot formation. In this work, brass wires introduced into a macro tubular reactor parallel to the walls are used to create the microstructure. Brass is chosen because of its composition – it contains copper and zinc catalyzing the reforming/oxidation of methanol – and for its high heat conductivity which ensures heat exchange improvements. Moreover the small diameter of reactor channels ensures narrow residence time distribution, leading to high selectivity, and a short residence time, improving reactors dynamic. The characteristics mentioned above are verified in chapter 4 for the reactor developed during this study. The hydrodynamic of this reactor is presented under the influence of wire diameter and catalyst preparation treatment. The flow in the microstructure is close to a plug flow: a Bodenstein number of 105 is obtained for brass wires with a diameter of 480μm. Concerning catalytic treatment, it doesn't appear to modify the hydrodynamic. Compared to a fixed bed reactor, the measured residence time distribution for our microstructured reactor is found to be much narrower; the pressure drops are also smaller. Chapter 5 focuses on the reaction conditions for SR of methanol, the reaction that generates hydrogen. These conditions are determined by using an industrial catalyst (based on copper –zinc – aluminium) and by comparing the catalytic activity measured in our conditions with the ones found in the literature. A molar water to methanol ratio of 1.2 is chosen in order to avoid carbon monoxide production, which is a poison for fuel cells. Chapter 6 deals with the development, the optimisation and the characterisation of brass based catalyst. Brass grids are first tested for SR: alloy composition, type and time of leaching are studied. The optimal catalyst is found to be a CuZn37 grid incorporated with aluminium and treated by an acid leaching during 20 minutes in order to increase its surface area (30m2/g). In order to test brass activity in presence of oxygen, partial oxidation of methanol is first carried out over this catalyst. Then the oxidative steam-reforming of methanol is studied: it is found that the modification of the support and hence of the treatment applied decreases the activity and essentially the stability. Deactivation is attributed to copper particles sintering and to oxidation of the catalyst which is not active for hydrogen production. A screening of additives is performed and it is shown that brass wires incorporated with aluminium and doped with chromium by an impregnation method is active, stable and selective: methanol conversion of 25.3% with a hydrogen selectivity of 42.6% are maintained during more than ten hours. Analyses indicate that this catalyst is not easily oxidised and reduced, due to the presence of spinels on the surface which modify electronic properties of copper. In chapter 7, the OSRM is analysed with focus on reaction mechanism and heat exchange. The reaction mechanism is quite complex due to reactions involved (partial oxidations, total oxidation, decomposition) and to their strong temperature dependence. A production of formaldehyde is observed at low temperature (T < 240°C), whereas at higher temperatures secondary reactions disappear and hydrogen production is initiated. Finally, measurements of the axial temperature profile allow to verify the isothermicity of our microstructured reactor: for a methanol conversion of 43% at T = 262°C a hot–spot of less than 3.5°C is measured and no cold–spot is observed. However, it is shown that the temperature profile is highly influenced by methanol conversion and by the oxygen quantity introduced in the reactor. Temperature variations in the entire reactor are nevertheless much lower than those developed in a fixed bed reactor, which is in agreement with our expectations and corresponds to our pursued objectives.

EPFL2005