Accessible triple-phase boundary length: A performance metric to account for transport pathways in heterogeneous electrochemical materials

The performance of materials for electrochemical energy conversion and storage depends upon the number of electrocatalytic sites available for reaction and their accessibility by the transport of reactants and products. For solid oxide fuel/electrolysis cell materials, standard 3-D measurements such as connected triple-phase boundary (TPB) length and effective transport properties partially inform on how local geometry and network topology causes variability in TPB accessibility. A new measurement, the accessible TPB, is proposed to quantify these effects in detail and characterize material performance. The approach probes the reticulated pathways to each TPB using an analytical electrochemical fin model applied to a 3-D discrete representation of the heterogeneous structure provided by skeleton based partitioning. The method is tested on artificial and real structures imaged by 3-D x-ray and electron microscopy. The accessible TPB is not uniform and the pattern varies depending upon the structure. Connected TPBs can be even passivated. The sensitivity to manipulations of the local 3-D geometry and topology that standard measurements cannot capture is demonstrated. The clear presence of preferential pathways showcases a non-uniform utilization of the 3-D structure that potentially affects the performance and the resilience to alterations due to degradation phenomena. The concepts presented also apply to electrochemical energy storage and conversion devices such as other types of fuel cells, electrolyzers, batteries and capacitors. (C) 2016 Elsevier B.V. All rights reserved.

Fabrication of structured anode-supported solid oxide fuel cell by powder injection molding

Antonin Faes, Jan Van Herle, Zacharie Wuillemin, Amédée Zryd

Powder Injection Molding (PIM) gives the possibility to produce, at an industrial rate, ceramic parts with fine details. It is thus a possible approach to reduce the fabrication costs of Solid Oxide Fuel Cells (SOFCs). This work presents fabrication and electrochemical characterization results of injection-molded structured anode-supported SOFCs. Planar anode-supported SOFCs with fine details have been produced by injection molding of a mixture of nickel oxide (NiO) and yttria-stabilized zirconia (YSZ). The channeling structure and support porosity ensure gas transport on the fuel side. Initial anode support characterizations like sintering shrinkage, porosity and electrical conductivity have been done on simple Ni-YSZ test bars. Thin YSZ electrolyte has been deposited by spin-coating on the PIM green body before half-cell co-sintering. Electrochemical testing was carried out with a lanthanum-strontium manganite (LSM)-YSZ cathode. The performance is comparable to tape-cast anode-supported cells, with 0.45 W cm(-2) obtained at 0.6 V and 810 degrees C. Medium-term galvanostatic testing shows a degradation rate of about 1.1% kh(-1). Post-test analysis with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) attribute this mainly to cathode degradation due to Cr and S poisoning. These results are therefore promising for using a PIM fabrication process in the SOFC field. (C) 2012 Elsevier B.V. All rights reserved.

2013

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

Ceramic Oxygen Separation Membranes and Mixed Conductors

Ferran Marqués, Peter Hugh Middleton, Jan Van Herle

A consortium was created to investigate electroceramic materials that show ionic or mixed ionic-electronic conduction (MIEC), for application as oxygen separation membranes and in solid oxide fuel cells (SOFC, electrolytes and cathodes). The focus is on dense structures as supported thin layers (5-20 µm) for reduced temperature operation (600-750°C). Under these conditions grain boundary effects are important. Zirconia, ceria and gallate are considered for SOFC electrolytes. The nature and the amount of the dopants will be varied, and sintering aids added. These parameters will influence the grain size and the grain boundary composition. The effects on microstructure, and conduction and strength properties will be examined by high resolution SEM/TEM, impedance spectroscopy and load testing under fast thermal cycling, respectively. A systematic study on the variation of the crystallite size of starting powders (prepared by ultrasonic spray pyrolysis - USP) and its effect on the grain boundary resistance is planned, for the case of zirconia electrolyte. Of special interest is also the compatibility between ceria and zirconia, when cofired at high temperatures: diffusion couples will be made, and studied by HR-SEM/EDX. Thin layers of these materials will be deposited on anode supports of planar shape, by tape casting, screenprinting or electrostatic spray pyrolysis (ESP). Dip coating and electrophoretic deposition (EPD), for example on tubular supports, is equally envisaged. Special attention is given to the conductivity measurement of such thin layers (in particular that of grain boundaries), and compared with that of bulky samples. Different results due to the thin layer microstructure have been observed. MIEC materials of the perovskite family La1-xSrx(Mn, Fe, Co)O3-x are considered as SOFC cathodes. Thin dense layers will be deposited by ESP on zirconia and ceria electrolytes. The effect of the annealing temperature, and therefore the grain size, on the electrochemical performance and the thermal-cycling performance of such cathode/electrolyte interfaces will be studied. Candidate MIEC Materials for oxygen separation membranes are intended to be fabricated as dense pellets (compaction) or tubes (extrusion). These can be single phase (e.g. La1-xBaxCo1-yFeyO3-d) or composites (e.g. ceria/La-chromite mixtures). They will be subjected to oxygen permeation measurements, i.e. under a large oxygen partial pressure gradient of for example CH4/air. Microstructural effects for permeation stability and mechanical stability will be of first importance: changes (grain size, grain boundaries, microporosity) will be undertaken. In view of their application, it is planned to try and deposit thin dense films of these materials on porous supports (e.g. of ceria): by ESP on porous plates, and by dip coating on porous tubes (made f.ex. by slip casting). Wherever possible, wet chemical synthesis routes are employed to obtain the ceramic starting powders, such as coprecipitation, citrate gel or dyanate complexation, colloidal synthesis or combustion synthesis.