Evolution of the Morphology Near Triple-Phase Boundaries in Ni–Yttria Stabilized Zirconia Electrodes Upon Cathodic Polarization

Microstructural changes in Ni–yttria stabilized zirconia (YSZ) near the YSZ electrolyte were examined by three-dimensional (3D) electron microscopy after electrolysis and fuel cell operation up to 10,700 h and 15,000 h, respectively. The depletion of Ni and three-phase boundaries (TPBs) close to the electrolyte was detected upon cathodic polarization. It corresponded to spatial variations of dihedral angles (θ) at TPBs and Ni surface curvature along the direction perpendicular to the electrolyte, which comport with electrowetting and Zener pinning theory on several aspects. θNi decreased by up to 6 deg next to the electrolyte after electrolysis but remained uniform after fuel cell operation. This is in line with predictions from electrowetting theory with capacitances measured by electrochemical impedance spectroscopy and distribution of relaxation times. The decrease in θNi was concurrent to transition toward concave Ni/pore interfacial shapes and lower genus of the Ni phase, which suggests the pinch-off of Ni ligaments following surface diffusion-controlled Rayleigh instability. The increase in absolute mean curvature near the electrolyte interface is a driving force for outward transport of Ni. The decrease in θYSZ further suggests that TPB lines relocate on YSZ surface features that provide higher Zener pinning force. In contrast, few localized contact losses between Ni and YSZ that can also occur under high cathodic polarization and trigger Ni depletion were detected. The results are expected to advance the understanding of the driving forces that cause Ni depletion near the electrolyte in electrolysis for the design of improved solid oxide cell electrode microstructures.

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

Marco Cantoni, Arata Nakajo, Jan Van Herle

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.

Elsevier Science Bv2016

Highly-Ordered Magneli Ti4O7 Nanotube Arrays as Effective Anodic Material for Electro-oxidation

Christos Comninellis, Caroline Miles

Pure Magneli Ti4O7 nanotube arrays (NTA) were successfully fabricated by reducing TiO2 NTA with hydrogen at 850 degrees C for 30 minutes. The microstructure, composition and electrochemical behavior of the prepared Ti4O7 NTA were characterized by means of X-ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray Photoelectron Spectroscopy, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy. The as-prepared Ti4O7 NTA had a highly-ordered tubular structure with high crystallinity, large electrochemical window of water electrolysis (2.4 V vs. Ag/AgCl, pH = 6.0) and low interfacial charge transfer resistance when they were employed as anode for electro-oxidation. Phenol was electro-oxidized on Ti4O7 particles and Ti4O7 NTA with the latter giving 20% more Chemical Oxygen Demand (COD) removal. Pure Ti4O7 NTA also displayed larger degradation coefficient as well as higher COD removal and current efficiency than Boron-doped Diamond and other types of Magneli NTAs. Cathodic polarization was found to be an effective way of restoring the electrochemical performance of oxidized Ti4O7 NTA as an anode. (C) 2014 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

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

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

EPFL2002