Imperial College London

DrEnriqueRuiz Trejo

Central FacultyEstates Division

Visiting Researcher



+44 (0)20 7594 9695e.ruiz-trejo




3.50Royal School of MinesSouth Kensington Campus





Publication Type

66 results found

Pereira VE, Dalwadi MP, Ruiz-Trejo E, Griffiths IMet al., 2021, Optimising the flow through a concertinaed filtration membrane, JOURNAL OF FLUID MECHANICS, Vol: 913, ISSN: 0022-1120

Journal article

Song B, Bertei A, Wang X, Cooper S, Ruiz-Trejo E, Chowdhury R, Podor R, Brandon Net al., 2019, Unveiling the mechanisms of solid-state dewetting in Solid Oxide Cells with novel 2D electrodes, Journal of Power Sources, Vol: 420, Pages: 124-133, ISSN: 0378-7753

During the operation of Solid Oxide Cell (SOC) fuel electrodes, the mobility of nickel can lead to significant changes in electrode morphology, with accompanying degradation in electrochemical performance. In this work, the dewetting of nickel films supported on yttriastabilized zirconia (YSZ), hereafter called 2D cells, is studied by coupling in-situ environmentalscanning electron microscopy (E-SEM), image analysis, cellular automata simulation and electrochemical impedance spectroscopy (EIS). Analysis of experimental E-SEM images shows that Ni dewetting causes an increase in active triple phase boundary (aTPB) length up to a maximum, after which a sharp decrease in aTPB occurs due to Ni de-percolation. Thismicrostructural evolution is consistent with the EIS response, which shows a minimum in polarization resistance followed by a rapid electrochemical degradation. These results reveal that neither evaporation-condensation nor surface diffusion of Ni are the main mechanisms of dewetting at 560-800 °C. Rather, the energy barrier for pore nucleation within the dense Ni film appears to be the most important factor. This sheds light on the relevant mechanisms and interfaces that must be controlled to reduce the electrochemical degradation of SOC electrodes induced by Ni dewetting.

Journal article

Song B, Ruiz-Trejo E, Brandon N, 2018, Enhanced mechanical stability of Ni-YSZ scaffold demonstrated by nanoindentation and electrochemical impedance spectroscopy, Journal of Power Sources, Vol: 395, Pages: 205-211, ISSN: 0378-7753

The electrochemical performance of Ni-YSZ SOFC anodes can quickly degrade during redox cycling. Mechanical damage at interfaces significantly decreases the number of active triple phase boundaries. This study firstly focuses on the sintering temperature impact on YSZ scaffold mechanical properties. The YSZ scaffold sintered at 1200 °C exhibited 56% porosity, 28.3 GPa elastic modulus and 0.97 GPa hardness and was selected for further redox cycling study. The Ni infiltrated YSZ scaffold operated at 550 °C had an initial stabilized polarisation resistance equal to 0.62 Ω cm2 and only degraded to 2.85 Ω cm2 after 15 redox cycles. The active triple phase boundary density was evaluated by FIB-SEM tomography, and degraded from 28.54 μm−2 to 19.36 μm−2. The YSZ scaffold structure was robust after 15 redox cycles, as there was no observation of the framework fracturing in both SEM and FIB-SEM images, which indicated that the mechanical stability of YSZ scaffold improves the anode stability during redox cycling. Nonetheless, Ni agglomeration could not be prevented within Ni-YSZ scaffolds and this needs further consideration.

Journal article

Cavallaro A, Pramana S, Ruiz Trejo E, Sherrell P, Ware E, Kilner J, Skinner SJet al., 2018, Amorphous-cathode-route towards low temperature SOFC, Sustainable Energy & Fuels, Vol: 2, Pages: 862-875, ISSN: 2398-4902

Lowering the operating temperature of solid oxide fuel cell (SOFC) devices is one of the major challenges limiting the industrial breakthrough of this technology. In this study we explore a novel approach to electrode preparation employing amorphous cathode materials. La0.8Sr0.2CoO3−δ dense films have been deposited at different temperatures using pulsed laser deposition on silicon substrates. Depending on the deposition temperature, textured polycrystalline or amorphous films have been obtained. Isotope exchange depth profiling experiments reveal that the oxygen diffusion coefficient of the amorphous film increased more than four times with respect to the crystalline materials and was accompanied by an increase of the surface exchange coefficient. No differences in the surface chemical composition between amorphous and crystalline samples were observed. Remarkably, even if the electronic conductivities measured by the Van Der Pauw method indicate that the conductivity of the amorphous material was reduced, the overall catalytic properties of the cathode itself were not affected. This finding suggests that the rate limiting step is the oxygen mobility and that the local electronic conductivity in the amorphous cathode surface is enough to preserve its catalytic properties. Different cathode materials have also been tested to prove the more general applicability of the amorphous-cathode route.

Journal article

Bertei A, Ruiz-Trejo E, Clematis D, Carpanese MP, Barbucci A, Nicolella C, Brandon Net al., 2018, A perspective on the role of the three-phase boundary in solid oxide fuel cell electrodes, Bulgarian Chemical Communications, Vol: 50, Pages: 31-38, ISSN: 0861-9808

© 2018 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria. Within composite electrodes for solid oxide fuel cells (SOFCs), electrochemical reactions take place in the proximity of the so-called three-phase boundary (TPB), the contact perimeter where the electron-conducting, the ionconducting and the porous phases meet. Strictly speaking, the TPB is a line and efforts have been made to increase its length per unit of electrode volume in order to reduce the activation losses. In this communication, by integrating physically-based modelling, 3D tomography and electrochemical impedance spectroscopy (EIS), a renovated perspective on electrocatalysis in SOFCs is offered, showing that the electrochemical reactions take place within an extended region around the geometrical TPB line. Such an extended region is in the order of 4 nm in Ni/Sc0.2Zr0.9O 2 .1 (Ni/ScSZ) anodes while approaches hundreds of nanometres in La0.8Sr0.2MnO 3 -x/Y0.16Zr0.92O 2 .08 (LSM/YSZ) cathodes. These findings have significant implications for preventing the degradation of nanostructured anodes, which is due to the coarsening of the fractal roughness of Ni nanoparticles, as well as for the optimisation of composite cathodes, indicating that the adsorption and surface diffusion of oxygen limit the rate of the oxygen reduction reaction (ORR). In both anodes and cathodes, the results point out that the surface properties of the materials are key in determining the performance and lifetime of SOFC electrodes.

Journal article

Song B, Ruiz Trejo E, Bertei A, Brandon Net al., 2017, Quantification of the degradation of Ni-YSZ anodes upon redoxcycling, Journal of Power Sources, Vol: 374, Pages: 61-68, ISSN: 0378-7753

Ni-YSZ anodes for Solid Oxide Fuel Cells are vulnerable to microstructural damage during redox cycling leading to a decrease in the electrochemical performance. This study quantifies the microstructural changes as a function of redox cycles at 800 °C and associates it to the deterioration of the mechanical properties and polarisation resistance. A physically-based model is used to estimate the triple-phase boundary (TPB) length from impedance spectra, and satisfactorily matches the TPB length quantified by FIB-SEM tomography: within 20 redox cycles, the TPB density decreases from 4.63 μm−2 to 1.06 μm−2. Although the polarisation resistance increases by an order of magnitude after 20 cycles, after each re-reduction the electrode polarisation improves consistently due to the transient generation of Ni nanoparticles around the TPBs. Nonetheless, the long-term degradation overshadows this transient improvement due to the nickel agglomeration. In addition, FIB-SEM tomography reveals fractures along YSZ grain boundaries, Ni-YSZ detachment and increased porosity in the composite that lead to irreversible mechanical damage: the elastic modulus diminishes from 36.4 GPa to 20.2 GPa and the hardness from 0.40 GPa to 0.15 GPa. These results suggest that microstructural, mechanical and electrochemical properties are strongly interdependent in determining the degradation caused by redox cycling.

Journal article

Chen J, Bertei A, Ruiz-Trejo E, Atkinson A, Brandon NPet al., 2017, Characterization of Degradation in Nickel Impregnated Scandia-Stabilize Zirconia Electrodes during Isothermal Annealing, Journal of The Electrochemical Society, Vol: 164, Pages: F935-F943, ISSN: 1945-7111

This study investigates the stability of nickel-impregnated scandia-stabilize zirconia composite electrodes during isothermal annealing at temperatures from 600 to 950°C in a humidified hydrogen atmosphere (3 vol % water vapor). Typically an initial rapid degradation of the electrode during the first 17 h of annealing is revealed by both an increase in polarization resistance and a fall in electronic conductivity. Secondary electron images show a shift in nickel particle size toward larger values after 50 h of annealing. The declining electrochemical performance is hence attributed to nickel coarsening at elevated temperatures. Nickel coarsening has two microstructural effects: breaking up nickel percolation; and reducing the density of triple phase boundaries. Their impact on electrode area specific resistance is explored using a physical model of electrode performance which relates the macroscopic electrochemical performance to measurable microstructural parameters.

Journal article

Ruiz-Trejo E, Bertei A, Maserati A, Boldrin P, Brandon NPet al., 2017, Oxygen Reduction, Transport and Separation in Low Silver Content Scandia-Stabilized Zirconia Composites, Journal of The Electrochemical Society, Vol: 164, Pages: F3045-F3054, ISSN: 1945-7111

Dense composites of silver and Sc-stabilized ZrO2 (Ag-ScSZ) are manufactured from ScSZ sub-micrometric particles coated with silver using Tollens’ reagent. A composite with 8.6 vol % of silver exhibits metallic conductivity of 186 S cm−1 and oxygen flux of 0.014 μmol cm−2 s−1 at 600°C for a 1-mm thick membrane when used as a pressure-driven separation membrane between air and argon. To gain insight into the role of oxygen transport in Ag and ScSZ, a dense non-percolating sample (Ag 4.7 vol%) is analyzed by impedance spectroscopy and the transport of oxygen through both phases is modelled. Oxygen transport takes place in both silver and ScSZ but it is still dominated by transport in the ionic conductor and therefore a large volume fraction of the ion conductor is beneficial for the separation. The oxygen transport in the silver clusters inside the composite is dominated by diffusion of neutral species and not by the charge transfer reaction at the interface between ScSZ and Ag, yet small silver particles on the surface improve the reduction of oxygen. Oxygen reduction is highly promoted by silver on the surface and there are no limitations of charge transfer at the interface between silver and ScSZ.

Journal article

Bertei A, Ruiz Trejo E, Kareh K, Yufit V, Wang X, Tariq F, Brandon Net al., 2017, The fractal nature of the three-phase boundary: A heuristic approach to the degradation of nanostructured solid oxide fuel cell anodes, Nano Energy, Vol: 38, Pages: 526-536, ISSN: 2211-2855

Nickel/zirconia-based nanostructured electrodes for solid oxide fuel cells suffer from poor stability even at intermediate temperature. This study quantifies the electrochemical and microstructural degradation of nanostructured electrodes by combining 3D tomography, electrochemical impedance spectroscopy (EIS) and mechanistic modeling. For the first time, the electrochemical degradation of nanostructured electrodes is quantified according to the fractal nature of the three-phase boundary (TPB). Using this hypothesis an excellent match between modeling and the electrochemical response is found. The origin of the degradation in microstructure and electrochemical performance can be found in the initial fractal roughness of the TPB at a length scale not detectable with state-of-the-art tomography at 30 nm resolution. This additionally implies that the hydrogen electro-oxidation takes place within 4 nm from the geometric TPB line, revealing that the electrochemical reaction zone cannot be regarded anymore as a one-dimensional line when dealing with nanoparticles.

Journal article

Boldrin D, Boldrin P, Ruiz-Trejo E, Cohen LFet al., 2017, Recovery of the intrinsic thermoelectric properties of CaMn0.98Nb0.02O3 in 2-terminal geometry using Ag infiltration, Acta Materialia, Vol: 133, Pages: 68-72, ISSN: 1359-6454

Oxide based thermoelectric (TE) materials offer several advantages over currently used intermetallic alloys due to their chemical and thermal stability at high temperatures, non-toxic elements, low cost and ease of manufacture. However, incorporation of oxides into thermoelectric generators (TEGs) is hindered by factors such as the requirement for polycrystalline materials over single crystals and the large electrode/ceramic contact resistances. The latter significantly limits the performance efficiency of a working TEG. Here we report the TE properties of Ag infiltrated polycrystalline CaMn0.98Nb0.02O3 ceramics. We demonstrate that by using this route the intrinsic TE properties of this material are fully recovered in 2-terminal geometry through Ag infiltration, thereby overcoming the electrode TEG contact problem. This synthetic route provides opportunities for bridging the performance gap between the intrinsic TE and TEG device properties of oxides.

Journal article

Jamil Z, Ruiz-Trejo E, Brandon NP, 2017, Nickel Electrodeposition on Silver for the Development of Solid Oxide Fuel Cell Anodes and Catalytic Membranes, Journal of The Electrochemical Society, Vol: 164, Pages: D210-D217, ISSN: 1945-7111

Nickel was electrodeposited on porous Ag/GDC (silver/Ce0.9Gd0.1O2-x) scaffolds and dense Ag/GDC composites for the fabricationof SOFC electrodes and catalytic membranes respectively. To control the distribution and amount of nickel deposition on the Ag/GDCsurfaces; first, a systematic cyclic voltammetry study of nickel electrodeposition from a Watts bath on silver foils was carried outto understand the influence of operating conditions on the electrodeposition process. From the cyclic voltammetry study, it can beconcluded that suitable operating conditions for nickel electrodeposition into porous Ag/GDC scaffolds and catalytic membranesare: 1.1 M Ni2+ concentration in Watts bath; deposition potential between −0.65 to −1.0 V vs. Ag/AgCl; a temperature at 55◦C;sodium dodecyl sulfate (SDS) as the surfactant; pH 4.0 ± 0.2 and an agitation rate of 500 rpm. It was observed that the nickel surfacemicrostructure changed with the deposition current densities due to the co-evolution of H2. Pulse and continuous electrodepositionmodes allow nickel to be deposited throughout porous Ag/GDC scaffolds and onto catalytic membranes. The pulse electrodepositionmode is favored as this is shown to result in an even Ni distribution within the porous scaffolds at minimum H2 pitting.

Journal article

Tariq F, Ruiz-Trejo E, Bertei A, Boldrin P, Brandon NPet al., 2017, Microstructural Degradation: Mechanisms, Quantification, Modeling and Design Strategies to Enhance the Durability of Solid Oxide Fuel Cell Electrodes, Solid Oxide Fuel Cell Lifetime and Reliability: Critical Challenges in Fuel Cells, Pages: 79-99, ISBN: 9780081011027

Electrode microstructure is one of the main factors determining the performance and durability of solid oxide fuel cells (SOFCs). The degradation is intimately linked to the microstructure, which in turn depends upon manufacturing and operation conditions. In this chapter we discuss the main causes for degradation of electrodes, concentrating mainly on the anode and present the techniques-both typical and state-of-the-art to follow these changes. We emphasize the need to quantitatively link the microstructural properties (e.g., triple-phase boundaries, porosity, and tortuosity) with the electrochemical responses measured and, most importantly, to link the change in microstructure to the performance degradation via suitable models. The knowledge gained must then be used to design new electrodes that can extend the lifetime of SOFCs once the critical parameters have been identified.

Book chapter

Brandon NP, Ruiz-Trejo E, Boldrin P, 2017, Solid Oxide Fuel Cell Lifetime and Reliability: Critical Challenges in Fuel Cells, ISBN: 9780081011027

Solid Oxide Fuel Cell Lifetime and Reliability: Critical Challenges in Fuel Cells presents in one volume the most recent research that aims at solving key issues for the deployment of SOFC at a commercial scale and for a wider range of applications. To achieve that, authors from different regions and backgrounds address topics such as electrolytes, contaminants, redox cycling, gas-tight seals, and electrode microstructure. Lifetime issues for particular elements of the fuel cells, like cathodes, interconnects, and fuel processors, are covered as well as new materials. They also examine the balance of SOFC plants, correlations between structure and electrochemical performance, methods for analysis of performance and degradation assessment, and computational and statistical approaches to quantify degradation. For its holistic approach, this book can be used both as an introduction to these issues and a reference resource for all involved in research and application of solid oxide fuel cells, especially those developing understanding in industrial applications of the lifetime issues. This includes researchers in academia and industrial R&D, graduate students and professionals in energy engineering, electrochemistry, and materials sciences for energy applications. It might also be of particular interest to analysts who are looking into integrating SOFCs into energy systems. Brings together in a single volume leading research and expert thinking around the broad topic of SOFC lifetime and durability. Explores issues that affect solid oxide fuel cells elements, materials, and systems with a holistic approach. Provides a practical reference for overcoming some of the common failure mechanisms of SOFCs. Features coverage of integrating SOFCs into energy systems.


Chen J, Ruiz-Trejo E, Atkinson A, Brandon NPet al., 2017, Microstructural and Electrochemical Characterisation of Degradation in Nickel Impregnated Scandia-stabilised Zirconia Electrode during Isothermal Annealing, 15th International Symposium on Solid Oxide Fuel Cells (SOFC), Publisher: ELECTROCHEMICAL SOC INC, Pages: 1125-1137, ISSN: 1938-5862

Conference paper

Bertei A, Tariq F, Yufit V, Ruiz Trejo E, Brandon Net al., 2016, Guidelines for the rational design and engineering of 3D manufactured solid oxide fuel cell composite electrodes, Journal of the Electrochemical Society, Vol: 164, Pages: F89-F98, ISSN: 0013-4651

The growth of 3D printing has opened the scope for designing microstructures for solid oxide fuel cells(SOFCs) with improved power density and lifetime. This technique can introduce structural modifications at a scale larger than particle size but smaller than cell size, such as by insertingelectrolyte pillars of ~5-100 µm. This study sets the minimum requirements for the rational design of 3D printedelectrodes based on an electrochemical model and analytical solutions for functional layers with negligible electronic resistanceand no mixed conduction. Results show that this structural modification enhances the power density when the ratio keffbetween effective conductivity and bulk conductivity of the ionic phase is smaller than 0.5. The maximum performance improvement is predicted as a function of keff. A design study on a wide range of pillar shapes indicates that improvements are achieved by any structural modification which provides ionic conduction up to a characteristic thickness ~10-40 µm without removing active volume at the electrolyte interface. The best performance is reached for thin (< ~2 µm) and long (> ~80 µm) pillars when the compositeelectrode is optimised for maximum three-phase boundarydensity, pointing towards the design of scaffolds with well-defined geometry and fractal structures.

Journal article

Boldrin P, Ruiz Trejo E, Mermelstein J, Bermudez Menendez J, Ramirez Reina T, Brandon Net al., 2016, Strategies for carbon and sulfur tolerant solid oxide fuel cell materials, incorporating lessons from heterogeneous catalysis, Chemical Reviews, Vol: 116, Pages: 13633-13684, ISSN: 1520-6890

Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies towards carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells.

Journal article

Ruiz Trejo E, Puolamaa M, Sum B, Tariq F, Yufit V, Brandon NPet al., 2016, New method for the deposition of nickel oxide in porous scaffolds for electrodes in solid oxide fuel cells and electrolyzers, Chemsuschem, Vol: 10, Pages: 258-265, ISSN: 1864-564X

A simple chemical bath deposition is used to coat a complex porous ceramic scaffold with a conformal nickel layer. The resulting composite is used as a Solid Oxide Fuel Cell electrode and its electrochemical response is measured in humidified hydrogen. X-Ray tomography is used to determine microstructural parameters of the uncoated and Ni-coated porous structure, among other, the surface area to total volume, the radial pore size and size of the necks between pores.

Journal article

Bertei A, Ruiz-Trejo E, Tariq F, Yufit V, Atkinson A, Brandon NPet al., 2016, Validation of a physically-based solid oxide fuel cell anode model combining 3D tomography and impedance spectroscopy, International Journal of Hydrogen Energy, Vol: 41, Pages: 22381-22393, ISSN: 1879-3487

This study presents a physically-based model for the simulation of impedance spectra in solid oxide fuel cell (SOFC) composite anodes. The model takes into account the charge transport and the charge-transfer reaction at the three-phase boundary distributed along the anode thickness, as well as the phenomena at the electrode/electrolyte interface and the multicomponent gas diffusion in the test rig. The model is calibrated with experimental impedance spectra of cermet anodes made of nickel and scandia-stabilized zirconia and satisfactorily validated in electrodes with different microstructural properties, quantified through focused ion beam SEM tomography. Besides providing the material-specific kinetic parameters of the electrochemical hydrogen oxidation, this study shows that the correlation between electrode microstructure and electrochemical performance can be successfully addressed by combining physically-based modelling, impedance spectroscopy and 3D tomography. This approach overcomes the limits of phenomenological equivalent circuits and is suitable for the interpretation of experimental data and for the optimisation of the electrode microstructure.

Journal article

Skinner SJ, Yanez-Gonzalez A, Ruiz-Trejo E, van Wachem B, Beyrau F, Heyes Aet al., 2016, Development of an optical thermal history coating sensor based on the oxidation of a divalent rare earth ion phosphor, Measurement Science & Technology, Vol: 27, ISSN: 1361-6501

The measurement of temperatures in gas turbines, boilers, heat exchangers and othercomponents exposed to hot gases is essential to design energy efficient systems and improvemaintenance procedures. When on-line measurements, such as those performed withthermocouples and pyrometers, are not possible or inconvenient, the maximum temperaturesof operation can be recorded and measured off-line after operation. Although thermal paintshave been used for many years for this purpose, a novel technique based on irreversiblechanges in the optical properties of thermographic phosphors, can overcome some of thedisadvantages of previous methods.In particular, oxidation of the divalent rare earth ion phosphor BaMgAl10O17:Eu(BAM:Eu) has shown great potential for temperature sensing between 700 °C and 1200 °C.The emission spectra of this phosphor change with temperature, which permits to define anintensity ratio between different lines in the spectra that can be used as a measurand of thetemperature. In this paper, the study of the sensing capabilities of a sensor coating based onBAM:Eu phosphor material is addressed for the first time. The sensitivity of the intensityratio is investigated in the temperature range from 800 °C to 1100 °C, and is proved to beaffected by ionic diffusion of transition metals from the substrate. The use of an interlayermade of zirconia proves efficient in reducing ionic diffusion and coatings with this diffusionbarrier present sensitivity comparable to that of the powder material.

Journal article

Jamil Z, Ruiz-Trejo E, Boldrin P, Brandon NPet al., 2016, Anode fabrication for solid oxide fuel cells: Electroless and electrodeposition of nickel and silver into doped ceria scaffolds, International Journal of Hydrogen Energy, Vol: 41, Pages: 9627-9637, ISSN: 1879-3487

A novel fabrication method using electroless and electrodeposited Ni/Ag/GDC for SOFC anodes is presented. First a porous Ce0.9Gd0.1O2−x (GDC) scaffold was deposited on a YSZ electrolyte by screen printing and sintering. The scaffold was then metallized with silver using Tollens' reaction, followed by electrodeposition of nickel from a Watt's bath. The electrodes (Ni/Ag/GDC) were tested in both symmetrical and fuel cell configurations. The microstructures of the Ni/Ag/GDC anodes were analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Nano-particles of Ni formed in the porous GDC scaffold provided triple phase boundaries (TPB). The electronic conductivity of the Ni/Ag/GDC (3.5/24.7/71.8 vol%) electrode was good even at relatively low Ni volume fractions. The electrochemical performance was examined in different concentrations of humidified hydrogen (3% H2O) and over a range of temperatures (600–750 °C). The total area specific resistance (ASR) of the anode at 750 °C in humidified 97 vol% H2 was 1.12 Ω cm2, with low-frequency polarization (R_l) as the largest contributor. The electrodes were successfully integrated into a fuel cell and operated in both H2 and syngas.

Journal article

Ruiz-Trejo E, Thyden K, Bonanos N, Mogensen MBet al., 2016, Conductivity and structure of sub-micrometric SrTiO3-YSZ composites, Solid State Ionics, Vol: 288, Pages: 82-87, ISSN: 1872-7689

Journal article

Francisco Gomez-Garcia J, Ramirez-de-Arellano JM, Ruiz-Trejo E, Tavizon G, de la Mora Pet al., 2016, On the mechanism of electrical conductivity in Ce1/3NbO3, Computational Materials Science, Vol: 111, Pages: 101-106, ISSN: 0927-0256

Using electrical conductivity measurements, we found the existence of different activation energy values for the electric transport in Ce1/3NbO3: 0.78 eV for T > 800 °C and 0.39 eV for T < 800 °C. Atomistic simulations have shown that the energy required to move the Ce3+ ions is around 8 eV, which is one order of magnitude higher of what is experimentally found. Additionally, electrical measurements at different partial pressures of oxygen show that the material has oxygen-ion conductivity, but the activation energy of 0.39 eV suggests other possible mechanisms of electrical conductivity. One of these possibilities is electronic transport, where the activation energy could be due to the band gap. To determine whether electronic conductivity is contributing in the low temperature regime, we performed ab-initio DFT electronic calculations to evaluate the gap, using the modified Becke–Johnson potential. Due to a Ce:4f level found in the gap, which obstructed the convergence of the calculation, we used the LDA + U approach on the Ce atoms, this moves the 4f levels out of the gap. We used the values U = 1, 2 and 3 eV, then extrapolated back to U = 0 to find the location of the Ce:4f state in the gap. The computed value of the activation energy for electronic conductivity is higher than the experimental one, and resembles more the optical gap value found in Ce1/3NbO3. Based on this result, it can be inferred that the electrical conductivity in Ce1/3NbO3 proceeds via anionic charge carriers in the entire temperature studied range.

Journal article

Kishimoto M, Lomberg M, Ruiz-Trejo E, Brandon NPet al., 2015, Numerical modeling of nickel-infiltrated gadolinium-doped ceria electrodes reconstructed with focused ion beam tomography, Electrochimica Acta, Vol: 190, Pages: 178-185, ISSN: 1873-3859

A one-dimensional numerical model of a nickel-infiltrated gadolinium-doped ceria (Ni-GDC) electrode has been developed to investigate the effects of electrode microstructure on performance. Electrode microstructural information was obtained with focused ion beam tomography and microstructural parameters were quantified, such as tortuosity factor, surface area and particle/pore sizes. These have been used to estimate the effective transport coefficients and the electrochemical reaction rate in the electrodes. GDC was considered as a mixed ionic and electronic conductor and hence the electrochemical reaction is assumed to occur on the GDC-pore contact surface, i.e. double-phase boundaries (DPBs). Sensitivity analysis was conducted to investigate the effect of electrode microstructure on both transport properties and electrochemical activity, including the effect of DPB density, GDC tortuosity factor and pore size. The developed model offers a basis to understand the microstructure-performance relationships and to further optimize the electrode microstructures.

Journal article

Yanez-Gonzalez A, Ruiz-Trejo E, van Wachem B, Skinner S, Beyrau F, Heyes Aet al., 2015, A detailed characterization of BaMgAl10O17:Eu phosphor as a thermal history sensor for harsh environments, SENSORS AND ACTUATORS A-PHYSICAL, Vol: 234, Pages: 339-345, ISSN: 0924-4247

Journal article

Turcaud JA, Bez HN, Ruiz-Trejo E, Bahl CRH, Nielsen KK, Smith A, Cohen LFet al., 2015, Influence of manganite powder grain size and Ag-particle coating on the magnetocaloric effect and the active magnetic regenerator performance, ACTA MATERIALIA, Vol: 97, Pages: 413-418, ISSN: 1359-6454

Journal article

Boldrin P, Ruiz Trejo E, Tighe C, Chang KC, Darr J, Brandon NPet al., 2015, Impregnation of nanoparticle scaffolds for syngas-fed solid oxide fuel cell anodes, ECS Conference on Electrochemical Energy Conversion & Storage with SOFC-XIV, Publisher: Electrochemical Society, Pages: 1219-1227, ISSN: 1938-6737

A strategy for fabrication of solid oxide fuel cell anodes with improved porosity and lower sintering temperatures by impregnation of nanoparticle-containing porous scaffolds of ceria-gadolinia (CGO) has been developed. The CGO scaffolds are fabricated using a screen-printed ink containing nanoparticles and commercial particles of CGO and polymeric pore formers. Scanning electron microscopy and in situ ultra-small angle X-ray scattering show that incorporation of nanoparticles increases the porosity by allowing a reduction in sintering temperature. Electrochemical characterisation of symmetrical cells shows that the cells sintered at 1000°C possess similar electrode polarisation compared to those sintered at 1300°C. Button cell testing showed that reducing the sintering temperature produced cells which perform better at 700°C and below in hydrogen, and performed better at all temperatures using syngas. This approach has the potential to allow the use of a wider range of nanomaterials, giving a finer control over microstructure.

Conference paper

Ruiz-Trejo E, Zhou Y, Brandon NP, 2015, On the manufacture of silver-BaCe0.5Zr0.3Y0.16Zn0.04O3−δ composites for hydrogen separation membranes, International Journal of Hydrogen Energy, Vol: 40, Pages: 4146-4153, ISSN: 1879-3487

Silver- BaCe0.5Zr0.3Y0.16Zn0.04O3−δ (Ag/BCZYZ) composites were investigated due to their potential application as hydrogen separation membranes, with emphasis on their fabrication and characterization. A precursor powder of BCZYZ was prepared via a wet chemical route and characterized by XRD, SEM and dilatometry. The precursor powder was coated with silver using Tollens reaction and then sintered under a variety of conditions. It was possible to obtain dense samples with a low level of non-percolating silver (2 vol%). Silver was present even if sintered at 1300 °C as it remained trapped in the ceramic matrix. The overall conductivity of a dense sample with 2 vol% of silver increased when compared to pure BCZYZ, and in particular the grain boundary resistance decreased considerably. A measurement of the open circuit voltage in fuel cell mode indicates the presence of mixed electronic-protonic conductivity in the composite.

Journal article

Ruiz-Trejo E, Boldrin P, Medley-Hallam JL, Darr J, Atkinson A, Brandon NPet al., 2015, Partial oxidation of methane using silver/gadolinia-doped ceria composite membranes, Chemical Engineering Science, Vol: 127, Pages: 269-275, ISSN: 1873-4405

Methane was partially oxidised to CO using oxygen permeated through a 1 mm thick silver/Ce0.9Gd0.1O2−x (Ag/CGO) composite membrane operating at 500–700 °C with air at 1 bar pressure. The membranes were fabricated by sintering ultrafine nanoparticles of gadolinia-doped ceria (<5 nm) coated with silver using Tollens׳ reaction. This unique combination led to dense composites with low content of silver (7 vol%), no reaction between the components and predominant metallic conductivity. When feeding 4% methane at 700 °C to a 1-mm thick Ag/CGO using Ni as reforming catalyst, the conversion reached 21% and the CO selectivity 92% with an estimated oxygen flux of 0.18 mL min−1 cm−2 (NTP). The samples were stable in carbon-containing atmospheres and under a large pO2 transmembrane pressure difference at temperatures below 700 °C for 48 h.

Journal article

Ruiz-Trejo E, Atkinson A, Brandon NP, 2015, Metallizing porous scaffolds as an alternative fabrication method for solid oxide fuel cell anodes, Journal of Power Sources, Vol: 280, Pages: 81-89, ISSN: 1873-2755

A combination of electroless and electrolytic techniques is used to incorporate nickel into a porous Ce0.9Gd0.1O1.90 scaffold. First a porous backbone was screen printed into a YSZ electrolyte using an ink that contains sacrificial pore formers. Once sintered, the scaffold was coated with silver using Tollens' reaction followed by electrodeposition of nickel in a Watts bath. At high temperatures the silver forms droplets enabling direct contact between the gadolinia-doped ceria and nickel. Using impedance spectroscopy analysis in a symmetrical cell a total area specific resistance of 1 Ωcm2 at 700 °C in 97% H2 with 3% H2O was found, indicating the potential of this fabrication method for scaling up.

Journal article

Kishimoto M, Lomberg M, Ruiz-Trejo E, Brandon NPet al., 2015, Towards the design-led optimization of solid oxide fuel cell electrodes, Pages: 2019-2028, ISSN: 1938-5862

A one-dimensional numerical model of a nickel-infiltrated gadolinium-doped ceria (Ni-GDC) electrode has been developed to investigate the effects of electrode microstructure on performance. Electrode microstructural information was obtained with focused-ion beam tomography and microstructural parameters were quantified. These have been used to estimate the effective transport coefficients and the electrochemical reaction rate in the electrode. GDC was considered as a mixed ionic and electronic conductor and hence the electrochemical reaction was assumed to occur on the GDC-pore contact surface, i.e. double-phase boundaries (DPBs). Sensitivity analysis was conducted to investigate the effect of electrode microstructure on both transport properties and electrochemical activity. The developed model offers a basis to understand the electrode-microstructure relationships and to further optimize the electrode microstructures.

Conference paper

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