Electrode Polarization Resistance Research Articles

Decreasing the Polarization Resistance of BaCo0.7Fe0.2Nb0.1O3–δ Cathodes by Infiltration of Ce0.8Y0.2O2–δ

AbstractCe0.8Y0.2O2–δ (YDC) was infiltrated into a BaCo0.7Fe0.2Nb0.1O3–δ (BCFN) cathode of intermediate temperature sold oxide full cells (IT‐SOFCs) in order to decrease its cathodic polarization resistance. BCFN and YDC infiltrated BCFN electrodes were fabricated on dense Ce0.8Gd0.2O2–δ (GDC) thin pellets to form symmetrical cells. The electrochemical impedance spectra of the symmetrical cells were investigated in this present study. Firstly, the thickness of BCFN electrodes was optimized, and controlled at 30 µm for further study. The effects of infiltrated YDC amount and firing temperature on electrode polarization resistance were studied. The symmetrical cells infiltrated with 30 μL YDC solution and fired at 900 °C exhibited the lowest electrode polarization resistance in all samples. It was suggested that infiltration of YDC resulted in more active sites and prolonged TPBs in electrodes, improving the surface oxygen exchange, and finally improved the electrode performance.

Chromium Resistive BaCeO3 Infiltrated La0.6Sr0.4Co0.2Fe0.8O3 Cathode of Solid Oxide Fuel Cells

Poisoning of cathodes due to deposition of chromium species from the Fe-Cr alloy interconnect is a major degradation mechanism of solid oxide fuel cell stacks. Our previous studies have shown that the infiltration of gadolinium doped ceria or BaO nanoparticles leads to substantially enhanced chromium resistive ability of La0.6Sr042Co0.2Fe0.8O3 (LSCF) cathodes (Electrochemistry Communications, 37, 84 (2013) and Physical Chemistry Chemical Physics, 17, 4870 (2015)). In this presentation we report BaCeO3 infiltrated LSCF electrodes by co-infiltrating BaO and CeO2 into the LSCF structure and investigate the electrode activity and stability under accelerated chromium poisoning conditions. The results show that 0.8 mg cm-2 BaCeO3 infiltrated LSCF has extraordinary electrocatalytic activity for the oxygen reduction reaction. For example, electrode polarization resistance is 0.12, 0.062, 0.014 and 0.002 Ω cm2 at 650, 700, 750 and 800 oC, respectively. More importantly, the BaCeO3-LSCF electrode is highly tolerant against Cr contamination. For the operation in the presence of bare Fe-Cr alloy without any protective layer, RE is as low as 0.07 Ω cm2 after the polarization at 200 mA cm-2 and 800 oC for 40 h, much smaller than 0.74 Ω cm2 of pristine LSCF cathode after the test under identical conditions. The results imply that the BaCeO3infiltration is an effective means to enhance the electrode activity and stability of LSCF electrodes for operation in the presence of Fe-Cr interconnect.

Relating Microstructure to Surface Exchange Kinetics Using in Situ Optical Absorption Relaxation

Reversible solid oxide cells (R-SOCs) are promising devices, which could directly convert chemical fuels to electrical power (Solid Oxide Fuel Cells, SOFCs) and store energy as chemical fuels (Solid Oxide Electrolysis Cells, SOECs). Nonetheless, the polarization resistance of the oxygen electrode, often originating in sluggish surface oxygen incorporation/ excorporation kinetics, causes large efficiency losses. Seeking to understand and ultimately improve surface exchange kinetics, many previous studies have focused on controlling the bulk defect chemistry (oxygen vacancy and/or electron content) or electronic structure through tailoring chemistry or strain. Fewer studies have focused on understanding the role of microstructural features in controlling both the surface exchange coefficient, k, and its deterioration over time (aging), especially for mixed ionic-electronic conductor (MIEC) electrode materials. SrTi0.65Fe0.35O3-δ (STF35) is a mixed conductor with fast k, which serves as a model system in our studies, to study the impact of microstructure (particularly crystallinity) on k and the aging behavior. In this work, a series of STF35 thin films were grown by Pulsed Laser Deposition (PLD) using different growth parameters (temperature, time) to vary structural aspects such as crystallinity in a well-defined and controlled manner. Structural characterization of the films’ thickness, orientation, crystal quality, and grain size was performed using X-ray reflectivity and diffraction (coupled scans, rocking curves) plus atomic force microscopy. The surface exchange kinetics (k) of the series of STF35 thin films were measured in situ and continuously using a novel optical transmission relaxation approach, to investigate the impact of the aforementioned structural features on the performance. The approach takes advantage of the different optical absorption signatures of Fe in different valence states. Both XRD and AFM results showed that the STF35 thin films grown on yttria-stabilized zirconia at temperatures up to 400 ºC are amorphous, while the films become crystalline and oriented around 500-550 °C, with larger grains at higher temperatures. There is no obvious optical response to changing gas content for the as-grown amorphous films below 500 °C; however, upon heating these films at 500-550 °C, where they begin to crystallize, rapid oxygen exchange could be observed optically in the same films. Subsequently, the films optically demonstrated an ability to exchange oxygen at temperatures as low as 350 °C. The results indicate that crystalline thin films showed faster oxygen exchange kinetics compared to the amorphous thin films. The aging of STF thin films’ surface exchange kinetics was also studied optically as a function of microstructure, which in every case demonstrated a rapid decline in k during the initial 50 hours of measurement. Implications for electrode design will be discussed.

La/Ba-based cobaltites as IT-SOFC cathodes: a discussion about the effect of crystal structure and microstructure on the O2-reduction reaction

This work discusses the effect of the crystal structure and microstructure on the electrocatalytic activity of Lanthanum-Barium cobaltite (LBC), evaluatedas IT-SOFC cathode. Two systems with similar microstructures (particle sizes dp≈1–5μm) are used to compare the effect of the crystal structure: the cubic La0.5Ba0.5CoO3-δ with La/Ba cations randomly distributed, and a tetragonal LaBaCo2O6-δ with La/Ba layered distribution. In addition, the effect of microstructure is studied by using a newsol-gel route which allows obtaining LaBaCo2O6-δ with smallest particle size (dp≈500nm). The electrode reaction isstudied by electrochemical impedance spectroscopy (EIS) as a function of temperature (T) and oxygen partial pressure (pO2). The electrode polarization resistance (RC,P) presents two contributions, a low frequency (RLF) and a high frequency Gerischer-resistance (RG). On one side, RLF, associated to the O2-gas diffusion across the gas layer boundary, presents low activation energy (Ea) and is proportional to pO2−1, presenting the same values regardless the microstructure.On the other side, RG is in agreement with anO2-reduction mechanismco-limited by O-surface exchange and O-bulk diffusion. RG is strongly influenced by structure (trough the diffusion coefficient DV) and microstructure (inversely proportional to dp). Besides, RG decreases by reducing dp and by modifying the La/Ba ordering from layered to randomly distributed.

Evolution of activity of impregnated with CeO2 Ni-SSZ anodes of fuel cells

The behavior of polarization resistance of 50% Ni + 50% Zr0.9Sc0.1O1.95 electrodes in contact with Zr0.84Y0.16O1.92 electrolyte both in their initial state and after the introduction of cerium oxide into their porous structure is studied by the method of electrochemical impedance. The 1500 h experiments carried out at 800°С in the 60% H2O + 40% H2 gas medium demonstrate that the degradation rate of the electrodes in their initial state is somewhat lower than after the introduction of CeO2, while the initial polarization resistance of thus modified electrodes is 5-times lower. It is found that the introduction of cerium oxide can lower down the polarization resistance of electrodes subjected to short-term 500 h exposures.

Perovskite LSCM impregnated with vanadium pentoxide for high temperature carbon dioxide electrolysis

Perovskite La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) has been used as a typical cathode for CO2 electrolysis in oxide-ion conducting solid oxide electrolyzers; however, the limited electrocatalytic activity still restricts electrode kinetic process. In this work, catalytic-active V2O5 nanocatalyst is impregnated in LSCM cathode to enhance electrocatalytic performance. Electrochemical measurements show that the loading of 2wt%V2O5 significantly improves electrode activity and accordingly reduces electrode polarization resistance from 2.6Ω•cm2 for LSCM to 1.2ΩCm2 for V2O5-LSCM in pure hydrogen. The current densities and Faradic efficiencies with V2O5-LSCM electrode are remarkably enhanced by 30% and 40% in contrast to bare LSCM electrode in the voltage region of 1.2-2.0V at 800°C for direct carbon dioxide electrolysis, respectively.

Effect of starting solution concentration in spray pyrolysis on powder properties and electrochemical electrode performance

Nano-composite powders consisting of Sr-doped SmCoO3 (SSC) and Sm-doped CeO2 (SDC) were synthesized by spray pyrolysis using different cation concentration solutions, and the effect of this cation concentration on the electrochemical performance of electrodes fabricated using the powders was investigated. Regardless of the cation concentration, spherical particles consisting of nano-order fine SSC and SDC crystallites were successfully obtained. Although the crystal size of all the synthesized powders was almost the same, the particle size increased with increasing cation concentration. The microstructure of the cathodes fabricated using the nano-composite powders clearly depended on the particle size of the starting powders; the microstructure became coarser with increasing particle size and the original particle shapes partially remained in the cathode network. Also, electrochemical measurements for solid oxide fuel cells with these cathodes revealed that the electrode performance was related to the microstructure. The electrode polarization resistance decreased with decreasing particle size, and a relatively fine microstructure cathode exhibited lower electrode polarization resistance, resulting in a high performance of 1.52Wcm−2 at 700°C. These results indicate that controlling the particle size, as well as controlling the nanostructure within the particles, is crucial to attain high performance electrodes.

A durable, high-performance hollow-nanofiber cathode for intermediate-temperature fuel cells

Hollow nanofibers of PrBa0.5Sr0.5Co2O5+δ (PBSC), created by an electrospinning process, are assembled into a three dimensional (3D) fibrous porous electrode, providing facile pathways for gas transport and excellent electrical conductivity for efficient charge transfer and, thus, greatly enhancing the rate of oxygen reduction reactions (ORR), as confirmed by the small electrode polarization resistance and low activation energy. A simple geometric modeling suggests that an electrode with longer fibers tends to be more efficient in facilitating mass and charge transfer under the conditions studied. A solid oxide fuel cell based on this 3D fibrous cathode demonstrates a peak power density of 1.11Wcm−2 at 550°C when humidified H2 was used as fuel and ambient air as oxidant. The fibrous architecture also shows excellent stability under the operating conditions. Further and in particular, the high-performance hollow-fiber electrodes are also applicable to other energy storage and conversion systems.

Effects of NiO on the conductivity of Ce0.85Sm0.15O1.925 and on electrochemical properties of the cathode/electrolyte interface

Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.925−0.5 at.% NiO (SDCN) are investigated as electrolytes for solid oxide fuel cells (SOFCs). Impedance spectroscopy measurements reveal that the grain boundary resistance can be significantly reduced by adding 0.5 at.% NiO to SDC. Symmetric cells of the BaCo0.7Fe0.2Nb0.1O3−δ (BCFN) electrode on SDC and SDCN electrolytes are fabricated and the electrochemical properties of the electrode/electrolyte interface are investigated. The polarization resistance of the BCFN electrode on the SDCN electrolyte is much lower than that of the BCFN electrode on the SDC electrolyte, mainly because of the increase in the electrolyte conductivity and the decrease in the Si content at the electrode/electrolyte interface. NiO is able to restrict the diffusion of the siliceous impurity from the electrolyte to the electrode/electrolyte interface. Single cells based on SDC and SDCN electrolytes are fabricated using Ni0.9Cu0.1Ox-SDC as the anode and BCFN as the cathode. At 800 °C, the maximum power density of the SDCN-based cell is 0.745 W cm−2, which is much higher than that of the SDC-based cell.

(Invited) High Performance Nano-Ceria Electrodes for Solid Oxide Cells

In solid oxide electrochemical cells, the conventional Ni-based fuel-electrodes provide high electrocatalytic activity but they are often a major source of long-term performance degradation due to carbon deposition, poisoning of reaction sites, Ni mobility, etc. Doped-ceria is a promising mixed ionic-electronic conducting oxide that could solve these issues if it can be integrated into an appropriate electrode structure. Two new approaches to obtain high-performance nanostructured doped-ceria electrodes are highlighted. The first is an infiltration-based architecture with Ce0.8Pr0.2O2-δ forming the active surfaces on a porous backbone with embedded electronic current collector material, yielding one of the highest performances reported for an electrode that operates either on fuel or oxidant. The second is a nano-Ce0.9Gd0.1O2-δ thin film prepared by spin-coating, which provides an unprecedented electrode polarization resistance of ~0.01 Ω cm2 at 650 °C in H2/H2O. These results demonstrate that nano-ceria has the ability to achieve higher performance than Ni-based electrodes and show that the main challenge is obtaining sufficient electronic current collection without adding too much inactive material.

Time Resolved DXAS Study on Micro and Nano NiO/Ce0.9Gd0.1O1.95 Cermets for Intermediate Temperature Solid Oxide Fuel Cells

The development of Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFC) as high efficient energy conversion systems requires the optimization of its components: cathode, anode and electrolyte. CeO2-based materials present excellent catalytic properties for the oxidation of H2 and CH4 fuels. Through the incorporation of metals oxides (Gd2O3, Sm2O3, Y2O3) in the CeO2 lattice the ionic conductivity can be significantly improved, becoming good electrolytes, particularly (Ce,Gd)O2- d (CGO) compound. Furthermore, the addition of metals such as Ni or Cu enhances the material electronic conductivity, thus enabling them as efficient IT-SOFC anodes [1]. It is recognized that the polarization characteristics of Ni-CGO cermet anodes are strongly affected by their preparation conditions, which determine the microstructure and morphology of these materials. For that reason, further exploration of new microstructures has to be done in order to improve anode performance [2]. In a previous work we presented a new modified sol-gel method to incorporate Ni into the precursors solution, which produced a powder with a very homogeneous Ni distribution resulted in smaller particle sizes with narrower distribution than the commercial cermet [3]. In this work, we present a study of the performance of these anodes sintered at different temperatures for the hydrogen oxidation reaction. The electrode polarization resistance is evaluated through Electrochemical Impedance Spectroscopy (EIS) on symmetric cells, while the cell power of Ni-CGO/CGO/LSCF configuration is measured in a home-made equipment. The structural and morphological characterization are analyzed by X-Ray Diffraction and Transmission and Scanning Electron Microscopy, respectively. The study of the oxidation state and coordination of Ce and Ni in these cermets, simulating in-operando conditions, was performed using synchrotron Dispersive X-ray absorption spectroscopy (DXAS) technique. The correlation between the electrochemical behavior, electronic properties and microstructural characteristics will be discussed. [1] W.C. Chueh, Y. Hao, W. Jung and S.M. Haile, Nature Materials 11 (2012) 155-161. [2] Z. Liu, B. Liu, D. Ding, M.F. Liu, F.L. Chen, C. R. Xia, Journal of Power Sources 237 (2013) 243-259. [3] A. Fernandez Zuvich, A. Soldati, S. Larrondo, M. Saleta, D.G. Lamas, L. B. Baqué, A. Caneiro and A. Serquis, ECS Transactions 64 (2014) 233-240.

Correlation Studies of Electrochemical and Crystallographic Properties on SOFC and Soec Related Materials By Simultaneous EIS and XRD in-Operando Measurements

The complexity of the process involved in the development of new technological devices requires the use of complementary characterization techniques in order to understand the whole phenomena. A usual approach to study materials for devices with non-ambient working conditions is to perform the experiment simulating the same environment (temperature, atmosphere, pressure, magnetic field, etc.). Although being a reasonable approach, this could fall short when real working conditions are more complex [1, 2]. Nowadays, the implementation of in-operando characterization techniques is a major field of capabilities development at world-wide large facilities [3-5] and, in lesser scale, at laboratory level [6]. The situation afore described is particularly applicable to the field of high temperature electrochemical devices such as Solid Oxide Fuel Cells (SOFC) and Solid Oxide Electrolyzer Cells (SOEC) where the different materials involved in the device are exposed to extreme environments and, moreover, could exists a strong correlation between different properties. For example, it is well known the dependence of the crystallographic parameters with several relevant electrochemical properties such as electrical conductivity, chemical compatibility, electrode reaction performance, etc [7, 8]. The simultaneous characterization of such properties in-operando condition allows gaining insight into some still not well understood phenomena. In this work, we present the simultaneous crystallographic and electrochemical characterization through X-ray Diffraction and Electrochemical Impedance Spectroscopy, respectively, of three different materials: a proton conductor electrolyte, an O2 electrode material, and a symmetrical (H2 and O2) electrode material. In order to study these materials we use a home-made sample holder specifically designed to study the electrical resistivity and the electrode polarization resistance evolution simultaneously with structural properties, under in-situ or in-operando conditions. Firstly, we followed a second order phase transition found on the proton conductor electrolyte material BaCe0.4Zr0.4Y0.2O3- δ at around 400°C under wet oxidizing and reducing atmospheres and its influence on its ionic conductivity. This phase transition (related to a crystallographic distortion coming from the hydration/dehydration process) affects the lattice parameters and, therefore, the ionic transport properties. The utilization of the in-operando sample holder allow us to get direct information about how this structural distortion affects the transport properties. Secondly, we studied the electrochemical performance of Nd2NiO4+ δ electrode material and its crystallographic parameters under an in-operando condition through the simultaneous EIS and XRD measurements of a half cell applying several electrode potentials. While their ionic conductivity combines the migration of interstitial oxygen (δ) in the rock-salt layer and the migration of oxygen vacancies in the perovskite layer, it is actually dominated by the former due to the higher mobility of these defects. Therefore, the O-content play a fundamental role in the O2 electrode activity (specially working under anodic polarization) and, moreover, is the driven force of a first order phase transition from an orthorhombic (Fmmm) to a tetragonal (I4/mmm) symmetry at 505 °C. Lastly, we also followed the second order phase transition exhibited by the symmetrical electrode La0.4Sr0.6Ti0.5Co0.5O3 at around 300 °C (in both oxidizing and reducing atmospheres), passing from a rhombohedral perovskite structure (R-3c) to the ideal cubic perovskite (Pm-3m) as temperature is increased. This phase transition is mediated by the rigid rotation of the oxygen octahedra (around a-a-a- direction, in Glazer’s notation) [9]. The correlation between the electrochemical properties and structural parameters on each system is discussed. 1. Zhang, C., et al., Nature Materials, 2010. 9(11): p. 944-949. 2. Kirtley, J.D., et al., Analytical Chemistry, 2012. 84(22): p. 9745-9753. 3. Peterson, V.K. and C.M. Papadakis, IUCrJ, 2015. 2(2): p. 292-304. 4. Gallo, E. and P. Glatzel, Advanced Materials, 2014. 26(46): p. 7730-7746. 5. Mueller, D.N., et al., Nature Communications, 2015. 6. 6. Brightman, E., et al., Review of Scientific Instruments, 2012. 83(5). 7. Imada, M., A. Fujimori, and Y. Tokura, Reviews of Modern Physics, 1998. 70(4 PART I): p. 1039-1263. 8. Fagg, D.P., et al., Solid State Ionics, 2003. 156(1-2): p. 45-57. 9. Napolitano, F., et al., ECS Transactions, 2013. 58(3): p. 185-193.

(Invited) High Performance Nano-Ceria Electrodes for Solid Oxide Cells

Solid oxide electrochemical cells (SOCs) show great promise to become a cornerstone of a renewable energy future, based on their ability to reversibly store electrical energy as green chemical fuels and to produce electricity from fuels. The conventional nickel and yttria-stabilized zirconia composite (Ni-YSZ) fuel-electrode provides high reaction rates for the H2O/CO2 reduction and H2/CO oxidation reactions, but it is often a major source of long-term performance degradation due to poisoning of reaction sites, carbon deposition, Ni mobility, and other degradation mechanisms. Alternative electrodes made of mixed ionic and electronic conducting (MIEC) oxides have been shown to avoid these issues, however they also typically show much lower electrochemical activity than that of state-of-the-art Ni-YSZ electrodes. During the last 10 years, DTU Energy has developed MIEC electrodes with nanostructured doped ceria (NDC) electrocatalysts as the active component. As doped ceria has low electronic conductivity, the initial electrodes were comprised of a small amount of NDC coated by infiltration onto the internal surfaces of porous electron-conducting backbones made of Sr(Ti,Nb)O3 and/or stainless steel. Although the electrochemical activity matches or even exceeds that of Ni-YSZ, insufficient long-term stability has proven to be a persistent issue with these electrodes. Recently we have developed new NDC based electrodes with no additional electron conductor or only a small volume fraction. Nanostructure optimization, primarily by adjusting heat treatment parameters, have yielded an unprecedented electrode polarization resistance (Rp) of ~0.01 Ω cm2 at 650 °C in a fuel gas mixture of H2/H2O 0.8/0.2 atm for a nanoporous Ce0.9Gd0.1O1.95-d thin film prepared by spin-coating. Initial results show highly promising long-term stability of the electrochemical reaction rates, although providing and maintaining sufficient electronic current collection is challenging due to the low in-plane conductivity of the film. We found that adding a small amount of a better electron conductor to the film minimizes ohmic resistance and maintains electronic contact. Similarly, more conventional structured porous electrodes with NDC surfaces and only a small amount of electronic conductor were found to show high activity and negligible electronic resistivity. We have also applied the same design principles to obtain NDC based electrodes that work almost equally well as either a fuel-electrode or oxygen-electrode, using CexPr1-xO2-d as an active surface component. High activity for both fuel and oxidant electrochemical reactions is possible because CexPr1-xO2-d is a MIEC in both the low oxygen partial pressure (pO2) of the fuel gas and high pO2of the oxidant gas. Electrochemical impedance and structural analyses of these highly defective, highly active MIEC electrodes will be presented, together with the impact of current collector components and a discussion of known limitations and future outlook. Figure: (a) Impedance spectra measured 42 h apart at 650 °C in a fuel gas mixture of H2/H2O 0.8/0.2 atm on a nanoporous Ce0.9Gd0.1O1.95-d thin film electrode. In between the measurements, the operating temperature was kept at 550 °C to 650 °C. (b) Scanning electron micrograph with backscattered electron detector on the same type of electrode. Figure 1

BaFe0.6Co0.3Ce0.1O3−δ as cathode materials for proton-conducting solid oxide fuel cells

Mixed ionic (O2−) and electronic conductor BaFe0.6Co0.3Ce0.1O3−δ (BFCC) has been prepared by the modified Pechini method and studied as cathode materials for proton-conducting solid oxide fuel cells with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) as the electrolyte. A new anode-supported SOFC material system Ni-BZCYYb|BZCYYb|BFCC is fabricated via a simple dry pressing/co-firing method. The performance of the button cell is tested and characterized under low temperature range from 550 to 650°C with humified H2 as fuel, oxygen as oxidant. The fabricated single button cell with about 20-μm-thick electrolyte exhibits promising performance, with maximum power densities of 451, 362, and 287mWcm−2 at 650, 600, and 550°C, respectively. The Nyquist plot of the impedance data at open circuit conditions shows that the overall electrode polarization resistances is 0.15, 0.28, 0.60Ωcm2 at 650, 600 and 550°C, respectively. No obvious cell degradation was observed during the 30-h durability test at a constant voltage of 0.5V at 650°C.

Li4Ti5O12–LiAlO2 Composite as High Performance Anode Material for Lithium-Ion Battery

A facile solid-state method to improve the rate performance of Li4Ti5O12 in lithium-ion batteries by LiAlO2 in situ modification is presented in this work. XRD shows that the LiAlO2 modification does not change the spinel structure of Li4Ti5O12 but forms Al-doped Li4Ti5O12–LiAlO2 composites, and little Al doping decreases the lattice parameter of doped Li4Ti5O12. SEM shows that all samples are composed of 1–2 μm primary particles with irregular shapes. Raman spectra reveal that the intensity of these lines for Li4Ti5O12–LiAlO2 composites obviously decreases caused by a modification of the LiAlO2 phase. CV and EIS tests indicate that the doping of Al3+ and the combination with in situ generated LiAlO2 on the surface of Li4Ti5O12 are favorable for reducing the electrode polarization and charge-transfer resistance, and then improve the reversibility and lithium ion diffusion coefficient of Li4Ti5O12, resulting in its relatively higher rate capacity. Charge–discharge tests reveals that Li4Ti5O12–LiAlO2 compos...

Probing novel triple phase conducting composite cathode for high performance protonic ceramic fuel cells

A key obstacle to practical operation of protonic ceramic fuel cells (PCFCs) is the development of high efficient cathode materials. In this study, we report the identification of Gd0.1Ce0.9O2-δ (GDC) infiltrated PrBaCo2O5+δ (PBC) – BaZr0.1Ce0.7Y0.2O3-δ (BZCY) materials as a novel triple phase (e−/H+/O2−) conducting composite cathode of PCFCs. This triple phase conducting composite cathode is confirmed to promise the extended electrochemical active sites and the accelerated oxygen reduction process. The polarization resistance (such as 0.051 Ω cm2 at 700 °C under open circuit voltage) of GDC infiltrated PBC-BZCY electrode is lower than other reported PCFCs, indicating the advanced electrochemical performance. The encouraging result is definitely a significant progress for the PCFCs field, which demonstrates that desirable electrode performance can be realized by applying triple phase conducting composite electrode.

Highly active nanoscale Ni - Yttria stabilized zirconia anodes for micro-solid oxide fuel cell applications

The optimum operating temperature of micro solid oxide fuel cells (μ-SOFCs) is < 600 °C, which normally results in an undesirably high internal resistance. While the resistance of the electrolyte (most commonly yttria stabilized zirconia, YSZ) can be lowered significantly simply by decreasing its thickness, minimization of the electrode resistance (thus maintaining rapid reaction kinetics) is not as straightforward. In this work, an ethylene glycol based polymeric precursor solution, which promotes the intimate mixing of the Ni, Y and Zr components prior to their crystallization as oxides, was spin-coated onto a YSZ disc, generating a nanocomposite NiO-YSZ thin film. The in-situ reduction of NiO phase within the dense NiO-YSZ film to Ni, in combination with the nanoscale size of the Ni and YSZ particles (ca. 25 nm), resulted in a nanoporous, Ni-YSZ anode morphology (thickness < 1 μm) with a homogeneous distribution of Ni and YSZ and a very high triple phase boundary length. Very small electrode polarization resistances of 0.65 Ω cm2 per electrode were obtained at 550 °C in humidified H2, the lowest values yet reported for SOFCs at this temperature. These highly active anodes are therefore very promising for use in next generation μ-SOFCs.

Reaction Kinetics of Protons and Oxide Ions in LSM/Lanthanum Tungstate Cathodes with Pt Nanoparticle Activation

Composite electrodes of La0.8Sr0.2MnO3 (LSM)/La28–xW4+xO54+3x/2 (x = 0.85, “LWO56”) on LWO56 electrolytes have been characterized by use of electrochemical impedance spectroscopy vs. pO2 and temperature from 900°C, where LWO56 is mainly oxide ion conducting, to 450°C, where it is proton conducting in wet atmospheres. The impedance data are analyzed in a model which takes into account the simultaneous flow of oxide ions and protons across electrolyte and electrodes, allowing extraction of activation energies and pre-exponential factors for the partial electrode reactions of protons and oxide ions. One composite electrode was infiltrated with Pt nanoparticles with average diameter of 5 nm, lowering the overall electrode polarization resistance (Rp) at 650°C from 260 to 40 Ω cm2. The Pt-infiltrated electrode appears to be rate limited by surface reactions with activation energy of ∼90 kJ mol−1 in the low temperature proton transport regime and ∼150 kJ mol−1 in the high temperature oxide ion transport regime. The charge transfer reaction, which makes a minor contribution to Rp, exhibits activation energies of ∼85 kJ mol−1 for both oxide ion and proton charge transfer.

Understanding degradation of solid oxide electrolysis cells through modeling of electrochemical potential profiles

Establishing the spatial distribution of the various chemical and electrochemical potentials in an operating SOEC is critical as several degradation mechanisms are tightly connected to them, but at the same time very challenging to achieve experimentally. Such distributions are presented here on the basis of a two dimensional bi-layer electrolyte SOC model including for both electrodes a description of activation, concentration, and conversion losses. An extensive parametric study is reported to illustrate the influence of the electrode polarization resistances, the ionic and electronic conductivities in the electrolyte, the gas composition, temperature, and pressure on the current density distribution over the cell and the oxygen activity distribution within the electrolyte. The developed model is further used to simulate long-term durability experiments during different stages of operation, thereby helping to rationalize microstructural and chemical changes observed in post-mortem analysis. Finally, measures to mitigate degradation by changing conditions of operation, material or electrode properties or overall cell geometry are suggested.

Mechanism and Kinetics of Ni-Y2O3-ZrO2 Hydrogen Electrode for Water Electrolysis Reactions in Solid Oxide Electrolysis Cells

Ni-Y2O3 stabilized ZrO2 (Ni-YSZ) cermet is the most commonly used hydrogen electrode for hydrogen oxidation reaction (HOR) under solid oxide fuel cell (SOFC) mode and water reduction reaction (WRR) under solid oxide electrolysis cell (SOEC) mode. Here we studied the electrocatalytic activity of Ni-YSZ electrodes as a function of Ni content, water concentration and dc bias for WRR and HOR under SOEC and SOFC modes, respectively. The activity of Ni-YSZ cermet increases significantly with the increase of YSZ content due to the enhanced three phase boundaries (TPB). The electrode activity for the WRR and in less degree for the HOR increases with the increase of steam concentration. The electrode polarization resistance, RE, for the WRR increases with the dc bias, while in the case of HOR, RE decreases with the dc bias, demonstrating that kinetically the WRR and HOR is not reversible on the Ni-YSZ cermet electrodes under SOFC and SOEC operation modes. The WRR can be described by two electrode processes associated with the H2O adsorption and diffusion on the oxygen-covered Ni or YSZ surface in the vicinities of TPB, followed by the charge transfer. The significant increase of high frequency electrode polarization resistance, RH and in much less extent low frequency electrode polarization resistance, RL with the dc bias indicates that the water electrolysis reaction is kinetically controlled by the reactant supply (e.g., the adsorbed H2O species) limited charge transfer process.