In-plane Conductivity Measurements Research Articles

Understanding the Effects of Anode Catalyst Conductivity and Loading on Catalyst Layer Utilization and Performance for Anion Exchange Membrane Water Electrolysis.

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

Mechanism of photo-ionic stoichiometry changes in SrTiO3

The impact of UV light (λ = 365 nm) on defect chemistry and composition of undoped SrTiO3 single crystals was investigated by means of impedance spectroscopy. In-plane conductivity measurements between 280 and 450 °C in air reveal a drastic increase of the bulk conductivity upon UV illumination. The measured time dependence of this increase is in accordance with oxygen chemical diffusion in SrTiO3. The corresponding photo-induced change in defect concentrations is caused by oxygen being pumped from the gas phase into the oxide under UV irradiation. This affects the entire SrTiO3 crystal rather than only the thin UV absorption zone and leads to very high oxygen chemical potentials with nominal oxygen pressures up to 106 bar. After switching the UV light off, the resulting conductivity increase relaxes extremely slowly due to slow surface exchange kinetics. A mechanistic model is introduced to explain the impact of UV light on the oxygen chemical potential as well as on the oxygen vacancy and hole/electron concentrations in semiconducting oxides, here SrTiO3. This model is based on the formation of oxygen quasi-chemical potentials in the illuminated region. It is discussed how the interplay of four kinetic parameters causes the observed time-dependent stoichiometry and thus conductivity changes.

Confined polaronic transport in (LaFeO3)n/(SrFeO3)1 superlattices

Functional oxide superlattices offer new and exciting possibilities for the exploration of emergent properties at the nanoscale. While the behavior of La1−xSrxFeO3 films has been extensively investigated at low temperatures, few studies have been carried out at high temperatures, particularly for LaFeO3/SrFeO3 superlattice systems. Here, we investigate the transport behavior and optical properties of (LaFeO3)n/(SrFeO3)1 superlattices at 373 K and above. Using optical spectroscopy, we observe a low energy excitation at ∼1 eV, attributable to charge transfer between the O 2p and Fe 3d states of the δ-doped single SrFeO3 layer. From in-plane conductivity measurements on the superlattices, we determine activation energies that are much lower than those of alloyed samples and vary with the total number of SrFeO3 layers. This suggests that polaronic transport is confined near the SrFeO3 regions, permitting mobilities significantly enhanced over those in alloyed thin films.

On the Properties and Long-Term Stability of Infiltrated Lanthanum Cobalt Nickelates (LCN) in Solid Oxide Fuel Cell Cathodes

Infiltration as a fabrication method for solid oxide fuel cells (SOFC) electrodes is offering significant improvements in cell performance at reduced materials and fabrication costs, especially when combined with co-sintering. However, important questions regarding the long-term performance and microstructural stability remain unanswered. Here, we present the results of a three-year project, where large footprint anode-supported SOFCs with a co-sintered cathode backbone and infiltrated La0.95Co0.4Ni0.6O3 (LCN) cathodes were developed and thoroughly characterized. The initial long-term performance and stability of this new cell type was investigated for 1500+ hours, coupled with STEM-EDS investigation of the microstructural changes in the infiltrated electrodes. Additionally, electrodes were further aged at elevated temperatures (750 - 900°C) for periods reaching up to 5000 hours, while following changes in the electrode properties using SEM, BET area, and in-plane conductivity measurements. Finally, the mechanical properties of co-sintered cathode backbone cells were determined in four-point bending tests carried out both at room temperature and at 800°C in air. Based on these results, degradation mechanisms were identified and recommendation for safe operation conditions in real life application could be formulated.

Long-Term Stability of Anode-Supported Solid Oxide Fuel Cells with a Co-Sintered Cathode Backbone and Infiltrated La0.95Co0.4Ni0.6O3 (LCN) Electro-Catalyst

Infiltration is a fabrication method that is offering potentially significant improvements in cell performance at reduced materials and fabrication costs, especially when combined with co-sintering. However, important questions regarding the long-term performance and microstructural stability of infiltrated electrodes, as well as the mechanical properties of such electrodes have remained unanswered.Here, we present the results of a three-year project, where large footprint (12 × 12 cm2) anode-supported solid oxide fuel cells with a co-sintered cathode backbone and infiltrated La0.95Co0.4Ni0.6O3 (LCN) cathode active phase were developed and thoroughly characterized. We report the initial and long-term (1500+ hours) performance of infiltrated cathode backbone cells hours running with 40% H2O/H2 as fuel and ambient air as oxidant, coupled with a scanning transmission electron microscopy - energy dispersive X-ray spectroscopy (STEM-EDS) investigation of microstructural changes in the infiltrated electrodes (Fig. 1). Some electrodes were further aged at elevated temperatures (750- 900°C) for periods reaching up to 5000 hours, while following changes in the electrode properties with scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area, and van der Pauw in-plane conductivity measurements. Finally, the mechanical properties of co-sintered cathode backbone cells were determined in four-point bending tests carried out both at room temperature and at 800°C in air. To our knowledge, this is the first time infiltrated cells have been characterized in such detail. The results of the study suggest that significant coarsening of the infiltrated phase takes place upon long-term testing and aging, resulting in decreased active surface area, reduced in-plane electronic conductivity, increased electrode impedance, as well as re-arrangement of the electro-catalyst in the porous backbone. The Weibull strength of the cells at room temperature was 400 MPa with a Weibull modulus of 12. Both the strength and the modulus were found to be lower at elevated temperature.This study was funded by EUDP (Danish Energy Agency) project 64012-0225 “SOFC Accelerated – Development to Accelerate Field Demonstrations”. Figure 1. STEM-EDS maps of infiltrated LCN cathodes after infiltration (left) and after (right) 1500-h cell-testing. Figure 1

Introducing Solubility Control for Improved Organic P-Type Dopants

To overcome the poor solubility of the widely used p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), we have synthesized a series of structure-modified, organic p-type dopants to include alkyl ester groups designed to enable solubility and miscibility control. UV–vis–NIR and cyclic voltammetry measurements show increased solubility of mono- and diester substituted dopants with only modest changes to acceptor strength. Using absorption spectroscopy, photoluminescence, and in-plane conductivity measurements, we demonstrate that the new dopants can successfully p-type dope poly(3-hexylthiophene-2,5-diyl) (P3HT). Monoester substituted dopants are characterized by only slightly reduced electron affinity relative to F4TCNQ, but greater doping effectiveness due to increased miscibility with P3HT. Diester substituted dopants undergo a dimerization reaction before assuming their doped states, which may help anchor dopants into position post deposition, thus decreasing the negative effect...

Epitaxy of Li3xLa2/3–xTiO3 Films and the Influence of La Ordering on Li-Ion Conduction

This paper reports on the epitaxial growth of Li3xLa2/3–xTiO3 thin films by means of pulsed laser deposition and their transport properties. A series of films obtained in this study have revealed that the thin film quality and Li-ion conductivity are highly sensitive to growth conditions as well as target composition. The epitaxial thin films are mostly c-axis oriented, containing a small amount of a-axis domains and antiphase boundaries. In-plane conductivity measurements investigating the relationship between these defects and Li-ion conductivity in Li3xLa2/3–xTiO3 show that the defects are not predominantly resistive against the ionic conduction.

Low temperature thermally stimulated current characterization of nanoporous TiO2 films

Nanoporous films of TiO2 have been obtained by deposition on an alumina substrate of a commercial colloid and Au contacts have been put on surface for in-plane conductivity measurements. Samples have been characterized by means of a series of sequential thermally stimulated currents scans, from nearly liquid He temperature up to 200 K, in order to get information on: conduction mechanisms, distribution of intragap density of states and recombination times of free an localized carriers. A model for hopping conductivity has been considered to properly take into account of localized states filling; it has been used to fit the experimental data, without a-priori assumptions on the nature of the conduction mechanisms. It is found that hopping conductivity prevails over free-carrier, in quasi-equilibrium regime, in the investigated temperature range.

‘Illusional’ nano-size effect due to artifacts of in-plane conductivity measurements of ultra-thin films

The nano-size effect, which indicates a drastic increase in conductivity in solid electrolyte materials of nano-scale microstructures, has drawn substantial attention in various research fields including in the field of solid oxide fuel cells (SOFCs). However, especially in the cases of the conductivity of ultra-thin films measured in an in-plane configuration, it is highly possible that the 'apparent' conductivity increase originates from electrical current flowing through other conduction paths than the thin film. As a systematic study to interrogate those measurement artifacts, we report various sources of electrical current leaks regarding in-plane conductivity measurements, specifically insulators in the measurement set-up. We have observed a 'great conductivity increase' up to an order of magnitude at a very thin thickness of a single layer yttria-stabilized zirconia (YSZ) film in a set-up with an intentional artifact current flow source. Here we propose that the nano-size effect, reported to appear in ultra-thin single layer YSZ, can be a result of misinterpretation.

Enhanced conductivity in pulsed laser deposited Ce0.9Gd0.1O2−δ/SrTiO3 heterostructures

Significant enhancement in the electrical conductivity of Ce0.9Gd0.1O2−δ (CGO) thin films (250 and 500 nm) deposited on MgO(001) substrate is observed by introducing ∼50 nm thin SrTiO3 buffer layer film. Introduction of the buffer layer is found to form epitaxial films, leading to minimal grain boundary network that results in a free conduction path with near-zero blocking effects perpendicular to current flow. The in-plane conductivity measurements confirm increase in conductivity with increase in compressive strain on CGO films.

Yttria-stabilized zirconia thin film electrolyte produced by RF sputtering for solid oxide fuel cell applications

Thin film (40–600 nm) yttria-stabilized zirconia (YSZ) electrolytes for solid oxide fuel cells (SOFC) were deposited on NiO-YSZ anodes and fused silica substrates by RF sputtering, using low applied power without the use of post deposition annealing heat treatment. YSZ film showed a nanocrystalline structure and consisted of the Zr .85Y .15O 1.93 (fcc) phase. The film was dense and the YSZ/anode interface was continuous and crack free. According to preliminary in-plane conductivity measurements (temperature range 550–750 °C) on the YSZ film, the activation energy for ionic conduction was found to be 1.18 ± 0.01 eV.

Conductivity measurement of different polymer membranes for fuel cells

In our work the through-plane and in-plane conductivity measurements were performed for commercial Nafion115 and original sulphonated poly(ether-ether-ketone) (SPEEK) membranes. It was found, that in-plane conductivity measurement method can be successively used for both types of membranes at maximal humidity (saturated water vapours) and temperatures till 100°C. The proton conductivities of home-made SPEEK membranes were found to be excellent in the order of 10−2 S/cm in the fully hydrated condition at temperatures 25−90°C.

Structural Effects in the Protonic/Electronic Conductivity of Dion-Jacobson Phase Niobate and Tantalate Layered Perovskites

The layered Dion-Jacobson phase perovskites HLaTa2O7·xH2O, HLaNb2O7·0.5H2O and structural variants of HLaNb2O7·0.5H2O were prepared and their AC and DC conductivities were measured as a function of temperature and atmosphere. The layered structure in HLaNb2O7·0.5H2O was modified by pillaring, exfoliation-restacking, and further annealing. Exfoliation followed by restacking markedly increased the ionic conductivity, either by creation of mesopores for hydration or by enhanced contact between layers, but pillaring had little effect. HLaNb2O7·0.5H2O was prone to reduction under hydrogen, yielding a mixed ionic-electronic conductor. In-plane conductivity measurements of an oriented film of HLaNb2O7·0.5H2O showed enhanced ionic conductivity attributed to the anisotropic conductivity of the sheets, but reduction under a hydrogen atmosphere was apparent. The reduction could be suppressed by adding water vapor to the ambient atmosphere or by replacing the B-site niobium with tantalum, which is less easily reduced.

Anomalous metal-to-insulator transition in FeSi films deposited on SiO2∕Si substrates

In-plane conductivity measurements of FeSi films deposited on boron-doped silicon substrates exhibited an anomalous metal-to-insulator transition near 250K. In the temperature range of 250–215K the resistance of the films increased by more than three orders of magnitude. For temperatures >250K, metallic conductivity consistent with the conductivity of the doped silicon substrate was observed. This indicates an ohmic contact between the film and the silicon substrate across the native SiO2 layer. Below the transition temperature (<250K), the temperature dependence of the resistance implies hopping conduction between localized states that is observed in disordered FeSi films. This metal-to-insulator transition observed in these films suggests switching of the current percolation path from substrate to the film due to a rapid increase in the interfacial resistance. The experimental results agree well with a three-layer model that incorporates an exponentially increasing interfacial resistance with decreasing temperature. The presence of a thin native oxide layer between the deposited film and the silicon substrate is essential for manifestation of the transition. Cross-sectional transmission electron microscopy analysis indicated diffusion of Fe through the oxide barrier and accumulation of Fe at the SiO2∕Si interface. The band bending at the interface resulting from Fermi level pinning due to interface states and the formation of (Fe+∕++B−)0∕+ pairs at the SiO2∕Si interface may be responsible for the observed transition.

Nanoscale effects on the ionic conductivity in highly textured YSZ thin films

The electrical conductivity of highly textured Yttria Stabilized Zirconia (YSZ) thin films deposited onto a MgO substrate can be enhanced significantly at thickness < 60 nm. The structure of these films is characterized by a negligible dislocation network coupled with only one film/substrate interface, which results in the absence of blocking effects. The in-plane conductivity measurements as a function of temperature and film thickness reveal a nanoscale effect with exceptionally high ionic conductivity for film thickness < 60 nm. The observed behavior is attributed to a significant contribution from interface conductivity with decrease in the film thickness. The analysis of presented results was performed using the rule of mixtures model and the lattice (grain) and interface related conductivities were separated. It was estimated that the thickness of interface layer is about 1.6 nm and interfacial conductivity is over three orders of magnitude larger than the lattice related conductivity, which illustrated the tremendous potential of nanoscaled materials.

Gas sensors made from Langmuir-Blodgett films of porphyrins

Langmuir-Blodgett multilayer films of porphyrins have been produced and in-plane conductivity measurements performed on the samples when exposed to the toxic gases NO2, CO and H2S. A number of porphyrins showed a large and rapid increase in conductivity when exposed to NO2. The response of the samples varied with film thickness, the partial pressure of the gas, the central metal ion of the porphyrin and also with the peripheral substituents on the porphyrin ring.