Electrical Transport Processes Research Articles

Novel Designs for 7-Layer Membrane Electrode Assembly in PEM Fuel Cells: A Systematic Investigation of Mass Transport and Charge Transfer Processes in PEM Fuel Cells

During the last decades, the focus of PEM fuel cell development in automobile applications has been mostly dedicated to the optimization of the catalyst-coated membrane (CCM). Nevertheless, the optimum design of other components like the gas diffusion layer, or gas flow fields can directly influence the overall performance of the PEM fuel cell. The gas diffusion layer (GDL) itself plays a crucial role in the water balance in a PEM fuel cell. Besides, an optimized combination of GDLs and gas flow fields leads to the efficient distribution of reactants on catalyst layers. Our main goal for the development of the 7-layer membrane electrode assembly in PEM fuel cells is to reduce the corresponding thickness. This will both improve the efficiency of mass, electrical, and heat transport processes and reduce materials and energy consumption during production. Thus, it will contribute in two ways to support the commercialization of PEM fuel cells.Further intentions of our new GDL designs are, on one hand, optimizing the capillary pressure gradient for a smooth discharge of saturated water between the catalyst layer and gas flow fields. On the other hand, enhancing the gas transport to the catalysts layer by reducing diffusion length. These could be obtained by innovative engineering of microporous structures in GDLs. Thus, following beyond the previous investigation of our research group to enhance the capillary pressure gradient, and subsequently, mass transport, in this study the design of the microporous layer is purposely varied by applying different mono disperse polymer particles as pore former. In addition, to reduce the cell width in a disruptive manner, two innovative architectures have been investigated. The first one consists in eliminating the gas diffusion medium (GDM) and thus keeping only the newly-designed MPLs. Consequently, in comparison with a conventional GDL, the uncompressed material thickness is reduced from approximately 200 µm to around 40 µm. The second cell architecture is based on the integration of the refined gas flow fields directly at the back of the GDL which enables us to get rid of “large” rib/channel designs used in state-of-art bipolar plates.In this contribution, the effect of new-designed MPLs, as well as the mechanically machined GDL on both mass transport and the charge transfer processes have been evaluated and significant performance gains have been obtained. Additionally, the scale-up possibility from the single-cell design to short-stack PEM fuel cells will be discussed.This work from the “DOLPHIN” project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under Grant Agreement No 826204. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research.

Minimum entropy production in inhomogeneous thermoelectric materials

Due to their potential applications in energy production based on waste heat, direct solar radiation or other energy sources, semiconductor materials have for years attracted the attention of theoretical and experimental researchers. The focus has been on improving the performance of thermoelectric devices through several strategies and special interest has been placed on materials with spatially inhomogeneous transport properties. Inhomogeneity can be achieved in various ways, all of them leading, to a greater or lesser extent, to an improvement of the thermoelectric performance. In this paper, general linear heat and electric charge transport processes in inhomogeneous materials are addressed. The guiding idea followed here is that there exists a relationship between inhomogeneity (structuring), minimum entropy production and performance which may be fruitfully exploited for designing more efficient thermoelectric semiconductor devices. We first show that the stationary states of such materials are minimum global entropy production states. This constitutes an extension of the validity of Prigogine’s minimum entropy principle. The heat and charge transport equations obtained within the framework of classical irreversible thermodynamics are solved to find the stationary profiles of temperature and self-consistent electric potential in a one-dimensional model of a silicon–germanium alloy subjected to an external temperature difference. This allows us to assess the effect of the spatial inhomogeneity on the thermoelectric performance. We find that, regardless of the value of the applied temperature difference, the system may efficiently operate in a regime of minimum entropy production and high efficiency.

Impedance behaviors under high pressure for DyFeO3 orthoferrite

An alternate current impedance spectrum up to 32.2 GPa was investigated to reflect the electrical properties of DyFeO3, where space charge plays the crucial role. Conductivity behaviors are determined by a universal dynamic response relationship. The band structure under various pressures was calculated by a density functional theory. The state density of d-shell electrons in conduction band varies with band gap, the reduction of which weakens the p-d coupling and leads to a conductivity transformation in the electrical transport process of O-phase. This work claims a reasonable electrical model for RFeO3 family, which is essentially impacted by high pressure.

Temperature effects on electric charge transport processes and electron emission in PBS–clay nanocomposites submitted to electron bombardment

The effects of isothermal conditions on the charge transport processes and electron emission properties of polybutylene succinate (PBS) nanocomposites with different nanoclay content are investigated. The temperature range used (20 °C–80 °C) is close to the working conditions of materials used in many applications (insulation, sensors). These effects are highlighted using an appropriate device placed in a scanning electron microscope, allowing measurement of induced current in an electron-irradiated specimen. The experimental results show that the rise in temperature involves a decrease in the charging ability and an increase in the conductivity of PBS nanocomposites. The mobility of charge carriers and its thermal dependency deduced during the discharge step were found to be lower for the nanocomposites. A qualitative analysis, expressed to understand these observations, is based on the determination of the electron trap energy distribution (i.e. density of occupied states). Secondary electron emission yield decreased with increasing temperature. The results help us envision a new class of insulators with improved electrical performance. The operating thermal stress, combined with the absence of trapped charges, allows a reduction of electrical and thermal aging and consequently ensures the long-term reliability of the insulators (for instance under High Voltage).

Microstructural Characterization of the Sol-Gel-Derived Sr2CrReO6 Powders and Their Magnetic and Electrical Transport Properties.

The double-perovskite (DP) Sr2CrReO6 (SCRO) oxide has gained much attention due to its high Curie temperature (∼635 K), high spin polarization, and strong spin-orbit coupling, which provides promising potential for room-temperature spintronic devices. In this work, we report on microstructures of a set of sol-gel-derived SCRO DP powders and their magnetic and electrical transport properties. The SCRO powders crystallize into a tetragonal crystal structure (space group of I4/m). X-ray photoemission spectroscopy spectra verify that the rhenium ions possess variable valences (Re4+ and Re6+) in the SFRO powders while chromium ions are presented as Cr3+. Ferrimagnetic behavior was observed in the SFRO powders at 2 K, and the saturation magnetization was evaluated to be 0.72 μB/f.u. and the coercive field to be 7.54 kOe. The Curie temperature was derived from susceptibility measurements to be 656 K at 1 kOe. Such ferrimagnetic behavior stems from the Cr3-Re4+(Re6+) super exchange interaction via intervening oxygen. Electrical transport measurements revealed that the SFRO ceramic grains were semiconducting and the electrical transport process was governed by the small polarons hopping with variable ranges. The hopping paths for these small polarons are provided by the hetero-valent Re ions in the SCRO ceramics. Negative magnetoresistance (MR) was observed in the SCRO ceramics, and the plot of MR vs magnetic field (H) exhibited a butterfly-like shape. The MR (2 K, 6 T) was measured to be -5.3%, due to the intergranular magneto-tunneling effect. The present results demonstrate a unique combination of high-temperature ferrimagnetism and intrinsic semiconducting nature of the sol-gel-derived SCRO oxides, which are highly attractive for oxide spintronics.

Study on pressure-induced isostructural phase transition and electrical transport properties of BiOBr.

In this work, we prepared a BiOBr powder sample by the coprecipitation method for in situ high-pressure AC impedance spectroscopy tests, in situ high-pressure Raman measurements and in situ high-pressure X-ray diffraction experiments to explore its structural properties and electrical transport processes under compression. Two pressure-driven isostructural phase transitions, T-T' and T'-T'' (T - tetragonal, T' - tetragonal 1 and T'' - tetragonal 2), were discovered at around 10.0 and 15.0 GPa, respectively. The pressure-induced changes in the crystal structure and electrical transport of BiOBr can provide a reference for explaining the mechanism of the isostructural phase transition of other similar compounds after compression.

Electrical conductivity and impedance spectroscopy studies of ceramic (Ba0.8Sr0.2)Ti0.95(Zn1/3Nb2/3)0.05O3 doped with Bi2O3

Polycrystalline samples of (Ba0·8Sr0.2)1–3x/2BixTi0.95(Zn1/3Nb2/3)0.05O3 (BixBSTBZN) (x = 0.00, 0.10), were prepared by the solid state reaction method. The dielectric impedance properties were studied over the range of frequency between 100 Hz and 1 MHz and in the temperature range of 420 °C–480 °C, using the modulus formalism. The impedance plot showed a first semicircle at high frequency which was assigned to the grain intrinsic effect and a second semicircle, at lower frequencies, which corresponds to grain boundary polarization (conduction phenomenon). A complex modulus spectrum was used to understand the mechanism of the electrical transport process, which indicates that a non-Debye type of multiple relaxations in the material. The values of the activation energy of the compound (calculated both from dc conductivity and the modulus spectrum) are very similar, suggesting that the relaxation process may be attributed to the same type of charge carriers. The frequency dependent conductivity plots exhibit double power law dependence suggesting three types of conduction mechanisms: low frequency conductivity owing to long range translational motion of electrons, mid-frequency conductivity due to short-range hopping, and high frequency conduction due to localized orientation hopping mechanism. Variation of ac conductivity as a function of frequency shows that the compound exhibits Arrhenius-type of electrical conductivity.

Structural, magnetic, and electrical transport properties of half-metallic double perovskite La2CrNiO6 oxides

We report on hydrothermal growth of double perovskite La2CrNiO6 (LCNO) powders, which are theoretically predicted to be half-metallic ferrimagnet. Their structural, magnetic, and electrical transport properties are comprehensively studied. The crystal structure of LCNO powders belongs to an orthorhombic crystal structure with Pbnm space group. The saturation magnetization of the LNCO powders was 0.32 μB/f.u. at 5 K (close to the theoretical value of 0.30 μB/f.u. calculated for a disordered system) and the coercive field was 3000 Oe. The negative Curie-Weiss (CW) temperature (θp = −35 K) indicates the predominant antiferromagnetic (AFM) interactions in the powders and ferrimagnetism with magnetic Curie temperature (TC) of 106 K. A spin glass-like behavior was observed in the LNCO powders as cooling the sample from paramagnetic state to the low-temperature ferro/ferrimagnetic state, which is originated from the magnetic interaction competitions via the AFM (Cr4+−O−Cr4+, Ni2+−O−Ni2+ or Ni2+−O−Cr3+) and FM (Ni2+−O−Cr4+ and Ni3+−O−Cr3+) magnetic paths. The observed sharply down deviation from the Curie-Weiss law below 290 K is decreased with the applied magnetic field increased, which leads to Griffiths-like phase in the powders. XPS spectroscopy reveals dual chemical states of Ni ions (Ni2+ and Ni3+) and Cr ions (Cr3+and Cr4+) in the powders, and two kinds of oxygen species exist in the forms of lattice oxygen and adsorbed oxygen. The electrical transport process of the LCNO powders is governed by small polaron variable range hopping mechanism, where the hopping path of localized electrons are provided by the B-site heterovalent metal Ni and Cr ions.

High breakdown electric field in (Ta1−xPrx)0.03Ti0.97O2 colossal permittivity ceramics through defect chemistry optimization

In this work, a novel (Ta1−xPrx)0.03Ti0.97O2 (x = 0.3, 0.5, 0.7 and 0.9) ceramic was fabricated via a solid-state reaction (SSR) method using an efficient defect engineering strategy, and the microstructure, breakdown electric field and charge transport processes were investigated in detail. All co-doped ceramics exhibit a pure rutile structure and the grain size is effectively refined by Pr/Ta ion doping. Notably, the ceramic sample with x = 0.9 displays a low tanδ of 0.018 and a large ε' of 1.74 × 104, enhancing temperature stability from RT to 200 ◦C (Δε′(T)/ε′30 = 1.5%, at 1 kHz) and Eb of 2.07 kV/cm. XPS and complex impedance spectroscopy analyses show that the excellent electrical properties of (Ta1−xPrx)0.03Ti0.97O2 ceramics originate from grains with electron-pinned defect dipoles (EPDD) and insulating grain boundaries. The (Ta1−xPrx)0.03Ti0.97O2 ceramics possess enhanced breakdown field strength and excellent giant dielectric properties, making them potential candidates for capacitors.

Pressure-directed mixed ionic–electronic to pure electronic conduction transition and enhanced grain boundary conductivity in solid electrolyte CdMoO4

Scheelite ABO4-type solid electrolytes have attracted much attention for potential applications as oxygen ionic conductors of solid oxide fuel cells. Herein, a systematic study was carried out on the electrical transport properties of CdMoO4 under high pressure by impedance spectroscopy measurements and theoretical calculations. The sequence of structural phase transitions at pressures was determined as I41/a → C2/c → P21/c by the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method. A pressure-induced conduction transition from mixed ionic–electronic to pure electronic conduction was observed. Below 25.6 GPa, O2− ions play a major role in the electrical transport process. The microscopic transport mechanism was analyzed with grain boundary energies and migration energy barriers. Above 26.9 GPa, the grain boundary response was weakened significantly after a pressure cycle, and the grain boundary conductivity increased by about three times due to pressure. These results provide guidelines for the optimization and application of scheelite ABO4-based oxygen ionic conductors in solid oxide fuel cells.

Impedance Spectroscopy of Pr-Doped BaBi2Nb2O9 Aurivillius Ceramics.

Herein this study, the polycrystalline nature of the Aurivillius type structure is studied; primarily, the main objective is to observe the influence of dopant Pr3+ at the Ba2+-site of BaBi2Nb2O9 (BBN) ceramics. The ceramics under investigation were fabricated via the conventional solid-state reaction method. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques were used to analyse their morphological structure. It was found that the chemical composition of the ceramic samples corresponds well to the initial stoichiometry of the ceramic powders. An increase in praseodymium content caused a slight decrease in the average size of the ceramic grains. The obtained ceramic materials are described by a tetragonal structure with the space group I4/mmm. The electrical properties of the material have been studied using complex impedance spectroscopy methods in wide temperature and frequency ranges. The analysis of obtained results showed grains and grain boundaries contribute to conductive processes in the material. A possible ’hopping’ mechanism for electrical transport processes in the system is evident from the analysis of results based on Joncher law.

Application of impedance spectroscopy in exploring electrical properties of dielectric materials under high pressure

Impedance spectroscopy (IS) is an indispensable method of exploring electrical properties of materials. In this review, we provide an overview on the specific applications of IS measurement in the investigations of various electrical properties of materials under high pressure, including electric conduction in bulk and grain boundary, dielectric properties, ionic conduction, and electrostrictive effect. Related studies are summarized to demonstrate the method of analyzing different electrical transport processes with various designed equivalent circuits of IS and reveal some interesting phenomena of electrical properties of materials under high pressure.

Li2O-ZnO-MoO3-SeO2 glass-nanocomposites and their crystalline counterparts: microstructure, electrical transport mechanism and first principle DFT analysis

Li2O doped glass-nanocomposites and their crystalline counterparts have been developed. Micro-structural study reveals the distribution of Li2Zn2(MoO4)3, ZnMoO4, Zn(MoO2)2, Li2Mo6O7 and Li2MoO3 nanorods in the glass-nanocomposites. Crystalline counterparts of them exhibit enhancement in sizes of nanophases. DFT and Density of States (DOS) spectra may be considered here to confirm the conducting nature of these nanophases. The ionic conductivity is found to be a function of frequency as well as temperature. In the small value of frequency, flat-conductivity may arise owing to the diffusional motion of Li+ ions whereas the ‘higher frequency dispersion’ may cause the nature of the motion of lithium ions with a tendency of sub-diffusive random trapping. As the crystalline counterpart is formed by controlled heating, ZnSeO3 chain-structure is expected to break by increasing the length and breadth of molybdate rod-like structures, which may lead to the formation of more voids (defects), where Li+ ions are supposed to be trapped. 10%–13% of the net Li+ ions are contributing to electrical transport processes.

Nonequilibrium thermodynamics of electrochemical processes

<p indent="0mm">Electrochemical energy storage and conversion involve the transport and reaction of electrolytes in the porous electrodes and membranes. Understanding the microscopic and coupling mechanisms plays a crucial role in guiding the screening of electrolytes and electrode materials, as well as the design of the electrode-electrolyte interface. However, exploring the micro- or even nano-scale phenomena and the coupling effects of multi-physical fields through traditional electrochemical thermodynamics and kinetics theory and experimental techniques remains a challenge, which requires a novel modeling approach. In this review, based on non-equilibrium thermodynamics, a multi-physics-coupled modeling approach for electrochemical processes is proposed, and the feasibility and theoretical value of this approach are illustrated from three typical problems: mass transfer under electrical field, reaction and transport processes, and thermal effect.

Molten Salt Flux Synthesis, Structure determination, Optical, Impedance and Modulus Spectroscopy Characterization of perovskite compound

The chemical Molten Salt Flux process was used for the synthesis of Ba0.97La0.02Ti1-xNb4x/5O3 polycrystalline sample (well-known as BLTi0.93Nb0.056). Doping with Niobium caused significant alterations in the physical properties of BLT ceramic. The XRD analysis revealed that Nb introduction forms a pure perovskite phase and crystallize in the P4/mmm-tetragonal symmetry. The average crystallite size was proven to decrease with the increase in Nb concentration. TEM image showed the formation of particles, where the average grain size decreased with Nb doping. The Nb incorporation into the BLT host lattice was also confirmed by the optical band gap (Eg) value. The optical properties were strongly influenced by Nb doping. The dielectric parameters were subsequently investigated as a function of frequency and d.c bias voltage. The observed dielectric dispersion was interpreted on the basis of Maxwell-Wagner interfacial model and Koop's phenomenological theory. Overall, Nyquist plot shows both bulk and grain boundary effect. The electrical relaxation process occurring in the material has been found to be d.c bias voltage dependent. Modulus analysis has established the possibility of hopping mechanism for electrical transport processes in the system.

Electrical Properties and Anisotropy of Schists and Fault Rocks from New Zealand’s Southern Alps under Confining Pressure

Magnetotelluric models spanning the Pacific–Australian Plate boundary in New Zealand’s South Island indicate a localized zone of low electrical resistivity that is spatially coincident with the ductile mid-crustal part of the Alpine Fault Zone (AFZ). We explored the source of this anomaly by measuring the electrical properties of samples collected from surface outcrops approaching the AFZ that have accommodated a gradient of systematic strain and deformation conditions. We investigated the effects of tectonite fabric, fluid saturated pore/fracture networks and surface conductivity on the bulk electrical response and the anisotropy of resistivity measured under increasing confining pressures up to 200 MPa. We find that porosity and resistivity increase while porosity and the change in anisotropy of resistivity with confining pressure (δ (ρ‖/ρ⊥)/δ (peff)) decreases approaching the AFZ, indicating the electrical response is controlled by pore fluid conductivity and modified during progressive metamorphism. Conversely, Alpine mylonites exhibit relatively low resistivities at low porosities, and lower δ (ρ‖/ρ⊥)/δ (peff) than the schists. These findings indicate a transition in both the porosity distribution and electrical charge transport processes in rocks that have experienced progressive grain size reduction and mixing of phases during development of mylonitic fabrics due to creep shear strain within the AFZ.

Highly sensitive strain sensors based on piezotronic tunneling junction

Piezotronics with capacity of constructing adaptive and seamless interactions between electronics/machines and human/ambient are of value in Internet of Things, artificial intelligence and biomedical engineering. Here, we report a kind of highly sensitive strain sensor based on piezotronic tunneling junction (Ag/HfO2/n-ZnO), which utilizes the strain-induced piezoelectric potential to control the tunneling barrier height and width in parallel, and hence to synergistically modulate the electrical transport process. The piezotronic tunneling strain sensor has a high on/off ratio of 478.4 and high gauge factor of 4.8 × 105 at the strain of 0.10%, which is more than 17.8 times larger than that of a conventional Schottky-barrier based strain sensor in control group as well as some existing ZnO nanowire or nanobelt based sensors. This work provides in-depth understanding for the basic mechanism of piezotronic modulation on tunneling junction, and realizes the highly sensitive strain sensor of piezotronic tunneling junction on device scale, which has great potential in advanced micro/nano-electromechanical devices and systems.

Effect of the substrate on the electrical transport and fluctuation processes in NbRe and NbReN ultrathin films for superconducting electronics applications

NbRe-based superconducting thin films recently received relevant interest in the field of low-temperature electronics. However, for these materials the electrical conduction mechanisms, in particular in the normal state, still need to be investigated in more detail. Here, NbRe and NbReN films of different thicknesses have been deposited on two different substrates, namely monocrystalline Si and text {SiO}_2 buffered Si. The films were characterized by DC electrical transport measurements. Moreover, a connection with the charge carriers fluctuation processes has been made by analyzing the electrical noise generated in the normal state region. Despite the films morphology seems not to be affected by the substrate used, a lower noise level has been found for the ones grown on text {SiO}_2, in particular for NbReN. From this study it emerges that both NbRe and NbReN ultrathin films are of very good quality, as far as the low-temperature electrical noise and conduction are concerned, with noise levels competitive with NbN. These results may further support the proposal of using these materials in a nanowire form in the field of superconducting electronics.

Double perovskite Sr2FeReO6 oxides: Structural, dielectric, magnetic, electrical, and optical properties

AbstractDouble perovskite Sr2FeReO6 (SFRO) powders were synthesized by so‐gel process and annealed in argon atmosphere. Their structural, dielectric, magnetic, electrical, and optical properties were comprehensively investigated. It was found that the SFRO powders possessed a tetragonal crystal structure with I4/m space group and exhibited spherical shapes with some agglomeration due to the magnetic interactions between particles of the powders. Quantitative energy dispersive X‐ray spectrometer data revealed the atomic ratio of Sr, Fe, Re, and O elements close to the nominal values of 2:1:1:6. X‐ray photoemission spectroscopy spectra reveal two species of Re5+ and Re6‐7+ coexist in the SFRO powders. Sr, Fe, and O elements are present as Sr2+, Fe3+, and lattice oxygen, respectively.Dielectric property measurements revealed a Maxwell–Wagner type dielectric dispersion in the SFRO ceramics. Ferromagnetic behavior was verified by the observed magnetic hysteresis loops in the SFRO powders at 2 K and 300 K. The remanent magnetization and coercive field at 2 K were 8.23 emu/g and 3152 Oe, respectively, and the saturated magnetization was estimated to be 21.8 emu/g (or 2.0 μB/f.u.), smaller than the theoretical value of 3.0 μB/f.u. owing to the presence of the anti‐site defects. Magnetic Curie temperature (TC) was estimated to be 432.3 K. Intergranular tunneling magnetoresistance and hysteresis phenomena were observed in the SFRO powders at low temperatures, and the MR (2 K, 6 T) was measured to be −15% and −10% for MR (100 K, 6 T). Electrical transport and optical absorption measurements demonstrate the semiconducting nature of the SFRO with optical band gap of 1.39 eV. The electrical transport process follows the small polaron variable range hopping theory. The unique combination of high TC ferromagnetism with the semiconductivity enables the SFRO to be a promising candidate for spintronic devices.

Advances in piezotronic transistors and piezotronics

• Advances in piezotronic transistors are reviewed for the first time. • Put forward new insights on the emerging field of piezotronics. • Give a perspective on further development of piezotronic transistors and piezotronics. Innovative technologies beyond or can augment the conventional metal-oxide-semiconductor (CMOS)-based technology to achieve adaptive and seamless interactions between electronics/machine and human/ambient are urgently needed for smart systems. Piezotronic transistors are a new type of devices designed using completely different principles from CMOS-based technology, which utilize inner-crystal piezoelectric potential as the gate controlling signal to control the electrical transport process, which have been widely demonstrated for the third-generation semiconductors (such as ZnO, GaN). Such devices are innovative in a way that the traditional external channel-width gating is replaced by an inner interface gating or inner channel-width gating. These nanodevices and integrated nanosystems show great potential to achieve complementary functionalities to the state-of-the-art CMOS technology. Coupling the piezoelectricity and the electrical transport process in piezoelectric semiconductors results in the emerging field of piezotronics, which show promises for advanced nano-electromechanical devices and micro/nano-electromechanical systems. This manuscript reviews the advances in piezotronic transistors, provides an overview of the development and gives in-depth understandings in the new area of piezotronics. A perspective is given about their further development and potential applications in energy harvesting, active electronics and intelligent sensing.