Homogeneous Current Density Distribution Research Articles

Design and Development of Flow Fields with Multiple Inlets or Outlets in Vanadium Redox Flow Batteries

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution.

On the coupling of magnetic moments to superconducting quantum interference devices

We investigate the coupling factor φ µ that quantifies the magnetic flux Φ per magnetic moment µ of a point-like magnetic dipole that couples to a superconducting quantum interference device (SQUID). Representing the dipole by a tiny current-carrying (Amperian) loop, the reciprocity of mutual inductances of SQUID and Amperian loop provides an elegant way of calculating ϕμ(r,eˆμ) vs. position r and orientation eˆμ of the dipole anywhere in space from the magnetic field BJ(r) produced by a supercurrent circulating in the SQUID loop. We use numerical simulations based on London and Ginzburg–Landau theory to calculate φ µ from the supercurrent density distributions in various superconducting loop geometries. We treat the far-field regime ( r≳a= inner size of the SQUID loop) with the dipole placed on (oriented along) the symmetry axis of circular or square shaped loops. We compare expressions for φ µ from simple filamentary loop models with simulation results for loops with finite width w (outer size A > a), thickness d and London penetration depth λ L and show that for thin ( d≪a ) and narrow (w < a) loops the introduction of an effective loop size aeff in the filamentary loop-model expressions results in good agreement with simulations. For a dipole placed right in the center of the loop, simulations provide an expression ϕμ(a,A,d,λL) that covers a wide parameter range. In the near-field regime (dipole centered at small distance z above one SQUID arm) only coupling to a single strip representing the SQUID arm has to be considered. For this case, we compare simulations with an analytical expression derived for a homogeneous current density distribution, which yields excellent agreement for λL>w,d . Moreover, we analyze the improvement of φ µ provided by the introduction of a narrow constriction in the SQUID arm below the magnetic dipole.

Reducing Irreversible Performance Losses via a Graphene Oxide Buffer Layer for Proton-Exchange Membrane Fuel Cells.

Alloy catalysts are promising for proton-exchange membrane fuel cells but are difficult to realize high-durability operation because the dissolution of Pt-M (M = Co, Ni, etc.) metals inevitably accelerates irreversible performance degradation. Here, we propose a buffer layer solution that inserts a trace layer of a graphene oxide (GO) film between the PEM and the alloy catalyst layer to mitigate the poison effect. To distinguish the irreversible and reversible losses, two typical recovery procedures (shutdown and JRC-based protocols) being part of a fuel cell dynamic load cycle durability test are characterized. The electrochemical evaluation reveals that GO-1 μg/cm2 enables a higher initial performance and stability. Furthermore, the GO buffer layer design allows the realization of membrane electrode assemblies with a highly homogeneous current density distribution and a low accessible mass transport resistance. Thanks to the ion sieving effect in the GO buffer layer, high anti-poisoning and stability during the accelerated stress test process are ensured.

Lithiophilic 3D VN@N-rGO as a Multifunctional Interlayer for Dendrite-Free and Ultrastable Lithium-Metal Batteries

It is still a big challenge to effectively suppress dendrite growth, which increases the safety and life of lithium-metal-based high energy/power density batteries. To address such issues, herein we design and fabricate a lithiophilic VN@N-rGO as a multifunctional layer on commercial polypropylene (PP) separator, which is constructed by a thin N-rGO nanosheet-wrapped VN nanosphere with a uniform pore distribution, relatively high lithium ionic conductivity, excellent electrolyte wettability, additional lithium-ion diffusion pathways, high mechanical strength, and reliable thermal stability, which are beneficial to regulate the interfacial lithium ionic flux, resulting in the formation of a stable and homogeneous current density distribution on Li-metal electrodes and hard modified separators that can resist dendrites piercing. Consequently, the growth of Li dendrite is effectively suppressed, and the cycle stability of lithium-metal batteries is significantly improved. In addition, even at a high current density of 10 mA cm-2 and cutoff areal capacity of 5 mAh cm-2, the Li|Li symmetric batteries with VN@N-rGO/PP separators still work very well even over 2500 h, exhibiting ultrahigh cycling stability. This work presents rational design ideas and a facile fabrication strategy of a lithiophilic 3D porous multifunctional interlayer for dendrite-free and ultrastable lithium-metal-based batteries.

Physical Features of Anodic Plasma Electrolytic Carburising of Low-Carbon Steels

This study considers some aspects of electrolytic plasma in the process of anodic carburising of steel, including the nature of the glow in a vapour gaseous envelope, its thickness under various hydrodynamic conditions; it also examines the formation patterns of a hardened layer after carburising along with quenching in the same electrolyte. The glow in a vapour gaseous envelope was examined with a spectrometer; its profile and thickness were determined by solving energy and mass balance equations in a pre-anode area. The structure of the carburised layer and hardness distribution were explored with an optical microscope and a microhardness tester. Carbon concentration in the carburised layer was determined by means of optical emission spectroscopy. The investigation has revealed that the glow in a vapour gaseous envelope under carburising is a continuous emission from heated bodies—vapour gaseous phase and the sample without any electric discharges. It has been theoretically derived, that in laminar approximation the layer has maximal thickness under certain hydrodynamic conditions. This conclusion has been confirmed by homogeneous distribution of current density throughout the surface of the sample during its carburising under condition of force hydrodynamics, i.e. the sample being flowed round with cooled electrolyte. Aerated stirring in electrolyte does not provide homogeneous current density distribution, which falls in vertical direction. Anodic carburising of steel in a glycerol electrolyte followed by quenching results in the formation of a martensitic layer up to 200 μm in thickness, within 5-min treatment, with maximal microhardness 1000 HV.

Local degradation effects in automotive size membrane electrode assemblies under realistic operating conditions

In automotive applications, the operational parameters for fuel cell (FC) systems can vary over a wide range. To analyze their impact on fuel cell degradation, an automotive size single cell was operated under realistic working conditions. The parameter sets were extracted from the FC system modeling based on on-road customer data. The parameter variation included simultaneous variation of the FC load, gas pressures, cell temperature, stoichiometries and relative humidity. Current density distributions and the overall cell voltage were recorded in real time during the tests. The current densities were low at the geometric anode gas outlet and high at the anode gas inlet. After electrochemical tests, post mortem analysis was conducted on the membrane electrode assemblies using scanning electron microscopy. The ex-situ analysis showed significant cathode carbon corrosion in areas associated with low current densities. This suggests that fuel starvation close to the anode outlet is the origin of the cathode electrode degradation. The results of the numerical simulations reveal high relative humidity at that region and, therefore, water flooding is assumed to cause local anode fuel starvation. Even though the hydrogen oxidation reaction has low kinetic overpotentials, “local availability” of H2 plays a significant role in maintaining a homogeneous current density distribution and thereby in local degradation of the cathode catalyst layer. The described phenomena occurred while the overall cell voltage remained above 0.3 V. This indicates that only voltage monitoring of fuel cell systems does not contain straightforward information about this type of degradation.

Variation of crystallinity of Cu and Cu2O nanowires arrays grown in various pores of porous alumina membrane

Various pore sizes of a porous alumina membrane were fabricated using H2SO4 and H2C2O4 electrolyte under different ionization voltages. Cu nanowire arrays with high aspect ratios, uniform pore size, and ordered pore arrangement were synthesized using the above porous alumina membrane (PAM). Moreover, Cu2O nanowire arrays were prepared through the oxidization of Cu metal nanowire arrays. From the microstructure and compositional analysis, it was observed that pores of different sizes, i.e. 20~30, 70~90 and 90~100 nm could be obtained by controlling various electrolytes and anodization voltage. The Cu nanowire synthesized with various pore sizes were found to be single crystal (20~90 nm) and polycrystalline (90~100nm) respectively. The single crystal Cu with (111) direction was occurred due to homogeneous current density distribution and relationship between current density (J) and nucleus radius (ro). After oxidation of Cu, the Cu2O nanowires with the pore sizes of 20~100 nm was found to be single crystal. The rearranged of Cu and O2 lattice sites promotes the enhancement of crystalline property.

Use of Segmented Cell Operated in Hydrogen Recirculation Mode to Detect Water Accumulation in PEMFC

AbstractAdequate water management is crucial to increase stability and durability of Polymer Electrolyte Membrane Fuel Cells. In this paper, a test rig suitable for water balance and nitrogen crossover studies was built around a hydrogen‐air segmented cell and used to indirectly assess flooding or drying conditions in specific zones of the active cell area. In particular, the anode of the segmented cell was operated in recirculation mode with continuous water removal. Current density distribution (CDD) diagrams were obtained for different anode operating parameters, namely, the recirculated gas flow rate, anode pressure, and time between purges. Water accumulation at the electrodes was assessed from CDD diagrams and confirmed using water balance and flow‐patterns calculations. It was concluded that lower recirculation flow rates led to flooding due to decreased water removal capabilities at the anode. For higher recirculation flow rates, drying was observed in one zone of the cell but homogeneous CDD in the other. Finally, the use of partially segment bipolar plates was proposed to increase the in‐plane electrical resistance between adjacent segments. The partial segmentation increased the segment to segment in‐plane electrical resistance between 14 and 21% and decreased the through‐plane to in‐plane resistance ratio by 17%.

OS5-4 PEFC内気液流動特性とその電流密度分布および電池性能に及ぼす影響(OS5 水素・燃料電池,循環型社会における動力エネルギー技術)

The objective of this paper is to observe water production behavior inside of the cell and to analyze its effect on current distributions over the reaction area. A fuel cell was made to allow direct observation of the phenomena in cathode side and local current density measurement in anode side. The experimental results showed that the cell performance and current density distribution of air operation is strongly affected by the condensed water compared with those of O_2 operation, although the drainage of air operation is superior. It was also shown from the comparison experiments of separator type that the serpentine separator has advantage to drain away the condensed water because of its high gas velocity, and may make gas paths deep in the gas diffusion layer. On the other hand, when the condensed water is drained efficiently, the straight separator can realize homogeneous current density distribution because of its shorter channel paths.

Evaluation of Skin Barrier Function Using Direct Current III: Effects of Electrode Distance, Boundary Length and Shape

The objective of this study was to propose a suitable electrode disposition and shape for iontophoretic drug delivery systems in consideration of a reduction in skin barrier function and a distribution of current density. The reduction in barrier function was evaluated with our proposed method, which measured the resistance in the short term. The distribution was estimated using an electromagnetic waves analysis program. Using rectangular electrodes, effects of distance between electrodes and boundary length of the electrode on the barrier function were examined. A distance of 2 mm decreased the barrier function effectively. The barrier function was reduced with increasing electrode boundary length. Furthermore, a surrounded square type electrode was more effective in the reduction of barrier function than a paired square type. With respect to the surrounded type electrodes, both square and circle types decreased the barrier function. However, percutaneous absorption using the circle type electrode was greater than with the square type. These phenomena are attributed to not only the electrode boundary length but also the homogeneous distribution of current density. Therefore, the surrounded circle type electrode was suitable for iontophoretic transdermal drug delivery systems.

Hexagon structures in a two-dimensional dc-driven gas discharge system

In a dc-driven planar gas discharge semiconductor system, self-organization phenomena have been investigated experimentally. The spatial structures we focus on appear as a spatial modulation of the transversal distribution of the electric current density. These patterns have been observed and recorded through a transparent electrode with spatial resolution via their corresponding light emission. When choosing appropriate experimental conditions, the initially homogeneous current density distribution undertakes a transition to a periodic hexagon and then to a stripe pattern. This behavior of the experimental system is investigated quantitatively and is discussed in the frame of a phenomenological model.

Electromagnetic properties of Bi-2223/Ag concentric tapes

It is well established that near the silver interface highly textured Bi-2223 layers compared to the inner ceramic core may be obtained. Thus Bi-2223 multilayered concentric tapes with silver matrix are very promising in increasing transport critical current densities. In the present work we report on the electromagnetic characterization of short tapes having in their cross-section a very thin HTS flattened ring with Ag inside and outside of it. The samples were prepared by the powder in tube method and have self-field critical current densities J/sub c/ of the order of 3/spl times/10/sup 4/ A/cm/sup 2/ at 77 K. Transport and SQUID magnetization measurements revealed weak link nature of the samples in low magnetic fields. Large transport J/sub c/(B) hysteresis was observed at 4.2 K and magnetic fields up to 20 T. Magnetic field profiles measurements with miniature Hall sensors are in qualitative accordance with model calculations supposing homogeneous current density distribution across the superconducting core.