Ohmic Limit Research Articles

Josephson Dynamics at High Transmissions: Voltage and Current Bias Limits

We establish a direct relation between theI–Vcurves for highly transparent Josephson junctions in the voltage- and current-biased regimes. We demonstrate that the presence of sub-Ohmic dissipation at subgap voltages and temperatures yields the linear dependence of the average voltageV¯on the bias currentIexceeding the critical oneIc, in contrast to the square root dependenceV¯∝I−Icin the Ohmic limit. Our predictions are compared with recent experimental findings.

Elimination of ferroresonance in the distribution zone by high ohmic reactor-shunt limiter

Electrical network components catastrophically fail as a result of aberrant system oscillations. Also, consequently leads to saturation in the ferromagnetic core elements. These factors induce distortions in the voltage and current waveforms as well as a large rise in voltage or current. This paper presents the role of distributed generation (DG) in contributing to ferroresonance investigation reduction in the distribution sector. A high ohmic reactor-shunt limiter (HOR-SL) is introduced as a novel technique based on a negative sequence component to mitigate ferroresonance. The proposed HOR-SL reduces the ferroresonance in distribution system (DS) by no more than 10.5 msec. A PSCAD/EMTDC software is used to model the ferroresonance phenomenon. The simulation results strongly show the effectiveness of using the proposed HOR-SL for limiting ferroresonant oscillations and creating stable orbit in the distribution sector. Case studies verified the effectiveness of the proposed technique in mitigating ferroresonance oscillations and keeping the security of the distribution zone.

Measuring the isotope effect on the gross beryllium erosion in JET

The isotope effect, hydrogen (H) versus deuterium (D), on the gross beryllium (Be) erosion yield has been measured in ohmic limiter plasmas in JET tokamak by spectroscopic means. A simplified method to extract the effective sputtering yield from the quotient of the radiances of the D α or D γ and the Be II lines at 527 nm was applied. A clear isotope effect has been found, the erosion yield of D being about a factor of 2 larger compared to H in the whole explored plasma density range. This is in agreement with physical sputtering data obtained with H+ and D+ ion beams and also with material surface computer simulations. The already published contribution of chemically assisted physical sputtering has been also identified here. Currently the study is being extended to tritium (T) and D–T plasmas and the effect of helium mixtures.

Membrane-Electrode-Assembly Design Parameters and Their Impact on CO2 Reduction

CO2 electrolyzers that operate at high current densities and selectivities are essential for commercializing CO2 electroreduction and for a sustainable, CO2-neutral green energy paradigm. As part of this, membrane-electrode assemblies (MEAs) are critical components that enable industrial-scale electrochemical CO2 conversion to value-added fuels and chemicals. They consist of humidified gaseous feeds at one or both electrodes and a solid, ion-conducting polymer (ionomer) as the electrolyte. Such assemblies can overcome the experimentally observed ohmic and mass-transfer limitations characteristic of aqueous gas-diffusion electrode (GDE) and planar systems, respectively, which limit their maximum achievable current densities and render these systems impractical for industrial implementation.Foundational design parameters that characterize the ionomer-based catalyst layers unique to MEA systems include the ionomer-to-catalyst (I:Cat) ratio, the catalyst loading, and the catalyst layer thickness. In this talk, the results of a comprehensive experimental study on a Ag-cathode CO2 reduction (CO2R) MEA will be reported, where we systematically analyzed the effects of these design parameters on CO2R MEA performance and selectivity. An optimum, intermediate I:Cat ratio was uncovered and rationalized based on electrochemically-active surface area and ionomer distribution arguments. The effect of decreasing the catalyst loading and/or thickness corroborates previous modeling work, resulting in an optimum range for catalyst layer utilization for CO2R. The effect of MEA anolyte/exchange solution concentration on system performance is also explored, yielding an efficient Ag-cathode CO2R MEA that operates at 200 to 1000 mA/cm2, at CO FEs of 78 to 91%, and with an area-specific resistance of 1 Ω*cm2.The scientific and experimental design learnings from our Ag-cathode MEA studies are subsequently applied to a Cu-cathode MEA, which has farther reaching and impactful applications due to its ability to produce value-added chemical precursors like ethylene and ethanol. The Cu-MEA cathodic I:Cat ratio and catalyst loading are fixed based on settings from the optimized Ag-MEA and additional catalyst layer parameters and operating conditions are examined. This systematic exploration provides deeper insights into the underlying interrelationships between physical phenomena in the Cu-MEA system. Overall, these studies provide a fundamental understanding of the relationships between critical design parameters and inherent operating and materials tradeoffs for practical CO2 reduction devices. Acknowledgements The authors gratefully acknowledge Lawrence Berkeley National Laboratory’s Laboratory Directed Research and Development (LDRD) Grant for funding.

Mesoscopic conductance fluctuations and noise in disordered Majorana wires

Superconducting wires with broken time-reversal and spin-rotational symmetries can exhibit two distinct topological gapped phases and host bound Majorana states at the phase boundaries. When the wire is tuned to the transition between these two phases and the gap is closed, Majorana states become delocalized leading to a peculiar critical state of the system. We study transport properties of this critical state as a function of the length $L$ of a disordered multichannel wire. Applying a non-linear supersymmetric sigma model of symmetry class D with two replicas, we identify the average conductance, its variance and the third cumulant in the whole range of $L$ from the Ohmic limit of short wires to the regime of a broad conductance distribution when $L$ exceeds the correlation length of the system. In addition, we calculate the average shot noise power and variance of the topological index for arbitrary $L$. The general approach developed in the paper can also be applied to study combined effects of disorder and topology in wires of other symmetries.

Current distribution in a slit connecting two graphene half planes

We investigate the joint effect of viscous and Ohmic dissipation on electric current flow through a slit in a barrier dividing a graphene sheet in two. In the case of the no-slip boundary condition, we find that the competition between the viscous and Ohmic types of the charge flow results in the evolution of the current density profile from a concave to convex shape. We provide a detailed analysis of the evolution and identify favorable conditions to observe it in experiment. In contrast, in the case of the no-stress boundary condition, there is no qualitative difference between the current profiles in the Ohmic and viscous limits. The dichotomy between the behavior corresponding to distinct boundary conditions could be tested experimentally.

Constraining the depth of the winds on Uranus and Neptune via Ohmic dissipation

ABSTRACT Determining the depth of atmospheric winds in the outer planets of the Solar system is a key topic in planetary science. We provide constraints on these depths in Uranus and Neptune via the total induced Ohmic dissipation, due to the interaction of the zonal flows and the planetary magnetic fields. An upper bound can be placed on the induced dissipation via energy and entropy flux throughout the interior. The induced Ohmic dissipation is directly linked to the electrical conductivity profile of the materials involved in the flow. We present a method for calculating electrical conductivity profiles of ionically conducting hydrogen–helium–water mixtures under planetary conditions, using results from ab initio simulations. We apply this prescription on several ice giant interior structure models available in the literature, where all the heavy elements are represented by water. According to the energy (entropy) flux budget, the maximum penetration depth for Uranus lies above 0.93 RU (0.90 RU) and for Neptune above 0.95 RN (0.92 RN). These results for the penetration depths are upper bounds and are consistent with previous estimates based on the contribution of the zonal winds to the gravity field. As expected, interior structure models with higher water abundance in the outer regions also have a higher electrical conductivity and therefore reach the Ohmic limit at shallower regions. Thus, our study shows that the likelihood of deep-seated winds on Uranus and Neptune drops significantly with the presence of water in the outer layers.

On the Resistances of a Slurry Electrode Vanadium Redox Flow Battery.

We studied the half‐cell performance of a slurry‐based vanadium redox flow battery via the polarization and electrochemical impedance spectroscopy methods. First, the conductive static mixers are examined and lower ohmic and diffusion resistances are shown. Further analyses of the slurry electrodes for the catholyte (VO2+−VO2 +) and anolyte (V3+−V2+) are presented for the graphite powder slurry containing up to 15.0 wt.% particle content. Overall, the anolyte persists as the more resistive half‐cell, while ohmic and diffusion‐related limitations are the dominating resistances for both electrolytes. The battery is further improved by the addition of Ketjen black nanoparticles, which results in lower cell resistances. The best results are achieved when 0.5 wt.% Ketjen black nanoparticles are dispersed with graphite powder since the addition of nanoparticles reduces ohmic, charge transfer and mass diffusion resistances by improving particle‐particle dynamics. The results prove the importance of understanding resistances in a slurry electrode system.

Modeling the effect of electrode thickness on the performance of lithium-ion batteries with experimental validation

Manufacturing process for composite electrodes in lithium-ion batteries generally results in non-uniform micro-structure geometrical distribution in the through-thickness direction of a porous electrode. Inhomogeneity of the porous electrode thus affects the lithium ions transport in the electrolyte and their diffusion in the active materials. Quantifying the relationship between mass transport-related parameters and the electrode thickness is very important for studying the effect of the electrode thickness on the performance of lithium-ion batteries. The objective of this work is to study how the Lix(Ni1/3Mn1/3Co1/3)O2 (NMC111) electrode thickness affects the battery performance by developing an improved physics-based electrochemical model. In this model, the Bruggeman coefficient and the effective diffusion coefficient of NMC111 cathode with various thicknesses ranging from 31 μm to 130 μm are estimated by comparing simulation results with experimental data. The proposed electrochemical model is validated experimentally for cells with various electrode thicknesses. The simulation results indicate that increasing electrode thickness reduces the rate capabilities of lithium-ion batteries. The reason is discussed by analyzing the ohmic and diffusion limitation for cells will various electrode thicknesses under different discharge C-rates.

Controlling Selectivity in the Electrochemical Reduction of Acrylonitrile: Towards a Solar-Driven Nylon 6,6 Production Process

Fossil fuel derived carbon dioxide (CO2) emissions have continuously increased over the past century and their impact in climate change is self-evident. The chemical industry is a large contributor to this increase, accounting for more than 5.5% of the global CO2 emissions. Implementing electrochemical routes for the production of large volume chemicals can allow for the direct integration of renewable energy sources into chemical manufacturing. In this way, solar- or wind-powered electrochemical processes could greatly contribute to the reduction of the carbon footprint of the chemical industry. Although inorganic electrosynthetic processes have been successfully implemented in industry at scale (i.e. chloro-alkali and Aluminum production), the implementation of electrochemical methods for the production of organic chemicals has been limited. Adiponitrile (ADN) is the largest-volume organic chemical product currently produced electrochemically (>1.5 million tons/year). ADN is a precursor used in Nylon 6,6 manufacturing and it can be obtained via the electrohydrodimerization of acrylonitrile (AN). Although the electrosynthesis of ADN is considered the most successful organic electrochemical process in industry, it still faces many challenges owing to its energy conversion efficiency and selectivity. Optimizing the composition of the electrolyte can result in significant performance improvements. The implementation of tetraalkylammonium (TAA) ions has been shown to increase the reaction selectivity towards adiponitrile, but there is limited understanding on their role in the mass transport and kinetic aspects of the transformation. In this study, we provide experimental insights into the effects of TAA ions in the selectivity and efficiency of the AN electrohydrodimerization reaction. TAA ions can limit the hydrogen evolution reaction by sterically hindering water molecules from approaching the electrode surface and locally increasing the concentration of organic reactants. These effects are believed to be responsible for the increase in selectivity towards the reduction of AN. In our study, we explored electrolytes containing TAA ions with varying lengths of the alkyl substituents (i.e., methyl, ethyl, butyl and hexyl) and assessed the effects of their molecular size, affinity towards AN, and concentration on the electrolyte conductivity and selectivity. Using a combination of cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry at a cadmium electrode, we showed that optimal performance is achieved for electrolytes containing tetrabutylammonium (TBA) ions. A maximum selectivity towards ADN of 80% was demonstrated for TBA concentrations ranging between 0.04 to 0.5 mM. Furthermore, we studied the effects of TAA ions on the ohmic and mass transport limitations under varying current densities. Our results suggest that (i) the local concentration of AN in the electrical double layer strongly depends on the nature of the TAA and the applied current density and (ii) that this concentration is important in determining the predominant reaction pathways and thus the product distribution between ADN and other electrochemical side products (e.g., propionitrile and 1,3,6 tricyanohexane). The results obtained in this study can be used to derive electrolyte design guidelines which will improve the efficiency of the electrochemical adiponitrile production process. Furthermore, understanding the effects of current density in the selectivity of the reaction allows for the design of operating protocols to directly drive electrochemical transformations with intermittent sources of energy such as solar power. These design guidelines can lead to highly efficient solar-ADN reactors and enable the clean manufacture of precursors for the polymer industry. The performance of proof-of-concept prototypes solar-AND devices will be discussed. Figure 1

Proton Ion Exchange Reaction in Li3IrO4: A Way to New H3+xIrO4 Phases Electrochemically Active in Both Aqueous and Nonaqueous Electrolytes

AbstractProgress over the past decade in Li‐insertion compounds has led to a new class of Li‐rich layered oxide electrodes cumulating both cationic and anionic redox processes. Pertaining to this new class of materials are the Li/Na iridate phases, which present a rich crystal chemistry. This work reports on a new protonic iridate phase H3+xIrO4 having a layered structure obtained by room temperature acid‐leaching of Li3IrO4. This new phase shows reversible charge storage properties of 1.5 e− per Ir atom with high rate capabilities in both nonaqueous (vs Li+/Li) and aqueous (vs capacitive carbon) media. It is demonstrated that Li‐insertion in carbonate LiPF6‐based electrolyte occurs through a classical reduction process (Ir5+ ↔ Ir3+), which is accompanied by a well‐defined structural transition. In concentrated H2SO4 electrolyte, this work provides evidence that the overall capacity of 1.7 H+ per Ir results from two additive redox processes with the low potential one showing ohmic limitations. Altogether, the room temperature protonation approach, which can be generalized to various Li‐rich phases containing either 3d, 4d or 5d metals, offers great opportunities for the judicious design of attractive electrode materials.

Optimizing the Design of Redox Flow Battery Electrodes Using Impedance Measurements

The reactions in redox flow batteries are not usually limited by electron transfer. Hence, the power density and efficiency are largely determined by mass transport and ohmic limitations. Consequently, electrode structure and flow field designs are important in optimizing efficiency. Therefore, assessment of various types of flow fields in combination with porous electrodes has been the subject of simulations and analysis by several developers of redox flow batteries. [1-3] The studies show the impact of flow field design on cell performance, simulated effects of channel design, and pressure drops across the electrode when used in combination with porous carbon paper. These studies highlight the importance of such analysis for the development of the redox flow batteries as a viable energy storage solution. Our focus on improving the performance of the aqueous organic redox flow battery has led us to qualify modified graphite felt structures in combination with flow-through, interdigitated flow fields and columnar flow fields. To this end, we have characterized the cell performance using highly reversible redox couples such as ferrocyanide/ferricyanide. Specifically, the measurements include the current-voltage curves under potentiostatic conditions at various flow rates to determine the limiting current, and electrochemical impedance spectroscopy over the range of 10 mHz to 10 kHz to determine mass transport parameters and surface area of the electrode. We have observed that the flow-through flow field configuration exhibited the lowest mass-transport resistance, highest limiting current, and the lowest electrical contact resistance with the electrode (Figure 1). However, the flow-through designs entail high pressure drop across the electrode compared to the columnar and interdigitated designs. For a given electrode structure, the diffusion layer thickness at the surface of the electrode was dependent on the flow configurations. The porosity and surface area of the electrode structures was modified by incorporation of nano-structured carbon coatings. An optimal design combines an electrode with the highest available surface area, low pressure drop, and minimal diffusion layer thickness. The rational design of such an optimal combination of electrode and flow properties is rendered possible by the measurements and mathematical analysis carried out in this study. Acknowledgement The Authors acknowledge the financial support for this research from ARPA-E Open-FOA program (DE-AR0000337), the University of Southern California, and the Loker Hydrocarbon Research Institute. 1. C. R. Dennison, E. Agar, B. Akuzum, and E. C. Kumbur, J. Electrochem. Soc. , 163, A5163–A5169 (2016) http://jes.ecsdl.org/content/163/1/A5163.abstract. 2. M. Skyllas-Kazacos, M. H. Chakrabarti, S. a. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011) http://jes.ecsdl.org/cgi/doi/10.1149/1.3599565. 3. C. Yin, Y. Gao, S. Guo, and H. Tang, Energy, 74, 886–895 (2014). Figure 1

(Invited) Polymer Electrolyte Fuel Cell Diagnostics

The analysis of polarization curves was a topic of interest to Dr. H. R. Kunz throughout his long career in fuel-cell research. In fact, Dr. Kunz’s first publication in JECS was on a method of using polarization curves to determine the catalytic activity of carbon-supported catalysts in Teflon-bonded porous electrodes [1]. This work was one of many important contributions that Dr. Kunz made to the development of phosphoric-acid fuel cell systems at United Technologies Corporation (UTC). Thirty years later, Dr. Kunz published a paper on a method to analyze polarization curves to determine six major sources of overpotential in polymer-electrolyte fuel cells (PEFCs) [2]. This analysis was subsequently utilized to compare the initial overpotential breakdown with conventional PEFCs and UTC’s porous-plates PEFCs, as well the change in overpotentials after durability testing [3].Analysis of polarization curves can be especially insightful since it can provide useful insights into the various sources of polarization over the entire operating range of cell under realistic conditions. Ideally, diagnostics should be as simple as possible in order to decrease the risk of errors and to minimize the time required to yield instructive results. One powerful simplification technique is to diagnose the changes in polarization curves under different operating conditions. This effectively eliminates the need to account for all polarization sources and focus on the overpotentials primarily responsible for changes in the performance. Polarization-change curve (PCC) analysis was first developed to help determine the root cause of performance losses due to degradation [4]. However, changes in cell performance that result from changes in operating conditions can readily utilize this same methodology, since whether the performance changes result from load hours or from operating conditions is irrelevant to PCC methodology.A PCC is constructed by taking the difference between two polarization curves measured on the same cell. These curves can be measured under two different operating conditions (e.g., under two different humidity levels if one is investigating RH sensitivity). Alternatively, these two curves can be measured under identical conditions before and after conducting accelerated-stress test (AST) protocols (e.g., if one is investigating cell durability). In any case, the general shape of the resulting PCC can be instructive by simply comparing it to the limiting cases that result if one assumes the changes are due to a change in a single type of polarization.In many cases, differentiating between ohmic and diffusive losses in the electrodes can be particularly challenging. In the case of the cathode catalyst layer, one can utilize the difference in the reaction order with respect to oxygen concentration within the double-Tafel region. For a cathode dominated by oxygen-diffusion limitations, first-order dependence is expected, while if ohmic limitations across the thickness of the cathode dominate, half-order dependence is expected [5]. However, transport losses may also be significant on length scales that are different from the thickness of the catalyst layer. Important length scales include the thickness of the ionomer film and agglomerate diameter, which may be especially important in PEFCs with ultra-low catalyst loadings with very thin catalyst layers. This talk will focus on the possible combination of these two simple analysis tools taught in [4] and [5]. An example of this type of graphical PCC analysis is illustrated in Fig. 1. A numerical model of the PEFC [6] with detailed physics is utilized to investigate the soundness of this type of graphical analysis, as well as the potential limitations of this simple technique, especially with state-of-the-art thin catalyst layers. The numerical model is used to probe different limiting case behaviors to analyze PEFC performance. Acknowledgements Thanks to the organizers of this special symposia in the memory of Dr. Kunz, as well as for the invitation to present. This material is based upon work supported by U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under contract numbers DE-AC02-06CH11357 and DE-EE0007652. References H. R. Kunz & G.A. Gruver, JECS, 122, 1279 (1975). M. V. Williams, H. R. Kunz, & J. M. Fenton, JECS, 152, A635 (2005). X. Wang, X. Huang, L. J. Bonville, H. R. Kunz, M. L. Perry, & D. Condit, JECS, 161, F761 (2014). M. L. Perry, R. Balliet, & R. Darling, “Experimental Diagnostics and Durability Testing Protocols,” in Modern Topics in Polymer Electrolyte Fuel Cell Degradation, M. Mench, et.al. (Eds.), Elsevier, Denmark (2011). M. L. Perry, J. Newman, & E. J. Cairns, JECS, 145, 5 (1998). I. V. Zenyuk, P. K. Das, & A. Z. Weber, JECS, 163, F691 (2016). Figure 1

Issues associated with modelling of proton exchange membrane fuel cell by computational fluid dynamics

Abstract The objective of the current study is to highlight possible limitations and difficulties associated with Computational Fluid Dynamics in PEM single fuel cell modelling. It is shown that an appropriate convergence methodology should be applied for steady-state solutions, due to inherent numerical instabilities. A single channel fuel cell model has been taken as numerical example. Results are evaluated for quantitative as well qualitative points of view. The contribution to the polarization curve of the different fuel cell components such as bi-polar plates, gas diffusion layers, catalyst layers and membrane was investigated via their effects on the overpotentials. Furthermore, the potential losses corresponding to reaction kinetics, due to ohmic and mas transport limitations and the effect of the exchange current density and open circuit voltage, were also investigated. It is highlighted that the lack of reliable and robust input data is one of the issues for obtaining accurate results.

Quantifying Mass Transfer Rates in Redox Flow Batteries

Engineering the electrochemical reactor of a redox flow battery (RFB) is critical to delivering sufficiently high power densities, as to achieve cost-effective, grid-scale energy storage. Cell-level resistive losses reduce RFB power density and originate from ohmic, kinetic, or mass transfer limitations. Mass transfer losses affect all RFBs and are controlled by the active species concentration, state-of-charge, electrode morphology, flow rate, electrolyte properties, and flow field design. The relationship among flow rate, flow field, and cell performance has been qualitatively investigated in prior experimental studies, but mass transfer coefficients are rarely systematically quantified. To this end, we develop a model describing one-dimensional porous electrode polarization, reducing the mathematical form to just two dimensionless parameters. We then engage a single electrolyte flow cell study, with a model iron chloride electrolyte, to experimentally measure cell polarization as a function of flow field and flow rate. The polarization model is then fit to the experimental data, extracting mass transfer coefficients for four flow fields, three active species concentrations, and five flow rates. The relationships among mass transfer coefficient, flow field, and electrolyte velocity inform engineering design choices for minimizing mass transfer resistance and offer mechanistic insight into transport phenomena in fibrous electrodes.

Multi-machine scaling of the main SOL parallel heat flux width in tokamak limiter plasmas

As in many of today’s tokamaks, plasma start-up in ITER will be performed in limiter configuration on either the inner or outer midplane first wall (FW). The massive, beryllium armored ITER FW panels are toroidally shaped to protect panel-to-panel misalignments, increasing the deposited power flux density compared with a purely cylindrical surface. The chosen shaping should thus be optimized for a given radial profile of parallel heat flux, in the scrape-off layer (SOL) to ensure optimal power spreading. For plasmas limited on the outer wall in tokamaks, this profile is commonly observed to decay exponentially as , or, for inner wall limiter plasmas with the double exponential decay comprising a sharp near-SOL feature and a broader main SOL width, . The initial choice of , which is critical in ensuring that current ramp-up or down will be possible as planned in the ITER scenario design, was made on the basis of an extremely restricted L-mode divertor dataset, using infra-red thermography measurements on the outer divertor target to extrapolate to a heat flux width at the main plasma midplane. This unsatisfactory situation has now been significantly improved by a dedicated multi-machine ohmic and L-mode limiter plasma study, conducted under the auspices of the International Tokamak Physics Activity, involving 11 tokamaks covering a wide parameter range with Measurements of in the database are made exclusively on all devices using a variety of fast reciprocating Langmuir probes entering the plasma at a variety of poloidal locations, but with the majority being on the low field side. Statistical analysis of the database reveals nine reasonable engineering and dimensionless scalings. All yield, however, similar predicted values of mapped to the outside midplane. The engineering scaling with the highest statistical significance, , dependent on input power density, aspect ratio and elongation, yields = [7, 4, 5] cm for = [2.5, 5.0, 7.5] MA, the three reference limiter plasma currents specified in the ITER heat and nuclear load specifications. Mapped to the inboard midplane, the worst case (7.5 MA) corresponds to mm, thus consolidating the 50 mm width used to optimize the FW panel toroidal shape.

Finite Element Simulation of Temperature and Current Distribution in a Superconductor, and a Cell Model for Flux Flow Resistivity—Interim Results

A critical problem arises when current distribution in a high-temperature superconductor and its stability against quench shall be predicted: is it correct to assume homogeneous temperature distribution in superconductors, in general or only in LHe-cooled devices? The finite element analysis presented in this paper shows that during the very first instants following a disturbance, like single Dirac or periodic heat pulses, or large fault currents, temperature distribution in a BSCCO 2223 conductor is highly inhomogeneous. This is because disturbances, of transient or continuous, isolated or extended types in conductor volumes, create hot spots of comparatively long life cycle. As a consequence, separation between Ohmic and flux flow current limiter types, or decisions on the mechanism that initialises current sharing, cannot be made definitely. A semi-empirical cell model is presented in this paper to estimate flux flow resistivity in multi-filamentary superconductors in a successive approximation approach. Weak links are modelled, as nano- and microscopic surface irregularities and corresponding resistances, in analogy to thermal transport. Though the model requests input of a large amount of data (dimensions, porosities, field-dependent quantities) that still have to be verified experimentally, it is by its flexibility superior to ideas relying on, for example, imagination of separate, non-interacting chains of strong and weak links switched in parallel. In particular, and in contrast to the standard expression to calculate flux flow resistivity, the cell model suggests to replace solid conduction by an effective resistivity, a method that is more appropriate for multi-filamentary conductors. The paper also discusses integration time steps in numerical simulations that have to be selected in conformity with several characteristic times of current and thermal transport.

Performance evaluation of a membraneless divergent electrode-flow-through (DEFT) alkaline electrolyser based on optimisation of electrolytic flow and electrode gap

A membraneless divergent electrode-flow-through (DEFT) alkaline electrolysis design and operating principle is investigated, which allows for the ohmic drop contribution and performance threshold limitations of a conventional membrane barrier to be overcome. Employing mesh electrodes of 30 mm diameter, operation of the electrolyser at an electrolytic flow velocity of 0.075–0.1 m s−1, resulted in an optimal electrode gap of ∼2.5 mm, while operating at greater velocities (>0.1–0.2 m s−1) allows for the employment of a smaller optimal gap of ∼0.8 mm. At an electrode gap of 2.5 mm and current densities of 3500 mA cm−2, hydrogen purity of 99.83% has been recorded. With pure nickel electrodes current densities of 101.19 mA cm−2 (at 1.80 VDC) and 326.33 mA cm−2 (at 2 VDC) have been achieved, while the use of superior catalysts, namely, RuO2/IrO2/TiO2 and Pt for the anode and cathode respectively, resulted in the current densities to increase to 219.99 mA cm−2 (at 1.8 VDC) and 474.40 mA cm−2 (at 2 VDC) at an electrode gap of 2.5 mm and a minimum flow velocity of 0.075 m s−1. The test rig is capable of generating hydrogen at a rate of 63.6 L/hr at normal temperature and pressure (NTP). The production rate follows current density linearly at high overpotentials.

Modeling of Low-Temperature Fuel Cell Electrodes Using Non-Precious Metal Catalysts

An electrode-scale, transport model for a proton-exchange-membrane fuel cell (PEMFC) cathode is presented. The model describes the performance of non-precious metal catalysts for the oxygen reduction reaction in a fuel cell context. Because of its relatively high thickness, emphasis is placed on phenomena occurring in the cathode layer. Water flooding is studied in terms of its impact on gas-phase transport and on electrochemically accessible surface area (ECSA). Although cathode performance in both air and oxygen are susceptible to ECSA loss, gas diffusion limitations at high current density in air are more significant. In oxygen, catalyst utilization at high current density is primarily limited by conductivity. For this reason, air fuel cell data is recommended over oxygen data for characterizing catalyst performance. Due to both ohmic and mass transport limitations, increased loading of low-cost catalysts does not necessarily lead to higher performance. Therefore, careful optimization of catalyst layer thickness is required.

Biofuel Cell Anodes Integrating NAD+-Dependent Enzymes and Multiwalled Carbon Nanotube Papers

This research aims to overcome kinetics, ohmic and mass transport limitations by improving the nanoarchitectured design of the catalytic layer and the mass transport system in order to improve the current output and stability of the bioanode. Carbon nanostructured material, the multiwalled carbon nanotube (MWNTs) paper (buckeye paper (BEP)), and the polymeric structure of chitosan-carbon nanotubes (CNTs) mixture are integrated in the design. Integration of these nanostructured materials with various enzymes-alcohol, lactate and glucose dehydrogenases (ADH, LDH and GDH respectively)-was performed and analyzed. Polymethylene green (PMG) is used as a catalyst toward the oxidation of NADH to NAD+. The GDH-based anodic interface showed the best performance in the static electrolytic cell as well as in the quasi-2D flow-fan cell. Michaelis-Menten analysis on the GDH-based bioanode showed an IMax = 3.382 ± 0.180 mA.cm−2 and KM = 17.5 ± 3.1 mM compared to IMax = 226.6 ± 10.1 μA.cm−2 and KM = 16.0 ± 2.5 mM for ADH and, IMax = 53.4 ± 5.1 μA.cm−2 and KM = 6.6 ± 3.3 mM for LDH respectively (electrolytic cell). Stability studies showed the GDH- bioanode conserved ∼80% of its initial performance at 15 days and ∼65% at 30 days of dry storage at 4°C.