Attainable Current Density Research Articles

Developing Catalysts Integrated in Gas-Diffusion Electrodes for CO2 Electrolyzers.

ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO2 electrolysis has become very active. Many catalysts are reported for converting CO2 into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO2 electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO2-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO2 reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO2 to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO2 to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO2, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO2 electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO2 electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.

Thermal Modeling and Experimental Validation of Mid-Conductor Winding Cooling

A direct cooling method for windings of electrical machines, mid-conductor winding cooling, is studied. Spaces between the wires are utilized as coolant channels, with a liquid being pumped through the winding along the length. These results in the elimination of thermal interface resistances, a high heat transfer area and heat transfer coefficient while maintaining the same cross-sectional area for the copper winding. A thermohydraulic model is made and validated to analyze the heat transfer rates and pressure drop. Validation measurements with a water-glycol mixture as coolant show that the modeled and measured pressure drop correspond within 0.07 bar and the modeled and measured winding temperature within 3 °C. When made relative to the temperature difference between winding and coolant, the deviation is equal to 12%. The validated model is used to analyze the performance when utilizing oil as coolant. For a winding temperature of 180 °C and a pressure drop of 1 bar, using the novel cooling method results in a maximal attainable current density equal to 39.4 A/mm2 which is 41% higher than that attainable with spray end winding cooling.

Oxygen reduction reaction kinetics on a Pt thin layer electrode in AEMFC

The study of the catalytic activity in a fuel cell is challenging, as mass transport, gas crossover and the counter electrode are generally interfering. In this study, a Pt electrode consisting of a thin film deposited on the gas diffusion layer was employed to study the oxygen reduction reaction (ORR) in an operating Anion Exchange Membrane Fuel Cell (AEMFC). The 2D Pt electrode was assembled together with a conventional porous Pt/C counter electrode and an extra Pt/C layer and membrane to reduce the H2 crossover. Polarization curves at different O2 partial pressures were recorded and the resulting reproducible ORR activities were normalized with respect to the active surface area (ECSA), obtained by CO stripping. As expected, decreasing the O2 partial pressure results in a negative shift in open circuit voltage (OCV), cell voltage and maximum attainable current density. For cell voltages above 0.8 V a fairly constant Tafel slope of 60 mV dec−1 was recorded but at lower voltages the slope increases rapidly. The observed Tafel slope can be explained by a theoretical model with an associative mechanism where charge- and proton-transfer steps are decoupled, and the proton transfer is the rate-determining step. A reaction order of 1 with respect to O2 was obtained at 0.65 V which corresponds well with the mechanism suggested above. Based on the obtained catalyst activities, the electrode performance is comparable to good porous electrodes found in the field. The methodology presented in this study is expected to be useful in future kinetic studies of other catalysts for AEMFC.

Synthesis of NiSe2 /Fe3 O4 Nanotubes with Heteroepitaxy Configuration as a High-Efficient Oxygen Evolution Electrocatalyst.

The rational design of high-efficient non-noble metal electrocatalysts for oxygen evolution reactions (OER) is of significance in electrochemical energy conversion. However, such low-cost but highly active electrocatalysts remain poorly developed because of the daunting synthetic challenge. Here, the synthesis of NiSe2 /Fe3 O4 nanotubes via a facile self-templating strategy, which manifests unique tetragonal morphology, asymmetric hollow interior, and unusual but adaptable heteroepitaxy structure, is reported. Benefiting from sufficient active sites and their improved activity around the heterointerface, accompanied by the good conductivity, the NiSe2 /Fe3 O4 nanotubes exhibit as a superior OER electrocatalyst, which affords the current density of 10mA cm-2 at a very small overpotential of 199mV, high attainable current density beyond 200mA cm-2 , and mass activity of 984.5 A g-1 , as well as excellent stability for 100 h in the alkaline media. This work provides a unique synthetic pathway to fabricate superior OER electrocatalysts by optimizing their composition and architecture.

Improved Quantum Efficiency by Advanced Light Management in Nanotextured Solution-Processed Perovskite Solar Cells

Light management strategies can increase the efficiency of perovskite single-junction and tandem solar cells. In this study, we present perovskite solar cells deposited on different shallow nanotextures by spin-coating. A morphological and optoelectronic analysis demonstrates a high quality of the perovskite absorber, regardless of the substrate. Using both, a nanotexture and a sodium fluoride antireflective coating, enables us to improve the power conversion efficiency by 1.0%abs to 19.7%, when compared to its planar reference. A characterization of the optical performance of nanotextured perovskite solar cells and rigorous optical simulations reveal that the gain in efficiency can be largely attributed to reduced reflection losses and therefore increased absorption in the perovskite. Our nanotextured perovskite solar cells reach 93.6% of the attainable current density, marking the highest reported value for perovskite single-junctions and approaching those of established photovoltaic technologies. Our results demonstrate that nanotextures can be applied to solution-processed perovskite solar cells and pave the way to increased power conversion efficiencies by light management not only in perovskite single-junction but also perovskite–silicon tandem solar cells.

Multi-objective steady-state optimization of two-chamber microbial fuel cells

Abstract A microbial fuel cell (MFC) is a novel promising technology for simultaneous renewable electricity generation and wastewater treatment. Three non-comparable objectives, i.e. power density, attainable current density and waste removal ratio, are often conflicting. A thorough understanding of the relationship among these three conflicting objectives can be greatly helpful to assist in optimal operation of MFC system. In this study, a multi-objective genetic algorithm is used to simultaneously maximizing power density, attainable current density and waste removal ratio based on a mathematical model for an acetate two-chamber MFC. Moreover, the level diagrams method is utilized to aid in graphical visualization of Pareto front and decision making. Three bi-objective optimization problems and one three-objective optimization problem are thoroughly investigated. The obtained Pareto fronts illustrate the complex relationships among these three objectives, which is helpful for final decision support. Therefore, the integrated methodology of a multi-objective genetic algorithm and a graphical visualization technique provides a promising tool for the optimal operation of MFCs by simultaneously considering multiple conflicting objectives.

(Invited) Engineering Modeling of PEM Water Electrolysis: A Survey

The conversion of electric energy into pressurized hydrogen by means of water electrolysis is currently discussed as promising option to handle intermittent electricity supply that occurs with the increasing use of renewable energy sources. Subsequently, this hydrogen could be (1) either reconverted into electricity, or (2) converted into other energetic molecules, such as methane (power to gas) or liquid fuels, or (3) utilized as a feed stock for the chemical industry. Besides conventional alkaline water electrolysis, a water electrolysis technology based on a Polymer Electrolyte Membrane (PEM) is discussed since several years, which is the focus of the present contribution. The particular advantages of PEM water electrolysis are higher attainable current densities, higher voltage efficiency, and higher load flexibility. Current disadvantages include far lower technical maturity, shorter life time, and higher specific costs compared to alkaline water electrolysis. The formulation of appropriate physico-chemical models for solid polymer electrolysis can help to contribute to the solution of the above mentioned problems. In the present contribution state of the art of modelling of solid polymer electrolyzers is summarized and discussed.

Engineering Modeling of PEM Water Electrolysis

The conversion of electric energy into pressurized hydrogen by means of water electrolysis is currently discussed as promising option to handle intermittent electricity supply that occurs with the increasing use of renewable energy sources. Subsequently, this hydrogen could be (1) either reconverted into electricity, or (2) converted into other energetic molecules, such as methane (power to gas) or liquid fuels, or (3) utilized as a feed stock for the chemical industry. Besides conventional alkaline water electrolysis, a water electrolysis technology based on a Polymer Electrolyte Membrane (PEM) is discussed since several years, which is the focus of the present contribution. The particular advantages of PEM water electrolysis are higher attainable current densities, higher voltage efficiency, and higher load flexibility. Current disadvantages include far lower technical maturity, shorter life time, and higher specific costs compared to alkaline water electrolysis. The formulation of appropriate physico-chemical models for solid polymer electrolysis can help to contribute to the solution of the above mentioned problems. In the present contribution state of the art of modelling of solid polymer electrolyzers is summarized and discussed.

Thermionic field emission from nanocrystalline diamond-coated silicon tip arrays

Thermionic field emission properties of nitrogen doped ultrananocrystalline diamond (UNCD) coated silicon tip arrays are examined using thermionic field emission electron microscopy (TFEEM). Nitrogen doping has been shown to enhance the emission properties of diamond by the introduction of a donor level $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band minimum. The field enhancing geometry of the films initiates accelerated electron emission at the tipped structures which may be beneficial to thermionic energy converter design where space charge effects can significantly limit attainable current densities. Two temperature regimes of electron emission are observed; $600--800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is enabled because of the H passivation and $900--1100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is attributed to tunneling from nitrogen related states through the barrier of a clean diamond surface.

Electron optical performance of electron beam lithography columns predicted by a simple model

A simple model is presented which predicts the maximum attainable current density and associated optimum brightness and semi‐angle in a variable shaped spot (VSS) and a Gaussian spot (GS) electron beam lithography system for a given resolution (edge width). For a GS the optimum source demagnification factor is also predicted. The contribution of the forward scattering in the resist is taken into consideration. The model allows the assessment of the sensitivity of the attainable current density to various design and operation parameters. The model is based on the quadratic sum of uncorrelated and uncorrectable aberrations and solves the expression for the sum for the current density using the reduced brightness as variable. VSS systems are characterized by an optimum brightness and a high sensitivity to stochastic trajectory displacement due to electron–electron interaction. The acceleration potential has a paramount effect on the attainable current density, still enhanced when forward scattering due to the resist is included. In GS systems the chromatic and geometrical aberrations limit the attainable density. The brightness attainable with Lanthanum–hexaboride emitters is satisfactory for VSS systems but GS systems require very high brightness guns (e.g., TFE) to exploit the potential for achieving a high current density.

Training and degradation of a model superconducting winding depending on cooling conditions and conductor cross-section

The process of training single-layer windings in an external magnetic field under various cooling conditions is described in the first part of this Paper. The character and duration of training under various cooling conditions showed the influence of cooling on the superconductor stability. Limiting currents corresponding to saturation of training were found to be the same in the pressurized and saturated He II and even in He I at a temperature near the λ point. A reduction of training in He II against He I was found. The lack of limiting current dependence on the electric insulation layer introduces an additional thermal resistance. The training process for various conductor cross-sections is described in the second part of the Paper under similar conditions to the case of He I cooling. The winding was made of four insulated parallel wires. By means of varying the electrical connections of separate wires one can simulate changing sizes of conductor cross-sections in the ratio 1:2:4, under otherwise equal conditions. An increase in the limiting attainable current density was found with increase of conductor cross-section, giving a simultaneous reduction of training. A simple method of calculating the theoretical limit of attainable current was formulated. Calculated values of current agree well with those obtained experimentally.

The relationship between space-charge-limited current and density of states in amorphous silicon

Abstract This paper is concerned with the nature of the current-voltage (J-V) characteristics of thin amorphous silicon layers under space-charge-limited conditions. J-V curves have been calculated by using a computer program which employs any arbitrary form of the density-of-states function N(E) as input. The use of simple analytical forms of N(E) illustrates the way in which J(V) depends on the form of N(E) and shows that, under certain conditions, it should be possible to observe the effects of carrier trapping both in deep states near the Fermi level and in tail states remote from the Fermi level. The possibility of observing tail-state trapping at attainable current densities is enhanced by low values of N(E), by working at low temperatures and by using thin samples. Calculations have also been made by using two experimentally determined density-of-states functions, one derived from field-effect conductance and the other from deep-level transient spectroscopy (DLTS) measurements. The corresponding J-V characteristics show wide variations. Comparison with experimental results in the literature shows no evidence for the deep minimum in N(E) derived from DLTS measurements.

Improvement of the catalytic properties of Ti-doted Raney-Nickel for H 2-Anodes of alkalione fuel cells by Ni(OH) 2-surface coating

This paper belongs to a series of three dealing with the latest improvements in the alkaline H 2O 2 fuel cells operating under mild conditions thanks to their Raney-Ni-catalysts. The first of these papers describes the benefication of a Ni(OH) 2 surface coating on the catalytic activity of Ti-doted Raney-Ni in supported electrodes. This Ni(OH) 2 surface coating is produced by carefully optimized oxidation. A Ni(OH) 2-content of 5 up to 6wt% increases the attainable current density by the factor 3–4. In addition, the exchange current density is markedly enlarged up to a Ni(OH) 2-fraction by 5%, but remains unchanged when further increasing the Ni(OH) 2 percentage. Thus, one may conclude that the Ni(OH) 2 surface coating improves markedly the charge exchange reaction. On the other side, the surface diffusion of H-atoms on the pore walls to the location of charge exchange reaction is hindered by too much Ni(OH) 2.

The design of a mass spectrometer ion source based on computed ion trajectories

Computed trajectories are presented for ions in an idealised ion source of the Nier type. It is shown how the presence of a “repeller” field can reduce the asymmetry of a set of trajectories and improve the imaging property of the source. By refoeussing, the ion beam can be made to have a current density approaching the theoretical maximum. The effects of practical limitations in a real source on the attainable current density are discussed.

Superconducting Magnet Materials

Superconducting Magnet Materials

Effect of Cathode Roughness on the Maximum Current Density in an Electron Beam

A discussion is presented of the limitation of current density by random irregularities of the cathode surface in an electron beam which is focused by a static electromagnetic field. The Langmuir-Pierce law for the maximum attainable current density is profoundly altered by the effects of cathode roughness. It is found that the minimum beam diameter attainable in typical cathode-ray tubes is unaffected by change of cathode temperature or proportional change in the potentials of all electrodes.

Limiting Current Densities in Electron Beams

In electron beam devices the thermal velocities of the electrons leaving the cathode limit the current density which can be attained with given cathode current density, cathode temperature, potential, and angular spread of arriving electrons. By using Liouville's theorem, limiting expressions are obtained which are valid for any point focus or line focus concentrating system using steady electric and magnetic fields. Current densities in perfectly focused beams are also evaluated. Curves are presented which relate the nearness of approach to the maximum attainable current density with the fraction of cathode current necessarily lost in the concentrating system. Use of the expressions obtained is illustrated by applying them to cathode ray tubes and electron multipliers.