Total Ohmic Resistance Research Articles

InAlGaN-based HEMT with very low Ohmic contact resistance regrown at 850 °C by MOVPE

Regrown Ohmic contacts have been widely studied for high millimeter-wave applications. However, few were applied to InAl(Ga)N-based HEMT despite the lattice match benefits with GaN channel because of the poor thermal stability of the quaternary barrier. In this article, we use relatively low temperature (850 °C) MOVPE technique for the regrowth of heavily Si doped GaN (1 × 1020 cm−3) to avoid deterioration of the channel's electrical characteristics. Moreover, high selectivity of the regrowth is obtained, thanks to large opening ratio of the hard mask. The state-of-the-art total Ohmic contact resistance Rc = 0.06 Ω mm is reached with high homogeneity on a 4-in. wafer. This result reflects the combined contribution of the doped GaN interfaces with both the metal contact and the channel. 2 × 50 μm transistors featuring 100 nm gate length with regrown Ohmic contacts present remarkable improvements of DC and RF performances. At 40 GHz, the highest Power Added Efficiency (PAE) of 54% is attained at an output power of 3.8 W mm−1, while the maximum output power of 9.4 W mm−1 is achieved at a PAE of 48%, corresponding to bias voltages of 15 and 30 V, respectively.

Interface Resistance Analysis in Solid Oxide Fuel Cells

A sophisticated design of the interface structure between the cathode and the electrolyte is essential to improve the performance of solid oxide fuel cells (SOFCs). It is because the interface is the place where it directly affects both the ohmic resistance and the polarization resistance as interface contact and interface reaction, respectively. To improve interface properties, electrolyte surface treatment or inserting interface functional layer between the cathode and the electrolyte have been applied. They improved cell performance by effectively enhancing the interfacial characteristics such as interface bonding and interface reaction area. However, although both ohmic and polarization resistance greatly contributed to the improved cell performance, a detailed analysis related to ohmic resistance compared to polarization resistance is insufficient. Unlike polarization resistance which can quantify interface reaction related resistance using distribution of relaxation time (DRT) model, there is no proper methodology to quantify interface resistance in the case of ohmic resistance. The interface resistance has been analyzed to the extent that it belongs to the remaining resistance except for the electrolyte resistance from total ohmic resistance in consideration of the ionic conductivity and thickness of the electrolyte, which still not completely separate the interface resistance from the total ohmic resistance. Furthermore, in recent years, the electrolyte thickness has gradually decreased to less than 5 μm for high-performance and the electrolyte resistance has been significantly reduced, increasing proportion of interface resistance in total ohmic resistance. Therefore, we need to reduce the interface resistance for further cell improvement. Also, quantifying the interface resistance and having a deeper understanding of the correlation between interface resistance and interface structure should be supported.Here, to realize the interface resistance from the ohmic resistance, we designed several types of interface structure using electrostatic spray deposition (ESD) which can precisely control the particle size and be used to fabricate a thin functional layer. Using the different interface properties and equivalent circuit models, we separate the interface resistance from the ohmic resistance quantitatively. Our results can suggest a simple and effective interface analysis method for achieving high performance SOFCs. Figure 1

Цифровая осциллография полимерных мембран с протонной проводимостью

Polyvinyl alcohol-based cast membranes have promising application in creating commonly available fuel cells and fuel synthesizers, as well as inexpensive water sources and microelectromechanical systems. In order to control the properties of such membranes a deep understanding of the charge transfer mechanisms and their relationship with the structure formed at a certain composition and manufacturing technology is required. A method for studying the structure of the proton-conductive polymer membranes using digital oscillography of ion currents excited by the low-frequency rectangular pulses with the amplitudes below the threshold voltage of the ionic conductivity in a dehydrated membrane, and for the analysis of the resulting ion current pulses (spikes) in the frames of the model of a proton pump acting in each layer of the membrane is proposed in this paper. Fast Fourier transform of these oscillograms reveals from 2 to 4 branches or spike sequences corresponding to the phases with different ionic conductivity and makes it possible to determine the thicknesses of both high-conductivity phase layers (7–30 µm) and low-conductivity phase interlayers (1–7 µm) formed in the process of polymerization. The reason of spike merging into bursts is described in terms of successively induced increase in the excited proton density over a threshold value in highly conductive layers. The resonance observed in dry proton membranes at the frequencies of about 2.2 to 3.0 kHz is interpreted as the burst merge point with the further increase in impedance due to proton lagging and respective decrease in the effective thickness of active layers. The effective charge carrier concentrations (as small as 1012 to 1013 cm−3) and the velocity (from 5 to 18 cm/s for the highly conductive phases which turned out to be much higher than those observed in solutions) are estimated. The asymmetry of the cast membranes, which becomes apparent at low frequencies and causes the generation of a direct ion current in response to excitation by a purely alternating current, is studied. It is found that the apparent conductivity determining contribution to the total ohmic resistance is made by a thin interlayer with a very low ion velocity, presumably surface layer, rather than the main layers. The conclusion on the optimization of the production technology and the composition of the proton membranes for various applications is made.

InAlN/GaN HEMT With n+GaN Contact Ledge Structure for Millimeter-Wave Low Voltage Applications

In this work, high performance InAlN/GaN HEMT based on the n+GaN regrown ohmic contact with n+GaN contact ledge structure is proposed. The regrown ohmic contact of InAlN/GaN HEMT is formed by MBE n+GaN regrowth and selfstopping etching, which makes the total ohmic contact resistance between the 2DEG channel and the ohmic metal decrease to 0.12 Ωmm and forms n+GaN contact ledge structure. Owing to the n+GaN contact ledge on the InAlN barrier, with the increasing of drain-source voltage (VDS), an additional current path comes into being between the n+GaN contact ledge on the InAlN barrier and the 2DEG channel, which can “shorten” the device effective drainsource distance, thus further reducing the parasitic resistance. Compared with regrown InAlN/GaN HEMT without n+GaN contact ledge structure, the peak transconductance (Gm.max) of regrown InAlN/GaN HEMT with n+GaN contact ledge structure increases from 747 mS/mm to 874 mS/mm, and the saturation current density (ID.max) increases from 2.6 A/mm to 2.9 A/mm. Besides, the self-stopping etching on the access region does not induce extra defects, and negligible current collapse is obtained. As the results of low parasitic resistance, high output current density, low knee voltage and negligible current collapse, poweradded-efficiency (PAE) of 44% together with output power density (Pout) of 2.5 W/mm is achieved at 30 GHz and VDS of 10 V, which indicates regrown InAlN/GaN HEMT with n+GaN contact ledge structure has great potential for millimeter-wave low voltage applications. Additionally, the transfer and schottky gate characteristics show negligible degradation after OFF/ON-state electrical stress tests.

Operando Neutron Radiography Measurements of a Zero-Gap Alkaline Electrolysis Cell

Low temperature alkaline water electrolysis is a well-known technology for hydrogen production and advantageous because less noble metals are needed compared to Proton Exchange Membrane (PEM) electrolysis. However, the two-phase flow behavior and especially the gas bubble distribution and flow during the operation in a zero-gap alkaline electrolysis cell have not been investigated in sufficient detail. Previous studies used e.g. transparent model electrolysis cells or focused on single flow phenomena e.g. gas bubble nucleation, growth or detachment.Recent publications show evidence that the ohmic resistance of zero-gap alkaline electrolysis cells is greater than the ohmic resistance of the used diaphragm, which has been believed the only term contributing to the ohmic resistance in zero-gap cells [1]. This suggests that the evolving gas bubbles might have a relation to the total ohmic resistance of the cell. As possible reasons nano bubbles in the pores of the diaphragm and a possible finite gap between electrodes and diaphragm is mentioned [2]. Haverkort et al. assume that the frontal area of the electrodes might be inactive [3].To prove the hypothesis and to deepen the general understanding of the two-phase flow inside an alkaline electrolysis cell we conducted operando cell measurements and visualized the electrolyte-gas bubble flow in an alkaline electrolysis cell by using neutron radiography. Thanks to the powerful neutron source at Institute Laue-Langevin in Grenoble, it was possible to observe the whole flow field with all flow channels at once with a sufficient temporal resolution to observe the movement of single gas bubbles, from nucleation to the moment of exiting the channel. The temporal resolution was 50 fps, which makes these measurement, to the best authors’ knowledge, the first to show the transient effects inside of a zero gap alkaline electrolysis cell. For the measurements, the NeXT (Neutron and X-Ray Tomograph) instrument was used which is the imaging station with one of the highest neutron fluxes of the world (1.4 E10 n/cm²/s at the end of the guide) [4]. The neutron beam was used to carry out through-plane measurements of the electrolysis cell.The measurements were conducted at a variety of operation conditions, varying the current density, the electrolyte volume flow and the operation temperature. At different operation conditions Electrical Impedance Spectroscopy (EIS) measurements have been conducted to measure the ohmic resistance of the cell. The used cell is an alkaline single cell out of a Nickel alloy with vertical parallel flow channel and a pocket milled to the backside of the cell in order to increase the transmission through the cell material.From the results of the measurements we observe a relation between current density (forming gas amount) and the ohmic resistance and thus establish a relation to the gas bubble behavior inside the electrolysis cell. Furthermore, we analyzed the velocity of the gas bubbles inside the channel depending on the different operation conditions, as well as the agglomeration behavior of the bubbles and their flow regime.[1] Phillips et al.,Minimising the ohmic resistance of an alkaline electrolysis cell through effective cell design, 2017[2] de Groot et al., Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm,2021[3] Haverkort et al., Voltage losses in zero-gap alkaline water electrolysis,2021[4] Tengattini et al., NeXT-Grenoble, the Neutron and X-ray tomograph in Grenoble, 2020 Figure 1

Fabrication and Characterization of Functional Ceramic Films by Solution Spray Pyrolysis: Correlations with the Thermal Decomposition Characteristics of the Constituent Salts

Thin films of ceramic or ceramic-metal (cermet) electrodes with appropriate structure and morphology are always sought in ceramic fuel cells and electrolyzers. These films can serve as either electrodes or electrolytes in solid oxide fuel cells (SOFCs) or electrolyzers (SOECs) requiring different morphological and structural characteristics for optimal function. Thus, electrode parameters such as thickness, porosity and tortuosity essentially determine the values of gaseous effective diffusivities and subsequently the extent of concentration polarization losses [1]. In addition, nanostructured thin film electrodes have shown enhanced conversion efficiencies compared to their bulk materials counterparts, essentially leading to low polarization losses [2].On the other hand, the electrolyte has to possess minimal porosity not allowing mixing between the anode and cathode compartments and adequately blocking electronic transport. Low thicknesses are desirable for minimization of the cell’s total ohmic resistance. However, at low electrolyte thicknesses, total ohmic cell polarization is affected by electrode microstructure to a greater extent than by electrolyte ohmic resistance due to contiguity factors between metal particles in the fabricated cermets and metal depletion in the anode functional layer [3].Generally, for a given set of chosen materials, all of the above microstructural requirements for electrodes and electrolytes have to be met by the adopted fabrication methods which have to be subsequently optimized with respect to their operational parameters. It is advantageous to resort to the so called innovative fabrication methods for the above, in particular for their functional components. The term refers to methods with the potential of producing nanostructural components and controlling in situ the film’s characteristics such as porosity, pore size, thickness and uniformity.The solution spray pyrolysis technique (SSP) belongs to this family of methods as it is based on molecular deposition and it involves spraying of a solution of suitable constituent metal salts onto a heated substrate [4].The technique is robust, environmentally friendly if water is used as solvent, easily upscalable and with easily controllable processing parameters. It also possesses the potential of producing very thin layers of components (i.e. < 1 μm) and therefore, reducing the subsequent sintering temperatures due to the more active precursor components of the constituent particles forming the final film.In a recent publication [4], we have shown that the technique can be optimized in terms of deposition temperature, time and concentration of the precursor salts in order to fabricate crack-free films of La0.75Sr0.25MnO3 (LSM) and CuO-CeO2 on dense YSZ substrates with thicknesses of the order of 0.65–3.5 μm. Cu-CeO2 constitutes a potential anode that combines the functionalities of copper ( an electronic conductor) and a good oxidation catalyst (i.e. CeO2) for potential hydrocarbon fuels fed into a SOFC while LSM is the typical state of the art cathode used with a YSZ electrolyte [5]. It was shown that lower deposition temperatures (but above the solvent boiling point) generally lead to more porous films and the precursor salt concentration is coupled with the optimal deposition temperature range and deposition time. In the present communication, we extend these findings in order to include films of Gadolinia (10mole%) stabilized Ceria (CGO10) on dense Zr0.92Y0.08O2 - x (YSZ) and La0.3Sr0.7FeO3 (LSF70) substrates. CGO10 is a typical solid electrolyte more suitable for intermediate temperature applications (i.e. 500-750°C) and it is amenable to being fabricated with lower porosities by SSP than YSZ [6]. Focus is on the thermal characteristics of the constituent salts with respect to correlations with the appropriate deposition temperatures as well as the effect of deposition time. The produced films are characterized by X-ray diffraction and scanning electron microscopy (SEM). CGO10 films were also characterized in terms of their electronic blocking capability in contact with the LSF70 substrates.

Comparative Study of the Performance of Syngas-Fueled Carbon-Air Solid Oxide Fuel Cells Using Anodes Based on M-Doped CeO2 (M: Mo or Mo-Cu)

A comparative analysis of difference in impedance spectra (ADIS) was performed for two SOFCs with different anode materials: CMO-YSZ and CMCuO-GDC, to determine the behavior of charge transfer and diffusive phenomena in both materials. This study showed a similar behavior for both materials, showing an increase of the diffusional limitations in the CMCuO-GDC anode with respect to the CMO-YSZ anode. The addition of copper reduced by 50% the total ohmic resistance of the cell with respect to the cell with CMO-YSZ anode, however, it was not possible to reduce the ASR value of the system. The maximum power densities obtained were 114 mW cm-2 and 120 mW cm-2 for the cells with CMO-YSZ and CuCMO-GDC anodes correspondingly, with CuCMO-GDC showing a slight increase in the electrocatalytic activity of the material.

Effect of current-induced ion transfer on the electrical resistance of reverse electrodialysis stack by chronopotentiometry

Reverse electrodialysis (RED) technology is a direct and effective technology for power generation by harvesting salinity gradient energy (SGE). The output current is one of the key operating parameters to control the power output of a RED stack. This research investigates the specific effect of the current-induced ion transfer on each electrical resistance component of the RED stack. The results showed that the internal resistance of the RED stack decreases with the increase of the current, because larger ion transfer induced by larger current would initiate the variation of concentrations of the feeding concentrated and diluted solutions along the flow path. It is worth highlighting that the ohmic resistance of the diluted solution component takes the dominant proportion in the total ohmic resistance of a RED stack, which decreases significantly with the increase of current. The non-ohmic resistance also decreases with the current increases. Therefore, the internal resistance of RED stack depends on the current. Moreover, it is found that the polarization curve and power density curve can be in good accordance with the experimental data if the influence of current on internal resistance is taken into consideration. The energy efficiency increases first and decreases with the current increase. The maximal energy efficiency of 3.4% was achieved when the RED stack operated in the maximum power density of 3.67 W/m2 at the current density of 44.9 A/m2. The coulombic efficiency increases with the increase of the current. The residual ratio of the SGE in effluent decreases with the increase of the current.

ПРОЕКТИРОВАНИЕ УСИЛИТЕЛЯ ЦИФРОВОГО СИГНАЛА МЕЖДУ КАМЕРОЙ И МОНИТОРОМ ПО КАНАЛУ СВЯЗИ В ОХРАННОМ ТЕЛЕВИДЕНИИ

The use of video surveillance systems is used in the areas of security, law and order, in the territories of protected objects, in monitoring the movement of road vehicles and in other areas. The main disadvantage of a video surveillance system is its susceptibility to weather influences (rain, fog, snowfall, etc.), which degrades the quality of the video system by reducing the signal level. Therefore, the urgency of finding new ways and possibilities to improve the quality of video signals is one of the priority areas of signal processing. The main task of this work was to determine the main parameters, simulate the transmission line and amplifier, and select the schematic diagram of the transmitting and receiving path with the voltage and current ratings. Both the receiver and the cable video transmitter have different means of adjusting to different transmission line lengths. The signal at the output of each receiver should be in the range from 0.9 to 1.1 V, and the spread of the total ohmic resistance of the wires of the video transmission line at the input of the receiver should be no more than 2 – 3%. Based on these parameters, the equipment is configured for transmitting video over the channel. The magnitude of the mismatch is regulated by potentiometers, which allow smooth adjustment of the video transmission equipment [1]. As a rule, video transmission over the channel is carried out at a distance of 50 to 1500 m. If it is necessary to transmit video at distances less than 50 m, additional resistances are connected in series at the receiver input so that the total line resistance is 30 - 50 Ohm [1].

Estimating charge-transport properties of fuel-cell and electrolyzer catalyst layers via electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is the most common experimental technique for measuring the charge-transport properties of catalyst layers. Such a measurement relies on fitting the impedance spectrum with analytical impedance expressions. To date, a study that examines the suitability of the available analytical models for such analysis does not exist. In this work, a numerical one-dimensional catalyst-layer model is used to assess the appropriateness of the analytical models for estimating catalyst-layer charge-transport properties. An ohmic-heating-based approach to computing ohmic resistance is used to examine the relationship between conductivity, resistance, and impedance of various catalyst layers representative of those used in proton-exchange-membrane fuel cells and water electrolyzers. The ohmic-heating analysis indicates that the most commonly used impedance expressions (Eikerling and Kornyshev, 1999; Makharia et al., 2005) result in a threefold overestimation of protonic resistance and may not be applicable to the proton-conductivity estimation for electrolyzer catalyst layers that exhibit low electronic conductivity. A more recent analytical model (Kulikovsky, 2017) is shown to produce protonic- and electronic-transport properties that agree with numerical simulations. The results of this work are used to provide recommendations for the selection of analytical impedance expressions for a given operating regime (H2/O2 or H2/N2) and an observed shape of the measured spectrum in order to achieve an accurate charge-transport characterization of a catalyst layer. A novel graphical approach to analyzing H2/N2 spectra is also proposed. Although the considered analytical models are not applicable to heterogeneous catalyst layers and no general algebraic conductivity-resistance relationship exists in that case, it is shown that the H2/N2 impedance measurement of heterogeneous catalyst layers provides the total ohmic resistance. A two-dimensional fuel-cell model is used to show that the anode catalyst layer may cause a distortion of the impedance spectrum at frequencies above 5 Hz that obstructs the charge-transport characterization with analytical models.

Ion Transport in Laser-Induced Graphene Cation-Exchange Membrane Hybrids.

Ion-exchange membranes hybridized with laser-induced graphene (LIG) might lead to membranes with functional surface effects such as antifouling, antibacterial, or joule heating effects; however, understanding the change in the electrical properties of the membrane is essential. Here we studied LIG-modified ion-exchange polymeric membranes using electrochemical impedance spectroscopy (EIS). The conductivity of the anionic sulfonated poly(ether sulfone) membranes and the effective capacitance of the membrane-electrolyte interface were obtained by fitting the EIS spectra to an electrochemical equivalent circuit and compared with LIG-modified nonionic poly(ether sulfone) films. The transport selectivity (as the relative permeability) of counterions (K+, Na+, Mg2+, Ca2+) across the membrane was quantified using the membrane's conductivities obtained from the EIS measurements. The total ohmic resistance of the membrane was directly correlated to the polymer thickness (with negligible contribution from the conductive LIG layer), thereby establishing EIS as a rapid, low-cost, and noninvasive method to accurately probe substrate usage in LIG modification.

Influence of the operation mode on PEM water electrolysis degradation

This paper compares the performance over time of seven different operation modes with the scope of investigating realistic degradation responses of a polymer electrolyte membrane water electrolysis (PEM WE) to different operation modes for grid-balancing services. Among these modes are constant current and constant voltage operations at different temperatures and current cycling operations, including a solar profile. It was found that faster current cycling improved the overall cell performance over the test period of 500 h, mainly due to a decrease in total ohmic resistance, which is also observed during the break-in phase preceding the experiments. Dynamic operation led to more severe fluoride emission from the catalyst binder, which improved the performance in terms of cell potential but could be a concern for long term degradation as membrane thinning promotes higher gas crossover. All other operation modes on the other hand suffered from an increase in total ohmic resistance, leading to an overall performance decrease, which is suspected to be due to the passivation of the Ti components. Higher operating temperatures were found to enhance cell performance, but are detrimental from a durability point of view since they exacerbate both membrane thinning and passivation processes.

Electrostatic spraying of membrane electrode for proton exchange membrane fuel cell

In order to improve the performance of proton exchange membrane fuel cell (PEMFC), the optimization of electrostatic spraying of membrane electrode was conducted. The influence of the spraying voltage on morphology, elemental composition of catalyst layer, and performance of the PEMFC were investigated. The results show that increasing spraying voltage could reduce agglomeration of the carbon-supported platinum particles, leading to more uniform pore distribution. High voltage did not accelerate oxidation of platinum catalyst. A high electrochemical active surface area of 26.18 m2/gpt was obtained when the platinum-carbon catalyst layer was deposited in cone jet mode. With further increasing spraying voltage, the total ohmic resistance and catalytic activity were changed slightly, whereas the charge transfer resistance was increased. Using the optimized electrostatic spraying parameters (injection rate = 100 μL min−1, spraying voltage = 8.5 kV, and working distance = 12 mm), a peak power density of 1.408 W cm−2 was obtained with an output voltage of 0.451 V.

Heat to H2: Using Waste Heat for Hydrogen Production through Reverse Electrodialysis

This work presents an integrated hydrogen production system using reverse electrodialysis (RED) and waste heat, termed Heat to H 2 . The driving potential in RED is a concentration difference over alternating anion and cation exchange membranes, where the electrode potential can be used directly for water splitting at the RED electrodes. Low-grade waste heat is used to restore the concentration difference in RED. In this study we investigate two approaches: one water removal process by evaporation and one salt removal process. Salt is precipitated in the thermally driven salt removal, thus introducing the need for a substantial change in solubility with temperature, which KNO 3 fulfils. Experimental data of ion conductivity of K + and NO 3 − in ion-exchange membranes is obtained. The ion conductivity of KNO 3 in the membranes was compared to NaCl and found to be equal in cation exchange membranes, but significantly lower in anion exchange membranes. The membrane resistance constitutes 98% of the total ohmic resistance using concentrations relevant for the precipitation process, while for the evaporation process, the membrane resistance constitutes over 70% of the total ohmic resistance at 40 ∘ C. The modelled hydrogen production per cross-section area from RED using concentrations relevant for the precipitation process is 0.014 ± 0.009 m 3 h − 1 (1.1 ± 0.7 g h − 1 ) at 40 ∘ C, while with concentrations relevant for evaporation, the hydrogen production per cross-section area was 0.034 ± 0.016 m 3 h − 1 (2.6 ± 1.3 g h − 1 ). The modelled energy needed per cubic meter of hydrogen produced is 55 ± 22 kWh (700 ± 300 kWh kg − 1 ) for the evaporation process and 8.22 ± 0.05 kWh (104.8 ± 0.6 kWh kg − 1 ) for the precipitation process. Using RED together with the precipitation process has similar energy consumption per volume hydrogen produced compared to proton exchange membrane water electrolysis and alkaline water electrolysis, where the energy input to the Heat to H 2 -process comes from low-grade waste heat.

GOLD-PLATED TITANIUM VS CARBON-IMPLANTED TITANIUM AS MATERIAL FOR BIPOLAR PLATES IN PEM FUEL CELLS

Three different types of current-collecting plates for air-hydrogen PEM fuel cell were manufactured and tested: unmodified titanium plates; gold-plated titanium plates and titanium plates treated by carbon ions implantation. It was shown that the applied surface modifications reduce contact resistance between titanium plate and carbon gas diffusion layer. Total ohmic resistance of fuel cell is reduced by 1.8 and 1.4 times in case of gold-plated titanium and carbon-implanted titanium, respectively, in comparison with uncoated titanium. Although gold plating turned out to be more profitable than carbon ion implantation in terms of electrical characteristics, in the last case, the performance enhancement was reached without using precious metals, which at mass production must play more important role. This technology promises to reduce the cost of bipolar plates manufacturing, while maintaining high electrical performance of PEM fuel cells.

Performance improvement of proton exchange membrane electrolyzer cells by introducing in-plane transport enhancement layers

Thin/tunable liquid/gas diffusion layers (TT-LGDLs) or porous transport layers (TT-PTLs), have exhibited superior multifunctional performance in proton exchange membrane electrolyzer cells (PEMECs), which can be attributed to their unique structures, such as planar surface, straight-through pores, thin thickness, etc. For achieving better PEMEC performance, TT-LGDLs with smaller pore size are desired. However, in this case, mass diffusion issues are brought in when some of the pores are covered by the flow field lands or shoulders. The coverage of the pores can lead to very high transport resistance, which may reduce the number of active oxygen evolution reaction sites, and therefore lower down the PEMEC performance. The in-plane transport enhancement layer for TT-LGDLs/PTLs are proposed to develop a dual-layer LGDL/PTL structure for improving the mass diffusion and the PEMEC performance. The results of this research reveal that the dual-layer LGDL/PTL structure exhibits smaller ohmic resistance and mass transport resistance, and therefore improve the PEMEC performance, without obvious changes in kinetic losses. The total ohmic resistance and mass transport resistance can be reduced by about 23% and 41%, respectively, with an ∼830 μm pore TT-LGDL/PTL stacking on a ∼100 μm pore TT-LGDL/PTL. The results indicate the feasibility of stacking the in-plane transport enhancement layer with large pore sizes onto a small pore TT-LGDLs/PTLs for high efficiency and low cost PEMEC practical applications.

High‐frequency InAlN/GaN HFET with f max over 400 GHz

Ultra-thin InAlN/GaN heterostructure field-effect transistors (HFETs) having high maximum oscillation frequency (f max) are fabricated by scaling lateral dimensions. A 3 nm GaN cap layer is adopted to reduce the electron density and suppress the short-channel effects. Non-alloyed regrown n+-GaN ohmic contacts with total ohmic resistance (R tot) of 0.13 Ω.mm is also introduced into the device, in which the virtual source-to-drain distance is 600 nm. T-shaped gate with 40 nm length is formed in the centre of the source-to-drain region by self-aligned e-beam lithography. The peak extrinsic transconductance (g m) reaches 956 mS/mm. Most of all, a high f max of 405 GHz is obtained, which is the highest value among the reported InAlN/GaN HFETs. These obtained results mean that the InAlN/GaN HFETs having reliability should be still suitable for G-band (140–220 GHz) power-amplifier application with further optimisation.

Electrophoretic Deposition of Gadolinium‐doped Ceria as a Barrier Layer on Yttrium‐stabilized Zirconia Electrolyte for Solid Oxide Fuel Cells

AbstractReplacing the electronically conductive (LaSr)MnO3±δ (LSM) cathode in the LSM/yttrium‐stabilized zirconia (YSZ) system with the mixed ion‐electron conductive (MIEC) (LaSr)(CoFe)O3–δ will promote cathode performance in solid oxide fuel cells (SOFCs) significantly. However, a barrier layer between LSCF and YSZ is necessary for preventing chemical reaction between these two components. In this study, a gadolinium‐doped ceria (GDC) barrier layer was deposited on the YSZ electrolyte by scalable and cost‐effective electrophoretic deposition (EPD). Polypyrrole (PPy) was coated on the YSZ surface as the conductive agent. A highly compact GDC green layer was obtained by the EPD process in an ethanol‐based suspension. GDC barrier layers ranging in thickness from 5 µm to 8 µm were successfully densified at temperatures as low as 1,300 °C. The performance of these cells was evaluated using a symmetrical cell configuration through electrochemical impedance spectroscopy (EIS). Ohmic resistance of the GDC barrier layer made by EPD versus the conventional spin‐coating method was reduced by 0.09 Ω cm2 at 750 °C, which generally accounts for 30% of the total ohmic resistance for the electrode‐supported fuel cells (0.30 Ω cm2). This result suggests that EPD is a highly desirable method for efficiently manufacturing an electrolyte barrier layer with improved performance.

A laminar flow-based single stack of flow-over planar microfluidic fuel cells

Abstract Power densities of microfluidic fuel cells are still not high enough for power source applications. In this study, we propose a novel planar stack to increase the total power of a microfluidic fuel cell. Electrical connections in serial or parallel are made within one channel by using multiple laminar flow. A planar structure with flow-over electrodes of platinum are adopted for easy integration with other planar micro devices. These structures are made by micromachining with a thin film process. Fuel cell performance and total ohmic resistances are measured experimentally with a formic acid-based fuel. The results show that the proposed single stacks provide more power density with a comparatively small total ohmic resistance and require less space than that of the fuel cell arrays. The peak volumetric power density improves by 97.5% and 39.3% using parallel and serial electrical connections, respectively, at a 300 μL min −1 flow rate. Utilizing this single stack, we believe that microfluidic fuel cells can be integrated into a compact planar configuration to achieve a power high enough for energy source applications.

Anomalous Ohmic Loss in Solid Oxide Fuel Cells with Doped Ceria Interlayers

The ohmic resistance of an anode-supported SOFC is usually composed of contributions from the following parts: anode, electrolyte, cathode, and contact between the cell components. All of the ohmic resistances of cell components should be as small as possible to achieve an efficient SOFC operation. It is often believed that most of the ohmic resistance attributes to the electrolyte since the ionic conductivity of the electrolyte is usually several orders of magnitude lower than the electrical conductivities of the electrodes. In an SOFC single cell with YSZ as the electrolyte, however, the contribution of the electrolyte to the total ohmic resistance of the anode-supported SOFC has not yet been fully understood despite of the numerous studies in this area. Several groups reported that the YSZ films in real SOFC environment showed lower ionic conductivities than that of the bulk YSZ material. This is especially true when the doped ceria is screen printed on YSZ-based single cell, in which the magnitude of the ohmic losses in the cell was observed more than three times greater than that for YSZ. The aim of this paper is to address the origin of the exaggerated ohmic resistance.