Small Interelectrode Distances Research Articles

(Invited) Understanding Potential Losses and pH Distribution in Electrochemical Nitrate Reduction Reaction to Ammonia

Electrochemical nitrate reduction reaction (NO3 -RR) to ammonia is a promising route to eliminate one of the major pollutants in surface and groundwater. When powered by renewable electricity, electrolysis provides a sustainable method to generate ammonia from nitrate ions, facilitating the transition from a linear to a circular economy. Optimizing the physical and chemical properties of electrolysis cells is crucial to make this process economically viable for widespread implementation. Here, we explore how the choice of current density, conductivity, pH, inter-electrode distance, membrane, catalyst, and buffer solution affect nitrate removal performance and efficiency. We develop a modeling framework to investigate the cell characteristics and fluid dynamics during electrochemical NO3-RR using both laminar and bubbly flows. To obtain more precise results, we employed the bubbly flow model (i.e., Multi-phase Fluid) to take into account how gas production near the electrode surface affects liquid velocity, pH distribution, and, ultimately, potential losses. We exploit mass transfer theory to include the current density effect on migration and diffusion. In the absence of a buffer solution, the Nernstian loss became a significant portion of the polarization loss, which increased with current density. We identified the positive effect of the membrane on energy efficiency to be more significant at smaller inter-electrode distances. This study provides insights into the origin of potential losses and pH distribution, enabling electrochemical cell optimization for renewable fuel synthesis. Next, we will discuss our recent work on designing single atom alloys (SAAs) for selective conversion of low concentration nitrate ions in natural wastewater streams from Dairy farm into ammonia. We benchmarked the catalyst's selectivity, activity, and stability following standard protocols established in the field of nitrogen fixation, including control experiments by using an electrolyte that does not contain nitrate ions and performing experiments under the same operating conditions using nitrogen isotopes (i.e., 14NO3 -, 15NO3 -).

Oxidation of organics in water by active chlorine performed in microfluidic electrochemical reactors: a new way to improve the performances of the process

Wastewater polluted by organics can be treated by using electro-generated active chlorine, even if this promising route presents some important drawbacks such as the production of chlorinated by-products. Here, for the first time, this process was studied in a microfluidic electrochemical reactor with a very small inter-electrode distance (145 μm) using a water solution of NaCl and phenol and a BDD anode. The potential production of chloroacetic acids, chlorophenols, carboxylic acids, chlorate and perchlorate was carefully evaluated. It was shown, for the first time, up to our knowledge, that the use of the microfluidic device allows to perform the treatment under a continuous mode and to achieve higher current efficiencies and a lower generation of some important by-products such as chlorate and perchlorate. As an example, the use of the microfluidic apparatus equipped with an Ag cathode allowed to achieve a high removal of total organic carbon (about 76%) coupled with a current efficiency of 17% and the production of a small amount of chlorate (about 30 ppm) and no perchlorate. The effect of many parameters (namely, flow rate, current density and nature of cathode) was also investigated.

Conversion of CO2 to formic acid in a microfluidic electrochemical cell with and without supporting electrolyte

Electrochemical reduction of carbon dioxide to formic acid or formate (FA) is considered an interesting route to valorize CO2 effluents. Here, we have performed the conversion of CO2 to FA in an undivided microchannel electrochemical reactor characterized by very small inter-electrode distances (75–250 μm) using Na2SO4 as supporting electrolyte (SE). It was found that the use of the microfluidic cell allows to work both in the presence and in the absence of SE with lower cell potentials with respect to conventional cells and to obtain significant conversions per pass of CO2 to FA. The effect of many parameters, such as distance between electrodes, flow rate, current density, concentration of Na2SO4 and pH, was studied. In particular, it was shown that the production of FA increases by reducing the concentrations of Na2SO4 and it presents the maximum value in the absence of it.

(Invited) Design and Modeling of Resistive Switching Devices for Neuromorphic Applications

Resistive switching devices are a critical component for the realization of computational systems that mimic biological neural systems. In biological systems, connection weights between the neurons are dependent on the time lapse between the neurons’ action potentials. Resistive switching devices (memristors) with multiple conductance states can mimic this biological spike-time-dependent plasticity (STDP) behaviour, with multiple conductance states being analogous to different synaptic weights. Additionally, memristors are one of the contenders as storage elements in non-volatile memories. The change in conductance in memristors can be broadly attributed to vacancy migration inside the switching layer (valence change memory) or the formation of metallic filaments inside the switching layer (electrochemical metallization).Electrochemical metallization-based memristors with vertical transport and small inter-electrode distances have been reported recently. Their device characteristics exhibit multiple conductance states with relatively low switching voltages, which makes them well-suited for low-power neuromorphic applications. Our work models the transport in these memristors, focusing on explaining and capturing their current-voltage characteristics. Our model also captures the switching dynamics (with an emphasis on estimating switching energies and delays) and explains the experimentally observed STDP behaviour in these devices. We have proposed models for filament growth and dissolution along one dimension (axial) and two dimensions (axial and radial). The simulation results obtained using our model (and implemented in Verilog-A) have been validated with experimental data from multiple sources. Our work demonstrates the flexibility of including different transport mechanisms in a unified framework.We have characterized valence change memory (VCM) memristors with HfO2 and HfZrO2 as switching layers. They exhibit bipolar switching, negative differential resistance, multiple conductance states, and STDP behaviour, which have been analyzed. We have also modeled the transport in these VCM-based memristors, and simulated the I-V and dynamic switching characteristics, and STDP behaviour in these devices. We have simulated arrays of, both, ECM and VCM devices using the proposed models, and tried to demonstrate a typical pattern classification problem using in-memory computation. We believe that our models provide useful insight into the design of memristors for neuromorphic and memory applications.

Toward noninvasive monitoring of plant leaf water content by electrical impedance spectroscopy

Plant water content is one key factor that affects the wellbeing of plants. Furthermore, detecting plant water needs in a timely manner is important when considering irrigation strategies (water management). A first step toward plant water monitoring is the availability of a portable and cost-effective system that yields reliable information. Electrical impedance spectroscopy (EIS) has previously been used invasively to provide such data and can be implemented at an acceptable cost. In this work, a cost-effective impedance analyzer capable of performing noninvasive four-electrode measurements was built. In addition, data fitting has been improved by using a modified equivalent circuit model, yielding relevant information about the plant water status. The results show that the resistor that models the extracellular fluid can track the change in the water content of a plant leaf while it is drying at room temperature. On the other hand, impedance data might provide information about the leaf tissue structure at small interelectrode distances.

Validity of the law of similarity for impulse and DC breakdown of gases in the vicinity of the Paschen’s minimum

The study discusses the validity of the law of similarity for the impulse breakdown of gases at low pressure and small inter-electrode distances. The research has been done theoretically, experimentally and numerically. The experiments were conducted under well-controlled laboratory conditions. The combined measurement uncertainty was less than 5%. The experiments were carried out with gases SF6, N2 and He. The electrodes were made of copper, but also of metals with different values of the work function. The obtained results are presented through the impulse characteristics for similar systems. Such a presentation only provided qualitative information. In order to obtain quantitative information, a modified Wilcoxon Rank Sum Test (U test) was performed. The obtained results show that the law of similarity is valid for the DC breakdown of gases, but it is not valid for the impulse breakdown of gases. It can be concluded that the law of similarity is inapplicable for the design of gas-insulated systems under low pressure at small values of the inter-electrode distance.

Simulation of Cold Atmospheric Plasma Generated by Floating-Electrode Dielectric Barrier Pulsed Discharge Used for the Cancer Cell Necrosis

A numerical simulation of a pulsed floating electrode dielectric barrier discharge (FE-DBD) at atmospheric pressure, used for melanoma cancer cell therapy, is performed using a plasma model in COMSOL Multiphysics software. Distributions of electron density, space charge, and electric field are presented at different instants of the pulsed argon discharge. Significant results related to the characteristics of the plasma device used, the inter-electrodes distance, and the power supply are obtained to improve the efficiency of FE-DBD apparatus for melanoma cancer cell treatment. The FE-DBD presents a higher sensitivity to short pulse durations, related to the accumulated charge over the dielectric barrier around the powered electrode. At higher applied voltage, more energy is injected into the discharge channel and an increase in electron density and electric consumed power is noted. Anticancer activity provided by the FE-DBD plasma is improved using a small interelectrode distance with a high electron emission coefficient and a high dielectric constant with a small dielectric thickness, allowing higher electron density, generating reactive species responsible for the apoptosis of tumor cells.

Comparing different bulk radiation damage models in TCAD simulations of small-pitch 3D Si sensors

Small-pitch, thin 3D Si sensors have been developed for the ATLAS and CMS experiment upgrades at the High Luminosity LHC. The pixel sizes are 50 × 50 µm2 with 1 readout column, and 25 × 100 µm2 with 1 or 2 readout columns (1E and 2E). Owing to the small inter-electrode distance, ranging from ∼28 µm to ∼51 µm in the considered layouts, these devices are expected to be extremely radiation hard. TCAD simulations by Synopsys Sentaurus, incorporating advanced radiation damage models, have been used for the design/optimization of these new 3D pixel sensors. In this study, we have compared the accuracy of different bulk damage models in predicting the signal efficiency of small-pitch 3D sensors irradiated at large fluences and its evolution with the bias voltage at different positions within the 3D cell. Selected simulation results will be reported in comparison to experimental data.

Effects of detection system parameters on cross-correlations between MUAPs generated from parallel and inclined muscle fibres

Abstract The aim of this study was to investigate the effects of inter-electrode distance (IED), electrode radius (ER) and electrodes configurations on cross-correlation coefficient (CC) between motor unit action potentials (MUAPs) generated in a motor unit (MU) of parallel fibres and in a MU of inclined fibres with respect to the detection system. The fibres inclination angle (FIA) varied from 0° to 180° by a step of 5°. Six spatial filters (the longitudinal single differential (LSD), longitudinal double differential (LDD), bi-transversal double differential (BiTDD), normal double differential (NDD), an inverse binomial filter of order two (IB2) and maximum kurtosis filter (MKF)), three values of IED and three values of ER were considered. A cylindrical multilayer volume conductor constituted by bone, muscle, fat and skin layers was used to simulate the MUAPs. The cross-correlation coefficient analysis showed that with the increase of the FIA, the pairs of MUAPs detected by the IB2 system were more correlated than those detected by the five other systems. For each FIA, the findings also showed that the MUAPs pairs detected by BiTDD, NDD, IB2 and MKF systems were more correlated with smaller IEDs than with larger ones, while inverse results were found with the LSD and LDD systems. In addition, the pairs of MUAPs detected by the LDD, BiTDD, IB2 and MKF systems were more correlated with large ERs than with smaller ones. However, inverse results were found with the LSD and NDD systems.

On the electrode positioning for bipolar EMG recording of forearm extensor and flexor muscle activity after transcranial magnetic stimulation

After stroke, motor pathways are often affected, leading to paresis. It remains difficult to reliably predict motor recovery of the upper extremity, for which transcranial magnetic stimulation (TMS) may add to clinical examination. Placement of the surface electromyography (sEMG) electrodes in TMS is essential for information about specific muscle groups and corticospinal pathways. This study primarily aimed to determine the optimal sEMG electrode positions for recording activity of forearm flexor and extensor muscles. The first goal was to optimize sensitivity in measuring any motor evoked potentials (MEP), because they may be reduced or absent in stroke patients. The second goal was adequate distinction between forearm flexor and extensor muscle groups. For optimal flexibility in choosing montages, a multichannel sEMG set-up with 37 electrodes encircled the forearm. The determination of optimal pairs was based upon electrical peripheral nerve stimulation. We found pairs with the highest compound nerve action potential (CMAP) amplitudes and pairs that optimally distinguished between the flexor and extensor muscles. Large interelectrode distances lead to responses with larger amplitudes and therefore sensitively measure any remaining corticomuscular connections. As a follow-up, specific muscle group responses can be targeted with smaller interelectrode distances. In conclusion, this study helps to identify better electrode locations for the use of clinical TMS studies.

The influences of the electrode dimension and the dielectric material on the breakdown characteristics of coplanar dielectric barrier discharge in ambient air

The influences of the angle of the V‐shaped electrode, the interelectrode distance, the thickness and the types of dielectric plate on the breakdown characteristics of air coplanar dielectric barrier discharge are investigated. When the angle of the electrode increases, the ignition voltage surprisingly decreases, while both the discharge current, and plasma area remain constant. With smaller interelectrode distance or thinner dielectric plate, lower ignition voltage is required. However, if either one of the them exceeds the threshold value, the discharge mode transits from steady to stochastic. In addition, the discharge appearance in the case of alumina is visually more diffuse with respect to the cases of mica and quartz, which may be caused by the surface charges with high density.

Long-term deconditioning of gas-filled surge arresters

ABSTRACTThe aim of this paper is to identify parameters that influence the long-term deconditioning effect of gas-filled surge arrester (GFSA) and to provide practical recommendations for mitigating this effect. Namely, after some period of time, on order of hours or days, during which there is no activation due to overvoltage, the deconditioning of GFSA occurs. This effect was observed experimentally within the paper. The observed parameters that could influence the long-term deconditioning effect were the following: shape of voltage load, gas type, gas pressure, interelectrode distance, electrode material, electrode surface topography as well as GFSA design such as two- or three-electrode configuration. According to the results obtained, it has been shown that the occurrence of long-term deconditioning in an insulating system, insulated by a noble gas at a subpressure and with small interelectrode distances, is a phenomenon that always occurs when the insulating system is at rest for about an hour. It has been found that the type of noble gas does not influence the long-term deconditioning. Analysis of such insulating systems' parameters, with a prospect of being used as GFSAs, has demonstrated that this phenomenon is less pronounced at higher pressures (for the same value of the pressure (p) and interelectrode distance (d) product) and for electrodes with microscopically embossed surfaces. According to the results that were obtained by noble gases and their mixtures, as well as the results that were obtained by mixtures of SF6 gas with noble gasses, it can be claimed with confidence that the effect of the long-term deconditioning is an electrode effect. It has also been established that the deconditioning effect does not depend on the electrode material except in the case of electrodes made out of noble metals, which reduce the effect. Based on these results, it can be recommended that the working point of GFSAs be set (according to the DC breakdown voltage value) at a pressure that is as high as possible (with pd = const), and that the electrode active surface should have a marked microscopic topography. In addition to this, an essential conclusion for GFSA manufacturers is that long-term system deconditioning is caused by impurities and adsorbed gases that appear at electrode during the state of rest. Out of these two causes, the influence of impurities is probably the dominant one, which is proved by considerably reduced long-term deconditioning in the case of noble metal electrodes, not susceptible to corrosion. This has also been confirmed by a less distinct effect of long-term deconditioning in the case of sandblasted electrodes that have a stronger tendency towards gas adsorption and a weaker tendency towards corrosion. However, it has been shown that adding of the third electrode (that is concentric to the main electrode system) on a free floating potential along with usage of sandblasted electrodes and with smaller interelectrode distance significantly reduces the effects of the long-term deconditioning.

Transient dynamics simulation of the electrical discharge-generated bubble in sinking EDM

The evolution of bubble generated by electrical discharge in sinking electrical discharge machining (EDM) occurs in a very short time period and in a very narrow space, thus making both observation and theoretical analysis extremely difficult. For this reason, the role of the discharge-generated bubble during the material removal process in sinking EDM has not to be understood very clearly. In this paper, the evolution process of the discharge-generated bubble was simulated in three dimensions and the influence of inter-electrode distance was studied. It was found that the evolution of the discharge-generated bubble was significantly affected by the inter-electrode distance. Smaller inter-electrode distances can contribute to a larger pressure and expansion velocity of the bubble as well as a larger force applied on the tool electrode and workpiece. The experimental results showed that the geometry characteristics of the crater were also significantly affected by inter-electrode distance. A smaller inter-electrodes distance can contribute to a larger volume of both melted and removed materials. The simulation results were discussed by relating to the results of single-pulse discharge experiments; some insightful views about the material removal mechanism of EDM were presented in this paper.

Hydrogen Production from Water Vapor Plasmolysis Using DBD-Corona Hybrid Reactor

Hydrogen is produced by plasmolysis of (1) demineralized water vapors (steam) and (2) water vapor and argon gas. In the current study, a custom-made dielectric barrier discharge (DBD)-corona hybrid reactor has been designed and used for steam plasmolysis. The objective is to show the feasibility of hydrogen production from water vapor plasmolysis at atmospheric pressure at a relatively smaller interelectrode distance and to characterize the plasma for its fundamental properties. The generated plasma properties have been characterized by applying spectrometric analysis, in which the measured plasma temperatures are demonstrated in the order Te > Texc > Trot, which confirms the nonlocalized thermal equilibrium (LTE) existence in the plasma bulk. Moreover, the estimated electron number density (1.765 × 1017 m–3) shows an exceptional energy for water vapor dissociation, which eventually results in a reasonable efficiency. The estimated energy efficiency and thermodynamic efficiency for water vapor plasmolysis along with steam was found to be 78.8% and 79.2%, respectively. Accordingly, the calculated production rate (20 g/kWh) and the predicted cost (£0.09/kWh) are shown to be competitive to the electrolysis process, which is increasingly applied for hydrogen production, with the benefit of reduced equipment size and low power consumption.

Design and Implementation of a Microelectrode Assembly for Use on Noncontact In Situ Electroporation of Adherent Cells

In situ electroporation of adherent cells provides significant advantages with respect to electroporation systems for suspension cells, such as causing minimal stress to cultured cells and simplifying and saving several steps within the process. In this study, a new electrode assembly design is shown and applied to in situ electroporate adherent cell lines growing in standard multiwell plates. We designed an interdigitated array of electrodes patterned on copper with printed circuit board technology and covered with nickel/gold. Small interelectrode distances were used to achieve effective electroporation with low voltages. Epoxy-based microseparators were constructed to avoid direct contact with the cells and to create more uniform electric fields. The device was successful in the electropermeabilization of two different adherent cell lines, C2C12 and HEK 293, as assessed by the intracellular delivery of the fluorescent dextran FD20S. Additionally, as a collateral effect, we observed cell electrofusion in HEK 293 cells, thus making this device also useful for performing cell fusion. In summary, we show the effectiveness of this minimally invasive device for electroporation of adherent cells cultured in standard multiwell plates. The cheap technologies used in the fabrication process of the electrode assembly indicate potential use as a low-cost, disposable device.

Ultrananocrystalline diamond film deposition by direct-current plasma assisted chemical vapor deposition using hydrogen-rich precursor gas in the absence of the positive column

We have investigated the ultrananocrystalline diamond (UNCD) deposition by direct-current plasma assisted chemical vapor deposition on 4 in. Si wafer using CH4H2 as well as CH4Ar gas chemistry containing additive nitrogen. CH4/N2/H2 (5%/0.5%/94.5%) and CH4/N2/H2/Ar (0.5%/5%/6%/88.5%) gas mixtures were compared as the precursor gas. Molybdenum and tungsten were compared as cathode material. Discharge voltage and current were 480 V/45 A and 320 V/60 A, for respective gas chemistry. Chamber pressure and substrate temperature were 110–150 Torr and 750–850 °C, respectively. The film was characterized by near edge x-ray absorption fine structure spectroscopy, x-ray diffraction, high-resolution transmission electron microscope, electron energy loss spectroscopy, and high-resolution scanning electron microscope. We have demonstrated that (1) elimination of the positive column, by adopting very small interelectrode distance, gave some important and beneficial effects; (2) the plasma stability and impurity incorporation was sensitive to the cathode material and the precursor gas; (3) using the conventional CH4/H2 precursor gas and tungsten cathode, the mirror-smooth 4 in. UNCD film of excellent phase-purity and grain size below 10 nm could be deposited even in the absence of the positive column. The high electric field in the unusually narrow interelectrode space and the consequent high electron kinetic energy, in conjunction with the unusually high electron current thereof, directed to the substrate, i.e., the anode, was proposed to be the source of the grain refinement to achieve UNCD at such high chamber pressure around 110–150 Torr, in the absence of the usual ion bombardment assistance.

Analysis of heating effect on the process of high deposition rate microcrystalline silicon

A possible heating effect on the process of high deposition rate microcrystalline silicon has been studied. It includes the discharge time-accumulating heating effect, discharge power, inter-electrode distance, and total gas flow rate induced heating effect. It is found that the heating effects mentioned above are in some ways quite similar to and in other ways very different from each other. However, all of them will directly or indirectly cause the increase of the substrate surface temperature during the process of depositing microcrystalline silicon thin films, which will affect the properties of the materials with increasing time. This phenomenon is very serious for the high deposition rate of microcrystalline silicon thin films because of the high input power and the relatively small inter-electrode distance needed. Through analysis of the heating effects occurring in the process of depositing microcrystalline silicon, it is proposed that the discharge power and the heating temperature should be as low as possible, and the total gas flow rate and the inter-electrode distance should be suitable so that device-grade high quality deposition rate microcrystalline silicon thin films can be fabricated.

Electrolysis in a closed electrochemical cell with a small inter-electrode distance. Metal dissolution/deposition in plain electrolytes

Abstract The present paper focuses on the specificity of electrolysis during metal dissolution/deposition in a closed electrochemical cell with a small inter-electrode distance. A mathematical model for the electrochemical process in such a cell has been developed. We give an analytical and a numerical solution to the system of mass transfer equations for such a cell, in binary solutions and solutions with supporting electrolyte. The dependence of metal deposition rate on the voltage applied to the cell and on the electrolysis time has been studied by numerical simulation. We also present the results of the experimental study of the rate of silver electrodeposition from an acid solution of AgClO4 in a cell of size 1 mm × 1 mm. Metal deposition rate in a closed electrochemical cell with a small inter-electrode distance has been demonstrated to be at least three times higher than that at recessed electrodes. The results obtained may be used in the LIGA processes for accelerating the filling of photoresist cavities with metal.

Recent advances in very high frequency plasma enhanced CVD process for the fabrication of thin film silicon solar cells

We have deposited amorphous silicon (a-Si) and nanocrystalline silicon (nc-Si) materials and the total p–i–n configurations for solar cells in a high vacuum multichamber system ASTER using very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) process. The deposition process is monitored and controlled by in-situ diagnostic tools to maintain reproducibility of the material quality. In this paper we show our recent results on single junction (amorphous silicon) and tandem (a-Si/nc-Si) cells on plastic foil using the Helianthos concept. The tandem cell efficiency on Asahi U-type SnO 2:F coated glass is ~ 12% and this is achieved by employing nc-Si deposited at high pressure ( p) conditions of 5 mbar and a small inter-electrode distance ( d) of 5 mm. The deposition scheme of this cell on glass was adapted for the SnO 2:F coated Al foil substrates from Helianthos b.v., especially taking into account the expansion of the foil during deposition. The inter-electrode distance d was one of the variables for this optimisation process. Depositions at four inter-electrode distances of 6 mm, 8 mm, 10 mm and 12 mm (keeping the pressure–distance product constant) revealed that the deposition rate increases at higher distances, reaching 0.6 nm/s at a d of 10 mm and pressure p of 3.0 mbar. The Raman crystalline ratio showed a monotonic increase with the combination of higher d and lower p. Tandem cells with an area of 2.5 cm 2 on plastic foil fabricated by the Helianthos concept and employing the above mentioned nc-Si made at 0.6 nm/s in the bottom cell and a-Si in the top cell, showed an efficiency of 8.12%, with a short circuit current density of 10 mA/cm 2. The combined deposition time of the photoactive silicon layers of the top and bottom cells amounted to only 85 min.

Estimating the measuring sensitivity of unipolar and bipolar ECG with lead field method and FDM models

New portable electrocardiogram (ECG) measurement systems are emerging into market. Some use nonstandard bipolar electrode montage and sometimes very small interelectrode distances to improve the usability of the system. Modeling could provide a straightforward method to test new electrode systems. The aim of this study was to assess whether modeling the electrodes’ measuring sensitivity with lead field method can provide a simple tool for testing a number of new electrode locations. We evaluated whether the actual ECG signal strength can be estimated by lead fields with two realistic 3D finite difference method (FDM) thorax models. We compared the modeling results to clinical body surface potential map (BSPM) data from 236 normal patients and studied 117 unipolar and 42 bipolar leads. In the case of unipolar electrodes the modeled measuring sensitivities correlated well with the clinical data ( r = 0.86, N = 117, p < 0.05). In the case of bipolar electrodes the correlation was moderate ( r = 0.62 between Model 1 and clinical data, r = 0.71 between Model 2 and clinical data, N = 42 and p < 0.05 for both). Based on this we can conclude that lead field analysis based on realistic thorax models provides a good initial prediction for designing new electrode montages and measurement systems.