Planar Electrodes Research Articles

Evaluating the impact of electrode planes on regional lung function assessment

ObjectiveThe aim of the study was to explore the influence of the measurement plane on regional lung function assessed via electrical impedance tomography (EIT).MethodsThe forced vital capacity (FVC) maneuver was prospectively performed in 30 healthy male volunteers. Simultaneously, EIT measurements were conducted at the 3rd, 4th, and 5th intercostal spaces (ICS). The EIT-based spirometry parameters are calculated in a similar manner to their original definitions. The spatial and temporal distributions of the corresponding functional images were assessed and compared among the measurement planes.ResultsAll subjects but one were able to perform the FVC maneuver according to the guidelines. Significant differences were found in 67% (6 out of 9) of the EIT-based parameters assessing the spatial and temporal distribution. The fEIT images were most homogeneous at ICS 4 compared to the other two measurement planes, except for the time required for 75% of FVC. The fEIT image FVCEIT distributed toward dorsal regions when the measurement planes moved from ICS 3 to ICS 5, whereas the identified lung areas became smaller.ConclusionThe spatial and temporal distribution of the regional lung function measured via EIT was influenced by the measurement planes. We recommend adhering to the same measurement plane for before–after comparison. ICS 4 was recommended for the sitting subjects performing lung function testing.

A simplified coaxial ion trap mass analyzer: Characterization of the simplified toroidal ion trap with a rectilinear ion guide

A new miniature coaxial ion trap mass analyzer with a rectilinear ion guide has been constructed using a combination of planar and cylindrical electrodes. The results reported here focus on characterizing the performance of the rectilinear ion guide and simplified toroidal ion trap components. The simplified toroidal ion trap was found to have an ion capacity in excess of 105 ions and mass spectral resolution of 0.5–0.6 when used as a mass analyzer. The ion storage efficiency within the toroidal trapping region was evaluated and found the stored ion population decreased exponentially with storage time. Ion losses depended slightly on the stored βz condition. Ion losses within the toroidal region are attributed primarily to field instabilities at the intersection point of the two components while charge exchange reactions were observed but considered a minor loss mechanism. The ability to mass selectively ejection ions of a specific mass from the toroidal trapping region was characterized and found to approach 100 % efficiency under appropriate ejection conditions.

Indentation-Free Resistance Spot Welding of SUS301L Stainless Steel

Paint-free bodywork has become an attractive alternative for rail vehicles, in the direction of easy maintainability and low manufacturing costs. However, conventional resistance spot welding inevitably leaves indentation marks to detrimentally reduce the optical homogeneity of the paint-free bodywork. In light of this, indentation-free resistance spot welding is proposed for joining SUS301L stainless steel sheets in order to achieve superior surficial integrity. A tiny SUS301L steel ball with a diameter of 1.5 mm was chosen as the intermediate filler between two steel sheets to avoid the formation of surficial indentation. The influence of welding current and welding time on the mechanical properties of joints was studied. The optimal parameters of the mechanical properties were obtained when the welding current was 8.0 kA, the welding time was 150 ms, the electrode pressure was 0.35 MPa, and the electrodes were cylindrical planar electrodes, which was determined by comparing the tensile shear test results. The surficial indentation depth was less than 1% of the plate thickness, and no observable indentations were seen on the surface of the optimized welding spots.

Deep cryogenic silicon etching for 3D integrated capacitors: A numerical perspective

One promising approach to increase the capacity density of integral microcapacitors, microsupercapacitors, and microbatteries is three-dimensional structure design, where electrodes are exposed in three dimensions instead of conventional in-plane electrodes. Such structures include nanowires, nanotubes, nanopillars, nanoholes, nanosheets, and nanowalls. In this work, a cryogenic silicon etching process suitable for fabrication of structures with high electrode area is proposed. A numeric model of this process is experimentally calibrated and used for pillar array structure sidewall area optimization. The use of adaptive Runge–Kutta–Fehlberg time integrator allows to achieve almost linear overall computation complexity as a function of simulated etching time, despite the linear increase in conductance computation complexity with depth. A rule for choosing optimal geometric structure parameters under technological constraints is formulated. An optimized trefoil-like structure is proposed, resulting in a total 5.5% increase in sidewall area with respect to the hexagonal array of circular pillars, resulting in 20.33 sidewall area per unit chip area for 30 min long etch or 31.80 for 60 min long etch.

Wave-shaped electrode design for enhanced nanowire alignment efficiency: A comparative analysis with interdigitated electrodes in two-dimensional plane

Wave-shaped electrode design for enhanced nanowire alignment efficiency: A comparative analysis with interdigitated electrodes in two-dimensional plane

Synergistic ZnO-NiO composites for superior Fiber-Shaped Non-Enzymatic glucose sensing

The rise in diabetes requires new glucose sensors, as traditional enzyme-based and planar electrodes are sensitive to the environment and hard to integrate into wearables. This study addresses these issues by developing a flexible, non-enzymatic glucose sensor using a co-sputtered ZnO: NiO (NZ) composite on PET fiber. This design enhances the tensile strength (60 mm at 3.2 kg.f) and conductance (0.23 S) of Cu-coated PET fiber, forming a durable sensing platform. The electrode’s enhanced electrochemical surface area (0.13 cm2) offers abundant active sites for glucose interaction, while the synergistic interface effect boosts ion and charge transport, improving glucose sensing. The sensor achieves high sensitivity (28.96 mA·cm−2·mM−1), fast response time (23 s), and a low detection limit (0.25 mM), while maintaining 78 % of its sensitivity after 500 bending cycles. These features, combined with good electrochemical stability—retaining 60 % of its initial performance after prolonged electrolyte exposure—mark a significant advancement in wearable glucose monitoring.

High-performance ZnO:CuO composite-based fiber-shaped electrode for non-enzymatic glucose sensing in biological fluids

The growing diabetes epidemic necessitates new glucose sensors, as traditional enzyme-based and planar electrodes face limitations in environmental stability and seamless integration into wearable technology. This research tackles these issues by designing a flexible, enzyme-free glucose sensor utilizing a co-deposited ZnO:CuO (CZ) composite on PET monofilament. This approach improves the tensile strength (55 mm at 2.7 kg.f) and electrical conductance (0.32 S), of the Ni-coated PET fiber, resulting in a strong and reliable sensing platform. The electrode's enlarged electrochemical surface area (0.11 cm2), offer a high density of active sites for glucose interaction, and the synergistic interface significantly enhances both ion and charge mobility. This results in exceptional sensitivity (35.05 mA.cm−2.mM−1), a rapid response (24 s), and a low detection limit (0.15 mM). Durability tests demonstrate that the sensor retains 80% of its sensitivity after 500 bending cycles, making it well-suited for wearable applications. Furthermore, the sensor accurately detects glucose in biological samples, such as saliva, highlighting its potential for non-invasive, real-time glucose monitoring in wearable healthcare systems.

Effects of Crumpling Stage and Porosity of Graphene Electrode on the Performance of Electrochemical Supercapacitor.

The performance characteristics of supercapacitors composed of crumpled graphene electrodes and aqueous NaCl electrolytes are investigated through Molecular Dynamics (MD) simulations using a newly developed crumpled graphene-based supercapacitor model. Results suggest that the three-dimensional configuration of crumpled graphene boosts electrolyte-electrode interaction. This improved interaction, which includes a larger ion-accessible zone, increases the specific capacitance of the supercapacitor by roughly 400% (16.4 μF/cm2) compared to planar graphene electrodes. Examining the effect of different stages of crumpling and the inclusion of pores on the electrode surface shows that the stages of crumpling substantially influence the supercapacitor performance. A smaller crumpling radius, meaning fully crumpled stage, improves the performance as increased crumpling leads to better packing efficiency, which aids in more ion separation. Furthermore, adding pores on the surface of crumpled graphene improves the ion accessibility by creating additional adsorption sites. An exceptional capacitance of 19.8 μF/cm2 is obtained for a porosity of 20%. However, the results suggest that the in-plane-porosity of the electrode needs to be optimized as there is a decline in specific capacitance after that point (20% porosity), indicating a suboptimal relationship between the charge distribution, specific surface area (SSA) and the porosity of the electrode.

A Comparison of DNA-DNA Hybridization Kinetics in Complex Media on Planar and Nanostructured Electrodes.

A comprehensive investigation into how nanostructures alter real-time DNA hybridization kinetics in both buffer and complex media and under a wide range of probe and target concentrations is currently lacking. In response, we use a real-time, wash-free, and in situ assay to study DNA hybridization kinetics by performing continuous electrochemical measurements in different media. We investigated the differences in hybridization kinetics under three regimes of probe density (low, medium, and high) and over three orders of magnitude of target concentrations (0.01-1 μM). Additionally, we compared the performance of planar and nanostructured electrodes in buffer, blood, urine, and saliva. Our experiments indicate that adding nanostructures to the transducer surface is only effective under a specific probe/target concentration regime. Additionally, we found that direct electrochemical readout is possible in the examined physiological media, with measurements in blood showing the highest and saliva showing the lowest signal magnitudes compared to buffer.

High‐Performance Single‐Microplate Perovskite Photodetectors Based on Polyvinyl Alcohol Filled Planar Electrodes for Weak‐Light Imaging and Pixel Integration

AbstractPhotoconductors featuring an individual micro/nano‐crystal as the photoactive channel hold the great promise for next‐generation integrated optoelectronics. Meanwhile, the gain aiming for signal amplification without an extra amplifier, the fast response for fast data transfer, and low dark current for weak signal acquisition are highly desirable in integrated optoelectronics, but are inherent trade‐off in traditional photoconductors. Herein, the study demonstrates a single‐microplate MAPbI3 photoconductor based on polyvinyl alcohol‐filled planar electrodes that can simultaneously achieve excellent performance on these parameters. The superior performance can be attributed to the exquisite design of filling the device channel gap with polyvinyl alcohol. This deep‐work‐function transparent polyvinyl alcohol can not only passivate surface defects of the perovskite microplate by hydrogen bonding, but also form a heterojunction with top perovskites to induce a depleted channel. Resultant single‐microplate photodetectors exhibit exceptional performance characteristics including ultra‐low noise‐equivalent power of 0.19 fW Hz−1/2, high specific detectivity of 3.68 × 1014 Jones, fast response of 3.2 µs, high EQE‐bandwidth over 107 Hz, as well as excellent stability. More importantly, single‐microplate MAPbI3 photoconductors demonstrate ultra‐weak‐light imaging ability surpassing silicon counterparts. Furthermore, the successful integration of their microplate pixels showcases the promising prospect for integrated optoelectronics.

Glucose Oxidase-Based Glucose Biosensing with a Simple Dual Ag/AgCl Probe Conductivity Readout.

We describe a conductometric assay of the enzymatic conversion of glucose to gluconic acid by dissolved glucose oxidase (GOx), using the generation of proton and gluconate from the reaction product dissociation for glucose detection. Simple basics of ionic conductivity, a silver/silver chloride wire pair, and a small applied potential translate glucose-dependent GOx activity into a scalable cell current. Enzyme immobilization and complex sensor design, involving extra nanomaterials or microfabrication of electrode structures, are entirely avoided, in contrast to all modern electrochemical glucose biosensors. Assay calibration showed a response linearity up to 500 μM, with a sensitivity of about 1.3 nA/μM. Selectivity tests excluded signals from sugars other than glucose, and glucose quantifications with recovery rates close to 100% were reached with a model sample and a beverage. Easy use of elementary physicochemical phenomena and a satisfactory performance are assets of the proposed non-amperometric glucose biosensing strategy. Assay integration into a planar dual electrode platform, with or without microfluidic application option, is feasible because of the simplicity of the sensor readout and suggests a route to affordable glucose analysis in beverage, food, and body fluid samples.

The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction.

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82-), and the strong reductant, oxalate (C2O42-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82- + C2O42- → 2 SO42- + 2 CO2 (ΔG0 = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S2O82-/C2O42- redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e- reduction of S2O82- to form S2O83•-, which rapidly undergoes bond cleavage to form SO42- and the highly oxidizing radical SO4•-. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•- can initiate an autocatalytic cycle that consumes both S2O82- and C2O42- in bulk solution. Several experimental demonstrations of S2O82-/C2O42- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S2O82- in solutions containing excess C2O42- is initiated by electrogeneration of immeasurably small quantities of S2O83•-. Capture of CO2 as BaCO3 during electrolysis additionally confirms the autocatalytic generation of CO2. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S2O82-/C2O42- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.

Unveiling Negative Differential Resistance and Superionic Conductivity: Water Anchored on Layered Materials.

Unravelling the perplexing nature of negative differential resistance (NDR) in 2D transition metal dichalcogenide (2D TMD) devices, especially regarding intrinsic properties, is hindered by experiments conducted in ambient environments. A thorough investigation is essential for unveiling the actual mechanism. In this study, we provide compelling evidence of the NDR effect with a remarkably high peak-to-valley current ratio and proton-diffused superionic conductivity in quantum-confined water molecules anchored to a thin film of 2D TMDs. Our investigation underscores the crucial role of ambient moisture for this robust NDR effect independent of underlying materials used. The bonding of water molecules to the existing sulfur defect sites on 2D TMD nanoflakes facilitates the formation of bridges between two planar metal electrodes, thus enabling superionic in-plane protonic conduction. During electrolysis of chemisorbed water, protons are liberated at the anode and migrate toward the cathode during bias voltage sweeping. Nevertheless, proton diffusion encounters increasing impedance beyond a certain applied bias, thereby restricting current flow even with higher biasing voltages, which is attributed to the interfacial Schottky energy barrier influenced by the Fermi level pinning effect. Our DFT simulations corroborate this mechanism, revealing minimal intermolecular interaction of H+ ions compared to OH- ions at distinct atomic sites on 2D TMD nanoflakes.

Soft, Multifunctional MXene-Coated Fiber Microelectrodes for Biointerfacing.

Flexible fiber-based microelectrodes allow safe and chronic investigation and modulation of electrically active cells and tissues. Compared to planar electrodes, they enhance targeting precision while minimizing side effects from the device-tissue mechanical mismatch. However, the current manufacturing methods face scalability, reproducibility, and handling challenges, hindering large-scale deployment. Furthermore, only a few designs can record electrical and biochemical signals necessary for understanding and interacting with complex biological systems. In this study, we present a method that utilizes the electrical conductivity and easy processability of MXenes, a diverse family of two-dimensional nanomaterials, to apply a thin layer of MXene coating continuously to commercial nylon filaments (30-300 μm in diameter) at a rapid speed (up to 15 mm/s), achieving a linear resistance below 10 Ω/cm. The MXene-coated filaments are then batch-processed into free-standing fiber microelectrodes with excellent flexibility, durability, and consistent performance even when knotted. We demonstrate the electrochemical properties of these fiber electrodes and their hydrogen peroxide (H2O2) sensing capability and showcase their applications in vivo (rodent) and ex vivo (bladder tissue). This scalable process fabricates high-performance microfiber electrodes that can be easily customized and deployed in diverse bioelectronic monitoring and stimulation studies, contributing to a deeper understanding of health and disease.

Detecting and Locating Corona Discharges in Low-Pressure Aircraft Environments Using Optical Sensors

Today, there is a clear trend toward electrification of transportation systems, including aircraft. Due to the enormous power requirements, they must operate at high voltages. However, the combined effect of higher voltage levels and low pressure environments is conducive to the occurrence of electrical discharges in electrical systems. Therefore, there is an urgent need to develop cost-effective systems to detect the discharges in the early stages before major failures can occur. This paper compares two optical sensors for early discharge detection in a simulated aircraft environment. The experimental study is performed in a low pressure chamber using a needle plane electrode. The pressure is changed from that corresponding to ground level to that corresponding to flight altitude, i.e., from 100 kPa to 20 kPa. The effect of the supply frequency is also studied, since modern aircraft operate in a wide range of frequencies up to about 800 Hz. Both variables, especially pressure, have shown significant effects on the CIV value. The results have also shown a similar sensitivity of both sensors for all the experimental conditions analysed, allowing a fast and sensitive detection and localization of incipient electrical discharge activity.

The Effect of Shape in Porous Electrodes on Cell Resistance and Chemo-Mechanical Stresses

In contrast to conventional planar electrodes, the use of electrodes with a three-dimensional (3D) shape has been shown to be an effective way to increase both the power and energy densities of a Li-ion battery. These 3D shapes allow batteries to overcome some of the capacity and rate performance trade-offs. For instance, they enable higher active-material loading, while keeping the transport pathways fixed and allowing for better material utilization. There are a variety of ways electrodes can be shaped: each electrode can be individually shaped and separated by a planar separator, or they can be completely intertwined with one another. In this talk, we consider both cases: a half cell with an electrode shape described by a sine wave and a full cell following an interdigitated design, where alternating planar fins protruding from the cathode and anode sandwich the electrolyte. First, we seek to understand how the electrode shape affects the electrical resistance of the cell based on a simple electrostatics model with a linearized Tafel expression. We identify the regimes of electronic to ionic conductivity ratios, Wagner numbers, and porosities that benefit most from electrode shaping, with a particular focus on low temperature electrolytes with reduced conductivity. Next, battery cycling is known to cause volumetric expansion and contraction of the electrodes. Therefore, we explore the chemo-mechanical stresses that arise in the same shaped electrodes with a solid electrolyte. To do so, we solve for the mechanical equilibrium for stress and deformation in the presence of an applied isotropic strain. By examining the stress concentrations at the electrode/electrolyte interface, we elucidate the likely failure mechanisms – interface delamination, crack propagation – of shaped electrodes in comparison to planar electrodes. Overall, we aim to provide design guidelines for how electrode shapes can be chosen to provide both low cell resistance and acceptable mechanical resilience.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number: LLNL-ABS-857536.

Investigation of the Gas Permeation Stream through Gas Diffusion Electrodes for CO2 Electrolysis and Possible Links to Selectivity

The electrochemical reduction of CO2 (eCO2R) is one promising attempt to create value added products from excess greenhouse gas with renewable energy. However, to become commercially relevant, some challenges have to be outgrown. To overcome the challenge of mass transport limitations on planar electrodes, Gas-Diffusion-Electrodes (GDE) have been implemented for eCO2R, allowing to reach sufficient reaction rates with the CO2 directly being delivered to the catalyst layer [1]. A major issue in the application of GDEs for eCO2R represents flooding of the mostly carbon based gas diffusion media by the alkaline electrolyte during application. This phenomenon results in a detrimental change of the 3-phase boundary and is accompanied by a loss of activity and a breakdown of performance [2]. A possible approach to avoid flooding of the GDE during eCO2R is to apply a backpressure in order to increase the capillary pressure inside the GDE, which needs to be balanced accurately, since excess backpressure can cause a CO2 crossover stream through the GDE into the electrolyte compartment [3]. In our work we investigate the permeation behavior of GDEs in a customized flow cell and investigate how to influence it via electrode preparation. A deeper understanding of the permeation characteristics of GDEs could lead to a better choice of operation points regarding flooding issues. However, the operation point itself and the preparation of GDEs could impact not only the flooding behavior of the electrode but also selectivity of the reaction. The eCO2R is taking place ~10 µm away from the gas-liquid boundary. The distribution of the two phases inside the GDE is crucial for the reaction pathway [4]. Therefore, we examine possible links of permeation behavior and electrode preparation on selectivity.In detail, our setup contains a customized flow cell in which the pressure in the gas compartment is adjusted with a needle valve and monitored via a pressure transmitter. The gas-crossover stream through the GDE is then registered by collecting the gas transferred to the liquid electrolyte in a measuring cylinder.For electrode preparation, commercial carbon based GDLs (25BC-Sigracet, H23C8-Freudenberg) are pretreated by either spray coating a carbon-ionomer-layer on top of the microporous layer or sputter coating with copper first. In the second step, a galvanostatic deposition of copper is carried out on the pretreated GDLs.The permeation behavior is tested for the non-coated commercial GDLs, the GDLs after the pretreatment step and after deposition of copper without and with current applied, respectively. In our experiments the crossover stream related to backpressure through the GDL H23C8 was lower compared to the crossover stream through GDL 25BC, most likely being linked to differences in the porosity and cracks of the MPL (Figure 1A). However, when comparing identically prepared GDEs based on different GDL types at the same crossover stream, only minor changes in selectivity can be observed (Figure 1B). It was also found that the crossover stream was influenced by the coating of the electrode. While sputter coating (spu) has a minor impact on gas permeation, spray coating (spr) leads to a significant change. These observations could be related to increased electrode wetting after spray- but not after sputter coating. The permeation behavior after galvanic deposition of copper is comparable, independent of the pretreatment procedure. Comparing the selectivity of GDEs with the different pretreatment procedure, a significant change in selectivity can be observed (Figure 1C). In the next steps, it will be crucial to investigate if this selectivity change is related to a different gas-liquid distribution in the GDE or to other factors as the composition of the coating layer to successfully influence operation stability and selectivity.

Modeling Bicarbonate Kinetics on Silver Catalysts for CO2 Electrolysis

Low-temperature CO2 electrolyzers transform captured CO2 into more useful chemicals, such as syngas (CO and H2) for systems with silver or gold catalysts and ethylene and other C2+ products for systems with copper catalysts. At low overpotentials, bicarbonate ions act as proton donors and have been shown to have enhanced activity1, leading to nonlinear Tafel slopes for the hydrogen evolution reaction (HER). We have developed a 1D planar electrode model for CO2 electrolysis, similar to the model developed by Gupta et al.,2 including a thin ionomer layer on top of a silver catalyst layer surface. The model includes buffer chemistry and ion transport in the boundary layer as well as the ionomer thin-film layer. Tafel parameters are fit for both bicarbonate and water as proton donors for HER using the model results to account for mass transport in the boundary layer. A comparison against a bare silver electrode shows that the Donnan potential across the ionomer/electrolyte interface has a significant effect on the ion concentrations at the catalyst layer surface, leading to enhanced HER from bicarbonate at low overpotentials. These results are then compared against Butler-Volmer parameters that are derived from a full microkinetic model. The Tafel kinetic parameters were then used in a 1D full-cell membrane electrode assembly (MEA) model, and we found good agreement with experimental data, illustrating that the ionomer in contact with the catalyst layer surface impacts the underlying kinetics both in planar electrodes and industrially-relevant MEA systems.References D. M. Koshy et al., J. Am. Chem. Soc., 143, 14712–14725 (2021) https://doi.org/10.1021/jacs.1c06212.N. Gupta, M. Gattrell, and B. MacDougall, J. Appl. Electrochem., 36, 161–172 (2006). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was partially supported by a Cooperative Research and Development Agreement (CRADA) between Lawrence Livermore National Laboratory, Stanford University, and TotalEnergies American Services, Inc. (affiliate of TotalEnergies SE) under Agreement No. TC02307 and Laboratory Directed Research and Development (LDRD) funding under project 22-SI-006.LLNL-ABS-857478 Figure 1

Optimizing the Performance of Nickel Electrocatalysts with Regular Arrays of Linear Ridge-like Features for Enhancing the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) represents a major source of inefficiency in the production of green hydrogen via electrocatalytic water splitting. At high overpotentials relevant to industrial rates of gas production, bubbles can contribute substantially to these inefficiencies by inhibiting the mass transport of electrolyte and partially obstructing the electrochemically active surface area (Aecsa) of an electrode.1 Arrays of microscale features have been shown to improve the efficiency of gas evolving reactions, including the OER. The performance of electrodes modified with linear ridge-like surface features has been previously observed to increase with the spacing between the ridges with a separation of up to 200 µm, but it was not known whether this trend continues indefinitely.2 The present study utilizes rapid prototyping techniques, including microscope projection photolithography and electrodeposition, to determine whether an optimal ridge separation exists between 200 and 450 µm. At a relatively high overpotential of 1.8 V (vs Hg/HgO) in an alkaline electrolyte, the performance of these ridges was observed to increase up to a critical separation, after which the performance decreased to a level comparable to a planar electrode at a wider separation between the ridges. Chronoamperometry measurements indicated that ridges with this optimal spacing attained a current response nearly 3× that of a planar electrode when normalized to (Aecsa). A better understanding of the reasons for this observed trend was pursued using high speed imaging. Direct observations of bubble production on the surfaces of the electrodes provided a deeper understanding of how the distribution of gas bubbles on an electrode affects performance.

Treatment of Se-Impacted Wastewater Using a Three-Dimensional Packed Bed Electrochemical Reactor

Selenium is an essential element for flora and fauna in trace amounts. It plays a crucial role in bolstering photosynthesis and facilitating thyroid hormone metabolism. However, excessive consumption of selenium can result in health concerns such as reproductive complications and cancer. Selenium usually coexists with minerals such as copper and coal in the natural environment. The associated activities, such as copper mining and coal combustion, have greatly accelerated the mobilization of selenium. The selenium released through coal combustion is mostly captured in the flue gas desulfurization (FGD) wastewater as inorganic selenite and selenate.Our lab previously validated the viability of direct electrochemical reduction (SeDER) to remove dissolved selenite from a simplified FGD water matrix using planar gold and graphite cathodes, where more than 94% of the aqueous selenite can be converted to elemental Se(0) deposited on the cathode surface. Graphite is eventually selected for its low material cost, decent removal performance, and minimum secondary pollution as compared to gold and other non-noble metal cathodes. This direct electrochemical deposition process provides many benefits that existing biological and indirect electrochemical methods lack, such as zero chemical addition and low to zero solid generation. However, this method is only suitable for wastewater with elevated temperatures necessary to generate conductive Se(0) for continuous reduction. The inclusion of a heating process adds complexity to the design, impeding the scalability of the SeDER system. Furthermore, the application of SeDER is constrained to Se-impacted wastewater with an elevated temperature.In this study, we propose a novel reactor design utilizing electrochemical reduction to treat Se-impacted wastewater at ambient temperature. The electrodes we used are made of graphite for the reasoning stated above. The reactor consists of a pair of planar graphite anode and cathode, and the inner chamber is filled with cylindrical graphite particle electrodes (PEs) separated by a nylon spacer to create an anodic chamber and a cathodic chamber. The PEs act as an extension of the planar electrodes, which offers numerous reaction sites and minimizes the travel distance for selenite ions to reach the electrode surface. In our 120-mL batch test with 0.1-mM selenite in 100-mM phosphate buffer, the exact system without filler removed 0% selenite at room temperature, while the filled 3D system removed on average 50% of the selenite in 3 hours. Using the same 3D system, we then investigated performance enhancement with a recirculating operation. The total working volume is fixed at 200 mL, while the flow rate and the applied voltage vary. The flow rate is set at the values with hydraulic retention time (HRT) fixed at 15 min, 30 min, and 60 min. The system-applied voltage is set at -1.9V (just enough for selenite reduction), -2.1V, and -2.3V. We found that the shortest HRT combined with -2.1V system applied voltage yielded the highest selenite removal performance in all tests, with an average of 47% removal in 3 hours. We hypothesized that the shorter HRT provides a faster flow rate that refreshes the PE’s surface from insulative Se(0) deposition, which provides sustainable reaction sites for the incoming selenite. At -2.1V, the potential is negative enough to allow selenite reduction while fewer parasitic reactions (e.g., hydrogen evolution reaction) occur compared to that under -2.3V.With the selected system voltage and flow rate, we eventually explored competing ion behavior and switched from a simplified water matrix to simulated Se-impacted wastewater. We found that of all the commonly found competing ions (i.e., sulfate, nitrate, and chloride), chloride significantly impacts selenite removal, with performance dropping to 10% removal in 3 hours. Nitrate slightly decreased the selenite removal, with an average Se removal performance of 23%. Sulfate, on the other hand, resulted in an increase in selenite removal after addition. We hypothesize that a high sulfate concentration in simulated wastewater could help alleviate the repulsing force from the cathode surface for the selenite ions. The double layer of the PEs could also be compressed by the high ionic strength, which decreases the capacitance and promotes selenite reduction. To confirm this hypothesis, we conducted an electrochemical impedance study to analyze the effect of competing ions on reaction kinetics and capacitance. We also performed surface analysis for the wasted planar electrodes and PEs to investigate the change in morphology and observed possible chemical residue. These characterizations will help us develop a regeneration protocol in long-term operation and help us scale up our prototype to manage actual Se-impacted wastewater.