Inert Electrode Research Articles

Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit.

The conduction of protons and other ions in nanoporous materials, such as metal-organic frameworks (MOFs), is intensively explored with the aim of enhancing the performance of energy-related electrochemical systems. The ionic conductivity, as a key property of the material, is typically determined by using electrochemical impedance spectroscopy (EIS) in connection with a suitable equivalent circuit. Often, equivalent circuits are used where the physical meaning of each component is debatable. Here, we present an equivalent circuit for the ionic conduction of electrolytes in nanoporous, nonconducting materials between inert and impermeable electrodes without faradaic electrode reactions. We show the equivalent circuit perfectly describes the impedance spectra measured for the ion conduction in MOFs in the form of powders pressed into pellets as well as for MOF thin films. This is demonstrated for the ionic conduction of an aprotic ionic liquid, and of various protic solvents in different MOF structures. Due to the clear physical meaning of each element of the equivalent circuit, further insights into the electrical double layer forming at the MOF-electrode interface can be obtained. As a result, EIS combined with the appropriate reference circuit allows us to make statements of the quality of the MOF-substrate interface of different MOF-film samples.

Electrochemically Active Biofilms as an Indicator of Soil Health

Soil health is a complex phenomenon that reflects the ability of soil to support both plant growth and other ecosystem functions. To our knowledge, research on extracellular electron transfer processes in soil environments is limited and could provide novel knowledge and new ways of monitoring soil health. Electrochemical activities in the soil can be studied by inserting inert electrodes. Once the electrode is polarized to a favorable potential, nearby microorganisms attach to the electrodes and grow as biofilms. Biofilms are a major part of the soil and play critical roles in microbial activity and community dynamics. Our work aims to investigate the electrochemical behavior of healthy and unhealthy soils using chronoamperometry and cyclic voltammetry. We developed a bioelectrochemical soil reactor for electrochemical measurements using healthy and unhealthy soils taken from the Cook Agronomy Farm Long-Term Agroecological Research site; the soils showed similar physical and chemical characteristics, but there was higher plant growth where the healthy soil was taken. Using carbon cloth electrodes installed in these soil reactors, we explored the electrochemical signals in these two soils. First, we measured redox variations by depth and found that reducing conditions were prevalent in healthy soils. Current measurements showed distinct differences between healthy and unhealthy soils. Scanning electron microscopy images showed the presence of microbes attached to the electrode for healthy soil but not for unhealthy soil. Glucose addition stimulated current in both soil types and caused differences in cyclic voltammograms between the two soil types to converge. Our work demonstrates that we can use current as a proxy for microbial metabolic activity to distinguish healthy and unhealthy soil.

Photoactive electrically switchable van der Waals semiconductor NbOI2

Room temperature van der Waals ferroelectric materials whose ferroelectricity may survive down to atomic layer limit are highly desirable for device miniaturization. In this article, we present the optically active reconfigurable room temperature rectification in a recently predicted ferroelectric van der Waals material NbOI2. NbOI2 devices with a thin (∼17-unit cells) single crystalline channel and inert graphite electrodes were assembled into two-terminal devices which showed >100 × photoresponse to 405 nm laser. By DC poling on a 1-μm-channel NbOI2 device, the photocurrent changed from symmetric to single-Schottky-diode type. The polarity of such rectification can be switched back and forth by DC poling along opposite directions. Such reconfigurability evidences the existence of in-plane room temperature ferroelectricity in thin NbOI2 and its potential in nonvolatile optoneuromorphic computing and nonvolatile technologies.

Physics-Based Stochastic Three-Dimensional Modeling for Metal–Oxide Resistive Random Access Memory

In this work, a fully physics-based stochastic 3-D modeling of metal–oxide-based resistive random access memory (RRAM) device has been conducted. In contrast to general 1-D or 2-D modeling of RRAM devices, 3-D modeling is able to provide a picture of the evolution of the 3-D random distributions of oxygen vacancies and temperature inside the device during the forming and SET/RESET processes. The modeling confirms that a tunneling gap between the top electrode (TE) which serves as an oxygen ion reservoir and the tip of the conductive filament may form at a high resistive state, which is assumed in the frequently used compact models of RRAM devices. The modeling shows that the sites with high temperature are concentrated in a cylindrical region at the center of the RRAM device, running from the inert bottom electrode to the TE. The modeling can reproduce both typical current-voltage characteristics of metal–oxide RRAM devices in forming and RESET and SET operations under a dc voltage sweep and the modulation of RRAM conductance by SET/RESET pulses.

Scanning electrochemical cell microscopy for the study of (nano)particle electrochemistry: From the sub‐particle to ensemble level

AbstractScanning electrochemical cell microscopy (SECCM) is a scanning‐droplet‐based technique that allows electrochemical fluxes and/or interfacial reactivity to be measured and visualized with high spatiotemporal resolution. This minireview spotlights the use of SECCM for studying the electrochemistry of (nano)particles, highlighting recent works spanning multiple timescales (i.e., microseconds to seconds) and lengthscales (i.e., nanometers to micrometers) to probe physicochemical phenomena at the sub‐particle, single particle, and/or (micro)ensemble levels. In SECCM, single (nano)particles (or small particle ensembles) are electrochemically interrogated—directly from colloidal solution (i.e., by investigating electrochemical nanoimpacts with a substrate electrode) or supported on an electrochemically inert support electrode—with a pipette probe that is operated in either a stationary (point measurement) or dynamic scanning/imaging mode. Nanoscale‐resolved electrochemical information (e.g., local rates of electron‐ or ion‐transfer, catalytic activity, corrosion resistance, etc.) from SECCM is readily related to (nano)particle structure and properties, collected at a commensurate scale with complementary, co‐located microscopy/spectroscopy techniques, allowing structure and function to be resolved directly and unambiguously down to the sub‐particle level. Understanding structure−function on this scale enables macroscopic “particle‐on‐support” electrode behavior to be rationalized and further predicted, guiding the discovery, design, and engineering of novel electromaterials with enhanced function, which is a “holy grail” in materials science.

Electrochemical activation of C\u2013H by electron-deficient W2C nanocrystals for simultaneous alkoxylation and hydrogen evolution

The activation of C–H bonds is a central challenge in organic chemistry and usually a key step for the retro-synthesis of functional natural products due to the high chemical stability of C–H bonds. Electrochemical methods are a powerful alternative for C–H activation, but this approach usually requires high overpotential and homogeneous mediators. Here, we design electron-deficient W2C nanocrystal-based electrodes to boost the heterogeneous activation of C–H bonds under mild conditions via an additive-free, purely heterogeneous electrocatalytic strategy. The electron density of W2C nanocrystals is tuned by constructing Schottky heterojunctions with nitrogen-doped carbon support to facilitate the preadsorption and activation of benzylic C–H bonds of ethylbenzene on the W2C surface, enabling a high turnover frequency (18.8 h−1) at a comparably low work potential (2 V versus SCE). The pronounced electron deficiency of the W2C nanocatalysts substantially facilitates the direct deprotonation process to ensure electrode durability without self-oxidation. The efficient oxidation process also boosts the balancing hydrogen production from as-formed protons on the cathode by a factor of 10 compared to an inert reference electrode. The whole process meets the requirements of atomic economy and electric energy utilization in terms of sustainable chemical synthesis.

Iron doped cobalt fluoride derived from CoFe layered double hydroxide for efficient oxygen evolution reaction

Water splitting plays an important role in overcoming the intermittent problems for sustainable energy development, and an efficient non-noble catalyst is highly desired to catalyze the more sluggish half-reaction of oxygen evolution reaction (OER). Herein, we demonstrated the Fe-doped CoF2 as a novel and efficient catalyst for OER via facile structural transformation derived from the CoFe layered double hydroxide. The crystal structure and surface chemical state analysis indicated the successful structural transformation from CoFe layered double hydroxide to the bulk crystal of CoF2 doped by Fe. The obtained Fe-CoF2 nanoflakes largely improved the utilization of active sites and effectively promoted mass transfer. The Fe-CoF2-300 showed a low overpotential of 230 mV to achieve the current density of 10 mA cm−2 when loaded on an inert glass carbon electrode, which is about 60 mV less than that of the pristine CoFe layered double hydroxide catalyst. It also exhibited a small Tafel slope of 41.9 mV dec-1 and super catalytic stability. High valence state change from the M−F bond to the M−O bond driven by the OER process was indicated after the long-term stability test indicating the facile active phase formation. This works provides an effective doping strategy to design novel catalysts for decreasing the overpotential and accelerating the kinetics of OER.

Effectiveness of Carbon Electrode Electrolysis Effluent Treatment System in Textile Dyeing

The electrolysis process is an easy alternative for the treatment of textile dyehouse wastewater. To study the effectiveness of the carbon electrode electrolysis method in dyehouse effluent treatment is the main objective of this research. In most cases, the metal electrode used in the electrolysis system becomes decayed and needs to be replaced after a certain time. In this study, an inert carbon electrode has been chosen to treat the cotton dyehouse wastewater that can function longer than a metal electrode. Hence, the process is named as Carbon Electrode Electrolysis (CEE) method. In this method, a combined effect of electro-coagulation and electro-reduction has occurred. Electro-coagulation happens by direct charge supplied from a carbon electrode and then neutralizing negative effluent charges using positive charge. The color chromophore becomes broken down by a reduction in the cathode. Through electrolysis of sodium chloride in the effluent, sodium hypochlorite is produced, which later oxidizes organic compounds and reduces BOD. The effects of different treatment parameters of the CEE method like salt concentration and operating time have been studied and found that higher salt concentration and around 65 minutes operating time is effective. The results are analyzed based on four criteria: Color change, Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Dissolved Solids (TDS). Results show that the CEE method minimizes effluents’ color and reduces BOD and COD within the acceptable limits of the Department of Environment (DoE), Bangladesh. Moreover, the CEE Effluent Treatment Plant (ETP) needs less space than the other existing biological and chemical ETP. However, this method is failed to remove TDS from wastewater.

Quantum-Dots Optimized Electrode for High-Stability Transient Memristor

Biomaterials such as fibroin have drawn wide attention as switching medium in transient memristors. However, due to unique degradability and inhomogeneous microstructure of fibroin film, fibroin-based memristor usually showed unstable resistive switching behaviors. Here, we show that using MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> quantum dots to modify inert electrodes can significantly reduce the randomness of the device, thereby achieving an excellent cycling endurance with narrow switching voltage distribution. Furthermore, we proposed a percolation model of conductive filaments to reveal the interrelationship between electrode morphology and switching characteristics. Our work reveals the microscopic origin of the instability of the transient memristor and exhibits effective approaches to improve the reliability of the device.

Redox behavior of indium in molten chlorides

An electrochemical study on the redox behavior of indium in the eutectic LiCl-KCl system at 450 °C was carried out with the transient techniques of cyclic voltammetry and chronopotentiometry on an inert molybdenum electrode. The reduction of In(III) was found to be a two-step process involving In(III)/In(I) and In(I)/In couples at the potentials of about −0.4 and −0.8 V versus Ag/AgCl, respectively. The redox mechanism was further confirmed by the theoretical evaluation of the number of transferred electrons based on cyclic voltammetry and characterizations of the precipitates generated by the potentiostatic electrolysis. The diffusion coefficients of indium ions in the eutectic LiCl-KCl melt at 450 °C were estimated by cyclic voltammetry and chronopotentiometry. The results obtained through the two methods are in fair agreement, delivering an average diffusion coefficient of approximately 1.8×10−5 cm2/s for In(III), and 1.4×10−4 cm2/s for In(I).

Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes.

To understand and control key electrochemical processes-metal plating, corrosion, intercalation, etc.-requires molecular-scale details of the active species at electrochemical interfaces and their mechanisms for desolvation from the electrolyte. Using free energy sampling techniques we reveal the interfacial speciation of divalent cations in ether-based electrolytes and mechanisms for their delivery to an inert graphene electrode interface. Surprisingly, we find that anion solvophobicity drives a high population of anion-containing species to the interface that facilitate the delivery of divalent cations, even to negatively charged electrodes. Our simulations indicate that cation desolvation is greatly facilitated by cation-anion coupling. We propose anion solvophobicity as a molecular-level descriptor for rational design of electrolytes with increased efficiency for electrochemical processes limited by multivalent cation desolvation.

Direct ink writing of fluoropolymer/CNT-based superhydrophobic and corrosion-resistant electrodes for droplet energy harvesters and self-powered electronic skins

Self-powered devices and systems that operate by harnessing environmental mechanical energies including raindrops and body motions have been extensively explored owing to their promising applications. In practical applications, these devices are often exposed to humid conditions or directly contact aqueous solutions. Here, we report the development of chemically inert and superhydrophobic electrode based on fluoropolymer (FP)/carbon-nanotube (CNT) that circumvents undesired metal electrode corrosion, deformation, and damage in harsh environments. The electrode surface can be patterned on flexible surfaces via direct ink writing (DIW), and no damage or corrosion is detected even being bent 10,000 times or immersed into salt/acid/alkaline solutions for 20 h. The integration of such robust electrodes with hydrophobic tribo-materials enables the construction of droplet-based electricity generators (DEGs) that exhibit an instantaneous current and power outputs of 2 mA and 0.12 W, respectively, and that light up 50 LEDs by one water droplet. Self-powered touch sensing function is also demonstrated on FP/CNT-based electronic skin, offering the broad applicability of the proposed electrode preparation strategy for self-powered devices. Chemically inert and superhydrophobic electrode based on fluoropolymer (FP)/carbon-nanotube (CNT) for self-powered devices working in the harsh environment has been developed. The FP/CNT electrode can be fabricated into various shapes via a facile direct ink writing approach. A droplet-based electricity generator (DEG) and self-powered electronic skin have been fabricated to demonstrate the feasibility of the FP/CNT electrode. • Superhydrophobic and corrosion-resistant electrodes are fabricated using direct ink writing (DIW) method. • Fluoropolymer (FP)/CNT ink is synthesized by a simple method of mixing FP solutions and CNTs. • FP/CNT electrodes are mechanically and chemically robust. • A droplet-based electricity generator (DEG) with DIW-fabricated FP/CNT electrodes can power 50 LEDs. • Self-powered touch sensing function is realized on an electronic skin device with DIW-fabricated FP/CNT electrodes.

Electrochemical behaviour of Lanthanum fluoride and Praseodymium fluoride on inert and reactive electrodes in molten LiF-CaF2

Electrochemical behaviour of La(III)/La and Pr(III)/Pr systems has been studied in LiF-CaF2 media (79.5%-20.5%) on inert and reactive electrodes in the 820-930°C temperature range using cyclic voltammetry, square wave voltammetry and open-circuit potentiometry. Experimental results show that La(III) and Pr(III) are reduced into metal in a one-step and diffusion controlled process, exchanging 3 electrons, at 0.08 and 0.15 V vs. LiF/Li respectively. Physico-chemical and thermodynamic data such as diffusion coefficients, standard potential, activity coefficients, have been determined. Effect of oxide additions have been investigated and leads to precipitation of LaOF and PrOF species. Alloys formation by reactive electrodeposition on Ni, Cu and Fe electrodes has also been evidenced and their Gibbs energies determined.

Numerical Modeling of Marine Self-Potential from a Seafloor Hydrothermal Ore Deposit

The redox field generated by electrically conductive minerals is one of the main constituents of self-potentials. It can be explained by electrochemical reactions in which conductors participate. The location and outline of seafloor hydrothermal ore deposits can be detected using marine self-potential anomalies that can be approximated through a marine geobattery model. The numerical modeling of marine self-potentials could be the foundation of corresponding data inversion and interpretation and improving the application effect of the self-potential method in detecting seafloor hydrothermal ore deposits. In this study, the inert electrode model and the on-land geobattery model are introduced to build the marine geobattery model, and the finite-infinite element coupling method is derived to deal with the truncated boundary problem effectively. Also, two tests are conducted to study the effect of model parameters on ground self-potential anomalies. A seafloor sulfide deposit model is built to study the self-potential characteristics. The numerical modeling results suggest that the precision and efficiency of the coupled method are superior to that of the traditional finite element method. Self-potential anomalies are greatly affected by medium resistivity, complex terrain, and amplitudes of embedded redox fields. Gradient changes in embedded redox fields do not cause significant self-potential anomalies but lead to mutations of current sources. The self-potential anomaly from the sulfide deposit model shows that the self-potential method can be effectively used to explore seafloor hydrothermal deposits that accompany negative self-potential anomalies above the ore bodies. The coupled method is quite suitable for multi-source models such as self-potential models.

Asymmetric Charged Conductive Porous Films for Electricity Generation from Water Droplets via Capillary Infiltrating.

Hydrovoltaic devices are proposed as an alternative way to directly generate electricity due to the ubiquity of water and its interaction with specific porous structures. At present, the output power density of the reported device is limited by its low current density arising from the low surface charge density and inferior charge transport capability of the active materials. In this work, an asymmetric structure consisting of positively charged conductive polyaniline (PANI) and negatively charged Ti3C2TX MXene is proposed to build a hydrovoltaic device to achieve high conductivity and surface charge density simultaneously. An extra polyvinyl alcohol layer is utilized between PANI and MXene to reserve the asymmetric structure and maintain a constant voltage output. As a result, a peak current density of 1.8 mA/cm2 is achieved, which is 18 times higher than the previous peak current density of the device with an inert electrode. Our work of incorporating an asymmetric structure provides an alternative way to target high-efficiency hydrovoltaic devices with a large current density.

Theory of nonvolatile resistive switching in monolayer molybdenum disulfide with passive electrodes

Resistive-memory devices promise to revolutionize modern computer architecture eliminating the data-shuttling bottleneck between the memory and processing unit. Recent years have seen a surge of experimental demonstrations of such devices built upon two-dimensional materials based metal–insulator–metal structures. However, the fundamental mechanism of nonvolatile resistive switching has remained elusive. Here, we conduct reactive molecular dynamics simulations for a sulfur vacancy inhabited monolayer molybdenum disulfide-based device with inert electrode systems to gain insight into such phenomena. We observe that with the application of a suitable electric field, at the vacancy positions, the sulfur atom from the other plane pops and gets arrested in the plane of the molybdenum atoms. Rigorous first principles based calculations surprisingly reveal localized metallic states (virtual filament) and stronger chemical bonding for this new atomic arrangement, explaining the nonvolatile resistive switching. We further observe that localized Joule heating plays a crucial role in restoring the popped sulfur atom to its original position. The proposed theory, which delineates both unipolar and bipolar switching, may provide useful guidelines for designing high-performance resistive-memory-based computing architecture.

Methodology for Determination of the Key Parameters of Conjugated Polymer Electrodeposition, Based on a Combination of Spectroelectrochemistry and Electrochemical Quartz Crystal Microbalance

Methodology based on a combination of experimental data obtained by in situ methods of spectroelectrochemistry and electrochemical quartz crystal microbalance has been proposed for determination of the key parameters of the conjugated polymer deposition on the electrode surface via monomer electrooxidation. These parameters are: the current efficiency of the process, the charge spent per an oxidized monomer molecule, the number of monomer units inside the deposited film, and the average number of valence bonds per one monomer unit inside the film. Besides, the electrochemical quartz crystal microbalance method applied to the discharge process of the electropolymerized film allows determining the average charging (oxidation) degree of the monomer unit at the polymerization potential and the degree of the solvent participation in the polymer’s redox transitions. The applicability of the proposed approach has been demonstrated by example of magnesium polyporphine films obtained by oxidation of unsubstituted magnesium porphine on inert electrode in acetonitrile solution at a low potential.

Electric conductivity of remotely heated Cu nanofilaments in Cu/TaOx/Pt ReRAM cells

A memory cell, initially set to a conductive state, of a resistive resistive random access memory array is heated remotely and controllably by a neighboring heat source cell, both cells having either a common active or an inert electrode. Upon switching off the heating of the heat source cell, the conductivity of the filament of the investigated cell is characterized by sufficiently small voltages to make sure that the filament does not suffer any structural reconstruction such as removal, addition, or permanent displacement of Cu atoms. One observes that immediately after the heating of the neighboring heat source device is switched off, the cell, initially set to an on state and characterized by a low resistance Ron, is found to be reset by remote heating to an off state, characterized by R off ≫ R on. In some instances, after a cooling off time of approximately 2 min or more, one observes the cell's spontaneous recovery from the Roff state to the original on state or very close to the initial Ron value. The process of the spontaneous recovery of the electric conductivity of the filament is interpreted as an attenuation of Cu atom vibrations in the filament with the attendant increase of electron tunneling effects for which the variation of the average tunneling distance between the vibrating Cu atoms is roughly proportional to the square root of the absolute temperature. At high temperatures, the average tunneling distance increases, leading to a sharp decrease of the tunneling probability and, consequently, to a sharp increase in transient resistance.

Influence of Fluoride Ions on the Speciation and Electrochemical Behavior of Th(IV) in Molten LiCl-KCl with a Copper Electrode

The speciation and electrochemical behavior of thorium (IV) fluoride in molten LiCl-KCl with a copper working electrode was investigated. In addition to the reduction of Th(IV) [ThCl6 2− and ThCl7 3−] to metallic thorium, there is a minor peak in the cyclic and square wave voltammagrams that results from the reduction of Th(IV) [ThCl6−xFx 2− and ThCl7−xFx 3−] to metallic thorium on the basis of the square wave voltammetry and X-ray diffraction results. This is also supported by the formation of metallic thorium deposits upon potentiostatic electrolysis. In addition, the diffusion coefficient and diffusion activation energy of Th(IV) with a copper electrode was determined, which show similarity in comparison to those obtained using inert working electrodes.

Non-activation synthesis and thermodynamic properties of ternary compounds of the Ag–Te–Br system

Based on the literature data on the phase formation of the Ag–Te–Br system in the part AgBr–Ag2Te–Te (I) and the results of the electromotive force (EMF) measurements of the electrochemical cells (ECCs) the alleged scheme division of (I) into 10 three-phase regions was analyzed. The overall potential-forming reactions were expressed for the phase regions AgBr–Ag19Te6Br7–Te, Ag19Te6Br7–Ag3TeBr–Te, Ag3TeBr–Ag10Te4Br3–Te, Ag3TeBr–Ag5Te2Br–Ag10Te4Br3, Ag5Te2Br–Ag2Te–Ag23Te12Br, and Ag10Te4Br3–Ag23Te12Br–Te. Reactions were performed by applying ECCs of the type (−) IE | Ag | Solid-state electrolyte | R(Ag+) | PE | IE (+), where IE is the inert electrode (graphite), Ag is the negative (left) electrode, PE is the positive (right) electrode, R(Ag+) is the region of Ag+ diffusion into PE. PEs of ECCs were prepared from finely ground non-equilibrium mixtures of pure components Ag, Te, and AgBr. The component ratios were determined from the equations of the potential-forming reactions in respective phase regions. Non-activation synthesis of the equilibrium set of phases was performed in the R(Ag+) region of ECCs. The Ag+ ions displaced from the left to the right electrode for thermodynamic reasons, act as the nucleation centers for stable compounds and as the catalysts for the low-temperature synthesis of an equilibrium set of phases. The synthesis duration of the initial mixture of phases with a particle size ∼5 μm and T = 500 K is under ≤5 h. Linear dependences ЕMF vs T of ECCs between T=(440 and 500) K were used for calculations of the standard values of the Gibbs energies, enthalpies, and entropies of equilibrium compounds in (I) consisting of Ag19Te6Br7, Ag3TeBr, Ag10Te4Br3, Ag5Te2Br, and Ag23Te12Br. The agreement calculated values of the Gibbs energy of the Ag23Te12Br compound in two different three-phase regions Ag5Te2Br–Ag2Te–Ag23Te12Br and Ag10Te4Br3–Ag23Te12Br–Te confirms the correctness of the alleged scheme division of (I).