Potential Of Reference Electrode Research Articles

Development of Direct Electron Transfer-Type Extended Gate Field Effect Transistor Enzymatic Sensors for Metabolite Detection.

In this work, direct electron transfer (DET)-type extended gate field effect transistor (EGFET) enzymatic sensors were developed by employing DET-type or quasi-DET-type enzymes to detect glucose or lactate in both 100 mM potassium phosphate buffer and artificial sweat. The system employed either a DET-type glucose dehydrogenase or a quasi-DET-type lactate oxidase, the latter of which was a mutant enzyme with suppressed oxidase activity and modified with amine-reactive phenazine ethosulfate. These enzymes were immobilized on the extended gate electrodes. Changes in the measured transistor drain current (ID) resulting from changes to the working electrode junction potential (φ) were observed as glucose and lactate concentrations were varied. Calibration curves were generated for both absolute measured ID and ΔID (normalized to a blank solution containing no substrate) to account for variations in enzyme immobilization and conjugation to the mediator and variations in reference electrode potential. This work resulted in a limit of detection of 53.9 μM (based on ID) for glucose and 2.12 mM (based on ID) for lactate, respectively. The DET-type and Quasi-DET-type EGFET enzymatic sensor was then modeled using the case of the lactate sensor as an equivalent circuit to validate the principle of sensor operation being driven through OCP changes caused by the substrate-enzyme interaction. The model showed slight deviation from collected empirical data with 7.3% error for the slope and 8.6% error for the y-intercept.

Factors Governing the Failure of Subsea Critical Connector Bolts

Impressed current cathodic protection (ICCP) systems are commonly used to shield offshore drilling rigs, pipelines, and subsea equipment in the oil and gas industry. In underwater service conditions, water temperature, salinity and velocity play a major role in the longevity of subsea applications. Interactions between the preceding factors can induce catastrophic failure to critical systems while the underlying cause is unclear. This paper proposes an approach for elucidating the corrosion process accompanying underwater applications. The service conditions of underwater application are simulated in a multidisciplinary system that records various parameters such as water temperature, reference-electrode potential, and electric current at five-minute intervals during the 21 d of the experiment. This novel, experimental, and inexpensive ICCP system was developed on an “Arduino” microcontroller and applied to an actual ASTM A193 B7 bolt tightened on an ASTM A105 flange at different torque levels. Experimental results indicate a direct relationship between the water day-night temperature profile and the cathodic protection performance. Specifically, the ICCP performance declines with increasing temperature. When the ICCP system was activated, gas bubbles are generated on the metal surface. Presumably these bubbles could induce hydrogen embrittlement cracks which were observed in scanning electron microscope images of the bolt cross-sections.

High resolution electrochemical monitoring of small pH changes

Usually pH measurements are carried out using potentiometric technique. However, sensitivity of this method is low due to limitation set by Nernst equation – the moderate resolution is a significant problem in today demand for tracking small pH changes in various application areas.We propose a novel approach that allows electrochemical monitoring of pH changes with resolution reaching below 0.01 units in alkaline, acidic or close to neutral media. The proposed method uses all-solid state pH sensitive electrode with syringaldazine (Syr) modified multi-walled carbon nanotubes as the reference electrode modulating amperometric signal of Ag|AgCl electrode used as the working one. Due to high slope of the current vs. potential dependence for low-polarizable silver / silver chloride electrode relatively small changes of reference (pH-sensitive) electrode potential are transformed to pronounced changes of the recorded current. Ultimately, this results in significant increase in resolution of traced pH changes. Moreover, the current recorded is found to be linearly dependent on changes of H+ or OH− ions concentration in a broad pH range, which additionally contributes to increased resolution readout. Advantages of herein proposed approach were fully verified by monitoring minute changes of solution pH resulting from dissolution of carbon dioxide.

Probing the thermodynamics and kinetics of ethylene carbonate reduction at the electrode-electrolyte interface with molecular simulations.

Understanding the formation of the solid-electrolyte interphase (SEI) in lithium-ion batteries is an ongoing area of research due to its high degree of complexity and the difficulties encountered by experimental studies. Herein, we investigate the initial stage of SEI growth, the reduction reaction of ethylene carbonate (EC), from both a thermodynamic and a kinetic approach with theory and molecular simulations. We employed both the potential distribution theorem and the Solvation Method based on Density (SMD) to EC solvation for the estimation of reduction potentials of Li+, EC, and Li+-solvating EC (s-EC) as well as reduction rate constants of EC and s-EC. We find that solvation effects greatly influence these quantities of interest, particularly the Li+/Li reference electrode potential in EC solvent. Furthermore, we also compute the inner- and outer-sphere reorganization energies for both EC and s-EC at the interface of liquid EC and a hydroxyl-terminated graphite surface, where total reorganization energies are predicted to be 76.6 and 88.9 kcal/mol, respectively. With the computed reorganization energies, we estimate reduction rate constants across a range of overpotentials and show that EC has a larger electron transfer rate constant than s-EC at equilibrium, despite s-EC being more thermodynamically favorable. Overall, this manuscript demonstrates how ion solvation effects largely govern the prediction of reduction potentials and electron transfer rate constants at the electrode-electrolyte interface.

A new method to determine AgCl(1% mol)/Ag electrode potential versus the standard chloride electrode potential in a LiCl-KCl eutectic

In this work, an innovative method was utilized to determine the AgCl(1% mol)/Ag reference electrode potential versus Cl2/Cl− at 673 K. Instead of directly measuring the chlorine electrode potential, the AgCl(1% mol)/Ag electrode potential versus Cl2/Cl− was calculated from the decomposition voltage of LiCl and the equilibrium potential of Li+/Li versus AgCl(1% mol)/Ag in a LiCl-KCl melt. The decomposition voltage of LiCl in the LiCl-KCl melt was calculated using the Nernst equation and the equilibrium potential of Li+/Li was determined by chronopotentiometry, open circuit chronopotentiometry and potentiodynamic scan techniques, and the deviation is within 5 mV. The AgCl(1% mol)/Ag electrode potential was determined to be −1.130 V vs Cl2/Cl− at 673 K. Since it does not involve direct injection of chlorine gas, this method is more convenient and safer than existing approaches. Furthermore, it also has the potential to be used to calibrate reference electrodes in a variety of melts.

An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Summary High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level.

Fabrication of a 3D-Printed Porous Junction for Ag|AgCl|gel-KCl Reference Electrode

Fused filament fabrication (FFF) is a 3D printing method that is attracting increased interest in the development of miniaturized electrochemical sensor systems due to its versatility, low cost, reproducibility, and capability for rapid prototyping. A key component of miniaturized electrochemical systems is the reference electrode (RE). However, reports of the fabrication of a true 3D-printed RE that exhibits stability to variations in the sample matrix remain limited. In this work, we report the development and characterization of a 3D-printed Ag|AgCl|gel-KCl reference electrode (3D-RE). The RE was constructed using a Ag|AgCl wire and agar-KCl layer housed in a watertight 3D-printed acrylonitrile butadiene styrene (ABS) casing. The novel feature of our electrode is a 3D-printed porous junction that protects the gel electrolyte layer from chloride ion leakage and test sample contamination while maintaining electrical contact with the sample solution. By tuning the 3D printing filament extrusion ratio (k), the porosity of the junction was adjusted to balance the reference electrode potential stability and impedance. The resulting 3D-RE demonstrated a stable potential, with a potential drift of 4.55 ± 0.46 mV over a 12-h period of continuous immersion in 0.1 M KCl, and a low impedance of 0.50 ± 0.11 kΩ. The 3D-RE was also insensitive to variations in the sample matrix and maintained a stable potential for at least 30 days under proper storage in 3 M KCl. We demonstrate the application of this 3D-RE in cyclic voltammetry and in pH sensing coupled with electrodeposited iridium oxide on a gold electrode. Our method offers a viable strategy for 3D printing a customizable true reference electrode that can be readily fabricated on demand and integrated into 3D-printed miniaturized electrochemical sensor systems.

Electrodeposition of Hydroxyapatite on a Metallic 3D-Woven Bioscaffold

In this study, we demonstrate that a uniform coating of hydroxyapatite (HAp, Ca10(PO4)6(OH)2) can be electrochemically deposited onto metallic 3D-woven bone scaffolds to enhance their bioactivity. The HAp coatings were deposited onto metallic scaffolds using an electrolyte containing Ca(NO3)2·4H2O, NH4H2PO4, and NaNO3. The deposition potential was varied to maximize the uniformity and adhesion of the coating. Using X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive spectroscopy (EDS), we found crystallized HAp on the 3D-woven lattice under all deposition potentials, while the −1.5 V mercury sulfate reference electrode potential provided the best local uniformity with a satisfactory deposition rate. The coatings generated under this optimized condition were approximately 5 µm thick and uniform throughout the internal and external sections of the woven lattice. We seeded and cultured both coated and uncoated scaffolds with human adipose-derived stromal/stem cells (ASCs) for 12 h and 4 days. We observed that the HAp coating increased the initial cell seeding efficiency by approximately 20%. Furthermore, after 4 days of culture, ASCs cultured on HAp-coated stainless-steel scaffolds increased by 32% compared to only 17% on the uncoated scaffold. Together, these results suggest that the HAp coating improves cellular adhesion.

Experimental and Theoretical Investigation of Lib-Active Materials Properties

Li-ion batteries (LIB) are currently used for energy storage in several applications. The design of LIB system must ensure the holding of performance for a given application during its entire life. Nowadays, Newman based models [1] are among the electrochemical models developed to compute the physical phenomena at the electrode scale, while predicting the behaviour of the entire cell. However, they require electrode’s components physical parameters. Finding out these physical parameters and their evolution during their lifetime is thus critical to simulate and predict with a high reliability the electrochemical response of the LIB via these models. Among the physical parameters, the equilibrium potential Eq , the solid diffusion Ds , and the exchange current density i0 of positive and negative active materials are affecting significantly this electrochemical response. Determining their expressions according to stoichiometry and their consistency based on thermodynamic approach [2] is an improvement for modelling Li-ion batteries. In this work, Ni0.6Mn0.2Co0.2O2 (NMC 622) is considered as the active material of the positive electrode and graphite for the negative one.In this paper, a methodology to extract these parameters from experimental data as function of the stoichiometry is presented. Then, an analytical model of their stoichiometry dependence is derived based on thermodynamic considerations. Eq and Ds have been measured using the Galvanostatic Intermittent Titration Technique (GITT) [3] on both half cells and a pouch cell with a lithium reference electrode. Results on the first design (half-cell) allow to quantify the active materials available at the positive and negative electrodes of the second design (pouch cell) and determine active materials stoichiometry as function of cell’s state of charge (SOC). i0 is determined from electrochemical impedance spectroscopy measurements (EIS) [4] performed at different cell’s state of charge on a specific experimental design with two references electrodes (Figure 1a). A lithium iron phosphate (LFP) reference electrode has been used to measure simultaneously the impedance spectra of both positive and negative electrodes. Lithium has been implemented as a second reference electrode to follow the LFP reference electrode potential, and the negative electrode potential at the different SOC of the EIS measurements. The exchange current densities are extracted using a Transmission Line Model (TLM) [5] and obtained results are presented (Figure 1b) for the two materials as function of the stoichiometry. A comparison of these results with the classical Butler-Volmer expression (classical BV) used in porous electrode theory shows a gap between them. This difference was highlighted by Bazant [6] where he presented an intercalation kinetics model dedicated to lithium iron phosphate intercalation, going beyond the elementary reaction considered in classical BV. With the classical BV, the molecular intercalations (repulsive or attractive forces) are neglected.The measured stoichiometry dependence of these three active material’ parameters is further analyzed, based on thermodynamic considerations. For the positive material, a model is proposed for this observed stoichiometry dependence, highlighting the non-ideal molecular behavior of lithium inside the host material.Finally, to validate the methodology and assess the predictability of Newman based LIB model, a comparative numerical simulation of the impedance spectra for the positive and negative electrodes with Newman electrochemical model with Eq , Ds , and i0 has been performed. Results at 20 and 80% SOC are presented Figure 1c for the negative electrode.

Far-field potentials in the compound muscle action potential.

It has long been believed that the compound muscle action potential (CMAP) in motor-nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This "reference electrode potential" is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far-field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle-tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.

New Reference Electrodes for Air and Moisture Stable Ionic Liquids

The use of room temperature ionic liquids (RTILs) for electrochemical applications such as fuel cells, batteries, dye-sensitized solar cells, and supercapacitors, has gained significant interest over the last decade. Despite the increase in attention, a universal reference electrode for use in RTIL electrochemistry has not been adopted. A stable, easy to prepare, and reproducible reference electrode, analogous to Ag/AgCl reference electrodes for aqueous electrochemistry, is crucial for accurately determining redox potentials as well as for comparing these potentials between RTILs and between research labs. Electrochemical work in RTILs often rely on quasi-reference electrodes (QREs),1 such as Ag or Pt wire, which must be calibrated by the addition of a redox couple (e.g. ferrocene/ferrocenium) after each electrochemical measurement. These quasi-reference electrodes are difficult to reproduce, likely due to the quality of the wire used and the presence of surface adsorbates. Furthermore, they have been known to exhibit potential drift during use, with 10s to 100s of mV drift often observed. Ferrocene is added after the experiment, however this one-point calibration of the QRE does not necessarily apply throughout the entire experiment, as the QRE may drift during the measurement.2 We report on reference electrodes designed for use in ionic liquids, which exhibit stable potential over several months of experimental use. One successful example is based on a silver wire coated with silver sulfide.3 The reference electrode potential is determined by the concentrations of Ag+ and S2 −, which are established by the solubility of the Ag2S coating on the Ag wire. Our reference electrode can be prepared and used in a normal air atmosphere, and does not need to be assembled and used in a glovebox, or protected from light. The reference electrode has been characterized by voltammetry measurements of ferrocene and cobaltocenium hexafluorophosphate, and was found to slowly drift to more positive potentials at a rate of <1 mV/day for five RTILs investigated over the course of 2+ months. Funding was provided by Lawrence Livermore National Laboratory Directed Research and Development (LDRD) Grant 17-ERD-047. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release LLNL-ABS-748348. (1) Electrochemical aspects of ionic liquids, 2nd ed.; Ohno, H., Ed.; John Wiley & Sons Inc., 2011. (2) Torriero, A. A. J.; Sunarso, J.; Howlett, P. C. Electrochim. Acta 2012, 82, 60–68. (3) Horwood, C.; Stadermann, M. Electrochem. commun. 2018, 88, 105–108.

More than a Liquid Junction: Effect of Stirring, Flow Rate, and Inward and Outward Electrolyte Diffusion on Reference Electrodes with Salt Bridges Contained in Nanoporous Glass

The Henderson equation is usually used to calculate liquid-junction potentials between miscible electrolyte solutions. However, the potentials of reference electrodes that comprise an electrolyte-filled nanoporous glass frit may also be affected by charge screening. As reported previously, when the Debye length approaches or surpasses the glass pore diameter, reference potentials depend on the composition of the bridge electrolyte, the pore size of the frit, and the concentration of electrolyte in the sample. We report here that stirring of samples may alter the reference potential as it affects the electrolyte concentration in the section of the nanoporous glass frit that is facing the sample solution. When the flow rate of bridge electrolyte into the sample is small, convective mass transport of sample into the nanoporous frit occurs. The depth of penetration into the frit is only a few nanometers but, despite the use of concentrated salt bridges, this is enough to affect the extent of electrostatic screening when samples of low ionic strength are measured. Mixing of sample and salt bridge solutions-and in particular penetration of sample components into the frit-was optically monitored by observation of a deeply colored Fe[(SCN)(H2O)5]2+ complex that formed in situ exclusively in the region where the sample and salt bridge mixed. Importantly, because flow through nanoporous frits is very slow, mass transport through these frits is dominated by diffusion. Consequently, over as little as 1 h, reference electrode frits with low flow rates become contaminated with sample components and undergo depletion of electrolyte within the frit to a depth of several millimeters, which can negatively affect subsequent experiments.

A 3rd generation Biosensor Towards the Continuous Long-Term Glucose Monitoring without Ag/AgCl Reference Electrode

Continuous glucose monitoring (CGM) is highly important, especially, for the diagnostics and control of Type I diabetes. [1] In recent years, automatic insulin pumps (artificial pancreas) are being applied and CGM is necessary for their operation. The insulin pump should deliver a correct dose of insulin based on real-time data about glucose levels in blood. Therefore, CGM sensors should be very selective and accurate, as well as reliable and stable. For this reason, CGM sensors are developed using selective enzymes – glucose oxidase or and FAD-dependent glucose dehydrogenase. [2] They usually operate by measuring either glucose oxidation current (amperometric biosensors) or potential (potentiometric biosensors). Historically, those biosensors are divided into three generations according to the electron acceptor: O2 dependent (1st), mediator depended (2nd) and direct electron transfer (3rd). The 3rd generation biosensors are the most attractive since they are most accurate, sensitive, operate at low potential and their response does not depend on the concentration of external mediator (either O2 or synthetic). However, to this date there are multiple unresolved problems which still hold the development of 3rd generation glucose biosensors for the CGM. Some of those problems include low stability (typically from a few hours to a few days) and miniaturization difficulties due to the usage of bulk reference electrodes for the control of the potential (Ag/AgCl, Hg/Hg2Cl2, etc.). Moreover, typical reference electrodes are poorly suited for long-time operation because they leak electrolyte and ions to the media. [3] A prolonged leakage of electrolyte solution changes the potential of reference electrode and may cause health risk if CGM biosensor in directly implanted in the patient. In this work, we present an open circuit potential (OCP) type 3rd generation glucose biosensor, which addresses the mentioned issues and operates in an electrolyte solution (pH 7.4) which mimics the composition of human serum. Our biosensor is composed of two electrodes: gold nanoparticle-glucose dehydrogenase (AuNPs-GDH) based bioanode and gold nanoparticles-laccase (AuNPs-LAC) based biocathode (Figure A). We have used AuNPs-LAC biocathode, which we have reported earlier [4], as a counter/reference electrode since its potential depends on oxygen reduction reaction. In the majority of solutions, O2 concentration is constant, thus it has a very stable OCP which is independent on glucose concentration and holds a constant value of 0.48 V vs.Ag/AgCl (Sat.) in an electrolyte solution. We have used AuNPs-GDH electrode as a working electrode since its OCP is dependent on glucose concentration raging from 10-6 M to 0.05 M. The OCP difference between two electrodes was -0.374 V and decreased depending on the concentration of glucose (10-6 M – 0.05 M) according to Nernst model (Figure B and C). The biosensor was calibrated in glucose range of 0.001 M to 0.01 M (typical glucose concentrations present in human blood) and a high correlation calibration curve was obtained (Figure D). Moreover, biosensor had showed an excellent stability and was able to operate at least for two weeks without a significant change in OCP. In conclusion, we have developed a promising biosensor for a constant long-term glucose monitoring which operates in an electrolyte solution, mimicking human serum conditions. A future challenge is to apply and optimize our biosensor to operate in serum and blood.

A universal quasi-reference electrode for in situ EC-STM

The use of an activated carbon based quasi-reference electrode for in situ electrochemical scanning tunneling microscopy was found to allow investigations of electrode/electrolyte interfaces under realistic electrochemical reaction conditions. The implementation of purely inert porous carbon electrode materials, both as reference and counter electrode, ensures a stable reference electrode potential over a wide range of temperatures and in different electrolytes, which renders this electrode ‘universal’. Furthermore, the high surface area carbon can reduce and even eliminate contaminations that might be present in the electrolyte. Activated carbon electrodes can therefore generally contribute to more reliable electrochemical measurements when investigating electrochemical reactions under complex experimental conditions, where the use of classical secondary reference electrodes is impossible or impractical. This is often the case when electrochemistry is combined with in situ interface analytics.

New Reference Electrodes for Air and Moisture Stable Ionic Liquids

The use of room temperature ionic liquids (RTILs) for electrochemical applications such as fuel cells, batteries, dye-sensitized solar cells, and supercapacitors, has gained significant interest over the last decade. Despite the increase in attention, a universal reference electrode for use in RTIL electrochemistry has not been adopted. A stable, easy to prepare, and reproducible reference electrode, analogous to Ag/AgCl reference electrodes for aqueous electrochemistry, is crucial for accurately determining redox potentials as well as for comparing these potentials between RTILs and between research labs. Electrochemical work in RTILs often rely on quasi-reference electrodes (QREs),1 such as Ag or Pt wire, which must be calibrated by the addition of a redox couple (e.g. ferrocene/ferrocenium) after each electrochemical measurement. These quasi-reference electrodes are difficult to reproduce, likely due to the quality of the wire used and the presence of surface adsorbates. Furthermore, they have been known to exhibit potential drift during use, with 10s to 100s of mV drift often observed. Ferrocene is added after the experiment, however this one-point calibration of the QRE does not necessarily apply throughout the entire experiment, as the QRE may drift during the measurement.2 We report on reference electrodes designed for use in ionic liquids, which exhibit stable potential over several months of experimental use. One successful example is based on a silver wire coated with silver sulfide.3 The reference electrode potential is determined by the concentrations of Ag+ and S2 −, which are established by the solubility of the Ag2S coating on the Ag wire. Our reference electrode can be prepared and used in a normal air atmosphere, and does not need to be assembled and used in a glovebox, or protected from light. The reference electrode has been characterized by voltammetry measurements of ferrocene and cobaltocenium hexafluorophosphate, and was found to slowly drift to more positive potentials at a rate of <1 mV/day for five RTILs investigated over the course of 2+ months. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release LLNL-ABS-748348. (1) Electrochemical aspects of ionic liquids, 2nd ed.; Ohno, H., Ed.; John Wiley & Sons Inc., 2011. (2) Torriero, A. A. J.; Sunarso, J.; Howlett, P. C. Electrochim. Acta 2012, 82, 60–68. (3) Horwood, C.; Stadermann, M. Electrochem. Commun. 2018, 88 (December 2017), 105–108.

Evaluation of a Ag/Ag2S reference electrode with long-term stability for electrochemistry in ionic liquids

We report on a reference electrode designed for use in ionic liquids, based on a silver wire coated with silver sulfide. The reference electrode potential is determined by the concentrations of Ag+ and S2−, which are established by the solubility of the Ag2S coating on the Ag wire. While potential shifts of >100 mV during an experiment have been reported when using silver or platinum wire quasi-reference electrodes, the reference electrode reported here provides a stable potential over several months of experimental use. Additionally, our reference electrode can be prepared and used in a normal air atmosphere, and does not need to be assembled and used in a glovebox, or protected from light. The reference electrode has been characterized by voltammetry measurements of ferrocene and cobaltocenium hexafluorophosphate, and was found to slowly drift to more positive potentials at a rate of <1 mV/day for five of the six ionic liquids investigated.

Activated Carbon Based Quasi-Reference Electrodes for Unconventional Lithium-Salt Containing Organic Electrolytes

Metallic lithium is the most frequently employed reference electrode in lithium ion battery research, but its high reactivity limits its utilization mainly to conventional carbonate based electrolytes. Novel concepts, like hybrid battery/supercapacitor devices, are often based on electrolytes with high ionic conductivity (e.g., acetonitrile). The utilization of these electrolytes brings up its own challenges as fundamental electrochemical characterizations are drastically aggravated by the high reactivity towards lithium metal. An alternative to Li/Li+ reference electrode for TEA-BF4 containing organic electrolytes is PTFE bound activated carbon as was shown in 2009.1 However, the latter approach yielded a pronounced drift of the reference electrode potential over time when a lithium-salt was employed. This effect is only poorly understood so far and a stable reference electrode for such systems is in high demand. Based on these issues, we provide a comprehensive study to establish (modified) carbon as a stable quasi-reference electrode for advanced lithium-salt containing electrolytes.2 In our work, we investigate the suitability of different carbons as quasi-reference electrodes (QREs) for lithium-salt containing electrolytes. As a reference point, we directly use the lithium intercalation/de-intercalation reaction of nanoparticulate Li4Ti5O12. Low surface area carbon-based QREs are impaired by high rates of potential shift (140 mV after 5 d). All high surface area carbon-based QREs show very low, uniform, and reproducible rates of potential shift (40-50 mV after 5 d). Only a negligible influence of carbon pore size distribution, electrolyte solvent, and binder on the reference electrode stability was observed. A distinct influence of the electrolyte anion on the quasi-reference electrode stability was found (Figure 1). The stability of pristine activated carbon decreases in the following order LiTFSI>LiClO4>LiPF6>LiBF4. We also studied the influence of carbon modification on the QRE stability. Activated carbon was de-functionalized (thermal annealing in vacuum or hydrogen) and functionalized (HNO3 treated) in order to unveil coherences of reference electrode stability with surface functional groups. The potential shift of activated carbon is drastically suppressed if the carbon surface is saturated by oxygen and nitrogen functional groups (Figure 2). After 15 days, the potential of heavily functionalized activated carbon is only marginally altered by 10 mV. Therefore functionalized activated carbon is a well suited quasi-reference electrode for the electrochemical characterization of Li-salt containing electrolytes which are not stable versus lithium metal.

Perspective—The Correct Assessment of Standard Potentials of Reference Electrodes in Non-Aqueous Solution

The challenges facing metal-air batteries have prompted fundamental studies of non-aqueous electrochemistry. However it appears that contributors in the field are not aware that the potentials of Li/Li+, Na/Na+, K/K+, and Mg/Mg2+ electrodes depend on the nature of solvent due to the cation solvation. Therefore, it is imperative to define a clear potential scale that can be correlated in different solvents. Here we report on the strong effect of the solvent on the Li/Li+ redox potential and discuss the use of the ferrocene/ferrocenium couple as internal or external standard for the measurements in non-aqueous solvents in lithium-ion and lithium-02 battery systems.

A Microfluidic Electrochemical Cell with Integrated Palladium Hydride Reference Electrode

Microfluidic devices have potential to be a great addition to the aqueous electrochemistry toolbox, for instance as an analog to the rotating ring-disk electrode, with an enhanced control of mass transport due to laminar flow. The photolithography process makes fabrication of various cell designs relatively fast, and it is possible to make electrodes of many different materials, including electrocatalyst particles. While the restricted volume of a microchannel provides excellent mass transport control, it introduces a very high solution resistance. This will create a significant shift in solution potential in the channel even at moderate currents, and makes the measurement of electrode potentials in the microchannel a challenge. In order to accurately measure potentials of electrodes in a microfluidic cell it is essential to characterize and minimize the effects of this potential distribution. Using a traditional reference electrode, and placing it in an outlet chamber essentially maximizes the uncompensated solution resistance, due to the necessity of having the counter electrode downstream of the working electrode. To solve the problems with the potential distribution, we fabricated a microfluidic electrochemical cell with an integrated palladium hydride reference electrode in a side channel upstream of the working electrode. A schematic of a cell is shown in Figure 1a. The cell design is relatively simple, and does not require salt bridges or ion conducting membranes in the cell to provide electrical contact to the reference electrode. This configuration results in low uncompensated resistance for single electrode experiments, and minimizes it for multiple electrode experiments. The placement of the reference electrode in a stagnant side channel eliminates influence of the species in the main channel on the reference electrode potential. Using PdH as the reference electrode makes the use of a wide range of acid and buffered electrolytes possible, and the presence of unfavorable ions such as chlorine can be eliminated. The reference electrode was charged in situ from a thin-film palladium electrode, and was able to hold a stable potential for up to eight hours of experiments. The relationship between the potential shift at one electrode, the cell geometry, and current through electrodes located upstream was probed using the half-wave potential of Ru(bpy)3 3+ oxidation and oxygen evolution as model reactions. Figure 1b shows how the potential for oxidation of Ru(bpy)3 3+ shifts at the SE at different anodic currents through the WE. This is the best case scenario for a cell with 100 µm wide electrodes and a 100 µm gap between the electrodes. If we allow for potential shifts up to 5 mV at the SE, the maximum current at the WE becomes 4 µA, using a 0.1 M Na2SO4 supporting electrolyte. Figure 1

Electrochemical Measurement of Li2O in Molten LiCl Salt

Nuclear fuel reprocessing is an important technology designed to increase the sustainability of nuclear power generation, as well as minimize long-lived radioactive waste production. There are multiple reprocessing schemes, most notably PUREX and pyroprocessing. While PUREX and related aqueous separation processes technically have superior maturity due to large scale implementation in France, Japan, Great Britain, and Russia, pyroprocessing has attracted substantial interest in recent years largely based on the perception that it represents a much lower proliferation threat and is ideally suited for metallic fast reactor fuel recycle. The Republic of Korea in particular has invested a tremendous amount of research funding over the last decade to develop pyroprocessing for spent fuel from commercial pressurized water reactors. Pyroprocessing utilizes a high temperature electrochemical cell called an electrorefiner to separate actinides from fission products. To apply this technology to oxide fuel, as in the case of the Korean spent fuel, an oxide reduction step must precede electrorefining. The current leading technology for oxide reduction is a high temperature electrolytic process carried out in molten lithium chloride salt. Electrolytic reduction relies on a lithium cycle, which consists of oxidation of lithium metal to lithium oxide at the cathode and reduction of the lithium oxide back to lithium metal at the anode. The net result is that uranium oxide is reduced to uranium metal, and oxygen gas is evolved. Lithium oxide concentration in the molten salt electrolyte is an important process variable. If it decreases, it can be a sign of lithium oxide accumulation in the fuel basket and will result in increased anode potential that ultimately leads to anode degradation. Currently, the only proven way of measuring lithium oxide concentration in lithium chloride involves sampling followed by acid-base titration. Measuring lithium oxide concentration in real time is an important objective to maintain optimal process control. The technique being developed for this purpose involves measuring the change in the open circuit potential (OCP) between a molten pool of a lead-lithium alloy and a Ni/NiO reference electrode. Because OCP measures equilibrium potential, this method can be combined with the Nernst Equation to continuously measure lithium oxide concentration. Mixtures of lithium chloride and lithium oxide were prepared with lithium oxide concentrations ranging from 0.00 to 2.75 wt%. A lead-lithium pool containing 2.0 wt% lithium was placed below the molten salt pool. After the salt melted it was allowed an hour to equilibrate, then an OCP measurement was obtained. As expected, the OCP increased with increasing concentration of Li2O. The measured sensitivity was about 200 mV for a Li2O concentration change from 0 to 2.5 wt%. Achieving repeatable and stable reference electrode potential is key to making accurate measurements from this method. The reference electrodes were made of high-density MgO tubes with low density, porous MgO plugs. Nickel wire and NiO powder were inserted into these tubes. The reference potential varied significantly between different sets of reference electrodes, which is hypothesized to be due to variations in the thickness of the porous MgO plugs. Methods for standardizing the reference electrodes have been attempted, and the results will be discussed. Another factor that may effect this measurement is the solubility of lithium metal in the zero oxidation state in the molten LiCl. Lithium solubility measurements were made in LiCl-Li2O with varying concentrations of Li2O. The results varied from 0.7 to 1.5 mole%. This low level of solubility is expected to cause minimal loss of lithium from the lead-lithium sensor. And experiments with different volumes of salt over a fixed volume of lead-lithium confirmed that the measured OCP is relatively insensitive to any decrease in the lithium concentration in the lithium-lead pool.