Electromotive Force Measurements Research Articles

Cathodic Decomposition Electrodes as Standard Reference Electrodes for Molten Salts: Example of the Lithium Eutectic Electrode for the LiCl-KCl Eutectic

Alternatives to the widely-used standard anodic decomposition reference electrodes in molten salts are necessary to enable more easily reproduced thermochemical and electrochemical data in molten salt electrolytes. The class of standard reference electrodes called cathodic decomposition electrodes (CDEs) are easily constructed and can be used to make thermochemical measurements in molten salts more directly compared to anodic decomposition electrodes. The lithium eutectic electrode (LEE) was chosen as a sample test case for validation and was applied to thermochemical measurements of electroactive species in molten LiCl-KCl eutectic. Transient measurements were made to measure the Li+/Li reduction potential at zero current in pure LiCl-KCl eutectic relative to a Li-alloy reference electrode to validate the reference potential of the LEE. Literature-reported electromotive force measurements against Li-alloy reference electrodes were used to generate a relationship between the LEE and the standard chlorine electrode and this relationship was used to evaluate measured and reported formal potential measurements for the LiCl-KCl-GdCl3 system. This work demonstrates the general framework for defining CDEs for any molten salt system and a method for calibrating external reference electrodes against a CDE standard reference electrode, improving the ease of obtaining thermochemical and electrochemical measurements in any molten salt system.

Activity Coefficients of the System {yNaCl + (1 − y)NaH2PO4}(aq) AT T = 298.15 K Determined by Electromotive Force Measurements

Activity Coefficients of the System {yNaCl + (1 − y)NaH2PO4}(aq) AT T = 298.15 K Determined by Electromotive Force Measurements

Study of the solid-phase equilibria in the GeTe-Bi2Te3-Te system and thermodynamic properties of GeTe-rich germanium bismuth tellurides

A set of self-consistent thermodynamic parameters of the GeTe-rich germanium-bismuth tellurides were determined using an electromotive force (EMF) method with a glycerol electrolyte in a temperature range from 300 to 450 K. The solid-phase equilibrium diagram of the GeTe-Bi2Te3-Te system at 400 K was constructed using X-ray diffraction (XRD) and scanning electron miscroscope (SEM) techniques of synthesized electrode alloys, as well as available literature data. It is found that all telluride phases in GeTe-Bi2Te3 pseudo-binary section have a tie-line connection with elemental tellurium. The relative partial thermodynamic functions of GeTe in alloys were calculated using data from EMF measurements of concentration cells relative to the GeTe electrode. These findings together with the corresponding thermodynamic functions of GeTe and Bi2Te3 were used to calculate the relative partial molar functions of germanium in alloys, and also the standard thermodynamic functions of formation and standard entropies of the ternary compounds, namely Ge2Bi2Te5, Ge3Bi2Te6 and Ge4Bi2Te7.

Comparative study of thermodynamic & kinetic parameters measuring techniques in lithium-ion batteries

Quantitative analysis of the aging process of lithium-ion batteries by using electrochemical thermodynamic and kinetic parameters such as electrochemical potential, Li stoichiometry, and Li inventory loss is a key research topic in the development of Li-ion battery for electric vehicles and smart grids. It is generally known the above parameters can be acquired through the analysis of Electromotive Force (EMF) or Open-Circuit Voltage (OCV) curves. In this work, we proposed and applied five EMF measurement techniques to obtain the EMF-SoC relationships in LFP/Gr single-layer laminated pouch cells at different temperatures and States of Health (SoH), and comprehensively examined them from the viewpoints of eight evaluation dimensions. A Python program, named Degradation Modes Analysis (DMA), is used to diagnose the thermodynamic degradation modes of the battery automatically. Furthermore, electrochemical kinetic parameters were also extracted along with the depolarization process. For faster and more accurate aging diagnosis, we recommend an optimal blend of short relaxation time GITT (Short-Rest-GITT) and extrapolated EMF (Extrap-EMF) to reach the most precise EMF measurements and gain the most comprehensive information about battery thermodynamics and kinetics at the same time.

Thermodynamic Assessment of Molten Bix-Sn1-x (x = 0.1 to 0.9) Alloys and Microstructural Characterization of Some Bi-Sn Solder Alloys.

Properties such as lower melting temperature, good tensile strength, good reliability, and well creep resistance, together with low production cost, make the system Bi-Sn an ideal candidate for fine soldering in applications such as reballing or reflow. The first objective of the work was to determine the thermodynamic quantities of Bi and Sn using the electromotive force measurement method in an electrolytic cell (Gibbs' enthalpies of the mixture, integral molar entropies, and the integral molar excess entropies were determined) at temperatures of 600 K and 903 K. The second objective addressed is the comprehensive characterization of three alloy compositions that were selected and elaborated, namely Bi25Sn75, Bi50Sn50, and Bi75Sn25, and morphological and structural investigations were carried out on them. Optical microscopy and SEM-EDS characterization revealed significant changes in the structure of the elaborated alloys, with all phases being uniformly distributed in the Bi50Sn50 and Bi75Sn25 alloys. These observations were confirmed by XRD and EDP-XRFS analyses. Diffractometric analysis reveals the prevalence of metallic Bi and traces of Sn, the formation of the Sn0.3Bi0.7, Sn0.95Bi0.05 compounds, and SnO and SnO2 phases.

Electrochemical behavior and electromotive force measurements of U3+/U0 in molten MgCl2-NaCl

The electrochemical behavior of UCl3 was investigated in molten MgCl2NaCl at 550 °C. Using electromotive force measurements, we have measured the formal reduction potential (Eo’) at 550 °C of U3+/U0 as +0.276 V vs. Mg/MgCl2 (-1.400 V vs. Ag/AgCl 5 mol%) on the molal scale. We also present the design and construction of a sealed Mg/MgCl2 reference electrode and have determined this system demonstrates ideal nonpolarizable electrode behavior, confirming it as an appropriate choice of reference electrode in molten chloride salts.

Phase evolution, defect chemistry, and electrical properties of Na0.5Bi0.5TiO3-BiNi0.5Ti0.5O3 solid solutions

Solid solutions of Na0.5Bi0.5TiO3 (NBT) and BiNi0.5Ti0.5O3 (BNiT) were prepared by a solid-state reaction route, and their electrical properties investigated by a combination of impedance spectroscopy and electromotive force measurements to explore the possibility of developing mixed ionic-electronic conductors based on NBT. Phase analysis showed that BNiT has a large solid solution limit in NBT (60 mol% based on X-ray diffraction), and the room temperature crystal structure changes from rhombohedral to pseudo-cubic with increasing BNiT content. Neutron diffraction revealed the coexistence of rhombohedral and tetragonal phases when the BNiT content ≥ 40 mol%. Electrically, incorporation of BNiT induces p-type electronic conduction into NBT by hopping of holes between Ni2+ (NiNix) and Ni3+ (NiNi·), and therefore changes the electrical conduction mechanism systematically from predominant oxide-ion conduction to mixed ionic-electronic conduction and then to predominant p-type electronic conduction. The total conductivity of the solid solutions showed a “V-shape” variation with increasing BNiT content. Possible mechanisms for the phase evolution and the conductivity-composition relationships are discussed. Achieving high levels of ionic and electronic conductivity simultaneously in NBT by introducing elements with variable oxidation states remains challenging due to the competition between an enhanced electronic component and a suppressed ionic component. Low levels of BNiT incorporation are, however, beneficial to reducing the dielectric loss of NBT for dielectric applications.

Solid – phase equilibria and thermodynamic properties of the Sb-Te-S system

The powder X-ray diffraction (PXRD) analysis and the electromotive force (emf) measurements have been used to investigate the Sb–Te–S system in the Sb2S3-Sb2Te3-Te-S composition region at 300–450 K temperatures interval. Relative partial molar functions of antimony in alloys have been calculated and obtained data have been used to get self-consistent sets of the standard Gibbs free energy, standard enthalpy, and standard entropy of the Sb2S3 and Sb2Te2S compounds, as well as the Sb2Te2,4S0,6 and Sb2Te2,7S0,3 solid solutions. The data obtained for Sb2S3 have been compared to the ones available in the literature. Thermodynamic functions of the Sb2Te2S compound, as well as, the Sb2Te2,4S0,6 and Sb2Te2,7S0,3 solid solutions have been determined for the first time.

Metrological Support in the field of DC Voltage and EMF Measurements: from Normal Elements to a Quantum Standard

The article deals with the issues of metrological support on the example of the highest level of the State Verification Scheme for measuring instruments of direct current voltage and electromotive force. The origin and development of reference measurements of DC voltage and electromotive force in the Russian Empire, the USSR, and modern Russia is briefly reviewed. The results of work on improving the State Primary Standard GET 13 completed in 2022 are presented.

THERMODYNAMIC STUDY OF THE CdSb COMPOUND BY THE ELECTROMOTIVE FORCE MEASUREMENTS METHOD

Thermodynamic properties of CdSb binary compound were determined using an electromotive force measurement (e.m.f.) method with a liquid electrolyte in the ~300-450K temperatures interval. Equilibrium alloys from the CdSb+Sb phase region of the Cd-Sb system was analyzed by means of the e.m.f. measurements. Phase compositions of all samples were controlled using powder X-rays diffraction method. The partial molar functions of cadmium in alloys, the standard thermodynamic functions of formation, as well as the standard entropy of the CdSb compound were calculated. A comparative analysis of obtained results with literature data was performed.

Electrochemical investigation and thermodynamic assessment of the Mg-Pd system

This paper presents the results of electromotive force measurements using coulometric titration technique for liquid and solid Mg-Pd alloys with concentrations up to 0.70 Mg mole fraction and at 918 K. These measurements made it possible to verify the homogeneity range of the occurring phases and phase boundaries in the investigated system. Based on the obtained results and other thermodynamic data found in the literature, the phase diagram of the Mg-Pd system was optimized using the CALPHAD method and ab initio calculations. The thermodynamics of the intermetallic phases, the liquid phase, and the solid solutions were described by the correct equations, which can be used in the future to calculate multicomponent systems with the Mg-Pd system as one of the binary components. The thermodynamic calculations successfully reproduced the Mg-Pd phase diagram.

Effect of Additions of Mixtures of Sodium Sulfite and Formic and Acetic Acid on the Photocatalytic Reduction of Water with Cadmium Sulfide Suspensions

The photochemical reduction of water with cadmium sulfide suspensions containing mixtures of sodium sulfite and formic and acetic acid solutions has been studied by electromotive force measurements and gasometry. The sulfite ions are oxidized on the anode in the sulfite–acetate suspensions; the sulfite ions and formic acid, in the sulfite–formate solutions. Hydrogen peroxide was found among the products of the photochemical reaction; it is involved in the oxidation of the radicals of sulfite ions and molecular sulfur of CdS particles and prevents the reduction of water.

Activity Coefficients of the System {yNaCl + (1 – y)Na2HPO4}(aq) at T = 298.15 K Determined by Electromotive Force Measurements

Activity Coefficients of the System {yNaCl + (1 – y)Na2HPO4}(aq) at T = 298.15 K Determined by Electromotive Force Measurements

Thermodynamic properties of Nd-Fe alloys via emf measurements in LiCl-KCl-NdCl3 electrolyte

The thermodynamic properties of Nd-Fe alloys with varying mole fractions of Nd (xNd = 0.05–0.78) were investigated via electromotive force (emf) measurements in molten chloride at 773–1073 K. The Nd-Fe alloys were heat-treated at 923 K for 7 days to stabilize equilibrium phases, and the phase transition properties were confirmed by X-ray diffraction, electron microscopy, and differential scanning calorimetry. Employing less reactive Nd-Sn alloys (xNd = 0.10, L + NdSn3) as a reference electrode, the measured emf values were consistent between alloy compositions, as shown by equivalent thermal trajectories in distinct two-phase regions of [Fe + Nd2Fe17], [Nd2Fe17 + Nd5Fe17], [Nd5Fe17 + Nd], [L + Nd2Fe17], and [L + Nd5Fe17]. The measured emf results were further validated using open-circuit potentiometry in which Nd was deposited onto an Fe electrode at 973 K, resulting in a difference of less than 18 mV for the identical two-phase alloy between the two methods.

(Invited) Thermodynamic Properties of Rare-Earth Alloys Relevant for Efficient Rare-Earth Recovery in Molten Salts

Electrode materials of Fe, liquid Bi, and Sn were investigated to enhance the electrochemical recovery of rare-earth elements in molten salts (eutectic LiCl-KCl) at 500‒700 °C, leveraging their chemical interactions with rare-earth elements. Using Nd as a prototypical example, the degree of interactions was quantified based on electromotive force (emf) measurements of Nd alloys (Nd-Fe, Nd-Bi, and Nd-Sn). Temperature-dependent emf values of the Nd alloys were used to determine the thermodynamic properties such as the partial molar Gibbs energy (chemical potential), entropy, and enthalpy of Nd. In addition, the phase behavior of Nd alloys was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) to determine transition temperatures and microstructures of Nd alloy electrodes. Based on these fundamental properties of Nd alloys, the Nd-Sn alloy at mole fraction, x Nd = 0.10 was selected as the reference electrode and employed for electrochemical measurements in three-electrode cells in molten chloride salt (LiCl-KCl-NdCl3). The stability of Nd-Sn reference electrode was maintained less than 1 mV deviation between identical constructs during entire measurements, originating from the two phase (liquid + NdSn3) stability of the binary Nd-Sn alloy at 500‒700 °C. Finally, electrodeposition of Nd onto these interacting electrode materials was conducted to assess the recovery efficiency of Nd from molten chloride salt.

Analytical treatment of ion-exchange permselectivity and transport number measurements for high accuracy

We analyse electromotive force measurements of concentration cells using non-equilibrium thermodynamics, and determine the transference coefficients of ion-exchange membranes in aqueous KCl solutions. By taking advantage of the analytical expression for the permselectivity, we extract transport coefficients with high accuracy. The transport number of K+ and the transference coefficient of water in the Selemion CMVN cation-exchange membrane are found to be 100tK+=99.59±0.56 and tw=3.69±0.40 respectively, while for the Selemion AMVN anion-exchange membrane they are 100tCl−=100.21±0.37 and tw=−3.75±0.27. These results suggest that the membranes are perfectly selective to the target ion, and that each ion carries 3-4 water molecules through the membrane, which reduces the membrane permselectivity. In these concentration cells, the electrical potential contribution of the membrane alone was more easily isolated with bare Ag/AgCl electrodes without reference solutions and liquid junction plugs. Additionally, we find a large contribution to the measured concentration cell voltage from concentration gradients across the porous plug of the reference electrode, which cannot be explained by Henderson’s equation alone. For most of the concentration range, the transport number of the porous plug is determined to be 100tK+=49.43±0.78 with negligible water transport, similar to literature values for bulk electrolyte. In dilute electrolyte solutions with concentrations below 0.1 mol kg−1, the plug shows anomalous behaviour consistent with an increase in K+ selectivity and water co-transport.

Application of the Solute-Solvent EMF Cell to Measure Activity of NiF2 in Molten FLiNaK

Electromotive force (emf) measurements made using a combination of solute- and solvent-based electrodes were used to determine the activity of NiF2 in molten FLiNaK eutectic at 823 K across a concentration range of x NiF2 = 5.2 × 10–4–1.0 × 10–2. The solute emf values were measured using electrodes consisting of Ni wires immersed in FLiNaK with dissolved NiF2 contained in graphite crucibles. The measured emf values were then converted to the FLiNaK Eutectic Potassium Electrode (FEKE) potential and used to quantify the activity of dissolved NiF2. This quantification was based upon comparative measurements of a reference solvent electrode consisting of a K-Bi alloy immersed in pure FLiNaK contained in a boron nitride crucible and a solute electrode. Short cell lives were characteristic of the measurements due to the corrosive nature of the fluoride salts. Quantifying the activity of NiF2 will improve the utility of Ni2+/Ni reference electrodes in molten fluoride salts, which are notoriously difficult electrolytes to work with because of their reactivity. This work demonstrates the general nature of the solute-solvent approach as a repeatable, easily employed method for measuring the activity values of electroactive species in a variety of molten salts to improve understanding of the electroactive species behavior in these systems.

Novel instrumentation for high-impedance electromotive force (EMF) measurements

Instrumentation is reported for measuring electromotive force (EMF) from a high-impedance solid electrolyte-based electrochemical cell. Generally, concentration galvanic cells are used to study the high-temperature thermochemical properties of materials. The thermochemical results are determined by precise measurement of the high-impedance EMF and temperature of the electrochemical cell with suitable user interface software for data acquisition. The data are conditioned and processed using the unique design of the hardware and sent to a computer through the serial port. Online graphical displays of the data, averaging, trend viewing, and storage, are performed using a graphical user interface (GUI). The instrumentation was validated by conducting experiments with different materials using an electrochemical cell and its performance was demonstrated to be on par with commercial instruments. This work has enables low-cost sophisticated EMF measurements for determining the thermochemical properties of materials.

Optically Induced Spin Electromotive Force in a Ferromagnetic-Semiconductor Quantum Well Structure.

Hybrid structures combining ferromagnetic (FM) and semiconductor constituents have great potential for future applications in the field of spintronics. A systematic approach to study spin-dependent transport in a GaMnAs/GaAs/InGaAs quantum well (QW) hybrid structure with a few-nanometer-thick GaAs barrier is developed. It is demonstrated that a combination of spin electromotive force measurements and photoluminescence detection provides a powerful tool for studying the properties of such hybrid structures and allows the resolution of the dynamic FM proximity effect on a nanometer scale. The method can be generalized to various systems, including rapidly developing 2D van der Waals materials.

Gibbs Energy of Formation of U2Mo and U2Ti Determined by EMF Measurements in LiCl-KCl-UCl3 Melt

Gibbs energy of formation of U2Mo and U2Ti intermetallic phases in U-Mo and U-Ti binary systems, respectively was determined employing electromotive force measurements in LiCl-KCl-UCl3 melt. U-Mo and U-Ti alloys corresponding to two-phase field U2Mo+Mo and U2Ti+Ti were prepared by vacuum arc-melting and characterized by X-ray diffraction. Single phase U2Mo and U2Ti intermetallics were also prepared and characterized to support the indexing of Bragg peaks in two-phase alloys. Temperature dependence of ΔfGU2Mo∘ and ΔfGU2Ti∘ were, respectively expressed asΔfGU2Mo∘(kJ/mol-atom)=(−144.9±1.5)+(63±3)×10−3T(K)ΔfGU2Ti∘(kJ/mol-atom)=(−116.2±1.7)+(60±3)×10−3T(K)Since there were no other experimental data available that could be used to compare present correlations, results by DFT calculations from earlier works were compared and discussed. LiCl-KCl-UCl3 melt was analyzed after completion of electromotive force runs by cyclic voltammetry and no identifiable redox features due to Mo or Ti species in melt were observed. This suggested that both alloys remained stable in the melt even after 100 h duration of electromotive force runs and there was no chemical leaching of Mo or Ti from the alloy.