This work was supported by the US Department of Energy, Office of Nuclear Energy&#39;s Nuclear Energy University Programs (Award No. DE-NE0008425); Integrated University Program Graduate Fellowship (Award No. DE-NE0000113); and the Ministry of Trade, Industry &#38; Energy, Republic of Korea, the Energy Efficiency &#38; Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20142020104190). Publication of this article was funded in part by The Pennsylvania State University Libraries Open Access Publishing Fund.

1. Fabrication of Electrochemical Cell Components
<ol>
	<li>Fabrication of the binary CaF2-AF2 electrolyte
	<ol>
		<li>Calculate the mass required for each component of the binary electrolyte for a 350.0 &#177; 5.0 g mixture with 97 mol% CaF2 and 3 mol% AF2 (e.g., 333.4 g of CaF2 and 16.6 g of SrF2).</li>
		<li>Measure out and pour the salt mixture into a 1.5 L plastic bottle, along with approximately 1.3 kg of yttria-stabilized zirconia milling media (3 mm diameter) and 25.0 &#177; 0.1 g of polyvinyl alcohol (PVA, organic binder). Then add isopropyl alcohol (IPA) until the bottle is 4/5 full. Close the bottle and manually shake its contents for approximately 1 min to evenly distribute the components of the mixture.</li>
		<li>Place the plastic bottle with the salt mixture on the ball mill (two-rollers, 12.5&#34; length). Set the speed of the ball mill to 250 revolutions per minute (RPM) and mill for 24 h.</li>
		<li>Pour the mixture through a sieve (10 mesh) into a pan to separate the milling media and the salt mixture. Using a squeeze bottle, rinse the sieve lightly with 10 mL of IPA to capture the remaining mixture.</li>
		<li>Dry the ball-milled homogeneous mixture in a fume hood for ~24 h and then grind the mixture into a fine powder using a mortar and pestle. 
		NOTE: If the drying process needs to be accelerated, the pan can be placed on a hot plate set to 373 K.</li>
		<li>Measure out 130.0 &#177; 1.0 g of electrolyte powder and load the powder uniformly into a pellet die (75 mm diameter, 60 mm height).</li>
		<li>Using a die press, uniaxially press the powder with 30 MPa of pressure for 2 mins to form a green pellet 75 mm in diameter and 17 mm thickness. To remove the pellet from the pellet die, invert the pellet die, place a stainless-steel ring (101 mm outer diameter (OD), 35 mm height, 4.8 mm thickness) centered on top of the pellet die with the pellet centered within the ring. Carefully press the pellet die punch with ~1.0 bar of pressure to remove the pellet from the die.</li>
		<li>Use a small drill bit (1 mm diameter) to create tapping holes (~0.5 mm in depth) in the green pellet, one in the center and six evenly spaced 25.4 mm between drill centers. Then use the large drill bit (11.2 mm diameter) to manually drill seven wells centered on the tapping holes, each one approximately 12 mm deep (approximately &#190; of the way through the pellet).</li>
		<li>For each of the six electrolyte caps required, measure out 4.5 &#177; 0.5 g of electrolyte powder and load the powder uniformly into a pellet die (19 mm diameter, 50 mm height).</li>
		<li>Uniaxially press the electrolyte powder with 7.5 MPa for 1 min into a green pellet 19 mm &#215; 10 mm in diameter and thickness. To remove the pellet from the pellet die, invert the pellet die, place a stainless-steel ring (37.5 mm OD, 30 mm height, 3.5 mm thickness) centered on top of the pellet die with the pellet centered within the ring. Carefully press the pellet die punch with ~1.0 bar of pressure to remove the pellet from the die. Use a small drill bit (2 mm diameter) to manually drill a center hole through each cap. 
		NOTE: Green pellets from 1.1.8. and 1.1.10. are ready for sintering to form a single-phase solid electrolyte in the following steps.</li>
		<li>For each large electrolyte pellet and set of six small electrolyte caps, lightly cover an alumina plate (10 cm diameter, 4.65 mm thick) with coarse alumina powder to facilitate the separation of the sintered pellet from the alumina plate. Place the electrolyte pieces on top of the alumina powder such that they do not touch each other.</li>
		<li>Place the above assembly from 1.1.11 in a high temperature box furnace. Sinter the pieces with the following heating profile: 393 K for 12 h to remove moisture, 823 K for 12 h to burn out the PVA, and 1,273 K for 3 h to sinter, all with heating rates of 5 K/min. Then cool to 298 K at a rate of 2.5 K/min.</li>
	</ol>
	</li>
	<li>Fabrication of the alkaline-earth alloy electrodes
	<ol>
		<li>In an argon-filled glovebox, measure out the mass of the two components of the binary alloy, with a combined mass of at least 6.0 g (e.g., 5.6 g of Bi and 0.4 g of Ba for Ba-Bi alloy at barium mole fraction xBa = 0.10). Place in a tray and remove from the glovebox. 
		NOTE: Reactive metals are stored under mineral oil to prevent oxidation. To remove the mineral oil, sonicate the alkaline-earth metal pieces in acetone for 10 s.</li>
		<li>Place all the metal pieces on the center of the arc-melter stage and secure the stage.</li>
		<li>Pull vacuum on the chamber for 3 min until a vacuum of approximately -1.0 bar (gauge pressure) is achieved, and then fill with argon to 0.0 bar (gauge pressure). Repeat this procedure at least three times to ensure an inert argon atmosphere during the arc-melting process.</li>
		<li>Close the eye-protection shield on the arc-melter unit and turn on the current to create a stable electric arc between the stage and the tungsten tip of the arc-melter. Melt the metal pieces into a single homogeneous piece by exposing the pieces to the electric arc. Sufficient melting can be confirmed by the absence of observable separate phases in the alloy piece. 
		NOTE: If highly reactive elements are being melted, using a high current for longer than ~5 s can result in vaporization of material and cause inconsistencies in alloy composition.</li>
		<li>After melting the pieces into a single alloy, turn off the current and the arc-melter. Unscrew the stage from the chamber, flip the alloy, and screw the stage back into the chamber of the arc-melter. Repeat 1.2.3 - 1.2.5 three times to form a homogeneous alloy.</li>
		<li>After re-melting the alloy, unscrew the stage from the chamber again and break or cut the alloy into approximately 3 to 6 smaller pieces. Place the pieces on the stage and screw the stage back into the chamber of the arc-melter. Re-melt the pieces into a single piece according to steps 1.2.3 - 1.2.5.</li>
		<li>Allow the system to cool for 3 - 5 min. Detach the stage from the arc-melter system, and store the alloy in a plastic bag. Place the bag under an inert argon atmosphere (e.g., glovebox) until the final electrochemical cell assembly. 
		NOTE: For each experiment, two reference electrode alloy pieces and up to four working electrode alloy pieces of different compositions will be required.</li>
	</ol>
	</li>
	<li>Preparation of the tungsten electrical leads and thermocouple
	<ol>
		<li>Cut 6 tungsten wires (1 mm diameter) 46 cm in length. Manually sand along the length of each wire to remove surface contaminants, such as an oxide layer, using 100 grit emery paper. Clean the wire surface using wipes wetted with acetone.</li>
		<li>Insert the wire into an alumina tube (6.35 mm diameter, 30.5 cm long) that will prevent electrical shorting between the electrical leads and stainless-steel test chamber during electrochemical measurement. Leave approximately 12.7 cm at one end (bottom) as bare wire for making contact with the electrodes, and 2.5 cm at the other end (top) for electrical contact with the potentiostat leads.</li>
		<li>Mix approximately 3 g of quick cure epoxy and hardener for 1 min using the end of a wood applicator stick.</li>
		<li>With the wire in the tube, apply approximately 3 g of the epoxy to the top end of the tube to seal it. Lay the tube and wire vertically using a laboratory stand and allow the epoxy to cure for 15 min. Repeat for each tungsten wire (electrical lead).</li>
		<li>Insert the bottom end of a 45 cm thermocouple (type-K) into the top of a new 30.5 cm long alumina tube and seal the gap between the thermocouple and alumina tube using a quick cure epoxy similar to Step 1.3.4, leaving ~5 mm of the thermocouple exposed at the top. Allow the epoxy to cure for 15 min.</li>
	</ol>
	</li>
</ol>
2. Assembly of the Electrochemical Cell
<ol>
	<li>Cleaning of the electrochemical cell assembly components
	<ol>
		<li>Before the assembly of the electrochemical cell, thoroughly sand the inner surface of the stainless-steel test chamber with 100 grit emery paper until there is no visible contamination on the stainless-steel surfaces. Clean the test chamber, chamber cap, and alumina crucible (8.2 cm diameter, 3.0 cm height) using de-ionized water and rinse with IPA.</li>
		<li>Sonicate the parts of the vacuum fittings and the o-rings in isopropanol for ~10 min and allow them to dry inside the drying oven at ~373 K. Apply a thin coating of vacuum grease to o-rings for improved vacuum quality. Then move all electrochemical setup components into an argon filled glovebox for assembly.</li>
	</ol>
	</li>
	<li>Loading of the electrochemical cell assembly
	<ol>
		<li>Place the sintered electrolyte (step 1.1) at the center of the alumina crucible located in the test chamber carrier.</li>
		<li>Load enough electrode material into each well such that the top of the material is flush with the surface of the electrolyte. Fill two wells with reference electrode materials (e.g., Ba-Bi (xBa = 0.05)) of identical composition. Then fill four wells with working electrode material, each well having a different composition (Figure 2). In this step, shape arc-melted electrode materials close to the cylindrical shape of the electrolyte wells and drill a center through hole (2 mm diameter) for electrical lead insertion using machining tools (e.g., mini-lathe, drill bits, etc.). 
		NOTE: Minimize the exposure duration of samples to air to mitigate oxidation. Extensive oxidation is indicated by the presence of a non-lustrous (dull) surface layer on the samples. To remove the oxide layer, sand the surface of the sample(s) using 100 grit emery paper and clean with a dry wipe.</li>
		<li>Insert the electrical lead assembly (tungsten wire with alumina tube in 1.3) through the vacuum fitting port of the chamber cap, the baffle plates of the chamber, the hole in an electrolyte cap, and in the hole in an alloy electrode. Repeat this procedure for all six electrodes. Then insert the thermocouple through the last vacuum fitting port and into the seventh well at the center. Firmly touch the electrolyte surface with the alloy. A complete assembly is shown in Figure 2 and Figure 3. 
		NOTE: Each tungsten wire must firmly touch the electrolyte surface. If the alloy is too brittle to be machined, the tungsten wire can be held pressed against the alloy by pressing the wire against the alloy and securing its placement by tightening the vacuum fitting on the vacuum fitting port.</li>
		<li>Place the large o-ring in the groove of the top of the stainless-steel vacuum chamber. Carefully lower the assembled electrochemical cells into the test chamber. Securely tighten all vacuum-seal components and the clamp of the test chamber.</li>
	</ol>
	</li>
	<li>Removing moisture and oxygen from electrochemical cell assembly for emf measurement
	<ol>
		<li>Load the assembled test chamber into a crucible furnace. Place two overlapping layers of fiberglass insulation around the exposed surface of the vacuum chamber that is not in the furnace to ensure a uniform temperature distribution in the electrochemical cell and prevent the failure of the epoxy seals at the top of the test chamber.</li>
		<li>Attach the cooling water lines to the cooling tube inlet and outlet ports on the test chamber (Figure 3 and Figure 4).</li>
		<li>Attach the vacuum/argon line to the inlet port of the test chamber and close the outlet port valve. Evacuate the test chamber until the vacuum gauge reading is below 10 mtorr. 
		NOTE: If the vacuum level cannot achieve less than 10 mtorr, check seal components of the test chamber, including o-rings, clamps, tube fittings, and epoxy seals.</li>
		<li>Under active vacuum (&#60;10 mtorr), increase the furnace temperature to 373 K at a heating rate of 5 K/min and hold for 10 h; increase to 543 K at the same heating rate and hold for 10 h. NOTE: The drying procedure takes about 20 h.</li>
		<li>Once the above drying procedure is completed, purge the chamber with ultra-high purity argon. Repeat evacuation (&#60;10 mtorr) and argon purge (~1 atm) at least three times to ensure an inert atmosphere for operation at elevated temperatures.</li>
		<li>After the last argon purge, open both the inlet and outlet valves of the test chamber and adjust the flow meter to maintain continuous argon flow of 50.0 mL/min at ambient atmospheric chamber pressure (~1 atm).</li>
	</ol>
	</li>
</ol>
3. Electrochemical Measurements
<ol>
	<li>Establishing electrical contact between cell assembly and potentiostat
	<ol>
		<li>Connect the counter electrode cable and the reference electrode cable from the potentiostat once the furnace has reached 543 K. 
		NOTE: Each electrode cable (e.g., reference electrode, counter electrode, working electrode, sensing electrode cables) has a plug at the end of the cable that allows electrical connection.</li>
		<li>Attach an alligator clip to the end of the reference electrode cable from the potentiostat and clip it onto the electrical lead of the reference electrode from the cell assembly.</li>
		<li>Plug in five working electrode cables, one into each port 1 through 5, on the multiplexing (MUX) switch box. Attach an alligator clip to each working electrode cable and connect each alligator clip to the electrical lead for each working electrode from the electrochemical cell assembly, allowing for sequential voltage measurements of the other five electrodes relative to the reference electrode. 
		NOTE: One working electrode should possess the same composition as the reference electrode. The voltage difference between these two identical electrodes should be approximately zero and should be monitored during the entire measurement. A voltage difference less than 2 - 3 mV indicates the stability and reliability of the reference electrode system for accurate emf measurements.</li>
		<li>Attach one end of a ground cable to the stainless-steel test chamber and plug the other end directly into the ground port of an electrical outlet. 
		NOTE: This procedure effectively suppresses the electrical noise coming from the furnace heating elements because the stainless-steel test chamber serves as a Faradaic cage during the electrochemical measurements.</li>
		<li>Create a program using electrochemical software to measure the open circuit potential (OCP) for each working electrode sequentially using the potentiostat software under galvanostatic mode. 
		NOTE: The custom program, available by request, measures and records the OCP of each working electrode, rotating through each working electrode sequentially over time, with each rotation lasting for 15 min. The program should rotate through the set of working electrodes to record OCP measurements at each temperature increment.</li>
		<li>Increase the furnace temperature from 543 K to 1,073 K at 5.0 K/min, where the electrolyte becomes ionically conductive for emf measurements. 
		NOTE: At 1,073 K, the reference electrode should be fully molten to establish stable electrical contact with the electrolyte and electrical lead for enhanced stability of reference electrode potentials during the emf measurements.</li>
	</ol>
	</li>
	<li>Setting the thermal profile of the furnace for thermal cycling during emf measurement 
	NOTE: The temperature range is contingent upon the phase transition behavior of the electrode compositions (e.g., melting temperature) as well as the reactivity of the alloy composition. A typical temperature profile for the study of the Ba-Bi and Sr-Bi alloy systems, between 723 and 1,073 K, is given below.
	<ol>
		<li>Program the furnace controller to decrease the furnace temperature from 1,073 K to 723 K and to increase from 723 K to 1,073 K in 25 K intervals at a ramping rate of &#177; 5 K/min. At each temperature step (each 25 K interval), hold the temperature for 1 - 2 h to allow components to reach thermal and electrochemical equilibrium. 
		NOTE: Thermal equilibrium is reached once the cell temperature remains steady within &#177; 1 K at each temperature step as denoted by the thermocouple data displayed by the associated data acquisition (DAQ) system.</li>
	</ol>
	</li>
	<li>Temperature and emf data collection
	<ol>
		<li>Record the temperature of the electrochemical cell during the entire thermal cycle using a thermocouple DAQ system; include the upper and lower temperature limits of the furnace program and the type of thermocouple. Begin the emf measurement program at the same time as the temperature recording. 
		NOTE: The OCP measurement of each working electrode is measured against the reference electrode. The OCP measurement between the two reference electrodes should be less than 2-3 mV.</li>
		<li>Use the cell temperature and the OCP measurements of each working electrode to determine the emf values of each alkaline earth-liquid metal alloy as a function of temperature. The OCP values at each temperature are the emf values between the working and reference electrodes.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Ipser, H., Mikula, A., Katayama, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Overview:+The+emf+method+as+a+source+of+experimental+thermodynamic+data.">Overview: The emf method as a source of experimental thermodynamic data.</a> CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 34 (3), 271-278 (2010).</li><li>Kim, H., Smith, N., Kumar, K., Lichtenstein, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+Separation+of+Barium+into+Liquid+Bismuth+by+Controlling+Deposition+Potentials.">Electrochemical Separation of Barium into Liquid Bismuth by Controlling Deposition Potentials.</a> Electrochim. Acta. 220, 237-244 (2016).</li><li>National Research Council. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Electrometallurgical Techniques for DOE Spent Fuel Treatment: Final Report. , (2000).</li><li>Simpson, M. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Projected+Salt+Waste+Production+from+a+Commercial+Pyroprocessing+Facility.">Projected Salt Waste Production from a Commercial Pyroprocessing Facility.</a> Sci. Technol. Nucl. Install. 2013, 1-8 (2013).</li><li>Lichtenstein, T., Smith, N. D., Gesualdi, J., Kumar, K., Kim, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+properties+of+Barium-Bismuth+alloys+determined+by+emf+measurements.">Thermodynamic properties of Barium-Bismuth alloys determined by emf measurements.</a> Electrochim. Acta. 228, 628-635 (2017).</li><li>Kim, H., Boysen, D. A., Ouchi, T., Sadoway, D. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium-bismuth+electrodes+for+large-scale+energy+storage+(liquid+metal+batteries).">Calcium-bismuth electrodes for large-scale energy storage (liquid metal batteries).</a> J. Power Sources. 241, 239-248 (2013).</li><li>Delcet, J., Delgado-Brune, A., Egan, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coulometric+Titrations+Using+CaF2+and+BaF2+Solid+Electrolytes+to+Study+Alloy+Phases.">Coulometric Titrations Using CaF2 and BaF2 Solid Electrolytes to Study Alloy Phases.</a> Symp. Calc. Phase Diagrams Thermochemistry Alloy Phases. 275, 275-287 (1979).</li><li>Roine, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Outokummpu HSC Chemistry 5.1. Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database. , (2002).</li><li>Smith, N. D., Lichtenstein, T., Gesualdi, J., Kumar, K., Kim, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+Properties+of+Strontium-Bismuth+Alloys+Determined+by+Electromotive+Force+Measurements.">Thermodynamic Properties of Strontium-Bismuth Alloys Determined by Electromotive Force Measurements.</a> Electrochim. Acta. 225, 584-591 (2017).</li><li>Kim, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+properties+of+Calcium-Bismuth+alloys+determined+by+emf+measurements.">Thermodynamic properties of Calcium-Bismuth alloys determined by emf measurements.</a> Electrochim. Acta. 60, 154-162 (2012).</li><li>Okamoto, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Ba-Bi (Barium-Bismuth). , (1990).</li><li>Lichtenstein, T., Gesualdi, J., Nigl, T. P., Yu, C. T., Kim, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+Properties+of+Barium-Antimony+Alloys+Determined+by+Emf+Measurements.">Thermodynamic Properties of Barium-Antimony Alloys Determined by Emf Measurements.</a> Electrochim. Acta. , (2017).</li><li>Wagner, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Limitation+of+the+Use+of+CaF2+in+Galvanic+Cells+for+Thermodynamic+Measurements+due+to+the+Onset+of+Electronic+Conduction+under+Reducing+Conditions.">Limitation of the Use of CaF2 in Galvanic Cells for Thermodynamic Measurements due to the Onset of Electronic Conduction under Reducing Conditions.</a> J. Electrochem. Soc. 115 (9), 933-935 (1968).</li></ol>

The authors have no conflicts of interest to disclose concerning the material in the publication.

The emf cell in this work uses a CaF2-based solid electrolyte and electrode materials at fixed compositions, compared to an emf cell that uses a coulometric titration technique where the electrode composition is changed at a constant temperature. With coulometric titration, the electrode composition is determined by Faraday&#39;s law, assuming perfect coulombic efficiency. However, highly reactive alkaline-earth metals are moderately soluble (e.g., Ba ~15 mol% solubility in BaCl2) in their own halide salts, which can promote electronic conduction through the electrolyte and prevent accurate control of the composition of the electrode during coulometric titration7,13. The electrochemical cell in this work operates with electrode materials at fixed compositions, thus eliminating uncertainty in compositional accounting by coulometric titration, and enables accurate emf measurements of alkaline-earth alloys. Furthermore, the unique electrochemical cell in this work measures the emf values of four alloy compositions simultaneously within the same experiment to accelerate the evaluation of the thermodynamic properties over a wide range of compositions and temperatures.
As the arc-melter is used to fabricate the binary alloys, it is possible that the final composition of the alloys can deviate from the initial composition due to the high temperature of the electric arc and the high vapor pressures of the metals. To accurately report the emf-temperature relationship of the binary alloys, their composition was confirmed using inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in Table 3 for the Ba-Sb system12.
Before drying the electrochemical cell components according to step 2.3.4, difficulties in obtaining high-quality vacuum (&#60;10 mtorr) may occur. The o-ring in the vacuum chamber setup may not be seated correctly in its stainless-steel groove. There may also be a gap in the epoxy seals of the alumina tubes, to which additional epoxy may be applied to plug possible leaks. During emf measurements, if the electrical leads lose contact with the A-B alloys and large fluctuations in emf values are observed, contact may be re-established with the alloys by gently twisting the alumina tube, thereby wetting the liquid alloy to the lead.
Occasionally the emf values may exhibit a large hysteresis between the cooling and heating cycles. In general, a hysteresis of emf values between cooling/heating cycles may originate from (1) the degradation of electrolyte with reactive electrode compositions, especially at high alkaline-earth concentrations; (2) the degradation of electrode materials due to vaporization at elevated temperatures and oxidation with the residual oxygen inside the test chamber; or (3) non-equilibrium phase behavior of electrode materials, including undercooling effects and the formation of metastable phases during the cooling cycle.
When the degradation reaction between the electrode and the electrolyte is evident, the experimental set-up can be modified to mitigate the degradation of the electrochemical cell by decreasing the maximum operating temperature. In the presence of undercooling effects, emf values obtained during the heating cycle may be utilized in determining equilibrium thermodynamic properties. When the formation of metastable phases causes a hysteresis in emf measurements, the phase behavior of electrode materials requires scrutiny through complementary techniques, e.g., structural characterization by XRD, analysis of phase constituents by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and phase transition temperatures by DSC. Phase transition data may also be difficult to obtain with the described emf measurement technique above 1,223 K, as the CaF2-AF2 electrolyte may begin to degrade.
The emf measurement technique in this work can be used to determine the empirical thermodynamic properties of binary alkaline earth-liquid metal alloys, including activity, partial molar entropy, partial molar enthalpy, and phase transition temperatures. These thermodynamic data are utilized as an experimental basis for refining binary phase diagrams of alkaline-earth alloys with complementary techniques (XRD, DSC, and SEM), as exemplified in Figure 85. Based on the activity values of each alkaline-earth metal (A = Ca, Ba, and Sr) in liquid metals (B = Bi and Sb), the strength of atomic interactions between alkaline-earth elements and liquid metals can be leveraged to electrochemically separate out alkaline-earth fission products from molten salt solutions.

Electromotive force (emf) measurements can directly determine the partial molar Gibbs free energy change of a chemical reaction and provide accurate thermodynamic properties such as activity, partial molar enthalpy, and partial molar entropy1. The acquisition of thermochemical data is crucial to a variety of research topics in the materials community, from the refinement of multi-component phase diagrams, to the experimental validation of first-principle materials modeling, to the synthesis of new intermetallic species with advantageous properties. Recently, Kim et al. utilized emf measurements to assess the viability of using liquid metal electrodes to separate alkaline-earth species from molten salt electrolytes2.
Electrochemical separation using molten salts (e.g., LiCl-KCl) is a promising technology for separating uranium and transuranic metals from used nuclear fuel for recycling3. As used fuel is processed as an anode in the molten salt, fission products with lower standard reduction potentials than uranium are oxidized and accumulate in the molten salt as dissolved ions (e.g., Ba2+, Sr2+, Cs+, and rare-earth metal cations)4. Consequently, the molten salt electrolyte must be periodically replaced and/or processed further to separate the accumulated fission products4. Of particular concern are alkali/alkaline-earth fission products (Ba2+, Sr2+, and Cs+) because these ions exhibit the lowest standard reduction potentials among the constituent cations, making them difficult to separate from the molten salt solution.
However, Lichtenstein et al. recently demonstrated that barium exhibits low thermodynamic activity in liquid bismuth (8.7 x 10-12 at barium mole fraction xBa(in Bi) = 0.05, 1,123 K), implying strong atomic interactions between barium and bismuth5. Kim et al. observed that these interactions caused a shift in the deposition potentials of barium ions into a liquid bismuth electrode (-3.74 V to -2.49 V vs. Cl-/Cl2(g)), resulting in a preferential deposition of barium from the electrolyte solution (BaCl2-LiCl-CaCl2-NaCl, 16-29-35-20 mol%) at 773 - 973 K6. This shift in deposition potential could be leveraged by using liquid metal electrodes to selectively separate alkali/alkaline-earth fission products from the electrolyte used for electrochemical processing of used nuclear fuel. To determine the viability of separating alkali/alkaline-earth fission products from molten salt electrolyte, the thermodynamic properties of these elements in the prospective liquid metals (e.g., Bi, Sb) must be determined.
In previous studies, Delcet et al. utilized coulometric titration to determine the thermodynamic properties of binary alloys (e.g., Ba-Bi, Ba-Sb, Ba-Pb)7. For Ba-Bi alloys up to xBa = 0.50, they employed coulometric titration using a single-crystal BaF2 electrolyte at 1,123 K and observed comparable activity values of barium in bismuth (2.4 x 10-12 at xBa(in Bi) = 0.05, 1,123 K). However, it was reported that the results were inaccurate due to the uncertainty regarding barium content in the binary alloys. Barium metal is highly reactive and soluble in its halide salts (~15 mol% in BaCl2 at 1,163 K), which can cause increased electronic conduction in the halide salt at higher temperatures and lead to inaccurate compositional accounting during coulometric titration. To determine the thermodynamic properties (e.g., excess partial molar Gibbs free energy, partial molar enthalpy, partial molar entropy) of binary alloys containing highly reactive elements, the emf method described in this protocol was used.
Thermochemical properties of binary alloys can be determined by measuring the equilibrium cell potential Ecell (i.e., emf) of an alloy (A-B) relative to the reference potential of the pure metal A. Then, the cell potential is directly related to the change in partial molar Gibbs free energy (or chemical potential) of the cell reaction according to the Nernst relation (<img alt="Equation 1" src="/files/ftp_upload/56718/56718eq1.jpg" />).
For emf measurements of alkaline-earth alloys in this work, the fluoride-ion conducting CaF2 is chosen as the base electrolyte because the Ca2+/Ca redox potential (E0 = -5.59 V) is more negative than other alkaline-earth redox potentials (e.g., <img alt="Equation 2" src="/files/ftp_upload/56718/56718eq2.jpg" />, <img alt="Equation 2" src="/files/ftp_upload/56718/56718eq2.jpg" /> versus F-/F2(g) at 873 K) in the fluoride system8. This implies that CaF2 is more chemically stable than the other alkaline-earth fluoride AF2 (A = Sr or Ba), and that Ba2+ or Sr2+ ions are the electroactive species in the CaF2-BaF2 and CaF2-SrF2 electrolyte, respectively. Utilizing the high stability of CaF2, which minimizes side reactions with Ba or Sr alloys as well as the ionic conductivity of CaF2 at elevated temperatures, the single-phase binary CaF2-AF2 electrolyte was successfully employed to accurately measure the emf of binary alkaline earth-liquid metal alloys. Confirmation of the formation of the single-phase binary electrolyte is confirmed with x-ray diffraction (XRD) analysis in Figure 19.
To measure the cell potential of an alkaline-earth alloy, the following electrochemical cell was implemented using a solid-state binary CaF2-AF2 (97 mol% CaF2, 3 mol% AF2) electrolyte10:
<img alt="Equation 4" src="/files/ftp_upload/56718/56718eq4.jpg" />,
where the pure alkaline-earth metal A (A = Ca, Sr, or Ba) acts as the reference electrode (RE), solid CaF2-AF2 as the electrolyte, fixed composition A-B alloys as working electrodes (WE), and B is a candidate liquid metal such as Bi or Sb. The half-cell reactions in the electrochemical cell are: 
<img alt="Equation 5" src="/files/ftp_upload/56718/56718eq5.jpg" /> 
<img alt="Equation 6" src="/files/ftp_upload/56718/56718eq6.jpg" />
and the overall cell reaction is: 
<img alt="Equation 7" src="/files/ftp_upload/56718/56718eq7.jpg" />
where e- is an electron exchanged in the cell reactions and z is the number of electrons exchanged (z = 2 for alkaline-earth elements). For this overall reaction, the change in partial molar Gibbs free energy of the A metal, <img alt="Equation 8" src="/files/ftp_upload/56718/56718eq8.jpg" />, is given by: 
<img alt="Equation 9" src="/files/ftp_upload/56718/56718eq9.jpg" /> 
where <img alt="Equation 10" src="/files/ftp_upload/56718/56718eq10.jpg" /> is the partial molar Gibbs free energy of A metal in the metal B, <img alt="Equation 11" src="/files/ftp_upload/56718/56718eq11.jpg" /> is the standard Gibbs free energy of pure A metal, R is the ideal gas constant, T is the temperature in Kelvin, and aA is the activity of A in the metal B. The measured cell emf, Ecell, is directly related to the change in partial molar Gibbs free energy of A by the Nernst equation, 
<img alt="Equation 12" src="/files/ftp_upload/56718/56718eq12.jpg" /> 
where F is the Faraday constant.

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determination of thermodynamic properties of alkaline earth-liquid metal alloys using the electromotive force technique

A novel electrochemical cell based on a CaF2 solid-state electrolyte has been developed to measure the electromotive force (emf) of binary alkaline earth-liquid metal alloys as functions of both composition and temperature in order to acquire thermodynamic data. The cell consists of a chemically stable solid-state CaF2-AF2 electrolyte (where A is the alkaline-earth element such as Ca, Sr, or Ba), with binary A-B alloy (where B is the liquid metal such as Bi or Sb) working electrodes, and a pure A metal reference electrode. Emf data are collected over a temperature range of 723 K to 1,123 K in 25 K increments for multiple alloy compositions per experiment and the results are analyzed to yield activity values, phase transition temperatures, and partial molar entropies/enthalpies for each composition.

Figure 5 displays emf measurements made upon cooling and reheating an electrochemical cell: Ba-Bi(xBa = 0.05)|CaF2-BaF2|Ba-Bi(xBa = 0.05, 0.10, and 0.20), where a Ba-Bi alloy at xBa = 0.05 serves as the reference electrode5.
The potential difference between the two identical Ba-Bi alloys at xBa = 0.05 remains less than 2 mV during the entire measurement, demonstrating the stability and reliability of the reference electrode. For alloy compositions at xBa = 0.10 and xBa = 0.20, a symmetric emf profile is obtained during the heating and cooling cycles, indicating reproducible emf values during thermal cycling. At each temperature step (each 25 K interval), the cell temperature and the cell emf value reach thermal and electrochemical steady-state in less than 1 - 2 h (Figure 5)5.
To determine thermodynamic properties of the Ba-Bi alloys against the standard state of pure Ba(s), the emf values of the Ba-Bi (xBa = 0.05) alloy reference electrode must be calibrated with respect to pure Ba. The emf values of the reference electrode with respect to pure Ba are determined by using a separate electrochemical cell: Ba(s)|CaF2-BaF2|Ba-Bi(xBa = 0.05) and the results are presented in Figure 6. Using the linear fit of this measurement at xBa = 0.05 (Figure 6), the emf values of Ba-Bi alloys (Ecell) are determined relative to pure Ba metal5.
The emf values of Ba-Bi alloys, relative to pure Ba(s), are plotted as a function of temperature at each electrode composition, as shown in Figure 7 for selected Ba-Bi alloys (xBa = 0.05 - 0.25)5. From linear fits of the emf data plotted versus temperature, the change in partial molar entropy was calculated using the following thermodynamic equation: 
<img alt="Equation 13" src="/files/ftp_upload/56718/56718eq13.jpg" /> 
and the partial molar enthalpy of barium in bismuth can be calculated using thermodynamic relations such as the Gibbs-Helmholtz equation, as shown below. The results are summarized in Table 15. 
<img alt="Equation 14" src="/files/ftp_upload/56718/56718eq14.jpg" />
The activity of barium was also determined using the collected emf values and the Nernst equation: 
<img alt="Equation 15" src="/files/ftp_upload/56718/56718eq15.jpg" /> 
The results are summarized in Table 25.
Emf values for Ba-Bi alloys (xBa = 0.05 - 0.80) were also used to determine the phase transition temperatures for each alloy composition. In conjunction with differential scanning calorimetry (DSC) phase transition data, inductively coupled plasma atomic emission spectroscopy (ICP-AES) composition data, similar to that displayed in Table 312, and crystal structure data from XRD analysis, emf data were used to refine the most recent Ba-Bi phase diagram reported by Okamoto (Figure 8)5,11.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56718/56718fig1.jpg" /> 
Figure 1: Single-phase CaF2-SrF2 electrolyte XRD spectra. XRD spectra (normalized to the most intense peak for each spectrum) of CaF2-SrF2 electrolyte before and after sintering. Pure (*) CaF2 and SrF2 diffraction patterns are provided for comparison. This figure has been modified from Smith et al.9 <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56718/56718fig2.jpg" /> 
Figure 2: Electrochemical cell of A-B alkaline earth-liquid metal alloys. A schematic of the electrochemical cell assembly used for emf measurements with electrolyte, electrolyte caps, electrode materials, tungsten leads, and thermocouple (TC). Two of the 6 A-B alloys are reference electrodes and 4 are working electrodes. <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56718/56718fig3.jpg" /> 
Figure 3: Electrochemical setup for emf measurements. An illustration of the electrochemical cell components and the associated components for proper operating conditions. <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56718/56718fig4.jpg" /> 
Figure 4: Instrumentation diagram of experimental setup. A schematic of cooling water (solid, bold), argon (solid, thin), and vacuum (dashed) fluid flow through the emf measurement system. <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56718/56718fig5.jpg" /> 
Figure 5: Electrochemical emf measurements of Ba-Bi alloys (xBa = 0.05 - 0.20). Electromotive force (E1) and temperature measured as a function of time upon cooling and reheating a Ba-Bi(xBa = 0.05)|CaF2-BaF2|Ba-Bi(xBa = 0.05, 0.10, and 0.20) cell. This figure has been modified from Lichtenstein et al.5 <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56718/56718fig6.jpg" /> 
Figure 6: Pure Ba vs. Ba-Bi alloy (xBa = 0.05) emf value calibration. Electromotive force (EII) measured as a function of temperature using a Ba(s)|CaF2-BaF2|Ba-Bi(xBa = 0.05) cell. This figure has been modified from Lichtenstein et al.5 <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56718/56718fig7.jpg" /> 
Figure 7: Emf measurements of Ba-Bi alloys (xBa = 0.05 - 0.25). Electromotive force (Ecell) as a function of temperature for Ba-Bi alloys at xBa = 0.05, 0.10, 0.15, 0.20, and 0.25 based on a Ba(s)|CaF2-BaF2|Ba-Bi(XBa = 0.05 - 0.25), where solid lines represent linear fits. This figure has been modified from Lichtenstein et al.5 <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56718/56718fig8.jpg" /> 
Figure 8: Ba-Bi phase diagram. Experimentally determined Ba-Bi phase diagram based upon emf measurements in complement with DSC and XRD characterization of Ba-Bi alloys, where (rt) and (ht) represent room temperature and high temperature, respectively. This figure has been modified from Lichtenstein et al.5 <a href="https://cloudflare.jove.com/files/ftp_upload/56718/56718fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>x Ba</td>
			<td>T (K)</td>
			<td>&#8706;Ecell/&#8706;T (&#956;V K&#8210;1)</td>
			<td>&#8706;(Ecell/T)/&#8706;(1/T) (mV)</td>
			<td><img alt="Equation 16" src="/files/ftp_upload/56718/56718eq16.jpg" /> (J mol&#8210;1 K&#8210;1)</td>
			<td><img alt="Equation 17" src="/files/ftp_upload/56718/56718eq17.jpg" />&#160;(kJ mol&#8210;1)</td>
		</tr>
		<tr>
			<td>0.05</td>
			<td>707-938</td>
			<td>197 &#177; 6</td>
			<td>1011 &#177; 5</td>
			<td>38</td>
			<td>-195.1</td>
		</tr>
		<tr>
			<td>0.1</td>
			<td>704-1048</td>
			<td>137 &#177; 1</td>
			<td>1031 &#177; 1</td>
			<td>26.4</td>
			<td>-199</td>
		</tr>
		<tr>
			<td>0.15</td>
			<td>728-1048</td>
			<td>125 &#177; 2</td>
			<td>1005 &#177; 2</td>
			<td>24.1</td>
			<td>-193.9</td>
		</tr>
		<tr>
			<td>0.2</td>
			<td>809-1048</td>
			<td>94 &#177; 7</td>
			<td>984 &#177; 6</td>
			<td>18.1</td>
			<td>-189.9</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>881-1048</td>
			<td>73.4 &#177; 5</td>
			<td>961 &#177; 5</td>
			<td>14.2</td>
			<td>-185.4</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>704-881</td>
			<td>-480 &#177; 14</td>
			<td>1448 &#177; 13</td>
			<td>-92.6</td>
			<td>-279.4</td>
		</tr>
	</tbody>
</table>
Table 1: Thermodynamic properties of Ba-Bi alloys (xBa = 0.05 - 0.25). Change in partial molar entropies (<img alt="Equation 16" src="/files/ftp_upload/56718/56718eq16.jpg" />) and partial molar enthalpies (<img alt="Equation 17" src="/files/ftp_upload/56718/56718eq17.jpg" />) for Ba-Bi alloy compositions xBa = 0.05 to xBa = 0.25 calculated from linear fits of emf values, where the slopes and intercepts are <img alt="Equation 18" src="/files/ftp_upload/56718/56718eq18.jpg" /> and <img alt="Equation 19" src="/files/ftp_upload/56718/56718eq19.jpg" />, respectively. This table has been modified from Lichtenstein et al.5
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td rowspan="2">x Ba</td>
			<td colspan="3">E (V)</td>
			<td colspan="3">ln a Ba</td>
		</tr>
		<tr>
			<td>773 K</td>
			<td>873 K</td>
			<td>973 K</td>
			<td>773 K</td>
			<td>873 K</td>
			<td>973 K</td>
		</tr>
		<tr>
			<td>0.05</td>
			<td>1.164</td>
			<td>1.183</td>
			<td>1.203</td>
			<td>-35</td>
			<td>-31.5</td>
			<td>-28.7</td>
		</tr>
		<tr>
			<td>0.10</td>
			<td>1.137</td>
			<td>1.15</td>
			<td>1.164</td>
			<td>-34.1</td>
			<td>-30.6</td>
			<td>-27.8</td>
		</tr>
		<tr>
			<td>0.15</td>
			<td>1.101</td>
			<td>1.114</td>
			<td>1.127</td>
			<td>-33</td>
			<td>-29.6</td>
			<td>-26.9</td>
		</tr>
		<tr>
			<td>0.20</td>
			<td>1.075</td>
			<td>1.066</td>
			<td>1.076</td>
			<td>-32.2</td>
			<td>-28.3</td>
			<td>-25.7</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>1.075</td>
			<td>1.027</td>
			<td>1.032</td>
			<td>-32.2</td>
			<td>-27.3</td>
			<td>-24.6</td>
		</tr>
	</tbody>
</table>
Table 2: Measured emf values (E) and the natural log of activity of barium in bismuth (ln aBa). The measured emf values of Ba-Bi alloys (xBa = 0.05 - 0.25) versus Ba(s) and the natural log of activity of barium in bismuth at 773 K, 873 K, and 973 K. This table has been modified from Lichtenstein et al.5
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td colspan="2">Mole fraction, x Ba</td>
		</tr>
		<tr>
			<td>Nominal</td>
			<td>Measured</td>
		</tr>
		<tr>
			<td>0.03</td>
			<td>0.03</td>
		</tr>
		<tr>
			<td>0.05</td>
			<td>0.05</td>
		</tr>
		<tr>
			<td>0.10</td>
			<td>0.09</td>
		</tr>
		<tr>
			<td>0.15</td>
			<td>0.14</td>
		</tr>
		<tr>
			<td>0.20</td>
			<td>0.20</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>0.25</td>
		</tr>
		<tr>
			<td>0.30</td>
			<td>0.30</td>
		</tr>
	</tbody>
</table>
Table 3: Nominal and measured barium content of Ba-Sb binary alloys. The nominal and measured barium content of Ba-Sb binary alloys. Barium content of Ba-Sb alloys was confirmed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). This table has been modified from Lichtenstein et al.12

Watch this Scientific Journal Video about Determination of Thermodynamic Properties of Alkaline Earth-liquid Metal Alloys Using the Electromotive Force Technique at JoVE.com

Determination of Thermodynamic Properties of Alkaline Earth-liquid Metal Alloys Using the Electromotive Force Technique

This protocol describes the measurement of the electromotive force of alkaline-earth elements in liquid metal alloys at high temperatures (723-1,123 K) to determine their thermodynamic properties, including activity, partial molar entropy, partial molar enthalpy, and phase transition temperatures, over a wide composition range.

A novel electrochemical cell based on a CaF2 solid-state electrolyte has been developed to measure the electromotive force (emf) of binary alkaline ...

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13.1K Views.  The Pennsylvania State University. This protocol describes the measurement of the electromotive force of alkaline-earth elements in liquid metal alloys at high temperatures (723-1,123 K) to determine their thermodynamic properties, including activity, partial molar entropy, partial molar enthalpy, and phase transition temperatures, over a wide composition range.

Video: Determination of Thermodynamic Properties of Alkaline Earth-liquid Metal Alloys Using the Electromotive Force Technique

Metal-silicate Partitioning at High Pressure and Temperature: Experimental Methods and a Protocol to Suppress Highly Siderophile Element Inclusions

Estimates of the primitive upper mantle (PUM) composition reveal a depletion in many of the siderophile (iron-loving) elements, thought to result from their extraction to the core during terrestrial accretion. Experiments to investigate the partitioning of these elements between metal and silicate melts suggest that the PUM composition is best matched if metal-silicate equilibrium occurred at high pressures and temperatures, in a deep magma ocean environment. The behavior of the most highly siderophile elements (HSEs) during this process however, has remained enigmatic. Silicate run-products from HSE solubility experiments are commonly contaminated by dispersed metal inclusions that hinder the measurement of element concentrations in the melt. The resulting uncertainty over the true solubility and metal-silicate partitioning of these elements has made it difficult to predict their expected depletion in PUM. Recently, several studies have employed changes to the experimental design used for high pressure and temperature solubility experiments in order to suppress the formation of metal inclusions. The addition of Au (Re, Os, Ir, Ru experiments) or elemental Si (Pt experiments) to the sample acts to alter either the geometry or rate of sample reduction respectively, in order to avoid transient metal oversaturation of the silicate melt. This contribution outlines procedures for using the piston-cylinder and multi-anvil apparatus to conduct solubility and metal-silicate partitioning experiments respectively. A protocol is also described for the synthesis of uncontaminated run-products from HSE solubility experiments in which the oxygen fugacity is similar to that during terrestrial core-formation. Time-resolved LA-ICP-MS spectra are presented as evidence for the absence of metal-inclusions in run-products from earlier studies, and also confirm that the technique may be extended to investigate Ru. Examples are also given of how these data may be applied.

We present a procedure to determine the metal-silicate partitioning of siderophile elements, emphasizing techniques that suppress the formation of metal inclusions in experiments for the noble metals. The results of these experiments are used to demonstrate the effect of core-formation on the highly siderophile element composition of the mantle.

Estimates of the primitive upper mantle (PUM) composition reveal a depletion in many of the siderophile (iron-loving) elements, thought to result from ...

Ultrasound Velocity Measurement in a Liquid Metal Electrode

A growing number of electrochemical technologies depend on fluid flow, and often that fluid is opaque. Measuring the flow of an opaque fluid is inherently more difficult than measuring the flow of a transparent fluid, since optical methods are not applicable. Ultrasound can be used to measure the velocity of an opaque fluid, not only at isolated points, but at hundreds or thousands of points arrayed along lines, with good temporal resolution. When applied to a liquid metal electrode, ultrasound velocimetry involves additional challenges: high temperature, chemical activity, and electrical conductivity. Here we describe the experimental apparatus and methods that overcome these challenges and allow the measurement of flow in a liquid metal electrode, as it conducts current, at operating temperature. Temperature is regulated within &#177;2 &#176;C using a Proportional-Integral-Derivative (PID) controller that powers a custom-built furnace. Chemical activity is managed by choosing vessel materials carefully and enclosing the experimental setup in an argon-filled glovebox. Finally, unintended electrical paths are carefully prevented. An automated system logs control settings and experimental measurements, using hardware trigger signals to synchronize devices. This apparatus and these methods can produce measurements that are impossible with other techniques, and allow optimization and control of electrochemical technologies like liquid metal batteries.

Ultrasound velocimetry is used to study mixing by fluid flow in liquid metal electrodes. The focus of this manuscript is to illustrate the methods used for making precise, spatially-resolved ultrasound measurements while limiting oxidation and controlling and monitoring temperature, applied current, and the heater power being supplied.

A growing number of electrochemical technologies depend on fluid flow, and often that fluid is opaque. Measuring the flow of an opaque fluid is inherently ...

Engineering

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

Metal Corrosion and the Efficiency of Corrosion Inhibitors in Less Conductive Media

Material corrosion can be a limiting factor for different materials in many applications. Thus, it is necessary to better understand corrosion processes, prevent them and minimize the damages associated with them. One of the most important characteristics of corrosion processes is the corrosion rate. The measurement of corrosion rates is often very difficult or even impossible especially in less conductive, non-aqueous environments such as biofuels. Here, we present five different methods for the determination of corrosion rates and the efficiency of anti-corrosion protection in biofuels: (i) a static test, (ii) a dynamic test, (iii) a static test with a reflux cooler and electrochemical measurements (iv) in a two-electrode arrangement and (v) in a three-electrode arrangement. The static test is advantageous due to its low demands on material and instrumental equipment. The dynamic test allows for the testing of corrosion rates of metallic materials at more severe conditions. The static test with a reflux cooler allows for the testing in environments with higher viscosity (e.g., engine oils) at higher temperatures in the presence of oxidation or an inert atmosphere. The electrochemical measurements provide a more comprehensive view on corrosion processes. The presented cell geometries and arrangements (the two-electrode and three-electrode systems) make it possible to perform measurements in biofuel environments without base electrolytes that could have a negative impact on the results and load them with measurement errors. The presented methods make it possible to study the corrosion aggressiveness of an environment, the corrosion resistance of metallic materials, and the efficiency of corrosion inhibitors with representative and reproducible results. The results obtained using these methods can help to understand corrosion processes in more detail to minimize the damages caused by corrosion.

The testing of processes associated with material corrosion can often be difficult especially in non-aqueous environments. Here, we present different methods for short-term and long-term testing of corrosion behavior of non-aqueous environments such as biofuels, especially those containing bioethanol.

Material corrosion can be a limiting factor for different materials in many applications. Thus, it is necessary to better understand corrosion processes, ...

Environment

Synthesis of Ionic Liquid Based Electrolytes, Assembly of Li-ion Batteries, and Measurements of Performance at High Temperature

The chemical instability of the traditional electrolyte remains a safety issue in widely used energy storage devices such as Li-ion batteries. Li-ion batteries for use in devices operating at elevated temperatures require thermally stable and non-flammable electrolytes. Ionic liquids (ILs), which are non-flammable, non-volatile, thermally stable molten salts, are an ideal replacement for flammable and low boiling point organic solvent electrolytes currently used today. We herein describe the procedures to: 1) synthesize mono- and di-phosphonium ionic liquids paired with chloride or bis(trifluoromethane)sulfonimide (TFSI) anions; 2) measure the thermal properties and stability of these ionic liquids by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA); 3) measure the electrochemical properties of the ionic liquids by cyclic voltammetry (CV); 4) prepare electrolytes containing lithium bis(trifluoromethane)sulfonamide; 5) measure the conductivity of the electrolytes as a function of temperature; 6) assemble a coin cell battery with two of the electrolytes along with a Li metal anode and LiCoO2 cathode; and 7) evaluate battery performance at 100 &#176;C. We additionally describe the challenges in execution as well as the insights gained from performing these experiments.

Here, we describe protocols to prepare phosphonium-based ionic liquid and lithium bis(trifluoromethane)sulfonimide salt electrolytes, and assemble a non-flammable and high temperature functioning lithium-ion coin cell battery.

The chemical instability of the traditional electrolyte remains a safety issue in widely used energy storage devices such as Li-ion batteries. Li-ion ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

In Situ Lithiated Reference Electrode: Four Electrode Design for In-operando Impedance Spectroscopy

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate multiple processes responsible for long-term performance decay. Given the complexity of physical processes occurring inside the cell, it is often challenging to achieve a full understanding of the root causes of this performance degradation. This difficulty arises in part from the fact that any electrochemical measurement of a battery will return the combined contributions of all components in the cell. Incorporation of a reference electrode can solve part of the problem, as it allows the electrochemical reactions of the cathode and the anode to be individually probed. A variation in the voltage range experienced by the cathode, for example, can indicate alterations in the pool of cyclable lithium ions in the full-cell. The structural evolution of the many interphases existing in the battery can also be monitored, by measuring the contributions of each electrode to the overall cell impedance. Such wealth of information amplifies the reach of diagnostic analysis in Li-ion batteries and provides valuable input to the optimization of individual cell components. In this work, we introduce the design of a test cell able to accommodate multiple reference electrodes, and present reference electrodes that are appropriate for each specific type of measurement, detailing the assembly process in order to maximize the accuracy of the experimental results.

The incorporation of reference electrodes in a Li-ion battery provides valuable information to elucidate degradation mechanisms at high voltages. In this article, we present a cell design that accommodates multiple reference electrodes, along with the assembly steps to assure maximum accuracy of the data obtained in electrochemical measurements.

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate ...

Identification and Quantification of Decomposition Mechanisms in Lithium-Ion Batteries; Input to Heat Flow Simulation for Modeling Thermal Runaway

The risks and possible accidents related to the normal use of lithium-ion batteries remain a serious concern. In order to get a better understanding of thermal runaway (TR), the exothermic decomposition reactions in anode and cathode were studied, using a Simultaneous Thermal Analysis (STA)/Gas Chromatography-Mass Spectrometry (GC-MS)/Fourier Transform Infrared (FTIR) spectrometer system. These techniques allowed the identification of the reaction mechanisms in each electrode, owing to the analysis of evolved gaseous species, the amount of heat released and mass loss. These results provided insight into the thermal events happening within a broader temperature range than covered in previously published models. This allowed the formulation of an improved thermal model to depict TR. The heat of reaction, activation energy, and frequency factor (thermal triplets) for each major exothermic process at material level were investigated in a Lithium Nickel-Manganese-Cobalt-Oxide (NMC (111))-Graphite battery cell. The results were analyzed, and their kinetics were derived. These data can be used to successfully simulate the experimental heat flow.

This work aims at determining the reaction kinetics of Li-ion battery cathode and anode materials undergoing thermal runaway (TR). Simultaneous Thermal Analysis (STA)/Fourier Transform Infrared (FTIR) spectrometer/Gas Chromatography Mass Spectrometry (GC-MS) were used to reveal thermal events and to detect evolved gases.

The risks and possible accidents related to the normal use of lithium-ion batteries remain a serious concern. In order to get a better understanding of ...

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion.
Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation.
Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Kelvin probe force microscopy (KPFM) measures surface topography and differences in surface potential, while scanning electron microscopy (SEM) and associated spectroscopies can elucidate surface morphology, composition, crystallinity, and crystallographic orientation. Accordingly, the co-localization of SEM with KPFM can provide insight into the effects of nanoscale composition and surface structure on corrosion.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...

Determination of Thermodynamic Properties of Alkaline Earth-liquid Metal Alloys Using the Electromotive Force Technique

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