We are grateful for the design and fabrication assistance of D. De La Cruz, for equipment borrowed from M. Zahn, and for insightful discussions with D. R. Sadoway and the talented electrochemists of his group.

1. System Setup and Assembly
<ol>
	<li>Clean the ultrasound transducer with isopropanol.</li>
	<li>Load the glovebox.
	<ol>
		<li>Load required equipment and materials (including ultrasound transducer, ePbBi, stir stick, and thermocouples) into the glovebox, following the instructions of the glovebox manufacturer to minimize ingress of oxygen and moisture.</li>
		<li>Keep porous materials under vacuum in the glovebox antechamber for 12 hr before entering the glovebox.</li>
	</ol>
	</li>
	<li>Tune the PID controller (first time only).
	<ol>
		<li>Place the same quantity of solid ePbBi into the battery vessel that will be used in experiments (840 g).</li>
		<li>Place the furnace insulation around the battery vessel if it is not already there, and place the lid atop the battery vessel, along with the negative current collector and thermocouples.</li>
		<li>Make all electrical connections for thermocouples and furnace power, as shown in Figure 2B.</li>
		<li>Initiate automatic tuning of the PID controller, using 150 &#176;C as the set point. Note: the details of this step will differ, depending on the PID controller manufacturer and model. The controller used here auto-tunes by controlling four full thermal cycles, from RT to operating temperature, over a course of hours.
		<ol>
			<li>Use the arrow keys to adjust the set point (shown by default after tuning the controller) to 150 &#176;C.</li>
			<li>Press and hold the loop button for 3 sec to enter the hidden loop. Then press the loop button repeatedly until the controller screen shows &#8220;tUnE&#8221;. Use the arrow keys to change it to YES.</li>
		</ol>
		</li>
		<li>Insert a thermocouple and use the workstation to monitor and log temperature.</li>
		<li>Once auto-tune is complete, record the Proportional, Integral, and Derivative parameters that the PID controller has automatically selected by using the controller interface, according to the manufacturer&#8217;s instructions.</li>
	</ol>
	</li>
</ol>
2. Sound Speed Measurement
<ol>
	<li>Use the furnace to melt enough ePbBi for the experiment, at least 400 g. Note: required amount will vary for different equipment, and ePbBi melts at 125 &#176;C.
	<ol>
		<li>If necessary, remove excess oxide by skimming it from the top surface of the ePbBi using a stir stick.</li>
		<li>Insert an ultrasound transducer into the sound speed measurement device and tighten the swage connection to prevent leaks, then insert a thermocouple and use the workstation to monitor and log temperature.</li>
	</ol>
	</li>
	<li>Transfer molten metal to the sound speed measurement device.
	<ol>
		<li>Place the sound speed measurement device on the furnace base and leave it there for 2 min to gradually increase the temperature and avoid thermal shock.</li>
		<li>Prepare for a safe transfer by removing heat-sensitive equipment or materials from the area.</li>
		<li>Add small amounts of molten metal at a time, because thermal shock can damage the ultrasound transducer. Add ePbBi until the transducer face and micrometer head are both completely submerged.</li>
		<li>Wait until the temperature remains stable within 1 &#176;C for at least 5 min before beginning measurements, since sound speed depends on temperature.</li>
	</ol>
	</li>
	<li>Measure ultrasound echoes at two locations.
	<ol>
		<li>Set the micrometer tip to an arbitrary but known location. Record ultrasound echo measurements, following the instructions provided by the instrument manufacturer.</li>
		<li>Using the micrometer dial, move the micrometer tip by a known distance. Record ultrasound echo measurements.</li>
	</ol>
	</li>
	<li>Remove the molten metal from the sound speed measurement device and store it in a heat-tolerant container.</li>
	<li>To determine sound speed, plot echo amplitude as a function of echo time for each of the two measurements. Locate the echoes by fitting a Gaussian curve to each echo peak, as in Figure 4. Calculate sound speed by dividing displacement distance by the difference in echo peak times.</li>
</ol>
3. Ultrasound Velocity Measurement
<ol>
	<li>Melt enough ePbBi for the experiment (840 g), removing excess oxide if necessary. Note: For best results, use the same quantity of ePbBi that was used to tune the PID controller.
	<ol>
		<li>Insert an ultrasound transducer into the battery vessel and tighten the swage connection to prevent leaks, ensuring that the furnace base is level.</li>
	</ol>
	</li>
	<li>Transfer molten metal to the battery vessel.
	<ol>
		<li>Place battery vessel on furnace base and leave it there for 5 min to gradually increase the temperature and avoid thermal shock. Prepare for a safe transfer by removing heat-sensitive equipment or materials from the area.</li>
		<li>Add small amounts of molten metal at a time, because thermal shock can damage the ultrasound transducer.</li>
		<li>Wait until the temperature reaches 150 &#176;C before beginning measurements, since sound speed depends on temperature.</li>
	</ol>
	</li>
	<li>Finish assembling the apparatus.
	<ol>
		<li>Place the furnace insulation around the battery vessel if it is not already there. Place the lid atop the battery vessel, along with the negative current collector and thermocouples. Be sure that all are positioned precisely and repeatably; shaft collars work well for this.</li>
		<li>Make all electrical connections for both power and signals, as shown in Figure 2B. Use an ohmmeter to verify that no unintended electrical paths are present, i.e., check that the electrical resistance between the negative current collector and all signal leads is at least 1 M&#937;.</li>
	</ol>
	</li>
	<li>Begin making measurements.
	<ol>
		<li>Begin logging and monitoring temperature, heater power, battery voltage, and battery current. Note: Here, a workstation running custom LabView code was used to log all measurements, with corresponding timestamps.</li>
		<li>Adjust the ultrasound instrument settings as necessary.
		<ol>
			<li>Be sure to set the sound speed, using the appropriate temperature, according to an accepted model19. For ePbBi at 150 &#176;C as used below, set the speed to 1,760 m/sec.</li>
			<li>Adjust the pulse repetition frequency such that echo depths are closely spaced (typically 0.25 mm).</li>
			<li>Adjust the gate count such that the strong echo from the far wall of the vessel appears in the last few gates; it provides a useful sanity check for troubleshooting signal strength issues.</li>
			<li>Using instructions provided by the manufacturer, set the instrument for hardware triggering.</li>
		</ol>
		</li>
		<li>Begin logging and monitoring velocity with the ultrasound instrument by initiating triggering from the workstation. Record four velocity profiles per second for 30 min.</li>
	</ol>
	</li>
	<li>Set the battery current to 5 A, wait 5 min for the flow to stabilize, and then record four velocity profiles per second for 30 min.</li>
	<li>Repeat step 3.5 for 10 A, 15 A, 20 A, 25 A, and 30 A. 
	Note: Many other experimental plans are also possible, including temperature variations and smooth changes in current. An atmosphere low in oxygen and moisture allows experiments with good signal quality for hours or more.</li>
	<li>Once the experiments are complete, stop logging data and turn off the furnace. Disconnect electrical connections and remove the furnace lid. Remove the molten metal from the battery vessel, using the same procedures for safe transfer that were used when filling the vessel. Store the molten ePbBi in a heat-tolerant container. Add extra argon to the glovebox; its pressure will drop as its atmosphere cools.</li>
</ol>

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Soc. 134, 1895-1897 (2012).</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&#8211;bismuth+alloys+determined+by+emf+measurements.">Thermodynamic properties of calcium&#8211;bismuth alloys determined by emf measurements.</a> Electrochim. Acta. 60, 154-162 (2012).</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&#8211;bismuth+electrodes+for+large-scale+energy+storage+(liquid+metal+batteries).">Calcium&#8211;bismuth electrodes for large-scale energy storage (liquid metal batteries).</a> J. Power Sources. 241, 239-248 (2013).</li><li>Cairns, E. J., Crouthamel, C. E., Foster, A. K., Foster, M. S., Hesson, J. 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The authors have nothing to disclose.

Ultrasound techniques can produce velocity measurements at hundreds or thousands of locations in a transparent or opaque fluid, many times per second. Applied to a liquid metal electrode, ultrasound techniques encounter challenges of high temperature, chemical activity, and electrical conductivity. The methods for overcoming those challenges and measuring flow in active liquid metal electrodes have been described. First, an electrode material subject to the same physics as high-temperature liquid metal battery electrodes...

Liquid metal batteries are a promising technology for providing large-scale energy storage on worldwide electrical grids1. These batteries offer high energy density, high power density, long cycle life, and low cost, making them ideal for grid-scale energy storage3. Introducing liquid metal batteries to the energy grid would allow peak shaving, improve grid stability, and enable much more widespread use of intermittent renewable sources like solar, wind, and tidal power. Liquid metal batteries are composed of two liquid metal electrodes separated by a molten salt electrolyte, as described in greater detail in prior work1. Though many different combinations of metals and electrolyte can result in a working liquid metal battery, the principles of operation remain the same. The metals are chosen such that it is energetically favorable for them to form an alloy; thus alloying discharges the battery, and de-alloying charges it. The salt layer is chosen so that it allows metal ions to pass between the two electrodes, but blocks transport of neutral species, thereby affording electrochemical control of the system.
This work will advance liquid metal battery technology by quantifying and controlling mass transport effects. The methods described here are informed by electrochemical methods developed for liquid metal batteries by Sadoway et al.1&#8211;4 as well as earlier liquid metal battery work at Argonne National Laboratory5,6, and the work of the broader electrochemical community (Bard and Faulkner7 provide many relevant references). The methods described here also build upon prior fluid dynamics studies. Ultrasound velocimetry was developed and first used in water8,9 and has since been applied to liquid metals including gallium10,11, sodium12,13, mercury14, lead-bismuth15, copper-tin15, and lead-lithium16, among others. Eckert et al. provide a useful review of velocimetry in liquid metals17.
Recent work using methods similar to the ones described here18 has shown that battery currents can enhance mass transport in liquid metal electrodes. Because mass transport in the positive electrode is the rate-limiting step in charge and discharge of liquid metal batteries, mixing therefore allows faster charge and discharge than would otherwise be possible. Moreover mixing prevents local inhomogeneities in the electrode, which can form solids that limit the cycle life of a battery. In ongoing work, we continue to study the role of fluid flow in the positive electrode of the liquid metal battery, which arises because of thermal and electromagnetic forces. Thermal gradients drive convective flow through buoyancy, and battery currents drive flow by interacting with the magnetic fields induced by the battery currents themselves. In experiments using the methods described below, we have observed flows with Reynolds number 50 &#60; Re &#60; 200, calculated from the electrode depth and root-mean-square velocity. A thorough experimental characterization is being undertaken and will use the resulting data set to build predictive battery models. The focus of this manuscript is on the experimental design and procedures required to produce such data. Ultrasound velocimetry provides the bulk of the measurements, and experimental conditions must be carefully controlled in order to use ultrasound successfully in liquid metal. High temperature, chemical activity, and electrical conductivity must all be managed carefully.
First, liquid metal batteries necessarily operate at high temperature, because both metals and the salt that separates them must be molten. One promising choice of materials, which uses lithium as the negative electrode, lead-antimony as the positive electrode, and a eutectic mix of lithium salts as the electrolyte, requires temperatures around 550 &#176;C. Measuring the flow of an opaque fluid at such high temperatures is quite difficult. High-temperature ultrasound transducers, which separate the delicate electro-acoustic components from the test fluid with an acoustic waveguide, have been demonstrated15 and commercialized. However, because the transducers have insertion loss near 40 dB, and because of the general difficulty of working at such temperatures, a surrogate system has been chosen for initial study: a liquid metal battery may also be made using sodium as the negative electrode, eutectic 44% lead 56% bismuth (hereafter, ePbBi) as the positive electrode, and a triple eutectic mix of sodium salts (10% sodium iodide, 38% sodium hydroxide, 52% sodium amide) as the electrolyte. Such a battery is entirely molten above 127 &#176;C, making it much more amenable to laboratory study. Because it is composed of three liquid layers separated by density, it is subject to the same physics as other liquid metal batteries. And it is compatible with readily available ultrasound transducers, which are rated to 230 &#176;C, involve no waveguide losses, and cost much less than high-temperature transducers. These experiments typically take place at 150 &#176;C. At that temperature, ePbBi has viscosity &#957; = 2.79 x 10-7 m2/sec, thermal diffusivity &#954; = 6.15 x 10-6 m2/sec, and magnetic diffusivity &#951; = 0.8591 m2/sec, such that its Prandtl number is Pr = &#957;/&#954; = 4.53 x 10-2 and its magnetic Prandtl number is Pm = &#957;/&#951; = 3.24 x 10-7.
Though this low-temperature liquid metal battery chemistry makes flow studies much easier than they would be in hotter batteries, temperature must nonetheless be managed carefully. Being delicate electro-acoustic devices, ultrasound transducers are susceptible to damage by thermal shock, and therefore must be heated gradually. High-quality ultrasound measurements also require careful temperature regulation. Ultrasound velocimetry works like sonar, as shown in Figure 1: the transducer emits a beep (here, the frequency is 8 MHz), then listens for echoes. By measuring the time of flight of the echo, the distance to the echoing body can be calculated, and by measuring the Doppler shift of the echo, one component of the body&#8217;s velocity can also be calculated. In water, tracer particles must be added to produce echoes, but no tracer particles are required in liquid metals, a fact that is not understood in detail but is typically attributed to the presence of small metal oxide particles. Each measurement is an average over all tracer particles in an interrogation volume; in this work, its minimum diameter is 2 mm, at a distance 30 mm from the probe. Though oxidation may eventually limit the duration of experiments, using the methods described below, we have made measurements continuously for as long as 8 hr.
Calculating either distance or velocity requires knowing the speed of sound in the test fluid, and that speed varies with temperature. The work described here focuses on flow in the ePbBi negative electrode, where the speed of sound is 1,766 m/sec at 150 &#176;C, 1,765 m/sec at 160 &#176;C, and 1,767 m/sec at 140 &#176;C 19. Thus inadequate temperature control would introduce systematic errors in the ultrasound measurements. A device was constructed to measure the speed of sound in ePbBi, finding values consistent with those published and accepted by the Nuclear Energy Agency19 (see below). Finally, since thermal convection is a primary driver of flow in liquid metal batteries, both the mean temperature and the temperature difference between the top and bottom of the ePbBi electrode directly affect observations. For consistent results, precise thermal control is essential.
Accordingly, temperature is measured continually with at least three K-type thermocouples, logging their measurements electronically with a computer-based acquisition device and a custom-written LabView program. The program also controls the power supply that provides battery current, via a USB connection; logs the battery current and voltage; and sends trigger pulses to the ultrasound instrument, so that its data can be synchronized with the other measurements. A system diagram is shown in Figure 2. Heat is provided by a custom-built furnace (also shown in Figure 2), which contains two 500-W industrial heating elements powered by a relay switched by a proportional-integral-differential (PID) controller. The base plate that supports battery cells is made of solid aluminum; because its thermal conductivity is an order of magnitude higher than the thermal conductivity of the stainless steel battery cell vessel and the ePbBi it contains19, the temperature of the furnace floor is approximately uniform. Moreover the aluminum base doubles as a path for the electrical currents passing through the electrode. Its electrical conductivity is also an order of magnitude higher than that of stainless steel or ePbBi, so the voltage of the furnace floor is also approximately uniform. Insulating legs separate the base from the bench top below, preventing burns and shorts. The sides of the battery vessel are insulated with silica ceramic insulation, cut to fit the vessel closely but leave room for accessing the cell&#8217;s ultrasound port. Finally, a polytetrafluoroethylene (PTFE) lid insulates the cell from above and holds the negative current collector and thermocouples in place. Though commercially-available hot plates can achieve the temperatures required for these experiments, our custom-built furnace maintains temperature with an order of magnitude less variation, and also allows us to measure heat power directly.
In addition to challenges associated with temperature, there are challenges associated with chemical activity. At 150 &#176;C, an ePbBi positive electrode is chemically compatible with many common materials. A sodium negative electrode, however, corrodes many materials, oxidizes readily, and reacts vigorously with moisture. A lithium negative electrode is also aggressive, especially because lithium-based liquid metal batteries typically run at much higher temperatures. Though those higher-temperature systems are outside the scope of this work, many of the same measures for managing chemical activity are used here as in those systems. All experiments described here take place in an argon-filled glovebox containing only trace amounts of oxygen or moisture. The battery vessel is made from alloy 304 stainless steel, which corrodes minimally even with lithium at 550 &#176;C. The thermocouples and negative current collector are also made from stainless steel. The vessel geometry is chosen to match vessels used for electrochemical testing of liquid metal batteries, to model as closely as possible the systems that are being commercialized. The vessel, shown in Figure 2, is cylindrical, with an 88.9 mm inner diameter and a 67 mm depth. All vessel walls are 6.4 mm thick. The vessel differs from those used for earlier experiments, however, in that it has an ultrasound port. The port passes through the side wall along a horizontal diameter of the cylinder, and the center of the port is 6.6 mm above the vessel floor. The port is 8 mm in diameter to accommodate an 8 mm ultrasound transducer, and seals around the transducer with a swage. In these experiments, the liquid metal electrode is just deep enough to cover the ultrasound transducer, typically 13 mm.
In order to achieve strong ultrasound signals, one requires good acoustic transmission between the ultrasound transducer and the fluid it probes (ePbBi). Maximum acoustic power is transmitted when the acoustic impedance of the transducer material and the test fluid are identical; when the impedances differ, signals suffer. Placing an ultrasound transducer in direct contact with clean ePbBi (as made possible by the port described above) provides ample signal, often for hours at a time. Metal oxides, however, have very different impedance, and may also interfere with wetting by altering the surface tension. If the ePbBi is substantially oxidized, ultrasound signals degrade and soon disappear. Again, an inert atmosphere is essential. If trace amounts of oxygen cause some oxidation nonetheless, the surface of the metal oxide is skimmed before transferring ePbBi into the battery vessel.
Finally, these experiments present challenges because of the presence of electrical currents. Though the currents are our central scientific and technological interest, they are large enough (30 A) to cause damage if incorrectly routed. Ungrounded thermocouples ensure that harmful electrical currents do not pass through the data acquisition device or the computer that supports it, because ungrounded thermocouples have no internal electrical connection from the protective sheath to either signal wire. Likewise it is essential to use ungrounded ultrasound transducers (Signal-Processing SA, TR0805LTH) to prevent stray current from damaging the valuable ultrasound instrument (Signal-Processing SA, DOP 3010). As mentioned previously, the base of the furnace serves to conduct electrical current, and must also be electrically isolated from its surroundings.
In the ePbBi electrode, current causes ohmic heating, potentially disrupting the temperature. Thus the automated thermal control system must be able to adjust to changes in heat input. Figure 3 shows how the temperature of the ePbBi electrode varies as current flows through it, and how the PID controller adjusts to compensate. Maintaining steady temperature with large currents (50 A = 800 mA/cm) would require additional cooling, but at the lower currents more realistic for liquid metal batteries in industrial applications (typically 17 A = 275 mA/cm 1), the controller is able to compensate for ohmic heating and hold temperature variation to 2 &#176;C.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>K Type Thermocouple Probe</td>
			<td>McMaster-Carr</td>
			<td>3856K83</td>
			<td><a href="http://www.mcmaster.com/">http://www.mcmaster.com/</a></td>
		</tr>
		<tr>
			<td>Red Lion PID Controller</td>
			<td>Red Lion</td>
			<td>P1610000</td>
			<td>http://store.redlion.net/store/p16.html</td>
		</tr>
		<tr>
			<td>Measurement Computing Data Acquisition Device</td>
			<td>Measurement Computing Corporation</td>
			<td>USB-2408</td>
			<td>http://www.mccdaq.com/index.aspx</td>
		</tr>
		<tr>
			<td>Power Supply</td>
			<td>TDK-Lambda</td>
			<td>GEN 8-90-USB-U</td>
			<td>http://us.tdk-lambda.com/hp/</td>
		</tr>
		<tr>
			<td>Ultrasound Instrument</td>
			<td>Signal Processing SA</td>
			<td>DOP3010</td>
			<td>http://www.signal-processing.com/index.html</td>
		</tr>
		<tr>
			<td>Ultrasound Transducer</td>
			<td>Signal Processing SA</td>
			<td>TR0805LTH</td>
			<td>http://www.signal-processing.com/index.html</td>
		</tr>
		<tr>
			<td>Bismuth-Lead Eutectic</td>
			<td>VWR</td>
			<td>AA40949-P2</td>
			<td>https://us.vwr.com/</td>
		</tr>
	</tbody>
</table>

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.

The procedure for measuring sound speed (described in detail above) was adapted from methods used by Signal-Processing SA. In principle, sound speed can easily be obtained by measuring the time of flight of an echo from a wall at known range. But precisely measuring the effective location of the transducer face is difficult, so instead one can measure time of flight twice, using a micrometer to displace the wall by a known distance between measurements. That displacement distance, and the difference in the measured time ...

Watch this Scientific Journal Video about Ultrasound Velocity Measurement in a Liquid Metal Electrode at JoVE.com

Ultrasound Velocity Measurement in a Liquid Metal Electrode

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 ...

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11.6K Views.  University of Rochester. 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.

Video: Ultrasound Velocity Measurement in a Liquid Metal Electrode

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

Development of a 3D Graphene Electrode Dielectrophoretic Device

The design and fabrication of a novel 3D electrode microdevice using 50 &#181;m thick graphene paper and 100 &#181;m double sided tape is described. The protocol details the procedures to construct a versatile, reusable, multiple layer, laminated dielectrophoresis chamber. Specifically, six layers of 50 &#181;m x&#160;0.7 cm x&#160;2 cm graphene paper and five layers of double sided tape were alternately stacked together, then clamped to a glass slide. Then a 700 &#956;m diameter micro-well was drilled through the laminated structure using a computer-controlled micro drilling machine. Insulating properties of the tape layer between adjacent graphene layers were assured by resistance tests. Silver conductive epoxy connected alternate layers of graphene paper and formed stable connections between the graphene paper and external copper wire electrodes. The finished device was then clamped and sealed to a glass slide. The electric field gradient was modeled within the multi-layer device. Dielectrophoretic behaviors of 6 &#956;m polystyrene beads were demonstrated in the 1 mm deep micro-well, with medium conductivities ranging from 0.0001 S/m to 1.3 S/m, and applied signal frequencies from 100 Hz to 10 MHz. Negative dielectrophoretic responses were observed in three dimensions over most of the conductivity-frequency space and cross-over frequency values are consistent with previously reported literature values. The device did not prevent AC electroosmosis and electrothermal flows, which occurred in the low and high frequency regions, respectively. The graphene paper utilized in this device is versatile and could subsequently function as a biosensor after dielectrophoretic characterizations are complete.

A microdevice with high throughput potential is used to demonstrate three-dimensional (3D) dielectrophoresis (DEP) with novel materials. Graphene nanoplatelet paper and double sided tape were alternately stacked; a 700 &#956;m micro-well was drilled transverse to the layers. DEP behavior of polystyrene beads was demonstrated in the micro-well.

The design and fabrication of a novel 3D electrode microdevice using 50 &#181;m thick graphene paper and 100 &#181;m double sided tape is described. The ...

Non-aqueous Electrode Processing and Construction of Lithium-ion Coin Cells

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational analysis. Testing of lithium-ion batteries in an academic setting has taken on several forms, but at the most basic level lies the coin cell construction. In traditional LIB electrode preparation, a multi-phase slurry composed of active material, binder, and conductive additive is cast out onto a substrate. An electrode disc can then be punched from the dried sheet and used in the construction of a coin cell for electrochemical evaluation. Utilization of the potential of the active material in a battery is critically dependent on the microstructure of the electrode, as an appropriate distribution of the primary components are crucial to ensuring optimal electrical conductivity, porosity, and tortuosity, such that electrochemical and transport interaction is optimized. Processing steps ranging from the combination of dry powder, wet mixing, and drying can all critically affect multi-phase interactions that influence the microstructure formation. Electrochemical probing necessitates the construction of electrodes and coin cells with the utmost care and precision. This paper aims at providing a step-by-step guide of non-aqueous electrode processing and coin cell construction for lithium-ion batteries within an academic setting and with emphasis on deciphering the influence of drying and calendaring.

Non-aqueous electrode processing is central to the construction of coin cells and the evaluation of new electrode chemistries for lithium-ion batteries. A step-by-step guide to the basic practices needed as an electrochemical engineer working with batteries in an academic experimental setting is furnished.

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational ...

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.

Optimized sampling protocols and the development of new wipe materials can be facilitated by standardized measurements of collection efficiency from wipe-sampling. Our approach for sampling trace explosives uses an automated device to control speed, force, and distance during wipe-sampling followed by extraction of collected explosives.

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common ...

Three-electrode Coin Cell Preparation and Electrodeposition Analytics for Lithium-ion Batteries

As lithium-ion batteries find use in high energy and power applications, such as in electric and hybrid-electric vehicles, monitoring the degradation and subsequent safety issues becomes increasingly important. In a Li-ion cell setup, the voltage measurement across the positive and negative terminals inherently includes the effect of the cathode and anode which are coupled and sum to the total cell performance. Accordingly, the ability to monitor the degradation aspects associated with a specific electrode is extremely difficult because the electrodes are fundamentally coupled. A three-electrode setup can overcome this problem. By introducing a third (reference) electrode, the influence of each electrode can be decoupled, and the electrochemical properties can be measured independently. The reference electrode (RE) must have a stable potential that can then be calibrated against a known reference, for example, lithium metal. The three-electrode cell can be used to run electrochemical tests such as cycling, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). Three-electrode cell EIS measurements can elucidate the contribution of individual electrode impedance to the full cell. In addition, monitoring the anode potential allows the detection of electrodeposition due to lithium plating, which can cause safety concerns. This is especially important for the fast charging of Li-ion batteries in electric vehicles. In order to monitor and characterize the safety and degradation aspects of an electrochemical cell, a three-electrode setup can prove invaluable. This paper aims to provide a guide to constructing a three-electrode coin cell setup using the 2032-coin cell architecture, which is easy to produce, reliable, and cost-effective.

Three-electrode cells are useful in studying the electrochemistry of lithium-ion batteries. Such an electrochemical setup allows the phenomena associated with the cathode and anode to be decoupled and examined independently. Here, we present a guide for construction and use of a three-electrode coin cell with emphasis on lithium plating analytics.

As lithium-ion batteries find use in high energy and power applications, such as in electric and hybrid-electric vehicles, monitoring the degradation and ...

Method Development for Contactless Resonant Cavity Dielectric Spectroscopic Studies of Cellulosic Paper

The current analytical techniques for characterizing printing and graphic arts substrates are largely ex situ and destructive. This limits the amount of data that can be obtained from an individual sample and renders it difficult to produce statistically relevant data for unique and rare materials. Resonant cavity dielectric spectroscopy is a non-destructive, contactless technique which can simultaneously interrogate both sides of a sheeted material and provide measurements which are suitable for statistical interpretations. This offers analysts the ability to quickly discriminate between sheeted materials based on composition and storage history. In this methodology article, we demonstrate how contactless resonant cavity dielectric spectroscopy may be used to differentiate between paper analytes of varying fiber species compositions, to determine the relative age of the paper, and to detect and quantify the amount of post-consumer waste (PCW) recycled fiber content in manufactured office paper.

A protocol for the non-destructive analysis of the fiber content and relative age of paper.

The current analytical techniques for characterizing printing and graphic arts substrates are largely ex situ and destructive. This limits the amount of ...