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Hydrogen produced through low-temperature water electrolysis using anion exchange membranes (AEM) combines the benefits of liquid-electrolyte alkaline electrolysis and solid-polymer proton exchange membrane electrolysis. The anion conductive ionomers in the oxygen-producing anode and hydrogen-producing cathode are a critical part of the three-dimensional electrodes. The ionomer in the hydrogen-producing cathode facilitates hydroxide ion conduction from the cathode catalyst to the anode catalyst, and water transport from the anode to the cathode catalyst through the AEM. This ionomer also binds the catalyst particles to the porous transport layer. In this study, the cathode durability was improved by use of a self-adhesive cathode ionomer to chemically bond the cathode catalyst particles to the porous transport layer. It was found that the cathode ionomers with high ion exchange capacity (IEC) were more effective than low IEC ionomers because of the need to transport water to the cathode catalyst and transport hydroxide away from the cathode. The cathode durability was improved by using ionomers which were soluble in the spray-coated cathode ink. Optimization of the catalyst and ionomer content within the cathode led to electrolysis cells which were both mechanically durable and operated at low voltage.

Alkaline fuel cells and electrolyzers are of interest because they have potential advantages over their acid counterparts. High-conductivity anion conducting membranes were analyzed and used in alkaline hydrogen/oxygen fuel cells. The membranes were composed of reinforced block copolymers of poly(norbornenes) with pendant quaternary ammonium head-groups. It was found that membranes with light cross-linking provided excellent mechanical stability and allowed very high ion exchange capacity polymers to be used without penalty of excessive water uptake and swelling. The optimum membrane and fuel cell operating conditions were able to achieve a peak power density of 3.4 W/cm2 using hydrogen and oxygen. The performance increase was greater than expected from minimizing ohmic losses. Mechanical deformations within the membrane due to excess water uptake can disrupt full cell operation. Cells were also run for over 500 h under load with no change in the membrane resistance and minimal loss of operating voltage.

Anion exchange membrane water electrolyzers (AEMELs) have recently received significant attention due to their potential advantages over proton exchange membrane electrolyzers (PEMELs). However, some AEMELs feed an aqueous salt solution to the cell where PEMELs typically feed deionized (DI) water. DI water is preferred to keep the system and maintenance costs low. Because of this, many AEMEL researchers report performance both in the salt solution (typically KOH) and DI water. However, the methodology for switching between KOH and DI water is often poorly defined, and it is unclear what impact the residual salt has on cell performance after switching from salt to DI water. Having a fully deionized environment is important because the presence of salts in the water feed increase the effective electrochemical surface area of the catalyst in the three-dimensional electrode and residual salt remaining after switching to DI water feed can have a misleading transient effect on cell performance. This paper focuses on understanding the transition from KOH to DI water testing in AEMELs. It is shown that when switching from salt to DI water feed, a large volume of DI water must be fed over several hours to achieve true DI-water performance. It is also shown that starting AEMELs from the beginning with DI water feed (without any KOH ever being fed to the cell) results in better cell durability. Lastly, a cell is demonstrated having operated exclusively on DI water at 1.0 A cm−2 for 500 h at an operating voltage of ca. 2 V and a low degradation rate.

Anion exchange membrane fuel cells (AEMFCs) have been widely touted as a low-cost alternative to existing proton exchange membrane fuel cells. However, AEMFCs operating on air suffer from a severe performance penalty caused by carbonation from exposure to CO2. Many approaches to removing CO2from the cathode inlet would consume valuable energy and complicate the systems-level balance-of-plant. Therefore, this work focuses on an electrochemical solution where CO2removal would still generate power, but not expose an entire AEMFC stack to carbonation conditions. Such a system consists of two AEMFCs in series. The first AEMFC, which acts as an anion exchange CO2separator (AECS), is relatively small and serves to scrub CO2from the air. The AECS is powered by the hydrogen bleed from the second (i.e., main) AEMFC. A small amount of hydrogen is bled from the recycled hydrogen used in the main AEMFC to mitigate impurity build-up, including nitrogen gas from diffusion across its membrane. The second, main AEMFC operates on the purified air output from the AECS and fresh H2. This work shows that it is possible to use an AECS to lower the CO2concentration in the AEMFC input air stream to values low enough that the main AEMFC can be operated stably for extended periods, 150 h in this demonstration. Also, in this study, AEMFCs are operated on AECS-purified air without experiencing a performance penalty. Lastly, the relative geometric active area of the AEMFC and AECS devices are evaluated and discussed.

Green hydrogen produced through anion exchange membrane water electrolysis is a promising, low-cost chemical storage solution for intermittent renewable energy sources. Low-temperature electrolysis using anion exchange membranes (AEM) combines the benefits of established water electrolysis technologies based on alkaline electrolysis and proton exchange membrane electrolysis. The anion conductive ionomers (ACI) used in the AEM electrolyzer (AEMEL) electrodes has been investigated. The ACI serves two primary purposes: (i) facilitate hydroxide conduction between the catalyst and bulk electrolyte and (ii) bind the catalyst to the porous transport layer and membrane. High ion exchange capacity (IEC) ACIs are desired, however, high IEC can cause excessive water uptake (WU) and detrimental ACI swelling. Proper water management is a key factor in obtaining maximum performance in AEM-based devices. In this study, a series of poly(norbornene)-based ACIs were synthesized and deployed in hydrogen evolving AEMEL cathode electrodes. A balance between ionic conductivity, WU and ionomer swelling was achieved in the ACI by varying the IEC and degree of polymer cross-linking. It was found that higher IEC ACIs with light crosslinking are preferred in the HER electrode. Such a configuration fine-tuned the WU and ionomer swelling to achieve optimum cell performance and reduce cell operating voltages.

Water electrolysis using an anion conductive, solid polymer electrolyte is an attractive method for point-of-use hydrogen production. Recent advances in catalysts and anion exchange membranes (AEM) have made alkaline devices increasingly competitive with their acidic counterparts. However, less attention has been paid to the anion conductive ionomers (ACI) used in the fabrication of electrodes for AEM electrolyzers. The ACI contributes to ion conduction between the catalyst and bulk electrolyte and serves as a binder for adhering the catalyst to the gas diffusion layer and AEM. Ionic conductivity, water uptake and ionomer swelling are critical properties for electrode performance. High ion exchange capacity (IEC) in the ionomer is desired for reduced electrode resistance, however, it can lead to excess water uptake (WU) and disruptive ACI swelling. In this study, a series of poly(norbornene)-based ionomers were synthesized, characterized and used to fabricate oxygen evolving anodes for low-temperature AEM water electrolysis. The IEC of the ionomers (0 to 4.73 meq g−1) was adjusted by controlling the ratio of ion conducting to non-ion conducting norbornene monomers in the ACI tetrablock copolymers. Low conductivity ionomers are shown to yield the best-performing oxygen evolution electrodes, in the absence of ACI polymer cross-linking because they do not experience excessive water swelling. Light cross-linking within the anode ACI was used as a means to independently lower WU of the ionomer without compromising ionic conductivity. This control over water swelling allows higher ionic conductivity within the ACI to be used in water-fed electrolyzer applications. Other methods of water management were compared including the use of hydrophobic additives and adjustment of the ionomer concentration in the electrode. It was shown that the cell performance greatly benefits from a highly conductive ionomer in the oxygen evolution reaction electrode if the WU is managed.

Solid polymer electrolytes (sPE) offer a pathway for safer, less flammable lithium batteries. However, developing a polymer that provides high Li+ mobility as well electrochemical stability remains a challenge, because ion conductive functional units in the polymer main-chain (e.g., polycarbonate and polyether) usually suffer from poor electrochemical stability at high and low potentials. Herein, an sPE with pendent carbonate on a hydrocarbon backbone has been designed and synthesized to overcome conductivity and electrochemical stability problems. This pendant polycarbonate is different from conventional polycarbonate electrolytes because the carbonate moiety is in the sidechain, which mitigates polycarbonate backbone stability problems while still providing high ionic conductivity when used with a plasticizer Conductivity as high as 1.1 mS/cm at 22oC was obtained. Stable lithium metal plating using and stripping using the sPE was observed for 1,200 hours and electrochemical stability up to 4.6 V vs Li+/Li has been demonstrated. The low interfacial resistance (

High ionic conductivity membranes can be used to minimize ohmic losses in electrochemical devices such as fuel cells, flow batteries, and electrolyzers. Very high hydroxide conductivity was achieved through the synthesis of a norbornene-based tetrablock copolymer with an ion-exchange capacity of 3.88 meq/g. The membranes were cast with a thin polymer reinforcement layer and lightly cross-linked with N,N,N',N'-tetramethyl-1,6-hexanediamine. The norbornene polymer had a hydroxide conductivity of 212 mS/cm at 80°C. Light cross-linking helped to control the water uptake and provide mechanical stability while balancing the bound (i.e. waters of hydration) vs. free water in the films. The films showed excellent chemical stability with

Lithium dendrites are needle-like structures that form during the electrodeposition of lithium metal. These whiskers complicate the use of lithium metal as an anode in lithium batteries because they can puncture the separator and short circuit the battery. In addition, the large surface area and poor adhesion of the deposit contributes to loss of coulombic efficiency. The effect of alkali and alkaline earth metal ions on the morphology of electrodeposited lithium metal has been studied. Varying concentrations of alkali and alkaline earth metal ions were added to a 1 M lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) trimethylbutylammonium bis(trifluoromethanesulfonyl)imide (N1114-TFSI) electrolyte. Lithium metal was electrodeposited from each electrolyte and examined ex-situ by scanning electron microscopy (SEM). Alkali metal ions, with the exception of sodium, had little or no effect on the deposited lithium morphology. However, alkaline earth metal ions at 0.05 M concentration significantly reduced the occurrence of dendrites. When the concentration of the alkaline earth metal ions was increased to 0.1 M, dendrites were completely eliminated and lithium was deposited in a sphere-like morphology. Energy dispersive X-ray spectroscopy (EDX) showed that no alkaline earth metals were found in the sphere-like deposits, suggesting that dendrite mitigation occurred through an adsorption mechanism. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0301409jes] All rights reserved.

The properties of the ionomer used to construct electrodes for direct methanol anionic fuel cells are critically important to the fuel cell performance. In this study, a new polymer backbone with a higher degree of fluorination for increased hydrophobicity has been shown to improve both the alkaline anode and cathode performance. It was also shown that decreasing the molecular weight of the ionomer improves fuel cell performance. Finally, higher fuel cell performance was observed with quinuclidinium head groups on the anionic ionomer, in comparison to the more traditional trimethyl ammonium cation. The improvements in performance appear to be due to improved mass transport of reactants and products through the electrode as a result of increased free volume within the electrode and a more efficient construction of the electrical double layer at the ionomer/catalyst interface.

Higher energy density batteries are desired, especially for mobile electronic devices. Lithium metal anodes are a possible route to achieving high energy and power density due to their light weight compared to current graphite anodes. However, whisker growth during lithium electrodeposition (i.e. charging) represents a serious safety and efficiency concern for both lithium metal batteries and overcharging of graphite anodes in lithium-ion batteries. The initial morphology of deposited lithium nuclei can have a significant impact on the bulk material deposited. The nucleation of lithium metal from an organic ethylene carbonate: dimethyl carbonate (EC:DMC) and an ionic liquid (trimethylbutylammonium bis(triflouromethanesulfonyl)imide) electrolyte has been studied. Whisker extrusion and tip-based dendrite growth was observed ex-situ, and confirmed by in-situ optical microscopy experiments. The nucleation of a non-dendritic sodium co-deposit is also discussed. A model based on nuclei geometry is provided which gives insight into the deposition rate at constant overpotential.

Electroless copper deposition was investigated on epoxy laminate substrates (Isola 185HR) using a silver-based catalyst, and a nonroughening surface treatment method based on sulfuric acid. The current challenges in electroless copper deposition include (i) high cost of Pd-based catalysts, (ii) deterioration of electrical performance of deposited metal at high frequency due to electron scattering at the roughened surface, and (iii) limited adhesion strength of electroless layers to substrates. We investigated an electroless copper deposition procedure composed of a H2SO4 surface pretreatment, two-step Sn/Ag nano-colloidal catalyst seeding, and immersion in a traditional formaldehyde-containing electroless copper bath. The H2SO4 pretreatment activated the epoxy surface for electroless deposition. Other strong acids did not lead to deposition. The H2SO4 treatment cleaned the substrate and provided the adhesion of the catalyst and electroless copper without increasing the surface roughness. XPS results showed a decrease in the carbonyl groups (C=O), and acid/ester functionalities (O-C=O) at the surface. Adsorbed sulfate on the substrate from the H2SO4 treatment led to Sn(II) sensitization. The tin-silver activation step resulted in Sn(IV) and Ag(0) products in the form of a Sn/Ag nano-colloidal catalyst. The Sn/Ag colloid acted as a catalyst for electroless copper deposition on the epoxy laminate substrates. © 2013 The Electrochemical Society. [DOI: 10.1149/2.039312jes] All rights reserved.

The effect of bis-(3-sulfopropyl)-disulfide (SPS) on the void-free electroless pillar-to-pillar bonding process has been investigated. Two dome-shaped Cu pillars were joined using electroless copper deposition with the addition of a suppressor to achieve solid, compliant Cu-to-Cu bonding without high temperature or pressure. SPS was added to the electroless copper plating bath which has strong suppression to the electroless plating in order to avoid the creation of an unbonded seam between the two Cu structures being bonded. The bath suppresses the deposition of copper near the entrance of the gap between the two pillars, while allowing high deposition rate in the geometrically restricted area between the copper structures being bonded. This phenomenon is due to diffusion of SPS through the narrow gap between the pillars being bonded. At adequate concentration of SPS, the two pillars were successfully joined without any remaining seam between the joined structures, by growth of copper from the center of the gap to the outside. a fillet of material between the two components being joined so that non-planar surfaces can be joined without having to be made flat or co-planar. Excessive force would have to be applied to copper pillars, or other I/O structures if direct copper-copper bonding were used to join components. The use of electroless deposition of metal is an alternate approach to flip-chip bonding. 9-11 The yield strength of the electroless bond- ing was high enough to successfully bond pillars together; however, defects or voids in the electroless metal between mated pillars is an issue. 12 Trapped voids could be an origin of various failure modes, such as rupture during thermally induced stress, electromigration due to current crowding around the voids, and high-frequency electrical noise. Voids can be created during the joining of pillars due to in- adequate plating in the narrow gap between the two pillars surfaces. For example, pillar-to-pillar gaps as wide as 5 to 20 μm may be en- countered in flip-chip bonding. The formation of dome-shaped pillars to reduce the initial gap between the two surfaces and the use of a surfactant to stimulate plating in the narrow gap were suggested to im- prove the bonding process. 13 These actions provide better deposition of the fillet between pillars compared to flat-top pillars with unmod- ified plating baths. 13 However, there remain fundamental issues in bonding large diameter pillars due to void trapping in the electroless metal joining the pillars. If the deposition rate difference in the gap at the center of the pillars being joined is less than that of the mass transfer preferred region at the edge of the pillars, then voids can be created. Thus, it is highly desirable to control of the deposition rate of the electroless bath to suppress deposition at the spatially favored regions, such as the pillar edge, so as to maintain access to the center of the pillar as the gap is filled. Organic additives can be added to electroless baths to suppress the deposition rate. Trench filling in geometrically restricted areas has been accom- plished in copper electrodeposition through the use of accelerators act- ing at the bottom of a trench with suppression at the top of the trench. A similar approach was taken in filling through silicon vias (TSV) by use of levelers. It is of interest to extend this concept to electroless copper deposition, such as used to fill trenches and bonding of pillars.

In this study, the electroless deposition of copper and silver was investigated on epoxy and silicon dioxide-based substrates. A cost-efficient, Sn/Ag catalyst was investigated as a replacement for the Sn/Pd catalyst currently used in board technology. The surface of the epoxy based polyhedral oligomeric silsesquioxane (POSS) films was modified by plasma and chemical etching for electroless activation without the creation of a roughened surface. The electroless copper deposited on the modified POSS surface exhibited excellent adhesion when annealed at 180 ◦ C in nitrogen for 90 min or at room temperature for 24 hr. Electroless copper deposition was also demonstrated on oxidized silicon wafers for through silicon via sidewall deposition.

Photoelectrochemical etching of silicon in nonaqueous solutions without the use of free-fluoride or HF has been demonstrated. Silicon was electrochemically oxidized and dissolved using stable, fluoride containing salts in acetonitrile solutions. The current-voltage behavior of silicon in acetonitrile solutions containing tetrabutylammonium tetrafluoroborate or tetrabutylammonium hexafluorophosphate were similar. The voltammetry of silicon in solutions containing tetrabutylammonium trifluoromethylsulfonyl, potassium hexafluoroarsenate, or sodium hexafluoroantimonate showed large hysteresis indicating the presence of silicon oxide on the surface. Although the dissolution rate of silicon dioxide was negligible in the absence of free-fluoride or HF, the oxidation and dissolution of silicon could be maintained, even when thin oxides were formed. Photocurrent oscillations associated with the buildup and removal of oxides were observed when traces of water were present.

The electrochemical behavior and surface chemistry of anodic silicon etching in nonaqueous electrolytes was studied. Etching of single‐crystal p‐type and n‐type (100) silicon was carried out in acetonitrile and propylene carbonate with hydrofluoric acid (HF) or tetrafluoroborate providing fluoride to complex the oxidized silicon. Electrolytes containing HF resulted in tetravalent dissolution, and photocurrent quadrupling was observed. Electrolytes containing also resulted in tetravalent dissolution; however, calculated quantum efficiencies were lower depending upon the electrolyte. Current‐voltage behavior indicates the presence of surface states which affect both the onset potential for oxidation and the current multiplication. In situ multiple internal reflection Fourier transform infrared analysis confirms that silicon surfaces etched in electrolytes containing HF remain hydride‐terminated throughout etching; however, silicon etched in based electrolytes loses the initial hydride termination at the onset of etching. © 1999 The Electrochemical Society. All rights reserved.

Sodium hydrosulfite is a major commodity reagent currently manufactured mainly by chemical means. A new, lower cost, electrochemical pathway is being used in some cases. One of the key determinants in the marketability of sodium hydrosulfite is the level of sodium thiosulfate in the product. At present, the level of thiosulfate from the electrochemical process is higher than that from the chemical process. Here a critical evaluation is presented of cathode materials for the electrochemical manufacture of sodium hydrosulfite nearly devoid of thiosulfate. It is demonstrated that the yield, selectivity, and efficiency of hydrosulfite and thiosulfate production depends upon the applied potential, pH of the feedstream, buffering capacity, and cathode material. Several planar cathodes for the electrosynthesis of hydrosulfite have been critically evaluated. We demonstrate that five main process parameters contribute to high hydrosulfite to thiosulfate ratios; NaHSO 3 reactant concentration, feedstream pH, buffer capacity, cathode material, and applied potential. By control of these parameters, we demonstrate that hydrosulfite solutions devoid of thiosulfate can be electrochemically produced.

The electrolytic manufacture of sodium hydrosulfite is affected by feedstock composition, electrode morphology, and electrode material. Here we show that porous graphite cathodes can provide the highest one-pass conversion of sodium bisulfite to hydrosulfite, without contamination by sodium thiosulfate. For example, at current densities up to 0.90 kA/m 2 , thiosulfate-free hydrosulfite can be produced at 66% current efficiency. At higher current densities, both hydrogen and thiosulfate are produced. In acetate-buffered bisulfite, these rise to 1.35 kA/m 2 at nearly 100% current efficiency. In unbuffered bisulfite feedstocks, variations in product quality depend solely on cathode composition. That is, the electrode kinetics for each of the reductions are a strong function of the electrode material. However, we show that the rate of bisulfite consumption is controlled by diffusional mass transfer in the porous electrode and can be predicted through use of a simple packed-bed, plug-flow reactor model.

Sodium hydrosulfite is an important industrial reducing agent widely used in the textile, clay, and pulp and paper industries. Electrochemically produced sodium hydrosulfite contains levels of sodium thiosulfate contamination that are of concern for many applications. In this report, a novel online process monitor specifically designed for use under the conditions of industrial scale, electrolytic sodium hydrosulfite manufacture is described. This monitor enables the determination of (i) hydrosulfite under acidic conditions, (ii) hydrosulfite in the presence of a large excess of bisulfite, and (iii) trace quantities of thiosulfate in the presence of a thousand-fold excess of hydrosulfite. Analyte concentrations are determined by square wave voltammetry in a flowing stream.

Porous silicon samples were prepared by anodizing p-doped Si(100) substrates in both aqueous (HF/H 2 O, HF/CH 3 OH, HF/CH 3 OH/H 2 O, HF/C 2 H 5 OH/H 2 O) and nonaqueous (MeCN/HF) media. The time-dependent porous silicon photoluminescence (PL) was monitored during the etch (in situ) and after removal from the etch solution (ex situ). Correlation of the ex situ and in situ PL indicates that the composition of the etchant solution plays an extremely important role in the onset, time-dependent intensity, and lifetime of the emission, both in and out O solution. The effect of etchant solution additives (ethylene glycol, CH 3 OH, C 2 H 5 OH, NaF, HCl, and NaCl) on the porous silicon PL both during and following the etching cycle, was also determined. The distinct and different correlations found between aqueous and nonaqueous etchants provide insights into the mechanism of PL. These results, when considered in the context of quantum chemical modeling, strongly suggest surface-bound silicon oxyhydride moieties as the source of the porous silicon PL.

Silyl ether modified polynorbornene is a new class of dielectric polymer materials of interest in electronic packaging and other applications. This work is focused on the reactive ion etching of polynorbornenes in oxygen, oxygen/fluoroform, and oxygen/fluoroform/argon plasmas. The etch rate, amount and nature of the residue, and etch undercut rate have been investigated for different plasma parameters, such as pressure, plasma power, and gas composition. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the residue. Several techniques were used to reduce the etch residue and undercut.

The electrochemical reduction of gold thiosulfate has been studied and compared to the reduction of gold cyanide. Gold thiosulfate is a potential replacement for gold cyanide in electro and electroless plating baths. Gold thiosulfate has a more positive reduction potential than gold cyanide and eliminates the use of cyanide. The standard heterogeneous rate constant, transfer coefficient, and diffusion coefficient for gold thiosulfate reduction were found to be 1.58 x 10 -3 cm/s, 0.23 and 7 x 10 -6 cm 2 /s, respectively. The effect of sulfite as an additive to gold thiosulfate solutions was examined.

Room temperature molten salts consisting of 1-methyl-3-propylimidazolium chloride and aluminum chloride have been examined as possible electrolytes for a room temperature design of the sodium/metal chloride battery; however, the coulombic efficiency of the sodium couple is less than 95%. This work examines the reduction and oxidation efficiency of the sodium couple from a 1-methyl-3-propylimidazolium chloride/aluminum chloride neutral melt. Most of the work was performed on a tungsten substrate using cyclic voltammetry. The coulombic efficiency of the sodium couple was improved by treating the melt with gaseous HCl using a closed electrochemical cell which allowed for quantification of the effect on HCl on the electrochemical behavior of sodium. Thionyl chloride was also found to induce sodium plating and stripping in 1-methyl-3-propylimidazolium chloride/aluminum chloride melts. Optical microscopy was used to examine the surface of the tungsten electrode during sodium deposition, open-circuit periods, and sodium stripping. In comparison to the stability of sodium in two other imidazolium melts, (1,2-dimethyl-3-propylimidazolium chloride and 1-methyl-2-ethylimidazolium chloride) the 1-methyl-3-propylimidazolium chloride system was found to have the widest stability window.

Room temperature molten salts consisting of 1-methyl-3-ethylimidazolium chloride (MEIC) and aluminium chloride (AlCl{sub 3}) have been examined as possible electrolytes for a room temperature design of the sodium/iron(II) chloride battery. This work examines the conditions required to achieve efficient reduction and oxidation of sodium from a sodium chloride buffered, neutral melt. Two substrates were examined, tungsten and 303 stainless steel, using both cyclic voltammetry and chronopotentiometry. Melts were protonated using a closed electrochemical cell to allow quantification of the effect of dissolved HCl on the efficiency of the sodium couple. A threshold of approximately 6 Torr HCl partial pressure was observed for sodium plating-stripping. Below this threshold, the sodium couple was not observed. The results, show that the sodium plating-stripping efficiency increases with increasing current density; however, the efficiency reaches a maximum and is adversely affected by high over potentials and extended exposure of the sodium to the melt. It appears that some passivation occurs as even a very thin layer of plated sodium exhibits a steady open-circuit voltage over long periods in the melt.