REFERENCE ELECTRODES FOR MOLTEN SALT SYSTEMS, AND RELATED METHODS AND ELECTROCHEMICAL SYSTEMS

Abstract

Various embodiments of the disclosure provide reference electrodes for use in electrochemical systems (e.g., electrochemical cells) that use molten salt media as the electrolyte of choice. The reference electrodes include a metal core with an outer, solid layer of the metal's oxide, silicide, or carbide. The oxide, silicide, or carbide outer layer may be formed uniformly and with sufficient durability to withstand exposure to molten salt material. The outer layer may be formed by processes configured to form (e.g., grow) the oxide, silicide, or carbide layer directly on the outer surface of the metal core with uniformity of the layer's composition and thickness all along the outer surface of the metal core. Related electrochemical systems are also disclosed.

Claims

1. A reference electrode for a molten salt system, the reference electrode comprising: a metal core; and a metal-based compound layer along an outer surface of the metal core, the metal-based compound layer comprising an oxide, a silicide, or a carbide of the metal of the metal core, wherein the metal core and the metal-based compound layer are each solid.

2. The reference electrode of claim 1, wherein the metal comprises at least one of nickel, aluminum, tin, titanium, tungsten, hafnium, zirconium, niobium, iridium, platinum, ruthenium, palladium, rhenium, scandium, or calcium.

3. The reference electrode of claim 1, wherein the metal-based compound layer defines a uniform thickness on the outer surface of the metal core.

4. The reference electrode of claim 1, wherein the reference electrode is wholly solid.

5. The reference electrode of claim 1, wherein the reference electrode does not include a liquid ionic solution.

6. The reference electrode of claim 1, wherein the reference electrode does not include a powder.

7. The reference electrode of claim 1, wherein the metal-based compound defines a thickness along the outer surface of the metal core of less than about 500 m.

8. A method for fabricating a reference electrode for molten salt systems, the method comprising: growing a metal-based compound on an outer surface of a metal structure to form an oxide, silicide, or carbide layer uniformly coating the outer surface.

9. The method of claim 8, wherein growing the metal-based compound comprises growing the oxide layer uniformly coating the outer surface.

10. The method of claim 9, further comprising further comprising annealing the oxide layer.

11. The method of claim 9, wherein growing the oxide layer comprises growing the oxide on the outer surface of the metal structure by thermal oxidation.

12. The method of claim 9, wherein growing the oxide layer comprises growing the oxide on the outer surface of the metal by anodization.

13. The method of claim 10, wherein annealing the oxide layer comprises annealing the oxide layer for more than one hour.

14. The method of claim 10, wherein annealing the oxide layer comprises annealing the oxide layer for more than five hours.

15. An electrochemical system, comprising: an electrolyte comprising a molten salt media; and a reference electrode comprising a metal-oxide layer, a metal-silicide layer, or a metal-carbide layer on a metal core.

16. The electrochemical system of claim 15, wherein the molten salt media comprises one or more of a chloride-based molten salt media, a fluoride-based molten salt media, or a bromide-based molten salt media.

17. The electrochemical system of claim 16, wherein the molten salt media comprises a eutectic LiClKCl molten salt media.

18. The electrochemical system of claim 15, wherein: the reference electrode comprises the metal-oxide layer, the metal-oxide layer comprises NiO, and the metal core comprises Ni.

19. The electrochemical system of claim 15, wherein: the reference electrode comprises the metal-oxide layer, and the metal-oxide layer comprises TiO.sub.2 or TiO.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a top and front, perspective, schematic illustration of a reference electrode, in accordance with embodiments of the disclosure, wherein the reference electrode has a rod-like shape.

[0023] FIG. 2 is a top plan, cross-sectional, schematic illustration of a reference electrodesuch as the reference electrode of FIG. 1in accordance with embodiments of the disclosure, wherein the view of FIG. 2 is taken along section line A-A of FIG. 1.

[0024] FIG. 3 is a top, front, and right-side, isometric, schematic illustration of a reference electrode, in accordance with embodiments of the disclosure, wherein the reference electrode has a plate-like shape.

[0025] FIG. 4 is a top plan, cross-sectional, schematic illustration of a reference electrodesuch as the reference electrode of FIG. 3in accordance with embodiments of the disclosure, wherein the view of FIG. 4 is taken along section line B-B of FIG. 3.

[0026] FIG. 5 is a right-side, elevational, schematic illustration of a reference electrodesuch as the reference electrode of FIG. 1 or of FIG. 3in accordance with embodiments of the disclosure, wherein the view of FIG. 2 and/or of FIG. 4 may be taken along section line B-B of FIG. 5.

[0027] FIG. 6 is a top, front, and right-side, isometric illustration of a reference electrode, in accordance with embodiments of the disclosure, wherein the reference electrode has a rod-like shape in an upper portion and a plate-like shape in a lower portion, such that the view of FIG. 2 may be taken along section line A-A of FIG. 6 and the view of FIG. 4 may be taken along section line B-B of FIG. 6.

[0028] FIG. 7 is a right-side, elevational, schematic illustration of a reference electrodesuch as the reference electrode of FIG. 6in accordance with embodiments of the disclosure, such that the view of FIG. 2 may be taken along section line A-A of FIG. 7 and the view of FIG. 4 may be taken along section line B-B of FIG. 7.

[0029] FIG. 8 is a simplified, elevational, schematic illustration of an electrochemical system, in accordance with embodiments of the disclosure, in which a reference electrodesuch as any of the reference electrodes of FIG. 1 through FIG. 17may be used.

[0030] FIG. 9A through FIG. 13 relate to Examples, discussed below, in accordance with embodiments of the disclosure, wherein:

[0031] FIG. 9A is a photograph showing an un-oxidized (as-received) 1 mm nickel wire.

[0032] FIG. 9B is a photograph showing the wire of FIG. 9A with a change in the coloration of the surface nickel after thermal oxidation (in static air) at 600 C. for a total duration of eight hours and subsequently annealing the partially oxidized wire in argon at 700 C. for about six hours.

[0033] FIG. 10 is a chart showing the measurementusing the thermally oxidized and annealed nickel wire of FIG. 9Bof the open-circuit potential (OCP) for different durations and/or cycles in a eutectic LiClKCl electrolyte.

[0034] FIG. 11 is a chart showing cyclic voltammetry measurementsusing the thermally oxidized and annealed nickel wire of FIG. 9B, post the OCP measurements for FIG. 10in the eutectic LiClKCl melt (e.g., electrolyte) containing 2 wt. % YCl.sub.3 and 1 wt. % ScCl.sub.3 at a temperature (of the LiClKCl melt) of 500 C.

[0035] FIG. 12A is a photograph of a titanium wire anodized, at 9.0V, to have a surface TiO.sub.2 layer.

[0036] FIG. 12B is a photograph of a titanium wire anodized, at 108V, to have a surface TiO.sub.2 layer.

[0037] FIG. 13 is a chart showing measurements collected using the anodized titanium wire of FIG. 12B.

DETAILED DESCRIPTION

[0038] Disclosed are reference electrodes, and methods for fabricating such reference electrodes, configured for use with molten salt systems, e.g., molten salt electrochemical systems. A reference electrode, according to embodiments of the disclosure, comprises a metal core with an outer layer of the metal's oxide, silicide, or carbide (collectively or individually referred to herein as a metal-based compound). By forming the reference electrode according to embodiments of the disclosure, the metal-based compound layer forms uniformly (e.g., with uniform thickness) along the outer surface of the metal core and forms with sufficient tenacity to survive contact with harsh, molten salt media.

[0039] As used herein, the term molten salt refers to molten media, which may or may not be wholly ionic or derived from simple salts.

[0040] As used herein, the term molten salt systems refers to electrochemical systems utilizing a molten salt electrolyte (e.g., a molten salt bulk electrolyte).

[0041] As used herein, the term metal core refers to a monolithic structure (e.g., a rod, a wire, or the like) comprising, consisting essentially of, or consisting of at least one metal.

[0042] As used herein, the term solid, when referring to a material or structure, refers to such referenced material or structure being in a firm and stableor substantially firm and stablephase, as opposed to being in a liquid or other flowable phase. A solid material or structure may also be nonporous.

[0043] As discussed above, conventional reference electrodes intended for use with molten salt systemssuch as conventionally-structured and conventionally-fabricated Ni/NiO reference electrodeshave shown signs of random fluctuations in measured voltages across a wide range of measurements, particularly when used in long-term measurements. It is contemplated that these problems may arise because of the way the conventional reference electrode is fabricated. That is, a conventional Ni/NiO reference electrode may be fabricated by disposing a bare (e.g., uncoated nickel wire) in its oxide (e.g., nickel-oxide), which oxide may be in finely powdered form (e.g., 1 m to 10 m particle size). In other words, a conventional Ni/NiO reference electrodelike many other conventional reference electrodesis fabricated as a ceramic tube that contains, therein, a solid structure (e.g., a wire) at least partially immersed within a reference solution (e.g., a reference electrolyte), such as the powdered NiO, which reference solution contains ions, such as oxide ions. However, in reference electrodes fabricated in this manner, that may be insufficient surface area contact, between the metal and its oxide (e.g., oxide ions), to accurately measure electric potentials.

[0044] Disclosed herein are new methods to fabricate reference electrodes configured for use within molten salt systems. The reference electrodes and methods disclosed herein take into consideration the metal-based-compound-formation ability (e.g., the oxide-formation ability, the silicide-formation ability, or the carbide-formation ability) of the metals of the reference electrodes and the tenacity of adherence of the metal-based compound (e.g., the oxides, the silicides, or the carbides) to the surface of the metal(s). The disclosed reference electrodes have an improved design and process due, at least in part, to the avoidance of the physical contact between the metal (e.g., the M of the M/M.sub.xO.sub.y) and the M.sub.xO.sub.y. Instead, an intimate surface metal-based compound (e.g., oxide, silicide, or carbide) layer is provided. The metal-based compound layer may function as the ion conductive medium of the reference electrode. So, by embodiments disclosed herein, at least two things are enabled. First, the finished reference electrode need not include an ionic liquid or powder solution (e.g., liquid or powdered reference solution) immersing the metal core. Instead, the metal-based compound (e.g., oxide, silicide, or carbide) layer is formed directly on the metal core, and the reference electrode is formed as a monolithic structure with its external surface provided by a uniform metal-based compound (e.g., oxide, silicide, or carbide) layer. So, the conventional immerse-in-reference-solution (e.g., metal immersed in reference electrolyte of liquid or powder form) technique for fabricating reference electrodes, which may limit the establishment of good physical contact between the metal of the metal core and its oxide, may be avoided. Second, the outer, metal-based compound (e.g., oxide, silicide, or carbide) layer may be formed by methods that not only provide a uniform contact between the metal-based compound and its underlying metal of the metal core (separated, perhaps, by only a relatively thin interface), but that also form the metal-based compound layer to be in sufficiently-strong attachment to the metal core (e.g., the metal wire) so that the metal-based compound layer can resist the attack of a molten salt media. For example, a tenacious M.sub.xO.sub.y layer (e.g., a metal-oxide layer) may be formed on the metal (M), e.g., by an anodization process, prior to the deployment of the M/M.sub.xO.sub.y reference electrode in a molten salt electrochemical cell. As another example, a tenacious M.sub.xC.sub.y layer (e.g., a metal carbide layer) may be formed on the metal (M), e.g., by forming an M.sub.xO.sub.y layer and then heating the structure with graphite powder in the presence of hydrogen or method followed by carburization. As still another example, a tenacious M.sub.xSi.sub.y layer (e.g., a metal silicide layer) may be formed on the metal (M), e.g., by polishing the metal with silicon carbide paper, cleaning in alcohol, ultrasonic cleaning and drying, burying in a pack mixture that includes silicon, and heating.

[0045] The metal (M), of the metal/metal-oxide (M/M.sub.xO.sub.y) reference electrodes disclosed herein, may comprise, consist essentially of, or consist of one or more of nickel, aluminum, tin, titanium, tungsten, hafnium, zirconium, niobium, iridium, platinum, ruthenium, palladium, rhenium, scandium, and/or calcium. Such reference electrodes may exhibit good performance characteristics, such as no significant drift in measured electric potentials. In contrast, conventional reference electrodes may exhibit significant drift.

[0046] The metal (M), of the metal/metal-carbide (M/M.sub.xC.sub.y) reference electrodes disclosed herein, may comprise, consist essentially of, or consist of one or more transition-group metal(s) and/or one or more platinum-group metal(s).

[0047] The metal (M), of the metal/metal-silicide (M/M.sub.xSi.sub.y) reference electrodes disclosed herein, may comprise, consist essentially of, or consist of one or more transition-group metal(s) and/or one or more platinum-group metal(s).

[0048] With reference to FIG. 1, in some embodiments a reference electrode 100 may be formed in the shape of a rod (e.g., a wire). With reference to FIG. 2, the reference electrode 100 may include a metal core 102 (e.g., the metal M described above) surrounded by a metal-based compound outer layer 104 (e.g., the M.sub.xO.sub.y, the M.sub.xC.sub.y, or the M.sub.xSi.sub.y described above).

[0049] With reference to FIG. 3, in additional embodiments, a reference electrode 300 may be formed in the shape of a plate or other planar structure. With reference to FIG. 4, the reference electrode 300 may include the metal core 102 surrounded by the metal-based compound outer layer 104. For example, with reference to FIG. 5, the metal-based compound outer layer 104 may be formed on all vertical sidewalls, on a top surface, and/or on a bottom surface of the reference electrode 300.

[0050] With collective reference to FIG. 6 and FIG. 7, in additional embodiments, a reference electrode 600 may be formed in a shape that includes a cylindrical (e.g., rod-like) portionsuch as near a top of the structureas well as a planar (e.g., plate-like) portionsuch as near a bottom of the structure. Again, the reference electrode 600 includes the metal core 102 with the metal-based compound outer layer 104, which may wholly enclose the metal core 102.

[0051] The disclosure is not limited to the particular configurations or shapes of reference electrodes illustrated in FIG. 1 through FIG. 7. In still other embodiments, the metal core 102 may be more complexly shaped and the metal-based compound outer layer 104 conformally formed on any or all outer surface(s) of the metal core 102.

[0052] The thickness of the metal-based compound outer layer 104 (e.g., dimension normal to a surface of the metal core 102 upon which the metal-based compound outer layer 104 is formed) may be tailored according to the molten salt environment in which the reference electrode (e.g., the reference electrode 100 of FIG. 1, the reference electrode 300 of FIG. 3 and FIG. 5, the reference electrode 600 of FIG. 6 and FIG. 7). In some embodiments, the thickness of the metal-based compound outer layer 104 may be less than about 500 m with the width (e.g., diameter or other lateral dimension) of the metal core 102 being between about 3 mm and about 5 mm.

[0053] The metal-based compound outer layer 104 (e.g., the oxide layer (e.g., the metal-oxide layer); the silicide layer (e.g., the metal-silicide layer); or the carbide layer (e.g., the metal-carbide layer)) of the reference electrodes (e.g., the reference electrode 100 of FIG. 1, the reference electrode 300 of FIG. 3 and FIG. 5, the reference electrode 600 of FIG. 6 and FIG. 7) may be formed (e.g., grown) on the metal core 102 by, e.g., a conformal formation, growth, and/or printing (e.g., screen printing, additive manufacturing) processes.

[0054] For example, in embodiments in which the metal-based compound outer layer 104 comprises, consists essentially of, or consists of an oxide, the oxide layer may be formed by thermal oxidation (a thermal process), by anodization (an electrochemical process), or a combination thereof. In some embodiments, forming the oxide layer by anodization may grow the oxide with the desired uniformity and durability. After forming the oxide layer on the metal corewhether by thermal or by electrochemical processesthe oxide may be exposed to a reducing/oxidizing atmosphere and/or annealed (e.g., for more than one hour, e.g., more than five hours, e.g., for more than one day, e.g., for several days) to remove residual stresses and to make the oxide layer uniform, in both composition and in thickness, and pore-free (e.g., substantially pore-free) across the surface of the metal core. In some such embodiments, further processing (e.g., post processing) may be carried out to ensure nominal density and stoichiometry of the coating (e.g., the oxide of the metal-based compound outer layer 104).

[0055] In embodiments forming the metal-based compound outer layer 104 as an oxide layer by anodization, the anodization forms the M.sub.xO.sub.y layer on the M core (e.g., the metal core 102) with adequate stability. For example, a platinum/platinum oxide (Pt/PtO.sub.2) reference electrodein which the platinum oxide has been formed by anodization of a platinum core (e.g., a nonporous platinum core)may be useful as a solid-state reference electrode, e.g., by virtue of its near Nernstian behavior, low hysteresis, and rapid response. However, in other embodiments, even an inexpensive metal may prove to be a better, cost-effective alternative.

[0056] In some embodiments, a surface oxide layeras the metal-based compound outer layer 104 of the reference electrode (e.g., the reference electrode 100 of FIG. 1, the reference electrode 300 of FIG. 3 and FIG. 5, the reference electrode 600 of FIG. 6 and FIG. 7)may be formed by both anodization and a thermally-activated process (e.g., thermal oxidation). During thermal activation (e.g., thermal oxidation), the metal may be kept in a furnace and heated at a slow enough heating rate to a desired temperature (e.g., up to about 500 C.) for several minutes to form a tenacious M.sub.xO.sub.y surface layer on the metal (e.g., the metal core 102).

[0057] In embodiments in which the metal-based compound outer layer 104 is formed as a carbide, the carbide layer may be formed by first forming an oxide (e.g., M.sub.xO.sub.y) on the metal core 102, by any of the techniques described above (e.g., by anodization and/or thermal treatment). The M/M.sub.xO.sub.y structure may be heated with graphite powder in hydrogen and/or in a chamber filled with methane up to a temperature of about 1000 C. The heat-treated structure may then be carburized at about 1000 C. for several hours.

[0058] In embodiments in which the metal-based compound outer layer 104 is formed as a silicide, the silicide layer may be formed by sizing the metal core 102 to the desired shape and dimensions (e.g., to a desired length, such as if the metal core 102 is a wire) and polishing the metal core 102 with silicon carbide paper (e.g., about 1200 grit silicon carbide paper). The polished metal core 102 may be cleaned in alcohol followed by an ultrasonic cleaning and drying process. The cleaned and dried metal core 102 may, then, be buried in a pack mixture in a crucible (e.g., an alumina crucible). The pack mixture may comprise, consist essentially of, or consist of, e.g., Si, NaF, and Al.sub.2O.sub.3 (e.g., about 12 wt. % Si, about 5 wt. % NaF, and about 83 wt. % Al.sub.2O.sub.3). Then, the buried metal core 102 may be heated to a temperature of about 1300 C. for several hours (e.g., about eight hours) to form the silicide (M.sub.xSi.sub.y) metal-based compound outer layer 104 on the metal core 102.

[0059] With reference to FIG. 8, illustrated is an electrochemical system 800 in which a reference electrode 802 (e.g., any of the aforementioned reference electrodes, such as the reference electrode 100 of FIG. 1, the reference electrode 300 of FIG. 3 and FIG. 5, the reference electrode 600 of FIG. 6 and FIG. 7) may be used. At least a portion of the reference electrode 802 may be exposed to (e.g., immersed in, submerged in) a molten salt environment 804 (e.g., a bulk electrolyte (e.g., a bulk electrolyte solution) comprising, consisting essentially of, or consisting of a molten salt), which may comprise any one or more of the molten salt electrolytes or other molten salt media described above or below (e.g., a chloride-based molten salt media, a fluoride-based molten salt media, a bromide-based molten salt media, or some combination of any of the foregoing).

[0060] The fabricated reference electrode(s) 802 (e.g., the reference electrode 100 of FIG. 1, the reference electrode 300 of FIG. 3 and FIG. 5, the reference electrode 600 of FIG. 6 and FIG. 7) maydepending on the molten salt environment 804 in which it is to be used, such as the temperature of the electrolytemay be used either directly (e.g., with direct contact between the metal-based compound outer layer 104 and the molten salt environment 804) or indirectly (e.g., with a sheath included around the metal-based compound outer layer 104 and the molten salt environment 804). For example, in embodiments in which the molten salt environment 804 of the electrochemical system 800 is at a relatively-lower temperature (e.g., less than about 500 C.), the reference electrode 802 may be used in the molten salt environment 804 without a sheath. As another example, in embodiments in which the molten salt environment 804 of the electrochemical system 800 is at a relatively-higher temperature (e.g., greater than about 500 C.) and the reference electrode 802 is to be used for a significant period of time (e.g., relatively-longer-term exposure), the reference electrode 802 may be used in the molten salt environment 804 with a suitable inert sheathing material (e.g., formed of and including glass, ceramic, and/or porcelain).

Examples

[0061] A 1 mm nickel wire was selected as a metal core (e.g., the metal core 102) for a reference electrode (e.g., the reference electrode 100 of FIG. 1). A nickel-oxide layer (e.g., the metal-based compound outer layer 104) was formed on the 1 mm nickel wire thermal oxidation. After forming the nickel-oxide layer, the structure was annealed for several days. Metal-metal-oxide reference electrodes were also formed using metal cores of aluminum, tin, titanium, and tungsten.

[0062] The annealed metal-metal oxide reference electrodes were examined under an optical microscope to ascertain the growth of the oxide (e.g., metal-oxide) layers. The fabricated electrodes were subsequently immersed in two molten chloride salts (e.g., the molten salt environment 804 of FIG. 8) (LiClKCl eutectic and CaCl.sub.21 wt. % CaO respectively) for the measurement of open circuit potentials. These measurements were compared with a variety of other reference electrodes, used in chloride melts.

[0063] FIG. 9A is a photograph showing the aforementioned 1 mm nickel wire before the oxidation stage. FIG. 9B is a photograph showing the wire after surface oxidation in which the NiO layer was grown by thermal oxidation. More particularly, the thermal oxidation was conducted in static air at 600 C. for a total duration of 8 hours, and then the structure was annealed in argon at 700 C. for about 6 hours. Comparing the two figures, a change in coloration is evident, and this color change indicated the formation of a smooth and adherent surface oxide (e.g., NiO) layer (e.g., as the metal-based compound outer layer 104) on the nickel surface (e.g., the surface of the nickel metal core, as the metal core 102).

[0064] FIG. 10 shows the measurement of the open-circuit potential (OCP) for different duration and/or cycles when the reference electrodes were used in a eutectic LiClKCl molten salt electrolyte (e.g., as the molten salt environment 804 of FIG. 8). The measurements indicated fairly stable potentials (with minimum drifts), particularly for measurement numbers at 1, 4, 6, and 7. This shows that the annealing steps can be optimized to obtain fairly stable voltages across the entire measurement duration.

[0065] With reference to FIG. 11, the OCP measurements were followed by cyclic voltammetry measurements. More particularly, cyclic voltammetry measurements were taken with the eutectic LiClKCl melt containing 2 wt. % YCl.sub.3 and 1 wt. % ScCl.sub.3 (e.g., as the molten salt environment 804 of FIG. 8) at a melt temperature of 500 C. As can be seen from FIG. 11, there was hardly any shift in the anodic and cathodic peaks recorded at different scans, which provides a strong indication in favor of the potential deployment of the new reference electrode. That is, the two sets of cathodic deposition peaks (first for Sc at about 1.7V and second for Y at about 2.05V, respectively, with their corresponding anodic stripping peaks at about 1.6V for Sc and about 1.8V for Y) indicate the robustness in the measured values in that the peak values did not shift at different scanning rates (0.025 Vs.sup.1 to 0.175 Vs.sup.1). Such types of voltammograms have been recorded with conventional reference electrodes, such as Ag/AgCl reference electrodes.

[0066] Experiments were also performed to grow oxide layers on titanium and tungsten by an anodization process. FIGS. 12A and 12B show the in situ formed oxide layers on titanium wire (e.g., a metal coresuch as the metal core 102 of FIG. 1consisting of titanium), wherein the titanium oxide layer (e.g., as the metal-based compound outer layer 104 of FIG. 2) was formed by anodization at two different applied voltages: 9V for FIG. 12A and 108V for FIG. 12B.

[0067] FIG. 13 shows the OCP measurements taken using the oxidized Ti wire as the reference electrode. The general flat (e.g., horizontal) trend of the plotted curves indicates the tested reference electrode exhibits good stability. As compared to the oxidized nickel (Ni/NiO) reference electrode, the oxidized titanium wire (Ti/TiO.sub.2) reference electrode showed better performance characteristics.

[0068] Preliminary measurements with Sn/SnO.sub.2 as a reference electrode and with W/WO.sub.3 as a reference electrode have also shown some initial promise. So, Ti/TiO (i.e., titanium-titanium monoxide) was also evaluated as a potential reference electrode material.

[0069] Accordingly, disclosed are reference electrodes and methods of forming reference electrodes that are conducive for use in electrochemical systems that use a molten salt as an electrolyte. The reference electrodes do not require physical contact between a naked (i.e., uncoated) metal core (e.g., wire) and its oxide, silicide, or carbide, which eliminates measurement uncertainties experienced with conventional reference electrodes. The disclosed reference electrodes are effective, even with a wide variety of metals used as the metal of the reference electrodes. The fabricated electrodes can be used in a variety of molten salt media (e.g., a variety of bulk electrolyte solutions), such as chloride molten salt media, fluoride molten salt material, and/or bromide molten salt media (e.g., as the molten salt environment 804 of FIG. 8). And, the reference electrodes of embodiments of the disclosure show promise for successful use in fundamental and applied electrochemical measurements. Moreover, the reference electrodes may be formed by processes that may form (e.g., grow) the oxide (e.g., metal-oxide), the silicide (e.g., metal-silicide), or the carbide (e.g., metal-carbide) layers on the metal cores in such a manner that the oxide, silicide, or carbide layers are uniform over the surface of the metal core and durable enough (e.g., adhering sufficiently to the metal core) to withstand exposure to even the generally-corrosive molten salt media.

[0070] While the disclosed apparatus and methods are susceptible to various modifications and alternative forms in implementation thereof, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure encompasses all modifications, combinations, equivalents, variations, and alternatives falling within the scope of the present disclosure as defined by the following appended claims and their legal equivalents.