AN ELECTRODE AND A PSEUDO-CAPACITOR BASED ON THE ELECTRODE

Abstract

The invention provides a process for preparing an electrode, comprising: electrodeposition of metallic ruthenium/ruthenium oxide (Ru.sup.(0)/RuO.sub.2) coating onto a progressively etched nickel surface; and partial electrochemical oxidation of said metallic ruthenium to ruthenium oxide. The electrode produced and a pseudo-capacitor based on the electrode are also disclosed.

Claims

1. A process for preparing an electrode, comprising: electrodeposition of metallic ruthenium/ruthenium oxide (Ru.sup.(0)/RuO.sub.2) coating onto a progressively etched nickel surface; and partial electrochemical oxidation of said metallic ruthenium to ruthenium oxide.

2. A process according to claim 1, wherein the electrodeposition and subsequent partial electrochemical oxidation are achieved with the aid of cyclic voltammetry.

3. A process according to claim 2, wherein the electrodeposition is achieved with the aid of cyclic voltammetry of a nickel working electrode in Ru.sup.3+-containing deposition solution in the presence of added sulfate salt, between switching potentials in the range from 0 to −1.1 V versus reference electrode.

4. A process according to claim 1, comprising the steps of: providing a two-electrodes or three-electrodes arrangement suitable for cyclic voltammetry, wherein the working electrode is a nickel-containing substrate immersed in an aqueous solution of ruthenium salt in the presence of a salt additive; applying a potential across the working electrode and a reference electrode and sweeping the potential negatively between a first value and a second value versus the reference electrode, reversing the scan to the positive direction, and repeating the potential scan for many cycles, wherein the scan rate is not less than 1 V/s to deposit Ru.sup.(0)/RuO.sub.2-containing layer; electrochemically oxidizing Ru.sup.(0) to ruthenium oxide, thereby creating a nickel electrode with Ru.sup.(0)/RuO.sub.2 coating thereon.

5. A process according to claim 4, wherein the additive is an alkali sulfate salt.

6. A process according to claim 4, comprising scanning the potential range between 0 V and −1.1V versus reference electrode, wherein the scan rate and number of cycles are adjusted to etch the surface of the nickel base.

7. A process according to claim 6, wherein the scan rate is of not less than 5 V/s for and the number of cycles is at least 1000.

8. A process according to claim 3, wherein the deposition solution further comprises Ni.sup.2+ source, such that Ni.sup.(0) is co-deposited alongside Ru.sup.(0), forming Ni.sup.(0)Ru.sup.(0) alloy phase.

9. A process according to claim 8, wherein the deposition solution comprises Ru.sup.3+ and Ni.sup.2+ at molar ratio in the range from 3:5 to 5:3.

10. An electrode comprising: a nickel-containing base; and a coating comprising metallic ruthenium/ruthenium oxide (Ru.sup.(0)/RuO.sub.2) applied onto the nickel-containing base.

11. An electrode according to claim 10, wherein the nickel base consists of 5 to 100 pm thick nickel foil and the Ru.sup.(0)/RuO.sub.2-containing coating is up to 500 nm thick.

12. An electrode according to claim 10, comprising Ru.sup.(0) layer interposed between the nickel base and an outermost RuO.sub.2-containing layer, such that the ruthenium oxide is preferentially located atop of said Ru.sup.(0) layer.

13. An electrode according to claim 10, wherein the proportion between Ru.sup.0 and RuO.sub.2 on the surface of the coating, as determined by the intensity of peaks assigned to Ru.sup.0 and RuO.sub.2 in a deconvoluted X-ray photoelectron emission spectrum (XPS), varies from 7:1 to 1:7.

14. An electrode according to claim 13, wherein Ru.sup.0/RuO.sub.2 are proportioned in the range from 3:1 to 1:3.

15. An electrode according to claim 10, wherein the coating is an electrodeposited coating which underwent post electrodeposition electrochemical oxidation.

16. An electrode according to claim 15, wherein the coating is prepared by electrodeposition step of a mixed Ru.sup.(0)/RuO.sub.2 layer onto the nickel base through cyclic voltammetry in an aqueous solution of ruthenium salt in the presence of a salt additive; and a post-deposition step which consists of electrochemical oxidation of said a mixed Ru.sup.(0)/RuO.sub.2 layer whereby the oxide content is increased.

17. An electrode according to claim 10, wherein the coating comprises Ni.sup.(0)Ru.sup.(0) alloy phase applied onto the nickel-containing base, with RuO.sub.2 surface layer on said alloy.

18. An electrode according claim 10, wherein the coating further comprises electrodeposited Ni.sup.(0), forming Ni.sup.(0)Ru.sup.(0) alloy phase with RuO.sub.2 surface layer thereon.

19. An electrode according to claim 17, wherein the Ni.sup.(0)Ru.sup.(0) alloy phase is ruthenium-rich.

20. A pseudo-capacitor comprising a pair of spaced apart electrodes, a separator disposed in the space between said electrodes and an electrolyte, wherein at least one of said electrodes is as defined in claim 10.

21. A pseudo-capacitor according to claim 20, which is a symmetric pseudo-capacitor.

22. A pseudo-capacitor capacitor according to claim 20, wherein the electrolyte is an alkali sulfate solution.

23. A pseudo-capacitor capacitor according to claim 20, wherein the separator consists of a film of polymeric microfibers.

24. A pseudo-capacitor according to claim 22, wherein the separator consists of carboxylated electrospun polystyrene microfibers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0022] The process of etching/deposition is presented in FIG. 1. The thickness of the nickel current collector onto which deposition takes place may vary from 1 to 50 μm, and the active area to be coated is from 0.1 to 20 cm.sup.2, chiefly restricted by the dimensions of the deposition set-up. The deposition solution is an aqueous solution comprising dissolved Ru ion source, e.g., Ru.sup.3+ salt such as ruthenium chloride (e.g., in hydrated form RuCl.sub.3.xH.sub.2O) at concentration from 0.001 to 0.1 M, e.g., from 0.005 to 0.015 M.

[0023] A key feature of the present invention is that the nickel electrode is etchable under the conditions of the electrodeposition method, to achieve ruthenium deposition onto a progressively etched nickel surface. Cyclic voltammetry has been shown to be useful for this purpose on condition that a salt additive such as alkali sulfate is present in the deposition solution. Experimental results reported below indicate that cyclic voltammetry using a nickel foil as a working electrode in 1M Na.sub.2SO.sub.4 solution has led to changes in the surface morphology of the working electrode. Similar changes were not observed in the absence of added sulfate salt (i.e., cyclic voltammetry in water). That is, the pronounced etching and enhanced surface area of the nickel electrode induced by the consecutive CV cycles account for the good performance at higher charge/discharge rates. Notably, as shown in FIG. 1 and comparative data presented below, attempts to induce surface etching of gold working electrode through cyclic voltammetry conducted in sulfate solution by consecutive CV cycles were unsuccessful. This difference underscores the important role of the current collector in affecting electrochemical properties of supercapacitors, i.e., nickel as opposed to gold. That is, when Ni electrode is used during the deposition process, an etching process of the Ni electrode occurs due to instability of the Ni in the deposition voltage window in the presence of suitable salt additive in the deposition solution.

[0024] Suitable salt additives which are present in the deposition solution include sulfate salts, e.g., alkali sulfate, owing to their electrochemical inertness across the voltage window used in the deposition step. The concentration of the sulfate salt in the deposition solution may vary from 0.1 to 1.5 M, for example, from 0.25 to 1 M.

[0025] As mentioned above, the deposition solution may advantageously include Ni.sup.2+ salt, such as NiSO.sub.4 (e.g., in hydrated forms, NiSO.sub.4.7H.sub.2O) at typical concentration ranging from 0.005 to 0.015 M. To achieve efficient co-deposition of both metals and creation of the Ni.sup.(0)Ru.sup.(0) alloy, roughly equimolar amounts (from 3:5 to 5:3, e.g., about 1:1 molar ratio of the two metal precursors) are present in the solution. For example, deposition solutions which contains from 0.0075 M to 0.0125 M of each salt can be used.

[0026] To produce the electrodes of the invention through cyclic voltammetry, a three-electrode set-up can be used, in which the nickel is the working electrode, the counter electrode may be a platinum wire or coil and the reference electrode, versus which potential is determined, may be Ag|AgCl (3M KCl). Other reference electrodes can also be used, such as saturated calomel electrode.

[0027] As mentioned above, potential sweep occurs by sweeping the potential negatively between a first value (for example, 0 V) and a second value (for example, the second value is from −1.0 to −1.2V, e.g. −1.1V) versus the reference electrode, reversing the scan to the positive direction, and repeating the potential scan for many cycles. The scan rate and number of cycles are adjusted to etch the surface of the nickel base. For example, A suitable scan rate is usually not less than 5 V/s, e.g., from 7 to 12 V/s, such 10 V/s, and the number of cycles is at least 1000, for example, from 1000 to 5000, e.g., from 2000 to 3000.

[0028] Regarding the conditions of the post-deposition oxidation step, whereby electrodeposited ruthenium is converted to ruthenium oxide, it is preferably accomplished electrochemically, conveniently with the aid of cyclic voltammetry (or alternatively, under constant potential or constant oxidation current).

[0029] To this end, the three-electrode set up mentioned above can be used, with Ni.sub.(base)/Ru.sup.(0) or Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0) serving as the working electrode, platinum as the counter electrode and Ag|AgCl (3M KCl) as the reference electrode. Regarding the electrolyte solution, it should be noted that acidic solutions are precluded, due to the instability of nickel in acidic environment. But other than this constraint, salt solutions can be used, such as the alkali sulfate solution mentioned above, e.g., Na.sub.2SO4 (1M). The working voltage window for ruthenium oxide generation is preferably from 0 and +0.8V. Sweeping the potential across this window for at least 1k cycles, for example 5k cycles, at a scan rate in the range from 1 to 20 V/s, for example, 10 V/s, leads to efficient creation of RuO.sub.2 layer.

[0030] Next, a pair of electrodes of the invention are assembled to produce a symmetric supercapacitor (asymmetric supercapacitors utilizing just one electrode of the invention are also contemplated). In general, the active area of each electrode is from 0.1 to 0.5 cm.sup.2. The electrolyte disposed in the space between the electrodes is preferably an aqueous (non-acidic as explained above) electrolyte solution, such as Li.sub.2SO.sub.4 and Na.sub.2SO.sub.4. As to the separator, major considerations in choosing a separator include nonconductivity, chemical resistance to the electrolyte solution, mechanical resistance and good wettability. Cellulose paper and polymer-based separators (possessing either fibrous structure or consisting of monolithic networks with pores) may be used. Especially preferred separator film in the supercapacitor of the invention is based on carboxylated electrospun polystyrene microfibers film. That is, the separator comprises polystyrene (PS) fibers which were electrospun from a polystyrene solution, and were then carboxylated with the aid of an oxidizer. For example, PS fibers were electrospun, e.g., in an organic solvent such as dimethylformamide (30% w/v), on a glass for not less than min (under a 20 kV voltage and 22 cm between needle and collector) and were annealed at 100° C. for at least 15 min. Following electrospinning, carboxylation of the PS microfibers was carried out by placing the PS fibers under heating (e.g., at 70° C.) in a 0.6 M H.sub.2SO.sub.4 solution containing an oxidant such as KMnO.sub.4 (for example, at concentration of 50 g L.sup.−1) for 3 h. The oxidation product, namely, MnO.sub.x precipitate, can be removed by immersing the PS fibers in a 6 M HCl for 24 h. The film was than rinsed with water 3 time for several hours each time. Until use, the separator film is kept in the electrolyte solution (e.g., 1 M solution of Na.sub.2SO.sub.4 electrolyte solution) to avoid drying of the film.

[0031] The performance of symmetrical capacitors based on a pair of Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2 electrodes, or a pair of Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 electrodes, with a neutral (e.g., 1M Na.sub.2SO.sub.4) electrolyte solution disposed between the pair of electrodes and carboxylated electrospun polystyrene microfibers film serving as separator, was investigated using cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance measurements. For example, the Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2-based supercapacitor demonstrates excellent capacitor behavior at high frequencies with capacitance of not less than 1.8 (e.g., 1.87) mF cm.sup.−2 at a current density of 10 mA cm.sup.−2, a near rectangular shape at a scan rate of 1000 V s.sup.−1, and a phase angle of −79.8° at 120 Hz. The Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2-based supercapacitor can operate at power densities above 1500 mW cm.sup.−2 (88 kW cm.sup.−3) with maximum energy densities exceeding 0.58 μWh cm.sup.−2 (34 mWh cm.sup.−3).

[0032] The Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2-based supercapacitor shows even better results, e.g., capacitance of not less than 2.1 (e.g., 2.29) mF cm.sup.−2 at a current density of 10 mA cm.sup.−2. In addition, the Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 based device can deliver energy densities as high as 0.71 μWh cm.sup.2 and power densities as high as 3000 mW cm.sup.−2.

[0033] Possible designs of electrochemical capacitors, fabrication methods and applications thereof are known in the art and are described, for example, in “Electrochemical Supercapacitors for Energy Storage and Conversion (Kim et al.; Handbook of Clean Energy Systems published by John Wiley & Sons (2015)]. That is, several capacitors are often combined in serial and parallel circuits, depending on whether higher voltage or higher power is needed. On account of their ability to be charged and discharged rapidly, showing good stability and high capacitive retention over repeated cycling, and high frequency response, the capacitors of the invention can be integrated in many applications such as high energy pulses and alternating current line-filtering, where aluminum electrolytic capacitors are currently being used commercially.

EXAMPLES

Methods

[0034] XPS analysis was carried out using Thermo Fisher ESCALAB 250 instrument with a basic pressure of 2×10.sup.−9 mbar. The samples were irradiated in two different areas using monochromatic Al Kα, 1486.6 eV X-rays, using a beam size of 500 μm. The high energy resolution measurements were performed with pass energy of 20 eV. The core level binding energies of the Ru 3d peaks were normalized by setting the binding energy for the C1s at 284.8 eV.

[0035] HRTEM samples were prepared using focus ion beam. HRTEM images were recorded on a 200 kV JEOL JEM-2100F.

[0036] Scanning electron microscopy (SEM) images were recorded on Verios 460L FEI (Czech Republic).

[0037] X-ray diffraction (XRD) data was obtained using Panalytical Empyrean powder diffractometer (PANalytical, Almelo, Netherlands) equipped with a parabolic mirror on incident beam providing quasi-monochromatic Cu Kα radiation (λ=1.54059 Å) and X'celeator linear detector. Data were collected in the grazing geometry with constant incident beam angle equal to 1° in a 20 range of 30-80° with a step equal to 0.05°.

[0038] Lamellas for cross section TEM imaging were fabricated using a Helios G4 UC dual beam focus ion beam (FIB)/SEM (Thermo Fisher Scientific). The sample was covered with 0.5 μm of carbon using electron deposition followed by another 1 μm of carbon ion deposition. Next, Ga ion beam was used to mill around the protective layer and an Easylift (Thermo Fisher Scientific) micromanipulator was used to lift out the lamella from the bulk and attached it to a TEM grid. Further reduction of thickness and cleaning of the lamella was done with 30 kV Ga ion beam probe from both sides to a thickness of −150 nm and then with 5 kV probe until the thickness was around 50 nm.

[0039] Electrochemical measurements: CV was conducted at voltage ranges between 0-1V. Galvanostatic charge/discharge measurements were conducted at current density in the range of 10-2000 mA cm.sup.−2 in a voltage window of 1.5V. Electrochemical impedance measurements were conducted between 1 Hz-100 kHz with a sinus amplitude of 5 mV. Cycle stability measurements were conducted in a scan rate of 10 V s.sup.−1 in a voltage window of 0-1 V or 0-1.5 V for 1-3 million cycles. The electrochemical measurements were conducted in two-electrodes configuration on either a CH instrument 760C (Austin, Tex.) or a Bio-Logic SP-150 (Claix, France).

Example 1

Ni.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrode Fabrication

[0040] The deposition of Ru was conducted on a commercial 20 μm thick Ni foil. The deposition solution was 0.01M RuCl.sub.3+1M Na.sub.2SO.sub.4 solution. The deposition was conducted using a 3-electrodes configuration with Ni as the working electrode, Pt wire as the counter electrode and Ag/AgCl (3 M KCl) as the reference electrode. The deposition was achieved using CV cycles between 0 V and −1.1V vs reference electrode at a scan rate of 10 V/s. Deposition for 1k, 2.5k and 5k cycles were checked. After deposition of Ru, oxidation to RuO.sub.2 is obtained by running CV for 5k cycles at a scan rate of 10 V/s in a voltage window of 0 and 0.8V for single electrode in a 1M Na.sub.2SO.sub.4 solution. The deposition/oxidation was conducted on a SP-150 Bio-Logic device (Claix, France).

Example 2 (Comparative)

Au.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrode Fabrication

[0041] The deposition/oxidation procedure of Example 1 was repeated, but this time the Ru/RuO.sub.2 coating was applied on electrode consisting of silicon wafer with 200 nm evaporated Au [200 nm Au on top of 50 nm evaporated Ti].

Example 3

Effect of Cyclic Voltammetry in Water or in Sodium Sulfate Solution on the Surface of Nickel Foil

[0042] Cyclic voltammetry measurements of a nickel electrode conducted in 1M Na.sub.2SO.sub.4 aqueous solution were compared to cyclic voltammetry in water (using three-electrode configuration, with Ni serving as the working electrode, Pt as the counter electrode and Ag|AgCl as the reference electrode), to investigate the effect of Na.sub.2SO.sub.4 on the morphology of the surface of a bare nickel foil under the conditions of the deposition process described in previous examples, i.e., CV cycles between 0 V and −1.1V versus reference electrode at a scan rate of 10 V/s after 1, 1k, 2.5k, and 5k cycles.

[0043] The results shown in FIGS. 2A and 2B indicate the stability of the nickel foil when the electrolyte is water (FIG. 2A, where only small changes are noted in the cyclic voltammograms), as opposed to increase in signal clearly seen in FIG. 2B, indicating that the surface of the Ni foil has been undergoing changes during cyclic voltammetry in 1M Na.sub.2SO.sub.4 solution, i.e., under the experimental condition of the deposition process illustrated in Example 1. SEM images shown in FIGS. 3A and 3B of a commercial Ni foil and the same foil after having been subjected to 5000 CV cycles, respectively, attest to the increased surface roughness of the nickel foil after the cyclic voltammetry in 1M Na.sub.2SO.sub.4 solution, in line with the cyclic voltammograms of FIG. 2B. Further support can be found in atomic force microscopy (AFM) 3D topography maps of Ni surface before (FIG. 3C) and after (FIG. 3D) CV 5k cycles in 1M Na.sub.2SO.sub.4 solution, showing pronounced etching and increased surface of the nickel resulting from the consecutive CV cycles.

Example 4 (Comparative)

Effect of Cyclic Voltammetry in Sodium Sulfate Solution on the Surface of Gold Foil

[0044] The results of cyclic voltammetry measurements of a bare Au current collector, namely, silicon wafer with 50 nm and 200 nm evaporated Ti and Au, respectively, in 1M Na.sub.2SO.sub.4 solution using the same set-up and experimental conditions as in Example 3 are shown in FIG. 4. The nearly negligible change in current indicates that Au electrode is stable under the deposition conditions. That is, the surface of the Au electrode is resistant to etching when subjected to cyclic voltammetry in sodium sulfate solution.

Example 5

Characterization of Ni.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrode (of the Invention) and Au.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrode (Comparative)

[0045] Scanning electron microscopy (SEM) image showing the surface morphology of the coated Ni electrode (produced after 2500 deposition cycles followed by oxidation) is shown in FIG. 5, indicating the electrodeposited layer, i.e., formation of Ru/RuO.sub.2-coated Ni electrode.

[0046] X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the composition of the as-deposited coating (spectra is shown in FIG. 6A) and the composition of the coating produced through the post-deposition electrochemical oxidation (spectra is shown in FIG. 6B). The red, green and blue are products of deconvolution and represent Ru, RuO.sub.2 and RuO.sub.2.xH.sub.2O components of the film surface, respectively. Ru.sup.(0) content of the deposited film is 63% (6A), and is decreased to 32% after oxidation (6B). Hence, the concentration of ruthenium oxide and its hydrated form is ˜40% prior to electrochemical oxidation, reaching ˜70% after oxidation. It is seen that Ru.sup.(0) still remains a significant component of the coating layer in the final electrode.

[0047] High resolution transmission electron microscopy (HRTEM) images shown in FIGS. 7A and 7B lend further support to the conclusion that Ru.sup.(0) is present in the coating layer after the CV oxidation cycles. HRTEM image of the cross section the coating layer confirm the presence of crystalline Ru.sup.(0), showing only signal of metallic ruthenium, indicating that the bulk of the coating consists essentially of Ru.sup.(0). On the other hand, no RuO.sub.2 crystalline peaks were observed in the diffraction analysis, indicating the amorphous nature of the RuO.sub.2 component of the coating layer.

[0048] To illustrate the differences between Ru/RuO.sub.2-coated nickel and Ru/RuO.sub.2-coated gold electrodes, TEM images of the cross-section of the electrodes are given in FIGS. 8A and 8B respectively, revealing the interface between the metal current collector (Ni or Au) and the ruthenium coating layer deposited thereon. The ruthenium layer on the Au electrode seems to be denser and more uniform, while the interface region between the Ni current and the ruthenium coating has more voids, which can explain the lower resistance and better performance at high scan rates, as shown by the electrochemical studies reported below.

Example 6

Assembly of Symmetric Supercapacitors Composed of a Pair of Ni.SUB.(base)./Rum/RuO.SUB.2 .Electrodes (of the Invention) or a Pair of Au.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrodes (Comparative)

[0049] Two symmetric supercapacitors were fabricated, one based on a pair of the electrodes of Example 1 and the other based on a pair of the comparative electrodes of Example 2. Coated electrodes produced following 2500 deposition cycles were used for creation of the symmetric supercapacitors.

Step 1: Spacer Fabrication

[0050] Polystyrene (PS) fibers were electrospun from a polystyrene solution in dimethylformamide (30% w/v), on a glass for 40 min (under a 20 kV voltage and 22 cm between needle and collector) and were annealed at 100° C. for 15 min. Following electrospinning, carboxylation of the PS microfibers was carried out by placing the PS fibers at 70° C. in a 0.6 M H.sub.2SO.sub.4 solution containing KMnO.sub.4 (50 g L.sup.−1) for 3 h. MnO.sub.x precipitate was removed by immersing the PS fibers in a 6 M HCl for 24 h. The film was than rinsed with water 3 time for several hours each time. Finally, the film was kept in a 1 M solution of Na.sub.2SO.sub.4 prior to use to avoid drying of the film.

Step 2: Device Assembly

[0051] The device (i.e., symmetric supercapacitor) was composed of two electrodes with the same area pressed between the PS spacer socked in 1 M Na.sub.2SO.sub.4 solution. The device was encapsulated using scotch tape and was pressed using plastic clamps. For the cycle stability measurements, the device was soaked in a 1 M Na.sub.2SO.sub.4 solution, during the measurement, to avoid evaporation of electrolyte solution. Measurements were conducted on a CH instrument excluding cycle stability which was conducted on Bio-Logic instrument.

Example 7

Electrochemical Properties of Symmetric Supercapacitors Composed of a Pair of Ni.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrodes (of the Invention) or a Pair Au.SUB.(base)./Ru.SUP.(0)./RuO.SUB.2 .Electrodes (Comparative)

[0052] The symmetric supercapacitors of Example 6 were tested using different electrochemical techniques.

Cyclic Voltammetry Measurements

[0053] FIGS. 9A to 9F show CV curves recorded for symmetric supercapacitors devices assembled as described in Example 6, i.e., using 2.5k deposition cycles and post-deposition electrochemical oxidation to create Ru/RuO.sub.2 coatings either on nickel (black curves) or gold (orange curves) current collectors. Cyclic voltammetry measurements were conducted with a two-electrode configuration at the voltage range of 0 to 1 volt at different scan rates (10 V/s, 50 V/s, 200 V/s, 500 V/s, 1000 V/s and 2000 V/s, which correspond to FIGS. 9A to 9F, respectively in 1M Na.sub.2SO.sub.4 electrolyte solution.

[0054] The data in FIG. 9 A to F attest to the excellent capacitive properties of the supercapacitor consisting of the Ru/RuO.sub.2-coated Ni electrodes even at high scan rates, as well as the superior properties of the Ni-deposited electrodes compared to the gold-based SC. Specifically, rectangular shapes of the CV curves, indicating near-ideal capacitor behavior, were maintained even at scan rates approaching 1000 V s.sup.−1 (FIGS. 9A-D). That is, the CV data in FIGS. 9 A-F reveal that supercapacitors constructed from Ru/RuO.sub.2-coated Au electrodes exhibited significantly lower capacitive behavior in high scan rates compared to the Ni.sub.(base))/Ru.sup.(0)/RuO.sub.2 devices.

[0055] To demonstrate the differences in the capacitive properties of the two supercapacitors, i.e., in their capacitance retention, capacitance versus scan rate plots were created. The capacitance from the cyclic voltammetry (CV) curves is calculated based on the following equation:

[00001]$C=\frac{\int I.Math.\mathrm{dV}}{2v.Math.A.Math.V}$

where the I is the current and ∫IdV is the area of the CV curve, v is the scan rate, A is the area of the electrode and V is the voltage window. Capacitance versus scan rate plot is given in FIG. 10, for the symmetric supercapacitors that are based Ru/RuO.sub.2-coated Ni electrodes and Ru/RuO.sub.2-coated Au electrodes. It is seen that capacitance drop is faster for the gold-based device compared to the Ni-based device. Specifically, approximately 75% capacitance retention was recorded at a scan rate of 200 V s.sup.−1 and excellent 35% retention was achieved at 2000 V s.sup.−1 compared to the capacitance calculated at 10 V s.sup.−1.

[0056] To better appreciate the role of the current collector metal in achieving good performance of the supercapacitor of the invention, a different gold-based supercapacitor was fabricated according to the procedure of Example 6, by assembling gold electrodes produced by 1000 deposition cycles (and post-deposition electrochemical oxidation), namely, with lower loading of active material applied onto the gold current collector. Still, as shown in FIG. 10, even at lower (1000 instead of 2500) deposition cycles, the Ru/RuO.sub.2-coated Au device shows faster capacitance decrease than the Ru/RuO.sub.2-coated Ni device, highlighting the role of the nickel in the supercapacitors of the invention.

[0057] To check the stability of the Ru/RuO.sub.2 film, cyclic voltammetry measurements of 25k cycles at a scan rate of 1 V/s was conducted and the voltammogram is presented in FIG. 11. From the graph it is seen that no change in capacitance was observed even after 25k cycles showing the great potential of the device for prolonged operation.

[0058] Cyclic voltammetry of a symmetric supercapacitor (assembled using electrodes produced with 2.5k deposition cycles and post-deposition electrochemical oxidation to create Ru/RuO.sub.2 coating on nickel) was conducted to determine peak currents as a function of scan rates across different voltage windows. Results are shown in FIG. 12A (circles: 1.0 V, rhombuses: 1.5 V, triangles: 1.7 V and squares: 2.0 V). It is seen that voltage windows above the water decomposition voltage of 1.23V were investigated. FIG. 12A demonstrates that at scan rates above 1 V s.sup.−1, the current increased linearly with the scan rate, indicating capacitive based current, while no faradaic currents (resulting from water decomposition) were detected even at a voltage window as high as 2 V (e.g. linear slopes). This is crucial to prevent gas formation and damage to the SC due to voltage spikes, which are common in AC signals.

[0059] Additionally, capacitance retention percentage is plotted against the number of consecutive charge/discharge cycles, spanning the range up to 3 million cycles. Excellent capacitance retention of the Ru/RuO.sub.2-nickel coated SC is observed under the experimental conditions, i.e., across a voltage window of 1.5 V and a scan rate of 10 V s.sup.−1. The increase in capacitance following initial cycling (above nominal 100%) likely reflects enhanced oxidation of the metallic Ru, combined with more effective utilization of the Ru/RuO.sub.2 surface, overall contributing to more efficient occurrence of the redox processes at the electrode surface. Notably, even after 3 million cycles, capacitance retention of 98% was observed. This is in line with the SEM image shown in FIG. 13, showing the surface of the Ru/RuO.sub.2-coated nickel electrode after 3 million charging cycles at a potential window of 1.5 V and a scan rate of V s.sup.−1. No significant structural changes are noted in the coating.

Impedance Spectroscopy Measurements

[0060] Impedance spectroscopy analysis was conducted and results are presented in FIGS. 14-17.

[0061] Nyquist plot for both the Au.sub.(base)/Ru.sup.(0)/RuO.sub.2 device and the Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2 device shows a nearly vertical line for each case, with a negligible charge transfer resistance (FIG. 14). The inset shows a magnification of the high frequency region. The lower resistance of the Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2 device, at high frequencies, can be attributed to the higher surface area of the electrode. From the graph, and assuming a model of a resistor and capacitor in series, we can calculate the RC time constant to be 643 is and 245 for the Au.sub.(base)/Ru.sup.(0)/RuO.sub.2 and Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2 devices, respectively.

[0062] We can write the impedance of the system as follows:

[00002]$Z=Z_R+Z_C=R+\frac{1}{i.Math.2\pi f.Math.C}=R-\frac{i}{2\pi f.Math.C}$

[0063] From the equation we can calculate the capacitance as a function of the frequency using the following equation:

[00003]$C=\frac{-1}{2\pi .Math.f.Math.A.Math.Z^{″}\left(f\right)}$

where C is the areal capacitance, f is the frequency, A is the area of one electrode and Z″ is the imaginary value of the impedance.

[0064] The calculated capacitance is presented in FIG. 15, showing that the decrease in performance is much more pronounced in the Au.sub.(base)/Ru.sup.(0)/RuO.sub.2 device compared to the Ni.sub.(base)/Ru.sup.(0)/RuO.sub.2 device.

[0065] The relaxation time, which is the minimum discharge time required for achieving more than 50% efficiency, was determined by calculating the imaginary part of the areal capacitance using the following equation:

[00004]$C^{″}=\frac{-Z^{\prime }\left(f\right)}{2\pi .Math.f.Math.A.Math.{.Math.Z\left(f\right).Math.}^2}$

where C″ is the imaginary areal capacitance, Z′ is the real part of the impedance, |Z| is the impedance vector magnitude. The results presented in FIG. 16, where the relaxation time is 1/f at the maximum point of the curve. The relaxation time of the Au/Ru/RuO.sub.2 device is 3.9 ms, which is almost an order magnitude higher than the relaxation time of the Ni/Ru/RuO.sub.2 device (0.68 ms).

[0066] Next, to assess the capacitance behavior of the device at high frequencies, the phase angle was calculated according to the following equation:

[00005]$\mathrm{Phase}\mathrm{angle}=\arctan \left(\frac{Z^{″}}{Z^{\prime }}\right)$

where a phase angle of −90° indicates a pure capacitor and a phase angle of 0° indicates a pure resistor. The two values that are important are the phase angle at the 120 Hz, which is the frequency at which a signal is coming out from a diode bridge in AC line rectifier, and the frequency at a phase angle of −45°, which is the frequency at which the device behaves equally as a capacitor and resistor. The phase angle as a function of the frequency is presented in FIG. 17, which is called Bode plot. From the Bode plot, one can see that the angle at 120 Hz for Au/Ru/RuO.sub.x and Ni/Ru/RuO.sub.x devices are −65° and −80° respectively. The frequencies, at a phase angle of −45°, for the Au/Ru/RuO.sub.2 and Ni/Ru/RuO.sub.2 devices are 328 Hz and 1678 Hz respectively. The results demonstrate the superior behavior of the Ni/Ru/RuO.sub.2 device at high frequencies.

Galvanostatic Charge/Discharge Measurements

[0067] Galvanostatic charge/discharge curves were recorded at a voltage window of 1.5 V in a current density range of 10-1000 mA cm.sup.−2 (0.59-59 kA cm.sup.−3). The curves are shown in FIG. 18 A (at current densities of 10, 20, 40, 50, 75, and 100 mA cm.sup.−2) and 18 B (at current densities of 200, 400, 500, 750, and 1000 mA cm.sup.−2). The linear curves indicate good capacitor behavior even at high current densities.

[0068] The data obtained from the galvanostatic charge/discharge curves were used to evaluate the capacitance and energy density properties of the Ru/RuO.sub.2-coated nickel supercapacitor prepared by 2.5k CV cycles and the results are graphically presented in FIGS. 19 A and B.

[0069] In FIG. 19A, areal (black; squares—see left ordinate) and volumetric (red; circles—see right ordinate) capacitance density as a function of current density are shown (volume was calculated from the thickness of the active layers of two electrodes). Specifically, a volumetric capacitance of 50 F cm.sup.−3 was measured for a current density of 59 kA cm.sup.−3 (red-circles curve, right ordinate). The high areal capacitance is 1.88 mF cm.sup.−2 at a current density of 10 mA cm.sup.−2 (black-squares curve, left ordinate). Notably, the device displays excellent capacitance retention with increasing current density, retaining 45% of the initial capacitance when the current density increased to 1000 mA cm.sup.−2 (0.85 mF cm.sup.−2).

[0070] In FIG. 19B, areal (black; squares—see left ordinate) and volumetric (red; circles—see right ordinate) energy densities are plotted as a function of power density (volume was calculated from the thickness of the active layers of two electrodes). Due to the high current density reached (1000 mA cm.sup.−2) high areal power density of 1500 mW cm.sup.−2 can be obtained, retaining a high areal energy density of 0.27 μWh cm.sup.−2.

Example 8

Ni.SUB.(base)./Ni.SUP.(0).Ru.SUP.(0)./RuO.SUB.2 .Electrode Fabrication

[0071] Deposition of Ni, Ru and NiRu was carried out on a commercial Ni foil. A 1 M Na.sub.2SO.sub.4 deposition solution which contained RuCl.sub.3 (0.01 M) and NiSO.sub.4*7H.sub.2O (0.01 M) was used. The deposition was conducted using a 3-electrodes configuration with the Ni foil as the working electrode, Pt wire as the counter electrode and Ag|AgCl (3 M KCl) as the reference electrode. The deposition was accomplished using cyclic voltammetry (2.5k cycles) between 0 V and −1.1V vs reference electrode at a scan rate of 10 V s.sup.−1. Subsequent oxidation of the Ru to RuO.sub.2 was carried out by running 5k CV cycles at a scan rate of 10 V s.sup.−1 in a voltage window of 0 and 0.8V using a 3-electrodes configuration with either Ni/Ru or Ni/NiRu as the working electrode, Pt wire as the counter electrode and Ag|AgCl (3 M KCl) as the reference electrode in 1M Na.sub.2SO.sub.4 solution. The deposition/oxidation was conducted on a SP-150 Bio-Logic (Claix, France).

Example 9

Characterization of Ni.SUB.(base)./Ni.SUP.(0) .Ru.SUP.(0)./RuO.SUB.2 .Electrode

[0072] Scanning electron microscopy (SEM) image showing the surface morphology of the NiRu/RuO.sub.2 layer is presented in FIG. 20, indicating formation of dense NiRu/RuO.sub.2 nanostructures. Cross-section scanning transmission electron microscopy (STEM) images depicted in FIGS. 21Bi&Ci (B and C are the first and second rows, respectively) show the structure of the electrodeposited layer before (FIG. 21Bi) and after (FIG. 21Ci) electrochemical oxidation of the Ru. The dendric like morphology of the Ni.sup.(0)Ru.sup.(0) alloy clearly shows the effect of the Ni on the structure of the final electrodeposited layer compared to structure grown only from Ru precursor described herein in FIG. 8B. In addition, the STEM (FIGS. 21Bi&Ci) images show no change in the structure due to the electrochemical oxidation process. Electron dispersion spectroscopy (EDS) elemental map shows a uniform distribution of both Ni (FIGS. 21Bii&Cii) and Ru (FIGS. 21Biii&Ciii) across the deposited film.

[0073] X-ray photoelectron spectroscopy (XPS) spectra are shown in FIG. 22A, of the ruthenium 3d core level before and after electrochemical oxidation and in FIG. 22B of the Ni 2p core level before and after electrochemical oxidation. XPS analysis of the Ru 3d core level peaks indicates that the as-deposited Ru on the surface has already been oxidized, with only 32.5% of the Ru on the surface being in the Ru.sup.(0) state (FIG. 22A, upper spectrum). After electrochemical oxidation (FIG. 22A, lower spectrum) the percentage of the Ru.sup.(0) on the surface is reduced to 17% and most of the RuO.sub.2 on the surface is anhydrous (67%, peak at 280.73 eV).

[0074] X-ray diffraction (XRD) spectra is shown in FIG. 23. XRD spectra of as deposited Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0) (black) and after electrochemical oxidation Ni.sub.(base/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 (red) is presented on a logarithmic scale. (bold circles peaks correspond to hexagonal NiRu (Ru rich, PDF No. 01-072-2536), X peaks correspond to hexagonal Ru (PDF No. 00-006-0663), and bold triangles peaks correspond to cubic Ni arising from the Ni substrate (PDF No. 00-004-0850). XRD analysis lends support to the finding that the XPS signal at binding energy (BE) of 280.18 eV arises from the hexagonal NiRu phase, indicating that the NiRu phase is Ru rich. This is consistent with the higher reduction potential of Ru, compared to Ni, which supports electrochemical deposition of a Ru-rich NiRu phase. In addition, no hexagonal Ru phase was detected, supporting that the formation of RuO.sub.2 due to electrochemical oxidation is through oxidation of the Ru from the hexagonal NiRu phase. XPS analysis of the Ni 2p core-level peaks, prior to the Ru electrochemical oxidation (FIG. 22B top spectrum), indicates a high-intensity satellite peak arising at 855.7 eV which was previously shown for the formation of NiO layer on top the NiRu alloy. After the electrochemical oxidation of the Ru the satellite peak indicating NiO is removed and two peaks at 855.1 eV and 853.0 eV indicating the exitance of Ni.sup.(0) in NiRu alloy appeared (FIG. 2B bottom spectrum).

[0075] Analysis of the Ni.sup.(0)Ru.sup.(0) and Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 layers was conducted on a cross-section lamella prepared through a focus ion beam (FIB). High-resolution transmission electron microscopy (HRTEM) image of the as-deposited NiRu film revels that the film is composed of crystalline NiRu nanoparticles (FIG. 22C). The HRTEM image indicates that Ru is predominantly present in the NiRu phase whereas the exitance of RuO.sub.2 is mainly in the surface (see also XPS results, FIG. 22A top spectrum). The inset in FIG. 22C shows a lattice spacing of 0.208 nm corresponding to (002) plane of hexagonal NiRu, which means that the phase is Ru rich. Diffraction pattern of the film, FIG. 22D, showing the (100), (101), (110), (102), and (103) crystal plane of hexagonal RuNi (PDF card No. 01-072-2536), lends further support to the observation that the film does not contain metallic Ni nor metallic Ru phases. EDS point analysis conducted across the film shows a roughly constant atomic ratio of 5:3:2 for Ni:Ru:O, respectively. The high content of Ni, in contradiction to the Ru rich NiRu phase, is due to the existence of the NiO indicated by the XPS results. After the electrochemical oxidation of Ru, NiO is removed (XPS, FIG. 22B bottom spectrum), and the content of Ni becomes lower than Ru, which is consistent with the presence of the hexagonal NiRu phase.

[0076] In addition, the analysis of the NiRu/RuO.sub.2 layer (after electrochemical oxidation), presented in FIGS. 22E&F, reveals that the core is still composed mainly of hexagonal NiRu, where the addition of amorphous RuO.sub.2 is limited to the surface. Interestingly, while both XRD and HRTEM diffraction results show hexagonal NiRu phase, a close inspection of the NiRu particles also reveals the existence of a small amount cubic NiRu, which indicates a Ni-rich NiRu phase.

Example 10

Assembly of Symmetric Supercapacitor Composed of a Pair of Ni.SUB.(base)./Ni.SUP.(0).Ru.SUP.(0)./RuO.SUB.2 .Electrodes

[0077] A symmetric supercapacitor was fabricated, based on a pair of Ni.sub.(base)/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 electrodes of Example 8. Coated electrodes produced following 2500 deposition cycles from 0.01M RuCl.sub.3+0.01 M NiSO.sub.4+1M Na.sub.2SO.sub.4 deposition solution were used for creation of the symmetric supercapacitor.

Step 1: Spacer Fabrication

[0078] Polystyrene (PS) fibers were electrospun from a polystyrene solution in dimethylformamide (30% w/v), on 2.5 cm 7.5 cm glass for 40 min (under a 20 kV voltage with a 22 cm between needle and collector) and were annealed at 100° C. for 15 min. Following the annealing, carboxylation of the PS microfibers was carried out by exposing the PS fibers to air plasma for 2 min under vacuum at 85 W. Finally, the film was kept in a solution of 1M Na.sub.2SO.sub.4 prior to use.

Step 2: Device Assembly

[0079] Symmetric supercapacitor was assembled from of a pair of Ni.sub.(base))/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 electrodes (with area between 0.1 and 0.3 cm.sup.2) and the PS spacer soaked in 1 M Na.sub.2SO.sub.4 solution. The device was then wrapped using scotch tape.

Example 11

Electrochemical Properties of Symmetric Supercapacitor Composed of a Pair of Ni.SUB.(base)./Ni.SUP.(0).Ru.SUP.(0)./RuO.SUB.2 .Electrodes

[0080] The symmetric supercapacitor of Example 10 was tested using different electrochemical techniques.

Cyclic Voltammetry Measurements

[0081] FIGS. 24A to 24E show CV curves recorded for symmetric supercapacitors device assembled as described in Example 10. Cyclic voltammetry measurements were conducted with a two-electrode configuration (one electrode is connected to the working electrode terminal and the other to both the counter and reference electrode terminals) at the voltage range of 0 to 1 volt at different scan rates (10 V/s, 200 V/s, 500 V/s, 1000 V/s and 2000 V/s, which correspond to FIGS. 24A to 24E, respectively) in 1M Na.sub.2SO.sub.4 electrolyte solution. The instrument used was CH instrument.

[0082] It is seen that the rectangular shape is retained for high scan rates such as 10, 200, and 500 V s.sup.−1 (FIG. 24A-C). A semi-rectangular shape still exists at 1000 V s.sup.−1. Notably, 2200 mA cm.sup.−2 areal current density is achieved at 2000 V s.sup.−1 (FIG. 24E). FIG. 24F presents the discharge peak current density as a function of the scan rate, showing a linear behavior up to 1000 V s.sup.−1.

[0083] The capacitance, calculated from the CV curves, is plotted against scan rate in FIG. 25. A high capacitance of 1.93 mF cm.sup.−2 at 10 V s.sup.−1 is noted, a 0.79 mF cm.sup.−2 at 1000 V s.sup.−1 (41% retention), and 0.46 mF cm.sup.−2 at 2000 V s.sup.−1 (24% retention).

Impedance Spectroscopy Measurements

[0084] The performance of the symmetric supercapacitor of Example 10 was assessed through electrochemical impedance spectroscopy measurements.

[0085] A Nyquist plot in a frequency range of 100 kHz-1 Hz is shown in FIG. 26A. In view of the nearly perpendicular line and in the absence of any visible semicircular Nyquist plot, one can model the device as a capacitor and a resistor connected in series. In addition, the nearly perpendicular line indicates a negligible current leakage from the device, demonstrating ideal capacitor behavior. The resistance at the frequency of 120 Hz extracted from the Nyquist plot is 0.260 Ω cm.sup.2.

[0086] The capacitor behavior of the devices was assessed by calculating the phase angle as a function of the frequency (FIG. 26B, black circles forming the upper curve). A phase angle of −90° indicates a perfect capacitor, while a phase angle of 0 indicate a perfect resistor. Because the phase angle decreases with increasing capacitance, the −76° calculated phase angle according to FIG. 26B is considered a reasonably high value. A phase angle of −45°, which indicates an equal capacitor and resistor behavior, was measured at a frequency of 966 Hz. The minimum point, in the graph of the overall impedance as a function of the frequency (FIG. 26B, blue circles forming the lower curve), indicates the equivalent series resistance (ESR) of the device and has a value of 0.122 Ω cm.sup.2. This low value is important in order to reach high power densities.

[0087] The capacitance which was calculated from the imaginary part of the impedance is plotted against the frequency in FIG. 26C. A high capacitance (1.31 mF cm.sup.−2) is calculated at 120 Hz. From the capacitance and the resistance, a RC time constant (τ.sub.RC) of 0.31 ms is calculated. This results shows that the NiRu structure has high efficient electron conduction and high surface area which results in low resistance and high capacitance arising from the thin layer of RuO.sub.2 on the surface of the NiRu.

Galvanostatic Charge/Discharge Measurements

[0088] Galvanostatic charge/discharge curves were recorded at a voltage window of 1.5 V across a current density range of 10-2000 mA cm.sup.−2. The curves are shown in FIG. 27A (at current densities of 10, 20, 50, 75, and 100 mA cm.sup.−2) and 27B (at current densities of 200, 500, 750, 1000 and 2000 mA cm.sup.−2). The linear curves indicate good capacitor behavior even at high current densities.

[0089] The data obtained from the galvanostatic charge/discharge curves were used to evaluate the capacitance and energy density properties of the Ni.sub.(base))/Ni.sup.(0)Ru.sup.(0)/RuO.sub.2 supercapacitor of Example 10 and the results are graphically presented in FIGS. 27C and 27D.

[0090] In FIG. 27C, areal capacitance density as a function of current density is shown. Specifically, high areal capacitance of 2.29 mF cm.sup.−2 at a current density of 10 mA cm.sup.−2 is noted. Notably, the device displays excellent capacitance retention with increasing current density, retaining 47% of the initial capacitance when the current density increased to 2000 mA cm.sup.−2 (1.07 mF cm.sup.−2).

[0091] In FIG. 27D, areal energy density is plotted as a function of power density.