WO2009011684A1 - Nanoscale manganese oxide on ultraporous carbon nanoarchitecture - Google Patents
Nanoscale manganese oxide on ultraporous carbon nanoarchitecture Download PDFInfo
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- WO2009011684A1 WO2009011684A1 PCT/US2007/019547 US2007019547W WO2009011684A1 WO 2009011684 A1 WO2009011684 A1 WO 2009011684A1 US 2007019547 W US2007019547 W US 2007019547W WO 2009011684 A1 WO2009011684 A1 WO 2009011684A1
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 title claims abstract description 40
- 229910052799 carbon Inorganic materials 0.000 title claims description 38
- AMWRITDGCCNYAT-UHFFFAOYSA-L hydroxy(oxo)manganese;manganese Chemical compound [Mn].O[Mn]=O.O[Mn]=O AMWRITDGCCNYAT-UHFFFAOYSA-L 0.000 title description 5
- NUJOXMJBOLGQSY-UHFFFAOYSA-N manganese dioxide Chemical compound O=[Mn]=O NUJOXMJBOLGQSY-UHFFFAOYSA-N 0.000 claims abstract description 121
- 230000008021 deposition Effects 0.000 claims abstract description 25
- 238000000576 coating method Methods 0.000 claims abstract description 19
- 239000011148 porous material Substances 0.000 claims abstract description 18
- 239000011248 coating agent Substances 0.000 claims abstract description 15
- 239000002131 composite material Substances 0.000 claims abstract description 13
- 239000008209 carbon nanofoam Substances 0.000 claims description 25
- 229910021400 carbon nanofoam Inorganic materials 0.000 claims description 24
- 239000003990 capacitor Substances 0.000 claims description 23
- 239000003792 electrolyte Substances 0.000 claims description 11
- 230000007935 neutral effect Effects 0.000 claims description 11
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims description 9
- 239000004966 Carbon aerogel Substances 0.000 claims description 7
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 claims description 5
- 229910052938 sodium sulfate Inorganic materials 0.000 claims description 5
- 238000000429 assembly Methods 0.000 claims description 4
- 230000000712 assembly Effects 0.000 claims description 4
- 229910021393 carbon nanotube Inorganic materials 0.000 claims description 4
- 239000002041 carbon nanotube Substances 0.000 claims description 4
- 150000001768 cations Chemical class 0.000 claims description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 3
- 239000007788 liquid Substances 0.000 claims 5
- 230000003139 buffering effect Effects 0.000 claims 2
- 239000002134 carbon nanofiber Substances 0.000 claims 2
- 150000003839 salts Chemical class 0.000 claims 1
- 235000011152 sodium sulphate Nutrition 0.000 claims 1
- 238000000151 deposition Methods 0.000 description 23
- 239000000758 substrate Substances 0.000 description 11
- 239000000243 solution Substances 0.000 description 9
- WOCIAKWEIIZHES-UHFFFAOYSA-N ruthenium(iv) oxide Chemical compound O=[Ru]=O WOCIAKWEIIZHES-UHFFFAOYSA-N 0.000 description 8
- 239000008208 nanofoam Substances 0.000 description 7
- 206010039509 Scab Diseases 0.000 description 6
- 230000002378 acidificating effect Effects 0.000 description 6
- 238000013461 design Methods 0.000 description 6
- 238000001878 scanning electron micrograph Methods 0.000 description 6
- 239000011572 manganese Substances 0.000 description 5
- 230000007246 mechanism Effects 0.000 description 5
- 239000007832 Na2SO4 Substances 0.000 description 4
- 239000002253 acid Substances 0.000 description 4
- 238000009826 distribution Methods 0.000 description 4
- 238000004146 energy storage Methods 0.000 description 4
- 239000010408 film Substances 0.000 description 4
- 238000011068 loading method Methods 0.000 description 4
- 239000002243 precursor Substances 0.000 description 4
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 238000013459 approach Methods 0.000 description 3
- 239000007864 aqueous solution Substances 0.000 description 3
- 230000008901 benefit Effects 0.000 description 3
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- 238000002484 cyclic voltammetry Methods 0.000 description 3
- 238000002848 electrochemical method Methods 0.000 description 3
- 238000004070 electrodeposition Methods 0.000 description 3
- 238000010348 incorporation Methods 0.000 description 3
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- 239000000872 buffer Substances 0.000 description 2
- 150000001721 carbon Chemical class 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000000354 decomposition reaction Methods 0.000 description 2
- 238000001764 infiltration Methods 0.000 description 2
- 230000008595 infiltration Effects 0.000 description 2
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- 238000000034 method Methods 0.000 description 2
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- 239000002121 nanofiber Substances 0.000 description 2
- 239000002071 nanotube Substances 0.000 description 2
- 239000000843 powder Substances 0.000 description 2
- 238000006722 reduction reaction Methods 0.000 description 2
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- -1 RuO2 Chemical class 0.000 description 1
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 1
- 238000004833 X-ray photoelectron spectroscopy Methods 0.000 description 1
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- 229910021386 carbon form Inorganic materials 0.000 description 1
- 239000002717 carbon nanostructure Substances 0.000 description 1
- OKTJSMMVPCPJKN-BJUDXGSMSA-N carbon-11 Chemical compound [11C] OKTJSMMVPCPJKN-BJUDXGSMSA-N 0.000 description 1
- 238000005234 chemical deposition Methods 0.000 description 1
- 239000003638 chemical reducing agent Substances 0.000 description 1
- 230000002860 competitive effect Effects 0.000 description 1
- 239000002322 conducting polymer Substances 0.000 description 1
- 229920001940 conductive polymer Polymers 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000005137 deposition process Methods 0.000 description 1
- ZOMNIUBKTOKEHS-UHFFFAOYSA-L dimercury dichloride Chemical class Cl[Hg][Hg]Cl ZOMNIUBKTOKEHS-UHFFFAOYSA-L 0.000 description 1
- 238000000840 electrochemical analysis Methods 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000006260 foam Substances 0.000 description 1
- 229910021397 glassy carbon Inorganic materials 0.000 description 1
- 238000001566 impedance spectroscopy Methods 0.000 description 1
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- 238000011835 investigation Methods 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 239000011244 liquid electrolyte Substances 0.000 description 1
- 229940052961 longrange Drugs 0.000 description 1
- 230000014759 maintenance of location Effects 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- PPNAOCWZXJOHFK-UHFFFAOYSA-N manganese(2+);oxygen(2-) Chemical class [O-2].[Mn+2] PPNAOCWZXJOHFK-UHFFFAOYSA-N 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 230000003278 mimic effect Effects 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 239000008363 phosphate buffer Substances 0.000 description 1
- 230000010287 polarization Effects 0.000 description 1
- 239000012286 potassium permanganate Substances 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 238000006479 redox reaction Methods 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000002829 reductive effect Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000027756 respiratory electron transport chain Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
- 238000004626 scanning electron microscopy Methods 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 239000011232 storage material Substances 0.000 description 1
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- 230000008685 targeting Effects 0.000 description 1
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- 238000006276 transfer reaction Methods 0.000 description 1
- 229910052723 transition metal Inorganic materials 0.000 description 1
- 229910000314 transition metal oxide Inorganic materials 0.000 description 1
- 150000003624 transition metals Chemical class 0.000 description 1
- 230000032258 transport Effects 0.000 description 1
- 229910021642 ultra pure water Inorganic materials 0.000 description 1
- 239000012498 ultrapure water Substances 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/30—Electrodes characterised by their material
- H01G11/32—Carbon-based
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/30—Electrodes characterised by their material
- H01G11/32—Carbon-based
- H01G11/36—Nanostructures, e.g. nanofibres, nanotubes or fullerenes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/30—Electrodes characterised by their material
- H01G11/46—Metal oxides
Definitions
- Electrochemical capacitors are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Energy storage in an electrochemical capacitor is accomplished by two principal mechanisms: double-layer capacitance and pseudocapacitance.
- Double-layer capacitance arises from the separation of charge that occurs at an electrified interface. With this mechanism the capacitance is related to the active electrode surface area, with practical capacitances in liquid electrolytes of 10—40 ⁇ F cm "2 .
- Electrochemical capacitors based on double-layer capacitance are typically designed with high- surface-area carbon electrodes, including carbon aerogels, foams, nanotube/nanofiber assemblies, and papers. Carbon aerogels and related porous carbons are particularly attractive due to their high surface areas, high porosities, and excellent conductivities (>40 S cm " '). Although the high-quality porosity of such carbon nanoarchitectures supports rapid charge- discharge operation, the overall energy-storage capacities of carbon electrodes are ultimately limited by their reliance on the double-layer capacitance mechanism.
- Pseudocapacitance broadly describes faradaic reactions whose current-voltage profiles mimic those of double-layer capacitors. Because this mechanism involves true electron-transfer reactions and is not strictly limited to the electrode surface, materials exhibiting pseudocapacitance often have higher energy densities relative to double-layer capacitors.
- the two main classes of materials being researched for their pseudocapacitance are transition metal oxides and conducting polymers.
- Electrodes based on disordered, hydrous RuO 2 yield specific capacitances up to 720F g '1 .
- the application OfRuO 2 is limited by the high costs of the ruthenium precursors.
- Manganese oxides have recently gained attention as active materials for electrochemical capacitors, primarily due to their significantly lower cost relative to hydrous RuO 2 .
- Electrodes involve electrode structures incorporating carbon nanotubes (an expensive carbon substrate) or alternative MnO 2 deposition methods (e.g., electrodeposition, sputtering), which are more complicated, costly and more difficult to control.
- MnO 2 deposition methods e.g., electrodeposition, sputtering
- Figure 1 shows a schematic of a hybrid electrode structure comprising a highly porous carbon nanostructure coated with nanoscopic MnO 2 deposits.
- FIG. 2 shows a schematic of electrodeposition on an ultraporous electrode structure in (i) a poorly controlled manner in which the pores are ultimately blocked by the growing film and (ii) a controlled, self-limiting deposition.
- Figure 3 shows scanning electron micrographs of (a and b) 4-h acid-deposited MnO 2 - carbon, (c and d) 4-h neutral-deposited MnO 2 - carbon, and (e and f) bare carbon nanofoam.
- Figure 4 shows a scanning electron micrograph (top) for the cross-section of a 4-h neutral-deposited Mn ⁇ 2 -carbon, and corresponding elemental mapping images (middle and bottom) of this same area of the sample for carbon and manganese content, respectively.
- Figure 5 shows cyclic voltammograms at 2 and 20 mV s "1 in 1 M Na 2 SO 4 for bare carbon nanofoam ( — ) and 4-h MnO ⁇ -carbon nanofoam deposited from acidic permanganate solution ( ) and neutral permanganate solution (• • •).
- Figure 6 shows a Nyquist plot and a capacitance vs. frequency profile for acid- deposited, neutral-deposited Mn ⁇ 2 -carbon nanofoam and bare carbon nanofoam.
- Figure 7 shows scanning electron micrographs of the surfaces of 4-h neutral-deposited Mn ⁇ 2 -carbon nanofoams, where AgMnO 4 is the deposition precursor, both with (top) and without (bottom) the addition of a pH 6.9 phosphate buffer.
- Nanostructured Mn ⁇ 2 -carbon nanoarchitecture hybrids can be designed as electrode structures for high-energy-density electrochemical capacitors that retain high power density. Homogeneous, ultrathin coatings of nanoscale MnO 2 can be incorporated within porous, high- surface-area carbon substrates (such as carbon nanofoams) via electroless deposition from aqueous permanganate under controlled pH conditions. The resulting hybrid structures exhibit enhanced gravimetric, volumetric, and area-normalized capacitance when electrochemically cycled in aqueous electrolytes. This design can be extended to other mesoporous and macroporous carbon forms possessing a continuous pore network.
- MnO 2 - carbon composites have been reported and primarily focused on incorporating nanoscale MnO 2 deposits onto carbon nanotubes using a variety of approaches including simple physical mixing of the components, chemical deposition using such precursors as KMnO 4 , and electrochemical deposition.
- the incorporation of MnO 2 improves the capacitance of the electrode structures that contain the MnO 2 -modified carbon nanotubes; however, the overall specific capacitance for the composite structures is typically limited to ⁇ 200 F g "1 , even for electrodes with high weight loadings of MnO 2 .
- Templated mesoporous carbon powders have also been used as a substrate for MnO 2 deposition as demonstrated by Dong et al., who used the reaction of permanganate with the carbon substrate to generate nanoscale MnO 2 deposits directly on the mesopore walls. Electrochemical testing of the resulting MnC> 2 -mesoporous carbon structures revealed that the MnO 2 deposits themselves exhibited a specific capacitance of -600 F g ⁇ ⁇ which approaches the 700 F g "1 reported by Pang et al. for nanometers-thick MnO 2 films.
- the overall specific capacitance of the hybrid MnC> 2 -mesoporous carbon structure was limited to 200 F g "1 , due to the relatively low weight loading (up to 26%) OfMnO 2 .
- the extent OfMnO 2 deposition within the mesoporous carbon substrate can be limited by the inherently small pore size ( ⁇ 3 nm) of the carbon.
- Carbon nanofoam papers were either purchased from a commercial source or prepared in-house.
- Mn ⁇ 2 -carbon nanoarchitecture hybrids were created based on the reductive decomposition of permanganate from aqueous solutions.
- the carbon nanoarchitecture surface can serve as a sacrificial reductant, converting the aqueous permanganate to insoluble MnO 2 .
- Carbon nanofoam substrates ( ⁇ 170- ⁇ m thick) were first wetted in an aqueous solution of controlled pH (0.1 M H 2 SO 4 , 0.1 M Na 2 SO 4 , or 0.1 M NaOH) by vacuum infiltration. The samples were then soaked in 0.1 M NaMnO 4 at each respective pH for a period of 5 min to 4 h. The Mn ⁇ 2-carbon nanofoam papers were rinsed thoroughly with ultrapure water and subsequently dried at ⁇ 50 0 C under N 2 for 8 hours and then under vacuum overnight.
- controlled pH 0.1 M H 2 SO 4 , 0.1 M Na 2 SO 4 , or 0.1 M NaOH
- Control of the permanganate reduction reaction can be required to achieve nanoscale MnO 2 deposits throughout the carbon nanoarchitecture as well as to inhibit the formation of thick MnO 2 coatings on the outer boundary of the carbon electrode.
- Preliminary results suggest that pH can be a critical factor in determining the quality of the MnO 2 deposition.
- the MnO 2 mass uptake (up to -60% for a 24-h deposition) can be relatively independent of the solution pH.
- the SEM analysis further confirmed that the porous texture of the initial carbon nanofoam can be largely retained following MnO 2 deposition (see Figure 3d and 3f).
- the retention of the nanofoam' s high-quality pore structure can result in better electrochemical performance under high-rate charge -discharge operation.
- Example 2 Electrochemical characterization of hybrid structures.
- MnO 2 -carbon nanofoam electrodes were wetted with 1 M Na 2 SO 4 under vacuum for electrochemical analysis and characterized in a conventional three-electrode electrochemical cell using techniques such as cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge measurements.
- Representative cyclic voltammograms of the bare carbon aerogel, 4-h acid-deposited, and 4-h neutral-deposited MnO 2 - carbon nanofoam electrodes in 1 M Na 2 SO4 at 2 and 20 mV s "1 are presented in Figure 5.
- a saturated calomel reference electrode (SCE) and reticulated vitreous carbon auxiliary electrode were used in all electrochemical measurements. At 2 mV s "1 , all curves exhibit a nearly symmetrical rectangular shape, indicative of relatively low uncompensated electrode or solution resistance.
- the gravimetric (normalized to total sample mass), volumetric, and area- normalized capacitance values calculated from these curves between 0.1 and 0.6 V vs. SCE are presented in Table 1. Both the total gravimetric and volumetric capacitance values increase for the acid- and neutral-deposited samples. Notably, the gravimetric capacitance increases by a factor of 2 for the neutral-deposited sample, while the volumetric capacitance is over 4 times greater. It is important to note that in the case of homogeneous, nanoscopic MnO 2 deposits like those in the neutral-deposited hybrid electrode, the incorporation of MnO 2 can contribute additional capacitance without increasing the bulk volume of the electrode structure.
- the area-normalized energy- storage capacity should also be considered.
- the area-normalized capacitance is often not reported for MnO ⁇ /carbon composites, it is usually around 10-50 mF cm "2 .
- the present hybrid electrode design maintains the advantages of a nanoscopic electrode/electrolyte interface while projecting the electrode structure in three dimensions with a limited footprint, such that the area-normalized capacitance for the neutral-deposited MnOr-carbon hybrid electrodes is orders of magnitude greater at >2 F cm "2 .
- the upper and lower limits of capacitance attributed to MnO 2 in Table 1 were estimated using one of two assumptions: (1) all capacitance arises from the MnO 2 phase (upper limit) or (2) the total sample capacitance was the sum of the carbon double-layer capacitance and the MnO 2 capacitance (lower limit). Although the capacitance attributable to the MnO 2 phase for the acid- and neutral-deposited samples likely falls within this range, the capacitance contribution from the carbon is expected to be different for the acid and neutral case because of the variation in the MnO 2 spatial distribution. For example in the acid case, the double-layer capacitance contribution of carbon should be largely unaffected due to the limited MnO 2 deposition in the electrode interior. Thus, the Mn ⁇ 2 -normalized capacitance is likely near the lower estimated limit, while that for the neutral sample is expected to be higher as a result of extensive MnO 2 coating the carbon on the electrode interior.
- the uncompensated solution resistance (Rn) of each electrode is similar, as shown in the Nyquist plot ( Figure 6a).
- the large hemispherical component for the MnO 2 -carbon nanofoam electrode deposited under acidic conditions is indicative of polarization as expressed by a charge-transfer resistance (Rp) of about 15 ⁇ .
- the profile for the neutral-deposited sample is more similar to that of the bare carbon nanofoam, with an R p of ⁇ 1 ⁇ .
- the capacitance vs. frequency profile of the neutral-deposited sample in Figure 6b shows that from about 0.01 to 1 Hz, the MnO 2 component can increase the capacitance of the bare carbon nanofoam.
- the capacitance for both electrodes begins to decrease, falling to below 1 F g "1 around 200 Hz.
- the initial capacitance increase for the acid-deposited sample at 0.01 Hz, with respect to the bare nanofoam, can be much lower than that for the neutral-deposited sample and begins to decrease between 0.1 and 1 Hz, falling below 1 F g '1 at 30 Hz.
- the electroless deposition described herein can be a simple, cost-effective, and scaleable approach for synthesizing MnC> 2 -carbon hybrid nanoarchitectures with electrochemical capacitance that is superior to unmodified carbon substrates.
- This disclosure demonstrates that by controlling solution pH during the deposition process, homogeneous MnO 2 deposits are achieved throughout macroscopically thick porous carbon templates.
- MnO 2 deposition there can be many benefits of homogenous MnO 2 deposition as can be evident when such structures are electrochemically analyzed.
- Mn ⁇ 2 -carbon hybrids that exhibit uniform MnO 2 distribution also exhibit higher overall gravimetric and volumetric capacitance, and higher MnO ⁇ -specific capacitance than acid-deposited MnO 2 - carbon hybrids, in which the MnO 2 is primarily deposited as a crust on the outer boundaries of the electrode.
- Uniform deposition within the interior of the carbon nanoarchitecture also can result in greater enhancement when the volumetric capacitance is considered, as the addition of the MnO 2 component contributes additional capacitance without increasing the bulk volume of the electrode structure.
- the gravimetric capacitance is increased by a factor of 3.3, while the volumetric capacitance is increased by a factor of 4.1.
- electroless deposition conditions e.g., varying the solution temperature, precursor concentration, permanganate counterion — including transition metal speciation, constituents that define the acidic or neutral medium including buffers
- carbon template pore structure particularly targeting larger pore sizes (100-200 nm) and higher overall porosity, which should result in higher mass loadings OfMnO 2 -
- . ⁇ O 2 deposits is affected by the presence or absence of buffering agents. Without buffer, the oxide coating is more nodular and preferentially deposited on the outer boundary,, while from buffered medium, the deposit is more uniformly distributed, not nodular, and less thick (as seen in Figure 7).
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Abstract
A composite comprising a porous carbon structure comprising a surface and pores and a coating on the surface comprising MnO2, wherein the coating does not completely fill or obstruct a majority of the pores, and wherein the coating is formed by self-limiting electroless deposition.
Description
Nanoscale Manganese Oxide on Ultraporous Carbon Nanoarchitecture
[001] This application claims the benefit of U.S. provisional application No. 60/847,399 filed on September 11 , 2006.
[002] Electrochemical capacitors (also denoted as supercapacitors or ultracapacitors) are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Energy storage in an electrochemical capacitor is accomplished by two principal mechanisms: double-layer capacitance and pseudocapacitance.
[003] Double-layer capacitance arises from the separation of charge that occurs at an electrified interface. With this mechanism the capacitance is related to the active electrode surface area, with practical capacitances in liquid electrolytes of 10—40 μF cm"2. Electrochemical capacitors based on double-layer capacitance are typically designed with high- surface-area carbon electrodes, including carbon aerogels, foams, nanotube/nanofiber assemblies, and papers. Carbon aerogels and related porous carbons are particularly attractive due to their high surface areas, high porosities, and excellent conductivities (>40 S cm"'). Although the high-quality porosity of such carbon nanoarchitectures supports rapid charge- discharge operation, the overall energy-storage capacities of carbon electrodes are ultimately limited by their reliance on the double-layer capacitance mechanism.
[004] Pseudocapacitance broadly describes faradaic reactions whose current-voltage profiles mimic those of double-layer capacitors. Because this mechanism involves true electron-transfer reactions and is not strictly limited to the electrode surface, materials exhibiting pseudocapacitance often have higher energy densities relative to double-layer capacitors. The two main classes of materials being researched for their pseudocapacitance are transition metal
oxides and conducting polymers.
[005] At present, some of the best candidates for electrochemical capacitors are based on nanoscale forms of mixed ion— electron conducting metal oxides, such as RuO2, which store charge via a cation— electron insertion mechanism. RuIVO2 + x e" + x H+ ^^ HxRum xRuIV,.xθ2 [1]
Electrodes based on disordered, hydrous RuO2 yield specific capacitances up to 720F g'1. However, the application OfRuO2 is limited by the high costs of the ruthenium precursors.
[006] Manganese oxides have recently gained attention as active materials for electrochemical capacitors, primarily due to their significantly lower cost relative to hydrous RuO2. A survey of recent publications, combined with research findings at the NRL, shows that when prepared in traditional electrode configurations, such as micron-thick films or in composite electrodes containing carbon and binders, MnO2 delivers a specific capacitance of -20OF g"1, which is competitive with carbon supercapacitors, but far short of the 720 F g"1 obtained with hydrous RuO2. However, as reported independently by Pang et al. and Toupin et al., when MnO2 is produced as a very thin film (tens of nanometers or less) on a planar current collector, specific capacitances of 700 F g" 1 and 1380 F g"1 can be achieved, respectively. This disparity in measured capacitance can be attributed to poor long-range electronic and/or ionic conductivity for MnO2, which can inhibit the charge-discharge process in conventional electrode designs. Although ultrathin films of MnO2 deliver high specific capacitance, this configuration can limit the area-normalized capacitance for practical EC devices.
[007] Alternatives involve electrode structures incorporating carbon nanotubes (an expensive carbon substrate) or alternative MnO2 deposition methods (e.g., electrodeposition, sputtering), which are more complicated, costly and more difficult to control.
[008] Brief Description of the Figures
(0091 Figure 1 shows a schematic of a hybrid electrode structure comprising a highly porous carbon nanostructure coated with nanoscopic MnO2 deposits.
[001 OJ Figure 2 shows a schematic of electrodeposition on an ultraporous electrode structure in (i) a poorly controlled manner in which the pores are ultimately blocked by the growing film and (ii) a controlled, self-limiting deposition.
[0011] Figure 3 shows scanning electron micrographs of (a and b) 4-h acid-deposited MnO2- carbon, (c and d) 4-h neutral-deposited MnO2- carbon, and (e and f) bare carbon nanofoam.
[0012] Figure 4 shows a scanning electron micrograph (top) for the cross-section of a 4-h neutral-deposited Mnθ2-carbon, and corresponding elemental mapping images (middle and bottom) of this same area of the sample for carbon and manganese content, respectively.
[0013] Figure 5 shows cyclic voltammograms at 2 and 20 mV s"1 in 1 M Na2SO4 for bare carbon nanofoam ( — ) and 4-h MnO∑-carbon nanofoam deposited from acidic permanganate solution ( ) and neutral permanganate solution (• • •).
[0014] Figure 6 shows a Nyquist plot and a capacitance vs. frequency profile for acid- deposited, neutral-deposited Mnθ2-carbon nanofoam and bare carbon nanofoam.
[0015] Figure 7 shows scanning electron micrographs of the surfaces of 4-h neutral-deposited Mnθ2-carbon nanofoams, where AgMnO4 is the deposition precursor, both with (top) and without (bottom) the addition of a pH 6.9 phosphate buffer.
Disclosure
[0016] Nanostructured Mnθ2-carbon nanoarchitecture hybrids can be designed as electrode structures for high-energy-density electrochemical capacitors that retain high power density. Homogeneous, ultrathin coatings of nanoscale MnO2 can be incorporated within porous, high- surface-area carbon substrates (such as carbon nanofoams) via electroless deposition from
aqueous permanganate under controlled pH conditions. The resulting hybrid structures exhibit enhanced gravimetric, volumetric, and area-normalized capacitance when electrochemically cycled in aqueous electrolytes. This design can be extended to other mesoporous and macroporous carbon forms possessing a continuous pore network.
[0017] The performance limitations Of MnO2 for electrochemical capacitors can be addressed with a hybrid electrode design, by incorporating discrete nanoscale coatings or deposits OfMnO2 onto porous, high-surface-area carbon structures (see Figure 1). In such a configuration, long- range electronic conduction is facilitated through the carbon backbone and solid-state transport distances for ions through the MnO2 phase can be minimized by maintaining a nanoscopic carbon 11 MnO211 electrolyte interface throughout the macroscopic porous electrode. Such a design can be realized using various types of porous carbon substrates including but not limited to aerogels/nanofoams, templated mesoporous carbon, and nanotube/nanofiber assemblies.
[0018] The synthesis and electrochemical characterization of MnO2- carbon composites has been reported and primarily focused on incorporating nanoscale MnO2 deposits onto carbon nanotubes using a variety of approaches including simple physical mixing of the components, chemical deposition using such precursors as KMnO4, and electrochemical deposition. In these cases, the incorporation of MnO2 improves the capacitance of the electrode structures that contain the MnO2-modified carbon nanotubes; however, the overall specific capacitance for the composite structures is typically limited to < 200 F g"1, even for electrodes with high weight loadings of MnO2. One exception was reported by Lee et al., who demonstrated specific capacitances of up to 415 F g"1 as normalized to the MnO2 of the composite structure. However, those results were achieved only for micron-thick electrode structures containing MnO2- modified carbon nanotubes, again a configuration that limits energy density.
[0019] Templated mesoporous carbon powders have also been used as a substrate for MnO2 deposition as demonstrated by Dong et al., who used the reaction of permanganate with the
carbon substrate to generate nanoscale MnO2 deposits directly on the mesopore walls. Electrochemical testing of the resulting MnC>2-mesoporous carbon structures revealed that the MnO2 deposits themselves exhibited a specific capacitance of -600 F g~\ which approaches the 700 F g"1 reported by Pang et al. for nanometers-thick MnO2 films. Despite the high MnO2- normalized capacitance, the overall specific capacitance of the hybrid MnC>2-mesoporous carbon structure was limited to 200 F g"1, due to the relatively low weight loading (up to 26%) OfMnO2. The extent OfMnO2 deposition within the mesoporous carbon substrate can be limited by the inherently small pore size (~3 nm) of the carbon.
[0020] The investigations of Dong et al. and Lee et al. demonstrate that nanoscopic deposits of MnO2 on high-surface-area substrates do deliver high specific capacitance. To further optimize the performance of MnO2-carbon hybrid structures for electrochemical capacitor applications, at least three design parameters must be addressed: (i) achieving high weight loadings OfMnO2
(>50 wt. %); (ii) fabricating electrode structures with macroscopic thickness (tens to hundreds of microns); and (iii) retaining a through-connected pore network in 3D and with pores sized at >5 nm.
[0021] The use of thick carbon substrates, as opposed to dispersed carbon powders, presents new challenges for achieving homogeneous MnO2 deposition throughout the electrode structure, while preserving the native pore structure of the carbon template. A high-quality pore structure is vital for high-rate EC operation, facilitating electrolyte infiltration and ion transport. These properties can be achieved by using coating methods that are inherently self-limiting as shown schematically in Figure 2. Described in this disclosure is the self-limiting electroless deposition of nanoscale MnO2, based on the redox reaction of aqueous permanganate (MnO-f) and carbon aerogel/nanofoam substrates. The MnO2 prepared by this protocol is a complex structure incorporating cations and water; this material will be designated as MnO2 in the body of this application.
[0022] Example 1 Electroless deposition OfMnO2 on carbon nanofoams.
[0023] Carbon nanofoam papers were either purchased from a commercial source or prepared in-house. Mnθ2-carbon nanoarchitecture hybrids were created based on the reductive decomposition of permanganate from aqueous solutions. The carbon nanoarchitecture surface can serve as a sacrificial reductant, converting the aqueous permanganate to insoluble MnO2.
[0024] Carbon nanofoam substrates (~ 170-μm thick) were first wetted in an aqueous solution of controlled pH (0.1 M H2SO4, 0.1 M Na2SO4, or 0.1 M NaOH) by vacuum infiltration. The samples were then soaked in 0.1 M NaMnO4 at each respective pH for a period of 5 min to 4 h. The Mnθ2-carbon nanofoam papers were rinsed thoroughly with ultrapure water and subsequently dried at ~ 500C under N2 for 8 hours and then under vacuum overnight.
[0025] Control of the permanganate reduction reaction can be required to achieve nanoscale MnO2 deposits throughout the carbon nanoarchitecture as well as to inhibit the formation of thick MnO2 coatings on the outer boundary of the carbon electrode. Preliminary results suggest that pH can be a critical factor in determining the quality of the MnO2 deposition.
[0026] As shown by the scanning electron micrographs (SEM) in Figure 3 a and 3b, under acidic conditions, permanganate reacts with carbon nanofoams to primarily form thick crusts of MnO2 on the outer boundary of the carbon electrode, presumably due to the autocatalytic decomposition of permanganate in acid. A cross-sectional image of the acid-deposited MnO2 crust, shown in the inset of Figure 3a, reveals that the crust thickness was ~4 μm for a 4-h deposition. By contrast, permanganate reduction in neutral or basic pH solutions results in homogeneous MnO2 deposits (neutral sample, Figures 3c and 3d) that are nearly indistinguishable from the bare carbon aerogel (Figures 3e and 3f) with no MnO2 crust formation at the outer boundary of the nanofoam electrode.
(0027] The MnO2 mass uptake (up to -60% for a 24-h deposition) can be relatively independent of the solution pH. The SEM analysis further confirmed that the porous texture of
the initial carbon nanofoam can be largely retained following MnO2 deposition (see Figure 3d and 3f). The retention of the nanofoam' s high-quality pore structure can result in better electrochemical performance under high-rate charge -discharge operation.
[0028] The cross-sectional SEM and elemental mapping images of the Mnθ2-carbon nanofoam synthesized under neutral conditions in Figure 4 show that the Mn can be evenly distributed throughout the thickness of the electrode structure. Incorporation of the MnO2 domains within the porous carbon nanoarchitectures in such a homogeneous, conformal fashion can result in hybrid electrode structures with superior performance relative to the less ideal structures obtained under acidic deposition conditions. X-ray photoelectron spectroscopy was used to verify that Mn deposits were primarily in the form of Mnlll/Ivθ2, with no residual MnO4 ".
10029] Example 2 Electrochemical characterization of hybrid structures.
[0030] The MnO2-carbon nanofoam electrodes were wetted with 1 M Na2SO4 under vacuum for electrochemical analysis and characterized in a conventional three-electrode electrochemical cell using techniques such as cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge measurements. Representative cyclic voltammograms of the bare carbon aerogel, 4-h acid-deposited, and 4-h neutral-deposited MnO2- carbon nanofoam electrodes in 1 M Na2SO4 at 2 and 20 mV s"1 are presented in Figure 5.
[0031] A saturated calomel reference electrode (SCE) and reticulated vitreous carbon auxiliary electrode were used in all electrochemical measurements. At 2 mV s"1, all curves exhibit a nearly symmetrical rectangular shape, indicative of relatively low uncompensated electrode or solution resistance. The gravimetric (normalized to total sample mass), volumetric, and area- normalized capacitance values calculated from these curves between 0.1 and 0.6 V vs. SCE are presented in Table 1. Both the total gravimetric and volumetric capacitance values increase for the acid- and neutral-deposited samples. Notably, the gravimetric capacitance increases by a factor of 2 for the neutral-deposited sample, while the volumetric capacitance is over 4 times
greater. It is important to note that in the case of homogeneous, nanoscopic MnO2 deposits like those in the neutral-deposited hybrid electrode, the incorporation of MnO2 can contribute additional capacitance without increasing the bulk volume of the electrode structure.
[0032] When pulse power is required in a footprint- or area-limited configuration, as in microelectromechanical (MEMS) based and on-chip devices, the area-normalized energy- storage capacity should also be considered. Although the area-normalized capacitance is often not reported for MnO∑/carbon composites, it is usually around 10-50 mF cm"2. In contrast, the present hybrid electrode design maintains the advantages of a nanoscopic electrode/electrolyte interface while projecting the electrode structure in three dimensions with a limited footprint, such that the area-normalized capacitance for the neutral-deposited MnOr-carbon hybrid electrodes is orders of magnitude greater at >2 F cm"2.
[0033] The upper and lower limits of capacitance attributed to MnO2 in Table 1 were estimated using one of two assumptions: (1) all capacitance arises from the MnO2 phase (upper limit) or (2) the total sample capacitance was the sum of the carbon double-layer capacitance and the MnO2 capacitance (lower limit). Although the capacitance attributable to the MnO2 phase for the acid- and neutral-deposited samples likely falls within this range, the capacitance contribution from the carbon is expected to be different for the acid and neutral case because of the variation in the MnO2 spatial distribution. For example in the acid case, the double-layer capacitance contribution of carbon should be largely unaffected due to the limited MnO2 deposition in the electrode interior. Thus, the Mnθ2-normalized capacitance is likely near the lower estimated limit, while that for the neutral sample is expected to be higher as a result of extensive MnO2 coating the carbon on the electrode interior.
[0034] Although the total capacitance enhancement for the acid and neutral-deposited MnO2 samples presented in Table 1 is similar, the difference in the spatial distribution of MnO2 for the two samples results in a sloping voltammetric curve for the boundary-crusted, acid-deposited
Mnθ2— carbon nanofoam at 20 mV s"1 due to increasing resistance that results from non-uniform MnO2 deposition. This increased resistance is confirmed by electrochemical impedance analysis (EDC = 200 mV vs. SCE) presented in Figure 6 (similar results were observed at 0 and 600 mV).
[0035] At high frequencies, the uncompensated solution resistance (Rn) of each electrode is similar, as shown in the Nyquist plot (Figure 6a). However, the large hemispherical component for the MnO2 -carbon nanofoam electrode deposited under acidic conditions is indicative of polarization as expressed by a charge-transfer resistance (Rp) of about 15 Ω. In contrast, the profile for the neutral-deposited sample is more similar to that of the bare carbon nanofoam, with an Rp of ~ 1 Ω. The capacitance vs. frequency profile of the neutral-deposited sample in Figure 6b shows that from about 0.01 to 1 Hz, the MnO2 component can increase the capacitance of the bare carbon nanofoam. As the frequency increases, the capacitance for both electrodes begins to decrease, falling to below 1 F g"1 around 200 Hz. The initial capacitance increase for the acid-deposited sample at 0.01 Hz, with respect to the bare nanofoam, can be much lower than that for the neutral-deposited sample and begins to decrease between 0.1 and 1 Hz, falling below 1 F g'1 at 30 Hz.
[0036] The higher resistance and lower capacitance for the acid-deposited sample is likely due to the thick MnO2 crust that forms on the electrode exterior, hindering electron and ion transport, while the more ideal homogeneous distribution of MnO2 in the sample deposited under neutral conditions results in electrochemical characteristics more similar to the bare nanofoam.
[0037]
Table 1.
Specific MnO2-specific Volumetric Area-normalized capacitance capacitance range capacitance capacitance (F cm" (F g~'θMno2) (F g~'Mno2) (Fcm"3) V*
Bare nanofoam 53 — 20 0.56
Acid-deposited* 92 150 - 220 81 1.4
Neutral-deposited* 110 170 - 230 90 1.5
These data are derived for 4-h depositions from acidic or neutral permanganate solutions.
* Normalized to the geometric area of one face of the nanofoam electrode.
[0038] The electroless deposition described herein can be a simple, cost-effective, and scaleable approach for synthesizing MnC>2-carbon hybrid nanoarchitectures with electrochemical capacitance that is superior to unmodified carbon substrates. This disclosure demonstrates that by controlling solution pH during the deposition process, homogeneous MnO2 deposits are achieved throughout macroscopically thick porous carbon templates.
[0039] There can be many benefits of homogenous MnO2 deposition as can be evident when such structures are electrochemically analyzed. For example, Mnθ2-carbon hybrids that exhibit uniform MnO2 distribution (neutral-pH deposition) also exhibit higher overall gravimetric and volumetric capacitance, and higher MnO∑-specific capacitance than acid-deposited MnO2- carbon hybrids, in which the MnO2 is primarily deposited as a crust on the outer boundaries of the electrode.
[0040] Uniform deposition within the interior of the carbon nanoarchitecture also can result in greater enhancement when the volumetric capacitance is considered, as the addition of the MnO2 component contributes additional capacitance without increasing the bulk volume of the electrode structure. For example, with a carbon nanofoam coated under neutral-pH conditions
the gravimetric capacitance is increased by a factor of 3.3, while the volumetric capacitance is increased by a factor of 4.1. Even greater enhancements in electrochemical performance for these hybrids can be realized with further optimization of the electroless deposition conditions (e.g., varying the solution temperature, precursor concentration, permanganate counterion — including transition metal speciation, constituents that define the acidic or neutral medium including buffers) and also by varying the carbon template pore structure, particularly targeting larger pore sizes (100-200 nm) and higher overall porosity, which should result in higher mass loadings OfMnO2-
[0041] In an example of two modifications to the deposition protocol, carbon nano foams were soaked in nominally neutral aqueous solutions of commercially available AgMnO4 (substituted for NaMnO4) under buffered and unbuffered conditions. The morphology of the resulting
AgxMnm/IV|.χO2 deposits is affected by the presence or absence of buffering agents. Without buffer, the oxide coating is more nodular and preferentially deposited on the outer boundary,, while from buffered medium, the deposit is more uniformly distributed, not nodular, and less thick (as seen in Figure 7).
[0042] Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles "a," "an," "the," or "said" is not construed as limiting the element to the singular.
Claims
1. A composite comprising a porous carbon structure comprising a surface and pores; and a coating on the surface comprising MnO2; wherein the coating does not completely fill or obstruct a majority of the pores, wherein the coating is formed by self-limiting electroless deposition.
2. The composite of claim 1, wherein cations and water are incorporated within the MnO2.
3. The composite of claim 2, wherein the structure is a carbon aerogel.
4. The composite of claim 2, wherein the structure is selected from the group consisting of carbon nanofoam, xerogel, templated mesoporous carbon, templated macroporous carbon, and carbon nanotube/nanofiber assemblies.
5..The composite of claim 1, wherein the pores have an average diameter of from about 2 nm to about 1 μm.
6. The composite of claim 1, wherein the coating has a thickness of less than about 50 nm.
7. The composite of claim 1, wherein the coating has a thickness of less than about 10 nm.
8. A capacitor comprising an anode, a cathode, and an electrolyte, wherein the anode, the cathode, or both comprise: a composite comprising a porous carbon structure comprising a surface and pores; and a coating on the surface comprising MnO2; wherein the coating does not completely fill or obstruct a majority of the pores; and a current collector in electrical contact with the composite.
9. The capacitor of claim 8, wherein cations and water are incorporated within the MnO2.
10. The capacitor of claim 9, wherein the structure is a carbon aerogel.
11. The capacitor of claim 9, wherein the structure is selected from the group consisting of carbon nanofoam, xerogel, templated mesoporous carbon, templated macroporous carbon, and carbon nanotube/nanofiber assemblies.
12. The capacitor of claim 8, wherein the pores have an average diameter of from about 2 nm to about 1 μm.
13. The capacitor of claim 8, wherein the coating has a thickness of less than about 50 nm.
14. The capacitor of claim 8, wherein the coating has a thickness of less than about 10 nm.
15. The capacitor of claim 8, wherein the electrolyte comprises aqueous sodium sulfate.
16. The capacitor of claim 8, wherein the electrolyte comprises a liquid selected from the group consisting of an aqueous, nominally neutral (pH 6-8) electrolyte with or without buffering components.
17. The capacitor of claim 8, wherein the electrolyte comprises a liquid selected from the group consisting of an aqueous, basic (pH >8) electrolyte with or without buffering components.
18. The capacitor of claim 8, wherein the electrolyte comprises a liquid selected from the group consisting of a nonaqueous liquid of sufficient dielectric constant to dissociate salts soluble in the nonaqueous liquid.
19. The capacitor of claim 8, wherein the coating is formed by self-limiting electroless deposition.
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US9146342B2 (en) | 2010-01-13 | 2015-09-29 | 3M Innovative Properties Company | Illumination device having viscoelastic lightguide |
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