WO2017181247A1 - An electrochemical capacitor and an integrated energy-generation and energy-storage device - Google Patents
An electrochemical capacitor and an integrated energy-generation and energy-storage device Download PDFInfo
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- WO2017181247A1 WO2017181247A1 PCT/AU2017/050369 AU2017050369W WO2017181247A1 WO 2017181247 A1 WO2017181247 A1 WO 2017181247A1 AU 2017050369 W AU2017050369 W AU 2017050369W WO 2017181247 A1 WO2017181247 A1 WO 2017181247A1
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- Prior art keywords
- metal oxide
- capacitor
- hierarchically
- energy
- nanostructured
- Prior art date
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- 239000003990 capacitor Substances 0.000 title claims abstract description 107
- 238000004146 energy storage Methods 0.000 title claims abstract description 37
- 229910044991 metal oxide Inorganic materials 0.000 claims abstract description 62
- 150000004706 metal oxides Chemical group 0.000 claims abstract description 62
- 239000004020 conductor Substances 0.000 claims abstract description 19
- 238000007743 anodising Methods 0.000 claims abstract description 17
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- 238000000034 method Methods 0.000 claims description 34
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- 239000002071 nanotube Substances 0.000 claims description 19
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 18
- 239000010936 titanium Substances 0.000 claims description 18
- 229910052719 titanium Inorganic materials 0.000 claims description 18
- 229910052751 metal Inorganic materials 0.000 claims description 16
- 239000002184 metal Substances 0.000 claims description 16
- 238000000137 annealing Methods 0.000 claims description 12
- 239000002127 nanobelt Substances 0.000 claims description 10
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 6
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- HLLICFJUWSZHRJ-UHFFFAOYSA-N tioxidazole Chemical compound CCCOC1=CC=C2N=C(NC(=O)OC)SC2=C1 HLLICFJUWSZHRJ-UHFFFAOYSA-N 0.000 claims 3
- 238000003860 storage Methods 0.000 abstract description 12
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 25
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- 230000008569 process Effects 0.000 description 13
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- 238000004519 manufacturing process Methods 0.000 description 10
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 9
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- 239000011733 molybdenum Substances 0.000 description 9
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- 230000003139 buffering effect Effects 0.000 description 5
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- 238000001069 Raman spectroscopy Methods 0.000 description 3
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- 230000003647 oxidation Effects 0.000 description 3
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- 239000010409 thin film Substances 0.000 description 3
- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 2
- 229910015711 MoOx Inorganic materials 0.000 description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 description 2
- 230000003466 anti-cipated effect Effects 0.000 description 2
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- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 2
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- 238000004544 sputter deposition Methods 0.000 description 2
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- DDFHBQSCUXNBSA-UHFFFAOYSA-N 5-(5-carboxythiophen-2-yl)thiophene-2-carboxylic acid Chemical compound S1C(C(=O)O)=CC=C1C1=CC=C(C(O)=O)S1 DDFHBQSCUXNBSA-UHFFFAOYSA-N 0.000 description 1
- 239000004966 Carbon aerogel Substances 0.000 description 1
- RAXXELZNTBOGNW-UHFFFAOYSA-O Imidazolium Chemical compound C1=C[NH+]=CN1 RAXXELZNTBOGNW-UHFFFAOYSA-O 0.000 description 1
- 229910021152 Li0.5TiO2 Inorganic materials 0.000 description 1
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- 238000001237 Raman spectrum Methods 0.000 description 1
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 229910021607 Silver chloride Inorganic materials 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
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- LDDQLRUQCUTJBB-UHFFFAOYSA-N ammonium fluoride Chemical compound [NH4+].[F-] LDDQLRUQCUTJBB-UHFFFAOYSA-N 0.000 description 1
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- 238000002484 cyclic voltammetry Methods 0.000 description 1
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- 230000035515 penetration Effects 0.000 description 1
- VLTRZXGMWDSKGL-UHFFFAOYSA-M perchlorate Inorganic materials [O-]Cl(=O)(=O)=O VLTRZXGMWDSKGL-UHFFFAOYSA-M 0.000 description 1
- VLTRZXGMWDSKGL-UHFFFAOYSA-N perchloric acid Chemical compound OCl(=O)(=O)=O VLTRZXGMWDSKGL-UHFFFAOYSA-N 0.000 description 1
- 238000007747 plating Methods 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 229920002451 polyvinyl alcohol Polymers 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000010248 power generation Methods 0.000 description 1
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- HKZLPVFGJNLROG-UHFFFAOYSA-M silver monochloride Chemical compound [Cl-].[Ag+] HKZLPVFGJNLROG-UHFFFAOYSA-M 0.000 description 1
- 229910052938 sodium sulfate Inorganic materials 0.000 description 1
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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
- H01G11/84—Processes for the manufacture of hybrid or EDL capacitors, or components thereof
- H01G11/86—Processes for the manufacture of hybrid or EDL capacitors, or components thereof specially adapted for electrodes
-
- 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
- H01G11/26—Electrodes characterised by their structure, e.g. multi-layered, porosity or surface features
- H01G11/28—Electrodes characterised by their structure, e.g. multi-layered, porosity or surface features arranged or disposed on a current collector; Layers or phases between electrodes and current collectors, e.g. adhesives
-
- 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
-
- 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
- H01G11/66—Current collectors
- H01G11/70—Current collectors characterised by their structure
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/0029—Processes of manufacture
- H01G9/0032—Processes of manufacture formation of the dielectric layer
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/004—Details
- H01G9/04—Electrodes or formation of dielectric layers thereon
- H01G9/048—Electrodes or formation of dielectric layers thereon characterised by their structure
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/04—Processes of manufacture in general
- H01M4/0438—Processes of manufacture in general by electrochemical processing
- H01M4/044—Activating, forming or electrochemical attack of the supporting material
- H01M4/0442—Anodisation, Oxidation
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/13—Energy storage using capacitors
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/70—Energy storage systems for electromobility, e.g. batteries
Definitions
- the present invention relates generally to the field of electrochemical capacitors suitable for energy storage and methods for forming electrochemical capacitors. Further, the present invention relates to integrated energy- generation and energy-storage devices comprising
- Energy storage is one of the crucial features in modern energy systems .
- Energy storage connected to an energy grid allows mitigating problems related to peak energy delivery.
- Off-grid energy generation systems for example photovoltaic systems, use storage to provide energy when sunlight is not available.
- Reliable and fast energy storage is also crucial for modern and future transportation systems, for example EV and hybrid
- Modern batteries such as lithium ions batteries and fuel cells, are capable of high energy densities. However, their capability of delivering high power densities required for short burst of power is limited. Electrochemical capacitors can be charged rapidly, can be cycled more frequently and provide higher power densities than rechargeable batteries. Therefore, they are being extensively used for commuter buses, airplanes emergency exit doors, seaport cranes, cordless power tooIs and provide backup power for CMOS memories .
- the present invention provides an electrochemical capacitor comprising: a first electrode element having a nano-porous or micro-porous conductive material and a hierarchically- nanostructured metal oxide region; the hierarchically- nanostructured metal oxide region being formed by
- the electrical charge is stored in the hierarchically-nanostructured metal oxide region by one or more of surface redox reactions, double layer capacitance effects and ion intercalation.
- the conductive material comprises a micro-porous metal or metal alloy foam.
- the micro-porous metal or metal alloy foam may be conformally-coated with a plurality of metal oxide nanostructures .
- nanostructures comprise metal oxide nanotubes that are formed by anodising a surface portion of the micro-porous metal or metal alloy foam.
- the micro-porous metal foam comprises titanium and a plurality of TiO x nanotubes are formed by anodising a surface portion of the foam.
- the plurality of ⁇ nanotubes may have an average tube diameter between 20 nm and 40 nm.
- the conductive material comprises a first conductive region and a second conductive region, the second conductive region being physically or
- the hierarchically- nanostructured metal oxide region is formed by anodising a portion of the second conductive region.
- the first conductive region may comprise nanostructured or micro-structured porous carbon.
- the hierarchically-nanostructured metal oxide region comprises a first micro-or nanostructure and a second nanostructure, formed in a hierarchical
- the hierarchically- nanostructured metal oxide region may comprise a
- the second nanostructure comprises a plurality of nano-belt or nano-tube structures .
- the nano- belt structures may have a width between 100 nm and 300 nm and a length between 100 nm and 700 nm.
- the hierarchically-nanostructured metal oxide region is arranged such that a charge discharge curve has a predetermined shape and charging can be performed at a voltage within 10% of the capacitor peak voltage .
- inventions provides a method for forming an electrochemical capacitor comprising the steps of: providing a first electrode having a conductive material; the conductive region having a portion with a first nanostructure or microstructure; anodising a portion of the surface of the nanostructured or microstructured conductive material in a manner such that a hierarchically-nanostructured metal oxide region is formed; annealing the metal oxide region in a manner such that a crystalline metal oxide is formed; providing an electrolyte in contact with at least a portion of the metal oxide region; and providing a second electrode element; wherein, when a voltage is applied between the first and the second electrode elements, electrical charge is stored in the hierarchically-nanostructured metal oxide region of the first electrode.
- a plurality of TiO x nanotubes is formed during formation of the hierarchically- nanostructured portion .
- the method further comprises the step of depositing a metal layer on a portion of the conductive material with a first nanostructure or microstructure .
- the step of annealing the metal oxide region is performed in a manner such that aplurality of nano-belt structures are formed in the hierarchically- nanostructured portion.
- the present invention provides an integrated energy-generation and energy- storage device comprising: a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; an electrochemical capacitor in accordance with the first aspect; wherein the conductive region of the
- electrochemical capacitor comprises a portion of the back contact of the photovoltaic cell.
- the device comprises three electrical termina Is : a first terminal in electrical contact with the front contact of the photovoltaic cell; a second terminal in electrical contact with the back contact of the photovoltaic cell and the first electrode element of the electrochemical capacitor; and a third terminal in electrical contact with the second electrode element of the electrochemical capacitor.
- invention provides a method for forming an integrated energy-generation and energy-storage device, the device comprising a photovoltaic cell and an electrochemical capacitor, the method comprising the steps of: providing a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; and forming an electrochemical capacitor in accordance with any one of claims 1 to 14 on a portion of the back contact of the photovoltaic cell; wherein the conductive region of the
- the present invention provides an integrated energy-generation and energy- storage device comprising: a photovoltaic module that includes at least one photovoltaic cell; and a circuitry assembly arranged to control the electrical performance of the integrated energy-generation and energy-storage device; the circuitry assembly
- the electrochemical capacitor in accordance with the first aspect; wherein, in use, the electrochemical capacitor allows compensating for irregular energy generation due to solar irradiation variance.
- Embodiments provide an integrated the three-terminal monolithic architecture that demonstrates the potential for high-performance hybrid energy harvesting-storage using the most commonly-manufactured solar cell technology worldwide .
- a key element of the device architecture is the shared (industrial) Al electrode which: (i) greatly simplifies the fabrication process; (ii) shortens the charge transfer path and reduces the associated energy losses in this transfer; (iii) acts as an electronic barrier preventing degradation of the solar cell's open-circuit voltage during the capacitor's electrode fabrication; (iv) effectively increases the surface area of the electrode though its rough screen-printed granular texture.
- junction box' of a photovoltaic module This variation places some restrictions on the volume of the energy storage element, however it provides the advantage that if the lifetime of either or both of the electronics or electrochemical capacitor storage system is less than that of the photovoltaic module, then the junction box can be periodically replaced without affecting the module.
- Figures 1, 2, 6, 7 and 9 show schematic diagrams of an electrochemical capacitor, SEM images of portions of the capacitor, and energy-generation and energy-storage devices in accordance with embodiments;
- Figures 3, 4 and 5 show XRD, cyclic voltammetry,
- Figures 8 and 10 are flow-diagrams illustrating methods for forming a capacitor and an integrated energy- generation and energy-storage device respectively;
- FIGS 11 and 12 show schematic circuits of integrated energy-generation and energy-storage devices connected to load during charging and discharging
- Figure 13 shows an example of voltage and current profiles and a voltage-charge characteristic respectively for devices manufactured in accordance with embodiments
- Figure 14 shows a configuration of photovoltaic devices and electrochemical capacitors in accordance with embodiments; and Figure 15 shows a schematic of an integrated energy-harvesting-storage device comprising a Si module, a junction box and an electrochemical capacitor in
- electrochemical capacitors suitable for energy storage and methods for forming electrochemical capacitors .
- embodiments of the invention provide integrated energy-generation and energy-storage devices comprising photovoltaic cells and electrochemical capacitors .
- the electrochemical capacitors may be located in a
- New designs of electrochemical capacitors can provide good energy and power density properties simultaneously.
- capacitors described herein high energy density and high power density are achieved by forming electrodes with a high surface area using hierarchical- nanostructures .
- methods to manufacture these structures which have been derived from solar cell fabrication and allow creating better
- the capacitors can be also manufactured in tandem with photovoltaic devices, taking advantage of well-established solar cell manufacturing processes. For example, herein there is disclosed a technique which uses light-induced anodisation and exploits the metallic material that forms the back contact of a solar cell to produce an integrated energy-generation and energy-storage device that comprises a photovoltaic cell and an electrochemical capacitor.
- Capacitor 100 comprises a first electrode element 104 and a second electrode element 106.
- the first electrode element comprises a conductive region 108 and a metal oxide region 110 that comprises a
- the capacitor also comprises an electrolyte 112 between the first and the second electrode elements and in contact with metal oxide region 110.
- the electrical charge can be stored in the hierarchically- nanostructured portion of the metal oxide region by one or more mechanisms.
- ion intercalation is an important mechanism for charge storing.
- the conductive region 108 of the first electrode element comprises a micro-porous titanium foam.
- the porous nature of the titanium foam provides a structured template to form a metal oxide hierarchical nanostructure .
- Macroscopic view 102 schematically shows the hierarchically-nanostructured portion 104. This is formed using an anodisation process of region 108.
- the anodisation process of the micro-porous titanium foam allows forming sub-stoichiometric titanium oxide (TiO x ) nanotubes that provide inter-octahedral sites for lithium ions insertion.
- the micro-porous titanium foam is cleaned by ultrasonication for 10 min in acetone,
- deionized water sequentially and then immersed in 6M hydrochloric acid (HC1) for 30 min.
- the titanium foam is then anodised in a high-purity glycerol (99.5%) solution containing 0.45 M of ammonium fluoride (NH 4 F) and 2.5 (v/v) of water.
- the anodisation process is performed using a three-electrode configuration with the titanium foam as working electrode, a platinum coil as pseudo reference electrode and a titanium plate as counter electrode .
- anodised titanium foams are rinsed in deionised water and dried in air before they are annealed in air at 600°C for 1 hour.
- Figure 1 also shows SEM images of the titanium foam before
- Figure 1 (d) shows a detail of the formed TiO x nanotubes .
- the titanium foam After deposition, the titanium foam has a 3-D micrometre- sized porous structure with a relatively smooth surface. After anodisation, the titanium foam shows a significantly roughened surface due to the growth of nanotubular TiO x arrays on both of its outer and inner surfaces .
- the as- grown TiO x nanotubes have an average tube diameter between 20 nm and 40 nm. This provides a substantial increase in electrode surface area for the electrochemical capacitor. Referring now to figure 2, there are shown SEM images of electrodes anodised for 200s (a), 500s (b) , 1000s (c), 2000s (d) , and 3000s (e) .
- FIG. 3 there are shown XRD patterns of the anodised titanium foam before (a) and after (b) annealing. Before annealing the anodised foam only shows titanium peaks. This suggests that the as-grown TiO x film is amorphous .
- the annealed anodised titanium foam shows both anatase and rutile XRD peaks indicating coexistence of two crystal structures from the nanotube layer and a thin-layer of thermal oxide underneath (formed during annealing) .
- FIG. 4 there are shown CV curves of electrochemical capacitors manufactured in accordance with embodiments using different anodisation times: 200s (a), 500s (b) , 2000s (c) and 3000s (d) . Measurements were performed in a three-electrode configuration with various scan rates between 2 mV/s and 100 mV/s . The faster measurements show the broader hysteresis loop.
- the CV curves of Figure 4 exhibit a pair of reversible redox peaks (2/2' for reduction/oxidation) which appear at potentials of -1.7 V and -0.8 V respectively.
- FIG. 5 (a) there are shown a comparison of Raman signal obtained from electrodes with anodised titanium oxide non-annealed (bottom curve) and annealed (top curve) .
- the middle curve shows the signal from an electrode that has not been anodised but thermally treated.
- the results of Figure 5 are consistent with those from XRD measurements of Figure 3.
- the Raman spectra of the as-anodised titanium oxide (bottom) show no noticeable features indicating its amorphous nature.
- Figure 5 (b) shows GCD curves electrodes that have been anodised and annealed for different time intervals measured at a current density of 1 mA/cm 2 .
- the non- linearity is evident in the slopes of the charge/discharge curves indicating the charge storage is not solely due to electrical double layer storage.
- the potential plateau in the discharge curve is commonly observed in anatase Ti0 2 indicating a coexistence of Ti0 2 and Li 0 . 5 TiO 2 due to Li + insertion.
- the ATO electrode anodised for the longest duration exhibits the most prolonged discharge period (consistent with CV results) with more than 2 min being achieved at a discharge current density of 1 mA/cm 2 .
- FIG. 6 there is shown a macroscopic view 600 of an alternative electrode configuration.
- a carbon expanded foam 602 with a nanostructured surface is used as a conductive material.
- a metal oxide layer 604 is formed so that it has a good uniformity to the surface of the carbon layer 602 and conforms to its nanostructured surface.
- the surface of metal oxide layer 604 therefore has a structure which is similar to the nanostructure on the surface of the carbon.
- Figure 6 shows that metal oxide region 604 has a secondary nanostructured portion 606 hierarchically arranged with the nanostructure replicated from the surface of the carbon layer 602. These hierarchically arranged nanostructures allow maximising the surface area of the electrode and improve charge storage.
- the metal oxide region 604 is formed by anodising a metallic material, such as molybdenum, that is sputtered on the conductive region 602 and subsequently anodised.
- a metallic material such as molybdenum
- Nanostructures 606 have been formed by annealing the structure at 450°C for 2 hrs at a ramping rate of 2.5°C /min. This allows forming crystalline structured of ⁇ - ⁇ 0 3 in the form of nano-belt structures.
- Other materials can be used as alternatives to molybdenum, such as titanium.
- the capacitor can be manufactured in an asymmetrical configuration or symmetrical configuration replicating the features of electrode 600 on both electrodes of the capacitor.
- the charge and discharge curve can be designed so that charging can be performed at a voltage within 10% of the capacitor peak voltage.
- the electrochemical capacitors described above may be embedded at the rear of a solar cell to form a combined energy- generation and energy-storage device, as shown below with reference to Figure 9.
- Figure 6(c) shows an SEM image of a cauliflower-like ⁇ - ⁇ 0 3 electrode surface with distinctive levels of surface roughness: (i) the Al lumps had feature size between 1 to 5 ⁇ , (ii) the porous anodic ⁇ - ⁇ 0 3 coating on the lumps has a pore size up to 100 nm.
- Figure 7 there is shown a SEM image of the CX-M0O 3 electrode surface consisting of nano-belts the structures having a width between 100 nm and 300 nm and a length between 100 nm and 700 nm.
- the method comprises providing a first electrode having a conductive material; the conductive region having a portion with a first nanostructure or microstructure, step 805. At step 810 a portion of the surface of the
- nanostructured or microstructured conductive material is anodised in a manner such that a hierarchically- nanostructured metal oxide region is formed. Subsequently the structure comprising the hierarchically-nanostructured metal oxide region is annealed in a manner such that a crystalline metal oxide is formed, step 815.
- an electrolyte is provided in contact with at least a portion of the metal oxide region and, at step 825, a second electrode element is provided.
- the second electrode element may also comprise a
- Device 900 comprises a monolithically integrated screen- printed photovoltaic (PV) cell 902 and an electrochemical capacitor 904 similar to the device described above with reference to Figure 6. This is a three-terminal
- Device 900 allows integrating the energy generation properties of the photovoltaic cell (902) with the energy storage and delivery properties of the electrochemical capacitor (904) to provide an hybrid device which opens scope for a number of energy related applications .
- Figure 10 shows a flow-diagram 980 outlining steps that can be performed to manufacture device 900. These steps are compatible with the commercial production of
- a photovoltaic cell 902 is provided.
- the cell comprises a current rectifying portion 908 and 912, a conductive front contact 906 and a conductive back contact 914.
- electrochemical capacitor 904 is then formed at the back of the photovoltaic cell.
- the back contact 914 has a first nanostructure or microstructure .
- a metal layer is
- the metal layer can comprise molybdenum, tungsten, ruthenium or other transition metals, with the metal being deposited on the back contact of the photovoltaic cell using sputtering, evaporation (thermal or e-beam) or metal plating .
- Hierarchically-nanostructured metal oxide layer 916 can optionally be annealed to form a crystalline metal oxide.
- This annealing process which is performed at a temperature in the range of 300°C to 500 °C and more preferably in the range of 400°C to 450°C does not impact the operation of the photovoltaic cell 902 and can introduce the formation of further metal oxide nanostructures , such as metal oxide nanobelt structures that further increase the surface area of the metal oxide electrode.
- a conductive electrolyte 918 is provided in contact with at least a portion of the hierarchically-nanostructured metal oxide surface 916 and finally a second electrode element 920 is provided in contact with the electrolyte to complete the electrochemical circuit.
- the electrolyte can comprise an aqueous electrolyte such as ⁇ 0.1 M sodium sulphate, an organic electrolyte
- the second electrode 920 comprises of metal or another
- electrochemical capacitor material such as a porous carbon aerogel, conductive polymer or metal oxide.
- Structure 950 is an alternative hybrid integrated energy- generation and energy-storage device comprising a
- the photovoltaic cell 952 comprises a front contact 956, rectifying components 962 and 958 and a back contact 964.
- Capacitor 954 has a first electrode shared with the back contact 964 of the solar cell 952. In this case the first electrode 966 of capacitor 954 is formed by directly anodising a portion of the back contact of the
- carbon nanostructures 968 are
- the second electrode of the capacitor in this case comprises a hierarchical-nanostructure and is similar to the electrode shown in Figure 6.
- a metal electrode can be used for the second electrode as shown for hybrid device 900.
- the photovoltaic cell used to manufacture the hybrid integrated energy-generation and energy-storage device 900 can be prepared by industrial-standard solar cell
- the rear electrode 914 comprises a layer of screen printed
- the rear electrode surface comprises a microporous structure comprising sintered aluminium grains.
- anodising the metal layer is preferably performed by light-induced anodisation by exposing the photovoltaic cell to light and using the generated photocurrent to anodise the metal surface to form a metal oxide.
- a molybdenum thin film is sputtered on the screen-printed aluminium contact of the photovoltaic cell.
- the molybdenum thin film thickness is in the order of 1.5 ⁇ .
- the molybdenum thin film is then anodised to MoO x using the light-induced anodisation (LIA) process.
- LIA light-induced anodisation
- the photovoltaic cell is supported on an anodisation reactor and exposed to NaF aqueous electrolyte solution in a three-electrode
- the reference electrode immerse in the electrolyte.
- An LED light source is then used to generate the photocurrent for anodisation.
- An external bias can be applied to compensate for resistive losses in the electrochemical cell, and facilitate the oxidation process.
- the anodised sample is then dried in vacuum and subsequently calcined, for example at 450°C for 2 hrs at a ramping rate of 2.5°C /min, to form the crystalline form ⁇ - ⁇ 0 3 .
- Figure 11 shows the charging and discharging process of the integrated energy-generation and energy-storage device shown in Figure 9(a).
- the front contact of the solar cell is connected with the second electrode of the capacitor, and the charge can be stored in capacitor using the photovoltage of the solar cell under illumination.
- the discharging stage Figure 11 (b) , the capacitor is connected to the load, and the charge stored is released.
- Figure 11 (c) shows the cyclic light charge-galvanostatic discharge test results
- Each charge- discharge cycle comprised 30 s illumination under a 0.6 Sun (60 mW/cm 2 ) LED light followed by a 30 s discharge period in the dark under a constant current density of 1.7 ⁇ /cm 2 .
- This figure demonstrates the functionality of the device.
- Figure 12 shows the charging and discharging process of the integrated energy-generation and energy-storage device shown in Figure 9(b).
- the front contact of the solar cell is connected with the second electrode of the capacitor, and the charge can be stored in capacitor using the photovoltage of the solar cell under illumination.
- the discharging stage Figure 12 (b) , the capacitor is connected to the load, and the charge stored is released.
- Figure 13 (a) shows the voltage and current profiles for charge-discharge experiment of an electrochemical
- Figure 13 (b) shows a charge-discharge curve of a double layer and asymmetric electrochemical capacitor (with increased current density) .
- the arrow denotes the charging voltage for sustained power.
- the electrochemical capacitor 1502 is formed in the layered structure as described above but rolled into a compact cell that could be located into a junction box 1504 of the photovoltaic module 1506.
- This embodiment places some restrictions on the volume of the energy storage element, however it provides the advantage that if the lifetime of either or both of the electronics or electrochemical capacitor storage system is less than that of the photovoltaic module, then the junction box can be periodically replaced without
- any electronics would maximise the use of software or firmware so as to minimise the number of electronic components that could age.
- the electrical circuit within which the electrochemical capacitor is integrated preferably maximises the power generated by the module using one of the many available power maximising algorithms employed for PV modules . It could be designed to generate an AC power and thereby act as a micro-invertor , or could output DC power. In a further variation the power maximising/buffering
- electronics could be arranged at the end of each cell string of a module, and connected by a circuit which then connects to the external circuit (e.g., array
- the electrical components would include internet accessibility allowing remote control of the module' s power for voltage ramping purposes and remote control of the power buffering using the electrochemical capacitor element(s), the latter being informed by software which can predict the need to store and release energy based on weather events and/or shading events.
- electrochemical capacitor storage elements could be instructed to buffer preceding an anticipated shading events and to release energy on arrival of the shading event. These communicated predictions could permit the use of an electrochemical capacitor energy storage system with a lower response rate (instantaneous power) , as energy discharge can be anticipated ahead of time and the storage system can begin discharging before the illumination intensity was reduced significantly.
- inclusion of the electrochemical capacitor energy storage system would reduce the ramp rate of the power generated by the module to less than 10%. Being able to reduce the ramp rate of power generated by renewable resources is of particular value for large penetration of renewable energy.
- the ramp rate i.e., power smoothing
- the capacity of central energy storage systems can be reduced as these systems do not need to be sized such that they can respond quickly to intermittencies in power.
- the maximising and buffering of power at the module level can increase the efficiency of power generation resulting in increased energy generation capacity from an array.
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Abstract
The present disclosure provides an electrochemical capacitor with at least one electrode that has a micro-porous conductive material and a hierarchically-nanostructured metal oxide region for charge storage. The hierarchically-nanostructured metal oxide region is formed by anodising a surface portion of the conductive material. An electrolyte disposed in contact with at least a portion of the hierarchically-nanostructured metal oxide region. Further, the present disclosure provides a hybrid energy-generation and energy-storage device comprising a photovoltaic cell and an integrated electrochemical capacitor.
Description
AN ELECTROCHEMICAL CAPACITOR AND AN INTEGRATED ENERGY- GENERATION AND ENERGY-STORAGE DEVICE
TECHNICAL FIELD OF THE INVENTION The present invention relates generally to the field of electrochemical capacitors suitable for energy storage and methods for forming electrochemical capacitors. Further, the present invention relates to integrated energy- generation and energy-storage devices comprising
photovoltaic cells and electrochemical capacitors .
BACKGROUND OF THE INVENTION
Energy storage is one of the crucial features in modern energy systems . Energy storage connected to an energy grid, for example, allows mitigating problems related to peak energy delivery. Off-grid energy generation systems, for example photovoltaic systems, use storage to provide energy when sunlight is not available. Reliable and fast energy storage is also crucial for modern and future transportation systems, for example EV and hybrid
vehicles.
Modern batteries, such as lithium ions batteries and fuel cells, are capable of high energy densities. However, their capability of delivering high power densities required for short burst of power is limited. Electrochemical capacitors can be charged rapidly, can be cycled more frequently and provide higher power densities than rechargeable batteries. Therefore, they are being extensively used for commuter buses, airplanes emergency
exit doors, seaport cranes, cordless power tooIs and provide backup power for CMOS memories .
However, the limited energy density of electrochemical capacitors requires frequent charging. Frequent charging and discharging requires complex electronics to provide high charging efficiency.
There is a need in the art for electrochemical capacitors with an increased energy density.
SUMMARY OF THE INVENTION In accordance with the first aspect, the present invention provides an electrochemical capacitor comprising: a first electrode element having a nano-porous or micro-porous conductive material and a hierarchically- nanostructured metal oxide region; the hierarchically- nanostructured metal oxide region being formed by
anodising a surface portion of the conductive material; an electrolyte disposed in contact with at least a portion of the hierarchically-nanostructured metal oxide region; and a second electrode element; wherein, when a voltage is applied between the first and the second electrode elements, electrical charge is stored in the hierarchically-nanostructured metal oxide region of the first electrode. In embodiments, the electrical charge is stored in the hierarchically-nanostructured metal oxide region by one or
more of surface redox reactions, double layer capacitance effects and ion intercalation.
In some embodiments, the conductive material comprises a micro-porous metal or metal alloy foam. The micro-porous metal or metal alloy foam may be conformally-coated with a plurality of metal oxide nanostructures .
In embodiments, the plurality of metal oxide
nanostructures comprise metal oxide nanotubes that are formed by anodising a surface portion of the micro-porous metal or metal alloy foam.
In some embodiments, the micro-porous metal foam comprises titanium and a plurality of TiOx nanotubes are formed by anodising a surface portion of the foam. The plurality of Τίθχ nanotubes may have an average tube diameter between 20 nm and 40 nm.
In other embodiments, the conductive material comprises a first conductive region and a second conductive region, the second conductive region being physically or
chemically formed on a portion of the surface of the first conductive region and wherein the hierarchically- nanostructured metal oxide region is formed by anodising a portion of the second conductive region.
The first conductive region may comprise nanostructured or micro-structured porous carbon. In an embodiment, the hierarchically-nanostructured metal oxide region comprises a first micro-or nanostructure and a second nanostructure, formed in a hierarchical
arrangement with the first micro- or nanostructure; the second nanostructure being formed by anodising a portion
of the second conductive region. The hierarchically- nanostructured metal oxide region may comprise a
crystallised transition metal oxide in the orthorhombic phase . In embodiments, the second nanostructure comprises a plurality of nano-belt or nano-tube structures . The nano- belt structures may have a width between 100 nm and 300 nm and a length between 100 nm and 700 nm.
In embodiments, the hierarchically-nanostructured metal oxide region is arranged such that a charge discharge curve has a predetermined shape and charging can be performed at a voltage within 10% of the capacitor peak voltage .
In accordance with the second aspect, the present
invention provides a method for forming an electrochemical capacitor comprising the steps of: providing a first electrode having a conductive material; the conductive region having a portion with a first nanostructure or microstructure; anodising a portion of the surface of the nanostructured or microstructured conductive material in a manner such that a hierarchically-nanostructured metal oxide region is formed; annealing the metal oxide region in a manner such that a crystalline metal oxide is formed; providing an electrolyte in contact with at least a portion of the metal oxide region; and providing a second electrode element;
wherein, when a voltage is applied between the first and the second electrode elements, electrical charge is stored in the hierarchically-nanostructured metal oxide region of the first electrode. In embodiments, during formation of the hierarchically- nanostructured portion a plurality of TiOx nanotubes is formed .
In some embodiments, the method further comprises the step of depositing a metal layer on a portion of the conductive material with a first nanostructure or microstructure .
In some instances, the step of annealing the metal oxide region is performed in a manner such that aplurality of nano-belt structures are formed in the hierarchically- nanostructured portion. In accordance with the third aspect, the present invention provides an integrated energy-generation and energy- storage device comprising: a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; an electrochemical capacitor in accordance with the first aspect; wherein the conductive region of the
electrochemical capacitor comprises a portion of the back contact of the photovoltaic cell.
In embodiments, the device comprises three electrical termina Is : a first terminal in electrical contact with the front contact of the photovoltaic cell; a second terminal
in electrical contact with the back contact of the photovoltaic cell and the first electrode element of the electrochemical capacitor; and a third terminal in electrical contact with the second electrode element of the electrochemical capacitor.
In accordance with the fourth aspect, the present
invention provides a method for forming an integrated energy-generation and energy-storage device, the device comprising a photovoltaic cell and an electrochemical capacitor, the method comprising the steps of: providing a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; and forming an electrochemical capacitor in accordance with any one of claims 1 to 14 on a portion of the back contact of the photovoltaic cell; wherein the conductive region of the
electrochemical capacitor comprises a portion of the back contact of the photovoltaic cell. In accordance with the fifth aspect, the present invention provides an integrated energy-generation and energy- storage device comprising: a photovoltaic module that includes at least one photovoltaic cell; and a circuitry assembly arranged to control the electrical performance of the integrated energy-generation and energy-storage device; the circuitry assembly
comprising an electrochemical capacitor in accordance with the first aspect;
wherein, in use, the electrochemical capacitor allows compensating for irregular energy generation due to solar irradiation variance.
Embodiments provide an integrated the three-terminal monolithic architecture that demonstrates the potential for high-performance hybrid energy harvesting-storage using the most commonly-manufactured solar cell technology worldwide .
A key element of the device architecture is the shared (industrial) Al electrode which: (i) greatly simplifies the fabrication process; (ii) shortens the charge transfer path and reduces the associated energy losses in this transfer; (iii) acts as an electronic barrier preventing degradation of the solar cell's open-circuit voltage during the capacitor's electrode fabrication; (iv) effectively increases the surface area of the electrode though its rough screen-printed granular texture.
Embodiments also provide electrochemical capacitors that can be rolled into a compact cell and positioned a
Λ junction box' of a photovoltaic module. This variation places some restrictions on the volume of the energy storage element, however it provides the advantage that if the lifetime of either or both of the electronics or electrochemical capacitor storage system is less than that of the photovoltaic module, then the junction box can be periodically replaced without affecting the module.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
Figures 1, 2, 6, 7 and 9 show schematic diagrams of an electrochemical capacitor, SEM images of portions of the capacitor, and energy-generation and energy-storage devices in accordance with embodiments; Figures 3, 4 and 5 show XRD, cyclic voltammetry,
Raman and galvanostatic charge-discharge measurement results respectively;
Figures 8 and 10 are flow-diagrams illustrating methods for forming a capacitor and an integrated energy- generation and energy-storage device respectively;
Figures 11 and 12 show schematic circuits of integrated energy-generation and energy-storage devices connected to load during charging and discharging;
Figure 13 shows an example of voltage and current profiles and a voltage-charge characteristic respectively for devices manufactured in accordance with embodiments;
Figure 14 shows a configuration of photovoltaic devices and electrochemical capacitors in accordance with embodiments; and Figure 15 shows a schematic of an integrated energy-harvesting-storage device comprising a Si module, a junction box and an electrochemical capacitor in
accordance with embodiments.
DETAILED DESCRIPTION OF EMBODIMENTS Embodiments of the present invention provide
electrochemical capacitors suitable for energy storage and methods for forming electrochemical capacitors . In
addition, embodiments of the invention provide integrated
energy-generation and energy-storage devices comprising photovoltaic cells and electrochemical capacitors . The electrochemical capacitors may be located in a
photovoltaic panel junction box. Modern batteries are characterised by their good energy density properties, whilst modern capacitors or
supercapacitors provide good power density
characteristics. New designs of electrochemical capacitors can provide good energy and power density properties simultaneously.
In the capacitors described herein, high energy density and high power density are achieved by forming electrodes with a high surface area using hierarchical- nanostructures . Herein there are also described methods to manufacture these structures which have been derived from solar cell fabrication and allow creating better
nanostructures using more reliable techniques.
The capacitors can be also manufactured in tandem with photovoltaic devices, taking advantage of well-established solar cell manufacturing processes. For example, herein there is disclosed a technique which uses light-induced anodisation and exploits the metallic material that forms the back contact of a solar cell to produce an integrated energy-generation and energy-storage device that comprises a photovoltaic cell and an electrochemical capacitor.
The capacitors and the integrated energy-generation and energy-storage devices described herein are good
candidates for a wide range of applications, these include: mobility, electric vehicles, airplanes and wearable electronics amongst others.
Referring now to Figure 1, there is shown a schematic illustration of an electrochemical capacitor 100 (a) . A macroscopic view 102 which shows a section of capacitor 100 is also shown (b) . Capacitor 100 is shown in a planar configuration for convenience, however the capacitor could have many other configurations, for example it could be wrapped in a cylindrical shape. Capacitor 100 comprises a first electrode element 104 and a second electrode element 106. The first electrode element comprises a conductive region 108 and a metal oxide region 110 that comprises a
hierarchically-nanostructured portion formed by anodising a portion of the conductive region. The capacitor also comprises an electrolyte 112 between the first and the second electrode elements and in contact with metal oxide region 110.
When a voltage is applied between the first electrode element 104 and the second electrode element 106,
electrical charge moves towards and is stored in the hierarchically-nanostructured portion of the metal oxide region .
The electrical charge can be stored in the hierarchically- nanostructured portion of the metal oxide region by one or more mechanisms. In capacitor 100, ion intercalation is an important mechanism for charge storing.
In the embodiment of Figure 1, the conductive region 108 of the first electrode element comprises a micro-porous titanium foam. The porous nature of the titanium foam provides a structured template to form a metal oxide hierarchical nanostructure . Macroscopic view 102
schematically shows the hierarchically-nanostructured portion 104. This is formed using an anodisation process of region 108.
The anodisation process of the micro-porous titanium foam allows forming sub-stoichiometric titanium oxide (TiOx) nanotubes that provide inter-octahedral sites for lithium ions insertion.
Before anodisation, the micro-porous titanium foam is cleaned by ultrasonication for 10 min in acetone,
deionized water sequentially and then immersed in 6M hydrochloric acid (HC1) for 30 min. The titanium foam is then anodised in a high-purity glycerol (99.5%) solution containing 0.45 M of ammonium fluoride (NH4F) and 2.5 (v/v) of water. The anodisation process is performed using a three-electrode configuration with the titanium foam as working electrode, a platinum coil as pseudo reference electrode and a titanium plate as counter electrode .
The Applicants have investigated the performance of titanium foam anodised for different periods of time
(200s, 500s, 1000s and 2000s) to grow TiOx nanotubes on their surface. Afterwards, the anodised titanium foams are rinsed in deionised water and dried in air before they are annealed in air at 600°C for 1 hour.
Figure 1 also shows SEM images of the titanium foam before
(c) and after (d) anodisation and annealing. The inset of
Figure 1 (d) shows a detail of the formed TiOx nanotubes .
After deposition, the titanium foam has a 3-D micrometre- sized porous structure with a relatively smooth surface. After anodisation, the titanium foam shows a significantly roughened surface due to the growth of nanotubular TiOx
arrays on both of its outer and inner surfaces . The as- grown TiOx nanotubes have an average tube diameter between 20 nm and 40 nm. This provides a substantial increase in electrode surface area for the electrochemical capacitor. Referring now to figure 2, there are shown SEM images of electrodes anodised for 200s (a), 500s (b) , 1000s (c), 2000s (d) , and 3000s (e) .
Figure 2 shows the dependence between the surface
morphology and length of the TiOx nanotubes, and thus the storage capability of the electrode material. After anodisation for circa 200 s, a non-ordered porous layer (initiation layer) forms on the surface, as shown in Figure 2(a). For longer anodisation periods, the porous layer disappears and a layer of circa 600 nm long TiOx nanotubes is formed, as evident from figure 2 (b) . After the initiation phase is completed, the tubes grow longer with longer anodisation durations until 2000 s, after which the tubes may begin to aggregate possibly due to the continuous etching and thinning of the upper tube walls, as shown in Figure 2 (d) . Longer anodisation periods reveal more exacerbated tube morphology with an average tube length being reduced from circa 1.3 μιη for 2000 s
anodisation to less than 1 μιη for 3000 s anodisation, as shown in Figure (e) . Referring now to Figure 3, there are shown XRD patterns of the anodised titanium foam before (a) and after (b) annealing. Before annealing the anodised foam only shows titanium peaks. This suggests that the as-grown TiOx film is amorphous . The annealed anodised titanium foam shows both anatase and rutile XRD peaks indicating coexistence of two crystal structures from the nanotube layer and a
thin-layer of thermal oxide underneath (formed during annealing) .
Referring now to Figure 4 there are shown CV curves of electrochemical capacitors manufactured in accordance with embodiments using different anodisation times: 200s (a), 500s (b) , 2000s (c) and 3000s (d) . Measurements were performed in a three-electrode configuration with various scan rates between 2 mV/s and 100 mV/s . The faster measurements show the broader hysteresis loop. The CV curves of Figure 4 exhibit a pair of reversible redox peaks (2/2' for reduction/oxidation) which appear at potentials of -1.7 V and -0.8 V respectively. These additional peaks can be attributed to a reversible phase transition between Ti02 and LixTi02 due to the lithium ion insertion/de-insertion into anatase Ti02 nanotubes . This new pair of redox peaks become more apparent with
increased anodisation duration and eventually dominated the discharge half-cycle over the other oxidation peak 1' for electrodes with longer nanotubes (i.e., anodised for over 1000 s) .
Referring now to Figure 5 (a) there are shown a comparison of Raman signal obtained from electrodes with anodised titanium oxide non-annealed (bottom curve) and annealed (top curve) . The middle curve shows the signal from an electrode that has not been anodised but thermally treated. The results of Figure 5 are consistent with those from XRD measurements of Figure 3. The Raman spectra of the as-anodised titanium oxide (bottom) show no noticeable features indicating its amorphous nature.
Annealing in air at 600°C transforms ATO from amorphous to majorly anatase structure. However, the spectra of TTO
show different Raman peaks which correspond to rutile Ti02 phase .
Figure 5 (b) shows GCD curves electrodes that have been anodised and annealed for different time intervals measured at a current density of 1 mA/cm2. The non- linearity is evident in the slopes of the charge/discharge curves indicating the charge storage is not solely due to electrical double layer storage. The potential plateau in the discharge curve is commonly observed in anatase Ti02 indicating a coexistence of Ti02 and Li0.5TiO2 due to Li+ insertion. The ATO electrode anodised for the longest duration exhibits the most prolonged discharge period (consistent with CV results) with more than 2 min being achieved at a discharge current density of 1 mA/cm2. Referring now to Figure 6, there is shown a macroscopic view 600 of an alternative electrode configuration. In this alternative configuration a carbon expanded foam 602 with a nanostructured surface is used as a conductive material. A metal oxide layer 604 is formed so that it has a good uniformity to the surface of the carbon layer 602 and conforms to its nanostructured surface. The surface of metal oxide layer 604 therefore has a structure which is similar to the nanostructure on the surface of the carbon. Features sizes can be slightly different due to filling effect. Figure 6 shows that metal oxide region 604 has a secondary nanostructured portion 606 hierarchically arranged with the nanostructure replicated from the surface of the carbon layer 602. These hierarchically arranged nanostructures allow maximising the surface area of the electrode and improve charge storage.
In this embodiment, the metal oxide region 604 is formed by anodising a metallic material, such as molybdenum, that is sputtered on the conductive region 602 and subsequently anodised. This allows forming a very thin and very uniform layer of metal oxide. The Applicants have been able to achieve thicknesses below 100 nm and, in some cases, below 20 nm using this technique.
Nanostructures 606 have been formed by annealing the structure at 450°C for 2 hrs at a ramping rate of 2.5°C /min. This allows forming crystalline structured of α-Μο03 in the form of nano-belt structures. Other materials can be used as alternatives to molybdenum, such as titanium.
By controlling the properties of the hierarchical
nanostructures on the electrode the charge and discharge curve of the capacitor can be affected. The capacitor can be manufactured in an asymmetrical configuration or symmetrical configuration replicating the features of electrode 600 on both electrodes of the capacitor. The charge and discharge curve can be designed so that charging can be performed at a voltage within 10% of the capacitor peak voltage.
In accordance to an aspect of the present invention, the electrochemical capacitors described above may be embedded at the rear of a solar cell to form a combined energy- generation and energy-storage device, as shown below with reference to Figure 9.
Figures 6 (a) to 6(c) show sea
(SEM) images of material surf
stages of metal oxide electro
formation on a screen-printed
shows a SEM image of the rear aluminium surface of a screen-printed solar cell. After a layer of molybdenum is deposited on the aluminium surface of the solar cell using sputtering technique, vertically-aligned thin molybdenum plates with smooth surface and sharp edges are formed which are shown in Figure 7 (b) . After this, the process of anodisation of molybdenum to MoOx in NaF solution is carried out which is then followed by annealing. This forms a (X-M0O3 electrode consisting of a plurality of nano- belt structures. Figure 6(c) shows an SEM image of a cauliflower-like α-Μο03 electrode surface with distinctive levels of surface roughness: (i) the Al lumps had feature size between 1 to 5 μιη, (ii) the porous anodic α-Μο03 coating on the lumps has a pore size up to 100 nm. Referring now to Figure 7, there is shown a SEM image of the CX-M0O3 electrode surface consisting of nano-belts the structures having a width between 100 nm and 300 nm and a length between 100 nm and 700 nm.
Referring now to Figure 8 there is shown a flow-diagram with the steps used to manufacture capacitor 100. The method comprises providing a first electrode having a conductive material; the conductive region having a portion with a first nanostructure or microstructure, step 805. At step 810 a portion of the surface of the
nanostructured or microstructured conductive material is anodised in a manner such that a hierarchically- nanostructured metal oxide region is formed. Subsequently the structure comprising the hierarchically-nanostructured metal oxide region is annealed in a manner such that a crystalline metal oxide is formed, step 815. At step 820, an electrolyte is provided in contact with at least a portion of the metal oxide region and, at step 825, a
second electrode element is provided. In some instances, the second electrode element may also comprise a
hierarchically-nanostructured metal oxide region formed following a similar set of steps as described above. Referring now to Figure 9, there are shown integrated energy-generation and energy-storage devices 900 and 950. Device 900 comprises a monolithically integrated screen- printed photovoltaic (PV) cell 902 and an electrochemical capacitor 904 similar to the device described above with reference to Figure 6. This is a three-terminal
architecture where the PV cell and the electrochemical capacitor share one electrode 914 with no need to
compromise the performance of the PV cell.
Device 900 allows integrating the energy generation properties of the photovoltaic cell (902) with the energy storage and delivery properties of the electrochemical capacitor (904) to provide an hybrid device which opens scope for a number of energy related applications .
Figure 10 shows a flow-diagram 980 outlining steps that can be performed to manufacture device 900. These steps are compatible with the commercial production of
photovoltaic devices.
A photovoltaic cell 902 is provided. The cell comprises a current rectifying portion 908 and 912, a conductive front contact 906 and a conductive back contact 914. An
electrochemical capacitor 904 is then formed at the back of the photovoltaic cell. The back contact 914 has a first nanostructure or microstructure . A metal layer is
deposited onto the back contact of the photovoltaic cell and then anodised to form a metal oxide region 916 which
is conformal to the nanostructure or microstructure of the back contact and comprises a further nanostructure, hierarchically-arranged with respect to the nanostructure or microstructure on the back contact photovoltaic cell. The metal layer can comprise molybdenum, tungsten, ruthenium or other transition metals, with the metal being deposited on the back contact of the photovoltaic cell using sputtering, evaporation (thermal or e-beam) or metal plating . After this, the photovoltaic cell 902 with the
hierarchically-nanostructured metal oxide layer 916 can optionally be annealed to form a crystalline metal oxide. This annealing process which is performed at a temperature in the range of 300°C to 500 °C and more preferably in the range of 400°C to 450°C does not impact the operation of the photovoltaic cell 902 and can introduce the formation of further metal oxide nanostructures , such as metal oxide nanobelt structures that further increase the surface area of the metal oxide electrode. A conductive electrolyte 918 is provided in contact with at least a portion of the hierarchically-nanostructured metal oxide surface 916 and finally a second electrode element 920 is provided in contact with the electrolyte to complete the electrochemical circuit. The electrolyte can comprise an aqueous electrolyte such as ~0.1 M sodium sulphate, an organic electrolyte
comprising ~1 M Li perchlorate in acetonitrile , a polymer gel electrolyte such as sulphuric acid in a gel of polyvinyl alcohol, or an ionic liquid comprising
imidazolium or pyrrolidinium complexes with
tetrafluoroborate and hexafluorophosphate . The second
electrode 920 comprises of metal or another
electrochemical capacitor material such as a porous carbon aerogel, conductive polymer or metal oxide.
Structure 950 is an alternative hybrid integrated energy- generation and energy-storage device comprising a
photovoltaic cell 952 and an electrochemical capacitor 954. As with device 900, the photovoltaic cell 952 comprises a front contact 956, rectifying components 962 and 958 and a back contact 964. Capacitor 954 has a first electrode shared with the back contact 964 of the solar cell 952. In this case the first electrode 966 of capacitor 954 is formed by directly anodising a portion of the back contact of the
photovoltaic cell without depositing a further metal layer. In addition, carbon nanostructures 968 are
deposited on the electrode to improve performance. The second electrode of the capacitor in this case comprises a hierarchical-nanostructure and is similar to the electrode shown in Figure 6. Alternatively a metal electrode can be used for the second electrode as shown for hybrid device 900.
The photovoltaic cell used to manufacture the hybrid integrated energy-generation and energy-storage device 900 can be prepared by industrial-standard solar cell
fabrication processes . In this cell structure the rear electrode 914 comprises a layer of screen printed
aluminium paste. After a thermal treatment, the rear electrode surface comprises a microporous structure comprising sintered aluminium grains. The step of
anodising the metal layer is preferably performed by light-induced anodisation by exposing the photovoltaic
cell to light and using the generated photocurrent to anodise the metal surface to form a metal oxide.
In one embodiment of the process, a molybdenum thin film is sputtered on the screen-printed aluminium contact of the photovoltaic cell. The molybdenum thin film thickness is in the order of 1.5 μιη. The molybdenum thin film is then anodised to MoOx using the light-induced anodisation (LIA) process. In this process, the photovoltaic cell is supported on an anodisation reactor and exposed to NaF aqueous electrolyte solution in a three-electrode
arrangement with the working electrode connected to the n- type region of the solar cell 908 via an aluminium foil contact, a nickel counter electrode and a Ag/AgCl
reference electrode immerse in the electrolyte. An LED light source is then used to generate the photocurrent for anodisation. An external bias can be applied to compensate for resistive losses in the electrochemical cell, and facilitate the oxidation process. The anodised sample is then dried in vacuum and subsequently calcined, for example at 450°C for 2 hrs at a ramping rate of 2.5°C /min, to form the crystalline form α-Μο03.
Figure 11 shows the charging and discharging process of the integrated energy-generation and energy-storage device shown in Figure 9(a). In the charging stage, Figure 11(a), the front contact of the solar cell is connected with the second electrode of the capacitor, and the charge can be stored in capacitor using the photovoltage of the solar cell under illumination. In the discharging stage, Figure 11 (b) , the capacitor is connected to the load, and the charge stored is released. Figure 11 (c) shows the cyclic light charge-galvanostatic discharge test results
corresponding to Figure 11 (a) and (b) . Each charge-
discharge cycle comprised 30 s illumination under a 0.6 Sun (60 mW/cm2) LED light followed by a 30 s discharge period in the dark under a constant current density of 1.7 μΑ/cm2. This figure demonstrates the functionality of the device.
Figure 12 shows the charging and discharging process of the integrated energy-generation and energy-storage device shown in Figure 9(b). In the charging stage, Figure 12(a), the front contact of the solar cell is connected with the second electrode of the capacitor, and the charge can be stored in capacitor using the photovoltage of the solar cell under illumination. In the discharging stage, Figure 12 (b) , the capacitor is connected to the load, and the charge stored is released. Figure 13 (a) shows the voltage and current profiles for charge-discharge experiment of an electrochemical
capacitor .
Figure 13 (b) shows a charge-discharge curve of a double layer and asymmetric electrochemical capacitor (with increased current density) . The arrow denotes the charging voltage for sustained power. Although capacitors do not need to be charged at a constant voltage it is more efficient to do so and PV devices generate a near-constant voltage. By increasing the energy density of the
electrochemical capacitor, the discharge curve is
flattened and charging can be performed either at peak voltage (for power) or the "shoulder" voltage for energy. This can result in more efficient and flexible charging methods making possible more compact power supplies than if a separate electrochemical capacitor and a battery were combined in the circuit.
Figure 14 shows a possible configuration of
electrochemical capacitors and photovoltaic cells in accordance with embodiments in the charging and
discharging states. The configuration of figure 14 allows operating the capacitors using higher voltages by
connecting the cells in series and can be applied at a PV module level .
Referring now to Figure 15, there is shown an alternative configuration of the hybrid energy generation-energy storage system 1500. In the example of Figure 15, the electrochemical capacitor 1502 is formed in the layered structure as described above but rolled into a compact cell that could be located into a junction box 1504 of the photovoltaic module 1506. This embodiment places some restrictions on the volume of the energy storage element, however it provides the advantage that if the lifetime of either or both of the electronics or electrochemical capacitor storage system is less than that of the photovoltaic module, then the junction box can be periodically replaced without
affecting the module. Preferably any electronics would maximise the use of software or firmware so as to minimise the number of electronic components that could age.
The electrical circuit within which the electrochemical capacitor is integrated preferably maximises the power generated by the module using one of the many available power maximising algorithms employed for PV modules . It could be designed to generate an AC power and thereby act as a micro-invertor , or could output DC power. In a further variation the power maximising/buffering
electronics could be arranged at the end of each cell
string of a module, and connected by a circuit which then connects to the external circuit (e.g., array
electronics) . This arrangement is advantageous because it can allow a greater volume for the energy storage element allowing for a larger capacity. However, it can also replace the need to use bypass diodes in the circuit. If the string of buffering/power maximising electronic elements is housed in a linear junction box element which extends across the width of a module, then this element can also be periodically replaced as described for the single junction box above. The presents an advantage over current PV module technology in that the electronics associated with bypass diodes are removed from the long- lifetime silicon PV module. Consequently, providing the PV module is well encapsulated and the electronics associated with power maximisation and buffering are external to this encapsulation, modules may continue to function in the field for a longer period allowing module lifetimes to be extended over their current value of 25 years. Preferably the electrical components would include internet accessibility allowing remote control of the module' s power for voltage ramping purposes and remote control of the power buffering using the electrochemical capacitor element(s), the latter being informed by software which can predict the need to store and release energy based on weather events and/or shading events.
Knowledge of shading events could also be communicated by other modules in an array. For example, in the case that the PV modules comprise part of an array that was used on a road or fagade surface, then shading events caused by passing traffic could also be predicted and the
electrochemical capacitor storage elements could be
instructed to buffer preceding an anticipated shading events and to release energy on arrival of the shading event. These communicated predictions could permit the use of an electrochemical capacitor energy storage system with a lower response rate (instantaneous power) , as energy discharge can be anticipated ahead of time and the storage system can begin discharging before the illumination intensity was reduced significantly.
Ideally, inclusion of the electrochemical capacitor energy storage system would reduce the ramp rate of the power generated by the module to less than 10%. Being able to reduce the ramp rate of power generated by renewable resources is of particular value for large penetration of renewable energy. By reducing the ramp rate (i.e., power smoothing) at a module level, the capacity of central energy storage systems can be reduced as these systems do not need to be sized such that they can respond quickly to intermittencies in power. Furthermore, the maximising and buffering of power at the module level can increase the efficiency of power generation resulting in increased energy generation capacity from an array.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element,
integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Claims
1. An electrochemical capacitor comprising: a first electrode element having a nano-porous or micro-porous conductive material and a hierarchically- nanostructured metal oxide region; the hierarchically- nanostructured metal oxide region being formed by
anodising a surface portion of the conductive material; an electrolyte disposed in contact with at least a portion of the hierarchically-nanostructured metal oxide region; and a second electrode element; wherein, when a voltage is applied between the first and the second electrode elements, electrical charge is stored in the hierarchically-nanostructured metal oxide region of the first electrode.
2. The capacitor of claim 1 wherein electrical charge is stored in the hierarchically-nanostructured metal oxide region by one or more of surface redox reactions, double layer capacitance effects and ion intercalation.
3. The capacitor of claim 1 or claim 2 wherein the
conductive material comprises a micro-porous metal or metal alloy foam.
4. The
or met
of met
5. The
oxide nanostructures comprise metal oxide nanotubes that
are formed by anodising a surface portion of the micro- porous metal or metal alloy foam.
6. The capacitor of any one of claims 3 to 5 wherein the micro-porous metal foam comprises titanium and a plurality of TiOx nanotubes are formed by anodising a surface portion of the foam.
7. The capacitor of claims 6 wherein the plurality of TiOx nanotubes have an average tube diameter between 20 nm and 40 nm.
8. The capacitor of claim 1 wherein the conductive material comprises a first conductive region and a second conductive region, the second conductive region being physically or chemically formed on a portion of the surface of the first conductive region and wherein the hierarchically-nanostructured metal oxide region is formed by anodising a portion of the second conductive region.
9. The capacitor of claim 8 wherein the first conductive region comprises nanostructured or micro-structured porous carbon .
10. The capacitor of claim 9 wherein the hierarchically- nanostructured metal oxide region comprises a first micro- or nanostructure and a second nanostructure, formed in a hierarchical arrangement with the first micro- or
nanostructure; the second nanostructure being formed by anodising a portion of the second conductive region.
11. The capacitor of claim 10 wherein the hierarchically- nanostructured metal oxide region comprises a crystallised transition metal oxide in the orthorhombic phase.
12. The capacitor of claim 11 wherein the second
nanostructure comprises a plurality of nano-belt or nano- tube structures.
13. The capacitor of claim 12 wherein the nano-belt structures have a width between 100 nm and 300 nm and a length between 100 nm and 700 nm.
1 . The capacitor of any one of the preceding claims wherein the hierarchically-nanostructured metal oxide region is arranged such that a charge discharge curve has a predetermined shape and charging can be performed at a voltage within 10% of the capacitor peak voltage.
15. A method for forming an electrochemical capacitor comprising the steps of: providing a first electrode having a conductive material; the conductive region having a portion with a first nanostructure or microstructure; anodising a portion of the surface of the nanostructured or microstructured conductive material in a manner such that a hierarchically-nanostructured metal oxide region is formed; annealing the hierarchically-nanostructured metal oxide region in a manner such that a crystalline metal oxide is formed; providing an electrolyte in contact with at least a portion of the metal oxide region; and providing a second electrode element;
wherein, when a voltage is applied between the first and the second electrode elements, electrical charge is stored in the hierarchically-nanostructured metal oxide region of the first electrode.
16. The method of claim 15 wherein during formation of the hierarchically-nanostructured portion a plurality of TiOx nanotubes is formed.
17. The method of claim 16 wherein the method further comprises the step of depositing a metal layer on a portion of the conductive material with a first
nanostructure or microstructure .
18. The method of claim 17 wherein the step of annealing the metal oxide region is performed in a manner such that a plurality of nano-belt structures are formed in the hierarchically-nanostructured portion.
19. An integrated energy-generation and energy-storage device comprising: a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; an electrochemical capacitor in accordance with any one of claims 1 to 14; wherein the conductive region of the
electrochemical capacitor comprises a portion of the back contact of the photovoltaic cell.
20. The device of any one of claim 19 wherein the device comprises three electrical terminals : a first terminal in electrical contact with the front contact of the
photovoltaic cell; a second terminal in electrical contact with the back contact of the photovoltaic cell and the first electrode element of the electrochemical capacitor; and a third terminal in electrical contact with the second electrode element of the electrochemical capacitor.
21. A method for forming an integrated energy-generation and energy-storage device, the device comprising a photovoltaic cell and an electrochemical capacitor, the method comprising the steps of: providing a photovoltaic cell comprising a current rectifying portion, a conductive front contact and a conductive back contact; and forming an electrochemical capacitor in accordance with any one of claims 1 to 14 on a portion of the back contact of the photovoltaic cell; wherein the conductive region of the
electrochemical capacitor comprises a portion of the back contact of the photovoltaic cell.
22. An integrated energy-generation and energy-storage device comprising: a photovoltaic module that includes at least one photovoltaic cell; and a circuitry assembly arranged to control the electrical performance of the integrated energy-generation and energy-storage device; the circuitry assembly
comprising an electrochemical capacitor in accordance with any one of claims 1 to 14;
wherein, in use, the electrochemical capacitor allows compensating for irregular energy generation due to solar irradiation variance.
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Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
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US7150938B2 (en) * | 2001-03-30 | 2006-12-19 | Lithium Power Technologies, Inc. | Structurally embedded intelligent power unit |
US20130321983A1 (en) * | 2011-01-06 | 2013-12-05 | Sungkyunkwan University Foundation For Corporate Collaboration | Nano-porous electrode for super capacitor and manufacturing method thereof |
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2017
- 2017-04-21 WO PCT/AU2017/050369 patent/WO2017181247A1/en active Application Filing
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US7150938B2 (en) * | 2001-03-30 | 2006-12-19 | Lithium Power Technologies, Inc. | Structurally embedded intelligent power unit |
US20130321983A1 (en) * | 2011-01-06 | 2013-12-05 | Sungkyunkwan University Foundation For Corporate Collaboration | Nano-porous electrode for super capacitor and manufacturing method thereof |
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