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 TiO _x 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 TiO _x 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 (TiO _x ) 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 (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 .

The Applicants have investigated the performance of titanium foam anodised for different periods of time

(200s, 500s, 1000s and 2000s) to grow TiO _x 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 TiO _x 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 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) .

Figure 2 shows the dependence between the surface

morphology and length of the TiO _x 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 TiO _x 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 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) .

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 Ti0 ₂ and Li _x Ti0 ₂ due to the lithium ion insertion/de-insertion into anatase Ti0 ₂ 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 Ti0 ₂ 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/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 . 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-Μο0 ₃ 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 MoO _x in NaF solution is carried out which is then followed by annealing. This forms a (X-M0O ₃ electrode consisting of a plurality of nano- belt structures. Figure 6(c) shows an SEM image of a cauliflower-like α-Μο0 ₃ 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-Μο0 ₃ 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-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.

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 MoO _x 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-Μο0 ₃ .

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/cm ² ) LED light followed by a 30 s discharge period in the dark under a constant current density of 1.7 μΑ/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). 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.