HALLS JONATHAN EDWARD (GB)
WO2004025705A2 | 2004-03-25 |
US4022950A | 1977-05-10 | |||
US20090226639A1 | 2009-09-10 |
MATHAI AND E RABINOWITCH K G: "Studies of the Thionine-Ferrous Iron Reaction in a Heterogeneous System", JOURNAL OF CHEMICAL PHYSICS, AMERICAN INSTITUTE OF PHYSICS, NEW YORK, NY, US, vol. 66, 1 April 1962 (1962-04-01), pages 663 - 664, XP009169031, ISSN: 0021-9606
FRANCKOWIAK AND E RABINOWITCH D: "The Methylene Blue-Ferrous Iron Reaction in a Two-Phase System", JOURNAL OF CHEMICAL PHYSICS, AMERICAN INSTITUTE OF PHYSICS, NEW YORK, NY, US, vol. 70, no. 9, 1 September 1966 (1966-09-01), pages 3012 - 3017, XP009169030, ISSN: 0021-9606
CLAIMS A photogalvanic cell comprising: a first electrode and a second electrode spaced apart from the first electrode ; an aqueous medium arranged between the first and second electrodes and including a lyotropic liquid crystal existing in a structured phase such that the aqueous medium includes at least one surfactant pseudo-phase and at least one aqueous sub-phase; a photoactive first redox couple contained in the surfactant pseudo-phase and a second redox couple contained in the aqueous sub-phase; at least one electrolyte contained in the aqueous sub-phase and, optionally, at least one electrolyte contained in the surfactant pseudo-phase. The photogalvanic cell according to claim 1 wherein the lyotropic liquid crystal exists in a lamellar (La) phase. The photogalvanic cell according to claim 1 or 2, wherein the first electrode is transmissive of light and is in contact with the surfactant pseudo-phase. An assembly for incorporation into a photogalvanic cell, the assembly comprising a first fluid medium and a second fluid medium, said fluid media being separated at at least one boundary, the first fluid medium including a first redox couple consisting of species soluble in the first fluid medium and substantially insoluble in the second fluid medium and the second fluid medium including a second redox couple and wherein a component of the first redox couple is transformable on exposure to light into a species which can partition between the first and second fluid media by crossing the said boundary. An assembly as claimed in claim 4 wherein the first fluid medium and the second fluid medium comprise first and second liquid phases arranged in parallel and in mutual contact. An assembly as claimed in claim 5 comprising a plurality of said first and second liquid phases in an alternating arrangement. An assembly as claimed in claim 4, 5 or 6 wherein the second redox couple consists of species soluble in the second fluid medium and substantially insoluble in the first fluid medium. An assembly as claimed in any of claims 4 to 7 further comprising a first electrode in contact with the first fluid medium and a second electrode in contact with the second fluid medium, the respective electrodes being spaced apart and defining a volume therebetween. 9. An assembly as claimed in claim 8 wherein the first and second fluid media are defined by an aqueous medium including a lyotropic liquid crystal existing in a lamellar (La) phase such that the aqueous medium includes at least one surfactant pseudo- phase and at least one aqueous sub-phase, the photoactive first redox couple being contained in the surfactant pseudo-phase and the second redox couple being contained in the aqueous sub-phase. 10. A photogalvanic cell comprising an assembly as claimed in claim 8 or 9 and wherein the first electrode is transmissive of light and is in contact with the surfactant pseudo- phase. 11. The photogalvanic cell according to claim 3 or 10, wherein the second electrode is in contact with the aqueous sub-phase. 12. The photogalvanic cell according to any of claims 1 , 2, 3, 10 and 11 wherein the cell is chargeable by illuminating the first electrode. 13. The photogalvanic cell according to any of claims, 1 , 2, 3 or 10 to 12, wherein the first electrode is selected from the group consisting of indium tin oxide, FTO (fluorine doped tin oxide), other tin oxides, and PEDOT (poly(3,4-ethylenedioxythiophene) and its derivatives. 14. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 13, wherein the second electrode is a sacrificial electrode. 15. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 14, wherein the second electrode is an electropositive metal. 16. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 15, wherein the second electrode is selected from the group consisting of nickel, silver, aluminium or zinc. 17. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 16, wherein the second electrode is zinc. 18. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 17, wherein the photoactive first redox couple is selected from the group consisting of N- methylphenothiazine, dyes selected from the classes thiazines, phenazines, xanthenes, acridines, triphenylmethanes, triphenylamines, ferrocenes, phenylenediamines, cyanines, azo dyes, porphyrins, phthalocyanines, ruthenium (ll)tris(bipyridyl) and ruthenium(ll)tris(phenanthroline) or derivatives having an alternative metal ion centre or ligand. 19. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 18, wherein the photoactive first redox couple is /V-methylphenothiazine. 20. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 19, wherein the aqueous medium within which the photoactive redox couple is contained further contains an aromatic (or polyaromatic) halide or halogenated solvent, or halogenated environmental pollutant. 21. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 20, wherein the electrolyte is selected from the group consisting of potassium chloride, water-soluble sulphates, hydrogenphosphates, fluorides, chlorides, bromides, iodides, thiocyanates and cyanates. 22. The photogalvanic cell according to any of claims 1 , 2, 3 or 10 to 21 , wherein the electrolyte is potassium chloride 23. An electric charge storing device comprising a photogalvanic cell as claimed in any of claims 1 , 2, 3 or 10 to 22. 24. A power source for an item of personal electronic equipment including a photogalvanic cell as claimed in any of claims 1 , 2, 3 or 10 to 22. |
[0001] This invention relates to electrochemical cells and in particular to
photoelectrochemical cells, more especially photogalvanic cells. The invention relates further to systems, components and assemblies for making, or which are included in or incorporated into, such cells. Advantageous embodiments provide a flexible (non-rigid) photoelectrochemical cell and/or systems, components and assemblies for making, or which are included in or incorporated into, such a flexible cell. Such cells, systems, components and/or assemblies are, in preferred embodiments, lightweight, self- assembling and/or self-reconstructing. Photoelectrochemical cells according to the invention are particularly suitable for use in portable (e.g. personal) electronic devices.
BACKGROUND
[0002] The quest for renewable sources of energy has led to an increasing interest in photoelectrochemical cells because of their possible role as transducers of solar to electrical energy. Although numerous developments have been made in the field of solar- based energy sources, there remains considerable scope to improve the efficiency and versatility of these technologies.
[0003] Commonly used systems employing solar cells involve photovoltaic cells which directly convert sunlight into electric current. Photovoltaic cells are incapable of storing energy. The cell voltage is proportional to the amount of sunlight falling on the cell but is generally of the order of one volt. Solar energy reaching the surface of the earth from the sun is approximately 1 kilowatt per square metre and although solar cell conversion efficiencies have improved in recent years, commercially available photovoltaic panels typically have an efficiency of about 23% (equating to delivery of 230 W m "2 ). The best research-cell efficiencies approach 41 %, but these systems are considerably more expensive to construct.
[0004] An example of a photovoltaic cell is a dye-sensitized solar cell which is based on a semiconductor (e.g. silicon) formed between a photo-sensitized anode and an electrolyte. Following significant research in this area, recent developments have led to the emergence of Gratzel cells. In Gratzel cells particles of Ti0 2 , coated with a dye that absorbs at a wide range of wavelengths given off by sunlight, are placed between two electrodes in an electrolyte solution containing iodine ions. The cells generate electricity when the energy captured by the dye promotes electrons in the dye molecules from one orbital to a higher one. The electrons then transfer into the Ti0 2 conduction band and transport towards one electrode. The dye is regenerated through reaction with iodide ions to form iodine (present as the triiodide ion), which allows electrons to be efficiently carried to the counter electrode. [0005] Gratzel cells are considered to offer high efficiencies and the economics are promising because they are based on Ti0 2 which is a reasonably cheap and widely available material.
[0006] Photogalvanic cells, in contrast to photovoltaic cells, are able to generate a voltage from sunlight and subsequently harness this voltage to drive an electric current through an external circuit in a manner similar to that of a conventional battery. These power-producing systems operate through altering the chemical composition of a galvanic cell through a photochemically-induced redox reaction, so that, unlike photovoltaic systems, it is a Faradaic reaction at the electrode | electrolyte interface which gives rise to the observed photocurrents.
[0007] Despite the early promise of photogalvanic systems, they have not been adopted as a method for efficient solar-to-electrical energy conversion as the maximum theoretical power conversion efficiency of 18% for homogeneous systems has yet to be realized. Typically, efficiencies well-below 1 % are observed under laboratory conditions using monochromatic light. The inventors have reached the view that, in part, the impracticality of photogalvanic systems stems from a combination of (1) small transport rates requiring the cells to be microfabricated effectively and reproducibly, and (2) a requirement for the occurrence of differential electrode kinetics of the involved species.
[0008] Despite these current obstacles, there is considerable potential for photogalvanic cells to offer a cheaper and more versatile photoelectrochemical cell than that provided by current solar-based technologies.
BRIEF SUM MARY OF THE DISCLOSURE
[0009] The present invention seeks to overcome or ameliorate disadvantages of known photogalvanic cells. In particular embodiments, the present invention seeks to provide flexible (that is, non-rigid) cells and cell systems, components and assemblies that may be moulded to fit a shape required by a given application. In embodiments, the present invention seeks to provide photogalvanic cells, cell systems, components and assemblies suitable for powering portable electronic devices or personal technology systems, such as music players, mobile phones, calculators, navigation devices, hand-held computers and the like. In other embodiments the present invention seeks to provide photogalvanic cell systems, components and assemblies which are self-annealing and/or seal-healing.
[0010] In accordance with a first aspect of the present invention there is provided a photogalvanic cell comprising:
a first electrode and a second electrode spaced apart from the first electrode ; an aqueous medium arranged between the first and second electrodes and including a lyotropic liquid crystal existing in a structured phase such that the aqueous medium includes at least one surfactant pseudo-phase and at least one aqueous sub-phase;
a photoactive first redox couple contained in the surfactant pseudo-phase and a second redox couple contained in the aqueous sub-phase;
at least one electrolyte contained in the aqueous sub-phase and, optionally, at least one electrolyte contained in the surfactant pseudo-phase;
[0011] In preferred embodiments of this first aspect of the invention the lyotropic liquid crystal exists in a lamellar (La) phase.
[0012] Preferably in this first aspect of the invention the first electrode is transmissive of light and is in contact with the surfactant pseudo-phase.
[0013] According to a second aspect of the present invention there is provided an assembly for incorporation into a photogalvanic cell, the assembly comprising a first fluid medium and a second fluid medium, said fluid media being separated at at least one boundary, the first fluid medium including a first redox couple consisting of species soluble in the first fluid medium and substantially insoluble in the second fluid medium and the second fluid medium including a second redox couple and wherein a component of the first redox couple is transformable on exposure to light into a species which can partition between the first and second fluid media by crossing the said boundary.
[0014] Preferably in this second aspect of the invention the first fluid medium and the second fluid medium comprise first and second liquid phases arranged in parallel and in mutual contact.
[0015] Preferably the assembly comprises a plurality of said first and second liquid phases in an alternating arrangement.
[0016] Preferably in embodiments of this second aspect of the invention the second redox couple consists of species soluble in the second fluid medium and substantially insoluble in the first fluid medium.
[0017] Preferably the assembly further comprises a first electrode in contact with the first fluid medium and a second electrode in contact with the second fluid medium, the respective electrodes being spaced apart and defining a volume therebetween.
[0018] Preferably in this second aspect of the invention the first and second fluid media are defined by an aqueous medium including a lyotropic liquid crystal existing in a lamellar (La) phase such that the aqueous medium includes at least one surfactant pseudo-phase and at least one aqueous sub-phase, the photoactive first redox couple being contained in the surfactant pseudo-phase and the second redox couple being contained in the aqueous sub-phase.
[0019] According to a third aspect of the invention there is provided a photogalvanic cell comprising an assembly as defined in the second aspect of the invention and wherein the first electrode is transmissive of light and is in contact with the surfactant pseudo-phase.
[0020] Preferably in the first and/or third aspects of the invention the second electrode is in contact with the aqueous sub-phase.
[0021] Preferably in the first and/or third aspects of the invention the cell is chargeable by illuminating the first electrode.
[0022] Preferably in the first and/or third aspects of the invention the first electrode is selected from the group consisting of indium tin oxide, FTO (fluorine doped tin oxide), other tin oxides, and PEDOT (poly(3,4-ethylenedioxythiophene) and its derivatives.
[0023] Preferably in the first and/or third aspects of the invention the second electrode is a sacrificial electrode.
[0024] Preferably in the first and/or third aspects of the invention the second electrode is an electropositive metal.
[0025] Preferably in the first and/or third aspects of the invention the second electrode is selected from the group consisting of nickel, silver, aluminium or zinc.
[0026] Preferably in the first and/or third aspects of the invention the second electrode is zinc.
[0027] Preferably in the first and/or third aspects of the invention the photoactive first redox couple is selected from the group consisting of
N-methylphenothiazine,
dyes selected from the classes thiazines, phenazines, xanthenes, acridines, triphenylmethanes, triphenylamines, ferrocenes, phenylenediamines, cyanines, azo dyes, porphyrins, phthalocyanines, and
ruthenium (l l)tris(bipyridyl) and ruthenium(ll)tris(phenanthroline) or derivatives having an alternative metal ion centre or ligand.
[0028] Preferably in the first and/or third aspects of the invention the photoactive first redox couple is N-methylphenothiazine.
[0029] Preferably in the first and/or third aspects of the invention the aqueous medium within which the photoactive redox couple is contained further contains an aromatic (or polyaromatic) halide or halogenated solvent, or halogenated environmental pollutant. [0030] Preferably in the first and/or third aspects of the invention the electrolyte is selected from the group consisting of potassium chloride, water-soluble sulphates, hydrogenphosphates, fluorides, chlorides, bromides, iodides, thiocyanates and cyanates.
[0031] Preferably in the first and/or third aspects of the invention the electrolyte is potassium chloride
[0032] According to a fourth aspect of the present invention there is provided an electric charge storing device comprising a photogalvanic cell as defined in the first or third aspect of the invention.
[0033] According to a fifth aspect of the present invention there is provided a power source for an item of personal electronic equipment including a photogalvanic cell as defined in the first or third aspect of the invention
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Embodiments of the invention are further described hereinafter, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a view of a hypothetical typical biphasic cell;
Figure 2 is a schematic view of a cell according to the present invention;
Figure 3 illustrates various possible self-arranged structures of a surfactant;
Figure 4 shows variation of the open circuit voltage of a galvanic cell based on the L a phase of Brij 30 in the presence of chopped light at 350 nm (bandwidth 30 nm), of intensity 1.8 mW cm "2 and where the phase was doped with 0.1 mmol PMe and 0.1 mmol EuCI 3 . The dark electrode was constructed from ITO;
Figure 5 shows variation of the open circuit voltage of a galvanic cell based on the L a phase of Brij 30 in the presence of chopped light at 350 nm (bandwidth 30 nm), of intensity 1.8 mW cm "2 and where the phase was doped with 0.5 mmol PMe and 0.1 mmol AgN0 3 . The dark electrode was constructed from Ag;
Figure 6 shows variation of the open circuit voltage of a galvanic cell based on the L a phase of Brij 30 in the presence of chopped light at 350 nm (bandwidth 30 nm), of intensity 1.8 mW cm "2 and where the phase was doped with 0.6 mmol PMe and 0.15 mmol NiS0 4 . Oxygen was rigorously removed. The dark electrode was constructed from Ni;
Figure 7 shows variation of the open circuit voltage of a galvanic cell based on the L a phase of Brij 30 in the presence of chopped light at 350 nm (bandwidth 30 nm), of intensity 1.8 mW cm "2 and where the phase was doped with 0.4 mmol PMe, 0.4 mmol PMe +" CI " , 40 μηιοΙ ZnCI 2 . The aqueous sub-phase was doped with potassium chloride. Oxygen was rigorously removed. The dark electrode was constructed from Zn;
Figure 8 shows variation of the open circuit voltage of a galvanic cell based on the L a phase of Brij 30 in the presence of chopped light at 350 nm (bandwidth 30 nm), of intensity 1.8 mW cm "2 and where the phase was doped with 0.5 mmol PMe, 3.0 mmol AICI 3 . The aqueous sub-phase was doped with potassium chloride. Oxygen was rigorously removed. The dark electrode was constructed from Al;
In each of Figs 4 to 8 the light electrode was constructed from ITO.
Figure 9 shows chronopotentiograms illustrating the electrical characteristics of the cell in Figure 7 under conditions of charge (the first 100 s at 50.9 μΑ cm "2 ), and discharge at various current densities: 127.3 μΑ cm "2 (line (i)), 101 .9 μΑ cm "2 (line(ii)), 50.9 μΑ cm "2 (line (Hi)), 25.5 μΑ cm "2 (line (iv)), 10.2 μΑ cm "2 (line (v)), 5.1 μΑ cm "2 (line (vi)), 0.51 μΑ cm "2 (line (vii)), 0.05 (line (viii)).
Figures 10a and 10b show Ragone plots for the cell in Figure 7: Fig 10a - normalisation using a total mass of electroactive species of 339.1 μg; Fig 10b - re-normalisation of the data in Fig 10a using an effective mass of 38.1 μg. In these Figures circles correspond to dark charge (at 100 s), squares correspond to illuminated charge (to a steady open-circuit value, using 350 nm light (centre band, 30 nm bandwidth), of intensity 1.8 mW cm "2 .
DETAILED DESCRIPTION
[0035] The present invention relates to a new type of photogalvanic cell and to systems, components and assemblies used or incorporated in such a cell and/or for creating or fabricating such a cell.
[0036] Considering two redox reactions at electrode surfaces:
(1) B + e " reversibly goes to A; and
(2) D - e reversibly goes to C
Then, the overall formal cell reaction can be expressed as:
(3) B + D reversibly goes to A + C.
[0037] The forward reaction direction (B + D goes to A + C) is spontaneous provided that the formal potential of the A/B redox couple is more positive than the formal potential of the C/D redox couple.
[0038] The inventors have appreciated that it is possible to separate the reaction so that species A and B and species C and D respectively are in separate "compartments" (and more especially the "compartments" may be of nanometric thickness). Species A and B on the one hand and species C and D on the other hand may be contained in physically separate media. Species A and B may be contained in a medium M 1 and species C and D may be contained in a separate medium M2.
[0039] Media M 1 and M2 are preferably chemically distinct. Media M 1 and M2 may, for example, be immiscible.
[0040] The separation of the respective redox species may also be achieved by using a lamellar L a phase, as discussed in further detail below. In such an arrangement, medium M2 may, for example, be a surfactant sub-phase and medium M 1 may be an aqueous phase.
[0041] Media M 1 and M2 preferably share a boundary or interface, such as an interface between immiscible layers of M 1 and M2 respectively or between a surfactant sub-phase and an aqueous sub-phase.
[0042] Thus the respective embodiments can be seen as consisting of two parallel (but distinct) liquid phases in contact with one another, such an arrangement being
conveniently referred to as a framework medium or framework structure or as comprising framework materials.
[0043] Species A and B may be soluble in medium M 1 and substantially insoluble in medium M2. Species C and D may be soluble in medium M2 and substantially insoluble in medium M 1 .
[0044] One or both of medium M 1 and medium M2, as necessary or appropriate, may preferably contain an electrolyte.
[0045] In one example using immiscible media M 1 and M2, a first medium M1 is aqueous and a second medium M2 is an organic medium immiscible with the aqueous medium M l In one example the organic medium M2 may be dichloromethane. In another example the organic medium M2 may be a mixture of chloroform and acetonitrile, in particular a 50:50 vol % mixture thereof. In the latter example, the aqueous medium M 1 may be aqueous zinc chloride. In these examples, the organic media M2 may contain an electrolyte such as tetrabutyl ammonimum perchlorate.
[0046] The inventors have further appreciated that if species A is selected to be photochemically active it is possible, by exposure to light energy, to reverse the direction of the spontaneous cell reaction. Thus:
(4) A (with energy (i.e. incident light)) goes to A* (where A* represents a photo-excited state of A) and
(5) A* + C goes to B + D is now energetically favourable. [0047] This leads to the possibility generating a voltage from incident light (e.g. sunlight), in effect providing a means of capturing and "storing" solar-derived energy.
[0048] An example of a photochemically active species which may be contained, by way of example, in the organic medium M2 is PMe (/V-methylphenothiazine).
[0049] Thus, a galvanic cell according to the invention constructed from these concepts may be based on a soft interface (liquid/liquid or pseudo-phase/pseudo-phase) which relies on two redox couples which exist in the different phases (that is, the "nano" cells). This can in principle be achieved by mixing two immiscible chemical components.
Depending on the nature and relative amounts of the two components, nano cells or stacked layers can be formed with the respective redox reactions occurring in the different layers, phases, sections or "compartments". These layers may be micelle-like structures that are packed tightly together in a regular arrangement. The two redox couple in the different phases can be arranged between two electrodes, so that one of the different phases is in contact with a first electrode and the other of the different phases is in contact with a second electrode. In particular examples, one electrode may be coated with a hydrophobic layer and the other coated with a hydrophilic layer to promote this
arrangement. When the electrodes are connected, current can flow through an external circuit.
[0050] The photogalvanic cells of the present invention employ, in preferred
embodiments, a lamellar (L a ) phase of a surfactant solution, which can be considered to comprise alternating bands of surfactant and water. By providing that the redox reagents are differentially soluble in the different phases - so for example, if A and B are soluble in one phase (phase 1 ) and insoluble in the other (phase 2) and C and D are insoluble in phase 1 and are soluble in phase 2 - it is possible to separate out the charges. The surfactant solution having the lamellar phases can be arranged between two electrodes, so that one phase is in contact with the first electrode and the other phase is in contact with the second electrode. For example, as noted above, one electrode may be coated with a hydrophobic layer and the other coated with a hydrophilic layer to promote this arrangement. When the electrodes are connected, current can flow through an external circuit. The lamellar layers may be electroporated (that is, the structure may be (partially or locally) broken down on application of an electric field), so molecules can move between the successive layers in the same phase (e.g. from one surfactant layer to the other).
[0051] The photochemical electron transfer reaction per se may be confined to layers next to the electrode. The distances between the phases are nanometric.
[0052] Without wishing to be bound by theory, the inventors believe that electron transfer across the soft interface between two out of the four redox species involved in the two redox couples is thermodynamically favoured, allowing the system to charge and discharge. The kinetics of this interfacial process may in preferred examples be restricted either through exploitation of the inverted Marcus region (as discussed further below), or in other embodiments through the slow dissolution of a solid particle or porous film located at the soft interface between the surfactant and aqueous sub-phases
[0053] Because the phases (i.e. the redox species containing media) are liquid, they can be encased in a flexible housing, allowing for the creation of a flexible power source (battery).
Charge separation for photo-rechargeable cells can in principle be achieved using very thin cells or engineering separating membranes. Constructions of this sort are seriously disadvantaged because it is possible to solubilise dyes (which form the photoactive redox couple) only to 0.1 mM concentrations in water. High concentrations (mM concentrations) are needed for the solar cell to work effectively. At the moment the best research efficiency of cells of this type is a maximum of <1 %. In contrast, the cells according to the present invention enable high concentrations of dye to be employed as the dyes partition between the organic and aqueous medium, with preference to reside within the organic component of the framework medium.
[0054] In arriving at the present invention, the inventors have explored the theoretical constraints of photogalvanic cells in terms of a hypothetical biphasic battery (100) as illustrated in Figure 1.
[0055] The hypothetical biphasic battery of Figure 1 comprises two spaced apart electrodes (1 12), (1 14) arranged parallel to each other, sandwiching a dielectric system (1 16) comprised of two immiscible liquids (1 18, 120) (e.g. oil and water) that are mutually- saturated. Each of these liquids (1 18, 120) is loaded with (1 ) excess supporting electrolyte and (2) a respective redox-active couple. The liquid | liquid interface is considered to be ideally polarisable, so it is assumed that only one electron is exchanged between the redox pairs across the biphasic interface. In this model, the more dense (a) phase contains photoredox-active species A and its redox partner, B so that
β + e > A■ These species A and B are both insoluble within the other liquid phase (β), which contains the electroactive couple C and D so that + e ~ < > D . In considering this system it is assumed that the formal potential for the A/B couple is more positive than k
that of the D/C couple. Thus, at the interface, the equilibrium A + C <— sn →B + D is established. In this equilibrium k on represents the rate constant for the forward reaction in the presence of light (i.e. the charging rate constant) and k 0ff represents the rate constant of the reverse reaction in the absence of light (i.e. the discharge rate constant). The rates of the charging and discharge reactions may depend on various factors, such as the intensity of the incident light, the depth of the cell (and associated attenuation of light with depth), diffusion rates of the redox species, and the concentrations of the redox species.
[0056] The inventors have considered in detail the case where the diffusion coefficients of the species A and B are equal, and likewise the diffusion coefficients of species C and D are equal, with each of the species A, B, C and D having identical concentrations within the liquid electrolytes at zero time. With these constraints, the inventors have noted that maximum photo-recharging voltages for a cell as illustrated in Figure 1 occur when k on /k 0 ff is greatest. In other words, such maximum photovoltages occur when charging is fast and discharge is slow. If the ratio k 0 ff/k 0 n is maximized, the cell is always in discharge mode and cannot be charged irrespective of the light intensity. It is also noted that the cases where both charge and discharge rates are fast may be useful for light-to-electrical energy conversion (but not necessarily energy storage), but when both charge and discharge rates are slow the resulting cell is unsatisfactory for energy conversion and storage
[0057] In order for a hypothetical cell as noted above to operate successfully in both charge and discharge modes the inventors have deduced that k on must be very much bigger than k off , or if these rate constants are of similar size, then ΔΕ 0 □ (the formal electrode potential difference ) needs to be as large as possible. Importantly, the discharge kinetics should fall within the Marcus inverted region (the Marcus inverted region, arising from Marcus theory, being a concept well known to those skilled in the art).
[0058] In developing the concepts further to the case where the diffusion coefficients of the species A and B, and C and D, respectively, are not equal the inventors observe that smaller cell voltages are obtained. Also, although the cell system discharges more slowly, it also requires longer to charge to constant capacity.
[0059] Thus, the inventors' simulations based on the hypothectical biphasic cell have illustrated that the maximum value of the cell open circuit voltage occurs when (1) ΔΕ 0 □ is biggest and (2) when the dimensionless open circuit voltage (¾, c ) is largest. The dimensionless open circuit voltage (ξ 00 ) is derivable through the relationship: ξ ο£ = -( Ε E \ L^'
RT L D ) RT
[0060] where F is Faraday's Constant (96484.6 C mol "1 )
R is the molar gas constant (8.3145 J mol "1 K "1 )
T is the absolute temperature E L is the potential of the illuminated electrode
and E D is The potential of the dark electrode.
[0061] With regard to condition (2) above, the inventors have appreciated that the role of kinetics of the electron transfer event at the liquid | liquid interface is of importance. The inventors believe that the electron transfer event follows a Marcus-type relationship, in that as the driving force for electron transfer increases, the rate constant increases to a maximum, and then decreases with further increase in -ΔΘ 0 - the so called inverted Marcus region. It follows from this that for maximum cell emf under illumination, the redox couples in the phases a and β should be selected such that they exist within the Marcus- inverted region. In this case spontaneous discharge of the cell takes place as slowly as possible.
[0062] In the light of the above considerations, the inventors concluded that there are at least two ways in which practical cell systems can be pursued, that is, (1) where one of the redox couples has an intrinsic slowness for self exchange (for example, as a result of subtle influences such as electron transfer non-adiabaticity, large inner sphere
reorganization energies, changes in co-ordination/solvation geometry leading to non- Marcus-type effects, solvent non-adiabaticity and the like), and (2) where one redox reaction, during charge, involves a phase change, such as nucleation.
[0063] The inventors have further noted that, all other things being equal, both phases should have similar mass transport rates for the reacting species, so that the maximum open circuit voltage is observed for a fixed pair of redox couples. Nevertheless, this is not easy to accomplish in conventional liquid | liquid systems. Further, it is very difficult to construct the very thin liquid | liquid interfaces required by the model outlined above.
[0064] The inventors' further note, however, that diffusive transport in the cell system is not restricted to physical translation of reactant species and that charge carrier hopping is also a mechanism encompassed by the above considerations. In the case of charge carrier hopping, the diffusion coefficient is a function of analyte concentration, and increases with the analyte concentration above a critical percolation threshold. In a construction of this sort, varying the concentration of one redox couple allows for both alteration in the rate of the charge/discharge process and the ability to fine tune transport rates, so that the transport rates may match-up within both phases.
[0065] The inventors have also considered a scenario in which the photo-induced electron transfer process is reaction with the medium (e.g. the solvent) in the compartment adjacent to the illuminated electrode. This may be seen as in contrast to the scenario where there is an interfacial Faradaic reaction. In this scenario, there will be a change in the relative concentration of A and B within that compartment, causing a change in the open circuit voltage. The potential at the other electrode remains controlled by a non- photo-active redox couple. This is particularly important if the medium of the compartment adjacent the illuminated electrode contains an entity which is reactive with A* (the photo- excited state of A) such as an aromatic halide or is a chlorinated solvent. If species B is stable and its counter ion is unstable (for example because reduction has occurred dissociatively, either in concert with the electron transfer or due to fast stepwise kinetics) the inventors have recognised that a system of this type can be made to store light energy, and the system behaves as a photo-battery.
[0066] In arriving at the present invention, the inventors have realised that the above considerations can usefully and beneficially be applied to lyotropic liquid crystals "doped" with redox-active molecules. These lyotropic liquid crystals are three-dimensional, dynamic, nano-restricted liquid systems and are multi-stacked quasi-biphasic ensembles that can be optically clear (if aligned completely in mono-domains) and which may be diffusionally anisotropic. Thus the inventors have utilized these framework materials for energy conversion and storage through electrochemical means.
[0067] Referring now to Figure 2, there is shown a simplified and schematic illustration of a photogalvanic cell in accordance with an embodiment of the present invention. The illustration is not to scale, with parts greatly exaggerated for clarity. The cell (10) comprises a first electrode (12) and a second electrode (14) which are typically (but not essentially) planar electrodes arranged in parallel spaced apart relation. Between, and in contact with, the electrodes (12, 14) is an aqueous surfactant medium (20). The surfactant in the aqueous surfactant medium (20) is preferably (but not essentially) a non-ionic surfactant. Where the surfactant is uncharged, electrical migration of surfactant monomers can be assumed to be negligible. Examples of suitable non-ionic surfactants include ethoxylated fatty alcohols such as those within the Brij range of surfactants (available from Croda International pic), the Triton X series of surfactants (available from The Dow
Chemical Company), and any material with polyethylene glycol subunits which is neutral in solution. Of these, a particularly preferred surfactant is a polyethylene glycol dodecyl ether which is available commercially under the name Brij L4 (formerly Brij 30).
[0068] It is well known in the art that surfactants can self-assemble into various structures, depending on the concentration of the surfactant. As illustrated in Figure 3, with increasing concentration, a surfactant may initially self-assemble into micelles, which in turn assemble into an ordered (e.g. cubic) structure of the micelles. Further self- assembly with increasing concentration leads to a typically hexagonal arrangement of cylindrical structures and ultimately to a lamellar (L a ) structure. Examples of these self- assembled structures are illustrated schematically in Figure 3. It is with this lamellar (L a ) lyotropic liquid crystalline phase that preferred embodiments of the present invention are primarily concerned.
[0069] For forming the photogalvanic cell of the invention according to these
embodiments, the lamellar L a (20) phase must first be established and should be appropriately oriented with respect to the electrodes (12, 14), that is, so that each plane of the L a phase (20) is arranged at least substantially parallel to the plane of the electrodes (12, 14). In embodiments of the present invention the establishment and orientation of the L a phase (20) is achieved through the application of a magnetic field, as noted in more detail below.
[0070] Between the electrodes (12, 14) of the cell (10), the L a phase consists of "bands" alternately of surfactant (16) and water (18). Only one such pair of bands (16, 18) is shown in Figure 2, for simplicity of illustration. In practice, multiple alternating bands (16), (18) are present between the electrodes (12, 14). In the surfactant band (16) the surfactant molecules are arranged as in a bi-layer in a "tail-to-tail" configuration, that is, with the hydrophobic tail portions (24) of the surfactant molecules in the respective layers directed towards those of the opposing layer and the head portions (22) directed outwardly of the bi-layer. The surfactant band (16) may be referred to as the surfactant pseudo- phase or the surfactant sub-phase. For convenience, the term "surfactant pseudo-phase" is used herein. The aqueous band (18) may be referred to as the aqueous sub-phase or the aqueous pseudo-phase. For convenience, the term "aqueous sub-phase" is used herein. The aqueous sub-phase (18) is doped with a suitable salting-out electrolyte to ensure that the aqueous sub-phase (18) is electrically conductive. A preferred salting-out electrolyte is KCI. Other suitable salting-out electrolytes include those based on water- soluble sulphates, hydrogenphosphates, fluorides, chlorides, bromides, iodides, thiocyanates, cyanates.
[0071] The surfactant pseudo-phase may also be doped with such a salting-out electrolyte if required. Addition of a salting-out electrolyte can also be useful in
encouraging de-mixing of the surfactant pseudo-phase (16) and aqueous sub-phase (18) of aqueous surfactant medium (20).
[0072] In the construction illustrated in Figure 2, electrode (14) is the so-called "dark" electrode which is not exposed (or does not need to be exposed) to incident light. The electrode (14) can conveniently be prepared from materials such as ITO (indium tin oxide), platinum, silver, nickel, zinc or aluminium (aluminum) foil. Electrode (12) is the so-called "light" electrode and is constructed to be transmissive of incident light and is preferably substantially transparent. A suitable material for the light electrode is ITO. Other suitable materials include FTO (fluorine doped tin oxide), other tin oxides, PEDOT (poly(3,4- ethylenedioxythiophene)) and its derivatives.
[0073] The cell (10) is constructed so that the light electrode (12) contacts the surfactant pseudo-phase (16) and the dark electrode (14) contacts the aqueous sub-phase (18).
[0074] The cell (10) must, clearly, contain redox active species. The surfactant pseudo- phase (16), which has a hydrophobic inner (palisade) layer, is doped with a photoredox- active species (conveniently referred to as a "dye"), such species being soluble in the surfactant pseudo-phase (16). Numerous photoredox-active materials can be used, of which examples include dyes of the following classes: thiazine (e.g. thionone), phenazine (e.g. phenosafranin), xanthenes (e.g. rhodamine 6G), acridine (e.g. acridine orange), triphenylmethane (e.g. fuchsin), triphenylamine (e.g. tris(4-bromophenyl)amine), ferrocene (e.g. decamethylferrocene), phenylenediamine (e.g. Λ/,Λ/,Λ/',Λ/'-tetraphenyl-para- phenylenediamine), cyanines (e.g. merocyanine 540), azo dyes (e.g. sunset yellow FCF), porphyrins (e.g. chlorophyll a) phthalocyanines (e.g. copper phthalocyanine tetrasulfonic acid tetrasodium salt), and ruthenium (l l)tris(bipyridyl) and ruthenium(l l)tris(phenanthroline) or derivatives having an alternative metal ion centre or ligand.
[0075] A particularly preferred example of a photochemically active species (dye) is N- methylphenothiazine (conveniently referred to as PMe).
[0076] In neutral form PMe exhibits a pale green colour and is insoluble in water but soluble in organic (hydrophobic) media. However, in its cation radical form, PMe is pink and has some solubility in water. Thus, in its cation radical form PMe can partition between hydrophobic and aqueous phases. In the context of the present invention, the cation radical form of the PMe partitions between the surfactant pseudo-phase (16) and the aqueous sub-phase (18).
[0077] In homogeneous solution, the pink cation radical form of PMe absorbs green light to produce a powerful oxidising agent. The green (neutral) form of PMe absorbs violet light ultimately to produce a powerful reducing agent. The inventors observe that, in isotropic media, this dual and opposing photoredox system may deprive a cell based on this system of utility, since charging of the cell in the violet causes rapid discharging of the cell in the green. In a heterogeneous system such as that of the present invention, this significant disadvantage may be offset by kinetic asymmetry (due to the occurrence of partition equilibrium between the surfactant pseudo-phase (16) and the aqueous sub- phase (18)).
Experimental
[0078] All chemical reagents were purchased from Sigma-Aldrich or Alfa Aesar in the purest commercially available grade, and used as received. Water, with a resistivity of not less than 18 ΜΩ cm, was taken from an Elgastat system (Vivendi, Bucks. , U K). Argon was obtained from BOC Gases, U. K. Electrode materials were obtained from Alfa Aesar, Goodfellow or Advent (for metals), or UQG, Cambridge for tin-doped indium oxide (ITO, of resistance 20 Ω per square, of thickness 25 nm on soda lime float with Si0 2 layer). Metal electrodes were cleaned and polished using wetted carborundum paper (P1200 grade, Presi, France) and stored under degassed, distilled water prior to use; ITO electrodes were rinsed with both ethanol and water, and dried immediately prior to deployment.
[0079] The pink cation radical of /V-methylphenothiazine was prepared as the chloride salt through a liquid | liquid oxidation, using ammonium cerium (IV) sulfate in aqueous 0.1 M KCI, with the organic reagent in dichloromethane. The salt was isolated through separation of the organic phase followed by drying over magnesium sulfate, filtration and evaporation of the solvent.
[0080] The L a phase was prepared by mixing approximately 5 g of Brij 30 with 5 ml_ water, together with relevant masses of the electroactive dopants (the redox active reagents), followed by heating with stirring to approximately 345 K for approximately 2 h so as to allow the mixture to melt into the isotropic micellar and dilute surfactant phase, thereby achieving homogenisation. For the cases requiring electrolyte doping of the surfactant sub-phase, the water was replaced with an aqueous solution of 0.1 M potassium chloride - a salting-out electrolyte which is known to encourage the de-mixing between surfactant and aqueous sub-phases. For the cases where oxygen was required to be rigorously excluded, the micellar isotropic phase at 345 K was formed under a stream of argon. An aliquot of the hot isotropic mixture was then poured into a cavity that had been prepared by gluing a Teflon (of thickness 1 .0 mm or 5.0 mm) or silicone (of thickness 0.5 mm) separator, with known dimensions onto the dark electrode (ITO, platinum, silver, nickel, zinc or aluminium foil), and which had been positioned within an horizontal field from a ferromagnet. The field strength was determined to be 1 .1 gauss using a homemade Hall probe. This solution was allowed to cool slowly (typically over the course of two hours) to furnish the required phase, either on the open bench, or within a polythene bag filled with argon. This approach was used to encourage surfactant chains to lower the gas I solution surface tension (enabling the dark electrode to be in contact with the aqueous sub-phase), whilst also allowing for the magnetic alignment of the planar bilipid bilayers into the required phase. Optical microscopy (undertaken using an Olympus BH-2 polarising microscope) confirmed the existence of this phase through the characteristic textures. The phase was measured to have a density of 0.99 g ml_ "1 . Last, an ITO electrode was allowed to cover the phase, in contact with the surfactant pseudo-phase, and the two electrode cell sealed with a low melting depilatory wax. This electrode had been masked with magic tape, so as to only allow the exposure of the material within the cavity to the electrode surface. Electrical connection to both electrodes was achieved through the use of copper tape with conductive adhesive underside (RS).
[0081] All electrochemical experiments were undertaken at ambient conditions of temperature of 296 ± 2 K. Open circuit potentials, and chronopotentiograms were recorded using a computer-controlled potentiostat/galvanostat ^Autolab Type I II , Eco Chemie); current-voltage characteristics of the device under illumination were recorded manually by connecting the cell in parallel to a Farnell DM 141 multimeter acting as a voltmeter, and in series with a UN I-T UT50A multimeter acting as ammeter connected to a series variable load resistor. Readings were taken after both current and voltage had reached steady values. A 75 W Cairn Research Optosource xenon arc lamp equipped with monochromator was employed to introduce monochromated violet light (of centre- band 350 nm and bandwith 30 nm) to the system. The light intensity was monitored in each experiment using a radiometer calibrated at 365 nm (UVP, Cambridge, UK), and was attenuated by varying the current flowing in the lamp power supply. In typical experiments, the light was focused onto the electrode cavity, and under typical working distances, the light intensity was observed to be ~2 mW cm "2 .
* * * * [0082] Initially, a cell (10) was constructed in which the surfactant pseudo-phase (16) was doped with PMe and the aqueous sub-phase (18) was doped with Eu 3+ ions, with 0.1 M KCL in the aqueous sub-phase (18) as salting-out electrolyte and using ITO electrodes (12), (14). Significant effects were demonstrated on illumination of the cell, as is apparent from Figure 4. Specifically, an increase in the potential of the illuminated (light) electrode (12) with respect to the dark electrode (14) was demonstrated. This increase in potential is as expected for the heterogeneous reduction of photochemically generated PMe +" taking place at the light electrode (12). The magnitude of the photovoltage achieved varied with light intensity. However, although demonstrably clear photoeffects were achieved, the overall change in the open circuit voltage was small, at about 45 mV. Also, the photovoltage of the illuminated (light) electrode with respect to the dark electrode was seemingly small, at about -0.21V, in the context of a large difference between the formal electrode potentials:-
¾ JS , ~ ¥ vs. SHE in ® tvtx&nl®
mM sohstixm
In each case SHE represents Standard Hydrogen Electrode.
[0083] Nevertheless, it is noteworthy that the change in the cell emf as a result of illumination was rapid, reaching a pseudo-stationary state within 20 s, and that the open circuit decay of the photovoltage took place very slowly, typically requiring over 5 minutes for the cell to return to its initial potential, even after only 100 s of illumination.
[0084] Without wishing to be bound by theory, the inventors postulate that, in the above described cell, one possible process is the photo-induced electron transfer from 3 PMe to a redox active species present within the micellar palisade layer, to afford the PMe +" cation radical which will likely partition between both the surfactant pseudo-phase (16) and the aqueous sub-phase (18), and which can be reduced at the illuminated electrode, which then becomes positive. The consequences of this then become (1 ) the partitioning causes an offset in the anticipated photovoltage according to the Nernst equation, and (2) such processes are not restricted to the layer closest to the illuminated electrode - there is the possibility of pseudo-phase | pseudo-phase electron transfer between photogenerated 3 PMe* (in the surfactant pseudo-phase (16)) and PMe +" (in the aqueous sub-phase (18)). The discharge process when the light is turned off is simply the reverse process, with the source of the electrodes for the reduction of the cation radical being through back reaction with the initially-reduced redox species.
[0085] In the light of the above results, the inventors considered ways in which the open circuit voltage of the cell could be increased. The inventors have recognised that the constructed cells (10) were reasonably thick (ranging from -0.5 - 5.0mm), so that light will be absorbed primarily by those layers close to the illuminated electrode. It follows that the use of a sacrificial electrode as "dark" electrode (14) would allow for the cell emf to be dominated by its potential and that the size of the photovoltage should change with changing the metal of the dark electrode (14) (and the nature of the M n+ ions with which it exists in potential-determining equilibrium). Thus cells of the type M | M n+ (aq)/Brij30 (PMe) |ITO were constructed where M represents the metal of the sacrificial electrode (14) and M n+ represents the corresponding ion in the aqueous sub-phase (18). Cell emf vs time characteristics for these cells were recorded under open circuit conditions in the presence of chopped light as indicated in Figures 5 to 8 for electrodes constructed respectively from:
M - Ag (E° A g|Ag - 0.8 V vs. SHE);
M = Ni (E° Ni|Ni 2+ = -0.3 V vs SHE);
M = Zn (E°zn|zn 2+ = -0.8 V vs.SH E);
M = Al (E° AI|AI 3+ = ■ ■ 1 .7 V vs. SHE).
[0086] It is apparent from these data that, in general, the more electropositive the metal, the larger the dark cell emf, and the greater the stationary photovoltage under illumination, with the zinc system reaching open circuit voltages over 1.0 V under illumination.
[0087] Next, the inventors have explored the use of lyotropic liquid crystals as frameworks for flexible, redox batteries with reference to the zinc system, owing to its superior performance as a sacrificial photogalvanic cell. Considering only the masses of the electroactive components (PMe/PMe ,+ : 213.8 g mol "1 ; Zn 2+ /Zn: 65.4g mol "1 ), the theoretical specific capacity of this system is 96.0 mA h g "1 , which is comparable to that of the lead/acid accumulator (83.5 mA h g "1 ) and a lithium-ion battery comprising a lithiated graphite anode (LiC 6 ) and a LiMn 2 0 4 cathode (103.1 mA h g "1 ), based on the following processes: ehss&e: positive el tro : FMe - *>- £i P e'~ <«i
negative Sedrocfe + ½ZB <.«£]>:,
aiscsxai'se: negative eleciroi : ½Zss - ¾r ½Zs a ÷ SSK!,
positive electrode: FM ' e' ; * e ' FMe. so that,
_. _ ,.„ _, ·> ,. is.¾«e _ ,. t
ii¾½ + Z« noting that the formal open circuit voltage of this system is +1.62 V. Figure 9 illustrates chronopotentiograms corresponding to constant current electrical charging (in contrast to the photochemical charging described above) of the system at current densities of 51 μΑ cm "2 (-30% of the current density under illuminated short-circuit discharge), with discharge at current densities ranging from 51 nA cm "2 to 102 μΑ cm "2 . It is clear from these data that the charging potential (~1.8 V) is close to that expected from the Nernst equation for the effective concentrations of species employed. The inventors suggest that the occurrence of higher voltages under this mode of charge compared with the photochemical method is due to the breakdown of the lyotropic liquid crystal state during the charge and discharge processes by the electrical field - electroporation of lipid membranes is a well known phenomenon.
[0088] The shape of the discharge curves follows that anticipated in the light of the above discussion, with several regimes readily identifiable. Initially the potential decreases rapidly due to capacitative discharging, with abrupt changes due to Ohmic losses only observable for high discharge rates, and which is followed by exhaustive consumption of the electroactive species within the within the parts of the cell available to be electrolysed within the charging time occurring after potentials of ca. 1.5 V, leading to the tail in the cell voltage to zero, especially under conditions of deep discharge. All experiments were undertaken with the charge time limit to 100 s (a constant capacity of 0.28 μΑ h). Given that zinc ions are the limited reagent within this system, it is notable that this charging regime is only to -7% of the total capacity of the device (4.2 μΑ h). This indicates that, as in the performance of the device under illumination, it is only a fraction of the liquid crystal that is involved in the potential determining equilibria within these relatively thick (1.0 mm) devices. Discharge of the system to ca. 1 .5 V did not cause a significant change in the capacity of the system (typically -25 nA h), leading voltage efficiencies (the ratio of cell voltage between charge and discharge cycles) of 85.9 ± 4.6 % and power efficiencies (the ratio of power between discharge and charge processes) of ca. 83% for the above state of charge. Noting that this system is essentially static, these figures are reasonable for redox batteries.
[0089] In order to compare the performance of this system with other battery
technologies, Ragone plots were constructed through integrating the discharge curves between 1.5 and 1 .8 V to estimate the energy density, determining the average power density through normalization of the energy density by the discharge time, and these are illustrated in Figures 10a and 10b. Note that such plots are conventionally constructed in terms of specific energy and power densities, which relies on the masses of the
electroactive species involved in the potential-determining equilibria, and ignoring the electrode mass; this convention was followed, in spite of the sacrificial electrode employed since its activity is independent of its mass. However, this causes a potential issue, given the inventors' belief that less than 10% of the electroactive material contained within the cell engages in the Faradaic processes within these thick (up to about 1 mm) cell systems. Accordingly, the Ragone plots are illustrated when normalized by the total mass of the electroactive species (339.1 μg) in Figure 10a, with normalization in Figure 10b by 38.1 μg, a value obtained through consideration of a conservative estimate for a working
electroactive diffusion layer equivalent to 2νπϋτ where τ = 100 s, and D= 10-7 cm 2 s "1 , a value typical for redox active species within these types of system assuming
electroporation effects occur within the bilipid bilayers in the presence of an electrical field.
[0090] The data in Figure 10 all show a decrease in energy density at low loads (<1W kg "1 for Figure 10a; <10W kg "1 for Figure 10b, but appear to operate efficiently (no loss in energy density at between 0.1 - 1.0 W h kg "1 ) for power densities up to at least 0.1 kW kg "1 (Figure 10a) or 1 .0 kW kg "1 (Figure 10b).
[0091] Also shown in the Ragone plots are the performance of the devices under electrical discharge when 350 nm (centre band) light is employed to recharge the system. It is clear that this mode of recharge is not as efficient as electrical recharge, possibly as a result of fewer layers contributing to the cell response in both charge and discharge modes (there is little perturbation of the phrase nanostructure): the devices appear to operate efficiently with comparable energy densities (0.1 - 1.0 W h kg "1 ) with lower power densities (1.0 - 100 W kg "1 ).
[0092] Thus, bearing in mind the assumptions regarding the working mass of the device, considering the energy density of these micropower sources range from 10 "2 - 100 Whkg "1 , with power densities in the range 10 "1 - 10 3 W kg "1 , the systems, the devices fall into the case of a mid-ranging electrochemical capacitor (for which the energy density is typically in the range 10 "2 - 10 2 W h kg "1 , with power densities in the range 10 "1 - 10 6 W kg "1 ).
[0093] Accordingly, the inventors have estimated from these data that the specific pseudocapacitance of these devices are in the range 0.8 - 7.0 F g "1 (based on estimates of the mass as above), or around 1.4 mF cm "2 , when the data are normalized with respect to the surface area of the exposed electrodes, with volumetric pseudocapacitances, in the range 14 - 124 mF cm "3 .
[0094] The inventors have thus developed a rechargeable and flexible energy device, based in preferred embodiments on the use of a lyotropic liquid crystal framework, exploiting a photoactive redox probe as one redox compartment, and, in particularly preferred embodiments, a sacrificial counter electrode. The system has been
demonstrated to outperform conventional photogalvanic solar energy devices under monochromatic conditions, whilst additionally showing applicability, for use as a flexible electrochemical capacitor. The performance of the system disclosed herein is competitive with currently known flexible electrical power systems and photo-rechargeable batteries. Moreover, the inventors believe that a system which utilises three-dimensional soft matter nanosystems as multifunctional frameworks for energy conversion and storage is not previously known.
[0095] Importantly, the inventors have calculated that, at current prices, assuming all other things (electrodes, quantities of materials, costs of all non-photoactive materials) are comparable, the cost-normalised efficiency of Gratzel's dye-sensitised solar cell
(maximally ca. 15% efficient under AM 1 .5 conditions, with dye costs for the cheapest dye, N719, at £10561 .60 for 50 g) provides over five times less solar-to-electrical power- conversion-efficiency-per-pound-sterling compared with the system disclosed herein (maximally 2% efficient under monochromatic conditions of violet light that is about 50 times less bright than AM 1.5, with dye costs for /V-methylphenothiazine at £94.10 for 25 g), demonstrating that the disclosed system has potential for exploitation in cost-effective devices suitable for the personalised energy market.
[0096] In summary, the photogalvanic cells according to the invention permit the manufacture of a small, portable battery which would be powerful enough to power (for example) a personal music player or basic calculator or similar device and which is also cheap to manufacture. The resulting battery is easy to charge up simply by placing it in the light. In contrast, silicon solar cells must incorporate super pure silicon which is expensive.
[0097] The photogalvanic cell of the invention is equally suitable for charging using an appropriate DC current or through photochemical irradiation. This allows a system based on the cell of the invention to behave as a light rechargeable battery.
[0098] One embodiment of a battery according to the invention in which two electrodes (one metal foil, one transparent conducting oxide) are separated by a distance of about 1 mm, has a maximal light-power conversion efficiency of about 1 .8% with a fill factor of 0.15, giving rise to a battery emf of about 1 .1 V and, a current density of around 34 μ\Λ/ cm "2 . The voltage efficiency of the battery in the dark is close to 85%, and the power efficiency is about 80%.
[0099] A battery incorporating a cell according to the invention is advantageous in that it charges up quickly and discharges slowly. In this system, the optimal wavelength for charging it up is 350 nm.
[00100] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
[00101] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
[00102] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.