Energy storage apparatus

Abstract

The present invention relates to electrical energy storage apparatus, such as rechargeable electrical energy storage devices such as batteries. We describe an electrochemical cell comprising: a chamber containing an electrolyte and a porous membrane dividing the chamber into a first compartment and a second compartment. The cell includes a first electrode, associated with the first compartment; and a second electrode, associated with the second compartment. The first compartment contains a first triphasic gas storage material in contact with the first electrode; and the second compartment contains a second triphasic gas storage material in contact with the second electrode. The first compartment further contains hydrogen gas, and the second compartment contains oxygen gas. In preferred examples, the first and / or the second triphasic gas storage material is a material selected from a polymer of intrinsic microporosity, a metal-organic framework, a zeolite or a porous silicate.

Description

[0001] ENERGY STORAGE APPARATUS

[0002] The present invention relates to energy storage apparatus, in particular to apparatus for storing electrical energy, or energy accumulators. More particularly, the present invention relates to rechargeable electrical energy storage devices such as batteries.

[0003] The current state of the art in electrical energy storages devices is based around reversible intercalation of lithium cations into electronically-conducting solids - so-called lithium ion batteries. Such batteries have a very wide utility, from small consumer devices through to battery storage power plants.

[0004] However, the mining of lithium ore and the extraction of lithium from lithium ore are considered to have significant environmental impacts. The use of flammable solvents adds hazards and fabrication challenges. Accordingly, there is a need to develop alternative electrochemical systems suitable for electrical energy storage.

[0005] In its broadest aspect, in a first aspect, the present invention provides an electrochemical cell comprising: a chamber containing an electrolyte; a porous membrane dividing the chamber into a first compartment and a second compartment; a first electrode, associated with the first compartment; and a second electrode, associated with the second compartment. The first compartment contains first particles comprising or consisting of a first triphasic gas storage material in contact with the first electrode. The second compartment contains second particles comprising or consisting of a second triphasic gas storage material in contact with the second electrode. The first compartment further contains hydrogen gas, and the second compartment contains oxygen gas.

[0006] In some examples, the particles are nanoparticles or microparticles or hierarchical structures.

[0007] In some embodiments, the particles have a diameter or mean diameter in the range of 20 to 200 nm.

[0008] In certain embodiments, the first triphasic gas storage material is a material selected from a polymer of intrinsic microporosity, a metal-organic framework, a zeolite or a porous silicate.

[0009] In certain embodiments, the second triphasic gas storage material is a material selected from a polymer of intrinsic microporosity, a metal organic framework, a zeolite or a porous silicate. In certain embodiments, the material of the first triphasic gas storage material is the same as the material of the second triphasic gas storage material. In other embodiments, the material of the first triphasic gas storage material is different from the material of the second triphasic gas storage material.

[0010] In certain embodiments, the triphasic gas storage material is a metal-organic framework, optionally a zinc-based MOF or a zeolitic imidazolium framework.

[0011] In certain examples, the metal-organic framework is a metal-organic framework selected from MOF-5, MOF-210, ZIF-8 and ZIF-67.

[0012] In certain embodiments, the triphasic gas storage material is a porous silicate, optionally silicate-1.

[0013] In preferred embodiments, the triphasic gas storage material is a polymer of intrinsic microporosity, optionally a polymer of intrinsic microporosity selected from PIM-1 , PIM-7 and PIM-PY.

[0014] In certain embodiments, the triphasic gas storage material is a material in nanoparticulate or hierarchical form.

[0015] In preferred embodiments, the porous membrane is a proton-conducting or hydroxide- conducting membrane. Optionally, the porous membrane is a proton-conducting ionomer membrane.

[0016] In certain embodiments, the electrode is formed of a material comprising a catalyst, optionally a platinum catalyst or a non-noble metal replacement catalyst.

[0017] In some examples, at least one of the first electrode and the second electrode is a porous carbon electrode or a porous nickel electrode.

[0018] In preferred embodiments, the electrolyte is water or an aqueous acid, optionally an aqueous acid having a concentration of about 0.1 M. In certain examples, the aqueous acid is an acid selected from HCI, H2SO4, HCIO4.

[0019] In preferred examples, a ratio of a volume of hydrogen gas in the first compartment to a volume of oxygen gas in the second compartment is about 2:1. In certain embodiments, the chamber is a sealed chamber at an internal pressure of about 1 atmosphere; or at an internal pressure of about 1 to about 5 atmospheres.

[0020] In preferred embodiments, the oxygen gas in the second compartment is present at a concentration of at least 10 mM. Optionally, the concentration of oxygen gas is at least 20 mM, at least 30 mM, at least 40 mM, at least 50 mM, at least 100mM, at least 150 mM, at least 200 mM, at least 250 mM, or at least 300 mM at atmospheric pressure.

[0021] In certain examples, hydrogen gas is present in the first compartment at a concentration of about 80mM or more at atmospheric pressure; and oxygen gas is present in the second compartment at a concentration of about 250 mM or more at atmospheric pressure.

[0022] Optionally, the sealed chamber is at an internal pressure of about 5 atmospheres and the oxygen gas is at an apparent (average) concentration of up to 5000 mM or higher.

[0023] Preferably, the first electrode forms an anode in a cell-charging configuration and the second electrode forms a cathode; and wherein the first electrode forms a cathode in a cell discharging configuration and the second electrode forms an anode.

[0024] In preferred embodiments, the or each triphasic gas storage material is a metal-free material.

[0025] In a second aspect, the present invention provides a method of storing electrical energy, the method comprising applying a source of direct current electricity to the first electrode of the electrochemical cell defined above.

[0026] In a third aspect, the present invention also provides a method of manufacturing an electrochemical cell as defined above. The method comprises providing a chamber, providing an electrolyte within the chamber; providing a porous membrane to divide the chamber into a first compartment and a second compartment; providing a first electrode, associated with the first compartment and a second electrode, associated with the second compartment; wherein the first compartment contains first particles comprising or consisting of a first triphasic gas storage material in contact with the first electrode. The second compartment contains second particles comprising or consisting of a second triphasic gas storage material in contact with the second electrode. The first compartment further contains hydrogen gas and the second compartment contains oxygen gas In a fourth aspect, the present invention provides a rechargeable battery comprising an electrochemical cell as defined above.

[0027] The above and other aspects of the present invention will now be described in further detail, by way of example only, with reference to the following examples and the accompanying drawings, in which:

[0028] Figure 1 is a schematic cross-sectional view of an embodiment of an electrochemical cell in accordance with the present invention;

[0029] Figure 2 is a schematic cross-sectional view illustrating an operation of the electrochemical cell of Figure 1 in charging and discharging modes;

[0030] Figure 3 is a scanning electron micrograph (SEM) of a cross-sectional view of the silicon wafer coated with PIM-1 nanoparticles (600 microgram on approx. 7 x 10'6m2);

[0031] Figure 4 is a SEM of a nanoparticle layer on a silicon substrate;

[0032] Figure 5 is a histogram of particle size data showing typically 35 nm diameter particles;

[0033] Figure 6 shows:

[0034] (A) Cyclic voltammograms (scan rate 50 mVs'1) for a 2 mm diameter Pt electrode immersed in 0.1 mol L'1phosphate buffer pH 7. Data are shown for the bare electrode (red) and for different amounts of PIM-1 nanoparticle deposits (20 - 600 pg);

[0035] (B) Cyclic voltammograms (scan rates 10 - 500 mV s-1) for 200 pg PIM-1 nanoparticles on a 2 mm diameter Pt disk. Insert: plot of peak currents of oxygen reduction versus square root of scan rate;

[0036] (C) Double-logarithmic plot of the peak current for oxygen reduction versus the scan rate; and

[0037] (D) Cyclic voltammograms (scan rate 50 mV s-1) for 200 mg PIM-1 nanoparticles on a 2 mm diameter Pt disk electrode in argon atmosphere (a), ambient air (b), and in pure oxygen atmosphere (c). Insert shows the same conditions but for a bare Pt electrode;

[0038] Figure 7 shows: (A) Chronoamperometric response for a 2 mm diameter Pt disk electrode bare or coated with PIM-1 nanoparticles (20, 50, 100, 200, 400, and 600 pg PIM-1) immersed in 0.1 mol L'1phosphate buffer pH 7 with applied potential -0.2 V vs. Ag / AgCI;

[0039] (B) Illustration of oxygen diffusion through a layer of PIM-1 nanoparticles;

[0040] (C) Oxygen reduction current transients versus logarithm of time for chronoamperometry at a bare Pt electrode and a PIM-1 nanoparticle covered Pt disk electrode (50 pg PIM-1); and

[0041] (D) a plot of the current ratio IRIM-1 |Pt / lpt s. Log t. Insert: first derivative to demonstrate the peak to identify the transition from inside diffusion to outside diffusion;

[0042] Figure 8 shows Cottrell plots (1 / I2versus time) for of different amounts of PIM-1 nanoparticles on a 2 mm diameter Pt disk electrode (for (A) 0, (B) 20, (C) 50, (D) 100, (E) 200, (F) 400, and (G) 600 mg PIM-1) immersed in a solution containing ambient 0.1 mol L'1of phosphate buffer pH 7. Black line: experimental data; red line: fitting of Cottrell line. (H) Plot of the apparent oxygen concentration versus PIM-1 nanoparticle thickness limiting at approx. Capp, oxygen = 50 mM; and

[0043] Figure 9 shows:

[0044] (A) Cyclic voltammograms (scan rate 50 mVs'1) for the reduction of ambient oxygen in 0.01 , 0.10, 0.25, and 0.50 M phosphate buffer pH 7 at a PIM-1 nanoparticle coated Pt disk electrode (400 pg PIM-1); and

[0045] (B) Cyclic voltammograms (scan rate 50 mVs'1) of 400 mg PIM-1 deposited on a 2 mm diameter Pt disk electrode immersed in 0.1 mol L'1of phosphate buffer pH 7 saturated with gas containing argon and oxygen. Insert shows Calibration plot of the sensor for oxygen determination.

[0046] Background

[0047] The present invention is based on the familiar equation of water electrolysis:

[0048] 2 H2O(aq) 2 H2(gas) + O2(gas).

[0049] Charging the apparatus of the present invention (ie. , storage of electrical energy) is based on the production of hydrogen and oxygen; and discharging (providing of electrical energy to an external device) is based on the reverse reaction, combining hydrogen and oxygen and generating an electric current. Construction

[0050] The construction of an exemplary apparatus is illustrated schematically in Figure 1 . Figure 2 illustrates the operation of the apparatus, showing the apparatus in charging (left view) and discharging (right view) modes. The active electrode area can be 1 cm2, 25 cm2, or higher. The thickness of compartments can be optimised for slower charging / discharging (higher water content) to faster charging / discharging (lower water content).

[0051] The apparatus 10 includes a sealed chamber 11 divided, by means of porous membrane 12, into a first compartment 13 and a second compartment 14. Each compartment contains particles 15 of a respective triphasic gas storage material and an electrolyte 20. First compartment 13 includes a first electrode 21 and second compartment 14 includes a second electrode 22.

[0052] First compartment 13 contains a concentration of hydrogen gas and second compartment 14 contains a concentration of oxygen gas.

[0053] Triphasic gas storage materials

[0054] In the absence of a gas storage material, a gas concentration of only about 1 to 2 mM can be maintained at ambient pressure in a liquid water phase. Accordingly, the present invention seeks to provide a means for increasing gas concentration by a significant amount. Based on our research, we have determined that the use of materials suitable as particulate triphasic gas storage materials allows an increase in gas concentration of up to about 250 mM at ambient (atmospheric) pressure and higher concentrations of up to 5000 mM at elevated pressures. By the term triphasic we describe materials which are, themselves, in a solid phase and are suitable for use in a liquid phase electrolyte with gas phase reactants of hydrogen and oxygen.

[0055] Erdosy et a / .(Erdosy, D.P.; Wenny, M.B.; Cho, J.; DeIRe, C.; Walter, M.V.; Jimenez-Angeles, F.; Qiao, B.F.; Sanchez, R.; Peng, Y.F.; Polizzotti, B.D.; De la Cruz, M.O.; Mason, J. A. Microporous water with high gas solubilities. Nature 2022, 608 (7924), 712-715) highlighted the ability of some microporous materials (such as metal-organic frameworks (MOFs) and zeolites) to enhance apparent oxygen solubility in aqueous media. Similarly, polymers of intrinsic microscopy (PIMs) provide materials suitable for localised triphasic gas storage (directly at the electrode surface) in aqueous media, for example for hydrogen (Marken, F.; Madrid, E.; Zhao, Y.Z.; Carta, M.; McKeown, N.B. Polymers of intrinsic microporosity in triphasic electrochemistry: perspectives. ChemElectroChem 2019, 6 (17), 4332 - 4342) and for oxygen (Madrid, E.; Lowe, J.P.; Msayib, K.J.; McKeown, N.B.; Song, Q.L.; Attard, G.A.; Duren, T.; Marken, F. Triphasic nature of polymers of intrinsic microporosity induces storage and catalysis effects in hydrogen and oxygen reactivity at electrode surfaces. ChemElectroChem 2019, 6 (1), 252 - 259).

[0056] In preferred embodiments, the triphasic gas storage materials are particular triphasic gas storage materials and have a particle size in the region of 20 to 200 nm. In certain examples, the materials have a particle size substantially in the range of 10 nm to 1000 nm. In preferred examples, the particle size is predominantly in the range of about 20 nm to about 50 nm (see Figure 5). In alternative examples, the triphasic gas storage material is formed as a foam, optionally a foam having a hierarchical structure.

[0057] Particularly suitable materials for our purposes include so-called polymers of intrinsic microporosity, MOFs, ZIFs, zeolites, or silicates. a) Polymers of Intrinsic Microporosity

[0058] Polymers of intrinsic microporosity are a group of polymers which exhibit properties of microporosity without possessing a network of covalent bonds.

[0059] Polymers of intrinsic microporosity (PIMs) are molecularly rigid and contorted polymers with high specific surface area and selective permeability and were reviewed in PIMs in Polymers of Intrinsic Microporosity - Neil B. McKeown - ISRM Materials Sciences 2012 (doi:10.5402 / 2012 / 513986). As a result of these properties, PIMs have been extensively studied for applications in separation membranes, catalysis, and in gas storage. PIMs have found new applications also in wet (electrochemical) conditions either in aqueous media for sensing, in gas diffusion electrodes, or when immersed in organic solvents for example in batteries (see, for example: Yang, Q.L.; Li, W.L.; Dong, C.; Ma, Y.Y.; Yin, Y.X.; Wu, Q.B.; Xu, Z.T.; Ma, W.; Fan, C.; Sun, K.N. Batteries PIM-1 as an artificial solid electrolyte interphase for stable lithium metal anode in high-performance batteries. J. Energy Chem. 2020, 42, 83

[0060] - 90; and Wang, L.N.; Zhao, Y.Z.; Fan, B.B.; Carta, M.; Malpass-Evans, R.; McKeown, N.B.; Marken, F. Polymer of intrinsic microporosity (PIM) films and membranes in electrochemical energy storage and conversion: A mini-review. Electrochem. Commun. 2020, 118, 106798 - (doi:10. 1016 / j.elecom.2020. 106798)). The use of PIMs in triphasic systems was reviewed in Polymers of Intrinsic Microporosity in Triphasic Electrochemistry: Perspectives - Marken et al

[0061] - ChemElectroChem - 6(17) (2019) doi:10.1002 / ceic.201900717). Studies have shown that PIM-1 nanoparticles are effective as components for multiphase electrode surfaces (Marken, F.; Carta, M.; McKeown, N.B. Polymers of intrinsic microporosity in the design of electrochemical multicomponent and multiphase interfaces. Anal. Chem.

[0062] 2021 , 93 (3), 1213 - 1220.). They can be utilized to store oxygen gas and enhance electrochemical signals for oxygen reduction (Beluomini, M.A.; Wang, Y.; Wang, L.A.; Carta,

[0063] M.; McKeown, N.B.; Wikeley, S.M.; James, T.D.; Lozano-Sanchez, P.; Caffio, M.; Stradiotto,

[0064] N.R.; Zanoni, M.V.B.; Marken, F. Polymer of intrinsic microporosity (PIM-1) enhances hydrogen peroxide production at Gii-Sens graphene foam electrodes. Electrochem. Commun.

[0065] 2022, 143, 107394.). These PIM-1 nanoparticles immersed in water can act as a reservoir for oxygen, providing additional oxygen for the electrochemical reduction. However, the key storage parameters, the apparent concentration of oxygen (Capp, oxygen) and the apparent diffusion coefficient (Dapp, oxygen) have not been evaluated. In particular the Capp, oxygen parameter is important as a “storage” parameter describing gas behaviour in aqueous media. By storing the gas evolved at an electrode, bubble formation can be avoided (Mahajan, A.; Bhattacharya, S.K.; Rochat, S.; Burrows, A.D.; Fletcher, P.J.; Rong, Y.Y.; Dalton, A.B.; McKeown, N.B.; Marken, F. Polymer of intrinsic microporosity (PIM-7) coating affects triphasic palladium electrocatalysis. ChemElectroChem 2019, 6 (16), 4307 - 4317.).

[0066] One PIM which we have demonstrated to be exemplary of this class of materials and particularly suitable for our purposes is PIM-1 : 2,3,5,6-tetrafluorophthalonitrile-3,3,3',3'- tetramethyl-1 ,1'-spirobisin dane-5,5',6,6'-tetrol co-polymer - (C29H2oN204)n:

[0067] Other suitable PIMs include PIM-7: and pyridine-containing PIMs, such as PIM-PY:

[0068] In certain examples, the PIM may be a functionalised PIM, functionalised to improve desorption / adsorption properties of the PIM for a particular gas. It will be appreciated that the first and second compartments may have different grades or functionalisations of PIM to reflect differences between the properties of hydrogen and oxygen. b) Zeolites

[0069] Zeolites are microporous aluminosilicate materials commonly used as adsorbents and molecular sieves. Zeolite microporosity derives from the materials having a framework structure. Some 250 zeolite frameworks have been characterised.

[0070] One such zeolite which we have determined to be suitable for our purposes is Zeolite ZSM-5, Zeolite Socony Mobil-5, used as a catalyst in hydrocarbon isomerisation reactions. Zeolite ZSM-5 is based on a pentasil structure and has the formula NanAlnSi96-nOi92' 16H2O, in which 0<n<27. c) Metal-organic frameworks

[0071] Metal-organic frameworks (MOF) are also particularly useful for our purposes, having a high surface area to volume. Suitable MOF include MOF-5 and MOF-210; and zeolitic imidazolium frameworks (ZIF), such as ZIF-8 and ZIF-67. d) Silicates

[0072] We have also determined silicate-1 , also known as silicalite-1 , to be suitable for our purposes. The synthesis of silicate-1 has been described by Ping Liu et al - Synthesis of hierarchically porous silicate-1 and ZSM-5 by hydrothermal transformation of SiO2 colloid crystal / carbon composites - Microporous and Mesoporous Materials, 262 (2018) 217-226 (https: / / doi.Org / 10.1016 / j.micromeso.2017.11.033) and its properties are discussed in the Erdosy paper mentioned above.

[0073] Porous membrane

[0074] The porous membrane is a proton-conducting membrane, allowing proton permeation through the membrane between the first and second compartments. Suitable materials are well-known in the art and include fluoropolymers such as Nation (a registered trade mark of DuPont). We have determined that Nation 117 or Nation 115 are particularly suitable for our purposes. In alternative examples, a hydroxide-permeable membrane may be used.

[0075] Electrodes

[0076] In preferred embodiments, each electrode is a gas diffusion electrode, suitably a porous carbon electrode. In some examples, the each electrode is a carbon paper electrode. Preferably, the material of the electrode includes an electrocatalyst, suitably platinum, preferably platinum nanoparticles (or non-noble metal replacement catalysts). The electrode may be in contact or partial contact with the porous membrane, or may be spaced from the porous membrane.

[0077] Hydrogen & oxygen

[0078] Both hydrogen in the first compartment and oxygen in the second compartment need to be available at the respective first and second electrode to maintain the electrolytic process. It will be appreciated that hydrogen and oxygen need to be present in an amount sufficient to provide a ratio of hydrogen to oxygen of about 2:1.

[0079] Each gas is present in a gas concentration of at least 10 mM. In some examples, the concentration is at least 50 mM, at least 100 mM, at least 150 mM, or at least 200 mM at ambient pressure, or about 250 mM. In other examples, the compartments are pressurised with the gases and the gases have a concentration of at least 1000 mM, at least 2000 mM, at least 3000 mM, at least 4000 mM or up to about 5000 mM or more. Operation of electrochemical cell

[0080] The operation of the inventive electrochemical cell as a rechargeable battery is illustrated in Figure 2. A charging mode is illustrated in the left hand view, in which an electric current is applied to the first electrode 21 of the first compartment. The current causes proton migration across the proton conducting ionomer membrane 12, electrolysing the water component of the electrolyte into molecular oxygen and molecular hydrogen.

[0081] The right hand view of Figure 2 illustrates a discharging mode, in which the electrochemical cell provides electrical power to an external device (not shown). In the discharging mode, protons migrate across the ionomer membrane 12 to the second compartment and a current flows from the cell through first electrode 21.

[0082] Examples

[0083] Reagents

[0084] Monobasic sodium phosphate (>98.0 %), dibasic sodium phosphate (>99.0 %), methanol and chloroform were obtained from Sigma-Aldrich and used without further purification. PIM-1 (2,3,5,6-tetrafluorophthalonitrile-3,3,3',3'-tetramethyl-1 , 1 '-spirobisindane-5,5',6,6'-tetrol copolymer, Sigma-Aldrich 918768, monomer molecular weight 460 g mol-1, molecular weight typically 70 KD) was synthesised using a method described in the literature - see Budd, P.M.; Elabas, E.S.; Ghanem, B.S.; Makhseed, S.; McKeown, N.B.; Msayib, K.J.; Tattershall, C.E.; Wang D. Solution-processed, organophilic membrane derived from a polymer of intrinsic microporosity. Adv. Mater. 2004, 16, 456 - 459. Argon and oxygen were purchased from BOC UK (Pureshield). Deionized and filtered water (18.2 MQ cm at 20 °C) obtained from a Thermo Scientific water purification system, was used to prepare solutions. All experiments were conducted at a room temperature of 20 ± 2 °C. A 0.1 mol L-1concentration of phosphate buffer solution of pH 7 was used as a background electrolyte solution for all experiments.

[0085] Instrumentation

[0086] A potentiostat system (Autolab GPSTAT12, EcoChemie, The Netherlands) was employed with a Pt wire counter electrode and a KCI-saturated calomel reference. The working electrode was a platinum disk electrode (2 mm diameter). A conventional three-electrode electrochemical cell was employed. The electrode modified with PIM-1 nanoparticles was characterized using a field emission scanning electron microscope (FE-SEM, Jeol JSM- 7900F) with an accelerating voltage of 5.0 kV. Particle size analysis was performed with Imaged software.

[0087] Procedures

[0088] PIM-1 nanoparticles were synthesized with typically 50 nm diameter using an anti-solvent precipitation method, as reported in Madrid, E.; Lowe, J.P.; Msayib, K.J.; McKeown, N.B.; Song, Q.L.; Attard, G.A.; Duren, T.; Marken, F. Triphasic nature of polymers of intrinsic microporosity induces storage and catalysis effects in hydrogen and oxygen reactivity at electrode surfaces. ChemElectroChem 2019, 6 (1), 252 - 259. Briefly, the PIM-1 polymer was dissolved in 2 mL of chloroform at a concentration of 1 mg mL-1. The solution was added dropwise into 20 mL of methanol with vigorous stirring for another 12 h. The PIM-1 solution was centrifuged for 30 min at 5000 rpm followed by the removal of excess methanol. The PIM-1 nanoparticles were subsequently re-dispersed in methanol using an ultrasonication process. To prepare nanoparticulate films, a volume of 5 pL (4 mg mL-1) of PIM-1 solution in methanol, equivalent to 20 pg PIM-1 (other quantities of PIM-1 used in this study were calculated proportionally), was drop-coated onto the platinum electrode to dry at room temperature.

[0089] Characterisation

[0090] Figure 3 shows a typical scanning electron microscopy (cross-sectional SEM) image of for a deposit of 600 pg of PIM-1 nanoparticles on an approximately 7 x 10'6m2area (employing a silicon wafer substrate to replicated the inlaid Pt disc electrode surface). A porous film of nanoparticles is observed covering the surface with a thickness of approximately 12 pm.

[0091] Figure 4 shows an SEM image of nanoparticles distributed on a surface. Image analysis allows the typical nanoparticle diameter to be estimated (Figure 5) as 35 nm. The value of the film thickness was employed to calibrate the variable thickness of films deposited onto the platinum disk electrode. Table 1 summarises the thicknesses of the nanoparticulate film for different amounts of PIM-1 deposited onto the Pt electrode surface (estimated, based on calibration with cross-sectional electron microscopy images; Figure 3):

[0092] TABLE 1

[0093] Results and Discussion

[0094] Using a practical construction of the apparatus shown in Figure 1 , with the gases within the sealed chamber at ambient pressure, a battery voltage of 1.23 V was obtained across the first and second electrodes. At higher pressures, higher voltages were obtained.

[0095] Background analysis and data collection

[0096] Detection of Oxygen Stored in Nanoparticulate PIM-1 : Cyclic Voltammetry

[0097] In order to evaluate the ability of PIM-1 nanoparticles to store oxygen gas, different amounts of PIM-1 nanoparticles (different thicknesses) were deposited onto a 2 mm diameter Pt disk electrode. A solution containing 0.1 mol L'1phosphate buffer pH 7 usually with ambient oxygen was employed in the voltammetric measurements. Cyclic voltammograms (50 mVs'1scan rate) in Figure 7A shows peaks for the oxygen reduction reaction at 0.0 V vs. Ag / AgCl. For the bare Pt electrode (red curve) a peak current of -8 mA is observed. In the presence of varying amounts of PIM-1 nanoparticles on the Pt disk electrode the reduction current is significantly enhanced. With 600 mg PIM-1 nanoparticles the current triples and the peak broadens. This increase in current (and charge) can be assigned either to faster diffusion or (perhaps more likely) an increased concentration of oxygen accumulated locally at the electrode surface (or to both) in the presence of the PIM-1 nanoparticles.

[0098] Effect of the scan rate on current peaks.

[0099] The influence of scan rate (□) on the oxygen reduction peak currents in the presence of PIM- 1 nanoparticles on Pt was investigated. The scan rate was varied from 10 to 500 mV s-1(Figure 6B). The current peak at 0.0 V vs. Ag / AgCl corresponds to oxygen reduction in phosphate buffer solution pH 7:

[0100] The peak currents increase with scan rate, and the peak potentials shift towards more negative potentials. There is a linear relationship between the peak current versus square root of the scan rate (Figures 6C and 6D) indicative of a diffusion-controlled process. The equation for peak current versus scan rate can be expressed as log I (pA) = 0.55 log u (mV s-1) + 0.25 (R = 0.998) consistent overall with a diffusion-controlled process.

[0101] Effect of gas composition.

[0102] Upon modifying the gas environment during the experiments (by 20 minutes gas purging with either oxygen or argon gas), significant (and anticipated) effects on the oxygen reduction reaction were observed. Figure 6D shows the data obtained from testing the PIM-1 nanoparticle coated Pt disk electrode. The oxygen reduction peak current is consistent with the oxygen content in the gas phase. The insert shows data for a bare Pt electrode for argon, ambient air, and for pure oxygen, revealing very similar changes in the reduction peak current linked to variations in oxygen concentration. Therefore, oxygen gas concentration equilibration occurs from the gas phase to the liquid phase and finally into the solid PIM-1 phase.

[0103] Detection of Oxygen Stored in Nanoparticulate PIM-1 : Chronoamperometry Effect of the amount of PIM-1 deposit.

[0104] Chronoamperometry data were recorded for approx. X0, 20, 50, 100, 200, 400, and 600 pg PIM-1 nanoparticles (see Table 1) deposited onto a 2 mm diameter Pt disk electrode immersed in 0.1 mol L'1phosphate buffer (pH 7) with an applied potential of -0.2 V vs. Ag / AgCI within the oxygen reduction potential region (Figure 6). Figure 7A shows chronoamperometry transient data over 200 s for the reduction of ambient oxygen. The presence of the PIM-1 nanoparticles clearly substantially increases the reduction current and even after 200 s the higher current remains significant indicative of more oxygen reaching the electrode surface. Replotting the data with a logarithmic time axis (Figure 3C) shows that a transition occurs where the PIM-1 nanoparticle coated electrodes transition from diffusion inside the PIM-1 film to diffusion outside of the PIM-1 nanoparticle film. The ratio of transient currents is plotted in Figure 7D clearly showing the transition from inside to outside diffusion.

[0105] By plotting the current data versus logarithm of time, the diffusional transport first within the PIM-1 nanoparticle region and second within the external electrolyte phase can be seen more clearly (Figure 7C). When plotting the ratio of current in the presence and in the absence of PIM-1 (as a dimensionless parameter), sigmoidally-shaped plots are observed (Figure 7D) that step from a value of approximately 2.2 (for short time) to 1.0 (for longer time). The transition from diffusion inside to outside is not smooth due to porosity and heterogeneity in the polymer film, but a first derivative of the dimensionless parameter plot shows a clear peak indicating the point in time where the transition occurs. It is possible to evaluate the transition time for each PIM-1 film thickness and then evaluate the apparent diffusion coefficient based on one dimensional Fickian diffusion:

[0106] Table 2 below summarises thickness, transition time, and Dapp, oxygen data. The diffusion coefficient for oxygen is decreased dramatically when compared to the diffusion coefficient in aqueous electrolyte. There is a three order of magnitude change / decrease and therefore diffusion of molecular oxygen in the PIM-1 nanoparticle film is extremely slow, comparable with diffusion in other types of polymers. For example, a diffusion coefficient of below 10'12m2s'1can be compared with that observed in solid poly-ethylene-terephthalate (PET).

[0107] By means of chronoamperometry, the currents flowing through the PIM-1 modified electrode during the early stages of the transient were investigated. Cottrell plots (1 / l2versus time) of the data (Figure 8) are fitted with the Cottrell line to match the early data points (red line). The Cottrell equation is employed to describe planar diffusion to the electrode surface:

[0108] In this equation F is the Faraday constant (96485 C mol-1), A is the geometric area of the electrode (3.14 x 10'6m2), C is the concentration of the oxygen species, n is the number of electrons for each oxygen molecule diffusing to the surface (n = 4), and D is the diffusion coefficient in m2s'1(as evaluated above). The equation is transformed to show that the slope in the Cottrell plots in Figure 8 provide access to the apparent concentration of oxygen:

[0109] 1 7T

[0110] T2 =n2F2A2C2Df

[0111] Table 2 summarises the data and Figure 8H shows a plot of the apparent oxygen concentration in the PIM-1 film limiting at appro. X50 mM which is more than 2 orders of magnitude higher when compared to the concentration of oxygen in aqueous solution under ambient conditions. The PIM-1 nanoparticles are able to store high amounts of gas (oxygen) locally at the electrode surface and this can affect the mechanism and activity of oxygen at the electrode surface. TABLE 2

[0112] Experimental data from chronoamperometry for oxygen reduction at Pt disk electrode with

[0113] PIM-1 nanoparticle deposits immersed in ambient 0.1 M phosphate buffer pH 7.

[0114] ** the observed transition is likely to be associated with convection effects associated with the bare electrode.

[0115] Effect salt concentration (PIM-1).

[0116] The effects of ionic strength (buffer concentration) on the triphasic gas storage capability of PIM-1 nanoparticles were explored. Figure 9A shows cyclic voltammetry data for the reduction of ambient oxygen in 0.01 , 0.10, 0.25, and 0.50 M phosphate buffer pH 7 at a PIM-1 nanoparticle coated Pt disk electrode (400 pg PIM-1). It is evident that the phosphate buffer concentration exerts some influence on the reduction peak position. However, the peak current and the apparent diffusivity / concentration of oxygen remained practically constant.

[0117] Detection of Oxygen Stored in Nanoparticulate PIM-1 : Sensor Electrodes

[0118] In order to demonstrate experimental detection of oxygen, the dissolved oxygen concentration in the 0.1 mol L'1of phosphate buffer pH 7 was controlled by mixing oxygen and argon gas prior to purging the solution. Different ratios of oxygen to argon (flowing into the solution to equilibrate for approximately 20 min) were controlled accurately using gas mass-flow controllers (MFC, Platon). The flow rate of oxygen gas was controlled to give 20%, 40%, 50%, 80%, and 100% partial pressure. The laboratory temperature was 21 ± 1 °C throughout all measurements. Figure 9B shows cyclic voltammograms for the reduction of oxygen at Pt disk electrode coated with 400 mg PIM-1 . An oxygen reduction the peak was observed at -0.05 vs. Ag / AgCl. The current increased with oxygen concentration and the peak potential shifted slightly with increasing oxygen concentration. A linear calibration plot demonstrates the fact that the PIM-1 enhanced reduction current signals are directly proportional to the partial pressure of oxygen in the gas phase. This can be interpreted in terms of both oxygen and argon binding into PIM-1 in the same ratio as given by the purging gas. In other words, other types of gases will interact with PIM-1 nanoparticles in a similar way as oxygen. Electrode processes based, for example, on hydrogen oxidation, nitrogen reduction, or carbon dioxide reduction should be affected in a similar way.

[0119] In terms of practical applications, the observation of enhanced oxygen reduction currents and significantly enhanced local concentrations of oxygen at the electrode surface may lead to benefits in electrochemical devices such as fuel cells, air-metal batteries, or oxygen sensors. Using PIM-1 nanoparticles to locally store oxygen can affect oxygen activity and reactivity at the electrode surface. Similar effects are anticipated for other types of gases.

[0120] The effects of PIM-1 nanoparticle deposits at a platinum electrode immersed in aqueous 0.10 M phosphate buffer pH 7 have been quantified and oxygen gas reduction at the electrode has been shown to be enhanced due to triphasic gas storage in PIM-1 . In the presence of PIM-1 nanoparticles (i) the apparent solubility of oxygen in the aqueous phase increases from 0.3 to 50 mM (by more than two orders of magnitude) and (ii) the apparent diffusivity of oxygen decreases from approximately x10-9to 10'12m2s-1(ie., by three orders of magnitude). Reactivity of oxygen gas at the electrode is considerably modified. We note that under 1 bar oxygen the apparent concentration of oxygen reaches 0.25 M.

[0121] Processes involving gas evolution such as hydrogen evolution and oxygen evolution are thought to be enhanced by use of the electrochemical cell of the present invention due to the “in situ storage” of gas close to the electrode surface. This phenomenon allows gaseous products to be generated and stored similarly to battery energy storage (a “hydrogen battery”). However, the electrochemical cell of the present invention avoids the need to include lithium in the battery or rely on intercalation reactions.

[0122] The content and disclosure of all documents referred to in this application are hereby incorporated by reference in their entirety.

Claims

CLAIMS1. An electrochemical cell comprising: a chamber containing an electrolyte; a porous membrane dividing the chamber into a first compartment and a second compartment; a first electrode, associated with the first compartment; and a second electrode, associated with the second compartment; wherein the first compartment contains a first triphasic gas storage material in contact with the first electrode; and the second compartment contains a second triphasic gas storage material in contact with the second electrode; further wherein the first compartment further contains hydrogen gas, and the second compartment contains oxygen gas.

2. An electrochemical cell as claimed in claim 1 wherein the first triphasic gas storage material and / or the second triphasic gas storage material are provided as particles or foams of triphasic gas storage material.

3. An electrochemical cell as claimed in claim 2 wherein the particles are nanoparticles or microparticles.

4. An electrochemical cell as claimed in claim 3 wherein the particles have a diameter or mean diameter in the range of 20 to 200 nm.

5. An electrochemical cell as claimed in any preceding claim wherein the triphasic gas storage material has a hierarchical structure.

6. An electrochemical cell as claimed in any preceding claim wherein the first triphasic gas storage material is a material selected from a polymer of intrinsic microporosity, a metalorganic framework, a zeolite or a porous silicate.

7. An electrochemical cell as claimed in any preceding claim wherein the second triphasic gas storage material is a material selected from a polymer of intrinsic microporosity, a metal organic framework, a zeolite or a porous silicate.

8. An electrochemical cell as claimed in any preceding claim wherein the material of the first triphasic gas storage material is the same as the material of the second triphasic gas storage material.

9. An electrochemical cell as claimed in any preceding claim wherein the first and / or second triphasic gas storage material is a metal-organic framework.

10. An electrochemical cell as claimed in claim 9 wherein the first and / or second triphasic gas storage material is a zinc-based MOF or a zeolitic imidazolium framework.

11. An electrochemical cell as claimed in claim 9 or claim 10 wherein the metal-organic framework is a metal-organic framework selected from MOF-5, MOF-210, ZIF-8 and ZIF-67.

12. An electrochemical cell as claimed in any preceding claim wherein the first and / or second triphasic gas storage material is a porous silicate.

13. An electrochemical cell as claimed in any preceding claim wherein the first and / or second triphasic gas storage material is or comprises silicate-1.

14. An electrochemical cell as claimed in any preceding claim wherein the first and / or second triphasic gas storage material is a polymer of intrinsic microporosity.

15. An electrochemical cell as claimed in claim 14 wherein the polymer is a polymer of intrinsic microporosity selected from PIM-1 , PIM-7 and PIM-PY.

16. An electrochemical cell as claimed in any preceding claim wherein the porous membrane is a proton-conducting or hydroxide-conducting membrane.

17. An electrochemical cell as claimed in claim 16 wherein the porous membrane is a proton-conducting ionomer membrane.

18. An electrochemical cell as claimed in any preceding claim wherein the electrode is formed of a material comprising a catalyst.

19. An electrochemical cell as claimed in claim 18 wherein the catalyst is a platinum catalyst or a non-noble metal replacement catalyst.

20. An electrochemical cell as claimed in any preceding claim wherein at least one of the first electrode and the second electrode is a porous carbon electrode or a porous nickel electrode.

21. An electrochemical cell as claimed in any preceding claim wherein the electrolyte is water or an aqueous acid, optionally an aqueous acid having a concentration of about 0.1 M.

22. An electrochemical cell as claimed in claim 21 wherein the aqueous acid is an acid selected from HCI, H2SO4, HCIO4.

23. An electrochemical cell as claimed in any preceding claim wherein a ratio of a volume of hydrogen gas in the first compartment to a volume of oxygen gas in the second compartment is about 2:1.

24. An electrochemical cell as claimed in any preceding claim wherein the chamber is a sealed chamber at an internal pressure of about 1 atmosphere; or at an internal pressure of about 1 to about 5 atmospheres.

25. An electrochemical cell as claimed in claim 24 wherein the sealed chamber is at an internal pressure of about 5 and the oxygen gas is at an apparent concentration of up to 5000 mM.

26. An electrochemical cell as claimed in any preceding claim wherein the oxygen gas in the second compartment is present at a concentration of at least 10 mM, at least 20 mM, at least 30 mM, at least 40 mM, at least 50 mM, at least 100mM, at least 150 mM, at least 200 mM, at least 250 mM, or at least 300 mM at atmospheric pressure.

27. An electrochemical cell as claimed in any preceding claim wherein the first electrode forms an anode in a cell-charging configuration and the second electrode forms a cathode; and wherein the first electrode forms a cathode in a cell discharging configuration and the second electrode forms an anode.

28. A method of storing electrical energy, the method comprising applying a source of direct current electricity to the first electrode of an electrochemical cell as claimed in any one of claims 1 to 27.

29. A method of manufacturing an electrochemical cell as claimed in any one of claims 1 to 27, the method comprising providing a chamber, providing an electrolyte within the chamber; providing a porous membrane to divide the chamber into a first compartment and a second compartment; providing a first electrode, associated with the first compartment and a second electrode, associated with the second compartment; wherein the first compartment contains first particles comprising or consisting of a first triphasic gas storage material in contact with the first electrode; wherein the second compartment contains second particles comprising or consisting of a second triphasic gas storage material in contact with the second electrode; and wherein the first compartment further contains hydrogen gas and the second compartment contains oxygen gas30. A rechargeable battery comprising an electrochemical cell as claimed in any one of claims 1 to 27.

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