SUPERCAPACITOR FOR ENERGY STORAGE

TECHNICAL FIELD

The present invention relates to the field of supercapacitors.

BACKGROUND

The use and development of suitable energy-storage systems have been heavily pursued over the last years. Supercapacitors are a type of electrochemical energy storage devices characterized by high power density and good cyclability, but with low energy density.

Supercapacitors can be classified in two types based on the charge storage mechanism, namely pseudocapacitive and double layer electrical capacitor (EDLC). In recent years, the development of hybrid storage supercapacitors has emerged as an advantageous strategy, combining both types of capacitor materials, such as carbonbased redox electrodes and/or redox electrolytes. Hybrid supercapacitors are designed to be able to deliver higher specific capacitance in comparison to the existing electric double layer capacitor (EDLC) and pseudocapacitors.

Navalpotro et al.(Navalpotro et al., Journal of Power Sources 306 (2016) 711-717), developed hybrid supercapacitors using electrodes made of an activated carbon paste or a carbon black paste deposited on a metallic mesh and a redox electrolyte based on para-benzoquinone (p-BQ) dissolved in the ionic liquid N-butyl-N-methylpyrrolidinium bis(- trifluoromethanesulfonyl) imide (PYR14TFSI). However, these hybrid systems present some drawbacks such as lower power density with rapid capacitance decay compared with electric double layer capacitors. In addition, the supercapacitors of Navalpotro et al. work at temperatures about 60°C due to the use of an ionic liquid in the electrolyte. Further, non-aqueous electrolytes can pose additional problems in terms of safety and eco-friendliness.

In this sense, there is still an ongoing need to develop hybrid storage systems with enhanced overall performance.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have developed a hybrid supercapacitor comprising at least one electrode consisting of a carbon felt and a carbon composition comprising activated carbon particles and an electrolyte comprising two different types of redox species, one in the catholyte and the other one in the anolyte, with better performance that other supercapacitors of the state of the art. The performance of the supercapacitor of the invention is significantly improved by the use of an electrode consisting of a carbon felt and a carbon composition comprising activated carbon particles. In addition, the authors have observed that the supercapacitor of the invention shows much better values of energy density and power density when using an electrolyte comprising a pair of redox species than with just one redox species or none redox species, in combination with the electrodes described above.

Thus, a first aspect of the invention is directed to a supercapacitor comprising:

- two electrodes; wherein at least one of the electrodes consists of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt;

- an electrolyte comprising: an anolyte comprising a first redox species and a catholyte comprising a second redox species; wherein the first and second redox species are different redox species; and wherein the second redox species is an organic or an organometallic compound;

- a separator; and

- means for connecting with a power/load source.

The supercapacitor of the present invention is configured to act as an energy storage and delivery system, i.e. it is configured to be reversibly charged and discharged.

Therefore, in a second aspect, the present invention is directed to an energy storage system comprising at least a supercapacitor as defined above.

In addition, in a third aspect, the present invention is directed to a method for storing electricity comprising the steps of: a) providing the supercapacitor of the present invention in any of its particular embodiments; b) applying an electric voltage or current between the electrodes of the supercapacitor of step (a) to obtain an electrode with positive charge and another electrode with negative charge, at the same time, oxidizing the redox species of the electrolyte to the corresponding oxidized state at the positive electrode while the redox active species of the electrolyte are reduced to the corresponding reduced state at the negative electrode; and c) storing charge at the positive electrode and at the negative electrode.

In an additional aspect, the present invention is directed to a method for delivering electricity comprising the steps of: a) providing the supercapacitor of the present invention in any of its particular embodiments; wherein one electrode has a positive charge and the other electrode has a negative charge; b) generating an electric voltage or current between the electrodes by reducing the oxidized redox species of the electrolyte to the corresponding reduced state at the positive electrode while the reduced redox active species are oxidized to the corresponding oxidized state at the negative electrode; and delivering the charge stored at the positive electrode and at the negative electrode.

Additional aspects of the present invention are directed to the use of the supercapacitor of the present invention in any of its particular embodiments or of the energy storage or delivery system of the present invention in any of its particular embodiments, to store or deliver energy.

FIGURES

Figure 1 shows the energy density values of (i) a supercapacitor with a non-redox electrolyte; (ii) a similar supercapacitor but with a redox electrolyte and having two electrodes with the same mass loading, and (iii) a similar supercapacitor as described above with redox electrolyte and having two electrodes with different mass loading.

Figure 2 shows a self-discharge graphic showing voltage (V) vs time (min) results of (i) a supercapacitor comprising two electrodes made of a graphite felt electrode coated and impregnated with a carbon composition, a separator and electrolyte having redox species; (ii) the same supercapacitor but with a non-redox electrolyte and (iii) a supercapacitor comprising stainless steel foils electrodes with the same carbon composition as used in the electrodes of (i). Figure 3 shows discharge energy density versus current density (mA/cm ² ) results of (i) a supercapacitor comprising two electrodes made of a graphite felt electrode coated and impregnated with a carbon composition, a separator and electrolyte having redox species; (ii) the same supercapacitor but with a non-redox electrolyte and (iii) a supercapacitor comprising stainless steel foils electrodes with the same carbon composition as used in the electrodes of (i) with redox electrolyte and (iv) without a redox electrolyte.

Figure 4 shows power density values versus current density for (i) a supercapacitor comprising two electrodes made of a graphite felt electrode coated and impregnated with a carbon composition, a separator and electrolyte having redox species; (ii) the same supercapacitor but with a non-redox electrolyte and (iii) a supercapacitor comprising stainless steel foils electrodes with the same carbon composition as used in the electrodes of (i), with redox electrolyte and (iv) without a redox electrolyte.

Figure 5 shows the voltage profile versus time results measured at 70 mA/cm ² of the supercapacitor having one redox active species or no active redox species dissolved into the electrolyte.

Figure 6 shows a Ragone diagram wherein power density versus energy density results measured at 100, 70 and 50 mA/cm ² of the supercapacitor having one redox active species or no active redox species dissolved into the electrolyte.

Figure 7 shows the voltage profile versus time results measured at 70 mA/cm ² of the supercapacitor having two redox active species or no active redox species dissolved into the electrolyte.

Figure 8 shows a Ragone diagram wherein the power density versus energy density results measured at 100, 70 and 50 mA/cm ² of the supercapacitor having two redox active species or no active redox species dissolved into the electrolyte.

Figure 9 shows a comparison of the energy density values measured at 50 mA/cm ² for each type of electrolyte tested.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. As used herein, the singular forms “a” “an” and “the” include plural reference unless the context clearly dictates otherwise.

As defined above, in a first aspect, the present invention refers to a supercapacitor comprising:

- two electrodes; wherein at least one of the electrodes consists of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt;

- an electrolyte comprising: an anolyte comprising a first redox species and a catholyte comprising a second redox species; wherein the first and second redox species are different redox species; and wherein the second redox species is an organic or an organometallic compound;

- a separator; and

- means for connecting with a power/load source.

Electrodes

At least one of the electrodes of the supercapacitor of the invention consists of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt.

In an embodiment, both of the electrodes of the supercapacitor of the invention consist of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt.

In a particular embodiment, the carbon composition is embedded in the carbon felt and is also coating the surfaces of the carbon felt.

In the context of the present invention, the term “activated carbon” refers to what is commonly understood in the art by said expression, it might be also called “activated charcoal”. Activated carbon is usually porous and has high surface-area-to-volume ratio. In a particular embodiment, activated carbon is obtained from a chemical or physical activation process of carbonaceous material such as charcoal. In a more particular embodiment, the chemical activation process of the carbonaceous material comprises the steps of: i) mixing the carbonaceous material with a dehydrating agent such as an acid, and ii) heating at high temperatures to obtain activated carbon. In a more particular embodiment, the physical activation process of a carbonaceous material comprises the steps of: i) treating the carbonaceous material with a mixture of combustion gases and water vapor under heat to obtain activated carbon.

In a particular embodiment, the carbon felt is a graphite felt. In an embodiment, the carbon felt has a thickness of between 1 and 10 mm; preferably of between 2 and 8 mm; more preferably a thickness of about 2.5, 4.6 or 6 mm.

The carbon composition comprises activated carbon particles. In an embodiment, the activated carbon particles have a specific surface area of between 500 and 4000 m ² /g; more preferably of between 1500 and 3000 m ² /g; even much more preferably of between 2000 and 2600 m ² /g; even much more preferably of about 2400 m ² /g; wherein the specific area has been measured by a gas adsorption technique. In an embodiment, any gas adsorption technique known in the art is suitable for measuring the surface area of the activated carbon particles; non-limiting examples of gas adsorption techniques are dynamic flow gas adsorption technique or a volumetric gas adsorption technique applying Brunauer, Emmett and Teller (BET) adsorption isotherm.

In an embodiment, the activated carbon particles are in the carbon composition in at least a 60 wt% of the total weight of the carbon composition; preferably at least in a 65 wt%; more preferably in at least a 70 wt%.

In an embodiment, the activated carbon particles are in the carbon composition in between a 60 wt% and a 95 wt% of the total weight of the carbon composition; more preferably in between a 65 wt% and a 90 wt%; even more preferably in between a 70 wt% and a 85 wt%; even much more preferably about 80 wt%.

In a more particular embodiment, the carbon composition further comprises electrically conductive particles; preferably carbon-based electrically conductive particles; more preferably carbon-based electrically conductive particles selected from carbon black particles, graphene, carbon fibers, carbon nanotubes and mixtures thereof; even more preferably carbon black particles selected from acetylene carbon black particles, channel carbon black particles, furnace carbon black particles, lamp carbon black particles and thermal carbon black particles; even much more preferably acetylene carbon black particles. In the context of the present invention, the expression “carbon black” refers to what is commonly understood in the art by said expression, i.e. a form of paracrystalline carbon that has a high surface-area-to-volume ratio, although lower than that of activated carbon.

In an embodiment, the electrically conductive particles are in the carbon composition in at least a 5 wt% of the total weight of the carbon composition; preferably at least in a 8 wt%; more preferably in at least a 9 wt%.

In an embodiment, the electrically conductive particles are in the carbon composition in between a 5 wt% and a 20wt% of the total weight of the carbon composition; more preferably in between a 8 wt% and a 18 wt%; even more preferably in between a 9wt% and a 15 wt%.

In a more particular embodiment, the carbon composition further comprises a binder; preferably a polymeric binder; more preferably a polyvinyl or a ethylene binder; even more preferably polyvinylidene fluoride or polytetrafluoroethylene (PTFE).

In an embodiment, the binder is in the carbon composition in at least a 5 wt% of the total weight of the carbon composition; preferably at least in a 8 wt%; more preferably in at least a 9wt%.

In an embodiment, the binder is in the carbon composition in between a 5 wt% and a 20 wt% of the total weight of the carbon composition; more preferably in between a 8 wt% and a 18 wt%; even more preferably in between a 9 wt% and a 15 wt%.

In an embodiment, the carbon composition consists of activated carbon particles, carbon black particles and a binder; preferably wherein the ratio of activated carbon particles, carbon black particles and binder is about 8:1:1.

The amount of carbon composition in the electrodes of the supercapacitor of the invention might be the same or different in each of the electrodes of the supercapacitor.

In an embodiment, when both of the electrodes of the supercapacitor of the invention consist of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt, the amount of carbon composition is different in each of the electrodes.

In a particular embodiment, the amount of carbon composition is different in each of the electrodes of the supercapacitor of the invention; preferably one electrode has between a 10 and a 500% in weight more amount of carbon composition than the other electrode; preferably between a 50 and a 200% in weight more amount of carbon composition than the other electrode; more preferably between 80 and 150% in weight more amount of carbon composition than the other electrode; even much more preferably about 100% in weight more amount of carbon composition than the other electrode.

In an embodiment, the carbon composition is in one or in the two electrodes of the supercapacitor of the invention in an amount over 1 mg/cm ² ; preferably over 10 mg/cm ² ; more preferably over 15 mg/cm ² ; even more preferably over 20 mg/cm ² ; even much more preferably of about 25 mg/cm ² .

In an embodiment, the carbon composition is in one or in the two electrodes of the supercapacitor of the invention in an amount of between 1 and 100 mg/cm ² ; preferably between 10 and 80 mg/cm ² ; more preferably between 15 and 70; even much more preferably between 20 and 50 mg/cm ² ; even much more preferably between 21 and 30 mg/cm ² ; even much more preferably of between 22 and 26 mg/cm ² ; particularly about 25 mg/cm ² .

In a particular embodiment, the at least one electrode of the supercapacitor of the invention has been obtained by the following method comprising the steps of

(i) providing a carbon felt; and a carbon slurry comprising at least a 60 wt% of the total weight of the carbon slurry of activated carbon particles;

(ii) coating and impregnating the carbon felt in the carbon slurry to obtain a carbon felt coated and impregnated with carbon slurry; and

(iii) heating the carbon felt with carbon slurry of step (i) at between 50 and 400 °C during between 1 and 10 h to obtain a carbon felt with a carbon composition.

In an embodiment, both of the electrodes of the supercapacitor of the invention have been obtained by the previous method.

In a particular embodiment, the carbon felt is a graphite felt.

In a more particular embodiment, the coating and impregnating of step (ii) of the previous method is performed by any coating and/or impregnation technique known in the art; preferably by spray coating or dip coating; more preferably by dip coating the carbon felt in the carbon slurry.

In a more particular embodiment, the carbon slurry comprises at least a 60 wt% of the total weight of the carbon slurry of activated carbon particles; at least a 5 wt% of carbon black particles; and a binder.

In a more particular embodiment, the carbon slurry further comprises a solvent; preferably an organic solvent; more preferably a polar organic solvent; even more preferably an alcohol; much more preferably isopropanol.

In a more particular embodiment, the carbon slurry consists of activated carbon particles; carbon black particles, a binder and a solvent.

In a more particular embodiment, the heating step (iii) is at between 80 and 300°C; preferably at between 100 and 200°C; more preferably at between 120 and 150°C.

In a more particular embodiment, the heating step (iii) is performed during between 2 and 8h; preferably between 3 and 7 h; more preferably about 4 h.

In a more particular embodiment, the carbon felt of step (i) is pretreated either by a heating step or by a chemically modifying step. In a more particular embodiment, the carbon felt of step (i) has been pretreated with a basic compound; preferably with a basic alkali compound; more preferably with NaOH.

In a more particular embodiment, the carbon felt of step (i) has been previously heated at between 200 and 600 °C during between 2 and 8 h; preferably between 300 and 500 °C during between 3 and 7 h; preferably between 350 and 450 °C during about 4 h.

In a particular embodiment, the electrodes of the supercapacitor of the invention are porous.

In a particular embodiment, the at least one of the electrodes of the supercapacitor of the invention consisting of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt; has a surface area of between 200 and 1000 m ² /g, preferably of between 300 and 900 m ² /g, more preferably of between 400 and 800 m ² /g, even more preferably of about 600 m ² /g. In an embodiment, any gas adsorption technique known in the art is suitable for measuring the surface area of the at least one electrode; nonlimiting examples of gas adsorption techniques are dynamic flow gas adsorption technique or a volumetric gas adsorption technique applying Brunauer, Emmett and Teller (BET) adsorption isotherm.

The authors of the present invention have observed that the use of the electrodes as described above in the supercapacitor of the present invention, lead to high values of power density. Moreover, the electrodes of the supercapacitor of the invention allow a facile flow of electronic charges and improve the interfacial properties between electrode and electrolyte.

Electrolyte:

The electrolyte of the supercapacitor of the invention comprises: i) an anolyte comprising a first redox species, and ii) a catholyte comprising a second redox species; wherein the first and second redox species are different redox species; and wherein the second redox species is an organic or an organometallic compound.

In a particular embodiment, the electrolyte consists of an anolyte comprising a first redox species and a catholyte comprising a second redox species.

In a more particular embodiment, the catholyte and the anolyte are not in physical contact with each other. In an alternative embodiment, the catholyte and the anolyte are in fluidic communication through the separator; preferably only some species of the anolyte and/or the catholyte are able to pass the separator.

In the context of the present invention the expression “redox species” is understood as “redox active species”. In the context of the present invention the terms “first” and “second” directed to the redox species, are used for the identification of different types of redox species, for example they could be substituted by the expressions “redox species A” and “redox species B”.

In an embodiment, the catholyte is in contact with one electrode of the supercapacitor, such as the cathode, and the anolyte is in contact with the other electrode of the supercapacitor, such as the anode.

In an embodiment, the separator is placed in the supercapacitor between the anolyte and the catholyte. In an embodiment, the anolyte and the catholyte are separated by the separator of the supercapacitor. In a particular embodiment, the separator comprises two opposite sides, and the anolyte is in contact with one side of the separator and the catholyte is in contact with the opposite side of the separator.

In a particular embodiment, the catholyte and the anolyte have a different composition.

In an embodiment, the first redox species is in the anolyte and the second redox species is in the catholyte. In a more particular embodiment, the anolyte only has one type of redox species that is the first redox species. In another particular embodiment, the catholyte only has one type of redox species that is the second redox species.

In a more particular embodiment, the anolyte consists of water, a salt and a first redox species. In an even more particular embodiment, the catholyte consists of water, a salt and a second redox species.

Redox species

In an embodiment, the first redox species is an organic compound. In a particular embodiment, the first redox species is an organic compound comprising a heteroaryl group; preferably an heteroaryl group having a nitrogen atom; such as a pyridyl group.

In a more particular embodiment, the first redox species is an organic compound comprising a pyridyl group; preferably comprising two pyridyl groups.

The terms "heteroaryl" and "heteroar-", used alone or as part of a larger moiety, e.g., heteroaralkyl, or "heteroaralkoxy", refer to groups having 5 to 14 ring atoms, preferably 5, 6, 9, or 10 ring atoms; having 6, 10, or 14 TT electrons shared in a cyclic array; and having, in addition to carbon atoms, from one to four heteroatoms. The term "heteroatom" refers to nitrogen, oxygen, or sulfur, and includes any oxidized form of nitrogen or sulfur, and any quaternized form of a basic nitrogen. Thus, when used in reference to a ring atom of a heteroaryl, the term "nitrogen" includes an oxidized nitrogen (as in pyridine N-oxide). Certain nitrogen atoms of heteroaryl groups also are substitutable, as further defined below. Heteroaryl groups include, without limitation, thienyl, furanyl, pyrrolyl, imidazolyl, pyrazolyl, triazolyl, tetrazolyl, oxazolyl, isoxazolyl, oxadiazolyl, thiazolyl, isothiazolyl, thiadiazolyl, pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, indolizinyl, purinyl, naphthyridinyl, and pteridinyl.

In some embodiments, two adjacent substituents on a heteroaryl ring, taken together with the intervening ring atoms, form an optionally substituted fused 5- to 6-membered aromatic or 4- to 6- membered non-aromatic ring having 0-3 ring heteroatoms selected from the group consisting of O, N, and S. Thus, the terms "heteroaryl" and "heteroar-", as used herein, also include groups in which a heteroaromatic ring is fused to one or more aryl, cycloaliphatic, or heterocyclyl rings, where the radical or point of attachment is on the heteroaromatic ring. Nonlimiting examples include indolyl, isoindolyl, benzothienyl, benzofuranyl, dibenzofuranyl, indazolyl, benzimidazolyl, benzthiazolyl, quinolyl, isoquinolyl, cinnolinyl, phthalazinyl, quinazolinyl, quinoxalinyl, 4H-quinolizinyl, carbazolyl, acridinyl, phenazinyl, phenothiazinyl, phenoxazinyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl, and pyrido[2,3-b]-1,4-oxazin-3(4H)-one. A heteroaryl group may be mono-, bi-, tri-, or polycyclic, preferably mono-, bi-, or tricyclic, more preferably mono- or bicyclic. The term "heteroaryl" may be used interchangeably with the terms "heteroaryl ring", or "heteroaryl group", any of which terms include rings that are optionally substituted. The term "heteroaralkyl" refers to an alkyl group substituted by a heteroaryl, wherein the alkyl and heteroaryl portions independently are optionally substituted.

In a particular embodiment, the first redox species is selected from diquat compounds such as 2,2'-bipyridylium; viologen compounds such as methyl viologen dichloride or 1 ,T -bis(3-sulfonatopropyl)-4,4’ -bipyridinium; and mixtures thereof.

In the context of the present invention a “diquat compound” is understood as commonly known in the art as 2,2'-bipyridylium (diquat) compounds, substituted 2,2'-bipyridylium compounds; derivatives of 2,2'-bipyridylium or salts thereof. A non-limiting example of a “diquat compound” is a 2 ,2'-bi pyridylium salt.

In the context of the present invention a “viologen compound” is understood as commonly known in the art as 4,4'-bipyridinium compounds, substituted 4,4'- bipyridinium compounds, derivatives of 4,4'-bipyridinium compounds or salts thereof.

Non-limiting examples of viologen compounds are heptyl viologen dibromide (HVBr2), methyl viologen dichloride (MVCh), methyl viologen dibromide (MVBr2), 1 ,T -bis(3- sulfonatopropyl)-4,4’ -bipyridinium, 1 ,1’-bis(3-hydroxypropyl) viologen; 1 ,1’-bis(2- hydroxyethyl) viologen, 1 ,1’-bis(6-hydroxyhexyl) viologen, 1 ,1 '-bis(3-phosphonopropyl)- [4,4 - bipyridine]-1 , 1 '-diium, bis-(trimethylammonio) propyl viologen and mixtures thereof.

In a particular embodiment, the viologen compound of the present invention is an alkylated viologen; preferably a C1-C10 alkyl viologen; more preferably an heptyl or methyl viologen (MV); even more preferably a methyl viologen salt selected from methyl viologen dichloride and methyl viologen dibromide.

In a particular embodiment, the viologen compound of the present invention is an alkyl substituted 4,4'-bipyridinium compound; preferably is a methyl viologen; more preferably is methyl viologen dichloride.

The term “alkyl” refers to a linear or branched alkane derivative containing from 1 to 10 (“C1-C10 alkyl”), preferably from 1 to 3 (“C1-C3 alkyl”), carbon atoms and which is bound to the rest of the molecule through a single bond. Illustrative examples of alkyl groups include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, heptyl. Preferably, it is methyl or ethyl. The term “alkylated” refers to a compound comprising alkyl groups.

In a more particular embodiment, the viologen compound of the present invention is a methyl viologen dichloride or a 1 ,T -bis(3-sulfonatopropyl)-4,4’ -bipyridinium.

The second redox species of the invention is an organic or an organometallic compound; particularly wherein said organic or organometallic compound is different from the first redox species. In an embodiment, the organometallic compound comprises iron.

In a more particular embodiment, the second redox species is selected from the group consisting of tetramethylpiperidine (TEMPO) compounds, ferrocene compounds, hydroquinone compounds and mixtures thereof.

In a more particular embodiment, the second redox species is selected from the group consisting of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, (ferrocenylmethyl)trimethyl- ammonium chloride, hydroquinone and mixtures thereof.

In a particular embodiment, the first redox species is an organic compound comprising a heteroaryl group; preferably an heteroaryl group having a nitrogen atom; such as a pyridyl group; and the second redox species is selected from the group consisting of tetramethylpiperidine (TEMPO) compounds, ferrocene compounds, hydroquinone compounds and mixtures thereof.

In a particular embodiment, the first redox species is selected from diquat compounds, viologen compounds and mixtures thereof; and the second redox species is selected from the group consisting of tetramethylpiperidine (TEMPO) compounds, ferrocene compounds, hydroquinone compounds and mixtures thereof.

In a particular embodiment, the first redox species is selected from diquat compounds, viologen compounds and mixtures thereof; and the second redox species is selected from the group consisting of 4-hydroxy-2, 2,6,6- tetramethylpiperidin-1-oxyl, (ferrocenylmethyl)trimethylammonium chloride, hydroquinone and mixtures thereof.

In a more particular embodiment, the first redox species is selected from a methyl viologen dichloride or a 1 ,T -bis(3-sulfonatopropyl)-4,4’ -bipyridinium; and the second redox species is selected from the group consisting of 4-hydroxy-2, 2,6,6- tetramethylpiperidin-1-oxyl, (ferrocenylmethyl)trimethylammonium chloride, hydroquinone and mixtures thereof.

In an embodiment, the electrolyte, such as the anolyte and the catholyte, of the supercapacitor further comprises a salt such as sodium sulfate or potassium chloride; or a hydroxide such as potassium hydroxide; more preferably a sulfate salt; more preferably sodium sulfate.

In an embodiment, the concentration of the redox species in the electrolyte, such as in the anolyte and the catholyte, is between 1 and 20 times less than the concentration of salt; preferably is between 5 and 15 times less; more preferably between 8 and 12 times less; more preferably is about 10 times less.

In an embodiment, the electrolyte, such as the anolyte and the catholyte, of the supercapacitor further comprises a solvent. In a particular embodiment, the electrolyte of the supercapacitor further comprises a polar solvent; more preferably water. In a particular embodiment, the electrolyte of the supercapacitor further comprises an organic solvent; preferably acetonitrile. In another particular embodiment, the electrolyte of the supercapacitor, such as the anolyte and the catholyte, further comprises an ionic liquid such as N,N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) alone or as a mixture with an organic sovent

In a more particular embodiment, the electrolyte such as the anolyte and the catholyte, of the supercapacitor further comprises a carbonate; particularly, wherein the carbonate acts as a solvent.

In a more particular embodiment, the anolyte consists of a carbonate, a salt and a first redox species. In an even more particular embodiment, the catholyte consist of a carbonate, a salt and a second redox species. Non-limiting examples of carbonates suitable for the electrolyte (anolyte and/or catholyte) of the invention are alkylcarbonates such as dimethylcarbonate, ethylmethylcarbonate or a mixture thereof.

Separator

The supercapacitor of the invention further comprises a separator. Any separator known in the art related to supercapacitors may be used in the present invention.

In a particular embodiment, the separator is made of an electrically insulating material. Non-limiting materials suitable for the separator are insulating polymers such as polycarbonate or polypropylene, cellulose based materials such as paper, etc.

In a particular embodiment, the separator is an ion exchange membrane; preferably a polymeric ion exchange membrane such as Nation™ perfluorinated membrane, or a selemion hydrocarbon-based ion-exchange membrane.

In a more particular embodiment, the separator comprises a cellulosic material; preferably a paper sheet. In a more particular embodiment, the separator is a paper sheet that has been previously soaked in the electrolyte.

The supercapacitor of the present invention comprises means adapted for connecting with a power/load source, such as electric connections, for example, electric wires.

The supercapacitor of the present invention comprises means adapted for connecting the electrodes with a power/load source; preferably each of the electrodes with a power/load source.

In an embodiment, the power/load source is any external electrical device such as an electrical grid, an electric vehicle, a domestic appliance or a sensor, that draws/transfers energy from/to the supercapacitor. In general, the power/load source have controllable voltages and/or current supplies or uptakes.

In an embodiment, the supercapacitor of the invention further comprises current collectors; preferably current collectors in electrical contact with the electrodes. In a more particular embodiment, the current collectors are made of a electrically conductive material.

In an embodiment, the supercapacitor of the invention comprises:

- two electrodes; wherein at least one of the electrodes consists of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt

- an electrolyte comprising: an anolyte comprising a first redox species and a a catholyte comprising a second redox species; wherein the first and second redox species are different redox species; and wherein the second redox species is an organic or an organometallic compound;

- a separator;

- means adapted for connecting with a power/load source; and

- optionally, current collectors.

In a more particular embodiment, the supercapacitor of the invention consists of:

- two electrodes; wherein at least one of the electrodes consists of a carbon felt and a carbon composition comprising activated carbon particles; wherein the carbon composition is coating and impregnating the carbon felt

- an electrolyte comprising: an anolyte comprising a first redox species and a a catholyte comprising a second redox species; wherein the first and second redox species are different redox species; and wherein the second redox species is an organic or an organometallic compound;

- a separator;

- means adapted for connecting with a power/load source; and

- optionally, current collectors.

In an embodiment, the electrodes in the supercapacitor are facing each other; preferably are parallel.

In an embodiment, the separator is between the two electrodes of the supercapacitor of the invention.

In a particular embodiment, the separator comprises two opposite sides and each of the electrodes of the supercapacitor is in contact with one opposite side of the separator.

In a more particular embodiment, the electrodes are embedded in the electrolyte; for example, one electrode is embedded in the catholyte and the other electrode is embedded in the anolyte.

In a more particular embodiment, the separator separates the catholyte and the anolyte.

The supercapacitor may be in any shape, such as button-shaped (i.e. substantially flat and round or oval surface), rectangular, cylindrical or prismatic.

In an embodiment, the supercapacitor is comprised within a housing such a case.

The authors have observed that the supercapacitor of the invention had a better performance when the electrolyte has at least one redox species than when the electrolyte has none redox active species. Surprisingly, the performance of the supercapacitor of the present invention was significantly improved when two different redox species were used in the electrolyte and, more particularly, when one of said redox species is a viologen compound.

Energy storage system

Therefore, in a second aspect, the present invention is directed to an energy storage system comprising at least one supercapacitor as defined above in any of their particular embodiments; preferably, at least two supercapacitors.

Methods

In addition, in a third aspect, the present invention is directed to a method for storing electricity comprising the steps of: a) providing the supercapacitor of the invention in any of their particular embodiments; b) applying an electric voltage or current between the two electrodes of the supercapacitor of step (a) to obtain an electrode with positive charge and the other electrode with negative charge, at the same time, oxidizing the redox species of the electrolyte to the corresponding oxidized state at the positive electrode while the redox active species of the electrolyte are reduced to the corresponding reduced state at the negative electrode; and storing charge at the positive electrode and at the negative electrode.

In an embodiment, step (b) comprises applying an electric current; preferably an electric constant current.

In a particular embodiment, the charge is stored by faradaic reactions and double layer mechanisms as known in the art.

In an additional aspect, the present invention is directed to a method for delivering electricity comprising the steps of: a) providing the supercapacitor of the present invention in any of its particular embodiments; wherein one electrode has a positive charge and the other electrode has a negative charge; b) generating an electric voltage or current between the electrodes by reducing the oxidized redox species of the electrolyte to the corresponding reduced state at the positive electrode while the reduced redox active species are oxidized to the corresponding oxidized state at the negative electrode; and c) delivering the charge stored at the positive electrode and at the negative electrode.

In an embodiment, step b comprises applying an electric current; preferably an electric constant current.

The working conditions of the supercapacitor of the present invention can be adjusted depending on the final application to increase the driving force of charge transfer during charging process or discharging process.

Uses

Additional aspects of the present invention are directed to the use of the supercapacitor as defined above in any of its particular embodiments or of the energy storage or delivery system defined above to store or deliver energy, particularly, in the renewable energy and electromobility sectors.

An aspect of the invention is directed to a method for storing or delivering energy comprising using the supercapacitor of the invention as defined in any of the particular embodiments.

To this end, the supercapacitor of the present invention may be used individually or in combination with other energy storage technologies (e.g., batteries, etc.) and may be integrated into or with various systems and/or devices to improve efficiency, address energy demands, etc.

Furthermore, the supercapacitor of the invention may be used in a variety of applications having different energy delivery and/or storage needs, including, but not limited to, very large scale applications (e.g., utilities, functioning as a green energy source for a smart grid, energy storage for use in combination with renewable energy resources such as wind and solar power, etc.) and smaller applications (e.g. backup power, residential power, electromobility sector, etc.).

The present invention will be described in further detail with reference to the following examples. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

EXAMPLES

EXAMPLE 1 - Supercapacitor cell preparation

A supercapacitor cell was prepared comprising two electrodes, a separator and an electrolyte, wherein the electrodes are made of a graphite felt electrode coated and impregnated with a carbon composition. The separator was a cellulosic paper impregnated with electrolyte located between the two electrodes. The electrodes were prepared as follows:

A carbon slurry was prepared in a ball mill with 80 wt% activated carbon (PICACTIF SuperCap BP10 type, PICA France, specific surface area = 2400 m ² g ^-1 , hereinafter AC), and 10 wt% carbon black (acetylene, 100% compressed, >99.9%, Alfa Aesar), 10 wt% polyvinylidene fluoride (PVDF, MTI) and isopropanol as a dispersive medium. The carbon slurry was then mixed and homogenized for 30 min at 400 rpm.

Graphite felts (GFD-grade, SGL carbon, Germany, hereinafter GFs) with different thicknesses (2.5, 4.6, and 6.0 mm) were used as a support for the carbon slurry. As received, GFs were activated for 4 h at 400 °C in a furnace. Next, these GFs were cut into pieces 1.2cm ² with circular shape and immersed in the AC carbon slurry by dip coating. Subsequently, the GF-AC electrodes were heated in a vacuum furnace at 140 °C for 4 h (hereinafter GF-ACx, where x is the GF thickness). The average mass loading of GF-AC2.5, was 25 mg of AC cm ^-2 for both electrodes. In addition two electrodes having a different mass loading (one with a 25 mg of AC cm ^-2 and the other one with 50 mg of AC cm ^-2 ) were tested in the supercapacitor.

Different aqueous redox electrolytes were prepared to be used in the supercapacitor. Each type of electrolyte was prepared by dissolving the corresponding mass of supporting salt and none or one redox active species into a solvent for achieving a specific concentration.

Figure 1 shows the energy density results of (i) the supercapacitor as described above with a non-redox electrolyte; (ii) the supercapacitor as described above with redox electrolyte and having two electrodes with the same mass loading, and (iii) the supercapacitor as described above with redox electrolyte and having two electrodes with the different mass loading. Results showed greater values for the energy density of the supercapacitor as described above with a redox electrolyte that for the supercapacitors with an electrolyte without redox species. In addition, better results were obtained for the supercapacitor when the electrodes have a different mass loading.

EXAMPLE 2 - Supercapacitor cell performance against other supercapacitor cells

The properties of the supercapacitor cell as described on EXAMPLE 1 using a redox electrolyte has been tested against a similar supercapacitor using non-redox electrolyte and against a supercapacitor comprising the same redox electrolyte but wherein the electrodes are made using the same carbon slurry as described in Example 1 but using stainless steel foils instead of graphite felts as supporting material. The redox electrolyte was an aqueous solution of a salt and containing one redox species and the non-redox electrolyte solution was an aqueous solution of a salt without redox species.

Galvanostatic charge-discharge (CD) testing was performed on the supercapacitors. Data was obtained from CD experiments over a range in voltage 0 to 1.2V and a temperature of 20°C.

Figure 2 shows a self-discharge graphic showing voltage (V) vs time (min) results of the (i) supercapacitor as described on Example 1 with redox electrolyte, (ii) the supercapacitor as described on Example 1 but with a non-redox electrolyte and (iii) a supercapacitor comprising stainless steel foils electrodes with a carbon composition as described above. Results showed reduced self-discharge values in the supercapacitor as described on Example 1 that in the other supercapacitors.

In addition, discharge energy density and power density versus current density were measured for the different supercapacitors. The discharge energy density power density ( ^p ™0 values were calculated applying the equations 1 y 2 showed below:

Wherein m is the mass loading of the electrode, I is the applied current, V is the voltage, tj represents the time at which the discharge starts and tf the time at which the discharges ends, respectively; t discharge is the total time that the supercapacitor discharge lasts at the applied current.

Figure 3 shows discharge energy density versus current density for the (i) supercapacitor as described on Example 1 with redox electrolyte, (ii) supercapacitor as described on Example 1 without a redox electrolyte (non-redox electrolyte) and for the supercapacitor comprising stainless steel foils electrodes with a carbon composition as described above (iii) with redox electrolyte and (iv) without a redox electrolyte. Results showed greater values for the energy density of the supercapacitor as described on Example 1 with redox electrolyte that for the other supercapacitors.

Figure 4 shows power density values versus current density for the (i) supercapacitor as described on Example 1 with redox electrolyte, (ii) supercapacitor as described on Example 1 without a redox electrolyte and for the supercapacitor comprising stainless steel foil electrodes with a carbon composition (iii) with redox electrolyte and (iv) without a redox electrolyte. Results showed greater values for the power density of the supercapacitor as described on Example 1 with redox electrolyte that for the other supercapacitors.

EXAMPLE 3 - Supercapacitor cell performance with different electrolytes

The properties of the supercapacitor cell as described on EXAMPLE 1 has been tested using different redox electrolytes.

Different aqueous redox electrolytes were prepared to be used with the supercapacitor. Each type of electrolyte was prepared by dissolving the corresponding mass of supporting salt and redox active species into a solvent (water) for achieving a specific concentration.

In particular, the electrolytes used were an aqueous solution of 1M Na2SC>4 with one, with two types of dissolved redox active species or without any redox active species (namely no-redox electrolyte).

The redox active species dissolved into the electrolytes tested were:

- methyl viologen dichloride (named as MV);

- 1 ,T -bis(3-sulfonatopropyl)-4,4’ -bipyridinium (also named as (SPr)2V)

- (ferrocenylmethyl)trimethylammonium chloride (named as FcNCI);

- 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (named as OHTEMPO);

- hydroquinone (named as H2Q).

The typical concentration of each redox active species in the electrolyte is 0.1 M.

When just one redox species was used in the electrolyte, the composition of each of the catholyte and the anolyte in the supercapacitor was (i) the same aqueous solution of 1 M Na2SC>4 comprising one redox species or (ii) in one of the anolyte or catholyte an aqueous solution of 1 M Na2SC>4 and, in the other one, the same aqueous solution but having a redox species.

The active redox pairs of species tested in the supercapacitor were: MV+ OHTEMPO, MV+ FcNCI, MV + H ₂ Q, ((SPr)2V)+ OHTEMPO, ((SPr)2V)+ FcNCI and ((SPr)2V) + H ₂ Q.

When a pair of redox species was used in the electrolyte, the composition of each of the catholyte and the anolyte in the supercapacitor was an aqueous solution of 1M Na2SC>4 comprising one redox species, however, the redox species in the catholyte was different to the one in the anolyte. In particular, the anolyte composition was an aqueous solution of 1M Na2SC>4 comprising an organic redox compound comprising a heteroaryl group having a nitrogen atom such as MV or ((SPr)2V. Moreover, the catholyte composition was an aqueous solution of 1M Na2SC>4 comprising a redox species selected from FcNCI, OHTEMPO or H2Q.

Figure 5 shows the voltage profile versus time results measured at 70 mA/cm ² of the supercapacitor having one redox active species or no active redox species dissolved into the electrolyte. Figure 6 shows a Ragone diagram wherein power density versus energy density results measured at 100, 70 and 50 mA/cm ² of the supercapacitor having one redox active species or no active redox species dissolved into the electrolyte. The active redox species tested were MV, FcNCI, OHTEMPO, H2Q.

Results of Figure 5 and 6 showed that the supercapacitor had a better performance when the electrolyte has a redox active species than when the electrolyte has none redox active species.

Figure 7 shows the voltage profile versus time results measured at 70 mA/cm ² of the supercapacitor having two redox active species or no active redox species dissolved into the electrolyte. Figure 8 shows a Ragone diagram wherein the power density versus energy density results measured at 100, 70 and 50 mA/cm ² of the supercapacitor having two redox active species or no active redox species dissolved into the electrolyte. The active redox species pairs tested were MV+ OHTEMPO, MV+ FcNCI, and MV + H ₂ Q.

Figure 9 shows a comparison of the Energy density values measured at 50 mA/cm ² for each type of electrolyte tested. In figure 8, the % of the energy density value obtained for the different electrolytes is expressed with respect to the value obtained without any redox active species which is considered as 100%. The active redox species tested were OHTEMPO, H2Q, MV, ((SPr)2V), FcNCI and the active redox species pairs tested were MV+ OHTEMPO, MV+ FcNCI, MV + H ₂ Q, ((SPr)2V)+ OHTEMPO, ((SPr)2V)+ FcNCI and ((SPr)2V) + H2Q. When a pair of active redox species was tested, the anolyte and the catholyte had a different redox species and the anolyte comprised either MV or ((SPr)2V).

The authors have observed that the supercapacitor of Example 1 had a better performance when the electrolyte has at least one redox active species than when the electrolyte has none redox active species. Surprisingly, Figure 9 results showed the supercapacitor performance is significantly improved when a pair of redox active species is used in the electrolyte. Results of figure 9 show a significant and unexpected increase in the energy density values of the supercapacitor when a pair of redox active species is used in the electrolyte in comparison with the use of just one redox active species in the electrolyte.