Regenerative fuel cell utilizing liquid hydrogen carrier

Abstract

The invention provides Pt 1-99- Ir1-99-Mo-99 aerogel useful as a bifunctional electrocatalyst in a unified regenerative fuel cell. Also provided is a unified regenerative fuel cell and a method of storing and producing energy with the aid of a liquid hydrogen carrier (LHC) as a fuel material in a unified regenerative fuel cell.

Description

[0001] Regenerative fuel cell utilizing liquid hydrogen carrier

[0002] Background of the invention

[0003] Climate change and its detrimental impact on our lives and environment increased the need to change the way we produce and use energy, i.e., move towards low-carbon electricity to reduce CO2 emission. Because hydrogen is a clean fuel material, there has been a growing trend over the last few decades towards switching to hydrogen economy. Yet for hydrogen economy to gain commercial acceptance, efficient storage technologies of hydrogen are needed.

[0004] The current hydrogen storage technologies are: (1) cryogenically liquifying H2 at 21 K and ambient pressure, (2) storing H2 in high pressure gas cylinders (200-1000 bar) , (3) adsorbing H2 on metals with high affinity to hydrogen, to form metal hydrides, and (4) in the case of ultra-large quantities, storage in salt caverns under mild pressure (usually below 100 bar) .

[0005] An alternative approach to storing hydrogen by compression, liquif ication and incorporation into metal hydrides is based on using liquid hydrogen carriers (LHC) . LHC are compounds (chiefly organic) which can switch reversibly from a reduced (hydrogenated) form to an oxidized (dehydrogenated) form. A recently published review [Tang et al. Journal of Materials Chemistry A, 26, 2024] lists different types of LHCs, based on alcohols, amines and aromatic compounds, including the quinone / hydroquinone redox couple, and their various applications.

[0006] The quinone / hydroquinone redox couple can act as an efficient LHC owing to its reversible electrochemical behavior, involving a reversible 2-electron, 2-proton reaction creating a system for hydrogen storage and generation of energy:

[0007] The terms quinone / hydroquinone are used herein to describe the couple depicted above, derivatives thereof, and any compound incorporating the quinone / hydroquinone nucleus, namely, condensed polycyclic aromatic compounds such as anthraquinones, e.g., 9,10- anthraquinone : and substituted anthraquinone derivatives. Also included are compounds featuring the quinone nucleus fused to five or six membered rings which contain heteroatoms, e.g., nitrogencontaining rings. A few examples are depicted below.

[0008] In addition to their beneficial reversible redox chemistry, quinones and hydroquinones are nonflammable and nontoxic and can be safely stored and transported in plastic containers, either dissolved in solution or in a solid form, i.e., they are easier and safer to handle compared to hydrogen gas or liquid. Their properties (e.g., water solubility and redox potential) can be modified by appropriate substitution of chemical groups on the aromatic rings. Hereinafter, Q and QH2 denote the deprotonated (oxidized) and protonated (reduced) forms of the quinone / hydroquinone couple, respectively. A fuel cell utilizing a Q / QH2 redox couple (a water-soluble sulfonated anthraquinone derivative: anthraquinone-2 , 7-disulfonic acid (AQDS) , depicted below in its reduced and oxidized forms was recently reported [Charvat, Mazur, Pocedic, Richtr, Mrlik, Kosek, Akrman and Kubac, Journal of Power sources 520 (2022) 230811; and Yurko and Elbaz, J. Am. Chem. Soc. 2023, 145, 2653- 2660] . The reduced AQDS was supplied to the anode (made of carbon felt and devoid of any catalyst) , with oxygen being fed to the cathode (where Pt was applied as a metal catalyst) . High power density was delivered by the cell upon oxygen reduction, outperforming the operation of a direct methanol fuel cell. A corresponding electrolysis cell was also studied, with water splitting (catalyzed by iridium oxide) occurring at the anode to produce O2, and reduction of the oxidized AQDS over the cathode, at cell potential held at 1.8V, to achieve full regeneration of the hydrogen carrier, restoring its reduced form. But the two functions were not joined into a single device, as seen from the experimental set-up that was tested by Charvat et al. (supra) , shown in Figure 1 below.

[0009] The invention

[0010] To benefit from the reversibility of the Q / QH2 redox couple in an efficient manner, we developed a unified regenerative fuel cell, i.e., an electrolyzer-fuel cell hybrid device. Not only this type of cell produces electricity from the reaction of the LHC and O2; when powered by electricity, it functions as an electrolytic cell to produce protons that regenerate the LHC and O2 by splitting water molecules. That is, a single device cycles between charge and discharge modes of operation, storing hydrogen and releasing it on demand. The hybrid device provided by the invention uses a bifunctional electrocatalyst composition applied on the oxygen electrode, which in the fuel cell mode, accelerates the oxygen reduction reaction ( 2 + 2H++ 2e~ H2O) , whereas in the electrolyzer mode, it accelerates the oxygen evolution reaction (H2O W2 + 2H++ 2e~) . The Q / QH2 redox pair fits well into regenerative fuel cells, to participate in the charge and discharge reactions shown below:

[0011] Charge mode (operation as an electrolyzer to store hydrogen) :

[0012] H20 t02+ 2H+ + 2e~;

[0013] (oxygen evolution reaction (OER) takes place on the anode, where water splits to oxygen and protons) .

[0014] Q + 2H+ + 2e- QH2;

[0015] (the oxidized form of the quinone is reduced (hydrogenated) at the cathode, whereby hydrogen is stored in the hydroquinone) .

[0016] Discharge mode (operation as fuel cell) :

[0017] QH2Q + 2H+ + 2e-

[0018] (protons are produced upon oxidation of the hydroquinone on the anode (the cathode in the previous mode) ) . t02+ 2H+ + 2e~ H20;

[0019] (oxygen / air is fed to the cathode (the anode in the previous mode) , where oxygen reduction reaction (ORR) takes place, as oxygen, protons and electrons combine to form water) .

[0020] It is seen that when the Q / QH2 redox couple serves as hydrogen carrier, hydrogen gas is in fact not produced, only protons which leave the QH2 upon its oxidation. A regenerative fuel cell utilizing the Q / QH2 redox couple according to the invention is therefore safe to handle, as the only byproduct in the system is pure water, making it nonpolluting.

[0021] The major components of the hybrid device are shown schematically in Figure 2. The electrodes are separated by a proton exchange membrane, allowing the passage of H+ions. The design is based on hydrogen and oxygen electrode setup, on the left and right sides of the cell of Figure 2, respectively. One electrode is associated with the Q / QH2 redox couple (named herein for simplicity "the quinone electrode" or "the LHC electrode") , and the other electrode with the O2 evolution and reduction reactions (named herein "the oxygen electrode") .

[0022] The feed stream supplied to the quinone side of the cell is either the Q (oxidized) or QH2 (reduced) form of the LHC, and the outgoing product stream consists of QH2 and Q, respectively. As pointed out above, a significant advantage of Q / QH2 redox couple is that the oxidation reaction where QH2 is stripped of hydrogen ions, advances effectively without the aid of any catalyst, e.g., over a simple porous carbon electrode. The presence of an electrocatalyst on the hydrogen side is unnecessary and the electrode surface is usually devoid of a catalyst layer.

[0023] Turning now the oxygen side, oxygen evolution reaction (when working in electrolyzer mode) and oxygen reduction reaction (when working in fuel cell mode) take place over the same electrode, with feed streams consisting of water or oxygen (e.g., atmospheric oxygen) , supplied to the electrode, respectively. This calls for installation of a corrosion-resistant electrode. In addition, both the ORR and OER need the help of an electrocatalyst. In its most general form, the bifunctional electrocatalyst applied on the surface of the oxygen electrode comprises at least two catalytically active metals of the platinum group. Platinum and iridium, which show strong catalytic activity towards ORR and OER, respectively, are preferably used in the invention. The electrocatalyst preferably includes at least one additional metal, e.g., a third metal, selected from the first row of transition metals, labeled herein M, e.g., nickel or cobalt. Owing to the bifunctionality of the electrocatalyst, the hybrid device shown in Figure 1 can switch between the two modes of operation. In electrolytic operation mode, surplus energy from sustainable energy sources such as solar panels can be used to power the cell, split water to oxygen and protons in the OER on the anode (catalyzed by Ir) , and protons can reduce the oxidized form Q on the cathode. In times when this stored energy is needed, the reduced form QH2 can be used in the same device to produce energy from the oxidation reaction of the QH2 and the reduction of oxygen coming from the air via the oxygen reduction reaction (ORR catalyzed by Pt) .

[0024] Experimental results reported below show that a binary (Pt-Ir) and preferably ternary (Pt-Ir-M; M is for example Ni or Co) alloy, e.g., in the form of an unsupported aerogel, exhibits strong electrocatalytic action, and performs better than Pt-IrO2 blend of obtained by mechanical mixing. Another aspect of the invention is therefore a process for preparing a bifunctional electrocatalyst, comprising combining in solution a source of Pt cation (e.g., Pt2+, Pt4+) , a source of Ir cation (e.g., Ir3+, Ir4+) and optionally a source of M cation, wherein M is a metal selected from the first row of transition metals (e.g., Ni2+) , reducing the metal cations in the solution to form the corresponding metals in elemental form and obtain metal-containing gel, soaking the gel with an organic solvent to displace water molecules by the organic solvent, and supercritical drying or freeze drying the gel to obtain P 11-99- 1 r 1-99- Mo-99 aerogel, e.g., Pts-gs-Irs-gs-Mo-so (atomic %; i.e., the general notation used herein is Ptx-Iry-Mz, with x+y+z=100. For example, in the case of Pts-gs-Irs-gs-Mo-so, the meaning is 5<x<95, 5<y<95, and 0<z<50; with the condition x+y+z=100) .

[0025] The general technique of preparing aerogels of transition metals is described in US 2015 / 0162622. Preferred sources of Pt4+, Ir4+and MP+, e.g., Ni2+, are water soluble salts or coordination complexes, e.g., halides (chlorides) , such as JhPtCle, J^IrCle and N1C12 (all available as hydrates) , that are dissolved in water, usually at concentrations of up to 50 mM. Other precursors to supply Pt4+, Ir4+and Ni2+to the solution include the corresponding halides, sulfate and carbonates, e.g., PtC12, IrCls, IrC14, N1SO4 and NiCOs. The relative amounts of the Pt2+ / 4+, ir3+ / 4+and Ni2+are adjusted to meet the desired elemental ratio, producing P 11-99“ I r 1-99- Mo-99 , e.g., Pt5-95_Ir5-95_Mo-5O, e.g., Ptl5-75_Iri5-75_M15-45 and specifically, Pt25-75-Iri5-35-Nii5-35, e.g., Pt45-55-Ir2o-3o-Ni2o-3o aerogel .

[0026] A reductant is added to the stirred solution, to obtain the zero- valent metals from their respective Pt4+, Ir4and Ni2+ions. For example, borohydride, such as sodium borohydride (NaBJh) , can be used. The latter is a powerful reductant, and the reduction reaction is completed swiftly under stirring at room temperature, with the gel precipitating almost immediately after the addition of the reductant. Other reductants (sodium ascorbate (CefbNaOe) , sodium hypophosphite monohydrate (NaH2PO2) and hydrazine monohydrate (N2H4) can also be used. Because the metals salts are often acids, the pH of the solution is acidic.

[0027] Hydrogelation is completed by allowing the reaction mixture to stand for sufficient time, usually not less than twenty-four hours (preferably in an oven) . The wet hydrogel gel is then isolated, e.g., by filtration or decantation, washed with water and soaked extensively in an organic solvent (e.g., acetone or ethanol) , to expel the water molecules from the hydrogel and replace them with the organic solvent, obtaining acetone (or ethanol) - containing gel .

[0028] Lastly, the acetone (or ethanol) - containing gel is subjected to supercritical drying or freeze drying. To this end, the gel is placed in a pressure vessel filled with liquid carbon dioxide. The C02molecules displace the acetone / ethanol molecules from the pores of the gel. The treatment lasts at least several hours, e.g., 12 hours, with fresh C02being supplied from time to time to the pressure vessel. Eventually, the vessel is heated to exceed the critical point of CO2. Then CO2 molecules leave the material and expand in the vessel. The vessel is depressurized to release the gas, affording a solvent-free Pti-99-Iri-99-Mo-99, e.g., Pt5-95-Irs-95- Mo-50, aerogel, e.g., with the composition set out above.

[0029] The method of supercritical drying preserves the porous structure of the product, resulting in a dark grey, self-supported Pt 5-95— I r 5-95— Ni 0-50 alloyed aerogel, possessing BET surface area of not less than 30 m2 / g, e.g., >40 m2 / g, >50 m2 / g, for example, from 30 to 70 m2 / g, e.g., from 50 to 70 m2 / g, e.g., ~ 60 m2 / g.

[0030] P 11-99- 1 r 1-99- Mo-99 , e.g., Pts-gs-Irs-gs-Mo-so, Pts-gs-Irs-gs-Ms-so, e.g.,

[0031] Pt 15-75- 1 r 15-75- M15-45 , for example, Pt25-75-Iri5-35-Mi5-35, such as Pt45-55- 1 r2o-3o- M20-30 alloy aerogel, where M is preferably Ni, forms another aspect of the invention. Specifically, the invention provides Pt45-55-Ir2o-3o-Ni2o-3o exhibiting X-ray diffraction peaks centered at ~ 40.5 20, 47.5 20 and 69.2 20 (±0.2 20) , indicating the alloy character of the aerogel (devoid of clear discrete peaks assigned to individual metallic particles, which would have been the case for multiphase formation) .

[0032] The electrochemical behavior of the Pts-gs-Irs-gs-Nio-so alloy aerogel (specifically, Pt45-55-Ir2o-3o-Ni2o-3o) was tested by linear sweep voltammetry in a three-electrode configuration, i.e., with the Pt 5-95— I r 5-95— Ni 0-50 applied on a rotating disc working electrode. The anodic and cathodic curves in the resulting voltammogram indicate good electrocatalytic action of Pts-gs-Irs-gs-Nio-so, towards OER and ORR, respectively, as shown in more detail below. In its most general form, the unified regenerative fuel cell of the invention comprises a first electrode and a second electrode joined to the opposite faces of a proton exchange membrane (PEM) , characterized in that the first electrode (the oxygen electrode) consists of (preferably corrosion resistant) liquid / gas diffusion electrode with the Pt-Ir-M (Pt-Ir-Ni) aerogel defined above as an electrocatalyst. The opposite face of the PEM is provided with the quinone electrode, which in the present case, owing to the unique character of the Q / QH2, preferably consists of a catalyst-free electrode, such as porous carbon felt. But electrodes made of noble metals can also be used on the quinone side.

[0033] On the oxygen side of the PEM, the Pts-gs-Irs-gs-Nio-so aerogel is used to form a liquid / gas diffusion electrode through application of the aerogel onto a corrosion resistant liquid / gas diffusion layer (LGDL) , made of titanium, e.g., titanium fiber felt, such as platinized titanium fiber felt, which is able to withstand the corrosive environment on the oxygen side due to the presence of water. Other forms of titanium, e.g., mesh and foam, can also be used as GDLs .

[0034] As pointed out above, the Pts-gs-Irs-gs-Nio-so aerogel is selfsupported and can be put to use without the help of regular catalyst supports. The Pts-gs-Irs-gs-Nio-so aerogel can be applied onto the surface of the LGDL to reach electrocatalyst loading of 0.5-2.0 mg per cm2by conventional methods, e.g., spraying or screen printing, with the aid of a binder, e.g., Nafion® (a copolymer consisting of tetrafluoroethylene backbone bearing perfluorinated vinyl-ether units, with sulfonic acid pendent groups) .

[0035] For example, the aerogel is suspended in a mixture of lower aliphatic alcohols (e.g., isopropanol) and water at concentration of 5 to 70% by weight in the presence of the Nafion ™ binder at levels of 5 to 40 % by weight relative to the total weight of the suspension. The suspension is used to coat the LGDL, namely, the platinum-plated titanium felt, by brushing or spraying, e.g., using an ultrasonic sprayer, to create a uniform coating of the electrocatalyst on the oxygen electrode / LGDL .

[0036] The PEM is usually about 5 to 300 pm thick and is made of a polymer bearing an acidic functionality (a sulfonic acid, a carboxylic acid, a phosphonic acid, a phosphoric acid) . The polymer is often fluorinated, e.g., poly (perfluorosulfonic acid) (abbreviated PFSA; Nafion®) . Non-reinf orced films, e.g., copolymers of PFSA and polytetrafluoroethylene (PTFE) , as well as reinforced fluorine- free films (with polyethylene terephthalate (PET) as a reinforcing agent) can be used. PEMs are available from various manufacturers, e.g., 3M, lonPower, and Fumatech.

[0037] The PEM, the oxygen electrode (consisting of Pts-gs-Irs-gs-Mo-so- coated LGDL, specifically titanium fiber felt or platinized titanium fiber felt with Pts-gs-Irs-gs-Mo-so layer deposited thereon) and the quinone electrode (preferably the catalyst-free porous carbon felt) , are assembled in the cell to form the membrane electrode assembly (MEA) .

[0038] Other cell components are assembled in a conventional manner, namely, gaskets sealing the MEA to achieve a desired cell compression and avoid leakage of the reactants, bipolar plates and end plates. Because the voltage produced by a single cell is usually insufficient, multiple cells are stacked (e.g. in series) with bipolar plates positioned to separate adjacent cells, made of metals (stainless steel, aluminum, titanium, nickel) or graphite. The bipolar plates allow flow patterns of different types, e.g., pin type, straight parallel, interdigitated, single-channel serpentine, spiral, and multiple-channel serpentine. Another approach consists of forming the MEA electrode assembly beforehand by laminating the oxygen electrode and the quinone electrode onto the opposing faces of the PEM, and hot pressing.

[0039] As pointed out above, the preferred Q / QH2 redox couple for use as a fuel material in the invention consists of anthraquinone / anthrahydroxyquinone, e.g., the 9, 1 O-anthraquinone having the structure of Formula (I) depicted above: wherein each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from: -hydrogen;

[0040] -halogen;

[0041] -hydroxyl ;

[0042] -thiol ;

[0043] -C (=0) OX1;

[0044] -N (X1)2;

[0045] -NO2 ;

[0046] -S (=0) 2OX1;

[0047] -P(=o) (ox1)2; wherein X1is H, C1-5 alkyl; and

[0048] - a group X2or -OX2wherein X2is C1-7 linear or branched alkyl or alkenyl chain, optionally interrupted with a heteroatom (N, 0) and bearing a group selected from -C(=o)ox1, -S(=O)2ox1and -P (=0) (ox1)2.

[0049] For example, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from hydrogen, hydroxyl, carboxylic acid -COOH, sulfonic acid -SO3H and phosphonic acid -PO3H2. For example, one or more of R1, R2, R3, R4, R5, R6, R7and R8is X2or OX2, wherein X2is C1-7 linear or branched alkyl chain terminated with COOH, SO3H, and PO3H2 such as: - (CH2)m-COOH (m=l-3) , -C ( CH3) 2-CH2-COOH, -0- ( CH2) m-PO3H2and -0- (CH2)m-SO3H.

[0050] 9.10-anthraquinone of Formula (I) are commercially available and can be prepared by methods known in the art. Major examples include

[0051] 9.10-anthraquinone-2 , 7-disulfonic acid (or its disodium salt) that was mentioned above, 1, 2-dihydroxy-9, 1 O-anthraquinone ; 1,4- dihydroxy-9, 1 O-anthraquinone ; 1, 8-dihydroxy-9, 1 O-anthraquinone ;

[0052] 2.6-dihydroxy-9, 1 O-anthraquinone ; 1, 2-dihydroxy-9, 10- anthraquinone-3-sulfonic acid (sodium salt) ; 3, 4-dihydroxy-9, 10- anthraquinone-2-sulfonic acid; 1, 3, 6, 7-tetrahydroxy-9, 10- anthraquinone ; anthraquinone-2 , 6, -propanoic acid; 1 , 8-dihydroxy-

[0053] 2.7-dicarboxymethyl-9, 1 O-anthraquinone and anthraquinone-2 , 6- diethylphosphinic acid. It should be noted that sulfonated 9,10- anthraquinone is available commercially in the form of their sodium salt and prior to use it is converted into the protonated form by passing the electrolyte solution through an ion exchange resin.

[0054] Anthraquinones of formula (I) for use in the invention can be obtained by condensing phthalic anhydride with benzene (or substituted benzene) and aluminum chloride, affording benzoly-2- benzoic acid, with ring closure effected by heating with sulfuric acid, to give the anthraquinone system, which is subsequently reacted as appropriate to introduce the desired chemical groups. Sulfonation of anthraquinone is achieved with fuming sulfuric acid. Nitration is accomplished by adding concentrated nitric acid to a solution of the anthraquinone in sulfuric acid. Introduction of amino groups is by reduction of nitro groups; efficient reductants are sodium sulfide, zinc dust and tin in HC1, or by treating hydroxylated derivative with ammonia / alkyl amine. The amino group can be alkylated with alkyl halide. Halogenated anthraquinone is prepared from the corresponding sulfonated derivative, as chlorine / bromine can easily replace sulfonic group at positions a, p (1,2; 4,3; 5, 6; 8,7) or by using chlorinated benzene compounds in the condensation reaction with phthalic anhydride. Hydroxylation of the anthraquinone is carried out using several methods: alkali metal hydroxide fusion of sulfonate salt or halogenated compound, thus replacing the sulfonic group or halogen atom by hydroxyl, respectively; converting an aromatic amine into a diazonium salt, with heating in sulfuric acid. Another efficient hydroxylation method consists of direct oxidation in sulfuric acid in the presence of boric acid. The general methods described above have been known for many years (see "The Chemistry of anthraquinone" by Max Phillips in Chem. Rev. 1929, 6(1) , 157- 174) . There are of course more recent examples, e.g. Mad j e et al. [Green Chemistry Letters and Reviews, 3:4, 269-273] showed the preparation of anthraquinone through the condensation reaction of phthalic anhydride and substituted benzene derivatives in water, in the presence of alum, reporting the synthesis of several anthraquinone in good yield, including the derivatives depicted below :

[0055] The liquid hydrogen carrier may also be selected from the class of polycyclic heteroaromatic quinone-based compounds (e.g., nitrogen heterocyclic derivatives) , for example: and from the following families (I-IV) :

[0056] Other examples of LHC of Formula I are depicted below (shown in the hydroquinone form) :

[0057] The unified regenerative fuel cell according to the invention can be used in a range of areas, such as renewable energy (storing solar / wind energy and generating electricity in the form of direct current on demand, e.g., to a DC load) and power supply (by being connected to the grid via an inverter, or off-grid for backup purposes) . Figure 3 shows the operation of a fuel cell system based on the unified regenerative fuel cell, switching between the electrolyzer and fuel cell functions. The red lines indicate the flow of reactant and product streams when the system operates as a fuel cell. The black lines indicate the flow of reactant and product streams when the system operates as an electrolyzer.

[0058] On the quinone side, reservoirs are installed, to hold the Q and QH2 forms separately (1 and 2, respectively) . Feed lines (Spueiceii and 3electrolyzer ) and discharge lines (4Fueiceii and 4eiectroiYzer and a pump 5 (e.g., a peristaltic / diaphragm pump) are also installed on the quinone side, to manage the flow of incoming QH2 and Q streams to the cell and outgoing streams to the tanks. For example, for applications such as small (household) to large energy storage devices (solar / wind farms) , which could be used for short (hours) , long (months, seasonal) and indefinite (strategic) times, the active area of the electrodes in one MEA can range from a cm2scale to a m2scale, the volume of each reservoir is usually at least 10 liter and can be a few orders of magnitude larger, and the liquid pumps delivering reactant feed streams to and withdrawing product streams from the regenerative fuel cell operate at a varied flow rate with the system operational parameters depending on the required power loads. A heat exchanger HEi is positioned in discharge line 4Fueiceii.

[0059] On the oxygen side, an air pump / compressor 6 pushes air through pipe 7 Fueiceii to the oxygen electrode. For example, suitable air pumps / compressor can supply from 10 to 20 liter per minute. Water is supplied to the oxygen electrode from a water tank 8 through a feed line 9 by a peristaltic / diaphragm pump 10. Water is discharged from oxygen side via line 11 connected to the water tank 8, with a heat exchanger HE2 positioned along line 11 upstream to tank 8. During operation in the fuel cell mode (load is connected) , QH2 incoming stream (consisting of a QH2 dissolved in solution at a concentration of > IM, e.g., 1-5 M, e.g., ~2M; either in water or in >1M sulfuric acid) flows via feed line 3Fueiceii, usually at a rate of 0.5-10 liter per minute, and is distributed over the quinone side of the fuel cell, where the QH2 Q + 2e~ + 2H+oxidation reaction takes place on the anode. The outgoing stream of oxidized form Q leaves the fuel cell via a discharge line Fueiceii, passes through the heat exchanger HEi to release heat, and is collected in tank 1. Atmospheric oxygen is simultaneously fed to the oxygen half-cell via pipe 7Fueiceii, where the 0.502 + 2H++ 2e~ H2O oxygen reduction reaction takes place over a corrosion resistant electrode coated with a layer of Pts-gs-Irs-gs-Mo-so aerogel (cathode reaction, catalyzed by the Pt (and Ni) component of the aerogel) .

[0060] During operation in the electrolytic cell mode (connected to a power source (e.g. DC) ) , the oxidized form Q flows from tank 1 via line 3eiectroiyzer , distributed in the fuel side(s) of the cell (s) , where the Q + 2e~ + 2H+ QH2 reduction reaction occurs (e.g., over a simple carbon electrode without any precious metal to catalyze the reaction) . Water is the reactant supplied to the oxygen electrode (s) from tank 8, where OER catalyzed by the Ir component of the aerogel takes place H2O 0.502 + 2H++ 2e. Thus, another aspect of the invention is a unified regenerative fuel cell comprising one or more individual cells (for example, a stack consisting of a plurality of individual cells) , wherein the individual cell comprises a first electrode and a second electrode joined to the opposite faces of a proton exchange membrane (PEM) , wherein the first (preferably corrosion resistant) electrode defining the oxygen side consists of liquid / gas diffusion layer (LGDL) with a bifunctional electrocatalyst applied thereon (e.g., Pt and Ir containing electrocatalyst, preferably in the form of Pts-95- 1 rs-95- Mo-50 aerogel, wherein M is a metal selected from the first row of transition metals, as shown above) , and the second electrode defining the fuel side is preferably catalyst-free.

[0061] The unified regenerative fuel cell is most preferably LHC-fueled, i.e., the fuel side is supplied by LHC such as QH2 dissolved in an aqueous solution on discharge, and Q dissolved in an aqueous solution on charge.

[0062] Thus, another aspect of the invention is a fuel cell system comprising :

[0063] A) the unified regenerative fuel cell described above (e.g., in a stack configuration) ;

[0064] B) a fuel supply and regeneration installation comprising separate aqueous solutions of the reduced and oxidized forms of a liquid hydrogen carrier.

[0065] Preferably, the fuel supply and regeneration installation comprises a first tank for holding a liquid hydrogen carrier (QH2) , a second tank for holding the oxidized form of the liquid hydrogen carrier (Q) , a first feed line connecting the first tank to a liquid inlet in the regenerative fuel cell; a first discharge line connecting a liquid output of the regenerative fuel cell to the second tank, a second feed line connecting the second tank to a liquid inlet in the regenerative fuel cell; a second discharge line connecting a liquid output of the regenerative fuel cell to the first tank; at least one pump for delivering liquid streams from the tanks to the fuel cell for distribution on the fuel sides (i.e., QH2 and Q on discharge and charge states, respectively) and returning liquid streams from the regenerative fuel cell back to the tanks .

[0066] The fuel cell system further comprises:

[0067] C) oxygen and water supply installation, comprising:

[0068] Cl) an air pump / compressor connected to the oxygen (e.g., air) inlet of the regenerative fuel cell to supply oxygen (e.g., air) to the oxygen sides on discharge, and optionally a vent to release oxygen evolving during charge (and optionally store it) ;

[0069] C2) a water tank; and a water flow path, joining 1) an outlet of the water tank to a liquid inlet of the regenerative fuel cell and 2) and optionally an inlet of the water tank to a liquid outlet of the regenerative fuel cell; and optionally wherein the water flow path is provided with a pump and a multidirectional valve having a first state and a second state to control water flow in the system and optionally store the water.

[0070] D) optionally a cooling installation, comprising one or more heat exchangers which can be used to produce heat for external use;

[0071] E) electrical installation to collect the electricity generated by the fuel cell to a load; and a power source (e.g., DC source) to power the electrolyzer.

[0072] The invention also provides a method of storing and producing energy with the aid of a liquid hydrogen carrier (LHC) as a fuel material in a unified regenerative fuel cell with LHC and oxygen electrodes design, comprising: charging the cell by supplying an oxidized form of the LHC to the LHC electrode and feeding water to the oxygen electrode, wherein the LHC electrode is optionally catalyst free and the oxygen electrode has a bifunctional electrocatalyst deposited on its surface, while passing electrical current across the cell, thereby splitting the water over the oxygen electrode to evolve oxygen and form protons which transport through a proton exchange membrane separating the electrodes to reduce the oxidized form of the liquid hydrogen carrier over the LHC electrode, and collecting the reduced form of the liquid hydrogen carrier; discharging the cell by supplying the reduced form of the liquid hydrogen carrier to the catalyst-free electrode and pushing oxygen to the oxygen electrode, to reduce the oxygen over the oxygen electrode having the bifunctional electrocatalyst deposited thereon and collecting the oxidized form of the liquid hydrogen carrier on the LHC side and optionally water in the oxygen side.

[0073] In the method of storing and producing energy described above, and in the fuel cell system, the bifunctional electrocatalyst is preferably the Pti-99-Iri-99-Mo-99, e.g., Pts-gs-Irs-gs-Mo-so,

[0074] Pts-95- 1 rs-95- M5-50 , e.g., Pti5-75-Iri5-75-Mi5-45, for example, Pt25-75-Iris- 35-M15-35, such as Pt45-55- 1 r2o-3o- M20-30 alloy aerogel presented above, where M is preferably Ni, i.e., the electrocatalysts described above, for example, Pt45-55-Ir2o-3o-Ni2o-3o exhibiting X-ray diffraction peaks centered at ~ 40.5 20, 47.5 20 and 69.2 20 (±0.2 20) .

[0075] In the method of storing and producing energy described above, and in the fuel cell system, the LHC is preferably based on the previously presented Q / QH2 redox couple consisting of anthraquinone / anthrahydroquinone, e.g., the 9, 1 O-anthraquinone of Formula I . In the drawings

[0076] Figure 1 shows an experimental set-up from the prior art [Charvat et al. (supra) employing separate fuel cell and electrolyzer units .

[0077] Figure 2 is a schematic representation of a unified regenerative fuel cell (the hybrid fuel cell-electrolyzer device) of the invention .

[0078] Figure 3 of shows the operation a fuel cell system based on the unified regenerative fuel cell of the invention.

[0079] Figure 4 is a photograph of the Pt-Ir-Ni aerogel of Example 1.

[0080] Figure 5 is the X-ray fluorescence (XRF) spectrum of Pt-Ni-Ir aerogel of Example 1.

[0081] Figure 6 is the X-ray powder diffraction pattern of the Pt-Ni-Ir aerogel of Example 1.

[0082] Figure 7 is a voltammogram generated by linear sweep voltammetry with the Pt-Ir-Ni aerogel of Example 1 applied on a rotating disc electrode (RDE) , in three-electrode cell set-up.

[0083] Figure 8 shows polarization curves of a reversible quinone fuel cell operating with a blend consisting of Pt nanoparticles and IrOx(black) as the catalyst and a reversible quinone fuel cell with a Pt-Ni-Ir aerogel of Example 1 as the catalyst (red) .

[0084] Figure 9 shows comparison of fuel cell performance operating with Pt-Ir-Ni (of the invention) or Pt-Pd (prior art, US 2015 / 0162622) aerogel catalysts at the cathode (oxygen side) .

[0085] Figure 10 shows comparison of electrolyzer performance operating with Pt-Ir-Ni (of the invention) and Pt-Pd (prior art, US 2015 / 0162622) aerogel catalysts at the anode (oxygen side) . Examples

[0086] Example 1 Preparation of Pt-Ir-Ni aerogel catalyst and characterization of its elemental composition and crystalline structure

[0087] For the platinum precursor, 65 mg of JhPtCle • 6H2O (1.26xl0~4mol) were dissolved in 2.5 mL of deionized water (DIW) . For the Ni precursor, 20 mg of N1C12 • 6H2O (8.4xl0~5mol) were dissolved in 15 mL of DIW. For the iridium precursor, 127 mg of H2lrCle hydrate (3.1xl0~4mol) were dissolved in 2.5 mL of DIW. The solutions were mixed together in a beaker for ten minutes. After ten minutes of stirring, 10 mL of aqueous solution containing 65.6 mg of NaBH4 were added to the beaker under vigorous stirring and stirred for an additional 5 minutes. Afterwards, the beaker was placed in an oven at 60 C’for two days, whereby a hydrogel was formed. The wet gel was washed with water several times. Next, the water was exchanged with acetone by removing the solvent and refilling with acetone several times. The washed wet gel was dried in a supercritical point drier in liquid CO2 which resulted in a dark grey aerogel, shown in Figure 4.

[0088] X-ray fluorescence (XRF) spectroscopy was conducted with the aid of Horiba XGT-7200 X-ray analytical microscope to determine the elemental composition of the aerogel at an X-ray tube voltage of 50 kV and a current of 1 mA and 100 pm spot diameter. The XRF spectrum is appended as Figure 5, and the results are set out in a tabular form below, indicating the formation of a trimetallic aerogel, with Pt-Ir-Ni 2:1:1 molar ratio (Ptsi-Ir26-Ni23) .

[0089] Table 1 A sample of the aerogel was analyzed by X-ray powder diffraction. X-ray powder diffraction pattern was recorded in a Bruker D8 Advanced diffractometer in the range of 10 to 80° 20 using CuKa (1.54060 A) radiation. The X-ray diffraction pattern is shown in Figure 6, exhibiting peaks centered at ~ 40.5 20, 47.5 20 and 69.2 20, with no clear isolated peaks assigned to individual metallic particles, thus attesting to the formation of an alloy. The BET surface area of the sample was 59 m2 / g. The surface area measurement was performed by gas adsorption analysis using a Quantachrome Autosorb iQ instrument with nitrogen as the adsorbate at 77 K. Surface area was calculated using the BET equation.

[0090] Example 2 Electrochemical characterization of Pt-Ir-Ni aerogel catalyst by linear seep voltammetry

[0091] The electrochemical characterization was performed in a three- electrode cell assembly with a rotating disc electrode (RDE) , to study the activity of the Pt-Ir-Ni aerogel for ORR and OER, using a potentiostat from Biologic (VSP) . To fabricate the working electrode, a catalyst slurry was prepared by combining 5 mg of the Pt-Ir-Ni aerogel of Example 1, 0.166 ml of 0.2 wt . % Nation solution in H2O and 0.34 ml of 0.2 wt . % Nation solution in isopropanol. The suspension formed was ultrasonicated for 1 hour and was drop- casted on the RDE (PINE E5 Fixed-Disk, 5 mm diameter) , giving a catalyst loading of 0.5 mg / cm2. Reversible hydrogen electrode (RHE) and a glassy carbon rod electrode were used as the reference and counter electrodes, respectively.

[0092] To test the electrocatalytic activity of the trimetallic aerogel, linear sweep voltammetry was performed in a 0.5 M H2SO4 aqueous solution stirred at 900 rpm, as follows: the potential range was scanned in the positive direction from 1.2 to 1.7 V vs RHE at a rate of 5 mV / sec to study the OER. the potential range was scanned in the negative direction from 1.2 to 0.05 V vs RHE at a rate of 5 mV / sec under oxygen environment to study the ORR.

[0093] The voltammogram is shown in Figure 7. From the anodic curve, it is determined that the onset potential for the OER is ca. 1.4 V vs RHE and the overpotential to reach current density of 10 mA cur2is 280 mV, indicating that the Ir metal component of the aerogel catalyzes the OER effectively. Turning to the cathodic curve, the Pt / Ni also show good ORR electrocatalytic activity with an onset potential above 1 V vs RHE.

[0094] Example 3

[0095] Dual electrocatalytic performance of a mixed metal catalyst in a regenerative fuel cell

[0096] The goal of the study was to evaluate the ability of two types of bifunctional mixed-metal catalysts: Ptsi-Ir26-Ni23 aerogel of

[0097] Example 1 and a blend of Pt / IrOx, to serve a dual function in a reversible quinone fuel cell: advance oxygen reduction reaction when working in a fuel cell mode and oxygen evolution reaction, when working in an electrolyzer mode.

[0098] The Ptsi-Ir26-Ni23 aerogel (20 mg) was mixed with 7.2 mg of Nation (36 mg Nation D2020 solution, TonPower) . The Pt / IrOxblend consisted of 10 mg of high surface area Pt black mixed with 10 mg of high surface area IrOx( FuelCellStore ) , combined with 7.2 mg of Nation (36 mg Nation D2020 solution, TonPower) .

[0099] Next, each of the electrocatalysts was formulated as a sprayable suspension, by addition to 3 mL H20 / isopropanol (1:2) solution. Each suspension was ultrasonicated in an ice bath for an hour, stirred overnight and again ultrasonicated in an ice bath for an hour prior to spraying.

[0100] A reversible quinone fuel cell was assembled by applying the suspension of the Ptsi-Ir26-Ni23 aerogel to a platinized titanium fiber felt via an ultrasonic spraying machine (Sonotek) and combining it with a reinforced proton exchange membrane (Fumasep FKS-PET-130) and a porous carbon felt (GFD2.5EA, SGL carbon) for the quinone side. A similar reversible quinone fuel cell was assembled, but with Pt / IrOx-containing suspension being applied on platinized titanium fiber felt.

[0101] In both cases, the loading of the electrocatalyst was 2 mg / cm2, with the active area being 5 cm2. The prepared electrode was assembled to a Scribner fuel cell fixture which was bolted diagonally to a torque of 7 N m.

[0102] The quinone solution consisted of 1 M of anthraquinone-2 , 7- disulfonic acid (AQDS) in DIW. The oxidized AQDS solution was charged with protons by the OER reaction occurring on the anode by applying a constant 2 A until a cutoff voltage of 2.5 V was reached. The fully reduced quinone solution was fed to the same half-cell and oxygen was fed instead of water to the second half cell in order to measure the performance of the cell in fuel cell mode (AQDS oxidation and ORR) .

[0103] The cells based on the Pt-Ir-Ni aerogel and the Pt / IrOxblend were operated under the same conditions of 20 mL / min quinone flow rate, 0.5 L / min O2 flow rate with 30 psi backpressure at 80 °C. The results are shown in the form of polarization curves in Figure 8. The Pt- Ni-Ir aerogel shows superior performance compared to the Pt / IrOxblend, with almost a twofold increase in power density. Example 4 Comparison between Pt-Ir-Ni aerogel catalyst of the invention and Pt-Pd aerogel catalyst of US 2015 / 0162622

[0104] The goal of the study was to compare between two types of bifunctional mixed-metal aerogel catalysts: Ptsi-Ir26-Ni23 aerogel of Example 1 and Pt-Pd aerogel of Example 1 of US 2015 / 0162622, i.e., to assess their functionality as dual catalysts in a reversible quinone fuel cell: advance oxygen reduction reaction when working in a fuel cell mode and oxygen evolution reaction, when working in an electrolyzer mode.

[0105] The experimental set-up (cell assembly and operation) is similar to the one described in Example 3, i.e., the electrocatalysts were formulated as sprayable suspensions, applied on at a platinized titanium fiber felt at 3 mg / cm2(the oxygen electrode) , and combined with Nafion® 212 (proton exchange membrane) and carbon felt (SGL GFD 2.5EA, the quinone electrode) . Active area was 5 cm2. That is, two identical reversible cells were assembled, which differed only in the type of catalyst on the platinized titanium fiber felt (the oxygen side) . The quinone solution consisted of 1 M of anthraquinone- 2 , 7-disulfonic acid (AQDS) in DIW.

[0106] Part A - fuel cell mode

[0107] The cell operated at 80°C with fully humidified oxygen flowing to the cathode at 0.5 SLPM with 2 atm backpressure. The AQDS was supplied to the anode at 0.02 SLPM.

[0108] The results are shown in Figure 9. From the polarization curves in fuel cell mode, it can be seen that the Pt-Ir-Ni is more active for the oxygen reduction reaction, in view of the higher OCV (0.85 V for Pt-Ir-Ni, 0.75 V for Pt-Pd) and lower activation polarization in the kinetic region. Part B - electrolyzer mode

[0109] The cell operated at 80°C with water flowing to the anode at 20 ml min-1and AQDS supplied to the cathode at 0.5 SLPM. No back pressure.

[0110] The results are shown in Figure 10. Pt-Ir-Ni shows significantly better performance with lower onset potential and higher currents, indicating that the Pt-Ir-Ni is much more efficient for oxygen evolution reaction than Pt-Pd.

Claims

Claims1) Ptx-Iry-Mzaerogel, wherein x, y and z indicate atomic percentage with x+y+z=100, and M is a metal selected from the first row of transition metals, wherein Ptx-Iry-Mzis Pti-99-Iri-99-Mo-99 aerogel.2) An aerogel according to claim 1, which is Pts-gs-Irs-gs-Mo-so .3) An aerogel according to claim 2, which is Pti5-75-Iri5-75-Mi5-45.4) An aerogel according to any one of claims 1 to 3, where the metal M is Ni .5) An aerogel according to claim 4, which is Pti5-75-Iri5-75-Nii5-45.6) An aerogel according to claim 5, which is Pt25-75-Iri5-35-Nii5-35.7) An aerogel according to claim 6, which is Pt45-55-Ir2o-3o-Ni2o-3o, exhibiting X-ray diffraction peaks centered at ~ 40.5 20, 47.5 20 and 69.2 20.8) A process for preparing a bifunctional electrocatalyst, comprising combining in solution a source of Pt cation, a source of Ir cation and optionally a source of M cation, wherein M is a metal selected from the first row of transition metals, reducing the metal cations in the solution to form the corresponding metals in elemental form and obtain metal-containing gel, soaking the gel with an organic solvent to displace water molecules by the organic solvent, and supercritically drying or freeze drying the gel to obtain the Pti-99-Iri-99-Mo-99 aerogel.9) An electrode comprising a liquid / gas diffusion layer (LGDL) coated with an electrocatalyst which is the aerogel of any one of claims 1 to 7, wherein the electrode is optionally corrosion resistant .10) A unified regenerative fuel cell, comprising a first electrode and a second electrode joined to the opposite faces of a proton exchange membrane (PEM) , wherein the first electrode consists of the electrode according to claim 9.11) A unified regenerative fuel cell according to claim 10, wherein in the first electrode, a Pts-gs-Irs-gs-Nio-so aerogel is deposited on a bare titanium fiber felt, mesh or foam, or platinized titanium fiber felt, mesh or foam.12) A unified regenerative fuel cell according to claim 11, wherein in the first electrode, a Pti5-75-Iri5-75-Nii5-45 aerogel is deposited on platinized titanium fiber felt, mesh or foam.13) A unified regenerative fuel cell to any one of claims 10 to 12, wherein the second electrode is catalyst-free.14) A unified regenerative fuel cell according to claim 13, wherein the second electrode is a carbon electrode.15) A unified regenerative fuel cell stack, consisting of a plurality of individual cells according to any one of claims 10 to 14 arranged in a stack, with adjacent cells being separated by bipolar plates and cell edges sealed with gaskets.16) A unified regenerative fuel cell comprising one or of individual cells, wherein an individual cell comprises a first electrode and a second electrode joined to the opposite faces of a proton exchange membrane (PEM) , wherein the first electrode defining the oxygen side consists of an electrode with a bifunctional electrocatalyst applied thereon and the second electrode defining the fuel side is optionally catalyst-free.17) A unified regenerative fuel cell according to claim 16, fueled by a liquid hydrogen carrier.18 ) A fuel cell system comprising :A) a uni fied regenerative fuel cell according to 17 ;B ) a fuel supply and regeneration installation comprising separate aqueous solutions of the reduced and oxidi zed forms of the liquid hydrogen carrier .19 ) A fuel cell system according to claim 18 , comprising :A) a uni fied regenerative fuel cell ;B ) a fuel supply and regeneration installation comprising a first tank for holding a liquid hydrogen carrier, a second tank for holding the oxidi zed form of the liquid hydrogen carrier, a first feed line connecting the first tank to a liquid inlet in the regenerative fuel cell ; a first di scharge line connecting a liquid output of the regenerative fuel cell to the second tank, a second feed line connecting the second tank to a liquid inlet in the regenerative fuel cell ; a second discharge line connecting a liquid output of the regenerative fuel cell to the first tank, at least one pump for delivering liquid streams from the tanks to the fuel cell for distribution on the fuel sides on discharge and charge states , respectively, and returning liquid streams from the regenerative fuel cell back to the tanks ;C ) oxygen and water supply installation, comprising :Cl ) an air pump / compressor connected to the oxygen inlet of the regenerative fuel cell to supply oxygen to the oxygen side on discharge ;C2 ) a water tank and a water flow path, j oining 1 ) an outlet of the water tank to a liquid inlet of the regenerative fuel cell and 2 ) and optionally an inlet of the water tank to a l iquid outlet of the regenerative fuel cell ; and optionally wherein the water flow path is provided with a pump and a multidirectional valve having a first state and a second state to control water flow in the system;D) optionally a cooling installation, comprising one or more heat exchangers ;E) electrical installation to collect the electricity generated by the fuel cell to a load; and a power source to power the electrolyzer .20) A fuel cell system according to claim 18 or 19, wherein the bifunctional electrocatalyst on the first electrode defining the oxygen side, comprises Pt and Ir.21) A fuel cell system according to claim 20, wherein the bifunctional electrocatalyst on the first electrode defining the oxygen side, consists of Pts-gs-Irs-gs-Mo-so aerogel, wherein M is a metal selected from the first row of transition metals.22) A fuel cell system according to claim 21, wherein the bifunctional electrocatalyst on the first electrode defining the oxygen side is Pt25-75-Iri5-35-Nii5-35 aerogel.23) A unified regenerative fuel according to claim 17, or a fuel cell system cell according to any one of claims 18 to 22, wherein the liquid hydrogen carrier is a hydroquinone.24) A fuel cell system according to claim 23, using the 9,10- anthraquinone / anthrahydroquinone redox couple having the structure of Formula (I) depicted below in the oxidized form:wherein each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from:hydrogen; halogen; hydroxyl ; thiol ;-C (=0) OX1;-N (X1)2;-NO2 ;-S (=0) 2OX1;-P(=o) (ox1)2; wherein X1is H and C1-5 alkyl; and a group -X2or -OX2wherein X2is C1-7 linear or branched alkyl or alkenyl chain, optionally interrupted with a heteroatom (N, 0) and bearing a group selected from -C(=o)ox1, -S(=O)2ox1and -P(=0) (ox1)2.25) A method of storing and producing energy with the aid of a liquid hydrogen carrier (LHC) as a fuel material in a unified regenerative fuel cell with LHC and oxygen electrodes design, comprising : charging the cell by supplying an oxidized form of the LHC to the LHC electrode and feeding water to the oxygen electrode, wherein the LHC electrode is optionally catalyst free and the oxygen electrode has a bifunctional electrocatalyst deposited on its surface, while passing electrical current across the cell, thereby splitting the water over the oxygen electrode to evolve oxygen and form protons which transport through a proton exchange membrane separating the electrodes to reduce the oxidized form of the liquid hydrogen carrier over the LHC electrode, and collecting the reduced form of the liquid hydrogen carrier; discharging the cell by supplying the reduced form of the liquid hydrogen carrier to the LHC electrode and pushing oxygen to the oxygen electrode, to reduce the oxygen over the oxygen electrode having the bifunctional electrocatalyst deposited thereon and collecting the oxidized form of the liquid hydrogen carrier on the LHC side and optionally water in the oxygen side.26) A method according to claim 25, wherein the bifunctional electrocatalyst is Pts-gs-Irs-gs-Mo-so aerogel, wherein M is a metal selected from the first row of transition metals.27) A method according to claim 26, wherein the bifunctional electrocatalyst is Pti5-75-Iri5-75-Nii5-45 aerogel.28) A method according to claim 27, wherein the bifunctional electrocatalyst is Pt25-75-Iri5-35-Nii5-35 aerogel.29) A method according to any one of claims 25-28, wherein the LHC is 9, 1 O-anthraquinone redox couple of Formula (I) as defined in claim 24.

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