Concentration cell and power generation system comprising a concentration cell

Abstract

Concentration cell and power generation system comprising a concentration cell. The concentration cell comprises two half-cells separated by a membrane permeable to a single ionic species, e.g. cations, and capable of containing the same electrolyte solution at two different concentrations in their respective half-cells. Both the first and second half-cells comprise a first and second gas electrode made of porous material. The power generation system of the invention further comprises an evaporator capable of receiving heat from the external environment.

Description

[0001] CONCENTRATION CELL AND POWER GENERATION SYSTEM COMPRISING A CONCENTRATION CELL

[0002] Field of the invention

[0003] The present invention relates to a concentration cell and a power generation system comprising a concentration cell.

[0004] In many industrial processes, a large amount of waste heat is generated at low temperatures. One thinks of industrial plants such as thermal power stations, iron and steel plants, petrochemical plants, the ceramics industry, and so on. In order to improve energy efficiency and reduce the carbon footprint of such activities, the problem arises of using this waste heat, e.g. for the production of electricity, to be reused in plants or to be fed into the grid. A similar problem is found in the exploitation of solar energy: current photovoltaic panels have efficiencies in the order of 10 per cent. In contrast, thermal machines of comparable efficiency would require very small temperature jumps: for example, a thermal machine working between two temperatures Tmax=130°C and Tmin=70°C would have a theoretical efficiency of 15 per cent. Such a maximum temperature is easily attainable, either by using waste heat from industrial processes or through the use of solar thermal panels, which are less expensive than photovoltaic panels.

[0005] State of the art

[0006] The technology currently available for recovering industrial waste heat or heat from solar thermal panels is based on thermal machines using the so-called organic Rankine cycle, i.e. steam turbines that use suitable organic solvents instead of common water. This thermal machine is then coupled to an alternator used to generate the electricity. This technology is very expensive, both in terms of construction, operation and maintenance. Over time, various alternative solutions have therefore been proposed.

[0007] For example, a promising approach is illustrated in the article by A. Carati, M. Marino, D. Brogioli, ' Thermodynamic study of a distiller - electrochemical cell system for energy production from low-temperature heat sources', Energy 93, pages 984-993 (2015), in which the use of an evaporator as a thermal machine and a concentration cell as an electricity generator is proposed.

[0008] With regard to concentration cells in particular, several solutions are known.

[0009] US 3,671 ,322 shows a concentration cell with aluminium electrodes, and an electrolyte containing sodium chloride and aluminium chloride that is activated at temperatures in the range from about 125 °C to about 180 °C. Such a cell is capable of producing a voltage of approximately 0.2 volts.

[0010] US 3,414,437 shows a battery comprising two cells with separate anode and cathode mixtures, which are collected for subsequent disposal.

[0011] US 3,887,399 describes a mercury-bonded aluminium anode with an aqueous electrolyte in which the cathode is porous carbon or porous metal. The aluminium anode can be activated by the addition to it of metals such as gallium, cadmium, indium or thallium.

[0012] US 4,064,327 illustrates a concentration cell having an aluminium anode, an inert metal cathode and a two-layer pellet, with the electrolyte sandwiched between them. The electrolyte can be a combination of aluminium chloride with one or more of sodium chloride, lithium chloride, potassium chloride and tetra-substituted ammonium chloride. Cell potentials of approximately 0.6 volts are reported.

[0013] US 4, 145,483 shows accumulators comprising aluminium electrodes, aluminium halide and mixtures of alkali metal halides and metal derivatives in a graphite matrix.

[0014] EP 0 613 199 A1 proposes using hydrogen electrodes to produce electricity from two solutions, one basic and one acidic, which are connected by a membrane permeable only to H+ ions. This electrochemical device, however, is not a true concentration cell, but a simple galvanic cell. In fact, two different electrolytes are used in the two halfcells, and the chemical reaction that supports current generation is that of acid-base neutralisation.

[0015] M. Marino, L. Misuri, A. Carati, D. Brogioli, in " Proof of concept of a zincsilver battery for the extraction of energy from a concentration difference", Energies 7, 3664-3683 (2014), illustrate a concentration cell in which, in the halfcell with a lower concentration, the electrode material is consumed by entering solution, while in the half-cell with a higher concentration, there is a precipitation of material on the electrode. Thus, during the work cycle, the electrode structure changes, degrading the performance of the cell to the point where operation is prevented.

[0016] In I. Facchinetti, E. Cobani, D. Brogioli, F. La Manila, R. Ruffo, ' Thermally regenerable redox flow battery', ChemSusChem 13, issue 20, p. 5460-5467 (2020), the use of redox flow concentration cells was pioneered, in which in each of the two half-cells, the same chemical species in solution is reduced or oxidised respectively, while the electrode structure remains unchanged. This ideally allows an unlimited service life of the cell.

[0017] Object of the invention

[0018] The currently known solutions can be further improved.

[0019] In particular, the object of the present invention is to provide an innovative concentration cell for generating electric current from solutions with different concentrations.

[0020] Another object of the present invention is to provide a concentration cell with a high efficiency.

[0021] A further object of the present invention is to provide a concentration cell with a high service life.

[0022] Another object of the present invention is to provide a concentration cell that can operate with different types of electrolyte.

[0023] A further object of the present invention is to provide a concentration cell that is inexpensive and easy to maintain. Another object of the present invention is to provide a system for generating electricity comprising a concentration cell, which is suitable for generating electricity using low-temperature heat.

[0024] A further object of the present invention is to provide a system for generating electricity comprising a concentration cell with a high efficiency.

[0025] Another object of the present invention is to provide a system for generating electricity comprising a concentration cell, which is suitable for operation both at atmospheric pressure and at pressures below atmospheric pressure. Yet another object of the present invention is to provide a system for generating electricity comprising a concentration cell, which is inexpensive in both construction and operation. A further object of the present invention is to provide a system for generating electricity comprising a concentration cell, which is inexpensive and easy to maintain.

[0026] Summary of the invention

[0027] The Applicant has found that these and further objects are achieved by a concentration cell comprising a first halfcell containing an electrolyte solution at a first concentration and a second half-cell containing the same electrolyte solution at a second concentration lower than the first concentration. The concentration cell of the invention further comprises a membrane permeable to an ionic species positioned between the first and second half-cells. Both the first and second half-cells comprise a first and a second gas electrode, respectively. Said first and second gas electrode are made of porous material.

[0028] According to an embodiment, in each of said first and second electrodes, said porous material has nanometersized pores and micrometer-sized pores.

[0029] According to an embodiment, both the first and the second gas electrode are hydrogen electrodes.

[0030] According to an embodiment, the electrolyte solution comprises a strong base as solute.

[0031] According to one aspect, such a strong base can be, for example, sodium hydroxide (NaOH), potassium hydroxide (KOH), barium hydroxide (Ba(OH)2), calcium hydroxide (Ca(OH)2), magnesium hydroxide (Mg(OH)2), lithium hydroxide (LiOH).

[0032] According to a preferred embodiment, the strong base is either sodium hydroxide or potassium hydroxide. According to an embodiment, the electrolyte solution comprises water (H20) as a solvent.

[0033] According to an embodiment, the membrane of the concentration cell of the invention is only permeable to cations, e.g. it is only permeable to Na+or to K+.

[0034] According to an embodiment, the gas electrodes of the concentration cell of the invention comprise nickel (Ni).

[0035] According to a preferred embodiment, the gas electrodes of the concentration cell of the invention are obtained by sintering from a powder with ternary composition.

[0036] According to one aspect, the ternary composition powder comprises a nickel powder, a powder of a metal partially soluble in nickel (e.g. aluminium) and an inert powder (e.g. MgO, Ca2C, BaCO3, K3PO4).

[0037] The present invention also relates to an electrical power generation system comprising a concentration cell as described above. Said system for generating electrical energy further comprises a first reservoir of electrolytic solution at the first concentration, capable of supplying the first half-cell of the concentration cell, and a second reservoir of electrolytic solution at the second concentration, lower than the first concentration, capable of supplying the second half-cell of the concentration cell.

[0038] The concentration cell of the power generation system of the invention is capable of feeding an evaporator with an electrolyte solution having an intermediate concentration with respect to the first concentration and the second concentration. The evaporator is also capable of feeding the first reservoir with the electrolyte solution at the first concentration and the second reservoir with the electrolyte solution at the second concentration.

[0039] According to an embodiment, the evaporator is capable of receiving heat from an external environment. According to an embodiment, the evaporator is capable of feeding a condenser with steam obtained from the evaporation of the electrolyte solution at the intermediate concentration.

[0040] According to one aspect, the condenser is able to supply the second reservoir with the electrolyte solution at the second concentration, which is lower than the first concentration.

[0041] According to an embodiment, the evaporator is able to feed the second reservoir with an electrolyte solution comprising only the solvent. In other words, according to this embodiment, the vapour sent from the evaporator to the condenser is solute-free.

[0042] According to an embodiment, the concentration cell is able to send the electrolyte solution, at the intermediate concentration, to the evaporator.

[0043] In the following, by way of non-limiting example, a description of some preferred embodiments of the present invention will be set out. Other embodiments, not described but nevertheless within the reach of the person skilled in the art, are also included.

[0044] Brief description of the drawings

[0045] This description will be set out below with reference to the accompanying drawings, which are provided for illustrative purposes only and are therefore not limiting in scope:

[0046] Figure 1 schematically shows a concentration cell according to the invention;

[0047] Figure 2 shows schematically a system for generating electricity according to the invention;

[0048] Figure 3 shows a diagram illustrating the electrical voltage achievable with a particular design of the concentration cell according to the invention.

[0049] Detailed description of a preferred embodiment of the invention

[0050] Figure 1 shows a concentration cell 1 with an envelope 2 that defines, together with a membrane permeable 3 to an ionic species, a first half-cell 4 and a second half-cell 5.

[0051] A first hydrogen electrode 6 is positioned inside the first half-cell 4 and a second hydrogen electrode 7 is positioned inside the second half-cell 5.

[0052] Hydrogen electrodes are a special type of gas electrode and, in recent times, their use has become increasingly widespread in the field of alternative energies. In hydrogen electrodes, the reaction takes place:

[0053] One of the most critical parameters of hydrogen electrodes is the so-called exchange current density jo , which determines the current flowing through the electrode when a potential difference q is applied.

[0054] Table 1 shows some values of jo for different metals:

[0055] The values given in Table 1 are really only indicative, since the exchange current density jo depends not only on the type of material the electrode is made of, but also on the composition of the specific electrolyte solution in which it operates. In any case, it is well known that all noble metals perform better (up to three orders of magnitude) than other materials, and even using e.g. platinum, large surface areas (of the order of one square metre) would be required to obtain currents of the order of a few amperes. There is therefore a need to increase the surface area of the electrodes as much as possible: this requires that the electrodes are porous, i.e. the true surface area is several orders of magnitude greater than the apparent surface area. In particular, the smaller the pore size, the greater the true surface area. Since the use of nickel (cheaper and more readily available than noble metals) tends to be preferred in the manufacture of electrodes, it is necessary that the pores are of nanometer size. However, with pores that are too small, the gas may not diffuse unless it is subjected to pressures of hundreds of atmospheres. It is therefore necessary to have pores with two different size scales: pores with a size in the order of a micron, to allow the hydrogen to diffuse into the electrode, and pores with a size in the order of a nanometre, to have an adequate electrode surface and thus significant current production.

[0056] Thus, the porous material of the first and second electrodes 5, 6 has both nanometer-sized and micrometer-sized pores.

[0057] The Applicants note that nano-sized pores are preferably understood to mean pores with a size between 1 nm and

[0058] 100 nm.

[0059] The Applicants note that micrometer-sized pores are preferably understood to mean pores with a size between 1 μm and 100 μm.

[0060] To achieve these purposes, the hydrogen electrodes of the invention are obtained by sintering from a powder of ternary composition. A nickel powder combined with a powder of a metal partially soluble in nickel (e.g. aluminium), and an inert powder (MgO, Ca2C, BaCO , K33PO4) are used. The powder is then sintered in an oven with an inert atmosphere (Ar or N2) to obtain a compact electrode of the desired shape. The inert powder is then removed using an appropriate solvent (a solution of water and glycerine, or water and a weak acid, depending on the type of powder), resulting in a structure with micrometer-sized pores. By then subjecting the electrode to a strong base, the second metal is removed, thus forming the second nanometer-sized pore structure. The first hydrogen electrode 6, located in the first half-cell 4, and the second hydrogen electrode 7, located in the second half-cell 5, are for example both made using such a technique. They are immersed in an electrolyte solution. Preferably, water is used as the solvent and a strong base, e.g. sodium hydroxide (NaOH) or potassium hydroxide (KOH), as the solute. In each case, the solution is the same in both half-cells, although at different concentrations: in the first half-cell 4, the concentration of the solution is higher than in the second half-cell 5.

[0061] In the case of using NaOH and a membrane permeable 3 to cations only (i.e., in this case, to Na+), the operation of the concentration cell 1 is as follows: first, a flow of ions Na+proceeds from the half-cell 4 containing the electrolyte solution with the higher concentration to the half-cell 5 containing the electrolyte solution with the lower concentration; at the same time, the following reactions occur at the two hydrogen electrodes 6, 7: the reaction H2→ 2H++ 2e_at the electrode immersed in the solution a higher concentration; the reverse reaction 2H++ 2e_→ H2at the electrode immersed in the solution at a lower concentration.

[0062] The net result is a transfer of NaOH from the solution of higher concentration to that of lower concentration, and correspondingly a current is generated in an external circuit connecting the electrodes (obviously with an open circuit the reactions do not proceed). The hydrogen produced at the electrode immersed in the solution of lower concentration is consumed at the other electrode. In this case, the hydrogen performs the function of the salt bridge found in conventional galvanic cells.

[0063] During the operation of the concentration cell, both the first and second electrodes are constantly supplied with hydrogen via appropriate conduits 12, 13. Similarly, the first half-cell 4 is constantly supplied with high concentration solution via a first adduction conduit 8, while the second half-cell 5 is constantly supplied with low concentration solution (at the limit, pure solvent) via a second adduction conduit 9. Simultaneously, an intermediate concentration solution is extracted from the first and second half-cells 4, 5 by respective outflow ducts 10, 11.

[0064] As an example, figure 3 shows the voltage obtainable with a concentration cell 1 of the type described above, comprising a GEFC 112 membrane, permeable in this case only to cations, which is commercially available and used, for example, in fuel cells. The concentration of the semi-cell 4 at the higher concentration is fixed at 0.3 M, and in figure 3 the potential difference at the ends of the electrodes as the concentration of the solution at the lower concentration varies is shown in the ordinates, whereas the concentration of the solution at the lower concentration is shown in the abscissas.

[0065] A system 100 for generating electricity according to the invention is illustrated in Figure 2. Such a system comprises an evaporator 101 containing an electrolyte solution 102 having an intermediate concentration. In particular, the evaporator 101 contains the same electrolyte solution present in the first and second half-cells 4, 5; the electrolyte solution present in the evaporator 101 has intermediate concentration with respect to the first concentration (i.e., the concentration of the solution present in the first half-cell 4) and the second concentration (i.e., the concentration of the solution present in the second half-cell 5). The evaporator 101 operates by receiving heat from an external source 200, for example from an industrial waste heat source. This heat generates, at the outlet of the evaporator, a flow of electrolytic solution at high concentration 112, or in any case at a higher concentration than the electrolytic solution 102 at intermediate concentration. This flow is conveyed, via the conduit 110, to the first reservoir 111 and from there, via the first adduction conduit 8, the high concentration solution 112 is fed inside the first half-cell 4 of the concentration cell 1 .

[0066] The external heat source also generates, on leaving the evaporator 101 , a flow of steam which is sent, via a conduit 104, to a condenser 105. The steam is condensed therein and, via a conduit 107, is collected, in the form of pure solvent or low concentration solution 109, in the second reservoir 108. From the second reservoir 109, the pure solvent or low concentration solution is fed, via the second inlet conduit 9, into the second half-cell 5 of the concentration cell 1 .

[0067] The concentrations of the electrolyte solutions contained within the first and second reservoirs can be calibrated as desired prior to their introduction into the first and second half-cells, in order to optimise the performance of the concentration cell 1.

[0068] The operation of concentration cell 1 generates a current in an external electric circuit (not shown in the figures) connecting electrodes 300.

[0069] Flow ducts 10 and 11 , exiting the first and second half-cells of the concentration cell respectively, collect an electrolyte solution of intermediate concentration 102 which is conveyed via duct 117 to the evaporator 101.

[0070] In order for the thermodynamic efficiency of the evaporator to be high, the boiling point of the electrolyte solution must be as high as possible compared to the boiling point of the pure solvent. This is why it is preferable to use, as described above, strong electrolytes, e.g. concentrated solutions of sodium hydroxide (NaOH) or potassium hydroxide (KOH), which show a boiling temperature rise (compared to pure water) of more than 60 °C at atmospheric pressure. Clearly, to increase the evaporator efficiency of the system, it is also possible to work at lower pressures than atmospheric. Overall, the system of the present invention makes it possible to achieve ideal thermodynamic efficiencies of the order of 15%.

[0071] Reference list

[0072] 1 concentration cell

[0073] 2 envelope

[0074] 3 permeable membrane

[0075] 4 first half-cell

[0076] 5 second half-cell

[0077] 6 first hydrogen electrode

[0078] 7 second hydrogen electrode

[0079] 8 first adduction duct

[0080] 9 second adduction duct

[0081] 10 first outflow duct 11 second outflow duct

[0082] 12 first hydrogen filling line

[0083] 13 second hydrogen filling line

[0084] 100 power generation system

[0085] 101 evaporator

[0086] 102 intermediate concentration electrolyte solution

[0087] 104 steam flow duct

[0088] 105 condenser

[0089] 107 conducted pure solvent or low concentration solution

[0090] 108 second reservoir

[0091] 109 pure solvent or low concentration solution

[0092] 110 conducted high concentration solution

[0093] 111 first reservoir

[0094] 112 highly concentrated solution

[0095] 117 conducted intermediate concentration solution

[0096] 200 external heat source

[0097] 300 electrodes

Claims

AMENDED CLAIMS received by the International Bureau on 09 May 2025 (09.

05. 2025)1. Concentration cell (1) comprising a first half-cell (4) containing an electrolyte solution at a first concentration; a second half-cell (5) containing said electrolyte solution at a second, lower concentration than the first concentration; a membrane permeable (3) to an ionic species positioned between said first and second half-cells, characterised by the fact that the first half-cell (4) comprises a first gas electrode (6) and the second half-cell (5) comprises a second gas electrode (7), and from the fact that the first gas electrode (6) and the second gas electrode (7) are made of porous material, and by the fact that in each of said first and second electrodes (5, 6), said porous material has pores of nanometer size and pores of micrometer size.

2. Concentration cell (1) according to claim 1 , in which said first and second electrodes (5, 6) are obtained by sintering from a powder of ternary composition.3 . Concentration cell (1) according to any of the preceding claims, in which the first gas electrode (6) and the second gas electrode (7) are hydrogen electrodes.

4. Concentration cell (1) according to any of the preceding claims, in which the electrolyte solution comprises a strong base as solute.5 . Concentration cell (1) according to claim 4, in which the strong base is selected from the group consisting of sodium hydroxide, potassium hydroxide, barium hydroxide, calcium hydroxide, magnesium hydroxide.6 . Concentration cell (1) according to any of the above claims, in which the membrane permeable (3) to an ionic species is permeable to cations.7 . System (100) for the generation of electric power comprising a concentration cell (1) according to any of the preceding claims, further comprising a first reservoir (111) of electrolyte solution at the first concentration, capable of feeding the first half-cell (4) of the concentration cell; a second reservoir (108) of electrolyte solution at the second concentration lower than the firstconcentration, capable of supplying the second half-cell (5) of the concentration cell, characterised by the fact that the concentration cell (1) is capable of feeding an evaporator (101) with said electrolyte solution having an intermediate concentration with respect to the first and second concentration, said evaporator (101) being capable of feeding the first reservoir (111) with the electrolyte solution at the first concentration and the second reservoir (108) with the electrolyte solution at the second concentration.8 . System (100) for generating electricity according to claim 7, wherein the evaporator (101) is capable of receiving heat from an external environment (200).9 . System (100) for generating electrical energy according to any one of claims 7-8, wherein the evaporator (101) is capable of feeding a condenser (105) with steam obtained from evaporation of the electrolyte solution at the intermediate concentration, and wherein the condenser (105) is capable of feeding the second reservoir (108) with the electrolyte solution at the second concentration.10 . System (100) for generating electricity according to any one of claims 7-8, wherein the evaporator (101) is capable of supplying a condenser (105) with steam obtained from evaporation of the electrolyte solution at the intermediate concentration, and wherein the condenser (105) is capable of supplying the second reservoir (108) with pure solvent.

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