"System and method for generating gaseous hydrogen on demand"

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TEXT OF THE DESCRIPTION

Field of the invention

The present invention relates to a system and a method for generation of gaseous hydrogen.

Prior art and general technical problem

The rapid exhaustion of fossil fuels and, at the same time, the increase of pollution and of the corresponding environmental risks have promoted the search for new energy sources.

In line with the recent development on a large scale of renewable energy resources (solar energy, wind power, tidal power), there has arisen an evergrowing need to develop an efficient technology for energy storage in such a way as to render as stable as possible supply of the energy converted starting from these sources, as well as to be able to modulate the supply of energy during periods of maximum and minimum consumption (in particular, as regards electrical energy).

The improvements made available in the field of renewable energy sources hence constitute the starting point for the search for improved forms of energy storage, and have likewise given rise to the need for effective storage solutions also for energy coming from non-renewable sources.

The development of energy- storage technologies may be essential, for example, also in view of a transition towards sustainable production of energy. In particular, energy storage enables energy production to be rendered independent of energy consumption, thus reducing the need for a constant monitoring and prediction of the user peak demand.

Energy storage provides tangible economic benefits in so far as it enables reduction of energy production of a plant. In other words, the target of production in energy terms would be that of an average demand instead of a peak demand.

The energy-transmission lines, along with the associated equipment, can thus be sized to meet average power demands. In addition, energy storage mitigates some problems associated to the intermittence in generation of energy from renewable sources (primarily, solar energy and wind and tidal power).

Renewable energy sources with efficient solutions of energy storage can guide transition from the traditional centralized production of power such as that from coal-fired, gas-fired, nuclear-fired plants, which require a grid for longdistance transmission of energy, towards systems with DER (Distributed Energy Resource) systems.

DER systems include, for example, small power generation sources located in the proximity of the place of use of the electrical energy (e.g., for domestic or professional use). An efficient energy-storage solution hence provides an alternative to the costly improvement of the traditional electric-power distribution grid.

Recourse to DER systems constitutes a nimbler and more economic option as compared to the construction of large centralized power-generation plants and associated high-voltage power-transmission lines.

DER systems in fact offer users the possibility of benefiting from reduced costs, a higher reliability of the service, a higher quality of the power generated, a greater energy efficiency, as well as also energy independence.

The use of renewable technologies for generation of electrical energy of a distributed type, together with the so-called "green energies", such as wind power, solar (photovoltaic) energy, geothermal energy, energy from biomasses, or energy from hydroelectric production, can also provide a significant environmental benefit.

Energy-distribution devices in DER systems comprise a panorama of technologies including fuel cells, micro-turbines, reciprocating motors, and technologies for reduction of the load and for energy management. DER technology also includes electronic power interfaces, as well as communication and control devices for the distribution and efficient operation of the individual generation units, multiple packet systems, and aggregated supply blocks.

The primary fuel for many distributed generation systems is natural gas, but hydrogen is a natural candidate for future uses.

Gaseous hydrogen is an effective energy carrier with a very high specific energy content (approximately, 120 MJ/kg). It has been demonstrated that hydrogen can be used for transport, heating, and electric-power generation, and can replace all fuels commonly used in currently available applications.

Hydrogen is the lightest and most abundant element on Earth. However, unlike oxygen, hydrogen is not available in free form in nature in significant concentrations. Hydrogen is produced using both renewable energy sources and non-renewable energy sources, exploiting a wide range of process technologies. The technologies available for the production of hydrogen include reforming of natural gas, gasification of carbon from biomasses, and water splitting by electrolysis, photo-electrolysis, photo-biological production, thermochemical cycles of water splitting, and high-temperature decomposition.

The main processes for the production of hydrogen are in general water electrolysis and reforming of natural gas.

Photo-electrolysis, photo-biological production, and high-temperature decomposition are - instead - in their initial development stage. Consequently, there are still required many research stages to render these technologies mature for commercial applications, a fact that does not make it an attractive solution in the short term.

At the moment, various technologies for storage of hydrogen are available. Some of them will be briefly described in what follows.

The simplest technology consists in storage of gaseous hydrogen in compressed form. It is a solution that can be pursued at ambient temperature, and management of the steps of compression and release of hydrogen is likewise very simple. However, the storage density (understood in terms of energy density) is very low if compared to other processes.

Another technology available is storage of hydrogen in liquid form. However, it should be borne in mind that the energy expenditure for liquefying gaseous hydrogen ranges from 25% to 45% of the energy stored, which already in part limits the convenience of the process.

The storage density of hydrogen (understood in terms of energy density) is very high. However, the practical efficiency of the process is further jeopardized by the fact that hydrogen has a boiling point of approximately -253°C, which renders necessary provision of cumbersome layers of insulating material to keep the temperature below this value (otherwise, hydrogen simply passes into the vapour phase).

There are then the technologies of storage of hydrogen in substrates of metal hydrides. These technologies envisage that metals in powder form absorb hydrogen at high pressures. The storage process is moreover associated to the release of heat, and is rendered reversible by reducing the pressure and supplying heat.

The main problem of the above technologies consists in the weight of the material that provides the absorbent substrate. A storage tank with substrate of metal hydrides would weigh approximately 600 kg, an enormous weight if compared to the 80 kg of a tank of compressed hydrogen of comparable size.

Yet a further hydrogen-storage technology exploits absorption of hydrogen by carbon. By applying a given pressure hydrogen binds to porous carbon materials, such as for example nanotubes.

However, the latter solution is particularly burdensome from the standpoint economic and particularly complex in management for the majority of the commercial applications of interest.

Object of the invention

The object of the invention is to solve the technical problems mentioned previously. In particular, the object of the invention is to provide an effective and economic solution for energy storage aimed at the production of hydrogen, also with recharging cycles that will be characterized by a high energy-storage density and that will likewise enable release of hydrogen on demand.

Summary of the invention

The object of the present invention is achieved by a system and a method having the features that form the subject of the appended claims, which form an integral part of the technical disclosure provided herein in relation to the invention.

In particular, the object of the invention is achieved by a system for generating gaseous hydrogen on demand, including:

- a first storage environment for a first electrolytic solution, said first electrolytic solution including water and a first alkaline electrolyte;

- a first supply unit for said first electrolytic solution;

- at least one deposition electrolytic cell, which includes an anode and a cathode and is configured for processing a flow of the first electrolytic solution supplied, during operation, by means of said first supply unit;

- a second storage environment configured for collecting a metal that, during operation, is deposited at the cathode of the at least one deposition electrolytic cell; and

- at least one generation galvanic cell configured for processing a flow of a second electrolytic solution and metal collected in the second storage environment and releasing gaseous hydrogen and electrical energy.

The object of the invention is moreover achieved by a method for generating gaseous hydrogen on demand, the method comprising: - a step of supply of a first electrolytic solution from a first storage environment to at least one deposition electrolytic cell, said first electrolytic solution including a first alkaline electrolyte and water, each deposition electrolytic cell including an anode and a cathode and being controlled for processing said first electrolytic solution to obtain deposition of a metal, in particular zinc, on said cathode;

- a step of storage of the metal deposited on said cathode in a second storage environment; and

- a step of supply of the metal stored in the second storage environment and of a second electrolytic solution that includes a second alkaline electrolyte and water to at least one generation galvanic cell, each generation galvanic cell being controlled for processing the flow of said second electrolytic solution and said metal stored in the second storage environment for the release of gaseous hydrogen and electrical energy.

Brief description of the drawings

The invention will now be described with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

- Figure 1 is a schematic block diagram of a system according to the invention;

- Figures 2A and 2B are two schematic circuit representations of two sections of the system according to the invention, where Figure 2B is the prosecution of Figure 2 A starting from the reference A marked with a dashed-and- dotted line in Figure 2A;

- Figure 3 is a schematic sectioned perspective view of an deposition electrolytic cell of the system according to the invention;

- Figure 4 is cross-sectional view corresponding to that of Figure 3;

- Figure 5 is a schematic sectioned perspective view of a generation galvanic cell of the system according to the invention;

- Figure 6 is cross-sectional view corresponding to that of Figure 5;

- Figures 7 and 8 are views similar to that of Figure 6 and illustrate further equipment of the galvanic cell;

- Figure 9 substantially corresponds to the view of Figure 2 A, but regards a further embodiment of the system;

- Figure 10A and Figure 10B illustrate schematic cross-sectional views of a component of Figure 9;

- Figure 11 illustrates an exploded perspective view of a galvanic cell used in yet a further embodiment of the system according to the invention;

- Figure 12 is a cross-sectional view of the cell of Figure 11;

- Figure 12A illustrates a variant of the circuit associated to a specific embodiment of the galvanic cells;

- Figure 13 is a schematic view of a battery (the so-called stack) of galvanic cells of the type of Figure 12;

- Figure 14 is a schematic view of an embodiment of an electrode of the cell of Figures 11 to 13; and

- Figure 15 is an equivalent electrical diagram of a generation section including a stack made up, by way of example, of four generation cells.

Detailed description of the invention

The reference SY in Figure 1 identifies a system for the generation of gaseous hydrogen on demand according to various embodiments of the invention. The system SY includes an energy-storage section designated by the reference ST and a section for generation of hydrogen and electrical energy designated by the reference GE.

As may be seen from the diagram of Figure 1, the energy-storage section is supplied with a first electrolytic solution taken from a first storage environment designated by the reference 1. The first storage environment 1 is supplied, albeit not necessarily in an exclusive way, by means of the products of reaction of the generation unit GE.

The storage section ST releases gaseous oxygen into the atmosphere and releases as product of reaction a second electrolytic solution and a metal in substantially pure form, which can be stored, in combination or alternatively, in a second storage environment designated by the reference 2.

The second storage environment 2 provides the supply for the generation section GE, which yields electrical energy, gaseous hydrogen that supplies a fuel cell FC (or in general a user device, such as a boiler or an internal-combustion engine), and a third electrolytic solution, which preferably has the same composition as the first electrolytic solution (substantially there is a reintegration of electrolyte, and hence a reintegration of the first electrolytic solution) and supplies the first storage environment 1.

As may be seen in Figure 1, the fuel cell FC releases water (H ₂ 0), which is preferentially supplied again into the storage section ST (as an alternative to the generation section GE) to constitute or reintegrate the first electrolytic solution (in this case as solvent, and not as electrolyte).

Given this initial circuit and functional description of the system SY, consider now the following detailed descriptions of the storage section ST and of the generation section GE. With reference to Figure 2A, the generation section ST includes the first storage environment 1 for the first electrolytic solution. In the preferred example considered herein, the first electrolytic solution is an aqueous solution including water and a first alkaline electrolyte, in particular potassium zincate (K ₂ Zn(OH) ₄ ).

The first storage environment 1 is preferentially a service tank, in which a certain amount of the first electrolytic solution is accumulated. This amount may possibly be integrated with the contribution of an external tank of larger size, separate from the system SY.

The first storage environment 1 is in fluid communication with a first supply unit PI, in particular a pump, the intake port of which is in fluid communication with an outlet port of the storage environment 1.

The storage section ST further includes at least one deposition electrolytic cell, which is configured for processing the first electrolytic solution coming from the tank 1. Even though embodiments may be envisaged in which just one deposition electrolytic cell is present, in the preferred embodiment illustrated herein there are present a plurality of electrolytic cells (three in this case) CI, C2, Cn, where the last reference Cn indicates that the number may even be different from (greater or smaller than) three, which is the number represented.

With combined reference to Figure 2 A and to Figures 3 to 4, each cell CI,

C2, Cn is an electrolytic cell, which includes an anode AN_C, in this case a positive electrode, and a cathode CT C, in this case a negative electrode, and preferably has a cylindrical geometry.

This means that the anode AN C is obtained as an elongated cylindrical element surrounded by a cylindrical shell constituting the cathode.

The anode AN C may be made of nickel, nickel alloys, nickel-based composite materials (e.g., activated carbon + powdered nickel + PTFE as binder, or else Raney nickel + PTFE as binder), or stainless steel. As an alternative, it may comprise a support (e.g., made of steel), applied on which are metals such as platinum and/or palladium, having the function of catalytic components. The cathode CT C may be made, for example, of zinc, stainless steel, or nickel or its alloys. More in general, the cathode CT C is made of a conductive material resistant to the alkaline solution and on which the zinc will have low adherence.

Defined between the anode AN C and the cathode CT C is a toroidal reaction volume, designated by the reference VC, where electrolysis takes place. The anode AN C and the cathode CT C are moreover connected to an electric- power supply source, in particular a voltage generator, preferentially an electronically controlled voltage generator.

Associated to the plurality of deposition electrolytic cells CI, C2, Cn are a first supply manifold Ml and a first discharge manifold M2. The terms used herein are not to be understood in an exclusive or limiting way with respect to the technical function thereof. As will be seen hereinafter, the supply manifold Ml and the discharge manifold M2 may occasionally reverse their function depending upon the step of the method of generation of hydrogen that is in progress.

The first supply manifold Ml comprises a first working port Mi l configured for receiving a flow of the first electrolytic solution from the first supply unit PI and further includes a plurality of second working ports Ml 2, present in which is one port M1 2 for each cell CI, C2, Cn. Each working port M1 2 is in particular in fluid communication with the reaction volume VC and is configured for conveying the first electrolytic solution into the corresponding deposition electrolytic cell.

The first discharge manifold M2 in turn includes a first working port M2 1 for each deposition electrolytic cell CI, C2, Cn, where each working port M2 1 is configured for receiving a flow of product of reaction from the deposition cells CI, C2, Cn. The product of reaction of the cells CI, C2, Cn includes a second electrolytic solution, which in turn includes water and a second alkaline electrolyte (potassium hydroxide - KOH), as will be described hereinafter.

The first discharge manifold M2 further includes a second working port M2 2 configured for discharging the flow of product of reaction of the deposition cells CI, C2, Cn.

The first working port Mi l of the first supply manifold Ml and the first supply unit PI, in particular a delivery port of the pump that defines the unit PI, are in fluid communication regulated by a first valve VI, which can be switched between an open position and a closed position. The second working port M2 2 of the first discharge manifold M2 is in fluid communication with a circuit node J, and the fluid communication of the port M2 2 with the circuit node J is regulated by a second valve V2, which can be switched between an open position and a closed position.

The first discharge manifold M2 further includes a third working port

M2 3 in fluid communication with the first supply unit PI, in particular with the aforesaid delivery port. The fluid communication between the third working port M2 3 and the first supply unit PI is regulated by a third valve V3, which can be switched between an open position and a closed position.

Finally, the first supply manifold Ml in turn includes a third working port

M1 3, which is also in fluid communication with the circuit node J. The fluid communication between the port M1 3 and the circuit node J is regulated by a fourth valve V4, which can be switched between an open position and a closed position.

The circuit node J is moreover in fluid communication with:

- the first storage environment 1, in particular with an inlet port thereof, in which the fluid communication is regulated by a fifth valve V5, which can be switched between an open position and a closed position; and

- the second storage environment 2, in particular an inlet port thereof, in which the fluid communication is regulated by a sixth valve V6, which can be switched between an open position and a closed position.

The storage unit ST further includes a filter element Fl set downstream of the circuit node J, in particular set between the circuit node J and the fifth valve V5, and the circuit node J and the sixth valve V6. The filter element Fl is substantially located upstream of a bifurcation that starts therefrom and connected to which are the valves V5 and V6.

The generation section GE will now be described with reference to Figure 2B, which provides, for clearer reference, also a portion of the section already illustrated in Figure 2A and corresponding to the circuit section comprised between the references A and B.

The generation section GE includes at least one generation galvanic cell configured for release of gaseous hydrogen and electrical energy. Even though embodiments may be envisaged that include just one generation galvanic cell, in the preferred embodiment illustrated herein a plurality of galvanic cells are present (three in this case) Dl, D2, Dn, where the last reference Dn indicates that the number may also be different from three (either greater or smaller), which is the number represented.

With reference to Figures 5 and 6, each generation galvanic cell includes an anode AN D (in this case a negative electrode) and a cathode CT D (in this case a positive electrode) and is configured for processing an electrolytic solution including an alkaline electrolyte and a metal dispersed therein, corresponding to the metal deposited by electrolysis in the reaction cells CI, C2, Cn. Following upon processing, each cell is configured for releasing gaseous hydrogen, electrical energy, and a third electrolytic solution. It should be noted that the anode AN D, as will be described in what follows, is physically and functionally constituted at the moment of introduction of zinc (or in general of a metal deposited on the cathode CT C and subsequently removed) within the cell, it being defined by the zinc itself.

In the case where, as in the preferred embodiment illustrated herein, the flow of chemical species that supplies the cells Dl, D2, Dn corresponds to a number of products of reaction of the deposition electrolytic cells, and in particular those stored in the storage environment 2 (i.e., not only the metal deposited by electrolysis, but also the second electrolytic solution of water and potassium hydroxide - KOH), the third electrolytic solution has the same composition as the first and functions as reintegration of the electrolytic solution in the first storage environment 1.

With reference to Figures 5 and 6, preferably the cells Dl, D2, Dn are cylindrical cells that include a hollow cylindrical shell and an elongated cylindrical element that is coaxial to the cylindrical shell and defines the cathode CT D. The cathode CT D is preferably obtained from a porous element made of nickel foam (or else a nickel mesh, or a stainless-steel mesh) (the so-called "hydrogen evolving electrode"). The cylindrical shell functions as current collector and may be made of metal material or even of plastic material, with application of a copper plate within it, preferably enriched with tip collectors BR.

The cells Dl, D2, Dn are moreover closed at a base by a blind bottom plate BT. Set in the toroidal volume comprised between the cathode CT D and the cylindrical shell is a cylindrical porous membrane PM, preferentially a polymeric membrane. The membrane PM is porous in regard to liquids and gases, but withholds the powdered zinc. In this way, it will be appreciated, the anode AN D may be constituted, which corresponds to a toroidal volume of powdered zinc contained within the volume VD1.

There are thus identified two notable volumes of the galvanic cell Dl, D2, Dn identified by the references VD1 and VD2, where these volumes are concentric toroidal volumes (VD1 is external with respect to VD2), which are configured for exchange of mass and energy with the external environment.

With reference to Figure 2B, associated to the plurality of galvanic cells for generating gaseous hydrogen are a second supply manifold M3 and a second discharge manifold M4.

The second supply manifold M3 includes a first working port M3 1 in fluid communication with the second storage environment 2, in particular with an outlet port thereof, where the fluid communication is regulated by a seventh valve V7, which can be switched between an open position and a closed position.

The second supply manifold M3 further includes a second working port M3 2 for each generation cell Dl, D2, Dn, configured for supplying the corresponding generation cell with the electrolytic solution and the metal dispersed therein, in the embodiment illustrated the second electrolytic solution and the metal stored in the environment 2. For this purpose, each working port M3 2 is in fluid communication with the volume VD1, as illustrated in Figures 5 and 6.

The presence of conductive electrolyte (KOH) is a cause of energy losses due to the onset of shunt currents between adjacent cells Dl, D2, Dn. To overcome this drawback, a "mechanical" solution may be adopted, by inserting a device for physical separation between the manifold and the cells, such as a rotary feed tube distributor of the type illustrated in US 6,162,555 (which can in general be applied regardless of the geometry of the cells, but is preferably applied in the case of plane cells, which will be described hereinafter), or else an "electrical" solution using the so-called shunt resistors. This corresponds to the formation of paths for intake and discharge of the second electrolytic solution (considered with evolving composition) into/from the corresponding generation cell, where the length and section are chosen in such a way as to increase the electrical resistance of the electrolytic solution contained therein and limit the dispersed current. An example of equivalent electrical diagram of the system is illustrated in Figure 15, where the shunt resistances are indicated with the following notation:

Rmi, Rmo are the shunt resistances of the inlet manifold (M3) and discharge manifold (M4); Ri, Ro are the shunt resistances of the channels at inlet to and outlet from the cells Dl, D2, D3, Dn; and

Rc is the internal resistance of the cell (Vo is the voltage to the electrodes of the cell; LD is the electrical load applied to the stack of cells).

The second discharge manifold M4 includes a first working port M4 1 for each generation cell Dl, D2, Dn, which is configured for receiving gaseous hydrogen and the third electrolytic solution from the corresponding cell. For this purpose, there exists a fluid communication between the volume VD2 and the corresponding port M4 1, as illustrated in Figures 5 and 6.

The membrane PM is permeable in regard to the electrolytic solution that supplies the galvanic cell, but is impermeable in regard to the metal dispersed in the electrolytic solution itself, which in this way remains confined between the cylindrical shell and the membrane PM. The electrolytic solution can, instead, traverse the membrane PM and invade the volume VD2.

The anode AN D and the cathode CT D of the galvanic cells Dl, D2, Dn are preferentially electrically connected together by means of two-position switches (open/closed, see switch SW1 in Figure 7) or rheostatic switches (see switch SW2 in Figure 8), which can be activated by the user, according to the demand for electrical energy, by means of an ordinary manual or electronic command. In the case of cells Dl, D2, Dn connected together in series, the terminals of the rheostatic switch coincide with the terminals of the series.

As has been mentioned, the cylindrical shell of the cells Dl, D2, Dn is moreover preferentially provided, on part or all of its inner surface, with an array of tip electrodes BR, which, as will be seen hereinafter, are configured for conveying within the anode AN_D the electricity that enters the cell itself.

The second discharge manifold M4 further includes a second working port M4 2 configured for discharging the third electrolytic solution towards an intake environment of a second supply unit P2, which in particular includes a second pump, which is configured for circulation of the third electrolytic solution within the generation section GE.

Finally, the second discharge manifold M4 includes a third working port M4 3, configured for discharging gaseous hydrogen towards the outside, regulated by a valve G2. To the valve G2 there may preferentially be connected the inlet of the fuel cell FC.

The second supply unit P2, in particular a delivery port of the corresponding second pump, is moreover in fluid communication with the first working port M3 1 of the second supply manifold M3, where the fluid communication is regulated by an eighth valve V8, which can be switched between an open position and a closed position.

Furthermore, the supply unit P2, in particular a delivery port of the corresponding second pump, is in fluid communication with the first storage environment 1, in particular the inlet port thereof. The fluid communication is regulated by a ninth valve V9, which can be switched between an open position and a closed position.

Operation of the system SY is described in what follows. With the aid of all the figures described herein, stored within the first storage environment 1 is a certain amount of the first electrolytic solution, which in the preferred embodiment considered herein includes water and a first alkaline electrolyte consisting of potassium zincate (K ₂ Zn(OH) ₄ ).

The method for generation of gaseous hydrogen on demand according to the invention comprises a first step of supply of the first electrolytic solution from the first storage environment 1 to the one or more deposition electrolytic cells CI, C2, Cn.

During this step, the valves VI, V2 and V5 are kept in the open position, whereas the valves V3, V4 and V6 are kept in the closed position. This means that an obligate path is defined for the electrolytic solution, set in circulation by means of the supply unit PI, which envisages traversal of the valve VI, invasion of the manifold Ml through the port Mi l, and entry into the cells CI, C2, Cn, through the ports Ml_2.

The electrolytic solution that enters the volume VC of the cells CI, C2, Cn is then subjected to electrolysis. For this purpose, the cells CI, C2, Cn are electrically supplied with a voltage applied between the anode AN C and the cathode CT C so as to trigger the following reaction:

K ₂ Zn(OH) ₄ → 2KOH + Zn + H ₂ 0 + (l/2)0 ₂ .

The partial reactions that develop at the cathode CT C and at the anode AN_C are instead the following:

Cathode

Zn ²⁺ + 2e ^" → Zn Anode

20H ^" → H ₂ 0 + (l/2)0 ₂ + 2e ^"

From the reactions referred to above it is evident how the first electrolytic solution is electrolysed, producing as first product of reaction a second alkaline electrolyte, potassium hydroxide (KOH), which is useful in the subsequent step of generation of hydrogen in the section GE.

A second product of reaction is (basically) pure metallic zinc, which deposits on the cathode in the spongy aggregate form of metal particles.

A third product of reaction is water (H ₂ 0), which, together with potassium hydroxide (KOH), will provide - according to the modalities that will be described shortly - the second electrolytic solution referred to above.

A fourth product of reaction is gaseous oxygen (0 ₂ ), which is released into the atmosphere through the port M2 4 and the valve Gl .

The person skilled in the art will on the other hand appreciate that operation of the cells CI, C2, Cn envisages an axial flow (either purely axial or with a prevalently axial component) from one end to the other end of the cells. The supply (ports M1 2) is preferably from beneath, and the outlet of the liquid products of reaction (second electrolytic solution) and gaseous products of reaction (oxygen) through the ports M2 1 is consequently from above.

The reaction of electrolysis occurs substantially in a regime of permanent flow. The circulation of the electrolytic solution of potassium zincate (K ₂ Zn(OH) ₄ ) through the cells CI, C2, Cn yields a second electrolytic solution, which, owing to its recirculation through the cells CI, C2, Cn by the unit PI, is progressively richer in potassium hydroxide (KOH) and progressively poorer in potassium zincate (K ₂ Zn(OH) ₄ ).

The reaction referred to above, and therewith the supply of the electrolytic solution (originally, first electrolytic solution, progressively, second electrolytic solution) by the supply unit PI to the cells CI, C2, Cn proceeds as long as the concentration of potassium zincate (K ₂ Zn(OH) ₄ ) in the second electrolytic solution (with evolving concentrations of the chemical species) that supplies the cells CI, C2, Cn reaches minimum values, in particular lower than a threshold value. The criterion for determining the threshold value is of an energy nature. The expenditure in terms of energy per mass of zinc deposited increases as the concentration of zincate ions (Zn(OH) ₄ ²⁺ ) decreases. The threshold value is chosen for the purpose of limiting the specific energy expenditure below a given value.

Moreover, it should be considered that the space between the electrodes AN C and CT C during deposition tends to decrease on account of the growth of the layer of zinc. It is necessary to prevent shorting between the electrodes and prevent the passageway by the flow of electrolyte from being blocked (in addition to preventing detachment of the zinc during deposition).

In this case, the next step is that of storage of the metallic zinc deposited on the cathode CT C in the second storage environment 2. In this step, the valves VI, V2, and V6 are switched into the closed position, whereas the valves V3, V4, and V5 are switched into the open position. This means - among other things - that the functions of the inlet and discharge manifolds Ml, M2 are temporarily reversed, as has previously been mentioned. In particular, it is now the manifold M2 that receives a flow of the second electrolytic solution with a low concentration of potassium zincate (K ₂ Zn(OH) ₄ ) and a high concentration of potassium hydroxide (KOH) through the valve V3 and the port M2 3. From here, the aforesaid electrolytic solution invades the manifold M2 and through the ports M2 1 invades the cells CI, C2, Cn with an axial flow (from top to bottom) opposite to the axial flow (from bottom to top) with which the electrolytic cells are supplied for the reaction of electrolysis. In this step, moreover, the supply of electricity to each of the cells CI, C2, Cn is interrupted.

Circulation of the second electrolytic solution with low concentration of potassium zincate (K ₂ Zn(OH) ₄ ) and high concentration of potassium hydroxide (KOH) within the cells CI, C2, Cn will provide flushing of the cathode CT C, with consequent detachment of the layer of spongy metallic zinc deposited thereon. As an alternative there may be envisaged a mechanical system for detachment of zinc, for example an ultrasound vibration system.

The zinc flushed off in this way is then collected, together with the second electrolytic solution with a low concentration of potassium zincate (K ₂ Zn(OH) ₄ ) and a high concentration of potassium hydroxide (KOH), within the manifold Ml, and from this sent on through the valve V4 and the circuit node J into the filter Fl, where the spongy metallic zinc deposits. The filter Fl separates the solid fraction constituted by metallic zinc from the liquid fraction (water, KOH, K ₂ Zn(OH) ₄ ). If the liquid fraction contains potassium zincate (K ₂ Zn(OH) ₄ ) in concentrations that can still be exploited for electrolysis, the valves are switched back into the initial configuration (VI, V2, V5 open, V3, V4, V6 closed), and the step of supply of the electrolytic solution restarts, which ceases when the concentration of potassium zincate (K ₂ Zn(OH) ₄ ) reaches values that render it no longer exploitable, with a further switching of the valves (VI, V2 and V6 closed, V3, V4 and V5 open) and a subsequent step of discharge of the zinc into the filter Fl in addition to the zinc that was deposited previously. The aqueous solution obtained following upon this step corresponds to the second electrolytic solution, including water and potassium hydroxide (KOH) (in addition to minimum amounts of potassium zincate - K ₂ Zn(OH) ₄ ), i.e., in which the evolution of the concentrations of potassium hydroxide (KOH) and potassium zincate (K ₂ Zn(OH) ₄ ) has reached a condition that renders it exploitable in another process within the system SY, in particular in the generation section GE.

There may then be started storage of the zinc deposited on the cathode CT C and collected in the filter Fl via emptying thereof into the storage environment 2 through the valve V6, which is for this purpose switched into the open position. Preferably, seeing that the concentration of potassium hydroxide (KOH) has reached a condition that renders it exploitable in the generation section GE, also the second electrolytic solution is stored in the environment 2, together with the zinc deposited by electrolysis, which in this way remains dispersed in the aqueous solution (without constituting a solute proper).

It should be noted that, in general, although this is a preferred option, it is not necessary to store either the second electrolytic solution or the zinc collected in the filter Fl in the storage environment 2. In some embodiments, storage of just the zinc collected in the filter Fl inside the tank 2 is envisaged, with separate storage of the second electrolytic solution in a second tank.

The reason for this, according to an advantageous aspect of the invention, is that the storage section ST and the generation section GE can be installed in different places. In this connection, the storage environment 2 is obtained as energy storage tank that can be separated and transferred in a completely safe way for a subsequent use as supply for the galvanic cells Dl, D2, Dn.

In fact, the tank 2 does not contain any gas under pressure, and in particular does not contain gaseous hydrogen under pressure (which would be highly inflammable). Instead, the tank 2 contains an inert material, such as zinc, which is moreover perfectly eco-compatible.

The zinc can be then transferred elsewhere in the tank 2 for use in the generation section GE with the only addition - at the moment of installation in the section GE - of water and an alkaline electrolyte, preferably potassium hydroxide (KOH), which already constitutes the second electrolytic solution. As an alternative, it is possible to transport the zinc and the electrolytic solution together.

The person skilled in the art will thus appreciate the extreme convenience of the system SY according to the invention: potassium hydroxide (KOH) is a compound commercially available at a low cost and in a form that is easily transportable (pellets or powder). Water may normally be taken from the water mains so that it is possible to recreate separately the same conditions of supply proper to the system SY when there exists physical integration (in addition to functional integration) between the two sections ST and GE, i.e., when both the metallic zinc and the second electrolytic solution are stored inside the tank 2.

With reference to Figure 2B, whatever the species stored in the tank 2, once the tank 2 is set in fluid communication with the generation section GE (opening of the valve V7), the process of generation of gaseous hydrogen and simultaneous production of electrical energy can be started.

The above process envisages in general a step of supply of the metal (zinc) stored in the second storage environment 2 and of the second electrolytic solution - whether it is already present in the environment 2 or added subsequently - to the one or more generation galvanic cells Dl, D2, Dn.

This preferentially occurs in two further steps. There is in particular envisaged a step of charging of the circuit in which the valve V7 and the valve V8 are kept in the open position, whereas the valve V9 is kept in the closed position. In the charging step, which lasts for a pre-set time, during which the valve V7 is kept open, the ensemble of second electrolytic solution and zinc invades the circuit of the generation unit GE. Once the circuit is charged, following upon the aforesaid pre-set time, the valve V7 is switched into the closed position, the valve V9 is kept in the closed position, and just the valve V8 remains open. It will consequently be appreciated, also with reference to the directions of flow illustrated in Figure 2B, that a closed circuit is defined, in which: - the supply unit P2 supplies a flow of second electrolytic solution and zinc to the second supply manifold M3; and

- from the manifold M3, the flow of second electrolytic solution and zinc enters the cells Dl, D2, Dn through the ports M3_2, is processed, and from the cells Dl, D2, Dn themselves there exits a flow of gaseous hydrogen (H ₂ ) and of a third electrolytic solution.

Processing of the flow in question occurs completely on demand. The user starts the process of generation of hydrogen simply by closing the switch SW1 or SW2 that equips each cell (the entire process can conveniently be governed electronically). This sets up the reaction that is the reverse of the reaction of electrolysis referred to above, namely,

Zn + 2KOH + 2 H ₂ 0→ K ₂ Zn(OH) ₄ + H ₂ . where:

Reaction at the cathode

2H ₂ 0 + 2e ^" → H ₂ + 20H Reaction at the anode

Zn + 40H ^" → Zn(OH) ₄ ^2" + 2e

With the aid of Figures 5 to 8, within each cell Dl, D2, Dn, in greater detail, the phenomena described below occur.

From the port M3 2 a flow of the second electrolytic solution and metallic zinc invades the toroidal volume VDl . The electrolytic solution wets the porous membrane PM and the metallic zinc, but only the electrolytic solution passes through the porous membrane PM.

The zinc hence remains concentrated in the toroidal volume VDl and is in contact with the current collector (cylindrical metal shell or copper plate with tip collector BR), which is configured for conveying the flow of electrical charges within the layer of zinc.

Upon closing of the circuit between the anode AN D and the cathode CT D the flow of electrons between them is set up, as well as the reaction referred to above, with simultaneous generation of electrical energy and gaseous hydrogen.

The third electrolytic solution continues, progressively enriching itself with potassium zincate (K ₂ Zn(OH) ₄ ) as it is recirculated to the supply of the galvanic cells Dl, D2, Dn, whereas potassium hydroxide (KOH), as a result, converts by reaction with the zinc to form, among other things, potassium zincate (K ₂ Zn(OH) ₄ ).

The third electrolytic solution invades the second discharge manifold M4 through the ports M4 1, and gaseous hydrogen is released through the port M4 3 and the valve G2, whereas the electrolytic solution is recirculated to intake of the unit P2 through the port M4 2. The presence of the bottom plate BT moreover determines a semi -toroidal flow of chemical species in the cells Dl, D2, Dn, (preferably) with inlet from above and outlet from below.

Also in this case, several cycles of the flow of the third (originally second) electrolytic solution and (progressively residual) metallic zinc are required to reintegrate the initial concentration of the electrolyte (K ₂ Zn(OH) ), namely the concentration that characterizes the first electrolytic solution in the tank 1.

When this condition is reached, the third electrolytic solution substantially has the same composition as the first electrolytic solution, and - above all - substantially has the same concentration of potassium zincate (K ₂ Zn(OH) ₄ ) that characterizes the first electrolytic solution in the tank 1 (in addition to minimal amounts of potassium hydroxide - KOH).

If, in the case of the sections ST and GE being located in different places, instead of potassium hydroxide (KOH) a different alkaline electrolyte is used (e.g., sodium hydroxide - NaOH - or lithium hydroxide - LiOH), then the third electrolytic solution will not have the same composition as the first electrolytic solution of potassium zincate (K ₂ Zn(OH) ₄ ) for the simple reason that the sodium hydroxide that is consumed gives rise to sodium zincate (Na ₂ Zn(OH) ₄ ). More in general, in the case where the aim is to use the third electrolytic solution as reintegration of the first electrolytic solution without envisaging further treatment stages, it is necessary for the alkaline metal in the electrolyte (zincate) of the first electrolytic solution to coincide with the alkaline metal of the electrolyte (hydroxide) of the second electrolytic solution. It should moreover be noted that, if there exists simultaneity between deposition of the zinc on the cathode CT C of the cells CI, C2, Cn and consumption of hydrogen by a utilizer, for example the fuel cell FC, a further energy integration can be obtained between the utilizer (FC) and the system SY by sending the oxygen released from the manifold M2 to the supply of the utilizer itself.

During the step of production of hydrogen and electric current in the section GE, the reaction at the anode leads to formation of zincate ions (Zn(OH) ₄ ^2"

)·

In actual fact, the zincate (Zn(OH) ₄ ^2" ) is an intermediate and metastable product of the reaction, which very slowly proceeds further according to the following reaction:

Zn(OH) ₄ ^2" → ZnO + H ₂ 0 + 20H ^"

This second reaction leads to formation of zinc oxide (ZnO), water, and hydroxide ions OH ^" . In preferred embodiments, the aim is to keep the zinc within the zincate ion, preventing the reaction from proceeding: in this way, there is obtained formation of a supersaturated solution of zincate ions. The advantage over allowing the reaction to proceed is the following: the limit of solubility of zinc oxide (ZnO) in alkaline solutions of potassium hydroxide (KOH) is in the region of 60 g/1 of Zn, whereas it is possible to obtain concentrations even 3-4 times higher for the solubility of zincate (Zn(OH) ₄ ^2" ) (formation of supersaturated solutions of zincate with even 300 g/1 of Zn).

This enables use of a smaller amount of solution, given the same amount of Zn (and hence of precursor) to be processed.

In particular applications, however, it could be convenient to use a very limited amount of electrolyte (to the advantage in terms of weight and volume occupied) so as to get the zinc oxide (ZnO) to precipitate. Clearly in these cases, it will be necessary to provide a drainage outlet from the cells Dl, D2, Dn to remove all the precipitated solute (ZnO). The storage section ST must moreover be supplied with a first electrolytic solution in which the concentration of Zn is linked to the solubility of the zinc oxide (approximately 60 g/1). The latter application presents an advantage in the case where the section GE is separated from the section ST and there are moreover requirements of containment of weights and spaces occupied.

The person skilled in the art will hence appreciate the extreme advantage of the present invention. In particular, the energy is stored without gas under pressure and without hydrogen-retention substrates. The energy is simply stored, so to speak, in the form of a precursor consisting of metallic zinc.

There are thus solved all the problems of size, process technologies, and encumbrance that characterize the substrates or processes for energy storage of a known type. Zinc presents no difficulties or restrictions to transport, is eco- compatible, and can be easily be combined with an electrolytic solution obtained at the moment of use on a generation section GE located elsewhere with respect to the storage section ST.

The system may moreover present different power levels according to the size and/or the number of the cells CI, C2, Cn that define the maximum power that can be stored.

The electric power at output depends instead upon the size of the hydrogen-evolving electrode that constitutes the cathode CT D of each cell Dl, D2, Dn. The capacity of the system, i.e., the amount of energy storable per unit time depends, instead, solely upon the size of the tank 1, which defines the amount of the first electrolytic solution circulating in the system.

By varying the size of the tank 1, without varying the size of the machine it is possible to work with a higher capacity. This means that it is possible to obtain an extremely compact system as compared to the systems of a known type, which can be installed in any environment, including a domestic one.

Furthermore, according to an advantageous aspect of the present invention, the rate of the electrochemical reaction within the galvanic cells Dl, D2, Dn can be significantly improved with a preheating of the second electrolytic solution to a temperature of approximately 40-80°C, which is a preheating temperature that can be typically obtained by means of integration of the system SY with a thermal solar plant, or else by means of exploitation of the waste heat of the fuel cell FC.

In fact, at room temperature - approximately 20°C - the reaction in the cells Dl, D2, Dn proceeds at a lower rate. Instead, with preheating to 40-80°C the reaction proceeds in a stable and fast way.

The choice of an alkaline environment for the reactions in the cells CI, C2,

Cn enables minimization of the size of the machine since the usable current density (as regards peak values; in steady-state conditions, the values are lower) is of approximately 30-40 A/dm ² ; consequently, it is possible to obtain cells with smaller electrodes.

If it were to operate in an acid environment, as in the case of known systems, the maximum usable current density would be approximately one order of magnitude lower (3-4 A/dm ² ), beyond which complete electrolysis would be obtained, with generation of hydrogen.

Within the cells CI, C2, Cn, the zinc can be left immersed in the electrolytic solution (KOH + H ₂ 0) without this producing hydrogen, especially if the zinc is pure. It is possible to resume zinc deposition subsequently, and it is moreover possible to store zinc and electrolytic solution even together in the cells CI, C2, Cn. This would be impossible in an acid environment since this would erode the zinc. As compared to known solutions that envisage zinc deposition in an acid environment (not in powder form), the solution according to the invention affords an economic advantage: in the aforesaid known solutions, there is the need to increase the number of cells to increase the capacity of the system, whereas in the solution according to the invention it is sufficient to increase the size of the tank.

Of course, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined by the annexed claims.

In this connection, even though in the embodiments described the metal that functions as precursor for energy storage is zinc, in alternative embodiments it is possible to use other metals, for example iron or manganese.

With reference to Figure 9, there will now be illustrated an embodiment of the system SY according to the invention, which includes a variant of the storage section so far illustrated.

The storage section ST' includes the tank 1 for the first electrolytic solution, once again an aqueous solution including water and a first alkaline electrolyte, in particular potassium zincate (K ₂ Zn(OH) ₄ ).

The tank 1 is in fluid communication with the pump PI, in particular a pump the intake port of which is in fluid communication with an outlet port of the tank l .

The storage section ST' further includes at least one deposition electrolytic cell, which is configured for processing the first electrolytic solution coming from the tank 1. Even though embodiments may be envisaged in which a single deposition electrolytic cell is present, in the preferred embodiment illustrated herein a plurality of electrolytic cells are present (three in this case) CI ', C2', Cn', where the last reference Cn indicates that the number may also be different from (greater or smaller than) three, which is the number represented. The electrolytic cells CI ', C2', Cn' are immersed in a containment volume VS, which in this embodiment is functionally equivalent to the storage environment (or tank) 1 for the section ST.

The volume VS includes a supply port C IN, which receives fluid from the pump PI, and a discharge port C OUT, which is configured for disposing of the (solid and fluid) products contained in the volume VS. On the casing of the volume VS there is moreover provided the valve Gl described previously. The port C OUT is in fluid communication with the inlet port of the second storage environment 2, where the fluid communication is regulated by the valve V6, as described previously.

With combined reference to Figure 9 and to Figures 10A, 10B, each cell CI ', C2', Cn' has the same structure as the cells CI, C2, Cn (electrolytic cell including the anode AN C, positive, and the cathode CT C, negative, and preferably having a cylindrical geometry). Consequently, the corresponding description already presented in regard to the aforesaid cells altogether applies here, provided that this does not regard aspects that are manifestly incompatible.

Operatively associated to each deposition electrolytic cell CI ', C2', Cn' is an impeller EVIl, EVI2, EVIn housed in a corresponding control volume and functionally constituting a supply unit configured for circulation of the first electrolytic solution through the corresponding deposition electrolytic cell CI ', C2', Cn'. In this sense, the function of the impellers EVIl, EVI2, EVIn is similar to that of the pump PI in the section ST.

Each control volume includes one or more ports PT in communication with the inside of the containment volume for:

- intake of the first electrolytic solution by the impeller FMl, EVI2, EVIn and sending of the first electrolytic solution into the reaction volume VC of the corresponding deposition electrolytic cell CI ', C2', Cn' when the impeller EVIl, EVI2, EVIn is driven in a first direction of rotation (Figure 10A); and

- delivery of a flow of the first electrolytic solution taken in from the reaction volume VC of the corresponding deposition electrolytic cell CI ', C2', Cn' when the impeller IM1, EVI2, EVIn is driven in a second direction of rotation, opposite to the first (Figure 10B).

The first direction of rotation corresponds to a step of deposition of metal on the cathode CT C of the deposition electrolytic cell, whereas the second direction of rotation corresponds to a step of scavenging/washing off of the metal deposited by the cathode CT C.

Each of the impellers EVIl, IM2, EVIn can be driven in rotation independently of the other impellers.

Set in the proximity of the bottom of the volume VS is a shielding filter Fl . In this connection, it should be noted that the geometrical height of the impellers EVIl, EVI2, EVIn - and as a result of the cells CI ', C2', Cn' that are coaxial and located above the impellers EVIl, IM2, EVIn - is equal to or higher than the height of the shielding filter FT.

The pump PI merely integrates operation of the impellers EVIl, EVI2, EVIn. It loses its function of circulation of the first electrolytic solution (which it possesses, instead, in the section ST) during the step of deposition or scavenging of the cells CI ', C2', Cn' for removal of the zinc, but functions as a simple pump for loading the first electrolytic solution into the volume VS.

In this connection, during the step of deposition of the zinc, the impeller or impellers are driven in the first direction of rotation, as illustrated in Figure 10A. The valve V6 is in the closed position, whereas the pump PI is stationary. During the step of scavenging of the cells to remove the zinc deposited on the cathode CT C, given the same operating conditions of the valve V6 and of the pump PI, the impellers EVIl, EVI2, EVIn are driven in rotation in the opposite direction. The powdered zinc that is emptied out through the ports PT migrates towards the bottom of the volume VS by gravity, traversing the shielding filter F l '.

The latter prevents any re-intake of the metal powders from the bottom of the volume VS in the case where a new step of zinc deposition is set under way (provided that the concentration of zinc in the first electrolytic solution - evolving towards the composition of the second electrolytic solution - so enables). The metal powders simply remain resting on the bottom, without being entrained in suspension by the motion of the fluid in the volume VS.

At the end of the step (or steps) of deposition (deposition and flushing) of zinc, the valve V6 is switched into the open position, and the ensemble of the second electrolytic solution and the powdered zinc is emptied into the tank 2. As an alternative, the liquid fraction can be emptied into a separate storage environment, leaving in the tank 2 just the powdered zinc as described previously.

The main advantage of a system SY including the generation section ST' consists in the fact that it is possible to eliminate the inlet and discharge manifolds Ml, M2, together with the corresponding valves VI to V5 (with considerable simplification), and likewise in the fact that it is possible to vary the amount of energy - or rather of precursor - stored, simply by varying the number of impellers that are simultaneously active.

With reference to Figures 11 to 14, there will now follow a description of an alternative embodiment of the generation cells. These cells can be used in the section GE both in the case where it co-operates with a storage section of the same type as the section ST and where it co-operates with a storage section of the same type as the section ST' just described.

With reference to Figures 11 and 12, instead of the cylindrical generation cells Dl, D2, Dn, it is possible to use generation cells Dl ', D2', Dn' which have a plane geometry.

In particular, each cell Dl ', D2', Dn' includes:

- a current collector CC D;

- an inlet body CM Dl coupled to the current collector CC D and configured for receiving said second electrolytic solution and the metal (zinc) deposited in the deposition electrolytic cells, where the metal (in powder form) defines an anode AN D' of the cell;

- a porous membrane PM;

- a cathode CT D'; and

- a discharge body CM D2, which is configured for collecting gaseous hydrogen and - according to the structure of the cathode CT D' - a second electrolytic solution, and is in fluid communication with the first working port M4 1 of the second discharge manifold M4 for release of gaseous hydrogen and the second (evolving) electrolytic solution; in certain embodiments, as will be seen hereinafter, hydrogen and the second electrolytic solution can be released in separate environments.

Furthermore:

- the current collector CC D is operatively associated to the inlet body

CM Dl so as to offer an area of contact with the metal that can be received in the inlet body CM Dl, thus enabling conveyance of electrical charges through the metal itself; this means that the collector CC D can be set within the body CM Dl in a position opposite to the membrane PM or in an intermediate position, or else again can be set in contact with or in the strict proximity of the membrane PM, in which case the collector CC D will have to provide areas of passage for the second electrolytic solution (e.g., it may be made like a copper mesh);

- the porous membrane PM is set between the cathode CT D' and the inlet body CM Dl (in certain embodiments, the membrane PM is laminated with the cathode, see hereinafter); and

- the discharge body CM D2 is set on an opposite side of the cathode CT D' with respect to the porous membrane PM; in this way, the two bodies CM Dl and CM D2 are located in end positions of the cell.

The cells Dl ', D2', Dn' are well suited, as a result of their plane geometry, to the constitution of a stack of cells, which will provide not only an electrical connection in series between the galvanic cells (as on the other hand is in any case obtained in the cells Dl, D2, Dn), but also a mechanical connection.

See in this connection Figure 13, which illustrates a stack of cells Dl '-D8'. It should be noted how the electrical interface of each module of the stack is located between the current collector CC D of one cell and the cathode CT D' of the immediately preceding cell, whereas the mechanical interface is between the bodies CM D2 and CM Dl of the next cell.

The cells Dl ', D2', Dn' are functionally identical to the cells Dl, D2, Dn in so far as they perform, macroscopically, the same function.

They differ, however, from the cells Dl ', D2', Dn' for some details of operation, namely:

- the intake of the powdered zinc and of the second electrolytic solution into the volume VD1 can be done from above as illustrated in Figures 11 and 12 (e.g., from the manifold M3 through the corresponding ports M3 2, or else from a hopper that receives zinc and the second electrolytic solution from the manifold M3 or functions itself as manifold M3, with corresponding valve for supply of the cells), or else can be done by means of passages orthogonal to the plane of the cell (i.e., that have a direction that proceeds from the collector CC D to the discharge body CM D2), which themselves constitute the inlet and discharge manifolds M3, M4;

- the current collector CC D, which is plane and not cylindrical, can be installed in a slightly inclined position to favour descent of the zinc particles; and

- the cathode CT D' can be obtained in a double-layered configuration, including a hydrophilic layer and a hydrophobic layer, where the hydrophilic layer faces the porous membrane PM, whereas the hydrophobic layer faces the volume VD2.

As an alternative, the cathode CT D' can be obtained with a triple-layered structure, as illustrated in Figure 14. An electrode of this sort comprises, all laminated together, from right to left as viewed in the figure:

- the membrane PM;

- a hydrophilic layer HF made of activated carbon, nickel, and PTFE as binder, or else Raney nickel + PTFE; this layer is permeable to the liquid (second electrolytic solution in this case) and to the gas (H ₂ in this case);

- a current collector CR (functionally the cathode CT D') made of porous nickel, which is permeable to liquid (second electrolytic solution in this case) and gas (H ₂ in this case);

- a hydrophobic layer FIB made of the same material as the layer HF, which is permeable only to gas (H ₂ in this case); and

- a separation layer SL, which is permeable only to gas (H ₂ in this case). It should moreover be borne in mind that, in the absence of integral lamination of the membrane PM with the remaining layers, the structure becomes identical to that of the double-layered electrode referred to above.

From the point of view of operation, the cells Dl ' D2', Dn' operate in a way similar to the cells Dl, D2, Dn. The user starts the process of generation of hydrogen simply by closing the electrical circuit at the terminals of the electrodes of the individual cell (or of the series of cells). The anode AN D', constituted by the powdered zinc in the volume VD1, is gradually eroded to form an electrolytic solution progressively richer in potassium zincate (K ₂ Zn(OH) ₄ ), with simultaneous generation of gaseous hydrogen at the cathode CT D' and simultaneous generation of electrical energy.

The electrolytic solution that enters the volume VD1 (having a substantially parallelepipedal shape) wets the porous membrane PM and the metallic zinc (the anode AN D'), but only the electrolytic solution passes through the porous membrane PM.

The zinc thus remains concentrated in the volume VD1 and is in contact with the current collector (e.g., a copper plate with collector tips BR), which is configured for conveying the flow of electrical charges within the layer of zinc.

The third electrolytic solution is progressively enriched with potassium zincate (K ₂ Zn(OH) ₄ ) as it is recirculated to the supply of the galvanic cells Dl ', D2', Dn', whereas potassium hydroxide (KOH) is consequently converted by reaction with the zinc to form, among other things, potassium zincate (K ₂ Zn(OH) ₄ ).

The third electrolytic solution enters the second discharge manifold M4 through the ports M4 1, and gaseous hydrogen is released through the port M4 3 and the valve G2, whereas the electrolytic solution is recirculated to the intake of the unit P2 through the port M4 2.

In the case of the double-layered or triple-layered cathode CT D', the second electrolytic solution that is recirculated through the cells Dl ', D2', Dn' penetrates the porous membrane PM and the cathode CT D', but in the latter only as far as the end of the hydrophilic layer.

The hydrophobic layer can be traversed just by the gaseous hydrogen H ₂ . In this way, there will be two separate outlet ports for the products of the electrochemical reaction within the cells Dl ', D2', Dn', i.e.,

- a first outlet port, which collects the liquid (second electrolytic solution) that wets the membrane PM and the cathode CT D' and functions as manifold for the second electrolytic solution, giving out on the outside preferentially at the bottom of the body CM Dl; and

- a second outlet port, which communicates with the volume VD2 and with the corresponding port M4 1 of the manifold M4, where the gaseous hydrogen H ₂ collects.

With reference to Figure 12 A, a circuit diagram of the generation section of the system SY is illustrated in the case of double- layered or triple-layered electrode. The references that are identical to the ones previously used designate the same components.

In this embodiment, to which the previous description applies where not manifestly incompatible, the manifold M4 is split into a first manifold and a second manifold. In particular, the first manifold (which maintains the simple reference M4) carries the same connections with the rest of the circuit already described. However, the first manifold M4 loses the valve G2, and is now located on an opposite side with respect to the manifold M3. The ports M4 1 now collect just the second electrolytic solution that is collected in the cavity inside the electrode CT D' and is schematically represented with a dashed line in Figure 12A. The port M4 2 is regularly in fluid communication with the intake of the pump P2.

The second manifold, identified by the reference M4' includes first working (inlet) ports M4'_l in fluid communication with the volume VD2. These ports collect the gaseous hydrogen that is discharged at the cathode CT D'. The valve G2 is now located on the manifold M4' and conveys gaseous hydrogen on the outside towards a user device, for example the supply of the fuel cell FC.

Like the cells Dl, D2, Dn, several cycles of the flow of third (originally second) electrolytic solution and the (progressively residual) metallic zinc are required to reintegrate the initial concentration of the electrolyte K ₂ Zn(OH) ₄ , namely the concentration that characterizes the first electrolytic solution in the tank 1.