METHOD FOR THE PRODUCTION OF HYDROGEN

Abstract

A method for electrolysis of water using a device having a reversible cell for the production of hydrogen and oxygen is disclosed. The method includes preparing an aqueous alkaline solution of KOH having a concentration of potassium hydroxide (KOH) of between 35% and 55% w/v in a mixer; heating the aqueous alkaline solution to between 150 C. and 374 C.; increasing the pressure to maintain the aqueous alkaline solution in the liquid phase at a minimum pressure condition of between 2.2 bar and 129.1 bar; passing the water through the hydrophobic layer of each electrode and reaching the catalyst layer to catalyse a reduction phase of the hydrogen or an oxidation phase of the oxygen and the porous electrode layer to carry out the reduction and oxidation; and collecting the released gaseous hydrogen and oxygen.

Claims

1. A method for the electrolysis of water to hydrogen and oxygen by means of an alkaline electrolyser wherein the electrolyser comprises a reversible cell for electrolysis including a channel; a mixing tank for mixing water and potassium hydroxide (KOH) to form an aqueous alkaline solution of KOH; feeding means for feeding said aqueous alkaline solution of KOH under pressure into said channel from said mixing tank; control means for controlling the pressure inside the channel and a heat exchanger for heating the aqueous alkaline solution of KOH; wherein said reversible cell for electrolysis comprises at least two multilayer electrodes, wherein said multilayer electrodes are arranged facing each other to define the channel for the passage of the aqueous alkaline solution of KOH, and wherein each of said multilayer electrodes comprises, starting from said channel, a hydrophobic layer, a catalyst layer to catalyse a reduction step of said hydrogen or an oxidation step of said oxygen, a porous electrode layer to bring about said reduction or said oxidation, and a bipolar plate, wherein said hydrophobic layer comprises aluminosilicate geopolymers or polytetrafluoroethylene polymers (PTFE) or perfluoroalkoxy polymers (PFA) or a 1 m-3 m film obtained by a PACVD process (Plasma-Assisted Chemical Vapour Deposition); and wherein said channel has an inlet connected to said supply means, and an outlet, and wherein said mixing tank, said supply means, said control means for controlling the pressure, said heat exchanger and said reversible electrolysis cell is connected in series to form a closed circuit; and wherein said aqueous alkaline solution of KOH has in the channel a concentration of potassium hydroxide (KOH) of between 35% and 55% w/v, is at a temperature of between 150 C. and 374 C., and is in a condition of minimum pressure between 2.2 bar and 129.1 bar, and wherein the method comprises I) preparing in the mixing tank the aqueous alkaline solution of KOH having a concentration of potassium hydroxide (KOH) of between 35% and 55% w/v; II) heating said aqueous alkaline solution of KOH to a temperature of between 150 C. and 374 C.; III) increasing the pressure to keep the aqueous alkaline solution of KOH in the liquid phase at a minimum pressure condition of between 2.2 bar and 129.1 bar; IV) forcing the water into said channel so that the water passes through the hydrophobic layer of each electrode and reaches the respective catalyst layer to catalyse a reduction phase of said hydrogen or an oxidation phase of said oxygen and the porous electrode layer to bring about said reduction and said oxidation respectively; and V) collecting gaseous hydrogen and oxygen released from step IV).

2. The method according to claim 1, wherein said aqueous alkaline solution of KOH is in one of the conditions: at a temperature of 150 C. the minimum pressure is between 2.2 bar at a KOH concentration of 55% w/v and 3.3 bar at a KOH concentration of 35% w/v; at a temperature of 160 C. the minimum pressure is between 2.9 bar at a KOH concentration of 55% w/v and 4.2 bar at a KOH concentration of 35% w/v; at a temperature of 170 C. the minimum pressure is between 3.6 bar at a KOH concentration of 55% w/v and 5.4 bar at a KOH concentration of 35% w/v; at a temperature of 180 C. the minimum pressure is between 4.5 bar at a KOH concentration of 55% w/v and 6.8 bar at a KOH concentration of 35% w/v; at a temperature of 190 C. the minimum pressure is between 5.6 bar at a KOH concentration of 55% w/v and 8.5 bar at a KOH concentration of 35% w/v; at a temperature of 200 C. the minimum pressure is between 6.8 bar at a KOH concentration of 55% w/v and 10.5 bar at a KOH concentration of 35% w/v; at a temperature of 210 C. the minimum pressure is between 8.2 bar at a KOH concentration of 55% w/v and 12.7 bar at a KOH concentration of 35% w/v; at a temperature of 220 C. the minimum pressure is between 9.8 bar at a KOH concentration of 55% w/v and 15.4 bar at a KOH concentration of 35% w/v; at a temperature of 230 C. the minimum pressure is between 11.6 bar at a KOH concentration of 55% w/v and 18.4 bar at a KOH concentration of 35% w/v; at a temperature of 240 C. the minimum pressure is between 13.6 bar at a KOH concentration of 55% w/v and 21.8 bar at a KOH concentration of 35% w/v; at a temperature of 250 C. the minimum pressure is between 15.9 bar at a KOH concentration of 55% w/v and 25.7 bar at a KOH concentration of 35% w/v; at a temperature of 260 C. the minimum pressure is between 18.4 bar at a KOH concentration of 55% w/v and 30 bar at a KOH concentration of 35% w/v; at a temperature of 270 C. the minimum pressure is between 21.3 bar at a KOH concentration of 55% w/v and 34.9 bar at a KOH concentration of 35% w/v; at a temperature of 280 C. the minimum pressure is between 24.4 bar at a KOH concentration of 55% w/v and 40.4 bar at a KOH concentration of 35% w/v; at a temperature of 290 C. the minimum pressure is between 27.9 bar at a KOH concentration of 55% w/v and 46.5 bar at a KOH concentration of 35% w/v; at a temperature of 300 C. the minimum pressure is between 31.8 bar at a KOH concentration of 55% w/v and 53.3 bar at a KOH concentration of 35% w/v; at a temperature of 310 C. the minimum pressure is between 36 bar at a KOH concentration of 55% w/v and 60.8 bar at a KOH concentration of 35% w/v; at a temperature of 320 C the minimum pressure is between 40.6 bar at a KOH concentration of 55% w/v and 69.1 bar at a KOH concentration of 35% w/v; at a temperature of 330 C. the minimum pressure is between 45.6 bar at a KOH concentration of 55% w/v and 78.1 bar at a KOH concentration of 35% w/v; at a temperature of 340 C. the minimum pressure is between 51.1 bar at a KOH concentration of 55% w/v and 88.1 bar at a KOH concentration of 35% w/v; at a temperature of 350 C. the minimum pressure is between 57 bar at a KOH concentration of 55% w/v and 98.9 bar at a KOH concentration of 35% w/v; at a temperature of 360 C. the minimum pressure is between 63.4 bar at a KOH concentration at 55% w/v and 110.8 bar at a KOH concentration at 35% w/v; at a temperature of 370 C. the minimum pressure is between 70.4 bar at a KOH concentration of 55% w/v and 123.6 bar at a KOH concentration of 35% w/v; or at a temperature of 374 C. the minimum pressure is between 73.3 bar at a KOH concentration of 55% w/v and 129.1 bar at a KOH concentration of 35% w/v.

3. The method according to claim 2, wherein said aqueous alkaline solution of KOH is in one of the conditions indicated in the table: TABLE-US-00010 % KOH (p/v) 35 45 55 T [ C.] P min [bar] P min [bar] P min [bar] 150 3.3 2.7 2.2 160 4.2 3.6 2.9 170 5.4 4.5 3.6 180 6.8 5.7 4.5 190 8.5 7.1 5.6 200 10.5 8.7 6.8 210 12.7 10.6 8.2 220 15.4 12.7 9.8 230 18.4 15.2 11.6 240 21.8 18 13.6 250 25.7 21.2 15.9 260 30 24.7 18.4 270 34.9 28.8 21.3 280 40.4 33.2 24.4 290 46.5 38.2 27.9 300 53.3 43.7 31.8 310 60.8 49.8 36 320 69.1 56.5 40.6 330 78.1 63.9 45.6 340 88.1 71.9 51.1 350 98.9 80.7 57 360 110.8 90.2 63.4 370 123.6 100.6 70.4 374 129.1 105 73.3

4. The method according to claim 1, wherein in said aqueous alkaline solution of KOH at the concentration of 35% w/v of KOH, the minimum pressure varies between 3.3 and 129.1 bar between 150 C. and 374 C., at the concentration of 45% w/v of KOH the minimum pressure varies between 3.3 and 105 bar between 150 C. and 374 C., and at the concentration of 35% w/v of KOH, the minimum pressure varies between 2.2 and 73.3 bar between 150 C. and 374 C.

5. The method according to claim 1, wherein said aqueous alkaline solution of KOH has a KOH concentration at 35%-50% w/v, with a temperature of 200 C. and a minimum pressure of 8.7 bar, temperature of 250 C. and minimum pressure of 21.2 bar, or temperature of 300 C. and minimum pressure of 43.7 bar.

6. The method according to claim 1, wherein said control means are a valve and are configured to maintain a differential pressure condition between the inside of the channel and the electrode layer.

7. The method according to claim 1, wherein said hydrophobic layer (3) comprises aluminosilicate geopolymers or a 1 m-3 m film obtained by means of a PACVD (Plasma-Assisted Chemical Vapour Deposition) process.

8. The method according to claim 1, wherein said hydrophobic layer comprises aluminosilicate geopolymers.

9. The method according to claim 1, wherein said hydrophobic layer (3) comprises a 1 m-3 m film obtained by a PACVD (Plasma-Assisted Chemical Vapour Deposition) process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention and the following detailed description of the preferred embodiments can be better understood with reference to the following drawings:

[0028] FIG. 1a shows a graph of a phase transition of the KOH-based alkaline solution (with different percentages of KOH by weight).

[0029] FIG. 1b shows another graph of the phase transition of the KOH-based alkaline solution (with different percentages of KOH by weight).

[0030] FIG. 2 shows a graph relative to the phase transition of the water.

[0031] FIG. 3 shows a voltage-temperature graph of an electrolyser operating at high temperature and pressure maintaining the process fluid in the liquid state.

[0032] FIG. 4 shows a conductivity-temperature graph with different quantities by weight of KOH in the alkaline solution.

[0033] FIG. 5 shows a voltage-pressure graph of a generic electrolyser.

[0034] FIG. 6 shows a voltage-temperature graph at different pressures.

[0035] FIG. 7 shows a cell voltage-current density graph.

[0036] FIG. 8 shows an efficiency-temperature graph referred to various approaches to calculating efficiency.

[0037] FIG. 9 shows a schematic view of a multilayer electrode according to an embodiment of the invention.

[0038] FIG. 10 shows a schematic view of a device for the electrolysis of water according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The invention relates to a method for the production of hydrogen and oxygen by electrolysis of water. With reference to the drawings and in particular FIGS. 9 and 10, the electrolysis takes place in a reversible cell 1 for the electrolysis of water. The cell 1 comprises two multilayer electrodes 1a and 1b, wherein said multilayer electrodes are arranged facing each other to define a channel 2 for the passage of an aqueous alkaline solution.

[0040] The aqueous alkaline solution comprises an electrolyte and at least ions comprising hydrogen and oxygen. Each of said electrodes 1a, 1b comprises a hydrophobic layer 3, an electrode layer 4, a catalyst layer 5 and a bipolar plate 6. The hydrophobic layer is a layer facing towards the channel 2. For this reason, starting from channel 2, each multilayer electrode 1a, 1b, comprises the hydrophobic layer 3, the electrode layer 4, the catalyst layer 5 and a bipolar plate 6. In practice, the cell has two identical opposite electrodes, for each electrode starting from the channel the hydrophobic layer, the electrode layer, the catalyst layer and the bipolar plate.

[0041] Preferably, to allow a continuous passage of the aqueous alkaline solution, the channel 2 has an inlet 2a for the aqueous alkaline solution and an outlet 2b. The hydrophobic layer 3 is configured to oppose the passage of the aqueous alkaline solution towards the electrode layer 4 of the electrodes and said passage of process water is allowed when the pressure of the electrolyte is greater than the pressure present in the electrodes. In other words, the hydrophobic layer allows the passage of process water only under certain pressure conditions, but if these pressure conditions do not occur, it opposes the passage, in its entirety, of the aqueous solution. The process water will drip through the hydrophobic layer, reaching the catalytic layer, which when polarized will give rise to the electrolysis process.

[0042] According to the invention, said passage is allowed when the differential pressure is of a certain value depending on the operating conditions which depend on the pressures in place, on the electrolyte concentrations (that is, of the KOH solution), on the temperature and on the nature of the hydrophobic layer.

[0043] The catalyst layer 5 is configured to catalyse a reduction phase of said hydrogen or an oxidation phase of said oxygen and the porous electrode layer 4 to carry out said reduction or said oxidation.

[0044] The bipolar plate 6 acts as a current distributor and provides a mechanical support for the electrodes.

[0045] According to the context of the present disclosure, the term electrolysis cell or electrolytic cell refers to a particular electrochemical cell which allows the conversion of electrical energy into chemical energy during the electrolysis process.

[0046] The adjective reversible refers to the possibility of the cell to operate both as an electrolyser and as a fuel cell, that is, a cell having the purpose of producing energy by exploiting hydrogen.

[0047] The electrical energy necessary to carry out the process is supplied by an external electrical circuit, connected to the poles of the cell, so the electrolysis process does not occur spontaneously. The electrolysis of water, from which H.sub.2 and O.sub.2 is obtained, takes place with a significant contribution of thermal energy, with a consequent increase in voltage efficiency, and allows the production of gas at high pressure.

[0048] The heat of reaction H.sub.298.sup., evaluated at the temperature of 25 C. and at the pressure of 1 Atm, is a function of the entropy of reaction S.sub.298.sup. and of the Gibbs free energy G.sub.298.sup., according to the relationship:

[00001]$\begin{array}{cc}{H}_{298}^{}=G_{298}^{}+T{S}_{298}^{}& \left(1\right)\end{array}$$\mathrm{With}:$$\begin{array}{cc}{H}_{298}^{}=68.32\frac{\mathrm{kcal}}{\mathrm{mol}}& \left(2\right)\end{array}$$\begin{array}{cc}{G}_{298}^{}=56.71\frac{\mathrm{kcal}}{\mathrm{mol}}& \left(3\right)\end{array}$$\begin{array}{cc}{S}_{298}^{}=38.96.Math.{10}^{-3}\frac{\mathrm{kcal}}{\mathrm{mol}K}& \left(4\right)\end{array}$

[0049] The electrical work required defines the minimum, or reversible, voltage at which water can be split by electrolysis. At the temperature of 25 C. and at the pressure of 1 Atm such voltage is:

[00002]$\begin{array}{cc}{U}_{REV}=\frac{G_{298}^{}}{2.Math.F}=\frac{56.71.Math.{10}^{-3}.Math.4,186}{2.Math.96500}=1.23V& \left(5\right)\end{array}$

where two Faraday units (F), equal to 2.Math.96500, are equivalent to approximately 53.6 Ah (the Ampere-hour, symbol Ah, is a practical unit of electric charge and represents the quantity of charge required to deliver an ampere of current per hour).

[0050] The minimum work required to split the water molecule into hydrogen and oxygen is a known value from thermodynamics. Water splitting is an endothermic chemical reaction, which requires a supply of heat from the outside to be conducted at a constant temperature. This can be achieved, in a simple manner, by exploiting the heat generated by the inevitable ohmic resistances opposed to the passage of the current: it is necessary to raise the cell voltage to the thermoneutral voltage.

[00003]$\begin{array}{cc}{U}_{\mathrm{THERMONEUTRAL}}=U_{TN}=1.49V& \left(6\right)\end{array}$

[0051] Below this voltage the cell tends to cool down, due to the heat dissipation by the reaction. If, on the other hand, the internal resistances are such as to require a higher voltage, the cell must be equipped with a heat removal system.

[0052] It is generally of interest to evaluate the energy costs necessary to carry out the electrolysis. Referring for convenience to Nm.sup.3, the standard unit of gas volume under standard conditions, the quantity of current required to produce a Nm.sup.3 of hydrogen in one hour can be evaluated as follows:

[00004]$\begin{array}{cc}1\frac{{\mathrm{Nm}}^3}{h}.Math.\frac{1}{22.414}\frac{\mathrm{kmol}}{{\mathrm{Nm}}^3}.Math.53.6\frac{\mathrm{Ah}}{\mathrm{mol}}=2393A& \left(7\right)\end{array}$

which at the thermoneutral voltage leads to a specific electricity consumption, equal to:

[00005]$\begin{array}{cc}{W}_{\mathrm{id}}=\frac{2393A.Math.1.49V}{1{\mathrm{Nm}}^3/h}=3.56\frac{\mathrm{kWh}}{N{m}^3}& \left(8\right)\end{array}$

[0053] However, this value will necessarily be greater than the neutral value due to the losses, so it is customary to indicate as the voltage efficiency nu the relationship:

[00006]$\begin{array}{cc}{}_U=\frac{U_{\mathrm{thermoneutral}}}{U_{\mathrm{real}}}& \left(9\right)\end{array}$

[0054] The current flow is also greater than the ideal minimum: a part of the flow, in fact, escapes the electrolysis by traveling through the distribution system of the alkaline solution to the individual cells. The current yield .sub.I is defined as follows:

[00007]$\begin{array}{cc}{}_I=\frac{I_{\mathrm{ideal}}}{I_{\mathrm{real}}}& \left(10\right)\end{array}$

[0055] The real specific consumption of electricity is:

[00008]$\begin{array}{cc}W=\frac{3.56\frac{\mathrm{kWh}}{{\mathrm{Nm}}^3}}{{}_U.Math.{}_I}& \left(11\right)\end{array}$

[0056] According to the context of the present disclosure, the term aqueous solution refers to a solution in which the solvent includes water.

[0057] Aqueous solutions that contain strong electrolytes conduct the electric current efficiently, conversely, those with weak electrolytes are poor conductors. The term strong electrolytes means substances that are completely ionized in water, while weak electrolytes are only partially ionized.

[0058] According to the context of the present disclosure, the term hydrophobic refers to the physical property of a chemical species and/or material to repel, not absorb, retain and/or not interact in any way with water.

[0059] Hydrophobic chemical species and/or materials tend to be electrically neutral and apolar.

[0060] According to the context of the present disclosure, the term catalyst means a chemical species which is capable of lowering the activation energy of the electrolysis reaction, increasing its speed, and which remains unchanged.

[0061] In the context of the present disclosure, the term electrode refers to a conductor used to establish an electrical contact with a non-metallic part of a circuit, such as, for example, an electrolyte.

[0062] According to the context of the present disclosure, the terms oxidation and reduction refer to reactions in which the oxidation number of at least one chemical species varies, in particular, in the case of oxidation the oxidation number increases because the chemical species loses electrons, while in the reduction the oxidation number decreases because the chemical species acquires electrons. The chemical species that acquires electrons is called the oxidant, while the one that donate electrons is called the reducing agent.

[0063] The equation that describes the electrolysis process of water is an oxidation-reduction reaction, which is represented below:

2H.sub.2O.sub.(aq).fwdarw.2H.sub.2(g)+O.sub.2(g)

[0064] Water acts simultaneously as an oxidizing species and a reducing species, causing the reduction of hydrogen and the oxidation of oxygen. In particular, the electric current dissociates the water molecule into the H.sup.+ and OH.sup. ions. At the cathode, the hydrogen ions (H.sup.+) acquire electrons, reducing and leading to the formation of molecular hydrogen (H.sub.2). At the anode, the hydroxide ions (OH.sup.) release electrons, oxidizing and leading to the formation of molecular oxygen (O.sub.2).

[0065] The term bipolar plate refers to an electrically conductive, gas-tight plate that separates individual cells into a single cell or in a stack, acting as a current distributor and providing mechanical support for the electrodes.

[0066] The hydrophobic layer 3 of the multi-layer electrodes 1a, 1b comprises aluminosilicate geopolymers or polytetrafluoroethylene polymers (PTFE) or perfluoroalkoxy polymers (PFA) or a 1-3 m film obtained by means of the PACVD (Plasma-Assisted Chemical Vapour Deposition) process.

[0067] Preferably, the hydrophobic layer 3 of the multilayer electrodes 1a, 1b comprises aluminosilicate geopolymers or a 1-3 m film obtained by means of the PACVD (Plasma-Assisted Chemical Vapour Deposition) process. Even more preferably, the hydrophobic layer 3 of the multilayer electrodes 1a, 1b comprises aluminosilicate geopolymers.

[0068] Even more preferably, the hydrophobic layer 3 of the multilayer electrodes 1a, 1b comprises a 1-3 m film obtained by means of the PACVD (Plasma-Assisted Chemical Vapour Deposition) process.

[0069] According to the context of the invention, the term aluminosilicate geopolymers means a category of synthetic materials based on aluminosilicates, species in which some aluminium (Al) atoms replace silicon (Si) atoms. These are inorganic materials obtained from the reaction between a solid aluminosilicate and an activating aqueous alkaline solution. The aluminosilicate can be of natural origin, synthetic or an industrial by-product and the activating aqueous alkaline solution contains silicates and alkaline hydroxides. The main features are: high resistance to the bases, a necessary condition for the invention in object, high heat resistance and impermeability, as the geopolymer matrix is endowed with mesoporosity, that is, it has many but very small pores, so it is impermeable to the passage of water or other large molecules. In order to deposit the geopolymer on porous elements, the Deep Coating technique is used (e.g. 1 cm/min).

[0070] The term polytetrafluoroethylene polymer (PTFE) refers to a polymer belonging to the class of perfluorocarbons deriving from the homopolymerization of tetrafluoroethene. It can be described as a hydrophobic, inert, smooth and high temperature resistant plastic material. It is characterised by a very low friction coefficient.

[0071] The perfluoroalkoxy polymer (PFA) is a fluoropolymer with properties similar to those of PTFE; in fact it is also characterised by hydrophobia, resistance to high temperatures, a low friction coefficient and is inert, however it is less easy to find. With the PACVD process (Plasma Assisted-Chemical Vapour Deposition) a colourless and thin film of a few micrometres (about 1-3 m) of amorphous carbon-silicon coating (a-C:H:Si) is obtained; the high-energy but low-temperature plasma makes the treated surfaces water-repellent, gives the electrodes high thermal stability (500 C.) and excellent resistance to the chemical agents.

[0072] In order to carry out the electrolysis of water, the cell, or preferably a stack of cells, are arranged inside a circuit to form a device for the electrolysis of water. Said electrolysis device, called an electrolyser comprises in its entirety, preferably supported in a frame: [0073] a mixing tank 8 for mixing the electrolyte of the aqueous solution; [0074] feeding means 9 for feeding said aqueous alkaline solution under pressure into said channel 2, in particular at the inlet 2a of said channel 2; [0075] control means for controlling the pressure 10 inside the channel 2 in order to control the passage of water through the hydrophobic layer 3 of the multilayer electrodes; [0076] a heat exchanger 11 for heating the aqueous solution; [0077] at least one electrolysis cell 1 according to any one of the embodiments described herein.

[0078] Preferably, as mentioned above, the device comprises a plurality of electrolysis cells such as the one described above which are fixed to each other in a stack. The mixing tank 8, the feeding means 9, the control means 10 for controlling the pressure, said heat exchanger 11 and said electrolysis cell 1 are connected in series to form a closed circuit.

[0079] Preferably, the mixing tank 8 has a capacity in litres according to the characteristics of the machine.

[0080] The concentration of potassium hydroxide (KOH) is between 35% and 55% w/v. The alkaline solution has a conductivity of up to 4 S/cm.

[0081] Preferably, according to the embodiments described herein, the feeding means 9 is a pump.

[0082] The term under pressure refers to a condition wherein the operating pressure is higher than atmospheric pressure, at values such as those indicated above, and in general between 2.2 and 129.1 bar. This is a minimum pressure to keep the solution in liquid form.

[0083] The operating pressure depends mainly on the temperature and the concentration of KOH, and its purpose is to keep the solution in the aqueous state. A person skilled in the art can control and regulate the pressure and temperature, as well as the concentration of KOH to keep the solution in the liquid state, according to the preferred indications mentioned above.

[0084] Still preferably, the control means for controlling the pressure (10) is configured in such a way as to maintain a differential pressure of at least 0.2 bar inside the channel 2. More preferably, in accordance with all the embodiments described herein, the control means for controlling the pressure 10 is configured in such a way as to maintain a pressure of 80 bar inside the channel 2.

[0085] Preferably, according to any one of the embodiments described here, the control means for controlling the pressure 10 comprise a valve.

[0086] The term heat exchanger refers to an equipment in which a thermal energy exchange of a heat transfer fluid is carried out with another having a different temperature. The heat exchanger 11 allows operation at a temperature of between 150 C. and 374 C.

[0087] The device 7 can be called a high temperature and pressure alkaline electrolyser (AWE-HTP).

[0088] In this high temperature and pressure alkaline electrolyser, the use of a diaphragm for the separation of gases is not envisaged, but of multilayer electrodes as shown in FIG. 9. The hydrophobic layer 3 of the electrodes maintains the flow of the electrolyte between the electrodes themselves. The process water will pass the hydrophobic layer 3 in a controlled manner, reaching the catalytic layer which, when polarized, will give rise to the electrolysis process, only if the pressure of the electrolyte is greater than the pressure present in the electrodes (FIG. 9).

[0089] The above-mentioned method comprises the following steps: [0090] i. making process water available; [0091] ii. mixing the process water and KOH to give an aqueous alkaline solution; [0092] iii. heating said aqueous alkaline solution; [0093] iv. increasing the pressure to maintain the aqueous alkaline solution in the liquid phase of KOH; [0094] v. making the water drip through the hydrophobic state 3 of the multilayer electrode; [0095] vi. electrolysis of water; [0096] vii. collecting gaseous hydrogen and oxygen released from step vi.

[0097] As mentioned above, according to the invention, the aqueous solution is an alkaline solution. The term alkaline solution refers to a basic aqueous solution, that is, an aqueous solution with a pH of between 7.1 and 14.

[0098] According to the present disclosure, the alkaline solution comprises water and potassium hydroxide (KOH) and, even more preferably, the concentration of the potassium hydroxide (KOH) is between 35% and 55% w/v.

[0099] The concentrations of potassium hydroxide (KOH) shown in the description are expressed as percentage mass on volume (w/v), i.e., percentage of grams of KOH with respect to 100 g of solution. The percentage of KOH inside the solution impacts on the phase transition conditions, as shown in FIG. 1. As the concentration in weight of KOH increases, the conditions for remaining in the liquid range are favourable with respect to pure water (the water phase diagram graph is shown in FIG. 2).

[0100] In general, the alkaline solution can be at a temperature of between 150 C. and 374 C. The temperature has a positive effect on the voltage, since, as it increases, the voltage required to carry out the electrolysis process decreases, as the contribution of thermal energy increases as per equation 13 (derived from the combination of formulas (1) and (5)).

[00009]$\begin{array}{cc}{U}_{REV}=\frac{G}{2.Math.F}=\frac{H-TS}{2.Math.F}& \left(12\right)\end{array}$

[0101] As the temperature increases, the component (TS) increases linearly leading to a decrease in U.sub.REV, as shown in FIG. 3.

[0102] Another preferred embodiment, according to any one of the embodiments described, is the alkaline solution under operating conditions with a conductivity up to 4 S/cm.

[0103] The conductivity [S/cm] of the alkaline solution is a function of the temperature and the concentration of potassium hydroxide (KOH), according to equation 12 and is shown in FIG. 4.

TABLE-US-00001 = C1 .Math. T.sup.3 + C2 .Math. T.sup.2 + C3 .Math. T + C4 T[ C.] C1, C2, C3, C4 coefficients Coeficiente value: wt % 35 45 55 C1 2.15E07 1.78E07 3.30E07 C2 6.44E05 7.04E05 1.06E04 C3 0.007675 0.005973 0.005205 C4 0.329863 0.270575 0.21049

[0104] In order to increase the conductivity and, therefore, also the efficiency of the process, it is necessary to select an appropriate percentage by weight of KOH. Furthermore, as indicated above, the alkaline solution is in a condition of pressure such that the pressure of the electrolyte is greater than the pressure present in the electrodes.

[0105] The effect of pressure P on the U.sub.REV is stated in equation (14) (which can be obtained from equation (13) and the breakdown of S) and dependent on element (15)

[00010]$\begin{array}{cc}{U}_{\mathrm{REV}}=\frac{G}{2.Math.F}=\frac{H}{2.Math.F}-\frac{T}{2.Math.F}\left(S_{\mathrm{React}}^{\mathrm{Ref}}+\underset{P-R}{.Math.}{z}_{i,k}{\mathrm{Cp}}_{i,k}\ln \left(\frac{T}{T_{\mathrm{ref}}}\right)-R\underset{P-R}{.Math.}{z}_{i,k}\ln \left(\frac{P}{P_{\mathrm{ref}}}\right)\right)& \left(14\right)\end{array}$

[0106] The effect of the pressure is logarithmic and a function of the sign of the component (15):

[00011]$\begin{array}{cc}\underset{P-R}{.Math.}{z}_{i,k}& \left(15\right)\end{array}$$\mathrm{wherein}:$$\begin{array}{cc}{z}_{i,k}=\frac{\mathrm{moles}\mathrm{of}\mathrm{species}k\mathrm{in}\mathrm{mix}i}{\mathrm{moles}\mathrm{of}\mathrm{fuel}}.fwdarw.\underset{P-R}{.Math.}{z}_{i,k}\{\begin{array}{c}>0\mathrm{the}\mathrm{reaction}\mathrm{increases}\mathrm{no}.\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{gas}\\ =0\mathrm{the}\mathrm{reaction}\mathrm{maintains}\mathrm{no}.\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{gas}\\ <0\mathrm{the}\mathrm{reaction}\mathrm{decreases}\mathrm{no}\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{gas}\end{array}& \left(16\right)\end{array}$

Defining P as Products and R as Reagents, observing the chemical formula of electrolysis (17):

[00012]$\begin{array}{cc}\underset{R}{\underset{}{H_{2}O}}.fwdarw.\underset{P}{\underset{}{\frac{1}{2}{O}_2+H_2}}& \left(17\right)\end{array}$$\mathrm{Gives}:$$\begin{array}{cc}\underset{P-R}{.Math.}{z}_{i,k}>0& \left(18\right)\end{array}$

[0107] As the pressure increases, the process voltage increases logarithmically according to equation 14). The voltage-pressure relationship is shown in FIG. 5.

[0108] In conclusion, as T increases, V decreases linearly (FIG. 3) and as P increases, V increases logarithmically (FIG. 5). Hence, the linear contribution of temperature, with respect to the logarithmic one of pressure, is greater. Hence, according to the initial intuition of the authors, at the indicated KOH concentrations, having a high temperature and pressure remains a favourable situation as shown in FIG. 6.

[0109] The following elements contribute to the value of the cell voltage (as shown in FIG. 7): reversible voltage (U.sub.REV), voltage referred to the ohmic resistances of the cell (R.sub.OHM.Math.I.sub.Cell) and activation (.sub.ACT).

[0110] The effects of the activation and ohmic resistances can be considered almost independent on the temperature, instead U.sub.REV, (as in FIG. 3) decreases as the temperature increases. In conclusion, as the temperature increases, the cell voltage also decreases.

[0111] The efficiency can be calculated in the following ways:

[00013]$\begin{array}{cc}{}_{\mathrm{electric}}=\frac{U_{\mathrm{TN}@\mathrm{STP}}}{U_{\mathrm{CELL}}}& \left(19\right)\end{array}$$\begin{array}{cc}{}_{\mathrm{LHV}}=\frac{U_{\mathrm{REV}@\mathrm{STP}}}{U_{\mathrm{CELL}}}& \left(20\right)\end{array}$$\begin{array}{cc}{}_{\mathrm{HHV}}=\frac{U_{\mathrm{TN}@T,P}}{U_{\mathrm{CELL}}}& \left(21\right)\end{array}$$\begin{array}{cc}{}_{E\mathrm{rev}\left(T,P\right)}=\frac{U_{\mathrm{REV}@T,P}}{U_{\mathrm{CELL}}}& \left(22\right)\end{array}$

[0112] In conclusion, as the temperature increases, the U.sub.CELL decreases, consequently each of the efficiencies increases according to FIG. 8. In light of the above explanation, the electrolysis cell does not comprise a polymeric membrane and/or a ceramic membrane.

[0113] On the basis of the above explanation and the diagrams of FIGS. 1a and 1b, valid operating conditions can be established. For example, any of the following operating conditions may be foreseen.

[0114] At a temperature of 150 C. the minimum pressure is between 2.2 bar at a KOH concentration of 55% w/v and 3.3 bar at a KOH concentration of 35% w/v.

[0115] At a temperature of 160 C. the minimum pressure is between 2.9 bar at a KOH concentration of 55% w/v and 4.2 bar at a KOH concentration of 35% w/v.

[0116] At a temperature of 170 C. the minimum pressure is between 3.6 bar at a KOH concentration of 55% w/v and 5.4 bar at a KOH concentration of 35% w/v.

[0117] At a temperature of 180 C. the minimum pressure is between 4.5 bar at a KOH concentration of 55% w/v and 6.8 bar at a KOH concentration of 35% w/v.

[0118] At a temperature of 190 C. the minimum pressure is between 5.6 bar at a KOH concentration of 55% w/v and 8.5 bar at a KOH concentration of 35% w/v.

[0119] At a temperature of 200 C. the minimum pressure is between 6.8 bar at a KOH concentration of 55% w/v and 10.5 bar at a KOH concentration of 35% w/v.

[0120] At a temperature of 210 C. the minimum pressure is between 8.2 bar at a KOH concentration of 55% w/v and 12.7 bar at a KOH concentration of 35% w/v.

[0121] At a temperature of 220 C. the minimum pressure is between 9.8 bar at a KOH concentration of 55% w/v and 15.4 bar at a KOH concentration of 35% w/v.

[0122] At a temperature of 230 C. the minimum pressure is between 11.6 bar at a KOH concentration of 55% w/v and 18.4 bar at a KOH concentration of 35% w/v.

[0123] At a temperature of 240 C. the minimum pressure is between 13.6 bar at a KOH concentration of 55% w/v and 21.8 bar at a KOH concentration of 35% w/v.

[0124] At a temperature of 250 C. the minimum pressure is between 15.9 bar at a KOH concentration of 55% w/v and 25.7 bar at a KOH concentration of 35% w/v.

[0125] At a temperature of 260 C. the minimum pressure is between 18.4 bar at a KOH concentration of 55% w/v and 30 bar at a KOH concentration of 35% w/v.

[0126] At a temperature of 270 C. the minimum pressure is between 21.3 bar at a KOH concentration of 55% w/v and 34.9 bar at a KOH concentration of 35% w/v.

[0127] At a temperature of 280 C. the minimum pressure is between 24.4 bar at a KOH concentration of 55% w/v and 40.4 bar at a KOH concentration of 35% w/v.

[0128] At a temperature of 290 C. the minimum pressure is between 27.9 bar at a KOH concentration of 55% w/v and 46.5 bar at a KOH concentration of 35% w/v.

[0129] At a temperature of 300 C. the minimum pressure is between 31.8 bar at a KOH concentration of 55% w/v and 53.3 bar at a KOH concentration of 35% w/v.

[0130] At a temperature of 310 C. the minimum pressure is between 36 bar at a KOH concentration of 55% w/v and 60.8 bar at a KOH concentration of 35% w/v.

[0131] At a temperature of 320 C the minimum pressure is between 40.6 bar at a KOH concentration of 55% w/v and 69.1 bar at a KOH concentration of 35% w/v.

[0132] At a temperature of 330 C. the minimum pressure is between 45.6 bar at a KOH concentration of 55% w/v and 78.1 bar at a KOH concentration of 35% w/v.

[0133] At a temperature of 340 C. the minimum pressure is between 51.1 bar at a KOH concentration of 55% w/v and 88.1 bar at a KOH concentration of 35% w/v.

[0134] At a temperature of 350 C. the minimum pressure is between 57 bar at a KOH concentration of 55% w/v and 98.9 bar at a KOH concentration of 35% w/v.

[0135] At a temperature of 360 C. the minimum pressure is between 63.4 bar at a KOH concentration at 55% w/v and 110.8 bar at a KOH concentration at 35% w/v.

[0136] At a temperature of 370 C. the minimum pressure is between 70.4 bar at a KOH concentration of 55% w/v and 123.6 bar at a KOH concentration of 35% w/v.

[0137] At a temperature of 374 C. the minimum pressure is between 73.3 bar at a KOH concentration of 55% w/v and 129.1 bar at a KOH concentration of 35% w/v.

[0138] The expression minimum pressure means the lowest pressure which allows the aqueous alkaline solution to be maintained in the liquid state. It is to be understood that the operating pressure can therefore be even greater, and that the minimum pressure indicated is the minimum for the system to operate.

[0139] Further combinations of temperature, pressure, and concentration of KOH and relative uses can be deduced from the table below,

TABLE-US-00002 % KOH (w/v) 35 45 55 T [ C.] P min [bar] P min [bar] P min [bar] 150 3.3 2.7 2.2 160 4.2 3.6 2.9 170 5.4 4.5 3.6 180 6.8 5.7 4.5 190 8.5 7.1 5.6 200 10.5 8.7 6.8 210 12.7 10.6 8.2 220 15.4 12.7 9.8 230 18.4 15.2 11.6 240 21.8 18 13.6 250 25.7 21.2 15.9 260 30 24.7 18.4 270 34.9 28.8 21.3 280 40.4 33.2 24.4 290 46.5 38.2 27.9 300 53.3 43.7 31.8 310 60.8 49.8 36 320 69.1 56.5 40.6 330 78.1 63.9 45.6 340 88.1 71.9 51.1 350 98.9 80.7 57 360 110.8 90.2 63.4 370 123.6 100.6 70.4 374 129.1 105 73.3

[0140] Considering the interdependence of the variables, all the combinations and ranges of said table for concentration, temperature and pressure must be considered valid within the present disclosure. Therefore, for example, at the concentration of 35% w/v of KOH, the minimum pressure varies between 3.3 and 129.1 bar between 150 C. and 374 C., at the concentration of 45% w/v of KOH the minimum pressure varies between 3.3 and 105 bar between 150 C. and 374 C., and at the concentration of 35% w/v of KOH, the minimum pressure varies between 2.2 and 73.3 bar between 150 C. and 374 C.

[0141] If the hydrophobic layer is based on aluminosilicate geopolymers, the differential pressure is preferably at least 0.2 bar inside channel 2.

[0142] According to a preferred embodiment, with any of the hydrophobic layers indicated above, with a KOH concentration of 35%-50% w/v, preferably of 45% w/v, the operating conditions can be 200 C. and the minimum pressure can be 8.7 bar, temperature of 250 C. and the minimum pressure can be 21.2 bar, or temperature of 300 C. and the minimum pressure can be 43.7 bar.

Operational Process

[0143] The process diagram of the AWE-HTP is represented, in its main components, in FIG. 10: [0144] At least one AWE-HTP water electrolysis cell (1); [0145] Feeding means (Pump) 9 of the alkaline solution (for example based on potassium hydroxide (KOH)) in the liquid state and process water in the liquid state [0146] Tank for collecting the alkaline solution and the eventual supply of process water. [0147] Heat exchanger 11 for the supply of thermal energy to the process, from an external source [0148] Control means (Valve) 10 configured to control, downstream of the electrolyser, the pressure and the dripping process.

[0149] The invention provides at least one electrolysis cell operating with an alkaline solution (for example KOH) in the liquid state.

[0150] The alkaline solution will circulate without coming into contact with the layers of the catalysed electrodes 5; this is because, instead of the membrane/diaphragm, the invention provides for the use of multilayer electrodes in which the last layer (the one in contact with the solution) is hydrophobic 3.

[0151] The electrolytic process will be possible, when the water can reach, in a controlled way, the electrode layer catalysed 5 and polarized 4 crossing the hydrophobic layer 3. In order to achieve this, the pressure of the alkaline solution must be increased, as it must overcome the resistance of the hydrophobic layer 3 itself. In order to benefit from the above-mentioned advantages, the solution is heated in a heat exchanger 11 placed in series with the tank 8 and the pump 9.

[0152] In order to keep the alkaline solution and process water in the liquid phase, it will be necessary to increase and maintain the pressure of the cell, within the limits set according to the phase diagram of the alkaline solution (FIG. 3). Consequently, in order to ensure the passage of water through the hydrophobic layer 3, the pressure of the alkaline solution increases further and this can be achieved by controlling the valve.

[0153] Further examples are shown below.

Example 1

TABLE-US-00003 T = 150 C. % wt KOH = 55% (percentage optimising conductivity at the defined temperature) P = 5 bar (P min @55% wtKOH = 2.2 bar) T [ C.] 150 n electric 94.1% % wt KOH 55% n LHV 78.1% P [bar] 5.0 n HHV 92.7% P min [bar] 2.2 n E rev(T, P) 72.6%

Example 2

TABLE-US-00004 T = 200 C. % wt KOH = 45% (percentage optimising conductivity at the defined temperature) P = 10 bar (P min @45% wtKOH = 8.7 bar) T [ C.] 200 n electric 102.2% % wt KOH 45% n LHV 84.8% P [bar] 10.0 n HHV 99.9% P min [bar] 8.7 n E rev(T, P) 76.8%

Example 3

TABLE-US-00005 T = 250 C. % wt KOH = 45% (percentage optimising conductivity at the defined temperature) P = 25 bar (P min @45% wtKOH = 21.2 bar) T [ C.] 250 n electric 111.8% % wt KOH 45% n LHV 92.8% P [bar] 25.0 n HHV 108.1% P min [bar] 21.2 n E rev(T, P) 82.4%

Example 4

TABLE-US-00006 T = 300 C. % wt KOH = 45% (percentage optimising conductivity at the defined temperature) P = 50 bar (P min @45% wtKOH = 43.8 bar) T [ C.] 300 n electric 123.5% % wt KOH 45% n LHV 102.5% P [bar] 50.0 n HHV 117.3% P min [bar] 43.8 n E rev(T, P) 89.4%

Example 5

TABLE-US-00007 T = 350 C. % wt KOH = 45% (percentage optimising conductivity at the defined temperature) P = 85 bar (P min @45% wtKOH = 80.7 bar) T [ C.] 350 n electric 137.8% % wt KOH 45% n LHV 127.2% P [bar] 85.0 n HHV 127.2% P min [bar] 80.7 n E rev(T, P) 98.8%

Example 6

TABLE-US-00008 T = 374 C. % wt KOH = 45% (percentage optimising conductivity at the defined temperature) P = 110 bar (P min @45% wt KOH = 105.1 bar) T [ C.] 374 n electric 146.0% % wt KOH 45% n LHV 121.2% P [bar] 110.0 n HHV 132.1% P min [bar] 105.1 n E rev(T, P) 104.7%

[0154] In the table a summary of the data for the various operating conditions of the above examples is provided.

TABLE-US-00009 T % wt P P min n n n n E [ C.] KOH [bar] [bar] electric LHV HHV rev(T, P) 1 150 55% 5.0 2.2 94.1% 78.1% 92.7% 72.6% 2 200 45% 10.0 8.7 102.2% 84.8% 99.9% 76.8% 3 250 45% 25.0 21.2 111.8% 92.8% 108.1% 82.4% 4 300 45% 50.0 43.8 123.5% 102.5% 117.3% 89.4% 5 350 45% 85.0 80.7 137.8% 114.4% 127.2% 98.8% 6 374 45% 110.0 105.1 146.0% 121.2% 132.1% 104.7%