EP4665890A1

EP4665890A1 - Electrolyzer and method for decoupled water electrolysis

Info

Publication number: EP4665890A1
Application number: EP24708881.8A
Authority: EP
Inventors: Marco Matteini; Alessandro Tampucci; Giacomo RONTINI; Lorenzo BROGI
Original assignee: New Mobility Electric System SRL
Current assignee: New Mobility Electric System SRL
Priority date: 2023-02-09
Filing date: 2024-02-07
Publication date: 2025-12-24
Also published as: IT202300002205A1; WO2024166004A1

Abstract

Electrolytic cell (10) for the decoupled electrolysis of water comprising a first electrode (11), for the production of hydrogen (H2) and oxygen (O2), alternatively; a second electrode (12); an alkaline aqueous solution (13) in which the first electrode and the second electrode are immersed; polarity reversal means operatively connected to both the first electrode and the second electrode; the first electrode and the second electrode are polarized in the opposite way to each other in all operating conditions; the second electrode (12) comprises porous conductive carbon.

Description

“ELECTROLYZER AND METHOD FOR DECOUPLED WATER ELECTROLYSIS”

TECHNICAL SECTOR

[001], The object of the present invention is an electrolytic cell for decoupled water electrolysis.

[002], The present invention also refers to a method for decoupled water electrolysis.

STATE OF THE ART

[003], Hydrogen, and in particular green hydrogen such as the one produced from water electrolysis starting from renewable energy sources, represents a promising candidate to reduce dependence on fossil fuels and reduce carbon dioxide emissions.

[004], The technologies available at the moment use electricity to carry out the water electrolysis reaction, simultaneously producing hydrogen gas at the cathode (“Hydrogen Evolution Reaction”, or “HER”, according to a commonly adopted English terminology) and oxygen gas at the anode ( “Oxygen Evolution Reaction”, or “OER”, according to a commonly adopted English terminology). The process consists of two half-reactions that usually occur in two different cells containing the two electrodes and separated by an ion exchange membrane or a diaphragm.

[005], A type of conventional devices for the production of hydrogen through electrolysis (electrolysers) are the PEM electrolysers (acronym for: "Proton Exchange Membrane"), which use cation exchange polymeric membranes to divide the two half-cells and allow the passage of ions . They work in an acidic environment and use noble metal-based catalysts.

[006], Another type of conventional electrolytic devices are alkaline electrolysers. These can be of two types: the first uses porous separator diaphragm instead of polymeric membranes and works in an alkaline environment. This makes it possible to reduce corrosion problems due to the acidic environment of the previous case and it is also possible to use non-noble metals as catalysts.

[007], Another conventional type of alkaline electrolysers are the alkaline membrane electrolysers (or "AEM", acronym for "Anion Exchange Membrane") which, unlike previous systems, use an anion exchange polymeric membrane. The presence of a polymer membrane prevents the mixing of hydrogen and oxygen produced during electrolysis in the event of a process interruption. This makes this technology particularly suitable for use in association with discontinuous renewable energy sources (e.g. photovoltaic, wind, etc.). [008], One of the main problems of conventional electrolysers is the so-called gas cross-over which, by allowing gaseous hydrogen to permeate towards the anode, although in small quantities, can give rise to the formation of dangerous and explosive gaseous mixtures.

[009], In alkaline membrane electrolysers, the use of a polymeric membrane instead of the separating diaphragm reduces the problem of gas cross-over and allows the modulation of the pressures inside the two half-cells.

[010], Despite this, numerous efforts are underway to try to further minimize or eliminate the gas cross-over problem. A possible known solution consists in carrying out the water electrolysis reaction in different times or containers (i.e. decoupled electrolysis or "decoupled water splitting") in order to have a temporally or mechanically separate production of hydrogen and oxygen (in two different electrolytic cells).

[011], The prior art document WO-2021-099986-A1 in the name of the same Applicant describes a process of electrolysis decoupled in time which uses a copper or silver electrode (or their alloys), on which the zinc deposition reaction in charging phase or the dissolution of zinc during discharging phase. Oxidationreduction thus occurs on the electrode, preventing the reactions of simultaneous development of hydrogen and oxygen.

[012], However, this known solution, although advantageous from many points of view, is not at all free of drawbacks.

[013], In fact, the repeated deposition of zinc on an electrode inevitably leads to the formation of dendrites on the electrode itself, with a consequent drop in performance of the electrolytic cell over time and in particular with a clear differentiation of charging times from discharge times as the deposition of zinc on the electrode due to the dendrites is much faster than the release of zinc from the electrode to the solution.

[014], Another well-known decoupled electrolysis solution consists in the use of redox mediators, substances that are reduced in place of hydrogen during the charging phase, so as not to allow the HER reaction, and are oxidized in place of oxygen in discharge phase, not allowing the OER reaction.

[015], These redox mediators can be of different types, and in particular they can be liquid, such as aqueous solutions containing poly-oxometalates, transition metal salts (vanadium and cerium), hydroquinones or iron complexes, or solid, such as NiOOH/ Ni(OH)2, metal oxides or organic solids.

[016], However, the use of redox mediators imposes operational constraints on the electrolytic cell and therefore limits the ranges of use in terms of pH and voltages. In fact, the oxidation potential of redox mediators must necessarily be lower than the OER potential, and on the contrary the reduction potential must be higher than the HER potential. This greatly limits the choice of redox mediators and the working operating voltage.

[017], Yet another solution for decoupled electrolysis is described in document FR 3 111 918 A1 (Total SA [FR]) which proposes the use of a first electrode intended for the production of hydrogen while at the second electrode there is a conductive carbon in fluid form, so that the positive electrode acts as a supercapacitor by accumulating positive charge. A membrane is necessarily provided to separate the positive electrode compartment from the negative electrode compartment to prevent the supercapacitive particles (carbon) in suspension from touching the negative electrode and discharging. In a solution of this type the supercapacitive particles have very long self-discharge times (from 5 to 10 hours) or a selfdischarge system must be provided which involves structural complications as a third electrode must be provided towards which the superconducting particles can circulate in suspension via a special recirculation system. The electrolytic cell described is intended for the production exclusively of hydrogen gas and the oxygen evolution reaction does not occur because the working electrode (first electrode) always remains negative and the supercapacitor (second electrode) always positive and therefore not capable of producing oxygen.

[018] The need is therefore felt to provide an improved solution capable of carrying out the electrolysis of water in a decoupled manner in which the evolution of both hydrogen and oxygen occurs, avoiding the use of membranes and other structural complications.

SYNTHESIS OF THE INVENTION

[019], An object of the present invention is to overcome the drawbacks highlighted with reference to the state of the art.

[020], A further particular aim of the present invention is to propose an electrolyser for decoupled electrolysis capable of operating effectively without the use of redox mediators and therefore in particular overcoming the limits relating to the operating voltage.

[021] Another object of the invention is to propose an electrolyser for decoupled electrolysis which does not require a separation membrane between the electrodes.

[022] Another object is to propose an electrolyser in which both gaseous hydrogen and gaseous oxygen are produced in a relevant electrolytic cell.

[023], This and other objectives are achieved with an electrolytic cell according to claim 1 , as well as with a method according to claim 8. [024], Some advantageous embodiments are the subject of the dependent claims.

[025], According to a general embodiment, an electrolytic cell (or electrolyser) for the decoupled electrolysis of water comprises a first electrode, for the production of hydrogen and oxygen, alternatively, a second electrode, an alkaline aqueous solution in which they are immersed the first electrode and the second electrode, and polarity reversal means operatively connected to both the first electrode and the second electrode.

[026], The first electrode and the second electrode are oppositely polarized with respect to each other in all operating conditions, and the second electrode is made of solid porous conductive carbon stably fixed to an electrode support.

[027], The second porous conductive carbon electrode works as a supercapacitor, accumulating electrical charge in the form of an electrochemical double layer. For this reason, two important characteristics are: electrical resistivity (the lower the resistivity, the greater the electrical conductivity) and the surface area (the greater the surface area for the same weight, the greater the electrical capacity). According to one embodiment, the porous conductive carbon of the second electrode has a resistivity less than or equal to 10 Ohm*cm and a surface area per unit weight that belongs to the range 2000-3000 m2/g.

[028], According to one embodiment, the first electrode is made of metal or metal alloy. Preferably, said metal and said metal alloy comprise a metal belonging to the group consisting of: iron, cobalt, nickel, molybdenum, manganese, titanium, platinum, silver, combination of the previous ones.

[029], According to one embodiment, the second electrode comprises conductive carbon doped with at least one of the following: manganese, nickel, iron, cobalt, polypyrrole, polyaniline, polyethyl dioxythiophene doped with polystyrenesulfonate, preferably with a concentration in the range 1-10% by weight relative to the weight of the conductive carbon.

[030], According to one embodiment, the alkaline aqueous solution comprises one or more alkaline inorganic salts and/or one or more hydroxides of alkaline and/or alkaline earth metals, in concentrations in the range 1-6 mol/l. Preferably, the operating temperature is less than 70°C.

[031], According to one embodiment, the alkaline aqueous solution comprises a potassium-based alkaline electrolyte in a concentration of approximately 5 mol/l; and the alkaline aqueous solution is contained in a container measuring 3 cm x 1.7 cm x 7.0 cm; and the first electrode is made of platinum on titanium and has dimensions of 4cm x 2cm; and the second electrode has a surface area per unit of weight of approximately 2500 m2/g and has dimensions of 4 cm x 2 cm. [032], According to one embodiment, the first electrode and/or the second electrode is/are associated with an electrically conductive metal support, for example a sheet, a mesh, a sponge. Preferably, the electrically conductive metal support is made of a metallic material belonging to the group consisting of: nickel, copper, iron, titanium, combination of the previous ones.

[033], According to a general embodiment, a method for the decoupled electrolysis of water comprising the following steps of (i) providing at least one electrolytic cell, according to any of the previously described embodiments; (ii) performing a charging step that produces hydrogen via the first electrode, wherein the second electrode is polarized with a positive charge; and subsequently, perform a discharge phase that produces oxygen via the first electrode, in which the second electrode is polarized with a negative charge.

[034], According to an embodiment of the method, during the charging phase, the hydroxyl ions migrate by electrostatic attraction to the second electrode, forming an electrochemical double layer on the surface of the second electrode; and during the discharge phase, positive ions, for example potassium ions, migrate to the second electrode forming an electrochemical double layer on the surface of the second electrode.

[035], According to one embodiment of the method, during the charging phase, the second electrode is charged from 0 Volts to 1 Volt relative to a third reference electrode (Reference Hydrogen Electrode, RHE, not shown), and during the charging phase discharged, the second electrode is discharged from 1 Volt to 0 Volt compared to the reference electrode (Reference Hydrogen Electrode, RHE).

[036], Thanks to the proposed solutions, it is possible to propose an electrolyser for decoupled electrolysis capable of functioning effectively without the use of redox mediators and therefore exceeding in particular the limits relating to the working operating voltage.

[037], According to one embodiment, a second electrode (counter electrode) based on conductive carbon, which essentially behaves like a super-capacitor, is used for the generation of hydrogen and oxygen in a decoupled mode in the electrolytic cell.

[038], Thanks to the Thanks to the possibility of inverting the polarity which allows the alternating discharge of H⁺ protons and OH' anions, the production of both hydrogen and oxygen occurs at a single first bifunctional electrode (which acts as a "Gas Evolution Electrode").

[039], In accordance with one embodiment, the electrolytic cell is made up of a "Gas Evolution Electrode" (the first electrode) and a suitably treated conductive carbon counter-electrode (second electrode), operating in an alkaline environment. At the second electrode, which acts as a supercapacitor, the discharge of neither cations nor anions can occur and its polarity is changed cyclically so as to always be opposite to that of the first electrode.

[040], Thanks to the fact that the conductive carbon is integral with the support of the second electrode, it is not necessary to use any separator placed between the electrodes.

Brief description of the drawings

[041], Further characteristics and advantages of the invention will appear from the following description of embodiments, given by way of example and not by way of limitation, with reference to the attached figures, in which:

- figure 1 is a schematic view of an electrolytic cell illustrated in the charging phase, according to one embodiment;

- figure 2 is a schematic view of the electrolytic cell of figure 1 , illustrated in the discharge phase;

- figure 3 is a graph showing the trend of the voltage (voltage) over time of repeated charging and discharging cycles of the electrolytic cell of figures 1 and 2, in which the trend of the voltage of a first electrode, the trend of the voltage of a second electrode comprising conductive carbon is shown in a fine dotted line, and the trend of the total voltage (sum of the previous ones) is shown in a solid line;

- figure 4 is a graph showing the trend of the voltage (voltage) over time of the electrolytic cell of figures 1 and 2 for 500 consecutive charge and discharge cycles.

Detailed description of some embodiments

[042], In accordance with a general embodiment, an electrolytic cell 10 or electrolyser 10 for decoupled electrolysis of water comprising a first electrode 11 , to alternatively produce both hydrogen H2 and oxygen 02.

[043], The electrolytic cell 10 further comprises a second electrode 12 and an alkaline aqueous solution 13 in which the first electrode 11 and the second electrode 12 are immersed. The alkaline solution can comprise potassium hydroxide (KOH).

[044], The electrolytic cell 10 is furthermore associated with polarity inversion means, which are operatively connected to both the first electrode 11 and the second electrode 12, for the purpose of switching between the charging phase and the discharging phase. For example, said electrical polarity inversion means comprise an inverter. An electronic control device may be provided to operate the polarity reversal means. When in operating conditions, the first electrode 11 and the second electrode 12 are oppositely polarized with respect to each other.

[045], The second electrode 12 is made of porous conductive carbon in solid form stably fixed to a support of the second electrode via a binder.

[046], As shown for example in figure 1 , the electrolytic process occurs in two consecutive phases, in which in the first phase (charging phase) the semi-reaction takes place: which leads to the production of hydrogen H2 at the first electrode 11. In particular, when in operating conditions, the hydrogen H2 produced bubbles from the electrolytic solution 13 while the hydroxyl OH- ions migrate by electrostatic attraction to the second electrode 12 of conductive carbon, forming an electrochemical double layer EDL (“Electrical Double Layer”) on its active surface. In this phase the conductive carbon counter electrode is positively polarized up to a pre-established voltage, which is preferably less than or equal to 1 Volt with respect to Reference Hydrogen Electrode, RHE, as higher voltages lead overtime to chemical reactions that can deteriorate the electrode.

[047], As shown for example in figure 2, in the second phase of the process (discharge phase) oxygen is produced at the first electrode 11 according to the half-reaction:

[048], In this second discharge phase the positive ions, for example potassium ions (K+) migrate towards the second conductive carbon electrode 12 forming an electrochemical double layer EDL on its active surface. In this second phase, this second electrode 12 is negatively polarized so that the positive charge previously stored in it is discharged until it reaches a pre-established voltage, which is preferably 0 Volt (with respect to Reference Hydrogen Electrode, RHE).

[049], It is therefore possible to carry out a process of decoupled electrolysis of water through the use of a counter-electrode which acts as a super-capacitor (second electrical electrode 12) and a Gas Evolution Electrode (first electrode 11).

[050], As mentioned above, the first electrode 11 works as a negative electrode in the charging phase and as a positive electrode in the discharging phase, producing hydrogen and oxygen in an uncoupled manner, through HER and OER type reactions alternating overtime.

[051], The first electrode 11 can be formed from metals or metal alloys formed from non-noble metals, such as for example: iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo), manganese (Mn), titanium

(Ti), or noble metals, to a limited extent platinum (Pt) and silver (Ag). [052], The second conductive carbon electrode 12 is polarised, during operation, in the opposite manner to the first electrode 11 , therefore positively charged during charging and negatively charged during discharging.

[053], Both the first electrode 11 and the second carbon electrode 12 can are advantageously deposited thanks to a binder on an electrically conductive electrode support thereof (not shown) so as to make them resistant, conductive and well supported at the same time. Such electrode supports can be highly conductive metal sheets, nets or sponges such as, for example, made of nickel (Ni), copper (Cu), iron (Fe), titanium (Ti).

[054], From an analysis conducted by the inventors, it emerged that the use of a conductive carbon with resistivity less than or equal to 10 Ohm*cm and a surface area between 2000 and 3000 m2/g easily allows the accumulation of charge on its surface, managing to create an EDL electrochemical double layer with the ions generated within the solution. In practice, the high conductivity and polarizability, together with the correct size of the pores (defined by the surface area) allow electric charges to be effectively attracted (potassium K+ ions and hydroxyl OH- ions, alternately).

[055], The conductive carbon of the second electrode 12 can be doped with manganese, nickel, iron, cobalt, poly-pyrrole, poly-aniline, poly-ethyledioxythiophene doped with poly-styrenesulphonate, in a concentration range that varies from 1% to 10% in weight compared to the weight of the treated coal. The appropriate doping of the electrode allows the working voltage to be modulated.

[056], The electrolytic cell 10 preferably operates at a temperature lower than 70°C, using an alkaline electrolyte (alkaline inorganic salts or alkaline/alkaline earth metal hydroxides) in concentrations between 1 mol/l and 6 mol/l.

[057], Some implementation examples are described below.

Example 1 - creation of electrolytic cell 10.

[058], The electrolytic cell is, according to this example, 3.0cm x 1.7cm x 7.0cm in size and is made up of a second conductive carbon electrode 12 with a surface area equal to 2500 m2/g, dimensions 4cm x 2cm x 0.1cm , from a first bifunctional electrode 11 in platinum on titanium with dimensions 4cm x 2cm x 0.1cm, in which the alkaline electrolyte in solution is potassium hydroxide (KOH) in a concentration of 5 mol/L

Example 2 - charge I discharge cycles

[059], The electrolytic cell 10 described in example 1 was tested with repeated galvanostatic chargedischarge cycles (as shown in figure 3, which refers to a time window of approximately 1 hour). Typically, the voltage is measured between the two working electrodes of the cell and this provides an indication of the quality of the cell as a whole: it is known that to obtain electrolysis the overall voltage cannot be less than 1 .23 V, however it is desirable to get as close as possible to this value because the higher the voltage, the higher the energy consumption for the same amount of hydrogen produced. Furthermore, even if the theoretical voltage of 1 .23 V is the minimum voltage for carrying out electrolysis (oxygen on one side and hydrogen on the other) in the cell of the invention the two reactions take place at two different times therefore the necessary condition is that in the two phases (charging and discharging) the voltage is higher than the minimum gas production voltage in that phase. Only the total voltage between one phase and the other must be greater than 1.23V.

[060], It is therefore important to also be able to evaluate the contribution of the individual electrodes in the total voltage. For this, an RHE (Reversible Hydrogen Electrode) reference electrode is used which allows the working voltage of each electrode to be measured with respect to this reference (which takes the hydrogen evolution reaction as zero value point).

[061], With reference to Figure 3, the second conductive carbon electrode 12 is charged from 0 V to 1 V (relative to the reference electrode, RHE) during charging and discharging from 1 V to 0 V (still relative to the reference electrode RHE) during discharging; and the first bifunctional electrode 11 produces gaseous hydrogen H2 in the charging phase and gaseous oxygen 02 in the discharging phase. The voltage (voltage) of the first electrode 11 and the second electrode 12 are detected using a three-electrode cell relative to a third reference electrode (RHE, not shown).

Example 3 - durability test overtime

[062], The electrolytic cell 10 referred to in examples 1 and 2 was tested with 500 (five hundred) consecutive charge and discharge cycles and, as shown in figure 4, no loss of cell performance was noted.

[063], Thanks to the characteristics described above, provided separately or jointly with each other in particular embodiments, it is possible to respond to the aforementioned needs, obtaining the aforementioned advantages, and in particular:

- unlike traditional electrolytic hydrogen production devices, the decoupling of the water electrolysis reaction brings notable advantages such as the production of pure hydrogen and oxygen at separate times and/or places, thus avoiding contamination problems of gases due to crossover through septa or membranes; This avoids the formation of explosive mixtures, the use of expensive polymeric membranes and the need to purify the gases obtained; - as previously mentioned, devices operating through solid or liquid redox mediators are known, which allow working in a decoupled manner thanks to their reduction and oxidation in alternating mode (instead of the production of hydrogen and oxygen respectively). However, their use is strongly limited by the voltage and pH ranges at which they can operate, and in fact they are generally stable in narrow pH ranges and their working voltage does not allow a broad spectrum modulation of the currents at which the system can work . On the contrary, the use of a conductive carbon of suitable porosity capable of accumulating electrical charges without carrying out oxidation-reduction reactions allows the operating conditions to be separated from the type of redox mediator used. In particular, activated carbon having the parameters described above is suitable for the purpose. The charging phase and the discharging phase can follow one another for a number of consecutive cycles exceeding 500 without significant deterioration in performance;

- the fact that the conductive carbon is stably fixed to the electrode support makes it possible (i) to make the use of a separation membrane between the electrodes superfluous, (ii) to be able to invert the polarity of the electrodes while always maintaining the polarity of the two electrodes inverse to each other, (iii) to not need to recirculate the conductive carbon outside the cell to allow it to be discharged with consequent structural simplification and cost reduction.

[064], Of course, the combinations of characteristics of the attached claims form an integral and integral part of the present description.

[065], To the embodiments described above, a person skilled in the art will be able to make numerous modifications, adaptations and replacement of elements with functionally equivalent elements without however departing from the scope of the attached claims.

Claims

1 . Electrolytic cell (10) for the decoupled electrolysis of water comprising:

- a first electrode (11), for the production of hydrogen (H2) and oxygen (02), alternatively;

- a second electrode (12);

- an alkaline aqueous solution (13) in which the first electrode and the second electrode are immersed;

- polarity inversion means operatively connected to both the first electrode and the second electrode; in which:

- the first electrode and the second electrode are polarized in the opposite way to each other in all operating conditions;

Said electrolytic cell (10) being characterized in that said second electrode (12) is made of solid porous conductive carbon stably fixed to an electrode support thereof.

2. Electrolytic cell according to claim 1 , wherein the porous conductive carbon of the second electrode (12) has electrical resistivity less than or equal to 10 Ohm*cm and surface area per unit weight belonging to the range 2000- 3000 m²/g.

3. Electrolytic cell according to claim 1 or 2, wherein the first electrode (11) is made of metal or metal alloy; and in which, preferably, said metal and said metal alloy comprise a metal belonging to the group consisting of: iron, cobalt, nickel, zinc, molybdenum, manganese, titanium, platinum, silver, combination of the previous ones.

4. Electrolytic cell according to any one of the preceding claims, wherein the second electrode (12) comprises conductive carbon doped with at least one of the following: manganese, nickel, iron, cobalt, polypyrrole, poly-aniline, doped poly-ethyldioxythiophene with polystyrenesulfonate, preferably with a concentration in the range 1-10% by weight relative to the weight of the conductive carbon.

5. Electrolytic cell according to any one of the previous claims, wherein the alkaline aqueous solution (13) comprises one or more alkaline inorganic salts and/or one or more hydroxides of alkaline and/or alkaline earth metals, in concentrations within the range 1-6 mol/l; and in which, preferably, the operating temperature is less than 70°C.

6. Electrolytic cell according to any preceding claim, wherein

-the alkaline aqueous solution (13) includes a potassium-based alkaline electrolyte in a concentration of approximately 5 mol/l;

- the alkaline aqueous solution (13) is contained in a container (14) measuring 3 cm x 1 .7 cm x 7.0 cm; - the first electrode (11) is made of platinum on titanium and has dimensions of 4 cm x 2 cm;

- the second electrode (12) has a surface area per unit of weight of approximately 2500 m²/g and has dimensions of 4 cm x 2 cm.

7. Electrolytic cell according to any preceding claim, wherein the first electrode and/or the second electrode is associated with an electrically conductive metallic support, for example a sheet, a mesh, a sponge; in which, preferably, the electrically conductive metallic support is made of a material belonging to the group consisting of: nickel, copper, iron, titanium, zinc, combination of the previous ones.

8. Method for the decoupled electrolysis of water comprising the following steps:

- providing at least one electrolytic cell (10) according to any preceding claim;

- carrying out a charging phase that produces hydrogen through the first electrode (11), in which the second electrode (12) is polarized with a positive charge;

- subsequently, carrying out a discharge phase which produces oxygen through the first electrode (11) with a positive charge, in which the second electrode (12) is polarized with a negative charge in order to discharge the positive charge previously stored in it.

9. Method according to claim 8, wherein:

- during the charging phase, hydroxyl ions (OH ) migrate by electrostatic attraction to the second electrode (12) forming an electrochemical double layer (EDL) on the surface of the second electrode;

- during the discharge phase, positive ions (K⁺), for example potassium ions, migrate to the second electrode (12) forming an electrochemical double layer (EDL) on the surface of the second electrode (12).

10. Method according to claim 8 or 9, wherein:

- during the charging phase, the second electrode (12) is charged from 0 Volt to 1 Volt with respect to the RHE (Reversible Hydrogen Electrode);

- during the discharge phase, the second electrode (12) is discharged from 1 Volt to 0 Volt, with respect to the RHE (Reversible Hydrogen Electrode).