WO2018189491A1

WO2018189491A1 - Energy converter comprising an electrochemical cell

Info

Publication number: WO2018189491A1
Application number: PCT/FR2018/050928
Authority: WO
Inventors: Mickael PRUVOST; Annie Colin; Philippe Poulin; Cécile MONTEUX
Original assignee: Paris Sciences Et Lettres - Quartier Latin; Centre National De La Recherche Scientifique - Cnrs -; Ecole Supérieure De Physique Et De Chimie Industrielles De La Ville De Paris; Universite Pierre Et Marie Curie (Paris 6)
Priority date: 2017-04-12
Filing date: 2018-04-12
Publication date: 2018-10-18
Also published as: FR3065315A1

Abstract

An energy converter (1) comprising at least one electrochemical cell (3) and a member (5) forming a switch. The electrochemical cell comprises a chamber (31), two electrodes (33; 35), each being arranged at least in part in the chamber, and an electrolyte (37) contained in the chamber between the two electrodes. The composition and the structure of each of the two electrodes and of the electrolyte are selected conjointly in such a way that: - each of the electrodes is chemically inert in the electrolyte, and - in the absence of external polarisation, the electrodes have a non-zero difference in potential with respect to each other. The member forming a switch is electrically connected between the two electrodes. The member is arranged to switch alternately between an open state and a closed state by means of an external energy.

Description

ENERGY CONVERTER COMPRISING AN ELECTROCHEMICAL CELL

The invention relates to the field of energy conversion. In particular, the invention belongs to the field of low power energy management.

Conventionally, accumulators, or batteries, convert a stored energy in chemical form into electrical energy. Most accumulators form a faradic system. Oxidation-reduction reactions take place and include charge exchanges between electrodes and an electrolyte. Some of these accumulators can be recharged by an external source of electrical energy, for example by a mains connection via a transformer, or by means of a source of mechanical energy converted by an alternator, for example activated manually. Providing an externally generated current reverses the chemical oxidation-reduction reactions. There are also supercapacitors formed of two electrodes generally identical and separated by an electrolyte. Supercapacitors form so-called capacitive systems: by applying a potential difference between the two electrodes (during recharging), the anions and the cations of the electrolyte agglutinate respectively to the two of them. electrodes. When the capacitive energy is released (in use), a current can be obtained by connecting between the two electrodes. Once the capacitive energy has been consumed and until a charging voltage is applied again between the two electrodes, the supercapacitor becomes inactive and unusable.

Accumulators and supercapacitors require external power supply between each full use of the previously stored energy. The devices thus equipped can be considered autonomous only over limited durations.

Devices allow, under laboratory conditions, to create a voltage between two electrodes movable relative to each other and between which a drop of water is trapped. Such an assembly is for example described in the following article: Pak and Al, "Electrical power generation by mechanically modulating electrical double layers", Nature Communication, 2013. Such a device is difficult to implement reliably out of the conditions of 'a laboratory. Energy recovery devices operating based on a deformation of piezoelectric material are also developed. The following article gives some examples: "A piezoelectric vibration based generator for wireless electronics"; S. Roundy and PK Wright; INSTITUTE OF PHYSICS PUBLISHING; 1131-1142 (2004). The reliability over time of such devices is not entirely satisfactory.

The invention improves the situation.

The Applicant proposes an energy converter comprising at least one electrochemical cell and a switch member. The electrochemical cell comprises an enclosure, at least two electrodes, each of the electrodes being disposed at least partially in the enclosure, and an electrolyte contained in the enclosure between the two electrodes. The composition and structure of each of the two electrodes and the electrolyte are jointly selected so that:

each of the electrodes is chemically inert in the electrolyte, and

- In the absence of external bias, the electrodes have a non-zero potential difference with respect to each other.

The switch member being electrically connected between the two electrodes of the electrochemical cell, said member being arranged to switch alternately between an open state and a closed state under the effect of an external energy.

Such a converter makes it possible to convert an external energy into an electrical energy by means of a capacitive energy and in a substantially continuous manner. The converter does not require a prior power supply from an external power source. Electrical energy can be consumed immediately or stored for later use.

In addition, such a converter may have various shapes and dimensions, which makes it possible to adapt to many uses and conditions, particularly in terms of compactness, reliability, type and intensity of the external energy available at the input, and quantity of energy to output. In applications relating to portable objects, sometimes called "wearables", the converter can be arranged to integrate in an environment for which the size and / or mass must be particularly low. The switch subjected to external energy can be deported and / or detachable from the electrochemical cell. The cell may have a substantially flat shape. Several cells can be combined, for example in a stack wherein an electrode may be common to two superimposed cells. No movement of the electrodes relative to each other is necessary for operation, which contributes to the reliability of the electrochemical cell. The absence of a voluntary chemical reaction in the chamber makes it possible to confer on the electrochemical cell an infinite theoretical lifetime. Neither the electrodes nor the electrolyte are consumed. In addition, and compared to known energy storage devices and comparable dimensions, the capacitive energy stored in the electrochemical cell is small in quantity and short duration: the successions of openings and closures of the switch allow to cumulatively provide a quantity of energy of substantial external origin. This mode of operation makes it possible to qualify the entire energy converter device as electrical energy rather than as an energy storage device. The incoming energy taken locally activates the switch member. The switch member is open / closed repeatedly. The converter may be coupled to an energy storage device, for example known in itself, to consume the external energy when it is available and to restore the electrical energy when it is necessary.

According to a second aspect, the applicant proposes a kit comprising the following elements:

an electrochemical cell comprising an enclosure, at least two electrodes, each of the electrodes being disposed at least partly in the enclosure, and an electrolyte contained in the enclosure between the two electrodes, and

- A switch member adapted to be electrically connected between two electrodes of an electrochemical cell. Said member is arranged to switch alternately between an open state and a closed state under the effect of an external energy.

The composition and structure of each of the two electrodes and the electrolyte are jointly selected so that:

each of the electrodes is chemically inert in the electrolyte,

The switch member being adapted to be electrically connected between the two electrodes of the electrochemical cell.

The kit may further comprise a control circuit adapted to be electrically connected to the two electrodes of the electrochemical cell, the control circuit being arranged so that, once connected to the electrochemical cell, the capacitive energy of the electrochemical cell is converted into electrical energy capable of supplying an energy storage device when the switch member changes to a closed state.

In another aspect, the Applicant proposes a sensor comprising an energy converter as defined herein. Such a sensor may be at least partly autonomous, that is to say rendered independent of an external power supply. The energy required for the operation of the sensor can be entirely removed from its immediate environment in a form other than electrical and converted into electrical energy through the converter. The implementation of a converter as defined herein is particularly advantageous in the context of sensors, especially small. Such sensors can be easily arranged in locations that are difficult to access. The installation of a wiring for a sensor power supply and / or access to maintenance to exchange a battery are made superfluous. In the case of sensors capable of communicating wireless data (so-called "communicating" sensors), the sensors may be devoid of any external physical connection.

The following features may optionally be implemented. They can be implemented independently of each other or in combination with each other:

The composition and the structure of each of the two electrodes and the composition of the electrolyte are jointly selected so that the ions accumulated near each of the two electrodes are of opposite signs. This facilitates the distribution of anions / cations in the electrolyte. At least one of the electrodes comprises metal. Metal electrodes are reliable over time, can deform while remaining functional and are inexpensive.

At least one of the electrodes comprises metal. The composition of the metal and the pH of the electrolyte are jointly selected so that the at least one electrode has an external passivation layer. Passivation protects the electrodes from unwanted chemical reactions, in particular by oxidation-reduction.

At least one of the electrodes comprises porous carbon. Porous carbon has good chemical stability and can be combined with various other electrodes and electrolytes. - The first electrode comprises porous carbon. The second electrode comprises zinc, magnesium or aluminum. The electrolyte comprises a saline solution in liquid or gel form. The various combinations tested by the applicant have shown good performance.

The composition and the structure of each of the two electrodes and of the electrolyte, and the mutual arrangement of the electrodes in the enclosure, are jointly selected so that, in the absence of external polarization, the capacitive load between the two electrodes in a stable regime is between 0.01 and 10 F.cnï ² . Such charges can be achieved with particularly compact structures while being sufficient to power the least energy consuming objects.

- The energy converter further comprises:

a control circuit electrically connected to the two electrodes of the electrochemical cell and including the switch member, and

- An energy storage device electrically connected to the two electrodes via the control circuit.

The control circuit is arranged so that the capacitive energy of the electrochemical cell is converted into electrical energy supplying the energy storage device when the switch member is in the closed state. Such a converter can equip a device by making it autonomous. It becomes unnecessary to recharge or replace the battery. - The switch member comprises at least one transistor. This improves reliability by replacing at least some of the mechanical components.

Other features, details and advantages of the invention will appear on reading the detailed description below, and on the analysis of the appended drawings, in which:

FIG. 1 shows a diagram of a converter according to the invention,

FIGS. 2 and 3 each show a diagram equivalent to an operating state of a converter according to the invention,

FIG. 4 shows a measurement of a voltage across a measurement resistor disposed between the electrodes at the closing of a switch of a converter according to the invention, FIG. 5 shows a measurement of a voltage across a measurement resistor disposed between the electrodes of a converter according to the invention,

FIG. 6 shows experimental results of a voltage measurement as a function of time,

FIG. 7 is a schematic representation of an experimental setup,

FIG. 8 shows two superimposed curves of a measured voltage and a resistance value of a switch over time, and

FIGS. 9 and 10 are two views of an experimental zinc electrode.

The drawings and the description below contain, for the most part, elements of a certain character. They can therefore not only serve to better understand the present invention, but also contribute to its definition, if any.

In the following, the terms "voluntary exchanges" are used in connection with ions. Voluntary exchange here means to distinguish itself from known devices for which the oxidation-reduction and electrolysis phenomena are necessary, and therefore voluntary, to supply a current. In the present context, such exchanges are in theory non-existent. In practice, defects or impurities can lead to involuntary ion exchanges between the electrodes and the electrolyte. Such exchanges are considered negligible. Figure 1 shows a converter 1. The converter 1 comprises at least one electrochemical cell 3 and a switch member 5.

The cell 3 comprises an enclosure 31, or enclosure, a first electrode 33, a second electrode 35 and an electrolyte 37.

Each of the electrodes 33, 35 is arranged at least partly in the chamber 31. In the example described here, each of the electrodes 33, 35 is entirely contained in the enclosure 31. Each of the electrodes 33, 35 is electrically connected to a conductive element projecting from the enclosure 31. Thus, the parts projecting from the enclosure 31 form connectors, or terminals, of the cell 3 through which each of the electrodes 33, 35 may be electrically connected to 3. Alternatively, at least one of the electrodes 33, 35 protrudes from the cell 3 through the enclosure 31. Such an electrode 33, 35 can be directly connected to external elements at the same time. cell 3. The enclosure 31 has sealing properties compatible with the electrolyte 37 and able to prevent its leakage to the outside of the enclosure 3. The enclosure 31 further protects the contents of the cell 3 against the external environment so as to limit the risks of contamination which could lead to unwanted chemical reactions, in particular oxidation phenomena.

In the operating example described next, and according to what is shown in the figures, the first electrode 33 forms an anode 33 while the second electrode 35 forms a cathode 35. In operation, the cell 3 is devoid of exchange voluntary ion between the electrodes 33, 35 and the electrolyte 37. It is stated that the use of the terms "anode" and "cathode" is usually reserved for faradic systems, for example in which the anode is able to give way electrons by chemical reaction with a substrate while the cathode captures electrons by chemical reaction with a substrate. In the present context, in the absence of a voluntary chemical reaction, the terms "anode" and "cathode" are used by analogy to facilitate the distinction between the two electrodes 33, 35. In accordance with FIG. 1, the anode 33 releases electrons in the circuit while the cathode 35 captures the electrons of the circuit.

The electrolyte 37 is contained in the enclosure 31. The electrolyte 37 is contained at least between the two electrodes 33, 35 and in contact with each of the two electrodes 33, 35. In the example described here, the electrodes 33, In a variant, the electrodes 33, 35 may form at least part of the enclosure 31. In a variant, an enclosure may contain a plurality of pairs of electrodes. For example, the electrodes can form walls compartmentalizing the inside of the enclosure 31. The enclosure then houses an alternation of anode and cathode, an electrode that can be paired with two separate electrodes.

The composition and the structure of each of the two electrodes 33, 35 and the electrolyte 37 are jointly selected so that:

each of the electrodes 33, 35 is chemically inert in the electrolyte 37,

in the absence of external polarization, the electrodes 33, 35 each have a non-zero potential difference with the electrolyte 37, so that an accumulation of ions of the electrolyte 37 is generated near each of the two electrodes 33, 35, and

in the absence of external polarization, the electrodes 33, 35 have a non-zero potential difference between them, so that the accumulation of ions near each of the two electrodes 33, 35 is asymmetrical relative to to the other.

The electrodes 33, 35 may each have an apparent standard potential, the two standard potentials being different from each other. This corresponds in particular to an example two different metal electrodes. The terms "apparent standard potential" are used by analogy with oxidation-reduction systems. However, the oxidation-reduction reactions are prevented in the converter 1. The electrodes may have, especially at the surface, excess sites or deficient in charges. This corresponds in particular to the case of a porous carbon electrode described below.

Such potential differences are enabled by the selection of the nature (structure and composition) of the electrodes and the electrolyte. The potential differences of electrodes 33, 35 and electrolyte 37 and the accumulation of ions (anions or cations) at the resulting electrode-electrolyte interfaces (adsorption phenomena) makes it possible to form an electrical double layer on the surface. each electrode 33, 35 in contact with the electrolyte 37. The ions form a screen on the surface of the electrode. Each of these interfaces then stores a capacitive energy. The asymmetry of the accumulation between the anode 33 and the cathode 35 causes a charge imbalance between the two electrodes 33, 35. The electrolyte 37 allows the transport of the ions but not the electrons. In an open state of the switch member 5, the flow of charges (electrons) from one to the other of the electrodes 33, 35 is thus prevented. An electrical imbalance is voluntarily created. In the example described here, a first of the electrodes, here the anode 33, comprises a porous material. The porosity makes it possible to reach a large adsorption interface with the electrolyte 37 for a small occupied volume of the electrode. When only one of the two electrodes is porous, a difference in interface area is created between two electrodes of similar shapes and dimensions. This contributes to the asymmetry of the charges between the two poles of the cell 3. The electrode has, for example, a free porosity greater than 60% by volume. Porosity is measured, for example, by mercury porosimeter. The porosity also makes it possible to reduce the mass of the electrode. In the example described here, porous carbon is used. The electrode is made electrically conductive by the addition of carbon black. Such an electrode is inert with at least some electrolytes used in the examples described below, easy to implement and inexpensive. Methods of manufacturing such electrodes are known and for example described in the following article: Salitra, G., Soffer, A., Eliad, L., Cohen, Y. & Aurbach, D. Carbon electrodes for double -layer capacitors . I. "Relations between ion and pore dimensions". J. Electrochem. Soc. 147, 2486-2493 (2000). In the example described here, the second of the electrodes, here the cathode 35, comprises a metal. The cathode 35 comprises zinc (Zn). The nature of the metal and the pH of the electrolyte 37 are jointly selected or adjusted to promote the creation and / or preservation of an outer layer of oxide on the electrode. The oxide layer protects the electrode from oxidation, in particular by oxygen. The oxide layer renders and / or maintains the electrode chemically inert and insensitive to oxy-reduction reactions in the electrolyte (passivation). In the example of a zinc electrode, the pH of the electrolyte 57 is preferably chosen to be between 9 and 11.25, for example about 10.7, in order to promote the existence of a hydroxide layer. Protective Zn (OH) _{2 (S)} .

Zinc has a low mass, good flexibility, and is inexpensive compared to other metals. Alternatively, other metals may be used, for example aluminum (Al) or gallium (Ga). In these cases, the composition of the electrolyte 37, and in particular its pH, are adapted to render the electrode chemically inert.

The composition of the electrolyte 37 is chosen so as to allow the mobility of the ions while being electrically insulating. By electrical insulation is meant here that the composition of the electrolyte, the inter-electrode distance and the expected operating voltages are compatible to prevent the appearance of an electric arc between the two electrodes (breakdown). The electrolyte 37 may comprise an ionic liquid such as an aqueous solution or an organic solvent, an ionized gas or any other fluid containing mobile ions. In the examples described herein, the electrolytes 37 include ionic salt solutions. Such as NaCl, KCl, KNO ₃ , MgCl ₂ . The pH of the solution can be adjusted by adding an acid or a base. In the example of a porous carbon anode 33 and a zinc cathode, an electrolyte 37 comprising an aqueous solution with a salt such as sodium chloride NaCl and sodium hydroxide NaOH may for example be used.

The electrolyte 37 here takes a liquid form. Alternatively, the electrolyte may take the form of a solid, a gel, a gas or even a plasma. A solid form or gel reduces the risk of leakage in case of leakage of the enclosure 31.

The following table shows some examples of possible combinations.

2 Porous carbon Magnesium NaCl> 13

3 Porous carbon Aluminum NaCl 5 - 7

4 Porous Carbon Zinc Aqueous Gel + NaCl 9 - 11.25

The switch member 5 is electrically connected between the two electrodes 33, 35 of the cell 3. Said member 5 has an open state, that is to say electrically isolating the electrodes 33, 35 from each other , and a closed state, that is to say allowing the flow of charges (electrons) from one electrode to another.

In an experimental example, the switch member 5 is controlled by a piezoelectric actuator. Thus, an open / close frequency can be chosen precisely. For example, frequencies between 0.1 and 10 Hz can be applied. The member 5 may comprise, for example, a switch with a flexible blade ("Reed switch"). Such a switch can be activated remotely via magnetic fields. Alternatively, the member 5 may comprise a bimetal switch, which can be activated by temperature variations. The member 5 may also include a transistor. The switch function can, for example, be provided by a transistor rather than a mechanical switch. The transistor alternately passes from a passing state to a blocking state and vice versa under the effect of external stimulation. The member 5 is adapted according to the type of energy supplied to the input of the converter 1 and the expected operating frequencies.

In the open state of the member 5, the imbalance of charges between the two electrodes 33, 35 described above is maintained. Capacitive energy is stored at the interface of the anode 33 with the electrolyte 37 and at the interface of the cathode 35 with the electrolyte 37. The current flowing in the circuit is zero. This state is equivalent to the electrical diagram shown in FIG. 2 and to a charged capacity. A potential difference can be measured between the two electrodes 33, 35, for example by means of a voltmeter V. This potential difference results from the potential difference between the two electrodes. The value of the difference in potential can be chosen by playing on the combination of the nature (for example the composition) of the two electrodes 33, 35, their structure (for example porous or not), their dimensions, their positions and mutual orientations. Upon closure of the switch member 5, the electrodes are short-circuited. The double ionic layers at the two electrolyte-electrolyte interfaces are being restructured. Such an ionic reorganization is concomitant with the circulation of charges (the electrons) in the circuit. By the effect of the electrical rebalancing, electrons move from the anode 33 to the cathode 35. The current is non-zero. This state is equivalent to the electrical diagram shown in FIG. 3 and to a capacitance that discharges into a circuit. The flow of the current can be measured by means of a voltmeter V and by the interposition of a resistor R between the two electrodes 33, 35 (Ohm's law).

The graph of FIG. 4 represents the voltage (in volts) at the terminals of a 100 ohm resistor connected between the two electrodes 33, 35 as a function of time (in seconds). The transition from the open state to the closed state of the switch member 5 is caused once at time t = 15 seconds. At T = 6000 seconds, the voltage is substantially stable at about 20 mV.

The graph in FIG. 5 represents the voltage (in volts) at the terminals of a 100 ohm resistor connected between the two electrodes 33, 35 as a function of time (in seconds). The switch member 5 changes from the open state to the closed state at a frequency of 6 hertz, and from the closed state to the open state at a frequency of 6 hertz (ie a change of state in a sense or in the other at an average frequency of 12 hertz). The frame referenced 71 corresponds to an open state, that is to say that no current flows in the circuit and the potential difference between the two electrodes 33, 35 increases gradually under the effect of the reorganization of the ion interfaces , while the frame referenced 73 corresponds to a closed state, that is to say that a current flows in the external circuit to the cell 3 and the potential difference between the two electrodes 33, 35 decreases progressively under the effect of the reorganization of the ion interfaces. The transition from the open state to the closed state of the switch member 5 generates the creation of a current. Thus, the external energy supplied to the converter 1 via the switch member 5 is converted into electrical energy. The electrical energy supplied to the terminals of the converter 1, or to the two electrodes 33, 35, can be consumed immediately by suitable electrical elements. This energy can also be stored by a storage device such as a battery or a supercapacitor. In the case of a coupling of the cell with a storage device, the storage device is connected to the terminals of the cell 3 via a control circuit. The control circuit is arranged for:

allow the current to be transmitted to the storage device during the periods when the switch member is closed, and - To prohibit the unloading of the storage device to the cell 3 when the switch member 5 is open. The control circuit comprises for example a diode arrangement. The circuit may further comprise a voltage optimization circuit. Such a converter makes it possible to make certain electric power consuming devices autonomous by capturing an energy available in the immediate environment. Such collected external energy may be available in the form of vibrations, heat, temperature variations, motions, magnetic fields, pressures, flows or light. The converter outputs capacitive rather than faradic energy. By this aspect, the converter is different from the usual batteries and can recall the supercapacitors. The converter captures the external energy and transforms it into electrical energy in a substantially continuous manner. Unlike a supercapacitor, it does not need to be switched on beforehand to work (neither initially nor to be recharged). This would not make any sense because, in some embodiments, the converter does not store energy in the usual sense of the term (electrochemical cell and associated switch member, devoid of storage device): the capacitive energy is accumulated in the electrodes only transiently, during a period of opening of the switch, that is to say a duration inversely proportional to the frequency of change of state of the switch. The operation and structure of such converters make them more reliable for use than devices in the piezoelectric field.

The power provided by such a converter can be estimated according to the intensity provided. The power corresponds to the area under the voltage curve during the phases during which the switch member 5 is in a closed state (frame 73 of FIG. 5). The power output depends in particular on the dimensions of the cell and in particular the useful surfaces of the electrodes and the energy that can be captured in the environment. The applicant has calculated the power provided by prototypes and estimates that the power as a function of the useful surface is between 0.01 and 10 mW.cm ^-2 , preferably between 0.1 and 1 mW.cm ^-2 , example about 0.3 mW.cm ^"2. Those skilled in the art includes that in such power lines are adapted to supply low-energy electronic devices, particularly in the area of the connected objects. in such case, and in particular when such devices are worn by a user, the body heat, vibrations and / or movements can act as external energy for the converter.The switch member is adapted according to the energy to be captured. . The nature of the electrodes and the electrolyte can be chosen according to the type and properties of the energy of external origin that can be captured in the environment of the converter. For example, the density of the maximum capacitive load of the cell 3 may be voluntarily clamped so that the amount of energy required to open the switch member is less than the amount of energy received by the switch member. member 5 forming a switch.

Such a converter can be provided in spare parts, for example in kit form. Preferably, such a kit comprises at least the electrochemical cell 3 and the switch member 5 adapted to be connected to each other. The kit may further comprise, for example, a power storage device and a compatible control circuit. Each of the elements of the kit is able to be connected operationally with the other elements.

Such a converter finds particularly advantageous applications in the fields of portable electronics, connected objects (or IoT for "Internet of Things"), sensors and in particular autonomous sensor networks. Such a converter responds particularly well to the constraints of miniaturization, mobility and low energy consumption. The converter can be combined with storage means for unsynchronized energy use with the availability of the local energy source taken from the environment. In other cases, the storage means are absent: the energy taken from the environment and converted is immediately consumed. For example, in the case of sensors, the environmental energy and the physical phenomenon to be detected may have a common source. A vibration sensor can both detect / measure vibrations while using the mechanical energy of said vibrations to power the sensor via the converter. Presence sensors, temperature variations, humidity or pressure (tactile interfaces) may for example be equipped with a converter as defined herein.

The present description is based on experimental tests carried out by the applicants and illustrated in FIGS. 6 to 10. During the tests, the two electrodes (one of platinum, the other of carbon) are immersed in a 6.1 M aqueous NaCl solution, the pH of which is adjusted to 10 with NaOH. A switch is associated with these two electrodes (electrically interposed between the two electrodes). The switch allows the connection or disconnection of the electrodes. A load resistor, referenced Ri _oad in FIG. 7, of 100 Ω is integrated in the circuit for measuring current using Ohm's law. FIG. 6 shows an extract of the signal measured in the load resistor Ri _oad by opening and closing the switch at a frequency of 1 Hertz. The signal was recorded for ten days without editing or significant signal variation. The power recorded during these 10 days is constant and equal to 1μλ ¥. The resistance used is 100 Ω. The average current intensity is 0.1 mA.

In the (false) assumption that usual oxy-reduction phenomena were involved, the result of the measurements would correspond to a consumption of 10 ^-3 moles of electrons or redox component, but none of the components present in the system (platinum, water, salt, oxygen, carbon) can not act as a reducing agent in the range of the measured potential.

After undergoing the above treatment, careful transmission electron microscopy (TEM) analysis of the platinum electrode was conducted. No degradation of the platinum electrode was detected. It can be deduced that no consumption of the platinum electrode is detectable. The potential of the platinum electrode is equal to 0.3 V / ESH (ESH for "Standard Hydrogen Electrode"). The potential of the carbon electrode is equal to 0.46 V / ESH. The pH is equal to 10.

In the hypothesis (false) in which the phenomena involved would be faradic, they could only result in impurities (materials other than platinum, water, salt, oxygen, and carbon). However, the sample volume was 100 ml, which would imply that the species unknown (impurity) has a concentration of at least 10 ^"2 mol / L. The samples were prepared with the usual precautions and the surfaces of the electrodes have been cleaned, so it is highly unlikely that an impurity will be present in such a large concentration, and such an impurity would obviously have been detected, so the hypothesis of faradic phenomena is not likely.

The power harvested under the aforementioned conditions of the experiments is low and equal to approximately 1 μλ ¥ per cm ² . To increase the value of the power, additional experiments have been carried out in which the platinum electrode is replaced by a zinc electrode (whose characteristics are otherwise similar to that of platinum). The two electrodes (zinc and carbon electrodes) are immersed in an aqueous ionic solution (6.1M NaCl, pH = 10 adjusted with NaOH). A switch is associated with these two electrodes for their connection or disconnection. A 10 Ω Ri _oad load _resistor is integrated in the circuit to measure current using Ohm's law. FIG. 8 shows the signal measured in the load resistance Ri _oad by opening and closing the switch at a frequency of 1 Hertz. The recorded power is substantially equal to 1 mW, which corresponds to a power density of 0.4 mW / cm ² or 10 mW / cm ³ . Such a value is three orders of magnitude greater than that recorded by the so-called "electrostrictive" materials and an order of magnitude greater than the best piezoelectric systems known to the inventors. "Electrostrictive" materials are materials having particular properties of electrostriction, that is to say having significant changes in shape under the effect of an electric field.

As for the aforementioned systems (with platinum electrode), it was verified that the measured intensity did not come from faradic phenomena. These can theoretically occur at zinc electrodes, zinc being oxidizable to Zn ²⁺ . However, at a pH of 10, zinc is protected from oxy-reduction phenomena by an oxide layer that is not very permeable to chemical exchanges. Such phenomena can be slow. To verify the absence of faradic phenomena, even slow, 30-hour experiments were conducted during which the power is recorded. To do this a zinc sheet of 0.7 grams has been used. It has been measured and recorded, in a resistance of 100 Ω, a constant power of lmW which corresponds to a load of 300C over 30h. In the (false) hypothesis where oxy-reduction phenomena at the zinc level would be responsible for this charge, this would imply a consumption of zinc equal to 0.1g. However, after 30h, the mass of the zinc sheet is unchanged (with a measurement accuracy strictly less than one-tenth of a gram). After drying the zinc sheet, the 0.7 gram mass remained unchanged.

The zinc electrode was then cut. Careful transmission electron microscopy (TEM) analysis of the zinc electrode was, in turn, conducted. A protective layer of zinc on the surface is clearly visible. No trace of oxidation was detected (areas that appeared in black) in the body of the electrode. No degradation was detected. Figures 9 and 10 are clichés made on this occasion. Zinc is not, therefore, attacked by faradic phenomena. According to the inventors, the only reasonable interpretation is that the energy observed does not come from faradic processes but from capacitive processes. When the two electrodes are not connected, they have a mixed potential, ie two potentials different from each other. These potentials depend on the nature of the solvent and the electrodes. When they are electrically connected to each other (via the switch in the closed position), some electrons circulate from the low electric potential to the highest, that is to say from the platinum to carbon or from zinc to carbon for the two examples mentioned above. This induces desorption of the counter ions on the surface of the electrodes. When the two electrodes are again disconnected from each other (by the opening of the switch), the counter-ions return to their previous position. Opening the switch consumes (mechanical) energy. The relaxation of the voltage is related to the reconstruction of the double layer of electrons and could be slow. The recovered energy probably comes from the work provided by opening the switch and heat related to the adsorption-desorption process of the counter ions on the electrodes. The measured energy is therefore not derived from faradic phenomena.

The invention is not limited to the examples of converters, kits and sensors described above, only by way of example, but it encompasses all the variants that may be considered by those skilled in the art within the scope of the present invention. sought protection.

Claims

claims

An energy converter (1) comprising at least one electrochemical cell (3) and a switch member (5), the electrochemical cell (3) comprising an enclosure (31), at least two electrodes (33; 35), each of the electrodes (33; 35) being disposed at least partly in the enclosure (31), and an electrolyte (37) contained in the enclosure (31) between the two electrodes (33; 35), the composition and the structure of each of the two electrodes (33; 35) and the electrolyte (37) being jointly selected so that:

each of the electrodes (33; 35) is chemically inert in the electrolyte (37), and

in the absence of external bias, the electrodes (33; 35) have a non-zero potential difference with respect to each other,

the switch member (5) being electrically connected between the two electrodes (33; 35) of the electrochemical cell (3), said member (5) being arranged to switch alternately between an open state and a closed state under the effect of external energy.

The energy converter (1) according to claim 1, wherein the composition and structure of each of the two electrodes (33; 35) and the composition of the electrolyte (37) are furthermore jointly selected so that the ions accumulated near each of the two electrodes (33; 35) are of opposite sign.

3. Energy converter (1) according to one of the preceding claims wherein at least one (35) of the electrodes (33; 35) comprises metal.

4. Energy converter (1) according to one of the preceding claims, wherein at least one (35) of the electrodes (33; 35) comprises metal, the composition of the metal and the pH of the electrolyte ( 37) being jointly selected so that said at least one electrode (35) has an outer passivation layer.

5. Energy converter (1) according to one of the preceding claims wherein at least one (33) of the electrodes (33; 35) comprises porous carbon.

6. Energy converter (1) according to one of the preceding claims wherein

the first electrode comprises porous carbon, the second electrode comprises zinc, magnesium or aluminum and the electrolyte comprises saline solution in liquid form or gel.

7. Energy converter (1) according to one of the preceding claims, wherein the composition and the structure of each of the two electrodes (33; 35) and the electrolyte (37), and the mutual arrangement of the electrodes. (33; 35) in the enclosure (31), are jointly selected so that, in the absence of external bias, the capacitive load between the two electrodes (33; 35) in a stable regime is between 0.01 and F.cn1 ² .

8. Energy converter (1) according to one of the preceding claims, further comprising:

- a control circuit electrically connected to the two electrodes (33; 35) of the electrochemical cell (3) and including the member (5) forming a switch, and

an energy storage device electrically connected to the two electrodes (33; 35) via the control circuit,

the control circuit being arranged so that the capacitive energy of the electrochemical cell (3) is converted into electrical energy supplying the energy storage device when the switch member (5) is in the closed state.

9. Energy converter (1) according to one of the preceding claims, wherein the member (5) forming a switch comprises at least one transistor.

10. Kit comprising the following elements:

an electrochemical cell (3) comprising an enclosure (31), at least two electrodes (33; 35), each of the electrodes (33; 35) being disposed at least partly in the enclosure (31), and an electrolyte ( 37) contained in the enclosure (31) between the two electrodes (33; 35), and

- a member (5) forming a switch adapted to be electrically connected between the two electrodes (33; 35) of the electrochemical cell (3), said member (5) being arranged to switch alternately between an open state and a closed state under the effect of an external energy, the composition and the structure of each of the two electrodes (33; 35) and the electrolyte (37) being jointly selected so that:

each of the electrodes (33; 35) is chemically inert in the electrolyte (37),

in the absence of external bias, the electrodes (33; 35) have a non-zero potential difference with respect to each other.

Kit according to claim 10, further comprising: a control circuit including the switch member and able to be electrically connected to the two electrodes (33; 35) of the electrochemical cell (3), the control circuit being arranged so that, once connected to the electrochemical cell (3), the capacitive energy of the electrochemical cell (3) is converted into electrical energy capable of supplying an energy storage device when the member (5) forming a switch is in a closed state.

12. Sensor comprising a power converter according to one of claims 1 to 9.