WO2016182439A1

WO2016182439A1 - Method and system for performing a capacitive deionisation and/or capacitive electrical energy generation

Info

Publication number: WO2016182439A1
Application number: PCT/NL2016/050333
Authority: WO
Inventors: Pieter Maarten Biesheuvel; Hubertus Victor Marie Hamelers
Original assignee: Stichting Wetsus, European Centre Of Excellence For Sustainable Water Technology
Priority date: 2015-05-11
Filing date: 2016-05-10
Publication date: 2016-11-17
Also published as: NL2014788B1; NL2014788A

Abstract

The present invention relates to a method and system for capacitive de-ionization or capacitive electrical energy generation processes. The method according to the invention comprises the step of: - providing a system with a first capacitive electrode and a second capacitive electrode that are connected with a circuit; - chemically modifying at least one of the first and second electrodes with charged groups providing an asymmetric electrode system, wherein the modifying is performed such that the equilibrium point, defined as the Voltage at which positive and negative charges are equal, is above +150 m V or below -150 m V; and - applying a Voltage to perform capacitive deionisation or applying flows with different charges to perform electrical energy generation.

Description

METHOD AND SYSTEM FOR PERFORMING A CAPACITIVE DEIONISATION AND/OR CAPACITIVE ELECTRICAL ENERGY GENERATION

The present invention relates to a method for performing capacitive deionisation and/or capacitive electrical energy generation. More specifically, the method relates to an efficiency improvement of such operations.

In conventional capacitive deionisation (CDI) water is desalinated by charging two parallel, closely spaced, porous carbon electrodes with a fluid flowing between the electrodes. The cell voltage between the two electrodes (V_ceu) is the voltage of the anode minus cathode. CDI involves a cyclic operation wherein cell voltage is operated at two values, i.e. charging voltage (V_ch typically 1.2 V) and discharge voltage (V_disch typically 0 V). In the charging phase the fluid is desalinated. After the electrodes have been saturated with ions the cell voltage is reduced from V_ch to V_disch such that the electrodes release the ions in the discharge state, wherein electric current flows in the reverse direction. CDI can be applied to either produce fresh water or to harvest salt ions as described in WO 2011/155839, for example.

Electrical energy generation with capacitive electrodes uses concentration differences between two fluids, for example sea water with a salt content of about 30 g/1 and diluted or river water with a salt content of about 1 g/1. Other flows with concentration differences can also be used, such as industrial salt water streams and waste water treatment plants. A capacitive electrical energy generation process is described in WO 2010/062175, for example. Other capacitive energy generation approaches use capacitive electrodes in combination with a membrane stack enabling the performance of capacitive reverse electrodialysis energy generation (such as Capmix). Also, methods for generating energy may involve methods to harvest energy from a gas flow with the use of capacitive electrodes. This enables energy generation from a flue gas with C0₂, for example.

These conventional methods with capacitive electrodes operate at least at two states, a charging state and a discharging state. Limitations, including efficiency limitations of the capacitive electrodes, influence the overall performance of these operations.

An object of the invention is to obviate the above mentioned problems and to provide a method for performing such operations more effectively and/or efficiently.

This object is achieved with the method for performing capacitive deionisation and/or capacitive electrical energy generation according to the invention, the method comprising the steps of:

providing a system with a first capacitive electrode and a second capacitive electrode that are connected with a circuit;

- chemically modifying at least one of the first and second electrodes with charged

groups providing an asymmetric electrode system, wherein the modifying is performed such that the equilibrium point, defined as the Voltage at which positive and negative charges are equal, is above +150 mV or below -150 mV; and

applying a Voltage to perform capacitive deionisation or applying flows with different charges to perform electrical energy generation.

Providing a system with at least a first capacitive electrode and a second capacitive electrode enables operating the system in at least a charging and a discharging state. By providing a voltage to the electrodes a deionisation process can be performed. Providing different flows with different concentrations enables electrical energy generation.

Chemically modifying at least one of the electrodes, and preferably two or more electrodes, with charged groups provides a so-called asymmetric system. In operations with a capacitive electrode system, the equilibrium point is defined as the voltage at which positive and negative charges in a capacitive electrode are equal. In conventional systems with unmodified capacitive electrodes the equilibrium point is at a voltage of about 0 mV. It is known that operations that operate with a cell voltage (V_ceU) around this equilibrium point perform relatively ineffective. In practice, for this reason the equilibrium point is avoided as much as possible. This significantly limits the AV_cen between the charging state and the discharging state.

By chemically modifying the electrodes with charging groups the anode is provided with negative chemical groups and/or the cathode is provided with positive chemical groups. In a presently preferred embodiment both electrodes are chemically modified. According to the present invention, the modifying of one or more of the electrodes is such that the equilibrium point lies above +150 mV or below -150 mV. This movement of the equilibrium point, as compared to conventional (capacitive) electrode systems, enables enlarging the AV_ceii between the charging and discharging state, while avoiding the equilibrium point. This may significantly increase the cycle time between switching from the charging state to the discharging state and vice versa, thereby reducing the switching frequency between the different states. This reduces efficiency losses due to the switching between the states. Furthermore, efficiency is increased as the loss of production time is significantly reduced due to the reduction of the switching frequency. This renders the capacitive operations associated with deionisation and electrical energy generation more effective and more efficient.

With reference to figures 1A and IB manipulating the equilibrium point can be explained in more detail. The movement of the equilibrium point will be illustrated in relation to a CDI cell.

In a similar manner this also applies to capacitive energy generation processes.

The electrical potential between anode and cathode in a cell is called the cell voltage, V_cell.

This cell voltage changes in time between certain (end-point) values. A cycle in operating a CDI cell, and similarly a cell for capacitive energy generation, comprises two steps. In the charging step, the cell voltage is set to (go to) a certain value called the charging voltage, V_ch. In the discharge step, the cell voltage is set to (go to) the discharge voltage, V_disch- How V_cell changes in time defines the cycle. The cell voltage is either suddenly changed from V_ch to V_disch, and back, in "constant-voltage" operation, or alternatively more slowly in "constant current" or other charging schemes; then V_ch and V_disch must be considered "end-voltages" defining the end of a step. The salt adsorption capacity (SAC) of the CDI cell per cycle can be expressed in mg salt removal per gram of both electrodes combined (c_ions,_mi (mM)).

Each cell, both for capacitive energy generation and CDI, can be defined by two special values of the cell voltage, which we call equilibrium points. So, we have two equilibrium points, one related to a first electrode acting as anode (EPl) and one related to a second electrode acting as cathode (EP2). These are values of the cell voltage.

For conventional electrodes without chemical charge, both EPl and EP2 are zero. When using these conventional electrodes (performing an operation going from V_ch to V_di_SCh and back), efficient operation can be achieved when both V_ch and V_di_SCh are on the same side of each of the Eps, therefore above or below 0 V.

As an example, with one modified electrode by adding chemical charge, one value of EP will change, e.g., to -1.2 V. The other EP stays close to 0 V. Then both V_ch and V_di_SCh must be above 0 V, or both must be below -1.2 V. However, when working below -1.2 V, we run into the risk of water splitting. In general, we want to keep both V_ch and V_di_SCh within the range of cell voltage -1.23 V to +1.23 V, to avoid water splitting. So, in this example we can work in the range between 0 and +1.2 V. In this example performance is similar to classical operation of unmodified electrodes which both have zero chemical charge (for which EP1=EP2=0).

For improved performance in this example, both electrodes are modified to move both EPl and EP2 in the same direction away from zero, e.g. to negative values. For example, by adding +1 M chemical charge in the anode (by adding positive groups into the carbon pores, e.g., amine), and adding -1 M chemical charge in cathode (e.g., carboxylic group, or sulfate, phosphate, silica), both EPs may go to negative values such as -1.2 V. This enables operating a CDI cycle or capacitive energy generation cycle in the entire window from -1.2 V to +1.2 V, without risking water splitting.

With a lower concentration of chemical charge (e.g. 0.5 M), the window is somewhat smaller, e.g. from -0.6 V to 1.2 V, but still larger than without chemical charge. The two EPs don't need to be the same, however, the electrode with the lowest chemical charge (in magnitude) limits the window. E.g., the one that causes its EP to be -0.6 V (the other being -1.2 V) limits the operation window to -0.6 V to 1.2 V.

The increase in operational window in this example is possible when one electrode has negative chemical charge, and the other positive chemical charge. In CDI, the increase in operational voltage window leads to a significant increase in the salt adsorption capacity in a cycle. For example, an increase of up to 50% at EP1=EP2= - 0.6 V and 100% at EP1=EP2= - 1.2 V can be achieved.

In Capacitive energy generation, when both EPs can be shifted in the same direction, a window opens of operational voltages around

where the operation can run without using membranes and without using an external battery. This provides a further advantageous effect of the present invention.

In a preferred embodiment of the invention at least one of the electrodes, and preferably all electrodes, are chemically modified such that the equilibrium point is modified and charge and discharge voltages are of a different sign. In addition, the discharge voltage in this embodiment is unequal to zero and above +150 mV or below -150 mV. For example, V_ch is positive and V_disch is negative, while remaining on the same side of the equilibrium point. This enlarges the operational window and increases the SAC, thereby improving the process performance. Particularly, the combination of using a negative V_disch and modified electrodes achieves the described combination of effects. Without chemical modification of electrode(s) adsorption would be negatively influenced when using a negative V_disch-

The advantageous effect of the present invention can be illustrated with a further example. When both electrodes are modified correctly, e.g., bringing both EPs to -1.2 V, this allows to enhance the operational window, i.e., to set V_ch to 1.2 V and V_disch to -1.2 V, for example. In this way the operation window is from -1.2 V to +1.2 V, and therefore amounts to 2.4 V. This is twice the window compared to classical operation at

V (here, window is 1.2 V). Due to the doubled window, the salt adsorption capacity per cycle is doubled as well. The energy consumption for the cycle also doubles, thus the energy consumption divided by ion removal ("energy per ion removed") stays about the same comparing operation in the enhanced window compared to the standard window of 1.2 V.

Also, with the chemically modified electrodes (each EP equal to, or lower than, -0.6 V) cycles can be performed with a "conventional" window of 1.2 V, however with much reduced energy use, namely by using

V). Compared to conventional operation the same salt adsorption capacity is achieved, however the energy consumption (expressed in energy required per mole of ions removed) is reduced by a factor of 2.

In a further example, figure 1A illustrates results with a conventional operation of a CDI cell with V_di_SCh = 0 and varying values of V_ch. Typically V_ch is 1.2 V which is a natural limit to avoid water splitting. In this case the SAC is defined by the difference between points indicated with 1 and indicated with 0 in figure 1A. With the modified electrodes the situation shown in figure IB can be achieved, wherein the equilibrium point (indicated with 2) is at about -500 mV. In figure IB, at V_di_Sch of 0 mV the SAC is already higher as compared to the conventional system, thereby improving the overall efficiency of the process. However, the efficiency can be even further improved by using a discharge voltage of V_di_Sch is -0.4 V, indicated with point 2 in figure IB, thereby significantly increasing the SAC. As compared to the results with the unmodified electrodes this results in an improvement of over 50%.

In a presently preferred embodiment according to the invention the modification is performed to position the equilibrium point above +250 mV or below -250 mV, preferably above +350 mV or below -350 mV, more preferably above +500 mV or below -500 mV, even more preferably above +750 mV or below -750 mV, and most preferably above +1000 mV or below - 1000 mV.

By further removing the equilibrium point from the 0 mV value a larger AV_cen is available, thereby improving the overall efficiency of the process. Preferably, the electrodes are modified to move the equilibrium point beyond the water splitting voltage thereby maximizing AV_cen.

Chemical modifying a capacitive electrode is possible with chemical pre-treatments, including activation, HN0₃ treatments, incorporation of charge groups using TEOS, for example.

Chemically modifying one or more of the capacitive electrodes may involve providing one of the electrodes acting as an anode with carboxyl-groups. It is shown that such chemical modification of the carbon capacitive electrodes is feasible. In addition or as an alternative thereto, modifying one or more of the electrodes acting as cathodes is performed by providing amine- groups. Such modification is also feasible. As already mentioned, in a presently preferred embodiment all electrodes are chemically modified.

In a presently preferred embodiment of the invention providing specific groups to the electrode involves synthetizing one or more of the electrodes with the active group, such as a carboxyl-group and/or amine-group. Synthetizing the electrodes achieves that that the active group(s) is located in the pores of the electrode, including relatively small pores of such electrode.

In a presently preferred embodiment according to the present invention the chemically modified capacitive electrodes are provided with a membrane, optionally embodied as a membrane coating that is provided on the electrodes after -modifying the electrodes. Alternatively, no membrane is applied.

In a presently preferred embodiment according to the present invention the upper Voltage limit is adjusted in time to compensate electrode change(s) during use.

By adjusting the upper Voltage limit changes in the electrode characteristics and the electrode performance can be compensated for. This significantly improves the overall process performance over time. In practice, electrode changes can be caused by oxygen that is present in the fluid. Preferably, the upper voltage limit is adjusted in response to a measurement determining the equilibrium point and/or the change of this equilibrium point. Measuring the equilibrium point enables an adjustment in time of the operating conditions, such as the applied cell voltage, and thereby achieving effective compensation of the electrode changes during use. This renders the overall process more effective.

Although adjustments can be made on basis of (theoretical) expectations or predictions of the electrode changes during use, performing actual (in-line) measurements may further improve the overall performance. Preferably, the equilibrium point is determined by changing the operating conditions. By manipulating the operating conditions the process behaviour indicates the location of the equilibrium point. A change in this location indicates changes in the electrode behaviour, such that compensation is enabled. Alternatively, or in addition thereto, an impedance measurement can be performed on at least one of the electrodes. This also provides information on electrode changes during use, thereby enabling compensation, and contributing to the overall performance of the operations.

In a presently preferred embodiment according to the present invention the method performs a capacitive deionisation process. In a capacitive deionisation process, by applying a method according to the present invention, the SAC can be increased significantly, for instance 50% as compared to non-modified electrodes, as was described earlier. This is achieved with the chemically modified capacitive electrodes that behave asymmetrically due to the presence of chemical charge reciting in micro pores of the capacitive electrodes. This may enhance desalination significantly resulting in an improved process performance.

In a presently preferred embodiment of a CDI process the method comprises the step of selecting a non-zero discharge Voltage. This increases the AV_ceii, thereby improving the overall efficiency of the process.

In a further preferred embodiment according to the present invention the method performs a capacitive electrical energy generating process.

Chemically modifying the capacitive electrodes and moving the equilibrium point as mentioned earlier increases the effective range for AV_cen, thereby improving the overall process performance.

The movement of the equilibrium point is preferably to a value below -250 mV or above

+250 mV, more preferably below -350 mV or above + 350 mV, even more preferably below -500 mV or above +500 mV, even more preferably below -750 mV or above +750 mV, and most preferably below -1000 mV or above +1000 mV. This improves the electrical energy generation process. This can be applied to a system with two capacitive electrodes, optionally provided with a membrane, in a reactor or compartment. Also, the method can be applied to a so-called capacitive Reverse Electrode Dialysis (RED) with capacitive electrodes. Such process is disclosed in document WO2013/147593, for example. This document illustrates the system component method for such capacitive Reverse Electrode Dialysis (RED) involving the method according to the present invention.

Also, the method according to the resent invention can be applied to generating energy form a gas flow, preferably a flue gas comprising C0₂. Such process is described in document WO 2014/182167, for example. Also, this process can be improved with the method according to the present invention.

The invention further relates to a system capable of performing the method as described herein.

Such system provides the same effects and advantages as those mentioned in relation to the method. These advantages include an effective deionisation and/or electrical energy generation using chemically modified capacitive electrodes to manipulate the equilibrium point .

Further advantages, features and details of the invention are elucidated on basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

figure 1 A and B illustrates the effect of manipulating the equilibrium point according to the present invention;

figure 2 illustrates a CDI process with the method according to the present invention; figures 3A and B illustrate a comparison of performing a CDI process with modified and unmodified capacitive electrodes;

figure 4 illustrates a capacitive electrical energy generating process with the method according to the present invention;

figure 5 illustrates a capacitive RED process with the method according to the present invention; and

- figure 6 illustrates an electrical energy generation process involving flue gas with the method according to the present invention.

Apparatus 2 (figure 2) capable of performing a CDI process comprises a process compartment 4. Compartment 4 comprises a fluid compartment 6 and modified porous carbon electrode 8 that is separated from fluid compartment 6 by anion exchange membrane 10. Electrode 8 is provided with the current collector 12. Furthermore, compartment 4 comprises a second modified porous carbon electrode 14 acting as cathode that is separated from the fluid

compartment 6 by cation exchange membrane 16. Electrode 14 is provided with a current collector 18. Current collectors 12, 18 enable connection to external circuitry enabling the provision of charged electrodes thereby driving the specific ion recovery from the fluid. In the illustrated embodiment membrane layers 10,16 involve coated membrane layers selective for one ion species only. In an alternative embodiment membranes 10,16 are omitted. Optionally, apparatus 2 comprises sub-compartments as disclosed in WO 2011/155839. Experiments with the apparatus 2 (figure 2) are performed by measuring the conductivity of the solution as a measure for the salt concentration. In the first charging state the concentration decreases as the electrodes adsorb the ions. After the concentration equals the concentration of the incoming fluids and the electrodes are saturated with the ions, in the discharging state the potential over the electrodes is reversed such that the absorbed ions are released. This illustrates the operation of CDI in general, including the possibility for adsorbing the salts and releasing them afterwards, thereby obtaining the possibility for recovery of a specific ion.

Application of the diagrams illustrated in figure 1A and B to apparatus 2 (figure 2) that is provided with modified and unmodified electrodes 8, 14 shows the effect of such chemical modification. Results are shown in figure 3 A and B, both for modified and unmodified electrodes 8, 14. Figure 3 A shows salt adsorption as function of charging voltage (V_disch=0 V) and figure 3 B shows salt adsorption as function of discharge voltage (charging voltage V_ch=-1.2 V). At a discharge voltage of -0.45 V, the modified electrodes have a maximum in salt adsorption (o_{chem A} = -o_chem>c=0.4 M, c_sa„=20 mM).

When recalculating to SAC in mg/g, as is common in the field of CDI, modified electrodes show lower values of SAC at negative V_ch, but slightly higher values of SAC at positive values of V_ch, for instance -13.2 mg/g instead of 11.8 mg/g at V_ch=1.2 V. Taking that value of V_ch, which is a natural limit to avoid water splitting, the value of V_di_Sch can be tuned or selected to an optimal value. For example, using a discharge voltage of V_di_Sch~ 0.4 V, SAC now significantly increases, to a maximum of -18.2 mg/g, which compared to the maximum for unmodified electrodes is an increase of over 50%. This improvement is obtained at a rather small chemical charge of 0.4 M.

Next, some possible set-ups of energy generating systems will be illustrated as examples. In these examples, and other embodiments that can be envisaged, the method according to the invention as illustrated earlier in relation to a CDI process can be applied to improve the overall efficiency of the process.

An energy generating system 102 (figure 4) comprises a first modified electrode 104 and a second modified electrode 106 of active carbon with modifications. For illustrative purposes, schematically the provision of anion-exchange material 108 and cation-exchange material 110 as membrane layer is shown. Alternative embodiments with other membrane configurations or without membranes can be envisaged. In the illustrated embodiment in electrode 104,

schematically drawn is current collector 112, 114. Container 116 is filled with a fluid 118. Fluid 118 is alternately a solution with a low concentration of salt (about 0 g/1 NaCl) and a high concentration (30 g/1 NaCl), for example. Fluid 118 is stirred with mixer 120. For enabling the generation of energy in system 102 the current collectors 112,114 are connected via wires 122 with electrical circuit 124. In the illustrated embodiment the electrical circuit comprises resistance 128, for example of 2 kQ. To measure the amount of energy generated in system 102, Voltage meter 126 is provided over the circuit 124. Some alternative embodiments are shown in WO

2010/062175. Application of the method according to the present invention improves the overall process performance.

Alternative energy generating system 202 (figures 5A-B) comprises a first modified capacitive electrode 204 that is placed in electrode compartment 206. An electrolyte compartment 208 is separated from first electrode compartment 206 by membrane 210. In the illustrated embodiment membrane 210 is a cation exchanging membrane.

In the first energy generating state (figure 5 A) a concentrated salt solution 212 flows through electrolyte compartment 208. Cations 216 migrate through cation exchange membrane 210 while anions 214 migrate through an anion exchanging membrane 218. A diluted salt solution 219 flows through electrolyte compartment 220. Membrane 218 separates electrolyte compartment 208 from electrolyte compartment 220. In the illustrated embodiment a second electrode compartment 222, wherein a second modified capacitive electrode 224 is placed, is separated by membrane 210. Compartments 208, 220 and two membranes 210, 218, one of each type, together form cell 226. Electrodes 204, 224 are externally connected via circuit 228 wherein load 30 is provided.

Switching means 232 switches system 202 between a first state (figure 5A) and a second state (figure 5B). In the second state flows 212, 219 change position. This means that in a second state diluted salt solution 219 flows through electrolyte compartment 208 and concentrated salt solution 212 flows through electrolyte compartment 220. This means that the flow of anions and cations 214, 216 tend to move in opposite direction as compared to the first state. Also the flow direction of the electrons in circuit 28 is in an opposite direction.

In a first state (figure 5A), flows 212, 219 are provided. Ions tend to move through membranes 210, 218. This results in a charge of electrodes 204, 224. Capacitive electrode 4 is being charged with anions 214 and second capacitive electrode 224 is charged with cations 216.

Electrons flow through circuit 228 via load 230 from first capacitive electrode 204 towards second capacitive electrode 224. After the capacitive electrodes 204, 224 have been charged switching means 232 switch system 202 to a second state (figure 5B) wherein flows 212, 219 change position. The net flow of cations 216 and anions 214 is in opposite direction as compared to the first state such that the direction of the flow of electrons in circuit 28 is also opposite. First, capacitive electrodes 204, 224 are being discharged and, next, electrodes 204, 224 are charged with cations for capacitive electrode 204 and anions for capacitive electrode 224.

The use of capacitive electrodes 204, 224 improves the overall process performance. The method according to the invention can also be applied to alternative systems 202 that can be envisaged and of which some are illustrated in WO 2013/147593. Application of the method according to the present invention improves the overall process performance. Another application that benefits from the use of modified electrodes in accordance with a method according to the present invention is electrical energy generation from a gas flow, such as a flue gas. System 302 (figure 6) comprises flow channel 304, first modified electrode 306 and second modified electrode 308. Electrodes 306, 308 comprise a conductive material 310 with a high capacitance and are modified according to the present invention. In the illustrated

embodiment conductive material 310 comprises porous carbon. Current collectors 312 of electrodes 306, 308 are in contact with conductive material 310. At the relatively large internal surface area within the porous carbon 310 ions can be stored or adsorbed. Current collectors 312 are connected by electrical circuit 314. In the illustrated embodiment the ions are stored next to the electrical charge, thereby forming a so-called electrical double layer as mentioned earlier wherein at the carbon/water interface the electronic charge can only be in the carbon and ions (ionic charge) can only be in the water. With an electrical charge of a negative sign the electrode will attract and adsorb cations in the water-filled micro pores in the carbon. This electrode behaves as a cathode. In the opposite electrode the processes are reversed and this electrode behaves as an anode. Electrode 306 is separated or sealed from flow channel 304 with cation exchange membrane 316. Electrode 308 is separated and/or sealed from flow channel 304 with anion exchange membrane 318. In the illustrated embodiment flow channel 304 is filled with a liquid 320, in the illustrated embodiment water. Gas, such as flue gas, flows through liquid 320 in the form of bubbles 322. Liquid 320 provides an ionic connection between the at least two electrodes 306, 308.

Operation of system 302 starts with providing gas at inlet 324. In flow channel 304 the reactions R1-R4 as described in WO 2014/182167 take place. The cations and anions diffuse towards the electrodes 306, 308 passing the selective membranes 316, 318. The ions are adsorbed and electric energy is generated in electrical circuit 314. When electrodes 306, 308 are saturated, system 302 is switched from the adsorption state to the desorption state. Next, desorption will take place wherein energy is generated. From the desorption an amount of energy is generated and/or an amount of separated C02 is being generated with optionally an amount of energy being provided to enable C02 separation. The amount of energy can be provided by the generated energy in the adsorption step and/or desorption step. Alternatively, energy is provided by an external source. Alternative embodiments are shown in WO 2014/182167. Application of the method according to the present invention improves the overall process performance.

The present invention is by no means limited to the above described preferred

embodiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.

Claims

1. Method for performing capacitive deionisation and/or capacitive electrical energy generation, comprising the steps of:

- providing a system with a first capacitive electrode and a second capacitive electrode that are connected with a circuit;

chemically modifying at least one of the first and second electrodes with charged groups providing an asymmetric electrode system, wherein the modifying is performed such that the equilibrium point, defined as the Voltage at which positive and negative charges are equal, is above +150 mV or below -150 mV; and

2. Method according to claim 1 , wherein the modification is performed to position the equilibrium point above +250 mV or below -250 mV, preferably above +350 mV or below -350 mV, more preferably above +500 mV or below -500 mV, even more preferably above +750 mV or below -750 mV, and most preferably above +1000 mV or below -1000 mV.

3. Method according to claim 1 or 2, wherein chemically modifying comprises synthetizing one of the electrodes acting as an anode with carboxyl-groups.

4. Method according to claim 1, 2 or 3, wherein chemically modifying comprises synthetizing one of the electrodes acting as cathode with amine-groups.

5. Method according to one or more of the foregoing claims, wherein the upper Voltage limit is adjusted in time to compensate electrode change during use.

6. Method according to claim 5, wherein the upper Voltage limit is adjusted in response to a measurement determining the equilibrium point.

7. Method according to claim 6, wherein the equilibrium point is determined by changing the operating conditions.

8. Method according to claim 6, wherein the equilibrium point is determined by performing an impedance measurement of at least one of the electrodes.

9. Method according to one or more of the foregoing claims, wherein the method performing a capacitive deionisation process.

10. Method according to claim 9, comprising the step of selecting a non-zero discharge Voltage.

11. Method according to one or more of the foregoing claims, wherein the method performing a capacitive electrical energy generation process.

12. Method according to claim 11, wherein the equilibrium point is below -250 mV or above +250 mV, more preferably below -350 mV or above + 350 mV, even more preferably below -500 mV or above +500 mV, even more preferably below -750 mV or above +750 mV, and most preferably below -1000 mV or above +1000 mV.

13. Method according to claim 11 or 12, wherein the method performing an electrical energy generating capacitive reverse electrodialysis process.

14. Method according to claim 11, 12 or 13, wherein the method performing an electrical energy generating capacitive flue gas treatment process.

15. System capable of performing a method according to one or more of the foregoing claims.