WO2021070023A1

WO2021070023A1 - A treatment device for liquid effluents

Info

Publication number: WO2021070023A1
Application number: PCT/IB2020/059258
Authority: WO
Inventors: Manuel Lai; Giulia Molinari; Alessandro CEDRINO
Original assignee: IRIS Srl
Current assignee: IRIS Srl
Priority date: 2019-10-07
Filing date: 2020-10-02
Publication date: 2021-04-15
Anticipated expiration: 2022-04-07
Also published as: EP4041688A1; IT201900018146A1

Abstract

Described herein is a treatment device for liquid effluents comprising a casing (2) defining a treatment volume (V) arranged in which is an array of electrodes. The electrodes (4) are provided as plate electrodes with perforated structure and characteristic ratio between the sum of the edge lengths (L30) and the electrode area (A22, A24) comprised between 0.1 and 1.5 mm/mm2.

Description

"A treatment device for liquid effluents"

★★★★

TEXT OF THE DESCRIPTION Field of the invention

The present invention relates to devices and systems for the treatment of liquid effluents, in particular to devices and systems for treatment obtained by application of an electrical field.

Prior art and general technical problem

Treatment of liquid effluents by application of an electrical field has proven a very effective technique in so far as it presents a relatively high disinfectant effectiveness as compared to conventional methods based upon the use of decontaminating agents, and can moreover be combined to the use of the latter so as to enhance the effectiveness of the treatment itself.

Typically, the treatment is carried out in cells equipped with plane electrodes for generation of the electrical field, where the plane electrodes are arranged in an array with mutually parallel orientation.

It may, however, be noted how the effectiveness of the treatment of the liquid effluents by application of an electrical field is markedly dependent upon the arrangement of the electrodes, in particular upon the accuracy in the arrangement itself.

There may in particular be noted a marked deterioration of the effectiveness of the treatment in the case where the plate electrodes are mounted within the treatment cell with even contained positioning errors, especially errors of parallelism, which imposes provision of extremely costly treatment cells in order to ensure acceptable conditions of parallelism. In any case, the inventors have noted how also by carrying out a strict and systematic control of construction of the treatment cells, plate electrodes generally give rise to a surface charge density that is uniform and on average moderate and as such is very sensitive to the variation of the boundary conditions.

Object of the invention

The object of the present invention is to solve the aforementioned technical problems. In particular, the object of the invention is to provide a treatment device for liquid effluents based upon application of an electrical field, will not require an extremely precise and costly construction of the treatment cell itself and in which the treatment electrodes will be substantially insensitive in regard to variations of relative positioning, with an effectiveness of treatment that is in general improved.

Summary of the invention

The object of the present invention is achieved by a treatment device and by an electrode having the characteristics that form the subject of the ensuing claims, which provide an integral part of the technical disclosure provided herein in relation to the invention.

Brief description of the drawings

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

- Figure 1 is a perspective view of a treatment device according to the invention;

- Figure 2 is a top plan view according to the arrow II of Figure 1; - Figure 3 is an orthogonal view according to the arrow III of Figure 1;

- Figure 4 is an orthogonal view according to the arrow IV of Figure 1;

- Figure 5 is a top plan view of an electrode according to the invention;

- Figure 5A is a schematic representation of the structure of the electrode according to the invention;

- Figure 6 is a cross-sectional view according to the trace VI-VI according to the invention;

- Figures 6A, 6B are schematic representations of characteristics of the electrode according to the invention;

- Figures 7 to 13 illustrate respective embodiments of the electrode according to the invention;

- Figure 14 illustrates yet a further embodiment of the electrode according to the invention;

- Figure 15 illustrates a detail according to the arrow XV of Figure 14; and

- Figure 16 and Figure 17 are a cross-sectional view and a top plan view, respectively, of the electrode of Figure 14.

Detailed description

The reference number 1 in Figures 1 to 4 designates as a whole a treatment device for liquid effluents according to the invention. The treatment device 1 comprises a casing 2 defining a treatment volume V designed to receive a liquid effluent to be subjected to the treatment. The device 1 further comprises a plurality of electrodes 4 arranged according to an array in the treatment volume V, where each electrode 4 is provided as plate electrode. In the view of Figure 1 and in the view of Figure 2 the electrodes 4 are represented with a long- and double- short-dashed line (the so-called phantom line) since the structure of the electrodes will be described in greater detail with reference to the subsequent figures.

In a preferred embodiment of the invention, the electrodes 4 are arranged parallel to one another as occurs in treatment devices of a known type. The intra electrode distance, i.e., the distance between pairs of adjacent electrodes 4, is preferably constant and chosen in the range between 1 mm and 50 mm. In certain embodiments, the distance can be varied along the array, for example in the direction of the flow of effluent (when the flow of effluent is orthogonal to the array).

With reference once again to Figures 1 to 4, the casing 2 comprises a base 6 that provides a bottom of the volume V and a first side wall 8, a second side wall 10, a third side wall 12, and a fourth side wall 14 that rise from said base to define the treatment volume V. The walls 8 and 12 are parallel and identical to one another, while the walls 10 and 14 are also parallel and identical to one another. The casing 2 is moreover provided with at least two working mouths, in particular one or more mouths for intake of the effluent, and one or more mouths for discharge of the effluent through which the effluent to be treated enters the treatment volume V and leaves it following upon treatment.

On the outer surface of the walls 10 and 14 there are conveniently provided ribs 16, 18 configured to receive coupling elements that connect in a battery a number of devices 1. Within the volume V, in particular on the inner surface of the walls 8, 12, there are instead provided reliefs 20, which identify gaps 21 that separate the adjacent reliefs 20 and are in pairs aligned between the wall 8 and the wall 12, meaning thereby that each gap 21 on the wall 8 is aligned with a corresponding gap 21 on the wall 12 in such a way as to create globally a guide for insertion of the electrodes 4 into the volume V.

With reference to Figures 5, 5A, and 6, each electrode 4 is provided as a plate electrode having a thickness t4, where each plate electrode 4 comprises a first electrode surface 22 and a second electrode surface 24, which are opposite and parallel to one another, as well as being separated from one another by the electrode thickness t4.

Each electrode 4 further comprises preferably an electrical contact 26 optionally provided with a hole 28 for connection of a vice. When the electrical contacts 26 are inserted in corresponding pairs of gaps 21 within the volume V, they can enable a fast connection of the electrodes 4 in series and/or in parallel according to the needs.

According to the invention, each electrode 4 comprises a plurality of through holes 30, each extending from the electrode surface 22 to the electrode surface 24 through the electrode thickness t4. With reference in particular to Figure 5A, each through hole comprises an edge length L30 corresponding to a perimeter of the hole at the intersection of the hole 30 itself with one of the electrode surfaces 22 and 24, meaning thereby that the edge length - for the purposes of the present description - is a length measured on just one surface and is not the sum of the two lengths L30 on the surfaces 22 and 24.

In addition, the first electrode surface and the second electrode surface comprise a respective electrode area A22 and A24, which corresponds to the area of the electrode surface 22, 24 external to the through holes 30. In other words, the electrode area corresponds to the area of the "full" portion of the electrode 4, i.e., to the total area of the electrode 4 understood as area subtended by the geometrical perimeter net of the area occupied - and hence emptied - by the through holes 30 (in this sense, the edge length L30 encloses the "empty" portion of the electrode 4).

According to the invention, the through holes 30 are preferably arranged according to an orderly grating, even more preferably according to a grating with equilateral triangular mesh, where each hole 30 has an axis X30 (along which the hole 30 develops) that is centred in a corresponding node of the aforesaid equilateral triangular mesh. The grating with equilateral triangular mesh is an example of regular and isotropic arrangement of the holes 30: the pitch, i.e., the distance between centres of adjacent holes is constant in all directions, and the hole density along the grating is uniform.

Also possible, of course, are anisotropic and/or irregular arrangements, i.e., with meshes having a preferential direction of extension and/or a hole density variable along the grating, as likewise possible are embodiments in which the mesh of the grating is square or rectangular according to the needs. Anisotropic or irregular arrangements of holes 30 are used, for example, in the case of supply of the treatment volume V with a flow rate of effluent that has a direction of flow parallel to the plane of the electrodes. In general, the flow of the effluent within the volume can have a direction orthogonal (or in general incident) with respect to the electrodes 4, or else parallel to the electrodes 4 themselves. There may hence be arrays of electrodes 4 with anisotropic and/or irregular arrangements of the holes 30, and/or that have electrodes 4 not evenly spaced apart according to the direction of flow of the effluent.

According to the invention, a ratio d between the sum of the edge lengths L30 of the through holes 30 and one of the surfaces A22, A24 (either surface, given that the surfaces are substantially identical) is comprised between 0.1 and 1.5 mm/mm². For the purposes of the present description, this is a characteristic ratio of the electrode 4 and is measured via averaging over the entire surface A22 or A24 (in this way, the definition is unique both for regular and isotropic meshes and for irregular and/or anisotropic meshes).

With reference to Figures 7 to 13, these represent a set of alternative embodiments of the electrode 4 in which parameters such as the shape and arrangement of the holes, the electrode thickness t4, the diameter of the holes 30, and the pitch of the holes 30, i.e., the distance between centres of adjacent holes, are varied individually or in combination.

Embodiment of Figure 7

In the embodiment of Figure 7 the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 0.6 mm, whilst the pitch of the holes is 2.3 mm (constant along the entire electrode according to the geometry of the mesh).

In the embodiment of Figure 7 the empty/full ratio is 6.12%, with a density of holes 30 of 2173.91/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 7 is 7.42 kg/m².

With reference to Figure 6A (and this applies to the entire description), in the case of arrangement of the axes X30 of the holes at the nodes of a grating with equilateral triangular mesh, the "full" portion of the electrode 4 (and of each surface 22, 24) is distinguished by a hatching with parallel lines and by the reference P, with A22, A24 in brackets. The "full" portion P corresponds to the portion of the equilateral triangular surface comprised between sides L of the mesh (i.e., the lines joining the nodes of the mesh itself, and hence the axes X30) and edges of the holes 30 comprised between two consecutive sides L. The "empty" portion is distinguished by the reference V and by a hatching with asterisks, and corresponds to the portion of hole 30 comprised between two consecutive sides L and delimited by the edge that delimits also part of the "full" portion P.

Figure 6B (and also this applies to the entire description) summarizes the case of a grating with square/rectangular mesh and of a grating with quincuncial mesh (hole 30 in brackets). In this case, the "full" portion P is delimited by sides L of the mesh (once again, segments joining the nodes of the mesh and hence the axes X30) and:

- for the simple quadrangular (square/rectangular) mesh, the edge portions of the holes 30 delimited by two adjacent sides L; with reference to Figure 6B, the "full" portion P corresponds to the sum of the area with hatching with parallel lines and of the area of the hole 30 indicated in brackets;

- for the quincuncial mesh, the edge portions of the holes 30 delimited by two adjacent sides L and the entire edge of the hole 30 in brackets, set at the intersection of two diagonals D joining opposite vertices of the mesh; with reference to Figure 6B, the "full" portion P corresponds to just the area with hatching with parallel lines. Generalizing, whatever the polygonal mesh of the grating according to which the holes 30 are arranged, the aforesaid holes 30 have axes X30 arranged at the nodes of the grating, and for each mesh of the grating the electrode area A22, A24 is delimited by sides L of the mesh and by edge portions of the holes 30 comprised between consecutive sides L. The electrode area of the mesh defines the "full" portion P of the mesh. The "empty" portion V of the mesh is delimited by the areas of the holes 30 comprised between pairs of consecutive sides L of the mesh and by the respective edge portions comprised between the same consecutive sides L. In the case of quincuncial meshes, the "empty" portion is increased by the contribution of the central hole of the quincunx, whereas the "full" portion is reduced by the contribution itself.

Embodiment of Figure 8

In the embodiment of Figure 8 the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 0.5 mm, whilst the pitch of the holes is 1.25 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 14.4%, with a density of holes 30 of 7360/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 8 is 6.76 kg/m².

Embodiment of Figure 9

In the embodiment of Figure 9, the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 0.7 mm, whilst the pitch of the holes is 1.5 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 19.6%, with a density of holes 30 of 5111.11/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 9 is 6.35 kg/m².

Embodiment of Figure 10

In the embodiment of Figure 10, the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 1 mm, whilst the pitch of the holes is 2.5 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 22.5%, with a density of holes 30 of 1840/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 10 is 6.12 kg/m².

Embodiment of Figure 11

In the embodiment of Figure 11, the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 1 mm, whilst the pitch of the holes is 2 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 22.5%, with a density of holes 30 of 2875/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 11 is 6.12 kg/m². It should be noted that the electrode of Figure 11 has the same empty/full ratio and weight per unit surface as the electrode of Figure 10, and this shows that, with certain parameters remaining the same, completely different electrodes may be obtained.

Embodiment of Figure 12

In the embodiment of Figure 12 the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 1.5 mm, whilst the pitch of the holes is 2.5 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 32.4%, with a density of holes 30 of 1840/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 9 is 5.34 kg/m². It should be noted that the electrode of Figure 12 has the same hole density as the electrode of Figure 10.

Embodiment of Figure 13

In the embodiment of Figure 13 the holes 30 have axes X30 arranged at the nodes of a grating with equilateral triangular mesh, the electrode 4 is made of AISI304 stainless steel, the electrode thickness t4 is 1 mm, the diameter of the holes 30 is 3 mm, whilst the pitch of the holes is 4 mm (constant along the entire electrode according to the geometry of the mesh). The empty/full ratio is 50.63%, with a density of holes 30 of 718.75/dm². The weight per unit surface of the electrode 4 in the embodiment of Figure 13 is 3.9 kg/m². It should be noted that the electrode of Figure 12 has the same hole density as the electrode of Figure 10.

All the electrodes 4 of the figures fall within the operating limits of the ratio d that are envisaged for the invention, evidence of the fact that it is possible to give rise to a practically infinite variety of electrode structures 4 specialized according to the requirements, i.e., according to the effectiveness of treatment required (for example, the time required for bacterial abatement) and/or of the effluent to be treated. See in this connection the table that is presented in the ensuing description.

Operation of the treatment device 1 is described in what follows.

As in known treatment devices, a liquid effluent that is to undergo treatment (for example, a liquid contaminated by bacteria and/or micro-organisms) enters the volume V through the intake mouth or mouths of the casing 2 and invades the intra-electrode regions 4. Applied to the electrodes 4 is a variable voltage (typically, the supply is a pulse-train supply) that distributes according to the scheme of connection of the electrodes 4 themselves (series, parallel, or mixed series-parallel) .

Provision of the electrodes 4 as perforated plates has the crucial result of modifying the electrostatic properties of the electrodes 4 themselves, and in particular of modifying the local density of electric charge.

The reason for the above is that each hole 30 has the edge L30 that acts as a concentrator of electric charges by the edge effect, accumulating charge in an amount sufficient to generate electrical discharges of a non-thermal type (e.g., due to the corona effect) or due to luminescence that present values of local electrical field that are decidedly higher than the electrical field that can be generated with a non- perforated plate electrode.

For the purposes of the solution to the technical problem underlying the invention the result is that of mitigating the sensitivity of the electrodes 4 to the imprecisions of positioning thereof, in particular to the local variations of electrical field due to errors of parallelism. In other words, the alteration in the distribution of electric charges caused by possible errors of parallelism between adjacent electrodes 4 are of a degree considerably lower than the local concentration of electric charges, and hence of electrical field, caused by the through holes 30.

This makes it possible to provide the casing 2 without excessive precision, thus containing the costs of design and production of the device 1. Moreover, the effect of concentration of local charge is so marked as to render effectiveness of the treatment and homogeneity of the treatment of the effluent independent of the dimensions of the electrodes 4 themselves.

Nonetheless, the presence of plate electrodes with perforated structure evenly distributes the velocity of the liquid effluent that passes through the electrodes 4 during treatment, orienting it in a direction perpendicular to the perforated surface of the electrodes 4. Application of a voltage across the series, the parallel, or the series/parallel of the electrodes 4 triggers the treatment with removal of the noxious species.

It has in fact been noted that the type of electric discharge generated by means of the electrical field generated as a result of the edge effect by the through holes 30 of the electrodes 4 according to the invention is particularly effective for disinfection from micro-organisms, in so far as this discharge causes an electroporation of the cell membrane of the contaminating micro-organisms, rendering the membrane more vulnerable to the attack by traditional oxidizing agents. It is thus possible to reduce considerably (or eliminate altogether) the oxidizing agent to be administered to the liquid effluent during treatment since this simply has direct access into the cells. The inventors have moreover noted how the electrical field generated by the electrodes 4 stimulates the polar portion of the cell membrane, in effect rendering it pervious to entry of oxidizing agents. According to the invention, treatment is preferably administered by supplying the electrodes 4 with a pulsed voltage at low frequency (typically of the order of 10 Hz). The first pulse train administered through the electrodes 4 has the result of opening the pores in the cell membrane, whereas the subsequent pulse trains maintain the pores open for the subsequent entry of the oxidizing agents.

Consequently, the treatment carried out via the device 1 has the effect of rendering the contaminating micro-organisms very vulnerable, reducing the environmental impact due to the use of disinfectants or oxidizing agents in general, so improving globally the effectiveness of the treatment.

Thanks to the provision of the electrical contacts 26, it is possible to vary the scheme of connection of the electrodes 4 very rapidly, optimizing it according to the specific pollutant and the concentration itself of the pollutant. In addition, by varying the distance between the electrodes 4 by means of removal or addition of electrodes 4 in the gaps 21 (which may be in a number much higher than the one represented schematically in the figures), it is possible to specialize further the treatment according to the nature of the pollutant.

The treatment process may be further optimized by modulating the ratio d (characteristic parameter for the electrodes 4) according to the requirements, since the ratio d is the expression of the concentration of electric charge per unit surface, given that the concentration of charge is proportional to the edge effect, and the latter is proportional to the length of the edge itself. The modulation of the ratio d alone can bestow on the electrodes 4 markedly different levels of performance. By way of example, provided in what follows is a comparative table of the performance (useful time for bacterial abatement) of electrodes 4 provided according to the invention (Refs. 1, 2, 3, 4, 5, 5c, 5d) and of a non-perforated plane electrode

(Ref. 0).

With reference to Figures 14 to 17, the electrodes 4 may present, in some embodiments, even higher performance, this being obtained by superposing on the plurality of through holes 30 a distribution of tips 32, which amplify further local electric-charge concentration by the point effect. As may be seen in Figure 14 and 17, the tips 32 may be preferably provided as conical tips with axis X32 parallel to the axes X30 of the holes 30, and can be fixed at the nodes of the grating with equilateral triangular mesh according to which the holes 30 are arranged, thus replacing some holes 30. It is preferable also for the tips 32 to be arranged according to an orderly scheme so that in the case of Figures 14 to 17 an embodiment is illustrated where a row of holes 30 is modified by alternating a hole 30 with a tip 32. Preferably, in various embodiments, the tips 32 can replace from 20% to 70% of the holes present on the electrode 4, it being thereby understood that on a hypothetical reference corresponding to an electrode 4 with one hundred (100) holes 30, the resulting electrode will have from 20 to 70 holes replaced by tips 32. In yet other words, every one hundred (100) positions of holes, on the above replacement hypothesis, from 20 to 70 will be occupied by tips, and - in a dual way - from 80 to 30 will be occupied by holes, whence the ratio between the number of holes and the number of tips will be comprised between 1:4 (20:80) and 7:3 (70:30).

Notwithstanding the alteration brought about by provision of the tips 32, the characteristic ratio d does not change for the embodiment illustrated in these figures, and always remains within the operating limits detailed above that the inventors have found as providing the best results in terms of effectiveness of the treatment. The presence of the tips 32 on the other hand amplifies the benefits introduced by the holes 30, since they operate as further - and more powerful - concentrator of electric charge, improving the effectiveness of the treatment and mitigating further the sensitivity of the electrodes 4 to possible errors of parallelism in positioning thereof.

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

Claims

1. A treatment device (1) for liquid effluents, comprising:

- a casing (2) defining a treatment volume (V) designed to receive a liquid effluent to be treated; and

- a plurality of electrodes (4) arranged according to an array in said treatment volume (V), each electrode (4) being a plate electrode, said plate electrode comprising a first electrode surface (22) and a second electrode surface (24) opposite to one another and separated by an electrode thickness (t4), wherein:

- each electrode (4) comprises a plurality of through holes (30), each extending from said first electrode surface (22) to said second electrode surface (24) through said electrode thickness (t4);

- each through hole (30) comprises an edge length (L30) corresponding to a perimeter thereof at the intersection with one of said first electrode surface (22) and said second electrode surface (24);

- the first electrode surface (A22) and the second electrode surface (24) have a respective electrode area (A22, A24) corresponding to the area of the electrode surface external to the through holes (30) of said plurality; and

- a ratio (d) between the sum of the edge lengths (L30) of the through holes (30) of said plurality and the electrode area (A22, A24) of one of said first electrode surface (22) and said second electrode surface (24) is comprised between 0.1 and 0.5 mm/mm².

2 . The treatment device (1) according to Claim 1, wherein the through holes (30) of said plurality have an axis (X30) set at the nodes of a grating with equilateral triangular mesh.

3. The treatment device (1) according to Claim 1, wherein said electrode thickness (t4) is comprised between 0.2 mm and 3 mm.

4. The treatment device (1) according to Claim 1, further comprising a plurality of tips (32) projecting from at least one of said first electrode surface (22) and said second electrode surface (24).

5. The treatment device (1) according to Claim 4, wherein a ratio between the number of tips (32) and the number of the through holes (30) of said plurality is comprised between 1:4 and 7:3.

6. The treatment device (1) according to Claim 1, wherein the electrodes (4) are arranged parallel to one another.

7. The treatment device (1) according to Claim 6, wherein the distance between the electrodes is comprised between 1 mm and 50 mm.

8. The treatment device (1) according to any one of the preceding claims, wherein the through holes (30) are arranged according to an anisotropic scheme.

9. The treatment device (1) according to any one of the preceding claims, wherein the holes (30) have axes (X30) arranged at the nodes of a grating with polygonal mesh, and wherein for each mesh the electrode area (A22, A24) is delimited by sides (L) of said mesh and by edge portions of the holes (30) of said mesh comprised between consecutive sides (L), said electrode area defining a full portion (P) of the mesh, and wherein moreover an empty portion of the mesh is delimited by the areas of the holes (30) comprised between pairs of consecutive sides (L) of the mesh and by the respective edge portions comprised between the same consecutive sides.

10. An electrode (4) for a treatment device (1) according to any one of the preceding claims, the electrode (4) comprising a first electrode surface (22) and a second electrode surface (24) opposite to one another and separated by an electrode thickness (t4), wherein:

- the electrode (4) comprises a plurality of through holes (30), each extending from said first electrode surface (22) to said second electrode surface (24) through said electrode thickness (t4); - each through hole (30) comprises an edge length

(L30) corresponding to a perimeter thereof at the intersection with one of said first electrode surface (22) and said second electrode surface (24);

- the first electrode surface (22) and the second electrode surface (24) have a respective electrode area

(A22, A24) corresponding to the area of the electrode surface external to the through holes of said plurality (30); and