The invention relates to a three-dimensional electrode according to the preamble of claim 1. Such an electrode, primarily intended for performing electrochemical reactions, is mainly comprised of an electrically conducting electrode material mixed with an ion-exchange electrolyte

The invention also concerns an apparatus according to the preamble of claim 9 for performing electrochemical reactions and the use of such an apparatus in the reduction of oxygen, metals, hydrogen peroxide and organic compounds.

Three-dimensional electrodes are used in such applications of electrochemistry that require a large electrode surface. The most typical of these is the precipitation of metals from waste liquors. Other applications are to be found in, e.g., organic syntheses and electrochemical oxygen removal

Three-dimensional electrodes differ from conventional planar-surface electrodes therein that the electrode also has a depth-direction dimension which permits water (or other liquid) to pass through the electrode. In this way, the reactive surface area of the electrode becomes manyfoid as compared with that of a conventional electrode.

Different types of three-dimensional electrode structures are disclosed in, e.g., US Pat. No. 4,118,305, FI Pat Appl. 922,485 and SU Inventor ^' s Certificate 966,026.

Three-dimensional electrodes have some disadvantages. A particular hindrance to their wider use is posed by their need for good electrical conductivity in the liquid to be treated To make an electrochemical cell functional, the solution under treatment must contain free ions which carry the charge between the electrodes

The requirement of good electrical conductivity is accentuated in three-

dimensional electrodes, since the ions must also form a conductive path within the three-dimensional structure of the electrode

It is an object of the present invention to overcome the drawbacks of the above- described technology and to provide an entirely novel type of cathode structure

The goal of the invention is achieved by complementing the electrode material with such an ion-exchange resin mixed therein that can form a current path for ions travelling from one electrode to another Hence, free ions are not needed and even deionized water can be treated electroche ically The counter-ions of the ion-exchange resin perform as the charge carriers of the electric current The counter-ions can be either amons or cations For example, in cation-exchange resins, the hydrogen ion, alkali metal ions (Na , K ) and the earth alkali metal

2+ 2+ ιons (Ca , M ) can perform as counter-ions, while anion-exchange resins have hydroxyl, chloride or sulfate ions acting as the counter-ions The opposite-polarity charges, that is, the ionic groups of the ion-exchange resin remain immobile

ln tests performed by the inventors it has been found that simple mixing of an ion- exchange resin into the electrode material does not provide a satisfactory performance Namely, sufficient conductivity of the ion exchange resin does not alone render a good performance to an electrode comprised of an ion-exchange electrolyte and electrode particles, but additionally, the resin particles must have an appropriate geometry Actually, conventional spherical ion-exchange resin particles do not offer sufficient contact area between the particles, whereby the ions cannot move adequately from one particle to another

In the present invention, the ion-exchange electrolyte is made up of a fine-particle resin in which at least the shortest dimensions of the particles are smaller than the dimensions of the particles of the electrode material itself Resultingly, the iσn- exchange resin particles can better adapt themselves into the lnterparticle spaces of the electrode material thus establishing a good contact with each other

More specifically, the electrode structure according to the invention is character¬ ized by what is stated in the characterizing part of claim 1.

The invention also concerns an apparatus for performing electrochemical reactions in liquid phase. The apparatus comprises an electrochemical cell having at least one first, three-dimensional, particulate electrode. Said electrode is formed from an electrode material with fine-particle ion-exchange material mixed thereto, wherein at least the smallest-diameter solid resin particles are smaller than the particles of the electrode material itself. The apparatus also includes at least one second electrode called the counterelectrode, inlet nozzles for feeding the liquid to the three-dimensional structure of said first electrode and outlet nozzles for removing the liquid with the reaction products formed in the three-dimensional electrode structure from said electrode structure.

More specifically, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 9.

In the context of the present invention, the term "particle" of the ion-exchange electrolyte material refers to the particles of the ion-exchange resin which form the basic material structure on the basis of which the material is macroscopically characterized by its fine structure into such classes as "spherical", "granular" or "fibrous". These particles may either have an exactly definable three-dimensional geometric shape, or alternatively, a randomly-shaped outer surface. Examples of particle structures are spherical and granular material particles, respectively. Additionally, in a particularly advantageous embodiment, fibrous ion-exchange electrolyte particles are used. Here, while the shapes of the fibrous particles may vary, their length is generally at least about 6 times their thickness. Conventionally, the length of the fibers is from 10 to 100,000 times their cross- sectional diameter. Particularly advantageously, in a three-dimensional cell according to the invention such fibers are used that the length of the fiber is shorter than the thickness of the cell. In practical implementations, the length of the fibers is generally shorter than about 30 mm, conventionally about 1 - 5 mm.

In the spirit of the invention, it is essential that the ion-exchange resin particles can adapt themselves into spaces remaining between the electrode particles, thereby effectively enclosing the latter and maintaining good contact between the ion-exchange resin particles For this reason, the resin particles must have at least their shortest dimension smaller than the dimensions of the electrode particles The

"shortest" dimension is determined by the shape of the particles The shortest dimension of spherical particles is equal to their diameter, while for fibrous particles it is equal to the smallest dimension of the fiber cross section perpendicular to the longitudinal axes of the fibers For particles with undefined geometrical shape, the shortest dimension is defined as the smallest cross-sectional diameter of a three-dimensional particle in the direction of its x-, y- or z-axis

As the particle sizes of commercially available ion-exchange resins as well as those of electrode materials vary, resulting in a certain particle size distribution in commercial materials, it is obvious that the resin grades used in the invention contain particles larger than the electrode material particles According to the invention, however, at least 50 %, advantageously at least 80 % and particularly advantageously at least about 90 % of ion-exchange resin particles are smaller than the average size of the electrode material particles Of the electrode material particles, respectively, at least 50 %, advantageously at least 80 % and particularly advantageously at least about 90 % are larger than the average size of the ion- exchange resin particles ln practice, such an ion-exchange resin grade is selected that according to the manufacturer's specification has a smaller particle size (e g , in the range 0 15 - 0 3 mm) than the corresponding size of the electrode particles (e g , having a size distribution of >0 3 - 2 mm)

While ion-exchange resins are conventionally manufactured in spherical particles of 0 3 - 1 2 mm diameter, also smaller-diameter resin particles are available As an ion-exchange material, resin spheres of 0 15 - 0 3 mm are optimal The resin spheres are formed by a polymer network which when wetted is capable of retaining water thus forming a gelled material The most common ion-exchange polymer, which is also suitable for use in the invention, is styrene-divinylbenzene

In this polymer, the backbone is formed by styrene A few percent of divinyl- benzene (DVB) is added to provide branching points in the polymer chain To each benzene ring is attached an ionic group capable of bonding counter-ions In a strong-acid-based cation exchanger, for instance, the active group generally is a sulfonic acid group, -SO _^ - Both strong cation-exchange and strong anion- exchange resins are suitable for use in the invention The chemical resistance of cation-exchange resins is slightly better than that of anion-exchange resins

As an example of commercially available spherical ion-exchange resins can be mentioned Amberiite IR-120 and Dowex 50 X 8, both of which are available in different particle sizes Both of these belong to the category of strong cation exchangers The former is manufactured by Rohm & Haas and the latter by DOW Chemical

In fibrous ion-exchange resin, the diameter of the fiber cross section is in the range 0 01 - 1 mm For use as an ion-exchange electrolyte material, the most suitable fiber has its cross-sectional diameter essentially in the range of about 0 02 - 0 10 mm Such thin fibers of the ion-exchange electrolyte can adapt themselves in the spaces between the electrode particles and intertwine about them The mutual configuration of these media is also illustrated in annexed

Fig 2

Fibrous ion-exchange electrolytes are conventionally formed from polyolefin fibers having thereto grafted polystyrene chains containing acid or base groups An example of commercial fibrous cation-exchange resins is S opex 101 , in which the fiber cross-sectional diameter is about 0 02 - 0 04 mm This product is made by SmopTech, Turku, Finland

The electrode material used in the invention is comprised of granular metal par- tides or carrier particles coated with a conducting metal Particularly advanta¬ geously the conducting particles are comprised of pelletized or milled silver par¬ ticles When a lower cost is desirable, silver can be replaced by copper or stainless

steel Also particles coated with a precious metal such as platinum, gold or silver can be used, whereby the carrier can be a lower-cost material such as graphite

The conducting electrode material is conventionally in spherical or granular form The dimensions of its particles are determined by the same design rules as those of the ion-exchange material Typically, the conducting particles are spherical having their diameters greater than about 0 3 mm

According to the invention, the electrode material and the ion-exchange resin are prepared into a mixture which is packed into the electrode space of an electro¬ chemical cell The volume mixing ratio of the electrode material and the ion- exchange resin is typically about 1 10 - 10 1 , advantageously about 3 4 - 4 3

The present invention provides significant benefits The fine-particle ion-exchange resin material used in the invention has a better conductivity than conventional, larger-particle resin Additionally, it has been found that the lower the conductivity of the liquid being treated the higher the benefit achievable by means of the cathode bed according to the invention over an ion-exchange bed packed with larger-size resin particles The specific advantage of spherical resin material is its easy mixability with the electrode material The benefit of fibrous resin is best evidenced in the treatment of deionized or distilled water This is because the ions can more freely move from one fiber to another than from one sphere to another due to the intertwined structure of the fibrous ion-exchange electrolyte The fibrous resin also stays well fixed in a three-dimensional electrode and has no tendency to separate from the electrode material

The electrode according to the invention can be adapted in different fashions into an electrochemical cell Since the electrode does not support itself, some kind of support structure is inevitably required In the simplest embodiment, the electrode material is placed in the electrode space on some kind of quid-transmissive support surface such as a grid Depending on the application, the electrode structure must be surrounded at its sides by some type of fixed structures and/or

membranes that are impermeable to liquids When the electrode according to the invention is used for performing reactions in which it is desirable to prevent the transport of ions or other species between the electrode spaces of the cell, the three-dimensional electrode structure is most advantageously separated from the counterelectrode by means of a diaphragm or membrane that simultaneously acts as a mechanical housing for the entire system Most advantageously such a membrane is an ion-exchange membrane or similar semipermeable diaphragm Examples of suitable membrane types are cation-exchanger membranes marketed under trade names Nafion and lonac To accommodate the liquid flow into the electrode and out therefrom, the cell must be provided with inlet nozzles for the liquid to be treated and outlet nozzles for the treated liquid

The electrochemical cell according to the invention can be used for removal of oxygen from water or aqueous solutions It can also be used for reduction of hydrogen peroxide and organic compounds An example of the latter application is the reduction of acetaldehyde to ethanol The invention may also be applied to the separation of metals from waste liquors, whereby metal ions such as transition metal (e g , iron) or heavy metal ions are reduced into elemental metal and precipitated

In the following, the invention will be examined in more detail by means of a detailed description and exemplifying embodiments with reference to the attached drawings, in which

Figure 1 is a diagrammatic longitudinally sectioned side view of an electro¬ chemical cell according to the invention,

Figure 2 is an illustration of a mixture of fibrous ion-exchange electrolyte with spherical electrode particles as an about 50x enlargement of the natural size, Figure 3 is a graph of the conductivity of two ion-exchange electrolytes (Amberlite IR-120 and Smopex 101 , H -form) in a three-dimensional electrode, and

Figure 4 is a graph of the conductivity of two ion-exchange electrolytes (Amberlite IR-120 and Smopex 101 , Ca -form) in a three-dimensional electrode

Referring to Fig 1 , the electrochemical cell shown therein comprises a housing 1 to which are adapted an inlet channel 2 for the liquid to be treated and an outlet channel 3 for the treated liquid Further, the housing includes anode compartments 4, 5 and a cathode bed 6 These spaces are separated from each other by means of membranes 11 , 12 through which ions can pass between the working electrode and the counterelectrode

The housing 3 is made of a durable material such as sheet steel Inside the housing 1 there is one pair of side plates 7, 8, made of, e g , a polymer sheet and performing as an interior lining for the anode compartments 4, 5 on their sides facing the housing On the other side of the anode compartments there are the actual anodes 9, 10 which are typically made of an insoluble material The anodes are adapted to rest against the cathode bed 6, however so that the anodes are separated from the cathode bed by membranes 11 , 12 serving as a barrier between these adjacent elements The anode can be formed into a mesh, grid or planar structure that in latter alternative is advantageously provided with flow channels for the anolyte The membranes are most advantageously of the semipermeable type, whereby they allow ions to pass between the electrodes The cathode compartment is filled with conducting granular material 13, which is mixed with the ion-exchange resin 14 The electrode itself is formed into an elongated, at least essentially planar layer

The anode and the cathode are connected by conductors (not shown) to a power supply suited for feeding the cell with an electric current

ln the above-described embodiment, the ion-exchange resin particles are essentially spherical with a diameter smaller than that of the electrode material particles thus being able to fill the mterparticle spaces of the electrode material, yet maintaining a good contact with each other

As shown in Fig 2, the fibers 15 of the ion-exchange material become intertwined about the electrode material particles 16, simultaneously also crisscrossing with each other Thus, a reticular structure is formed in which the segregation of the electrode particles from the fibers is efficiently inhibited

The invention will be elucidated with the help of the following exemplifying embodiments

Example 1 Conductivity tests of ion-exchange electrolyte materials

The conductivities of ion-exchange electrolytes were determined with a conductivity meter type Knick 702 using a 4-electrode cell by the same manu¬ facturer Glass beads of 0 5 mm diameter were used as the "electrode material" in the tests, since the electron conductivity of a real electrode material would have caused an error in the ionic conductivity measurement All tests measured the effect of the liquid conductivity on the conductivity of the ion-exchange electrolyte The variables of the test series were the type of the ion-exchange resin and the counter-ion The tests were commenced with the most concentrated solution (starting from a conductivity of 100-400 mS/cm) and proceeding therefrom by dilution toward the purity of distilled water (having a conductivity of about 0 004 mS/cm)

Cation-exchange resins were investigated using three different cations, H , Na and Ca 2+ , as the counter-ions Comparison was made between two different types of strong cation-exchange resins, namely, a conventional grade marketed under the trade name Amberlite IR-120, of the large-sphere type with a particle diameter of

0 3 - 1.2 mm (manufacturer Rohm & Haas) and a fibrous grade marketed under the trade name Smopex 101 (manufacturer SmopTech)

The conductivity of the ion-exchange electrolyte is dependent on the type of the counter-ion In Figs 3 and 4 is shown the conductivity of the ion-exchange

electrolyte as a function of the liquid conductivity when the hydrogen ion and the calcium ion are used as the counter-ions

For easier evaluation of the different ion-exchange resin types, only two param¬ eters were chosen for the comparison

1 ) ion-exchange electrolyte conductivity at a water conductivity of 0 2 mS/cm which value is approximately equal to that of tap water, and

2) ion-exchange electrolyte conductivity at a water conductivity approaching zero (distilled water)

The results of the test are given in Table 1

Table 1. Conductivity of ion-exchange electrolyte material in a three-dimensional electrode.

The conductivity of the spherical-particle resin IR-120 mixed with glass beads was low The reason thereto was that the size of the resin particles was equal to that of the glass beads Therefore, the glass beads adapted themselves between the resin spheres thus preventing conductivity along the ion-exchanger particles By contrast, the thin Smopex 101 fibers could easily adapt themselves in the spaces remaining between the glass beads and intertwine about them Resultingly, a high conductivity was attained

Accordingly, the ion-exchange resin must not only possess a sufficiently high conductivity, but additionally, the geometry of its particles must be suitable Small size and fibrous shape of the resin particles are advantageous properties

Example 2

Removal of dissolved oxygen from water

In corrosion prevention, removal of dissolved oxygen from water is of utmost importance Next, a practicable embodiment of an electrochemical cell intended for electrochemical oxygen removal and its use are described The structure of the cell is similar to the diagrammatic illustration of Fig 1 The description below is related to a preferred embodiment that by suitable modifications may be applied to other electrochemical reactions, too

Except for its cathode, the basic structure of the cell is a modified version of the MP cell by ElectroCell Ab The cell is assembled from a number of separate mod¬ ular units which are pressed together The units are shaped so that between the adjacently assembled units are formed contiguous flow channels through which the liquid is passed to the anode and cathode compartments The capacity of the cell assembly can be increased by adding a larger number of the modular units in a parallel flow configuration

The mixture made from the electrode material and the ion-exchange material (volume mixing ratio 1 1 ) is packed in the cathode compartment of the oxygen- removal cell housing The membranes and grid-like oxygen-evolution anodes are placed on both sides of the cathode The anodes press the membranes against the cathode, whereby the shape of the cathode is retained Gaskets are mounted between the membranes, anodes and electrode frames

The cathode of the oxygen-removal cell is a three-dimensional bed with a volume of about 100 cm The cathode material is comprised of irregularly shaped copper grains with a diameter ranging from greater than 0 3 mm to about 1 2 mm Electric current to the three-dimensional cathode is taken over a copper conductor adapted to pass through the cathode frame Both the inlet and outlet channels of the cathode compartment are provided with a fine-mesh plastic screen serving to prevent the escapement of the copper grains and the resin particles from the cathode compartment

The anodes of the cell are DSA (dimensionally stable anode, supplied by

ElectroCell Ab, Sweden) electrodes particularly developed for oxygen evolution Such anodes are made from titanium mesh coated with an lπdium-oxide-based material Indium oxide is an electroactive material catalyzing the oxygen evolution reaction The surface area of the anode is about 230 cm The electrolyte in the

-J anode compartment is a 0 1 mol/dm HNO-, solution

13

The membranes in the oxygen-removal cell are lonac MC-3470 cation-exchange membranes (manufactured by Sybron Chemicals, Inc ) They are semipermeable polymer membranes whose mechanical durability is improved by means of a support net The overall thickness of the membrane is about 0 4 mm When the embodiment according to the invention is used for oxygen removal from water or aqueous solutions, the membrane serves to prevent the diffusion of oxygen molecules from the anode compartment to the cathode compartment

The electrodes of the cell are connected to a DC power supply equipped with an output current regulator

In the operation of the above-described apparatus the water or aqueous solution to be treated is passed via the liquid infeed nozzle of the cell inlet channel to the flow-through space formed by the three-dimensional cathode, whereby the oxygen contained in the water or aqueous solution is reduced to water on the metal particles of the cathode electrode The electric current passes between the cathode and the anode so that the hydrogen ions formed simultaneously on the anode travel through the membrane to the cathode compartment and move along the resin particles to the surface of the metal particles of the cathode

Accordingly, the cathode reaction of the oxygen-removal process is the reduction of dissolved oxygen

0 ₂ + 4H + 4e ^~ 2H ₂ 0 (1)

The anode reaction generates oxygen gas that is released to the atmosphere

2H ₂ 0 - 0 ₂ + 4H ⁺ + 4e ^~ (2)

To evaluate the significance of the particle size in ion-exchange resin material, two oxygen-removal tests were performed The tests were carried out using the same resm grades as in Example 1, that is, Amberlite IR-120 and Smopex 101

Calcium ion was used as the counter-ion in the resins Raw tap water without any additions was treated in the oxygen-removal cell The principal ions of the water were Ca 2+ , CI — and SO^ 2— , and its conductivity was about 0 2 mS/cm

The goal of the tests was to determine the water treatment capacity (maximum flow rate) of the cell achievable without compromising the oxygen-removal efficiency The cell was expected to deliver treated water at an oxygen content less than 3 μg/kg Simultaneously, the cell voltage necessary to attain this capacity was determined The current efficiency was set at 90 % The test results are given in Table 2

Table 2. Oxygen removal test results.

Resin Flow rate Cell voltage Cell current 0 content 0 ₂ content at inflow at outflow

IR- 120 73 g/min 1 72 V 0 13 A 8 mg/kg < 3 μg/kg

Smopex 101 282 g/mm 2.32 V 0 52 A 8 mg/kg < 3 μg/kg

As can be seen from the test results, the IR-120 resin can offer only a marginal efficiency This resin grade is hampered by the inferior contact between the resin spheres Smopex 101 offered a clearly superior efficiency

An additional benefit of the fibrous Smopex resin is that it stays locked in the three-dimensional electrode without a tendency of segregating from the electrode material as easily as the spherical resin grades

Example 3

Reduction of hydrogen peroxide

Hydrogen peroxide is a strong oxidizer Hence, its oxidizing power is utilized in a great number of bleaching processes Using an electrochemical reaction, hydrogen peroxide can be reduced into water at the cathode

H ₂ 0 ₂ + 2H ,+ + 2e ^" - 2H ₂ 0 (3)

Reduction of hydrogen peroxide was tested in the same cell as used in Example 2

The anode reaction in test was oxygen evolution (refer to Formula 2)

In the test, hydrogen peroxide was added in tap water The infeed solution conductivity was about 0.2 mS/cm The hydrogen oxide content of the water treat- ed in the cell was measured The current efficiency of the cell was set at 90 %

The test was run only using the Smopex 101 resin The test results are given in Table 3

Table 3. Hydrogen peroxide reduction test results.

Resin Flow rate Cell Cell H ₂ 0 ₂ content H ₂ 0 ₂ content voltage current at inflow at outflow

Smopex 101 120 g/min 2.35 V 0.94 A 50 mg/kg < 0.1 mg kg