FIELD OF THE INVENTION

The present invention is in the field of a system for elec trolysis and in particular for electrolysis of water into hydro gen and oxygen, the system comprising an anode, a cathode and electrolyte and an electrical potential/power supplier, as well as an input and output. The produced gas or liquids may be stored, such as in a vessel.

BACKGROUND OF THE INVENTION

Electrolysis is a technique using an electrical potential difference to provide a direct electric current (DC) for driving a chemical reaction. Without the electrical potential difference the reaction would not occur. Electrolysis may be important for separation of elements or splitting of molecules, such as ¾ and O ₂ from H ₂ O, or for separation from naturally occurring sources. The voltage that is needed for electrolysis to occur is called the decomposition potential.

During actual electrolysis a direct electric current is pro vided through an electrolyte. The current enters in the form of electrons in the cathode where a reduction reaction creates a current carried by ions that travel to the other electrode where an oxidation reaction supplies again electrons. The electrolyte has the function of carrying an ionic current. The reac

tants/products may form separate phases or they may be dissolved in the electrolyte in case of a liquid electrolyte. By chemical reactions at the electrodes compound may be decomposed. Main components in electrolysis are an electrolyte, an electrical current, and electrodes, and in particular a cathode and anode. The electrolyte conducts ions either through a solid or a liq uid. An applied voltage provides for the thermo-dynamical energy that is necessary for the reaction. In the external circuit the electric current is carried by electrons. Further typically two electrodes are present per electrochemical cell, that are elec trical conductors. They provide a physical interface between electrolyte and the electrical circuit that provides the energy. Electrodes may be made of various materials, such as metals, graphite, and semiconductor material. Typically in electrolysers many electrochemical cells are electronically connected in par- allel, a monopolar configuration, or in series, a bipolar con figuration .

The amount of electrical energy that is added is considered to be equal to the change in (thermo-dynamical) Gibbs free en ergy of the reaction. Due to losses in a system the required en ergy is always higher and often much higher, for instance due to a release of heat.

Electrolysis of water produces hydrogen according to

2 H ₂ 0(1) 2 H ₂ (g) + 02(g); E ₀ = +1.229 V.

The required cell potential at standard conditions is > 1.229 V. It is noted that energy efficiency of water electrolysis may vary widely. The efficiency of an electrolyser is often defined as the enthalpy contained in the hydrogen, compared to electrical energy provided. Heat/enthalpy values for hydrogen are about 144 MJ/kg. Some reports quote water electrolysis efficiencies between 50% and 70% for alkaline electrolysers ; higher efficiencies are available, such as 95% efficiency. Production costs for 1 kg of hydrogen are about €5 in the near term and €2 in the long term.

Only a small fraction of hydrogen gas produced worldwide is generated by electrolysis, and normally used onsite. The generated hydrogen may be used for subsequent reaction, such as for forming ammonia.

A disadvantage with prior art systems is that the current density is still rather low. For instance, in alkaline electrolysis {with an aqueous electrolyte containing usually potassium hydroxide to make it more conductive) the current densities are typically of the order of 3000 A/m ² in industrial electrolysers. In polymer electrolyte membrane (PEM) electrolysers this can be up to an order of magnitude higher. A reason for the lower current densities in alkaline electrolysis are larger Ohmic losses. By Ohms law the losses are proportional to the current density (A/m ² ) and the distance over which the current travels. The main focus is therefore on making thinner membranes that reduce the distance between the anode and cathode, such as thinner than 0.5 m thickness.

A way of reducing Ohmic losses may be reducing the current density. However this directly reduces the amount of gas (hydro gen) produced thus making the electrolyser more expensive per kg of hydrogen produced per unit of time.

Various documents recite electrode systems, such as US

2015/292094 Al) , WO 2015/085363 Al) , US 2012/222954 Al, US

6,254,741 Bl, US2014/284209 Al, US 2015/075977 Al and WO

2017/170664 Al .

The present invention therefore relates to a system for elec trolysis, which solves one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates to an electrolysis system, in particular for generation of gaseous species, such as hydrogen and oxygen, and for generation of lower alcohols, such as metha nol, and ethanol, for carboxylic acids or ions thereof, such as formic acid and formate, methane, CO, and N¾ . The present inven tion relates to an innovative geometric electrode array, provid ing higher current densities (at the same or similar overpoten tial) , such as two times higher than comparable prior art elec trolysis systems comprising planar electrodes, with a same Ohmic voltage drop, and thus comparable or less Ohmic losses. In the present system current flows in two substantially independent (such as perpendicular) directions so that for the same current per unit volume a two times lower current density may be applied and thus a two times lower Ohmic voltage drop may be obtained, compared to in a planar configuration. Another advantage is that e.g. gas may be removed through a centre of an electrode rather than at the back (in zero-gap systems, or in front of in tradi tional systems with a gap between the electrode and the membrane) . This avoids a need for separate flow channels {other than just making a hole in the center of the electrode) . It is also a better suited configuration for miniaturization than the traditional sandwich model which tends to be bulky. Finally a good prospect for membrane-less operation is provided, i.e. the present array may also be operated without a membrane, with a chimney-like action providing effective separation of e.g. gases being produced, such as hydrogen and oxygen. Buoyant rise of the gaseous species allows for easy collection, whereas if a liquid would be produced a pump may be applied.

The present array comprises elongated, slender, (pillar like, or column like) shaped electrodes, in addition to a an array of said electrodes. The height of the electrodes is preferably at at least two times as large as a cross-sectional dimension thereof. The cross-sectional dimension may relate to a (largest) diameter in case of largely circular electrodes, to a longest diagonal in case of multigonal electrodes, to a longest perpendicular line in case of a triangle, and so on. Typically two or more anodes are present and two or more cathodes, preferably [3- m] anodes (wherein m may be up to 2 ²⁰ ) , and preferably [3-n] cathodes (wherein n may be up to 2 ²⁰ ) . The cathodes and anodes are typically alternating, that is a cathode is in at least two direction adjacent to two anodes, and vice versa. A material, typically a fluid material, in between said at least one cathode and at least one anode comprises an electrolyte and is suited for transfer of electrolyte species typically being ions, or precursors thereof, such as a solution comprising a polar solvent, such as water. For performing electrolysis the at least one anode and at least one cathode are at least partly in flu ^¬ idic contact the material. This contact may be direct or indirect, i.e. a further medium may be provided in between an elec trode and fluid material. For performing the electrolysis also an electrical potential/power supplier, typically of a direct current type (DC), is provided. As electrolyte may need to be replenished, or refreshed, or re-supplied, such in a continuous mode, in a semi-continuous mode, in a batch-wise mode, an input and an output are provided, as well as an optional outlet for gaseous species, typically one outlet per gaseous species.

Thereby the present invention provides a solution to one or more of the above mentioned problems and drawbacks.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a system according to claim 1.

In an exemplary embodiment of the present system at least one electrode is made from a metal, such as Pt, Ir, Ni, Cu, Zn, Ru, Ti, Fe, NiFe/NiO, graphite, a semiconductor, or combinations thereof. The Electrode material may be provided on a support.

In an exemplary embodiment of the present system a width of the at least one cathode is 0.9-1.1 times a length thereof.

In an exemplary embodiment of the present system

a width of the at least one anode is 0.9-1.1 times a length thereof.

In an exemplary embodiment of the present system a width of the at least one anode is 0.1-5 cm.

In an exemplary embodiment of the present system a width of the at least one cathode is 0.1-5 cm.

In an exemplary embodiment of the present system a height of the cathode may be at least 1 m, such as at least one cm.

In an exemplary embodiment of the present system a height of the anode is at least 1 mm, such as at least one cm.

In an exemplary embodiment of the present system an electrode may comprise a current collector, typically parallel to the

height of the electrodes. The current collector provides an im proved conductance and may increase a current applied to the electrodes .

In an exemplary embodiment of the present system the solution may be an aqueous solution or a melt.

In an exemplary embodiment of the present system the solution may comprise anions and cations, preferably selected from at least one of Li ⁺ , Rb ⁺ , K ⁺ , Cs ⁺ , Ba ²⁺ , Sr ²⁺ , Ca ²⁺ , Na ⁺ , Mg ²⁺ , S0 ₄ ^2- , HCO3 , and OH-, preferably K ⁺ and OH- for water electrolysis and preferably HCO3- and K ⁺ for CO2 electrolysis.

In an exemplary embodiment of the present system may comprise 2-2 ²⁰ cathodes and 2-2 ²⁰ anodes, preferably 4-2 ¹⁵ cathodes and 4- 2 ¹⁵ anodes, more preferably 10-2 ¹² cathodes and 10-2 ¹² anodes, even more preferably 16-2 ¹⁰ cathodes and 16-2 ¹⁰ anodes, such as 32-2 ⁹ cathodes and 32-2 ⁹ anodes, wherein the array comprises at least one symmetry axis, such as a two-fold axis, a three-fold axis, and a four-fold axis.

In an exemplary embodiment the present system may be for electrolysis of water, wherein the voltage supplier in operation provides a potential difference over the electrodes of about 1.23-2.2V, such as 1.48-2 V, and optionally providing an overpo tential .

In an exemplary embodiment of the present system a cross sec ^¬ tion of the at least one anode and at least one cathode may be independently selected from square, rectangular, circular, el lipsoidal, multigonal, such as pentagonal, hexagonal, octagonal, and combinations thereof. As such a large freedom of design is provided by the present system.

In an exemplary embodiment of the present system the solution and/or electrolyte may flow parallel to a height of the anode or cathode .

In an exemplary embodiment of the present system the height of the at least one cathode may be substantially equal to the height of the at least one anode, i.e. both are of equal height or almost equal height.

In an exemplary embodiment of the present system the width of the at least one cathode may be substantially equal to the width of the at least one anode, i.e. an array of electrodes appears to substantially regular in one direction of a top view.

In an exemplary embodiment of the present system the length of the at least one cathode may substantially equal to the length of the at least one anode, i.e. an array of electrodes appears to substantially regular in one direction of a top view. Each individual electrode may be structured as indicated.

In an exemplary embodiment the present system may comprise at least one storage vessel for fluids, such as for replenishing or for storage of gases being produced.

In an exemplary embodiment of the present system at least one electrode may be hollow. In addition thereto also a fluid material may be provided at both an inside and an outside of the electrode. Therewith both an internal and an external surface, as well as the surface in the preferred pores and channels, may be used for electrolysis.

In an exemplary embodiment of the present system at least one electrode may be obtained by 3D-printing. Below an example thereof is given.

In an exemplary embodiment of the present system at least one membrane (M) may be provided in between at least one cathode and at least one anode, preferably in between any cathode and any neighboring anode. The membrane preferably may have a thickness of 0.001-0.1 cm, such as 0.01-0.05 cm.

In an exemplary embodiment of the present system at least one electrode comprises an indention (U-profile) , preferably an in dentation over a full height thereof, or is hollow (D-profile) , or is Y-shaped, or is H-shaped, and combinations thereof.

In an exemplary embodiment of the present system at least one electrode may have a wall thickness of 0.1-3 mm. In case of a hollow electrode the wall thickness of the electrode and pillar like structure may fully coincide, whereas in case of a beam like structure the outer part of the electrode may function as an electrode, and the inner part thereof as a core or support, or the full electrode may be considered to have the wall thickness .

In an exemplary embodiment of the present system at least one cathode and at least one anode may be separated by a membrane or porous substrate, such as a bipolar membrane, and are thus chem ^¬ ically separated from one and another.

The anode may be at an outside of the electrode and a cathode may be at an inside, or wherein the anode may be at an inside of the electrode and a cathode may be at an outside thereof. If a common electrode plate is used the cathode may extend slightly more to a top side (or likewise bottom side) , and the anode slightly more to a bottom side (or likewise top side) .

In an exemplary embodiment of the present system at least one anode may be in electrical connection to a common anode plate.

In an exemplary embodiment of the present system at least one cathode may be in electrical connection to a common cathode plate .

In an exemplary embodiment of the present system a cathode or a group of cathodes may be in electrical connection to a first common cathode plate and an anode or a group of anodes may be in electrical connection to a first common anode plate, and so on, wherein the first common cathode plate may be electrically connected in series or in parallel to the first common anode plate, which in turn may be connected to a second common cathode plate, and so on. In an exemplary embodiment of the present system in the array anodes and cathodes may be alternating.

In an exemplary embodiment of the present system at least one electrode may comprise pores, such as pores with a diameter of 1 nm-10 mm.

In an exemplary embodiment of the present system pores may have a heterogeneous size, that is very small sized pores may be present, such as of 1-100 ran, slightly larger size pores may be present, such as of 0.1-100 p , and relatively large pores may be present, such as of 100 pm-l mm. Therewith gas transport and liquid transport can be controlled well, and a pore size can be adapted to specific species, such as gas species, such as ¾ and O2, allowing one species to pass, and to block another.

In an exemplary embodiment of the present system a pore den sity may be 1-10 ⁹ pores/cm ² , such as 100-10 ⁶ pores/cm ² .

In an exemplary embodiment of the present system a porosity (volume open/volume solid) may be from 0.1-0.95, preferably 0.5- 0.9. The porosity can be measured in various ways, such as by using a water evaporation method and calculating accordingly (pore volume = (weight of saturated sample - weight of dried sample) /density of water) .

In an exemplary embodiment of the present system a distance between an anode and neighboring cathode may be 0.001-5 cm, preferably 0.01-0.3 cm. depending on design and further boundary conditions the distance may vary and may be adapted.

In an exemplary embodiment the present system may comprise at least one of sealing side covers, a support, such as supporting pillars, a first gas outlet for a first gaseous species, such as H2, a second gas outlet for a second gaseous species, such as O2, a pump, and a fluid material chamber.

The one or more of the above examples and embodiments may be combined, falling within the scope of the invention.

EXAMPLES

The below relates to examples, which are not limiting in nature .

A conventional- and a matrix electrode were both 3D-printed to produce two similar electrodes in terms of pore structure and material. The reaction studied were

4H ₂ 0 + 4e- -> 2H ₂ 0 + 40H Ec = -0.826V

40H ^~ -> +O2 + 2H ₂ 0 + e Ea = 0.404V

The performance of the electrode has been measured in the range of 1.48 to 3.0 V, with minimal step size of the used power source being 0.1 V.

The used electrolyte is KOH dissolved in water. Experiments have been done for 0.1 M, 1 M and 3 M KOH. The material used for the electrode is 316-L stainless steel, which was used for the 3-D print. The material contains about 10 % Nickel ( FeO .9Nio.i ) and is used as a cathode material. The material choice for the 3D printed plastic parts is Acrylonitrile Butadiene Styrene

(ABS) , in view of its strength and that it does not react with high concentrations KOH.

The electrode wall thickness was 2 mm. The porosity of the material was about 50 percent. The current density expected was about 2000 A/m ² . The pores/channels in this printed structure have a diameter of around 250 p , which is found sufficient for the bubbles to escape in view of sizes of the gas bubbles in electrolysis. These channels are set under an angle of 45 de grees. Hollow electrodes, reminiscent of chimneys, are designed to be able to remove gas with an almost zero gap between the electrodes and the membrane. The chimney width and length were about 4 mm. For a good comparison a 2 mm gap behind each elec trode in the conventional configuration was provided for the re moval of bubbles.

The anode plate is connected to 18 electrodes and the cathode plate is connected to 17 electrodes. This systems has 58 sur ^¬ faces of 8x20 mm, which oppose an electrode of opposite polar ^¬ ity. The 0.5 m thick membrane (Zirfon Perl, Agfa} is folded in the 0.7 mm gap between the anodes and the cathodes to separate the oxygen gas and hydrogen gas.

For every step in potential, a waiting time of 2 minutes is used in order to have the reaction reach an equilibrium state.

The chimneys from the array electrodes produced clouds of bub ^¬ bles above the electrode. This was most visible on chimneys from the cathode. For every emerging oxygen molecule, 2 hydrogen molecules are produced, which explains the difference in chimney activity.

The main result is shown in Figures 7 and 8. Figure 7 shows that the required overpotential to obtain a certain current per unit external electrode area is similar between the conventional and array configurations. This is expected since similar electrodes and membrane are used. Figure 8 shows a roughly 50% in ^¬ crease in current per unit volume at the same potential compared to the conventional configuration. This is related to the roughly 1.5 times higher surface area per unit volume of the array electrolyser . When upscaling to more anodes and cathodes, this factor can be shown to approach approximately 2 for the used dimensions.

An average performance increase of 27% has been achieved ( Concentration Conventional Matrix

3M 700 804

(current densities in A per m ² of external anode area at 2.2V)) . Also the Tafel slope in mV/decade shows a clear improvement ( Concentration Conventional Matrix

3M 116 79) .

Further the exchange current density in 10 ^-6 A/cm ² shows a clear improvement (Concentration Conventional Matrix

3M 3.5 3.1).

The invention is further detailed by the accompanying figures, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

FIGURES

The invention although described in detailed explanatory con text may be best understood in conjunction with the accompanying figures .

Fig. 1 shows a conventional alkaline water electrolysis set ^¬ up .

Fig. 2 shows a traditional (left) and zero gap cell (right) layout .

Fig. 3a and 3c show a prior art layout and fig. 3b and 3d a layout of the present invention.

Figs. 4a-e show a layout of a 3D-printed array.

Fig. 5 shows a chimney design.

Fig. 6 shows a layout with concentric electrodes.

Figs. 7 and 8 show a comparison between current densities of a prior art system and the present system.

DETAILED DESCRIPTION OF THE FIGURES

In the figures:

100 present system 10 electrode array

51 common Anode plate

52 common Cathode plate

A Anode

C Cathode

e electrode

E electrolyte

M membrane

S support

V Voltage

Fig. 1 shows a conventional alkaline water electrolysis setup.

The figures have been detailed throughout the description.

Fig. 2 shows a traditional (left) and zero (right) gap cell layout .

Fig. 3a and 3c show a prior art layout and fig. 3b and 3d a layout of the present invention.

Figs. 4a-e show a layout of a 3D-printed array. Fig. 4a shows a worked open top view of a 7 by 5 array with alternating cath odes (18) and anodes (17) with a membrane m in between the elec trodes, each electrode having a chimney layout (see fig. 5).

Fig. 4b shows a worked open side view with alternating electrodes producing 0 ₂ and Fk, respectively. Also a common anode plate 51 and common cathode plate 52 are shown, as well as supports S. Gas outlets are provided at a top side. Optionally, by making only outlets in the top plate for the cathodes (or the anodes), the products produced at the anodes (or the cathodes) may be collected from outlets below the top plate (for instance at a location (schematically indicated with an "X"). Fig. 4c shows a top section of fig. 4b. Figs. 4d and 4e show a worked open version of the present system and a closed system thereof, respectively. Also the side covers ssc are shown.

Fig. 5 shows a chimney design of a 3D-printed electrode.

Fig. 6 shows a layout with concentric electrodes.

Fig. 7 shows a comparison between current densities (A per m ² of external electrode area) of a prior art system and the pre ^¬ sent system with a 3 M KOH.

Fig. 8 shows a comparison between volumetric current densi- ties {A per m ³ of total electrolyser volume) of a prior art sys tem and the present system with a 3 M KOH.