EP4713297A1

EP4713297A1 - Water treatment loop for connection to an electrolysis stack

Info

Publication number: EP4713297A1
Application number: EP24742490.6A
Authority: EP
Inventors: Henrik TÆKKER MADSEN; Jens KRARUP
Original assignee: Grundfos Holdings AS
Current assignee: Grundfos Holdings AS
Priority date: 2023-07-18
Filing date: 2024-07-08
Publication date: 2026-03-25
Also published as: WO2025016765A1

Abstract

The invention relates to a water treatment loop (20) for connection to at least one electrolysis stack (8) of a hydrogen producing electrolysis plant (40), comprising: a water inlet section (21) into which water drained from at least one electrolysis stack (8) can be recirculated; an ion exchanger (2) arranged downstream of the water inlet section (21); a water outlet section (22) arranged downstream of the ion exchanger (2) and adapted to supply water treated by the ion exchanger (2) to said at least one electrolysis stack (8); and a catalytic surface (23) arranged downstream of the water inlet section (21) and upstream of the ion exchanger (2), so that water recirculated via the water inlet section (21) is made to contact the catalytic surface (23) prior to interaction with the ion exchanger (2), whereby oxidants such as peroxides are at least partly removed from the water, prior to being treated by the ion exchanger (2).

Description

Water treatment loop for connection to an electrolysis stack

TECHNICAL FIELD OF THE INVENTION

The invention relates to a water treatment loop for connection to at least one electrolysis stack, such as an electrolysis stack of a hydrogen producing electrolysis plant .

BACKGROUND OF THE INVENTION

In the electrolysis of water, an electrolysis stack is used to split water into oxygen and hydrogen gas by electrolysis . The hydrogen gas can then be used as hydrogen fuel . The water supplied to the electrolysis stack must be of a suf ficient quality ( such as suf ficiently pure ) such that no contaminants accumulate in the electrolysis stack and such that the electrolysis stack does not get damaged . To this end, the water is treated in a side-stream refinement loop by an ion exchanger before it enters the electrolysis stack . The ion exchanger exchanges ions dissolved in the water for hydrogen ions (H+ ) and hydroxide ions ( OH- ) , which combine to form water (H2O) . Thus , high purity of water can be ensured .

However, because of the process conditions in the electrolyzer, the ion exchanger has a complex design . For example , the ion exchanger comprises complex resins such that no unwanted substances are leached from the ion exchanger into the water . Such (high grade, in particular nuclear grade ) resins are time consuming and complex to produce and, thus , very expensive . Thereby, the hydrogen production is complex and expensive .

Thus , it is an obj ective to simpli fy the production of hydrogen . SUMMARY OF THE INVENTION

The obj ect of the present invention is achieved by the solution provided in the enclosed independent claims . Advantageous implementations of the present invention are further defined in the dependent claims .

According to a first aspect , the invention relates to a water treatment loop for connection to at least one electrolysis stack, wherein the water treatment loop comprises : a water inlet section into which water drained from at least one electrolysis stack can be recirculated; an ion exchanger arranged downstream of the water inlet section; a water outlet section arranged downstream of the ion exchanger and adapted to supply water treated by the ion exchanger to said at least one electrolysis stack; and a catalytic surface arranged downstream of the water inlet section and upstream of the ion exchanger, so that water recirculated via the water inlet section is made to contact the catalytic surface prior to interaction with the ion exchanger, whereby oxidants such as peroxides are at least partly removed from the water, prior to being treated by the ion exchanger .

This achieves the advantage that contact of the ion exchanger with harmful oxidants is avoided or at least signi ficantly reduced . A part or all of the oxidants are removed ( such as decomposed, reduced, and/or adsorbed) by the catalytic surface upstream of the ion exchanger, so that the water flowing downstream of the catalytic surface includes no or at least a signi ficantly reduced amount of oxidants , thereby having a reduced negative influence on the ion exchanger . Thus , the ion exchanger can be designed in a simpler manner, such as by using cheaper resins for ion exchange and/or by downsi zing the ion exchanger . For example , a part of the ion exchanger, e . g . its resin, can be simpli fied in such a way that it is not required to withstand the chemically aggressive condition brought about by oxidants , which may be formed in the upstream electrolysis process . In other words , the arrangement of the upstream catalytic surface opens the possibility to design parts of the ion exchanger to withstand a reduced number of chemically aggressive conditions , such as high pressure and pressure swings , and/or to focus on a speci fic interaction such as the removal of pollutants from the water . Accordingly, the production of hydrogen is simpli fied, in particular less expensive .

In the context of the present invention, "water" is to be understood as a composition that includes pure water (H2O) and that may include additional substances such as oxidants . By (pre- ) treating the water, the amount of the additional substances can be reduced . The water may be part of a water stream of a continuous hydrogen production process .

The oxidants may comprise hydrogen peroxide . For example , the catalytic surface may remove the hydrogen peroxide by reacting the hydrogen peroxide (H2O2 ) to water (H2O) and oxygen ( O2 ) . Accordingly, the ion exchanger does not need to be designed to withstand hydrogen peroxide , whereby the ion exchanger can be simpli fied in a particularly well manner . The catalytic surface can accelerate the reaction of hydrogen peroxide to water and oxygen, in some cases almost explosively owing to the rapid release of heat .

The catalytic surface may comprise one , more or all of : a charcoal, such as activated charcoal; a transition metal; metallic oxide, such as manganese oxide, e.g. manganese dioxide (MnCt) , iron oxide, e.g. iron(III) oxide (Fe2Oa) , and/or cobalt oxide, e.g. cobalt ( 11 , 111 ) oxide (C03O4) , wherein the metallic oxide is, optionally, treated and/or provided in a granulated form; a noble metal, such as platinum and/or palladium.

In an embodiment, the catalytic surface is adapted to remove oxidants without leakage of harmful substances such as metallic cations, organic matter and/or fluorides. Thereby, the design of the ion exchanger can be further simplified. By "harmful substances" it is meant substances that risk damaging or minimizing the efficiency of the electrolysis stack.

The catalytic surface may be adapted to operate under ASTM Type I conditions. The "ASTM Type I conditions" may be according to the standard "ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520/D1193-99E01" .

In an embodiment, at least part of the ion exchanger and at least part of the catalytic surface are arranged in a same (common) container. This achieves the advantage that the catalytic surface and the ion exchanger are arranged in a compact manner. For example, each of the ion exchanger and the catalytic surface may be completely arranged in the same (common) container.

In an embodiment, at least part of the ion exchanger and at least part of the catalytic surface are arranged in different containers. Accordingly, the ion exchanger and the catalytic surface can be better maintained. For example, this may open the possibility that parts of each of the ion exchanger and of the catalytic surface can be replaced without interference with the catalytic surface respectively ion exchanger. For example, each of the ion exchanger and the catalytic surface may be completely arranged in a respective container.

The container (s) may be vessel (s) .

The water treatment loop may comprise a treatment device, such as a filter, in particular a metallic filter and/or a polymeric filter. Thus, the catalytic surface is combined with an additional treatment step, thereby further minimizing the risk of leaks. The filter may be designed to withhold particles, e.g. by pores, that may support leakage.

The ion exchanger may comprise a resin for ion exchange. The resin may be of a so-called mixed bed type. For example, the resin comprises a mixed-bed unit that comprises, or consists of, a mix of cation and anion exchange resins (positive and negative charged) .

In an embodiment, at least part of the catalytic surface is arranged on, and preferably in direct contact with, a part of the ion exchanger, wherein preferably said part comprises, or is, the resin. This achieves the advantage of saving space.

In one embodiment, one or more sensors are placed downstream of the catalytic surface and/or ion exchanger to continuously monitor the performance of the catalytic surface and/or ion exchanger. This is beneficial as it allows to monitor the catalytic effect of the catalytic surface and ensure timely maintenance .

In one embodiment, there is a side stream arranged downstream of the catalytical surface and/or the ion exchanger from where water samples can be taken for the purpose of surveillance of water quality .

According to a second aspect, the invention relates to a hydrogen producing electrolysis plant comprising a loop as described above and at least one electrolysis stack.

In an embodiment, the loop and the electrolysis stack are integrated with one another.

In an embodiment, the at least one electrolysis stack is of the proton exchange membrane (PME) type. Additionally or alternatively, the at least one electrolysis stack may be of a different type, such as of the Solid Oxide Electrolysis Cell (SOEC) type.

According to a third aspect, the invention relates to the use of a catalytic surface in a water treatment loop of a hydrogen producing electrolysis plant upstream an ion exchanger.

According to a third aspect, the invention relates to a method for treatment of water for a hydrogen producing electrolysis plant (e.g. by using a water treatment loop or a hydrogen producing electrolysis plant as described above) , wherein the method comprises the steps of: a) recirculating water from an electrolysis stack; b) at least partly removing, by a catalytic surface, oxidants such as peroxides from the recirculated water, thereby obtaining pre-treated water; c) treating, by an ion exchanger, the pre-treated water, thereby obtaining treated water; and d) supplying the treated water to a water outlet section connected to the electrolysis stack.

In an embodiment, the method further comprises the step of supplying the treated water to, or into, the electrolysis stack.

In an embodiment, prior to entering the electrolysis stack the treated water undergoes one or more further treatments such as one, more or all of: filtering by a filter; UV-treatment by an UV-lamp; oxygen separation by an oxygen separator; cooling by a cooler. The cooler may be a heat exchanger.

In an embodiment, the method further comprises the step of pumping, by a pump, the treated water. The pump may be a circulation pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in the following together with the figures.

Fig. 1 shows a schematic view of a hydrogen producing electrolysis plant;

Fig. 2 shows a schematic view of a water treatment loop;

Fig. 3 shows a schematic view of a water treatment loop according to an embodiment; and

Fig. 4 shows a schematic flow chart of a method for treatment of water for a hydrogen producing electrolysis plant according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a schematic view of a hydrogen producing electrolysis plant 40 according to an embodiment.

The hydrogen producing electrolysis plant 40 comprises a water treatment loop 20 (also referred to as a so-called water refinement loop) with a water inlet section 21 into which water drained from at least one electrolysis stack 8 (e.g., an electrolysis cell) of the plant 40 can be recirculated. For example, the water inlet section 21 comprises, or is connected to, a vessel 1 such as a holding tank. On its upstream side, the water inlet section 21 is fluidly connected, directly or indirectly, to the electrolysis stack 8, e.g. at least via one or more lines (pipes, etc.) . The water inlet section 21, e.g. the vessel 1, may comprise an absorber 10 for absorbing carbon dioxide within the water inlet section 21, in particular in the vessel 1. For example, the absorber 10 may be a carbon dioxide scrubber .

The water treatment loop 20 further comprises an ion exchanger 2, e.g. a so-called ion exchange polisher, arranged downstream of the water inlet section 21, and a water outlet section 22 arranged downstream of the ion exchanger 2 and adapted to supply water treated by the ion exchanger 2 to said at least one electrolysis stack 8. The ion exchanger 2 is adapted to remove contaminants, in particular ionic species, from the water. For example, the ion exchanger 2 comprises a resin for ion exchange.

The electrolysis stack 8 is provided downstream of the water outlet section 22. The water outlet section 22 may be connected directly or indirectly to the electrolysis stack 8. If the water outlet section 22 is connected indirectly to the electrolysis stack 8, the water supplied by the water outlet section 22 may undergo one or more further (preferably consecutive) treatments before entering the electrolysis stack 8. For example, the one or more further treatments may comprise filtering by a filter 3, UV-treatment by an UV-lamp 4, oxygen separation by an oxygen separator 5, cooling by a cooler 6.

The hydrogen producing electrolysis plant 40 may comprise a pump 7 that is arranged to pump at least the treated water supplied by the water outlet section 22 into the electrolysis stack 8. The pump 7 may be arranged downstream of the cooler 6.

In the electrolysis stack 8, water at least treated by the ion exchanger 2 is split, by using electricity, into oxygen (O2) and hydrogen (H2) .

The hydrogen is then released from the electrolysis stack 8, e.g. via a first outlet line connected to the electrolysis stack

8. The first outlet line may be connected to a downstream hydrogen separator 9 where the hydrogen is separated from other substances such as water. The separated hydrogen can then be released from the hydrogen separator 9, e.g. for being used as a fuel. The separated water, which is thus drained from the electrolysis stack 8, can be released from the hydrogen separator

9, e.g. for being recirculated into the water inlet section 21 of the water treatment loop 20.

The oxygen is released from the electrolysis stack 8, e.g. via a second outlet line connected to the electrolysis stack 8 and preferably parallel to the first outlet line. The second outlet line may be connected to a (or the) oxygen separator 5 arranged downstream of the second outlet line. In the oxygen separator 5 the oxygen is separated from other substances such as water. The separated oxygen can then be released from the oxygen separator 5, e.g. for being stored in a container. The separated water, which is thus drained from the electrolysis stack 8, can be released from the oxygen separator 5, e.g. for being recirculated into the water inlet section 21 of the water treatment loop 20. By at least the second outlet line, a return stream of water can thus be sent to the water inlet section 21.

Accordingly, the water inlet section 21, in particular its vessel 1, may be connected directly or indirectly to the electrolysis stack 8, wherein the indirect connection may be provided by at least the hydrogen separator 9 and the first outlet line and/or by at least the oxygen separator 5 and the second outlet line.

The electrolysis stack 8 is not limited to a particular type of electrolysis stack and/or to a particular number of electrolysis stacks. Preferably, the electrolysis stack 8 comprises, or is, an electrolysis stack of the proton exchange membrane (PEM) type and/or an electrolysis stack of the Solid Oxide Electrolysis Cell (SOEC) type. Additionally or alternatively, the electrolysis stack 8 may comprise an electrolysis stack of the alkaline water electrolysis (AWE) type .

As shown in Fig. 1, and in Fig. 3 in more detail, the water treatment loop 20 further comprises a catalytic surface 23 arranged downstream of the water inlet section 21 and upstream of the ion exchanger 2. Thereby, water, which is recirculated via the water inlet section 21, can be made to contact the catalytic surface 23 prior to interaction (e.g., at least contact and/or reaction) with the ion exchanger 2. The water recirculated via the water inlet section 21 includes oxidants (such as peroxides) which may be harmful for the ion exchanger. Thus, the catalytic surface 23 ensures that these oxidants are at least partly removed from the water, prior to being treated by the ion exchanger 2. In other words, the amount of oxidants (measured in wt . % or mg/1) in the water downstream of the catalytic surface 23 is lower than the amount of oxidants in the water upstream of the catalytic surface 23, e.g. at least 90% lower, preferably at least 95% lower, so that the water interacting with the ion exchanger 2 includes the lower amount of oxidants, such as no or substantially no oxidants.

Thereby, the design of the ion exchanger 2 can be simplified, e.g. by using a cheaper resin for ion exchange. In other words, the ion exchanger 2 is, by the presence of the upstream catalytic surface 23, not, or at least significantly less, exposed to oxidants, so that the ion exchanger 2 is not required to be designed in a complex manner in order to maintain its chemical stability in the presence of oxidants.

The oxidants present in the recirculated water may be produced during operation of the plant 40, in particular in the electrolysis stack 8 and/or other components (equipment) which come into contact with the water. During operation, the water may be also contaminated with other contaminants, which may be produced within the electrolysis stack 8, such as fluoride ions (F~) , e.g. released by a proton exchange membrane (DEM) , or sulf ate/sulfonic acid and organics. One of the reasons for the release of contaminants is the formation of oxidants in the electrolysis process. A further cause of contamination is from leaching of the materials used for the whole plant. The metals and polymers used to build the plant can leach ions and organics into the water, which, if not removed by the ion exchanger 2, will accumulate and damage the electrolysis stack 8. For example, metal ions can inhibit the transfer of protons while organics can foul the electrodes/proton exchange membrane and even poison the catalysts. Organics can also be oxidized completely to CO2, which negatively affect the performance of the system.

Fig. 2 shows an ion exchanger 2, wherein, however, upstream of the ion exchanger 2 no catalytic surface 23 is provided. Thereby, this ion exchanger 2 interacts with the water drained from the electrolysis stack 8 and thus with water including oxidants. For this reason, the ion exchanger 2 of Fig. 2 must be designed in a more complex manner than the ion exchanger 2 according to Fig. 3, namely such that it does not degrade due to contact with the oxidants present in the water. Accordingly, compared to the ion exchanger 2 shown in Fig. 2, by the ion exchanger 2 shown in Fig. 3 the hydrogen production can be simplified. The single step treatment as shown in Fig. 2 is thus replaced by a two-step treatment as shown in Fig. 3, i.e. by at least a first step for at least partly removing oxidants (pre-treatment ) and a second step for interaction with the ion exchanger 2 (treatment) , wherein the latter step is simplified.

For example, the oxidants, which are at least partly removed by the catalytic surface 23 from the water, prior to the water being treated by the ion exchanger 2, may comprise hydrogen peroxide. When protons move through a membrane, such as a PEM membrane, of the electrolysis stack, they create an osmotic force that drags some water across the membrane. This water is saturated with oxygen formed at the anode, and when oxygen arrives at the cathode, platinum catalysts can facilitate the reduction of oxygen to hydrogen peroxide :

O₂ + 2H⁺ + 2e~ -► H₂O₂

Hydrogen peroxide is a strong oxidant , but it can also produce the highly reactive hydroxyl radicals :

H₂O₂ -► 20H •

These oxidants can degrade parts of the electrolysi s stack, such as a fluorinated polymer of the PEM membrane , which can release both fluoride and organics together with sulphonic acid groups that can be further oxidi zed to sulphate .

Hydrogen peroxide may also be formed at the anode .

In the water treatment loop shown in Fig . 2 , which has no catalytic surface 23 upstream of the ion exchanger 2 , the water including oxidants such as the hydrogen peroxide interacts with the ion exchanger 2 in such a way that the presence of the oxidants , such as of the hydrogen peroxide and corresponding radicals , cannot oxidise with functional parts of the ion exchanger, such as with resins for ion exchange ; in this way, leakage of organics and functional sul fonic acid groups into the water is avoided . To achieve this , however, the ion exchanger 2 , in particular its resin, is required to have a complex, big design that is time consuming and expensive to produce .

In particular, to achieve high stability towards thermal and high degradation, and thus to reduce unwanted leaching, the ion exchanger 2 in Fig . 2 comprises a high grade resin that has a particularly low content of metals /metal ions and organics . This ef fect is particular important during start-up of new resins . After a (mixed) resin has been standing idle for a while , metal ions and organics from the centre of the resins beads will have had time to di f fuse to and accumulate at the resin surface . Once the resin is brought into operation, this can create a large spike in concentration of metals and TOC in the water coming from the resins , and i f this water is sent to the electrolyser , it will momentarily be exposed to concentration far exceeding the quality control limits .

By contrast , the water treatment loop 20 shown in Fig . 3 makes it possible to design the ion exchanger 2 simpler . For example , with the water treatment loop 20 shown in Fig . 3 , the catalytic surface 23 arranged upstream of the ion exchanger 2 removes the hydrogen peroxide from the aqueous solution ( i . e . from the water ) through a process known as catalytic decomposition . The catalytic surface 23 may provide an active side where the hydrogen peroxide (H2O2 ) molecules can undergo a reaction that breaks them down into harmless water and oxygen . The catalytic decomposition of hydrogen peroxide is typically an exothermic reaction, and the presence of a suitable catalyst surface enhances the reaction rate signi ficantly . By facilitating this decomposition, the catalytic surface 23 helps in the ef fective removal of hydrogen peroxide from the aqueous solution . For example , the catalytic surface 23 may comprise a metal catalyst such as manganese dioxide (MnCt ) and platinum, whereby the hydrogen peroxide is decomposed into water and oxygen :

As can be seen, the catalyst does not form a chemical product with the hydrogen peroxide but only facilitates the decomposition .

Subsequently, the (pre-treated) water flowing downstream of the catalytic surface 23 includes no oxidants ( such as hydrogen peroxide ) , or at least a signi ficantly reduced amount of the same , whereby this water does not pose a risk to degrade the ion exchanger 2 by including oxidants . Consequently, the ion exchanger 2 can be simpli fied compared to the one shown in Fig . 2 , e . g . by using a lower grade resin and thus not a high grade resin that has a leakage factor 5x less leakage of TOC ( total organic carbon) then a standard resin in the same conditions . By the water treatment loop 20 , in particular the release of organic substances from the ion-exchanger 2 can be simply reduced or even prevented . As such, the water treatment loop 20 makes it possible to prolong li fetime and capacity as well as to use resins that are optimi zed for refinement of ultrapure water without the added need for chemical stability with respect to oxidants .

The catalytic surface 23 is not limited to a particular catalyst . For example , the catalytic surface 23 may comprise one , more or all of : a charcoal ( e . g . in the form of a charcoal-based filter, wherein by the term "charcoal-based filter" is meant any type of carbon-containing filter that has been treated to have an increased surface area ) , such as activated charcoal (i.e. a so-called activated carbon, which is characterized by having a large number of pores, whereby its adsorption capacity is particularly increased) , wherein the charcoal is preferably treated, e.g. so as to provide activated charcoal , a transition metal, metallic oxide (whereby the surface 23 may be a metallic oxide surface) , such as o manganese oxide, e.g. manganese dioxide (MnCt) , o iron oxide, e.g. iron(III) oxide (Fe2Oa) , and/or o cobalt oxide, e.g. cobalt ( I I , I I I ) oxide (C03O4) , o wherein the metallic oxide is, optionally, treated and/or provided in a granulated form (thus increasing efficiency) , and/or wherein, optionally, a column is made from the (solid) metallic oxide, a noble metal, such as platinum and/or palladium, wherein the noble metal may be provided as a thin coating and/or in the form of metallic particles.

Activated charcoal (carbon) can be fabricated from a variety of sources, e.g. from organic waste under a large variety of conditions e.g., chemical impregnation, steam and/or dry conditions, ranges of temperatures, etc.

For example, if the catalytic surface 23 comprises charcoal, the hydrogen peroxide can adhere to the surface of the carbon, it binds to it and reacts with it: H⁺ H

Carbon surface) C-O... OH +H OOH — >)C-O...OOH +H2O

Carbon surface) C-O...OOH + H2 O2 — » ) C-O...OH + H2 O + O2

H

By using charcoal (in particular activated carbon, e.g. granulated) as the catalyst of the catalytic surface 23, also a sufficient decomposition of oxidizing agents can be ensured.

The catalytic surface 23 and the ion exchanger 2 are not limited to a particular arrangement relative to one another as long as the catalytic surface 23 is arranged upstream of the ion exchanger 2, so that water comes first into contact with the catalytic surface 23 before interacting, in a state with at least removed oxidants, with the ion exchanger 2. For example, the water treatment loop 20 may comprise a (single) container, such as a vessel or tank, in which both the catalytic surface 23 and the ion exchanger 2 are arranged (accommodated, housed, received, etc.) ; in other words, the ion exchanger 2 and the catalytic surface 23 may be arranged in a same container. Alternatively, each of the ion exchanger 2 and the catalytic surface 23 may be arranged in a respective (i.e., own) container, such as a vessel, so that the ion exchanger 2 and the catalytic surface 23 are arranged in different containers that may be spaced apart .

The catalyst and the ion exchange resin may be spatially separated. When arranged spatially separated, the catalyst is arranged upstream of the ion exchange resin. As can be seen from Figures 1 and 3, the catalytic surface 23 is spatially separated from the ion exchanger 2 with the catalytic surface 23 being arranged upstream of the ion exchanger 2.

The catalytic surface 23 may be distanced from the ion exchanger 2, e.g. by at least a ( f luid/liquid) line, such as a conduit, via which water can flow towards the ion exchanger 2. Alternatively, the catalytic surface 23 may not be distanced from the ion exchanger 2, in particular from parts of the ion exchanger 2 interacting with the water. In particular, at least part of the catalytic surface 23 may be arranged on, and preferably in direct contact with, a part of the ion exchanger 2, in particular a part adapted to interact with the water having the at least partly removed oxidants, such as the resin for ion exchange. In such a configuration, the ion exchanger 2 and the catalytic surface 23 may be arranged in the same container. Despite from being arranged in the same container, the catalyst and the ion exchange resin may be spatially separated. The resin of the ion exchanger 2, e.g. a surface of the resin, may comprise an upstream section and a downstream section. The catalyst of the catalytic surface 23, e.g. a surface of the catalyst, may comprise an upstream section and a downstream section. In particular, the downstream section of the catalytic surface 23 is arranged on the upstream section of the ion exchanger 2. More particularly, the downstream section of the catalytic surface 23 is arranged on the upstream section of the ion exchanger 2 , wherein the catalyst and the ion exchange resin are spatially separated. The resin of the ion exchanger 2, e.g. a surface of the resin, may comprise an upstream section and a downstream section, wherein the catalytic surface 23 is arranged on the upstream section, so that at least the downstream section is, by the catalytic surface 23 (a reactive surface) protected, i.e. not exposed to oxidants. The catalytic surface 23 may be coated onto the resin of the ion exchanger.

Fig. 4 shows a schematic flow chart of a method for treatment of water for the hydrogen producing electrolysis plant 40, wherein the method 100 comprises at least steps 101-104.

In step 101, water is recirculated from the electrolysis stack 8. This water may be water drained from the at least one electrolysis stack 8, such as from an outlet line of, or connected to, the electrolysis stack 8, e.g. the first and/or second outlet lines. Thus, this water drained from the at least one electrolysis stack 8, i.e. "recirculated water", includes oxidants such as peroxides (hydrogen peroxide, etc.) and may have an ASTM Type II grade ("ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520/D1193- 99E01") .

In step 102, oxidants are at least partly removed from the recirculated water by the catalytic surface 23, such as by the oxidants contacting and thus interacting with the catalytic surface 23, whereby the oxidants may react to (such as decompose into, etc.) harmless components such as (pure) water (H2O) and oxygen. Thereby, pre-treated water is obtained at the downstream end of the catalytic surface 23.

In step 103, the pre-treated water is treated by interaction with the ion exchanger 2, thereby obtaining treated water at the downstream end of the ion exchanger 2. The treated water may have a defined quality or purity, e.g. such that requirements according to the ASTM Type I standard ("ASTM Standard D1193- 99el, 2017, "Standard Specification for Reagent Water", 10.1520/D1193-99E01") are fulfilled.

In step 104, the treated water is supplied to the water outlet section 22 (which is fluidly connected to the electrolysis stack 8) , wherefrom the treated water may be directly or indirectly supplied, e.g. by using the pump 6, into the electrolysis stack 8. In the electrolysis stack 8, the at least treated water can then be split, by using electricity, into hydrogen and oxygen.

The method 100 may comprise one or more further (treatment) steps, such as one or more steps before the treated water enters the electrolysis stack 8, i.e. one or more steps carried out upstream of the electrolysis stack 8. For example, these steps may comprise one, more or all of: filtering by the filter 3; UV- treatment by the UV-lamp 4; oxygen separation by the oxygen separator 5; cooling by the cooler 6. At least downstream of the UV-lamp 4 the water may have such a quality that the requirements according to the ASTM Type I standard (ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520/D1193-

99E01) are fulfilled.

Claims

1. A water treatment loop (20) for connection to at least one electrolysis stack (8) of a hydrogen producing electrolysis plant (40) , comprising:

- a water inlet section (21) into which water drained from at least one electrolysis stack (8) can be recirculated,

- an ion exchanger (2) arranged downstream of the water inlet section (21) ,

- a water outlet section (22) arranged downstream of the ion exchanger (2) and adapted to supply water treated by the ion exchanger (2) to said at least one electrolysis stack ( 8 ) , and

- a catalytic surface (23) arranged downstream of the water inlet section (21) and upstream of the ion exchanger (2) , so that water recirculated via the water inlet section (21) is made to contact the catalytic surface (23) prior to interaction with the ion exchanger (2) , whereby oxidants such as peroxides are at least partly removed from the water, prior to being treated by the ion exchanger ( 2 ) .

2. The water treatment loop (20) according to claim 1, wherein the oxidants comprise hydrogen peroxide.

3. The water treatment loop (20) according to claim 1 or 2, wherein the catalytic surface comprise one, more or all of:

- a charcoal, such as activated charcoal,

- a transition metal,

- metallic oxide, such as manganese oxide, e.g. manganese dioxide (MnCt) , iron oxide, e.g. iron(III) oxide (Fe2Oa) , and/or cobalt oxide, e.g. cobalt ( I I , I I I ) oxide (C03O4) ,

• wherein the metallic oxide is, optionally, treated and/or provided in a granulated form,

- a noble metal, such as platinum and/or palladium.

4. The water treatment loop (20) according to any one of the preceding claims, wherein the catalytic surface (23) is adapted to remove oxidants without leakage of harmful substances such as metallic cations, organic matter and/or fluorides.

5. The water treatment loop (20) according to any one of the preceding claims, wherein the catalytic surface (23) is adapted to operate under ASTM Type I conditions.

6. The water treatment loop (20) according to any one of the preceding claims, wherein at least part of the ion exchanger (2) and at least part of the catalytic surface (23) are arranged in a same container.

7. The water treatment loop (20) according to any one of claims 1-5, wherein at least part of the ion exchanger (2) and at least part of the catalytic surface (23) are arranged in different containers .

8. The water treatment loop (20) according to any one of the preceding claims, further comprising a treatment device, such as filter (3) , in particular a metallic filter and/or a polymeric filter .

9. The water treatment loop (20) according to any one of the preceding claims, wherein the ion exchanger (2) comprises a resin for ion exchange.

10. The water treatment loop (20) according to any one of the preceding claims, wherein at least part of the catalytic surface (23) is arranged on, and preferably in direct contact with, a part of the ion exchanger (2) , wherein preferably said part comprises, or is, the resin.

11. A hydrogen producing electrolysis plant (40) comprising a loop (20) according to any one of the preceding claims and at least one electrolysis stack (8) .

12. The hydrogen producing electrolysis plant (40) of claim 11, wherein the at least one electrolysis stack (8) is of the proton exchange membrane (PEM) type.

13. Use of a catalytic surface (23) in a water treatment loop (20) of a hydrogen producing electrolysis plant (40) upstream an ion exchanger (2) .

14. A method (100) for treatment of water for a hydrogen producing electrolysis plant (40) , comprising the steps of: a) recirculating (101) water from an electrolysis stack (8) ; b) at least partly removing (102) , by a catalytic surface (23) , oxidants from the recirculated water, thereby obtaining pre-treated water; c) treating (103) , by an ion exchanger (2) , the pre-treated water, thereby obtaining treated water; and d) supplying (104) the treated water to a water outlet section (22) connected to the electrolysis stack (8) .

15. The method (100) according to claim 14, further comprising the step of supplying the treated water to or into the electrolysis stack (8) .

16. The method (100) according to claim 14 or 15, wherein prior to entering the electrolysis stack (8) the treated water undergoes one or more further treatments such as one, more or all of: filtering by a filter (3) ,

- UV-treatment by an UV-lamp (4) ,

- oxygen separation by an oxygen separator (5) , - cooling by a cooler (6) .

17. The method (100) according to any one of claims 14-16, further comprising the step of pumping, by a pump (7) , the treated water.