WO2021160759A1

WO2021160759A1 - Electrochemical cell for the synthesis of hydrogen peroxide

Info

Publication number: WO2021160759A1
Application number: PCT/EP2021/053376
Authority: WO
Inventors: Rajath SATHYADEV RAJMOHAN; Rasmus FRYDENDAL; Arnau VERDAGUER CASADEVALL; Ziv Gottesfeld
Original assignee: Hpnow Aps
Priority date: 2020-02-11
Filing date: 2021-02-11
Publication date: 2021-08-19

Abstract

A configuration of electrolyzer enabling electrochemical hydrogen peroxide production is disclosed. This electrolyzer consists of an anode, a cation exchange membrane, a porous proton-conducting layer and a cathode, assembled into a dedicated housing. A voltage difference is induced between anode and cathode to drive electrochemical reactions. Oxygen-containing gas is fed to the cathode, while the anode generates protons (typically using water as a proton source). Protons generated at the anode cross a cation exchange membrane and a porous proton-conducting layer, and they combine with oxygen at the surface of the cathode electrode to generate hydrogen peroxide. Generated hydrogen peroxide is extracted through the porous proton-conducting layer, in a process that can be aided with a water flow through the proton-conducting layer, which allows for a high faradaic efficiency and throughput.

Description

ELECTROCHEMICAL CELL FOR THE SYNTHESIS OF HYDROGEN PEROXIDE

Introduction

The use of electrochemical devices is increasingly in focus for both power generation in fuel cells and production of chemicals as electrolysis cells. Electrolysis cells offer unique advantages in generating chemicals in decentralized facilities with the use of electricity as input energy rather than requiring large chemical production plants. Advantages include generation of chemicals where they are needed, thereby removing the need for transportation, and use of energy produced by sustainable means such as wind power and solar.

Traditionally, the use case for electrolysis has been hydrogen production for either energy storage or direct use in the chemical industry. A more recent example of an electrolysis application is electrochemical CO₂ reduction into syngas which subsequently can be used to produce a range of common organic compounds.

Similarly, electrochemical production of hydrogen peroxide can offer advantages over the traditional Anthraquinone process that takes place in centralized chemical facilities. Hydrogen peroxide, H₂O₂, is used as an oxidizer in important industrial processes as well as for water treatment and disinfection globally. The production of hydrogen peroxide in the Anthraquinone process involves a large amount of energy and chemical waste. With electrochemical production, oxygen from the atmosphere can react with water to form H₂O₂ and only electricity is required as energy input, meaning that a fully sustainable process can be achieved. Today hydrogen peroxide is produced in large concentrations at centralized production plants and shipped around the world in drums of 30-70 wt% solutions. Besides driving up the price, shipping of high-concentration hydrogen peroxide is a hazard as the chemical in these concentrations is explosive. Therefore, the product requires processes for transport, storage and handling. These concerns are directly addressed by producing the chemical on-site. Furthermore, in many applications, low concentrations (below 3 wt%) are needed which means safety concerns can be avoided altogether if the solutions produced are kept low from the beginning.

Background of the invention

The original development of electrolyzers for hydrogen peroxide production has been focused towards delivering an alternative to the Anthraquinone process with the goal of producing highly concentrated solutions at a central facility and shipped to end users. Typically, liquid electrolytes for ionic transport between the electrodes were used, which is problematic since the produced hydrogen peroxide is then mixed with either alkaline or acidic solutions and has a low purity. An example of such configuration can be seen in patent application US 8 591,719 B2, where a porous diaphragm between anode and cathode electrodes contains an acid or salt electrolyte providing ionic conductivity. This has the disadvantage of producing hydrogen peroxide mixed with the electrolyte, and a consumable-based electrolyte is required in the first place.

This present invention explicitly negates the need to use a liquid electrolyte, which allows for higher purities of hydrogen peroxide without using consumables. More recently, state-of-the-art electrolyzers producing hydrogen peroxide have utilized Membrane Electrode Assemblies, MEA, in which the ionic transport between electrodes, anode and cathode, is handled by solid state membranes (see for example Handbook of Fuel Cells: Fundamentals, Technology and Applications, Wiley VCH,

2014). Using such MEA’s provide for more compact devices and the membranes and electrodes can be manufactured in thin layers that are often pressed together under heat and pressure to provide a good adhesion. A good MEA ensures that the electrodes are given appropriate access to 1. Ion conductivity 2. Electric conductivity and 3. The chemical reactant. In the case of hydrogen peroxide synthesis, the chemical reactant is oxygen which is reduced at the cathode. On the anode, water is oxidized to produce the protons that cross through the membrane towards the cathode where they react with O2. To address the access of chemical reactant an MEA based cell has been proposed where oxygen containing gas is delivered directly to the cathode, wherein a H₂O₂ and water mixture is formed in the catalyst layer (see GB patent application 2012/052316). However, with only gas supplied to the electrode the hydrogen peroxide solution stays in close vicinity of the catalyst and can decompose again with high rates. This challenge was addressed by operating an MEA directly in a water solution providing for better hydrogen peroxide extraction (see Neutral H2O2 Synthesis by Electrolysis of Water and O2, Angewandte Chemie 2008). The strategy of operating in liquid water means that the chemical reactant, oxygen, now has limited access to the electrode, since oxygen solubility in water is low and the result is a low overall throughput of hydrogen peroxide.

The balance between providing good chemical reactant transport while facilitating removal of the product could be improved by introducing oxygen in close vicinity to the hydrogen peroxide producing electrode while the MEA is still fully immersed in water (see US patent application 62/309,655, US patent application 2014/0131217 A1). In this setup oxygen delivery is better than relying on dissolved oxygen and the hydrogen peroxide removal into the bulk water solution is facilitated.

With the above inventions reaction rate vs. decomposition rate is improved but further development has indicated that hydrogen peroxide concentration in the output could be increased by impeding hydrogen peroxide decomposition (see Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte, Xia et al, Science 2019). In this setup the cathode is in alkaline conditions and in direct contact with an anion exchange membrane. At the cathode OOH- ions are produced and subsequently conducted through the anion exchange membrane. On the other side of the alkaline membrane there is a porous layer with ionic conducting properties where the OOH- ions are combined with protons that are produced at the anode to form H2O2. This approach relies on the alkaline exchange membrane conducting OOhT anions to the porous layer. Between the anode and the porous layer there is a cation exchange membrane. By using two membranes and a porous layer, the decomposition of hydrogen peroxide can be minimized. However, this setup comes with issues of stability on the anion exchange membrane as well as increased ohmic losses due to the presence of two membranes and a porous layer. In particular, alkaline membranes suffer from the presence of CO2, which contributes to their loss of functional groups and has a negative impact on their ionic conductivity. The presence of hydrogen peroxide is also detrimental for alkaline membranes as it can lead to oxidation, and alkaline ionomers easily dissolve in aqueous solution which also results in loss of ionic conductivity (Handbook of Fuel Cells: Fundamentals, Technology and Applications, Wiley VCH, 2014). For these reasons the use of alkaline exchange membranes is impractical for most applications, including for electrochemical hydrogen peroxide generation and would severely limit their implementation in a commercial product.

In this invention we disclose an electrochemical cell with improved hydrogen peroxide removal using a single type of ion exchange membrane.

In this setup, a cathode electrode is in direct contact with a proton-conducting porous layer that has cation exchange properties and allows for an aqueous solution path in the plane of the porous layer. The other side of the porous layer is in direct contact with a cation exchange membrane separating the anode electrode from the porous layer. In this cell configuration, protons made at the anode have a facile path towards the cathode where they react with oxygen to form hydrogen peroxide, and the presence of aqueous solution in the porous layer facilitates removal of generated hydrogen peroxide and minimizes its decomposition. Oxygen containing gas can be fed into the cathode in gaseous phase for optimal reactant transport. This same gas also helps to extract produced hydrogen peroxide from the cathode into the porous layer. That way, the anion exchange membrane is eliminated altogether, which results in a simpler system with lower costs and longer lifetimes of the electrochemical cells. Detailed description of the invention

Fig. 1 shows a schematic view of the inventive electrochemical cell. The present invention relates to a novel electrochemical cell design for the electrochemical generation of hydrogen peroxide. The electrochemical cell 1 consists of an anode 5, a membrane 4, a porous proton-conducting layer 3, and a cathode 2. The electrochemical cell is enclosed in a dedicated housing which provides the required mechanical and electrical environment.

The overall cell reaction is the synthesis of hydrogen peroxide through the following reaction:

2 H₂0 + 0₂ ® 2 H₂0₂

This is split into the anode and the cathode half-cell reactions:

Anode: 2 H₂0 ® 0₂ + 4 H⁺ + 4e

Cathode: 20₂ + 4 H⁺ + 4e^~ ® 2 H₂0₂

The anode 5 acts as a proton source for the cathode, and while water is the most common reactant other proton sources such as alcohols (methanol, ethanol...) or molecular hydrogen could be used without affecting the nature of the invention. If other proton sources are used the overall cell reaction and half-cell reaction are accordingly affected.

Anodes for water oxidation to oxygen are well-known to those versed in the art. These consist of an anode catalyst layer and a current collector. Anodes are in intimate contact with the cation exchange membrane 4. The cation exchange membrane needs to be proton conducting, and common types include Nafion. The thickness of the cation exchange membrane is generally between 10 pm to 500 pm, preferably between 20 to 150 pm. On the anode, the current collector is also known as a porous transport layer, and it is typically a Titanium felt or foam. The Titanium felt or foam can also be coated with other materials such as Platinum or Gold to improve electrical contact. Iridium oxide nanoparticles act as a catalyst for water oxidation, and can be combined or replaced with ruthenium oxide, platinum and other metals. Deposition of the nanoparticles can take place via spray coating, tape casting or other suitable methods, and can be done directly at the membrane or at the current collector. Usually, nanoparticles are made into an ink, which can contain ionomer and solvents such as water or alcohols. Once deposited, the nanoparticles form the anode catalyst layer. Following the deposition step, the polymer exchange membrane, the anode catalyst layer and the current collector are joined together so they have intimate contact with each other. This process can be aided with the application of heat and pressure, in some cases with the objective of achieving an anode with all components forming an ensemble that cannot be separated.

The cathode 2 reacts oxygen into hydrogen peroxide. Oxygen could come from air, an oxygen concentrator or from a bottle of compressed gas. Cathodes consist of a gas diffusion layer and a cathode catalyst layer. The gas diffusion layer is usually made of carbon cloth or fibers, Titanium felt, or other suitable conductive materials. The gas diffusion layer can be coated with Polytetrafluoroethylene (PTFE) particles, other fluoropolymers or other suitable materials to modify its properties. In particular, PTFE coating could provide the gas diffusion layer with hydrophobic properties, which would prevent water from going deeper in the gas diffusion layer and potentially blocking the gas path. The main objectives of the gas diffusion layer are to provide electrical and mechanical support for the catalyst layer, while at the same time allowing an even distribution of gaseous reactants. The cathode catalyst layer is where the oxygen reduction to hydrogen peroxide takes place. The cathode catalyst layer can be formed by applying a catalyst ink onto the gas diffusion layer. It is also possible to apply the catalyst ink on the porous proton-conducting layer, and adding the gas diffusion layer in a subsequent step. Catalyst ink is applied through spray coating, screen printing or other suitable processes. The catalyst ink contains a suitable catalyst, usually a high surface area material with good selectivity for the target reaction, ionomer, providing adhesion and ionic conductivity, mixed with a suitable solvent to facilitate ink formation and homogeneous dispersion. Optionally the catalyst ink may also contain FIFE particles, which could give it hydrophobic properties that could be beneficial. Typical solvents include alcohols and/or water. Suitable cathode catalysts should be selective towards oxygen reduction to hydrogen peroxide, and include Pt-Hg, Pd-Hg, Cu-Hg, Ag- Hg, Ag, Au, carbon, graphene, nitrogen doped carbon, Sulphur doped carbon, Cobalt porphyrins and phthalocyanines, transition metal sulfides and nitrides and any combinations thereof.

A porous proton-conducting layer 3 is placed on the side of the membrane that is not facing the anode. The porous proton-conducting layer should allow the extraction of hydrogen peroxide from the cathode to the outside of the electrochemical cell, and as such it should allow the passage of water in the same direction as the cathode surface plane. The easier the water passage, the easier the extraction of hydrogen peroxide. At the same time, the porous proton-conducting layer should provide ionic conductivity from the membrane to the cathode to allow transport of protons from anode to cathode. The higher the ionic conductivity, the lower the voltage losses in the electrochemical cell, and the lower the heat generation. Therefore, there is a balance between achieving a high proton conductivity and facilitating water passage which has an optimum. Water could flow through the porous proton conducting layer, and the flow could be intermittent, or preferably continuous. Water flow is one of the parameters that determines output hydrogen peroxide concentration (the higher the flow, the lower the concentration). It is preferred that water flows the cathode electrode plane, and in a homogeneous way across the whole surface. Water flow through the porous proton conducting layer enables continuous extraction of produced hydrogen peroxide. One advantage of using a porous proton conducting layer is that the proton transport between anode and cathode does not require of any liquid electrolyte, and water can be used to extract produced hydrogen peroxide. This configuration does not require of any consumable salts or electrolytes, and it results in a very high purity of hydrogen peroxide generated.

The porous proton-conducting layer may consist of ion conducting spheres or particulates of various shapes, such as cubes or irregular, but it can also be a mesh-like structure, a foam-like structure, a sponge-like structure or plate of sintered spheres or particles, all of which have at least a part of the surface treated to be ion conducting. An example of suitable materials could be ion exchange resin made of styrene- divinylbenzene treated with sulfonic acid functional groups. The diameters of the particles should be in the range of 5 to 1000 pm, preferably 200-600 pm. The surface of the spheres can be coated with an ion conducting material, such as an ionomer or surface treated to be ion conducting so as to create a network of ion conducting material that is porous but stretches from cation exchange membrane all the way to the cathode catalyst material on the cathode electrode. Coating of the spheres can be done via immersion or casting of ion exchange resin spheres into an ionomer solution, via spraying or other suitable techniques. Deposition of the porous cation conducting material onto the membrane can be aided by application of temperature and/or pressure. Gaps between the ion conducting spheres or particulates can be between 0 to 1000 pm and preferably 0-200 pm. The resulting porous proton-conducting layer should allow easy passage of water through, with a preferred porosity between 5 and 90 %, and even more preferred between 15 and 60 %. There could be one or more layers of ion conducting spheres stacked or in a packed structure, and after preparation they could be coated with ionomer solution.

Alternatively, the porous proton-conducting layer could be formed by coating a porous substrate in ionomer solution. For example, the porous substrate could be a sheet of porous PTFE, polyethylene or other plastics, porous forms of carbon or porous titanium or other metals, which is coated in ionomer solution. Coating could be done by spraying, dip coating or other suitable methods. Yet another approach to make the porous proton-conducting layer could be to form a suitably porous proton conducting membrane. This could be done for example through electrospinning (as an example described by J.W. Park et al, Journal of Membrane Science, 2017), membrane casting processes (for instance as described in US patent application US10236527B2) or additive manufacturing.

Advantageous manufacturing can be done with electrospinning or other techniques which results in a felt or fiber mat type membrane with fibers. For such a configuration, fibers should be in the range of 0.5 pm to 50 pm in diameter, Nafion or another ionomer content of 30-100 %, added polymer for mechanical strength of 10-70%, thickness from 0.1 mm to 5 mm and proton conductivity should be above 0.02 S/cm while allowing for water flow in-plane.

The total thickness of the porous proton-conducting layer can range between 0.02 mm to 5 mm, preferably between 0.03 mm and 1 mm. With this preferred thickness a good balance between water transport and proton conductivity is achieved. It is important that the thickness of the proton-conducting porous layer is low to decrease ohmic drop, but at the same time there needs to be enough space for water to be able to cover the whole surface of the cathode.

The cathode 2 is placed with its catalyst layer facing the porous proton-conducting layer 3. The cation exchange membrane 4 is placed in intimate contact with the porous proton-conducting layer, and the anode 5 is placed in intimate contact with the face of the cation exchange membrane not in contact with the porous proton-conducting layer. Optionally, the cathode or the cation exchange membrane or both may be attached to the porous proton-conducting layer with the help of heat and pressure, which in some cases could help establish proper ionic contact. In other embodiments, the cathode could be prepared directly on one of the sides of the porous proton-conducting layer. The assembly of cathode electrode, porous proton-conducting material, cation conducting membrane and anode electrode could be done outside the housing and potentially aided by applying pressure and/or temperature when assembling. The electrochemical cell is placed in a suitable housing, typically made of Aluminum, Titanium, graphite and/or plastic materials. Typically operation takes place at room temperature, and the pressures involved in the process are < 2 bar. The cell housing may also include gaskets of rubber or other suitable material for sealing purposes. The assembly of the electrochemical cell into the housing is shown in Figure 2. The housing is composed of a cathode plate and an anode plate. The cathode plate 7 allows for electrical contact to the cathode of the electrochemical cell, as well as delivery of gas and water to the cathode. Gas is delivered through gas inlet 8 and is delivered to the side of the cathode electrode not in contact with the porous proton-conducting layer 3.

In some embodiments, it would be beneficial to place a fluidizing media in contact with the cathode gas diffusion layer electrode 2 to facilitate dispersion of the gas into the gas diffusion layer. This fluidizing media may be incorporated into the cathode plate 7. Optionally the fluidizing media can also be electrically conductive to facilitate current collection from the cathode gas diffusion layer. Examples of suitable fluidizing media are metals in porous form, such as steel, nickel or titanium, carbon materials such as graphite, or porous plastics. The fluidizing media may also incorporate coatings which modify the hydrophobic properties of the surface and such coating could, as examples, consist of Teflon based materials or high surface area carbon materials.

A preferred pore size of the fluidizing media is between 0 to 200 pm, and even more preferred between 1 and 20 pm. Gas composition needs to contain oxygen, which could be from air, oxygen concentrator or any container of oxygen. Oxygen-containing gas crosses the cathode gas diffusion layer, and it reaches the cathode catalyst layer, where it is reduced to hydrogen peroxide. Typically, there will be an over-stoichiometry of gas, which will cross the fluidizing media and the cathode, assisting in the removal of hydrogen peroxide from the cathode catalyst layer. This is a distinct advantage of this cell configuration which has not been achieved in prior art, where flow of gas was blocked by an alkaline exchange membrane. Liquid water is inserted through water inlet 9 and is directed through the porous proton conducting layer 3 between cathode electrode 2 and membrane 4. Optionally water could also contain electrolytes, such as alkali hydroxides or sulfuric acid. Preferably water is in deionized form. Having water filling the pores in the porous proton conducting layer facilitates extraction of hydrogen peroxide that is generated at the cathode electrode. Hydrogen peroxide solution is extracted through outlet 10. Liquid solution passes through the proton-conducting porous layer and outside of the cell. There are three ways that liquid is introduced to the proton-conductive porous layer where it dilutes the generated Hydrogen Peroxide:

1 ) Water is generated at the cathode (through oxygen reduction or hydrogen peroxide reduction),

2) it is transported from the anode through the cation exchange membrane and

3) it can be introduced between cathode and membrane through a dedicated inlet to deliberately dilute and/or cool the system.

It is important that the liquid is in contact with the cathode electrode surface to facilitate Hydrogen Peroxide extraction, and that there is no dead volume where generated Hydrogen Peroxide can accumulate without access to the liquid flow. External liquid introduced between cathode and membrane can be from one or both of the laterals, from the bottom or from the top of the cell. In some embodiments it may be advantageous to introduce the liquid from the bottom to ensure the whole compartment is filled with water thanks to gravity. Water and hydrogen peroxide exit the cell housing through outlet 10 which can be positioned in one of the laterals, top or bottom of the cell housing. More than one inlet and/or outlet could also be used without affecting the nature of the invention. The anode plate 11 directs water, preferably in the deionized form, to the anode to be used for water oxidation reaction. Water is inserted through water inlet 12 and exits together with products from the anode reaction (typically oxygen if water oxidation is carried out) through outlet 13. It is also possible to set a water path that takes water (and potentially oxygen) from the anode plate, and this same water is then used in the cathode plate, which may be beneficial in certain circumstances.

For the sake of creating a stack of cells connected serially, an embodiment of the cell housing includes connected cathode and anode plates or one combined bipolar plate without changing the above-described functionality. With a stack of cells, production capacity is increased proportionally to the number of cells while increasing the necessary voltage applied to run the reaction, as is known from hydrogen electrolysers.

Following assembly of the electrochemical cell into the housing, gas is introduced at the cathode with a flow between 0.01 to 100 ml_/min/cm² of electrode area, to obtain a pressure between 0.01 and 10 bar. Water is introduced to the anode with a water flow between 0.01 to 50 ml/min/cm². This flow can be continuous or pulsating so as to only refill the anode compartment periodically. Water is also introduced between the cathode electrode and the ion exchange membrane through the proton-conducting porous layer in a suitable flow.

Voltage is applied between the cathode and anode electrodes, between 0.6 and 20 V per cell, preferably between 1.2 and 5 V and even more preferably between 1.2 and 3.5 V. Current from the cell ranges between 20 mA/cm² to 1500 mA/cm². This results in hydrogen peroxide being generated at the interface between cathode and ion conducting spheres. The generated concentration is between 200 mg/L to 200000 mg/L, preferably between 5000 to 30000 mg/L. The output concentration can be varied depending on the applied current and the water flow in the compartment containing ion conducting spheres. It is also possible to combine one or more cells in series, in parallel or a combination of to generate higher throughputs. The generated solution can be stored in a reservoir for subsequent use or be directly injected in a pipe. Examples of suitable uses are within wastewater treatment, irrigation water treatment or cooling tower water treatment, or other applications where hydrogen peroxide is used as an oxidant, biocide and / or oxygen source, where the electrochemical cell can generate hydrogen peroxide on-site. Generated hydrogen peroxide can also be combined with UV light, Fenton-like agents (such as iron ions) or ozone to create OH radicals, which have a higher oxidation potential and are the basis for advanced oxidation processes. It can also be combined with acetic acid on-site to generate peracetic acid.

Drawings

Fig. 1 shows a schematic representation of a hydrogen peroxide producing electrochemical cell;

Fig. 2 shows schematic representation of a hydrogen peroxide producing electrochemical cell and related housing;

Fig. 3 shows a polarization curve of a hydrogen peroxide electrolyzer;

Fig. 4 shows the Faradaic efficiency over time;

Fig. 5 shows the energy consumption for produced Hydrogen peroxide as function of porous proton conducting layer thickness;

Fig. 6 shows Energy consumption for produced hydrogen peroxide as function of ionomer content in the porous proton conducting layer. Figure 1 is a schematic representation of a hydrogen peroxide producing electrochemical celM . 2 is the cathode, 3 is the porous proton-conducting layer, 4 is the cation exchange membrane and 5 is the anode.

Figure 2. is a schematic representation of a hydrogen peroxide producing electrochemical cell and related housing 6. 7 is the gas plate, 8 the gas inlet, 2 is the cathode, 3 is the porous proton-conducting layer, 9 is the cathode inlet/outlet, 10 is the cathode inlet/outlet, 4 is the cation exchange membrane, 5 is the anode, 11 is the anode plate, 12 is the anode inlet and 13 is the anode outlet.

To demonstrate a preferred embodiment of this invention, an electrochemical cell was set up as an electrolyzer to produce hydrogen peroxide on the basis of the following half-cell reactions:

Anode: 2 H₂0 ® 0₂ + 4 H⁺ + 4 e^~

Cathode: 20₂ + 4 H⁺ + 4e^~ ® 2 H₂0₂

It is also important to minimize hydrogen peroxide decomposition, which can take place chemically:

2 H₂0₂ ® 2 H₂0 + 0₂

Or electrochemically:

H₂0₂ + 2 H⁺ + 2e^~ ® 2 H₂0

Other proton sources could be used at the anode without affecting the nature of the invention; these include ethanol, methanol, hydrogen and others, which will be apparent to those versed in the art.

The anode was prepared by depositing Iridium oxide nanoparticles on a cation polymer exchange membrane. The thickness of the polymer exchange membrane was 135 pm but thicker or thinner membranes could be used without affecting the nature of the invention. Preferably the membrane has a thickness between 5 and 500 pm, and more preferably between 20 and 200 pm. A suitable current collector was placed on the anode side of the membrane in direct contact with the iridium oxide nanoparticles. The material of the current collector is selected to withstand oxidizing conditions and is preferably made of Titanium and/or its oxides, tantalum and/or its oxides, gold, carbon, stainless steel or platinum among others. The current collector material may also be of an electrically conducting material, coated with Platinum, Iridium and its oxides,

Titanium and its oxides or tantalum and its oxides. The purpose is to obtain suitable electrical contact to the anode catalyst, which could be facilitated by the application of pressure and/or temperature during the process.

Cathodes were obtained by coating a gas diffusion layer with a suitable catalyst material. Gas diffusion layers could be hydrophilic or hydrophobic and contain coatings of PTFE or other substances in order to control the hydrophobicity. Coating was done by dispersing suitable catalyst nanoparticles in ethanol, water and ionomer to form a catalyst ink, which can then be sprayed or deposited by other means onto the gas diffusion layer. Suitable cathode catalysts should be selective towards oxygen reduction to hydrogen peroxide, and include Pt-Hg, Pd-Hg, Cu-Hg, Ag-Hg, Ag, Au, carbon, graphene, nitrogen doped carbon, Sulphur doped carbon, Cobalt porphyrins and phthalocyanines, transition metal sulfides and nitrides and any combinations thereof.

Anodes with current collector medium were placed in an electrolyzer housing, where a water flow is facilitated on the same side as the anode electrode. On the opposite side of the cation exchange membrane, a porous proton-conducting layer material that allows for a suitable degree of porosity was deposited. Preferably the proton-conducting porous layer consists of ion conducting spheres or particulate of various shapes, such as cubes or irregular, but it can also be a mesh like structure, a foam like structure or plate of sintered spheres or particles, all of which have a part of the surface treated to be ion conducting. The ion conducting material for this example was ion exchange resin with approximately 400 pm diameter spheres but can be composed of ion exchange resin material or inert spheres with diameters in the range of 5 to 1000 pm, preferable 200-600 pm, coated with an ion conducting material, such as an ionomer or surface treated to be ion conducting so as to create a network of ion conducting material that is porous but stretches from cation exchange membrane all the way to the cathode catalyst material on the cathode electrode. Coating of the spheres was for this example done via immersion of ion exchange resin spheres into an ionomer solution where the ionomer concentration was 10 wt%. Deposition of the porous ion conducting material onto the membrane can be aided by application of temperature and/or pressure.

Gaps between the ion conducting spheres or particulates can be between 0 to 1000 pm and preferably 0-200 pm. There could be one or more layers of ion conducting spheres stacked or in a packed structure, and after preparation they could be coated with ionomer solution. The cathode was then placed on the ion conducting material with the cathode catalyst layer facing the ion conducting material. In this example the cathode was not pressed onto the ion conducting material but in another embodiment the assembly of cathode electrode, porous ion conducting material, ion conducting membrane and anode electrode could be done outside the cell and potentially aided by adding pressure and/or temperature when assembling.

The housing is positioned to allow for gas input into the cathode gas diffusion layer. Between the anode and cathode housing a separate gasket or plate may be inserted to allow for liquid input and/or output in a way that takes the liquid through the porous ion conducting material between cathode electrode and membrane. The components that allow for liquid transport can also be incorporated internally into anode or cathode housing.

The cathode electrode was fed with a gas flow of 22 ml/min/cm², normalized to electrode area, with a preferred range of 0.01 to 100 ml_/min/cm². The pressure is set between 0.01 and 10 bar. The anode was fed water flow at 0.3 ml/min/cm2, normalized to electrode area and can be varied in the range of 0.01 to 50 ml/min/cm². Water was also fed in between the cathode electrode and the ion exchange membrane in a suitable flow to produce a hydrogen peroxide concentration of 1000 to 3000 mg/L, and preferably concentration can be set between 200 mg/L to 50000 mg/L, even more preferable between 5000 to 30000 mg/L. The current density was set to 55 mA/cm² but can preferably be set in the range of 10 to 500 mA/cm². The potential corresponding to the 55 mA/cm² was measured to 1.95 V. The resulting polarization curve is shown in Figure 3.

Figure 4 shows Faradaic efficiency over time for a hydrogen peroxide electrolyzer.

An important parameter for the performance of this type of configuration is the amount of energy spent for the hydrogen peroxide generated and the design should aim to minimize this while ensuring high throughput.

Figure 5 shows energy consumption in Wh per gram of produced H2O2 as function of thickness for the porous proton-conducting layer. The optimal thickness was found to be between 0.2 to 1.2 mm, and preferably between 0.2 to 0.8 mm since this results in the lowest energy consumption per gram of H2O2. Thickness lower than 0.2 results in higher energy consumption, due to a loss in faradaic efficiency, and higher thickness results also in higher energy consumption due to a higher applied voltage. These results emphasize the importance of the thickness of the porous proton conducting layer.

The energy consumption parameter was also found to depend on the ionomer content in the porous proton conducting layer.

Figure 6 shows the optimum ionomer content to be between 1 to 60 % of ionomer content. Preferably, ionomer content would be between 20 to 50 %. This is explained because at lower ionomer content, water flows easier through the porous proton conducting layer, enhancing faradaic efficiency for the production of hydrogen peroxide, but voltage required to operate the cell at a given current will be higher since ion transport is harder. In contrast, with high ionomer content water path will be harder, decreasing faradaic efficiency for production of hydrogen peroxide, and ion transport will be easier, decreasing voltage required to operate the cell at a given current.

Claims

Claims:

1. Apparatus or device producing hydrogen peroxide by electrochemical means, comprised of one or more electrochemical cells (1), with each of the electrochemical cells comprising at least one cathode with suitable catalysts (2), at least one porous proton-conducting layer (3), at least one ion exchange membrane (4), at least one anode with suitable catalysts (5) where the anode (5) is pressed against one side of the ion exchange membrane (4), and the other side of the ion exchange membrane is in direct contact with a porous proton-conducting layer (3) allowing the flow of water, and wherein the cathode (2) is adjacent to the other side of the porous proton-conducting layer.

2. Apparatus or device of claim 1 wherein the thickness of the porous proton conducting layer is between 0.2 to 0.8 mm.

3. Apparatus or device of claim 1 wherein oxygen-containing gas is delivered to the side of the cathode opposite to the porous proton-conducting layer.

4. Apparatus or device of claim 1 wherein water-containing liquid is delivered to the anode.

5. Apparatus or device of claim 1 wherein the applied voltage is between 0.5 and 5 V per cell.

6. Apparatus or device of claim 1 wherein the porous proton-conducting layer is in intimate contact with the ion exchange membrane.

7. Apparatus or device of claim 1 wherein liquid water is flowing through the porous proton-conducting layer.

8. Apparatus or device of claim 1 wherein the flow of water through the porous proton-conducting layer is between 0 and 100 ml_/m in/cm/cm.

9. Apparatus or device of claim 1 wherein the porous proton-conducting layer is comprised of spheres with an outer surface with ion-conducting properties.

10. Apparatus or device of claim 9 wherein ionomer solution is added to the spheres comprising the porous proton-conducting layer.

11. Apparatus or device of claim 1 wherein the porous solid electrolyte is comprised of a porous material coated with ionomer solution.

12. Apparatus or device of claim 1 wherein the cathode catalyst layer is deposited directly on the porous proton-conducting layer.

13. Apparatus or device of claim 1 wherein ionomer content is between 1 to 40 %.