The invention is related to an electrochemical cell with improved properties for production of chemicals from a gas-phase reactant in a liquid environment. The electrochemical cell has a membrane electrode assembly configuration and is comprised of an anode, an ion exchange membrane and a cathode. The membrane electrode assembly is in direct contact with a liquid solution to facilitate extraction and handling of the produced chemicals, and the gas reactant is delivered through the liquid to the membrane electrode assembly.
The present invention also relates to a novel process and electrochemical cell for use in the synthesis of chemicals. The present invention relates to the electrocatalysts used and their incorporation into a membrane electrode assembly, MEA. The present invention also relates to the electrodes used in a membrane electrode assembly, in particular the gas diffusion layer, and a method for transport of gas-phase reactants into a liquid-immersed membrane electrode assembly.
Electrochemical methods are a viable strategy to synthesize chemicals on-site, on-demand using compact reactors. For example, they are employed today for the synthesis of chlorine-based compounds (e.g. sodium hypochlorite) used in water treatment and biocide applications, or for the synthesis of CO from CO2.
Typically electrochemical cells are composed of three parts: anode, electrolyte and cathode. An oxidation reaction takes place at the anode, while charged species are transported through the electrolyte and a reduction reaction happens at the cathode.
Of particular interest is the development within the last decades of electrolytes based on ion exchange membranes. These substitute liquid electrolytes, which require extensive handling, can be hazardous and ultimately complicate the separation of generated products. The term electrochemical cell in this document refers to a membrane electrode assembly, whereby the electrolyte is an ion exchange membrane. Standard techniques to build membrane electrode assemblies including catalyst-coated membranes and gas diffusion electrodes are well-known to practitioners in the field (see for example Handbook of Fuel Cells: Fundamentals, Technology and Applications, Wiley VCH, 2014). Membrane electrode assemblies can be comprised of more than one membrane and more than one gas diffusion layer on either side.
The use of ion exchange membranes as electrolytes is widespread for energy applications, for instance in fuel cells and electrolyzers, but thus far has seen limited use for the synthesis of chemicals despite allowing easy separation of reaction products. One of the reasons for such limited use is that chemicals synthesized at the catalyst layer of the electrochemical cell need to be extracted from the membrane electrode assembly. This is particularly challenging when gas-phase reactants need to be fed into the cell, and most solutions proposed in the literature rely purely on diffusion of reactants and products (see for example Handbook of Fuel Cells: Fundamentals, Technology and Applications, Wiley VCH, 2014).
To facilitate the extraction of products some practitioners have turned into immersing the electrodes into liquid solutions. That way, products dissolve into the liquid and diffuse through it. This is widely practiced in liquid electrolytes (see for example U.S. Pat. No. 6,712,949, Gopal), but not utilized in membrane-based electrochemical cells, despite the advantages these electrodes represent in terms of scalability and product separation.
In membrane electrode assembly configurations products formed in the vicinity of the ion exchange membrane need to be extracted through the gas diffusion layer. Particles, for instance from catalyst dispersions, can be present in the gas diffusion layer and decompose products on their way out by chemical or electrochemical means.
One of the key reasons why membrane electrode assemblies are not utilized in a liquid environment is that gas reactants, such as oxygen, hydrogen, CO or CH4 have very low solubility in most liquids, including aqueous solutions. For example, oxygen solubility in water is only of 40 mg/L under normal conditions. In turn, this restricts availability of reactants at the electrode and achievable current densities, which are typically limited to a few mA per cm2. This is exemplified in the report by Li et al (Drinking Water Purification by Electrosynthesis of Hydrogen Peroxide in a Power-Producing PEM Fuel Cell, ChemSusChem 2013), where current densities reach values up to 30 mA/cm2.
Yamanaka et al. report an attempt to mitigate the problem addressed at oxygen reduction to hydrogen peroxide (Neutral H2O2 Synthesis by Electrolysis of Water and O2, Angewandte Chemie 2008). In this work, half of the electrochemical cell cathode is exposed to gaseous oxygen, and the other half is immersed in liquid electrolyte. Using this method current densities of 20 to 60 mA/cm2 can be achieved. However, faradaic efficiency to hydrogen peroxide is 26%, which could indicate that hydrogen peroxide removal from the electrode is difficult.
The usual method adopted by practitioners in the art to solve the problems associated with the limited mass transport of gasses in liquids is to operate the electrochemical cells in a fully gaseous environment, with limited liquid interactions.
Examples of this strategy for the production of chemicals are found in GB application 2012/052316. In this document a fuel cell aimed at producing hydrogen peroxide is described. Hydrogen peroxide is produced at the cathode of the reactor using pressurized humidified oxygen, without the presence of liquid water (other than water from the membrane or produced at the catalyst). Current density in this case approaches 80 mA/cm2, but faradaic efficiency to hydrogen peroxide is not stated.
A similar example is U.S. Pat. No. 7,892,408 B2, disclosing an electrolyzer cell for hydrogen peroxide production using gaseous oxygen or air feeds to the cathode, without the presence of liquid water. Current densities in this case are 200-300 mA/cm2.
The present invention was made in view of the prior art described above, and its objective is to facilitate electrochemical production of chemicals in a membrane electrode assembly configuration. Using this invention it is possible to enhance extraction of products from the membrane electrode assembly. The invention also enables an improved utilization of gaseous reactants.
In a first aspect, the present invention provides enhanced mass transport of gas phase reactants to the electrochemical cell and easy extraction of products. This is very useable for the electrochemical synthesis of products from a gaseous reactant in a membrane electrode assembly type electrochemical cell. The gas is delivered to the electrochemical cell by convection through the liquid solution and the gas diffusion layer.
The presence of a liquid solution in immediate contact with the anode and/or cathode of the cell eases extraction of products by facilitating their diffusion to the liquid.
In a second aspect, the invention provides a catalyst-coated membrane with improved properties for hydrogen peroxide generation. This allows for a very thin catalyst layer adjacent to the membrane, which decreases residence time of products in the catalyst layer and facilitates their extraction.
In a third aspect, the invention provides a way to optimize gas usage in one or several electrochemical cells.
In a fourth aspect, the electrochemical cells generate hydrogen peroxide by cathodic oxygen reduction, and the generated hydrogen peroxide is introduced into a water line for uses including but not limited to disinfection and bleaching. Hydrogen peroxide generated using this method can also be vaporized for room or surface disinfection, or combined with other disinfection methods such as ultra-violet light or ozone for enhanced disinfection properties.
The present invention relates to a novel electrochemical cell design in the membrane electrode assembly configuration and a method for improved transport of reactants. A membrane electrode assembly consists of a cathode and an anode which are pressed against either sides of a membrane, shown in
Accordingly, the present invention provides a liquid solution in the immediate vicinity of the electrodes to facilitate extraction of the products from the electrochemical cell. The liquid solution can be static or flowing, preferably flowing. The liquid solution is preferably composed of more than 90% of water. The temperature of the liquid solution is between −20 and 500° C., preferably between 0 and 100° C.
The invention also provides a method to deliver a gaseous reactant through the liquid solution and into the electrode in a manner that gas solubility and diffusion in the liquid is not limiting current density in the cell. The gas is delivered to the electrochemical cell through a gas disperser. An example of a suitable gas disperser could be a tube or other suitable gas conduit, which may have a suitable fluidizing media in contact with the liquid solution or electrochemical cell to facilitate dispersion of the gas. Preferably, the gas disperser is placed between 0 and 1 cm of the electrode, even more preferably between 0 and 0.3 cm, in a manner that gas is directed into the electrode. Using this invention, gas diffusion in the liquid is overcome by convection and mass transport of reactants is enhanced. Suitable gas reactants are well known to those versed in the art, several examples include hydrogen, carbon monoxide, carbon dioxide, methane, propene, oxygen and others. The preferred electrode area covered by the gas disperser is between 10 and 90%, even more preferred between 30 and 70%.
The present invention also relates to a device for the electrochemical production of chemical compounds in an electrochemical cell consisting of a membrane electrode assembly. The membrane electrode assembly is at least partly immersed in a liquid solution, at least on the side of the membrane requiring a gas reactant. For clarity, the membrane electrode assembly may be partly or fully immersed in the liquid solution.
Another aspect of the invention, which could be combined with the method allowing for improved gas delivery, is the utilization of a gas diffusion layer facilitating extraction of the products generated at the electrode. At least one of the gas diffusion layers in the membrane electrode assembly is made of carbon, titanium or other suitable materials, and presents openings (for example through holes) in a pattern, which can be regular, semi-regular or random. The shape of the holes is not of importance to the present invention, and their dimensions are preferably between 1 μm and 5000 μm.
Another strategy to increase removal efficiency of products from the catalyst layer, which could be combined with the previously described inventions, is to selectively deposit catalyst on a gas diffusion layer (forming a gas diffusion electrode) or on a membrane (forming a catalyst-coated membrane) in selected regions to facilitate extraction of the products generated at the electrode. This can also decrease residence time of products at the gas diffusion layer, minimizing their possible decomposition by the electrode. This selective coating can be achieved for instance by using a pattern when depositing the catalyst ink. As an example, the pattern could have between 5 and 80% of open area and a distance between coated areas between 10 and 10000 μm.
In yet another aspect of the invention, the catalyst is applied as a catalyst coated membrane. That way, catalyst material is concentrated in the vicinity of the membrane and has good ionic contact. Importantly, using this method the catalyst layer is thin, which is favorable because it minimizes decomposition of chemical compounds produced at the catalyst during the extraction process. Using a catalyst coated membrane for the production of chemical compounds, especially if such compounds are in the liquid form, is beneficial to decrease residence time of the compounds in the catalyst layer, and it also allows for improved catalyst utilization and reduced loadings. A schematic representation of a catalyst coated membrane and a gas diffusion electrode is shown in
Electrochemical synthesis of hydrogen peroxide
Electrochemical synthesis of hydrogen peroxide is promising for distributed generation of the chemical closer to the point of use. This would overcome limitations of the current method for industrial hydrogen peroxide synthesis, the anthraquinone process, a large-scale process, which can only be efficiently conducted at large scale chemical plants. From the production plant hydrogen peroxide is concentrated to 30-70% solutions (labelled as a hazardous material) and transported to the point of use, where it typically needs to be diluted to manageable concentrations. These could be in the ppm to few-% range. The transportation and dilution process results in extensive logistics and complexity for end-users, who despite the attractive properties of hydrogen peroxide as an effective and environmentally friendly chemical turn to other alternatives such as chlorine or formaldehyde for their processes. A method to generate hydrogen peroxide on-site would simplify use of the chemical by eliminating the logistics burden, and facilitate application of hydrogen peroxide directly at the required low concentrations and at high purity. This would also improve safety of the processes as the concentration of hydrogen peroxide would not exceed dangerous levels. Hydrogen peroxide synthesized this way could also be used to enhance dissolved oxygen levels in water or other liquids, while accurately controlling concentration.
Some applications where hydrogen peroxide is used, and where its distributed generation can provide strong benefits, include bleaching in the pulp and paper industry, as a biocide or disinfectant in the agricultural and food industries (in aquaculture, animal drinking water, irrigation water treatment), in municipal and residential water treatment, in waste water treatment, in soil remediation, in swimming pools, or in healthcare for personal hygiene and as a surface and room disinfectant. Hydrogen peroxide produced locally can be introduced directly into a water line or stored in a reservoir prior to introduction into a water line, as shown in
Hydrogen peroxide can be synthesized on-site through an electrochemical process using water and oxygen as reactants. Electrochemical generation can be achieved in compact devices generating on-site, on-demand hydrogen peroxide directly at the diluted concentrations required. Such a device has three distinct parts: anode, membrane and cathode. Water oxidation takes place at the anode, resulting in oxygen and protons. The protons are transported through an ion exchange membrane to the cathode, where they recombine with oxygen to yield hydrogen peroxide at the surface of an appropriate catalyst. The ion exchange membrane is typically a polymer electrolyte membrane, but could also be a hydrocarbon membrane or other suitable cation exchange membranes.
The half-cell reactions are as follows:
Anode: 2H2O→O2+4H++4e−
Cathode: 2O2+4H++4e−→2H2O2
It is important to minimize hydrogen peroxide decomposition, which can take place chemically or electrochemically:
2H2O2→2H2O+O2
H2O2+2H++2e−→2H2O
Other proton sources than water can be used at the anode, for example hydrogen, methane, methanol etc. These are evident to those skilled in the art.
Using the present invention in an electrochemical cell for hydrogen peroxide synthesis shows improved properties. The presence of a liquid near and through the cathode gas diffusion layer decreases residence time of hydrogen peroxide in the vicinity of the electrode, which minimizes its decomposition. Simultaneously, an oxygen containing gas is delivered to the cathode electrode, as depicted in
In addition, if liquid is flowing near and/or through the gas diffusion layer it further favors fast mass transport, and it is preferred over non-flowing liquid. Preferably the liquid is composed of more than 90% of water, and even more preferably it is water.
In another aspect of the invention, air (or another suitable oxygen source) is fed to the cathode side of the cells in the close vicinity, in a manner that gas is directed into the gas diffusion layer and to the catalyst layer, depicted in
In yet another aspect, the gas diffusion layer used at the cathode side is modified to facilitate extraction of the produced hydrogen peroxide, shown in
In another aspect, the cathode electrode consists of a catalyst coated membrane, as seen in
In another aspect, the oxygen-containing gas is delivered to the cathode and exits the housing through an outlet. At this point the gas may be directed back into the cathode of the same cell, or to the cathode of another cell, as described in
The oxygen gas produced at the anode of the electrochemical cells can also be merged with the cathode gas feed at any point of the gas lines. This may enable even higher oxygen concentrations at the cathode gas feeds.
In an example cell, the cathode is made of an ink consisting of a suitable catalyst, ionomer solution, a suitable solvent (typically an alcohol) and water. The solid content of the ink has a catalyst ratio of between 1:5 to 4:5, an ionomer ratio between 1:5 to 4:5 and may contain PTFE in ratios between 0 to 4:5, preferably between 1:20 to 4:5. The ink may also contain a suitable surfactant, such as Triton, a quaternary ammonium compound or other suitable polymers in ratios between 0 to 4:5. The ink is applied onto a gas diffusion layer (with the resulting electrode known as gas diffusion electrode). Further, the coating can take place on one side or on both sides of the gas diffusion layer. Alternatively, the cathode can also consist of a catalyst coated membrane, where the ink is sprayed directly on an ion exchange membrane and a gas diffusion layer is added afterwards.
The anode in this example is a water oxidation anode, and is prepared in the catalyst coated membrane configuration well-known to those versed in the art (see for example Handbook of Fuel Cells: Fundamentals, Technology and Applications, Wiley VCH, 2014). Other anode reactions than water oxidation, including suitable materials, could be used without affecting the present example.
Anode and cathode are pressed on either side of an ion exchange membrane and incorporated into the electrochemical cell housing. The compartments of the housing are filled with water, and an oxygen-containing gas is injected into the cathode side of the system. In this example the distance between the gas disperser and the cathode gas diffusion layer is <1 mm. The experiment takes place at room temperature, and the pressure of the gas can vary between 0 and 50 bar, preferably between 0 and 5 bar. In a typical experiment a potential difference between 0.5 and 2.5 V, preferably between 1.5 and 2.2 V, is applied between anode and cathode, and current is observed indicating electrochemical reactions are taking place. Preferably the current is above 30 mA/cm2, and even more preferably above 50 mA/cm2. The experiment is let run for a certain amount of time, while water is flowing through both anode and cathode compartments, and the hydrogen peroxide concentration produced at the cathode is determined via a suitable method, which could be permanganate titration or indicator strips. This hydrogen peroxide can be accumulated in a reservoir for its later use, or utilized directly.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2017/055710 | 3/10/2017 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
62309655 | Mar 2016 | US |