WO2016151628A1

WO2016151628A1 - Anion exchange membrane and method for manufacturing same

Info

Publication number: WO2016151628A1
Application number: PCT/JP2015/001683
Authority: WO
Inventors: Stephen M. LYTH; Thomas Bayer
Original assignee: Kyushu University, National University Corporation
Priority date: 2015-03-24
Filing date: 2015-03-24
Publication date: 2016-09-29
Also published as: JP2018514371A; JP6475862B2

Abstract

An anion exchange membrane includes oxidized carbon that contains oxygen-containing functional groups. Hydrogen cations in at least a portion of the oxygen-containing functional groups are substituted by alkali or alkali earth metal cations, such that the membrane has anion conductivity. The alkali or alkali earth metal cations preferably include Li⁺ and/or K⁺, and the oxidized carbon preferably includes a single or multilayer graphene oxide.

Description

ANION EXCHANGE MEMBRANE AND METHOD FOR MANUFACTURING SAME

The present invention relates to an anion exchange membrane and a manufacturing method thereof. In particular, the present invention relates to an oxidized carbon-based anion exchange membrane with high ionic conductivity, high mechanical strength and high gas barrier properties usable in a solid polymer electrolyte membrane type fuel cell (PEMFC) (also called a polymer electrolyte fuel cell (PEFC)), electrolysis cell (or electrolyzer), battery, humidifier, etc. and a manufacturing method thereof.

Fuel cells are a promising technology for future energy supply due to their high efficiency and power density, and because the only exhaust gas is water if hydrogen is used as fuel. Alkaline fuel cells (AFC) have several advantages compared to proton-conducting, acid-based fuel cells (e.g. Nafion (registered trademark of E.I. du Pont de Nemours & Co., Inc.)-based fuel cells). The cathode reaction kinetics are much faster, thus allowing the use of inexpensive non-noble metal catalysts (e.g. nickel at the anode and silver at the cathode). The alkaline environment is less corrosive to the catalyst and accelerates methanol oxidation. AFCs have lower fuel crossover compared to Nafion-based polymer electrolyte membrane fuel cells (PEMFCs). Further, it is possible to use higher alcohols such as ethanol and propanol as fuel, increasing the system's energy density.

However, problems can occur as AFCs use mainly liquid electrolytes. Aqueous electrolytes are easily poisoned by CO₂, for which reason AFCs are usually operated with pure O₂, and this increases cost. Another problem is flooding of the gas electrodes. Thus, replacing the liquid electrolyte by an anion exchange membrane (or anion conducting electrolyte membrane) with good mechanical and chemical stability as well as good ionic conductivity is of great interest. Some anion exchange membranes are available, but their major problem is that their conductivity and mechanical strength are quite low compared with proton conducting membrane Nafion or other proton conducting electrolyte membranes.

Meanwhile, it is proposed in JP 2014-216059A to use graphene oxide (which may also be referred to as oxidized graphene) as a solid electrolyte in a secondary battery. Graphene oxide is comprised of a single layer of graphitic carbon covered with oxygen-containing functional groups (which may also be referred to as oxygen functional groups or oxygen-groups), including carboxyl acid groups, hydroxyl groups, epoxy groups, etc., as schematically shown in Fig. 1A. Graphene oxide can be produced in large quantities, can be dispersed in water, and forms mechanically strong films (refer to the scanning electron microscopy image shown in Fig. 1B), which are highly impermeable to everything but water. The surface oxygen functional groups can be used to further chemically functionalize the surface to form new compounds. These properties make graphene oxide an ideal material to use in membrane technologies.

However, JP 2014-216059A discloses only graphene oxide having proton conductivity, and there is no disclosure in JP 2014-216059A of modifying graphene oxide such that it has anion conductivity.

US 8,715,610 B2 discloses a process for the preparation of a stable graphene dispersion which comprises reducing a graphene oxide dispersion by adding a reducing agent in the presence of a base. However, US 8,715,610 B2 discloses nothing of a process for modifying graphene oxide to have anion conductivity.

In view of such problems of the prior art, a primary object of the present invention is to provide an oxidized carbon-based anion exchange membrane and a method for forming such an anion exchange membrane in a simple manner.

A second object of the present invention is to provide an anion exchange membrane with high ionic conductivity, high mechanical strength and/or high gas barrier properties and a method for forming such an anion exchange membrane in a simple manner.

To accomplish such objects, one aspect of the present invention provides an anion exchange membrane, including oxidized carbon that contains oxygen-containing functional groups, wherein hydrogen cations in at least a portion of the oxygen-containing functional groups are substituted by alkali or alkali earth metal cations. Thereby, an oxidized carbon-based anion exchange membrane is provided.

Preferably, the hydrogen cations in at least a portion of the oxygen-containing functional groups are substituted by alkali metal cations, which preferably include at least one of Li⁺ and K⁺. These cations readily substitute hydrogen cations in the oxygen-containing functional groups in the oxidized carbons, thereby contributing to realizing an oxidized carbon-based anion exchange membrane having a high ionic conductivity.

According to another aspect of the present invention, there is provided a method of manufacturing an anion exchange membrane, including treating oxidized carbon that contains oxygen-containing functional groups in basic solution. According to this method, an oxidized carbon-based anion exchange membrane can be formed in a simple manner.

Preferably, a base of the basic solution is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali and alkali earth metals, and more preferably is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali metals, which may preferably include at least one of Li and K. The cations contained in such basic solution readily substitute hydrogen cations in the oxygen-containing functional groups in the oxidized carbons, thereby contributing to realizing an oxidized carbon-based anion exchange membrane having a high ionic conductivity.

According to yet another aspect of the present invention, there is provided an anion exchange membrane, including oxidized carbon that contains oxygen-containing functional groups, the oxidized carbon having been treated in basic solution. The oxidized carbon treated in basic solution assumes anion conductivity, and thus, provides an oxidized carbon-based anion exchange membrane.

In a preferred embodiment of the present invention, the oxidized carbon in the aforementioned anion exchange membrane or method for manufacturing it includes at least one of graphene oxide, graphite oxide, oxidized carbon nanotubes and oxidized fullerenes, and particularly preferably includes a single-layer or multilayer graphene oxide. As such oxidized carbons have high mechanical strength and high gas barrier properties, the anion exchange membrane formed therefrom can have high mechanical strength and high gas barrier properties. Moreover, the substitution of the hydrogen cations by the alkali metal or alkali earth metal cations can improve the gas barrier properties of the oxidized carbons.

The oxygen-containing functional groups typically include at least one of carboxyl acid groups, hydroxyl groups, and epoxy groups.

Fig. 1A is a schematic representation of the chemical structure of graphene oxide containing various functional groups including carboxylic acid functional groups. Fig. 1B is a scanning electron microscopy image of a cross section of a graphene oxide membrane. Fig. 2 is a schematic representation of a cation exchange process which takes place in a process of modifying graphene oxide according to an embodiment of the present invention. Fig. 3A is a graph showing a wide-span X-ray photoelectron spectrum of graphene oxide after potassium cation exchange in Example 1. Fig. 3B is a graph showing an X-ray photoelectron spectrum in the range of electron binding energies from 280 to 292 eV and shows C 1s signal. Fig. 3C is a graph showing an X-ray photoelectron spectrum in the range of electron binding energies from 526 to 538 eV and shows O 1s signal. Fig. 3D is a graph showing an X-ray photoelectron spectrum in the range of electron binding energies from 290 to 298 eV and shows K 2p signal. Fig. 4A is a graph showing dependence of anion conductivity of the graphene oxide membrane obtained in Example 1 on humidity at various temperatures. Fig. 4B is an Arrhenius plot showing the activation energy of the graphene oxide membrane obtained in Example 1 at different relative humidities. Fig. 4C is a graph showing dependence of activation energy of the graphene oxide membrane obtained in Example 1 on humidity. Fig. 5A is a graph showing gas chromatography - thermal conductivity detector (GC-TCD) signal of hydrogen permeated through four different types of membranes; 15 and 40 micrometers thick conventional graphene oxide, and 15 and 40 micrometers thick graphene oxide modified by cation exchange. Fig. 5B is a bar chart showing hydrogen permeability and permeance through the four different types of membranes. Fig. 6 is a graph showing a relationship between potassium content in the graphene oxide membrane obtained in Example 1 and stirring time. Fig. 7A is a graph showing dependence of anion conductivity of the graphene oxide membrane obtained in Example 2 on humidity at various temperatures. Fig. 7B is an Arrhenius plot showing the activation energy of the graphene oxide membrane obtained in Example 2 at different relative humidities. Fig. 7C is a graph showing dependence of activation energy of the graphene oxide membrane obtained in Example 2 on humidity. Fig. 8 is a graph showing ionic conductivity of LiOH-modified graphene oxide membrane obtained in Example 3 at different temperature and humidity. Fig. 9A is a schematic diagram showing an anion conducting membrane device (alkaline fuel cell) using a treated graphene oxide-based membrane as an electrolyte thereof. Fig. 9B is a graph showing a polarization curve and power density data of an alkaline fuel cell fabricated using a graphene oxide anion exchange membrane. Fig. 10A is a schematic diagram showing an alkaline membrane electrolyzer in which the anion exchange membrane of the present invention may be adopted. Fig. 10B is a schematic diagram showing an alkaline membrane redox flow battery in which the anion exchange membrane of the present invention may be adopted. Fig. 10C is a schematic diagram showing an alkaline membrane waste water treatment system in which the anion exchange membrane of the present invention may be adopted. Fig. 10D is a schematic diagram showing an alkaline membrane seawater desalination system in which the anion exchange membrane of the present invention may be adopted.

In the following, embodiments of the present invention will be described with reference to the drawings.

In a preferred embodiment of the present invention, an anion exchange membrane is synthesized by treatment of graphene oxide in basic solution containing alkali and/or alkali earth cations (for example, but not limited to, Li⁺, K⁺, Na⁺, Ca²⁺). As described above with reference to Fig. 1A, graphene oxide includes oxygen-containing functional groups, such as carboxylic acid functional groups or hydroxyl functional groups, bonded on its surface. Treating graphene oxide in basic solution causes some proportion of the oxygen-containing functional groups of graphene oxide to undergo cation exchange in the basic solution. Namely, hydrogen cations in the oxygen-containing functional groups of graphene oxide are substituted with alkali or alkali earth metal cations in the basic solution, such that oxygen-alkali metal (or alkali earth metal) bonds are formed.

As is well known, reacting carboxylic acid, for instance, with alkalis in solution results in a spontaneous neutralization reaction, for example:
HCO₂H(aq) + NaOH(aq) -> HCO₂Na(aq) + H₂O(l)
CH₃CO₂H(aq) + KOH(aq) -> CH₃CO₂K(aq) + H₂O(l)

In essence, a cation exchange reaction occurs, where the hydrogen cation in the carboxylic acid functional group is replaced by the alkali cation (such as Li⁺, K⁺, Na⁺), forming a carboxylate salt. The same or similar cation exchange reaction takes place involving the oxygen-containing functional groups of graphene oxide in the presence of strong base.

In an exemplary case of treating graphene oxide having carboxylic acid groups (and hydroxyl functional groups) immobilized on the surface thereof in a basic solution containing alkali cations, with for examples pH = 12, the same neutralization reaction occurs; namely, alkali cations in the basic solution react with the carboxylic acid groups. Thereby, the hydrogen cations in the carboxylic acid groups are replaced by the alkali cations, forming a new compound. Fig. 2 is a schematic diagram exemplarily showing the cation exchange in the carboxylic acid group of graphene oxide (oxidized carbon), where the base is LiOH and alkali cation is Li⁺.

Once the membrane is fabricated from the graphene oxide treated in the basic solution and dried, the alkali cations (or alkali earth cations) are relatively immobile, and allow for increased mobility of OH^- ions in the now basic environment. Thus, treating graphene oxide in basic solution to cause cation exchange in the graphene oxide renders the graphene oxide to have anion conductivity, and the membrane formed therefrom makes an anion exchange membrane.

Graphene oxide-based anion exchange membranes produced in this way offer higher mechanical strength, higher gas barrier properties with comparable and potentially higher conductivity to currently available commercial anion exchange membranes as well as cost benefits and high processability.

In the above-described process, graphene oxide of varying oxygen content can be used (for example, but not limited to, between 1 and 50 atomic percent (at%)). The graphene oxide can be dispersed in a solvent (for example, but not limited to, water, ethanol). Graphene oxide dispersion with different graphene oxide concentration can be used. The graphene oxide concentration is preferably greater than about 0.1 mg/ml, and more preferably from about 1 to about 10 mg/ml. It is to be noted that if the graphene oxide concentration is too low, evaporating or filtering so much solvent to obtain a graphene oxide-based anion exchange membrane would be too time consuming and energy-intensive for industrial processing. On the other hand, if the concentration is too high, the viscosity of the graphene oxide dispersion would be too high such that stirring, spraying, printing painting, filtering or the like of the graphene oxide dispersion would become difficult, and the anion-exchange membranes obtained from the graphene oxide dispersion tend to have uneven thickness.

Alkali and alkali earth metal salts (or bases) which can be used in basic solution for cation exchange in the embodiment of the present invention are, for example but not limited to, hydroxides, oxides, hydrides or carbonates of alkali or alkali earth metals such as lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium and radium. A mixture of the above may also be utilized.

In the embodiment of the present invention, graphene oxide dispersion (e.g. 5 mg/ml graphene oxide concentration, 20 at% oxygen content) is used. The anion exchange membrane is manufactured by chemical reaction between graphene oxide in water dispersion and basic solution, such as potassium hydroxide solution.

Basic solution containing alkali or alkali earth metal salts (such as hydroxide) in various concentrations can be mixed with graphene oxide dispersion and mixed for varying amounts of time at differing temperatures. During the mixing process, alkali (or alkali earth) cation exchange occurs, modifying the surface functional groups of the graphene oxide to include alkali or alkali earth cations. It is to be noted that a reducing agent such as hydrazine should not be used in this process as the use of reducing agent would remove most of the oxygen-containing functional groups in the graphene oxide (see US 8,715,610 B2).

Graphene oxide anion exchange membranes can be prepared by filtration of the alkaline-treated graphene oxide dispersion onto e.g. a polycarbonate filter. It is also possible to spray, print or paint the dispersion onto various kinds of substrate (for example, but not limited to, carbon paper, silicon wafer, glass, or plastic) in order to create an electrolyte layer (or anion exchange membrane) of varying thickness. In a preferred embodiment of the present invention, the membranes are prepared by vacuum-filtration onto polycarbonate filters. After filtration, the membranes are dried at e.g. room temperature for e.g. 48 h and peeled off the filter.

The thickness of the modified graphene oxide-based anion exchange (or electrolyte) membrane can be easily varied according to the concentration and volume of the dispersion. An electrolyte membrane within the range of, for example but not limited to, 5 to 80 micrometer thickness is usually used. The thickness of the electrolyte membrane used for the fuel cell of one embodiment of the present invention is e.g. 15 micrometers.

(Examples)
The following examples are intended to illustrate the invention, but are not to be construed as being limitations thereon.

Ionic conductivity (mS/cm) of the sample membranes obtained in each example was obtained in the following manner.

Through-plane ionic conductivity was determined using a membrane test system (Scribner Associates Inc. MTS 740) coupled with an impedance analyzer (Solatron SI1260). A 10 x 30 mm² membrane with a thickness of about 40 micrometers was measured in the frequency range of 30 MHz to 10 Hz with an AC amplitude of 100 mV in a 2-electrode / 4-terminal setup (the latter for minimizing lead resistance). Gas diffusion electrodes (E-TEK, High Temperature ELAT, 140E-W, 18 x 5 mm²) were attached on the platinum electrodes using conductive carbon paint (SPI Supplies, colloidal graphite, Part #05006-AB), and the sample graphene oxide-based membrane was interposed between the electrodes. Compression of the cell to obtain good electrode contact was performed using a pressure of 1.074 MPa. The impedance was measured from 30 to 80 degrees Celsius, with isothermal changes in relative humidity (RH) from 100 to 0%. Before impedance measurements, the sample was pretreated at 100% RH for 4 h, and then for 1 h between each different RH measurement. From the determined membrane resistance R (ohm), thickness L (cm) and the effective measurement area between the overlapping electrodes, A (0.55 cm²), conductivity (usually denoted by a Greek letter, sigma) was calculated using the following formula:
Conductivity = L / (R x A).

(Example 1: KOH-modified graphene oxide membranes)
210.8 mg of potassium hydroxide (KOH) was dissolved into 43.01 ml of graphene oxide water dispersion (5 mg/ml graphene oxide concentration, 20 at% oxygen content) and stirred with a magnetic stirrer for 24 hours at room temperature, at 500 rpm. The resulting dispersion (or basic solution) had a pH of 12.5. This was vacuum filtered onto polycarbonate filters, dried, and peeled off to obtain membranes. The membrane used for the conductivity measurement had a thickness of about 40 micrometers. Membranes for hydrogen permeability test were of about 15 and 40 micrometers thick, respectively, and were measured for comparison with two unmodified graphene oxide membranes with about 15 and 40 micrometers thickness, respectively.

X-ray photoelectron spectroscopy (XPS) (Fig. 3A-3D) reveals a carbon content of 81.6 at%, an oxygen content of 16.5 at%, and a potassium content of 5.7 at%. Ionic conductivity (anion conductivity) showed strong increase with increasing humidity and temperature (Fig. 4A). The highest conductivity was found to be 6.1 mS/cm at 70 degrees Celsius. The activation energy decreased with decreasing humidity and ranged from 0.44 eV at 100% RH to 0.01 eV at 0% RH (Figs. 4B and 4C). The activation energy at 100% RH is similar to that of commercially available Tokuyama A201 anion exchange membrane (Tokuyama Co., Japan).

To ascertain the gas barrier property of the modified graphene oxide membranes obtained as above, hydrogen permeability tests were conducted. Two modified graphene oxide membranes prepared for use in hydrogen permeability tests had thicknesses of 15 and 40 micrometers, respectively. For comparison, they were compared with two unmodified graphene oxide membranes with thicknesses of 15 and 40 micrometers, respectively.

The dry hydrogen permeation rate (in gas permeance units, GPU) through the membranes (diameter 1 cm, area 0.785 cm²) was measured at constant temperature of 30 degrees Celsius using a dry gas barrier analysis system (GTR-11A/31A, GTR Tec Corp., Japan). This system contains an automatic gas sampler unit connected to a gas chromatograph with a thermal conductivity detector. The total pressure difference between the feed and sweep sides of the membranes was equal to 200 kPa. The sample collection time after vacuuming the sweep side of the membrane was 30 minutes. The graphene oxide membranes functionalized by KOH-treatment showed around 5 times lower hydrogen permeability (or permeance) compared to pure (i.e. unmodified) graphene oxide membranes, indicating the increase of the gas barrier properties by KOH treatment (Figs. 5A and 5B). It is to be noted that in Fig. 5B, permeance is independent of pressure and depends on thickness, while permeability is ideally independent from thickness.

Further, the potassium content of the membrane prepared from the modified graphene oxide was measured after 10 mins, 30 mins and 24 hours alkali cation exchange reaction time, using XPS (Fig. 6). There was no trend in potassium content with increasing reaction time, indicating a very fast cation exchange reaction occurring spontaneously at room temperature, as reported in literature. Thus, it may be considered that the process of cation exchange reaction can complete in a few seconds if the treatment of the graphene oxide with the basic solution is performed in mass-production scale, though it may be influenced by various conditions such as the amount of graphene oxide dispersion, amount of alkali or alkali earth metal salt dissolved therein (or pH of the basic solution), stirring rate, etc. The values of the potassium content measured here (around 5.0 to 7.5 at%, Fig. 6) were close to the potassium content observed for the previous membrane (5.7 at%).

(Example 2: KOH-modified graphene oxide membranes)
98.6 mg potassium hydroxide (KOH) was dissolved into 40.24 ml graphene oxide dispersion (5 mg/ml graphene oxide concentration, 20 at% oxygen content), corresponding to approximately half the KOH concentration compared to Example 1. This was stirred for 24 hours at room temperature at 500 rpm. The resulting dispersion had a pH of 11.3. From this modified graphene oxide dispersion, six membranes were prepared by vacuum-filtration using 4 x 5 ml and 2 x 10 ml of the dispersion each. The membrane used for the ionic conductivity measurement had a thickness of 40 micrometers. The potassium content measured by XPS was 1.9 at%.

Ionic conductivity increased with increasing humidity and temperature (Fig. 7A). The highest conductivity was found to be 5.3 mS/cm at 70 degrees Celsius. This compares favorably with commercially available anion exchange membranes on the market today (e.g. 8 - 12 mS/cm through-plane conductivity for Tokuyama A201 membrane). The activation energy decreased with decreasing humidity and ranged from 0.38 eV at 100% RH to 0.04 eV at 0% RH (Figs. 7B and 7C).

The ionic conductivity of Example 1 was higher compared to Example 2, indicating that ionic conductivity depends on the concentration of KOH used for the cation exchange reaction. To achieve sufficient ionic (anionic) conductivity in the modified graphene oxide membrane, the concentration of KOH in the graphene oxide dispersion having KOH dissolved therein is preferably greater than about 1 mM, more preferably from about 10 mM to about 1 M. In other words, pH of the graphene oxide dispersion having KOH dissolved therein is preferably greater than about 11, more preferably from about 12 to about 14. It is to be noted that the reaction between graphene oxide and KOH will saturate at a certain KOH concentration where all of the oxygen-containing functional groups on the graphene oxide have reacted and no further reactions can occur. Therefore, it is considered preferable that the concentration of KOH does not exceed such a concentration considerably.

(Example 3: LiOH-modified graphene oxide membranes)
In Example 3, potassium hydroxide (KOH) was replaced by lithium hydroxide (LiOH) in order to demonstrate the general applicability of the alkali cation exchange method, regardless for the specific cation involved. LiOH was reacted for 24h with graphene oxide in dispersion. 0.55 at% Li was found in the sample of the modified graphene oxide as measured by XPS. This is slightly lower than for the case of KOH and may be explained by the fact that LiOH is a weaker base than KOH. The ionic conductivity of Li-modified graphene oxide was found to be lower than the conductivity observed for KOH-treated graphene oxide for all temperatures (around one third of the conductivity observed for KOH-treated graphene oxide at 30 degrees Celsius) (Fig. 8). This may be attributed to a difference in alkalinity between the two different alkali salts, or the difference in electronegativity of the alkali cations, changing the activation energy of the OH^- ions. Thus, anion exchange membranes according to the embodiment of the present invention can be fabricated utilizing various alkaline (or alkaline earth) salts not restricted to KOH.

(Example 4: Fuel Cell)
A membrane electrode assembly (MEA) was prepared using the base-treated, graphene oxide-based anion exchange membrane with a thickness of 15 micrometers obtained by the process outlined in Example 1. Catalyst ink was prepared by mixing Pt/C electrocatalyst (Tanaka Kikinzoku Kogyo K.K., 46.2 wt% Pt) with 5 wt% anion conducting polymer electrolyte solution (Tokuyama Co.), ethanol (Chameleon), and deionized water. The catalyst ink was stirred for 10 hours, then sonicated before use for 30 min (SMT Ultra Sonic Homogenizer UH-600). The catalyst ink was sprayed onto the anion conducting modified graphene oxide membrane by means of a spraying device (Nordson K.K. Spraying Device, C-3J) using a mask to create an electrode having a size of 0.5 cm² on each side of the graphene oxide membrane, with a catalyst loading of 0.3 mg Pt/cm² for both electrodes. Hydrophobic carbon paper (EC-TP1-060T) gas diffusion layers (GDLs) were precisely positioned over the electrocatalyst layers. This forms a graphene oxide-membrane electrode assembly (GO-MEA), as shown in Fig. 9A. The prepared GO-MEA was placed into a single cell test holder (1 cm²) and installed in a home-made fuel cell testing system. The resulting alkaline fuel cell was installed in an oven and heated up to 30 degrees Celsius. Hydrogen and air were flowed at 100 ml/min and 100% RH. The cell performance was investigated using a potentiostat (Solartron SI 1287). The resulting open circuit voltage was 0.88 V and the maximum power density was 1.13 mW/cm² at a current density of 2.6 mA/cm² (Fig. 9B). Thus, the graphene oxide-based anion exchange membrane according to the embodiment of the present invention can be favorably used as an electrolyte of an alkaline fuel cell.

(Example 5: Other Applications)
Beside application as alkaline fuel cell electrolytes, graphene oxide-based anion exchange membranes according to the present invention can be used in other membrane-based technologies. For example, the graphene oxide-based anion exchange membrane may be used as: an anion-conducting electrolyte in microbial fuel cells or enzymatic fuel cells; an anion exchange membrane (AEM) in alkaline polymer electrolyte electrolyzers (Fig. 10A); an ion exchange membrane (IEM) in redox flow batteries (Fig. 10B); an ion exchange membrane in alkaline membrane waste water treatment systems (Fig. 10C); and an AEM in alkaline membrane seawater desalination systems (Fig. 10D). Further, the graphene oxide-based anion exchange membrane according to the present invention may be used in reverse electrodialysis cells, alkaline batteries, metal-air batteries, gas barrier applications, humidification applications, and so on.

The present invention has been described above in terms of preferred embodiments thereof, but it is obvious to a person skilled in the art that the present invention is not limited to the embodiments and various alterations and modifications are possible without departing from the scope of the present invention.

For instance, though graphene oxide was used in the above embodiment, other oxidized carbons, such as graphite oxide, oxidized carbon nanotubes or oxidized fullerenes, preferably having high mechanical strength and/or high gas barrier properties may also be utilized in place of graphene oxide.

Further, in the foregoing embodiment, the anion exchange membrane was prepared by filtration of the base-treated graphene oxide dispersion onto a polycarbonate filter. However, the anion exchange membrane may be formed by printing, casting, pressing or spraying the dispersion, ink or paint containing the base-treated graphene oxide. Further, the anion exchange membrane may be a composite with a polymer, a fullerene, nanoparticles etc. formed by mixing the alkaline-modified graphene oxide dispersion with such materials before forming the membrane. Yet further, the base-treated graphene oxide may be subjected to further chemical surface functionalization as appropriate and/or desired.

Claims

An anion exchange membrane, comprising oxidized carbon that contains oxygen-containing functional groups, wherein hydrogen cations in at least a portion of the oxygen-containing functional groups are substituted by alkali or alkali earth metal cations.
The anion exchange membrane according to claim 1, wherein the hydrogen cations in at least a portion of the oxygen-containing functional groups are substituted by alkali metal cations.
The anion exchange membrane according to claim 2, wherein the alkali metal cations include at least one of Li⁺ and K⁺.
The anion exchange membrane according to claim 1, wherein the oxidized carbon includes at least one of graphene oxide, graphite oxide, oxidized carbon nanotubes and oxidized fullerenes.
The anion exchange membrane according to claim 1, wherein the oxidized carbon includes a single-layer or multilayer graphene oxide.
The anion exchange membrane according to claim 1, wherein the oxygen-containing functional groups include at least one of carboxyl acid groups, hydroxyl groups, and epoxy groups.
A method of manufacturing an anion exchange membrane, comprising treating oxidized carbon that contains oxygen-containing functional groups in basic solution.
The method of manufacturing an anion exchange membrane according to claim 7, wherein a base of the basic solution is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali and alkali earth metals.
The method of manufacturing an anion exchange membrane according to claim 7, wherein a base of the basic solution is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali metals.
The method of manufacturing an anion exchange membrane according to claim 9, wherein the alkali metals include at least one of Li and K.
The method of manufacturing an anion exchange membrane according to claim 9, wherein the oxidized carbon includes at least one of graphene oxide, graphite oxide, oxidized carbon nanotubes and oxidized fullerenes.
The method of manufacturing an anion exchange membrane according to claim 7, wherein the oxidized carbon includes a single-layer or multilayer graphene oxide.
The method of manufacturing an anion exchange membrane according to claim 7, wherein the oxygen-containing functional groups include at least one of carboxyl acid groups, hydroxyl groups, and epoxy groups.
An anion exchange membrane, comprising oxidized carbon that contains oxygen-containing functional groups, the oxidized carbon having been treated in basic solution.
The anion exchange membrane according to claim 14, wherein a base of the basic solution is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali and alkali earth metals.
The anion exchange membrane according to claim 14, wherein a base of the basic solution is at least one selected from the group consisting of hydroxide, oxides, hydrides and carbonates of alkali metals.
The anion exchange membrane according to claim 16, wherein the alkali metals include at least one of Li and K.
The anion exchange membrane according to claim 14, wherein the oxidized carbon includes at least one of graphene oxide, graphite oxide, oxidized carbon nanotubes and oxidized fullerenes.
The anion exchange membrane according to claim 14, wherein the oxidized carbon includes a single-layer or multilayer graphene oxide.
The anion exchange membrane according to claim 14, wherein the oxygen-containing functional groups include at least one of carboxyl acid groups, hydroxyl groups, and epoxy groups.