Patent Application
20250186987
Hydrogen as an energy vector for grid balancing or power-to-gas and power-to-liquid processes plays an important role in the path toward a low-carbon energy structure that is environmentally friendly. Water electrolysis produces high quality hydrogen by electrochemical splitting of water into hydrogen and oxygen; the reaction is given by Eq. 1 below. The water electrolysis process is an endothermic process and electricity is the energy source. Water electrolysis has zero carbon footprint when the process is operated by renewable power sources, such as wind, solar, or geothermal energy. The main water electrolysis technologies include alkaline electrolysis, proton exchange membrane (PEM) water electrolysis (PEMWE as shown in
As shown in
Water electrolysis reaction: 2 H2O→2 H2+O2 (1)
Oxidation reaction at anode for PEMWE: 2 H2O→O2+4 H++4 e− (2)
Reduction reaction at cathode for PEMWE: 4 H++4 e−→2 H2 (3)
AEMWE is a developing technology. As shown in
Ni alloy catalysts. At the negatively charged cathode 210, water is reduced to form hydrogen 225 and hydroxyl (OH−) ions by the addition of four electrons; the reaction is given by Eq. 4. The hydroxyl ions diffuse from the cathode 210 to the anode 205 through the AEM 215 which conducts hydroxyl ions. At the positively charged anode 205, the hydroxyl ions recombine as water and oxygen 230; the reaction is given by Eq. 5. The AEM 215 not only conducts hydroxyl ions from the cathode 210 to the anode 205, but also separates the H2 225 and O2 230 produced in the water electrolysis reaction. The AEM 215 allows the hydrogen 225 to be produced under high pressure up to about 35 bar with very high purity of at least 99.9%.
Reduction reaction at cathode for AEMWE: 4 H2O+4 e−→2 H2+4 OH− (4)
Oxidation reaction at anode for AEMWE: 4 OH−→2 H2O+O2+4 e− (5)
AEMWE has an advantage over PEMWE because it permits the use of less expensive platinum group metal-free catalysts, such as Ni and Ni alloy catalysts. In addition, much cheaper stainless steel bipolar plates can be used in the gas diffusion layers (GDL) for AEMWE, instead of the expensive Pt-coated Ti bipolar plates currently used in PEMWE. However, the largest impediments to the development of AEM systems are membrane hydroxyl ion conductivity and stability, as well as lack of understanding of how to integrate catalysts into AEM systems. Research on AEMWE in the literature has been focused on developing electrocatalysts, AEMs, and understanding the operational mechanisms with the general objective of obtaining a high efficiency, low cost and stable AEMWE technology.
Fuel cells, as a next generation clean energy resource, convert the energy of chemical reactions such as an oxidation/reduction redox reaction of hydrogen and oxygen into electric energy. The three main types of fuel cells are alkaline electrolyte fuel cells, polymer electrolyte membrane fuel cells, and solid oxide fuel cells. Polymer electrolyte membrane fuel cells may include proton exchange membrane fuel cells (PEMFC), anion exchange membrane fuel cells (AEMFC), and direct methanol fuel cells. PEMFC uses a PEM to conduct protons from the anode to the cathode, and it also separates the H2 and O2 gases to prevent gas crossover. AEMFC uses an AEM to conduct OH− from the cathode to the anode, and it also separates the H2 and O2 gases to prevent gas crossover.
The anode in an electrochemical cell is the electrode at which the predominant reaction is oxidation (e.g., the water oxidation/oxygen evolution reaction electrode for a water electrolyzer, or the hydrogen oxidation electrode for a fuel cell). The cathode in an electrochemical cell is the electrode at which the predominant reaction is reduction (e.g., the proton reduction/hydrogen evolution reaction electrode for a water electrolyzer, or the oxygen reduction electrode for a fuel cell). The membrane is one of the key materials that make up an electrolysis cell or a fuel cell and is an important driver for safety and performance. Some important properties for membranes for fuel cells and membrane electrolysis include high conductivity, high ionic permeability, high ionic exchange capacity (for ion-exchange membrane), high ionic/H2 and O2 selectivity (low H2 and O2 permeability/crossover), low price, low area resistance to minimize efficiency loss resulting from ohmic polarization, high resistance to oxidizing and reducing conditions, being chemically inert at a wide pH range, high thermal stability together with high proton conductivity, and high mechanical strength (thickness, low swelling).
Significant advances are needed in cost-effective, high performance, stable catalysts, membrane materials, as well as other cell stack components for AEM water electrolysis and AEMFCs with a wide range of applications in renewable energy systems.
Novel anion exchange polymers have been developed with both anion exchange functional groups such as N,N-dimethyl piperidinium groups and amine functional groups such as N-methyl piperidine groups for the preparation of anion exchange membranes (AEMs) for electrolysis, fuel cell, energy storage, and other electrochemical applications. The current invention also relates to methods of making and manufacturing the polymer, AEMs made from the polymer, and catalyst coated membranes (CCMs) made from the AEMs. The new polymers and AEM membranes are particularly useful for AEM water electrolysis (AEM-WE) to produce green H2. Green H2 is produced from water via a water electrolysis process using renewable energy as the energy source with zero CO2 emission as the only byproduct is O2 and no CO2 is produced from this process. As an example, the present AEM may comprise a new poly (p-terphenyl-phenanthrene-N,N-dimethyl-4-piperidinium bicarbonate)-co-poly (p-terphenyl-phenanthrene-N-methyl-4-piperidine) anion exchange polymer comprising both N,N-dimethyl piperidinium anion exchange functional groups and N-methyl piperidine functional groups as shown in
One aspect of the invention is an anion exchange polymer. In one embodiment, the polymer comprises a plurality of repeating units of formula (I)
In some embodiments, Ar1 and Ar2 are independently selected from the group consisting of
In some embodiments, Ar1 and Ar2 are independently selected from the group consisting of
In some embodiments, Ar1 and Ar2 are independently selected from the group consisting of
In some embodiments, X1 is
wherein R30 and R31 are each independently —H, —CH3, —CH2CH3, —CH(CH3)2, —C(CH3)3, —CH2—C6H5, or —CH2—CH(CH3)2;
In some embodiments, X2 is
In some embodiments, X2 is a mixture of
In some embodiments, the polymer comprising a plurality of repeating units of formula (I) is formed from a superacid catalyzed polyhydroxyalkylation reaction of monomers Ar1′, Ar2′, and X1′ followed by a Menshutkin reaction, wherein Ar1′ and Ar2′ are independently selected from the group consisting of:
In some embodiments, Ar1′ and Ar2′ are independently selected from the group consisting of:
In some embodiments, Ar1′ is selected from the group consisting of:
In some embodiments, Ar1′ and Ar2′ are the same and are selected from the group consisting of:
In some embodiments, X1′ is
In some embodiments, X1′ is a mixture of:
In some embodiments, X1′ is a mixture of:
wherein R30 is —H, —CH3, —CH2CH3, —CH(CH3)2, —C(CH3)3, —CH2—C6H5, or —CH2—CH(CH3)2; and
The anion exchange polymer comprising a plurality of repeating units of formula (I) may be synthesized by two steps: 1) a superacid catalyzed polyhydroxyalkylation reaction of monomers Ar1′ and Ar2′ with X1′, such as p-terphenyl as Ar1′ and phenanthrene as Ar2′ with N-methyl-4-piperidone as X1′, to form a neutral precursor polymer; and 2) a Menshutkin reaction to convert the neutral precursor polymer comprising piperidine-based groups to the anion exchange polymer comprising a plurality of repeating units of formula (I) with both piperidinium-based cation groups and piperidine-based groups. Optionally, the anion exchange polymer comprising a plurality of repeating units of formula (I) with piperidine-based groups, piperidinium-based cation groups and negatively charged halide ions is converted to an anion exchange polymer comprising a plurality of repeating units of formula (I) with piperidine-based groups, piperidinium-based cation groups and negatively charged bicarbonate ions by soaking the polymer in sodium bicarbonate or potassium bicarbonate solution before the polymer is made into a membrane.
The polyhydroxyalkylation reaction of monomers Ar1′ and Ar2′ with monomer X1′ followed by the Menshutkin reaction provides an anion exchange polymer with a polymer backbone free of ether bonds, which results in high chemical stability of the polymer. The incorporation of electron-rich monomers Ar1′ and Ar2′ into the anion exchange polymer provides a hydrophobic polymer backbone, and the incorporation of monomer X1′ into the anion exchange polymer provides the polymer with piperidinium or piperidinium salt-based anion exchange property that helps to achieve stable high OH− conductivity, as well as piperidine-based functional group that is important to reduce the polymer swelling and reduce the gas crossover. The combination of the hydrophobic polymer backbone, the piperidine-based functional group, and alkaline stable piperidinium or piperidinium salt-based cation functional groups provides the novel anion exchange polymer with high OH− conductivity, high chemical stability, high mechanical strength, low swelling in alkaline conditions, low gas crossover, and long-term performance stability. The molar ratio of Ar1′ monomer to Ar2′ monomer can be in a range of 20:1 to 1:20, or in a range of 10:1 to 1:10, or in a range of 5:1 to 1:5. The molar ratio of X1′ monomer to Ar1′ and Ar2′ monomers can be in a range of 1.2:1 to 1:1.2, or in a range of 1.1:1 to 1:1.1, or in a range of 1.05:1 to 1:1.05.
The superacid catalyzed polyhydroxyalkylation reaction can be carried out at 0° C. to 50° C., or at 10° C. to 30° C., or at 20° C. to 30° C. for 2 h to 72 h, or 10 h to 48 h, or 12 to 24 h. Suitable superacid catalysts include, but are not limited to, trifluoromethanesulfonic acid (CF3SO3H (TFSA)), methanesulfonic acid (MSA), fluorosulfuric acid (FSO3H), or mixtures thereof. Solvents for the polyhydroxyalkylation reaction are those that can dissolve one or more of the monomers. Suitable solvents include, but are not limited to, methylene chloride, chloroform, trifluoroacetic acid (TFA), or mixtures thereof.
The Menshutkin reaction is used to react the neutral precursor polymer comprising piperidine-based groups with an alkyl halide to convert about 50 mol % to 99 mol % of the neutral piperidine-based groups to piperidinium-based cation groups to form the anion exchange polymer comprising a plurality of repeating units of formula (I) with both piperidinium-based cation groups and piperidine-based groups. Suitable alkyl halides include, but are not limited to, alkyl iodides or alkyl bromides. The Menshutkin reaction can be carried out at 10° C. to 80° C., or at 20° C. to 30° C. for 2 h to 72 h, or 10 h to 48 h, or 12 to 24 h. Solvents for the Menshutkin reaction are those that can dissolve the neutral precursor polymer comprising piperidine-based groups. Suitable solvents include, but are not limited to, N-methylpyrrolidone (NMP), N,N-dimethyl acetamide (DMAC), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1,3-dioxolane, or mixtures thereof. To synthesize the anion exchange polymer comprising a plurality of repeating units of formula (I) with both piperidinium-based cation groups and piperidine-based groups, the molar ratio of the alkyl halide to the piperidine-based groups of the neutral precursor polymer should be controlled in a range of 0.5/1 to 1:1 for the Menshutkin reaction.
The anion exchange polymer comprising a plurality of repeating units of formula (I) has a weight average molecular weight in a range of 10,000 to 50,000,000 Daltons, or in a range of 50,000 to 40,000,000 Daltons.
Another aspect of the invention is an anion exchange membrane comprising the anion exchange polymer described above. The anion exchange membrane may be used in a wide variety of applications including, but not limited to, fuel cells, electrolyzers, flow batteries, electrodialyzers, waste metal recovery systems, electrocatalytic hydrogen production systems, desalinators, water purifiers, waste water treatment systems, ion exchangers, or CO2 separators.
In some embodiments, the anion exchange membrane comprises a nonporous symmetric dense film membrane, an integrally-skinned asymmetric membrane, a reinforced composite membrane, or a thin film composite membrane. By “dense” we mean that the membrane does not have pores larger than 1 nm.
In some embodiments, the integrally-skinned asymmetric membrane, the reinforced composite membrane, or the thin film composite membrane comprises a porous substrate material impregnated or coated with the anion exchange polymer described above. The porous substrate membrane may be prepared from a polymer different from the anion exchange polymer.
In some embodiments, the nonporous symmetric dense film membrane, the integrally-skinned asymmetric membrane, the reinforced composite membrane, or the thin film composite membrane may be a flat sheet membrane.
In some embodiments, the nonporous symmetric dense film anion exchange membrane is prepared using a method comprising: 1) dissolving the anion exchange polymer comprising a plurality of repeating units of formula (I) in a solvent to form a polymer casting solution; 2) casting the polymer casting solution on a nonporous substrate to form a uniform layer of the polymer casting solution; 3) drying the polymer casting solution layer to form a dried membrane on the nonporous substrate at 50° C. to 180° C., or at 50° C. to 120° C., or at 80° C. to 120° C.; and optionally 4) ion exchanging the halide anions of the anion exchange polymer in the membrane with hydroxide, bicarbonate, carbonate ions, or a combination thereof if the anion exchange polymer has halide anions before membrane fabrication to form the nonporous symmetric dense film anion exchange polymer membrane. The nonporous substrate is removed from the membrane when the membrane is used in a desired application. The solvent used to dissolve the anion exchange polymer can be selected from, but is not limited to, NMP, DMAc, DMF, DMSO, 1,3-dioxolane, acetic acid, trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TFSA), or mixtures thereof. The nonporous substrate used for the fabrication of the nonporous symmetric dense film membrane can be selected from, but is not limited to, glass plate, polyolefin film, polyester film, or fluorocarbon-based polymer film such as poly (tetrafluoroethylene) (PTFE) and poly (vinylidene fluoride) (PVDF) film.
In some embodiments, the integrally-skinned asymmetric anion exchange membrane is prepared using a method comprising: 1) making an anion exchange polymer membrane casting solution comprising the anion exchange polymer with formula (I), solvents which are miscible with water and can dissolve the anion exchange polymer, and non-solvents which cannot dissolve the anion exchange polymer; 2) casting a layer of the anion exchange polymer membrane casting solution onto a supporting substrate; 3) evaporating the solvent and non-solvent from the surface of the coated layer and then coagulating the coated polymer layer in a coagulating bath to form the integrally-skinned asymmetric membrane structure; 4) drying the membrane at 50° C. to 150° C., or at 50° C. to 120° C., or at 80° C. to 120° C.; and optionally 5) ion exchanging the halide anions of the anion exchange polymer in the membrane with hydroxide, bicarbonate, carbonate ions, or a combination thereof if the anion exchange polymer has halide anions before membrane fabrication to form the integrally-skinned asymmetric anion exchange polymer membrane. In some embodiments, the supporting substrate is removed from the membrane when the membrane is used in a desired application. In some embodiments, the supporting substrate is part of the final integrally-skinned asymmetric anion exchange polymer membrane. The supporting substrate may comprise polyolefin such as polypropylene and polyethylene, polyester, polyamide such as Nylon 6 and Nylon 6,6, cellulose, or fluorocarbon-based polymer such as PTFE and PVDF. The solvents for the preparation of the integrally-skinned asymmetric membrane include, but are not limited to, NMP, DMAc, DMF, DMSO, dioxanes, 1,3-dioxolane, acetic acid, trifluoroacetic acid (TFA), trilluoromethanesulfonic acid (TFSA), and mixtures thereof. The non-solvents for the preparation of the integrally-skinned asymmetric membrane include, but are not limited to, acetone, methanol, ethanol, tetrahydrofuran (THF), toluene, n-octane, n-decane, lactic acid, citric acid, isopropanol, and mixtures thereof. The integrally-skinned asymmetric membrane may have a thin nonporous dense layer less than 500 nm on a microporous support layer.
In some embodiments, the reinforced composite anion exchange membrane is prepared using a method comprising: 1) dissolving the anion exchange polymer comprising a plurality of repeating units of formula (I) in a solvent to form a polymer solution; 2) impregnating a porous matrix support membrane with the anion exchange polymer solution to fill the pores with the anion exchange polymer via dip-coating, soaking, spraying, painting, or other known conventional solution impregnating method; 3) drying the impregnated membrane at 50° C. to 150° C., or at 50° C. to 120° C., or at 80° C. to 120° C.; and optionally 4) ion exchanging the halide anions of the anion exchange polymer in the pores of the reinforced membrane with hydroxide, bicarbonate, carbonate ions, or a combination thereof if the anion exchange polymer has halide anions before membrane fabrication to form the reinforced composite anion exchange membrane with interconnected anion exchange polymer domains in a porous matrix. The solvents for the preparation of the thin film composite anion exchange membrane include, but are not limited to, NMP, DMAc, DMF, DMSO, dioxanes, 1,3-dioxolane, acetic acid, trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TFSA), and mixtures thereof. The porous matrix should have good thermal stability (stable up to at least 120° C.), high stability under high pH condition (e.g., pH greater than 8), high resistance to oxidizing and reducing conditions (insoluble and no performance drop under oxidizing and reducing conditions), high mechanical strength (no dimensional change under the system operation conditions), as well as other factors dictated by the operating conditions for electrochemical reactions. The porous matrix must be compatible with the electrochemical cell chemistry and meet the mechanical demands of cell stacking or winding assembly operations.
The polymers suitable for the preparation of the porous matrix can be selected from, but not limited to, polyolefins such as polyethylene and polypropylene, polyamide such as Nylon 6 and Nylon 6,6, polyester, cellulose acetate, polybenzimidazole, fluorocarbon-based polymer such as PTFE and PVDF, polycarbonate, cellulose, or combinations thereof. These polymers provide a range of properties such as low cost, high stability in alkaline water, good mechanical stability, and ease of processability for porous matrix fabrication.
The porous matrix can either a non-woven matrix or a woven matrix and have either a symmetric porous structure or an asymmetric porous structure. The porous matrix can be formed by an electrospinning process, a phase inversion membrane fabrication approach followed by direct air drying, or by phase inversion followed by solvent exchange methods. The porous matrix also can be fabricated via a dry processing of thermoplastic polyolefins or a wet processing of thermoplastic olefins. The dry processing of thermoplastic polyolefins utilizes extrusion to bring the polymer above its melting point and form it into the desired shape. Subsequent annealing and stretching processes may also be done to increase the crystallinity and orientation and dimension of the pores. The wet processing of polyolefin porous matrix is done with the aid of a hydrocarbon liquid or low molecular weight oil mixed with the polymer resin or a mixture of the polymer resin and inorganic nanoparticles in the melt phase. The melt mixture is extruded through a die similar to the dry processed separators. The thickness of the porous matrix can be in a range of 10-400 micrometers, or a range of 10-200 micrometers, or a range of 10-100 micrometers, or a range of 20-100 micrometers. The pore size of the porous matrix can be in a range of 1 micrometer to 500 micrometers, or a range of 10 micrometer to 200 micrometers, or a range of 50 micrometers to 100 micrometer.
In some embodiments, the thin film composite anion exchange membrane is prepared using a method comprising: 1) dissolving the anion exchange polymer comprising a plurality of repeating units of formula (I) in a solvent to form a polymer coating solution; 2) coating a layer of the anion exchange polymer coating solution on one surface of a microporous support membrane via dip-coating, meniscus coating, spin coating, casting, soaking, spraying, painting, or other known conventional solution coating technologies; 3) drying the coated membrane at 50° C. to 150° C., or at 50° C. to 120° C., or at 80° C. to 120° C.; and optionally 4) ion exchanging the halide anions of the anion exchange polymer in the coating layer with hydroxide, bicarbonate, carbonate ions, or a combination thereof if the anion exchange polymer has halide anions before membrane fabrication to form the thin film composite anion exchange membrane. The solvents for the preparation of the thin film composite anion exchange membrane include, but are not limited to, NMP, DMAc, DMF, DMSO, dioxanes, 1,3-dioxolane, acetic acid, trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TFSA), and mixtures thereof. The microporous support membrane should have good thermal stability (stable up to at least 120° C.), high stability under high pH condition (e.g., pH greater than 8), high resistance to oxidizing and reducing conditions (insoluble and no performance drop under oxidizing and reducing conditions), high mechanical strength (no dimensional change under the system operation conditions), as well as other factors dictated by the operating conditions for electrochemical reactions. The microporous support membrane must be compatible with the electrochemical cell chemistry and meet the mechanical demands of cell stacking or winding assembly operations.
The polymers suitable for the preparation of the microporous support membrane can be selected from, but not limited to, polyolefins such as polyethylene and polypropylene, polyamide such as Nylon 6 and Nylon 6,6, polyester, cellulose acetate, polybenzimidazole, fluorocarbon-based polymer such as PTFE and PVDF, polycarbonate, cellulose, or combinations thereof. These polymers provide a range of properties such as low cost, high stability in alkaline water, good mechanical stability, and ease of processability for membrane fabrication.
The microporous support membrane can have either a symmetric porous structure or an asymmetric porous structure. The asymmetric microporous support membrane can be formed by a phase inversion membrane fabrication approach followed by direct air drying, or by phase inversion followed by solvent exchange methods. The microporous support membrane also can be fabricated via a dry processing of thermoplastic polyolefins or a wet processing of thermoplastic olefins. The dry processing of thermoplastic polyolefins utilizes extrusion to bring the polymer above its melting point and form it into the desired shape. Subsequent annealing and stretching processes may also be done to increase the crystallinity and orientation and dimension of the micropores. The wet processing of polyolefin separators is done with the aid of a hydrocarbon liquid or low molecular weight oil mixed with the polymer resin or a mixture of the polymer resin and inorganic nanoparticles in the melt phase. The melt mixture is extruded through a die similar to the dry processed separators. The thickness of the microporous support membrane can be in a range of 10-400 micrometers, or a range of 10-200 micrometers, or a range of 10-100 micrometers, or a range of 20-100 micrometers. The pore size of the microporous membrane can be in a range of 10 nanometers to 50 micrometers, or a range of 50 nanometers to 10 micrometers, or a range of 0.2 micrometers to 1 micrometer.
Another aspect of the invention is a membrane electrode assembly (or called catalyst coated membrane). In one embodiment, the membrane electrode assembly (or called catalyst coated membrane) comprises: an anion exchange membrane comprising the polymer described above; a cathode comprising a cathode catalyst on a first surface of the anion exchange membrane; and optionally an anode comprising an anode catalyst on a second surface of the anion exchange membrane.
In some embodiments, the membrane electrode assembly further comprises: a cathode porous transport layer adjacent to the cathode; and an anode porous transport layer or an anode catalyst-coated anode porous transport layer adjacent to the second surface of the anion exchange membrane.
In some embodiments, the anode and the cathode catalysts are platinum group metal (PGM) electrocatalysts or PGM-free electrocatalysts. The anode and the cathode catalysts are for oxygen evolution reaction and hydrogen evolution reaction, respectively. The term “platinum group metal” (PGM) means the six noble, precious metallic elements including ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). In some embodiments, unsupported or supported iridium (Ir) based PGM electrocatalysts are used for the oxygen evolution reaction (OER) on the anode and platinum (Pt) or carbon supported platinum electrocatalyst (Pt/C) is used for the hydrogen evolution reaction (HER) on the cathode for AEM water electrolysis. Both Ir and Pt based PGM catalysts are very expensive and scarce. The anode and the cathode catalysts should have good electrical conductivity, and good electrocatalytic activity and stability. In some embodiments, PGM-free electrocatalysts are used for OER and/or HER for AEM water electrolysis. Suitable cathode catalysts can be selected from, but are not limited to, Ni-based alloys such as Ni—Mo, Ni—Al, Ni—Cr, Ni—Sn, Ni—Co, Ni—W, and Ni—Al—Mo, metal carbides such as Mo2C, metal phosphides such as CoP, metal dichalcogenides such as MoSe2, and mixtures thereof. Suitable anode catalysts can be selected from, but are not limited to, Ni—Fe alloy, Ni—Mo alloy, spinel CuxCo3xO3, Ni—Fe—W—B layered double hydroxide, Ni—Fe—Ce—B layered double hydroxide, Ni—Fe—Co—B layered double hydroxide, Ni—Fe layered double hydroxide nanoplates on carbon nanotubes, immobilized metal catalyst on conductive supports, and mixtures thereof.
In some embodiments, the anode comprising an anode catalyst on a first surface of the anion exchange membrane is formed by coating an anode catalyst ink on the first surface of the anion exchange membrane via meniscus coating, knife coating, spray coating, slot die coating, Mayer rod coating, gravure coating, comma coating, painting, or other known conventional ink coating technologies, followed by drying the coated anion exchange membrane.
In some embodiments, the cathode comprising a cathode catalyst on a second surface of the anion exchange membrane is formed by coating a cathode catalyst ink on the second surface of the anion exchange membrane via meniscus coating, knife coating, spray coating, slot die coating, Mayer rod coating, gravure coating, comma coating, painting, or other known conventional ink coating technologies, followed by drying the coated anion exchange membrane.
In some embodiments, the anode catalyst ink comprises the anode catalyst, an OH− exchange ionomer as a binder, and a solvent. In some embodiments, the cathode catalyst ink comprises the cathode catalyst, an OH− exchange ionomer as a binder, and a solvent. The OH− exchange ionomer binder creates OH− transport pathways between the membrane and the reaction sites within the electrodes and thus drastically improves the utilization of the electrocatalyst particles while reducing the internal resistance. The OH− exchange ionomer binder can have a chemical structure similar to the anion exchange polymer described above, so that the binder will allow low interfacial resistance and similar expansion in contact with water to avoid catalyst delamination, but OH− conductivity and high oxygen and hydrogen permeance. The solvent can be selected from, but is not limited to, water, alcohol, or a mixture thereof.
The anode gas diffusion layer and the cathode gas diffusion layer simultaneously transport electrons, heat, and products with minimum voltage, current, thermal, interfacial, and fluidic losses. The cathode gas diffusion layer can be made from, but is not limited to, stainless steel, titanium meshes, titanium felts, titanium foams, or carbon-based materials such as non-woven carbon paper, non-woven carbon cloth, or woven carbon cloth. The anode gas diffusion layer can be made from, but is not limited to, stainless steel, titanium meshes, titanium felts, or titanium foams.
The following examples are provided to illustrate one or more preferred embodiments of the invention but are not limited embodiments thereof. Numerous variations can be made to the following examples that lie within the scope of the invention.
Terphenyl (10.36 g, 45 mmol), phenanthrene (0.89 g, 5 mmol) and 1-methyl-4-piperidone (6.22 g, 55 mmol) were mixed with 40 mL of dichloromethane and cooled in icy water to 0° C. To the stirred mixture, trifluoroacetic acid (6 mL) and trifluoromethanesulfonic acid (40 mL) were added dropwise consecutively under stirring. The mixture was stirred for 11 hours while kept at 0° C. The reaction mixture was blended with 1 L of water to generate a slurry. The slurry was filtered and rinsed with 1 L of water, after which the filter cake was soaked in 1 L of water containing 5 g of sodium hydroxide overnight. The mixture was again filtered, and the filter cake was washed with water until pH neutral. The obtained solid poly(p-terphenyl-phenanthrene-N-methyl-4-piperidine) (abbreviated as PTPP) precursor polymer was then dried at 60° C. overnight, followed by drying under vacuum at 80° C. overnight.
The obtained PTPP polymer (about 50 mmol of N-methyl-4-piperidine units) was dissolved in 160 mL of dimethyl sulfoxide containing 4 mL of trifluoroacetic acid. To the solution, iodomethane (47.5 mmol) and anhydrous potassium carbonate (180 mmol) were added. While protected from light, the mixture was stirred for 16 hours. The reaction mixture was poured in 1 L of KHCO3 aqueous solution. The obtained mixture was filtered, and the filter cake was washed with KHCO3 aqueous solution and water several times until pH neutral. The obtained solid was dried at 60° C. for at least 24 h to obtain dried poly(p-terphenyl-phenanthrene-N,N-dimethyl-4-piperidinium bicarbonate)-co-poly (p-terphenyl-phenanthrene-N-methyl-4-piperidine) (abbreviated as PTPPB-PTPP) anion exchange polymer. 1H NMR analysis result for the PTPPB-PTPP polymer confirmed that the polymer had 83 mol % of N,N-dimethyl-4-piperidinium bicarbonate functional groups converted from the N-methyl-4-piperidine functional groups on the PTPP precursor polymer and had 17 mol % of unconverted N-methyl-4-piperidine functional groups.
Poly (p-terphenyl-phenanthrene-N,N-dimethyl-4-piperidinium bicarbonate)-co-poly(p-terphenyl-phenanthrene-N-methyl-4-piperidine) anion exchange membrane (abbreviated as PTPPB-PTPP AEM) was prepared by dissolving the PTPPB-PTPP anion exchange polymer (5.0 g) prepared in Example 1 in DMSO (20 g), casting the solution on a clean substrate, and drying at 60° C. overnight. The membrane was peeled off the substrate and further dried in a vacuum oven at 100° C. for 48 h to form PTPPB-PTPP AEM.
A poly (p-terphenyl-phenanthrene-N-methyl-4-piperidine) (abbreviated as PTPP) precursor polymer was synthesized using the procedure same as that described in Example 1. The PTPP polymer (about 50 mmol of N-methyl-4-piperidine units) was dissolved in 160 mL of dimethyl sulfoxide containing 4 mL of trifluoroacetic acid. To the solution, iodomethane (75 mmol) and anhydrous potassium carbonate (180 mmol) were added. While protected from light, the mixture was stirred for 24 hours. The reaction mixture was poured in 1 L of KHCO3 aqueous solution. The obtained mixture was filtered, and the filter cake was washed with KHCO3 aqueous solution and water several times until pH neutral. The obtained solid was dried at 60° C. for at least 24 h to obtain dried poly (p-terphenyl-phenanthrene-N,N-dimethyl-4-piperidinium bicarbonate) (abbreviated as PTPPB) anion exchange polymer. 1H NMR analysis result for the PTPPB polymer confirmed that the polymer had 100 mol % of N,N-dimethyl-4-piperidinium bicarbonate functional groups converted from the N-methyl-4-piperidine functional groups on the PTPP precursor polymer and no unconverted N-methyl-4-piperidine functional group was observed.
Poly(p-terphenyl-phenanthrene-N,N-dimethyl-4-piperidinium bicarbonate) anion exchange membrane (abbreviated as PTPPB AEM) was prepared by dissolving the PTPPB anion exchange polymer (5.0 g) prepared in Comparative Example 1 in DMSO (20 g), casting the solution on a clean substrate, and drying at 60° C. overnight. The membrane was peeled off the substrate and further dried in a vacuum oven at 100° C. for 48 h to form PTPPB AEM.
PTPPB-PTPP AEM and PTPPB AEM membranes as prepared in Example 2 and Comparative Example 2 were ion exchanged in 1 M KOH aqueous solution for 10 h at 80° C., respectively. The treated membranes were washed with ultra-pure water at least three times and then their in-plane OH− conductivities were measured using the a.c. impedance on a three-electrode cell with platinum electrodes at room temperature. The measurement cell was submerged in DI H2O at room temperature. Impedance measurements were carried out at open circuit over a frequency range of 10-100 kHz, with a Gamry Reference 600+ potentiostat/galvanostat. Both PTPPB-PTPP AEM and PTPPB AEM showed higher than 100 mS/cm high OH− conductivities. PTPPB-PTPP AEM showed a OH− conductivity of 113.9 mS/cm and PTPPB-PTPP AEM showed a OH− conductivity of 124.7 mS/cm at room temperature.
The areas and the weights of the dried PTPPB-PTPP AEM prepared in Example 2 and PTPPB AEM prepared in Comparative Example 2 were measured and then the membranes were soaked in ultra-pure water for 3 h at room temperature, respectively. The areas and the weights of the wet membranes were measured immediately after they were removed from the ultra-pure water. The area changes and the weight changes after the water soaking were summarized in Table 1. It can be seen from Table 1 that PTPPB-PTPP AEM showed significantly reduced swelling with only 5.5% area increase after 3 hours of soaking in ultra-pure water compared to PTPPB AEM with 23.9% area increase after 3 hours of soaking in ultra-pure water. PTPPB-PTPP AEM also showed much lower water uptake with 15.4% weight increase after 3 hours of soaking in ultra-pure water compared to PTPPB AEM with 41.7% weight increase after 3 hours of soaking in ultra-pure water. The significantly reduced swelling of the PTPPB-PTPP AEM allowed direct cathode and/or anode catalyst ink coating on the surface of the membrane for the fabrication of 2-layer or 3-layer catalyst coated membrane (CCM) for AEM water electrolysis. On the other hand, the PTPPB AEM wrinkled badly during the cathode and/or anode catalyst ink coating on the surface of the membrane, and the CCM could not be prepared successfully.
a Result from an average of three membrane samples;
b area change (%) = (membrane area after water soaking − membrane area before water soaking)/membrane area before water soaking;
c weight change (%) = (membrane weight after water soaking − membrane weight before water soaking)/membrane weight before water soaking.
A 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM was prepared by a direct wet coating method using a Mayer rod to coat a 40% Pt/C cathode hydrogen evolution reaction (HER) catalyst ink directly on one surface of the 20 PTPPB-PTPP AEM. A 40% Pt/C cathode catalyst ink was prepared by mixing the 40% Pt/C catalyst and PTPPB anion exchange polymer as an ionomer in ultra-pure water and alcohol such as ethanol. The mixture was finely dispersed using a combination of mixing and ultrasonicating. The Pt/C ink was coated onto one surface of the PTPPB-PTPP AEM using the Mayer rod for only about 5-10 min and then dried at 60-100° C. to form the 2-layer CCM comprising 40% Pt/C cathode 25 catalyst coating layer on one surface of the PTPPB-PTPP AEM.
A 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM was not successfully prepared by the direct wet coating method using a Mayer rod to coat a 40% Pt/C cathode HER catalyst ink directly on one surface of the PTPPB AEM because the membrane wrinkled badly during the coating process. Therefore, a spray coating method was used to prepare the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM. A 40% Pt/C cathode catalyst ink was prepared by mixing the 40% Pt/C catalyst and PTPPB anion exchange polymer as an ionomer in ultra-pure water and alcohol such as ethanol. The mixture was finely dispersed using a combination of mixing and ultrasonicating. The Pt/C ink was coated onto one surface of the PTPPB AEM using a spray-coating method at 80° C. to form the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM. The membrane did not wrinkle badly during the spray coating process because the membrane was heated at 80° C. and the uncoated side of the membrane was under vacuum during the whole coating process for about 2-3 h. However, this spray-coating process is not as feasible as the Mayer rod coating or slot die direct coating process due to the much slower coating process and required heating on the membrane to prevent wrinkles.
The water electrolysis performance and H2 crossover of the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM prepared in Example 5 and the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM prepared in Example 6 were evaluated using a single water electrolysis cell at atmospheric pressure in a Scribner unit. The 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM and the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM were sandwiched between a carbon paper (cathode PTL) and a 2-layer NiFeCeBOx anode O2 evolution catalyst-coated porous transport layer (PTL) to form membrane electrode assemblies (MEAs) inside the testing cells, respectively.
A water electrolysis test station (Scribner 600 electrolyzer test system), modified for testing with potassium hydroxide feed, was used to evaluate the water electrolysis performance of the membrane electrode assemblies (MEAs) comprising the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM or the 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM in a single electrolyzer cell with an active membrane area of 5 cm2. The test station included an integrated power supply, a potentiostat, an impedance analyzer for electrochemical impedance spectroscopy (EIS) and high-frequency resistance (HFR), and real-time sensors for product flow rate and cross-over monitoring. The testing was conducted at 80° C. under 15 psig pressure with a 1 wt % KOH feed supplied to the anode side of the test cell. The polarization curves for the MEAs at 80° C. showed that the MEA comprising 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM showed cell voltages (1.69 V at 0.8 A/cm2 and 1.88 V at 2.0 A/cm2) comparable to those of the MEA comprising 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM (1.69 V at 0.8 A/cm2 and 1.86 V at 2.0 A/cm2). The H2 crossover from the cathode side to the anode side of the test cell indicated by the H2 concentration (0.08 mol % H2 in O2) in O2 in the anode side of the test cell comprising the MEA with 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB-PTPP AEM was much lower than that in the anode side of the test cell comprising the MEA with 2-layer CCM comprising 40% Pt/C cathode catalyst coating layer on one surface of the PTPPB AEM (0.63 mol % H2 in O2), demonstrating that the PTPPB-PTPP AEM prepared from the new PTPPB-PTPP anion exchange polymer comprising both N,N-dimethyl piperidinium anion exchange functional groups and N-methyl piperidine functional groups has lower H2 crossover than the PTPPB AEM prepared from the PTPPB anion exchange polymer comprising N,N-dimethyl piperidinium anion exchange functional groups without any N-methyl piperidine functional group. The low H2 crossover for the new PTPPB-PTPP AEM prepared from the PTPPB-PTPP anion exchange polymer is important for safe electrolyzer operation and will improve the electrolyzer efficiency due to the lower loss of H2 product.
While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.
A first embodiment of the invention is an anion exchange polymer comprising a plurality of repeating units of formula (I)
and mixtures thereof; wherein R25, R26, R27, and R28 are each independently —H or —CH3; wherein p is 1 or 2; and wherein q is 0 or 1. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein Ar1 and Ar2 are independently selected from the group consisting of
wherein R30 is —H, —CH3, —CH2CH3, —CH(CH3)2, —C(CH3)3, —CH2—C6H5, or —CH2—CH(CH3)2; and wherein t is 1, 2, 3, 4, 5, or 6; wherein Y2− is HCO3−, OH−, I−, CF3SO3−, or
A second embodiment of the invention is an anion exchange membrane comprising the anion exchange polymer described above. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the anion exchange membrane is used in a fuel cell, an electrolyzer, a flow battery, an electrodialyzer, a waste metal recovery system, an electrocatalytic hydrogen production system, a desalinator, a water purifier, a waste water treatment system, an ion exchanger, or a CO2 separator. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the anion exchange membrane comprises a nonporous symmetric dense film membrane, an integrally-skinned asymmetric membrane, a reinforced composite membrane, or a thin film composite membrane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the integrally-skinned asymmetric membrane, the reinforced composite membrane, or the thin film composite membrane comprises a porous substrate material impregnated or coated with the anion exchange polymer described above.
A third embodiment of the invention is an apparatus comprising an anion exchange
membrane comprising the anion exchange polymer described above; a cathode comprising a cathode catalyst on a first surface of the anion exchange membrane; and optionally an anode comprising an anode catalyst on a second surface of the anion exchange membrane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph further comprising a cathode porous transport layer adjacent to the cathode; and an anode porous transport layer or an anode catalyst-coated anode porous transport layer adjacent to the second surface of the anion exchange membrane.
Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.
In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.
