Patent Application
20240342699
This invention is directed to ultra-thin anion exchange membranes incorporating anion exchange polymers and functional additives and its application as self-humidification membranes for electrochemical applications incorporating anion exchange membranes.
Anion exchange membrane fuel cells (AEMFCs) have intriguing features as compared to proton exchange membrane fuel cells (PEMFCs). One of their advantage is the possibility of using non-platinum-group catalysts due to faster oxygen reduction reaction (ORR) kinetics.
Anion exchange membrane (AEM) is a critical component of AEMFCs, where hydrogen and oxygen are used to generate electricity with water as a byproduct. In an AEMFC, the following electrochemical reactions occur in the catalyst layers:
Cathode: O2+2H2O+4e−→4OH− (1)
Anode: 2H2+4OH−→4H2O+4e− (2)
In AEMFCs, anions and water are transported across the AEMs. The AEM needs to stay in hydrated state to have high ion conductivity and durability, which requires proper humidification. The water management in the AEMFCs is more complex than in the PEMFCs, as water is a product at the anode and a reactant at the cathode, back-diffusion of water from the anode to the cathode is a key parameter for high cell performance. Also, flooding may occur at the anode of AEMFCs at the same time as water is consumed at the cathode. Both these conditions are undesirable, as flooding limits the mass transport of reactants, and drying out of the cathode limits the supply of water for the cathode reaction and decreases the oxygen reduction reaction.
AEMFC humidification methods may include direct water injection and gas bubbling humidifier methods. However, the amount of water injected in the cathode inlet gas flow has to be controlled precisely to avoid water flooding. In addition, certain applications such as automotive and electronics applications prefer self-humidification of the AEM as they have limited space requirements.
The back diffusion is sufficient to humidify the cathode side membrane only when the thickness of the membrane is thin enough. However, AEMs require a higher activation energy for anion transport compared to proton transport in PEMs. To achieve high ionic conductivity and hydrophilic-domain phase separation, AEMs are designed to have high ion exchange capacities (IECs). High IEC increases water uptake and hydrophilic-domain phase separation, leading to a reduction in mechanical strength and dimensional stability. Traditionally, to combat these issues, thicker membranes were used. Thicker membranes have higher ionic resistance which also reduces the water back-diffusion in AEMFCs thereby further hampering water management in AEMFCs.
Anion exchange membrane fuel cells (AEMFCs) continue to have limited use and adoption due to the problems listed above.
The present invention provides an anion exchange membrane that is ultra-thin and comprises functional additives to provide improved water management. An exemplary anion exchange membrane may be a composite anion exchange membrane comprising a porous scaffold support. An anion exchange polymer may be coupled to the porous scaffold, such as by being imbibed into the pores of the porous scaffold. The porous scaffold may enable the composite anion exchange membrane to be ultra-thin and therefore reduce water management issues. An ultra-thin anion exchange membrane may have a thickness of about 50 μm or less, and may even be as thin as about 5 μm or less. An exemplary anion exchange polymer may include a functional polymer based on a hydrocarbon backbone with quaternary ammonium functional groups and an inert porous scaffold material for reinforcement. The functional additives may contribute to increase water production, water retention, back-diffusion and reduce the gas crossover.
The combination of ultra-thin and functional additives provides a unique combination of features that enables anion exchange membranes to operate without supplemental humidification methods. A porous scaffold supports the anion exchange polymer to make the composite dimensionally stable and durable for processing. Very thin and ultra-thin anion exchange membranes may however be susceptible to high gas crossover rates across the membrane during operation. The gas crossover rates become a serious issue for ultra-thin AEMs because polymeric membranes have inherent gas diffusivity and permeability properties. These crossover rates are reduced by the addition of a functional additive. Reactant gases that permeate into the membrane react with the functional additive and produce water. The reactant gasses may include oxygen and/or hydrogen. Furthermore, reactant gasses can have other detrimental effects to the anion exchange membrane including reacting with the anion exchange polymer and/or the reacting with the porous scaffold to reduce strength and durability.
An exemplary functional additive may comprise a reactive species, including a catalyst that reacts with oxygen or hydrogen, a plasticizer, a hygroscopic material and/or a radical scavenger. The functional additives may be included in the anion exchange membrane in a concentration of about 1% by weight or less, about 0.5% or less and in some cases about 0.1 wt % or less, and any range between and including the concentrations provided. Too much of reactive functional additives will decrease the performance of the system.
An exemplary reactive species includes, but is not limited to, TiO2, CeO2, SiO2, carbon nanotubes, graphene, carbon black, sulfated zirconia and any combination of them. Note that the reactive species may be included as a particle in the anion exchange polymer and/or may be coupled to the porous scaffold, such as being bound in an expanded polytetrafluoroethylene, or coated onto a porous scaffold, wherein the reactive species is exposed to the anion exchange polymer at the interface between the ionomer and the porous scaffold.
The reaction of permeated may be catalyzed by the catalytic functional additive, such as platinum particles to generate water, which in turn is adsorbed and retained by the oxide particles (e.g. SiO2). Moreover, the addition of TiO2 enhances the back-diffusion of water that is generated by the electrochemical reaction from the cathode to the anode, owing to the hygroscopic property of TiO2. Thus, the membrane at the cathode side that is dried by the electro-osmotic drag can be humidified effectively. A catalytic functional additive is a subset of a reactive species but is not consumed in the reaction. A catalyst reduces activation energy for a reaction to occur.
Functional additives may include plasticizers that increase the amorphous content of the anion exchange polymer. This increases the free volume for water to occupy, which enhances ion conductivity. Too much plasticizer decreases the glass transition temperature. Exemplary Plasticizers are selected from the group consisting of 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 1-Hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (MC6), glycerol, and Nylon 6,6.
Functional additives may include fillers such as a hygroscopic material that retain water in the membrane to improve conductivity. Exemplary fillers may include Aluminium oxide (Al2O3), Heteropolyacid (HPA)-based inorganic filler, IL-functionalised carbon materials, Carbon black (CB), Multiwallcarbon nanotubes (MWCNTs) and Reduced Graphene Oxide (RGO), Phosphoric acid-modified silica, Imidazole-modified silica, Phosphate-modified titanium zirconium oxide, and Zeolitic imidazolate framework-8 (ZIF-8).
Functional additives may include radical scavengers that improve durability of the membrane because they convert radicals into non-harmful species, which protects the anion exchange polymer or ionomer. Exemplary radical scavengers include, but are not limited to metal oxides including cerium oxide, manganese oxide, and polymers including polydopamine.
The reaction of permeated may be catalyzed by the platinum particles to generate water, which in turn is adsorbed and retained by the oxide particles (e.g. SiO2). Moreover, the addition of TiO2 enhances the back-diffusion of water that generated by the electrochemical reaction from the cathode to the anode, owing to the hygroscopic property of TiO2. Thus the membrane at the cathode side that dried by the electro-osmotic drag can be humidified effectively.
Exemplary anion exchange polymers may have backbones selected from the group of Polysulfones, Polyphenylenes, Poly(arylene ether ketones), Poly(phenylene oxides), Polystyrenes, Polynorbornenes, Polybenzimidazoliums, Polypropylenes, Polyethylenes, and fluorinated hydrocarbon or carbon-based polymers. Exemplary anion exchange polymers may have functional groups selected from the group of quaternary ammonium, tertiary diamines, phosphonium, benz (imidazolium), sulphonium, guanidinium, metal cations, pyridinium.
Anion exchange membranes as described herein may be use in AEMFCs, electrolyzes, sensors, compressors, batteries, super capacitors.
In one embodiment, a membrane is prepared by dissolving the anion exchange polymer in isopropyl alcohol at a 3% weight ratio, i.e. 0.3 grams of polymer to 9.7 grams of solvent. Then 0.06 grams of TiO2, 0.2 grams of SiO2 and 0.01 grams Pt Black are added into the solution. The mixture was stirred until homogenous.
The polymer solution was then applied to a microporous polyethylene material tensioned around a chemically resistant plastic frame. The polymer solution was then poured on to the microporous scaffold. The frame was covered with a lid to slow solvent evaporation. The membrane was dried at room temperature. The final thickness of the membrane was 5 micrometers.
In another embodiment, a membrane is prepared by dissolving the polymer in isopropyl alcohol at a 5% weight ratio i.e. 0.5 grams of polymer to 9.5 g of solvent. Then 0.06 grams of CeO2, 0.25 grams of SiO2 and 0.01 grams carbon nanotubes are added into the solution. The mixture was stirred until homogenous.
The polymer was then applied to a microporous poly(tetrafluoroethylene) material with a doctor blade. The membrane was dried at room temperature. The final thickness of the membrane was 10 microns.
It will be apparent to those skilled in the art that the latter embodiment can be scaled up to a roll-to-roll, continuous process.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.
Corresponding reference characters indicate corresponding parts throughout the several views of the figures. The figures represent an illustration of some of the embodiments of the present invention and are not to be construed as limiting the scope of the invention in any manner. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.
As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Also, use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.
Certain exemplary embodiments of the present invention are described herein and are illustrated in the accompanying figures. The embodiments described are only for purposes of illustrating the present invention and should not be interpreted as limiting the scope of the invention. Other embodiments of the invention, and certain modifications, combinations and improvements of the described embodiments, will occur to those skilled in the art and all such alternate embodiments, combinations, modifications, improvements are within the scope of the present invention.
This application is a continuation of U.S. patent application Ser. No. 16/913,767, filed on Jun. 26, 2020 and currently pending, which claims the benefit of priority to U.S. provisional patent application No. 62/867,146, filed on Jun. 26, 2019 and currently pending; the entirety of both prior applications are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62867146 | Jun 2019 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16913767 | Jun 2020 | US |
| Child | 18649475 | US |
