BACKGROUND

With growing demand for clean and efficient energy, fuel cells have emerged as attractive electrochemical conversion devices due to their high energy density and their ability to produce energy more cleanly and efficiently compared to conventional systems, such as internal combustion engines. Proton exchange membrane fuel cells (PEMFCs) find use in many commercial applications. However, widespread production of PEMFCs is limited by the cost and durability of the materials used to produce them, such as platinum electrodes and electrolyte membrane.

In view of drawbacks associated with widespread production of PEMFCs, increased focus has been placed on alkaline fuel cells (AFCs), which operate by transporting hydroxide ions through the electrolyte under basic conditions. At elevated pH, oxygen reduction is more facile and lower overpotentials are required, thereby enabling the use of non-noble metal catalysts in AFCs. Indeed, the earliest examples of commercial fuel cells used aqueous potassium hydroxide solutions as the electrolyte medium to facilitate anion conduction. Unfortunately, the performance of these early fuel cells was compromised by exposure to carbon dioxide, a common component of feedstock gases, which reacts with hydroxide to produce carbonate salts. To overcome this issue, alkaline anion exchange membranes (AAEMs) can be used in alkaline fuel cells. AAEMs, which are generally comprised of organic cations covalently linked to a polymer backbone, are employed to prevent the formation of mobile salts and retain the conductive organic cation/hydroxide species.

Tetraalkylammonium cations have been appended to various polymer architectures to prepare AAEMs, including perfluorinated membranes, aromatic polysulfones, poly(arylene ethers), poly(arylene ether ketones), polyphenylenes, polystyrenes, and various aliphatic backbones. However, current ammonium cations (for example, the ubiquitous benzyl trimethylammonium (BTMA) cation) degrade rapidly under fuel cell operating conditions, which limits their utility and makes the improvement of AAEM stability a critical priority. Therefore, a need exists for moieties that offer enhanced stability under fuel cell operating conditions.

While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicant in no way disclaims these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was, at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.

SUMMARY OF THE INVENTION

Briefly, the present invention satisfies the need for moieties that offer enhanced stability under fuel cell operating conditions. The present invention may address one or more of the problems and deficiencies of the art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.

Certain embodiments of the presently-disclosed imidazoles and imidazolium cations, polymers incorporating imidazolium cations, and articles comprising the same, and related methods have several features, no single one of which is solely responsible for their desirable attributes. Without limiting the scope of the inventive compounds, polymers, articles, and methods as defined by the description and claims that follow, their more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section of this specification entitled “Detailed Description of the Invention,” one will understand how the features of the various embodiments disclosed herein provide a number of advantages over the current state of the art. These advantages may include, without limitation, providing imidazole and imidazolium cations (and polymers incorporating imidazolium cations) that are easy to make and/or, for the compounds, to incorporate into polymers, that are resistant to degradation via nucleophilic or basic attack by hydroxide or methoxide, and/or that maintain favorable conductivity and stability in fuel cell devices.

Imidazole compounds and/or imidazolium cations that are stable under basic conditions are extremely important for several applications, including, without limitation: organocatalysts, solar cell electrolytes, phase-transfer catalysis, as carbon material precursors, as semiconductors in OLEDs (organic light emitting diodes), as ionic liquids, and cocatalysts. In general, the inventive compounds and polymers will be useful wherever base-stable organic cations and their precursors are required or beneficial. Moreover, embodiments of the inventive imidazoles and imidazolium cations can be readily incorporated into polymer architectures using several polymerization techniques, including but not limited to ring opening-metathesis (ROMP), controlled radical polymerization, and polymerization of functionalized α-olefins. Imidazole compounds may also be attached to pre-formed polymers by reacting with electrophilic sites existing in the polymer. Polymers with base-stable cations appended to the backbone are in demand for applications such as fuel cell membrane electrolytes, electrolysis, gas separation, desalination, anion exchange resins, nuclear waste remediation, and as stimuli-responsive materials. The inventive polymers may be useful anywhere that polymers containing base-stable cations are currently employed.

In one aspect, the invention provides a compound of formula (I) or (II):


wherein:

R1is selected from C2-C16hydrocarbyl, wherein one carbon atom of the C2-C16hydrocarbyl may optionally be replaced by O;

R2is phenyl substituted with 0 to 3 substituents R6individually selected from C1-C3alkyl;

R3is selected from C2-C16hydrocarbyl;

R4and R5are individually selected from C1-C16hydrocarbyl, or, taken together, R4and R5, together with the carbon atoms to which they attached, form a ring selected from benzene, cyclooctene and norbornene; and

X−is a counterion.

In a second aspect, the invention provides a polymer comprising a plurality of imidazolium-containing repeating units (IRUs) of formula (III′):


wherein:

R2′is selected from C1-C6alkyl and R2;

R2is phenyl substituted with 0 to 3 substituents R6individually selected from C1-C3alkyl;

R3′is selected from hydrogen, methyl, and R3;

R3is selected from C2-C16hydrocarbyl;

R4and R5are individually selected from C1-C16hydrocarbyl, or, taken together, R4and R5, together with the carbon atoms to which they attached, form a ring selected from benzene, cyclooctene and norbornene;

X−is a counterion;

wavy lines indicate points of attachment to adjacent repeating units of the polymer;

W is a direct bond or C1-C10hydrocarbyl;

Y is a direct bond or C1-C10hydrocarbyl; and

Z is a direct bond or C1-C13hydrocarbyl, wherein one carbon atom of the C1-C13hydrocarbyl may optionally be replaced by O,

provided that the sum of carbon atoms in W, Y, and Z is 1-15.

In a third aspect, the invention provides a membrane comprising a polymer according to the second aspect of the invention.

In a fourth aspect, the invention provides a device comprising a polymer according to the second aspect of the invention or a membrane according to the third aspect of the invention.

In a fifth aspect, the invention provides a method of determining the stability of a compound, said method comprising:

preparing a solution of the compound in basified methanol-d3(KOH/CD3OH);

storing the solution; and

analyzing the solution by1H NMR spectroscopy for amount of compound remaining relative to an internal standard.

These and other features and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the appended claims.

