Hydrophilic side-chain polymer, electrolyte membranes

Abstract

Exemplary methods for the characterization of proton dissociation and transport for hydrophilic components of hydrated Polymer Electrolyte Membranes (PEM's) is described. Disclosed features and specifications may be variously implemented, controlled, adapted or otherwise optionally modified to improve differential hydrophilicity of the sidechain of any ionomeric PEM material. A representative embodiment of the present invention generally provides for the amelioration of electro-osmotic drag of water by protons, for example, in Direct Methanol Fuel Cells.

Description

FIELD OF INVENTION [0001] The present invention generally concerns fuel cell technology; and more particularly, in one representative and exemplary embodiment, the characterization of proton dissociation and transport for hydrophilic components of Polymer Electrolyte Membranes (PEM's). A PEM material comprising a novel differential sidechain chemical composition is also disclosed as inter alia preventing or otherwise ameliorating electro-osmotic drag as the membrane material experiences increased swelling upon permeation with, for example, aqueous methanol. BACKGROUND [0002] Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation is converted into electrical energy. The earliest fuel cells were first constructed by William Grove in 1829 with later development efforts resuming in the late 1930's with the work of F. T. Bacon. In early experiments, hydrogen and oxygen gas were bubbled into compartments containing water that were connected by a...

AI Technical Summary

Benefits of technology

[0013] The disclosed method for modeling of the dielectric saturation in Polymer Electrolyte Membranes (PEM's) demonstrates that with increased protrusion of the anionic groups within the membrane pores, the permittivity (i.e., dielectric constant) of the water generally decreases. The decrease in the dielectric constant (relative to bulk water) generally corresponds to the water being more constrained. Both the predictive values and experimental measurements have shown that under such conditions, electro-osmosis is minimized.

[0014] In application to NAFION and PEEKK membranes, at various levels of hydration, the present invention demonstrates improved predictive capability—without requiring the use of ‘fitting parameters’ and / or input from SAXS experiments or molecular orbital calculations. Diffusion coefficients determined in accordance with the method of the instant invention are generally observed to be in good agreement with experiment. Additionally, the instant system and method has demonstrated substantial sensitivity to the parameters of the domains (e.g., pores) within the PEM material where proton conduction generally occurs; including: sidechain length, length of pore, radius of pore, and distribution of sulfonic acid fixed sites within the pore.

[0016] Optimized geometries for the chemical species of interest were determined by means of both ab initio Hartree-Fock theory and second order Møller-Plesset electron correlation corrections, and density functional theory with Becke's three parameter hybrid method. A representative custom-designed ionomer is disclosed as comprising a chemical structure derived from a series of ab initio molecular based calculations and simulations; namely, a polymer electrolyte membrane material having a novel sidechain for preventing or otherwise ameliorating increases in electro-osmotic drag when the material experiences increased swelling upon permeation with, for example, aqueous methanol. In one representative aspect, the present invention involves the tailoring of the sidechain (through alteration of the chemical functionalization of the sidechain) such that hydrophilicity of the intermediate portion of the sidechain is increased beyond that typically found in presently available PEM materials.

[0017] One representatively practical example of the present invention involves the incorporation of an ether oxygen (or oxygens) along the length of the sidechain of NAFION to introduce or otherwise improve hydrophilicity (e.g., the ability to form hydrogen bonds with water molecules). Another example of how the structure of NAFION may be altered is disclosed as comprising an alteration of the sidechain where the groups vicinal, geminal, etc. to the ether oxygen are changed from CF2 to CH2. The disclosed system and method may be readily and more generally adapted for use in the optimization and / or chemical functionalization of any ionomeric material, whether now known or otherwise hereafter described in the art. The disclosed alteration of NAFION is shown inter alia as dramatically increasing the tendency of the ether oxygen to form a hydrogen bond with, for example, a water molecule.

[0018] The increased differential hydrophilicity of the sidechain generally manifests itself as a representatively practical benefit, for example, when the ion-containing domains or pores swell by increasing the protrusion of the sidechains within the pores. Characterization of the permittivity of the water in the pores, in accordance with the methods disclosed in the instant invention, demonstrates that with increased protrusion of the sidechains, the transport of water by the protonic current is inhibited, thereby preventing or otherwise ameliorating increases in electro-osmotic drag. In addition, because the hydrophilicity of the sidechains may be custom designed so as to make the terminal part more hydrophilic than along the length of the sidechain, the conductivity of the membrane at lower water concentrations generally will not be reduced with the increased hydrophilicity of the sidechain.

Problems solved by technology

As a standard battery operates, various chemical components of the electrodes are depleted over time.

Until recently, however, no molecular-level understanding was available for electro-osmosis.

During operation of the fuel cell, protonic current within the membrane drags water (e.g., ‘electro-osmotic drag’) from the anode to the cathode, which reduces the efficiency of the fuel cell by hindering the reduction reaction at the cathode (e.g., ‘cathode flooding’).

This, in turn, generally requires the utilization of relatively expensive water management techniques typically involving capture or return of water to the anode side of the DMFC.

Accordingly, despite the efforts of the prior art, one problem warranting resolution is the characterization of the increased electro-osmotic drag of water by protons in membranes used in the manufacture of PEMFC's.

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