High Heat Low Humidity Hydrogen Cyano Fullerene Containing Proton Conducting Membranes for Polymer Fuel Cells

The figure shows University of California Santa Barbra's azide addition of PEO-azide in a fullerene synthesis scheme utilized to produce exemplary C₆₀{(NCH₂CH₂O)nCH₃}m molecules, made with numbers of and various lengths of PEO chains which are used to make polymer proton conducting membranes for improved fuel cell operation at low humidity and high heat.

The Regents of the University of California (Oakland, CA) received U.S. Patent 7,588,824  for  high heat low humidity hydrogen cyano fullerene membranes used for improved polymer fuel cells.  

The components of and the proton conducting membrane (PCM) are produced from a host polymer and an attached, or physically blended in, hydrogen cyano fullerene proton-source agent, with the physical blending of the host polymer and hydrogen cyano fullerene further promoted by a poly(ethylene oxide) attached fullerene mixing agent.  

The subject PCMs, comprised of the novel subject components, possess an improved performance over existing PCMs under low humidity, <50% relative humidity (RH), and at high temperature (>120 degree  C.) in the operation of polymer electrolyte fuel cells (PEFC) say the inventors.

University of California Santa Barbra Chemistry Professor of Chemistry Fred Wudl and Professor of Chemistry Galen D. Stucky with Hengbin Wang, Bruno Jousselme,  Ken Tasaki and Arunkumar Venkatesan developed the novel proton conducting membranes (PCMs) and the components utilized to produce these PCMs.  The novel PCMs and their constituent components are comprised of  hydrogen cyano fullerenes (HC₆₀(CN)_x as a proton-source agent and often poly(ethylene oxide) attached fullerenes (C₆₀(PEO)_y) as mixing agents to facilitate PCM formation with a host polymer.

Generally, the PCM has a host polymer and a proton-source agent. The proton-source agent includes a carbon cluster derivative, wherein the carbon cluster is derivatized with both hydrogen and cyano moieties. The carbon cluster derivative comprises from about 0.01 wt % to about 80 wt % of the PCM and may be physically blended with the host polymer or attached to the host polymer.

Although any suitable carbon cluster (such as a fullerene family member or equivalent molecule such as a carbon nano-tube, open or closed carbon cage-molecule, and the like) that does not interfere with the structural and functional characteristics of the PCM may be used. The preferred carbon cluster is usually one of the family of carbon structures known as fullerenes and the carbon cluster derivative usually comprises a hydrogen cyano fullerene.

The membrane is utilized as a major component of a polymer electrolyte fuel cell (PEFC). PEFCs are generally comprised of three major components: the anode; the proton conducting membrane (PCM); and the cathode. The PCM plays a critical role of transporting a proton from the anode to the cathode. It has to be highly proton conductive and also mechanically, thermally, and chemically stable. Water is produced at the interface between the cathode and the membrane. This water can be problematic, as discussed below, in operation of a PEFC. Lack of suitable membrane availability has been hindering the commercialization of PEFC. Water management is one of the most difficult issues in operating a PEFC. The water in the PEFC is produced as a product at the cathode side in PEFC.

A breakdown in water balance between production and loss of water at the cathode side often results in water flood, while the anode interface with the membrane may suffer from water depletion due to water transportation toward the cathode side. Both the flood and the depletion may increase the cell over-potential which results in loss of power. Furthermore, the most commonly used PCMs are based on sulfonated perfluoropolymers that need to be fully humidified to be functional during the operation of the PEFC. Thus, these sulfonated perfluoropolymers not only require a humidifier, but also need an even distribution of water across the membrane which becomes an additional concern because of the membrane's high dependence on water.

Dry operation of PEFC may alleviate some of the water management problems. In fact, there is a strong demand in the auto industry as well as the distributed power generation industry for PEFC functional under low relative humidity (RH) (<50% RH). Currently, no commercially available PCM meets this demand. NAFION, the industrial standard PCM by DuPont, is widely used in PEFC; yet it is sensitive to humidity, a very undesirable characteristic. Other existing proton conducting membranes, commercially available or under development, are as good or even better than NAFION under fully humidified condition, but very few outperform NAFION under low humidity conditions.  

The hydrogen cyano fullerene containing proton conducting membranes are able to overcome the problems of low humidity at elevated temperatures.