Electrode Substrate for Electrochemical Cell From Carbon and Cross-Linkable Resin Fibers

Technical Field

This disclosure relates to carbon composite electrode substrates for fuel cells formed from a non-woven felt of chopped carbon fibers and chopped uncured cross-linkable resin fibers, prepared in either a dry-lay or wet-lay paper making process, pressed together and cured, the resin cross-linking to bind all of the fibers together, followed by heat treating to carbonize and to optionally partially graphitize the materials.

Background Art

Porous carbon-carbon composites as electrode substrates, which are also known as gas diffusion layers (GDL's), may be used in a variety of electrochemical cells such as proton exchange membrane fuel cells, phosphoric acid fuel cells, direct methanol fuel cells, as the cathode in cells used to electrolyze water and as electronic conductors in a variety of batteries. The carbon-carbon composites are typically formed from a mixture of carbon fibers and a thermosetting resin. Some substrates may contain carbon or graphite powders in addition to the carbon fibers and thermosetting resins. A good example of the structure of a fuel cell substrate, and a process used to form these substrates, are shown in U.S. patent 4,851 ,304. A typical method of making a substrate consists of: (1 ) forming a non-woven felt from a chopped carbon fiber and a temporary binder by a wet-lay paper making process in an aqueous suspension, (2) impregnating the felt with a phenolic resin dissolved in a solvent, followed by solvent removal without curing the resin so as to form the prepreg, (3) pressing one or more layers of prepreged felt to obtain both a desired thickness and a desired porosity, at a temperature sufficient to cure the resin, (4) heat treating in an inert atmosphere to between 750-1000 ⁰ C to convert the phenolic resin to carbon and (5) heat treating in an inert atmosphere to between 2000-3000 ⁰ C, which partially graphitizes the carbon, to improve thermal and electrical conductivities and to improve corrosion resistance.

The art as illustrated by the aforementioned patent is technically acceptable, but is unacceptably expensive. About sixteen percent of the cost of the aforementioned process is for impregnating the carbon fiber paper with phenolic resin. Attempts to add powdered phenolic resin into the paper- making process, have not been successful. This is partly due to the high resin content resulting in sticking of the felt to the rolls on the paper-making machine. The powdered resins migrate to the surface of the felt during the drying portion of the paper-making process. This results in a high surface concentration of resin which causes sticking. It is also partly due to the impossibility of removing all of the phenolic powder from the aqueous suspension, which leads to environmental problems.

Summary

The disclosed process provides an electrode substrate at a lower cost, by eliminating the need for a resin impregnation step. The elimination of resin impregnation is predicated on the realization that uncured, cross-linkable resin fibers commingled with carbon fibers will, when cured, cross link to sufficiently bind all of the fibers together, and is thereafter successfully heat treated to form carbon and optionally graphite.

The resin fibers are intermingled with the carbon fibers which immobilizes them during the drying process, thus eliminating the high surface concentration of resin that causes sticking.

The process herein comprises (1 ) creating an aqueous or dry mixture of chopped carbon fibers and chopped cross-linkable resin fibers that are still fuseable after the felt is formed, (2) forming a non-woven felt from an aqueous or air suspension by a non-woven felt forming process, (3) pressing one or more layers of felt to a desired thickness and a desired porosity, at a temperature sufficient to melt, cross-link and cure the resin and (4) heat treating in an inert atmosphere.

The process may be tailored by altering the length of carbon fibers and of the cross-linkable resin fibers to achieve desired properties.

Other improvements, features and advantages will become more apparent in the light of the following detailed description of exemplary embodiments.

Mode(s) of Implementation

The electrode substrate disclosed herein, and the method of making it disclosed herein, may have varying characteristics and be produced from a variety of components and adjustable process steps. The carbon fibers may be conventional, being derived from polyacrylonitrile, rayon, pitch, mesophase-pitch, or phenolic-based carbon fibers, with nominal diameters of between 4 microns and 10 microns, the diameters being selected to tailor the properties of the substrate. The length of the carbon fiber or of the uncured resin fiber may be altered to effect the properties of the finished substrate. Chopped fibers are generally considered to be greater than 1 mm in length while milled fibers are typically less than 1 mm in length. Milled fibers will result in a structure with more isotropic properties whereas those made from chopped fibers will have more anisotropic properties. The fibers may be sized, for instance, to make them easier to disburse in the suspension used in the wet-lay process.

Any of the thermoplastic phenol-formaldehyde resins formed with an excess of phenol in the process are suitable for use in cross-linkable resin fibers herein. In addition to uncured novolac resin fibers, partially cured novolac resin fibers are acceptable as long as they have sufficient flow left to bond the carbon fibers together during the hot pressing step. A truly thermoplastic resin, which is not curable to a semi-solid state, is not acceptable. Uncured, cross-linkable fibers are available from a variety of sources. For instance, uncured novolac fibers are available from Nippon Kynol with nominal diameters of 10-15 microns, although smaller diameter novolac fibers are acceptable. The uncured novolac fibers are typically 1.5 mm - 6 mm long and a mixture of lengths may be used either to support the manufacturing process (easier to disburse in the suspension) or to alter the characteristics of the final product.

The ratio of carbon fibers to uncured novolac fiber is approximately 50:50 with a range of between 35:65 by weight and 65:35 being acceptable for some substrate applications.

Novolac resins are any of the thermoset resins produced by the reaction between phenol and formaldehyde under acidic conditions with the

formaldehyde quantity being insufficient to effect polymer cross-linking. Novolac resins are two-stage thermoset resins which are not seif-reactive, and require a hardener or catalyst for curing, typically hexamethylene tetramine (referred to hereinafter as "hexa"). The novolac resins cross-link and cure in the presence of hexa at a temperature of about 15O ⁰ C to 200°C. Resole resins are one stage phenolic resins formed under alkaline conditions with excess formaldehyde. The residual formaldehyde reacts during the cure to form methylene bridge cross links. Partially cured resole resins and other single-stage phenolic resin fibers may be utilized in the process herein as long as the fibers are not fully cured and have sufficient flow left to bond the carbon fibers during the hot pressing step. Alternatively, carbon or graphite powders may be added to the mixture of carbon fibers, uncured novolac fibers, a temporary binder and the curing agent to effect the properties of the finished substrate. Other cross-linkable resin systems are acceptable. A resin system includes the resin and any commonly used curing agents. Suitable resin systems are epoxies, polyimides, polyesters, phenolics, polyurethanes, melamines and urea formaldehydes.

Novoiac and resole phenolic resins are preferred for their high carbon yield of approximately 50% during the heat-treat process. The ratio of uncured novolac fiber to hexa is typically about between

10:1 and 20:1 (by weight), but may be adjusted to permit melting and flow of the resin before the resin is cured within the selected temperature-time window. For continuous laminating processes, cure times of less than five minutes, and even less than one minute, are desirable. Higher hexa contents and higher cure temperatures are desirable for minimizing the cure time. The resin must cure during the hot pressing step to maintain the desired properties (thickness and porosity) during subsequent heat treating steps.

A temporary binder, such as polyvinyl alcohol, preferably in fiber form but acceptable in powdered form, is added to the mix of fibers and hexa, with a ratio of temporary binder to the sum of the carbon fibers and uncured novolac fibers being approximately 1 :10 (by weight). However, other temporary binders and the concentration of temporary binders may be selected to provide acceptable strength throughout the paper-making process.

The temporary binder must be one which is decomposed or converted to carbon during the heat treating processes

A preferred process includes creating an aqueous suspension of chopped carbon fibers, chopped uncured novolac fibers, a temporary binder such as polyvinyl alcohol fiber or powder, and a curing agent such as powdered hexa. Then forming a non-woven felt from the aqueous suspension by a wet-lay paper making process, de-watering the felt by a combination of removing water by gravity and removing water by vacuum against the wire screen, and drying the felt by heating the felt at a sufficiently low felt temperature, such as between 75°C and 125°C so that the resin does not cross link. Then, one or more layers of the felt are pressed to a desired thickness and a desired porosity at a temperature sufficient to melt and then cross link and cure the resin, such as between 150 ⁰ C and 200 ⁰ C, for a time of between about one minute and about five minutes. For instance, a single layer may provide a thin, highly porous substrate, while several layers may provide a thicker porous substrate or a thin substrate of low porosity, depending on pressure. Thickness and porosity of the substrate are tailored to the intended application, as is known. The substrate is then heat treated in an inert atmosphere to convert the phenolic resin to carbon, at a temperature between about 750°C and 1000°C. The substrate may be further heat treated in an inert atmosphere to partially graphitize the materials, at a temperature between about 1000°C and about 3000 ⁰ C to improve electrical and thermal conductivity and corrosion resistance.

However, a dry process may be used wherein a dry blend of chopped carbon fibers, chopped cross-linkable resin fibers, a temporary binder, such as polyvinyl alcohol powder, or preferably polyvinyl alcohol powder, and a curing agent, such as powdered hexa, is formed into a non-woven felt from a fluidized stream of the dry powder blend in a dry-lay non-woven felt forming process. The further steps of heating the felt at a temperature below a cross- linking temperature, pressing and heat treating follow, as described hereinbefore. In both the wet and dry processes, various cross-linkable resin systems referred to hereinbefore may be used, although novalac and resole phenolic resins are preferred for their high carbon yield of approximately 50% during the heat treating process.