US Patent 7238413 - Porous conductive film formed with carbon microfibers and carbon nanofibers

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The formation of gas diffusion layers for fuel cells requires thin, flexible, strong material that is electrically conductive. This patent discloses forming such a sheet material using both carbon microfibers and carbon nanofibers. Claim 1 reads:

1. A conductive microporous sheet material comprising primary carbon fibers having a cross-sectional dimension of at least 1 micron, secondary carbon fibers in the form of carbon nanofibers and a binding agent for binding said primary and secondary fibers, wherein the binding agent constitutes less than 10 wt % of the sheet material and the binding agent is selected from the group comprising thermoplastic resins and thermosetting resins; and wherein the sheet has a tensile strength of between 0.7 and 1.3 kN/m.