TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The carbon substrate has a density gradient in the thickness direction of the carbon substrate that decreases toward the second surface from the first surface of the carbon substrate. The first and second carbon fibers have a length of 3-25 mm, where a length difference between the first and second carbon fibers is 3-20 mm. A large pore having a width of 300-900 mu m, and a medium pore or small pore having a width of 10 mu m to less than 300 mu m are co-present in the second surface, and a medium or small pore having a width of 10-100 mu m is uniformly distributed in the first surface. The microporous layer has a porosity gradient in the thickness direction of the carbon substrate so that the pore size increases toward the second surface from the first surface. The microporous layer comprises carbon powder, thermoexpandable graphite, and a fluorine-based binder fused between the carbon powder and the thermoexpandable graphite. An amount of the thermoexpandable graphite increases toward the second surface from the first surface in the thickness direction of the carbon substrate, where the microporous layer comprises a sequential stack structure of a first microporous layer and a second microporous layer, and an amount of the thermoexpandable graphite in the first microporous layer is larger than that in the second microporous layer. The amount of the thermoexpandable graphite in the first microporous layer is 5-20 pts. wt. based on 100 parts by weight of a total weight of the carbon powder and thermoexpandable graphite. The amount of the thermoexpandable graphite in the second microporous layer is 5-10 pts. wt. based on 100 parts by weight of a total weight of the carbon powder and thermoexpandable graphite. The carbon substrate is porous and comprises many carbon fibers randomly disposed to form a non-woven fabric. An amount of the thermoexpandable graphite decreases in the thickness direction of the microporous layer toward the surface of the microporous layer opposite to and away from the surface adjacent to the carbon substrate. The microporous layer comprises a sequential stack structure of a first microporous layer and a second microporous layer, and an amount of the thermoexpandable graphite in the first microporous layer is larger than that in the second microporous layer. EXAMPLE - 70 g Carbon fiber having a carbon content of 95%, a diameter of 7 mu m, and a fiber length of 6 mu m, 30 g carbon fiber having a carbon content of 95%, a diameter of 7 mu m, and a fiber length of 12 mu m, 1 g TRITON X-100 (RTM: Nonionic, octylphenol ethoxylate surfactant), and 5 g polyvinyl alcohol staple fiber having a fiber length of 6 mu m were put into 200 l water, and mechanically mixed at a rate of 1500 rpm for 20 minutes to obtain a homogeneous dispersion for forming a carbon fiber pre-web. The dispersion was put into an agitator. 300 l Water was further added and stirred. The dispersion was poured into the inclined Fourdrinier-type paper-making machine equipped with a 120# wire mesh, followed by pumping with a vacuum pump to remove water from the dispersion through the wire mesh to deposit carbon fibers on the wire mesh, to form a carbon fiber pre-web in the form of a non-woven fabric. The carbon fiber pre-web was impregnated with a slurry (a weight ratio of a phenol resin to graphite particles of 50:50, a total solid content of 20 wt.%) of graphite particles dispersed in a phenol resin solution (a weight average molecular weight of 3000-5000, and N-methyl-2-pyrrolidone used as a solvent) to include 3 mg/cm2 of the slurry. The carbon fiber pre-web impregnated with the phenol resin and electroconductive graphite particles was heated at 1200-1600 degrees C in a nitrogen atmosphere to carbonize the phenol resin and PVA, to obtain a carbon fiber web including a carbide binding the carbon fibers. A PTFE emulsion as a fluorine-based resin having an average particle diameter of 0.2 mu m was dispersed in water to prepare a fluorine-based resin emulsion having a solid content of 10 wt.%. The carbon fiber web was impregnated with the fluorine-based resin emulsion, followed by removing the fluorine-based resin emulsion from a surface of the carbon fiber web, drying at 120 degrees C, and further thermal treatment in a 350 degrees C oven for 30 minutes, to obtain a hydrophobicized carbon fiber substrate.