NOVELTY - Modifying a carbonized substrate (1) involves providing carbonized substrate; providing a mixture of hydrophobic polymer (22) and carbonaceous material (21); applying the mixture onto at least one surface of the substrate; and thermally treating the substrate under the protection of an inert gas. The carbonized substrate is selected from carbon cloth, carbon paper and carbon felt. The mixture is applied onto both surfaces of substrate. The method further involves drying step before thermal treatment step. The inert gas is selected from nitrogen, helium and/or argon.    USE - For modifying a carbonized substrate used in gas diffusion layer of an electrode in a fuel cell (claimed).    ADVANTAGE - The method uses a simpler procedure to modify a carbonized substrate to provide a modified carbonized substrate with good conductivity, air permeability and hydrophobicity. The method provides modified carbonized substrate with desired hydrophobic property and levelness. The fuel cell containing the modified carbonized substrate effectively prevents the environmental problems caused by disposable non-rechargeable batteries; and eliminates the need of time-consuming recharging procedure required for conventional rechargeable battery. The modification process is simple and has reduced cost. The modified carbonized substrate does not only maintain the conductivity of the carbonized substrate due to the carbonaceous material contained in the leveling layer, but also exhibits the desired air permeability and an inherent hydrophobic property due to the thermally treated hydrophobic polymer (which forms a net-like structure having small pores) and hence is particularly suitable for use as a material for an electrode gas diffusion layer in a fuel cell. The fuel cell containing the modified carbonaceous substrate demonstrates a superior fuel cell performance e.g. current density.    DETAILED DESCRIPTION - Modifying a carbonized substrate (1) involves providing a carbonized substrate; providing a mixture of hydrophobic polymer (22) and carbonaceous material (21); applying the mixture onto at least one surface of the substrate; and thermally treating the carbonized substrate under the protection of an inert gas. The carbonized substrate is selected from carbon cloth, carbon paper and carbon felt. The applying step is performed by spraying, screen printing, coating and/or immersing. The mixture is applied onto both surfaces of substrate. The method further involves drying step before thermal treatment step. The inert gas is selected from nitrogen, helium and/or argon. The hydrophobic polymer is polytetrafluoroethylene (PTFE), polyhexafluoropropylene (PHFP), copolymers of hexafluoropropylene and tetrafluoroethylene (FEP), copolymers of tetrafluoroethylene and perfluoropropylvinylether (PFA), copolymers of tetrafluoroethylene and perfluoromethyl vinylether (MFA), homopolymers of chlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), poly(vinylfluoride) (PVF), copolymer of tetrafluoroethylene and ethylene (ETFT), and/or copolymer of vinylidene fluoride and hexafluoropropylene and tetrafluoroethylene (THV). INDEPENDENT CLAIMS are included for the following:    (1) a modified carbonized substrate (10) comprising carbonized substrate; and a leveling layer (2) directly disposed on at least one surface of the carbonized substrate; and    (2) a fuel cell comprising an anode and a cathode, where at least either the anode or the cathode contains the modified carbonized substrate.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of structure of a modified carbonized substrate.    Carbonized substrate (1)    Leveling layer (2)    Modified carbonized substrate (10)    Carbonaceous material (21)    Hydrophobic polymer (22)