Method for producing a diffusion layer for an electrochemical device, comprising the following steps: superposing a plurality of unidirectional webs (102a, 102b, 102c) of carbon filaments, the filaments of each web being deposited in parallel next to one another; needle punching the webs, piercing some of the filaments such that pierced portions of said filaments are entangled with other carbon filaments of the webs; cutting out part of the plurality of unidirectional webs, the carbon filaments forming an electrically conductive external surface of the diffusion layer, and in which method the needle-punching step punches right through the thickness of the plurality of webs, and/or through two opposed main faces (116, 118) of the plurality of webs, and/or with a density of impacts with the plurality of webs of between about 100 and 300 impacts/cm
2 .