The role of the CL is to act as a porous medium to allow for ideal gas transport and water management. The electron and mass transport rates in the MEA are strongly influenced by the CL microstructure. The fabrication of CLs begins with the addition of carbon supported catalyst powders and an ionomer to a water-alcohol solution, to form a catalyst ink. The ink is then deposited onto an appropriate substrate and dried to obtain an electrode. The morphology of the electrode can be controlled by altering the composition of the various components of the catalyst ink. The morphology of the CL is determined by : (i) ionomer conformation, (ii) aggregate size and distribution, (iii) hydrophobicity of the electrode, (iv) presence of free ionomer and (v) pore distribution .