Start-up of a proton exchange membrane (PEM) fuel cell from subzero temperatures, commonly referred to as cold start, remains a major challenge for automotive applications. In this work, we theoretically and experimentally study the effect of catalyst layer (CL) pore volume (or, more directly, CL thickness) on the cold-start performance of a PEM fuel cell for both isothermal and nonisothermal operations. Special attention is directed to determining the limits of a cold-start performance with an ultrathin CL (1 mu m). The cold-start product water or the operational time approaches a minimum nonzero asymptote as the CL is gradually made infinitesimally thin. For a PEM fuel cell with standard cell thermal mass, e.g., 0.4 J/cm(2) K, and with moderately low initial membrane water content (lambda(0)=7), successful start-up from -20 degrees C at 100 mA/cm(2) can be achieved for CL thicknesses of 10 mu m and above, whereas a CL thickness of 20 mu m is required for successful self-start-up from -30 degrees C. However, successful start-up can be achieved even with a 1 mu m thick CL, given certain adjustments to cell design and material properties. In particular, we study the effects of cell thermal mass and membrane water diffusivity and present a design map for self-start-up of a 1 mu m CL PEM fuel cell from various subfreezing temperatures.