cathode ECSA when the pressure is increased from 1.0 to
3.0 MPa. A further increase to 4.0 MPa lowers the ECSA value,
because a too high pressure may have caused the ionomer to
block some small pores and thus prevent the catalyst particles
inside them from participating in the fuel cell reactions. Hotpressing
under optimal pressure is a vital step to maximize
the three-phase boundary by improving the contact between
the catalyst particles and the ionomer. These results clearly
show that the high performance of the MEA fabricated at
3.0 MPa is a result of the lower charge transfer resistance and
the higher ECSA of its cathode.