However, sulfonated copolymers PEMs have some shortcomings in that they have relatively lower proton conductivity than PFSA membranes. In general, proton conductivities tend to increase with an increment in the IECs in membrane. Methanol permeability, on the other hand, has also been increased by enhancing IECs. As a result, it has been difficult to obtain PEMs that have both high proton conductivity and low methanol permeability by adjusting IECs.