The electrochemical energy conversion system include an anode, a cathode, and a proton exchange membrane disposed between the anode and the cathode. The proton exchange membrane includes a polymer having a hard block polymer, a soft block polymer, and one or more hydrophilic functional groups attached to the soft block polymer. The glass transition temperature of the hard block polymer is higher than a glass transition temperature of the soft block polymer, such that the hard block polymer is non-elastic and the soft block polymer is elastic at a desired operating temperature. The hydrophilic functional groups are attached to the soft block polymer via a thiol-ene reaction to modify double bonds in the soft block polymer. The swellable functional groups are selectively connected to the soft domains of the block copolymers, so that when the membrane swells (under hydration or gas adsorption), the stress is effectively absorbed by the soft domain and the impact on overall mechanical properties is minor, resulting in more durable membranes.