A novel aromatic graft polymer was synthesized by grafting highly sulfonated poly(phenylene sulfide sulfone) side chain to poly(arylene ether sulfone) main chain as a proton exchange membrane (PEM) for proton exchange membrane water electrolysis (PEMWE). Facile control over ion exchange capacity and hydrophilic domain morphology was achieved by varying the side chain molecular weight and grafting density. The hydrophilic-hydrophobic phase separation of graft polymer induced an optimized morphology of narrow, co-continuous channels of small hydrophilic ion domains for PEMWE, which was observed in transmission electron microscopy. The water-bearing hydrophilic channels function as the proton transport pathway and the hydrogen permeation pathway, in which the hydrogen can permeate with lower activation energy, making the morphology of hydrophilic channels, regarding the domain size, tortuosity, and dead-end fraction, the most critical factor in PEM. Balancing the trade-off between high conductivity and low gas permeability, in addition to maintaining durable chemical, mechanical, dimensional, and thermal stability are required under PEMWE operating condition where the membrane is in contact with liquid water at high temperature. The graft membranes showed durable dimensional and mechanical stabilities under the PEMWE-simulated condition. Furthermore, the unique morphology of graft polymer induced low hydrogen permeability owing to the small domain size, while maintaining high proton conductivity from the well-connected hydrophilic channels, balancing the trade-off between hydrogen permeability and proton conductivity while restricting excessive dimensional swelling. As a result, LG-1.95, showed an outstanding performance of 4009 mA cm<sup>-2</sup> at 1.8 V and long-term durability result, making it a promising candidate for PEMWE.

Synthesis of novel aromatic graft polymer with an optimized ion domain morphology for proton exchange membrane water electrolysis

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