Enhanced electrochemical hydrogen compression performance with a gradient water-retaining hybrid membrane

[Display omitted] •Water-retaining PCTS was synthesized via end-functionalized grafting of PBTCA and CTS.•MEAs with the hybrid membrane show improved H+ conductivity and I-V performance.•MEAs with the gradient hybrid membrane exhibit the best H2 compression performance.•PCTS additives can effectively decrease H2 crossover at the membrane. Stored hydrogen has become increasingly important as an efficient and clean energy source. Noiseless and highly efficient electrochemical hydrogen compressors (EHCs) have been employed in H2 fueling stations, refrigeration, and hydrogen purification. However, unequal water distribution in EHCs during operation significantly affects the compression performance. Herein, we developed a gradient hybrid membrane (PFSA/PCTS-5 + 2.5 + 1 %) for use in EHCs by synthesizing phosphorylated chitosan (PCTS) using chitosan, 2-phosphonate butane-1, 2, 4-tricarboxylic acid, and other water-retaining components. Advanced physical characterizations were employed to analyze the morphology, chemical structure, proton conductivity, water uptake ratio, hydrophilicity, thermal stability, and mechanical properties of the membranes. Furthermore, the membrane electrode assemblies (MEAs) fabricated with PFSA/PCTS hybrid membranes exhibited significant performance enhancement at a 50 % RH. The PFSA/PCTS-5 + 2.5 + 1 %-based MEA yields an EHC that can compress H2 up to 0.9 MPa in 419 s (at 50 % RH) and up to 1.0 MPa in 137 s (at 100 % RH) in compression testing. EIS analysis reveals that the ohmic resistance decreased markedly due to gradient hybrid structure of membrane. Such a remarkable performance is attributed to the gradient structure, high hydrophilicity of the functional groups (–OH, –NH2), and good proton conductivity arising from the synergetic effect of phosphate groups and carboxyl groups.