Two acidic coordination polymers containing uncoordinated carboxyl groups: Syntheses, crystal structures and proton conductivities in Nafion composite membranes

Highlights: • Two acidic CPs containing uncoordinated carboxyl groups have been synthesized and characterized. • Because of acidity, the two CPs exhibited good proton conducting behavior evaluated by AC impedance spectra. • Compound 2 exhibited higher proton conductivity because of the continuous hydrogen bonds in its crystal structure. Two coordination polymers [Cu(H₂L)(bpyBr)]_n (1) and [Ni(H₂L)(bpy)(H₂O)₂]_n (2) (H₄L=5,5'-(butane-1,4-diylbis (oxy)) diisophthalic acid, bpyBr=4,4′-dibromo −2, 2′-bipyridyl, bpy=4, 4′-bipyridyl) were synthesized at lower temperature and weaker acidity condition. Crystal structure studies revealed that both compounds exhibited 2-D coordination layer with uncoordinated protonated –COOH groups, which can promote the acidity of compounds, form hydrogen bonds and benefit to proton conducting behavior. So, their proton conductivities have been evaluated through incorporating with Nafion to form composite membrane. AC impedance spectra studies suggested that compounds 1–2 can promote the proton conductivity of the composite membrane about 15.97% and 24.61% higher than that of pure Nafion membrane, respectively. The higher proton conductivity of 2/Nafion should be attributed to the continuous hydrogen bonding chains in the crystal structure.