Hyperbranched polyester and its sodium titanate nanocomposites as proton exchange membranes for fuel cells

Abstract

A sulfonated hyperbranched polyester (HPES-SO₃H) was synthesized in order to prepare a proton exchange membrane. The modified hyperbranched polymer was characterized using ¹H and ¹³C NMR and GPC. Hybrids of the modified polymer and polyether sulfone were prepared to incorporate sufficient entanglements and hence increase the mechanical properties of HPES-SO₃H. The morphology of the films cast from the hybrids was studied using scanning electron microscopy (SEM), which confirmed the presence of a smooth surface with a controlled pore size. In order to enhance the proton conductivity and improve the mechanical properties of the obtained membrane, different loadings of sodium titanate nanowires (ST NWs), prepared hydrothermally, were added as fillers for the polymer hybrids. Membranes cast from pure polymer hybrids or nanocomposites were used to fabricate a membrane electrode assembly (MEA). It was found that a membrane composed of hyperbranched polyester with a 50% degree of sulfonation and immersed in H₂SO₄ for 2 h, had an open circuit potential of 0.9 V, compared with Nafion® which showed a potential of 0.85 V. Increasing the immersion time in H₂SO₄ solution to 12 h led to a sharp increase in current density to a value of 200 mA cm⁻² and a power density value of 35 mW cm⁻². Moreover, the results reveal that the addition of different amounts of ceramic material to the polymer hybrids has a negative influence on the membrane performance. Nevertheless, high proton conductivity was noticed in samples containing 1 wt% ST NWs. The good water swelling and mechanical properties of the obtained sulfonated polymeric hybrids, along with their cell performance being comparable to commercial membranes, make them promising candidates as proton exchange membranes for fuel cells.