Enhanced proton conductivity and stability of polybenzimidazole membranes at low phosphoric acid doping levels via constructing efficient proton transport pathways with ionic liquids and carbon nanotubes

Polybenzimidazole (PBI) membrane doped with phosphoric acid (PA) usually requires the high PA doping level (ADL) to ensure excellent proton conductivity, however, the loss of PA can lead to dramatical attenuation of fuel cell performance. Herein, we develop a facile way to enhance the proton conductivity and stability of poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole (OPBI) membranes at much low ADL (about 80% of OPBI membrane) via construction of proton transport pathways between ionic liquids (ILs), carbon nanotubes (CNTs) and PA. Owing to the anionic and cationic structures of ILs and the adsorption of CNTs on PA, abundant proton transport pathways can be constructed to guarantee superior proton conductivity and durability of hybrid OPBI membrane (ILs/CNTs/OPBI) with low ADL. The as-prepared 10%-ILs/NH2-CNTs/OPBI hybrid membrane exhibits the highest proton conductivity (130.8 mS cm−1) and stability (above 119 mS cm−1 after 228 h test) at 160 °C, much better than other ILs/CNTs/OPBI hybrid membranes and OPBI membranes. More importantly, a fuel cell based on ILs/NH2-CNTs/OPBI membrane achieves the highest power density of 508 mW cm−2 at 160 °C, 2.7 times higher than that of OPBI membranes (189 mW cm−2). Good durability of a single cell with ILs/NH2-CNTs/OPBI membranes is also confirmed in steady-state and restart tests.