Simultaneous in situ unzipping and sulfonation of multi-walled carbon nanotubes (MWCNTs) using potassium sulfate (K2SO4) and sodium dodecyl benzene sulfonate (SDBS) by a hydrothermal synthetic route is carried out to prepare sulfonated graphene nanoribbons (sGNR) as confirmed by various characterization techniques. Further, nanocomposite polymer electrolyte membranes of this with, sulfonated polyether ether ketone (SPEEK) show enhanced ion exchange capacity (IEC), proton conductivity and water uptake compared to that of pristine SPEEK membrane. Higher mechanical stability for these composite membranes is observed in comparison with pristine SPEEK membrane. Interestingly, these SPEEK/sGNR composite electrolyte membranes (0.1 wt% sGNR) while testing in a proton exchange membrane fuel cell (PEMFCs) test-bed, shows a current density of 840 mA cm(-2) at 0.6 V (peak power density of 660 mW cm(-2)) compared to the current density of 480 mA cm(-2) at 0.6 V (peak power density of 331 mW cm(-2)) for pristine SPEEK. The accelerated durability test for the membranes confirms that composite membranes of SPEEK/sGNR are highly durable even after 200 h with marginal drop in OCV with negligible fuel cross-over up to 175 h to suggest its potential applications in slew of future technologies including polymer electrolyte fuel cells, water electrolyzers and electrochemical sensors. (C) 2016 Elsevier B.V. All rights reserved.