The need for developing more sustainable electrochemical energy storage and conversion devices prompted us to develop novel bio-based membranes based on poly(lactic acid) (PLA) and imidazolium ionic liquids (ILs) to act as an electrolyte in energy conversion devices, such as fuel cells. PLA membranes based on [C 4 C 1 im][PF 6 ] and [C 4 C 1 im][BF 4 ] present different morphologies according to the anion nature and high thermal stability (>300 °C). Scanning electron microscopy and energy-dispersive spectroscopy analyses show that [C 4 C 1 im][BF 4 ] tends to be isolated inside small pore cavities, which prevents the IL from leaching out of the membranes. Owing to this improved retention capacity of [C 4 C 1 im][BF 4 ] and to the hydrophilic character of the IL anion, the PLA−[C 4 C 1 im][BF 4 ] blends present conductivity values higher than 0.01 S cm −1 at 98% RH and 94 °C. The work further demonstrates the operation of a PLA−[C 4 C 1 im][BF 4 ]-based oxygen/hydrogen fuel cell with an open-circuit voltage above 0.9 V.