On the efficiency of dual-chamber biocatalytic electrochemical cells applying membrane separators prepared with imidazolium-type ionic liquids containing [NTf₂]⁻ and [PF₆]⁻ anions

In this study, the dependency of energy recovery on separator characteristics applied in microbial fuel cells (MFCs) was sought by testing an emerging class of membranes (supported ionic liquid membranes (SILMs), prepared with [hmim][PF₆] and [bmim][NTf₂] ionic liquids) comparatively with well-known proton exchange (Nafion N115) and microfiltration (PVDF) counterparts. Crucial membrane features such as O₂ and substrate (acetate as the sole carbon source) crossovers were assessed and as a result, mass transfer as well as diffusivity coefficients of these compounds (k_O, k_A, D_O, D_A, respectively) were determined. The experiments showed that SILM-operated MFCs could work in a reliable way and among them, the [bmim][NTf₂]-based one produced higher specific energy yield (Y_S = 9.78 kJ g⁻¹_CODin m⁻²) than the Nafion-MFC (Y_S = 8.25 kJ g⁻¹_CODin m⁻²) used as an important reference. This outcome was found to be associated with the membrane-cross oxygen shuttle properties of the membranes (k_O = 1.25 cm s⁻¹ and 1.31 cm s⁻¹, respectively). As for the two SILMs, significant differences in terms of the energy yield, mass transfer and diffusion coefficients were noted, however, it has appeared from cell polarization measurements that the internal resistances of the SILM-MFCs were nearly the same. The evaluation of the SILM-operated MFCs’ power production was complemented by measuring the dielectric traits of ionic liquids that can be related with the ion conductivity of these materials. It turned out that the [hmim][PF₆] IL had an order of magnitude lower ionic conductivity.