Abstract
Hydrophobic ionic liquids (IL) may offer a special electrolyte in the form of supported ionic liquid membranes (SILM) for microbial fuel cells (MFC) due to their advantageous mass transfer characteristics. In this work, the proton and ion transfer properties of SILMs made with IL containing imidazolium cation and [PF6]− and [NTf2]− anions were studied and compared to Nafion. It resulted that both ILs show better proton mass transfer and diffusion coefficient than Nafion. The data implied the presence of water microclusters permeating through [hmim][PF6]-SILM to assist the proton transfer. This mechanism could not be assumed in the case of [NTf2]− containing IL. Ion transport numbers of K+, Na+, and H+ showed that the IL with [PF6]− anion could be beneficial in terms of reducing ion transfer losses in MFCs. Moreover, the conductivity of [bmim][PF6]-SILM at low electrolyte concentration (such as in MFCs) was comparable to Nafion.
Highlights
microbial fuel cells (MFC) belong to the family of bioelectrochemical technologies, where the bioelectrocatalytic activity of exoelectrogenic bacteria (EAB), which are known for their ability to transfer electrons extracellularly to the surface of an electrode, is utilized [1,2]
The supported ionic liquid membranes (SILM) with ionic liquids of different properties, e.g., viscosity (Table 1), were prepared, in which the capillary forces arising in the pores influenced SILM stability and the global mass transfer traits during operation
Coincides well with μH+ measured in water (3.62 × 10−7 m2 V−1 s−1) [30,35]. This finding indicates that water permeates through the IL and the proton transfer through the SILM is mediated by water diffusion
Summary
MFCs belong to the family of bioelectrochemical technologies, where the bioelectrocatalytic activity of exoelectrogenic bacteria (EAB), which are known for their ability to transfer electrons extracellularly to the surface of an electrode, is utilized [1,2]. Ionic liquids (ILs) are often referred to as the green solvents and electrolytes of the future due to their excellent properties, such as negligible vapor pressure, low volatility, non-flammability, excellent thermal stability, flexible solvation features, and tunability by varying the cation/anion pairs [4,5,6,7,8] Among others, they have been successfully used as non-conventional solvents for synthetic, hydrolytic, polymerization, (bio)catalytic, etc. Membranes containing imidazolium-type ILs could be applied effectively in MFCs by fabricating supported or polymer inclusion membranes [17,18,19] It resulted that, compared to Nafion, SILMs with hydrophobic hexafluorophosphate and bis(trifluoromethylsulfonyl)imide ([PF6 ]− and [NTf2 ]− ) anions and 1-butyl- or 1-hexyl-3methylimidazolium ([bmim]+ and [hmim]+ ) cations have beneficial features, including lower acetate mass transfer and diffusion coefficients, as well as lower oxygen permeation (in case of [NTf2 ]− ) [20]. To the best of our knowledge, such an approach to describe the possible proton and ion transfer behavior of SILMs made with these ILs is presented for the first time
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