Self-Standing, Robust Membranes Made of Cellulose Nanocrystals (CNCs) and a Protic Ionic Liquid: Toward Sustainable Electrolytes for Fuel Cells

Abstract

Energy-conversion devices based on the phenomenon of proton conduction, for example, polymer electrolyte membrane fuel cells (PEMFCs), require low cost and sustainable electrolytes with high ionic conductivity and good mechanical properties under anhydrous conditions and at temperatures up to 150 °C. Biopolymers possess an intrinsic thermomechanical stability but an insufficient proton conductivity in the dry state, which however may be imparted by a protic ionic liquid (PIL). This work presents the preparation and properties of composite membranes made of cellulose nanocrystals (CNCs) and a PIL. The membranes are thermally stable and display an ionic conductivity within the range 10–4–10–3 S/cm for temperatures between 120 and 160 °C. Moreover, the analysis of the biopolymer’s apparent dimensions at nanoscale reveals a dependence of the CNCs’ defects, twisting, and aggregation in the presence of the PIL. Preliminary tests using a simple fuel cell setup demonstrate a response of the membranes to the inlet of H2 gas, with a generation of electrical current. These findings provide a solid groundwork for further development and future studies of biopolymer/PIL electrolytes for energy applications.

Thermal and Electrical Properties of High-Temperature Proton-Conducting Cellulose Nanofibers Swollen with Protic Ionic Liquid

“…Aprotic ionic liquids (APILs) to prepare these materials are used. , We believe using PILs instead of APILs will be a better solution to obtain electrolytes showing high proton conductivity because PILs have exchangeable, mobile protons. Danyliv et al reported the first results for CNC-based composites with PILs. The composites showed good thermal stability up to 200 °C and conductivity values from 10 –2 to 10 –1 S/m under anhydrous conditions in the range of 120–160 °C.…”

Tunable Gas Permeation Behavior in Self-Standing Cellulose Nanocrystal-Based Membranes

“…20 The possibility to obtain a selfstanding and robust membrane even of the rigid CNCs in combination with an IL was demonstrated by Danyliv et al, who used such a membrane as a fuel cell polymer electrolyte. 21 To the best of our knowledge, gas permeation studies have not been conducted on CNC/IL blends.…”

“…A CNF film plasticized by the ionic liquid (IL) 1-ethyl-3methylimidazolium acetate showed excellent CO 2 selectivities and good permeability at the same time, illustrating the potential of such blends . The possibility to obtain a self-standing and robust membrane even of the rigid CNCs in combination with an IL was demonstrated by Danyliv et al, who used such a membrane as a fuel cell polymer electrolyte . To the best of our knowledge, gas permeation studies have not been conducted on CNC/IL blends.…”

Ionic Liquid–Poly(lactic acid) Blends as Green Polymer Electrolyte Membranes

“…Therefore, continuous research efforts have been made to develop cost-effective and eco-friendly PEMFCs from renewable sources. Natural-based polymers such as chitosan, alginate, carrageenan, starch, or cellulose are being investigated as “green” materials for fuel cells, owing to their biodegradability, nontoxicity, and low-cost. − With the significant exception of cellulose, most of these biopolymers (e.g., the widely reported chitosan or carrageenan) exhibit high water solubility and low mechanical strength, − obvious handicaps for their application in PEMFCs. An even greater handicap is the low ionic conductivity of these biopolymers.…”

“…An even greater handicap is the low ionic conductivity of these biopolymers. Among various strategies to improve the conductivity of these host biopolymers, the combination with a conducting ionic liquid (IL) into composite membranes has attracted considerable attention in the last years. − However, the application of these composites in fuel cells is still limited due to the losses of the unbound IL during cell operation …”