The Role of Superadsorbent Polymers on Covalent Organic Frameworks-Based Solid Electrolytes: Investigation of the Ionic Conductivity and Relaxation

Abstract

The scarcity of fossil fuels calls for immediate action toward the development of clean and renewable energy resources. In this context, proton exchange membrane fuel cells (PEMFCs) are gaining ever-increasing attention as clean technology. Although covalent organic frameworks (COFs) do not usually exhibit high intrinsic proton conductivity (σ), they have been recently proposed as solid polymer electrolytes in PEMFCs, thanks to their high crystallinity and stability to acids and bases. Here, a simple strategy is presented to improve the performance of poor COF-based proton conductors through addition of sodium polyacrylate (PANa) superadsorbent polymer. Electrochemical impedance spectroscopy investigations after activation at high temperature and relative humidity (RH) provide insights into the role of PANa, whose presence is key to preserve high σ at low RH. The humidity-dependent X-ray diffraction study reveals a strengthening of the stacking interaction along the COF (100) plane direction with increasing humidity, through the formation of H-bonding, thus promoting proton hopping. The study of the dielectric properties as a function of PANa content enables to determine a Debye relaxation regime for the COF/PANa blend with a maximum relaxation frequency of 1513 and 6606 Hz for the pristine COF and the COF/PANa blend, respectively, at their maximum operating temperatures.