Highly Proton Conductive Phosphoric Acid–Nonionic Surfactant Lyotropic Liquid Crystalline Mesophases and Application in Graphene Optical Modulators

Proton conducting gel electrolytes are very important components of clean energy devices. Phosphoric acid (PA, H₃PO₄·H₂O) is one of the best proton conductors, but needs to be incorporated into some matrix for real device applications, such as into lyotropic liquid crystalline mesophases (LLCMs). Herein, we show that PA and nonionic surfactant (NS, C₁₂H₂₅(OCH₂CH₂)₁₀OH, C₁₂E₁₀) molecules self-assemble into PANS–LLCMs and display high proton conductivity. The content of the PANS–LLCM can be as high 75% H₃PO₄·H₂O and 25% 10-lauryl ether (C₁₂H₂₅(OCH₂CH₂)₁₀OH, C₁₂E₁₀), and the mesophase follows the usual LLC trend, bicontinuous cubic (V₁)–normal hexagonal (H₁)–micelle cubic (I₁), by increasing the PA concentration in the media. The PANS–LLCMs are stable under ambient conditions, as well as at high (up to 130 °C) and low (−100 °C) temperatures with a high proton conductivity, in the range of 10^–2 to 10^–6 S/cm. The mesophase becomes a mesostructured solid with decent proton conductivity below −100 °C. The mesophase can be used in many applications as a proton-conducting media as well as a phosphate source for the synthesis of various metal phosphates. As an application, we demonstrate a graphene-based optical modulator using supercapacitor structure formed by graphene electrodes and a PANS electrolyte. A PANS–LLC electrolyte-based supercapacitor enables efficient optical modulation of graphene electrodes over a range of wavelengths, from 500 nm to 2 μm, under ambient conditions