Silicon hydrogensulfates: solid acids with exceptional 25 °C conductivities and possible electrochemical device applications

Abstract

Solid acids as a class are of much interest in areas such as acid catalysis, and exotic protonation chemistries. They include the strongest acids yet identified. A subclass exhibits very high protonic conductivity and its members have been investigated as possible fuel cell electrolytes, as first demonstrated by Haile's group in 2001 with CsHSO₄. These superprotonic plastic crystals bring a “true solid state” alternative to polymer electrolytes, operating at mild temperatures (150–200 °C) without the requirement of humidification. However, they suffer from the narrow operating temperature range, and other problems. Here we describe a new class of solid acids based on silicon, which are of general interest. Its members have extraordinary conductivities, as high as 21.5 mS cm⁻¹ at room temperature, orders of magnitude above any previous reported case. We discuss possible applications. Although the present electrolytes are not suitable for H₂/O₂ fuel cell applications due to hydrolizable components, a proof of principle short term fuel cell test is shown to produce a maximum (short circuit) current of 640 mA cm⁻² at 87 °C despite a low open circuit voltage.