Caesium bisulfate or caesium hydrogen sulfate is an inorganic compound with the formula CsHSO<sub>4</sub>. The caesium salt of bisulfate, it is a colorless solid obtained by combining Cs<sub>2</sub>SO<sub>4</sub> and H<sub>2</sub>SO<sub>4</sub>.
Above 141 ðC, CsHSO<sub>4</sub> is a superionic conductor. The rapid ionic conductivity arise especially in the range of these temperatures due to the high activity of protons.
Based on the results of X-ray crystallography, the structure consists of tetrahedral sulfate centers that bridge caesium ions. The proton is associated with the oxygen on sulfate.
CsHSO<sub>4</sub> goes through three crystalline phases that are referred to as phase III, II, and I. CsHSO<sub>4</sub> is initially existing in phase III at a room temperature of 21 ðC. Phase III ranges from 21 ðC to 90 ðC with a transition temperature of 90 ðC to 100 ðC between phase III and phase II. Phase II ranges from 90 ðC to 140 ðC. At 140 ðC, CsHSO<sub>4</sub> undergoes a phase shift from phase II to phase I.
Phase III (21 ðC to 90 ðC) and Phase II (90 ðC to 140 ðC) are referred to as the monoclinic phases, in which CsHSO<sub>4</sub> exhibits its lowest proton conductivity. As the crystalline structure's temperature is raised, it will show variations in the unit cell volume and the arrangement of its hydrogen bonds, which will alter the ability of a CsHSO<sub>4</sub> crystalline structure to allow the displacement of protons.
At 141 ðC, the CsHSO<sub>4</sub> crystal structure experiences a structural change from monoclinic phase II to a tetragonal phase, becoming phase I. Phase I has more elevated crystal symmetry and widened lattice dimensions. Phase I is noted as the superprotonic phase (strong conducting phase), which triggers an extreme growth in proton conductivity by four orders of magnitude, reaching 10 mS/cm. This makes the conductivity of CsHSO<sub>4</sub> ten-fold stronger than the conductivity of a sodium chloride aqueous solution. In the superprotonic phase, the movement of an SO<sub>4</sub> tetrahedron generates a disruption of the hydrogen bond network, which accelerates proton transfer. The tetragonal anions available in the structure are accountable for the arrangement of the hydrogen bonds with the moving protons.
The maximum conductivity of pure CsHSO<sub>4</sub> is 10 mS/cm, which is too low for practical applications. In composites with SiO<sub>2</sub>, TiO<sub>2</sub>, and Al<sub>2</sub>O<sub>3</sub>), the proton conductivity below the phase transition temperature is enhanced by a few orders of magnitude.
Unlike hydrated protonic conductors, the absence of water in CsHSO<sub>4</sub> provides thermal and electrochemical stability. Electromotive force (EMF) measurements in a humidified oxygen concentration cell verified the high ionic nature of CsHSO<sub>4</sub> in its superprotonic phase. Based on heat rotation, the voltage stayed the same for over 85 hours during the measurement, particularly at the high temperature. These results, demonstrate the thermal independence from humidity-type environments. Additionally, the crystal structure of CsHSO<sub>4</sub> allows for quick transport of smaller charged ions, resulting in efficient energy transfer in electrochemical devices.