Caesium hydrogen sulfate

Synthesis

CsHSO₄ is synthesized via a 1:1 molar reaction between Cs₂SO₄ and H₂SO₄. Both compounds are dissolved in DI water, and placed in a 60 °C oven with warm air flowing over the top of the mixing beaker. Once the solution is at half of its initial volume it is cooled to 5 °C. At this point there will be four molecules in the solution: Cs₂SO₄, H₂SO₄, CsHSO₄, and H₂O. At 5 °C only CsHSO₄ will precipitate into a crystalline solid. This makes for easy separation by filtration. The remaining wet CsHSO₄ can be dried in a vacuum dedicator.

The same reaction can be done using Cs₂CO₃ and H₂SO₄, but this reaction requires a 1:2 ratio.

Properties

At the critical temperature of 141 °C, CsHSO₄ is considered a superionic conductor.The rapid ionic conductivity arise especially in the range of these temperatures due to the high activity of protons.

CsHSO₄, a solid molecule, goes through three crystalline phases that are referred to as phase III, II, and I. CsHSO₄ is initially existing in phase III at a room temperature of 21 °C. Phase III ranges from 21 °C to 90 °C with a transition temperature of 90 °C to 100 °C between phase III and phase II. Phase II ranges from 90 °C to 140 °C. At 140 °C, CsHSO₄ undergoes a phase shift from phase II to phase I.

Phase III (21 °C to 90 °C) and Phase II (90 °C to 140 °C) are referred to as the monoclinic phases, in which CsHSO₄ 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 CsHSO₄ crystalline structure to allow the displacement of protons.

At 141 °C, the CsHSO₄ 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 CsHSO₄ ten-fold stronger than the conductivity of a sodium chloride aqueous solution. In the superprotonic phase, the movement of an SO₄ 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.

Application & Use as a Proton Conductor

The maximum conductivity of pure CsHSO₄ reaches 10 mS/cm, which is too low for practical applications. To overcome this disadvantage, the addition of oxide materials as supporting material have been applied experimentally. In the case of CsHSO₄/oxide (SiO₂, TiO₂, and Al₂O₃) composites, the proton conductivity below the phase transition temperature was successfully enhanced by a few orders of magnitude compared to pure CsHSO₄.

Unlike low temperature hydrated protonic conductors, the absence of water molecules in CsHSO₄ provides a great amount of stability both thermally and electrochemically. Electromotive force (EMF) measurements in a humidified oxygen concentration cell verified the high ionic nature of CsHSO₄ 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 CsHSO₄ allows for quick transport of smaller charged ions, resulting in efficient energy transfer in electrochemical devices. These properties carries many important utilizations of solid electrolytes in electrochemical field such as hydrogen partial pressure sensors, fuel cells, electrocatalytic reactors, batteries, displays, and super-capacitors.

Further research into solid state proton conductors is crucial towards the development of more efficient and powerful hydrogen partial pressure sensors, displays, supercapacitors, fuel cells, electrocatalytic reactors, and batteries. Solid state proton conductors rely on the movement of protons, which are commonly found in: hydrogen sulfates, hydrates, hydroxides, and polymers, to elicit currents. Solid state proton conductors can function at temperatures within the range of Earth’s natural climate. This makes them more efficient since relatively small amounts of energy are required to keep the conductor at operational temperatures.

Fuel Cells

Due to its ability to maintain high proton conductivity in intermediate temperatures (~200℃), CsHSO₄ is considered to be prime candidate for proton transferring agent in proton exchange membrane fuel cells (PEMFC). Transfer of proton across the catalyst layer is the primary limiting step of the cell reaction and electrolysis, thus the high proton conductivity of CsHSO₄ can contribute greatly to the cell’s performance. CsHSO₄ has advantage over typical low-temperature PEMs, which require water-rich environment limiting them to temperatures below 100 °C. CsHSO₄ combined with polymer has been shown to surpass even some other high-temperature PEMFC systems in performance.

The negative aspects of CsHSO₄ are its high gas permeability that hinders cell reaction and chemical instability under hydrogen-rich atmosphere. Also, CsHSO₄ in its superprotonic phase might be degraded to less proton-conductive Cs₂S₂O₇ and water, which will compromise cell performance.

H2S Electrolysis

The CsHSO₄ cells can be used to electrolyze hydrogen sulfide, which is a common toxic environmental waste. The current method of using the Claus Process produces carbon dioxide byproducts and is considered costly. A 2008 study shows successful electrolysis of H₂S gas using anode catalyst consisted of RuO₂/p-dichlorobenzene/CsHSO₄/Pt black, showing that high-performance CsHSO₄ composite system can be used in efficient decomposition of H₂S. The fact that sulfur has lower melting point (115℃) than phase transition temperature of CsHSO₄ (140℃) can also be an advantage since sulfur can be more easily removed and collected as liquid.