Structure and Dynamics of the Superprotonic Conductor Caesium Hydrogen Sulfate, CsHSO4

Abstract

We have investigated caesium hydrogen sulfate, CsHSO4, in all three of its ambient pressure phases by total scattering neutron diffraction, inelastic neutron scattering (INS) and Raman spectroscopies and periodic density functional theory calculations. Above 140 °C, CsHSO4 undergoes a phase transition to a superprotonic conductor that has potential application in intermediate temperature fuel cells. Total scattering neutron diffraction data clearly show that all the existing structures of this phase are unable to describe the local structure, because they have either partial occupancies of the atoms and/or non-physical O–H distances. Knowledge of the local structure is crucial because it is this that determines the conduction mechanism. Starting from one of the previous models, we have generated a new structure that has no partial occupancies and reasonable O–H distances. After geometry optimisation, the calculated radial distribution function is in reasonable agreement with the experimental data, as are the calculated and observed INS and Raman spectra. This work is particularly notable in that we have measured INS spectra in the O–H stretch region above room temperature, which is extremely rare. The INS spectra have the enormous advantage that the electrical anharmonicity that complicates the infrared spectra is absent and the stretch modes are plainly seen.

Net Zero and Catalysis: How Neutrons Can Help

“…The interest in these materials is that they undergo a phase transition at ~150 • C to a superprotonic state, where the conductivity increases by five orders of magnitude. The only report (by any method) of the observation of the key O-H stretch modes in the superprotonic phase of CsHSO 4 is by INS [107].…”

“…Reproduced from[101] with permission of The Institute of Physics.ITFC have been much less studied by neutron methods. The QENS and INS spectra of the candidate materials CsH2PO4[102,103] and CsHSO4[104][105][106][107] have been studied. The interest in these materials is that they undergo a phase transition at ~150 °C to a superprotonic state, where the conductivity increases by five orders of magnitude.…”

Boosting Proton Conductivity in Hydrogen-Bonded Organic Layers by Modulating Embedded Guest Molecules

“…Venkataraman’s work on squaric acid derivatives also gives us hints that increasing the spatial resistance of the hydrogen bonding system can promote the conduction of protons. Therefore, we deduce that hydrogen-bond networks composed of strong acid and base pairs may also achieve high proton conductivity even with a larger Δp K a . , Considering the fact that DMSO molecules can block the path of proton diffusion through it, the DMSO molecule in HOL-DMSO plays an important role like the big cations in proton conductive compounds based on oxide acids (CsHSO 4 for example) which could separate the structural protons more effectively and facilate subsequent structure reorganization. − At the same time, KHSO 4 has a significantly smaller conductivity compared to CsHSO 4 at low temperature due to its smaller cation radius, although they exhibit a HSO 4 – composed hydrogen bonding network. Therefore, it is speculated that this special aggregation manner with a separated layer structure greatly reduces the resistance of −SO 3 H-based C 3 -rotation, which mainly contributes to the high proton conductivity of HOL-DMSO .…”