A First-principles Study On Structure And Energetics For Additional H In Superionic Phase Of CsHSO₄

During the past decades, new energy forms and sources have been seeking because of the crises of energy and ecology on the earth. Fuel cell, the new energyform attracts more and more interests in the last few years because it can be taken as a new way to deal with the crises of energy and ecology. As for the H-O fuel cell, the key part is membrane electrode, which conducts protons as an electrolyte.In the past few years the proton conductors arose a great interest because of their applications in fuel cell. At relatively low temperatures (below 370 K), proton exchange membrane (PEM) can be applied for the electrolyte. At high temperature (-800 K-1200 K), some perovskite-type oxides exhibit good proton conductivity and can be used for this purpose. At intermediate operating temperature(from -370 to 800 K), however, there is very few candidates for a solid electrolyte. CsHSO₄ is a potential candidate since it exhibits extremely high protonconductivity above 415 K.Above 415 K, the CsHSO₄ transforms into the phase with a tetragonal structure,which is called superionic phase. It shows the extremely high proton conductivity(the order of 109 s^-1), which is 3-4 orders of magnitude greater than that in room temperature.Experimentally, proton conductivity in CsHSO₄ has been extensively studied.Theoretically, only a few calculations have been devoted to the microscopic mechanism of proton conduction in these materials. So far, very few first-principles calculations have been reported.In this paper, using the first principle theory, we have studied the structure of CsHSO₄ in superionic phase. Then we focused on the stability of a H-added CsHSO₄ . In our calculation we found the stable structure of CsHSO₄ in superi- onic phase. As a nondefective neutral crystal, the proton prefer to locate in the 16f site than any of other sites. The calculated results show that the energy of this structure is much lower than that of any other structures. It is no doubt that the structure with all H in 16f state is what we want to find. The structure is a ground state structure. It is an average structure or the most frequent structure due to the environment in which the material operates. As it is above 415 K, the H can reach the local minimum nearby easily.When the energetics of an additional H absorbed into the CsHSO₄ was studied,different charged states were taken into account. The charged states were calculated by using neutral(0), negative charged (+e), and positive charged (-e) supercells. we found the SO₄ tetrahedron is broken when the supercell is optimizedin either the neutral or the negative charged state. The calculated formationenergy for an additional neutral H in CsHSO₄ indicates that this H can not be absorbed into CSHSO4. In the positive charged state, the calculated formation energy indicates that many additional protons may be absorbed into CsHSO₄ to form a high chemical potential on one side. The charge for an additional H absorbedinto the CsHSO₄ in the different charged states has been calculated. The results show that the H in the positive charged state can be considered as a "proton",and the charge for the H in the negative charged state is almost equal to that for the H in the neutral state. The neutral or negative H absorbed into CsHSO₄ only gives local effect, but the positive H (proton) can affect the charges of H far from the local area.