Evidence for a Structural Transition to a Superprotonic CsH 2 PO 4 Phase Under High Pressure
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0929-II02-01
Evidence for a Structural Transition to a Superprotonic CsH2PO4 Phase Under High Pressure Cristian E. Botez1, Russell R. Chianelli2, Jianzhong Zhang3, Jiang Qian3, Yusheng Zhao3, Juraj Majzlan4, and Cristian Pantea5 1 Department of Physics, University of Texas at El Paso, El Paso, Texas, 79968 2 Department of Chemistry and Materials Research Institute, University of Texas at El Paso, El Paso, Texas, 79968 3 Los Alamos Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545 4 Institute of Mineralogy and Geochemistry, University of Freiburg, Freiburg, 79104, Germany 5 Materials Science and Technology - National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545
ABSTRACT We have used synchrotron X-ray powder diffraction (SXRPD) to investigate the structural behavior of CsH2PO4 upon heating. Temperature-resolved data collected at ambientpressure demonstrate that a transition from the room-temperature monoclinic phase (P21/m; a=7.90Å, b=6.39Å, c=4.87Å, and β=107.64º) to a high-temperature cubic phase (Pm3m; a=4.96Å) occurs at T=237°C. The high-temperature phase is not stable under ambient-pressure conditions, even in the absence of further heating. On the other hand, SXRPD measurements carried out under high-pressure (~1GPa) evidence a transition from monoclinic to a stable cubic phase (Pm3m, a=4.88Å) at a temperature within the 255°C-275°C range. A 1000-fold increase in the proton conductivity (indicating the transition to the superprotonic phase) was previously observed under the same non-ambient conditions [1]. Therefore, our results represent strong evidence that the superprotonic behavior in CsH2PO4 is associated with a monoclinic-to-cubic polymorphic structural transition and not with chemical modifications.
INTRODUCTION In the quest for alternative sources of energy fuel cell research plays a prominent role, as fuel cells hold great potential for a highly efficient and environmentally friendly way of producing electrical energy. The functioning of any fuel cell is based on the electrolyte’s ability to conduct ions between the electrodes, but not allow electrons to pass through. It has been recently shown that MHnXO4-type solid acids (M – monovalent cation, n=1,2 and X=S,P) can function as fuel cell electrolytes at temperatures between 150 and 300ºC [2,3]. Since polymer electrolyte fuel cells cannot operate within this temperature range, solid acids seem to be promising candidates for fuel cell applications where cell functioning at intermediate temperatures is desired (e.g. in the automobile industry). To function as fuel cell electrolytes, solid acids need to undergo a so-called superprotonic phase transition, where, upon heating, their proton conductivity sharply increases by up to four orders of magnitude.
CsHSO4 was the first solid acid used as a fuel cell electrolyte [2], but it was soon noticed that phosphate-based compounds (such as CsH2PO4 and RbH2PO4) would yield better and longerterm cell stability, in part by e
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