Characterization of Solid Acid Electrolyte CsH₅(PO₄)₂by NMR Spectroscopy

Abstract

The electrolyte is one of the most important constituents of fuel cells (FCs), and determines the application areas, the performance, and the operation temperature range. The high proton conductivities of solid acids in specific temperature ranges have made them as excellent candidates for FC electrolytes.1 Solid acids are both an acid and salt in a solid state, and can generally be described as M a H b (XO 4 ) c , where M is Cs, Rb, K, Na, or NH 4 and X is P, S, As, or Se. Ever since CsHSO 4 (CHS) electrolyte was demonstrated for FCs by Halie et al.1 various solid acid electrolytes for FCs have been reported.2,3 Even better performance than that with CHS was reported with CsH 2 PO 4 (CDP) only, or with CDP/ Si-oxide composites in humidified conditions.2,3 All of the CDP in the CDP/SiP 2 O 7 composite electrolyte was observed by X-ray diffraction (XRD) to convert to CsH 5 (PO 4 ) 2 (CPDP) at 220°C and higher, indicating that the good conductivity of this composite electrolyte over 150-250°C was mainly from CPDP, not CDP. 3Solid acid electrolytes have been characterized primarily by XRD. However, solid-state nuclear magnetic resonance spectroscopy (SS-NMR) can be more powerful. SS-NMR can be used to directly observe nuclei, detect light atoms and phases without a long-range order, and provide dynamics information such as ionic movement and molecular motion in more detail. Two hydrogens in CDP were identified by 1 H magic angle spinning (MAS) NMR spectroscopy. Cs MAS NMR spectra of the mixture of CDP and CPDP were reported.7 The peaks corresponding to each compound were assigned, but the 1 H peaks of the mixture of CDP and CPDP were not clearly assigned due to the possible overlapping of the signals for CPDP and CDP.7 In this work, the 1 H, 31 P, and 133Cs MAS NMR spectra of pure CPDP with clear peak assignments are presented, and the confirmation of the purity of CPDP is demonstrated, which is not possible with XRD techniques.Since the stoichiometric mole ratio of H 3 PO 4 and Cs 2 CO 3 for CPDP synthesis is 4:1, as described by 4H 3 PO 4 + Cs 2 CO 3 → 2CsH 5 (PO 4 ) 2 + H 2 O + CO 2↑ , CPDP was prepared as follows. First, 16 mmol H 3 PO 4 (85 wt %, d = 1.685 g/mL, Sigma Aldrich, USA) was placed into a 50-mL beaker, and 4 mmol Cs 2 CO 3 (99.995%, Sigma Aldrich) was added. All of the reagents were weighed for accurate stoichiometric matching. The reagent mixture slurry was stirred well with a glass bar prior to carefully adding 1 mL of distilled water into the slurry to dissolve all the Cs 2 CO 3 , and stirred more until the solution became transparent. The transparent solution was transferred to a clean beaker and kept at 100°C for 24 h. The synthesized CPDP powder was identified with X'Pert PRO-MPD XRD (Philips, Netherland). All NMR spectra were obtained on a 14.1 T unity INOVA NMR spectrometer (Varian, USA) using a MAS probe for 2.5-mm zirconia rotors spun at 20 kHz for Cs MAS NMR spectra of CsH5(PO4)2.

Electrical Behavior and Stability of K2HPO4-KH5(PO4)2-Ce0.9Gd0.1P2O7Composite Electrolytes for Intermediate Temperature Proton-Conducting Fuel Cells

“…The calcined powder was grinded with mortar and pestle, sieved through a 76 μm mesh test sieve, and the obtained powder was used for further characterization. The CGP substrates were prepared by molding the powder into disks 23 Briefly, H 3 PO 4 (85%, Dae Jung) was slowly added to K 2 CO 3 (99%, Alfa Aesar) aqueous solution with constant stirring using magnetic stirrer at room temperature. Then the solution was kept in oven at 100 • C for 12 h to get dry KH 5 (PO 4 ) 2 .…”