Phase diagram of ammonium perchlorate: Raman spectroscopic constrains at high pressures and temperatures

Adapting a continuous flow cryostat and a plate DAC to do high pressure Raman experiments at low temperatures

We present a method for modifying a continuous flow cryostat and a steel plate DAC (Diamond Anvil Cell) to perform high pressure micro-Raman experiments at low temperatures. Despite using a steel DAC with a lower specific heat capacity (∼335 J/kg K), this setup can routinely perform high pressure (∼10 GPa) measurements at temperatures as low as 26 K. This adaptation is appropriate for varying the temperature of the sample while keeping it at a constant pressure. We determined that the temperature variation across the sample chamber is about 1 K using both direct temperature measurements and finite element analysis of the heat transport across the DAC. We present Raman spectroscopy results on elemental selenium at high pressures and low temperatures using our modified setup.

Proton Conduction of Nafion Hybrid Membranes Promoted by NH3-Modified Zn-MOF with Host-Guest Collaborative Hydrogen Bonds for H2/O2 Fuel Cell Applications

It is of great significance to develop creative proton exchange membrane materials for proton exchange membrane fuel cells (PEMFCs). The strategy of doping metal-organic frameworks (MOFs) with guest molecules into the Nafion matrix is adopted to improve the electrochemical performance of Nafion hybrid membranes. Various and abundant hydrogen bonds can make a tremendous contribution to the proton conduction of hybrid membranes. In this work, we used high proton-conducting Zn-MOFs with the characteristics of host-guest collaborative hydrogen bonds as the filler to prepare Zn-MOF/Nafion hybrid membranes. Alternating current (AC) impedance tests show that when the doping amount of Zn-MOF is 5%, the proton conductivity reaches 7.29 × 10^-3 S·cm^-1, being 1.87 times that of the pure Nafion membrane at 58% relative humidity (RH) and 80 °C. In an attempt to prove the promotion effect of guest NH₃ on proton conductivity of Nafion hybrid membranes, Zn-MOF-NH₃ was filled into the Nafion matrix. Under the same conditions, its proton conductivity reaches the maximum value of 2.13 × 10^-2 S·cm^-1, which is 5.47 times that of the pure Nafion membrane. Zn-MOF-NH₃/Nafion-5 was used to fabricate a proton exchange membrane for application in H₂/O₂ fuel cells. The maximum power density of 212 mW cm^-2 and a current density of 630 mA cm^-2 reveal a respectable single cell performance. This study provides a promising method for optimizing the structure of MOF proton conductors and inspires the preparation of high-performance Nafion hybrid membranes.

Effects of pressure on the structure and lattice dynamics of ammonium perchlorate: A combined experimental and theoretical study

Ammonium perchlorate NH₄ClO₄ (AP) was studied using synchrotron angle-dispersive X-ray powder diffraction (XRPD) and Raman spectroscopy. A diamond-anvil cell was used to compress AP up to 50 GPa at room temperature (RT). Density functional theory (DFT) calculations were performed to provide further insight and comparison to the experimental data. A high-pressure barite-type structure (Phase II) forms at ≈4 GPa and appears stable up to 40 GPa. Refined atomic coordinates for Phase II are provided, and details for the Phase I → II transition mechanics are outlined. Pressure-dependent enthalpies computed for DFT-optimized crystal structures confirm the Phase I → II transition sequence, and the interpolated transition pressure is in excellent agreement with the experiment. Evidence for additional (underlying) structural modifications include a marked decrease in the Phase II b'-axis compressibility starting at 15 GPa and an unambiguous stress relaxation in the normalized stress-strain response at 36 GPa. Above 47 GPa, XRD Bragg peaks begin to decrease in amplitude and broaden. The apparent loss of crystalline long-range order likely signals the onset of amorphization. Three isostructural modifications were discovered within Phase II via Raman spectroscopy. A revised RT isothermal phase diagram is discussed based on the findings of this study.