Freezing Point Depression Of Water Research Articles

Effect of Water Content on Thermodynamic Properties of Compressed Hydrogen.

Force field-based molecular simulations were used to calculate thermal expansivities, heat capacities, and Joule–Thomson coefficients of binary (standard) hydrogen–water mixtures for temperatures between 366.15 and 423.15 K and pressures between 50 and 1000 bar. The mole fraction of water in saturated hydrogen–water mixtures in the gas phase ranges from 0.004 to 0.138. The same properties were calculated for pure hydrogen at 323.15 K and pressures between 100 and 1000 bar. Simulations were performed using the TIP3P and a modified TIP4P force field for water and the Marx, Vrabec, Cracknell, Buch, and Hirschfelder force fields for hydrogen. The vapor–liquid equilibria of hydrogen–water mixtures were calculated along the melting line of ice Ih, corresponding to temperatures between 264.21 and 272.4 K, using the TIP3P force field for water and the Marx force field for hydrogen. In this temperature range, the solubilities and the chemical potentials of hydrogen and water were obtained. Based on the computed solubility data of hydrogen in water, the freezing-point depression of water was computed ranging from 264.21 to 272.4 K. The modified TIP4P and Marx force fields were used to improve the solubility calculations of hydrogen–water mixtures reported in our previous study [RahbariA.;J. Chem. Eng. Data2019, 64, 4103−4115] for temperatures between 323 and 423 K and pressures ranging from 100 to 1000 bar. The chemical potentials of ice Ih were calculated as a function of pressure between 100 and 1000 bar, along the melting line for temperatures between 264.21 and 272.4 K, using the IAPWS equation of state for ice Ih. We show that at low pressures, the presence of water has a large effect on the thermodynamic properties of compressed hydrogen. Our conclusions may have consequences for the energetics of a hydrogen refueling station using electrochemical hydrogen compressors.

Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell

This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from −20 to −1 °C. It is found that the freezing-point depression of water in the cathode CL is 1.0 ± 0.5 °C and its effect on PEFC cold start under automotive conditions is negligible.