Solubility of Water in Hydrogen at High Pressures: A Molecular Simulation Study

Hydrogen is one of the most popular alternatives for energy storage. Because of its low volumetric energy density, hydrogen should be compressed for practical storage and transportation purposes. Recently, electrochemical hydrogen compressors (EHCs) have been developed that are capable of compressing hydrogen up to P = 1000 bar, and have the potential of reducing compression costs to 3 kWh/kg. As EHC compressed hydrogen is saturated with water, the maximum water content in gaseous hydrogen should meet the fuel requirements issued by the International Organization for Standardization (ISO) when refuelling fuel cell electric vehicles. The ISO 14687–2:2012 standard has limited the water concentration in hydrogen gas to 5 μmol water per mol hydrogen fuel mixture. Knowledge on the vapor liquid equilibrium of H₂O–H₂ mixtures is crucial for designing a method to remove H₂O from compressed H₂. To the best of our knowledge, the only experimental high pressure data (P > 300 bar) for the H₂O–H₂ phase coexistence is from 1927 [J. Am. Chem. Soc., 1927, 49, 65–78]. In this paper, we have used molecular simulation and thermodynamic modeling to study the phase coexistence of the H₂O–H₂ system for temperatures between T = 283 K and T = 423 K and pressures between P = 10 bar and P = 1000 bar. It is shown that the Peng-Robinson equation of state and the Soave Redlich-Kwong equation of state with van der Waals mixing rules fail to accurately predict the equilibrium coexistence compositions of the liquid and gas phase, with or without fitted binary interaction parameters. We have shown that the solubility of water in compressed hydrogen is adequately predicted using force-field-based molecular simulations. The modeling of phase coexistence of H₂O–H₂ mixtures will be improved by using polarizable models for water. In the Supporting Information, we present a detailed overview of available experimental vapor–liquid equilibrium and solubility data for the H₂O–H₂ system at high pressures.