A New Force Field for OH^– for Computing Thermodynamic and Transport Properties of H₂ and O₂ in Aqueous NaOH and KOH Solutions

Abstract

The thermophysical properties of aqueous electrolyte solutions are of interest for applications such as water electrolyzers and fuel cells. Molecular dynamics (MD) and continuous fractional component Monte Carlo (CFCMC) simulations are used to calculate densities, transport properties (i.e., self-diffusivities and dynamic viscosities), and solubilities of H2 and O2 in aqueous sodium and potassium hydroxide (NaOH and KOH) solutions for a wide electrolyte concentration range (0–8 mol/kg). Simulations are carried out for a temperature and pressure range of 298–353 K and 1–100 bar, respectively. The TIP4P/2005 water model is used in combination with a newly parametrized OH– force field for NaOH and KOH. The computed dynamic viscosities at 298 K for NaOH and KOH solutions are within 5% from the reported experimental data up to an electrolyte concentration of 6 mol/kg. For most of the thermodynamic conditions (especially at high concentrations, pressures, and temperatures) experimental data are largely lacking. We present an extensive collection of new data and engineering equations for H2 and O2 self-diffusivities and solubilities in NaOH and KOH solutions, which can be used for process design and optimization of efficient alkaline electrolyzers and fuel cells.

Interfacial Tensions, Solubilities, and Transport Properties of the H2/H2O/NaCl System: A Molecular Simulation Study

“…Figure a shows the decrease of the solubilities of H 2 at increasing molalities of NaCl (i.e., salting-out effect). The salting-out of nonpolar gases (e.g., H 2 , O 2 , and CO 2 ) in the presence of salts such as NaCl, KCl, and KOH is a well observed phenomenon. ,, As shown in Figure a, the models by Torín-Ollarves and Trusler and Chabab et al agree for H 2 solubilities in pure H 2 O and at NaCl molalities below 0.5 mol NaCl/kg H 2 O. For NaCl concentrations higher than 0.5 mol NaCl/kg H 2 O the two models predict different H 2 solubilities.…”

“…The raw data of these properties, as well as their statistical uncertainties, are listed in Table S10 of the Supporting Information. Both the Madrid-2019 81 and Madrid-Transport 77,82 capture the experimental data of density very accurately (within 1%). As shown in Figure 3a, the Madrid-Transport 77,82 force field yields a better agreement with the experimental data of viscosity compared to the Madrid-2019 81 force field.…”

“…To compute the probability of occurrence of each λ value, a histogram with 100 bins is used. The Boltzmann averaged probability distributions of λ ( p (λ)) can be computed using , p ( λ ) = false⟨ p obs ( λ ) exp [ − W false( λ false) ] false⟩ false⟨ exp [ − W false( λ false) ] false⟩ The Boltzmann sampled probability distribution of λ ( p (λ)) can be related to the infinite dilution chemical potential (μ ex ) using ,, μ ex = prefix− k normalB T .25em ln nobreak0em.25em⁡ p ( λ = 1 ) p ( λ = 0 ) where p (λ = 1) and p (λ = 0) are the Boltzmann averaged probability distributions of λ at 1 and 0, respectively.…”

“…To compute the probability of occurrence of each λ value, a histogram with 100 bins is used. The Boltzmann averaged probability distributions of λ (p(λ)) can be computed using 77,119 =…”

“…The Boltzmann sampled probability distribution of λ (p(λ)) can be related to the infinite dilution chemical potential (μ ex ) using 77,78,119…”