Electro-osmotic Drag and Thermodynamic Properties of Water in Hydrated Nafion Membranes from Molecular Dynamics

Abstract

One of the important parameters in water management of proton exchange membranes is the electro-osmotic drag (EOD) coefficient of water. The value of the EOD coefficient is difficult to justify, and available literature data on this for Nafion membranes show scattering from in experiments and simulations. Here, we use a classical all-atom model to compute the EOD coefficient and thermodynamic properties of water from molecular dynamics simulations for temperatures between 330 and 420 K, and for different water contents between λ = 5 and λ = 20. λ is the ratio between the moles of water molecules to the moles of sulfonic acid sites. This classical model does not capture the Grotthuss mechanism; however, it is shown that it can predict the EOD coefficient within the range of experimental values for λ = 5 where the vehicular mechanism dominates proton transfer. For λ > 5, the Grotthuss mechanism becomes dominant. To obtain the EOD coefficient, average velocities of water and ions are computed by imposing different electric fields to the system. Our results show that the velocities of water and hydronium scale linearly with the electric field, resulting in a constant ratio of ca. 0.4 within the error bars. We find that the EOD coefficient of water linearly increases from 2 at λ = 5 to 8 at λ = 20 and the results are not sensitive to temperature. The EOD coefficient at λ = 5 is within the range of experimental values, confirming that the model can capture the vehicular transport of protons well. At λ = 20, due to the absence of proton hopping in the model, the EOD coefficient is overestimated by a factor of 3 compared to experimental values. To analyze the interactions between water and Nafion, the partial molar enthalpies and partial molar volumes of water are computed from molecular dynamics simulations. At different water uptakes, multiple linear regression is used on raw simulation data within a narrow composition range of water inside the Nafion membrane. The partial molar volumes and partial molar excess enthalpies of water asymptotically approach the molar volumes and molar excess enthalpies of pure water for water uptakes above 5. This confirms the model can capture the bulklike behavior of water in the Nafion at high water uptakes.

Decoupled Water Electrolysis via VO2+/VO2+ Redox Mediator for 35 MPa High-Pressure Hydrogen Production

“…Electro-osmotic drag, which refers to the movement of water associated with the movement of H + from the anode to the cathode, was employed here. 34 The electroosmotic drag could compensate for the hydraulic water transport from the cathode to the anode, which mitigated the H 2 crossover to some extent. Given the electro-osmotic drag was reported as drastic at only high current densities (>1 A cm −2 ), this hypothesis needs further experimental and theoretical validation.…”

“…Nevertheless, there should be an explanation for the mitigation in H 2 crossover with the increased current densities, especially when comparing the r c values between open and closed circuits. Electro-osmotic drag, which refers to the movement of water associated with the movement of H + from the anode to the cathode, was employed here . The electro-osmotic drag could compensate for the hydraulic water transport from the cathode to the anode, which mitigated the H 2 crossover to some extent.…”

Tackling the proton limit under industrial electrochemical CO2 reduction by a local proton shuttle

“…During industrial eCO 2 RR, the great majority of anions are enriched in localized regions due to the rejection effect of “Donnan exclusion” of sulfonate groups in Nafion, 49,50 in which HCO 3 − acts as the main proton pool; however, a small proportion of protons is from H 3 O + . First, H + from HCO 3 − near the reactive sites interacts with −SO 3 − of the Nafion modifier, forming −SO 3 H. Then, −SO 3 H completely dissociates to yield H + , which then forms HCO 3 − and/or hydrated ions H 3 O + through hydrogen bonds at the reactive sites 51 . Therefore, the HCO 3 − and H 3 O + act as local proton donors, while −SO 3 − is the proton hopping site, enabling a denser hydrogen bonding network, in which protons quickly hop to the redox site through the hydrogen bonds by reorientation of the proton donor.…”

Carbonation in Low-Temperature CO2 Electrolyzers: Causes, Consequences, and Solutions

“…Creative GDE designs will be of paramount importance to overcome the flooding problem. , Multiscale/multiphysics modeling: CO2R at the electrode is a complex phenomenon that involves multiple phases (gas/liquid/solid) and reactions (homogeneous and heterogeneous), which are affected by the presence of electrolytes and electric field. Advanced modeling of the near-electrode and membrane environment will provide useful insights into the carbonation phenomena that is affected by mass transport, electrochemical and homogeneous reactions, and thermal effects. − Moreover, advanced thermodynamic modeling using electrolyte equations of state and molecular simulation are essential for predicting the key thermodynamic, transport, and structural properties of the relevant liquid systems. − Such properties are the mutual solubilities (e.g., gases in the aqueous electrolyte phase), transport coefficients (i.e., Maxwell-Stefan and self-diffusivities, ionic conductivities, viscosity), and partial molar properties. − Electrolyte-free electrolysis: The presence of electrolytes in CO 2 electrolyzers has several disadvantages. First, liquid products are contaminated with the electrolytes, which complicate the downstream process.…”