Thermodynamic analysis of electrochemical synthesis of ammonia in solid-state proton-conducting electrochemical reactors with considering interfacial potential steps

Ammonia (NH3) as a carbon-free hydrogen carrier shows great potential as fuel and its production at mild conditions is desired. NH3 synthesis at atmospheric pressure can be realized in solid-state...

Divalent Ion-Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

The second-order nonlinear susceptibility, chi(2), in the Stern layer, and the total interfacial potential drop, Phi(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined chi(2) value for NaCl and that of the alkali earth series, but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO4 2- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notable CaCl2 and MgSO4, but less so for BaCl2 and NaCl.

Potential Dependence of the Ionic Structure at the Ionic Liquid/water Interface Studied Using MD Simulation

<div> <p>The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis (nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL|W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and also unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL|W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL|graphene interface but different from the perpendicular orientation toward the vacuum side at the IL|vacuum interface. The comparison suggests that water behaves like a wall repelling IL ions like a solid electrode.</p></div>

Potential Dependence of the Ionic Structure at the Ionic Liquid/water Interface Studied Using MD Simulation

<div> <p>The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis (nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL|W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and also unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL|W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL|graphene interface but different from the perpendicular orientation toward the vacuum side at the IL|vacuum interface. The comparison suggests that water behaves like a wall repelling IL ions like a solid electrode.</p></div>

Absolute ion hydration free energy scale and the surface potential of water via quantum simulation

With a goal of determining an absolute free energy scale for ion hydration, quasi-chemical theory and ab initio quantum mechanical simulations are employed to obtain an accurate value for the bulk hydration free energy of the Na+ion. The free energy is partitioned into three parts: 1) the inner-shell or chemical contribution that includes direct interactions of the ion with nearby waters, 2) the packing free energy that is the work to produce a cavity of size λ in water, and 3) the long-range contribution that involves all interactions outside the inner shell. The interfacial potential contribution to the free energy resides in the long-range term. By averaging cation and anion data for that contribution, cumulant terms of all odd orders in the electrostatic potential are removed. The computed total is then the bulk hydration free energy. Comparison with the experimentally derived real hydration free energy produces an effective surface potential of water in the range −0.4 to −0.5 V. The result is consistent with a variety of experiments concerning acid–base chemistry, ion distributions near hydrophobic interfaces, and electric fields near the surface of water droplets.