Anhydrous Grotthuss mechanism for fast proton transport in a dense oxide-ion array of α-MoO3

Developing high-power battery chemistry is an urgent task to buffer fluctuating renewable energies and achieve a sustainable and flexible power supply. Owing to the small size of proton and its ultrahigh mobility in water via the Grotthuss mechanism, aqueous proton batteries are an attractive candidate for high-power energy storage devices. Although Grotthuss proton transfer usually occurs in hydrogen-bonded networks of water molecules, in this work, we discover anhydrous Grotthuss-type proton transport in a dense oxide-ion array of solid α-MoO3 even without structural water. The fast proton transfer and accumulation that occurs during (de)intercalation in α-MoO3 is unveiled using both experiments and first-principles calculations. Coupled with a zinc anode and a superconcentrated dual-ion Zn2+/H+ electrolyte, the solid-state anhydrous Grotthuss proton transport mechanism realizes an aqueous MoO3-Zn battery with both high energy and power densities.

Phosphinate MOF formed from tetratopic ligands as proton conductive materials

Metal organic frameworks (MOFs) are attracting attention as potential proton conductors. There are two main advantages of MOFs in this application: the possibility of rational design and tuning of the properties, and clear conduction pathways given by their crystalline structure. We hereby present two new MOF structures, ICR-10 and ICR-11, based on tetratopic phosphinate ligands. The structures of both MOFs were determined by 3D electron diffraction. They both crystallize in the P-3 space group and contain arrays of parallel linear pores lined with hydrophilic non-coordinated phosphinate groups. This, together with the adsorbed water molecules, facilitates proton transfer via the Grotthuss mechanism, leading to the proton conductivity up to 4.26∙10-4 S cm-1 for ICR-11.

The Grotthuss mechanism for bifunctional proton transfer in poly(benzimidazole)

Poly(benzimidazole) (PBI) has received considerable attention as an effective high-temperature polymer electrolyte membrane for fuel cells. In this work, the Grotthuss mechanism for bifunctional proton transfer in PBI membranes was studied using density functional theory and transition state theory. This study focused on the reaction paths and kinetics for bifunctional proton transfer scenarios in neutral ([PBI] 2 ), single (H + [PBI] 2 ) and double-protonated (H 2+ [PBI] 2 ) dimers. The theoretical results showed that the energy barriers and strength for H-bonds are sensitive to the local dielectric environment. For [PBI] 2 with ε = 1, the uphill potential energy curve is attributed to extraordinarily strong ion-pair H-bonds in the transition structure, regarded as a ‘dipolar energy trap’. For ε = 23, the ion-pair charges are partially neutralized, leading to a reduction in the electrostatic attraction in the transition structure. The dipolar energy trap appears to prohibit interconversion between the precursor, transition and proton-transferred structures, which rules out the possibility for [PBI] 2 to be involved in the Grotthuss mechanism. For H + [PBI] 2 and H 2+ [PBI] 2 with ε = 1, the interconversion involves a low energy barrier, and the increase in the energy barrier for ε = 23 can be attributed to an increase in the strength of the protonated H-bonds in the transition structure: the local dielectric environment enhances the donor–acceptor interaction of the protonated H-bonds. Analysis of the rate constants confirmed that the quantum effect is not negligible for the N–H + … N H-bond especially at low temperatures. Agreement between the theoretical and experimental data leads to the conclusion that the concerted bifunctional proton transfer in H 2+ [PBI] 2 in a high local dielectric environment is ‘the rate-determining scenario’. Therefore, a low local dielectric environment can be one of the required conditions for effective proton conduction in acid-doped PBI membranes. These theoretical results provide insights into the Grotthuss mechanism, which can be used as guidelines for understanding the fundamentals of proton transfers in other bifunctional H-bond systems.

Proton Transfer in Bulk Water using the Full Adaptive QM/MM Method: Integration of Solute- and Solvent-Adaptive Approaches

<div><div>The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that increases the accessibility of local electronic structures of large systems.</div><div> The technique combines the benefit of accuracy found in the QM method and that of cost efficiency found in the MM method.</div><div> However, it is difficult to directly apply the QM/MM method to the dynamics of solution systems, particularly for proton transfer. </div><div> As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ions and solvent water molecules. </div><div> Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. </div><div> Although several solvent-adaptive methods have been proposed, a full adaptive framework, which is an approach that also considers adaptation for solutes, remains untapped. In this paper, we propose a new numerical expression for the coordinates of the excess proton and its control algorithm.</div><div> Furthermore, we confirm that this method can stably and accurately simulate proton transfer dynamics in bulk water.</div></div>

Proton Transfer in Bulk Water using the Full Adaptive QM/MM Method: Integration of Solute- and Solvent-Adaptive Approaches

<div><div>The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that increases the accessibility of local electronic structures of large systems.</div><div> The technique combines the benefit of accuracy found in the QM method and that of cost efficiency found in the MM method.</div><div> However, it is difficult to directly apply the QM/MM method to the dynamics of solution systems, particularly for proton transfer. </div><div> As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ions and solvent water molecules. </div><div> Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. </div><div> Although several solvent-adaptive methods have been proposed, a full adaptive framework, which is an approach that also considers adaptation for solutes, remains untapped. In this paper, we propose a new numerical expression for the coordinates of the excess proton and its control algorithm.</div><div> Furthermore, we confirm that this method can stably and accurately simulate proton transfer dynamics in bulk water.</div></div>

Electric Migration of Hydrogen Ion in Pore-Voltammetry Suppressed by Nafion Film

Micro-hole voltammetry exhibiting rectified current-voltage curves was performed in hydrochloric acid by varying the lengths and the diameters of the micro-holes on one end of which a Nafion film was mounted. Some voltammetric properties were compared with those in NaCl solution. The voltammograms were composed of two line-segments, the slope of one segment being larger than the other. They were controlled by electric migration partly because of the linearity of the voltammograms and partly the independence of the scan rates. Since the low conductance which appeared in the current from the hole to the Nafion film was proportional to the cross section area of the hole and the inverse of the length of the hole, it should be controlled by the geometry of the hole. The conductance of the hydrogen ion in the Nafion film was observed to be smaller than that in the bulk, because the transport rate of hydrogen ion by the Grotthuss mechanism was hindered by the destruction of hydrogen bonds in the film. In contrast, the conductance for the current from the Nafion to the hole, enhancing by up to 30 times in magnitude from the opposite current, was controlled by the cell geometry rather than the hole geometry except for very small holes. A reason for the enhancement is a supply of hydrogen ions from the Nafion to increase the concentration in the hole. The concentration of the hydrogen ion was five times smaller than that of sodium ion because of the blocking of transport of the hydrogen ion in the Nafion film. However, the rectification ratio of H+ was twice as large as that of Na+.

Proton Transfer in Bulk Water by Full Adaptive QM/MM Method: Integration of Solute- and Solvent-Adaptive Approaches

The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that makes local electronic structures of large systems accessible.<br>It has the strengths of accuracy found in the QM method as well as the strengths of small<br>computational costs found in the MM method. However, it is severe to directly apply the<br>QM/MM method to dynamics of solution systems, particularly to proton transfer. As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ion and solvent water molecules. Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. Although there have been several solvent-adaptive methods proposed, a full adaptive framework, an approach that also takes into account of adaptation for solutes, still remains untapped. In this paper, we propose a new numerical expression for the coordinate of the excess proton and its control algorithm. Furthermore, we confirmed that this method can stably and accurately simulate proton transfer dynamics in bulk water.

Proton Transfer in Bulk Water by Full Adaptive QM/MM Method: Integration of Solute- and Solvent-Adaptive Approaches

The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that makes local electronic structures of large systems accessible.<br>It has the strengths of accuracy found in the QM method as well as the strengths of small<br>computational costs found in the MM method. However, it is severe to directly apply the<br>QM/MM method to dynamics of solution systems, particularly to proton transfer. As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ion and solvent water molecules. Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. Although there have been several solvent-adaptive methods proposed, a full adaptive framework, an approach that also takes into account of adaptation for solutes, still remains untapped. In this paper, we propose a new numerical expression for the coordinate of the excess proton and its control algorithm. Furthermore, we confirmed that this method can stably and accurately simulate proton transfer dynamics in bulk water.