Vehicular Diffusion Research Articles

Proton transport mechanisms in aqueous acids: Insights from abinitio molecular dynamics simulations.

The solvation and transport of protons in aqueous solutions of phosphoric acid (PA), sulfuric acid (SA), and nitric acid (NA) were studied using abinitio molecular dynamics simulations. Systems with acid-to-water ratios of 1:1 and 1:3 were examined to understand the similarities and differences in transport mechanisms. The solvation structure of H3O+ in these systems is similar to that in slightly acidic water, with variations in the strength of hydrogen bonds (H-bonds) accepted by acid molecules. In aqueous PA systems, strong H-bonds between PA molecules are slightly affected by water, leading to significantly greater H3O+ diffusion compared to aqueous SA and NA systems. This enhanced diffusion is attributed to the participation of PA molecules in H3O+ transport, where the PA molecule can shuttle a proton for H3O+, facilitating a large displacement via collective proton hopping. This shuttling mechanism is prominent in aqueous PA but rare in aqueous SA and absent in aqueous NA. Moreover, the decomposition of H3O+ diffusion into vehicular and structural components indicates that the higher diffusion in aqueous PA is primarily due to the structural mechanism with the aid of PA molecules. In the aqueous NA systems, the vehicular diffusion is dominant at low water contents and the increase in water content improves the structural diffusion by forming connected H-bonds within water molecules. Our findings elucidate the role of acid molecules in proton transport within their aqueous solutions, thereby advancing the fundamental understanding of proton transport mechanisms.

Facilitating the rapid Grotthuss diffusion of OH− in electrolyte for accelerated oxygen evolution reaction

Faster OH− transfer for efficient adsorption on catalysis is key to accelerating the oxygen evolution reaction (OER). The diffusion of OH− in electrolyte mainly involves two mechanism which is dependent on the water coordination number (CN): Grotthuss diffusion (4 CN) and vehicular diffusion (5 CN). The Grotthuss diffusion is much faster but the vehicular diffusion of OH− is predominant in the electrolyte. Herein, negatively charged kaolin, which are simply dispersed in the electrolyte, are utilized to provide surface electrostatic attraction to H2O and repulsion to OH− to reduce the OH− CN from 5 to 4 for rapid Grotthuss diffusion. This contributes to the accelerated OH− adsorption which facilitates the charge transfer, the formation and the deprotonation of *OOH leading to enhanced OER performance. The OH− CN recovery from 4 to 5 is proved quite slow after leaving the kaolin surface, breaking the limitation that kaolin must be on the catalyst surface. This strategy exhibits universality for various electrocatalysts with reduced overpotential by 10–246 mV at 10 mA cm−2. We provide a clean, simple and low-cost way to promote the industrial development of alkaline water electrolysis.

Nanoparticle Diffusion in Crowded Polymer Nanocomposite Melts.

This study examines nanoparticle diffusion in crowded polymer nanocomposites by diffusing small Al2O3 nanoparticles (NPs) in SiO2-loaded P2VP matrices. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measures Al2O3 NP diffusion coefficients within a homogeneous PNC background of larger, immobile SiO2 NPs. By developing a geometric model for the average interparticle distance in a system with two NP sizes, we quantify nanocomposite confinement relative to the Al2O3 NP size with a bound layer. At low SiO2 concentrations, Al2O3 NP diffusion aligns with the neat polymer results. In more crowded nanocomposites with higher SiO2 concentrations where the interparticle distance approaches the size of the mobile Al2O3 NP, the 6.5 nm Al2O3 NPs diffuse faster than predicted by both core-shell and vehicular diffusion models. Relative to our previous studies of NPs diffusing into polymers, these findings demonstrate that the local environment in crowded systems significantly complicates NP diffusion behavior and the bound layer lifetimes.

Proton Transport in Aqueous Phosphoric, Sulfuric, and Nitric Acids: An Ab Initio Study

Proton transport plays a pivotal role in various electrochemical processes and technologies. It occurs through two primary mechanisms: the vehicular mechanism where a protonic defect moves with the aid of a molecular entity; and the ‘Grotthuss mechanism’ where a proton diffuses by being transferred between host molecules via hydrogen bonds. Recently, results from ab initio molecular dynamics (AIMD) simulations combined with dielectric spectroscopy, light, and quasielastic neutron scattering revealed that protons in neat phosphoric acid indeed move by surprisingly short jumps (~0.5-0.7 Å)1,2. In the present work, we continue to delve into the solvation and transport of protons in aqueous acid using AIMD simulations.Aqueous systems of phosphoric acid (PA), sulfuric acid (SA), and nitric acid (NA) were examined at two different ratios of acid to water 1:1 and 1:3 to understand the similarities and differences in proton transport mechanisms in these aqueous acids. The solvation structure of H3O+ in these acid solutions resembles that in the slightly acidic water but varies in the strength of hydrogen bonds formed with the acid molecules. Our results demonstrate that the strong hydrogen bonds between the PA molecules are slightly influenced by the presence of water molecules. Furthermore, the simulations reveal that H3O+ diffuses significantly faster in the aqueous PA systems than in the solutions of either SA or NA due to the assistance of PA molecules. Proton transfer events in the process of H3O+ transport show that the acid molecule can accept a proton from H3O+, either enhancing the H3O+ diffusion by shuttling a proton through a collective proton hopping or impeding it via rattling. The shuttling event significantly contributes to the H3O+ diffusion in aqueous PA, rarely occurring in the aqueous SA and never in the aqueous NA. Moreover, decomposition of H3O+ diffusion into vehicular and structural diffusion components indicates that the higher diffusion in aqueous PA is primarily due to the structural mechanism aided by the PA molecules. In the aqueous NA systems, however, the vehicular diffusion is dominant at low water contents, while increasing water content enhances the structural diffusion via connected hydrogen bonds between the water molecules. These findings shed light on the role of acid molecules in proton transport within their aqueous solutions, advancing our understanding of these fundamental mechanisms.

Unraveling relationship between complex lifetimes and microscopic diffusion in deep eutectic solvents.

Aqueous mixtures of deep eutectic solvents (DESs) have emerged as a subject of interest in recent years for their tailored physicochemical properties. However, a comprehensive understanding of water's multifaceted influence on the microscopic dynamics, including its impact on improved transport properties of the DES, remains elusive. Additionally, the diffusion mechanisms within DESs manifest heterogeneous behavior, intricately tied to the formation and dissociation kinetics of complexes and hydrogen bonds. Therefore, it is imperative to explore the intricate interplay between bond kinetics, diffusion mechanism, and dynamical heterogeneity. This work employs water as an agent to explore their relationships by studying various relaxation phenomena in a DES based on acetamide and lithium perchlorate over a wide range of water concentrations. Notably, acetamide exhibits Fickian yet non-Gaussian diffusion across all water concentrations with Fickian (τf) and Gaussian (τg) timescales following a power-law relationship, τg∝τfγ, γ ∼ 1.4. The strength of coupling between bond kinetics and different diffusion timescales is estimated through various power-law relationships. Notably, acetamide-water hydrogen bond lifetime is linked to diffusive timescales through a single power-law over the entire water concentration studied. However, the relationship between diffusive timescales and the lifetime of acetamide-lithium complexes shows a sharp transition in behavior at 20 wt. % water, reflecting a change from vehicular diffusion below this concentration to structural diffusion above it. Our findings emphasize the critical importance of understanding bond dynamics within DESs, as they closely correlate with and regulate the molecular diffusion processes within these systems.

Unravelling the effect of molecular interactions on macroscale properties in Sustainion anion exchange membrane (AEM) under hydrated conditions using MD simulations

Sustainion is an anion exchange water electrolyzer membrane that has exhibited a scalable performance. In this work, we present a molecular model for the Sustainion membrane incorporating functionalization of the polymer in order to better mimic experimental conditions. Here in, we present a comprehensive exploration of its structural and transport properties like density, diffusivity and conductivity at various operating hydration levels using molecular dynamics simulations. The density exhibits a non-monotonic trend while the diffusivity showcases a non-linear behaviour with hydration. Furthermore, diffusion exhibits an Arrhenius-like dependence on temperature with the activation energies exhibiting a non-monotonic relationship akin to the density. It is concluded that the afore-mentioned properties of the Sustainion membrane are due to the counteracting influence of the enhancement in coordination number, and a reduction in the potentials of mean force of the different atomic pairs in the system. The effect of added salt on Sustainion properties is also determined and collated with experiments. Based on comparison with experiment, we deduce that vehicular diffusion is the predominant mechanism in the diffusion of the chloride ion, while it contributes approximately 15% to the diffusivity of the hydroxide ion.

Gravity field induced composite solid electrolytes enabling enhanced Li+ transport kinetics of lithium metal battery

Multilayer composite solid electrolytes (CSEs) exhibit many advantages over uniform monolayer CSEs but are hindered by high interlayer resistance and complex preparation methods. Herein, for the first time, a natural sedimentation strategy was developed to construct concentration gradient CSEs (GCSEs) for lithium-metal batteries (LMBs). This method utilizes intrinsic gravity and photopolymerization to achieve multiple functions in the monolayer, avoiding additional interlayer resistance and reducing preparation time. Owning to the concentration gradient structure, the Li+ transport on the PolyIL-rich side relies on the weak solvation of Li+ with EMIMTFSI, while the Li+ transport on the LLZTO-rich side follows the 'vehicular diffusion' mechanism with the aid of TFSI−, improving the Li+ transport and enhances the Li+ transference number, leading to the high stability to 2300 h for the Li//Li cell and stable operation at 4.3 V with 89.6 % capacity retention after 100 cycles for the assembled LMB. Moreover, compared with the monolayer uniform hybrid CSEs, the gradient structure alleviates uncoordinated thermal expansion between LLZTO and PolyIL, avoiding stress increase during cycling and battery capacity fade. This gradient strategy mitigates high interlayer resistance and offers a universal path to address the sluggish Li+ transportation in multilayer CSEs and improves compatibility between the electrolyte and electrodes in fabricating solid-state batteries.

Mechanisms of ion transport in lithium salt-doped zwitterionic polymer-supported ionic liquid electrolytes.

Recent experimental results have demonstrated that zwitterionic ionogel comprised of polyzwitterion (polyZI)-supported lithium salt-doped ionic liquid exhibits improved conductivities and lithium transference numbers than the salt-doped base ionic liquid electrolyte (ILE). However, the underlying mechanisms of such observations remain unresolved. In this work, we pursued a systematic investigation to understand the impact of the polyZI content and salt concentration on the structural and dynamic properties of the poly(MPC) ionogel of our model polyZI ionogel, poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)] supported LiTFSI/N-butyl-N-methylpyrrolidinium TFSI base ionic liquid electrolyte. Our structural analyses show strong lithium-ZI interaction consistent with the physical network characteristic observed in the experiments. An increase in polyZI content leads to an increased fraction of Li+ ions coordinated with the polyZI. In contrast, an increase in salt concentration leads to a decreased fraction of Li+ ions coordinated with the polyZI. The diffusivities of the mobile ions in the poly(MPC) ionogel were found to be lower than the base ILE in agreement with experiments at T > 300K. Analysis of ion transport mechanisms shows that lithium ions within the poly(MPC) ionogel travel via a combination of structural, vehicular diffusion, as well as hopping mechanism. Finally, the conductivity trend crossover between the poly(MPC) ionogel and the base ILE was rationalized via a temperature study that showed that the base ILE ions are influenced more by the variation of temperature when compared to the poly(MPC) ions.

On Relative Importance of Vehicular and Structural Motions in Defining Electrolyte Transport

Molecular motions of ionic and neutral electrolyte species determine transport properties at the continuum scale. These molecular motions can be classified as vehicular (e.g., cations moving with a solvation shell of neighboring solvent molecules) and structural (e.g., cations hopping from one solvation shell to another) motions. While literature studies have described the presence, and relative importance, of each of these motions in various electrolytes, a clear link to macroscopic transport properties has not been made. We herein establish this link by using the fluctuation-dissipation theorem to develop theoretical expressions connecting the molecular displacements to Stefan-Maxwell diffusivities. To illustrate the usefulness of the proposed equations, we study LiPF6 in propylene carbonate as an exemplar electrolyte. We show that its transport behavior improves at all concentrations when structural diffusion of cations is promoted. On the other hand, boosting the cation vehicular diffusion negatively affects the concentrated compositions. We extend this understanding to a generalized electrolyte of a salt dissolved in a solvent. Our theory suggests that while structural diffusion influences Stefan-Maxwell diffusivities globally, vehicular diffusion is only relevant under certain conditions. Such guidelines are critical for a bottom-up design of electrolyte transport.

Ultralean Electrolyte Li-S Battery by Avoiding Gelation Catastrophe.

Due to the poor electronic conductivity of solid sulfur and sulfides, the dissolution of Sα- (α = 0, 2/8, 2/6, 2/4) into a liquid electrolyte and the vehicular diffusion of Sα- to carbon black are necessary for the electrochemical activity of a sulfur cathode in lithium-sulfur (Li-S) batteries. However, exactly how much dissolution and diffusion are required for high sulfur utilization and how this may control the minimum electrolyte/sulfur ratio, (E/S)min, have not been quantitatively settled. In this work, we show experimentally that a dissolved polysulfide concentration which is too high (>10-20 MS) may gel the liquid electrolyte, leading to catastrophic loss of Sα- mobility, a failure mode that is especially susceptible in a high-donor-number (DN) electrolyte under a lean condition (low E/S), similar to a traffic jam, resulting in high electrochemical polarization and low sulfur utilization. In contrast, we show that a low-DN electrolyte, even with a low polysulfide solubility of 0.1-0.5 MS, will never encounter a gelation catastrophe even at extremely low E/S, leading to unprecedentedly high energy density. Specifically, high sulfur utilizations of 96% (1600 mAh g-1) and 78% (1300 mAh g-1) are reached in an electrolyte as lean as E/S = 2 and 1 μL mg-1 Li-S coin cells when DME1.6LiFSI-HFE of low solvation capability (DN = 13.9) is adopted, even paired against a high-sulfur-loading cathode (5 mg cm-2).

A Theoretical Investigation into pH of Neat Water at the Nanoscopic Scale Via Rexpon Force Field and Stochastic Simulations

Design of durable solar CO2 conversion devices requires a mechanistic understanding of molecular-level reactions at the electrode-electrolyte interface that lead to interfacial degradation.[1] This interface often incorporates such features as a porous and rough surface to maximize active area and enhance catalytic performance. However, this roughness may introduce local water pool inclusions, whose size may have an unforeseen, detrimental impact on device durability due to their local pH being very different from the bulk pH. It is well-recognized that for water pools containing less than molecules, the solution pH cannot be defined in the conventional sense because the pools are too small to support stable water ion concentrations near neutral pH. Rather, the water ion concentrations fluctuate to very high values for short periods of time. To better understand pH in small pools, we combine theoretical calculations using QM/MD and stochastic simulations [3]. We combined Ab initio MD and the accurate RexPoN force field [2] simulation to quantify the thermodynamic free energy to create water ion pairs through the reaction as a function of the water pool size. The free energies are used to calculate the equilibrium constants for this reaction and the ion pair generation and recombination rate constants. The rate constants are then used as inputs to the stochastic simulations to gain insights into ion pair generation frequency and lifetime statistics over extended periods of time (hours). Proton and hydroxide diffusion are also incorporated in the stochastic simulation to understand how far water ions may travel from where they are generated. The results show that the combination of Grotthuss and vehicular diffusion mechanisms for H3O+ and OH- can lead to unequal distributions at an adjacent interface. We will discuss how the stochastic generation and recombination of water ions influences buffer chemistry and dissolved CO2 chemistry.Reference[1] S. Nitopi, E. Bertheussen, S.B. Scott, X. Liu, A.K. Engstfeld, S. Horch, B. Seger, I.E.L. Stephens, K. Chan, C. Hahn, J.K. Nørskov, T.F. Jaramillo, I. Chorkendorff, Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte, Chem. Rev. 119 (2019) 7610–7672. https://doi.org/10.1021/acs.chemrev.8b00705.[2] S. Naserifar, W.A. Goddard, The quantum mechanics-based polarizable force field for water simulations, J. Chem. Phys. 149 (2018). https://doi.org/10.1063/1.5042658.[3] W.D. Hinsberg, F.A. Houle, Kinetiscope, (n.d.). http://www.hinsberg.net/kinetiscope.

Exchange-Mediated Transport in Battery Electrolytes: Ultrafast or Ultraslow?

Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and reformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional (2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence of solvent exchange gives rise to the same spectral signatures, hiding the desired processes. We employ 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiate chemical exchange and energy-transfer dynamics in a comprehensive series of Li+, Mg2+, Zn2+, Ca2+, and Ba2+ bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to the superconcentrated regime. No exchange phenomena occur within at least 100 ps, regardless of the ion identity, salt concentration, and presence of water. All of the observed spectral dynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solvent residence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations and conductivity measurements on the Li+ and Zn2+ systems, we discuss these results as a continuum of vehicular and structural modalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. These results hold broadly applicable to many battery-relevant ions and solvents.

Revealing the Anticorrelation Behavior Mechanism between the Grotthuss and Vehicular Diffusions for Proton Transport in Concentrated Acid Solutions.

In this study, we performed reactive molecular dynamics simulations to characterize proton solvation and transport in concentrated hydrochloric acid solutions. The correlation contribution to the total proton mean square displacement is found to be negative for all acid concentrations, indicating the anticorrelation between the Grotthuss and vehicular diffusions. For the vehicular diffusion, the hydronium ions tend to move freely toward the lone pair side independent of acid concentrations, whereas for the Grotthuss diffusion, the proton hopping direction is limited to one of the hydrogen-bonded water molecules on the opposite side of the lone pair region, which are specifically oriented with respect to the neighboring hydronium ion at higher acid concentrations. This result is justified by our findings of the higher fraction of proton rattling with the single hopping event and longer hydrogen bond lifetimes at higher acid concentrations. However, the angular distribution for both the vehicular and Grotthuss diffusions is found to be rather broad and comparable for all acid concentrations, and thus, the anticorrelation shows a minimal dependence on the acid concentration. Our results reveal that the anticorrelation behavior between the vehicle and Grotthuss diffusions is attributed to the amphiphilic nature of hydronium ions and thus is independent of the acid concentrations in solutions.

Translational Jump-Diffusion of Hydroxide Ion in Anion Exchange Membrane: Deciphering the Nature of Vehicular Diffusion.

Earlier, ab initio and reactive force-field-based molecular dynamics (MD) simulation studies suggested an overwhelming contribution of the vehicular diffusion in the total diffusion of hydroxide ions rather than structural diffusion. But does the vehicular diffusion occur via small-step displacement? This question is important to have an understanding of the real characteristics of vehicular diffusion. To answer this question, we perform a classical molecular dynamics simulation of a system containing a hydroxide ion exchange membrane polymer and hydroxide ion at different hydration levels and temperatures using the same molecular force field (Dubey, V. Chem. Phys. Lett. 2020, 755, 137802), which successfully captured the microscopic structure and dynamics of the system. We use the translational jump-diffusion approach, used previously in supercooled water for understanding the origin of breakdown of the Stokes-Einstein relation, to calculate the jump-diffusion coefficient of hydroxide ion and water in the anion exchange membrane. We have seen a significant role of hydration level and temperature in the mechanism of vehicular diffusion. In overhydrated membrane, both hydroxide ions and water molecules diffuse via both small- and large-step displacement. With decreasing hydration level and temperature, the diffusion is increasingly governed by the jump-diffusion mechanism. The larger contribution of jump-diffusion comes from the stronger caging of the diffusing species by the solvent at lower hydration levels and temperature. These results, therefore, suggest that the hydration level and temperature of the hydroxide ion exchange membrane determine the detailed mechanism of the vehicular diffusion of hydroxide ion, especially whether the diffusion follows hydrodynamics or not.

Electron and proton conducting framework organic salt single crystals

Organic molecular assemblies that can conduct both electrons and protons are attractive materials, as they have important applications in organic electronics and fuel cells. Herein, two three-dimensional conducting framework organic salt (CFOS) single crystals with excellent electron and proton conductivity have been successfully synthesized by a simple drop-wise addition and crystallization method. The packing structure of CFOSs can be controlled through changing the central atom of building unit from a single carbon atom to a silicon atom, which in turn affects the electron and proton conductivity of CFOSs. Furthermore, the theoretical calculation results show that electron conduction occurs through the 3D super stacked π-conjugated network pathways whereas the proton conduction occurs via a novel combination of Grotthus and vehicular diffusion. The general design strategy and intuitive results in this work will provide practically useful insights in the preparation of new conducting framework organic salt single crystals with tuneable structures for specific applications.

Development of coarse-grained force field to investigate sodium-ion transport mechanisms in cyanoborate-based ionic liquid

Sodium-ion batteries have a great potential for energy storage applications, however, its realization depends strongly on a deep atomistic understanding of the sodium-ion transport mechanisms, which is a key step to optimize its performance. A complete characterization of sodium-ion transport mechanism requires large time-scale calculations, which is a challenge even for classical atomistic force-field molecular dynamics simulations. Thus, in this work, we performed a calibration of a coarse-grained force-field to describe the thermodynamic and transport properties of sodium-ion for the particular case of cyanoborate-based ionic liquid using different molar fractions of sodium tetracyanoborate salt dissolved in 1-ethyl-3-methyl-imidazolium tetracyanoborate. The results from coarse-grained modeling were found to be in fair agreement with atomistic simulations and experimental data, and they captured concentration dependence of [Nax[Anion]y]x−y aggregate sizes. In particular, two distinct populations of sodium-ions were identified inside the [Nax[Anion]y]x−y aggregates: 1) a slow one in vehicular diffusion and 2) a fast one in hoping diffusion. Both of them contribute to sodium ion transport and occur preferentially at different sodium-ion concentrations. The present findings were obtained for a cyanoborate-based ionic liquid, however, we expect that our insights can be used to understand similar ionic liquids.

Study on the Nanostructure of Nafion Ultra-Thin Film at Pt/C Catalyst Layer in PEMFC: Molecular Dynamics Simulation Approaches

In this study, the hydrated Nafion film in the CL of the cathode for PEMFC is investigated using molecular dynamics simulation (MD) method, exhibiting different structural characteristics on the Pt and carbon surfaces. First, it is found that water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and Pt surface, whereas Nafion backbone chains are present in a high concentration at the interface between Nafion phase and carbon surface. Second, it is also found from pair correlation function analysis that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface, which is due to their strong attractive interactions with the Pt surface that makes the dimension of hydrated Nafion phase 4–7% thinner on the Pt surface. Third, it is observed from water-occupied volume analysis that water molecules on the carbon surface can form a large-size water phase between the Nafion phase and carbon surface because the Nafion-carbon interface is not tightly integrated due to their weak interaction. In these structural characteristics, it is demonstrated that the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in water phase as well as polymer phase in addition to the strong molecular interaction with the Pt surface, whereas the proton hopping diffusion is enhanced due to the well-developed organized water phase via hydrogen bonding network.

Predicting the solvation structure and vehicular diffusion of hydroxide ion in an anion exchange membrane using nonreactive molecular dynamics simulation

The anion exchange membranes (AEM) are generally simulated using ab initio or reactive molecular dynamics (MD) simulation approach. However, the applicability’s of these methods are still limited by computational cost, availability of force-field, transferability, etc. Empirical force-field based MD simulation can predict structure and dynamics at a lower computational cost. Considering polarizable models for hydroxide ion and water in combination with a non-polarizable model of AEM polymer, we simulate hydrated AEM. The simulated solvation structure and non-reactive dynamics are in reasonable agreement with literature. A detailed study has revealed that the diffusion of the hydroxide ion and water approximately follow hydrodynamics.

Water-Assisted Proton Transport in Confined Nanochannels

Hydrated excess protons under hydrophobic confinement are a critical component of charge transport behavior and reactivity in nanoporous materials and biomolecular systems. Herein, excess proton confinement effects are computationally investigated for sub-2 nm hydrophobic nanopores by varying the diameters (d = 0.81, 0.95, 1.09, 1.22, 1.36, 1.63, and 1.90 nm), lengths (l ∼3 and ∼5 nm), curvature, and chirality of cylindrical carbon nanotube (CNT) nanopores. CNTs with a combination of different diameter segments are also explored. The spatial distribution of water molecules under confinement is diameter-dependent; however, proton solvation and transport are consistently found to occur in the water layer adjacent to the pore wall, showing an “amphiphilic” character of the hydrated excess proton hydronium-like structure. The proton transport free energy barrier also decreases significantly as the nanopore diameter increases and proton transport becomes almost barrierless in the d > 1 nm nanopores. Among the nanopores studied, the Zundel cation (H5O2+) is populated only in the d = 0.95 nm CNT (7,7) nanopore. The presence of the hydrated excess proton and K+ inside the CNT (7,7) nanopore induces a water density increase by 40 and 20%, respectively. The K+ transport through CNT nanopores is also consistently higher in the free energy barrier than proton transport. Interestingly, the evolution of excess protonic charge defect distribution reveals a “frozen” single water wire configuration in the d = 0.81 nm CNT (6,6) nanopore (or segment), through which hydrated excess protons can only shuttle via the Grotthuss mechanism. Vehicular diffusion becomes relevant to proton transport in the “flat” free energy regions and in the wider nanopores, where protons do not primarily shuttle in the axial direction.

Proton Transport in [BMIM+][BF4-]/Water Mixtures Near the Percolation Threshold.

The incorporation of ionic liquids into existing proton exchange membrane (PEM) materials has been shown to enhance thermal stability and improve conductivity at reduced water content. Because proton transport is dictated by an interplay between vehicular diffusion and the Grotthuss mechanism, it is expected that the nanoscale structure of the resulting ionic liquid/water networks will sensitively influence transport properties. In this work, we study proton transport in [BMIM+][BF4-]/water mixtures of systematically varying water volume fraction, focusing on concentrations near the percolation threshold in which water networks are connected over macroscopic length scales. We utilize reactive molecular dynamics within the multistate empirical valence bond (MS-EVB) framework to explicitly model Grotthuss hopping processes. Excellent agreement with experimental conductivity data is obtained within the Nernst-Einstein approximation, indicating that proton transport proceeds in a largely uncorrelated manner even at pH <0. We additionally study the changing topology of the hydrogen-bonded water network in these mixtures using percolation and graph theory analysis. We find that the proton diffusion coefficient and forward hop rate increase linearly with water content at concentrations ranging from dilute through the percolation threshold; surprisingly, we find no deviation in this trend at the percolation transition. The high concentration of BF4- anions inherently alters the fraction of Eigen and Zundel proton states, producing a net detrimental effect on proton transport rates relative to bulk water. This mechanistic insight is useful for selecting ideal ionic liquid candidates and determining the optimal ionic liquid concentration to incorporate into PEM materials.