Proton Transport Mechanism Research Articles

Mechanisms of proton transport in aqueous acid solutions

Mechanisms of proton transport in aqueous acid solutions

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.

Large-scale floating polyacrylonitrile hybrid micro-/nanofiber membrane achieves efficient H/D isotope separation via photocatalytic proton transport

Advancements in isotope separation are both essential and challenging. The separation of water isotopes is vital in industrial production, biopharmaceuticals, and healthcare applications. This process is energy-intensive and complex due to the similarity of the isotopes. Pure polyacrylonitrile (PAN) is resistant to proton permeability, but neutral hydrogen radicals are capable of penetration. In this study, we report a novel approach using PAN macroscopic hybrid micro-/nanofibers in combination with a photocatalyst to separate hydrogen ion isotopes. Application of photocatalytic water splitting to generate protons results in significantly slower deuteron permeation in these fibers relative to protons, resulting in a separation factor of approximately 15 at room temperature. The composite PAN fibers exhibit a conductivity of 17.5 mS cm−1 at 25 °C and 95 % relative humidity. Our approach not only converts sunlight directly into storable hydrogen, but also enriches heavy water from H₂O/D₂O by H/D isotope separation. This study provides new insights into proton transport mechanisms in acidic media and demonstrates the significant potential of high molecular weight polymers for hydrogen isotope separation.

Comprehensive Analysis of the Gradient Porous Transport Layer for the Proton-Exchange Membrane Electrolyzer.

A reasonable porous transport layer (PTL) is crucial to decreasing the mass-transfer loss in proton-exchange membrane water electrolyzers (PEMWEs). In this study, it was experimentally demonstrated that the gradient porosity PTL is beneficial in improving the performance of electrolyzers. The research comprehensively investigates the impact of gradient porosity PTL structures on the performance of the PEMWE, considering mass transfer and interfacial contact. It offers insights into the two-phase (oxygen-water) flow transport mechanisms within the PTLs using a 2D numerical model based on the actual PTL geometry. At the microscopic level, it analyzes how the interfacial contact impacts proton and electron transport mechanisms, affecting not only the contact resistance but also the number of effective catalytic sites for the oxygen evolution reaction. Experimental results demonstrate that the cis-gradient porosity PTL leads to a performance enhancement of 9.3% at 2.2 A/cm2. Numerical simulations reveal that the drivers of oxygen transport include the surface tension of the fibers and the pressure drop influenced by the local PTL porosity. Further analysis indicates that the lower oxygen saturation in the bottom region of the PTL with cis-gradient porosity favors a lower oxygen coverage area in the catalyst layers (CL) since the narrower pore space and higher capillary pressure increase the number of water flow paths into the CL. Overall, this study provides valuable insights for designing high-performance PTLs for use in electrolyzers.

Proton Transport Scenarios in Sulfuric Acid Explored via Ab Initio Molecular Dynamics Simulations.

Sulfuric acid (H2SO4), a highly reactive reagent containing intrinsic protonic charge carriers, has been studied via ab initio molecular dynamics simulations. Specifically, we explore the solvation shell structure of the protonic defects, H1SO4- and H3SO4+, as well as the underlying proton transport mechanisms in both the neat and hydrated H2SO4 solutions. Our findings reveal a significant contraction of the dynamic hydrogen-bonded network around the protonic defects, which resembles features seen in water. The simulations provide estimates of the structural relaxation time scales for proton release from both the covalent O-H bonds (∼23 ps) and the hydrogen bonds (∼0.4 ps). In contrast to water, our analysis of the proton transfer scenarios in sulfuric acid reveals correlated events mediated by the formation of longer (up to four) hydrogen-bonded Grotthuss chains.

Emerging Two-Dimensional Materials for Proton-Based Energy Storage.

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Molecular and Dissociative Adsorbed Water Concentration and Surface Protonic Conduction in Nanocrystalline TiO2.

Surface protonic conduction in porous nanocrystalline oxides is commonly involved in catalytic processes. The configuration of surface adsorbed water on oxides plays a crucial role in surface protonic conduction. However, studies on the impact of complex surface adsorbed water configuration on the surface water concentration and diffusivity remain limited, and hinder an in-depth understanding of surface proton transport mechanisms, and the design of better surface proton conductors. Here, in situ Raman spectroscopy is utilized to quantitatively identify the contribution of dissociative and molecular adsorbed water components on porous nanocrystalline TiO2 surfaces between 25 and 200°C. The variations in molecular and dissociative adsorbed water concentration agree with the predominant surface proton conduction mechanisms at three different temperature stages. From 40 to 125°C, the reduced coverage of molecular adsorbed water layer results in the decreasing proton diffusivity. Water dissociation on the nanocrystalline TiO2 surface is easier in wet N2 than in wet O2, resulting in higher proton conductivity in wet N2; while the surface proton diffusivities in these two atmospheres are similar. The in situ spectroscopy technique enables the improvement of surface proton conducting oxides through quantitative evaluation and modulation of the surface proton concentration and diffusivity.

Mechanism of proton transport in SLC25 family proteins

Mechanism of proton transport in SLC25 family proteins

Uncoupling Protein 1: biochemical and biophysical studies of the proton transport mechanism

Uncoupling Protein 1: biochemical and biophysical studies of the proton transport mechanism

Neural-network-based molecular dynamics simulations reveal that proton transport in water is doubly gated by sequential hydrogen-bond exchange.

The transport of excess protons in water is central to acid-base chemistry, biochemistry and energy production. However, elucidating its mechanism has been challenging. Recent nonlinear vibrational spectroscopy experiments could not be explained by existing models. Here we use both vibrational spectroscopy calculations and neural-network-based molecular dynamics simulations that account for nuclear quantum effects for all atoms to determine the proton transport mechanism. Our simulations reveal an equilibrium between two stable proton-localized structures with distinct Eigen-like and Zundel-like hydrogen-bond motifs. Proton transport follows a three-step mechanism gated by two successive hydrogen-bond exchanges: the first reduces the proton-acceptor water coordination, leading to proton transfer, and the second, the rate-limiting step, prevents rapid back-transfer by increasing the proton-donor coordination. This sequential mechanism is consistent with experimental characterizations of proton diffusion, explaining the low activation energy and the prolonged intermediate lifetimes in vibrational spectroscopy. These results are crucial for understanding proton dynamics in biochemical and technological systems.

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.

(Invited) Computational Design and Analysis of Novel Battery Electrolytes Driven by the Grotthuss Mechanism

Candidate systems for next-generation electrolyte materials, such as deep eutectic solvents and ionic liquids, often suffer from high viscosities, which suppresses rates of charge transport and limits their performance characteristics. A strategy for circumventing this problem is to leverage the structural or Grotthuss mechanism of proton transport through hydrogen-bond networks, which can exhibit high proton diffusion rates even when rates of vehicular charge transport are poor. In this talk, I will describe a project aimed at leveraging computational design and simulation approaches, in combination with experimental synthesis and characterization, to develop a novel class of battery electrolytes based on the structural/Grotthuss diffusion mechanism. The workflow involves identification of a suitable proton-carrier liquid, a proton source, and a redox-active species capable of undergoing proton-coupled electron transfer (PCET) reactions. I will discuss the computational methods, which involve a combination of machine learning and first-principles simulation techniques, candidate chemical species for each of the components, first simulation and experimental protocols for combining these components into a high-performance electrolyte, preliminary results, and next steps in the evolution of the project. This work will serve to illustrate both the power of modern computational approaches in the design of electrochemical systems but also to broaden the perspective on what constitutes a “breakthrough” electrolyte. Figure 1

Expanding the π-system of Fatty Acid-Anion Transporter Conjugates Modulates Their Mechanism of Proton Transport and Mitochondrial Uncoupling Activity.

Mitochondrial uncoupling by small molecule protonophores is a promising strategy for developing novel anticancer agents. Recently, aryl urea substituted fatty acids (aryl ureas) were identified as a new class of protonophoric anticancer agents. To mediate proton transport these molecules self-assemble into membrane-permeable anionic dimers in which intermolecular hydrogen bonds between the carboxylate and aryl-urea anion receptor delocalise the negative charge across the aromatic π-system. In this work, we extend the aromatic π-system by introducing a second phenyl substituent to the aryl urea scaffold and compare the proton transport mechanisms and mitochondrial uncoupling actions of these compounds to their monoaryl analogues. It was found that incorporation of meta-linked phenyl substituents into the aryl urea scaffold enhanced proton transport in vesicles and demonstrated superior capacity to depolarise mitochondria, inhibit ATP production and reduce the viability of MDA-MB-231 breast cancer cells. In contrast, diphenyl ureas linked through a 1,4-distribution across the phenyl ring displayed diminished proton transport activity, despite both diphenyl urea isomers possessing similar binding affinities for carboxylates. Mechanistic studies suggest that inclusion of a second aryl ring changes the proton transport mechanism, presumably due to steric factors that impose higher energy penalties for dimer formation.

Ultrahigh proton conductivity of four ionic hydrogen-bonded organic frameworks based on functionalized terephthalates

Recently, the utilization of hydrogen-bonded organic frameworks (HOFs) with high crystallinity and inherent well-defined H-bonding networks in the field of proton conduction has received increasing attention, but obtaining HOFs with excellent water stability and prominent proton conductivity (σ) remains challenging. Herein, by employing functionalized terephthalic acids, 2,5-dihydroxyterephthalic acid, 2-hydroxyterephthalic acid, 2-nitro terephthalic acid, and terephthalic acid, respectively, four highly water-stable ionic HOFs (iHOFs), [(C8H5O6)(Me2NH2)]∙2H2O (iHOF 1), [(C8H5O5)(Me2NH2)] (iHOF 2), [(C8H4NO6)(Me2NH2)] (iHOF 3) and [(C8H5O4)(Me2NH2)] (iHOF 4) were efficiently prepared by a straightforward synthesis approach in DMF and H2O solutions. The alternating-current (AC) impedance testing in humid conditions revealed that all four iHOFs were temperature- and humidity-dependent σ, with the greatest value reaching 10-2 S·cm−1. As expected, the high density of free carboxylic acid groups, crystallization water, and protonated [Me2NH2]+ units offer adequate protons and hydrophilic environments for effective proton transport. Furthermore, the σ values of these iHOFs with different functional groups were compared. It was discovered that it dropped in the following order under 100 °C and 98 % relative humidity (RH): σ iHOF 1 (1.72 × 10−2 S·cm−1) > σ iHOF 2 (4.03 × 10−3 S·cm−1) > σ iHOF 3 (1.46 × 10−3 S·cm−1) > σ iHOF 4 (4.86 × 10−4 S·cm−1). Finally, we investigated the causes of the above differences and the proton transport mechanism inside the framework using crystal structure data, water contact angle tests, and activation energy values. This study provides new motivation to develop highly proton-conductive materials.

Dynamic Second Harmonic Imaging of Proton Translocation Through Water Needles in Lipid Membranes.

Proton translocation through lipid membranes is a fundamental process in the field of biology. Several theoretical models have been developed and presented over the years to explain the phenomenon, yet the exact mechanism is still not well understood. Here, we show that proton translocation is directly related to membrane potential fluctuations. Using high-throughput wide-field second harmonic (SH) microscopy, we report apparently universal transmembrane potential fluctuations in lipid membrane systems. Molecular simulations and free energy calculations suggest that H+ permeation proceeds predominantly across a thin, membrane-spanning water needle and that the transient transmembrane potential drives H+ ions across the water needle. This mechanism differs from the transport of other cations that require completely open pores for transport and follows naturally from the well-known Grotthuss mechanism for proton transport in bulk water. Furthermore, SH imaging and conductivity measurements reveal that the rate of proton transport depends on the structure of the hydrophobic core of bilayer membranes.

Construction of wide-temperature-range proton exchange membrane by regulating proton transfer channels based on phosphate-functionalized carbon nanotubes

Phosphoric acid (PA) doped polybenzimidazole (PBI) high-temperature proton exchange membrane is favored because it can achieve faster proton transfer at 180 °C without additional humidification. However, the PA-PBI membrane is faced with slow PA proton conduction kinetics below 100 °C, and failing rapid start-up of fuel cells. Herein, carbon nanotubes functionalized with phosphate groups (P-CNT) and sulfonated poly [2,2'-(p-hydroxydibenzene)-5,5′-benzimidazole] (SOPBI) composite membranes are prepared as proton exchange membranes with wide temperature range (40–180 °C). Based on the regulation of monophosphate and bisphosphate groups on P-CNT, the bisphosphate group can strengthen the vehicular mechanism of proton transport by accelerating the formation and diffusion of H3O+ at low temperature/high humidity, and the Grotthuss mechanism of proton transport by constructing dense hydrogen bond network at high temperature/low humidity. The 2P-CNT(6 %)/SOPBI composite membrane shows excellent proton conductivity (131.8 mS cm−1 at 80 °C/90%RH, 170.5 mS cm−1 at 180 °C/0%RH) and peak power density (378 mW cm−2 at 80 °C/100 % RH, 531 mW cm−2 at 160 °C/0 % RH) under wide temperature conditions, surpassing SOPBI membrane and other composite membranes. Furthermore, the 2P-CNT(6 %)/SOPBI composite membrane exhibits high proton conduction and fuel cell stability at both 80 °C/100 % RH and 160 °C/0 % RH.

An innovative method to identify structural change through ion-molecule collision, making use of Time-Of-Flight measurements and SIMION simulations.

Structural change of ions induced by collision with a neutral has been studied in a guided ion beam tandem mass spectrometer, using Time-Of-Flight measurements and SIMION simulation. The exothermic catalytic isomerization of HOC+ to HCO+ is used to explore the new methodology. Isomerization is catalyzed via a proton transport mechanism through the interplay of a neutral molecule, the catalyst. Four different potential catalysts, Ne, D2, CH4, and C18O, were studied at different collision energies. SIMION simulation of the ion path and collision in the instrument leads to the highlight of a specific signature related to the catalytic isomerization in the time-of-flight spectra. This signature is used to identify the experimental conditions where isomerization takes place. Only C18O, at low collision energies, gives a clear signature of catalytic isomerization, and a quantitative estimate of the catalyzed isomerization cross-section and rate constant is derived. This new methodology is sensitive to clear presence of catalyzed isomerization and can be used in instruments designed for cross-section measurements, provided low collision energy is used and ion bunching is available.

Tailored Porous Ferrocene-Based Metal-Organic Frameworks as High-Performance Proton Conductors.

Although crystalline metal-organic frameworks (MOFs) have gained a great deal of interest in the field of proton conduction in recent years, the low stability and poor proton conductivity (σ) of some MOFs have hindered their future applications. As a result, resolving the issues listed above must be prioritized. Due to their exceptional structural stability, MOFs with ferrocene groups that exhibit particular physical and chemical properties have drawn a lot of attention. This study describes the effective preparation of a set of three-dimensional ferrocene-based MOFs, MIL-53-FcDC-Al/Ga and CAU-43, containing both main group metals and 1,1'-ferrocene dicarboxylic acid (H2FcDC). Multiple measurements, including powder X-ray diffraction (PXRD), infrared (IR), and scanning electron microscopy (SEM), confirmed that the addition of ferrocene groups enhanced the thermal, water, and acid-base stabilities of the three MOFs. Consequently, their proton-conductive behaviors were meticulously measured utilizing the AC impedance approach, and their best proton conductivities are 5.20 × 10-3, 2.31 × 10-3, and 1.72 × 10-4 S/cm at 100 °C/98% relative humidity (RH), respectively. Excitingly, MIL-53-FcDC-Al/Ga demonstrated an extraordinarily ultrahigh σ of above 10-4 S·cm-1 under 30 °C/98% RH. Using data from structural analysis, PXRD, SEM, thermogravimetry (TG), and activation energy, their proton transport mechanisms were thoroughly examined. The fact that these MOFs are notably easy to assemble, inexpensive, toxin-free, and stable will increase the range of practical uses for them.

Fungal Plasma Membrane H+-ATPase: Structure, Mechanism, and Drug Discovery.

The fungal plasma membrane H+-ATPase (Pma1) pumps protons out of the cell to maintain the transmembrane electrochemical gradient and membrane potential. As an essential P-type ATPase uniquely found in fungi and plants, Pma1 is an attractive antifungal drug target. Two recent Cryo-EM studies on Pma1 have revealed its hexameric architecture, autoinhibitory and activation mechanisms, and proton transport mechanism. These structures provide new perspectives for the development of antifungal drugs targeting Pma1. In this article, we review the history of Pma1 structure determination, the latest structural insights into Pma1, and drug discoveries targeting Pma1.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction.

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.