Excess Proton Research Articles

Simulation-based study of local hydrogen crossover dynamics and their effects on proton exchange membrane fuel cells

The hydrogen (H2) crossover significantly impacts the performance and durability of proton exchange membrane fuel cells (PEMFCs). However, understanding the intricate dynamics of local H2 crossover in fuel cells is challenging due to complex transport and reactions. This study introduces advanced two-phase, three-dimensional PEMFCs transient models, incorporating H2 dissolution, diffusion/convection, and surface reactions, providing a comprehensive analysis of the interplay between H2 crossover and local charge, mass and heat. The study emphasizes the substantial influence of geometric and operational parameters on both H2 crossover flux and distribution uniformity by adjusting local membrane water and H2 concentrations. Elevated H2 crossover flux is often accompanied by non-uniform distribution, particularly noticeable under conditions of elevated temperature, backing pressure, and relative humidity (RH). Sensitivity analysis reveals load-dependent parameter effects, with RH exerting the most significant impact at lower loads compared to temperature and pressure, with its effect diminishing at higher loads. Additionally, H2 crossover results in 1–10 mV decrease in dynamic voltage loss but 20–57 mV increase in steady-state voltage loss. The steady voltage loss diminishes notably with increasing current load before slightly rising 1–3 mV, attributed to reduced activation overpotential for the oxygen reduction reaction utilizing excess electrons and protons from permeated H2. The subsequent increase is associated with heightened ohmic polarization losses due to membrane dehydration at elevated catalyst layer temperatures near the channel. This study contributes to advancing our understanding of H2 crossover, providing valuable insights for the development of mitigation strategies in fuel cell operation.

Bjerrum defects in s-II gas hydrate

The energy and structure of Bjerrum defects in structure II gas hydrates were investigated by using first-principle calculations for finite-size clusters and periodic 3D lattice systems. The formation energies of these defects were calculated for the first time when the cages of the structure II structure were completely empty and the large cage was filled with a THF molecule. Analogous to findings in ice structures, one of the hydrogen atoms forming the D defect was noted to orient toward the cage. If the excess proton resides in the large cage, it acts as an attraction center for the polar guest molecule, i.e., THF. Therefore, the large cage guest THF molecule stabilizes the D/L defect pair and isolated D/L defect formation energies by forming hydrogen bonds with the D defect. In such cases, the defect structure representing a D/L defect pair containing a THF molecule interacting with one of the hydrogen atoms of the D defect mirrors the guest-induced ones. Notably, the classical Bjerrum defect and the guest-induced Bjerrum defect exhibit a similar phenomenon in defective structures. Contrary to existing literature, it is evident that guest-induced Bjerrum defects involve both the L and D components. The insights gained from this study could potentially offer an alternative perspective to understand various experimental observations, such as those related to dielectric and NMR properties.

Metastable Evaporation of Molecules from Water Clusters.

We probe the stability of water clusters by means of their metastable decay probability extracted from two-dimensional reflectron time-of-flight mass spectra. Two different methods are used to ionize and potentially excite the clusters and trigger the evaporation: (i) attachment of electrons with near-zero energies, producing negatively charged clusters, and (ii) electron impact ionization, producing protonated (H2O)nH+ clusters. The electron attachment is a soft ionization and therefore provides information about the size distribution of the neutral clusters in the beam due to a very limited amount of post-ionization loss of water molecules. A dependence of metastable fractions on the conditions of neutral clusters production prior to the electron attachment is reported. For the cations, the higher energy electron impact ionization leads to a more extensive metastable loss of water molecules. The results are discussed in the light of neutral cluster excitation energy distributions and, for negative clusters, also in terms of binding energies. The experiments demonstrate clearly the role of the excess electron vs the excess proton in the two different charge states of the clusters around sizes N = 50-55, for which binding energies of the anions are derived from the data.

Abstract C041: Emerging role of P2X receptors as Golgi pH regulators in pancreatic cancer

Abstract Cancer cells reprogram their metabolism to meet their elevated energy demands, favoring energy production via glycolysis even with functional mitochondria. Upregulated glycolysis results in rapid production of lactate and protons in the cytoplasm. This would normally reduce the intracellular pH (pHi); however, cancer cells employ mechanisms such as ion transporters that remove excess protons from the cytoplasm to maintain an alkaline pHi, allowing for cell growth, evasion of cell death and cell proliferation. Our previous studies have shown that in pancreatic ductal adenocarcinoma (PDAC), the Golgi apparatus is among the intracellular compartments that can uptake excess protons, giving PDAC cells a cell fitness advantage. The underlying mechanisms that drive Golgi acidification in PDAC are incompletely understood. Here, we set out to comprehensively identify novel modulators that can affect this process. By using a fluorescent-protein-based ratiometric pH biosensor targeted to the Golgi membrane, we screened thousands of chemical compounds for an impairment in Golgi acidification, which would lead to pHi dysregulation and cell death. One of our hits from the screen was an inhibitor of the P2X family of receptors (P2XRs). P2XRs consist of seven subtypes (P2X1-7) and are ATP-gated ion channels that mediate the passage of non-selective cations through the plasma membrane and intracellular compartments. P2XRs have also been shown to regulate the pHi. We are elucidating the mechanism by which P2XRs may regulate pHi via the Golgi in PDAC. Our preliminary results show that concomitant with the alkalinization of the Golgi, P2XR inhibition leads to a pronounced decrease of pHi and results in cell death in PDAC while normal untransformed pancreas cells are unaffected. These data suggest that P2XRs play a role in pHi regulation and cell fitness maintenance through a function in the Golgi. Targeting this metabolic reprograming dependency may serve as a novel therapeutic tool for PDAC. Citation Format: Cheska Marie Galapate, Koen Galenkamp, Charles Fisher, Susanne Heynen-Genel, Cosimo Commisso. Emerging role of P2X receptors as Golgi pH regulators in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl_2):Abstract nr C041.

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.

Suppressing proton-induced HER and cathode fading by an aprotic hybrid electrolyte for long-cycle stability of aqueous Na||Zn hybrid batteries

Aqueous zinc metal batteries with unique advantages of intrinsic safety and low cost are considered as one of the most promising next-generation energy-storage technologies. However, their lifespan is significantly constrained by the electrolyte, specifically due to limitations caused by proton-induced HER and dendritic growth at the anode side and dissolution phenomena at the cathode. Unlike most studies on the protic electrolyte systems, herein, a new aprotic hybrid electrolyte of Na/Zn tetrafluoroborate-trimethyl phosphate with ultralow proton activity tuned by reconstruction of hydrogen bonding network and ultrahigh interfacial stability endowed by simultaneous formation of SEI and CEI were designed for the first time. Attributed to the synergistic effect of proton activity attenuation and ZnF2-Zn3(PO4)2 SEI formation, HER-trigged corrosion current was decreased by 96.52%, which leads to an exceptionally high coulombic efficiency up to 99.63%, and operational lifespan exceeding 2000 hours at 1 mA cm−2/ 1 mAh cm−2 for Zn||Cu asymmetric cell and Zn||Zn symmetric cell, respectively. Furthermore, the suppression of excess hydrated proton co-insertion reaction and construction of inorganic-organic CEI enabled the PBAs||Zn full cell a prolonged cycling life of 6000 cycles with an ultralow decay rate of 0.004% per cycle. The special electrolyte utilizing cheap tetrafluoroborate salts in conjunction with cost-effective PBA cathode and Zn anode provides a new choice for low-cost, high-safety, non-toxic batteries for practical applications.

Global Distribution of EMIC Waves and Its Association to Subauroral Proton Precipitation During the 27 May 2017 Storm: Modeling and Multipoint Observations

AbstractRecent simulation studies using the RAM‐SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the magnetosphere‐ionosphere coupling. In this study, we use the BATS‐R‐US + RAM‐SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave‐particle interactions. During the 27 May 2017 storm, Arase and RBSP‐A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk‐midnight sector. Simulation results show that H‐ and He‐band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP‐A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H‐band waves in the dusk sector are stronger than He‐band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS‐R‐US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm.

Two co-existing and opposing mechanisms of proton transfer in one-dimensional open-end water chains.

The proton transport in one-dimensional (1D) confined water chains has been extensively studied as a model for ion channels in cell membrane and fuel cell. However, the mechanistic understanding of the proton transfer (PT) process in 1D water chains remains incomplete. In this study, we demonstrate that the two limiting structures of the hydrated excess proton, H5O2+ (Zundel) and H3O+ (linear H7O3+), undergo a change in dominance as the water chain grows, causing two co-existing and opposing PT mechanisms. Specifically, H5O2+ is stable in the middle of the chain, whereas H3O+ serves as a transition state (TS). Except for this region, H3O+ is stabilized while H5O2+ serves as a TS. The interaction analysis shows that the electrostatic interaction plays a crucial role in the difference in PT mechanisms. Our work fills a knowledge gap between the various PT mechanisms reported in bulk water and long 1D water chains, contributing to a deeper understanding of biological ion channels at the atomic level.

Reshaping the QM Region On-the-Fly: Adaptive-Shape QM/MM Dynamic Simulations of a Hydrated Proton in Bulk Water.

Adaptive quantum mechanics/molecular mechanics (QM/MM) reclassifies on-the-fly a molecule or molecular fragment as QM or MM during dynamics simulations without abrupt changes in the energy or forces. Notably, the permuted adaptive-partitioning (PAP) algorithms have been applied to simulate a hydrated proton, with a mobile QM zone anchored at a pseudoatom called a proton indicator. The position of the proton indicator approximates the location of the delocalized excess proton, yielding a smooth trajectory of the proton diffusing via the Grotthuss mechanism in aqueous solutions. The mobile QM zone, which has been taken to be a sphere with a preset radius, follows the proton wherever it goes. Although the simulations are successful, the use of a spherical QM zone has one disadvantage: A large preset radius must be utilized to minimize the chance of missing water molecules that are important to proton translocation. A large radius leads to a large QM zone, which is computationally expensive. In this work, we report a new way to set up the QM zone, where one includes only the water molecules important to proton transfer. The importance of a given water molecule is quantified by its "weight" that depends on its relation to the reaction path of proton transfer. The weight varies smoothly, ensuring that a water molecule gradually appears in or disappears from the QM zone without abrupt changes, as required by the PAP method. Consequently, the shape of the QM zone evolves on-the-fly, keeping the QM zone as small as possible and as large as necessary. Test simulations demonstrate that the new algorithm significantly improves the computation efficiency while maintaining the proper descriptions of proton transfer in bulk water.

Vibrational signatures of dynamic excess proton storage between primary amine and carboxylic acid groups.

Ammonium and carboxylic moieties play a central role in proton-mediated processes of molecular recognition, charge transfer or chemical change in (bio)materials. Whereas both chemical groups constitute acid-base pairs in organic salt-bridge structures, they may as well host excess protons in acidic environments. The binding of excess protons often precedes proton transfer reactions and it is therefore of fundamental interest, though challenging from a quantum chemical perspective. As a benchmark for this process, we investigate proton storage in the amphoteric compound 5-aminovaleric acid (AV), within an intramolecular proton bond shared by its primary amine and carboxylic acid terminal groups. Infrared ion spectroscopy is combined with abinitio Molecular Dynamics (AIMD) calculations to expose and rationalize the spectral signatures of protonated AV and its deuterated isotopologues. The dynamic character of the proton bond confers a fluxional structure to the molecular framework, leading to wide-ranging bands in the vibrational spectrum. These features are reproduced with remarkable accuracy by AIMD computations, which serves to lay out microscopic insights into the excess proton binding scenario.

β3-Adrenoceptor as a new player in the sympathetic regulation of the renal acid-base homeostasis.

Efferent sympathetic nerve fibers regulate several renal functions activating norepinephrine receptors on tubular epithelial cells. Of the beta-adrenoceptors (β-ARs), we previously demonstrated the renal expression of β3-AR in the thick ascending limb (TAL), the distal convoluted tubule (DCT), and the collecting duct (CD), where it participates in salt and water reabsorption. Here, for the first time, we reported β3-AR expression in the CD intercalated cells (ICCs), where it regulates acid-base homeostasis. Co-localization of β3-AR with either proton pump H+-ATPase or Cl-/HCO3 - exchanger pendrin revealed β3-AR expression in type A, type B, non-A, and non-B ICCs in the mouse kidney. We aimed to unveil the possible regulatory role of β3-AR in renal acid-base homeostasis, in particular in modulating the expression, subcellular localization, and activity of the renal H+-ATPase, a key player in this process. The abundance of H+-ATPase was significantly decreased in the kidneys of β3-AR-/- compared with those of β3-AR+/+ mice. In particular, H+-ATPase reduction was observed not only in the CD but also in the TAL and DCT, which contribute to acid-base transport in the kidney. Interestingly, we found that in in vivo, the absence of β3-AR reduced the kidneys' ability to excrete excess proton in the urine during an acid challenge. Using ex vivo stimulation of mouse kidney slices, we proved that the β3-AR activation promoted H+-ATPase apical expression in the epithelial cells of β3-AR-expressing nephron segments, and this was prevented by β3-AR antagonism or PKA inhibition. Moreover, we assessed the effect of β3-AR stimulation on H+-ATPase activity by measuring the intracellular pH recovery after an acid load in β3-AR-expressing mouse renal cells. Importantly, β3-AR agonism induced a 2.5-fold increase in H+-ATPase activity, and this effect was effectively prevented by β3-AR antagonism or by inhibiting either H+-ATPase or PKA. Of note, in urine samples from patients treated with a β3-AR agonist, we found that β3-AR stimulation increased the urinary excretion of H+-ATPase, likely indicating its apical accumulation in tubular cells. These findings demonstrate that β3-AR activity positively regulates the expression, plasma membrane localization, and activity of H+-ATPase, elucidating a novel physiological role of β3-AR in the sympathetic control of renal acid-base homeostasis.

The oxidation of d‐galactose into mucic acid (galactaric acid): experimental and computational insights towards a bio‐based platform chemical

AbstractAldaric acids are becoming valuable platform chemicals in bioeconomy and circular economy approaches. Among them, mucic acid (meso‐galactaric acid) is gaining industrial attention as a bio‐based precursor of adipic acid found in polyamides, and of 2,5‐furan dicarboxylic acid, a valuable substitute for terephthalic acid in polyesters. A combination of detailed experimental protocols and conditions with DFT computational calculations for the synthesis of mucic acid from galactose are herein described. The product is obtained by reaction with diluted aqueous nitric acid as the oxidizing agent and the reaction medium. Optimized reaction conditions require 5 M nitric acid, 95 °C, galactose/HNO3 molar ratio 1 : 9, 100 g/L galactose concentration in the reaction medium. Computational calculations suggest that the oxidation mechanism may involve a contextual oxygen protonation and nitrate nucleophilic attack, leading to the oxidized product. The proposed mechanism requires a stoichiometry of 6 moles of nitric acid per galactose mole, albeit both experimental and computational data evidence that proton excess may be necessary to start the reaction. The reported study gives useful insights for the production of galactaric acid.

Speciation of the proton in water-in-salt electrolytes.

Water-in-salt (WiS) electrolytes are promising systems for a variety of energy storage devices. Indeed, they represent a great alternative to conventional organic electrolytes thanks to their environmental friendliness, non-flammability, and good electrochemical stability. Understanding the behaviour of such systems and their local organisation is a key direction for their rational design and successful implementation at the industrial scale. In the present paper, we focus our investigation on the 21 m bis(trifluoromethanesulfonyl)imide (LiTFSI) WiS electrolyte, recently reported to have acidic pH values. We explore the speciation of an excess proton in this system and its dependence on the initial local environment using ab initio molecular dynamics simulations. In particular, we observe the formation of HTFSI acid in the WiS system, known to act as a superacid in water. This acid is stabilised in the WiS solution for several picoseconds thanks to the formation of a complex with water molecules and a neighboring TFSI- anion. We further investigate how the excess proton affects the microstructure of WiS, in particular, the recently observed oligomerisation of lithium cations, and we report possible notable perturbations of the lithium nanochain organisation. These two phenomena are particularly important when considering WiS as electrolytes in batteries and supercapacitors, and our results contribute to the comprehension of these systems at the molecular level.

Analysis of the structural H/D isotope effect with an excess proton/deuteron in light/heavy water solvent using path integral molecular dynamics simulations

Abstract We analyzed the difference in the structural H/D isotope effect between an excess proton in light water (H-body) and an excess deuteron in heavy water (D-body), including the nuclear quantum effect, using path integral molecular dynamics simulations. We found that the second peak of the H-body is shorter than that of the D-body in the radial distribution function of O*–O, where O* is the oxygen atom of the H3O+/D3O+ fragment. The main reason for this would be the difference in the ratio of the Zundel structure with the sp3-like configuration, where the Zundel structure in the H-body (14.0%) is greater than that in the D-body (12.0%). We also found rare occurrences of double H3O+/D3O+ configurations, mainly including Zundel–Zundel-like structures such as H7O3+/D7O3+ and H9O4+/D9O4+. The ratios of such configurations appearing in our simulations are 0.89% and 0.20% for the H-body and the D-body, respectively.

Microenvironment-Driven Fenton Nanoreactor Enabled by Metal-Phenolic Encapsulation of Calcium Peroxide for Effective Control of Dental Caries.

Caries are one of the most common oral diseases caused by pathogenic bacterial infections, which are widespread and persistently harmful to human health. Using nanoparticles to invade biofilms and produce reactive oxygen species (ROS) in situ is a promising strategy for killing bacteria and disrupting the structure of biofilms. In this work, a biofilm-targeting Fenton nanoreactor is reported that can generate ROS responsive to the cariogenic microenvironment. The nanoreactor is constructed by metal-phenolic encapsulation of calcium peroxide (CaO2) followed by modification with a biofilm targeting ligand dextran. Within the cariogenic biofilm, the Fenton nanoreactor is activated by an acidic microenvironment to be decomposed into H2O2 and iron ions, triggering a Fenton-like reaction to generate ROS that can eliminate the biofilm by breaking down extracellular polymeric substances(EPS)and killing cariogenic bacteria. Meanwhile, the depletion of excess protons in biofilm leads to a reversal of the cariogenic microenvironment. The Fenton nanoreactor can effectively inhibit the biofilm formation of Streptococcus mutans on ex vivo human teeth and is effective in preventing caries meanwhile maintaining the oral microbial diversity in rat caries infection model. This work provides a novel and efficient modality for acid microenvironment-driven ROS therapy.

Ribosome profiling reveals the fine-tuned response of Escherichia coli to mild and severe acid stress.

Bacteria react very differently to survive in acidic environments, such as the human gastrointestinal tract. Escherichia coli is one of the extremely acid-resistant bacteria and has a variety of acid-defense mechanisms. Here, we provide the first genome-wide overview of the adaptations of E. coli K-12 to mild and severe acid stress at both the transcriptional and translational levels. Using ribosome profiling and RNA sequencing, we uncover novel adaptations to different degrees of acidity, including previously hidden stress-induced small proteins and novel key transcription factors for acid defense, and report mRNAs with pH-dependent differential translation efficiency. In addition, we distinguish between acid-specific adaptations and general stress response mechanisms using denoising autoencoders. This workflow represents a powerful approach that takes advantage of next-generation sequencing techniques and machine learning to systematically analyze bacterial stress responses.

Comparison of classical and abinitio simulations of hydronium and aqueous proton transfer.

Proton transport in aqueous systems occurs by making and breaking covalent bonds, a process that classical force fields cannot reproduce. Various attempts have been made to remedy this deficiency, by valence bond theory or instantaneous proton transfers, but the ability of such methods to provide a realistic picture of this fundamental process has not been fully evaluated. Here we compare an abinitio molecular dynamics (AIMD) simulation of an excess proton in water to a simulation of a classical H3O+ in TIP3P water. The energy gap upon instantaneous proton transfer from H3O+ to an acceptor water molecule is much higher in the classical simulation than in the AIMD configurations evaluated with the same classical potential. The origins of this discrepancy are identified by comparing the solvent structures around the excess proton in the two systems. One major structural difference is in the tilt angle of the water molecules that accept an hydrogen bond from H3O+. The lack of lone pairs in TIP3P produces a tilt angle that is too large and generates an unfavorable geometry after instantaneous proton transfer. This problem can be alleviated by the use of TIP5P, which gives a tilt angle much closer to the AIMD result. Another important factor that raises the energy gap is the different optimal distance in water-water vs H3O+-water H-bonds. In AIMD the acceptor is gradually polarized and takes a hydronium-like configuration even before proton transfer actually happens. Ways to remedy some of these problems in classical simulations are discussed.

Intra-cluster Charge Migration upon Hydration of Protonated Formic Acid Revealed by Anharmonic Analysis of Cold Ion Vibrational Spectra.

The rates of many chemical reactions are accelerated when carried out in micron-sized droplets, but the molecular origin of the rate acceleration remains unclear. One example is the condensation reaction of 1,2-diaminobenzene with formic acid to yield benzimidazole. The observed rate enhancements have been rationalized by invoking enhanced acidity at the surface of methanol solvent droplets with low water content to enable protonation of formic acid to generate a cationic species (protonated formic acid or PFA) formed by attachment of a proton to the neutral acid. Because PFA is the key feature in this reaction mechanism, vibrational spectra of cryogenically cooled, microhydrated PFA·(H2O)n=1-6 were acquired to determine how the extent of charge localization depends on the degree of hydration. Analysis of these highly anharmonic spectra with path integral ab initio molecular dynamics simulations reveals the gradual displacement of the excess proton onto the water network in the microhydration regime at low temperatures with n = 3 as the tipping point for intra-cluster proton transfer.

Layer defoliation and slippery of triformylphloroglucinol-covalent organic framework under pH stress

The triformylphloroglucinol covalent organic framework is a keto-enamine-based porous matrix generally presented as two-dimensional layers stacked to form a superstructure. This study aims to compare the layer stability for three Tp-COFs, two from triformylphloroglucinol and compounds containing the 2,5-diamino structure, TpPa(OH)2 and TpPaSO3Na, and another from triformylphloroglucinol and biuret, called TpBu. The pH −0.2 stresses moderately slip the TpPa(OH)2 layers, while pH 14 stress leads to severe layer slippery to deteriorate its long-range crystal structure. The excess protons protonate the sulfite group of TpPaSO3Na, weaken the H bonds between amine and S-O groups, and make the pH −0.2 stress layer slippery. The layer stability of Tp-COFs correlates with the H bonds formed with the amine groups. The TpBu has excessive amine groups, hence having the highest layer stability.

Developing machine-learned potentials to simultaneously capture the dynamics of excess protons and hydroxide ions in classical and path integral simulations.

The transport of excess protons and hydroxide ions in water underlies numerous important chemical and biological processes. Accurately simulating the associated transport mechanisms ideally requires utilizing abinitio molecular dynamics simulations to model the bond breaking and formation involved in proton transfer and path-integral simulations to model the nuclear quantum effects relevant to light hydrogen atoms. These requirements result in a prohibitive computational cost, especially at the time and length scales needed to converge proton transport properties. Here, we present machine-learned potentials (MLPs) that can model both excess protons and hydroxide ions at the generalized gradient approximation and hybrid density functional theory levels of accuracy and use them to perform multiple nanoseconds of both classical and path-integral proton defect simulations at a fraction of the cost of the corresponding abinitio simulations. We show that the MLPs are able to reproduce abinitio trends and converge properties such as the diffusion coefficients of both excess protons and hydroxide ions. We use our multi-nanosecond simulations, which allow us to monitor large numbers of proton transfer events, to analyze the role of hypercoordination in the transport mechanism of the hydroxide ion and provide further evidence for the asymmetry in diffusion between excess protons and hydroxide ions.