Collective Proton Dynamics at Highly Charged Interfaces Studied by

Article Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Collective Proton Dynamics at Highly Charged Interfaces Studied by Ab Initio Metadynamics Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Swati Vartak, Ata Roudgar, Anatoly Golovnev, and Michael Eikerling J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp308313v • Publication Date (Web): 11 Dec 2012 Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Just Accepted Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. Subscriber access provided by Brown University Library The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Collective Proton Dynamics at Highly Charged Interfaces Studied by Ab Initio Metadynamics Swati Vartak, Ata Roudgar, Anatoly Golovnev, Michael Eikerling Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A1S6 KEYWORDS: surface proton conduction, dense monolayer of anionic surface groups, proton-conducting polymers Supporting Information Placeholder ABSTRACT: Surface proton conduction is of utmost importance in biology, materials science and electrochemistry. Yet, experimental findings of ultrafast proton transport at densely packed arrays of anionic surface groups have remained controversial and unexplained. We present an ab initio molecular dynamics study of proton dynamics at sulfonic-acid terminated surface groups. Results furnish a highly efficient collective mechanism of hydronium ion translocations at a critical surface group separation of ~7 Å. Orientational fluctuations of SG trigger hydrogen bond breaking that sets off the hydronium ion motion. The activation free energy of this process is 0.3 eV (±0.1 eV). The soliton-like nature of this mechanism is owed to the trigonal symmetry of sulfonate anions and exceptionally strong interfacial hydrogen bonding. These insights should stimulate surface conductance studies at SG monolayers with sulfonic acid groups and they bolster efforts in designing proton conducting polymers conducive to fuel cell operation above ~ 100ºC.

INTRODUCTION Proton transfer via hydrogen-bonded water chains in proteins and at mitochondrial membranes is essential for biological energy transduction1. Similarly, operation of polymer electrolyte fuel cells (PEFC) hinges on high rates of proton transfer in water2-4. Exceptional energy efficiency, high power density, and ideal compatibility with hydrogen, render PEFC a primary solution to the global energy challenge5, 6. Leading automotive companies and energy suppliers are working towards the rollout of retail fuel cell vehicles from 2015. Recent progress in performance, durability and cost reduction is promising in view of this target. In PEFC, spatially separated redox processes generate a gradient in electrochemical potential that drives separate electron and proton fluxes. Anodic hydrogen oxidation produces a flux of electrons to perform electrical work in vehicle drive trains, portable electronics, and power generators. Electrons and protons recombine with oxygen to produce water at the cathode. A polymer electrolyte membrane (PEM) separates anodic and cathodic reactions and enables the rapid passage of protons3, 5, 6. Suitable PEM must exhibit high proton conductivity, low gas permeability, electronic insulation, and stability under strongly oxidizing conditions. The standard PEM in PEFC is Nafion® of Du Pont de NemoursTM 4, 7, 8. Immersion into water induces ionomer self-organization into a phaseseparated nanostructure (Fig. 1a). Protons move relative to polymer-water interfaces that are lined by surface groups (SGs) with head groups of sulfonate anions (Fig. 1b). The dilemma of Nafion-like PEM is their dependence on water. If soaked with liquid water, the combination of high proton concentration, exceptional proton mobility, and excellent pore network connectivity results in proton conductivity

of 0.1 Scm-1 that meets demands of fuel cell developers4, 5, 7, 8. Extensive loss of water and conductivity above 80ºC as well as the glass transition temperature of ~100ºC curtail, however, the operational range of these PEM9. Materials research explores multifarious avenues to expand the range of operation of proton-conducting materials4, 7, 8, 10-22 Anomalously high proton conductivity at ~150oC was observed by Schuster et al12 in PEM with high ion density; in this temperature range, proton transport must involve strongly bound interfacial water. A recent trend is the replacement of NafionTM by ionomer membranes with short side chains and high density of ion exchange sites; like Aquivion® by SolvayTM, which exhibits superior proton conductivity, water retention and thermal stability23. In biological systems, proton transfer in water surrounding acidic residues in protein channels and at mitochondrial membranes has drawn tremendous attention from the 1960s, starting with Mitchell’s chemiosmotic hypothesis24. Teissie et al25, using a pH-sensitive fluorescence probe to monitor changes in proton concentration at a lipid monolayer, had found a surface proton diffusion coefficient that was 20 times the value of bulk water (9.3*10-5 cm2s-1). This result was heavily disputed26. Zhang and Unwin27, using an SECM proton feedback method, found a value of 6*10-6 cm2s-1. Serowy et al28, using flash photolysis to produce protons and a lipid bound fluorescent dye to monitor changes in local pH, determined a surface diffusion coefficient of 5.8*10-5 cm2s-1. Morgan et al29 measured lateral proton conductance at the alkanoic acid and lipid monolayerwater interface as a function of area per molecule. They observed that during a surface compression cycle, surface potential and interfacial conductance of various monolayers exhibited pronounced increases below a critical surface area. Based on the results of their control experiments they concluded that the increase in lateral conductance during monolayer compression was due to the transport of protons along a two-


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dimensional hydrogen bond network formed between the monolayer headgroups and adjacent water molecules. Their conclusions are in agreement with those of Sakurai et al30 who measured lateral proton conductance along a phosphatidylcholine monolayer. Infrared spectroscopy studies of Leberle et al31 support the speculations of Morgan et al and Sakurai et al. Their results show that phosphatidylserine headgroups are hydrogen bonded with large proton polarizabilities, which helps the efficient proton transfer between proton donoracceptor groups by creating lateral electrochemical gradients. In spite of diverging predictions of surface proton diffusion coefficients, the quoted studies have consistently reported on the impact of monolayer composition, reduced dimensionality, and interfacial ordering on proton dynamics. Altogether, there is thus ample evidence for highly efficient surface proton transport, which is highly sensitive to packing density and chemical nature of acid headgroups. Surface pressure, surface electrostatic potential and lateral proton conductivity increase dramatically upon monolayer compression below a critical area of about 25 Å2 per SG that corresponds to a nearestneighbor separation distance of ~7 Å1, 32. Continuum dielectric theories and classical molecular dynamics (MD) studies have been employed to study proton transport in PEM33. These approaches treated charged interfaces as perturbations of bulk proton conduction mechanisms. They remained inconclusive with respect to the role of packing density, conformational fluctuations of sidechains, and

charge delocalization of anionic groups on proton conduction. MD simulations based on empirical valence bond (EVB) models have been applied to study proton dynamics in bulk water, biomolecular systems, and PEM34, 35. The EVB method accounts for hydrogen bond (HB) breaking and making in a highly efficient way; it relies, however, on heavy parameterization of proton transitions between predefined states. None of the mentioned approaches could describe surface proton conduction at high density of ion exchange sites. On the other hand, static quantum chemistry studies at the density functional theory (DFT) level that focus on proton solvation and dissociation of small polymeric fragments cannot capture the concerted dynamics that gives rise to high rates of surface proton conduction36. Few theoretical attempts, invoking mainly 1D soliton models, have focused explicitly on surface proton conductance at densely packed Langmuir films32, 37. In spite of the ubiquitous importance of proton conductance at highly charged interfaces, molecular-level understanding of underlying mechanisms is grossly lagging behind the understanding of proton transport in bulk water. For bulk water, essential details of the structural diffusion of protonic defects have been resolved by ab initio molecular dynamics (AIMD)38-40. In the work reported in this Article, we have adopted the ab initio Lagrangian metadynamics method of Laio and Parrinello41 to explore reaction pathways and free energy surfaces of interfacial transport.

FIGURE 1. Comb-shaped ionomer molecules self-assemble into fibrous aggregates that form the matrix of a hydrophilic pore network, containing water, hydronium ions and anionic head groups (a). Protons in pores move relative to polymer-water interfaces that are lined by anionic surface groups (SG) (b). The atomistic configuration of our basic model in (c) with SG of the type R-SO3H emulates the structure of the interface at high SG density. We fix terminating carbon atoms of SG at positions of a hexagonal lattice. In the basic configuration, we use R = CF3 (trifluoromethane sulfonic acid) and add one water molecule per SG to simulate minimal hydration. Specific SG corresponding to donor (D), acceptor (A) and spectator groups (S) of a hydronium ion transition are highlighted. Moreover the distances that define the collective variable of the H3O+ ion shift are shown, dCV = d12 – d23.

MODEL AND COMPUTATIONAL DETAILS Earlier findings of stable interfacial conformation at minimal hydration11, 23, 24 form the backbone of the present AIMD study of surface proton dynamics. The model system, depicted in Fig. 1b, consists of an array of SG of the type CF3SO3H with one water molecule per SG. Terminating C atoms of SG are anchored at positions of hexagonal lattice with lattice constant dCC. Initially, we selected the hexagonal arrangement based on symmetry considerations. Thereafter, explicit DFT calculations have shown that self-organization among hy-

dronium (H3O+) and sulfonate ions (SO3-) enforces hexagonal ordering at high surface group density (cf. Fig. 3 in Ref.[34]). We report results of ab initio metadynamics simulations performed at the density functional theory level using the CP2K package which implements a mixed Gaussian and plane wave method (GPW method42, 43). Using this methodology, we have determined the activation energy barriers and reaction pathways of relevant interfacial proton transitions. Metadynamics is an efficient method for exploring rare events in many body systems. It uses a coarse-grained dynamics in a space of a few time-dependent collective variables (CV) to simulate the system trajectory. Adding small Gaussian functions along the trajectory of the CV creates a history-

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dependent potential. Height and width of these Gaussian functions must be optimized in view of sampling efficiency and precision of free energy calculations. Furthermore, a harmonic coupling between real and fictitious CV in the metadynamics Lagrangian ensures uniform sampling of configurations in well regions of the free energy landscape. This leads to an even distribution of the history dependent potential. In this method, valence electrons are represented by doubleξ augmented Gaussian basis sets (DZVP-MOLOPT)44. The pseudopotential of Goedecker, Teter and Hutter (GTH) represents core electrons45. The energy cut-off of plane wave expansions was 300 Ry. Exchange and correlation energies were computed within the GGA approximation using the BLYP functional46. For every time step, the electronic structure was quenched to an accuracy of 10-7 Hartree. Starting each metadynamics run from the optimized geometry of the condensed state (Fig. 1c), the system was thermalized in the NVT ensemble for around 3 ps. The temperature was set to 300 K using a Nóse-Hoover thermostat. We used a time step of 0.3 fs and 0.5 fs for simulations of collective and local defect-type mechanisms (explained in detail below), respectively. In the metadynamics Lagrangian we used a coupling constant k = 0.5 a.u., 0.4 a.u. and a fictitious particle mass of M = 50 a.u., 75 a.u. for collective and local defect-type simulation respectively. Gaussian potentials with height h = 0.013 eV and width Δδ = 0.02 Å were used to create the history dependent potential. A Gaussian function was added to the history dependent potential every time when the displacement of the CV relative to the previous state reached 3Δδ/2.

INTERFACIAL TRANSITIONS

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Our previous calculations47 showed that for dCC below 9 Å, the minimally hydrated array of SG undergoes a transition to a completely dissociated state. Further densification of SG triggered a transition to the condensed 2D state at dCC ≈ 6.7 Å, shown in Fig. 1c. Upon addition of extra water we found a wetting transition from a hydrophilic for dCC > 7.5 Å to superhydrophobic for dCC < 6 Å. In the condensed superhydrophobic state, the number of interfacial HB saturates due to matching trigonal symmetries of H3O+ and SO3-; remarkably, the HB strength (~0.5 eV) is markedly enhanced compared to HB energies in bulk water (0.1 eV). In a previous article, we have reported results of systematic conformational studies for a series of surface groups with increasing length, namely -CF3CF2SO3H, -CF3CF2CF2SO3H and -CF3OCF2CF2SO3H48. Notably, the last surface group listed resembles the sidechains in Dow® and Aquivion® membranes. The same sequence of interfacial transitions, as for the shortest surface group (triflic acid), was found for the sequence of surface groups with increasing length. The value dCC ≈ 6.7 Å for the concomitant structural and wetting transitions agrees with the critical distance of dCC ≈ 7 Å for transitions in surface pressure and electrostatic potential in experimental monolayer studies27, 28, 32, 49. In spite of these consistent findings that affirm the validity of our model system, we duly note that disorder of the fixation points of surface groups could be an important aspect to evaluate in future studies.

FIGURE 2. Lattice configurations of the 2D two-component lattice of H3O+ ions and SO3- anions. The native 2D crystal structure in (a) is shown at the critical SG density of dCC ≈ 6.7 Å. In the introduced symbolic representation, green triangles indicate hydronium positions and red dots represent positions of sulfonate anions. The structure after completion of a single H3O+ ion move, shown in (b), creates a hydrogen bond deficit at the donor site and a hydrogen bond excess at the acceptor site. Structures obtained after collective H3O+ ion translocations that conserve the total number of hydrogen bonds are shown in (c) and (d), denoted, for obvious reasons, as single file and parallel file proton transfer. Occurrence of the different transitions depends on the hydrogen bond strengths at the interface. Stronger hydrogen bonds enforce collective proton transfer. INTERFACIAL PROTON TRANSITIONS Our foremost interest lies in understanding whether the highly ordered and exceptionally stable array of densely packed SG could efficiently promote long-range proton transfer. Fig. 2 introduces an intuitive graphical representation of possible interfacial H3O+ ion transitions on the 2D two-component lattice at dCC ≈ 6.7 Å. Filled green triangles represent mobile H3O+ ions. Vertices (red dots) correspond to SO3- anions. A local, defect-type H3O+ ion translocation (Fig. 2b) creates an

HB deficit at the donor SG and an HB excess at the acceptor SG. The HB structure of the two spectator SG is not affected. Any interfacial proton transfer on the lattice in Fig. 2a can be represented as a sequence of these elementary H3O+ ion translocations. Fig 2c and d show final structures after collective single file and multifile H3O+ ion transfers which are enforced by strong interfacial HB. These transitions conserve the number of interfacial HB due to concerted orientational motions of donor and acceptor SGs.


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In the previous work50, the potential energy surface (PES) of the concerted H3O+ ion transition in Fig. 2d was reconstructed using geometry optimization studies, based on Car-Parinello MD, in a space of three collective variables, corresponding to rotation and tilting of donor SG and lateral shift of the transferring H3O+ ion. The activation energy of the H3O+ ion transfer (~ 0.5 eV) calculated through this static approach is, however, imprecise, since it does not account for the full dynamic range of lattice reorganization during the transition. In our ab initio metadynamics study, reported here, we have investigated two extreme scenarios, corresponding to local defect-type (Fig. 2b) and highly collective H3O+ ion transitions

(Fig. 2d). In both cases, the initial structure was the perfectly ordered condensed state at dCC ≈6.7 Å (Fig. 2a). Proton transitions involve rapid HB fluctuations, orientational fluctuations of SG, and translational H3O+ ion motion. We have performed laborious metadynamics test runs to select appropriate CVs from the set of degrees of freedom and to fine-tune metadynamics parameters. As a conclusion from these studies, we have selected a single metadynamics CV, namely the lateral H3O+ ion shift, as defined by dCV = d12 – d23 in Fig. 1c.

FIGURE 3. Trajectories of the real (red) and fictitious (black) collective variable as a function of simulation time. The trajectory in (a) corresponds to a local defect-type H3O+ ion translocation, depicted in Figure 2b. The transition to this state was studied for a unit cell with 12 SG. The collective transfer in (b), illustrated schematically in Figure 2d, was studied for a unit cell of 3 SG. Local defect-type and collective transitions were simulated using unit cells of 12 and 3 SG, respectively. Fig. 3 shows trajectories of the CV for the two transitions. Transitions are completed within 140 ps and 45 ps for local and collective pathways. Fig. 4 shows reconstructed Helmholtz energy profiles as a function of the CV along with snapshots of configurations during transitions. Adding Gaussian potentials fills up

the potential well at the initial configuration and enforces H3O+ ion to shift away from the donor SG (D) towards the acceptor SG (A). Relocation distances of H3O+ ions during transitions are 3-4 Å.

FIGURE 4. Reconstructed free energy surfaces for (left panel, l) local defect-type and (right panel, r) collective H3O+ ion translocations, corresponding to the trajectories in Figure 3. Snapshots on the sides show structures at points A, B, C, Dn/c marked at the trajectories in Figure 3 as well as at the free energy profiles. Activation and reaction Helmholtz energies of the local defect-type transition (Fig. 4l) are ΔFa=0.6 eV and ΔFr=0.5 eV.

The final state of the transition is metastable. For the collective transition, activation and reaction Helmholtz energies are



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ΔFa=0.3 eV and ΔFr=0.0 eV. Considering the computational error in activation energy (0.1 eV), the value of ΔFa for the collective transition is 2-3 times larger than the activation energy of proton transport in bulk water (0.1 eV), as expected based on the increased HB strength. It is to be seen in further refinements of metadynamics simulations whether different choice of CVs and longer SG will reduce ΔFa. It is expected that the collective nature of hydronium ion translocations at the interface could give high total rates of proton transfer in spite of the higher activation energy. This possibility is evaluated in a soliton theory of collective proton transfer in our model system that is under development.

Figure 5. Stretching frequency spectrum of a spectator covalent O-H bond. The increased HB strength is manifested in the lowered stretching frequency of covalent O-H bonds in hydronium ions. HB formation weakens the covalent O-H bond thereby reducing its stretching frequency. We calculated O-H stretching frequencies by taking Fourier transform of velocity correlation function. HB fluctuations cause the covalent O-H bonds to elongate/compress, thereby giving rise to different absorption bands in the spectrum. We calculated frequency spectra for spectator O-H bond, O-H bond of a hydronium ion not undergoing flipping, and directly involved O-H bond i.e. the O-H bond of a hydronium ion undergoing flipping. As shown in Figure 5, we observed 3 bands for spectator O-H bonds in 100-4000 cm-1 range: (1) the most intense band lies in the 1301330 cm-1 range with a peak around 675 cm-1, (2) a band with lower intensity band is seen in the 1450-2260 cm-1 range with a peak at ~1720 cm-1 and (3) the lowest intensity band in lies in the 2450-3600 cm-1 range. An additional peak at ~3720 cm-1 was observed for the directly involved O-H bond. This peak appeared due to the added bias weakening HB by translating the hydronium ion away from the donor SG. These peaks can be assigned to different stretching modes by comparing our results with the experimental as well as computational studies reported in the literature for different hydrogen-bonded systems. Agostini et al51 simulated a vibrational spectrum for protonated water dimer and assigned peaks at 1317 cm-1 and 630 cm-1 to asymmetric and symmetric OH--O stretching modes respectively. Although hydrogen bonds in our system are not symmetric as in protonated water dimer, we can still roughly assign the first two peaks to ‘symmetric’ and ‘asymmetric’ OH--O stretching modes. Infrared study of interfacial water in nafion membranes52, model nafion sidechain molecule53 and highly concentrated solution of trifluoromethanesulfonic acid54 attributed a broad band (~2700-3400 cm-1) with a peak around 2750 cm-1 to the O-H stretching of hydronium ions associated with SO3- groups. Finally, the peak above 3600 cm-1 was assigned to the O-H stretching mode of weakly Hbonded water molecules52. This is in good agreement with our results as we see this peak only for directly involved O-H bond. Buzzoni et al55 proposed a model for solvated H5O2+ ion

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to better understand the IR spectrum of hydrogen bonded systems. They categorized the OH--O groups into ‘internal’, ‘external’ and ‘solvation’ groups according to the O-O distances. For internal groups, with O-O distance of 2.45-2.57 Å, the stretching absorption is expected to be in 1300-2200 cm-1 range. With O-O distances in the range of 2.6-2.7 Å, the external groups are expected to absorb radiations in 2500-3200 cm-1 region and the solvation layer groups with O-O distances greater than 2.7 Å would show a constant absorption in 33003400 cm-1. These expected stretching frequencies for different O-O distances are in line with our results. Relative intensities of different bands give information about relative prominence of different stretching modes. The largest intensity of lowest frequency band in the spectrum indicates the presence of larger number of configurations with strong HB in the system. Atomic configurations in Fig. 4 illustrate snapshots of intermediate structures during transitions, taken at the points AD marked at trajectories and Helmholtz energy profiles. Orientational fluctuations of the donor SG trigger HB breaking with the transiting H3O+ ion (A). HB breaking and reformation is responsible for the steep ascending and descending flanks of free energy along the CV. The H3O+ ion shift occurs in the saddle point region (B); it involves a flip of the H3O+ ion while its remaining two HB with spectator SG remain intact. Frame C shows the formation of the HB with the acceptor SG. Relaxation of the interfacial network occurs in the well region of the final state (Dn or Dc), involving further rotation and tilting of SG to optimize HB configurations. In the case of the collective transition, each donor SG rotates to accept another HB from the left, simultaneously with HB formation between H3O+ ion and acceptor SG. The final state (Dn) after the local transition shows HB defects at donor (undersaturated) and acceptor SG (oversaturated), rendering this state highly unstable. The metadynamics method allows one to use several CVs to reconstruct the free energy surface of a system under investigation. But the cost of simulations is strongly dependent on the number of CVs used. Our results show that even with the use of only one CV, the scenario of highly improbable proton transfer as obtained through static calculations (activation energy ~ 0.5 eV) changes to the one where the activation barrier for proton transfer is surmountable (~ 0.3 eV) and the collective nature of proton transfer will help in achieving high proton conductance. The activation energy is in reasonable agreement with activation energy of proton transport in Nafion under minimal hydration, as determined from Arrhenius plots of experimental conductivity data by Cappadonia et al56, 57. Thus the inclusion of dynamic effects in the simulations is of utmost importance and takes us a step further in understanding the proton transport mechanism.

CONCLUSION We have presented an ab initio molecular dynamics study of interfacial proton transfer at highly structured interfaces of protogenic surface groups. We introduced a graphical representation to categorize interfacial hydronium ion transitions. At a critical surface group density, strong hydrogen bonds in the network of H3O+ and SO3- ions enforce soliton-like motion of hydronium ions. The proposed mechanism is in line with numerous experimental observations of highly efficient concerted proton transfer at charged monolayers. The obtained value of the activation energy of proton transport is in agreement with relevant experimental data for minimally hydrated polymer electrolyte membranes. We expect that insights pro-


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vided in this manuscript will stimulate surface conductance studies at monolayers of surface groups with sulfonic acid head groups. Such studies could be highly insightful, since, as we have determined, the trigonal structure of sulfonate anions play a pivotal role for interfacial ordering, wettability and proton dynamics. Moreover, our results bolster efforts in designing PEM with controlled chemical architec-ture and morphology to enable high proton conductivity at temperature above 100ºC and minimal hydration11, 12, 23.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT We would like to acknowledge NSERC for funding and Westgrid for providing the necessary computational resources.

ABBREVIATIONS SG surface groups, HB hydrogen bonds, PEFC polymer electrolyte fuel cells, PEM polymer electrolyte membrane, MD molecular dynamics, EVB empirical valence bond, DFT density functional theory, AIMD ab initio molecular dynamics, CV collective variable, GPW Gaussian and plane wave.

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