Anurag Prakash Sunda

Molecular dynamics simulations of side chain pendants of perfluorosulfonic acid polymer electrolyte membranes

Perfluorosulfonic acid (PFSA) polymer electrolyte membranes like Dow, Aciplex and Nafion have similar backbones but different side chain pendants. The effect of hydration and temperature on the side chain pendant nanostructure, and water and hydronium ion dynamics, are investigated by employing classical molecular dynamics simulations at 300 K and 350 K. The 60% longer side chain pendant length in Aciplex compared to Dow results in phase segregation. The presence of an extra ether oxygen atom in the Nafion side chain pendant provides more flexibility (∼20% chain length contraction caused by flexibility and the hydrophobic force of the pendant CF3 group) where the sulfonate group tends to drift from the hydrophilic–hydrophobic domain, which gives rise to a hydrosphere region at higher hydration. The calculated structure factors and scattering intensities reproduce features of SANS and SAXS profiles for Dow and Nafion, and confirm the existence of spherical water aggregates in the rod shaped pendant nanostructure of Nafion. The effect of hydration on the mobility of hydronium ions at 300 K in Nafion is insignificant at higher hydration (λ ≥ 9), and trends are in agreement with experimental data. The activation energy of the diffusion of hydronium ions and water molecules in Nafion side chain pendant–water mixtures (14–25 kJ mol−1) validate experimental observations (16–22 kJ mol−1).

Molecular dynamics simulations of triflic acid and triflate ion/water mixtures: A proton conducting electrolytic component in fuel cells

Triflic acid is a functional group of perflourosulfonated polymer electrolyte membranes where the sulfonate group is responsible for proton conduction. However, even at extremely low hydration, triflic acid exists as a triflate ion. In this work, we have developed a force‐field for triflic acid and triflate ion by deriving force‐field parameters using ab initio calculations and incorporated these parameters with the Optimized Potentials for Liquid Simulations ‐ All Atom (OPLS‐AA) force‐field. We have employed classical molecular dynamics (MD) simulations with the developed force field to characterize structural and dynamical properties of triflic acid (270–450 K) and triflate ion/water mixtures (300 K). The radial distribution functions (RDFs) show the hydrophobic nature of CF3 group and presence of strong hydrogen bonding in triflic acid and temperature has an insignificant effect. Results from our MD simulations show that the diffusion of triflic acid increases with temperature. The RDFs from triflate ion/water mixtures shows that increasing hydration causes water molecules to orient around the SO3− group of triflate ions, solvate the hydronium ions, and other water molecules. The diffusion of triflate ions, hydronium ion, and water molecules shows an increase with hydration. At λ = 1, the diffusion of triflate ion is 30 times lower than the diffusion of triflic acid due to the formation of stable triflate ion–hydronium ion complex. With increasing hydration, water molecules break the stability of triflate ion–hydronium ion complex leading to enhanced diffusion. The RDFs and diffusion coefficients of triflate ions, hydronium ions and water molecules resemble qualitatively the previous findings using per‐fluorosulfonated membranes.

Atomistic simulations of structure and dynamics of hydrated Aciplex polymer electrolyte membrane

Aciplex is a perfluorosulfonic acid (PFSA) polymer electrolyte membrane, where its efficiency depends on hydration and temperature. In the present work, the nanostructure of the Aciplex membrane and transport of hydronium ions and water molecules are characterized using classical molecular dynamics simulations at varying hydrations and temperatures. An examination of radial distribution functions and scattering intensities shows that temperature has a negligible effect on membrane nanostructure at all hydration levels. The calculated structural factors and scattering intensities of water molecules closely resemble the experimental SAXS and SANS features of PFSA membranes. Further, for all hydration, the strong interactions between sulfonate groups of the pendant side chain arise only from inter-chain interactions. The stiffness of a pendant side chain limits the possibility of intra-chain interactions between sulfonate groups. The distance between adjacent sulfonate groups shows a variation of 3 A from an average distance of 25 A which shows a suitable orientation of the pendant side chain to maximize water–hydronium interactions with the sulfonate group. The radius of gyration shows an insignificant change with hydration and temperature which demonstrates that the membrane is a suitable electrolytic component for PEM fuel cells. The calculated diffusion coefficients of hydronium ions and water molecules are found to be in reasonable agreement with experimental data. The enlarged hydrophobic domains assisted by the rigid pendant side chain in a hydrated Aciplex membrane results in lower mobility of water molecules compared to Nafion.

Ammonium-based protic ionic liquid doped Nafion membranes as anhydrous fuel cell electrolytes

Polymer electrolyte membranes doped with Protic Ionic Liquids (PILs) can serve as promising materials for anhydrous proton conduction. In the present study, molecular dynamics simulations are performed to characterize the structure and dynamics of a diethylmethylammonium triflate ([dema][TfO]) PIL doped Nafion membrane at various PIL doping and temperatures. The polymer membrane PIL interface structure shows that the hydrogen bonding interactions predominantly exist between the acidic site (N–H) of the ammonium cation and SO3− group of the [TfO−] anion or Nafion. The distribution of the SO3− group (of the [TfO−] anion or Nafion) around the ammonium cation increases with PIL concentration. The existence of weak hydrogen bonding interactions between the alkyl hydrogen atoms of the [dema+] and the SO3− group of Nafion remains unaffected by the PIL concentration. An increase in the PIL doping level and temperature results in faster diffusion (higher ionic conductivity) which is in qualitative agreement with experiments.

Parametric dependence on shear viscosity of SPC/E water from equilibrium and non-equilibrium molecular dynamics simulations

A parametric dependent study is crucial for the accurate determination of transport coefficients such as shear viscosity. In this study, we calculate the shear viscosity of extended simple point charge water using a transverse current auto-correlation function (TCAF) from equilibrium molecular dynamics (EMD) and the periodic perturbation method from non-equilibrium molecular dynamics (NEMD) simulations for varying coupling time and system sizes. Results show that the shear viscosity calculated using EMD simulations with different thermostats varies significantly with coupling times and system size. The use of Berendsen and velocity-rescale thermostats in NEMD simulations generates a significant drift from the target temperature and results in an inconsistent shear viscosity with coupling time and system size. The use of Nosé–Hoover thermostat in NEMD simulations offers thermodynamic stability which results in a consistent shear viscosity for various coupling times and system sizes.

A molecular investigation of the nanostructure and dynamics of phosphoric–triflic acid blends of hydrated ABPBI [poly(2,5-benzimidazole)] polymer electrolyte membranes

Poly(2,5-benzimidazole) membranes (ABPBI) doped with phosphoric acid (PA) are known to serve as promising electrolytes in fuel cells. The addition of triflic acid, which exist in its dissociated form (TFA), is known to enhance the efficiency of PA-doped ABPBI membranes. In the present work, we employ classical molecular dynamic simulations to characterize the structure and dynamics of ABPBI + PA, ABPBI + TFA and ABPBI + PA + TFA blends with varying levels of hydration. The structural properties seen from the radial distribution functions (RDFs) show that the distance between two adjacent imidazole units on the polymer chain remain unaffected by hydration and the type of blend. The end-to-end polymer chain distance and radius of gyration are also unaffected by hydration and the type of blend, illustrating that the stability of polymer membranes under various hydrated acidic environments remains unaffected. The number of PA, TFA and water molecules in the cluster around the polymer membrane (skewed and extended form) is found to depend significantly on the extent of hydration. The lowest water mobility was obtained from the ABPBI + PA + TFA blend, which suggests that this blend could be the most effective in reducing acid leaching from the membrane matrix.

Polymer chain length, phosphoric acid doping and temperature dependence on structure and dynamics of an ABPBI [poly(2,5-benzimidazole)] polymer electrolyte membrane

An ABPBI [poly(2,5-benzimidazole)] membrane doped with phosphoric acid (PA) are promising electrolytes in high temperature fuel cells. In the present work, we employ Molecular Dynamics (MD) simulations to characterize the effect of polymer chain length using a dimer to hectamer. Results from our MD simulations (dimer to decamer) show the following trends: the inter-chain and intra-chain interactions in membrane are unaffected with polymer chain length and temperature, though a significant increase with PA doping is observed. The radius of gyration linearly increases with polymer chain length and remains unchanged with PA doping and temperature. However, the end-to-end distance deviates from linearity with polymer chain length which suggests increasing coiling of the membrane. The diffusion coefficient of PA increases with PA doping and temperature, but remains constant with polymer chain length. The activation energy of diffusion of PA decreases significantly with an increase in polymer chain length at low PA doping, but remains unaffected at higher PA doping. A comparison of the structural and dynamical properties of a decamer and a hectamer shows that the decamer represents the optimum polymer chain length beyond which no significant change in properties is observed.

Structure and dynamics of benzyl-NX3 (X = Me, Et) trifluoromethanesulfonate ionic liquids.

Ammonium-based benzyl-NX3 (X = methyl, ethyl) trifluoromethanesulfonate (TFA) ionic liquids (ILs) are low cost, nontoxic, thermally stable ion-conducting electrolytes in fuel cells and batteries. In the present study, we have characterized the structure and dynamics of these ILs using molecular dynamics (MD) simulations and ionic conductivity using electro-chemical impedance spectroscopy (EIS) at varying temperature and relative humidity (RH). Results from MD simulations predict that cation-cation and cation-anion interactions are stronger in benzyltrimethylammonium (BzTMA) compared to benzyltriethylammonium (BzTEA) that diminish with increase in RH. Further, the BzTMA cations show both C-H/Ph (center of mass of phenyl ring) and cation-Ph interactions whereas BzTEA cations show only strong cation-Ph interactions. The C-H/Ph interactions (ψ ≥ 90°, d(H-Ph) ≤ 4 Å, θ < 50° and d(C-Ph) ≤ 4.3 Å) in BzTMA cations increase with RH and are highest at RH = 90%. The cumulative impact of electrostatic, cation/Ph, and C-H/Ph interactions results in lower conductivity of BzTMA-TFA IL compared to BzTEA-TFA IL. The EIS measurements show that the trends in ionic conductivity of ILs at RH = 30 and 90% are qualitatively similar to the Nernst-Einstein conductivity from MD simulations. The ionic conductivity of BzTEA-TFA IL is ~3 times higher than BzTMA-TFA IL at 353 K and RH = 90%.

Molecular dynamics simulations of ammonium/phosphonium-based protic ionic liquids: influence of alkyl to aryl group

The variation of the center atom in the cation from an N to a P-atom leads to improved physiochemical properties of protic ionic liquids (PILs) which are suitable for electrolyte applications. We present an atomistic simulations study to compare the effect of an alkyl or aryl group on trioctylammonium triflate ([HN(Oct)3][TFO]) and triphenylammonium triflate ([HN(Ph)3][TFO]) with their phosphonium analogues. We have computed the binding energy from quantum chemical calculations and physical properties such as the viscosity and the electrical conductivity of PILs from molecular dynamics simulations. The influence of the aromatic character in PILs is found to be significant to the physical properties. Gas phase quantum chemical calculations on clusters of ion pairs have revealed the presence of C-H/π interactions in aromatic PILs along with hydrogen bonding. The variation in strength of the ion-pair affinities is examined using electric-current correlation and velocity autocorrelation functions. The qualitative differences observed are due to the aromatic rings and change in the central atom of the quaternary cation from an N to a P-atom, substantiated quantitatively by diffusion coefficients and electrical conductivities. The relatively weaker ion-pair interactions and low binding energy (-73.34 kcal mol-1) lead to the highest electrical conductivity in [HP(Ph)3][TFO].

Nanoscale defolding influence of polypeptides in the charge-transfer process through an organic–inorganic nanohybrid system

A biologically important polypeptide [with an alternate sequence of alanine (ALA) and 2-aminobutyric acid (AiB)] is used as a linker molecule to investigate the charge-transfer phenomenon between CdSe nanoparticle (NP) (diameter ∼6-7 nm) assemblies and gold (Au) substrates. The (ALA-AiB)n polypeptides, with varying chain lengths n = 5, 8, 11, were attached to the surface to form self-assembled monolayers (SAMs) through a thiol group located either at the N-terminal or C-terminal of the sequence. Temperature dependent photoluminescence (PL) spectra showed anomalous behavior in the quenching regime of CdSe NPs in the 237 K to 290 K region. In principle, the fluorescence intensity of any fluorophore decreases with a gradual increase in temperature, due to dominating non-radiative relaxation over radiative relaxation. PL spectral intensity follows this general trend from 77 K to 237 K for all chain lengths. For chain length n > 5 (n = 5 showed a kink, but the extent of the kink is negligible in comparison with n > 5) polypeptide-based monolayers, there is a sudden increase in fluorescence intensity above 237 K. This sudden increase is probed using molecular dynamics simulations which reveals that this unprecedented behavior arises due to interchain polypeptide interactions. An insertion of an alkyl chain with an almost similar length of peptide along with polypeptide (in a 3 : 1 ratio, in terms of concentration) diminishes the interchain polypeptide hydrogen-bonding interactions and manifests the normal trend of PL spectra.