Collective Proton Dynamics at Highly Charged Interfaces Studied by Ab Initio Metadynamics

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 ~6.5 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 ~100 °C.

Water sub-diffusion in membranes for fuel cells

“…Advanced spectroscopy and scattering techniques are powerful tools in this direction 5 , 6 , since they allow to explore structure 7 – 14 and dynamics 15 – 23 of matter at those scales. This effort has been efficiently integrated by numerical work, employing techniques such as ab-initio calculations 24 – 27 , Molecular Dynamics (MD) 11 , 28 – 40 and DPD 41 , 42 simulations, and continuum approaches 43 , 44 . Insight into properties and elementary processes of PFSAs as structure 8 , 13 , 23 , 31 , ionization 20 , 24 , 25 , sorption 9 , 34 , 45 , proton conduction 2 , 26 , 27 , 35 , 38 , 39 , 44 , 46 , water diffusion 15 , 17 , 28 , 35 , 37 , 47 , 48 , polymer relaxation 16 , 21 , has been accumulated.…”

“…This effort has been efficiently integrated by numerical work, employing techniques such as ab-initio calculations 24 – 27 , Molecular Dynamics (MD) 11 , 28 – 40 and DPD 41 , 42 simulations, and continuum approaches 43 , 44 . Insight into properties and elementary processes of PFSAs as structure 8 , 13 , 23 , 31 , ionization 20 , 24 , 25 , sorption 9 , 34 , 45 , proton conduction 2 , 26 , 27 , 35 , 38 , 39 , 44 , 46 , water diffusion 15 , 17 , 28 , 35 , 37 , 47 , 48 , polymer relaxation 16 , 21 , has been accumulated. The impact of the nature of polymer backbone 4 , 40 , 49 and ionic side-chains 18 , 40 , 41 , the nature of acidic sites 14 , the density of charge 11 , 18 , 42 , elaboration methods 10 , and other factors 12 , 19 , 30 , 50 have also been studied.…”

“…Advanced spectroscopy and scattering techniques are powerful tools in this direction, [5,6] since they allow to explore structure [7][8][9][10][11][12][13][14] and dynamics [15][16][17][18][19][20][21][22][23] of matter at those scales. This effort has been efficiently integrated by numerical work, employing techniques such as ab-initio calculations [24][25][26][27], Molecular Dynamics (MD) [11,[28][29][30][31][32][33][34][35][36][37][38][39][40] and DPD [41,42] simulations, and continuum approaches [43,44]. Insight into properties and elementary processes of PFSAs as structure [8,13,23,31], ionization [20,24,25], sorp...…”

“…This effort has been efficiently integrated by numerical work, employing techniques such as ab-initio calculations [24][25][26][27], Molecular Dynamics (MD) [11,[28][29][30][31][32][33][34][35][36][37][38][39][40] and DPD [41,42] simulations, and continuum approaches [43,44]. Insight into properties and elementary processes of PFSAs as structure [8,13,23,31], ionization [20,24,25], sorption [9,34,45], proton conduction [2,26,27,35,38,39,44,46], water diffusion [15,17,28,35,37,47,48], polymer relaxation [16,21], has been accumulated....…”

Electrokinetics: insights from simulation on the microscopic scale

“…Another challenge for the modelling of electrokinetics is the description of proton transport near interfaces, e.g. in membranes [139][140][141], at the surface of metal electrodes [142,143], of minerals [144] or of biological molecules [145], and for which quantum effects must be accounted for.…”