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.
This article presents an ab initio metadynamics study of elementary hydronium ion transitions at dense arrays of surface groups with sulfonic acid head groups. Calculations simulate minimally hydrated conditions of the interfacial ionic system. The specific focus is on the influence of the surface group density on hydronium ion transport. Results reveal a high sensitivity of the activation free energy of hydronium translocations to the surface group density. A spontaneous concerted transition with low activation barrier is found at a surface group separation of 6.8 Å. When hydroniums translocate concertedly, the activation barrier of the transition drops by more than a factor of two to the value of 0.25 eV. An approach is presented to determine interaction constants of hydronium ions and anionic surface groups as well as the surface group flexibility from the analysis of frequency spectra. These properties are discussed in the context of a recently developed soliton theory of interfacial proton transport.
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