Transport phenomena in proton conducting media

Abstract

Proton transport phenomena are of paramount importance for acid-basechemistry, energy transduction in biological organisms, corrosion processes, andenergy conversion in electrochemical systems such as polymer electrolyte fuelcells. The relevance for such a plethora of materials and systems, and theever-lasting fascination with the highly concerted nature of underlying processesdrive research across disciplines in chemistry, biology, physics and chemicalengineering.A proton never travels alone. Proton motion is strongly correlated with itsenvironment, usually comprised of an electrolyte and a solid or soft host material.For the transport in nature's most benign proton solvent and shuttle, water that is,insights from ab initio simulations, matured over the last 15 years, have furnishedmolecular details of the structural diffusion mechanism of protons. Excess protonmovement in water consists of sequences of Eigen–Zundel–Eigen transitions,triggered by hydrogen bond breaking and making in the surrounding waternetwork. Nowadays, there is little debate about the validity of this mechanism inwater, which bears a stunning resemblance to the basic mechanistic picture putforward by de Grotthuss in 1806.While strong coupling of an excess proton with degrees of freedom of solventand host materials facilitates proton motion, this coupling also creates negativesynergies. In general, proton mobility in biomaterials and electrochemical protonconducting media is highly sensitive to the abundance and structure of the protonsolvent. In polymer electrolyte membranes, in which protons are bound to movein nano-sized water-channels, evaporation of water or local membranedehydration due to electro-osmotic coupling are well-known phenomena thatcould dramatically diminish proton conductivity.Contributions in this special issue address various vital aspects of theconcerted nature of proton motion and they elucidate important structural anddynamic effects of solvent, charge-bearing species at interfaces and porous hostmaterials on proton transport properties. As a common thread, articles in thisspecial issue contribute to understanding the functionality provided by complexmaterials, beyond hydrogen bond fluctuations in water. The first group of articles(Smirnov et al, Henry et al, Medvedev and Stuchebrukhov) elucidates variousaspects of the impact of local structural fluctuations, hydrogen bonding andlong-range electrostatic forces on proton transfer across and at the surface ofmitochondrial membranes. The second group of articles (Ilhan and Spohr,Allahyarov et al and Idupulapati et al) employ molecular dynamics simulations torationalize vital dependencies of proton transport mechanisms in aqueous-basedpolymer electrolyte membranes on the nanoporous, phase-separated ionomermorphology, and on the level of hydration. The articles by Gebel et al, Boillat etal, and Aleksandrova et al employ small angle neutron scattering, neutronradiography, and electrochemical atomic force microscopy, respectively, to obtaindetailed insights into the kinetics of water sorption, water distribution, watertransport properties, as well as spatial maps of proton conductivity in fuel cellmembranes. The contribution of Paschos et al provides a comprehensive reviewof phosphate-based solid state protonic conductors for intermediate temperature fuel cells. The topic of proton conductive materials for high-temperature,water-free operation of fuel cells is continued in the article of Verbraeken et alwhich addresses synthesis and characterization of a proton conducting perovskite.The guest editor wishes to acknowledge and thank all contributing authors fortheir commitment to this special issue. Moreover, I would like to thank the staff atIOP Publishing for coordinating submission and refereeing processes. Finally, forthe readers, I hope that this special issue will be a valuable and stimulating sourceof insights into the versatile and eminently important field of transportphenomena in proton conducting media.Complex dynamics of fluids in disordered and crowded environments contentsElectrostatic models of electron-driven proton transfer across a lipid membrane Anatoly Yu Smirnov, Lev G Mourokh and Franco NoriMolecular basis of proton uptake in single and double mutants of cytochrome c oxidase Rowan M Henry, David Caplan, Elisa Fadda and Régis PomèsProton diffusion along biological membranes E S Medvedev and A A StuchebrukhovAb initio molecular dynamics of proton networks in narrow polymer electrolyte pores Mehmet A Ilhan and Eckhard SpohrA simulation study of field-induced proton-conduction pathways in dry ionomers Elshad Allahyarov, Philip L Taylor and Hartmut LöwenMolecular structure and transport dynamics in perfluoro sulfonyl imide membranes Nagesh Idupulapati, Ram Devanathan and Michel DupuisThe kinetics of water sorption in Nafion membranes: a small-angle neutron scattering study Gérard Gebel, Sandrine Lyonnard, Hakima Mendil-Jakani and Arnaud MorinUsing 2H labeling with neutron radiography for the study of solid polymer electrolyte water transport properties P Boillat, P Oberholzer, B C Seyfang, A Kästner, R Perego, G G Scherer, E H Lehmann and A WokaunSpatial distribution and dynamics of proton conductivity in fuel cell membranes: potential and limitations of electrochemical atomic force microscopy measurements E Aleksandrova, S Hink, R Hiesgen and E RodunerA review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells O Paschos, J Kunze, U Stimming and F MagliaA structural study of the proton conducting B-site ordered perovskite Ba3Ca1.18Ta1.82O8.73 Maarten C Verbraeken, Hermenegildo A L Viana, Philip Wormald and John T S Irvine