Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba₂In₂O₅ Thin Films

Abstract

Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba2In2O5 (BIO) exhibits both oxygen ion and proton conduction, in dry and humid environment, respectively. In dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers.To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here the proton and oxygen ion conductivity along different crystallographic directions.Using epitaxial thin films with different crystallographic orientations the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained through the layered structure of the oxygen sublattice in brownmillerite BIO. The anisotropic proton conduction however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chain-like or layered structures.

Designing UiO-66-Based Superprotonic Conductor with the Highest Metal–Organic Framework Based Proton Conductivity

“…Meanwhile, a lot of effort has also been devoted within last decade to contribute to the understanding of proton conduction mechanism. [28][29][30][31][32][33][34][35][36] The most common method of proton conduction is solvent assisted proton transfer. 1,2 Similar mechanism has been found to operate in most of the Brønsted acids, common protonic solvents and functionalized polymers.…”

“…But they are less popular as proton conductors due to low efficiency at ambient conditions. [30][31][32] In this scenario, metal organic frameworks (MOFs) stand as a connecting bridge between inorganic cluster containing compounds and organic polymers in terms of detailed structural information from crystal structure and efficiency of proton conduction. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] These MOFs, being polymeric hybrid of organic linker molecule and metal ions, cultivate some of the useful properties of both.…”

“…Plenty of reports on new and efficient proton conductors in the past 5 years clearly establish the increasing importance of this field in recent times. − Meanwhile, a lot of effort has also been devoted within the past decade to contribute to the understanding of the proton conduction mechanism. − The most common method of proton conduction is solvent-assisted proton transfer. , A similar mechanism has been found to operate in most of the Brønsted acids, common protonic solvents, and functionalized polymers. ,,, Apparently, protonic solvents and Brønsted acids have the simplest structures among all; however, on the contrary, they are found to follow a complicated conductive mechanism. ,− Since the past few decades, organic polymeric compounds (e.g., Nafion) have been well established as the supreme proton conductors. They possess conductivity of the order of 10 –1 S cm –1 under humidification .…”

Applying the Influence of Conformational Freedom on the Network Topologies Showing Impressive Proton Conductivity

“…The quest for an alternate environmentally benign, ecofriendly synthetic fuel has tantalized the attention of energy researchers for the last few decades. − The present scenario with over reliance on fossil fuels has severe consequences with regard to air pollution and global warming. − Very recently, proton exchange membrane fuel cells (PEMFCs) have emerged, inter alia , as highly promising systems involving proton conductivity. − One such widely used proton exchange membrane (PEM), with a remarkably high proton conductivity value, would be Nafion. − However, several disadvantages including high cost, high solubility, loss of proton conductivity above 80 °C temperature (due to dehydration at high temperature), and noncrystallinity limit its economic viability. − Notwithstanding, an escalating research interest has been triggered regarding the design of low-cost robust crystalline materials that can transport protons at high humidity and high temperature. − In this regard, crystalline metal–organic frameworks (MOFs) have emerged as stand-alone materials for developing PEMFCs. − MOFs, with their tunable pore size, remarkable stability at high thermal and humidity, low cost, and easy synthetic methods, offer excellent solutions to all the prerequisites of a PEMFC. − In general, the basic prerequisite for proton conductivity is the presence of functional groups that can lead to the formation of infinite hydrogen bonded networks. − For MOFs, typically this criterion can be fulfilled in three different ways: infinite hydrogen bonded networks (a) among the protic solvent molecules entrapped inside channels or voids of structural framework, (b) among the strong hydrogen bond forming immobilized functional groups grafted inside the structural framework, or (c) a combination of (a) and (b), combining protic solvent as well as immo...…”

One-Dimensional Growth of Li2NiPO4F Single Crystals from Intermediate LiNiPO4 Crystal Surface Using KCl–KI Fluxes

“…Innovations in materials science are desirable objectives for achieving a sustainable society. The shape-controlled syntheses of inorganic materials influence many geometry-sensitive physical and chemical properties, including luminescence, − catalytic activity, − surface energy, − solubility, , conductivities, − and adsorption , of the material. Elementary reactions during crystal growth are fully reflected in the morphological characteristics of the crystals.…”