Proton-conducting oxides are materials of huge interest for a variety of applications, such as electrolytes in membrane reactors or in proton-conducting fuel cells (PCFC) operating in the intermediate temperature range, T = 200 – 500°C. A limiting factor pertaining to the performance of present-day PCFC devices is a too low proton conductivity of the electrolyte material. The proton conductivity is characterised by a macroscopic diffusion coefficient, which is closely linked to the microscopic diffusion mechanism, which in turn is characterised by the nature of local coordination environments of the protons, proton diffusion pathway and energy barriers for proton migration, etc. Inelastic neutron scattering (INS) and ab initio molecular dynamics (AIMD) simulations provide a powerful mean to investigate the vibrational dynamics and, in particular, the local coordination environments of protons in these materials. Such investigations are essential to gain insights into the microscopic mechanisms of proton diffusion, which allows for the rational design of materials with higher proton conductivity; a crucial requirement for the development of new technologies based of PCFCs.
The hydrated phase of barium indate, Ba2In2O5(H2O)x is a very promising “candidate” proton-conducting material for future PCFCs devices. Structurally, it is related to the well-known, cubic perovskite structure, but differs by the presence of extended static distortions. These distortions lead to the stabilisation of, at least, two distinct protons sites, H(1) and H(2), in a structure referred to as “pseudo-cubic” (Fig. 1). Upon dehydration, oxygen vacancies tend to segregate to ultimately form the brownmillerite phase, Ba2In2O5.
Figure 1. Figure 1 - Schematic representation of the distorted layer of Ba2In2O5(H2O)x, exhibiting hydrated “pseudo-cubic” domains (left) and brownmillerite-structured domains (right), respectively. InOX polyhedra in blue, oxygen atoms in red, H(1) protons in white, H(2) protons in yellow.
We performed INS measurements on the IN1-Lagrange spectrometer at the Institut Laue-Langevin, France, on fully- and partially-hydrated Ba2In2O5(H2O)x phases. We also performed AIMD simulations from which we calculated the neutron scattering function, S(Q,ħω). Combined analyses of the experimental and theoretical data established an assignment of the vibrational modes of the fully-hydrated phase (Fig. 2) in terms of fundamental and higher-order overtone of the hydroxide wag modes, δ(O-H), and stretch modes, ν(O-H). The features marked (b1) and (b2) in the infrared (IR) spectra of Ba2In2O5(H2O)0.92 (BIO92) and of the related, accepted-doped BaZr0.5In0.5O3H0.5 (50InBZO) proton-conducting oxide [from Mazzei et al., J. Mater. Chem. A 7 (2019) 7360—7372], are signatures, not of δ(O-H(2)) overtones, but of fundamental ν(O-H(2)) modes associated to a small population of protons in particularly strong hydrogen-bonding configuration.
Figure 2 - INS (black line and circles) and IR (blue line) spectra of BIO92, and calculated scattering function of the fully-hydrated model (red line). The IR spectrum of 50In/BZO (green line) from Mazzei et al., J. Mater. Chem. A 7 (2019) 7360—7372, is also represented.
From the AIMD simulations, we correlated this specific H(2) population to configurations where the O(3)–O(3) distance of the O(3)–H(2)---O(3) pattern is < 2.6 Å, close to the transient state of H(2) proton transfer along its hydrogen bond (see Fig. 3c), which indicates that H(2) has a propensity for proton-transfer events. We also determined that two local environments for H(1) coexist on the H(1) site due to a difference in the amplitude of displacement of the acceptor oxygen atom [O(1), O(3)] the H(1) proton is hydrogen bonded to (see Fig. 3a). The difference in the amplitude of displacement is notably due to the alignment of the covalent bond on the edge of the O(1)4O(2)2 octahedra (see Fig. 3b). Fig. 3b also illustrates the propensity of the O-H(1) group to re-orientate (fourfold rotation).
Figure 3 - Schematic drawing of the pseudo-cubic phase (a) showing the amplitude of displacement of O(1) and O(3) under the pull of the hydrogen bonds of H(1) protons, and (b) showing the alignment of the O(2)–H(1) covalent bond with the edges of the O(1)4O(2)2 octahedra. (c) Correlation between the bond lengths of the O(3)–H(2) covalent bond (below dashed line), the H(2)---O(3) hydrogen bond (above dashed line) and the O(3)–O(3) distance.
For the partially-hydrated phases a similar spectra is obtained, which suggests that hydrogen and oxygen atoms segregate and form hydrogen-rich oxygen-rich domains that maintain the pseudo-cubic structure.
By studying the proton vibrational dynamics we have evidence that the H(1)-type protons have a propensity to “rotate” (O-H(1) reorientation) and the H(2) proton to “jump” (proton transfer). As a next step, we are interested in exploiting the quasielastic neutron scattering (QENS) technique, which has a good synergy with the AIMD simulations, to obtain information on the macroscopic diffusion coefficients and the activation energies for both the rotational and translational diffusion processes in Ba2In2O5(H2O)x. In particular, we aim to develop the fundamental understanding of the proton conduction process needed to develop design criteria for new proton conductors with higher conductivities than those available today.
This work was conducted by Adrien Perrichon, Laura Mazzei, Seikh M. Habibur Rahman, and Maths Karlsson# from Chalmers University of Technology, Göteborg, together with Mónica Jiménez-Ruiz from the Institut Laue-Langevin, Grenoble, France.
Further information: A. Perrichon, M. Jiménez-Ruiz, L. Mazzei, S. M. H. Rahman, and M. Karlsson, J. Mater. Chem. A 7 (2019) 17626—17636