Theoretical insights into inorganic-organic intercalation products of the layered perovskite HLaNb2O7: perspectives for hybrid proton conductors.

The modification of metal oxide surfaces with organic moieties has been widely studied as a method of preparing organic-inorganic hybrid materials for various applications. Among the inorganic oxides, ion-exchangeable layered perovskites are particularly interesting, because of their appealing electronic and reactive properties. In particular, their protonated interlayer surface can be easily functionalized with organic groups allowing the production of stable hybrid materials. As a further step in the design of new inorganic-organic hybrid proton conductors, a combined experimental and theoretical study of two intercalated compounds (propanol and imidazole) in HLaNb2O7 is presented here. A generally very good agreement with the available experimental data is found in reproducing both structural features and 13C-NMR chemical shifts, and marked differences between the two considered intercalated compounds are evidenced, with possible important outcomes for proton conduction. Notably, the free imidazole molecules are easily protonated by the acidic protons present in the interlayer spacing, thus inhibiting an efficient charge transport mechanism. In order to overcome this problem, a model system has been considered, where the imidazoles are bound to the end of a butyl chain, the whole being intercalated between two perovskite layers. The obtained theoretical data suggest that, in such a system, proton transfer between two adjacent imidazoles is a barrierless process. These results could then open new perspectives for such hybrid proton conductors.

Interface Diffusion and Compatibility of (Ba,La)FeO3-δ Perovskite Electrodes in Contact with Barium Zirconate and Ceria.

Ba1-xLaxFeO3-δ perovskites (BLF) capable of conducting electrons, protons, and oxygen ions are promising oxygen electrodes for efficient solid oxide cells (fuel cells or electrolyzers), an integral part of prospected large-scale power-to-gas energy storage systems. We investigated the compatibility of BLF with lanthanum content between 5 and 50%, in contact with oxide-ion-conducting Ce0.8Gd0.2O2-δ and proton-conducting BaZr0.825Y0.175O3-δ electrolytes, annealing the electrode-electrolyte bilayers at high temperature to simulate thermal stresses of fabrication and prolonged operation. By employing both bulk X-ray diffraction and synchrotron X-ray microspectroscopy, we present a space-resolved picture of the interaction between electrode and electrolyte as what concerns cation interdiffusion, exsolution, and phase stability. We found that the phase stability of BLF in contact with other phases is correlated with the Goldschmidt tolerance factor, in turn determined by the La/Ba ratio, and appropriate doping strategies with oversized cations (Zn2+, Y3+) could improve structural stability. While extensive reactivity and/or interdiffusion was often observed, we put forward that most products of interfacial reactions, including proton-conducting Ba(Ce,Gd)O3-δ and mixed-conducting (Ba,La)(Fe,Zr,Y)O3-δ, may not be very detrimental for practical cell operation.

Solid-Solid Interfaces in Protonic Ceramic Devices: A Critical Review.

The literature concerning protonic ceramic devices is critically reviewed focusing the reader's attention on the structure, composition, and phenomena taking place at solid-solid interfaces. These interfaces play a crucial role in the overall device performance, and the relevance of understanding the phenomena taking place at the interfaces for the further improvement of electrochemical protonic ceramic devices is therefore stressed. The grain boundaries and heterostructures in electrolytic membranes, the electrode-electrolyte contacts, and the interfaces within composite anode and cathode materials are all considered, with specific concern to advanced techniques of characterization and to computational modeling by ab initio approaches. An outlook about future developments and improvements highlights the necessity of a deeper insight into the advanced analysis of what happens at the solid-solid interfaces and of in situ/operando investigations that are presently sporadic in the literature on protonic ceramic devices.

Cation Diffusion and Segregation at the Interface between Samarium-Doped Ceria and LSCF or LSFCu Cathodes Investigated with X-ray Microspectroscopy.

The chemical compatibility between electrolytes and electrodes is an extremely important aspect governing the overall impedance of solid-oxide cells. Because these devices work at elevated temperatures, they are especially prone to cation interdiffusion between the cell components, possibly resulting in secondary insulating phases. In this work, we applied X-ray microspectroscopy to study the interface between a samarium-doped ceria (SDC) electrolyte and lanthanum ferrite cathodes (La0.4Sr0.6Fe0.8Cu0.2O3 (LSFCu); La0.9Sr0.1Fe0.85Co0.15O3 (LSCF)), at a submicrometric level. This technique allows to combine the information about the diffusion profiles of cations on the scale of several micrometers, together with the chemical information coming from space-resolved X-ray absorption spectroscopy. In SDC-LSCF bilayers, we find that the prolonged thermal treatments at 1150 °C bring about the segregation of samarium and iron in micrometer-sized perovskite domains. In both SDC-LSCF and SDC-LSFCu bilayers, cerium diffuses into the cathode perovskite lattice A-site as a reduced Ce3+ cation, whereas La3+ is easily incorporated in the ceria lattice, reaching 30 atom % in the ceria layer in contact with LSFCu.