In Situ Study of Hydrogen Permeable Electrodes for Electrolytic Ammonia Synthesis Using Near Ambient Pressure XPS

Hydrogen permeable electrodes can be utilized for electrolytic ammonia synthesis from dinitrogen, water, and renewable electricity under ambient conditions, providing a promising route toward sustainable ammonia. The understanding of the interactions of adsorbing N and permeating H at the catalytic interface is a critical step toward the optimization of this NH3 synthesis process. In this study, we conducted a unique in situ near ambient pressure X ray photoelectron spectroscopy experiment to investigate the solid gas interface of a Ni hydrogen permeable electrode under conditions relevant for ammonia synthesis. Here, we show that the formation of a Ni oxide surface layer blocks the chemisorption of gaseous dinitrogen. However, the Ni 2p and O 1s XPS spectra reveal that electrochemically driven permeating atomic hydrogen effectively reduces the Ni surface at ambient temperature, while H2 does not. Nitrogen gas chemisorbs on the generated metallic sites, followed by hydrogenation via permeating H, as adsorbed N and NH3 are found on the Ni surface. Our findings suggest that the first hydrogenation step to NH and the NH3 desorption might be limiting under the operating conditions. The study was then extended to Fe and Ru surfaces. The formation of surface oxide and nitride species on iron blocks the H permeation and prevents the reaction to advance; while on ruthenium, the stronger Ru N bond might favor the recombination of permeating hydrogen to H2 over the hydrogenation of adsorbed nitrogen. This work provides insightful results to aid the rational design of efficient electrolytic NH3 synthesis processes based on but not limited to hydrogen permeable electrode

Revising the role of chromium on the surface of perovskite electrodes Poison or promoter for the solid oxide electrolysis cell performance?

Perovskite materials are typically used as oxygen electrodes of solid oxide fuel and electrolysis cells (SOC). The high stability of the perovskite structure in oxidative environments makes it a good candidate as a cathode electrode for steam electrolysis SOC as well. In this work, we investigate SOC with La0.75Sr0.25Cr0.9Fe0.1O3 perovskite cathodes employing near ambient pressure X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies combined with online electrochemical measurements. Based on operando experimental evidences the surface state of the perovskite electrode is directly associated with the electrocatalytic performance of the cell. The results indicate that under steam electrolysis operating conditions the well-known Sr surface enrichment is accompanied by Cr segregation and formation of SrCrO4-like oxide. In contrast to the common perception of the role of surface chromites, we show here that its presence does not induce cell deactivation, but on the contrary, is beneficial for cell performance. (C) 2019 Elsevier Inc. All rights reserved

Can surface reactivity of mixed crystals be predicted from their counterparts? A case study of Bi1 xSbx 2Te3 topological insulators

The behavior of ternary mixed crystals or solid solutions and its correlation with the properties of their binary constituents is of fundamental interest. Due to their unique potential for application in future information technology, mixed crystals of topological insulators with the spin locked, gapless states on their surfaces attract huge attention of physicists, chemists and material scientists. Bi1 amp; 8722;xSbx 2Te3 solid solutions are among the best candidates for spintronic applications since the bulk carrier concentration can be tuned by varying x to obtain truly bulk insulating samples, where the topological surface states largely contribute to the transport and the realization of the surface quantum Hall effect. As this ternary compound will be evidently used in the form of thin film devices its chemical stability is an important practical issue. Based on the atomic resolution HAADF TEM and EDX data together with the XPS results obtained both ex situ and in situ, we propose an atomistic picture of the mixed crystal reactivity compared to that of its binary constituents. We find that the surface reactivity is determined by the probability of oxygen attack on the Te Sb bonds, which is directly proportional to the number of Te atoms bonded to at least one Sb atom. The oxidation mechanism includes formation of an amorphous antimony oxide at the very surface due to Sb diffusion from the first two quintuple layers, electron tunneling from the Fermi level of the crystal to oxygen, oxygen ion diffusion to the crystal, and finally, slow Te oxidation to the 4 oxidation state. The oxide layer thickness is limited by the electron transport, and the overall process resembles the Cabrera Mott mechanism in metals. These observations are critical not only for current understanding of the chemical reactivity of complex crystals, but also to improve the performance of future spintronic devices based on topological material