Roadmap on exsolution for energy applications

Over the last decade, exsolution has emerged as a powerful new method for decorating oxide supports with uniformly dispersed nanoparticles for energy and catalytic applications. Due to their exceptional anchorage, resilience to various degradation mechanisms, as well as numerous ways in which they can be produced, transformed and applied, exsolved nanoparticles have set new standards for nanoparticles in terms of activity, durability and functionality. In conjunction with multifunctional supports such as perovskite oxides, exsolution becomes a powerful platform for the design of advanced energy materials. In the following sections, we review the current status of the exsolution approach, seeking to facilitate transfer of ideas between different fields of application. We also explore future directions of research, particularly noting the multi-scale development required to take the concept forward, from fundamentals through operando studies to pilot scale demonstrations

Electrochemical Stability Window and Electrolyte Breakdown Mechanisms of Lithium Lanthanum Titanate

Perovskite-type La$_{0.57}$Li$_{0.29}$TiO$_3$ (LLTO) is a promising solid electrolyte material with high Li-ion conductivity. However, its experimental electrochemical stability window is not precisely known, and thus the compatibility with potential electrode materials is partly unclear. In this contribution, we present results from electrochemical and analytical experiments to elucidate the stability of LLTO when being polarized with Li-ion-blocking Pt electrodes. Above 2.5 V, a darkened color front starts moving from the cathode to the anode, leading to electrolyte degradation. While first-principles calculations predict the appearance of new phases as decomposition products, we find zones with modified defect chemical properties originating from the anode and cathode. The darkened zone forming at the cathode contains Ti$^{3+}$ polarons with high mobility, which leads to a mixed ion-electron conductivity, already for a very small Li excess concentration. Next to the anode a spatially very confined, weakly conductive Li depletion zone forms. The spatially confined but substantial Li depletion near the anode could be quantified by analytical laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In contrast to first-principles calculations, no new phases were found near the anode, according to synchrotron-based grazing incidence XRD measurements

Nano-scale oxide formation inside electrochemically-formed Pt blisters at a solid electrolyte interface

We report on platinum oxide formation during electrochemical anodic polarization of a platinum film on yttriastabilizedzirconia (YSZ) under electrochemical oxygen potential control. The electrochemical potential drivesoxygen through the YSZ electrolyte towards a nominally 175 nm thin Pt film, which we found to locally delaminatefrom the substrate by forming nano-scale blisters. High resolution scanning electron microscopy (SEM)and energy dispersive spectroscopy (EDX) mapping of focused-ion beam (FIB)-prepared cross-sections of singlebubbles of a few micrometers in diameter reveal them to be hollow and enclosed by a Pt outer and a few tens ofnanometers thick PtOx inner shell. The oxide shell presumably formed due to the increase of local oxygenchemical activity under the applied process conditions (723 K, 500 mbar O2, bias voltage +100 mV). Interface Xraydiffraction indicates that the solid electrolyte surface morphology is largely unaffected by the process suggestingthat the YSZ surface is stable on the atomic scale under application relevant oxygen transport conditions.Platinum is known to be rather stable towards oxidation, even at elevated oxygen pressure, leading to oxidescalethicknesses of the order of 1 nm. Our results however indicate that many of the kinetic barriers for oxidationduring the nano-confined blistering process are lowered. This may have implications in general for themechanism how oxygen is stored in an electrode at such an internal metal - oxide/metal - gas interface, which isimportant for the functionality of many solid-state electrochemical and memresistive devices

Reversible Ultrathin PtOx_x Formation at the Buried Pt/YSZ(111) Interface Studied In Situ under Electrochemical Polarization

Three different platinum oxides are observed by in situ X-ray diffraction during electrochemical potential cycles of platinum thin film model electrodes on yttria-stabilized zirconia (YSZ) at a temperature of 702 K in air. Scanning electron microscopy and atomic force microscopy performed before and after the in situ electrochemical X-ray experiments indicate that approximately 20% of the platinum electrode has locally delaminated from the substrate by forming pyramidlike blisters. The oxides and their locations are identified as (1) an ultrathin PtO$_x$ at the buried Pt/YSZ interface, which forms reversibly upon anodic polarization; (2) polycrystalline β-PtO$_2$, which forms irreversibly upon anodic polarization on the inside of the blisters; and (3) an ultrathin α-PtO$_2$ at the Pt/air interface, which forms by thermal oxidation and which does not depend on the electrochemical polarization. Thermodynamic and kinetic aspects are discussed to explain the coexistence of multiple phases at the same electrochemical conditions

Understanding electrochemical switchability of perovskite-type exsolution catalysts

Exsolution of metal nanoparticles from perovskite-type oxides is a very promising approachto obtain catalysts with superior properties. One particularly interesting property of exsolutioncatalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized La$_{0.6}$Sr$_{0.4}$FeO$_{3-δ}$ thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high andlow activity state is accompanied by a phase change of exsolved particles between metallic $α$-Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on La$_{0.6}$Sr$_{0.4}$FeO$_{3-δ}$ electrodes affect the H$_2$ oxidation and H$_2$O splitting mechanism and why the particle size plays a minor role