Understanding the chemical evolution at the interface of organic light-emitting diodes (OLEDs) under working conditions is critical for addressing device failure and further improving performance. In this work, an in-operando approach was developed employing synchrotron-based X-ray absorption spectroscopy (XAS) to investigate the electronic structures and chemical degradation mechanisms of a model tris(8-hydroxyquinoline) aluminum (Alq3)-based OLED device under working condition. The results identify that Mg atoms from the electrode migrate into the Alq3 organic layer under a potential bias and replace Al atom sites, forming the unstable Mgq3 species which lead to device degradation. The findings from the classic and simple model device elucidate the degradation mechanisms occurred at the interface of OLED devices, which may facilitate the development of more efficient and stable OLED devices with complex structures.

The observation of a spontaneous electron enrichment at the apexes of bismuth vanadate (BiVO4) nanopyramidal arrays is achieved by performing synchrotron-based angular dependent X-ray absorption spectroscopy. In accordance with density function theory calculations, the formation of (112) facets enriched apexes is proposed. Such intrinsically electron-enriched (112) facets at the apexes produce a potential facilitating the diffusion of photoexcited holes, which not only mitigates the recombination with the photoexcited electrons, but also provide an active reaction site for the photoassisted selective growth of cobalt phosphate (CoPi), a well known and highly active co-catalyst for water oxidation at the apex of BiVO4 nanopyramids. Benefiting from the element-resolved properties of synchrotron-based X-ray spectroscopy, 3d electrons on the vanadium site are directly resolved by resonant inelastic X-ray scattering measurements. Due to this unique morphology, the charge at the apex is expected to induce a strong interaction with CoPi, as a result of the metal-to-ligand charge transfer spectroscopy feature observed from the cobalt site. Substantial evidences are collected by means of comprehensive soft X-ray spectroscopy techniques with high spectral resolution revealing the atomic-scale origin and the nature of the synergy as well as the strong correlation between its unique structural properties and the electronic structure of this novel highly-ordered hybrid photocatalyst and its significantly improved photoelectrochemical performance.

Lithium/sulfur (Li/S) cells have attracted much attention due to their higher theoretical specific capacity and energy compared to those of current lithium-ion cells. However, the application of Li/S cells is still hampered by short cycle life. Sulfur-graphene oxide (S-GO) nanocomposites have shown promise as cathode materials for long-life Li/S cells because oxygen-containing functional groups on the surface of graphene oxide were successfully used as sulfur immobilizers by forming weak bonds with sulfur and polysulfides. While S-GO showed much improved cycling performance, the capacity decay still needs to be improved for commercially viable cells. In this study, we attempt to understand the capacity fading mechanism based on an ex situ study of the structural and chemical evolution of S-GO nanocomposite cathodes with various numbers of cycles using scanning electron microscopy (SEM), near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). It is found that both the surface morphologies and chemical structures of the cathode materials change considerably with increasing number of cycles. These changes are attributed to several unexpected chemical reactions of lithium with S-GO nanocomposites occurring during the discharge-charge processes with the formation of Li2CO3, Li2SO3, Li2SO4, and COSO2Li species. These reactions result in the loss of recyclable active sulfur on the surface of the electrode, and thus capacity fades while coulombic efficiency is near 100%. Moreover, the reaction products accumulate on the cathode surface, forming a compact blocking insulating layer which may make the diffusion of Li ions into/out of the cathode difficult during the discharge-charge process and thus lead to lower utilization of sulfur at higher rates. We think that these two observations are significant contributors to the capacity and rate capability degradation of the Li/S-GO cells. Therefore, for the rechargeable Li/S-GO cells, we suggest that the content of oxygen-containing functional groups on GO should be optimized and more stable functional groups need to be identified for further improvement of the cycling performance. The information we gain from this study may provide general insights into the fundamental understanding of the degradation mechanisms of other rechargeable Li/S cells using similar oxygen-containing functional groups as sulfur immobilizers.

Lithium nitrate (LiNO3) has been the most studied electrolyte additive in lithium-sulfur (Li-S) cells, due to its known function of suppressing the shuttle effect in Li-S cells, which provides a significant increase in the cell's coulombic efficiency and cycling stability. Previous studies indicated that LiNO3 participated in the formation of a passive layer on the lithium electrode and thus suppressed the redox shuttle of the dissolved polysulfides. However, the effects of the LiNO3 on the positive electrode materials have rarely been investigated. By combining scanning electron microscopy, element-selective X-ray absorption spectroscopy, and electrochemical characterizations, we performed a comprehensive study of how the LiNO3 altered the properties of the sulfur electrode/electrolyte interface in Li-S cells and thus influenced the cell performance. We found that LiNO3 is a double-edged sword in the Li-S cell: on one hand, it increased the consumption of the active sulfur; on the other hand, it promoted the survival of the carbon matrix constituent in the sulfur electrode. These two competitive effects indicated that a proper moderate concentration of LiNO3 is required to achieve an optimized cell performance.

The generation of chemical fuel in the form of molecular H2 via the electrolysis of water is regarded to be a promising approach to convert incident solar power into an energy storage medium. Highly efficient and cost-effective catalysts are required to make such an approach practical on a large scale. Recently, a number of amorphous hydrogen evolution reaction (HER) catalysts have emerged that show promise in terms of scalability and reactivity, yet remain poorly understood. In this work, we utilize Raman spectroscopy and X-ray absorption spectroscopy (XAS) as a tool to elucidate the structure and function of an amorphous cobalt sulfide (CoSx) catalyst. Ex situ measurements reveal that the as-deposited CoSx catalyst is composed of small clusters in which the cobalt is surrounded by both sulfur and oxygen. Operando experiments, performed while the CoSx is catalyzing the HER, yield a molecular model in which cobalt is in an octahedral CoS2-like state where the cobalt center is predominantly surrounded by a first shell of sulfur atoms, which, in turn, are preferentially exposed to electrolyte relative to bulk CoS2. We surmise that these CoS2-like clusters form under cathodic polarization and expose a high density of catalytically active sulfur sites for the HER.

We describe a new experimental technique that allows for soft x-ray spectroscopy studies (∼100-1000 eV) of high pressure liquid (∼100 bars). We achieve this through a liquid cell with a 100 nm-thick Si₃N₄ membrane window, which is sandwiched by two identical O-rings for vacuum sealing. The thin Si₃N₄ membrane allows soft x-rays to penetrate, while separating the high-pressure liquid under investigation from the vacuum required for soft x-ray transmission and detection. The burst pressure of the Si₃N₄ membrane increases with decreasing size and more specifically is inversely proportional to the side length of the square window. It also increases proportionally with the membrane thickness. Pressures > 60 bars could be achieved for 100 nm-thick square Si₃N₄ windows that are smaller than 65 μm. However, above a certain pressure, the failure of the Si wafer becomes the limiting factor. The failure pressure of the Si wafer is sensitive to the wafer thickness. Moreover, the deformation of the Si₃N₄ membrane is quantified using vertical scanning interferometry. As an example of the performance of the high-pressure liquid cell optimized for total-fluorescence detected soft x-ray absorption spectroscopy (sXAS), the sXAS spectra at the Ca L edge (∼350 eV) of a CaCl₂ aqueous solution are collected under different pressures up to 41 bars.