AVS2010 Session IS+SS-TuM: In Situ Spectroscopy – Interfacial Chemistry/Catalysis

XPS on a micro liquid jet has been used to study solutions of monoethanolamine (MEA), which is commonly used in gas stream scrubbing for carbon dioxide capture. It is likely that interactions between CO₂ and the aqueous MEA solution at the gas/liquid interface are important to this process, yet there is little information available concerning the spatial distribution of species at the interface of such solutions. In the present work, aqueous solutions of MEA with a range of pH values as well as solutions of MEA reacted with CO₂ have been measured using tunable synchrotron radiation from the BESSY facility in Berlin, where the photoelectron kinetic energy can be varied to obtain depth dependent composition information. N1s photoemission spectra allow for the identification of protonated verses unprotonated MEA by the different binding energies of the two species, and likewise, C1s spectra allow for the determination of CO₂-reacted verses unreacted MEA. Depth profiling reveals that deprotonated MEA is more surface active than both protonated MEA and the CO₂-reacted species. The mechanism of the reaction of CO₂ with aqueous solutions of monoethanolamine will be discussed in light of our results.

Electrochemical technologies will be increasingly used to supply energy to the world without contributing to climate change. These technologies can store and convert energy with unsurpassed efficiencies through, for example, the charging and discharging of batteries or the inter-conversion of electrical and chemical energy via fuel cell and electrolyzer. Perhaps the most important phenomena to understand in electrochemical energy storage/conversion is how electric charge is transferred across interfaces and subsequently stored in material phases and/or double layers. Currently, detailed knowledge is lacking of critical pathways such as which chemical reactions are responsible for charge transfer, what species are involved, and where charge transfer reactions occur in heterogeneous devices. These limitations arise in no small degree from the physical complexities of these devices, which consist of a variety of electrified materials undergoing chemical reactions. Lacking this knowledge, development proceeds largely using engineering approaches.

To help answer these questions we have spectroscopically characterized electrochemical charge-transfer and storage as it occurs. This is accomplished by primarily using a new diagnostic based on synchrotron X-ray spectroscopies that we have been developing at the Advanced Light Source (ALS, LBNL, Berkeley, CA ). Photoelectrons are used as a contact-less probe for the direct measurement of the electric inner potential everywhere in a Ni-YSZ based electrochemical cell operating at near ambient pressure. This information, in addition to space-resolved chemical characterization of the surface species showing phase changes relevant to Ni-metal-hydride batteries, will be discussed. The experimental configuration consists of a thin-film Ni electrode that is electrochemically modified by injection of O2- ions. During an applied bias, charge is stored in the electrode by the conversion of the Ni to NiOOH. This leads to dramatic changes in the XPS spectra as well as the existence of a constant discharge potential plateau resulting from the equilibrium of NiOOH with two other phases, Ni and H2O. Thus, our approach has the ability to identify the phases that store charge, which are only stable under electrical bias. This rich data will provide new understanding on how electrochemically driven phases form.

The use of hard X-ray excitation in the range from about 2 to 15 keV for photoelectron spectroscopy (HAXPES) is a rapidly emerging technique at synchrotron sources worldwide since it significantly widens the range of applications, in particular in the study of complex materials and buried nano-structures or interfaces. Due to the increased electron inelastic mean free paths, it becomes possible to probe chemical composition and electronic structure in the bulk of materials with considerable sensitivity, down to typically 10-20 nm at 10 keV kinetic energy. This not only is essential in the study of complex correlated materials which often exhibit a modified surface electronic structure but also is very relevant for technologically interesting multi-layered materials with buried interfaces. As an additional benefit, "as-grown" materials can be measured without need for prior surface treatment.

A drawback is the rapidly decreasing photoelectric cross section in the hard X-ray range, especially for shallow core levels and valence states, and - until recently - the limited availability of suitable high-voltage electron spectrometers. The latter are available meanwhile and the high X-ray flux at undulator beamlines has been shown to effectively compensate the cross section decrease. In addition, the X-ray tunability over a large energy range not only allows to significantly vary the electron probing depths and the photoelectric cross sections, but also enables the study of resonance phenomena at deep inner-shell thresholds. Further, the excitation of X-ray standing wave fields within single crystals or multi-layered structures can be effectively used to correlate geometric and electronic structure information.

In the past five years, HAXPES activities at synchrotron laboratories worldwide have increased dramatically and the trend continues as new instruments are currently being built and planned. A brief overview is given on the current activities worldwide, both on instrumental developments and results obtained. At DESY, HAXPES experiments routinely use a dedicated spectrometer at an X-ray wiggler with moderate energy resolution (~0.5 eV), well suited for many core level studies. Very recently, high-resolution studies also became possible at DESY with the availability of PETRA III, a new storage ring source providing the highest X-ray brilliance.