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Soft- and Hard X-ray PES Investigation of PEM Fuel Cell Catalysts
Vladimir Matolin1 Iva Matolinova1, Hideki Yoshikawa2, Keisuke Kobayashi2
1 Charles University in Prague, V Holesovickach 2, 18000 Prague, Czech Republic
2 NIMS Beamline station BL15XU, SPring-8, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan
The possibility of co-fabricating a power source on the same substrate as the electronic circuit offers unique advantages, including a reduction in size and weight, improved signal integrity because of fewer interconnections, increased processing efficiency, and lower cost. However, at the very heart of the -FC reactor we are facing a great chemical challenge: As for any other FC, the catalyst is the key to performance and, therefore, the most critical component. However, standard wet-process techniques for powder catalyst are incompatible with on-chip -FC technology. Planar on-chip -FCs, therefore, require novel catalysts prepared by thin-film deposition.
For one thing, planar technologies are generally considered incompatible with the preparation of sufficiently large surface areas. For another, catalytic loading of high-surface-area materials is deemed impossible because thin-film deposition techniques themselves often (not always by far) yield compact low-surface-area films. These arguments, however, do not really hamper because high-surface-area nanoporous catalyst films can indeed be prepared by suitable deposition techniques. One example is the deposition of active materials on nanostructured substrates, e.g. carbon nanotubes (1,2) or nanoporous carbon films. Thin film catalysts could be an option with the potential of low loading and tailor made catalysts and beside on-chip structure can be successively used in standard proton exchange membrane fuel cells (PEMFC), too.
Chemical composition of porous large surface catalyst films can be investigated by hard and soft x-ray synchrotron radiation photoelectron spectroscopy (HAXPES, SXPES) by taking advantage of high brilliance and high resolution of synchrotron beamlines. We have shown that in spite of low surface sensitivity HAXPES can be successfully used for investigation of catalyst surface chemical state in the case of porous systems due to its high information depth. On such systems we can obtain information which cannot be gained using SXPES or laboratory XPS. SXPES on the other hand can provide information from top parts of the catalyst films and can be used as a complementary technique, particularly in the case of model catalyst study.
We have shown that Pt-doped sputtered cerium oxide films contained high concentration of cationic platinum Pt2+ and Pt4+ which were highly active species for hydrogen dissociation to protonic hydrogen H+ in PEMFCs(1,2). Because of porous structure of the catalyst films with active sites at the surface and boundaries of grains inside the pores and the formation of multiple chemical states of dopants and Ce atoms (Ce3+,4+), HAXPES experiments supplied unique chemical information. SXPES provided information about the surface composition. The experiments showed that Pt2+/Pt4+ and Ce3+/Ce4+ ratios were key parameters of the catalyst activity.
Understanding corrosion inhibition through photoelectron spectroscopy
Corrosion and Protection Centre, School of Materials,
The University of Manchester, Sackville Street, Manchester, M13 9PL, UK
Corrosion inhibitors are substances that, when introduced into a corrosive environment, reduce the corrosion rate of a metallic substrate by inducing a change at or near its surface, without significantly changing the concentration of corrosive species. This approach to corrosion control is well established, and has long been the focus of significant research activity. Nevertheless, despite such interest, corrosion inhibitor selection and development for technological application essentially remains an empirical process, involving trial-and-error performance testing of candidates. Such an approach partially arises from the complex nature of the parameter space influencing inhibitor functionality, but is also a result of a lack of mechanistic knowledge. In this presentation, I will discuss our efforts to address this issue through the employment of photoelectron spectroscopy. More specifically, I will focus on the protection of carbon steel in different environments by inorganic and organic species.
Probing s electron states in metal oxides with hard X-ray photoemission
University of Oxford, Inorganic Chemistry Laboratory, S. Parks Road, Oxford OX1 3QR, UK
The cross sections for ionization of highly penetrating 5s or 6s orbitals decrease with increasing photon energy much less rapidly that the O 2p cross section. This results in selective relative enhancement in the intensity of states with significant metal s character in valence band hard x-ray photoemission (HAXPES) of post transition metal oxides. Application of this simple principle will be illustrated by discussion of work on PbO2 (1), In2O3(2-5), In4Sn3O12 (6) and SnO2.
Most attention will focus on In2O3 where comparison between HAXPES and conventional photoemission spectra led to the realization that the fundamental bandgap of this prototype transparent conducting oxide was almost 1 eV lower than the widely quoted value of 3.75 eV (2). This in turn helped to establish that an electron accumulation layer presents itself on most In2O3 surfaces (3-5).
Figure 1: Valence band Al Kα XPS and HAXPES of In2O3, SnO2 and In4Sn3O12
Electronic Structure of Transparent Conducting Oxide Semiconductors
Department of Physics, University of Warwick, Coventry CV4 7AL UK.
Oxide semiconductors have become of great interest lately with enormous opportunities for new uses that will potentially improve existing materials and device applications. The fact that some of these materials, such indium tin oxide (ITO), have been around for some time and, in a relatively low quality form have seen significant industrial use as transparent conductors, has perhaps contributed to the belated recognition of the possibilities as semiconductors in their own right. Here, the surface and bulk electronic properties of epitaxially grown high-quality oxide semiconductors (In2O3, CdO and ZnO) will be discussed and the effects of modifying these surfaces by controlled adsorption and surface treatment. Optical, electronic and structural properties of these semiconducting oxide films will be presented. The valence band density of states and the surface electronic properties of these oxide semiconductors have been studied using high-resolution angle-resolve photoemission spectroscopy (ARPES) and compared with theoretical band structure calculations (1-3). A common property of these oxide semiconductors is found to be the presence of a surface electron accumulation layer, in marked contrast to the electron depletion generally observed at the surfaces of conventional semiconductor materials. Additionally, hydrogen is found to be a donor and any native defects have a propensity to be donors in already n-type material. The origins of this phenomenon will be discussed in terms of the band structure and intrinsic properties of these materials.