X-ray Absorption Spectroscopy Cell Research Articles

In-Situ XAFS Study to Monitor the Degradation of an Fe/N/C Cathode Catalyst in PEMFC

Understanding the degradation mechanism of Fe/N/C cathode catalysts in proton exchange membrane fuel cells (PEMFCs) is extremely important. In this study, an Fe/N/C catalyst prepared from polyimide nano-particles were placed in an in-situ cell for X-ray absorption spectroscopy, and its Fe K-edge adsorption spectra were recorded during a continuous operation of the fuel cell. The Fe/N/C catalyst used in this study was prepared by pyrolyzing Fe-containing polyimide nano-particles, which were synthesized by the precipitation polymerization of Pyromellitic dianhydride and 1,3,5-tris(4-aminophenyl)benzene. The Fe content after the preparation procedure was 0.6 wt%. This catalyst was used to fabricate a membrane and electrode assembly (MEA) with a Pt/C anode and a Nafion 212 membrane. This MEA was installed in a test cell for in-situ X-ray adsorption fine structure (XAFS) measurements, and operated at 0.5 V and 80°C for 8 h monitoring its Fe K-edge adsorption spectra. Fig. 1 shows the changes in the spectra during the fuel cell operation. The valence of the Fe specie at the initial stage was 3+. The spectra gradually changed during the fuel cell operation, suggesting that the dissolution of the Fe species from the active sites. This change could be relevant to the degradation of the cathode catalyst. Acknowledgements This study is financially supported by NEDO. The in-situ XAFS study was carried out at BL36XU in SPring-8 (No. 2016B7907, 2018A7840 and 2018B7907). Figure 1

In-Situ XAFS Study to Monitor the Degradation of an Fe/N/C Cathode Catalyst in PEMFC

Understanding the degradation mechanism of Fe/N/C cathode catalysts in proton exchange membrane fuel cells (PEMFCs) is extremely important. In this study, an Fe/N/C catalyst prepared from polyimide nano-particles was placed in an in-situ cell for X-ray absorption spectroscopy, and its Fe K-edge adsorption spectra were acquired during a continuous operation of the fuel cell. The valence of the Fe specie at the initial stage was 3+. The spectra gradually changed during the fuel cell operation, suggesting that the dissolution of the Fe species from the active sites.

X-ray Absorption under Operating Conditions for Solid-Oxide Fuel Cells Electrocatalysts: The Case of LSCF/YSZ

We describe a novel electrochemical cell for X-ray absorption spectroscopy (XAS) experiments during electrical polarization suitable for high-temperature materials such as those used in solid oxide fuel cells. A half-cell LSCF/YSZ was then investigated under cathodic and anodic conditions (850 °C and applied electrical bias ranging from +1 V to −1 V in air). The in situ XAS measurements allowed us to follow the LSCF degradation into simple oxides. The rapid deterioration of LSCF is ascribed to the formation of excess of oxygen vacancies leading to the collapse of the mixed perovskite structure.

Low-cost vacuum compatible liquid cell for hard X-ray absorption spectroscopy

We present the design, fabrication, and commissioning of a new liquid cell for X-ray absorption spectroscopy, compatible with measurement in both transmission and fluorescence modes. The design consists of easily demountable and replaceable parts: body, windows, elastomer spacer, washers and a holding screw. The pathlength of the liquid chamber can be adjusted by simply selecting the appropriate thickness of the spacer, making the cell extremely customizable to overcome experimental constraints. The cell has probed to be leak tight under low vacuum conditions. The compact and simple design makes the cell compatible with cryogenic applications. Finally, the possibility of manufacturing the cell by 3D printing makes it readily available and very competitive from a cost point of view (each unit is estimated less to be than 10 €).

XAS spectroelectrochemistry: reliable measurement of X-ray absorption spectra from redox manipulated solutions at room temperature.

The design and operation of a low-volume spectroelectrochemical cell for X-ray absorption spectroscopy (XAS) of solutions at room temperature is described. Fluorescence XAS measurements are obtained from samples contained in the void space of a 50 µL reticulated vitreous carbon (sponge) working electrode. Both rapid electrosynthesis and control of the effects of photoreduction are achieved by control over the flow properties of the solution through the working electrode, where a good balance between the rate of consumption of sample and the minimization of decomposition was obtained by pulsing the flow of the solution by 1-2 µL with duty cycle of ∼3 s while maintaining a small net flow rate (26-100 µL h(-1)). The performance of the cell in terms of control of the redox state of the sample and minimization of the effects of photoreduction was demonstrated by XAS measurements of aqueous solutions of the photosensitive Fe(III) species, [Fe(C2O4)3](3-), together with that of the electrogenerated [Fe(C2O4)3](4-) product. The current response from the cell during the collection of XAS spectra provides an independent measure of the stability of the sample of the measurement. The suitability of the approach for the study of small volumes of mM concentrations of protein samples was demonstrated by the measurement of the oxidized and electrochemically reduced forms of cytochrome c.

Novel spectro-electrochemical cell for in situ/operando observation of common composite electrode with liquid electrolyte by X-ray absorption spectroscopy in the tender X-ray region

A novel spectro-electochemical cell for X-ray absorption spectroscopy in the tender X-ray region (TX-XAS) was designed and fabricated to investigate the electrochemical behavior of common battery materials with liquid electrolytes under in situ/operando conditions. The cell has several unique features, such as high X-ray transmittance, high signal to noise ratio, and high vacuum tightness. These features enable us quick and reliable XAS measurements. Operando P K-edge XAS measurements of an olivine-type LiFePO4 composite positive electrode were carried out to clarify its phosphorus environment during the electrochemical charging process. Results of spectral analysis show that there is no significant change in the oxidation state of phosphorus and in the coordination of the phosphate anions in the charging process, but a closer look of the consecutive XAS spectra suggests the shrinkage of the PO4 cage during the charging process, and the structural changes in a biphasic manner. These results demonstrate the usefulness of the cell for in situ/operando TX-XAS observations of light elements in practical batteries.

Operando XANES study of simulated transient cycles on a Pd-only three-way catalyst

A model Pd-only three-way catalyst has been subjected to simulated driving conditions of natural gas and gasoline operation in an operando reactor cell for X-ray absorption spectroscopy that included alternated, but longer than real oscillations, rich and lean periods and a high temperature surge (850–900 °C). The X-ray absorption near edge structure (XANES) spectra indicated that metallic palladium is observed in the whole temperature range investigated (up to 900 °C) and irrespective of the air/fuel ratio. In both natural gas and gasoline cycles, the XANES data show that the PdO reduced in the rich periods cannot be restored in the lean periods. With this background, activity for methane abatement in the high temperature regime is greatly affected by the oxidation state of palladium rather than by the change of air/fuel ratio. In the case of propene oxidation, while Pd also remains predominantly in the reduced state, activity is dictated by the oxygen concentration in the feedstock. Comparison between the two hydrocarbons demonstrates that the oxidation state of Pd may be responsible for observed methane emissions under realistic operating circumstances. Moreover, the experiments demonstrate that reduced Pd may be continuously present during operation in agreement with observations on real catalytic converters. Although this may be the average oxidation state of Pd, more advanced probes are certainly necessary to capture variations of oxidation state under the fast oscillatory conditions needed to imitate real operation.

High throughput cell for X-ray absorption spectroscopy applied to study the effect of Au on Rh-catalyzed partial oxidation of methane

Abstract A parallel reactor cell for catalytic in situ X-ray absorption spectroscopy (XAS) measurements was developed. The cell facilitates the simultaneous catalytic and structural investigation of six catalysts under different feed gas conditions. A two-dimensional X-ray sensor was used for spectra collection. Gas compositions were measured by on-line mass spectrometry. The potential and limitations of the high throughput XAS cell are discussed. The heterogeneously catalyzed partial oxidation of methane (CPO) was chosen as a test reaction. Alumina-supported Rh and Au/Rh catalysts with different metal loadings (0.5–2.5 wt%) were applied and prepared via different preparation routes using flame spray pyrolysis (fsp) and colloid adsorption (col). For comparison the same catalysts were also investigated in a fixed-bed capillary reactor heated by a gas blower (also for XAS measurements) and in an eight-fold parallel gas phase reactor using similar reaction conditions (6% CH 4 –3% O 2 –He, 250–500 °C). Similar catalytic results were obtained in all three reactor types, confirming the suitability of the parallel reactor XAS cell for catalytic measurements. The catalysts showed different activity, selectivity and reducibility depending on metal loading, preparation route and Au/Rh ratio. The analysis of the catalytic data, STEM images and the in situ XANES experiments of the various catalysts indicated the following characteristics for the CPO reaction: sufficient heat production by combustion of methane, total conversion of oxygen, and reduction of a certain fraction of the catalyst. The overall catalytic behavior was in line with a two-zone model of the catalyst bed where catalytic combustion dominates in the front zone and reforming reactions become favored in the second part of the catalyst bed.