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AP-XPS Measures MIEC Oxides in Action Print

Oxide materials with mixed ionic-electronic conductivity (MIEC) can conduct both electrons and oxygen ions. MIEC oxides have broad applications, including use in solid-oxide fuel cells, high-temperature electrolysis for synthetic fuel production, and oxygen-separating membranes for chemical processes or NOx-free combustion; however, their surface activity under reaction conditions has been difficult, if not impossible, to ascertain, until recently. A team from the University of Maryland and Sandia National Laboratories joined ALS scientists on Beamlines 9.3.2 and 11.0.2 to overcome the vacuum limitations of conventional XPS instruments using ambient-pressure x-ray photoelectron spectroscopy (AP-XPS), providing the first in situ measurements of local surface oxidation states and electric potential in active MIEC electrodes.

Making Fuel from Water

This work explored model materials that may be used in high-temperature water splitting to make hydrogen gas (H2). Understanding how the surfaces react under different conditions provides a critical foundation for designing efficient processes for H2 production from steam using renewable solar and wind energy.

The unique capabilities of the ambient-pressure XPS stations on Beamlines 9.3.2 and 11.0.2 allow for observation of how these material surfaces behave under the far-from-equilibrium conditions during high-temperature activation. These first-of-a-kind measurements provide valuable insight into the surface kinetics and how they couple with ionic and electronic transport during water splitting and H2 oxidation at high temperatures.

The insights achieved here lay the groundwork for developing models of these materials for use in designing more efficient hydrogen gas production processes, like electrolysis, from renewable power sources, like water, allowing for either a sustainable hydrogen economy or sustainable production of synthetic hydrocarbon fuels.

The effectiveness of MIEC oxide material in electrochemical cells and membrane reactors depends largely on their surface activity under reacting conditions far from equilibrium. Because laboratory-scale x-ray sources have traditionally required ultrahigh vacuum for measurements, it was previously impossible to measure the surface oxidation states of active electrode surfaces using x-ray photoelectron spectroscopy (XPS); therefore, the research community has lacked quantitative data on these materials’ surface redox cycles and associated oxide vacancy formation under electrochemical activation.

 

Top: Single-chamber solid-oxide electrochemical cell for AP-XPS measurements with two thin-film CeO2-x working electrode with an Au current collector and a Pt counter electrode patterned onto a polycrystalline YSZ substrate. This geometry exposes all cell components to the x-ray beam. During operation, the high-temperature cell is positioned close to the detector aperture in 1-mbar H2 and H2O. Bottom: A schematic representation of the spatially resolved photoelectron detection.

By constructing single-chamber electrochemical cells with MEIC electrode components in a planar arrangement, x-rays could reach all components while the cell was operating. This advancement, combined with the use of AP-XPS at temperatures and pressures representative of reaction conditions, allowed researchers to study MIEC oxides’ active electrode surfaces.

Studying one MIEC material, undoped cerium oxide (CeO2-x), in H2/H2O mixtures provided new insight into how MIEC electrodes distribute electrochemical reactions and associated overpotentials (or voltage penalties) during H2O electrolysis or H2 oxidation. To study the behavior of CeO2-x electrodes during H2O electrolysis and H2 oxidation, dense thin-film CeO2-x electrodes with Au current collectors were deposited onto YSZ electrolytes with a thin-film Pt counter electrode. The AP-XPS endstations at the ALS were operated at 750 ºC in 1-mbar reactant gases H2 and H2O.

 

A 250-nm-thick ceria anode at a +1.2 V bias converts H2O to H2 and O2- ions in a broad region, as revealed by the in situ AP-XPS measurerments of local surface potentials (red squares) and relative shifts of Ce oxidation state (green circles) from equilibrium in this region.

XPS measurements of either the Ce3d or Ce4d core electron spectra can be fitted to provide quantitative measurements of Ce oxidation states, which complement local surface potential measurements based on kinetic energy shifts of these core-level photoelectron spectra. Simultaneous measurements provide a basis for correlating overpotentials with changes in oxidation states between Ce4+ and Ce3+, revealing regions of surface activity as a function of electrical bias.

By comparing Ce3+ surface fractions over a range of electrical biases with fractions at equilibrium conditions, electrochemical performance and surface behavior were assessed for a range of CeO2-x film thicknesses, operating temperatures, and gas compositions. Measurements reveal that active regions on the CeO2-x extend 150 μm from the Au current collectors. This indicates the effectiveness of MIEC oxides to spread electrochemical reactions for both fuel oxidation and electrolysis.

 

Ce4d spectra obtained with a 2-D area detector (ALS Beamline 9.3.2) reveal the 150-μm active region of the ceria electrode. Plots (a)–(c) show spatially resolved XPS spectra of the Ce4d region at +1.2, 0, and -1.2 V applied potential recorded with 490-eV photon energy.

In general, oxidation states show significant increases in Ce3+ fractions only in the active regions under positive cell biases to drive H2O electrolysis. Negative biases to drive H2 oxidation decrease Ce3+ from equilibrium values and require higher overpotentials for comparable currents, suggesting that CeO2-x serves as a more effective electrode material for H2O electrolysis than H2 oxidation.

This study has laid the groundwork for expanded studies to look at other complex MIEC oxide materials for O2 separation and high-temperature electrochemical cells using AP-XPS and other in situ measurements. Additionally, new cell configurations involving two-chamber cells will study electrodes at higher current fluxes comparable to practical high-power-density devices. These forthcoming studies will provide fundamental insight into the nature of inefficiencies in high-temperature electrochemical cells, allowing the research community to develop improved strategies and materials to move these technologies forward for clean power generation and fuel production.

 


 

Research conducted by C. Zhang, S. DeCaluwe, G. Jackson, and B. Eichhorn (University of Maryland); M.E. Grass, Z. Liu, H. Bluhm, and Z. Hussain (Berkeley Lab); and A. McDaniel, F. El Gabaly, K. McCarty, R. Farrow, and M. Linne (Sandia National Labs).

Research funding: Office of Naval Research and the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publications about this research: C. Zhang, M.E. Grass, A.H. McDaniel, S.C. DeCaluwe, F. El Gabaly, Z. Liu, K.F. McCarty, R.L. Farrow, M.A. Linne, Z. Hussain, G.S. Jackson, H. Bluhm, and B.W. Eichhorn, "Measuring fundamental properties in operating electrochemical cells by using in situ X-ray photoelectron spectroscopy", Nat. Mater. 9, 949 (2010); S.C. DeCaluwe, M.E. Grass, C. Zhang, F. El Gabaly, H. Bluhm, Z. Liu, G.S. Jackson, A.H. McDaniel, K.F. McCarty, R.L. Farrow, M.A. Linne, Z. Hussain, and B.W. Eichhorn, "In situ characterization of ceria oxidation states in high-temperature electrochemical cells with ambient pressure XPS," J. Phys. Chem. C 114, 19853 (2010).

ALS Science Highlight #227

 

ALSNews Vol. 320