Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H 2 O + 2e − → H 2 + O 2− ) and electro-oxidation of hydrogen (H 2 + O 2− → H 2 O + 2e − ) at ∼700°C in 0.5 Torr of H 2 /H 2 O on ceria (CeO 2−x ) electrodes. The experiments reveal a transient buildup of surface intermediates (OH − and Ce 3+ ) and show the separation of charge at the gas−solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH − and incorporated O 2− differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH − and O 2− shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H 2 O + Ce 3+ ⇔ Ce 4+ + OH − + H • ) is rate limiting in both the forward (water electrolysis) and reverse (H 2 electrooxidation) reactions. This separation of potentials reflects an induced surface dipole layer on the ceria surface and represents the effective electrochemical double layer at a gas−solid interface. The in situ XPS data and DFT calculations show that the chemical origin of the OH − /O 2− potential separation resides in the reduced polarization of the Ce−OH bond due to the increase of Ce 3+ on the electrode surface. These results provide a graphical illustration of the electrochemically driven surface charge transfer processes under relevant and nonultrahigh vacuum conditions. ■ INTRODUCTIONUnderstanding the mechanisms of charge separation and charge transfer at electrochemical interfaces is essential for the rational development of electrochemical devices, such as batteries, fuel cells, electrolyzers, and supercapacitors. 1,2 However, the materials and operating conditions employed in real world applications of these technologies are usually quite different from those used in surface science studies on model systems (i.e., the "pressure and materials gap"). 3−5 This disconnect is particularly problematic with high temperature electrochemical energy conversion devices with multicomponent materials (e.g., solid oxide fuel cells, electrolyzers, and electrocatalytic fuel processors) 6 for which in situ surface experiments at cell operating temperatures (typically >500°C) are challenging. 7 Because of the experimental constraints of most surface science experiments, the knowledge and understanding of the surface processes at relevant conditions are limited and rely on extrapolations from ultrahigh vacuum (UHV) conditions and modeling studies. 8 As a result, the electrochemical surface processes are not well understood. For example, the nonFaradaic electrochemical modification of catalytic activity (NEMCA or EPOC) 9 can significantly enhance the rates of catalytic transformation of over 100 reactions, 3,10 yet the origins of this enhancement are not fully understood. 3 Even the mechanism ...

Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oxidation states and local electric potentials of thin-film ceria electrodes operating in solid oxide electrochemical cells for H 2 O electrolysis and H 2 oxidation. Ceria electrodes which are 300 nm thick are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H 2 /H 2 O mixtures. Between 635 and 740 °C, equilibrium (zero-bias) near-surface oxidation states between 70 and 85% Ce 3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H 2 O electrolysis on ceria and further increase the percentage of Ce 3+ on the surface over 100 µm from an Au current collector, signifying broad regions of electrochemical activity due to mixed ionic-electronic conductivity of ceria. Negative biases to drive H 2 oxidation decrease the percentage of Ce 3+ from equilibrium values but with higher electrode impedances relative to H 2 O electrolysis. Additional tests indicate that increasing H 2 -to-H 2 O ratios enhances ceria activity for electrolysis.

Spatially resolved ambient pressure X-ray photoelectron spectroscopy has been used to measure and visualize regions of electrochemical activity, local surface potential losses, overpotentials, and oxidation state changes on single sided ceria/yttria-stabilized zirconia (YSZ)/Pt solid oxide electrochemical cells. When hydrogen electro-oxidation (negative applied bias) or water electrolysis (positive applied bias) is promoted on the ceria electrocatalyst, the Ce 3+ /Ce 4+ ratios shift away from equilibrium values and thereby demarcate electrochemically active regions on the ceria electrode. In addition to the ceria oxidation state shifts, inactive surface impurities with high photoelectron cross sections, such as Si, can provide local markers of activity through chemical and surface potential mappings under various electrochemical conditions. Localized removal of chemically active carbonaceous surface impurities also reveals regions of electrochemical oxidation activity on the ceria electrode. Finally, we show that electrochemical polarization of solid oxide electrochemical cells under different gas environments is used to control the ceria surface chemical state and oxygen vacancy density.

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Fuchen Chemical Reagents Factory), potassium permanganate (KMnO 4 , Beijing Yongjia Boyuan Trading Co. Ltd.), potassium carbonate (K 2 CO 3 , ≥99%, Beijing Chemical Works). Porous α-Al 2 O 3 disks, (Angtai Electronic Ceramics Co. Ltd.) 30 mm in diameter, 3 mm in thickness, were used as supports. Modification of the support surface: Porous α-Al 2 O 3 disks were selected as the supports of membranes with the most probable pore size of ca. 160 nm and about 35% porosity. One side of the support was polished using 2000 grit SiC sandpaper to obtain a smoother surface for membrane growth, ultrasonically cleaned using abundant deionized water to clean off the impurities. 1,2 Then the α-Al 2 O 3 discs were soaked in saturated NaOH solution for 24 h to get rid of the oil of support surfaces and also increase the concentration of free hydroxyl groups. 3 After that the supports were ultrasonically dealt with abundant deionized water, then dried at 100 °C for 1 h. 4 The supports were then exposed to the vinyltrimethoxysilane (CH 2 =CH Si(OCH 3) 3 for 20 h, followed by rinsing with deionized water and drying. The terminating vinyl group of the vinyltrimethoxysilane self-assembled organic monolayer (SAM) was oxidized by immersing the supports into an aqueous solution (20 ml) of 0.002 g KMnO 4 , 0.084 g NaIO 4 and 0.007 g K 2 CO 3 according to the methods in literature. 5-7 After 20 h the supports were removed from the oxidation solution, rinsed with water and dried.

Highly diverse structures and pore sizes make metal−organic frameworks (MOFs) good candidates for the fabrication of gas separation membranes. The synthesis of continuous MOF membranes still remains a challenge. In this work, an integrated Cu-BTC membrane was successfully prepared on the novel potassium hexatitanate support for the first time by in situ solvothermal growth. This kind of support was found to be more suitable for the growth of Cu 2+ -containing MOF membranes than other traditional supports, such as a porous alumina support. The permeation results of Cu-BTC membranes obtained in this work show moderate separation selectivities of helium over other small gas molecules, including CO 2 , N 2 , and CH 4 . Compared to other MOF membranes, the Cu-BTC membrane exhibits higher ideal selectivity for helium under the condition of similar helium permeance, while it has higher helium permeance with similar ideal selectivity.

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