In Situ Characterization of Ceria Oxidation States in High-Temperature Electrochemical Cells with Ambient Pressure XPS

Abstract

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.

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…In the fuel environment, the activity of the MIEC/H2,H2O 2PB, as observed for CGO and other low-pO2 MIECs, considerably increases when reduced by cathodic polarisation and decreases when oxidized by anodic polarisation, corresponding to the pO2 changes 29,60,64,65 . Interestingly, MIECs are often activated by redox cycles, even those that are chemically and dimensionally stable both in air and fuel environments.…”

Fast vacancy-mediated oxygen ion incorporation across the ceria–gas electrochemical interface

“…For example, density functional calculations predicted reduced defect formation energy on the low-index surfaces of ceria from that of the bulk [20][21][22][23] , which has been validated experimentally 24 . In operando techniques such as in-situ X-ray photoelectron spectroscopy (XPS) [25][26][27][28][29] have also been applied to ceria 16,[30][31][32] . However, many of these experiments adopted electrochemical cell geometries that are bulk-diffusion-limited, rather than surface-reaction-limited.…”

Probing surface & transport phenomena in energy materials under operating conditions.

“…19 Our larger data set allows determining the energetics without assumptions. The negative entropic contribution to the chemical potential of surface oxygen ( o TS  ) compared to that of the bulk oxygen establishes that the preferential reduction of the surface becomes greater with decreasing temperature, behavior also evident directly from Figure 4.…”

“…15 Understanding the behavior of surface reactive species is crucial towards establishing the reaction mechanism. Ce 3+ and oxygen vacancies are thought to be the active sites on ceria surfaces [16][17][18] in reactions such as hydrolysis: ( 2 Several studies have shown that the surface concentrations of these active species can be higher than the bulk, potentially explaining ceria's high chemical activity [16][17][18][19] However, the behavior of these active surface species is not well-understood, in part because studies involving mixed-valent materials are complicated by the fact that both the bulk and the surface participate in the catalytic reactions.…”

“…However, the surface area was shown to be unstable during the course of the experiment 30 . Some in situ studies have emerged recently, 13,19,31 though none have successfully achieved the desired surface and bulk sensitivities. The accepted view on the behavior of reactive Ce 3+ species, mostly based on theoretical calculations 17,18,[32][33][34][35][36][37][38][39] , is that the segregation of Ce 3+ species to the surface is driven by a change in the oxidation enthalpy.…”