Abstract
In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.
Highlights
Solid oxide electrochemical cells (SOCs) offer a means for clean and efficient conversion of chemical energy into electricity as fuel cells or a means of storing excess electrical energy into chemical fuels as electrolysis cells
Negative Vcell − IRbulk indicated H2 oxidation on the working electrodes (WEs) whereas positive Vcell − IRbulk indicated H2O electrolysis on the WE. Since both WEs shared the same yttria-stabilized zirconia (YSZ) electrolyte support and Pt counter electrode (CE), differences between V–I curves in figures 3(a) and (b) at the same operating conditions were attributed to differences in electrochemical behaviour of the Ni/Gadolinia-doped ceria (GDC) and Au/GDC
The presented study explored surface potentials, active surface species, and associated electrochemical performance of the Ni/GDC and Au/GDC electrodes in H2 oxidation and H2O electrolysis
Summary
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Keywords: in operando, ambient-pressure XPS, solid oxide electrochemical cell, GDC, charge transfer, Ni electrocatalysis
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