On the Active Surface State of Nickel‐Ceria Solid Oxide Fuel Cell Anodes During Methane Electrooxidation

Abstract

Solid oxide fuel cells (SOFCs) have grown in recognition as a viable technology able to convert chemical energy directly into electricity, with higher efficiencies than conventional thermal engines. Direct feeding of the SOFCs anode with hydrocarbons from fossil or renewable sources, appears more attractive compared to the use of hydrogen as a fuel. The addition of mixed oxide-ion/electron conductors, like gadolinium-doped ceria (GDC), to commonly used nickel-based anodes is a well–known strategy that significantly enhances the performance of the SOFCs. Here we provide in situ experimental evidence of the active surface oxidation state and composition of Ni/GDC anodes during methane electroxidation using realistic solid oxide electrode assemblies. Ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies (APPES and NEXAFS respectively) combined with on line electrical and gas phase measurements, were used to directly associate the surface state and the electrocatalytic performance of Ni/GDC anodes working at intermediate temperatures (700°C). A reduced anode surface (Ce3+ and Ni), with an optimum Ni to Ce surface composition, were found to be the most favorable configuration for maximum cell currents. Experimental results are rationalized on the basis of first principles calculations, proposing a detailed mechanism of the cell function

Effect of Au and/or Mo Doping on the Development of Carbon and Sulfur Tolerant Anodes for SOFCs—A Short Review

“…The latter has been proven in Papaefthimiou et al (2013), both experimentally (APPES) and theoretically by the use of DFT calculations. Specifically during the course of the experiment depicted in Figure 4 excess oxygen was observed owing to current application which did not correspond to the lattice oxygen of the partially reduced Ce 3+ .…”

“…The dynamic nature of the Ni/GDC anode surface has been elucidated in Papaefthimiou et al (2013) by the use of Ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies (APPES and NEXAFS respectively). These surface techniques are combined with on line electrochemical and gas phase measurements.…”

“…The main features of Ni L 3 -and Ce M 5 -edge NEXAFS spectra that are being recorded during the evolution of the experiment and are depicted in Figure 4. In addition to the spectra evolution the inset of Figure 4 depicts the relative concentration of the Ce 3+ species with respect to the overall ceria spectrum (see Supporting Information S5 for details of Papaefthimiou et al, 2013). Initially and upon the application of the constant voltage +1 V the anode is still largely oxidized with Ni and Ce being at 2+ and 4+ respectively (Ni 2+ , and mainly Ce 4+ ), while the current flow is negligible.…”

“…In the extreme cases where the electrode is oxidized NiO is accumulated on the surface thus screening GDC, while at highly reductive atmospheres the metallic Ni particles are encapsulated by the GDC (Papaefthimiou et al, 2013).…”

“…In this direction our research group with collaborators (Niakolas et al, 2010Papaefthimiou et al, 2013;Souentie et al, 2013;Neofytidis et al, 2015) attempted to study and modify NiO/YSZ and NiO/GDC powder with Au and/or Mo nano-particles. These modifications resulted in electrodes with high tolerance and improved electrocatalytic activity under both carbon forming and sulfur poisoning conditions.…”