Transition metal (oxy)carbides for protonic ceramic cells: Opportunities and challenges of molybdenum (oxy)carbide

Transition metal (oxy)carbides are promising electrode materials in protonic ceramic cells (PCCs) for operation in hydrocarbon atmospheres due to their structural stability under these conditions, which addresses a known issue in Ni-based cermets, and their satisfactory electrical properties. In this work, several (oxy)carbides — Mo 2 C x O y , ZrC x O y , WC x O y , and NbC x O y — are evaluated, all exhibiting phase stability in methane. The chemical compatibility of the materials with the BaCe 0.9 Y 0.1 O 3- δ (BCY) electrolyte is also examined, revealing that only the molybdenum-based composition does not form secondary phases. The application of the molybdenum (oxy)carbide in biogas is constrained by its susceptibility to oxidation in CO 2 , with thermodynamic and kinetic studies indicating resistance to oxidation only at low temperatures, where the reforming process is limited. Notably, the Mo 2 C x O y -BCY composite electrode shows considerable improvement after one week in pure (humidified) CH 4 at 800 °C, with polarisation resistance decreasing by ~82%, highlighting the beneficial effect of prolonged exposure to the hydrocarbon atmosphere. Elemental analysis and thermogravimetric measurements suggest an increased C/O ratio after methane treatment, as also confirmed by X-ray photoelectron spectroscopy (XPS) and corroborated by a slight expansion of the crystal lattice observed by X-ray diffractometry (XRD), contributing to the electrical and catalytic properties. A stable open-circuit voltage (OCV) of ~0.8 V at 800 °C for one week demonstrates that the anode is not degraded in CH 4 . However, a decrease to ~0.5 V at 700 °C indicates a limitation in low-temperature internal reforming. Finally, variation of hydrogen partial pressure further reveals that the electrode, with a p H 2 − 0.37 dependence, is rate-limited by hydrogen adsorption/desorption and surface diffusion kinetics, which should occur on the (oxy)carbide surface. • Mo 2 C x O y is chemically compatible with yttria-doped barium cerate, while ZrC x O y and NbC x O y are not. • CO 2 exposure drives oxidation of (oxy)carbides, which limits their applicability in biogas. • The anionic sublattice of Mo (oxy)carbide is recarburised in methane. • Long-term CH 4 operation enhances electrochemical activity by ~82%. • The electrochemical mechanism of the electrode is outlined.