Unexpected Reactivity of Zirconium Carbide on Nickel/Yttria-Stabilized Zirconia Boosts CO ₂ /H ₂ O Co-electrolysis to Methane

The high-temperature co-electrolysis of CO₂ and H₂O in solid oxide cells is a promising avenue toward renewable energy storage and climate change mitigation. The present study identifies fundamental electrochemical pathways toward methane under syngas-rich electrochemical conditions and differentiates the specific reactivities of the observed carbon deposits. A thin-film electrode comprising Ni on yttria-stabilized zirconia (YSZ) was evaluated by operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and mass-spectrometric product detection under strongly cathodic electrolysis conditions with predominantly CO and H₂ in the gas phase. Under these conditions, the reduction of CO toward elemental carbon species takes place. A hitherto unknown methane-forming mechanism via polarization-induced carbon spillover from Ni to YSZ was identified, causing ZrC _x formation at the electrolyte surface near the Ni/YSZ boundary. Hydrolysis of this ZrC _x species by added traces of steam enhances the methane selectivity. Our experimental results, corroborated by density functional theory (DFT) modeling, enable better control of high-temperature CO₂-H₂O co-electrolysis for product-selective renewable energy storage.