Unexpected Reactivity of Zirconium Carbide on Nickel/Yttria-Stabilized Zirconia Boosts CO₂/H₂O Co-electrolysis to Methane

Abstract

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 ZrCₓ formation at the electrolyte surface near the Ni/YSZ boundary. Hydrolysis of this ZrCₓ 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.