Temperature promotes selectivity during electrochemical CO₂ reduction on NiO:SnO₂ nanofibers

Electrolyzers operate over a range of temperatures; hence, it is crucial to design electrocatalysts that do not compromise the product distribution unless temperature can promote selectivity. This work reports a synthetic approach based on electrospinning to produce NiO:SnO₂ nanofibers (NFs) for selectively reducing CO₂ to formate above room temperature. The NFs comprise compact but disjoined NiO and SnO₂ nanocrystals identified with STEM. The results are attributed to the segregation of NiO and SnO₂ confirmed with XRD. The NFs are evaluated for the CO₂ reduction reaction (CO₂RR) over various temperatures (25, 30, 35, and 40 °C). The highest faradaic efficiencies to formate (FE_HCOO⁻) are reached by NiO:SnO₂ NFs containing 50% of NiO and 50% SnO₂ (NiOSnO50NF), and 25% of NiO and 75% SnO₂ (NiOSnO75NF), at an electroreduction temperature of 40 °C. At 40 °C, product distribution is assessed with in situ differential electrochemical mass spectrometry (DEMS), recognizing methane and other species, like formate, hydrogen, and carbon monoxide, identified in an electrochemical flow cell. XPS and EELS unveiled the FE_HCOO⁻ variations due to a synergistic effect between Ni and Sn. DFT-based calculations reveal the superior thermodynamic stability of Ni-containing SnO₂ systems towards CO₂RR over the pure oxide systems. Furthermore, computational surface Pourbaix diagrams showed that the presence of Ni as a surface dopant increases the reduction of the SnO₂ surface and enables the production of formate. Our results highlight the synergy between NiO and SnO₂, which can promote the electroreduction of CO₂ at temperatures above room temperature.