Efficient Electrochemical Reduction of CO 2 to HCOOH over Sub‐2 nm SnO 2 Quantum Wires with Exposed Grain Boundaries

Electrochemical reduction of CO 2 could mitigate environmental problems originating from CO 2 emission. Although grain boundaries (GBs) have been tailored to tune binding energies of reaction intermediates and consequently accelerate the CO 2 reduction reaction (CO 2 RR), it is challenging to exclusively clarify the correlation between GBs and enhanced reactivity in nanostructured materials with small dimension (<10 nm). Now, sub‐2 nm SnO 2 quantum wires (QWs) composed of individual quantum dots (QDs) and numerous GBs on the surface were synthesized and examined for CO 2 RR toward HCOOH formation. In contrast to SnO 2 nanoparticles (NPs) with a larger electrochemically active surface area (ECSA), the ultrathin SnO 2 QWs with exposed GBs show enhanced current density (j), an improved Faradaic efficiency (FE) of over 80 % for HCOOH and ca. 90 % for C1 products as well as energy efficiency (EE) of over 50 % in a wide potential window; maximum values of FE (87.3 %) and EE (52.7 %) are achieved.