. XRD patterns of Sn-QDs/CN and CN

. ¹H NMR spectrum of the cathodic electrolyte after CO₂RR (a), and linear relationship between HCOOH concentration and relative peak area ratio (vs. DMSO) (b)

. TEM images at different magnifications (a, b) and corresponding EDS line scanning spectra (c) of Sn-p/CN; TEM image (inset: magnified image) (d), HRTEM image (e) and Sn-QDs size distribution (f) of Sn-QDs/CN

. High-resolution N1s XPS spectra of Sn-QDs and CN

. i-t curves of CO₂RR on Sn-QDs/CN at different applied potentials (a), and Faradaic efficiencies of HCOOH, CO and H₂ at different applied potentials on the Sn-QDs/CN electrode (b)

. High-resolution Sn3d (a) and O1s (b) XPS spectra of Sn-QDs/CN

. LSV curves of the Sn-QDs/CN electrode in Ar-(dotted line) and CO₂-saturated (solid line) 0.1 mol·L^-1 KHCO₃ electrolyte at a scan rate of 30 mV·s^-1 (a), and Faradaic efficiencies of HCOOH on Sn-QDs/CN and Sn-p/CN at a series of potentials (b)

. Faradaic efficiencies of CO and H₂ at different applied potentials on CN

. Charging current density differences plotted against scan rates (a), electrochemical impedance spectra with inset showing the corresponding equivalent circuit (b), Tafel plots for HCOOH production on Sn-QDs/CN and Sn-p/CN (c), and the stability of Sn-QDs/CN catalyst at -1.0 V for 24 h in CO₂-saturated 0.1 mol·L^-1 KHCO₃ (d)

. LSV curves of Sn-p/CN and Sn-QDs/CN in CO₂-saturated 0.1 mol·L^-1 KHCO₃ electrolyte at a scan rate of 30 mV·s^-1

. Proposed possible reaction pathway of CO₂-to-HCOOH conversion on Sn-QDs/CN

Table 0. Comparison of various Sn-based catalysts for CO₂-to-HCOOH conversion

Table 0. Fitted data of EIS for Sn-QDs/CN and Sn-p/CN