Tunable Electronic Properties of Graphene Quantum Dots Guide the CO2-to-Formate Conversion Efficiency on SnO2 Nanosheet

SnO2 has been proved to be a promising catalyst for the electrochemical conversion of CO2 to formate. However, slow interfacial charge transfer and the unoptimized binding of intermediates to active sites result in large barriers and slow reaction kinetics for formate formation. Herein, SnO2 was selected as a model catalyst and decorated with graphene quantum dots (GQDs) embedding different functional groups (SnO2/R-GQD, with "R" being -NH2, -OH, or -SO3) to systematically explore the variation of interfacial effect on the behavior of intermediates adsorption and CO2 reduction reaction (CO2RR) activity. Experimental results show that the CO2RR performance was dependent on the electron-donating property of functional groups on GQDs. The SnO2/NH2-GQD composite can selectively convert CO2 to formate with a Faraday efficiency (FEformate) as high as 92.9% at -1.3 V vs RHE and a partial electric current density (jformate) as high as 16.2 mA cm-2. Experimental results and density functional theory (DFT) calculations indicate that the performance of CO2RR depends on the electron-donating properties of the functional groups on the GQDs. The strong electron-donating effect of -NH2 optimizes the adsorption free energy of the key intermediate and accelerates the desorption of *HCOOH, thus improving the catalyst activity. This study not only shows a series of advanced CO2RR electrocatalysts but also provides a feasible strategy for the rational design of catalysts for other proton-coupled electron-transfer reactions.