Efficient Aqueous Electroreduction of CO₂ to Formate at Low Overpotential on Indium Tin Oxide Nanocrystals

Abstract

Electroreduction of CO2 to formate powered by renewable energy offers an alternative pathway to producing carbon fuels that are traditionally manufactured using fossil fuels. However, achieving simultaneously high partial current density (j HCOO –), high product selectivity (Faradaic efficiency (FEHCOO –)), and low overpotentials (η) remains difficult due to the lack of suitable catalysts. Here, we report the electroreduction of CO2 on Sn-doped indium oxide (ITO) nanocrystal catalysts in an alkaline flow electrolyzer. Colloidally synthesized monodisperse 20 nm ITO nanocrystals (NCs) with various Sn-doping levels (0, 1, 5, 6.5, 8, and 12 atom %) were studied. We find that ITO NC catalysts exhibit a high selectivity for production of HCOO– over CO and H2 (approximately 87% HCOO–, 1–4% CO, and 2–6% H2 at −0.85 V vs RHE), an onset potential for HCOO– as early as −0.21 V vs RHE, and a high partial current density for HCOO– up to 171 mA/cm2 at a cathode potential of −1.08 V vs RHE. The main difference between the catalysts’ performances resides in the onset potential for formate production. The onset of formate production occurred at cell and cathode overpotentials of only −440 and −143 mV, respectively, by the 12% ITO. Analysis of the ITO electrodes before and after electrolysis suggests that no changes in surface composition, crystal structure, or particle size occur under the reduction conditions. Tafel slopes of ITO NC catalysts range from 27 to 52 mV per decade, suggesting that the rate-determining step is likely the proton-coupled electron transfer to CO2 ●–* to form HCOO–*.

Investigating the Role of Surface Depletion in Governing Electron-Transfer Events in Colloidal Plasmonic Nanocrystals

“…7 Similar investigations using chemical titrations to interrogate other doped metal oxide NC compositions will shed light on the relevance of surface depletion in understanding properties across this emerging class of plasmonic nanocrystals, with implications for their potential applications in catalysis, sensing, therapeutics, and more. 5,10,26,29,48 ■ EXPERIMENTAL SECTION Chemicals. For the synthesis and characterization of Sn-doped In 2 O 3 (Sn:In 2 O 3 ) nanocrystals (NCs), all chemicals were acquired commercially and employed without further purification prior to their use.…”

Investigating the Role of Surface Depletion in Governing Electron Transfer Events in Colloidal Plasmonic Nanocrystals

“…25 Therefore, modulation of surface sensitive properties like LSPR, catalysis, sensing and electrical conductivity in degenerately doped materials such as doped In2O3 NCs becomes inherently dependent on the presence of this surface depletion layer. [25][26][27][28][29] Indeed, studying electrochemical modulation of LSPR in thin films of Sn-doped In2O3 (Sn:In2O3) NCs showed that the extent to which the LSPR shifts during charging is controlled by the fraction of the NC volume occupied by the depletion layer at the surface, which depends on both NC size and Sn dopant concentration. 30 However, in electrochemically charged thin films, net LSPR shift of doped metal oxide NCs is dependent on a competition between depletion effects and changes in plasmonic coupling between NCs, which makes it difficult to precisely identify the contribution of depletion effects in controlling LSPR modulation of doped metal oxide NCs.…”

“…7 Similar investigations using chemical titrations to interrogate other doped metal oxide NC compositions will shed light on the relevance of surface depletion in understanding properties across this emerging class of plasmonic nanocrystals, with implications for their potential applications in catalysis, sensing, therapeutics, and more. 5,10,26,29,48 Experimental Section: Chemicals: For the synthesis and characterization of Sn-doped In2O3 (Sn:In2O3) nanocrystals (NCs), all chemicals were acquired commercially and employed without further purification prior to their use. Indium(III) acetate (STREM, ≥99.99%), tin(IV) acetate (Sigma-Aldrich 99.99%), oleyl alcohol (Sigma-Aldrich, 85%) and oleic acid (Sigma-Aldrich, 90%) were used for the synthesis of different Sn-doped In2O3 NCs.…”

CO2 Electroreduction over Metallic Oxide, Carbon-Based, and Molecular Catalysts: A Mini-Review of the Current Advances

“…When CO is the major product, there is no need to separate the gaseous CO from the liquid electrolyte because it will spontaneously separate. To this date, we can rank three distinct groups of monometallic catalysts: (1) CO selective metals such as (Au, Ag, Pd, Ga, and Zn) [41,42]; (2) metals that mainly produce HCOOH (e.g., Pb, Cd, Sn, In, and Ti) [43][44][45][46]; (3) metals that form hydrocarbons such as CH 4 and C 2 H 4 (e.g., Cu) [47].…”