High current density electroreduction of CO2 into formate with tin oxide nanospheres

Abstract

In this study, we demonstrate three-dimensional (3D) hollow nanosphere electrocatalysts for CO₂ conversion into formate with excellent H-Cell performance and industrially-relevant current density in a 25 cm² membrane electrode assembly electrolyzer device. Varying calcination temperature maximized formate production via optimizing the crystallinity and particle size of the constituent SnO₂ nanoparticles. The best performing SnO₂ nanosphere catalysts contained ~ 7.5 nm nanocrystals and produced 71-81% formate Faradaic efficiency (FE) between -0.9 V and -1.3 V vs. the reversible hydrogen electrode (RHE) at a maximum formate partial current density of 73 ± 2 mA cm_geo^-2 at -1.3 V vs. RHE. The higher performance of nanosphere catalysts over SnO₂ nanoparticles and commercially-available catalyst could be ascribed to their initial structure providing higher electrochemical surface area and preventing extensive nanocrystal growth during CO₂ reduction. Our results are among the highest performance reported for SnO₂ electrocatalysts in aqueous H-cells. We observed an average 68 ± 8% FE over 35 h of operation with multiple on/off cycles. In situ Raman and time-dependent X-ray diffraction measurements identified metallic Sn as electrocatalytic active sites during long-term operation. Further evaluation in a 25 cm² electrolyzer cell demonstrated impressive performance with a sustained current density of 500 mA cm_geo^-2 and an average 75 ± 6% formate FE over 24 h of operation. Our results provide additional design concepts for boosting the performance of formate-producing catalysts.

Figure 1

(a) Scheme illustrating the synthesis of 3D hollow SnO₂ nanospheres by a combined sol–gel and templating method. (b,c) Representative FE-SEM and (d) HR-TEM images of SnO₂ nanosphere calcined at 500 °C. (e) XRD crystallite size as function of calcination temperature.

Figure 2

(a) Representative Faradaic efficiency for formate, CO, and H₂ vs. cathodic potentials for SnO₂ nanospheres calcined at 500 °C. (b) Potential-dependent formate partial current density as function of calcination temperature of SnO₂ nanospheres (aka SnO₂ crystallite size). (c) Comparison of CO₂RR performance for the best-performing SnO₂ nanospheres with previously reported Sn, SnO₂ and SnO₂-carbon electrocatalysts tested in a H-cell with bicarbonate electrolyte (mixed metal oxides, alloys, and doped systems are excluded): Sn dendrite, nanoporous SnO₂, SnO₂ porous nanowires, chainlike mesoporous SnO₂, Sn/SnO_x thin film, Sn/SnO/SnO₂ nanosheets/carbon cloth, wire-in-tube SnO₂ nanofibers, SnO₂ nanoparticles, ultrathin SnO₂ quantum wires, SnO₂/carbon nanotubes, Sn quantum sheet/graphene, SnO₂/carbon aerogel, SnO nanoparticles/carbon black, mesoporous Sn/SnO_x, wavy SnO₂/carbon black. (d,e) Comparison of (d) geometric formate partial current density and (e) ECSA-normalized formate current density for commercially-available SnO₂ nps (com-SnO₂ nps), non-templated SnO₂ nps, and the best-performing SnO₂ nanospheres calcined at 500 °C.

Figure 3

(a) Long-term CO₂RR performance of the best performing SnO₂ nanospheres and non-templated SnO₂ nps at −1.2 V vs. RHE. The experiments were run intermittently over multiple 5-h electrolysis periods. (b) Back-scattered SEM and (c,d) TEM images of SnO₂ nanospheres electrode after 35 h of operation. (e) SEM image of non-templated SnO₂ nps after 20-h electrolysis. (f) Time-dependent synchrotron-based XRD profiles of SnO₂ nanospheres collected at −1.2 V vs. RHE.

Figure 4

(a) MEA full cell polarization curves collected before and after a 24-h electrolysis at 500 mA cm_geo⁻². (b) Corresponding formate FEs vs. geometric current density. The MEA full cell contained a 5 cm × 5 cm cathode GDE decorated with SnO₂ nanospheres, a Ni foam anode, and a bipolar membrane with aqueous 0.4 M K₂SO₄ catholyte and aqueous 1 M KOH anolyte.