Electrochemical reduction of carbon dioxide on tin/tin oxide composites and oxide-derived nanocrystalline metals [electronic resource]
Keywords: Chemistry.

The ability to recycle CO2 into fuel using renewable energy inputs could, in principle, substantially reduce fossil fuel use and slow the rise of the atmospheric CO2 level. A versatile strategy for renewable fuel synthesis is to use electricity from solar, wind or hydroelectric sources to power an electrolytic device that converts H2O and CO2 into O2 and fuel. Arguably the greatest challenge to this approach is developing electrocatalysts that reduce CO2 efficiently. Extensive studies of both polycrystalline and single-crystal metal electrodes have yet to reveal a metal surface that is a sufficiently active CO2 reduction catalyst for fuel synthesis. All suffer from one major shortcoming: they require large overpotentials in order to reduce CO2 with high selectivity (Faradaic efficiency) and suppress competitive H+ reduction. Large overpotentials compromise the energetic efficiency of electrolysis and can lead to rapid electrode deactivation. In light of these results, it is imperative to identify new surfaces that are effective for CO2 reduction and develop synthetic strategies for preparing particles that expose these surfaces. In this work, I describe two new designs for heterogeneous CO2 reduction catalysts: metal/metal oxide composites and "oxide-derived" nanocrystalline metals. I present three materials embodying these designs that exhibit significantly reduced overpotentials and improved electrode stability compared to previous state-of-the-art heterogeneous catalysts. One explanation for the large overpotentials required to reduce CO2 on metal surfaces is that the surfaces afford poor stabilization of CO2[TM]−, the product of initial one-electron transfer to CO2. We hypothesized that the interface between a metal and metal oxide would provide Lewis or Brønsted acidic functionality to stabilize CO2[TM]− and therefore lower the barrier to reduction. Sn was selected to test this idea because of the large kinetic stability of Sn oxides (SnOx) under reducing conditions. The effect of tin oxides (SnOx) on the efficiency of CO2 reduction on Sn electrodes was evaluated by comparing the activity of Sn electrodes that had been subjected to different pre-electrolysis treatments. In aqueous NaHCO3 solution saturated with CO2, a Sn electrode with a native SnOx layer exhibited potential-dependent CO2 reduction activity that is consistent with previously reported activity. In contrast, an electrode etched to expose fresh Sn0 surface exhibited higher overall current densities but almost exclusive H2 evolution. Subsequently, a thin film catalyst was prepared by simultaneous electrodeposition of Sn0 and SnOx on a Ti electrode. This Sn/SnOx composite exhibited up to 8-fold higher partial current density and 4-fold higher Faradaic efficiency for CO2 reduction than a Sn electrode with a native SnOx. These results implicate the participation of the native SnOx in the CO2 reduction pathway on Sn electrodes and suggest that increasing the interface between Sn and SnOx improves the efficiency of CO2 reduction over H2 evolution. The results for Sn prompted an investigation of other metal/metal oxide composite materials. The stability of SnOx during reduction conditions proved to be exceptional; in general, metal oxide layers on metal electrodes were rapidly and completely reduced at the potentials required for CO2 reduction. However, it was discovered that the reduction of thick metal oxide layers resulted in metallic thin films ("oxide-derived" metals) with dramatically improved selectivity for CO2 reduction over H+ reduction at low overpotential. In-depth studies were performed for oxide-derived Au (OD-Au) and oxide-derived Pd (OD-Pd). OD-Au electrodes reduced CO2 to CO with quantitative Faradaic efficiency at less than 200 mV of overpotential, while polycrystalline Au, commercial Au nanoparticles and other nanostructured Au electrodes required > 400 mV of overpotential. Similarly, OD-Pd attained > 50% Faradaic efficiency for CO2 reduction to CO across a 500 mV potential range for which polycrystalline Pd and commercial Pd nanoparticles exhibited > 97% Faradaic efficiency for H+ reduction. Electrokinetic measurements for OD-Au supported a novel CO2 reduction mechanism that consists of rapid, reversible formation of electrode-adsorbed CO2[TM]−, followed by a rate-limiting H+ transfer from HCO3--. The results suggest that the improved Faradaic efficiency on this material is the result of enhanced stabilization of CO2[TM]−. In contrast, electrokinetic measurements revealed that the major contributor to the high Faradaic efficiency on OD-Pd was a > 100-fold suppression of the intrinsic (surface area--normalized) current density for H+ reduction relative to other Pd materials. X-ray diffraction and transmission electron microscopy revealed that OD-Au and OD-Pd are comprised of continuous networks of nanocrystals separated by grain boundaries (i.e. they are nanocrystalline materials). Structure-activity comparisons between OD-metals, thermally annealed OD-metals, and other nanostructured electrodes showed that selectivity for CO2 vs H+ reduction is not correlated with crystallite size or electrode morphology. The results motivate additional studies to examine the relationship between grain boundary networks and electrocatalytic activity.