Photocatalytic CO2 Reduction with Dissolved Carbonates and Near-Zero CO2(aq) by Employing Long-Range Proton Transport.

Photocatalytic CO2 reduction (CO2R) in ∼0 mM CO2(aq) concentration is challenging but is relevant for capturing CO2 and achieving a circular carbon economy. Despite recent advances, the interplay between the CO2 catalytic reduction and the oxidative redox processes that are arranged on photocatalyst surfaces with nanometer-scale distances is less studied. Specifically, mechanistic investigation on interdependent processes, including CO2 adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO2R in ∼0 mM CO2(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO3 (aq) of pH 7 but without continuously bubbling CO2, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrOx nanoparticles that are supported on a coating-protected GaInP2 photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H2, even with copious protons co-generated nearby. CO2 flux to the Ag@CrOx CO2R sites enhances CO2 adsorption, probed by in situ Raman spectroscopy. CO is produced with local protonation of dissolved inorganic carbon species in a pH as high as 11.5 when using fast electron donors such as ethanol. Isotopic labeling using KH13CO3 was used to confirm the origin of CO from the bicarbonate solution. We then employed COMSOL Multiphysics modeling to simulate the spatial and temporal pH variation and the local concentrations of bicarbonates and CO2(aq). We found that light-driven CO2R and CO2 reactive transport are mutually dependent, which is important for further understanding and manipulating CO2R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO2, thereby achieving CO2 capture and conversion without purifying and feeding gaseous CO2.

Strain-Stabilized Ceramic-Sulfide Electrolytes.

Ceramic-sulfide solid electrolytes are a promising material system for enabling solid-state batteries. However, one challenge that remains is the discrepancy in the reported electrochemical stability. Recent work has suggested that it may be due to the sensitivity of ceramic sulfides to mechanically induced stability. Small changes in ceramic-sulfide microstructure, for example, have been shown to cause substantial differences in the electrochemical stability. In this work, a rigorous theoretical framework is constructed to enable the simulation of such mechanically induced stability for a generalized constraint mechanism. It is shown that the susceptibility for voltage widening in ceramic sulfides can be significantly influenced by the choice of different decay morphology models. This results in a less intrusive microstructure requirement for improved stability, which stems from the tendency of sulfides to decay via inclusions rather than homogeneously. This predicted decay morphology is experimentally confirmed. Li10 GeP2 S12 is stabilized by a thin amorphous shell, which prior models predict is too thin for stabilization. The generality of this framework is discussed in light of stabilization methods beyond microstructure, such as on the battery cell level. The relation of our picture to the observed lithium metal formation in ceramic sulfides is also discussed.