(Invited) Defect-Rich CO2 Reduction Catalysts

The discovery of new structural motifs that promote high electro-reduction activity is essential for the development of electrochemical fuel synthesis. Polycrystalline Au is the most active bulk material for electrochemical CO₂ reduction to CO and serves as a useful model system to evaluate new structures [1]. We recently developed a catalyst called oxide-derived Au that has even higher selectivity than bulk Au for CO₂ reduction to CO at very low overpotentials [2]. Oxide-derived Au is a nanocrystalline material with a dense grain boundary network. Quantifying the effect of the grain boundaries in oxide-derived Au is challenging because extracting TEM samples from the oxide-derived Au films is inefficient. Here we describe the synthesis of grain-boundary rich Au catalysts by vapor deposition. The catalysts can be directly studied by TEM with minimal sample preparation. We compared the CO₂ reduction activity of as-deposited catalysts to catalysts that were annealed at various temperatures. The annealing process has very little impact on the electrochemical surface area or the distribution of Au surface facets, as determined by Cu and Pb underpotential deposition studies. Annealing does, however, reduce the grain boundary density, which is quantified by counting boundaries in a large number of individual particles using TEM. In the low overpotential regime (–0.3 V to –0.4 V vs RHE), there is a linear correlation between grain boundary density and specific current density (surface area normalized) for CO₂ reduction to CO. This quantitative correlation between defect density and catalytic activity suggests that grain boundary engineering is a fruitful avenue to explore for the development of practical CO₂ reduction catalysts.

References:

(1) Hori, Y. In Modern Aspects of Electrochemistry; Vayenas, C. G., White, R. E., Gamboa-Aldeco, M. E., Eds.; Springer: New York, 2008; Vol. 42, p 89.

(2) Chen, Y.; Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 19969−19972.