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Research reported in this publication was supported by the
National Institutes of Health (NIH) under award number
R01 GM103558-04 (D. W.C.M., Y. Y.L., and K.N.). Y. Y.L. thanks the
Agency for Science, Technology and Research (A*STAR) for a
graduate fellowship. K.N. thanks the Japan Society for the
Promotion of Science for an overseas postdoctoral fellowship.
Additional funding was provided by kind gifts from Merck, Abbvie,
BMS, and Janssen. The content is solely the responsibility of
the authors and does not necessarily represent the official views of
the NIH. Additional data supporting the conclusions are available
in the supplementary materials.
Materials and Methods
Figs. S1 to S5
Tables S1 and S2
14 September 2017; accepted 30 October 2017
Published online 9 November 2017
Selective increase in CO2
electroreduction activity at
grain-boundary surface terminations
Ruperto G. Mariano,1 Kim McKelvey,2 Henry S. White,2 Matthew W. Kanan1†
Altering a material’s catalytic properties requires identifying structural features that give rise to
active surfaces. Grain boundaries create strained regions in polycrystalline materials by
stabilizing dislocations and may provide a way to create high-energy surfaces for catalysis that
are kinetically trapped. Although grain-boundary density has previously been correlated with
catalytic activity for some reactions, direct evidence that grain boundaries create surfaces
with enhanced activity is lacking. We used a combination of bulk electrochemical measurements
and scanning electrochemical cell microscopy with submicrometer resolution to show that
grain-boundary surface terminations in gold electrodes are more active than grain surfaces for
electrochemical carbon dioxide (CO2) reduction to carbon monoxide (CO) but not for the
competing hydrogen (H2) evolution reaction. The catalytic footprint of the grain boundary is
commensurate with its dislocation-induced strain field, providing a strategy for broader
exploitation of grain-boundary effects in heterogeneous catalysis.
Bulk defects such as grain boundaries (GBs) and dislocations have been used extensively to control properties such as mechanical strength, plasticity, and conductivity (1), but relatively little is known about how
defects alter catalytic properties. Bulk defects
create substantial structural perturbations in their
vicinity and are often trapped by large kinetic
barriers (2), but it is unclear whether bulk de-
fects can create regions of enhanced catalytic
activity when they terminate at a surface. We
previously proposed that the high densities of
GBs present in nanocrystalline “oxide-derived”
catalysts were responsible for their improved
CO2-to-CO and CO-to-fuels electrocatalytic ac-
tivity (3–5). More recently, we showed quantita-
tive correlations between electrocatalytic activity
and GB density for catalysts composed of dis-
crete Au and Cu nanoparticles (NPs) (6, 7 ). Other
recent studies have also reported GB effects on
both electrochemical and chemical catalysis with
nanostructured materials (8–11). Although these
correlations are consistent with the formation of
active regions at GB surface terminations, direct
evidence requires spatially resolved measurements.
In addition, the use of GBs as catalyst design
elements will require an understanding of how
activity depends on GB structure. The short length
scale of nanostructured catalysts makes it ex-
tremely challenging to quantify GB structure
distributions and to correspondingly elucidate
the structural disorder and unusual reactivity in-
duced in their vicinity.
Here, we investigate GB effects on flat, polycrystalline Au electrodes with large grain sizes,
which permit spatial resolution of the GB surface-termination regions (Fig. 1). Using bulk electrochemical measurements, we show that CO2
reduction activity increased with GB density
on these electrodes, but there was no correlation
with H2 evolution activity. By probing the local
electrocatalytic activity across GBs using scanning
electrochemical cell microscopy (SECCM) (12–15),
we show that the origin of this effect is a selective
increase in CO2 reduction activity at the GB surface terminations. The magnitude of the increase and the width of the region exhibiting
increased activity depended on the GB geometry, which determined the concentration of
1Department of Chemistry, Stanford University, 337 Campus
Drive, Stanford, CA 94305, USA. 2Department of Chemistry,
University of Utah, 315 S 1400 E, Salt Lake City, UT 84112, USA.
*Present address: School of Chemistry, Trinity College Dublin,
Dublin 2, Ireland.
†Corresponding author. Email: email@example.com