Real-time
changes in the composition and structure of bismuth electrodes
used for catalytic conversion of CO2 into CO were examined
via X-ray absorption spectroscopy (including XANES and EXAFS), electrochemical
quartz crystal microbalance (EQCM), and in situ X-ray reflectivity
(XR). Measurements were performed with bismuth electrodes immersed
in acetonitrile (MeCN) solutions containing a 1-butyl-3-methylimidazolium
([BMIM]+) ionic liquid promoter or electrochemically inactive
tetrabutylammonium supporting electrolytes (TBAPF6 and
TBAOTf). Altogether, these measurements show that bismuth electrodes
are originally a mixture of bismuth oxides (including Bi2O3) and metallic bismuth (Bi0) and that the
reduction of oxidized bismuth species to Bi0 is fully achieved
under potentials at which CO2 activation takes place. Furthermore,
EQCM measurements conducted during cyclic voltammetry revealed that
a bismuth-coated quartz crystal exhibits significant shifts in resistance
(ΔR) prior to the onset of CO2 reduction
near −1.75 V vs Ag/AgCl and pronounced hysteresis in frequency
(Δf) and ΔR, which suggests
significant changes in roughness or viscosity at the Bi/[BMIM]+ solution interface. In situ XR performed on rhombohedral
Bi (001) oriented films indicates that extensive restructuring of
the bismuth film cathodes takes place upon polarization to potentials
more negative than −1.6 V vs Ag/AgCl, which is characterized
by a decrease of the Bi (001) Bragg peak intensity of ≥50%
in [BMIM]OTf solutions in the presence and absence of CO2. Over 90% of the reflectivity is recovered during the anodic half-scan,
suggesting that the structural changes are mostly reversible. In contrast,
such a phenomenon is not observed for thin Bi (001) oriented films
in solutions of tetrabutylammonium salts that do not promote CO2 reduction. Overall, these results highlight that Bi electrodes
undergo significant potential-dependent chemical and structural transformations
in the presence of [BMIM]+-based electrolytes, including
the reduction of bismuth oxide to bismuth metal and changes in roughness
and near-surface viscosity.
The ability to synthesize value-added chemicals directly from CO2 will be an important technological advancement for future generations. Using solar energy to drive thermodynamically uphill electrochemical reactions allows for near carbon-neutral processes that can convert CO2 into energy-rich carbon-based fuels. Here, we report on the use of inexpensive CuSn alloys to convert CO2 into CO in an acetonitrile/imidazolium-based electrolyte. Synergistic interactions between the CuSn catalyst and the imidazolium cation enables the electrocatalytic conversion of CO2 into CO at −1.65 V versus the standard calomel electrode (SCE). This catalyst system is characterized by overpotentials for CO2 reduction that are similar to more expensive Au- and Ag-based catalysts, and also shows that the efficacy of the CO2 reduction reaction can be tuned by varying the CuSn ratio.
I n row 18 of Table S1, under the "Parameter (units)" column, it read "Bismuth, crystalline layer roughness (Å)" but the correct parameter name is "SiC substrate surface roughness (Å)". The correct Table S1 is shown below.
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