Hybrid materials consisting of semiconductors and cocatalysts have been widely used for photoelectrochemical (PEC) conversion of CO 2 gas to value-added chemicals such as formic acid (HCOOH). To date, however, the rational design of catalytic architecture enabling the reduction of real CO 2 gas to chemical has remained a grand challenge. Here, we report a unique photocathode consisting of CuS-decorated GaN nanowires (NWs) integrated on planar silicon (Si) for the conversion of H 2 Scontaining CO 2 mixture gas to HCOOH. It was discovered that H 2 S impurity in the modeled industrial CO 2 gas could lead to the spontaneous transformation of Cu to CuS NPs, which resulted in significantly increased faradaic efficiency of HCOOH generation. The CuS/GaN/Si photocathode exhibited superior faradaic efficiency of HCOOH = 70.2% and partial current density = 7.07 mA/cm 2 at −1.0 V RHE under AM1.5G 1 sun illumination. To our knowledge, this is the first demonstration that impurity mixed in the CO 2 gas can enhance, rather than degrade, the performance of the PEC CO 2 reduction reaction.
Electrochemical reduction of carbon
dioxide (CO2) is
a promising method toward carbon recycling. Highly selective bimetallic
catalysts have been extensively demonstrated, while efforts to understand
the compositional and geometrical effects have been limited. Here,
we studied the relationship between the catalytic activity of bimetallic
Cu–Sn catalysts with their composition and geometry through
the fabrication of three-dimensional hierarchical (3D-h) Cu nanostructure
and the solution-based coating of Sn nanoparticles (NPs). As the coating
time of Sn NPs was increased from 1 to 60 s, Sn NPs with a larger
size and a higher surface density were coated onto the 3D-h Cu, thus
the surface atomic ratio of Cu/Sn gradually decreased. This compositional
change in bimetallic Cu–Sn catalysts remarkably shifted the
faradaic efficiency (FE) of carbon monoxide (CO) from 90.0 to 23.4%
at −0.6 VRHE. Moreover, we found that the catalytic
performance increases as the geometric structure becomes complex in
the order of flat, rods, and 3D-h Cu–Sn. The 3D-h Cu–Sn
began to produce CO at a low potential of −0.15 VRHE and showed the maximum FECO of 98.6% at −0.45
VRHE. This study reveals that the synergetic effects of
composition and nanoscale geometry are significant for the CO2 reduction reaction.
We investigated the relationship between grain boundary (GB) oxidation of Cu−Ag thin-film catalysts and selectivity of the (photo)electrochemical CO 2 reduction reaction (CO 2 RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH 4 . However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu−Ag catalysts. The optimized Cu (100 nm)−Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FE CO = 79.8%) and CH 4 (FE CH4 = 59.3%) at a reductive potential of −1.0 and −1.4 V RHE , respectively. Moreover, the Cu−Ag thin film was used as a cocatalyst for photo-electrochemical CO 2 reduction by patterning the Cu−Ag thin film and a SiO 2 passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH 4 under light illumination (100 mW/cm 2 ).
The
electrochemical CO2 reduction in aqueous media is
a promising method for both the mitigation of climate changes and
the generation of value-added fuels. Although many researchers have
demonstrated selective and stable catalysts for electrochemical reduction
of pure CO2 gas, the conversion of industrial CO2 gas has been limited. Here, we fabricated the copper sulfide catalysts
(CuS
x
), which were spontaneously formed
by dipping a Cu foil into a laboratory-prepared industrial CO2-purged 0.1 M KHCO3 electrolyte. Because industrial
CO2 contains H2S gas, sulfur species dissolved
in the electrolyte can easily react with the Cu foil. As the concentration
of dissolved sulfur species increased, the reaction between the Cu
foil and sulfur enhanced. As a result, the average size and surface
density of CuS
x
nanoparticles (NPs) increased
to 133.2 ± 33.1 nm and 86.2 ± 3.3%, respectively. Because
of the larger amount of sulfur content and the enlarged electrochemical
surface area of CuS
x
NPs, the Faradaic
efficiency (FE) of formate was improved from 22.7 to 72.0% at −0.6
VRHE. Additionally, CuS
x
catalysts
showed excellent stability in reducing industrial CO2 to
formate. The change in FE was hardly observed even after long-term
(72 h) operation. This study experimentally demonstrated that spontaneously
formed CuS
x
catalysts are efficient and
stable for reducing the industrial CO2 gas to formate.
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