1 3
Topics in Current Chemistry (2018) 376:41
of ethylene and liquid products was observed. AgCu showed a weaker CO binding energy due to the compressive strain of surface Cu. The authors suggested that
reduced hydrogen and oxygen binding energies relative to CO could be the reason
for improved performance.
Nie et al. reported DFT calculations for a promising CuFe electrocatalyst for producing C 2 hydrocarbons [58]. The combination of Cu and Fe enhanced CO 2 conversion by lowering the kinetic barriers, and consequently altered the selectivity preference to more C 2 H 4 (or C 2 H 6 with further hydrogenation) from CH 4 coupling on the
surface of monometallic Fe.
Hoang et al. synthesized nanoporous Cu–Ag alloys on GDL. It was tested in a
flow cell [53]. The Cu 94 Ag 6 sample showed 60% ethylene and 25% ethanol FE at
− 0.7 V with a total current density of 300 mA/cm
2
. The high selectivity towards C2
products was determined from the enhanced stabilization of the Cu 2 O overlayer and
the increased CO intermediate availability by adding Ag. The high pH of solvent
was the possible reason for the high FE of ethylene.
Zhang et al. showed that Cu 2 Pd nanoalloy on the polymeric film could produce up
to 50% CH 4 as the reduction product [52]. This was tested in CO 2 -saturated 0.1 M
TBAPF 6 /CH 3 CN solutions with 1 M added H 2 O. Interestingly, when considering the
state-of-the-art work on CO 2 to CH 4 , monometallic Cu NPs can achieve 80% FE
for CH 4 [63], suggesting that current bimetallic catalysts did not show a significant
improvement for methane production.
2.3 Oxygenate Selective Bimetallic Electrocatalysts
Previous studies showed that if the binding energy of *COOH intermediate is weak
on a catalyst surface, formate will be produced. Another pathway suggests that
CO 2 adsorbs as *OCHO, this can explain some formate formation at more negative
potentials [10]. Table 3 summarizes some of the bimetallic electrocatalysts with a
high selectivity towards formate.
Hahn et al. investigated electrocatalytic properties of thin film AuPd alloys for
CO 2 RR [72]. They found that using an electron-beam co-deposition method could
produce phase-pure and uniform AuPd alloy films with different compositions
(Fig. 6). Electrocatalytic results showed that both the selectivity and activity of the
as-synthesized AuPd alloy films were higher for HCOO
−
than either pure Au or Pd
metals, indicating that both metals can act synergistically. The Pd sites had reduced
binding energy of CO, which allowed increased amounts of adsorbed H and consequently increased the probability of *H adjacent to COOH*.
Cu-modified Pd NPs synthesized by underpotential deposition showed a formate
FE higher than 80% and a good stability [25]. The results were attributed to decreasing the adsorption strength of CO via modification of the Pd d-band, leading to an
improved CO tolerance and an increased FE. In a related study, Ruud et al. established a new Pd-based electrochemical catalyst system, converting CO 2 to formic
acid with a low overpotential [67]. Pd was electrodeposited on a polycrystalline Pt
electrode. The onset potential for the formation of formic acid was − 0.15 V vs.
RHE at pH values of 2 and 6.7, compared with a − 1.2 V onset potential of the bulk
Reprinted from the journal
115
Topics in Current Chemistry (2018) 376:41
of ethylene and liquid products was observed. AgCu showed a weaker CO binding energy due to the compressive strain of surface Cu. The authors suggested that
reduced hydrogen and oxygen binding energies relative to CO could be the reason
for improved performance.
Nie et al. reported DFT calculations for a promising CuFe electrocatalyst for producing C 2 hydrocarbons [58]. The combination of Cu and Fe enhanced CO 2 conversion by lowering the kinetic barriers, and consequently altered the selectivity preference to more C 2 H 4 (or C 2 H 6 with further hydrogenation) from CH 4 coupling on the
surface of monometallic Fe.
Hoang et al. synthesized nanoporous Cu–Ag alloys on GDL. It was tested in a
flow cell [53]. The Cu 94 Ag 6 sample showed 60% ethylene and 25% ethanol FE at
− 0.7 V with a total current density of 300 mA/cm
2
. The high selectivity towards C2
products was determined from the enhanced stabilization of the Cu 2 O overlayer and
the increased CO intermediate availability by adding Ag. The high pH of solvent
was the possible reason for the high FE of ethylene.
Zhang et al. showed that Cu 2 Pd nanoalloy on the polymeric film could produce up
to 50% CH 4 as the reduction product [52]. This was tested in CO 2 -saturated 0.1 M
TBAPF 6 /CH 3 CN solutions with 1 M added H 2 O. Interestingly, when considering the
state-of-the-art work on CO 2 to CH 4 , monometallic Cu NPs can achieve 80% FE
for CH 4 [63], suggesting that current bimetallic catalysts did not show a significant
improvement for methane production.
2.3 Oxygenate Selective Bimetallic Electrocatalysts
Previous studies showed that if the binding energy of *COOH intermediate is weak
on a catalyst surface, formate will be produced. Another pathway suggests that
CO 2 adsorbs as *OCHO, this can explain some formate formation at more negative
potentials [10]. Table 3 summarizes some of the bimetallic electrocatalysts with a
high selectivity towards formate.
Hahn et al. investigated electrocatalytic properties of thin film AuPd alloys for
CO 2 RR [72]. They found that using an electron-beam co-deposition method could
produce phase-pure and uniform AuPd alloy films with different compositions
(Fig. 6). Electrocatalytic results showed that both the selectivity and activity of the
as-synthesized AuPd alloy films were higher for HCOO
−
than either pure Au or Pd
metals, indicating that both metals can act synergistically. The Pd sites had reduced
binding energy of CO, which allowed increased amounts of adsorbed H and consequently increased the probability of *H adjacent to COOH*.
Cu-modified Pd NPs synthesized by underpotential deposition showed a formate
FE higher than 80% and a good stability [25]. The results were attributed to decreasing the adsorption strength of CO via modification of the Pd d-band, leading to an
improved CO tolerance and an increased FE. In a related study, Ruud et al. established a new Pd-based electrochemical catalyst system, converting CO 2 to formic
acid with a low overpotential [67]. Pd was electrodeposited on a polycrystalline Pt
electrode. The onset potential for the formation of formic acid was − 0.15 V vs.
RHE at pH values of 2 and 6.7, compared with a − 1.2 V onset potential of the bulk
Reprinted from the journal
115