1 3
Topics in Current Chemistry (2018) 376:41
made from a simple electrodeposition method for converting CO 2 into CO and
HCOO
−
[33]. For the Cu 87 Sn 13 electrode, high selectivity toward CO formation was observed at all applied potentials with maximum FE of 60% for CO
at − 0.99 V. Furthermore, good selectivity toward HCOO- with maximum FE
of 90% at − 1.09 V was obtained on the Cu 55 Sn 45 electrode. These results indicated the importance of the appropriate tuning of binding energies for the active
intermediate species on the catalyst surface. The selectivity for HCOO
−
formation was affected by the surface oxidation state of Sn, while the selectivity for
CO was mainly affected by the crystal structures such as intermetallic compound
formation.
Lu et al. showed that Pd 83 Cu 17 bimetallic aerogels presented high FE (80.0%)
and high current density (31.8 mA/cm
2
) for methanol when a molar mixture of
25% [Bmim]BF4 and 75% water was used as the electrolyte [74]. The superior
performance was attributed to the high Pd
0
/Pd
2+
and Cu
1+
+Cu
0
/Cu
2+
ratios and
Cu/Pd grain boundaries from the nanochain structure.
Zhang et al. synthesized 2D hierarchical Pd/SnO 2 nanosheets for CO 2 RR. They
showed high FE (54.8%) for CO 2 to methanol at -0.24 V with a total current density of − 2.5 mA/cm
2
. [75] The formation of the Pd–O–Sn interface was considered to be the key for the improved methanol formation.
Wen et al. demonstrated a highly selective Bi-Sn catalyst for CO 2 to formate
[76]. The Bi NPs were deposited on Sn nanosheets. The catalyst could reach 96%
FE and 0.74 mmol/h/cm
2
at − 1.1 V. The stability test was conducted for 100 h
with almost no FE loss. The Bi NPs could modify the density of states of Sn,
resulting in a stronger binding of the HCOO* intermediate that was attributed as
a potential reason for high formate FE.
An alternative strategy to produce C 2+ products in CO 2 RR is to design a tandem micro-environment where one metal converts CO 2 to CO and the other metal
does the further reduction of CO to form C 2+ products. Oxide-derived Cu–Zn
catalysts were reported to have higher ethylene and ethanol yield based on this
strategy [73]. Ren et al. synthesized oxidized Cu–Zn films onto a polished Cu
disk galvanically. Oxide-derived Cu–Zn with different compositions were tested
in 0.1 M KHCO 3 electrolyte. The electrode surface was determined to be covered
with phase segregated Cu
0
and Zn
0
crystallites during CO 2 RR process (Fig. 8a).
Operando Raman spectroscopy measurements revealed that Zn adsorbed CO
weakly. The reduced CO on Zn could diffuse and spill over onto the Cu sites,
leading to 29.1% FE for ethanol formation at − 1.05 V (Fig. 8b) on Cu 20 Zn. This
concept was also applied on catalyst systems of CuNi and CuAg (Fig. 8c, d) by
Lee et al. [70]. On Ag–Cu biphasic boundaries, CO on Ag could interact with the
intermediate on the neighboring Cu site to form C 2 H 5 OH.
For Cu-based bimetallic catalysts, utilization of a second metal to strengthen
CO adsorption energies was also studied. Zhang et al. placed Cu overlayers on
tetrahexahedral Pd NP surfaces [79]. The resulting high index Cu (310) surface
could achieve a 0.3–0.4 V stronger binding for CO. This could result in a better
selective ethanol formation. Jia et al. showed that FE of methanol (up to 15%) on
nano-Cu 63.9 Au 36.1 (with nanoporous Cu as a support, dealloyed from Cu–Zn) was
Reprinted from the journal
119
Topics in Current Chemistry (2018) 376:41
made from a simple electrodeposition method for converting CO 2 into CO and
HCOO
−
[33]. For the Cu 87 Sn 13 electrode, high selectivity toward CO formation was observed at all applied potentials with maximum FE of 60% for CO
at − 0.99 V. Furthermore, good selectivity toward HCOO- with maximum FE
of 90% at − 1.09 V was obtained on the Cu 55 Sn 45 electrode. These results indicated the importance of the appropriate tuning of binding energies for the active
intermediate species on the catalyst surface. The selectivity for HCOO
−
formation was affected by the surface oxidation state of Sn, while the selectivity for
CO was mainly affected by the crystal structures such as intermetallic compound
formation.
Lu et al. showed that Pd 83 Cu 17 bimetallic aerogels presented high FE (80.0%)
and high current density (31.8 mA/cm
2
) for methanol when a molar mixture of
25% [Bmim]BF4 and 75% water was used as the electrolyte [74]. The superior
performance was attributed to the high Pd
0
/Pd
2+
and Cu
1+
+Cu
0
/Cu
2+
ratios and
Cu/Pd grain boundaries from the nanochain structure.
Zhang et al. synthesized 2D hierarchical Pd/SnO 2 nanosheets for CO 2 RR. They
showed high FE (54.8%) for CO 2 to methanol at -0.24 V with a total current density of − 2.5 mA/cm
2
. [75] The formation of the Pd–O–Sn interface was considered to be the key for the improved methanol formation.
Wen et al. demonstrated a highly selective Bi-Sn catalyst for CO 2 to formate
[76]. The Bi NPs were deposited on Sn nanosheets. The catalyst could reach 96%
FE and 0.74 mmol/h/cm
2
at − 1.1 V. The stability test was conducted for 100 h
with almost no FE loss. The Bi NPs could modify the density of states of Sn,
resulting in a stronger binding of the HCOO* intermediate that was attributed as
a potential reason for high formate FE.
An alternative strategy to produce C 2+ products in CO 2 RR is to design a tandem micro-environment where one metal converts CO 2 to CO and the other metal
does the further reduction of CO to form C 2+ products. Oxide-derived Cu–Zn
catalysts were reported to have higher ethylene and ethanol yield based on this
strategy [73]. Ren et al. synthesized oxidized Cu–Zn films onto a polished Cu
disk galvanically. Oxide-derived Cu–Zn with different compositions were tested
in 0.1 M KHCO 3 electrolyte. The electrode surface was determined to be covered
with phase segregated Cu
0
and Zn
0
crystallites during CO 2 RR process (Fig. 8a).
Operando Raman spectroscopy measurements revealed that Zn adsorbed CO
weakly. The reduced CO on Zn could diffuse and spill over onto the Cu sites,
leading to 29.1% FE for ethanol formation at − 1.05 V (Fig. 8b) on Cu 20 Zn. This
concept was also applied on catalyst systems of CuNi and CuAg (Fig. 8c, d) by
Lee et al. [70]. On Ag–Cu biphasic boundaries, CO on Ag could interact with the
intermediate on the neighboring Cu site to form C 2 H 5 OH.
For Cu-based bimetallic catalysts, utilization of a second metal to strengthen
CO adsorption energies was also studied. Zhang et al. placed Cu overlayers on
tetrahexahedral Pd NP surfaces [79]. The resulting high index Cu (310) surface
could achieve a 0.3–0.4 V stronger binding for CO. This could result in a better
selective ethanol formation. Jia et al. showed that FE of methanol (up to 15%) on
nano-Cu 63.9 Au 36.1 (with nanoporous Cu as a support, dealloyed from Cu–Zn) was
Reprinted from the journal
119