2.1 Noble metal
Noble metal-based electrocatalysts (Au, Ag, and Pd) usually show
efficient catalytic activity and selectivity, especially in
CO2 to CO conversion. Hsieh et al. fabricated Ag
nanocoral catalysts via an oxidation-reduction process using chloride
anions to promote the intrinsic electrocatalytic activity of bulk
Ag.[45] As a result, the Ag nanocoral catalyst
demonstrated a high CO2RR performance with FE for CO
(FECO) of 95 % at a low overpotential of 0.37 V vs.
reversible hydrogen electrode (RHE), and high long-term stability for 72
h at −0.6 V vs. RHE. These results indicate that the presence of
chloride anions plays a significant role in the enhancement of
CO2 reduction performance. Vertically standing Ag
nanowire arrays (NWAs) with outstanding electrocatalytic performance
have been reported using an easy nanomolding method (Figure
1(A)).[46] This facile method increases the
electrical conductivity of electrocatalysts and facilitates the charge
transfer process, leading to a more efficient CO2RR. Two
different types of anode aluminum oxide (AAO) templates were used to
fabricate Ag-200 nm and Ag-30 nm NWAs. As shown in Figure 1(B), the
as-synthesized Ag-200 nm NWA exhibited nanoarray morphologies. As
compared to Ag-30 nm NWAs, Ag-200 nm NWAs showed excellent activity and
selectivity toward CO with a lower onset potential of
~200mV, maximum FECO of 91 % at −0.6 V
vs. RHE, and current density for CO (jCO) of up to 4.47
mA cm−2 (Figure 1(C) and (D)).
Zhu et al. fabricated ultrathin Au nanowires (NWs) using seed-mediated
growth method for reducing CO2 to
CO.[47] Electrochemical measurements indicated
that the Au NWs demonstrated high activity and selectivity toward CO
with an onset potential of −0.2 V vs. RHE, FE up to 94 % at −0.35 V vs.
RHE, and great stability of 6 h at −0.35 V vs. RHE in the
CO2-saturated 0.5 M KHCO3. Chen et al.
demonstrated facile EC alloying/dealloying of polycrystalline Au with a
mixture of ethylene glycol and ZnCl2 for the fabrication
of three-dimensional (3D) nanoporous Au electrocatalysts (Figure 1(E)).[48] Further acid treatment with
H2SO4 followed by calcination at 150°C,
resulted in enhanced catalytic performance due to the creation of new
active sites. The EC measurements indicate that the nanoporous Au
exhibited excellent electrocatalyst performance toward CO with a maximum
FE of 95.86 % at the applied potential of −0.6 V vs. RHE in a
CO2-saturated 0.1 M NaHCO3 solution
(Figure 1(F)). Shao et al. proposed twisted Pd–Au NWs with a unique
core–shell using a simple template-free
approach.[49] The optimized Pd-Au NWs displayed
outstanding performance compared with Pd nanoparticles with a maximum CO
FE of 94.3 % at −0.6 V vs. RHE, and a low overpotential of 90 mV.