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.