2.2 Non-noble metal
Among non-noble metals, transition metals such as Ni, Zn, and Fe are the most popular.
Luo et al. electrodeposited Zn2+ on Cu mesh to fabricate highly porous Zn catalysts.[50] scanning electron microscopy (SEM) images of the electrodeposited Zn catalysts revealed that the corresponding electrocatalysts had highly porous structures, resulting in superior performance (Figure 1(G)). Specifically, Zn electrocatalysts exhibit FECO of 95 % and a current density of 27 mA cm−2 at −0.95 V vs. RHE in H-type cell, (Figure 1(G)) and 84 % FECO and current density of 200 mA cm−2 in a flow cell (Figure 1(I)). These outstanding performances are attributed to the highly porous structure of P–Zn, which increases the active site density and enhances the local pH effect, further surpassing the hydrogen evolution reaction (HER). Jiang et al. synthesized surface-regulated Ni nanoparticles supported on N-doped CMK-3 via pyrolysis of Ni2+ and Zn2+ pre-absorbed CMK-3 and urea in argon.[51] By coordinating with N and O, the as-synthesized electrocatalyst exhibits electronic properties different from metallic Ni, resulting in an exceptional CO FE of 97 %, a high CO partial current density of 13.01 mA cm−2, and a turnover frequency of 4.25 s−1. Li et al. demonstrated a PCN-222(Fe)/CNTs catalyst by loading PCN-222(Fe) onto CNTs through an in situ solvothermal synthesis process, resulting in an unprecedented high activity and selectivity for CO2reduction.[52] Due to the synergistic effect between PCN-222(Fe) and CNTs, the electrocatalyst exhibited exceptional electrocatalytic performance with a FECO of 95.5 %, TOF of 448.76 h−1, and high durability over 10 h at −0.6 V vs. RHE.
Metal-organic frameworks (MOF) are promising nanomaterials for the CO2RR because of their large specific surface area, adjustable porosity, and composition.[53-54] Kang et al. used zeolitic imidazolate frameworks (ZIFs), which are a subclass of MOFs, as electrocatalysts for the reduction of CO2 to CO.[55] Three different ZIF-8 catalysts were synthesized using various Zn sources: ZnSO4, Zn(NO3)2, and Zn(AC)2. EC measurements demonstrated that ZIF-8 prepared with ZnSO4, showed excellent CO selectivity under a wide potential range from −1.5 to −1.9 (V vs. SCE), reaching a maximum FE of 65.5 % at −1.8 (V vs. SCE). Similarly, ultrasmall ZIF-8s were fabricated using a facile sol-gel method by Wang et al., and SEM images revealed that the as-synthesized ZIF-8 exhibited an orthododecahedral structure with a size of 80 nm. Ultrasmall ZIF−8 electrocatalysts display FECO of 90 % at −1.5 V vs. RHE) with a partial current density of ~5 mA cm−2 and long-term stability of 12.5 h at −1.8 (V vs. RHE.[56]
Various metals, including Bi, Sn, and Cu, are considered efficient electrocatalysts for the conversion of CO2 to formate (HCOO). Among them, Bi has attracted much attention in CO2RR owing to its large HER overpotential and the strong binding energy of *OCHO species. For example, Zhang et al. fabricated a high-performance Bi-Zn bimetallic catalyst by surface modification of a Zn catalyst through a hydrothermal procedure with different concentrations of Bi(NO3)3solution.[57] The optimized Bi-Zn bimetallic catalyst showed maximum formate FE of 94 %, a current density of 3.8 mA cm–2, and long-term stability of 7 h under −0.8 V vs. RHE with CO production of less than 10 % over the entire potential area (Figure 2(A) and (B)). Bifunctional interfaces between the bimetal and grain boundaries were attributed to the superb selectivity toward formate by favoring *OCHO intermediate adsorption on the catalyst surface (Figure 2(C)). Lee et al. demonstrated a novel approach for the construction of Bi nanoflakes using the pulse-electrodeposition method for the conversion of CO2 to formate.[58] Various Bi nanostructures have been obtained using different electrodeposition methods. As shown in Figure 2(D), nanodot-shaped Bi particles and dendrite-shaped Bi were obtained using DC-60s and DC-120s, respectively, and Bi nanoflakes were synthesized using pulse deposition for six cycles (PC-6c). The Bi nanoflakes electrocatalysts exhibited exceptional CO2RR performance with formate FE of 79.5 % at −0.4 V vs. RHE and achieved a maximum FE up to ~100 % at −0.6 V vs. RHE (Figure 2(E)). Moreover, Bi nanoflakes showed excellent long-term durability of 10 h at −0.8 V vs. RHE without significant decay of FECO. Recently, nanoporous bismuth (np-Bi) with a 3D ligament-channel network structure was synthesized using a chemical dealloying approach.[59]Mg92Bi8 consisting of Mg and Mg3Bi2 was selected as the precursor, and Mg completely dissolved in tartaric acid, while Bi maintained stability during the process. Consequently, np-Bi exhibited an outstanding formate FE of 94 % at −0.9 V vs. RHE with a maximum partial current density of 62 mA cm–2 −1.2 V vs. RHE in an H-cell and remarkable current density of ∼500 mA cm–2 at a low overpotential of 420 mV in a flow cell.