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.