3.2 Non-noble metal cocatalysts
Due to the high structural stability of CO2 molecules,
many semiconductors face challenges in selectively facilitating
CO2 reduction. In such scenarios, cocatalysts play a
crucial role in minimizing the overpotential required for
CO2 reduction and enhancing the CO2reduction kinetics to improve overall selectivity. Therefore, it is
essential to integrate semiconductor materials with appropriate
cocatalysts to accelerate surface catalytic conversion and enhance both
PEC activity and selectivity. The electrical conductivity of the
cocatalyst, along with its adsorption and desorption capabilities based
on binding energies with relevant reaction species—especially those
with moderate binding affinities for key intermediates—can lead to
higher intrinsic CO2 reduction
activity.[43-45] This, in turn, facilitates the
promotion of CO2 reduction at lower overpotentials,
thereby enhancing onset potentials.
Noble metals, such as Ag and Au, remain efficient catalysts for
converting PEC energy into CO2. However, the use of
noble metal-based catalysts faces significant limitations due to their
high cost and limited availability, which constrain commercial
implementation.[27,46] Consequently, extensive
research efforts have been directed toward developing alternative
catalysts for CO2 reduction based on non-noble metal
materials, including transition metals. These materials, which encompass
metal oxides, metal NPs, metal-organic frameworks (MOFs), and metal
molecular catalysts, aim to provide low-cost, high-activity, and
long-term stability for CO2 reduction, addressing the
challenges associated with noble metal
cocatalysts.[27, 47-49]
Among these, cobalt molecular catalysts are widely used as cocatalysts
for the photocathode in PEC CO2RR. For instance, Shang
et al. prepared a p-type silicon photocathode incorporating a cobalt
phthalocyanine molecular catalyst immobilized on graphene oxide
(GO/CoPc).[50] Initially, CoPc molecules were
immobilized on GO through ultrasonication in a DMF solution, taking
advantage of GO’s conductive properties and capability to facilitate
electron transfer, as illustrated in Figure 3A. Subsequently, p-Si
wafers, coated with a protective TiO2 layer
(Si-TiO2), were treated with a solution of
(3-aminopropyl)triethoxysilane (APTES), as depicted in Figure 3B.
Following this treatment, the Si-TiO2-APTES (STA)
substrate was immersed in a GO/CoPc aqueous dispersion, forming a
monolayer-like coating on the surface through electrostatic and hydrogen
bonding interactions between the amine groups of APTES and the
carboxylic acid groups of GO. The
STA-GO/CoPc achieved a
photocurrent density of 0.7 mA cm−2, while
simultaneously attaining a maximum FECO of 86% at −0.28
V vs. RHE, as observed in Figure 3C. Moreover, STA-GO/CoPc
exhibited a remarkably low onset potential of −0.36 V vs. RHE,
displaying a FECH3OH of 8% at −0.62 V vs. RHE.
Roy et al. constructed a hybrid photocathode structure comprising a
cobalt phthalocyanine catalyst with four phosphonic acid anchoring
groups (CoPcP) immobilized on mesoTiO2, which coated a
p-Si photocathode
(Si|mesoTiO2|CoPcP).[51]Incorporating the four phosphonic acid anchoring groups facilitated the
immobilization of the CoPcP catalyst on meso TiO2. To
assess the impact of the CoPcP catalyst on mesoTiO2,
they synthesized a mesoTiO2|CoPcP hybrid
electrode. The resulting mesoTiO2|CoPcP
photocathode demonstrated highly selective CO2 to CO
conversion, achieving a turnover number for CO (TONCO)
of 1949 ± 5 after 2 h of controlled-potential electrolysis at a 550 mV
overpotential in a 0.5M KHCO3 aqueous electrolyte. In
addition, when combined with a p-Si photocathode, the
Si|mesoTiO2|CoPcP exhibited a
TONCO of 939 ± 132 with 66% CO selectivity under 0.5M
KHCO3 conditions. Wen et al. prepared
CoII (BrqPy) (BrqPy=4’,4”-bis(4-bromophenyl)-2,2’
:6’,2” :6”,2”’-quaterpyridine) molecular catalysts with multiwalled
carbon nanotubes (CNT) on TiO2-protected p/n-Si
photocathodes
(Si|TiO2|CNT|CoII(BrqPy)),[52] as illustrated in Figure 3D. First,
a p/n-Si wafer was covered with TiO2 through
atomic-layer deposition (ALD). Then, a CNT layer was drop-cast onto the
Si|TiO2 layer. Finally, a DMF solution
containing CoII (BrqPy) molecular catalysts was
drop-cast, and the catalysts were immobilized on the CNT layer through
π–π stacking interactions. The hybrid photocathodes, benefiting from
the highly conductive nature of CNT, achieved a remarkable photocurrent
density of up to −1.4 mA cm–2 at −0.11 V vs.RHE, as seen in Figure 3E. Furthermore, due to the exceptional activity
and selectivity of the cobalt molecular catalyst for CO2reduction, the total FE and FECO reached 99% and 97%,
respectively, as revealed in Figure 3F. These results surpassed other
hybrid photocathodes employing more complex configurations. Leung et al.
employed cobalt(Ⅱ) bis(terpyridine) molecular catalysts (CotpyP) as
cocatalysts to construct a highly efficient photocathode for
CO2 reduction.[53] This hybrid
photocathode consisted of a p-type silicon photocathode coated with a
mesoporous TiO2 layer with anchored CotpyP catalysts
(Si|mesoTiO2|CotpyP). The mesoporous
TiO2 layer protected the Si, allowing for high loading
of CotpyP catalysts and ample surface contact with the electrolyte due
to its high surface area. Photoelectrochemical tests indicated CO and
formate (HCOO–) production in aqueous acetonitrile
(MeCN) with 0.1M tetrabutylammonium tetrafluoroborate
(TBABF4) and pure CO2-saturated 0.1M
KHCO3. The turnover number for CO2reduction reached 381 during a 24 h test in aqueous MeCN.
Furthermore, nanowires (NWs), NPs, metal oxides, and MOFs, which consist
of non-noble metals, are also widely used as photocathodes in PEC
CO2RR. For example, Dong et al. designed a photocathode
comprising CuS-covered GaN NWs on silicon wafers (CuS/GaN/Si) to
efficiently convert H2S-containing CO2mixture gas to HCOOH.[54] The fabrication involved
thermally evaporating Cu NPs and plasma-assisted molecular beam epitaxy
(MBE) of GaN NWs on Si wafers. PEC experiments were conducted in a 0.1M
KHCO3 solution purged with CO2 and
H2S mixture gas under 1-sun illumination, as shown in
Figure 4A. Notably, Cu NPs spontaneously transformed into CuS NPs during
the PEC experiment. Compared to other photocathodes of Cu/Si, CuS/Si,
and Cu/GaN/Si, the CuS/GaN/Si photocathode revealed an outperforming
faradaic efficiency for HCOOH (FEHCOOH) of 70.2% at
−1.0 V vs. RHE and achieved a maximum current density for HCOOH
of 7.07 mA cm–2, as shown in Figure 4B,C. Zhou et al.
fabricated Bi NP cocatalysts supported on GaN NWs photocathodes for PEC
CO2RR to HCOOH.[55] Focusing on
the electronic interaction between Bi NPs and GaN NWs grown on a planar
Si wafer (Bi/GaN/Si), the strong electronic interaction enhanced
CO2 conversion due to electron sharing between Bi NPs
and GaN NWs. The Bi/GaN/Si photocathode demonstrated outstanding
selectivity for HCOOH, achieving a FEHCOOH of
~98% at −0.3 V vs. RHE, a high current density
of 10.3 mA cm–2 at −0.6 V vs. RHE, and
sustained stable operation for 12 h under 1-sun illumination. This
underlines the critical role of electronic interactions between
photocathodes and cocatalysts in enhancing PEC performance. Dong et al.
showcased a novel nanoarchitecture with a Sn NP/GaN NW/Si photocathode
for aqueous PEC reduction of CO2 to
HCOOH.[56] Integrating defect-free GaN NWs grown
on a planar Si via MBE with electrodeposited Sn NPs, the photocathode’s
PEC testing revealed exceptional performance, unprecedented TOF of 107
min–1, a total current density of 17.5 mA
cm–2, and a high FEHCOOH of 76.9% at
a low potential of −0.53 V vs. RHE under 1-sun illumination,
corresponding to a productivity of 201 μmol cm–2h–1. The photocathode also exhibited a high TON for
HCOOH production, reaching 64,000 during stable operation over 10 h. DFT
calculations suggested that the synergistic effects of covalent Ga-C
bonding and ionic-like Sn-O bonding played a crucial role in activating
CO2, contributing to the remarkable activity and
selectivity for CO2 reduction. Deng et al. developed a
Cu2O photocathode coated with MOFs named
Cu3(BTC)2 (BTC =
benzene-1,3,5-tricarboxylate) for PEC CO2RR to
CO.[57] The
Cu3(BTC)2 coating played multiple roles
in the PEC CO2RR process, including preventing
photocorrosion of the Cu2O layer, facilitating electron
transfer, and providing catalytic active sites for CO2reduction. PEC performance was measured in a
CO2-saturated MeCN containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6). Results indicated that the
Cu3(BTC)2/Cu2O/ITO
photocathode reached a maximum FECO of about 95% at
applied potentials ranging from −1.77 to −1.97 V vs.ferrocene/ferrocenium (Fc/Fc+), as displayed in Figure
4D. Furthermore, the solar-to-CO (STC) efficiency of this photocathode
reached 0.83% at −2.07 V vs. Fc/Fc+ under AM
1.5G illumination, as observed in Figure 4E. Long-term chronoamperometry
indicated that the photocathode maintained a nearly constant current
density under visible light and a steady photocurrent density under
chopped visible light, highlighting its stability and performance under
varying light conditions.
Wang et al. constructed a hybrid photocathode structure comprising CuO
adorned with asymmetric Cu-N sites (CuNx/CuO) for PEC
CO2RR.[58] The
CuNx/CuO photocathode exhibited a faradaic efficiency
towards C2 products (FEC2) of 15.2%,
accompanied by a photocurrent density of −1.0 mA cm–2at 0.2 V vs. RHE in CO2-saturated 0.1M
KHCO3 under AM 1.5G simulated sunlight. DFT calculations
demonstrated that the adsorption of OCCO* and*COCH2 intermediates on Cu-N sites,
crucial for the formation of C2 products, was more
favorable than on Cu-Cu sites, as presented in Figure 4F,G. Theoretical
calculations also indicated that the CuNx/CuO
photocathode had higher electron migration efficiency than CuO due to
the asymmetric d-p orbitals at Cu-N sites, which lowered the energy
barrier for C–C coupling. Roh et al. fabricated Si NWs connected with a
Cu NP ensemble to create photocathodes (Cu NPs/Si NWs) for PEC
CO2RR to
C2H4.[59] Assessed
under 100 mW cm−2 air mass (AM) 1.5 simulated sunlight
in CO2-purged 0.1M KHCO3, the
photocathode achieved a selectivity for CO2 reduction to
C2H4 with a faradaic efficiency
(FEC2H4) of approximately 25% and demonstrated activity
with partial current densities exceeding 2.5 mA cm−2at −0.50 V vs. RHE. Moreover, the Cu NPs/Si NWs photocathode
exhibited long-term stability, maintaining PEC CO2RR
under 50 h of continuous bias and illumination. Kim et al. constructed a
metal-insulator-semiconductor (MIS) structure comprising
Cu/TiO2/p-Si photocathodes for PEC CO2RR
to multicarbon products.[60] They also
investigated the effects of ionomer bilayer coatings, specifically
Nafion atop Sustainion, on the Cu surface of the
Cu/TiO2/p-Si MIS photocathodes. PEC testing indicated
that Cu/TiO2/p-Si photocathodes coated with Nafion on
top of Sustainion revealed partial current densities for
C2H4 ranging from −0.9 mA
cm−2 to −2.3 mA cm–2 in
CO2-saturated 0.1M CsHCO3 under wet-side
illumination, compared to the condition without ionomer bilayers on the
Cu surface. This underscores the enhancement of both activity and
selectivity for C2H4 due to the Cu
cocatalyst and bilayer coatings.