4.1 C1 product
In most cathode electrodes of the PV-EC CO2RR
combination, CO2RR and HER are
dominant[99-100] while the oxygen evolution
reaction is dominant at the anode electrode.[99]As described above, noble metal catalysts, including Au and Ag, with low
overpotentials capable of suppressing hydrogen formation as well as
active CO formation from CO2 have been utilized as
electrocatalyst cathodes. The CO2RR characteristics of
the catalyst were confirmed by FE, current density, and overpotential
measurements.[71,100]
Disordered Ag nanoparticles were used as the cathode to improve the
selectivity and durability for CO2 to CO formation, Pt
foil was employed as the anode, and electrical energy was provided by a
six-section a-Si PV cell. The SEM images show that the disordered Ag
nanoparticles have an irregular size distribution, while the 3, 5, and
11 nm Ag on the carbon support are uniform in
size.[101] To confirm the FECO and
jCO, the CO2RR performance of Ag
nanoparticles was evaluated in an H-cell filled with a 0.1 M
KHCO3 electrolyte. While a maximum FECOof 83%, 90%, and 95% were confirmed on 3, 5, and 11 nm of Ag
nanoparticles, respectively, disordered Ag exhibited
FECO of more than 90% over a wide range of potentials
–0.6 to –1.7 V vs. RHE (Figure 5A). In addition, the disordered Ag
nanoparticles had jCO of –16.7 mA
cm–2 higher than other uniform Ag nanoparticles at
–1.8 V vs. RHE (Figure 5B). In addition, linear sweep voltage (LSV)
measurements on disordered Ag were performed on CO2RR at
the cathode and OER at the anode to determine the required
light-inducing voltage for driving the PV-EC system. The results
indicated that a voltage of 2.4 V was required (Figure 5(C)).
Six-section a-silicon-PV, which has an area of 25 cm2providing 3.38 V of a circuit voltage, was combined with the EC system
to confirm the electrocatalytic performance of PV-EC (Figure 5(D)). As
shown in Figure 5(E), during the PV-EC system test, a potential of 0.75
V was observed at the cathode electrode. Owing to the active
proton-electron coupling transfer (PECT) process on disordered Ag, the
selectivity of PV-EC performance exhibited a FECO of
92.7%, which is significantly higher than other uniform Ag catalysts
(Figure 5(F)). Thus, owing to the excellent electrocatalytic properties
with an appropriate Tafel slope (128 mV dec–1) and
overpotential, disordered Ag noble metals show high STC efficiency when
combined with a-Si-PV.[102-103] Another promising
CO2RR catalyst, which has high efficiency in CO
formation when combined with a PV system, is Au, which is a noble metal.
For example, Wang et al. fabricated needle-like nano-Au on carbon paper
using a one-step electrodeposition method as a cathode for efficient
CO2RR and nanosheet-like NiFe hydroxide on Ni foam via a
hydrothermal method as an anode for oxygen
evolution.[104] Needle-like nano-Au exhibited
excellent electrochemical CO2RR performance through low
onset overpotential of less than 160 mV, a low Tafel slope of 47 mV
dec–1, and a maximum FECO of
~92% at –0.57 vs. RHE. To further explain the Tafel
value, catalysts with low Tafel slopes indicate that the initial
rate-determining chemical step is *COOH formation by facilitating the
equilibrium state for the adsorbed
CO2·– intermediate. To build a
complete PV-powered EC system efficiently, GaAs (InGaP/GaAs/Ge) was
adopted, recoding a high photoconversion rate of 37.9% and stable
durability of 24 h with an average FECO of 92% in a
CO2 saturated 0.5 M KHCO3 electrolyte
under continuous electrolysis. Lee et al. reported a carbon-supported
tungsten-seed-based 3D silver dendrite (W@AgD) as a
CO2RR catalyst for CO formation, by investigating a
zero-gap CO2 electrolyzer.[105] As
shown in Figure 6(A) of the scheme of the STC system, in order to
compose a complete PV-EC system, 3–6 silicon solar cells were assembled
in series as modules with a size of 10 cm X 12 cm, and as the OER
catalyst in charge of the anode part, Fe-doped Co foam, which exhibited
high catalytic activity in alkaline media, was used. The assembled PV-EC
system varied the number of silicon-based solar cells to confirm the
optimized I-V curves and exhibited a high STC conversion rate of
12.1% with a current of 1.1 A under AM 1.5 G, which is close to the
highest value among silicon-based PV-EC systems, and also exhibited
excellent FECO of 95% (Figure 6(B) and (C)). Kim et al.
reported another excellent PV-EC system that is advantageous for
generating CO. Their system consisted of an Au25 cluster
placed on carbon paper and was used as a cathode, NiFe inverse opal was
used as an anode, and Ga0.5In0.5P/GaAs
tandem PV cell was the serving solar energy (Figure
6(D)).[106] As displayed in Figure 6(E), theI-V characteristics of the individual series-connected
Ga0.5In0.5P/GaAs tandem cell and
CO2 electrolyzer with Au25 cluster, were
confirmed to match interaction of two curves at –14 mA at 1.63 V.
Moreover, the PV-EC system combined with the Au25cluster, NiFe inverse opal, and tandem solar cells exhibited an
excellent average of solar to CO efficiency of 18% under continuous
reaction for 12 h (Figure 6(F)). To attain a high STF efficiency, Chen
et al. fabricated boron-doped bismuth (Bi(B)) by anticipating the unique
electronic properties of Bi(B) that regulate the free energy of the
OCHO* intermediate by inducing the movement of the p-electron state to
the Fermi level.[107] As shown in Figure 6(G), the
STF efficiency was evaluated using Bi(B) as an efficient
CO2 catalyst and FeP nanosheets supported on Ni foam as
an OER catalyst with commercial GaInP/GaInAs/Ge solar cells for
efficient PV-EC devices. As a result, this PV-EC system achieved the
best record of STC of 11.8%, accompanied by high FE for formate of 93%
under the CO2RR system (Figure 6(H) and (I)).