5. PHOTOVOLTAIC-POWERED PHOTOELECTROCATALYTIC
CO2RR REACTOR
The PV-EC system is operated by a sufficient driving force through solar
energy from multi-junction PVs for CO2RR. However,
PV-PEC, which consists of a single-junction and multi-junction PV and a
photoelectrode, absorbs light energy to generate voltage and current
from the electrode and provides a redox reaction to the interfacing
electrode.[125]
As shown in Figure 8(A), Jang et al. designed the stacked tandem cell
structure of a PV-PEC system for efficient STC conversion efficiency
from a photoelectrode, which has excellent light harvesting of
higher-energy photons, and from a single junction of perovskite PV,
which has lower-energy photons.[126] In a concrete
structure, the gold-decorated triple-layer ZnO@ZnTe@CdTe (ZCT)
core-shell nanorod array with facilitated charge separation and a narrow
band gap with excellent catalytic efficiency was utilized as a
photocathode, a
CH3NH3PbI3 perovskite
solar cell in tandem was adopted for efficient light harvesting, and
Co-Ci was situated in a light-blocked place as an OER anode. The
absorption property, including incident photon-to-current conversion
efficiency (IPCE), of two light absorbers under AM 1.5 G indicated that
while the power density of Au nanoparticles decorated a ZCT (ZCT NR-Au)
photocathode was reduced to 55%, accompanied by unchanged open-circuit
voltage, most of light it used was below 550 nm, as shown in the Figure
8 (B) and (C). From the J-V characteristic, the operating point
at the current of 0.85 mA was confirmed by the intersection of the
perovskite solar cell and the photocathode in an unbiased tandem device
for spontaneous photoelectrochemical CO2RR (Figure
8(D)). Furthermore, the evolution of gaseous products containing CO and
H2 was measured for 3 h under 1 sun illumination by
chronoamperometry with unbiased external voltage, and a ZnO@ZnTe@CdTe-Au
photocathode with single-junction perovskite achieved FE of 74.9% for
CO in the CO2-purged KHCO3, which shows
excellent selectivity among induced corrosion of Te-based materials
(Figure 8(E) and (F)). Similarly, the production of value-added products
with tandem structures combined with solar cells and Cu-based
photocathodes has attracted considerable attention. For the effective
formation of HCOOH, one of the valuable C1 products, Kim
et al. introduced assembled FeOOH/BiO4/CIGS tandem devices that do not
require external bias such as 1.2 V. [127] For the
assembled tandem device, a single Cu(In,Ga)Se2 (CIGS)
solar absorber was utilized as a PV cell owing to its photo-advantages
such as adequate direct band gap (1.12 eV) and excellent durability in
an aqueous electrolyte under the CO2RR system (Figure
8G). In addition, FeOOH/BiVO4 adjusted in thickness,
retained high stability and optimal current as a photoanode situated at
the top cell, and a mesoporous indium tin oxide (meso ITO) cathode
was used as the working electrode. As shown in Figure 8(H) and (I),
photoelectrochemical profiles including J-V characteristics and
chronoamperometry measurement of the combined CIGS-based tandem device
compared to perovskite solar cell exhibited improved
photoelectrochemical performance in the following metrics: an
open-circuit voltage (0.64 V), short-circuit current density (35.68 mA
cm–2), a fill factor (65%), and power conversion
efficiency (15.01%). In contrast to the humidity-sensitive
perovskite-based tandem cells showing formate formation below 5 Mm under
relative humidity (RH) environments,
FOOH/BiVO4/CIGS/meso ITO CIGS-based system
produced approximately 6 mM of formate concentration regardless of RH
environment value of more than 80%. For efficient CO2RR
toward multi-carbon (C2+) formation by
photovoltaic-biased photoelectrocatalysis, Gurudayal et al. designed a
PV-PEC device utilizing an Ag-supported dendritic Cu as Si-photocathode
(bottom photoabsorber), IrO2 nanotube as an anode in
tandem with two series-connected semi-transparent halide perovskite
solar cell (top photoabsorber), which had a band gap of 1.58 eV under 1
sun illumination in 0.1 M CO2-purged
CsHCO3 (Figure 8(J)).[128] From
the J-V curve of the photocathode and two series-connected PV
intersections, operating currents of 2.1 to 2.9 mA depending on the
concentrations of the CsHCO3 electrolyte were confirmed
(Figure 8(K)). The outstanding FE of the system for diverse carbon
compounds by suppressing HER evolution and STC efficiency of 3.5% with
unbiased external voltage are depicted in Figure 8(L). The use of
perovskite solar cells, which are cost efficient, results in high STC
efficiency by combining tandem devices with an efficient photocathode.
The PV-EC and PV-PEC devices are listed in Table 3.