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.