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)).