# Experimental and Modeling Study for the Solar-Driven CO2 Electrochemical Reduction to CO
## 3 Results and Discussion
### PV-EC Integration  
Regarding the EC device, the performance (in terms of the FE, current density, etc.) has already been characterized and reported in the previous paper and summarized in the Supporting Information (Figures S1 and S2). For what concerns the PV-EC system, the first important step to thoroughly engineer the device is to establish the electrical operational point. Without any external imposed stimulation, the system finds its own operational point on the superimposition between the I-V characteristics of the PV and EC modules. In this case, as shown in Figure 4, the system should work at a V\({}_{\text{op}}\) = 3.1V and a I\({}_{\text{op}}\) = 8.6 mA, which  
Figure 3: Fit of the I–V characteristics (**a**) and the incident power dependencies of \(R_{s}\), \(R_{sh}\), and \(I_{ph}\) (**b**). corresponds to approximately 3.4 mA cm\({}^{-2}\), when illuminated with an incident power of 1 sun. This self-adjusting behavior, where the system balances the I-V characteristics of both the PV and EC modules, ensures efficient operation without external control, emphasizing the robustness of the device in varying environmental conditions. In previous works, [PERSON] et al. [25] exploited Ag nanoparticles in a flow cell configuration, which, coupled with a GaInP/GaInAs/Ge triple-junction solar cell, showed a J\({}_{\text{op}}\) = 13.8 mA cm\({}^{-2}\) at V\({}_{\text{op}}\) = 2.23V. Thanks to the gas diffusion electrode, in fact, it is possible to reach higher current densities, but the system will be limited by the solar cell's ability to provide sufficient electrical energy. With large active areas, on the other hand, it is possible to exploit the systems for higher production rates, demonstrating the scalability of these PV-EC systems. As an example, [PERSON] et al. [26] exploited Ag, as well, in a flow cell electrolyzer, with an active area of 25 cm\({}^{2}\), coupled with five Si-based solar cells of a total area A = 125 cm\({}^{2}\): they achieved a FE\({}_{\text{CO}}\) = 75.9%, at J\({}_{\text{op}}\) = 36.1 mA cm\({}^{-2}\) at V\({}_{\text{op}}\) = 2.71V, which means a I\({}_{\text{op}}\) = 0.9A. In Figure 5, the solar-driven electrochemical CO\({}_{2}\) reduction is performed to produce CO through solar energy. Not only does the device show the correct operational point at 1 sun of illumination, but it is also possible to understand the performance under different conditions. At lower light intensities, in fact, the current density decreases rapidly (Figure 5a) along with the selectivity toward the CO\({}_{2}\)RR. Interestingly, the overall voltage (Figure 5b) does not change, ranging from 3.05 V up to 3.10 V when using 0.6 and 1 sun, respectively. This trend could be related to the proximity of the operational point with respect to the maximum power point of the solar cells, where the potential variation is small. As the device operates close to the MPP, the voltage remains nearly constant, even with changes in the light intensity, indicating that the PV module is effectively maintaining its voltage output within a narrow range. The decrease in faradaic efficiency at lower light intensities, as observed in Figure 5a, suggests that the reduction in available energy leads to less efficient CO\({}_{2}\) reduction. A lower light intensity, in fact, results in reduced electron flow, thus lowering the overall efficiency of the electrochemical reduction process. Since no liquid-phase products were detected in the catholyte during these analyses, only the faradaic efficiencies related to H\({}_{2}\) and CO are reported. Lastly, the mathematical model was validated through the experimental result, as shown in Figure 6. By exploiting a time-dependent simulation, it has been possible to simulate a chronoamperometry of the PV-EC device with 1 sun illumination.