# Experimental and Modeling Study for the Solar-Driven CO2 Electrochemical Reduction to CO
## 1 Introduction  
Addressing climate change has become a pressing global necessity, as it impacts numerous facets of life, from food security to geopolitics and economic stability. The consequences of global warming (such as extreme weather, reduced crop yields, water scarcity, rising sea levels, and the extinction of multiple species) stem largely from greenhouse gases, particularly carbon dioxide. CO2 emissions are predominantly generated by fossil fuel combustion, which currently satisfies about 80% of the global energy demand, along with heat and electricity production. As energy consumption continues to climb, atmospheric CO2 levels are rising. In this framework, the electrochemical reduction of CO2 represents a significant advancement toward both environmental sustainability and energy production. In fact, as a response to the escalating global CO2 emissions primarily driven by fossil fuel combustion, the need for renewable energy and carbon-neutral processes has become critical. Electrochemical CO2 reduction (CO2 RR), which involves converting CO2 into valuable fuels or chemicals using electricity, offers an innovative pathway to address this issue [1]. This process has shown considerable promise for producing a range of high-value products, including carbon monoxide (CO), formic acid (HCOOH), methane (CH4), and other hydrocarbons [2], under relatively mild conditions of temperature and pressure. A unique advantage of CO2 electroreduction lies in its compatibility with renewable energy sources, especially solar power. By coupling photovoltaic (PV) cells with electrochemical (EC) cells, in fact, it is possible to achieve a self-sustaining process that converts sunlight directly into chemical energy [3; 4; 5; 6]. This approach not only leverages the abundant available solar energy but also mitigates the environmental impact associated with traditional energy production. The PV-EC integration, or \"artificial leaf\" concept, is an exciting area of research that exemplifies the potential for renewable-driven electrochemical systems for green fuel production. Although extensively utilized for solar-driven water splitting to produce green hydrogen [7; 8; 9], only one study in the literature shows the total integration between the PV and EC components for solar-driven CO2 conversion [10]. In the typical PV-EC connected configuration, the PV is externally connected to the EC electrodes, with a proper electrical matching between the two components: in this field, the research community has explored different PV and EC generations to maximize the solar-to-fuel efficiency nstc. For example, [PERSON] et al. exploited amorphous Si-based PV to power a flow cell, achieving a nstc = 4.9% at 99.2 mA [11], while [PERSON] et al. reached a higher nstc = 12.5%, thanks to the employment of third-generation perovskite solar cells connected to a H-type EC [12]. Concerning PV-EC systems with high current densities, [PERSON] et al. exploited a zero-gap reactor powered by a Si-based PV (with active area A = 120 cm\({}^{2}\)), achieving a nstc = 12.1% at 1.1 A [13]. From a modeling perspective, simulations have been widely exploited to understand the dynamics of the CO2 RR in the reactor. [PERSON] et al. defined a 1D model for the electrolyte-electrode boundary layer [14], predicting the surface concentration of chemical species and laying the foundation for more complex simulations. [PERSON] et al. introduced gas bubbles, and a diffusion-dependent boundary layer [15], and more recent studies introduced the full electrochemical cell, also benefiting from COMSOL(r) Multiphysics software 6.3 [16; 17]. Regarding the PV module, the majority of the work in the literature focuses on the extrapolation of the main electrical parameters from the one-diode model [18], emphasizing their dependence on the incident light intensity and temperature [19; 20].