Electrification of industrial chemical processes is on its way to being fully integrated into the existing chemical plants. In this context, hydrogen (H₂), one of the main utility gases, can be easily produced through water electrolysis or, even more promisingly, through water photo-electrolysis. According to the International Energy Agency (IEA), the demand for green hydrogen should rapidly increase in the near future, sustained by the large number of projects and investments being conducted worldwide [IEA (2023), Net Zero Roadmap].

The use of a photo-electrochemical (PEC) cell, rather than a conventional water electrolyser, is a valid alternative to meet the increasing green H₂ demand. Basically, a PEC cell is an improvement of a classic electrochemical cell by replacing one of the two electrodes with a semiconductor material capable of absorbing visible light to drive oxidation or reduction reactions. The main advantage of PEC technology lies in its ability to directly convert sunlight into chemical energy (e.g., H₂ or carbon solar fuels), avoiding the step of converting sunlight into electricity by a photovoltaic system, an advantage in terms of process intensification.

Despite its potential, the PEC approach is a novel technology that requires further development before being commercially viable. For CO₂ reduction reaction (CO₂RR), the integration of a photovoltaic system with an electrochemical reactor remains the more efficient route [1]. This contribution discusses the key aspects and current limitations of PEC cells, providing insights into critical parameters that must be considered for a correct evaluation of the process efficiency. The limitations of PEC cells will be assessed and step-by-step strategies will be proposed to mitigate or overcome these challenges, with the ultimate aim to enhance the overall efficiency of the process.

The discussion begins with the selection of the photoactive material, starting from titanium dioxide (TiO₂) being the most widely studied in the literature due to its suitable bandgap (3.2 eV), low toxicity, corrosion resistance, and abundance [2]. However, TiO₂ faces significant drawbacks that strongly limit its efficiency: 1) poor absorption of visible light, as it primarily absorbs only the UV portion of the solar spectrum, and 2) a high charge recombination rate. Through advanced synthesis techniques and electrode engineering, we will show how these limitations can be addressed to improve TiO₂ performance. The selection of different materials than TiO₂ will also be discussed, as well as the importance of electrode and cell design will be remarked [3]. We will discuss the importance of reactor configuration, specifically how to move from a classic H-type cell to a more advanced membrane electrode assembly (MEA) flow device to significantly reduce diffusivity and resistivity, enabling higher current densities that are more suitable for industrial applications. To achieve higher efficiencies, we examine the potential of novel 2D and 3D architecture electrodes and the possibility of replacing TiO₂ with alternative semiconductor materials that offer better light absorption and charge transport characteristics. Additionally, we will discuss the application of PEC devices for solar fuel production through the CO₂RR, focusing on key parameters to investigate in lab-scale systems to facilitate scale-up for commercialization [4].