Silicon Photoelectrodes Prepared by Low-Cost Wet Methods for Solar Photoelectrocatalysis

ConspectusThe electrochemical conversion of sunlight by photoelectrochemical cells (PECs) is based on semiconductor electrodes that are interfaced with a liquid electrolyte. This approach is highly promising, first, because it can be employed for the generation of a chemical fuel (e.g., H2) to store solar energy that can be used on-demand to generate electricity when the sun is not available. Second, it can be seen as a concept reminiscent of photosynthesis, where CO2 is converted into a valuable feedstock by solar energy. Thus, photoelectrochemical cells are sometimes referred to as "artificial leaves". Silicon, being the main semiconductor in the electronics and photovoltaic sector, is a prime candidate to be used as the light absorber and the substrate for building photoelectrochemical cells. However, Si alone has "poor-to-no photoelectrochemical performance". This is caused by its weak electrocatalytic activity for cathodic reactions (namely, the hydrogen evolution reaction (HER), the CO2 reduction reaction (CDRR), and the N2 reduction reaction) and by its deactivation in the anodic regime, prohibiting its use for the oxygen evolution reaction (OER). This latter reaction is essential for supplying electrons to generate a solar fuel. Due to these problems, layers that both protect and are catalytically active are typically employed on Si photoelectrodes but require rather sophisticated manufacturing processes (e.g., atomic layer or electron beam deposition), which hinders research and innovation in this field. Nevertheless, our group and others have demonstrated that these layers are not always required and that highly active and stable Si-based photoelectrodes can be manufactured using simple wet processes, such as drop casting, electroless deposition, or aqueous electrodeposition. In this Account, we first introduce the topic and the possible structures that can be easily obtained starting from commercial Si wafers. Then, we discuss strategies that have been employed to manufacture photocathodes based on p-type Si. Among these, we describe Si photocathodes coated with metal, inorganic compounds such as metal sulfides, and more original constructs, such as those based on macromolecules composed of a catalytic Mo3S4 core and a polyoxometallate macrocycle. Also, we discuss the elaboration and the advantages of Si photocathodes obtained by grafting organometallic catalysts which are promising candidates for reaching excellent selectivity for the CDRR. Then, the manufacturing of photoanodes based on n-Si is reviewed with an emphasis on those prepared by electrodeposition of a transition metal such as Ni and Fe. The effect of the catalyst morphology, density, and Si structuration is discussed, and future developments are proposed.