Using waste CO2 to produce essential amino acids for humans: An efficient photoelectrochemical route.

l-phenylalanine (l-Phe), an essential amino acid for humans, is widely used as building blocks. Currently, l-Phe is obtained via biosynthetic methods including microbial and enzymatic processes, but their tightly complex feedback regulation and lengthy reaction steps lead to a low practical yield of l-Phe. Here, we have designed a hierarchical Si-based photocathode for l-Phe synthesis by photoelectrochemical coupling of waste CO2 and nitrophenyl ethane, achieving a high yield rate of 37.5 μg·hour-1·cm-2 and a remarkable Faradaic efficiency of 21.2% at low applied potential under 1 sun illumination. The hierarchical structure with CuO-TiO2-C mixtures dispersed in amorphous TiO2 layer/n+p-Si creates an internal built-in electric field and forms plentiful conducting channels to efficiently realize the injection of electrons into Cu and Ti sites. These Cu and Ti sites adsorb and activate the CO2 and nitrophenyl ethane, respectively, cooperatively facilitating the l-Phe synthesis. This work introduces an environmentally friendly and highly efficient approach for converting solar energy into valuable amino acid products.