Using waste CO 2 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 CO 2 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-TiO 2 -C mixtures dispersed in amorphous TiO 2 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 CO 2 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.