Abstract: This disclosure provides systems, methods, and apparatus related to photochemical diodes. In one aspect, a device include a photoanode, a photocathode, and a bipolar membrane between the photoanode and the photocathode. The photoanode comprises a first semiconductor, the first semiconductor being N-type doped, a first catalyst disposed over the first semiconductor, and the photoanode being disposed in an anolyte. The photocathode comprises a second semiconductor, the second semiconductor being P-type doped, a second catalyst disposed over the second semiconductor, and the photocathode being disposed in a catholyte. The photoanode and the photocathode are in electrical contact. A hydrogen reduction reaction or a carbon dioxide reduction reaction occurs at the photocathode and a chemical oxidation reaction occurs at the photoanode when the photocathode and the photoanode are illuminated with light.