**3.2 Photoanode and biocathode systems**

Although the most common photobioelectrochemical systems that have been studied consist of bioanodes and photocathodes, there are systems in which semiconductors are used as anodes instead of cathodes, and microorganisms are used on the cathodes instead of the anodes. This allows the device to use an advanced oxidation process with the photoelectrochemically generated holes in the photoanode and, at the same time, perform reduction processes with the microorganisms at the biocathode.

At the time of this writing, only four publications that use this electrode configuration were found. The most recent one, published in 2017, demonstrated the successful oxidation of rhodamine B with more than 90% efficiency using a photoanode composed of TiO2 nanotubes with Ag nanoparticles and an oxygen-reducing biocathode, achieving a maximum power density of 0.318 W/m2 . This research highlighted that the biocathode eliminated the persistent kinetic limitations of abiotic cathodes, which ensured the stable operation of the photoanode. This was also the only study in which the cathodic microbial community was characterized [55].

Another research team developed a nitrogen removal strategy in a single reactor by combining photoelectrocatalytic oxidation of ammonium using an anode composed of AgI/TiO2 nanotubes and denitrification of nitrates using a cathode with an autotrophic biofilm (Q. [56]).

The two remaining publications were made at the same laboratory and focused on organic pollutant oxidation at the photoanode using TiO2 nanotubes with coupled nitrification of ammonia nitrogen at the biocathode. One of the publications compared this system with a conventional photoelectrochemical cell with a TiO2 photoanode and a Pt/C cathode, demonstrating that the photobioelectrochemical device does more pollutant removal and electricity generation [57], while the other focused on the parameter that influences the performance of this device such as pollutant types, electrolyte concentration and gas atmosphere of the photoanode [58].
