*1.3.1 Electrode materials*

Photoelectrochemical systems use a wide range of electrode materials. Conductor or semiconductor electrodes can be used directly or as a supporting matrix for catalysts such as chemotrophic microorganisms [19], prototrophic microorganisms [20], enzymes [21] or any other biotic material like cell membranes [22] or chlorophyll [23]. As seen in **Figure 4**, there are at least 10 combinations of electrodes that can be formed assuming that there are no combinations between catalysts and all the biotic materials that are not enzymes are grouped. Considering that an electrochemical cell requires at least two electrodes to work, there can be near 100 possible configurations of photobioelectrochemical systems, not counting systems with more than two electrodes or electrode pairs that are only bioelectrochemical or photoelectrochemical.

This extensive variety of electrodes and cell configurations makes the field of photobioelectrochemistry so large that doing a complete review that identifies all the systems that have been tested to date is a big challenge. As different conditions are required to preserve an enzyme, to maintain an active biofilm, or to keep other biotic materials that are not greatly affected by the environment, different cell designs and configurations are required according to the biological catalysts that they contain, which increases the difficulty for classifying all the possible photobioelectrochemical devices.
