**2. Methodology**

Given the considerable number of permutations that photobioelectrochemical systems can have, it is better to approach the analysis of this field in terms of subsets of devices that share the same cell designs and operating conditions for a selection of electrode materials and catalysts. It is convenient to group devices that share similarities in terms of the biological materials used. For instance, microbial photobioelectrochemical cells were reviewed in this study. These systems use chemotrophic and phototrophic microorganisms as the biological component, which means that their designs must have the conditions necessary to maintain microbial growth and to harness light.

### **2.1 Nomenclature system**

To avoid confusion and present this review in the clearest way possible, a nomenclature system was developed. The main purpose of this tool is to identify any photobioelectrochemical device, as well as the nature of its components, just by reading the constructed name. The prefixes, suffixes, and descriptive terms have been based on literature published, as well as the Compendium of Chemical Terminology of the International Union of Pure and Applied Chemistry (IUPAC) [44]. Although this nomenclature was developed for this review, it was designed to name any photobioelectrochemical system.

### *2.1.1 Electrode name building*

As the electrodes can be made from a considerable variety of materials, their nomenclature should be able to describe their composition as accurately as possible. To achieve this, two categories for supporting electrode materials and four for catalysts, as illustrated in **Figure 5**, will be considered. It is assumed that conductors are a component of all electrodes since they are connected to an electrochemical circuit, so no nomenclature will be added for these materials. The prefix "photo" will be used to refer to semiconductors and the prefix "bio" will be used for any biological material. To distinguish the four categories of biomaterials, the prefixed words "enzymatic" and "biotic" will be used to distinguish enzymatic catalysis from any other material with a biological origin that cannot be classified as microorganisms or enzymes, such as chlorophyll. The words "phototrophic" and "chemotrophic" will be used to indicate the type of microorganisms.

The prefixes of the compound name for an electrode are ordered to indicate the material that would be exposed to light and the words "cathode" or "anode" indicate the electrode where reduction or oxidation, respectively, takes place. The order is determined by looking at the electrode material layers from the outside into the center of the electrochemical cell, as illustrated in **Figure 6**, where a chemotrophic

**Figure 5.** *Categories for electrode materials.*

photo bioanode would indicate an anodic electrode that has a semiconductor side exposed to light and chemotrophic microorganisms on the other side.

### *2.1.2 System name building*

The name for a photobioelectrochemical system will follow the structure shown in **Figure 7** where, in the first section (**Figure 7**, I) the first word(s) indicate if the biological component of the device is enzymatic, microbiological, or any other biotic material (**Figure 7**, I, 1). The second part of the name (**Figure 7**, I, 2) has the prefixes "photobioelectro" to indicate that a system is an object of study from the field of photobioelectrochemistry and this word will end with the suffix "chemical" if the electrical current generated by the system is spontaneous, or the suffix "lytic" if it is non-spontaneous [44]. The third part of the name (**Figure 7**, I, 3) ends with

### **Figure 6.**

*Example of nomenclature for an electrode composed of a semiconductor exposed to light and chemotrophic microorganisms.*

*Microbial Photobioelectrochemical Systems: A Scoping Review DOI: http://dx.doi.org/10.5772/intechopen.99973*

**Figure 7.**

*Photobioelectrochemical device nomenclature.*

the words "fuel cell" to indicate electricity generation, "cell" for electrolysis-related reactions, and "system" for any other functionality that the device may have.

The second section (**Figure 7**, II) mentions the electrode and material combinations, following the corresponding nomenclature for electrodes. The complete name built with this nomenclature should describe all the characteristics of a device. For instance, a microbial and biotic photobioelectrolytic cell with chemotrophic bioanode and biophotocathode could refer to a two-electrode electrochemical chamber with a biofilm coated anode that does not need to be exposed to light and a chlorophyll-sensitized semiconductor cathode with the capacity to generate hydrogen from water using solar light. All the specific details would be described later here for any paper that reports the studies made on such a system.
