**2.2 Determination of electrode variety and cell permutations**

The subset of microbial photobioelectrochemical cells contains conductors and semiconductors, as well as chemotrophic and phototrophic microorganisms as electrode materials. **Table 3** shows the possible electrode variations found after estimating combinations of these materials. As the order in which these materials are added to the surface of the electrode can modify their performance, terms like photobioelectrode and biophotoelectrode would mean that the former has a surface with a semiconductor covered in a biofilm, while the latter has a biofilm coated with a semiconductor film.

Of the 16 possible electrode combinations shown in **Table 3**, the 9 that are in italics are exclusively photobioelectrodes because they either have phototrophic microorganisms or combine semiconductors with chemotrophic microorganisms. These electrodes would need to be exposed to light and be in an environment that meets the appropriate conditions to maintain cellular growth. Bioelectrodes and Photoelectrodes are already widely studied in their respective fields; however, photobioelectrochemical systems can also be built by combining them [39], so this review covers reported studies that either use photobioelectrodes or combines bioelectrodes and photoelectrodes.

### **2.3 Search strategy**

The articles were selected according to the following inclusion criteria:

a.Use of microorganisms and light: All references that state in their title or abstract that the studied device uses either phototrophic microorganisms and/ or semiconductors in addition to chemotrophic microorganisms would need


**Table 3.** *Microbial photobioelectrochemical electrodes. Each material on the first column is the surface of the electrode.* a similar cell design and operating conditions, which is within the scope of this review.


Likewise, articles were discarded according to the following exclusion criteria:


Other data such as year of publication and applications or subproducts of the studied devices were not limited as exclusion criteria because information about microbial photobioelectrochemical systems is very scarce and this was considered as an area of opportunity for this manuscript to harness all the available information.

Publications were collected from Web of Science (WoS), covering a period of 27 years from 1993 to 2020. The most recent search was executed on December 15th, 2020.

A preliminary search with the keywords "Microbial photoelectrochemical cells" was realized. After obtaining a set of terms, some of which were mentioned in **Tables 1** and **2**, the sampling was carried out by using 12 simultaneous WoS search fields, each referring to combinations of prefixes to name photobioelectrochemical devices or combinations of electrodes. All the search fields are linked by OR operators and are explained in the following points:


The search fields were set to find the defined queries in all fields; however, as the number of references initially found was 2009, the survey was refined by restricting the query to the title of the publications. The remaining reports were analyzed by their relevance to the topic of interest by screening their titles, abstracts, and the full texts, selecting them according to the eligibility criteria.

The data was charted by using a form developed on an Excel file. The spreadsheet was continuously updated in an iterative process when new information was found.

The data extracted from the references include general characteristics (such as digital object identifier, year, country, and keywords), electrodes that were used, and subproducts obtained (such as electricity, hydrogen, or other synthetic materials).

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

Data from research done using microbial consortiums or pure cultures, citing their names if they were reported in the publications was extracted too.

From the results reported in these studies, only the highest values regarding current density, power density, and efficiency of contaminant removal or product synthesis were included in this review. Also, the units in which values are reported were adjusted using the data available in the publications to report them in the most homogeneous way possible in order to make direct comparison feasible. In the cases where the cell compartments have different volumes, the unit standardization was done using the anolyte's volume. When a result is reported in a quantity per unit of area, it is based on the anode's projected area.

The studies were grouped according to the combination of electrodes and electrode materials used. Subgroups were made by classifying the subproducts generated such as electricity, hydrogen, or other synthetic compounds. If any review articles about photobioelectrochemical systems were found, an analysis of their references was performed to identify studies missed with the search strategy reported above in this manuscript.
