**3. Results**

The initial search, which included all the fields, resulted in 2009 publications. Refining the search to look by topic, 1273 results were obtained. Of these, 332 included the keywords in their titles. After a full-text analysis, 87 articles met the inclusion criteria. **Figure 8** illustrates the process of selection that was carried out. The selected studies were published after 2009.

It was found that the combinations of materials and electrodes that have been studied are chemotrophic bioanode and photocathode, photoanode and biocathode, phototrophic bioelectrodes, chemotrophic photobioelectrodes, and systems with three or more electrodes.

### **3.1 Chemotrophic bioanode and photocathode systems**

Microbial photobioelectrochemical cells with chemotrophic bioanodes and photocathodes are the most studied devices because their main function is to use light

as the source of energy to drive a hydrogen reduction reaction at the photocathode. Most of them can simultaneously degrade organic compounds at the bioanode, which can increase the energy efficiency and lower operating costs compared to conventional microbial electrolysis cells [45]. Aside from hydrogen production, these systems have been used to generate electricity or synthesize semiconductors, as well as to reduce heavy metals.

### *3.1.1 Hydrogen production*

A list of microbial photobioelectrochemical cells that focus on hydrogen production is summarized in **Table 4**. The semiconductor materials that are more frequently used in photocathodes are titanium dioxide [30, 38, 41, 46] and copper(I) oxide [14–16, 45, 47]. These tests have only been done with known substances such as synthetic nutrient mediums with acetate or trypticase soy broth. This limits the extrapolation of the results obtained because there are no tests done with wastewater that can prove the effectiveness of these devices for wastewater treatment.

There were studies in which the power density obtained by illuminating the photocathode was so low that an additional voltage had to be applied to obtain measurable amounts of electricity and hydrogen [14–16, 35, 37, 43, 47]. These references did not provide enough information to determine if the low current generation could be attributed to the semiconductor used, the electrode dimensions, or the concentration of organic matter. It should be noted that applying an additional voltage to a photobioelectrochemical system would defeat the purpose of using it as an alternative for a bioelectrochemical system.

The biofilms were mostly inoculated from wastewater and activated sludge, except for one that used a pure culture of Shewanella oneidensis MR-1 [45]. As the results reported on the reference do not include any information on energy efficiency, the amount of electrical power or hydrogen produced cannot be compared between different studies, as more information is needed to standardize the results and make a correct assessment.

### *3.1.2 Electricity generation*

A list of microbial photobioelectrochemical cells that focus on electricity generation is summarized in **Table 5**. These studies also use titanium dioxide [27, 32–34] and copper(I) oxide [32, 48, 51, 52] as the most common photocathode materials. There is a study that used n-type Cu2O, which sets it apart from other studies because photocathodes are usually made with p-type semiconductors and copper(I) oxide is normally a p-type material [52].

There was one study in which the device used anaerobic sludge as substrate. The increased generation of electricity in comparison to a regular microbial fuel cell demonstrates the potential for microbial photobioelectrochemical cells with chemotrophic bioanode and photocathode to be used in wastewater treatment plants [34].

The most used pure culture for these systems is Shewanella Oneidensis MR-1 [27, 32, 53], while microbial consortiums are mostly inoculated from wastewater or anaerobic sediments from large bodies of water [33, 51]. As the energy efficiency was not reported in any of these references, their results in terms of power generation cannot be properly compared.

### *3.1.3 Synthetic materials production*

Reduction of chromium (VI) and simultaneous electricity production was achieved using a photocathode built with graphite coated with titanium dioxide.


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


**Table 4.** *Microbial photobioelectrochemical cells with chemotrophic bioanode and photocathode that produce hydrogen.*


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

