**3.3 Phototrophic bioelectrodes**

Cyanobacteria, algae, and similar phototrophic microorganisms can be considered as part of photobioelectrochemical systems because they need light to grow and catalyze reactions. Phototrophic bioelectrodes can be anodic or cathodic and can be used for a wide variety of applications.

### *3.3.1 Phototrophic bioanodes*

Most of the studies related to phototrophic bioanodes have been aimed only at their potential for electricity generation. This is mainly because these microorganisms are photosynthetic and do not oxidize organic matter. Also, while

chemotrophic bioanodes are usually built with microbial consortiums, phototrophic bioanodes are assembled with pure culture biofilms. **Table 6** summarizes some of the publications found about these electrodes.

Unlike most of the works published about these electrodes, one published work used a phototrophic microbial consortium obtained from sediment and seawater, therefore, it can function as a self-assembled and self-repairing device to generate energy from sunlight. The measurements made on the cell showed that it can produce up to 0.017 W/m2 [64].

Also, a research team from Italy developed a photobioelectrochemical cell with phototrophic bioanode and chemotrophic biocathode that generate hydrogen by coupling the photosynthetic capabilities of cyanobacteria in the anode and the dark fermentation process of heterotrophic bacteria degrading an organic substrate. This research shows that cathodic biofilms can consume organic substrates similar to anodic biofilms [65].

Phototrophic bioanodes can be integrated into electronics to supply them with power, as demonstrated by a study that built an invasive ultramicroelectrode array and a microfluidic chamber using silicon microfabrication techniques to immobilize photosynthetic microorganisms and use them in a microtip array as a solar power generator to integrate into solar cells and sensors. The prototype registered a current of 250 pA and 45 mV [66].

### *3.3.2 Phototrophic biocathodes*

It is common to find microbial fuel cells with phototrophic biocathodes to be named microbial solar cells, as oxygenic microorganisms can produce the oxygen required for the device to generate electricity without aeration. This research shows that phototrophic cathodic biofilms made of mixed cultures have microorganisms that are either photosynthetically active or catalyzes the reduction of oxygen in the electrode [67]. An oxygenic phototrophic biocathode with a mixed culture of the cyanobacteria Synechococcus leopoliensis, Anabaena cylindrica, and the algae Chlorella pyrenoidosa has been used in a microbial fuel cell to generate two times more electricity in comparison to a regular carbon fiber veil cathode [68].

Phototrophic biocathodes can be inoculated without using pure cultures. One example is an experiment in which the biofilms were grown on a carbon fiber veil that was submerged in pond water for 2 months in a well-illuminated room. Unfortunately, the consortium was not characterized, but the resulting electrode increased the power generation of the microbial fuel cell by 42% in comparison to a regular carbon fiber veil electrode [69]. In other work, a biofilm grown with microorganisms obtained from surface soil from the base of a drainage ditch mixed with distilled water and exposed to sunlight for 1 month was reported. The resulting phototrophic biocathode allowed a sediment-type microbial fuel cell, in which the anode is a bed of sediment, to generate a maximum power density of 11 mW/m2 over 6 months without feeding [70].

Some electrodes have been investigated for many other applications. A microbial fuel cell in which both the anode and cathode were inoculated with anaerobic sludge from a wastewater treatment plant achieved a microbial fuel cell with phototrophic bioanode and phototrophic biocathode. The function of the bioanode was to dechlorinate 4-chlorophenol so it can be mineralized at the anode [71]. Another application is using the biomass generated on the cathodic compartment of a microbial fuel cell with chemotrophic bioanode and phototrophic biocathode to feed the anodic compartment, thus creating a device with self-sustainable electricity production when exposed to sunlight [20]. Another way to use these phototrophic biocathodes is to take the light/dark cycles of the microorganisms. The biocathode


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

**Table 6.**

 *Microbial photobioelectrochemical cells with phototrophic bioanodes.* can generate oxygen to be used as the electron acceptor when exposed to light and use nitrate as the electron acceptor (and reducing it to N2 in the process) in dark conditions [17].

### **3.4 Chemotrophic Photobioelectrodes**

Chemotrophic biofilms can be grown on semiconductors in a way that the bias generated by the photoelectric effect can enhance the current generated by the microorganisms. These electrodes are referred to as photobioelectrodes in the literature, which agrees with the nomenclature used in this document, except that the word chemotrophic is added here to clarify that the photoelectric effect is produced at the semiconductor and the electrical current is provided by the chemotrophic microorganisms. So far, only chemotrophic photobioanodes have been reported in the literature.

Microorganisms could serve as a protective layer for corrosive semiconductors while providing catalytic functionality. One example of these electrodes is a hematite nanowire electrode with a biofilm of Shewanella oneidensis strain MR-1, which shows a synergistic effect in which the power generation and substrate consumption is greater with live cells instead of dead cells or the hematite photoanode alone [53].

Titanium dioxide can be used to build chemotrophic photobioanodes. A TiO2/Ti electrode was operated with a biofilm inoculated from non-chlorinated Dutch tap water to reduce phenol with a removal efficiency of 62% after 4 hours of light irradiation [72].

It has been demonstrated that the use of a semiconductor such as α-Fe2O3 as chemotrophic biofilm support can reduce the resistance of the electrode, accelerate biofilm formation, enrich exoelectrogens, shorten the startup time for the microbial fuel cell and significantly increase the current produced [73]. Scientific studies show that a chemotrophic photobioelectrodes can consist of a stainless steel sheet substrate in which the semiconductor material is deposited on one side and the chemotrophic biofilm is grown on the other side, achieving the same benefits previously reported [16].

The synergistic effect of a chemotrophic photobioelectrode to produce electrical current was used in a desalination cell with anion-exchange membranes and cation-exchange membranes. The metallic anode of the system was modified on one side with nanostructured α-Fe2O3 and the other side was inoculated with the fresh anodic effluent of an anaerobic granulate sludge blanket reactor. The device achieved a maximum current of 8.8 A/m2 and a salt removal performance of at least 96%, which demonstrated the capacity for this system to generate electricity and desalinate water [15].

Although phototrophic bioelectrodes and photobioelectrodes have been studied, there are no available parameters that can measure its performance in terms of solar energy conversion. An appropriate place to begin building a diagnostic formula for phototrophic biofilms may be the solar-to-hydrogen efficiency formula used in photoelectrochemistry for water splitting, which can be written as the Eq. (1):

$$\varepsilon\_{SH} = \left[ \frac{\left| j\_{\rm CC} \left( \frac{mA}{cm^2} \right) \times \mathbf{1}.23 \, V \times \eta\_{\rm F} \right|}{P\_{\rm nad} \left( \frac{m \, W}{cm^2} \right)} \right]\_{AM1, \rm 5G} \tag{1}$$

In Eq. (1), jCC is the photocurrent density per electrode area, 1.23 V is the required voltage for water splitting, ηF is the faradaic efficiency for hydrogen production and Ptotal is the light energy per electrode area when the electrode is irradiated [46]. Results evaluated with this formula are standardized by doing

experiments with a solar simulator that can produce light resembling the AM 1.5G spectrum. It may be possible that the same relationship that is shown in Eq. (1) can be used to evaluate the energetic efficiency of phototrophic bioelectrodes, replacing the voltage for water splitting with the required bias for the reduction reaction that the microorganisms are performing.
