**1. Introduction**

The available data on worldwide wastewater treatment indicate that, on average, high-income countries treat 70% of their generated wastewater, while upper-middle-income countries treat 38%, lower-middle-income-countries treat 28% [1]; in contrast, 80 to 90% of the wastewater in low-income-countries is neither collected nor treated [2]. Developing countries do not perform enough wastewater treatment due to their high operating costs, high energy consumption, and low economic return [3]. It is for this reason that polluted water is constantly accumulating.

The energy contained in wastewater pollutants is 6 to 13 times greater than that required for its treatment [4, 5], thus, much of new research and development initiatives in this area revolves around the recovery of such energy to save or profit from wastewater treatment. Some of the technological developments that are applied to recover energy from wastewater are the combustion of organic solid waste to generate electricity and anaerobic digestion to generate biogas from pollutants [6].

Much of the technology that can harness energy from water pollutants are currently under research and development. Within this set, bioelectrochemical systems are particularly interesting because they can perform wastewater treatment and generate electricity, hydrogen, or high-value chemical compounds simultaneously; furthermore, they can be designed for other purposes such as desalinating water or removing nitrogen, producing electricity at the same time [7].

The number of studies about bioelectrochemical systems has increased by orders of magnitude in the last couple of decades [8]; however, there is no standardized parameter to report their performance. Many publications report results using different indicators, which, in turn, complicates direct comparison among results [7]. This is arguably one of the main factors that slow the optimization and advance of these devices.

There are bioelectrochemical systems that require the application of voltage to work, which reduces the net profits in their operation. It has been proven that the use of semiconductors or phototropic bacteria can generate the required potential difference using sunlight [9]. This new set of devices that use bacteria and semiconductors in electrochemical cells can be called photobioelectrochemical systems.

On the other side, there is little information available in the literature about this mix of photoelectrochemical and bioelectrochemical systems. However, the variety of combinations of materials, as well as subproducts and functions that these devices can perform is very large. The scoping review approach can open a way to explore this new research area to identify areas of opportunity that can help research teams work towards its evolution.

The main objective of the present work is to identify and map the existing literature on photobioelectrochemical systems, as well as to identify gaps in knowledge that can be useful to research and advance their development. Also, this paper proposes a way to organize the knowledge surrounding these devices, by systematically reviewing in the future when more literature will be available.

### **1.1 Photoelectrochemistry**

The term "photoelectrochemistry" refers to the area of electrochemistry that studies photoactive electrodes, also called photoelectrodes, exposed to light. Photoactivity is usually achieved by using semiconductor materials that, when irradiated by light, generate an electrochemical reaction that produces an electrical current, also known as photocurrent. This process represents the conversion of light energy into electric and chemical energy. Photoelectrochemical systems are widely studied because they can have potential applications for renewable energy generation and storage [10], as well as environmental applications. After all, their performance can be useful for the design of advanced oxidation processes [11].

The simplest form of a photoelectrochemical cell, illustrated in **Figure 1**, consists of a photoelectrode and a metallic electrode. A photoelectrode can behave as anode or cathode depending on the semiconductor nature. It is more common for photoanodes, made with n-type semiconductors, to be used because photocathodes, made with p-type semiconductors, tend to corrode in solution. In photoanodes, the voltage generated by the production of electron–hole pairs drives the photocurrent from the anode to the cathode. The most studied application

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

**Figure 1.** *Photoelectrochemical cell with photoanode.*

of photoanode devices is water splitting because they can store solar energy by generating hydrogen [10].

Another configuration of photoelectrochemical cells consists of two photoelectrodes, allowing the full use of sunlight. As the Fermi level of the photoanode, due to its n-type semiconductor properties, is higher than that of the photocathode, which tends to have a lower Fermi level due to its p-type semiconductor properties, the generated photocurrent between the electrodes is enhanced and stronger redox abilities of the electrons and holes on each photoelectrode can be achieved. Used as an advanced oxidation process, these systems are also known as photocatalytic fuel cells, and they are also widely studied as an environmentally friendly technique to oxidize nonbiodegradable compounds in polluted water [11].
