**4. Interactions between algae and electrodes**

### **4.1. Anolyte**

The anolyte used in AFC is rich in carbon source such as glucose formate and acetate being similar to other prepared sources like LB medium, *Scenedesmus* algae in powder form, fruit industry liquid waste and wastewater [9, 11, 20–24]. The factors affecting the generation of power depends on the types of anolytes used and their internal resistance. The efficiency and power production of AFC depend on the resistance of membrane on anolyte, high ion generation in the anodic chamber and oxygen crossover through the membrane. Some of the problems faced by AFC are membrane fouling, high COD and low pH of anolyte. To overcome these problems, membrane pretreatment and continuous monitoring of the internal conditions of the anodic chamber is necessary.

#### **4.2. Catholyte**

The commonly used catholyte in AFC is microalgae. Microalgae in cathode help in reducing the CO<sup>2</sup> emitted from bacterial metabolism, respiration providing economic and environmental sustainability. Blue green algae, *Chlorella vulgaris*, *Desmodesmus sp.*, etc. are some of the microalgae used in the cathode compartment of AFC. *Chlorella vulgaris* is one of the common microalgae which have been studied extensively as a catholyte by many researchers. It is influenced by several factors such as electron consumption by methanogenesis, aerobic respiration by the cathodic biofilm and oxygen crossover which is hindered during COD removal [25]. Moreover, algal biofilms can limit the diffusion of oxygen affecting the performance of AFC [26]. Researchers have reported 92% of COD removal and 90–80% removal of inorganic components with 2.2 mW−3 of power density [27]. The yielded biomass from AFC can be used as animal feed or for energy and bio-product generation [28].

#### **4.3. Electrode material**

Electrode materials play a vital role in AFC because of its overall cost effectiveness and the performance in power generation. Properties such as good electrical conductivity and low resistance, strong biocompatibility, chemical stability and anti-corrosion, large surface area and appropriate mechanical strength and toughness are to be considered in the selection of an electrode material. Commonly used anode materials are graphite plates and rods, carbon fiber brushes, carbon cloth, carbon paper, carbon felt, carbon nanotubes and granulated graphite [17]. Carbon electrode is used extensively due to its low cost when compared to other electrodes. Biofilm helps in trapping the electron with the help of electrode and algal substrate. Therefore, cathode graphite felt coated with platinum, 10% Teflon coated on carbon paper, etc. are preferred to increase biofilm formation on the cathode.

## **5. Membrane**

cost-effective photosynthetic microbial fuel cell design with highly reproducible electrochemical characteristics that can be used to screen algae and cyanobacteria for photosynthetic electrogenic activity. *Paulschulzia pseudovolvox* (*Chlorophyceae*) is identified as good electrogenic

The anolyte used in AFC is rich in carbon source such as glucose formate and acetate being similar to other prepared sources like LB medium, *Scenedesmus* algae in powder form, fruit industry liquid waste and wastewater [9, 11, 20–24]. The factors affecting the generation of power depends on the types of anolytes used and their internal resistance. The efficiency and power production of AFC depend on the resistance of membrane on anolyte, high ion generation in the anodic chamber and oxygen crossover through the membrane. Some of the problems faced by AFC are membrane fouling, high COD and low pH of anolyte. To overcome these problems, membrane pretreatment

and continuous monitoring of the internal conditions of the anodic chamber is necessary.

The commonly used catholyte in AFC is microalgae. Microalgae in cathode help in reducing

Electrode materials play a vital role in AFC because of its overall cost effectiveness and the performance in power generation. Properties such as good electrical conductivity and low resistance, strong biocompatibility, chemical stability and anti-corrosion, large surface area and appropriate mechanical strength and toughness are to be considered in the selection of an electrode material. Commonly used anode materials are graphite plates and rods, carbon fiber brushes, carbon cloth, carbon paper, carbon felt, carbon nanotubes and granulated graphite [17]. Carbon electrode is used extensively due to its low cost when compared to other electrodes. Biofilm helps in trapping the electron with the help of electrode and algal substrate. Therefore, cathode graphite felt coated with platinum, 10% Teflon coated on carbon

 emitted from bacterial metabolism, respiration providing economic and environmental sustainability. Blue green algae, *Chlorella vulgaris*, *Desmodesmus sp.*, etc. are some of the microalgae used in the cathode compartment of AFC. *Chlorella vulgaris* is one of the common microalgae which have been studied extensively as a catholyte by many researchers. It is influenced by several factors such as electron consumption by methanogenesis, aerobic respiration by the cathodic biofilm and oxygen crossover which is hindered during COD removal [25]. Moreover, algal biofilms can limit the diffusion of oxygen affecting the performance of AFC [26]. Researchers have reported 92% of COD removal and 90–80% removal of inorganic components with 2.2 mW−3 of power density [27]. The yielded biomass from AFC can be used

qualities among several cyanobacteria [19].

**4.1. Anolyte**

94 Microalgal Biotechnology

**4.2. Catholyte**

**4.3. Electrode material**

the CO<sup>2</sup>

**4. Interactions between algae and electrodes**

as animal feed or for energy and bio-product generation [28].

paper, etc. are preferred to increase biofilm formation on the cathode.

The membrane is the heart of this system which is highly expensive. This results in the increase of the overall cost of AFC. Membranes act as a separator for the anode and cathode compartments. The substrate that is used in this system tends to produce electrons and protons which are passed through the membrane for the separation of specific ions. Though the membranes are used as a barrier, it has some issues. The motion of ions from the anode to cathode chamber slowly increases the protons in the anode chamber and the negatively charged ions in the cathode chamber. This results in low and high pH in anode and cathode.

The overall performance of AFC can improve by a membrane separator having micellar porous structure separating the specific ions from anode chamber to cathode chamber. Proton exchange membrane and electron exchange membrane are the most preferred membranes due to their superior conductivity properties. But these are unsuitable for high power scale application due to their need for hydration and high cost. Some of the studies have explored the use of alternative membranes of low cost which are: cation exchange membranes such as sulfonated polyether ether ketone, sulfonated polystyrene-ethylene-butylene-polystyrene, CMI-7000 and Hyflon ion, anion exchange membranes such as AMI-7000, salt bridges and porous materials such as J-Cloth, glass fiber filters, nylon, nonwoven cloth, earthenware pot, ceramic, terracotta, compostable bags and latex glove. The use of these inexpensive membranes occasionally causes difficulties like high internal resistance.

### **6. Influence of carbon dioxide**

The healthy growth of algae in AFC is essential for efficient power production which is influenced by growth media, nutrient supplement and CO<sup>2</sup> . The optimal growth of microalgae is achieved when the cathodic chamber is bubbled with CO<sup>2</sup> or by diverting CO<sup>2</sup> produced in the anodic chamber which concludes that the microalgae is able to fix CO<sup>2</sup> by consuming the inorganic carbon in cathodic chamber and CO<sup>2</sup> produced in the anionic chamber which permeates through the membrane [23, 29]. The micro-algae also prefer to use CO<sup>2</sup> in the presence of light and organic carbon the result of which is the production of daytime electricity depending on the organic loading rate and light irradiation. In some cases, a higher concentration of CO<sup>2</sup> causes adverse effect on algae during the early stages of growth. The dissolved CO<sup>2</sup> eventually decreases the pH of the electrolyte and this pH of the algal inoculums must be high initially to overcome. Apart from this, CO<sup>2</sup> concentration also affects the lipid content of microalgae. The cells produce polyunsaturated fatty acids under high CO<sup>2</sup> concentrations. A 6% lipid content increase was observed accompanied by a 10–15% increase in CO<sup>2</sup> supply [30].

### **7. Influence of light source**

Algae and higher plants contain two major photosynthetic systems in thylakoid membrane. They are classified as photosystem I and photosystem II containing chlorophylls and carotenoid pigments respectively for light energy absorption [31, 32]. The chlorophyll pigment adsorbs wavelength between 650 and 750 nm in the red region while carotenoids pigment absorbs wavelength between 450 and 500 nm in the blue region. This mechanism of transferring excitation energy by both chlorophyll and carotenoids results in higher efficiency of photosynthesis over a wide range of wavelengths [32]. However, the absorption of wavelength by the pigments depends entirely on the type and history of microalgae [33].

expensive catalyst. The latest development in low cost catalyst like carbon based cathode delivers equivalent performance due to abundant pores and larger specific area. However, the main drawbacks of this porous structure are their low resistance to biological fouling. Therefore, ionic membranes and separators are used in AFC to reduce this effect on proton exchange layers.

Algal Fuel Cell

97

http://dx.doi.org/10.5772/intechopen.74285

Biofouling is caused by the bacteria attached to the surface of catalyst layer that releases extracellular polymeric substances. Biofouling on catalyst layer is similar to biofilm on membranes and separators. It is a thick layer developed on carbon based cathode that increases the diffusion resistance responsible for the declined performance during the long term. Further, it also decreases the activity of dopants on the surface of catalyst layer by the combined effect of biofilms with salt deposition. This was evident from the research of Zhang et al. [42] in which improved power density of cathode increase up to 29% was observed after removing the fouling by weak hydrochloric acid. But there are not clear and sufficient demonstrations regarding the individual effect of biofouling located on the surface of the catalyst layer and inside the layer.

Economic success of AFC is directly related to power generation, algal biomass production in combination with other application in a fully biotic cell. Even though there is enormous progress in the research in this area, there are still difficulties in practical applications. The overall power output of the system decreases with the increase in the dimension of AFC. This is mainly due to poor mixing and deprived configuration of electrodes. Laboratory scale reactors having a capacity less than 50 mL relatively generate high power densities greater than 500 Wm−3 whereas configurations having larger than 2 L normally produce a power density less than 30 Wm−3. The energy data of AFC are generally expressed in normalized energy recovery expressed in kilowatt hours per cubic meter based on the volume of reactor. Simple anode substrate produces more electricity than complex substrate due to easy degradation pathways. For instance, acetate produce much higher power densities than glucose (<0.03), sucrose (−< 0.01) and wastewaters (<0.01) which are complex. Similarly, average normalized energy recovery with acetate, glucose and wastewater are 0.25, 0.18 and 0.04 kWh m−3 respectively [43].

A good separation between the electrodes is necessary to prevent interaction between oxygen diffusion, anolyte, catholyte and other materials. This is facilitated by a solid electrolyte or an oxygen gradient. The commonly used solid electrolytes include cation exchange membrane, anion exchange membrane, proton exchange membrane and other materials like textiles, woven fabrics, eggshell, papers, glass wool, etc. [44]. These materials greatly affect energy recovery, performance and capital cost of AFC. Some of the researches show that ion exchange membranes have a lower normalized energy recovery 0.14 ± 0.40 kWh m−3 when

compared to the membrane-less system which has 0.23 ± 0.46 kWh m−3 (p < 0.05) [43].

Stacking AFC in parallel or series configuration helps to achieve preferred voltage and current output [45]. This shows some encouraging results for the technical feasibility of operating multiple AFCs. It is proven that a stack consisting of 40 identical 20 mL units can achieve an open-circuit voltage of 13.03 V [46]. Similarly, by shuffling the parallel and serial electric connections in a stack an external power management system can extract a power of ~200 mW

**9. Energy analysis**

which can drive a 60-W DC motor [47].

During photosynthesis, light energy absorbed by chlorophyll induces the transfer of electrons and hydrogen ions from water to an acceptor called NADP+ where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with a proton from which electrical power may be generated [34].

Photosynthesis rate can be increased by proper light source and light intensities leading to higher cell growth and generation of electrons. As a result, higher bioelectricities might be observed with an optimized light source installed in photo microbial fuel cells. However, only a few studies focusing on the influence of specific light supply or intensities upon power generation and cell growth of photosynthetic microorganisms have been carried out. Xing et al. [35] found that the exposure of AFC to incandescent light increased power densities by 8–10% for glucose fed reactors and 34% for acetate fed reactors when compared to the reactors operated under dark condition. But, Fu et al. [36] obtained a higher power density and open-circuit voltage when AFC was operated under dark condition by using *Spirulina platensis* as biocatalyst. Yeh et al. [37] had investigated the effect of the type and light intensity of artificial light sources on the cell growth of microalgae *Chlorella vulgaris*. They found that fluorescent light source was effective in indoor cultivation of these microalgae with an overall productivity of 0.029 g dry cell weight L−1d−1 and it was obtained by using light source having a light intensity of 9 W m−2. Similarly, *S. platensis* and *H. pluvialis* cultivated under red LED light condition showed better growth profile [35, 38, 39]. On the other hand, *Nannochloropsis sp.* showed a maximum specific growth rate of 0.64, 0.51, 0.54 and 0.58 d−1 when exposed to blue, red, green and white light respectively [40].
