Plant Microbial Fuel Cell

Abstract

Living plants can generate electricity with the help of the microbial fuel cell. This is a sustainable way to generate electricity as there is no chance of environmental pollution. In this chapter, plant microbial fuel cells will be discussed thoroughly including their design and mechanism for sustainable power generation through plant microbial fuel cells. This plant microbial fuel cell can provide the necessary bioenergy and the potential food supply at the same time. If we could get green energy and food together, then it would certainly increase the chance of the probable supply of the two important demands of the world, which are shortages of green energy and food for humanity.

1. Introduction

We need electricity and new sources of electricity in the world as there are around 1.6 billion people that still live in the dark at night. Many countries of the world can produce only half of their electricity demand. The demand for clean sources of energy has been increasing in recent times [1]. The global warming phenomenon is happening due to the excess amount of greenhouse gas emissions for electricity generation from fossil fuels and nonrenewable energies. Fossil fuel, which is not good for the environment, is used all over the world, and the remaining amount of this fossil fuel is decreasing day by day [2]. So, it is very much important for us to find new sources of green energy for a sustainable world. In this perspective, the plant microbial fuel cell (PMFC) is one of the technologies in which bioelectricity can be generated with the help of living plants and microbial actions. PMFC is an ecological solar cell in which solar energy helps plants in photosynthesis to produce carbohydrates [3]. In the photosynthesis process, living plants can use carbon dioxide from the atmosphere. As this is the main source of electricity generation, PMFC is a carbon-neutral option for clean energy production [4]. The soil microbes that live in the root area are very useful for electricity generation as they break the carbohydrates from the trees and can produce electrons [3]. These geobacteria serve as a biocatalyst resource for PMFCs [4]. Through placing an electrode near the roots, the root exudates can be oxidized by electro-active bacteria, which can transfer their electrons from anode to cathode [5, 6, 7]. Recently, the generation of bioelectricity via microbial fuel cells (MFCs) has been gaining notice due to its greener nature and simultaneously resolving the environmental problems. MFCs produce bioelectricity via the metabolic actions of electrochemically active microbes.

PMFCs are microbial fuel cells consisting of living plants, a supporting matrix, and a conductive anode inserted into the substrate and cathode placed in air or water to convert chemical energy into bioelectricity [8, 9, 10, 11, 12, 13]. According to Moqsud et al., two basic designs of PMFCs can be distinguished, namely, aquatic and nonaquatic plant microbial fuel cells [14, 15, 16, 17, 18]. Based on these two designs, other models such as tubular and flat-plate PMFCs have been developed and successfully applied [18, 19, 20, 21, 22, 23, 24, 25].

2. Advantages of plant microbial fuel cell

Plant microbial fuel cell is a sustainable source of electricity because they will not destroy any food products such as corn or soybeans for bioenergy production [26, 27, 28, 29, 30]. Both bioenergy and food can be received at the same time in this innovative system. So, there will be no competition between the green energy source and the food source for humans and animals [31, 32, 33, 34, 35]. Secondly, it is a renewable source of energy as only sunlight is used to generate bioelectricity with the help of living plants. There are trees all over the world, and the photosynthesis of these trees is the main source of energy for this system. Finally, unlike fossil fuel, it is a completely environmentally friendly approach to electricity generation. So, there is no further fear of greenhouse gas emissions and consequent global warming in the future. Plant microbial fuel cells can change the future need for green energy and a sustainable future by achieving sustainable development goals.

The other advantages of plant microbial fuel cells are that it is a carbon-neutral or carbon-negative approach for electricity generation, which is the future goal for energy harvesting [36, 37, 38, 39, 40].

Plant microbial fuel cells are not only a green way to generate electricity but also a safe way to get electricity. So, there is no danger of getting electricity from the living plants. PMFC can be used for wastewater treatment and bioremediation or phytoremediation of the contaminated soil and environment [41, 42, 43, 44, 45].

3. Mechanism of plant microbial fuel cells

The plant microbial fuel cell is a novel technology in which organic matter is converted into electricity using living plants and bacteria in the soil [46, 47, 48, 49, 50]. The green leaves of the plants can generate sugar, which is an organic matter due to photosynthesis. The plants use the carbon dioxide from the air and water from the soil to make the organic matter in the green leaves [51, 52, 53, 54, 55, 56]. However, about 60% of this organic matter has been deposited in the soil near the root zone. This sugar or organic matter is the food of the bacteria that live in the soil. So, the geobacteria and others gather in the root area and eat the sugar/organic matter and increase the number. While breaking down the organic matter, a significant amount of electrons is released from it. The anode and cathode connected with the external circuit can catch this electron and consequently get the electricity. This is the main mechanism of plant microbial fuel cells. Figure 1 shows the mechanism of plant microbial fuel cells.

Figure 1.

Mechanism of plant microbial fuel cell.

4. Types of plants used in PMFC research

Different types of plants have been used in the research so far in plant microbial fuel cells [51, 52, 53, 54, 55, 56, 57, 58, 59, 60]. However, most of the plants are aquatic plants that use a lot of water for their growth such as rice plants [61]. Table 1 summarizes the amount of power generation by using plant microbial fuel cells using different types of plants. It has been seen that it can be used for electricity generation for a long time while using different types of plants. The reason behind it is that bacteria can get food from living plants continuously. However, many factors can affect the bioelectricity production from the plant microbial fuel cells [62]. Weather factors are one of the main factors that can influence bioelectricity generation in cold regions. The bacterical activities are reduced during the lower temperature, and hence, the bioelectricity amount was also reduced [63].

Name of plant species	Duration of operation	Highest Power Density (PD) recorded and reported
Canna indica	140 Days	18 mW/m^2
Canna indica	N/A	320.08 mW/m^3
Typha latifolia	N/A	6.12 mW/m^2
Physcomitrella patens	70 Days	6.7 ± 0.6 mW/m^2
Lolium perenne	70 Days	55 mA/m^2
Oryza sativa	N/A	140 mA/m^2
Sporobolus arabicus	2 Months (60 Days)	120 mW/m^2
Typha latifolia	228 Days	93 mW/m^3
Elodea nuttallii	276 Days	184.8 ± 7.5 mW/m^3
Canna indica	90 Days	5.11 mW/m^3
Phragmites australis	3 months (90 Days)	0.15 mW/m^3
Phragmites australis	160 Days	22 mW/m^2
Spartina anglica	160 Days	82 mW/m^2
Cyperus involucratus R.	5 Days	5.99 mW/m^2
Brassica juncea	30 Days	69.32 mW/m^2
Trigonella foeumgraecum	30 Days	80.26 mW/m^2
Canna stuttgart	30 Days	222.54 mW/m^2
Sedum album	360 Days	0.0024 μW/m^2
Sedum sexangulare	360 Days	0.0084 μW/m^2
Sedum rupestre	360 Days	0.0155 μW/m^2
Sedum hybridum	360 Days	0.092 μW/m^2
Sedum reflexum	360 Days	>0.001 μW/m^2
Sedum kamtschaticum	360 Days	>0.001 μW/m^2
Sedum spurium	360 Days	>0.001 μW/m^2
Acorus tatarinowii	51 Days	21 mW/m^2
Vetiveria zizanioides Nash	N/A	242 ± 10.5 mA/m^2
Puccinellia distans	114 Days	83.7 mW/m^2
Chlorophytum comosum	100 Days	18 mW/m^2
Epipremnum aureum	60 Days	15.38 mW/m^2
Dracaena braunii	60 Days	12.78 mW/m^2
Chasmanthe floribunda	100 Days	0.21 mW/m^2
Papyrus cyperus	100 Days	1.083 mW/m^2
Chlorophytum comosum	100 Days	18 mW/m2
Spartina anglica	105 Days	1.04 mW/m2

Table 1.

Different plants and their ability to generate electricity.

For aquatic plants, the type of water has an influence on the power generation in PMFCs. It can be seen that seawater plants can generate more bioelectricity than freshwater plants. The reason behind this is that seawater is a more electricity-conductive material compared with fresh water. Soil properties are another important factor for bioelectricity generation. The organic soil shows more bioelectricity than the sandy soil as the organic soil contains more microbes and food for bacteria.

5. Future challenges in plant microbial fuel cell

Various technological and feasibility challenges are still there in plant microbial fuel cell application. The low voltage generation and the stability of voltage generation are the two main challenges for the future application of plant microbial fuel cell. Some plants have a short lifespan and can die if there are some lacks water and fertilizers. To resolve these problems, more research will be required. High power generation is another serious problem. Most of the small plants cannot generate high power. So, finding out the best plant is another challenge for practical application of plant microbial fuel cell. Most of the plant species used in the plant microbial fuel cells are wild grass type and do not have good economic value. However, using rice plants and wheat plants can change this situation. Some of the researchers are using vegetable plants and can generate electricity successfully. Another most important factor for future challenge is the internal resistance of the plant microbial fuel cell. Biofilm can be generated on the surface of the anode materials and can reduce the conductivity of the electricity. So, in the future research, this problem should be addressed properly. The second challenge for PMFC research is the standard design. It should be standardized the electrode materials, their shapes, sizes and other electrical properties. Most of the researchers are using different types of materials and different shapes for the electrode materials. This causes a serious problem to compare the data and the future development of this method. So, in some point, the standard materials should be used to support the future global development of this power-generating system. The reported plants, electrode materials, and design are different for plant microbial fuel cell that cause a serious problem for the future development of it. So, some experimental protocol should be developed.

6. Future aspects of PMFC

Plant microbial fuel cell is a promising technology for green energy harvesting. Bioelectricity can be generated from various types of plants, which may change the game of energy demand in the world soon. However, a lot of research is required to fulfill the needs of the optimum design and the sustainability of this system in different weather and different places in the practical application. The future of green energy depends on the final output of power generation and the standard design of the total system.

PMFC can be used in mangrove vegetation areas and remote areas for off-grid power supply.

PMFC can be used in the forest and in the slope areas to get more energy for powering the environmental monitoring sensors.

7. Conclusions

Plant microbial fuel cell is one of the new methods for getting green energy for future generations. Plants are available all over the world, and they can be used as a source of green bioelectricity. This system can change the world’s energy demand and the sustainable source of green energy. PMFC can also help to achieve carbon neutrality in the future. However, there is still a long way to go to use it for a practical purpose. The long life span trees and other plants should be introduced rather than grass type of plants only for the better outcome. PMFC can be a source of green energy in which both food and energy can be achieved simultaneously, which will make this new bioenergy system more sustainable and environment friendly. The effect of natural disasters on bioelectricity generation in PMFCs should be studied further for emergency uses as a source of electricity and green energy supply during the most needed time at evacuation shelters and distressed areas. However, most importantly, 25% of the world’s population is still living in the dark at night due to the lack of electricity, and PMFC can give a “Light of Hope” to them.

Conflict of interest

The author declares no conflict of interest.