Direct Methanol Fuel Cells (DMFCs

Direct methanol fuel cells are a kind of fuel cell, that consists of a polymer electrolyte membrane and operates at a temperature range between 70 and 100°C. The fuel used in DMFCs is liquid methanol, which is dissolved in water. Similar to PEMFCs, the DMFCs are also eco-friendly fuel cells. The polymer electrolyte ion-exchange membrane is the main part of DMFC, which is in direct contact with both anode and cathode, each of which consists of a three-layer structure consisting of a catalytic layer, a diffusion layer, and a backing layer [84]. The combination of platinum at the cathode and a platinum-ruthenium alloy at the anode, along with an ionomer, combines to form the catalytic layer. The ionomer membrane is composed of a perfluoro sulfonic acid polymer. The mixture of carbon and Teflon combines to form the diffusion layer which allows both the transportation of oxygen molecules to the catalyst layer of the cathode and the escape of CO, molecules from the anode.

DMFCs are simple in design and quite quiet, and consist of environmentally friendly technology without the hazard of explosions, making them appropriate for portable applications and generators. DMFCs also have applications in both civil and military environments, although DMFCs show the lowest efficiency, at 35%, of the fuel cells; this is the only drawback associated with DMFCs.

Alkaline Fuel Cells (AFCs

Alkaline fuel cells can work under different operative temperatures, ranging from 30°C to 250°C, which makes them useful in various areas like the space sector. The electrolyte used in the alkaline fuel cell is a solution of potassium hydroxide, with pure oxygen as the oxidant. Depending on the design and ability of electrolytes, each cell generates a voltage between 0.5 V to 0.9 V and an efficiency of up to 65% [84]. In addition, the AFCs have a simple cell structure, can perform at different operative temperatures, have high electrical efficiency, and require less maintenance.

AFC consists of a porous anode and a porous cathode, made with a nickel or silver catalyst, and these electrodes are separated by a liquid KOH solution, which acts as the electrolyte. In an AFC cell, oxygen is continuously fed into the cathode part, and hydrogen is continuously fed into the anode part. An electric current is developed when the ions are transported between the cathode and the anode [88]. Potassium titanate, ceria, asbestos, and zirconium phosphate gel are also being used in the microporous separator for AFCs, although asbestos is carcinogenic and it is not being used nowadays [88]. Based on the type of electrolyte, AFCs are classified as follows;

  • (i) Mobile electrolyte AFCs: In a mobile electrolyte AFC, the electrolyte is pumped inside the cell via an external circuit. The anode and cathode parts are hydrogen and air, respectively. The reaction between KOH and CO, in the air presents the main challenges facing mobile AFCs. This problem can affect the overall efficiency of an AFC.
  • (ii) Static electrolyte AFCs: In the case of a static electrolyte AFC, the anode and cathode are separated by an electrolyte that is held inside the asbestos. Injection of pure oxygen inside the cathode is necessary for the working of the AFC.
  • (iii) Dissolved fuel AFCs: The electrolyte w'hich separates the anode and cathode is combined with a fuel like hydrazine or ammonia; as a result, this type of AFC cannot be used for large pow'er generators. Also, it uses hydrazine, which is toxic, carcinogenic, and explosive.

Phosphoric Acid Fuel Cell (PAFC

Phosphoric acid fuel cells are the first type of fuel cell to be used and commercialized inside power applications. The development and growth of PAFCs occurred in the period 1960-1970. As the name of the fuel cell indicates, phosphoric acid is the electrolyte which is used inside the PAFC. For better function of PAFCs, the temperature must be maintained between 150 to 200°C due to the poor ionic conductivity of phosphoric acid. Pure hydrogen is not required in PAFC, however, the process takes place at at a high-temperature range. PAFC offers an overall efficiency between 37% and 42% [86].

The overall structure is made of a ceramic matrix of thickness 0.1-0.2 mm in which the porous electrodes are separated by the electrolyte, phosphoric acid. A platinum nanocatalyst is used to develop the diffusion electrodes and is then functionalized by high surface carbon, dispersed inside a layer made of carbon bonded with polytetrafluoro ethylene (PTFE). The phosphoric acid electrolyte provides the high thermal, chemical, and electrochemical stability required in order to be actively used inside the cell. Furthermore, the phosphoric acid does not react with C02, minimizing the problems associated with carbon monoxide and carbonate [89]. The characteristics of PAFCs includes:

  • • The operative temperature is between 150°C and 220°C, with an operating life cycle of more than 65,000 hours
  • • PAFCs achieve an efficiency of up to 40% and are capable of extending the efficiency up to 60% by means of combined heat systems
  • • There is less chance of carbon monoxide poisoning
  • • The se of PAFC reduces the cost of power generation.

Molten Carbonate Fuel Cells (MCFC

The base structure of MCFC is made of a ceramic matrix in w'hich the porous anode electrode is fueled by hydrogen and the porous cathode is usually fueled by oxygen. Two mixtures of molten carbonate salts, such as a combination of either lithium carbonate and potassium carbonate or lithium carbonate and sodium carbonate could be used as electrolytes. Ceramic powder and fibers are used inside the matrix to enhance the entire mechanical strength. Usually, nickel is used as the catalyst instead of an expensive catalyst like platinum [90, 91]. The cell operates at a temperature range of 650°C, since it needs high operative temperatures. Upon reaching this temperature, carbonate salts begin to melt and become conductive by carbonate ions (CO,2 ). These ions are then transported from the cathode to the anode and are collected at the anode part which leads to the generation of an electric current. The advantages of characteristics of the molten carbonate fuel cell are:

  • • MCFCs are capable of achieving an efficiency of up to 45%.
  • • Use of MCFCs reduces the costs of power generation
  • • Use of stainless steel and nickel-based alloys results in the reduction of the production cost of MCFCs

Microbial Fuel Cells (MFC

Microbial fuel cells (MFC) are also known as biological fuel cells. They are bio- electrochemical devices that convert chemical energy to electrical energy through electrochemical reactions, which involve bio-enzymatic catalysis and biochemical pathways. Here, organic matter is converted into electricity, using bacteria as the catalyst, providing the possibility of using a wide range of microbe-degradable organic or inorganic matter such as organic waste and soil sediments. The microorganisms carry out glycolysis, citric acid cycle, etc. which generate electrons and protons that are used for the generation of electricity. Microbial fuel cells operate at ambient temperature and atmospheric pressure and are currently used for generating energy from organic matter [92].

An MFC consists of three major components, namely an anaerobic anode chamber, an aerobic cathode chamber, and a separator connecting the two chambers. A simple schematic representation of MFCs is presented in Figure 1.11. The growth and the electron extraction from microorganisms take place in the anode chamber. Protons and electrons are produced by means of oxidative microbial metabolism in this chamber. Electrons are transferred by microbes to the anode part and flow to the cathode through a resistor, leading to the production of electricity.

An unmediated MFC is a type of microbial fuel cell that was developed in the 1970s, and bacteria in this type of MFC have electrochemically active redox proteins,

Structure and working principle (including the Krebs cycle) of microbial fuel cells

FIGURE 1.11 Structure and working principle (including the Krebs cycle) of microbial fuel cells.

such as cytochromes, on their outer membrane that can transfer electrons directly to the anode. Carbon dioxide, protons, and electrons are produced by organic electron donors in most of the MFCs. Sulfur compounds and hydrogen are the other reported electron donors. Since MFCs require very low power for action, this which makes them suitable for use in power generation applications as well as in wireless sensor networks, because MFCs use energy more efficiently than do standard internal combustion engines, which are limited by the Carnot cycle. Also, MFCs can be used in a biosensor for monitoring BOD [biological (biochemical) oxygen demand] values because an MFC- type BOD sensor can provide real-time BOD values.

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