U.S Department of Energy (2010) provides the description of different fuel cell technologies. The fuel cell technologies are differentiated according to their efficiency, operating temperatures, costs and application. The classifications are based on 6 major groups:
Alkaline fuel cell (AFC),
Phosphoric acid fuel cell (PAFC),
Solid oxide fuel cell (SOFC),
Molten carbonate fuel cell (MCFC),
Proton exchange membrane fuel cell (PEMFC);
Direct methanol fuel cell (DMFC).
Alkaline fuel cell (AFC)
The AFC generates electrical power using alkaline electrolyte KOH (potassium hydroxide) in water-based solution. The presence of hydroxyl ions within the electrolyte allows a circuit to extract electric energy. The illustration in Fig 2 reveals an alkaline fuel cell. As being revealed in Fig 2, two hydrogen gas molecules combines with 4 hydroxyl ions have a negative charge to release 4 electrons and 4 water molecules. The equation 4 reveals the reaction of oxidation that takes place. (Mark, 2003).
(Oxidation) 2H2 + 4OH H2O + 4e?
Fig. 2. AFC (Alkaline fuel cell)
Source: (Andujar et al. 2009).
Typically, electrons are released in this reaction and reach the cathode and react with water to generate (OH?) ions. Moreover, 2 water molecule and oxygen combine with 4 electrons to form 4 negatively charged hydroxyl ions.
The equation 5 below reveals that reaction:
(Reduction) O2 + 2H2O + 4e OH?
AFC generally performs better at a temperature between 60 and 90 "C. However, recent AFC design operates at temperature between 23 and 70 "C. Generally, AFC is a low cost catalyst, work at low temperature and the AFC electrical efficiency is approximately 60%, however its CHP efficiency is more than 80%, and has ability to generate electricity of up to 20kW.
NASA was the first organization that used AFC to generate electric power and supply drinking water during the space application. Based on the cost effectiveness of AFC, the AFC technology has now been used in boats, submarines, niche transportations, and forklift trucks applications. (Kordesch, 1999). Typically, AFC emits no green house gas and is very useful for space shuttle fleets and spacecrafts and operate with 70% efficiency. Despite the benefits derived from AFC, the technology could be easily poisoned with carbon dioxide. For example, when alkaline solution (KOH) in AFC electrolyte absorbs CO2, the chemical reaction will convert into potassium carbonate (K2CO3) which consequently poisons AFC. (Larminie & Dicks 2003). Typically, a small amount of CO2 could affect the cell operations. To make AFC more effective, there is a need to carry out the purification process. However, the purification process is very costly. AFC needs to be more cost effective to be effective used for commercial purpose.
Phosphoric acid fuel cell (PAFC)
PAFC uses H3PO4 (liquid phosphoric acid) electrolyte and carbon paper electrodes. The H3PO4 consist of:
Typically, H3PO4 is a clear colorless liquid used for food flavoring, detergents, fertilizers, and pharmaceuticals. The PAFC could operate at temperature ranging between 150 and 220 "C. The PAFC charge carrier is the hydrogen ion or proton.
"The hydrogen ions pass from the anode to the cathode through the electrolyte and the expelled electrons return to the cathode through the external circuit and generate the electrical current. At the cathode side, water is forming as the result of the reaction between electrons, protons and oxygen with presence of platinum catalyst to speed up the reactions." (Mekhilef et al. 2012 P. 983).
Illustration in Fig 3 reveals the hydrogen that expels at the anode splits into 4 electrons and 4 protons. At cathode, 4 electrons and 4 protons combine to form water as being revealed in equation 6 and equation 7.
Fig. 3. Operating Principle of Phosphoric Acid Fuel Cell
(Oxidation) 2H2 ? 4H+ + 4e?
(Reduction) O2 + 4H+ + 4e H2O
When protons and electrons pass through the electrolyte and the external circuit respectively, the reaction generates heat and electrical current. The heat could be used for heating water, or for steam generation. The PAFC is considered the first generation of fuel cell, and is one of the most mature fuel cells. Moreover, PAFC is the first fuel cell to be used commercially, and it is being used for stationary power generation. PAFC has also been used to power large vehicles.
However, steam reactions within the PAFC produce carbon monoxide (CO), which may poison the fuel cell and reduce the PAFC performance. However, solution to decline the CO absorption is to increase the tolerance of anode temperature. PAFC run on air and could be easily operate with reformed fossil fuels. Moreover, PAFC is very expensive, electrical efficiency of PAFC is between 40 and 50%, and its CHP efficiency is about 85%.
Solid Oxide Fuel Cell (SOFC)
Contrary to other fuel cell technologies, SOFCs high temperature fuel cells containing metallic oxide solid ceramic electrolyte. As being revealed in Fig 4, SOFC uses mixture of carbon monoxide and hydrogen to form air and hydrocarbon fuel. The oxidation process in SOFC is that the oxygen is oxidized and reacts with the cathode at 1000 "C. On the other hand, the fuel oxidation occurs at anode as being revealed in the following equation:
"(Oxidation) (1/2)O2 (g) + 2e
"(Reduction) O2? (S) + H2 (g) ? H2O (g) + 2e?"
Fig 4: Solid -- oxide Fuel Cell
SOFC has ability to generate large-scale power systems reaching the capacity of hundreds of MWs. The byproduct of the heat is used to generate electricity. The SOFC is considered advantageous because it can generate power at areas having no access to public grids. Moreover, the SOFC could be maintained with low costs and deliver noise free operation. The challenges of SOFC are that it produces high temperature corrosion.
Molten carbonate fuel cell (MCFC)
Molten carbonate fuel cells are generally high temperature fuel cell and use molten carbonate salt mixed with electrolyte and suspended in a porous solid electrolyte. The Illustration in Fig 5 presents a MCFC.
Fig. 5: Molten Carbonate Fuel Cell
Illustration in Fig 5 reveals that hydrogen fuel and carbonate ion react to water, carbon dioxide, and electrons. At anode level, reaction of methane Chapter 4 with water produce carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO) as being illustrated in the following equation:
CH4 + H2O ? CO + 3H2
CO + H2O ? CO2 + H2
Moreover, the oxidation reaction is presented in following equation
H2 + CO32
H2O + CO2 + 2e?
(Oxidation 2) CO + CO CO2 + 2e?
However, the reduction occurs at cathode and expels new carbonate ions from carbon dioxide (CO2) and oxygen (O2). Thus, cell voltage and electric current could be collected at electrodes as being revealed in the following equation:
(1/2)O2 + CO2 + 2e
Currently, MCFCs are being employed at coal-based and natural gas power plant and converted to electrical utility. The shortcoming of MCFC is that it is being operated with high operating temperature. However, MCFC does not require infrastructure development to install.
"PEMFC (Proton Exchange Membrane Fuel Cell)"
PEMFCs activate hydrogen by catalyst to form proton ion as well as ejecting electron at the anode. Typically, the protons are able to pass through the membrane and the electrons are forced to the external circuit to generate power. In PEMFC, electron interacts with proton ion and oxygen to form water. Fig 6 illustrates the chemical reactions occurring at PEMFC.
The benefits of PEMFC are that it is low temperature fuel cells and operating at temperature between 60 and 100°C. Typically, PEMFC is a lightweight compact system that is easy and cheaper to manufacture. From the efficient point of few, higher efficiency can be gained from using PEMFC. Typically, PEMFC's electrical efficiency is between 40 and 50% and its output can be as high as 250 kW. Moreover, PEMFC requires minimum maintenance
DMFC (Direct methanol fuel cell)
DMFC is suitable source of power that could be used for portable energy purposes due to its ability to generate low temperature. As being revealed in Fig 7, DMFC is a clean renewable energy from oxygen available in air. Equation 18 reveals that methanol is formed from carbon dioxide (CO2) and cathode steam is formed from oxygen in the air.
Fig. 7: DMFC (Direct methanol fuel cell)
CH3OH + H2O ? CO2 + 6H+ + 6e? (Anode)
(3/2)O2 + 6e? + 6H+ ? 3H2O (Cathode)
The benefit of DMFC is that "it does not have many of the fuel storage problems typical of some fuel cells because methanol has a higher energy density than hydrogen -- though less than gasoline or…