2: Recent Progress on Nafion-based Nanocomposite Membranes for
2.2 Fuel Cells
A fuel cell is an electrochemical device that converts chemical energy of a fuel into electrical energy.[2, 5, 6] Fuel cells can be classified based on the type of electrolyte used and are of five main types.[7, 8]
2.2.1 Alkaline Fuel Cells (AFCs)
AFCs operate on compressed hydrogen and oxygen. They generally use a solution of potassium hydroxide (KOH) in water as their electrolyte. Efficiency is about 70% and operating temperatures are 150–200C. The cell output ranges from 300W to 5kW. AFCs require neat hydrogen fuel and platinum as electrode catalysts.
2.2.2 Phosphoric Acid Fuel Cells (PAFCs)
PAFCs use phosphoric acid as the electrolyte. Efficiency ranges from 40–80% and operating temperatures are in the range of 150–200C. These types of cells have outputs of up to 200kW.
PAFCs tolerate a carbon monoxide concentration of about 1.5%, which actually broadens the choice of fuels. For these cells, platinum electrode catalysts are needed and internal parts must be able to withstand the corrosive acid.
2.2.3 Molten Carbonate Fuel Cells (MCFCs)
MCFCs generally use the high temperature metal carbonate salts, such as sodium or magnesium carbonates, as the electrolyte. The operating temperature is about 650C and the efficiency ranges from 60–80%. For these types of cells nickel electrode catalysts are used and are inexpensive compared to a platinum (Pt) based one. However, the high temperature operation limits the materials and safe uses of MCFCs. The reaction products from the electrolyte carbonate ions make it necessary to inject carbon dioxide to compensate.
2.2.4 Solid Oxide Fuel Cells (SOFCs)
SOFCs use metal oxides such as calcium oxide, zirconium oxide, etc., as the electrolyte. The performance efficiency is about 60% and the operating temperature is about 1000C. Cells output is up to 100 kV. One of the biggest advantages of SOFCs is that the electrolyte is solid;
however, the high temperature operation limits the applications of SOFCs.
2.2.5 Proton Exchange/Electrolyte Membrane Fuel Cells (PEMFC)
PEMFCs use a membrane that conducts only protons to the cathode side. They are inexpensive to operate and give the highest efficiency compared to other types of fuel cells discussed above.
These cells operate at a low enough temperature (80C) to make them suitable for homes and cars. However, their fuels must be purified and a platinum catalyst is used on both sides of the membrane, raising costs.
PEM fuel cells can be divided into fuel cells and direct methanol fuel cells (DMFC), depending on the nature of the fuel use. The hydrogen fuel cell uses H2 gas as fuel and provides excellent fuel cell performance and efficiency. While the direct methanol fuel cell uses liquid methanol (MeOH) as a fuel that provides relatively low performance, due to the methanol permeability through the membrane, it prevents the use of a reformer to produce hydrogen.[1–3, 9] The main components of a representative PEM fuel cell are:
Electrodes (anode and cathode)
Membrane electrode assembly (MEA)
Gas diffusion layer (GDL)
Collector graphite plates (CGP)
At the interface between the anode catalyst, which is typically Pt-based, and the electrolyte, fuels are converted into protons (H+) and electrons (e–). The protons enter through a PEM that prohibits electrons to the cathode side. This is the unique property of a PEM, and it only allows protons to diffuse, hence avoiding shorting out the circuit. The electrons (e–) are conducted
through the external circuit and delivers energy to do useful work such as turning a motor on the way to the cathode. At the cathode, the transferred protons and the energy-depleted electron combine with oxygen to form water. Theoretically, any substance capable of chemical oxidation and can be supplied continuously can be used as fuel. Also the oxidant can be any fluid that can be reduced at a sufficient rate. However cost, reactivity, and availability are the key issues in their selection. [9] A schematic diagram of a representative methanol PEM fuel cell is shown in Figure 2.1.
Figure 2.1 A schematic diagram of methanol PEM fuel cell. Reproduced from Ref. [9] with permission from John Wiley & Sons.
The chemical reaction takes place at the surface of the electrodes that are attached to a carbon paper or carbon cloth, called the GDL. The electrochemical reactions for a direct methanol PEM fuel cell are as follows:
Anode : (2.1)
Cathode : (2.2)
Net reaction : (2.3)
For hydrogen PEM fuel cell as follows:
Anode : ΔG° = 0.00 (2.4)
Cathode : ΔG° = –237 kJ/mol (2.5)
Net reaction : ΔG° = –237 kJ/mol (2.6)
The cell voltage is well related to the Gibbs energy change of a chemical reaction as:
ΔG = –nFV (2.7)
where n is the number of electrons involved in the reaction, F is the Faraday constant, and V is the cell voltage for thermodynamic equilibrium in the absence of current flow i.e., open circuit equilibrium.[10, 11]
The most widely used membrane in PEM fuel cells is typically a solid electrolyte called Nafion.
Nafion is a perfluorinated polymer that contains small proportions of sulfonic or carboxylic ionic functional groups. Its general chemical structure can be seen in Figure 2.2, where X is either a
2H 2e H2
O H e 2 2O H 1
2 2 2
O H 2O
H2 1 2 2
H O 6H CO 6e OH
CH3 2 2
O H 3 e 6 H 6 2O
3 2 2
2 2
2
3OH 32O 2H O CO
CH
sulfonic or carboxylic functional group, and M is either a metal cation in the neutralized form or an H+ in the acid form.
Figure 2.2 Chemical structure of Nafion perfluorinated ionomer.
This membrane allows protons to travel through but restrain the electrons from passing through it. The proton transfers through the membrane by virtue of the electric field created across the membrane. The performance of the fuel cell is depicted by current vs. voltage plots, known as the polarization curves. It is evident from the literature that Nafion is the leading candidate for PEM fuel cells, though other membranes such as poly(vinylidene-fluoride) (PVDF), poly(ether- ether-ether-ketone) (PEEK), and poly(tetrafluoroethylene) (PTFE) have also been investigated.[12] In addition to fuel cell applications, Nafion has been widely used in metal ion recovery as a super acid catalyst in organic reactions and different electrochemical devices. [13, 14]
Various properties of Nafion membranes are illustrated in Table 2.1.
Table 2.1 Properties of a standard Nafion membrane
Molecular weight (g/mol) 100 000
Glass transition temperature Tg (K) 339
Density (g/cm3) at 300K 2.074
Dielectric constant 3.04
O
b 2 a
2
2
CF ) ( CFCF ) CF
(