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FUEL CELLS Understanding
Fuel Cells Fuel cells are an energy producing alternative to conventional combustion engines. In a fuel cell, chemical energy is converted into a usable form of energy as fuel (e.g. hydrogen) and oxidant (e.g. oxygen) are fed continuously to the anode (positive electrode) and cathode (negative electrode) respectively. The electrochemical reaction at the anode produces a flow of electrons to an external circuit as shown in Figure 3.1. The DC current output is then used to supply power to some load, or if AC is needed, an inverter is used. The electrical circuit and chemical process is then completed when hydrogen, oxygen, and the returning electrons are brought together to form water. Listed below are a few terms to help better understand fuel cells: Catalyst – substance that speeds up the chemical process by lowering the amount of energy needed to cause the reaction. Ionization – a compound separating into ions (e.g. 2H2 ŕ 4H+ + 4e-) Anode – electrode where the chemical reaction produces positive ions (material itself can be a catalytic metal e.g. Palladium – Pd, or a mixture of substances to have the catalytic property to speed up the process) Cathode – electrode where negative ions are produced (also can be a catalytic metal or a mixture) Electrolyte – permits only the appropriate ions to pass (positive or negative, e.g. H+, O2-), this means that depending on the electrolyte used only certain ions will conduct through the material – that is to say, H+ may not always conduct through the electrolyte. NOTE: electrolyte does not react with the ions.
Figure 3.1 Diagram of a hydrogen-oxygen fuel cell where the electrolyte conducts H+ ions. How Fuel Cells Operate:
A Three Stage Process The complete fuel cell process may be divided into three stages; two of these stages involve the chemical reactions at the electrodes and the third, being the ion conduction through the electrolyte. The three stages of a fuel cell are as follows: (i)
At the hydrogen electrode: The reaction at the anode will involve the release of electrons from the hydrogen fuel that will then be conducted by the electrode to some load.
2H2 g
4H+ + 4e-
To get the process going, hydrogen fuel is fed to the anode where ionization occurs and releases electrons and H+ ions. The hydrogen arrives as a diatomic gas 2H2 where each adsorbed (ads) hydrogen molecule ionizes into four hydrogen protons (H+) and four electrons (e-). The rate of this process can be increased with the help of a catalyst. Because the chemical reaction at this electrode produces positive ions, it can be thought of as the positive electrode or the anode. The negatively charged electrons are then forced to flow from the conductive electrode (anode) to an external load before re-entering the fuel cell into the cathode (i.e. electrons will diffuse naturally from high concentration to low concentration of electrons). However, the hydrogen ions may or may not conduct through the electrolyte since this will depend on the type of electrolyte used, meaning the hydrogen ions may have to temporarily remain at the anode in a receptive state. (ii) At the
oxygen electrode: Meanwhile, oxygen molecules O2 are diffusing through the oxygen electrode (cathode). According to [3.1], the catalytic surface of the electrode (cathode) facilitates the separation of the adsorbed oxygen molecule (oxygen bonds are broken) into oxygen atoms which are held momentarily into a “receptive” state on the active catalyst. The returning electrons, oxidant, and cathode electrode together causes another reaction to occur where negative ions or products are produced. If these H+ ions are the free moving ions (again, this will depend on the electrolyte used), these positive ions will then be attracted to the negative ions generated at the cathode and will be conducted through the electrolyte and products (i.e. 2H2O) are formed completing the process. If however, H+ ions are not able to travel through the electrolyte, then the reaction at the cathode (negative electrode) must produce the free moving negative ions that will move through the electrolyte to combine with the H+ ions and complete the process. (iii)
Through the electrolyte medium The electronically-insulated (does not conduct free electrons) electrolyte serves as the physical barrier preventing the fuel and oxidant gas streams from directly mixing allowing only the appropriate ions to move freely across this layer of medium. This requires that one of the reactants must be able to form the ionized specie needed to complete the process and form the primary by-product water. Summary: The direct conversion of chemical energy into thermal and electrical energy is facilitated by the electrode-electrolyte structure of the fuel cell. Of the two electrodes (anode or cathode), one will produce the appropriate ions (can be thought of as free moving ions that are either positive or negative and can move through the electrolyte) needed to pass through the electrolyte. These ions generated at this electrode are then attracted to the opposite electrode (cathode or anode, respectively) and are conducted through the electrolyte to complete the process. Background on Fuel Cell Types There are basically five fuel cell types. Table 3.1 provided by [3.2] summarizes the differences in these fuel cells. The following fuel cells are described in order of operating temperature (ranging from 80 to 1000 degrees C): 1. Proton exchange membrane fuel cells (PEMFC) 2. Alkaline fuel cells (AFC) 3. Phosphoric acid fuel cells (PAFC) 4. Molten carbonate fuel cells (MCFC) 5. Solid oxide fuel cells (SOFC) 1. Proton exchange membrane fuel cells (PEMFC) are also known as solid polymer fuel cells operating at a temperature of 80°C, this results in the capability of bringing the cell to its operating temperature quickly. This type of electrolyte also has the advantage of preventing gas crossover like SOFCs. PEM fuel cells are said to be suited for quickly meeting shifts in power demand for applications that vary in output. However, because the membrane must be hydrated, the fuel cell must operate under conditions where the byproduct water does not evaporate faster than it is produced usually less than 120°C therefore water management issues in the membrane are critical for efficient performance. PEM fuel cells also require very pure hydrogen with minimal or no CO (a poison at low temperature) which puts heavy demands on the fuel processing unit. 2. Alkaline fuel cells (AFC) operate around 120°C to150°C using an aqueous solution of potassium hydroxide (KOH) as the electrolyte. Desirable attributes of the alkaline fuel cell include its high performance compared to other fuel cells and its flexibility to use a wide range of electrocatalysts. However, AFCs are intolerant to CO2 which reacts with the KOH and effectively degrades the cell performance. Even the smallest amount of CO2 in the oxidant would have to be scrubbed when considering the alkaline cell. This constraint requires that pure hydrogen and pure oxygen, not air, be used. 3.
Phosphoric acid fuel cells (PAFC) operate
at 200°C using
liquid concentrated phosphoric acid (H3PO4) as the
electrolyte. The relative stability
of concentrated phosphoric acid is high compared to other common acids;
consequently the PAFC is capable of operating at the high end of the acid
temperature range of 200°C.
The advantage of using PAFCs is its ability to tolerate some impurities
in the fuel stream broadening the choice of fuels they can use, but still
requires that hydrocarbon fuels be reformed.
Because of the higher operating environment, the rejected heat from the
cell is high enough in temperature to be used for heating.
Unfortunately, the electrochemical environment within the PAFC at
operating temperature is highly corrosive and can best be avoided by special
operating procedures. 4.
Molten carbonate fuel cells (MCFC)
typically use a mixture of alkali carbonates (Li2CO3
and K2CO3) retained in a ceramic matrix, for an
electrolyte. The high operating
temperature of 650°C
is needed to achieve sufficient conductivity in the electrolyte. The advantages
of having a high operating temperature include:
CHP capability where rejected heat can be recovered and used, flexibility
in the electrocatalyst used, and internal reformation where anode poisoning by
CO and, to a certain degree, by other reformer gas impurities is no longer an
issue. On the other hand,
high-temperature corrosion is a major problem for molten carbonate fuel cells
and requires the use of expensive materials and protective layers.
To date, MCFCs have been operated on hydrogen, carbon monoxide (converted
to hydrogen by rapid water-gas shift reaction inside the cell), natural gas,
propane, and other hydrocarbons when some pre-reforming is applied. 5. Solid oxide fuel cells (SOFC) have the same advantages (e.g. internal reformation, CHP capability) and disadvantages (e.g. thermal stress, unwanted sintering and corrosion) of high temperature operation as that of the MCFC. The electrolyte is comprised of a hard ceramic material of solid zirconium oxide and ytrria (yttrium-stabilized zirconia - YSZ), allowing operating temperatures to reach 1000°C. SOFCs are capable of generating enough power to be used in high-power applications including industrial and large-scale generating stations. Table 3.1 Summary of differences in fuel cell types
Process for Each
Fuel Cell Type Typically, all fuel cells function in the same basic way and are often characterized by the kind of electrolyte used. Note that the ion specie and its transport direction can differ in the electrolyte depending on the type of fuel cell used, influencing the site of water production and removal as shown in Figure 3.2 from [3.3]. The electrolyte may consist of a liquid solution or a solid material. In any case the electron-insulated electrolyte serves the vital function of ionic transfer. To get a better idea of how each fuel cell works, the simple hydrogen-oxygen reaction will be used to describe the process for each fuel cell type. When considering the use of other fuels, e.g. methane, the process would follow the same basic idea. Table 3.2 provides a summary of the ions and electrolyte found in the different fuel cell types. Table 3.3 provided by [3.4] shows the chemical reactions at the anode, cathode, and the overall chemical reaction for each fuel cell type using different fuels (i.e. H2, CO, and CH4).
Figure 3.2 Electrochemical reaction of the different types of fuel cells PEM: Hydrogen gas, 2H2 is fed to the anode and ionized into 4H+ ion and 4e-. These hydrogen ions are the free moving ions and will conduct through the solid polymer electrolyte. At the cathode, a reaction causes O2 molecules to separate into oxygen atoms which are held in a receptive state. The returning electrons meet the hydrogen ions and combine with the oxygen atoms resulting in the formation of water molecules, 2H2O at the cathode. PAFC: Same chemical process as the PEM fuel cell. Hydrogen gas, 2H2 is fed to the anode and ionized into 4H+ ion and 4e-. These hydrogen ions are the free moving ions and will conduct through the phosphoric acid electrolyte. At the cathode, a reaction causes O2 molecules, the returning electrons, and hydrogen protons to combine producing water molecules, 2H2O.
AFC: Hydrogen
is fed to the anode and ionized into 4H+ ion and 4e-.
These hydrogen ions are not free moving ions and will be held in the
receptive state at the anode. At the
cathode, oxygen O2 and water 2H2O plus
returning electrons from the circuit form hydroxide ions 4( MCFC: Formation of water will also occur at the anode. The hydrogen ions produced at the anode will not conduct through the electrolyte. The hydrogen gas fed to the anode ionizes into 4H+ ion and 4e-. The cathode process combines oxygen O2 and 2CO2 from the oxidant stream with electrons entering the cathode to produce carbonate ions 2CO3-2 which enter the electrolyte. The free moving carbonate ions conduct through the electrolyte and combine with the hydrogen ions at the anode forming water 2H2O and carbon dioxide 2CO2. The overall reaction shows that no extra carbon dioxide was produced since we put in 2CO2 at the cathode and got out 2CO2 at the anode. SOFC: Again, we have formation of water at the anode. Hydrogen gas is fed to the anode and ionized into 4H+ ion and 4e-. These hydrogen ions are not free moving ions and will be held in the receptive state at the anode. The cathode process with electrons entering the cathode forms oxygen ions 2O-2. The free moving oxygen ions then conduct through the electrolyte and combine with the hydrogen ions at the anode forming water molecules 2H2O. Table 3.2 Fuel cell type and ion-electrolyte summary Type
ions
Electrolyte PEM H+ solid polymer (e.g. Nafion) PAFC H+ phosphoric acid (H3PO4)
Alkaline
Solid Oxide O-2 yttrium-stabilized zirconia (YSZ) Molten Carbonate CO3-2 lithium and potassium carbonate mixture (Li2CO3 and K2CO3)
REFERENCES [3.1] D. Booth.
Understanding Fuel Cells. Home
Power, June-July 1993. [3.2] Renewable
Energy World. Renewable fuel cell
power from biogas.
http://www.jxj.com/magsandj/rew/2001_06/renewable_fuel_cell.html.
James & James Ltd, Nov-Dec 2001. [3.3]
Smithsonian
Institution. A
Basic Overview of Fuel Cell Technology.
http://fuelcells.si.edu/basics.htm. [3.4]
EG&G Services Parsons, Inc. Fuel
Cell Handbook 5th Ed. National
Energy Technology Laboratory, 2000. [3.5] K. R. Williams. An Introduction to Fuel Cells. Elsevier Publishing Company, 1966. Fuel Cell Potential The open circuit voltage is the maximum operating voltage
(when no current is flowing) of a fuel cell and is determined by the chemical
thermodynamics of the overall cell reaction.
The Nernst equation provides a relationship between the standard
potential (
Equation (4.1) is the general form of the Nernst equation for the overall cell reaction and it can be used to determine the open circuit voltage of any fuel cell.
where:
F = Faraday’s number, 96487 C/equiv
R = universal gas constant, 8.314 J/ °K-mol
T = absolute temperature of cell, °K
n = the number of electrons involved (equiv/mol) x, y = stoichiometric coefficients The open circuit voltage ET of
the cell by convention is measured as the potential difference between
the potential drop at the cathode and
Example 1:
We will leave the subscript T off momentarily for convenience purposes
only, but we will continue to associate the potential to some temperature T.
Using the hydrogen-oxygen fuel cell as an example, the standard potential
of the overall reaction of the cell has been divided between the two electrodes
such that
Anode: 2H2 D 4H+ + 4e- Cathode: O2 + 4H+ + 4e- D 2H2O Overall: 2H2(g) + O2(g) g 2H2O Using the equilibrium reaction at the anode and Equation (4.2) gives us
This is equivalent to,
Similarly, using the equilibrium reaction at the cathode and Equation (4.1):
This is equivalent to,
From Equation (4.2), E is the difference between Equation (4.4) and Equation (4.3):
Simplifying to,
This is equivalent to,
Alternatively, it can also be shown that the open-circuit voltage E between the electrodes of an ideal fuel cell can be directly obtained from the overall cell reaction using the Nernst equation assuming the number of electrons involved in the reaction is known. Example 2: From the overall cell reaction of the hydrogen-oxygen fuel cell and Equation (4.1) gives us:
This simplifies to,
Note that Equation (4.6) is exactly the same as Equation (4.5). By applying the Nernst equation, Equation (4.1), to an overall cell reaction, the open circuit voltage of any fuel cell can be determined. Table 4.1 provides the Nernst equations for various fuel cell reactions [4.2]. Table 4.1 Overall fuel cell reactions and the corresponding Nernst Equations
Standard Potential at other Temperatures The following section is the general procedure in which we can find the standard potential of a cell at any temperature. Once the standard potential is computed at the desired temperature it can then be used in the Nernst equation to compute the open circuit voltage at that temperature when the species involved are not at unit activity. When the ratio of activity of all the
species are at unity Preactants/Pproducts
= 1, then the open circuit voltage is equal to the standard potential of the
cell, that is
Solving
for
where:
Using the Matlab
script (energy_density.m), when the overall reaction occurs as shown in
Equations (2.7) and (2.8), the ideal standard potential for different fuel cell
types at their operating temperature can be obtained and are given in Table 4.2.
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