The Chemistry of Fuel Cells
Fuel cell can be described as it can directly convert the chemical energy that is carried by hydrogen fuel into electrical energy and it is suggested that as the final step in a hydrogen economy. Moreover, fuel cells are unique in terms of the variety of potential application, from small electronic devices up to power plants.
The first fuel cell was developed around 1840 by Welsh scientist Sir William Grove, but it was until the beginning of space travel in the 1960s that fuel cells saw their application. It was used to generate electricity and drinking water. Moreover, in 2007 the Nobel Prize in Chemistry was awarded to Gerhard Ertl for groundbreaking studies in surface chemistry, which is important to understand many phenomenon, including how fuel cells function.
Basically, fuel cell consist of 2 catalytic electrodes separated by an electrolyte, and there are many combinations of fuel (hydrogen, alcohol, hydrocarbons) and oxidant (air, chlorine, chlorine dioxide) exist. Unlike batteries, they area an open system in which the consumed reactants can be replenished. Additionally, the electrodes are catalytic and relatively stable.
Fuel cells can be classified primarily by the kind of electrolyte they employ. The uses of electrolytes determines a number of factors:
Generally, the component of fuel cell can be described as a schematic diagram above. Firstly, flow plates are used to maximise the flow of fuel and oxidant to the electrode. Hence, the power can be calculated as the product of the electrode area, the current density of the cell, and cell voltage. After that, the cell efficiency is affected by a number of factors:
Typical cell voltage (around 0.7 V) is too low for practical applications, so cells area stacked together. Then, the flow plates should have high conductance and act as gas separator between the cells and endplates are used to collect current. Hence, the stack power or voltage is simply the product of the number of cells and individual cell power or voltage. Later on, we will see the different types of fuel cell that mainly used in these days.
Basically, PEM is a thin, solid organic compound with the consistency of plastic and typically as thick as 2-7 sheets of paper and it has functions as an electrolyte, and it must be kept moist. One of the example of PEM is nafion and it has advantages such as highly conductive to cations and it resists chemical attack, and the structure is shown below.
Then, the anode is the negative electrode at which oxidation takes place and it is composed of Pt catalyst particles uniformly supported on C particles. Besides that, it is a porous electrode, so that hydrogen (or fuel) can pass through it. The cathode, which is the positive electrode at which reduction takes place also made from Pt catalyst supported on C particles, and as well it is porous to allow oxygen pass through it. Lastly, the flow plates transport hydrogen and oxygen to anode and cathode, and as well to remove water and heat. The reactions in PEM fuel cell are:
Hence, the overall reaction is shown below and it has standard e.m.f. is 1.23 V.
Moreover, the process when the fuel cell is connected to external circuit is shown below.
Then, PEM fuel cells are also several advantages and disadvantages as show below.
Advantages:
This type of fuel cell uses methanol as the fuel, instead of hydrogen; but DMFC has similar design to PEM cell. The anode is made from Pt catalyst on carbon and the anode is made from Pt/Ru catalyst on carbon. Generally, the electrode is still made from noble metals. Then, the reactions that occur in the cell are:
Hence, the overall reaction is:
The advantages of DMFC are it uses a liquid fuel, so it is easier to handle compare to the previous fuel cell which uses the hydrogen gas. Besides that, It also has similar design to PEM fuel cell. In the other sides, the disadvantages are it is high cost due to the noble metal electrode and also it has low efficiency (approximately 0.3 V less than PEM fuel cell) due to methanol leakage through membrane. Despite the disadvantages, DMFCs are used for electrical portable devices, such as laptop or MP3 player. Besides that, one of the type of DMFC, ultraCell XX25™ is used for the power source for computing, communications and sensing devices that are used in mobile and remote operations - such as military missions, emergency and disaster response, remote surveilance, and field research and exploration. It can delivers up to 25 W of continuous maximum power and its weight is 1.24 kg (2.7 lbs).
Alkaline fuel cells use aqueous potassium hydroxide solution, instead polymeric thin layer as PEM and DMFC use. The electrode in this cell is no longer using the noble metal, but it uses non-precious metal and Ni catalyst on carbon black and it uses hydrogen gas as the fuel. Then, the reactions that occur in this cell are:
Therefore, the overall reaction is:
Then, the advantages of alkaline fuel cells are it has high energy efficiency (up to 60%) and it has low operating temperature, approximately 25 - 80 °C. In the other sides, the disadvantages are the relatively low stability. The current systems have an operation lifetime of around 8 000 hours and it is required to exceed 40 000 hours to be viable in large-scale utility applications. Then, there is also a problem for the electrolyte which is easily poisoned by carbon dioxide, which reacts with electrolyte to form potassium carbonate. Lastly, the purification of hydrogen and oxygen to remove carbon dioxide is very costly. Hence, it can applied for the under the sea mission or space exploration.
In this cell, the electrolyte is liquid phosphoric acid which is contained in a teflon-bonded silicon carbide matrix. Then, the electrodes that is used in this fuel cell is porous carbon electrode containing a finely-dispersed Pt catalyst. After that, the reactions that occur in this fuel cell are:
Hence, the overall reaction is:
The advantages of this cell are:
However, in recent year Volkswagen have recently been developing a phosphoric acid fuel, but it still has slow start-up. In the other sides, there is significantly more powerful than current PEM fuel cells. The reason is the higher operating temperature, so there is a 33% size and weight reduction in cooling requirements. The VW's acid fuel cells could be a fifth more efficient than the water variety. The special membrane is soaked in phosphoric acid, then assembled in prototype fuel cell.
Solid oxide fuel cells use hard non-porous ceramic compounds of metal oxides such as ZrO2, CaO, CeO2. The anode is made from solid electrode, typically yttria-stabilised zirconia doped with Ni and the cathode is made from lanthanum manganite (LaMnO3) doped with rare earth metals (e.g. Sr, Ce, Pr). Hence, the reactions in this cell are:
and the overall reaction is:
Besides that, as the character of this fuel cell the advantages are:
In the other sides, the disadvantages are:
Hence, this fuel cell is suitable for static power plants and, due to their fuel flexibility, as auxiliary power plants in vehicles.
The carbonate fuel cells uses molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAlO2) matrix. Then, the anode is made of Ni with Cr or Al additives and the cathode is made of NiO with Mg or Fe additives. Besides that, the reactions that occur in this cell are:
Hence, the overall reaction is:
From this cell, the advantages are:
In the other sides, the disadvantages are high temperature operations speeds up corrosion in cell and results in a slow start-up, and the electrolyte is corrosive and it has complex management.
Hence, it can be applied as static power plants with power output approximately 2MW and also it is used for power source of ships. The advantages of this fuel cell for surface ships are:
Moreover, U31 "Wittenberge", type 212A submarine of the German Navy is powered by one diesel engine and an electric motor driven by nine fuel cells, it has advantage as making it harder to detect. Besides that, Hellenic Navy Class 214 non-nuclear submarine is developed to use a silent operating fuel cell plant that runs on nine 34 kW Siemens SINAVYCIS polymer electrolyte membrane (PEM) hydorgen fuel cells.
The efficiency of fuel cells is considerably superior to conventional internal combustion engines. Fuel cell is exhaust free and, beyond electricity, generate only water and heat. Besides that, due to low operating temperature of maximum of 80 °C, the modules radiate very little heat. Lastly, the comparatively low reaction consumption enables the submarine to remain submerged significantly longer.
To sum up, fuel cells are beginning to show great promise as electrical energy sources for small and large scale applications, as well to provide clean and efficient energy source. However, it still need a reliable and clean hydrogen (or other fuel) source. Hence, there is significant interest in developing the technologies for future application. Moreover, the table below is to summarise the type of fuel cells that we have discussed in this section.
 |
| Sir William R. Grove |
 |
| Grove's gas voltaic cell (early fuel cell) and fuel cell for shuttle orbiter |
Fuel cells can be classified primarily by the kind of electrolyte they employ. The uses of electrolytes determines a number of factors:
- kind of chemical reactions that take place,
- temperature range in which the fuel cell operates
- catalysts required.
 |
| Schematic diagram for single fuel cell |
Generally, the component of fuel cell can be described as a schematic diagram above. Firstly, flow plates are used to maximise the flow of fuel and oxidant to the electrode. Hence, the power can be calculated as the product of the electrode area, the current density of the cell, and cell voltage. After that, the cell efficiency is affected by a number of factors:
- Hydrogen crossover through electrolyte
- Some energy is converted to heat
- Catalyst poisoning
- Blocking of electrode pores (e.g. water)
Typical cell voltage (around 0.7 V) is too low for practical applications, so cells area stacked together. Then, the flow plates should have high conductance and act as gas separator between the cells and endplates are used to collect current. Hence, the stack power or voltage is simply the product of the number of cells and individual cell power or voltage. Later on, we will see the different types of fuel cell that mainly used in these days.
A. Proton Exchange Membrane (PEM) fuel cells
Advantages:
- Relatively low operating temperature, around 80 °C, so it allows quick start-up.
- Solid electrolytes reduces corrosion and electrolyte management problems.
- the noble-metal catalyst (Pt) is expensive.
- The catalyst is highly sensitive to fuel impurities.
 |
| Fuel cell powered London taxi |
B. Direct Methanol fuel cells (DMFCs)
This type of fuel cell uses methanol as the fuel, instead of hydrogen; but DMFC has similar design to PEM cell. The anode is made from Pt catalyst on carbon and the anode is made from Pt/Ru catalyst on carbon. Generally, the electrode is still made from noble metals. Then, the reactions that occur in the cell are:
 |
| UltraCell XX25™ |
C. Alkaline Fuel Cell
Alkaline fuel cells use aqueous potassium hydroxide solution, instead polymeric thin layer as PEM and DMFC use. The electrode in this cell is no longer using the noble metal, but it uses non-precious metal and Ni catalyst on carbon black and it uses hydrogen gas as the fuel. Then, the reactions that occur in this cell are:
D. Phosphoric Acid Fuel Cells
In this cell, the electrolyte is liquid phosphoric acid which is contained in a teflon-bonded silicon carbide matrix. Then, the electrodes that is used in this fuel cell is porous carbon electrode containing a finely-dispersed Pt catalyst. After that, the reactions that occur in this fuel cell are:
- when it is used for the co-generation of electricity and heat (operating temperature 175 - 200 °C), an overall energy efficiency of 80% is achieved.
- It is tolerant of impurities such as carbon dioxide that are present in reformed hydrogen. These impurities poison PEM-based fuel cells.
- The technoloy is well developed (in excess of 200 units sold).
- Expensive as PEM fuel cells due to Pt catalyst is used.
- Less efficient at generating electricity alone, having efficiency of around 40%.
- Less powerful than other fuel cells, given the same weight and volume.
However, in recent year Volkswagen have recently been developing a phosphoric acid fuel, but it still has slow start-up. In the other sides, there is significantly more powerful than current PEM fuel cells. The reason is the higher operating temperature, so there is a 33% size and weight reduction in cooling requirements. The VW's acid fuel cells could be a fifth more efficient than the water variety. The special membrane is soaked in phosphoric acid, then assembled in prototype fuel cell.
E. Solid oxide fuel cells
Solid oxide fuel cells use hard non-porous ceramic compounds of metal oxides such as ZrO2, CaO, CeO2. The anode is made from solid electrode, typically yttria-stabilised zirconia doped with Ni and the cathode is made from lanthanum manganite (LaMnO3) doped with rare earth metals (e.g. Sr, Ce, Pr). Hence, the reactions in this cell are:
- 50-60% efficient at converting fuel to electricity
- High temperature operation (750 - 1000 °C) removes the need for expensive catalyst
- Sulphur resistant and not poisoned by CO, which can even be used as a fuel
- Capturing heat (for example to fire a gas turbine) can result in overall energy efficiencies in excess of 80%.
In the other sides, the disadvantages are:
- High temperature operation means a slow start-up and significant thermal shielding required.
- Ceramic materials tend to be brittle and can crack under thermal stresses.
Hence, this fuel cell is suitable for static power plants and, due to their fuel flexibility, as auxiliary power plants in vehicles.
F. Molten carbonate fuel cells
The carbonate fuel cells uses molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAlO2) matrix. Then, the anode is made of Ni with Cr or Al additives and the cathode is made of NiO with Mg or Fe additives. Besides that, the reactions that occur in this cell are:
- Electrical efficiencies approaching 60%.
- High temperature operation (650 °C) means a variety non-precious metal catalysts can be used at the electrodes.
- Capturing heat (for example to fire a gas turbine) can result in overall energy fuel efficiencies as high 80%.
- Not prone to CO or CO2 poisoning. Moreover, can be used as fuels.
In the other sides, the disadvantages are high temperature operations speeds up corrosion in cell and results in a slow start-up, and the electrolyte is corrosive and it has complex management.
Hence, it can be applied as static power plants with power output approximately 2MW and also it is used for power source of ships. The advantages of this fuel cell for surface ships are:
- High efficiency compare with gas turbine and diesel powered naval vessels (37-52% compare with 12-16%)
- Reduced emission of all types
- Low vibration and sound levels
- Improved thermal efficiencies
- Reduced cost for fuel (30% less for navy)
- Ship design flexibility (modular units)
- Permits the use of alternative fuels.
 |
| U31 "Wittenberge" |
Moreover, U31 "Wittenberge", type 212A submarine of the German Navy is powered by one diesel engine and an electric motor driven by nine fuel cells, it has advantage as making it harder to detect. Besides that, Hellenic Navy Class 214 non-nuclear submarine is developed to use a silent operating fuel cell plant that runs on nine 34 kW Siemens SINAVYCIS polymer electrolyte membrane (PEM) hydorgen fuel cells.
 |
| NRP Tridente by Portuguese Navy |
The efficiency of fuel cells is considerably superior to conventional internal combustion engines. Fuel cell is exhaust free and, beyond electricity, generate only water and heat. Besides that, due to low operating temperature of maximum of 80 °C, the modules radiate very little heat. Lastly, the comparatively low reaction consumption enables the submarine to remain submerged significantly longer.
To sum up, fuel cells are beginning to show great promise as electrical energy sources for small and large scale applications, as well to provide clean and efficient energy source. However, it still need a reliable and clean hydrogen (or other fuel) source. Hence, there is significant interest in developing the technologies for future application. Moreover, the table below is to summarise the type of fuel cells that we have discussed in this section.
Comments