Alkaline Fuel Cell

Low temperature aqueous alkaline electrolyte cells have the advantage of being able to start up easily from cold, and operate usually at 60-80 °C, where the water vapour pressure of the electrolyte is appopriately high for a controlled removal rate. At these temperatures, highly active catalysts are required, usually of the platinum family. Silver and high-surface nickel have been used, however, as catalysts in these system; nickel is conventionally used as a conducting structural material.

Cheaper catalysts normally require higher operating temperatures; the Bacon cell is an example of a nickel catalyst used at 200-250 °C. At these temperatures, either a high pressure must be applied to the system or highly concentrated solutions must be used to prevent water loss. High-pressure systems are not suitable for air operation, due to the high pumping energy required, whereas high concentration may cause corrosion, wich restricts the choice of construction materials.

The intolerance of this type of cell to carbon dioxide is a major problem; it restricts the choice of fuel to pure hydrogen of hydrazine, and requires that the air filter removes 0.04% of the CO2 present in the air. The internal reforming cell is an attempt to get rid of this problem : the fuel electrode is made of palladium plus silver, and the fuel is either alcohol or a hydrocarbon which is reformed with steam on a nickel catalyst on one side of the electrode. The hydrogen formed passes through the electrode and reacts with the electrolyte, but the palladium prevents the CO2 to passes through and get into the electrolyte.

Anode reaction:	H2 + 2OH- → 2H2O + 2e-
Cathode reaction:	½O2 + H2O + 2e- → 2OH-
Overall reaction:	H2 + ½O2 → H2O

Sulfuric and Phosphoric Acid Fuel Cells

Acid electrolyte cells are more tolerant to CO2 and allow the use of normal air and non-pure hydrogen. But the corrosion problem restricts the choice of construction materials especially for the electrodes and the catalysts. The electrodes can be made out of gold, tatalum, titanium and carbon and only the platinum group metals can be used as catalysts. The acid used as the electrolyte must be non-volatile, such as sulfuric and phosphoric acids, so that only water is lost by evaporation.

The electrolyte in the PAFC is a paper matrix saturated with phosphoric acid, transporting the hydrogen ions. The operating temperature is around 200 °C. The operating temperature require platinum as catalyst which is supported being dispersed on graphite material. But platinum at this temperature is sensitive to CO-poisoning.

Cells wich use hydrocarbons directly as fuel around 150 °C have low efficiency and current density, thus have been restricted to research investigations. Alcohol fuels and impure hydrogen (containning CO and CO2 produced by reforming hydrocarbons) have been used by various compagnies.

In general, the performances of acid cells are much lower than that of alkaline cells, due to the poorer performances of the air electrode, probably because of the increased stability of formed perroxides in an acid environment. However there are many compromises that can be made between alkaline and acid fuel cells, considering the constructions and operating temperature and regarding the probable use of the desired cell.

Anode reaction:	H2 → 2H+ + 2e-
Cathode reaction:	½O2 +2H+ + 2e- → H2O
Overall reaction:	H2 + ½O2 → H2O

Solid Polymer Fuel Cell

An example : the direct methanol fuel cells (DMFC)

This type of fuel cell is based on solid polymer technology but uses methanol directly as a fuel. If it can be made to work, that would be a big step forward in the automotive area where the storage or generation of hydrogen is one of the big obstacles for the introduction of fuel cells. Prototypes exist, but the development is at an early stage. There are principal problems, including the lower electrochemical activity of the methanol as compared to hydrogen, giving rise to lower cell voltages and, hence, efficiencies. Also, methanol is miscible in water, so some of it is liable to cross the water-saturated membrane and cause corrosion and exhaust gas problems on the cathode side. Nevertheless, the direct methanol fuel cell is an interesting proposition and a number of places are working on it, including Siemens in Germany, the University of Newcastle and Argonne National Laboratory. There are also efforts to develop a low-temperature SPFC (500 °C) that would also allow the direct use of methanol, as well as using stainless steel components. The idea is still young but intriguing. The Imperial College in London is active in this area.

Anode reaction:	MeOH + H2O → CO2 + 6H+ + 6e-
Cathode reaction:	3(½O2) + 6H+ + 6e- → 3H2O
Overall reaction:	MeOH + H2O+ 3(½O2) → CO2 + 3H2O

Molten Carbonate Fuel Cell

In the molten carbonate fuel cell, the electrolyte consists of a molten mixture of potassium carbonate and lithium carbonate to transport carbonate-ions from the cathode to the anode. The CO32- transport needs supply of CO2 at the cathode side of the cell which is generally be obtained by recycling the anode off side gas. The operating temperature is about 850 °C which allows nickel to be used as catalyst material. The process occuring in a hydrogen-oxygen fuel cell operating at higher temperatures without an aqueous electrolyte might well be considered as oxide ions produces at the air electrode:

O2 + 4e- → 2O2-

which then move to the fuel electrode to oxidise the hydrogen:

H2 + O2- → H2O + 2e-

and it might therfore be considered that a molten ionic oxide would provide the best electrolyte to encourage this process. However, simple ionic oxides have melting points greater than 1000 °C and therefore attention has been focussed on salts melting at lower temperature. These salts are generally those with oxygen-containing anions, eg nitrates, sulfates, carbonates. At high temperature it is likely that the direct reaction of hydrocarbons at the fuel electrode is quite favorable and hence conversion of petroleum products to hydrogen or methanol is uneccessary. But consideration must be given to the effect of hydrocarbon oxidation at the fuel electrode on the choice of electrolyte. Carbon dioxide will be a major product that can be troublesome with some salts, for example:

CO2 + SO42- → CO32- + SO3

Hence it is most satisfactrory to consider as electrolyte a molten carbonate or mixture of carbonates. A mixture of salts may have a considerable advantage since it will have a lower melting point than either of its components. A convinient way of maintaining the carbonate composition of the electrolyte invariant is to remove carbon dioxide as a gaseous product from the fuel electrode and transfer it to the oxidant electrode in the air or oxygen stream. Thus for a fuel such as carbon monoxide the overall electrode processes will be:

O2 + 2CO2 + 4e- → CO32- and CO + CO32- → 2CO2 + 2e-

Thus carbonate ion transfert within the electrolyte may be balanced by carbon dioxid transfert outside it. A similar mechanism could operate even for cells using hydrogen as a fuel.

Anode reaction:	H2 + CO32- → CO2 + H2O + 2e-
Cathode reaction:	½O2 + CO2 + 2e- → CO32-
Overall reaction:	H2 + ½O2 → H2O

FUEL CELL TYPE	COMMON ELECTROLYTE	OPERATING TEMPERATURE	TYPICAL STACK SIZE	ELECTRICAL EFFICIENCY (LHV)	APPLICATIONS	ADVANTAGES	CHALLENGES
Polymer electrolyte membrane (PEM)	Perfluorosulfonic acid	<120°C	<1 kW–100 kW	60% direct H2; 40% reformed fuel	Backup power Portable power Distributed generation Transportation Specialty vehicles	Solid electrolyte reduces corrosion and electrolyte management problems Low temperature Quick start-up and load following	Expensive catalysts Sensitive to fuel impurities
Alkaline (AFC)	Aqueous potassium hydroxide soaked in a porous matrix, or alkaline polymer membrane	<100°C	1–100 kW	60%	Military Space Backup power Transportation	Wider range of stable materials allows lower cost components Low temperature Quick start-up	Sensitive to CO2 in fuel and air Electrolyte management (aqueous) Electrolyte conductivity (polymer)
Phosphoric acid (PAFC)	Phosphoric acid soaked in a porous matrix or imbibed in a polymer membrane	150°–200°C	5–400 kW, 100 kW module (liquid PAFC) <10 kW (polymer membrane)	40%	Distributed generation	Suitable for CHP Increased tolerance to fuel impurities	Expensive catalysts Long start-up time Sulfur sensitivity
Molten carbonate (MCFC)	Molten lithium, sodium, and/or potassium carbonates, soaked in a porous matrix	600°–700°C	300 kW–3 MW, 300 kW module	50%	Electric utility Distributed generation	High efficiency Fuel flexibility Suitable for CHP Hybrid/gas turbine cycle	High temperature corrosion and breakdown of cell components Long start-up time Low power density
Solid oxide (SOFC)	Yttria stabilized zirconia	500°–1,000°C	1 kW–2 MW	60%	Auxiliary power Electric utility Distributed generation	High efficiency Fuel flexibility Solid electrolyte Suitable for CHP Hybrid/gas turbine cycle	High temperature corrosion and breakdown of cell components Long start-up time Limited number of shutdowns

Question 2:
Using a plot with energy density and power density on the X and Y axis, arrange various battery types like lithium ion, lead acid, Nickel Cadmium, Ultracapacitor, fuel cell.

A Ragone plot is a plot used for comparing the energy density of various energy-storing devices. On such a chart the values of specific energy (in Wh/kg) are plotted versus specific power (in W/kg).

The vertical axis describes how much energy is available per unit mass (Energy density-Whr/kg), while the horizontal axis shows how quickly that energy can be delivered, otherwise known as power per unit mass (Power density- W/kg).

In the above plot, fuel cells have high energy density so that it can deliver small current for a long duration.

Among different types of batteries, lithium ion batteries have comparitively high energy density as well as power density which makes it suitable for automotive apllications.

whereas the ultracapacitors have high power density means it can deliver a large amount of current in a very short duration (typically in milliseconds). for this reason ultracapacitors are used to meet peak power requirement.