3.3- FUEL CELLS

A fuel cell is, by definition, an electrochemical energy converter device that transforms the energy released by hydrogen oxidation into electric power. In FCVs, this electric power is used to feed the electric engine.

3.3.1- TYPES OF FUEL CELLS

There are many different types of fuel cells, which are classified depending on the electrolyte they use and their operational temperature. Next an overview of the most relevant ones at present is exposed, contemplating their advantages and disadvantages on automotive application.

•  Proton exchange membrane fuel cell (PEMFC)

It offers a high power density and low operating temperatures, from 60 to 100 ºC, but requires platinum in its catalyst, which is really expensive and scarce and must work in purified hydrogen, given that this catalyst becomes corrupted with CO amounts superior to 10 ppm. It is contemplated as the most appropriated to work in FCVs (a chapter dedicated to this fuel cell will be later exposed).

•  Alkaline fuel cell (AFC)

it has low operating cost, and runs at temperatures between 70 and 120 ºC. Then, its catalyst is easily corruptible in presence of CO2, so it has to be fed with purified hydrogen and oxygen, and occupies great volumes. Currently, the AFC is mostly used in military programs.

•  Phosphoric acid fuel cell (PAFC)

It is considerably similar to the PEMFC, with the difference of a quite higher working temperature (from 190 to 200 ºC) and a minor potency per volume unit that makes it acceptable for small static generators, but less attractive for automotion. It must be heated from 190 to 200 ºC to work and fed with pure hydrogen, since CO can damage its catalyst.

•  Molten carbonate fuel cell (MCFC)

This fuel cell operates at high temperatures (from 600 to 700 ºC) and, along with the possibility of being fed with any hydrocarbon, is a good option for great-sized stationary generators, but not for mobile devices, in view of its mechanical fragility.

•  Solid oxide fuel cell (SOFC)

It works on temperatures between 500 and 1000 ºC and, like the MCFC, can work on hydrocarbons; in any case, SOFCs are not an interesting option for FCVs.

Furthermore, there are two types of fuel cells that, despite they are still being developed, may become promising alternatives for the FCV in the middle or long term.

•  Direct methanol fuel cell (DMFC)

As has been commented before in the biomethanol section, there is a type of fuel cell that can be fed directly with methanol instead of hydrogen. Actually, this is a variant of the PEMFC that works in a range of temperatures between 50 and 100 ºC and emits CO2 besides H2O. It requires a higher quantity of platinum in its catalyst, so it results more expensive than the anterior fuel cell, it has a lower efficiency than PEMFC and it is still being developed.

•  Direct ethanol fuel cell (DEFC)

Based on the DMFC technology, there is an experimental fuel cell that runs on ethanol instead of methanol. Currently, this prototype offers a very low efficiency and performance, but could be applied on FCVs in the very long term if it was developed.

3.3.2- OPERATION OF A PROTON EXCHANGE FUEL CELL

The PEMFC offers advantages that make it perfect for automotive appliances over any other fuel cell: first, it fits perfectly in small appliances and offers a good reliability in intermittent operation; then, its low operative temperature avoids waiting during the motor preheat and fire danger in case of accident. Those reasons lead this kind of fuel cell to be the only one that is currently being developed for car appliances by companies over the world.

Next a schematic explanation about how PEMFCs perform is exposed:

In first place, high-pressurized hydrogen is introduced in the cell, contacting the anode layer (negative) and the platinum catalyst that splits those atoms into single protons (H+) and electrons (e-). While protons migrate across the polymer electrolyte to the cathode (positive), electrons pass through an external electric circuit that feeds the electric engine and all the in-board gadgets in the car. Once both protons and electrons reach the cathode, they combine with oxygen from the air forming H2O.

The oxidation of hydrogen releases 286 kJ/mol of energy. In ideal conditions, a PEMFC produces 237 kJ of electricity per mol of hydrogen, wasting 49 kJ in form of heat. That is traduced in a theoretical efficiency of 83%. However, the real case is that common vehicle fuel cells work on less than half of this efficiency, around 35%.

For a PEMF to achieve a full performance it is necessary the adequate management of its internal heat and moisture. This fuel cell must work at about 85 ºC, mildly higher than the environmental temperature, so it must be preheated in order to produce electricity. Nevertheless, when the PEMF is working it generates residual warmth that may overheat this device, reducing its performance and damaging its components. To compensate this heat excess, the electrochemical system must be equipped with a coolant mechanism.

Referring to the moisture requirements, it is convenient to achieve the maximum humidification possible in order to increase the ion conductivity, avoiding the drying of the cell, what would reduce its service life. Though, the inundation of the membrane must be also prevented, since it would collapse the proton circulation and make the fuel cell stall. The regulation of the in-cell moisture requires complex expensive high-technology systems.

Currently, the cost of a complete car-designed PEMFC system is of about $3.000/kW, including all the previously described equipment, and its lifespan is of 2,000 hours or less than 160,000 km in comparison to the 15 years or about 272,000 km of an average ICE (internal combustion engine).

	Polymer exchange fuel cell operation scheme. Image source: <http://www.rsc.org/Education/EiC/issues/2007Nov/FuellingFutureSolidPhaseHydrogenStorage.asp>