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Operating principle of fuel cells

On this web page a overview of the operating principle of fuel cells, as well as the manufacturing of PEMFC will be presented. Fuel cells are galvanic elements that transfer chemical energy into electrical energy using a reversible electochemical process. On this occation a continous added fuel reacts with an oxidazing agent. Since the source of energy is not stored in the fuel cell, capacity and performance scale independently. The underlying redox reaction run seperated from each other at the anode and kathode. Therefore electrical energy instead of thermical energy is won. The most used components are Hydrogen (H2) and Oxygen (O2), whose underlying reactions are shown in the following. This redox reaction can be performed acid as well as alkaline catalysed.


Acidic electrolyt

Alkaline elektrolyt

Anode: 2 H2+ 4 H2O → 4 H3O+ + 4 e-
Cathode: O2+ 4 H3O+ + 4 e- → 6 H2O
Anode: 2 H2 + 4 OH- → 4 H2O + 4 e-
Cathode: O2 + 2 H2O + 4 e- → 4 OH-
Overall: 2 H2+ O2 → 2 H2O Overall: 2 H2+ O2 → 2 H2O

Using an alkaline as well as an acidc catalyst, hydrogen gas (H2) is converted at the anode. During this reactions, electrones are released. On this occation an acidic enviroment leads to protoniesed water (H3O+), while in an akaline enviroment hydroxid ions (OH-) react to water (H2O). At the cathode on the other hand Oxygen (O2) is converted to water (H2O) (acidic cathalyst) or hydroxid ions (OH-) (alkaline catalyst). Therefore the overall redox reaction showes the convertion of hydrogen and oxygen to water for both catalyst systems.

Although oxygen an hydrogen are the most common combination in a fuel cell, other hydrogen carrier like methanol (CH3OH), butane (C4H10) or natural gas (>75% methane) are worth considering as a fuel. Frequently Hydrogen is stored chemical as amonia (NH3). Amonia can be decomposed thermally on site using a hydrogen generator. The H2-generator is also called amonia splitter or rather amonia cracker, since amonia is decomposed to hydrogen and nitrogen in it as shown in the folloing equation:.

2 NH3 → H2+ 3 N2

Depending the used fuel cell type, sometimes nitrogen has to be sperated using a extra porzess.

Fuel cell construction

A fuel cell usually consits of two catalyst coated electrodes and one electrolyte. (Ion conductor). Mainly metal or carbon based systems with high surfaces are used as elektrodes (e.g. carbon felt (CF)). As a catalyst e.g. ruthenium respectively platin are common materials. In the prozess the electrolyt provides a spacial seperation of the educts and furthermore enables transport of charge between the electrodes. Hereby liquid as well as solid electrolyts are possible.

Typs of fuel cells

For the realisation of the spacial seperated oxidation processes of a hydrogenous fuel, various approaches are possible. Underlying fifferences of fuel cells are in particular based on operating temperatures, the type of electrolyts as well as the provided fuel und corresponding redox reactions. Low temperature fuel cells (LT-FC) are operating at up to 200 °C while high temperatur fuel cells (HT-FC) start at temperatures higher than 700 °C. Due to the temperatur, catalysts in low temperatur fuel cells have to be based on expensive metals of the platin group. Furthermore contamination of the educt gases with for example carbon monooxid can damage the prozess heavily. Using high temperature fuel cells, cheaper catalysts based on e.g. nickel are sufficient. In additon higher efficency is often possible. Moreover fuel cells can be distinguished by the used electrolytes. Polymer electrolyte membranes (e.g. proton exchange membrane fuel cell - PEMFC), aqueous alkaline electrolyts (e.g. alkaline fuel cell - AFC), aqueous acidic electrolyts (phosphoric acid fuel cells - PAFC), ionic electrolyte liquids (molten carbonate fuel cells - MCFC) as well as solid electrolyts (solid oxide fuel cell - SOFC) are the most commonly used types.
The most frequent types of fuel cells and their properties are shown in the following table.

Name Type Electrolyte Charge carrier Fuel gas (Anode) Oxidizing agent (Cathode) Temperature (°C) Efficiency Application
Polymer electrolyte membrane fuel cell
for hydrogen (PEMFC)
Acidic low temperature oxyhydrogen gas cell Proton-conducting polymer membrane
(PEM)
Hydronium ion (H3O+) Hydrogen (H2) Oxygen (O2) or air; humidified 60-70 Cell: 50-68 Production vehicles, thermal power stations,
Supplies for electronic
Polymer electrolyte membrane fuel cell
for alternative fuels
like methanol (DMFC), ethanol (DEFC) etc.
Low temperature oxyhydrogen gas cell Proton-conducting polymer membrane
(PEM)
Hydronium ion (H3O+) Methanol-Water-Mixture (CH3OH-H2O)
Methanol-Water-Mixture (C2H5OH-H2O)
etc.
Atmospheric oxygen (O2) 60-130 Cell: 20-30 Electric drives, battery usage
Solid oxide fuel cell (SOFC) High temperature oxyhydrogen gas cell Oxide ceramic electrolyte
(ZrO2 + Y2O3)
Oxide ion (O2-) Hydrogen (H2 from methane, coal, methanol, ...) Atmospheric oxygen (O2) 800-1000 Cell: 60-65 Thermal power stations (up to 250kW)
Galvanic fuel cell
with alkaline electrolyte e.g. (AFC)
Alkaline low temperature oxyhydrogen gas cell e.g. Potassium hydroxide solution, 30% Hydroxide ion (OH-) Pure hydrogen (H2) Pure oxygen (O2) 20-90 Cell: 60-70 Small plants (bis 150kW); Submarine drive
Galvanic fuel cell
with acidic electrolyte e.g. (PAFC)
Acidic low temperature oxyhydrogen gas cell e.g. Concentrated phosphoric acid Hydronium ion (H3O+) Hydrogen (H2)
(fossil fuel)
Atmospheric oxygen (O2) 150-220 Cell: 55 Stationary cogenerations of power and heat

The two most promising types of fuel cells acording to the current state of art, are the Polymer electrolyte membrane fuel celland the Solid oxide fuel cell.

Polymer electrolyte membrane fuel cell (PEMFC)

The "proton exchange membrane fuel cell" (PEMFC), is also known as polymer electrolyte fuel cell (PEFC)
This kind of fuel cells operate at temperatures between 10° - 100 °C (low temperature PMEFC), respectively at 130 - 200 °C (high temperatur PEFC) depending on the used electrolyte membrane. Both Applications reach an efficency of about 60%, using pure hydrogen gas (about 48% using fossil gas). As it is described at acidic electrolyte in the section Operatig_principle, at the cathode hydrogen, or a hydrogen sorce like hydrocarbons is converted at the anode, while oxygen e.g. atmospheric oxygen is converted at the cathode. The continous water supply of the anode is achieved using back diffusion through the membrane as well as the humidification of the educts.

Low temperatur PEMFC

Using low temperature polymer electrolyt fuel cells, usually a polymer membran consisting of Nafion®, a sulfonated tetrafluoroethylene based fluoropolymer-polymer, is used. By humidifying this membrane, it developes an acidic nature an therefore gets abe to carry protones. The conductivity scales with increasing water contend. The membrane is coated on both sides, usually using a porous carbon electrode that has an accordingly high surface. Commenly a cathalyst consiting of platin, respectively a mixture of platin and ruthenium, platin and nickel or platin and cobalt, is integrated. At this operating temperatures, paticular attention has to be paid on carbon monooxide impuritys in the hydrogen gas. For example CO can be a side product of the hydrogen production using natural fossil oil sorces and therefore reach the fuel cell. This is important, since even small proportions of 10 ppm carbon monooxid in the fuel gas can lead to catalyst poisoning and therefor an abortion of the reaction. The reason for that CO, haveing a high affinity, blocks catalytically aktive center of the membran. However flushing the fuel cell with inert gas respectively pure hydrogen, removes the poisonous CO again. A to high CO concentration can be prevented using the Shift reaction or the selective CO oxidation.
Using the reversibel shift reaction CO can be converted to CO2 and hydrogen by adding vapourised water
CO + H2O ⇌ CO2 + H2
In this process the equilibrium is shifted to product side correlating with higer temperatures.
ALso sulphur compounds and amonia in the fuel gas are catalyst poisening and therefore have to be kept in a low ppm section if possible.

High temperature-PEMFC

Using high temperature polymer electrolyt fuel cells, usually the polymer membran is made of polybenzimidazole. To increace the proton conductivity, phosphoric acid is incorporated in the polybenzimidazole-matrix. Storing water as it the case in LT-PEMFC is not necessary here. In addition reactions running at 130 - 200 °C are fare more restistent to the catalyst poisinging gas carbon monooxid, since CO desorbes faster and therefore stops blocking active catalytic centers.

Advantages PEMFC

Disadvantages PEMFC

State of developement PEMFC

At the moment first production vehicles (automobils, trucks and buses),smaller plants and cogeneration are run wit polymer electrolyte fuel cells. Furthermore there are applications in battery usage, portable electronic supply (e.g. notebooks). Applications in space travel or military use are also developed at the moment. Here perfomances between 5 and 250 kW can be achieved.

PEMFC manufactoring

For the manufactoring of a PEMFC, the fllowing parts have to be produced and attached:

The key component of the fuel cell, respectively the fuel cell stacks is the Membrane Electrode Assembly (MEA)
This can be manufactured using the so called Decal-prozess (abbreviation for decalcomania). Here the different layers are printed on a decal foil using a ink prozess combined with a hot press. In a multi-stage prozess two sub gaskets, two times the electrode material (carbon net) wit a cathalyst layer (platin) and the polymer membrane (Nafion®) are attached. Normally at least 3 steps are necessary to fuse the materals in this procedure. The prozess can be carried out continously and at atmospheric pressure. Using mechanical rolls small irregularities of the individual layers can occur. Therefore our partner Shindo Eng. Lab. has enhanced this process. Whin the scope of this procedure a one-stage prozess is possible. A laminating machine is used, which attaches foils using vacuum. Gas that is released during the fusing of the catalyst hot press is absorbed. Therefore a particular high uniformity of the layers can be achived. Using this process plant, a 3-layer (electrode + membrane; MEA), 5-layer (MEA + sub gasket) respectively 7 layer (MEA + sub gasket + gas diffusion layer) equipment can be manufactured as needed. Following graphic shows the order of the 7-layer arrangement, as well as the process plant of our partner Shindo Eng. Lab.

7-layer-PEMFC


MEA-Vacuum-Press

Crystec Technology Trading GmbH represents the company Shindo in europe and is gladly helpful procuring a corresponding plant.

Solid oxid fuel cell (SOFC)

Die "solid oxide fuel cell" (SOFC) is operating at 650 - 1000°C and therefore is part of the high temperature fuel cells. The solid electrolyte made of oxide ceramic is characteristic for the SOFC technique. The most commen used material here is yttrium oxide stabilised zirconium oxide (YSZ). An alternative strontium- and magnesium doped lanthan-galium oxid (LSGM) or gadolinium doped cerium oxide (CGO) can be used. Here an efficiency up to 70% can be achieved. The electrolyt as the core element of the fuel cell is manufactured as a tube or alternatively as planar membrane. It has to be designd quite thin, to garantee a low energy transport of oxygen ions. The cathode material can be made out of perovskites based on manganese e.g. La0.8Sr0.2MnO3 (LSM). These are attractive for their high durability and resistance to ageing. However they are vulnerable to chrome poisening, which can be set free from stacks connecting compounds made out of chrome steel and therevor reduce the life span significantly. At the anode in contrary materials like nickel and yttrium oxide stabalized zirconium oxide (Ni-YSZ) are used.

Advantages SOFC

Disadvantages SOFC

State of developement SOFC

At the moment experimental prototyps of block unit power stations for stationary electricity supply exist. Here perfomances between up to 250 kW can be achieved.


Nafion® is a registered trademark of the Chemours company FC LLC