Fuel cells: Operation principal; assembly; manufacturing; machines.
Fuel cell machines
On this web page an 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 electrochemical process. On this occasion a continuous added fuel reacts with an oxidizing agent. Since the source of energy is not stored in the fuel cell, capacity and performance scale independently. The underlying redox reaction run separated from each other at the anode and cathode. Therefore, electrical energy instead of thermal 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.
| 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 acidic catalyst, hydrogen gas (H2) is converted at the anode. During these reactions, electrons are released.
On this occasion an acidic environment leads to protonated water (H3O+), while in an alkaline environment hydroxide ions (OH-) react to water (H2O).
At the cathode on the other hand Oxygen (O2) is converted to water (H2O) (acidic catalyst) or hydroxide ions (OH-) (alkaline catalyst).
Therefore, the overall redox reaction shows the conversion of hydrogen and oxygen to water for both catalyst systems.
Although oxygen a 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 ammonia (NH3). Ammonia can be decomposed thermally on site using a hydrogen generator. The H2-generator is also called ammonia splitter or rather ammonia cracker, since ammonia is decomposed to hydrogen and nitrogen in it as shown in the following equation:
2 NH3 → H2+ 3 N2
Depending the used fuel cell type, sometimes nitrogen has to be separated using an extra process.
A fuel cell usually consists of two catalyst coated electrodes and one electrolyte. (Ion conductor). Mainly metal or carbon-based systems with high surfaces are used as electrodes (e.g. carbon felt (CF)). As a catalyst e.g. ruthenium respectively platin are common materials. In the process the electrolyte provides a spacial separation of the educts and furthermore enables transport of charge between the electrodes. Hereby liquid as well as solid electrolytes are possible.
For the realisation of the spacial separated oxidation processes of a hydrogenous fuel, various approaches are possible.
Underlying differences of fuel cells are in particular based on operating temperatures, the type of electrolytes 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 temperature fuel cells (HT-FC) start at temperatures higher than 700 ░C. Due to the temperature, catalysts in low temperature fuel cells have to be based on expensive metals of the platin group. Furthermore,
contamination of the educt gases with for example carbon monoxide can damage the process heavily. Using high temperature fuel cells, cheaper catalysts based on e.g. nickel are sufficient. In addition, higher efficiency 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 electrolytes (e.g. alkaline fuel cell - AFC), aqueous
acidic electrolytes (phosphoric acid fuel cells - PAFC), ionic electrolyte liquids (molten carbonate fuel cells - MCFC) as well as solid electrolytes (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
|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
|Hydronium ion (H3O+)||Methanol-Water-Mixture (CH3OH-H2O)
|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)
|Atmospheric oxygen (O2)||150-220||Cell: 55||Stationary cogenerations of power and heat|
The two most promising types of fuel cells according to the current state of art, are the Polymer electrolyte membrane fuel cell and the Solid oxide fuel cell.
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 temperature PEFC) depending on the used electrolyte membrane. Both Applications reach an efficiency of about 60%, using pure hydrogen gas (about 48% using fossil gas). As it is described at acidic electrolyte in the section Operating_principle, at the cathode hydrogen, or a hydrogen source like hydrocarbons is converted at the anode, while oxygen e.g. atmospheric oxygen is converted at the cathode. The continuous water supply of the anode is achieved using back diffusion through the membrane as well as the humidification of the educts.
Using low temperature polymer electrolyte fuel cells, usually a polymer membrane consisting of Nafion®, a sulfonated tetrafluoroethylene based fluoropolymer-polymer, is used. By humidifying this membrane,
it develops an acidic nature and therefore gets able to carry protons. 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. Commonly a catalyst consisting of platin, respectively a mixture of platin and ruthenium, platin and nickel or platin and cobalt, is integrated.
At this operating temperatures, particular attention has to be paid on carbon monoxide impurities in the hydrogen gas. For example, CO can be a side product of the hydrogen production using natural fossil oil sources and therefore
reach the fuel cell. This is important, since even small proportions of 10 ppm carbon monoxide in the fuel gas can lead to catalyst poisoning and therefor an abortion of the reaction. The reason for that CO, having a high
affinity blocks catalytically active centre of the membrane. However, flushing the fuel cell with inert gas respectively pure hydrogen, removes the poisonous CO again. A too high CO concentration can be prevented using the Shift
reaction or the selective CO oxidation.
Using the reversible 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 ammonia in the fuel gas are catalyst poisoning and therefore have to be kept in a low ppm section if possible.
Using high temperature polymer electrolyte fuel cells, usually the polymer membrane is made of polybenzimidazole. To increase 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 far more resistant to the catalyst poisoning gas carbon monoxide, since CO desorbs faster and therefore stops blocking active catalytic centres.
State of development PEMFC
At the moment first production vehicles (automobiles, trucks and buses), smaller plants and cogeneration are run with 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 performances between 5 and 250 kW can be achieved.
For the manufacturing of a PEMFC, the following 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-process (abbreviation for decalcomania). Here the different layers are printed on a decal foil using an ink process combined with a hot press. In a multi-stage process two sub gaskets, two times the electrode material (carbon net) with a catalyst layer (platin) and the polymer membrane (Nafion®) are attached. Normally at least 3 steps are necessary to fuse the materials in this procedure. The process can be carried out continuously 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 process 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 achieved. 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.
|Film supply method||Roll Feeding type|
|Cutting||Plate Knife Cutting (Sub-gasket, MEA, GDL)|
|Alignment Type||Vision Align (Pre Align, 2ndAlign , Punching Align)|
Crystec Technology Trading GmbH represents the company Shindo in Europe and is gladly helpful procuring a corresponding plant.
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 common used material here is yttrium oxide stabilised zirconium oxide (YSZ). An alternative strontium- and magnesium doped lanthan-galium oxide (LSGM) or gadolinium doped cerium oxide (CGO) can be used. Here an efficiency up to 70% can be achieved. The electrolyte as the core element of the fuel cell is manufactured as a tube or alternatively as planar membrane. It has to be designed quite thin, to guarantee 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 poisoning, which can be set free from stacks connecting compounds made out of chrome steel and therefore reduce the life span significantly. At the anode in contrary materials like nickel and yttrium oxide stabilised zirconium oxide (Ni-YSZ) are used.
State of development SOFC
At the moment experimental prototypes of block unit power stations for stationary electricity supply exist. Here performances between up to 250 kW can be achieved.