Skip to main content

Carbon Monoxide Poisoning in PEMFCs

In large PEMFC systems, the hydrogen fuel usually comes from a fuel reforming system. These systems always involve a reaction that produces carbon monoxide, like the reaction between steam and methane:

CH4 + H2O → 3H2 + CO

Fuel cells that work in high temperatures can use this carbon monoxide as fuel, which is different from FCs that employ platinum as part of the catalyst, since small amounts of the compound can negatively affect the anode. One of the strategies to overcome this limitation is the transformation of carbon monoxide to carbon dioxide through the increased insertion of steam:

 CO + H2O → H2 + CO2

Which is a reaction commonly known as the water gas shift reaction. However, this process never fully transforms all the CO to CO2. The best systems usually leave 0.25 to 0.5% of the original concentrations of CO in the PEMFC.

What the carbon monoxide does it to occupy platinum catalyst sites because of its relative affinity, which prevents the hydrogen fuel from reaching the platinum sites. Studies have found that concentrations of CO as low as 10 ppm have unacceptable effecst on the performance of PEMFC systems, which means that a reduction of the factor of CO levels from fuel reformation systems by at least 500 is needed.

Another way to circumvent the CO contamination is the addition of small quantities of oxygen or air to the fuel stream. The process results in a reaction with the carbon monoxide at the catalyst sites, thus removing it. One disadvantage of this strategy is that any oxygen that doesn't react with the CO can react with the hydrogen and waste fuel. Additionally, the mentioned method works only on CO levels that are as low as 10s or 100s ppm, meaning that the PEMFC system needs to have a previous system to reduce carbon monoxide levels. Another point to take in consideration is the fact that the rate of oxygen input needs to be carefully controlled and follow the rate of hydrogen inflow.

Usually, a CO gas clean-up system is used by itself, or a less effective one is employed in conjunction with air/oxygen feed to the fuel gas.

Lastly, the longer the molecule length of the hydrocarbon fuel to be reformed is, the worse the CO contamination problem is. The reformation of methane produces one CO molecule for each CH4, but a fuel such as C8H18 produces 8 CO molecules for each C8H18 molecule, for example.

Reference:

LARMINIE, James; DICKS, Andrew. Fuel Cell Systems Explained. 2. ed. West Sussex, England: Wiley & Sons Ltd., 2003. 418 p.

 


 
Labels:

Comments

Post a Comment

Popular posts from this blog

Photovoltaics: Band Diagram

In the previous post we discussed silicon, which is the most used material in photovoltaics. In this post, we introduce the band diagram, for which we will use silicon as an example. We will start our discussion of the band diagram with the Bohr model of the silicon atom. In semiconductor materials the outer shell of the atom, which is called the valence shell, is not completely filled. The outer shell of silicon contains 4 out of the possible 8 electrons, which we call valence electrons. As we discussed in the previous post, each silicon atom in a crystalline structure is bonded to four other silicon atoms. The bonds between the silicon atoms are called covalent bonds. These bonds actually consist of two valence electrons that are shared by two silicon atoms. All valence electrons are fixed in the lattice, forming covalent bonds, and are therefore immobile. However, at a temperature above absolute zero, thermal energy is supplied to these miconductor and some of the vale...
Post a Comment
Read more

Petroleum as Fuel for Fuel Cells (FCs)

Petroleum is made of gaseous, liquid, and solid hydrocarbon-based chemical compounds from sedimentary rock deposits around the planet. Crude petroleum, when refined, provides high-value liquid feeds, solvents, lubricants, and other products. Petroleum-based fuels make up almost one half of the energy supply in the world. Simple distillation is enough to make gasoline, diesel, aviation fuel and kerosene out of petroleum. How much is obtained, in terms of fractions, from the crude oil depends on the origin of the supply. When fuel cells are considered, it is important to understand the physical and combustion characteristics of the fuel, as well as its chemical composition (it is this factor that determines the fuel processing type).  Different technologies have to be employed to convert the many fraction types of the petroleum into hydrogen for FCs.  A special case is when the fuel is catalytically converted and generates various trace compounds that may be poisonous...
Post a Comment
Read more

Physical Description of the Gas Diffusion Layer (GDL)

Porous and electrically conductive material is the kind of material that is used for gas diffusion backings. Usually, the gas diffusion layer is single or composite, and the last means that there is a junction with a micro-porous layer. Water management and electrical properties can be improved with a treatment composed of fluoropolymer and carbon black . Diffusion of the reactant gases to the membrane/electrode assembly are improved with these material types. The structure is made it so that it spreads out the gas to maximize the contact surface area of the catalyst layer membrane. Carbon cloth and carbon paper are the most usually used GDL materials. The purpose of the GDL is to limit the amount of water that reaches the membrane/electrode assembly. Additionally, it helps with the removal of liquid water from the cathode side to prevent excess flooding. One required characteristic for this layer is that it has to be water-proofed so that the layer does not get clogged with wat...
Post a Comment
Read more