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.

Comments

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...

Environmental conservation must be a commitment of all

In the midst of the presidential race, historically, we have followed guidelines linked to environmental conservation placed in the background. The result of this stance are inoperative mandates regarding the gradual and uninterrupted degradation of our forests and natural resources. We need the elected candidates, both in the Executive and in the Legislative, to make real commitments to the preservation of our biodiversity, even though promises related to the environment do not attract so many votes. In view of the election of the National Electoral Council last month, the people's choices cannot depend on environmental issues alone, but also on the protection of our natural resources, the health of our environment and its environment, of our environment with respect to the people. Our objective is to create a culture of conservation, of environmental awareness, and of respecting the environment. Environmental awareness is a cornerstone of our campaign and is linked to envi...

Photovoltaics: Silicon

Welcome to this new post about semiconductor physics. Before we delve into all the important topics of semiconductor physics that are relevant to solar cells, we need to talk about silicon. Silicon is one of the most important materials when it comes to solar cells and we will be using it throughout this post series as an example for all the semiconductor concepts we will be going into. This will only be a short introduction to silicon, and there will be an extensive post series on silicon-based solar cells in the following course, Photovoltaic Technologies. Let’s start our discussion by looking at the reasons why silicon is the most used material for fabricating solar cells. The first successful silicon solar cell was fabricated in the Bell Laboratories in US in 1954. At present, the photovoltaic industry is dominated by silicon-based solar cells with 90%share of the market. Silicon is the most widespread material used for solar cells. But why is it that silicon is used so much?...

The Environmental Engineering Hub

Search This Blog