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.

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