Impact of Methanol Reformate on Fuel Cell Anode Performance and Durability
For many fuel cell system applications hydrogen has to be generated from locally available fuels due to the difficulty to store and transport hydrogen gas. Methanol is one of the high hydrogen content liquid fuels that is readily available in many geographic areas and provides a suitable FC fuel. A fuel processing system, including a reformation stage is usually used to convert the methanol to hydrogen gas to feed to the anode channels of the PEMFC stack. The fuel processor is required to meet specific technical and market demands, such as cost, fuel efficiency and purity levels (e.g., CO, methanol concentration) in its output fuel stream. Meeting those constraints requires a number of reformer and fuel cell stack design trade-offs. For this reason, every reformate based fuel cell system is the result of a series of compromises . In order to understand reformer requirements, a study was conducted on the impacts of a small amount of methanol present in the reformate stream. The goal of this communication is to discuss the impacts of methanol in the reformer output stream on the fuel cell anode performance and durability.
An artificial gas mixture of 71%H2, 20% CO2, 8% N2, and 1% methanol vapor was used to mimic the reformer output fuel stream. All the fuel cell tests were conducted on a Ballard standard research testing cell with an active MEA geometry area of 45cm2. The anode was made with a CO tolerant electrode with 0.1 mg/cm2 PtRu/C catalyst, and the cathode contains 0.4 mg/cm2 Pt/C. The cell operation temperature was 75oC, under 5 psig and 100% RH for both MEA sides. The gas flow rates were set to a very high level in order to keep the reactants concentrations approximately uniform across the MEA active area. In order to understand the methanol impact, a dynamic hydrogen electrode (DHE) was mounted onto some of the MEAs to examine the anode and the cathode potentials .
Methanol impact on MEA CO tolerance and the anode stability in an accelerating stress testing (AST) were investigated. This AST is used to accelerate the anode degradation in start-up/shutdown processes .
Results and Discussions
Figure 1 gives the MEA performances with and without the presence of 1% methanol vapor. The polarization behaviors of anode and cathode measured against the DHE are also given in the same plot. As we can see the introduction of 1 % methanol significantly decreased the MEA performance, and this performance loss is dominantly due to cathode performance loss, although the methanol was introduced in the anode side. In-situ cathode cyclic voltammograms were conducted and confirmed that a significant amount of methanol crossed over to the cathode side. The oxidation of the crossover methanol in the oxygen rich cathode occupied the oxygen reduction (ORR) sites and resulted in the decreased ORR activity. In addition, the methanol oxidation current also reduced the ORR current output efficiency.
A small stream of air is commonly introduced into the anode under reformed fuel operation in order to oxidize any carbon monoxide present, referred to as “air bleed”. An anode air bleed sensitivity test was used to evaluate the anode catalyst layer CO tolerance. We found that in spite of the lower performance in the presence of methanol, methanol did not significantly impact the air bleed sensitivity.
It has been found [3,4] that one of the MEA performance loss failure modes is Ru crossover to the cathode side in reformate tolerant anode designs, due to the use of Ru/C catalyst to improve CO tolerance. The impact of methanol on Ru stability in fuel cell start up /shutdown processes was also studied in this research. More detailed discussion will be presented.
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