Mar 07 2011
Polymer electrolyte membrane fuel cell is the most appropriate power source candidate for the next-generation electric vehicle and small-scale stationary power. In recent years, durability has been one of the most important issues to be solved before the commercialization of PEMFCs.
PEMFC performance loss under steady-state and cycling conditions has been attributed to the significant loss of electrochemical active surface area (EAS) of Pt catalyst, especially in the cathode, where it is subjected to low pH (< 1), high potential (0.6-1.2 V), high oxygen concentration and high temperature (50-90 °C). It was found that the Pt particle agglomeration could be accelerated by both potential cycling and steady-state processes. It has also been observed that the Pt electrode could dissolve to some extent in PEMFC operation process. Pt dissolution-deposition and agglomeration lead to the increase of Pt particle size (sintering), which results in the decrease of the Pt EAS. Thus, the fuel cell performance is decreased.
A variety of carbon materials with high surface areas are widely used as the supports for Pt catalyst. As the catalyst support, besides the enhanced catalytic activity, the support should show good corrosion resistance because the corrosion behavior might affect the performance and stability of the Pt catalyst, especially in the cathode of PEMFC. This is because that oxygen reduction reaction occurs at potentials closer to those where oxidation of carbon can also happen. When carbon is oxidized, some Pt particles may detach from the carbon support, resulting in a decrease of catalytic activity of the catalyst. And the interaction of Pt-support may be weakened. Thus, carbon material corrosion plays a negative effect on the stability of the Pt/C catalyst. Furthermore, the corrosion rate of carbon catalyst support is accelerated in the presence of Pt-containing catalysts.
The degradation of carbon on PEMFC electrodes has been reported as carbon corrosion recently. These carbon corrosion phenomena are fatal problems for the PEMFC commercialization because of the short life time. The developments of catalyst materials, cell designs and the management of operation condition are important to solve these problems.
Generally speaking, the carbon corrosion requires high overpotential in PEMFC because of slow kinetic properties, though the carbon might start to be oxidized even at 0.2 V (vs. RHE) based on thermodynamics. In steady PEMFC operations, the carbon materials on both electrodes are rarely exposed to such high potential. But, in transient conditions, the carbon materials on electrodes are often exposed to such conditions. Some cases are predicted: start up, after shut down, load change, partially hydrogen starvation and so on. In every case, it is predicted that the hydrogen deficiency or hydrogen and oxygen co-existence on anode electrode will cause a large enough overpotential between the solution and the cathode metal to oxidize the carbon supports on the cathode electrode. Fuel starvation on the anode side (i.e. low anode stoichiometry) is one of the most damaging operational modes of fuel cells and fuel cell stacks. In particular, high current densities are leading to critical conditions, especially during dynamic operation. Fuel starvation caused severe and permanent damage to the electrocatalyst of the PEMFC and it must be absolutely avoided even if the operation under fuel starvation is momentary.
Catalyst durability study in PEMFC is a difficult topic because of the lengthy duration of the test time required and the complexity of failure analysis. One strategy to reduce Pt/C catalyst performance degradation due to carbon corrosion is to use alternative more stable carbon support (for example carbon nanotube). Another approach for increasing Pt/C catalyst durability is alloying other transition non-precious metals with platinum.
Interaction between the carbon support and the Pt plays an important role in the properties of the Pt/C catalyst. It has been demonstrated that this interaction is attribute to the electron transferring from platinum to carbon support. Electronic structure change of platinum catalytic layer by the presence interaction leads to the change of the catalyst properties. Generally, this electronic interaction has the positive effect towards the enhancement of catalytic properties and the improvement of the catalysts’ stability in PEMFC operation. Oxygen surface groups are of greatest interest in the preparation of carbon-supported catalysts and can be obtained through surface treatment of carbon by chemical methods with different oxidants. Proper heat treatment of carbon support can increase the stability of Pt/C catalyst.
The promising strategies for the durability improvement of Pt/C catalysts are: (a) building proper surface functional groups (including surface oxygen functional groups) or increasing the basic sites on carbon supports to enhance the Pt-C interaction; (b) increasing surface stability of carbon support, e.g. increasing the hydrophobicity of carbon support through proper surface treatment; (c) preparing catalysts with high platinum uniformity and low platinum load.
In your opinion, what of these budding strategies plays the leading role in durability improvement of carbon supported platinum catalyst?
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