Challenges
in PEMFC technologies
The challenges and barriers to
commercializing PEM based
fuel cells are many. They have been outlined in a report by the
Department of
Energy, Basic
Research Needs for the Hydrogen Economy
and in a number of publications1-8 (see
References). We repeat the salient points
below.
PEM fuel cells have limited durability, especially when used in a frequent on-off mode and when the power output changes significantly and frequently, as would happen if the fuel cell were the only power source in a transportation vehicle or in fact in many portable applications. Currently, automotive fuel cells have difficulty reaching 1000 hours of use before failure. At least 5000 hours of use is needed for consumer acceptance (this translates to somewhat more than 100,000 miles of use averaged over typical driving modes and traffic conditions). The durability problems are related to shortcomings of the materials used in the MEA: the catalysts, the membrane and the catalyst support. Further, the highest efficiency fuel cells (see Technologies) can only use very pure hydrogen as the fuel. Such hydrogen is very expensive.
When there are impurities in the hydrogen, such as sulfur or carbon monoxide, the platinum anode catalyst is poisoned; that is, it is coated with the impurities and the catalytic activity is reduced or altogether lost. Such poisoning leads to further efficiency loss as well as complete cell failure.
At the cathode, the potentials are high enough to cause corrosion of the carbon black support as well as dissolution and re-precipitation of the platinum catalyst. When the carbon black corrodes, the catalysts become separated from the support. The corrosion is even thought to be accelerated by the presence of the catalyst. When the platinum re-precipitates, it often does so away from the electrode, such as in the membrane, where it can no longer contribute to the electrochemical reduction of oxygen. Even if it stays in the cathode, the particle size grows through this process. This causes a loss of surface are and reactivity when measured as activity per gram of platinum.
The membranes also chemically degrade, allowing large holes to form between the cathode and anode. This produces an explosion hazard as well as rapid cell failure.
The above problems can only be partially mitigated by
re-engineering the fuel cell system and the fuel cell controls. It is
generally
agreed that new materials are needed to address the shortcomings of
materials
presently used and to reach higher efficiency and lower fuel cell costs.
