Fuel Cells Hold
Key to Hydrogen-Powered
Vehicles


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Water Droplets


Perhaps the most widely anticipated alternative fuel is hydrogen, which has been the subject of research for decades. Hydrogen has some very attractive properties: it has the highest energy density by mass of any potential fuel and it burns cleanly, creating only water vapor during efficient combustion.

For hydrogen-powered transportation, the key component will be the proton exchange membrane fuel cell. PEM fuel cells use hydrogen and oxygen to generate electricity, producing water and heat as byproducts of the electro-chemical reaction. They are the fuel cell technology of choice for transportation because of their high power density, high efficiency, rapid startup capability, clean operation, and flexibility in terms hydrogen fuel sources.

While various fuel cell vehicles have been prototyped and some have been released in limited availability from General Motors and Honda, PEM fuel cells need to address performance, durability, and cost if they are to compete favorably with internal combustion engines and hybrid cars.

The cost of fuel cell stacks is primarily governed by the catalyst, membrane, and bipolar plates. In spite of a longstanding target of $40 per kilowatt for automotive fuel cells, a survey by the U.S. DOE revealed that the low-volume costs for fuel cells are still in excess of $1,800 per kilowatt.

Water management is considered one of largest issues for PEM fuel cells, particularly in cold climates. Water accumulates inside the cell and freezes following shutdown. Ice then blocks the reactant gas flow hindering startup.

Experiment

An experimental set-up at the PEM fuel cell research facility at the Rochester Institute of Technology (top) uses a high-speed camera to observe water formation in the channels of an operational fuel cell. Water droplets can clearly be seen in the bottom image.

Large research initiatives are searching for a better understanding of water transport dynamics within fuel cells in hopes of ultimately mitigating the startup problems. At Rochester Institute of Technology in New York, for instance, researchers use transparent fuel cells with high-speed imaging to study water production and transport within the cell. Meanwhile, General Motors Fuel Cell Research Laboratory in Honeoye, Falls, NY taps neutron radiography, a technique that passes a neutron beam through an operating fuel cell to reveal the 2-D measurement of water thickness.

If cost and performance issues can be resolved, then PEM fuel cells will likely be a core component of a hydrogen economy. These fuel cells are ideally suited to transportation purposes, because of their high efficiency and a refueling time comparable to petroleum fuels. As with electric vehicles, PEM fuel cells will reduce air pollution and CO2 emissions.

Skeptics claim that creating the necessary infrastructure to support a hydrogen-fueled transportation system will be too costly to be practical. Yet hydrogen can be produced from a variety of sources including natural gas reforming, biomass, and water splitting. Perhaps as much as 95 percent of hydrogen produced in the U.S. comes from natural gas feedstock.

Hydrogen from natural gas could provide a firm stepping stone as the energy system evolves away from petroleum. On-site hydrogen generation using the existing natural gas infrastructure could provide an efficient means to transport this low energy-to-volume ratio fuel.

Radiography

Neutron radiography reveals the simultaneous distributions of water, current, high-frequency resistance, and temperature within a PEM fuel cell.

Natural gas reforming is not considered a long-term option since carbon dioxide is produced in the process. However, when the reformed hydrogen is used in fuel cell vehicles, these greenhouse gas emissions are still considerably less than those produced by an internal combustion engine vehicle.

Another process, biomass conversion, typically requires high temperature, but through advances in genetic engineering, special microorganisms could be developed to commercially produce hydrogen by decomposing organic waste material. Such conversion would be carbon-neutral.

Electrolysis and artificial photosynthesis can also generate hydrogen by splitting water molecules. Electrolysis can be driven by a variety of energy sources, such as the electrical grid or renewable sources including solar and wind, for on-site or distributed hydrogen generation. Artificial photosynthesis splits water with sunlight to create hydrogen in a process analogous to photochemical reactions in plants. This ideal energy conversion process has shown great promise using an earth-abundant non-noble catalyst in neutral water at atmospheric temperature and pressure.

[Adapted from “Hydrogen Horizon” by Satish G. Kandlikar, Jacqueline Sergi, Jacob LaManna, and Michael Daino, for Mechanical Engineering, May 2009.]

Hydrogen from natural gas could provide a firm stepping stone as the energy system evolves away from petroleum.

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March 2011

by Satish G. Kandlikar, Jacqueline Sergi