Fueling a Cleaner Future Jonathan Tran

America consumes twenty percent of all oil produced in the world despite being the home of less than fi ve percent of the world’s population1. Our dependence on oil creates economic and political pressures, as we have seen with recently infl ated gas prices2. When these concerns are compounded by the damaging environmental effects of oil extraction, transport, and use, it is easy to understand why the U.S. is turning toward alternative fuel sources. Hydrogen fuel cells are one of the most promising alternatives that has been researched over the last few decades.

These fuel cells have several advantages over conventional fossil fuels3. One such advantage is the ability to generate power without combustion or carbon emissions. Even if one were to account for emissions due to production, when measured against gasoline vehicles, vehicles that utilize fuel cells are able to reduce their total carbon dioxide by half if the hydrogen is produced by natural gas and by 90% if the hydrogen is produced by renewable sources. Other upsides include the accessibility of hydrogen and oxygen in our atmosphere and the ability to be electrical grid-independent, which is especially important for critical load functions such as hospitals and military applications. These advantages have motivated researchers and engineers to explore and refi ne the processes and technologies that are essential to hydrogen fuel cells. Many different types of hydrogen fuel cells have been developed, including alkali fuel cells, molten carbonate fuel cells (MCFC), and proton exchange membrane fuel cells (PEMFCs)4. These are all variations of the same fundamental process of reacting hydrogen and oxygen to generate power. To better understand how these fuel cells work, one can study PEMFCs, which many scientists believe to be a promising candidate for use in homes and vehicles.

There are four main components to a PEMFC: the anode, the cathode, the electrolyte, and the catalyst5. Respectively, the anode and cathode are negative and positive electrodes (conductors that connect nonmetallic materials to a circuit). That is, both the anode and cathode are the mechanisms with which electrons are directed throughout the fuel cell. They also have channels which are responsible for distributing gases evenly across the catalyst6. Next, the electrolyte, commonly referred to as the proton exchange membrane, functions by conducting positive charges. Lastly, the catalyst is where the hydrogen and oxygen reactions occur. It is engineered from carbon paper or cloth coated with platinum nanoparticles.

Fueling A Cleaner Future Written by Jonathan TraN Designed by Phuong Ngo

H

HYDROGEN STATION

The actual process of producing energy begins with high pressure hydrogen gas entering the anode side5. This pressure forces the gas through the catalyst, which breaks the hydrogen into two protons and two electrons. These electrons fl ow through the anode, into an external circuit, and fi nally to the cathode. It is in the external circuit that the fuel cell is able to produce work from the electron fl ow.

On the cathode side, a similar process is simultaneously occurring, as oxygen gas is forced through the catalyst5 . The oxygen is broken into two oxygen anions, which then react with incoming hydrogen cations to form one of the products of this reaction—water.

This process generates very little voltage from a single reaction (about seven-tenths of a volt, which is about half of what a typical AA battery provides); however, when fuel cells are connected using bipolar plates, large amounts of voltage can be effi ciently produced by what is known as a fuel-cell stack.

Scientists are currently conducting extensive research into optimizing the effi ciency and practicality of hydrogen fuel cells. One prominent example is the study of refueling technologies, which is especially important for hydrogen fuel cell vehicles. As previously explained, these fuel cells require high pressure hydrogen gas as an input to the anode. However, engineers have found that the transfer of hydrogen at high pressures can cause a sudden increase in the temperature of storage systems7. This can decrease the fuel effectiveness, damage the fuel tanks, and create safety concerns. In response, a group of researchers led by Mengxiao Li at Chongqing University have explored numerous experiments and simulations to model ways in which this temperature change can be minimized. To approach this problem, they fi rst determined the parameters that affect the fi nal temperature of the gas, which include but are not limited to the initial temperature of the hydrogen, the initial pressure, and the fl ow rate. By employing mathematical models that relate gas parameters, Li and her team have been able to predict the effects of different refueling methods.

One of Li’s most signifi cant fi ndings was in the exploration of the different tank materials used for hydrogen storage. To date, there are two types of units considered most suitable for hydrogen storage—the difference between the two largely being in the inner layer of the tanks. One is made of aluminum, and the other is constructed from high density polyethylene, a plastic. By modeling the thermal conductivity of both tanks, Li reports a lower fi nal temperature for the aluminum tank because of its higher thermal diffusivity. The plastic absorbs heat slower which ultimately results in less heat dissipated during the refueling process. Because the plastic tank is worse at counteracting the increase in hydrogen temperature, the aluminum tank is superior in this regard. Li and her team to date continue to research such factors in hydrogen refueling in hopes of optimizing this complex process.

As the world continues to grapple with the effects of climate change and heavy dependence on fossil fuels, hydrogen fuel cells are becoming an increasingly important and appealing alternative. The government and the private sector are making enormous investments in this rapidly improving technology. Currently, states across the U.S. are vying for almost $10 billion in federal funding for hydrogen fuel cell projects8. As these investments continue to drive innovation and production, hydrogen fuel cells are becoming ever closer to drastically changing the way we think about energy in the U.S. and around the world.

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