Roller coaster gas prices and rising energy costs for the home have created uneasiness about the future of our fossil-fuel based economy. One near-term solution being pursued by researchers at the Biodesign Institute at ASU is a new fuel cell technology for renewable energy and the fledgling hydrogen economy.
Don Gervasio, associate research professor at the institute's Center for Applied NanoBioscience, is overseeing an ASU team that was awarded a $1.5 million grant by the Department of Energy (DOE) to develop new fuel cell components to more efficiently generate electrical power. The technology is designed for use in large fuel cells that can generate 100 kilowatts of power, which is enough for a car, a home or a remote power station.
Most power generators come with an unwanted side effect: heat. For a car, excess heat is handled by running coolant from the radiator through the engine block. To date, attempts to use a similar cooling technology with most fuel cells have been unsuccessful.
"Even though a fuel cell operates at a higher efficiency than a car engine, it still puts out a considerable amount of heat," said Gervasio. "For a long time people thought that a room temperature fuel cell would be ideal for automobiles, but it turns out that if you power an automobile using a fuel cell operating at room temperature, you need a radiator as big as the car."
Fuel Cell Sandwich
Fuel cells are generating significant interest because they offer a more efficient alternative to heat engines and avoid nasty pollutants like carbon monoxide, nitrogen oxides and ozone.
A typical fuel cell is an elaborate assembly of membranes sandwiched by electrodes and plates that send gases into the fuel cell to generate electricity. According to the DOE, the membrane and electrode parts of the fuel cell account for more than half of the costs of the fuel cell stacks. A reduction in those costs would make the fuel cell system more competitive with standard gasoline engines.
By designing a membrane that operates at high temperatures (a medium oven setting of 250 F, or 120 C), Gervasio and colleagues want to reduce both the amount of heat management needed to operate the fuel cell and its overall size, weight and costs.
A hydrogen powered fuel cell has positive and negative ends just like a battery. It works by splitting hydrogen gas into its component protons and electrons at the negative electrode, which react with oxygen from air at the positive electrode. This produces electricity while leaving only water as a byproduct.
The 'cheese' of the fuel cell stack, the fuel cell membrane, completes the electrical circuit by funneling protons through the membrane from one electrode to the other. Just as importantly, it also forces energized electrons to move across a circuit outside the membrane, producing an electron current to power devices such as a light bulb or electric motor.
Some commercial membrane designs reduce the operating voltage generated from the fuel cell by as much as 50 percent of its theoretical value, and most don't operate at temperatures much above room temperature, Gervasio said. This makes the development of a new membrane essential if fuel cells are to be used to reduce consumption of fossil fuels and have a central role in the hydrogen economy.
The Secret Sauce
Another aspect of the fuel cell membrane that ASU scientists hope to improve is its source of electrolytes, the salts that carry charge through the inside of the fuel cell. ASU Regents' Professor Austen Angell, a co-leader of the group, has been using protic ionic liquids to accelerate the movement of protons, which are essential for completing the circuit and generating electric power.
Currently, high temperature fuel cell systems use phosphoric acid in a polymer matrix as the membrane electrolyte, but the voltage generated is about half what could theoretically be achieved. One of the protic ionic liquids that Angell and his fellow researchers have experimented with has generated electric potentials approaching the theoretical limit, but has not yet been able to maintain the voltage at higher currents.
Another advantage of protic ionic liquids compared to the phosphoric acid system is that they don't contain any water. Water, the only byproduct of generating energy from hydrogen in a fuel cell, can often clog up the system and prevent the hydrogen and oxygen gases from flowing into the cell.
One protic ionic mixture being tested uses the combination of two ammonium salts, ammonium nitrate and ammonium bisulfate. "These are some of the cheapest chemicals on the market, and they work like a charm," said Angell.
Fashioning a water-free electrolyte system is not without its difficulties. Individually, the ammonium salts are solid at room temperature, like table salt. However, when the salts are combined at just the right ratio, they can melt into a liquid at the operating temperatures of the fuel cell, allowing incorporation into the membrane.
A Real Pickle
Another co-leader of the project, Jeff Yarger, professor of chemistry and biochemistry, is building an analytical system using a powerful tool, NMR spectroscopy, to both troubleshoot fuel cell development and help uncover the mechanisms of proton conduction across the fuel cell membrane.
"The whole point of the membrane is to get protons across as fast as possible," said Yarger. "If they are getting stuck in the membrane we want to be able to see where they are getting stuck and find a way to fix it."
Yarger and his team can measure how quickly the protons move across the membrane, which will aid in membrane design. "From a practical engineering perspective you'd want the membrane to be as solid as possible, but from a proton diffusion perspective you want the membrane to be as liquid as possible," said Yarger.
An assortment of different polymers will be used to absorb the protic ionic liquids to find the best combination of stability and conductivity. With continued optimization and a better understanding of how the fuel cells work, the researchers hope to break through the barriers that have limited widespread adoption of fuel cells. Funding for the fuel cell project will continue until 2011.