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While renewable energy sources help to fight the effects of global warming, they do have their drawbacks. Renewable energy cannot be produced as predictably as plants powered by oil, coal, or natural gas. Ideally, alternative energy plants would be paired with a huge energy storage system that would store and dispense power. Stanford School of Engineering is working to use reversible fuel cells to combat this storage issue. Fuel cells use oxygen and hydrogen to create electricity; if the process were reversed, the fuel cell could be used to also store electricity.
“You can use the electricity from wind or solar to split water into hydrogen and oxygen in a fuel cell operating in reverse,” said William Chueh, an assistant professor of materials science and engineering at Stanford and a member of the Stanford Institute of Materials and Energy Sciences at SLAC National Accelerator Laboratory. “The hydrogen can be stored, and used later in the fuel cell to generate electricity at night or when the wind isn’t blowing.”
Fuel cells are not a perfect solution. The chemical reactions that cleave water into hydrogen and oxygen or join them together are not completely understood – at least not to the degree necessary to make utility-grade storage systems. Chueh is working alongside researchers from SLAC, Lawrence Berkeley National Laboratory and Sandia National Laboratories to study the chemical reactions in fuel cells in a new way. In an article published in Nature Communications, Chueh and his team describe how they observed the hydrogen-oxygen reaction in a specific type of high-efficiency solid-oxide fuel cell. They also took atomic-scale photos of the process using a particle accelerator called a synchrotron. This type of analysis is first-of-its-kind and help lead to more efficient fuel cells that could eventually allow for utility-scale alternative energy systems.
In a traditional fuel cell, a gas-tight membrane separates the anode and cathode. Oxygen molecules are introduced at the cathode where a catalyst fractures them into negatively charged oxygen ions. These ions then make their way to the anode where they react with hydrogen molecules to form the cell’s primary “waste” product: pure water. To perform these reactions, electrons also need to make the journey. Normally, the electrons are drawn to the cathode and the ions are drawn toward the anode, but while the ions pass directly through the membrane, the electrons can’t penetrate it; they are forced to circumvent it via a circuit that can be harnessed to run anything from cars to power plants.
Because electrons do the designated “work” of fuel cells, they are thought of as the critical functioning component. But ion flow is just as important, said Chueh.
“Electrons and ions constitute a two-way traffic pattern in many electrochemical processes,” Chueh said. “Fuel cells require the simultaneous transfer of both electrons and ions at the catalysts, and both the electron and ion ‘arrows’ are essential.”
Electron transfer in electrochemical processes such as corrosion and electroplating is relatively well understood, Chueh said, but ion flow has remained unclear. This is due to the environment where ion transfer may best be studied — catalysts in the interior of fuel cells — is not conducive to inquiry.
Solid-oxide fuel cells operate at relatively high temperatures. Certain materials are known to make superior fuel cell catalysts. Cerium oxide, or ceria, is particularly efficient. Cerium oxide fuel cells can hum along at 600 degrees Celsius, while fuel cells incorporating other catalysts must run at 800 C or more for optimal efficiency. Those 200 degrees represent a huge difference, Chueh said. “High temperatures are required for fast chemical reactivity,” he said. “But, generally speaking, the higher the temperature, the quicker fuel cell components will degrade. So it’s a major achievement if you can bring operating temperatures down.”
How Does It Work
While cerium oxide established itself strong catalysts for fuel cells, it is unclear why it works so efficiently. What were needed were visualizations of ions flowing through catalytic materials. But putting an electron microscope into the pulsing, red-hot heart of a fuel cell running at full bore isn’t exactly possible. “People have trying to observe these reactions for years,” Chueh said. “Figuring out an effective approach was very difficult.”
In their Nature Communications paper, Chueh and his colleagues at Berkeley, Sandia and SLAC split water into hydrogen and oxygen (and vice versa) in a cerium oxide fuel cell. While the fuel cell was running, they applied high-brilliance X-rays produced by Berkeley Lab’s Advanced Light Source to illuminate the routes the oxygen ions took in the catalyst. Access to the ALS tool and the cooperation of the staff enabled the researchers to create “snapshots” revealing just why ceria is such aFuel Cell superior catalytic material: it is, paradoxically, defective. “In this context, a ‘defective’ material is one that has a great many defects — or, more specifically, missing oxygen atoms — on an atomic scale,” Chueh said. “For a fuel cell catalyst, that’s highly desirable.”
Such oxygen “vacancies,” he said, allow for higher reactivity and quicker ion transport, which in turn translate into an accelerated fuel cell reaction rate and higher power.
“It turns out that a poor catalytic material is one where the atoms are very densely packed, like billiard balls racked for a game of eight ball,” Chueh said. “That tight structure inhibits ion flow. But ions are able to exploit the abundant vacancies in ceria. We can now probe these vacancies; we can determine just how and to what degree they contribute to ion transfer. That has huge implications. When we can track what goes on in catalytic materials at the nanoscale, we can make them better — and, ultimately, make better fuel cells and even batteries.”