Solar power and other sources of renewable energy can help combat global warming but they have a drawback: they don’t produce energy as predictably as plants powered by oil, coal or natural gas. Solar panels only produce electricity when the sun is shining, and wind turbines are only productive when the wind is brisk. Ideally, alternative energy sources would be complemented with massive systems to store and dispense power – think batteries on steroids. Reversible fuel cells have been envisioned as one such storage solution.
Fuel cells use oxygen and hydrogen as fuel to create electricity; if the process were run in reverse, the fuel cells could be used to store electricity, as well. “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.”
But like the renewable energy sources they seek to complement, fuel cells also have a drawback: the chemical reactions that cleave water into hydrogen and oxygen or join them back together into water are not fully understood – at least not to the degree of precision required to make utility-grade storage systems practical. Now Chueh, working with researchers at SLAC, Lawrence Berkeley National Laboratory and Sandia National Laboratories, has studied the chemical reactions in a fuel cell in a new and important way.
In a paper published today 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 and took atomic-scale “snapshots” of this process using a particle accelerator known as a synchrotron.
The knowledge gained from this first-of-its-kind analysis may lead to even more efficient fuel cells that could, in turn, make utility-scale alternative energy systems more practical.
This image shows miniaturized fuel cells probed with high brilliance X-rays at the Advanced Light Source at Lawrence Berkeley National Laboratory. Credit: Chueh Lab, Stanford Engineering
Role of electrons
In a typical fuel cell, the anode and cathode are separated by a gas-tight membrane. 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 both of these reactions, electrons also need to make the journey: electrons are added to the cathode and removed from the anode. 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 popularly perceived as the critical functioning component. But ion flow is just as important, said Chueh, a center fellow at Stanford’s Precourt Institute for Energy.
“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.”