Researchers at Stanford, working with colleagues at SLAC, Lawrence Berkeley National Laboratory and Sandia National Laboratories, have observed the hydrogen-oxygen reaction in a cerium oxide (ceria) fuel cell and taken atomic-scale “snapshots” of this process using a synchrotron. Knowledge gained from this first-of-its-kind analysis, reported in Nature Communications, may lead to even more efficient fuel cells that could, in turn, make utility-scale alternative energy systems more practical, the team suggested.
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: water. To perform both of these reactions 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 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, a center fellow at Stanford’s Precourt Institute for Energy.
Electrons and ions constitute a two-way traffic pattern in many electrochemical processes. Fuel cells require the simultaneous transfer of both electrons and ions at the catalysts, and both the electron and ion “arrows” are essential.—William Chueh
Electron transfer in electrochemical processes such as corrosion and electroplating is relatively well understood, Chueh said, but ion flow has remained opaque. That’s because the environment where ion transfer may best be studied—catalysts in the interior of fuel cells—is not conducive to inquiry.
People have trying to observe these reactions for years. Figuring out an effective approach was very difficult.—William Chueh
In the 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 that ceria’s catalytic attributes derive from its numerous defects—specifically, missing oxygen atoms.
Those missing oxygen atoms are highly desirable for a fuel cell catalyst, Chueh said; the missing atoms 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. 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.—William Chueh
Zhuoluo A. Feng, Farid El Gabaly, Xiaofei Ye, Zhi-Xun Shen & William C. Chueh (2014) “Fast vacancy-mediated oxygen ion incorporation across the ceria–gas electrochemical interface,” Nature Communications 5, Article number: 4374 doi: 10.1038/ncomms5374