Fuel cells are gaining in importance as an alternative to electromobility. Their service life is limited because the catalytic converters corrode quickly. Specially layered nanoparticles avoid these disadvantages. Fuel cells use various redox reactions to generate electrical energy. The energy carrier, such as hydrogen, is oxidized and oxygen is reduced. Electrons flow. Platinum metals catalyze, i.e. accelerate and facilitate the reduction, otherwise the energy barrier would be too high. However, they make the cells expensive. Mobile systems with phosphoric acid as the electrolyte solution achieve around 5,000 operating hours, and their service life is very limited. Scientists at Brown University in Providence, Rhode Island, circumvent these disadvantages with specially designed multi-layer nanoparticles.
The researchers already knew from earlier experiments that alloys are more suitable than pure platinum, not least for economic reasons. The need for expensive precious metals is lower. However, under the conditions prevailing in fuel cells, less noble alloy components are quickly washed out. Phosphoric acid fuel cells operate at 135 to 200 °C. The electrolyte is extremely corrosive. In addition, catalysts are quickly “poisoned”, i.e. deactivated.
The scientists have now developed nanoparticles with a special structure. Their particles have a platinum outer shell. In the core are layers of platinum and cobalt that alternate with each other. Shouheng Sun, a chemistry professor at Brown University, rates the design as "key to catalyst reactivity and durability." The arrangement would make platinum atoms on the surface more reactive and cobalt atoms inside protected.
First tests in the laboratory.
After successfully completing their metallurgical work, the researchers investigated the catalyst's ability to reduce oxygen. The reaction determines how long-lived a catalyst is. Tests in the laboratory confirmed all expectations. The new catalyst was still active even after 30,000 voltage cycles, which corresponds to 5 years in a fuel cell car. A conventional catalyst for fuel cells showed significantly poorer results; Performance data collapsed after 30,000 cycles. The laboratory tests are an important indication, but only partially correspond to reality. The temperature is higher in commercial devices and the metals therefore corrode even faster.
Catalyst exceeds US Department of Energy requirements
So Sun sent his catalyst to Los Alamos National Laboratory (LANL) for further study. Engineers from the US Department of Energy installed the new alloy in commercially available fuel cells and conducted stress tests. According to Sun, this phase is important because he hopes for rapid application in cars.
Their results exceeded the requirements of the United States Department of Energy (DOE), i.e. the American Department of Energy. According to the DOE, by 2020 all catalysts in new fuel cells should have a minimum of 0.44 amps per milligram of platinum. After 30,000 voltage cycles, i.e. 5 years in the vehicle, the DOE requires at least 0.26 amps per milligram. The new alloy was 0.56 amps per milligram at the start of the current draw and 0.45 amps per milligram after 30,000 voltage cycles.
"Even after that time, our catalyst still exceeded the DOE target for activity," says Sun. "This kind of performance in a real fuel cell is really promising." He wants to continue work on the catalyst and has already applied for a patent. The corresponding market exists. In addition to questions about the safe transport of hydrogen or other gases, short-lived catalysts are among the greatest challenges for mobile fuel cells.