Argonne researchers develop new non-precious-metal fuel cell catalyst with performance comparable to platinum
Researchers at the US Department of Energy’s Argonne National Laboratory have developed a new fuel cell catalyst using earth-abundant materials with performance that is comparable to platinum in laboratory tests. The nanofibrous non-precious metal catalyst (NPMC) is synthesized by electrospinning a polymer solution containing a mixture of ferrous organometallics and metal-organic frameworks and then is thermally activated.
The resulting catalyst offers a carbon nanonetwork architecture made of microporous nanofibers decorated by uniformly distributed high-density active sites. As reported in an open access paper in Proceedings of the National Academy of Sciences (PNAS), in a single-cell test, the membrane electrode containing the catalyst delivered volumetric activities of 3.3 A⋅cm−3 at 0.9 V or 450 A⋅cm−3 extrapolated at 0.8 V, representing the highest reported value in the literature. The team also observed improved fuel cell durability.
|This figure shows the microstructural difference between conventional catalysts and the new reduced-platinum catalyst. (Image courtesy Di-Jia Liu) Click to enlarge.|
The high price of platinum—the predominant electrocatalyst in fuel cells—is a major cost barrier for large-scale implementation of polymer electrolyte membrane fuel cells. While non-precious metal catalysts (NPMCs) could be attractive low-cost alternatives, they suffer from significantly lower turnover frequency at the individual catalytic sites. This has made traditional carbon-supported NPMCs inadequate in reaching the desired performance delivered by platinum.
Platinum represents about 50 percent of the cost of a fuel cell stack, so replacing or reducing platinum is essential to lowering the price of fuel cell vehicles.—Di-Jia Liu, Argonne team leader
The Argonne NPMC replaces all the platinum in the fuel cell’s cathode, which usually requires four times as much platinum as the anode. The new electrode design also optimizes the flow of protons and electrons within the fuel cell and the removal of water.
Fuel cell catalysts must be densely packed with active sites that are uniformly distributed throughout the cathode and directly connected to the arriving protons and electrons, while maintaining easy access to oxygen, Liu said. The catalyst should also have an architecture that can readily channel away the produced water. No conventional method for preparing carbon-based platinum or non-precious metal catalysts can meet all these criteria, Liu added.
The performance of conventional carbon-supported catalysts is strongly influenced by the support morphology, which contains micropores, mesopores, and macropores. Whereas micropores host the majority of the active sites and macropores promote effective reagent/product mass transfer, mesopores contribute a limited role in both but occupy a significant fraction of the total pore volume. For catalytic applications where maximizing active site number and mass/charge transports with the highest possible catalyst density is essential, conventional carbon supports are no longer suitable. In this paper, we introduce a previously unidentified catalyst’s morphology with a high catalytic active surface concentrated nearly exclusively in micropores while transferring reactant/product via a macroporous nanofiber framework.—Shui et al.
The research was supported by the US Department of Energy’s Office of Science and the Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.
Jianglan Shui, Chen Chen, Lauren Grabstanowicz, Dan Zhao, and Di-Jia Liu (2015) “Highly efficient nonprecious metal catalyst prepared with metal–organic framework in a continuous carbon nanofibrous network” PNAS 112 (34) 10629-10634; doi: 10.1073/pnas.1507159112