Researchers at UCLA, with colleagues at UC Irvine, have designed a graphene-nanopocket-encaged platinum cobalt (PtCo@Gnp) nanocatalyst for fuel cells with good electrochemical accessibility and exceptional durability under a demanding ultralow PGM loading (0.070 mg_PGM cm^–2) due to the non-contacting enclosure of graphene nanopockets.

The PtCo@Gnp delivers a state-of-the-art mass activity of 1.21 A mg_PGM^–1, a rated power of 13.2 W mg_PGM^–1 and a mass activity retention of 73% after an accelerated durability test. With the greatly improved rated power and durability, the researchers project a 6.8 g_PGM loading for a 90 kW PEMFC vehicle; the loading approaches that used in a typical catalytic converter.

A paper on their work appears in the journal Nature Nanotechnology.

The graphene-wrapped alloy yielded 75 times more catalytic activity 65% more power about 20% more catalytic activity at the expected end of the fuel cell’s life about 35% less loss of power after testing that simulates 6,000 to 7,000 hours of use, beating the target of 5,000 hours for the first time. Credit: Huang Group UCLA

This has never been done before. This discovery involved some serendipity. We knew we were onto something that might make smaller particles stable, but we didn’t expect it to work this well.

—corresponding author Yu Huang, professor and chair of the Department of Materials Science and Engineering at the UCLA Samueli School of Engineering, and a member of the California NanoSystems Institute at UCLA

Today, half of the total global supply of platinum and similar metals is used for catalytic converters in vehicles powered by fossil fuels; somewhere between 2 and 8 grams of platinum are required per vehicle. By comparison, current hydrogen fuel cell technology uses about 36 grams per vehicle.

At the lowest load of platinum tested by Huang and her team, each hydrogen-powered vehicle would need only 6.8 grams of platinum.

The researchers broke up the platinum-cobalt alloy catalyst into particles an average of 3 nanometers long; smaller particles mean more surface area, and more surface area means more real estate where catalytic activity can occur. However, tinier particles are also far less durable, because they tend to pull off of a surface or crowd together into larger particles.

Huang and her colleagues addressed this limitation by armoring their catalyst particles in graphene nanopockets, which kept the particles from migrating. At the same time, the graphene allowed for a tiny gap of about 1 nanometer around each catalyst nanoparticle, which meant that key electrochemical reactions could occur.

This latest advance follows a recent collaborative study led by Huang that produced a model for predicting the catalytic activity and durability of a platinum-based alloy that can be used to guide the design of catalysts— the first of its kind. (Earlier post.) She and her team are working to translate their experimental results into practical technology that can be taken to the market.

The study’s co-first authors are postdoctoral researcher Zipeng Zhao and doctoral student Zeyan Liu, both of UCLA. Other UCLA authors are doctoral students Ao Zhang, Wang Xue and Bosi Peng; and Xiangfeng Duan, professor of chemistry and biochemistry at UCLA College and a member of the CNSI. UC Irvine faculty member Xiaoqing Pan and his postdoctoral researcher Xingxu Yan helped with imaging graphene nanopockets.

The research received funding from the US Office of Naval Research.

Resources

• Zhao, Z., Liu, Z., Zhang, A. et al. (2022) “Graphene-nanopocket-encaged PtCo nanocatalysts for highly durable fuel cell operation under demanding ultralow-Pt-loading conditions” Nat. Nanotechnol. doi: 10.1038/s41565-022-01170-9