High-performance, cost-effective nanoparticle electrocatalyst for fuel cells outperforms commercial Pt/C catalyst
Scientists at Korea’s Institute for Basic Science’s (IBS’) Center for Nanoparticle Research and colleagues at other institutions in Korea have synthesized highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell as fuel cell electrocatalysts.
The ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, the team demonstrated long-term stability in the membrane electrode assembly (MEA) for 100 hours without significant activity loss. A paper on their work is published in theJournal of the American Chemical Society.
The ordered fct-PtFe NPs—only a few nanometers in size—are obtained by thermal annealing of polydopamine-coated PtFe NPs. The N-doped carbon shell that is in-situ formed from dopamine coating effectively prevents the coalescence of NPs. It also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions.
By controlling the thickness of the shell below 1 nm, the team achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules.
|Synthesis of ordered intermetallic fct PtFe/C. Source: IBS. Click to enlarge.|
Nanoparticle-based electrocatalysts have been intensively investigated for fuel cell applications over the past decade, mainly motivated by their high mass activity. Many research groups have made great effort to utilize the high activity and surface area of nanoparticles (NPs) in order to make a breakthrough for fuel cell commercialization. However, practical use of nanomaterials for fuel cell electrocatalyst was impeded by their low physical and chemical stability. Under the standard fuel cell operating conditions, NPs are often oxidized, dissolved, or detached from the support and agglomerated into larger particles, losing their electrochemical catalytic activity during cycling. Moreover, according to theoretical calculations, the oxidation and dissolution potentials of NPs tend to decrease with their sizes.—Chung et al.
Ordered intermetallic NPs are considered one of the most promising candidates to deliver both high activity and stability in practical fuel cell applications. However, the synthesis of small-sized ordered fct-PtFe NPs with high mass activity has been rarely reported thus far, the researchers noted.
PtFe NPs have disordered fcc structure, and so require thermal annealing at ∼700 °C to transform them into an ordered fct structure. This process inevitably leads to the coalescence of the NPs. To prevent this, protective coating of the NPs with inorganic shells or physical barriers has been suggested. This approach, however, requires the additional step of removing the coating layer from the surface of the NPs to expose the active sites—increasing time and cost in production.
A different approach to improve long-term stability also uses a protective coating. Coating NPs deposited on the support with a protective layer can make the NPs more resistive toward detachment and agglomeration. Various coating layers, including a carbon shell, an inorganic barrier, and graphitic hollow spheres have been tried. The protective layer-coated NPs show better performance in terms of the long-term stability compared to the uncoated counterparts. N-doped carbon has very good affinity to the surface of metal NPs.
Being inspired by those two different approaches to improve the long-term stability of nanocatalysts, we designed a nanoparticle-based electrocatalyst that combines those approaches together and overcomes their limitations.
… We believe that this approach can open a new possibility for the development of high performance and cost-effective fuel cell catalysts in the near future.—Chung et al.
Dong Young Chung, Samuel Woojoo Jun, Gabin Yoon, Soon Gu Kwon, Dong Yun Shin, Pilseon Seo, Ji Mun Yoo, Heejong Shin, Young-Hoon Chung, Hyunjoong Kim, Bongjin Simon Mun, Kug-Seung Lee, Nam-Suk Lee, Sung Jong Yoo, Dong-Hee Lim, Kisuk Kang, Yung-Eun Sung, and Taeghwan Hyeon (2015) “Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction” Journal of the American Chemical Society Article ASAP DOI: 10.1021/jacs.5b09653