Owing to their high energy conversion efficiency and environmental benignity as well as their applicability for small electronic devices, residential power generation, and automobile transportation, polymer electrolyte fuel cells (PEFCs) have long been considered promising energy conversion devices. As electrocatalysts of PEFCs, carbon-supported platinum-based nanoparticles have been used predominantly in anode as well as cathode electrodes. However, even Pt-based electrocatalysts exhibit sluggish kinetics for the oxygen reduction reaction (ORR) at the cathodes of PEFCs. Moreover, they tend to sinter or agglomerate into larger particles during long-term fuel cell operation, resulting in a marked loss of activity. The prohibitively high cost and scarcity of Pt have also been bottlenecks that further impede the widespread use of PEFCs. Therefore, the high cost along with the low durability of Pt-based catalysts triggered the quest for low-cost, high-performance non-precious metal catalysts.

...Here we report on a simple approach to scalable and highly reproducible synthesis of a new family of non-precious metal catalysts—self-supported, transition metal-doped ordered mesoporous porphyrinic carbons (M-OMPCs)—which exhibit Pt-like catalytic activity for the ORR. The M-OMPC catalysts were prepared by nanocasting ordered mesoporous silica (OMS) templates with metalloporphyrin precursors, and were constructed with three-dimensional networks of porphyrinic carbon frameworks. Our synthetic strategy for the non-precious metal catalysts included a multitude of advantages that would be favorable to PEFC applications.

—Cheon et al.

Such advantages include:

• The synthetic route is amenable to simple and mild experimental conditions.

• The pore size and connectivity of the M-OMPC catalysts were tunable by utilizing OMS templates with different pore sizes and structures.

• The synthesis of the M-OMPC catalysts could be readily scaled up, with the preservation of structural integrity, to a few tens of grams in a single batch.

• The well-developed, hierarchical micro-mesoporosity would be advantageous for efficient transport of fuels and by-products.

• The M-OMPC catalysts showed very high surface areas, which could significantly increase the density of the catalytically active sites accessible to reactants.

Among the family of M-OMPC catalysts, the Fe and Co co-doped OMPC (FeCo-OMPC) showed an extremely high electrocatalytic activity for ORR in acidic media. To our knowledge, its ORR activity is one of the best among the non-precious metal catalysts reported in the literature, and even higher than the state-of-the-art Pt/C catalyst by 80% at 0.9 V (vs. RHE). In addition, the FeCo-OMPC showed superior long-term durability and methanol-tolerance in ORR, compared to the Pt/C...We attribute the high ORR activity of the FeCo-OMPC to its relatively weak interaction with oxygen as well as the high surface area design of catalyst.

—Cheon et al.

The materials developed by the UNIST research team were prepared by nanocasting ordered mesoporous silica (OMS) templates with metalloporphyrin precursors. In addition they were constructed with three dimensional networks of porphyrinic carbon frameworks.

Comparison of kinetic currents of Pt/C and FeCo-OMPC catalysts before and after 10,000 potential cycles. Cheon et al. Click to enlarge.

To investigate the durability of the FeCo-OMPC catalysts, the team cycled the catalysts 10,000 times between 0.6 and 1.0 V (vs. RHE) at a scan rate of 50 mV s⁻¹, following the accelerated durability test protocol of the US Department of Energy (DOE).

They found that, based on the changes in half-wave potentials as well as kinetic current densities between the cycling tests, the FeCo-OMPC catalyst exhibited superior durability over the Pt/C catalyst. (See diagram at right.)

The M-OMPC catalysts also showed enhanced poison-tolerance compared to the Pt/C catalysts.

The research was led by Sang Hoon Joo, professor of the School of Nano-Bioscience and Chemical Engineering at South Korea’s UNIST. Fellow authors include: Jae Yeong Cheon from UNIST; Gu-Gon Park from the Korea Institute of Energy Research (KIER); and Radoslav R. Adzic from the Chemistry Department of the Brookhaven National Laboratory.

Recent years have witnessed a rapid progress in the development of electrocatalysts for ORR. For instance, nanoparticles composed of a Pt monolayer on PdAu core, intermetallic Pt-Co nanoparticles, and mesostructured PtNi thin films have demonstrated significantly improved ORR activity and durability in acidic media. We believe that, along with these new catalysts, the M-OMPCs could emerge as highly promising ORR catalysts. Furthermore, the design concept towards enhanced electrocatalytic performances presented in this work could be extended to other electrocatalytic reactions in energy devices, such as metal-air batteries and electrolyzers.

—Cheon et al.

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, the support from Korea Institute of Energy Research, National Junior Research Fellowship, and Global Ph.D. Fellowship.

Resources

• Jae Yeong Cheon, Taeyoung Kim, YongMan Choi, Hu Young Jeong, Min Gyu Kim, Young Jin Sa, Jaesik Kim, Zonghoon Lee, Tae-Hyun Yang, Kyungjung Kwon, Osamu Terasaki, Gu-Gon Park, Radoslav R. Adzic & Sang Hoon Joo (2013) Ordered mesoporous porphyrinic carbons with very high electrocatalytic activity for the oxygen reduction reaction. Scientific Reports 3, Article number: 2715 doi: 10.1038/srep02715