Researchers at Johns Hopkins University, with colleagues at Purdue and Oak Ridge National Laboratory (ORNL), have plated a one nanometer thick coating of platinum on a core of cobalt to create a cost-effective and highly efficient fuel cell catalyst. A paper on their work was published last year in the ACS journal Nano Letters.
The specific electrochemically active surface areas (ECSA) estimated are 54 m2/gPt for Co@Pt and 43 m2/gPt for Pt. Although the overall particle size of Co@Pt is significantly larger than Pt (~5 nm), the Co@Pt catalyst has a higher specific surface area owing to the presence of non-precious cobalt in the particle core, the researchers said.
The mass activity of Co@Pt achieved 1.21 A/mgPt—an improvement factor of 8.1 versus the commercial Pt catalyst. In addition to the high catalytic activity observed in the initial electrochemical studies, the researchers observed further activation of the Co@Pt catalysts after potential cycling in the ORR-relevant regime.
After 5,000 cycles between 0.6 and 1.0 V, the specific activity was raised to 3.02 mA/cm2, while negligible loss was observed in ECSA. As a result, the mass activity achieved 1.45 A/mgPt after the potential cycling, corresponding to an improvement factor of 9.7 versus Pt. The loss in catalytic activity after further potential cycling was found to be insignificant (e.g., up to 10,000 cycles).—Wang et al.
The cobalt/platinum core/shell (denoted as Co@Pt) nanoparticles were synthesized via seed-mediated growth. The Co seeds were first synthesized by thermal decomposition of cobalt carbonyl, and the Pt shell was overgrown in situ by adding platinum acetylacetonate.
There’s a lot more cobalt out there than platinum. We’ve been able to significantly stretch the benefits of platinum by coating it over cobalt, and we even managed to enhance the activity of platinum at the same time.—lead author Lei Wang
Earlier attempts to plate precious metals on non-precious materials were largely stymied by galvanic replacement reactions—oxidation of the non-precious metal. In this study, the team successfully suppressed such reactions by introducing carbon monoxide, a gas molecule that strongly binds to cobalt, protecting it from oxidation.
The researchers said the enhanced catalytic activity—almost 10x Pt alone—resulted from both the maximized exposure of platinum atoms on the surface and from interactions between the two metals.
The intimate contact between cobalt and platinum gives rise to compressive strain. It shortens the distance between platinum atoms and makes the chemical reactions more feasible on the surface.—Lei Wang
Because platinum and other rare metals play key roles in many industrial applications, the implications of this work extend beyond fuel cells. Currently, the team is working on adapting their technique to other precious metals and non-precious substrates. New developments will target further applications of such materials in chemical conversions of hydrocarbons.
The research was supported by National Science Foundation grant DMR-1410175 and a Johns Hopkins University Catalyst Award. The researchers have obtained a provisional patent covering this technique through the Johns Hopkins Technology Ventures office.
Lei Wang, Zhenhua Zeng, Cheng Ma, Yifan Liu, Michael Giroux, Miaofang Chi, Jian Jin, Jeffrey Greeley, and Chao Wang (2017) “Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts” Nano Letters 17 (6), 3391-3395 doi: 10.1021/acs.nanolett.7b00046