Design of catalysts for energy conversion and storage

Yijin Kang, Materials Science Division, Argonne National Laboratory

My research interests and achievements focus on fundamental understanding of catalytic processes and development of highly active (or selective) and durable catalysts for the reactions of industrial importance, especially for energy conversion and storage. I establish a research system that connects fundamental investigation on well-defined extended surfaces (e.g. single crystal surfaces), extrapolation onto nanocrystals with highly controlled shape and size, exploration of interfacial interaction using novel nanocrystal superlattices as platform, and finally design of high performance catalysts in which all the possible beneficial properties from complex functional structures are implemented.
The fundamental understanding of correlationship between materials structure and catalytic properties is the key to design new catalysts with desired properties. The very first step would be to understand the most simplified surface processes using well-defined extended surfaces and computer-powered theoretical simulations. However, the real catalysts used in industrial processes are mainly nanomaterials that take advantage of their high surface-to-volume ratio. It is necessary to confirm that the desirable properties found in simplified cases are actually functioning in the nanomaterials. Therefore, well-defined nanocrystals

Figure 1. High quality Pt nanocrystals with various shapes. The size is tunable in the range

1

with exposed surfaces mimicking the simplified surfaces which are used in theoretical studies are highly valuable and appreciated. My work at this level

of 2-30nm.

focuses on providing high quality model materials at nanoscale. For instance, in order to study the surface sensitive reactions on
Pt surface at nanoscale, I prepared high quality Pt nanocrystals with various controlled shapes1,2, exposing selective surfaces (e.g.
{111} on octahedra and icosahedra, {100} on cubes, and {110} on rhombic dodecahedra). The library of such highly controlled nanocrystals with desired surface is not limited to single element metals3, but also include alloys4,5 and oxides6.
Besides desired surfaces, the interactions among the components within the catalysts are also important. For example, in the Pt-Ni alloy Ni atoms modify the electronic structure of Pt, weakening the adsorption of OHad spectator species on Pt surfaces, thus enhancing the oxygen reduction reaction (ORR) activity. Because of certain desired metal-oxide interfacial interactions that promote catalysis or introduce synergistic effect, metal catalysts supported on oxide supports are the other class of important catalysts that exploit beneficial interactions. In additional to well-controlled size and shape, the structures rendering the desired interactions are needed to carry out the investigation of interfacial effects. I established a platform using binary nanocrystal superlattices (BNSLs) to construct tunable interactions7,8. For example, by constructing Au-FeOx BNSLs with various structures (AB, AB2, AB13), I was able to control the population of Au-FeOx contacts. In the investigation of CO oxidation, a linear correlation between population of contacts and activity was found. Such correlation provided strong evidence that the interface of Au-FeOx contacts is responsible for the extraordinary activity toward CO oxidation, rather than small sized Au7.
Finally, with all the knowledge learned from theoretical studies and investigation on model materials, I am able to design catalyst that incorporates the beneficial properties in a single nanostructure. For example, my Berkeley collaborators observed a shape evolution from solid PtNi3 nanocrystal to hollow Pt3Ni nanoframes9. I took advantage of the hollow structure and Pt3Ni composition of the nanoframes. I treated the surface of nanoframes to promote the surface segregation, obtaining the surface structure mimicking the optimal Pt3Ni(111)-Pt-Skin for ORR. The hollow structure of nanoframes not only substantially increases the active surface area, but also provides a capillary force to hold a specific ionic liquid in which oxygen solubility is much higher than that in regular acid electrolyte. Therefore, combining all these beneficial properties for ORR in surface segregated Pt3Ni nanoframes, I was able to achieve a 36-fold enhancement in activity in comparison to state-of-the-art Pt/C catalysts9. The similar approach is applicable to other catalyst systems and for other important reactions. For instance, I made Pt-Pb core-shell structure with desirable Pt3Pb intermetallic structure, in order to overcome the poisoning problem thus to enhance the overall activity for formic acid oxidation10.
In summary, I have established an effective catalyst designing route that extracts knowledge from model study and theoretical computation, fabricates desirable surfaces and constructs beneficial

Figure 2. Nanocrystal superlattices, allowing programmable inter-particle

contacts.7

Figure 3. Pt3Ni catalysts for ORR (oxygen electrode of fuel cells) that incorporated all beneficial properties in one nanostructure.9

structures, and ultimately directs the production of real catalysts of industrial interests. I am seeking for a tenure-track faculty
position at R1 universities at the cycle of 2014-2015. I would like to focus my future research on energy conversion and storage, especially on solar-to-fuel conversion and advanced battery technologies.

Notes and references:

1. Kang et al. J. Am. Chem. Soc., 2013, 135 (7), 2741

2. Kang et al. ACS Nano, 2013, 7(1), 645

3. Kang et al. Angew. Chem. Int. Ed., 2010, 49(35), 6156

4. Kang et al. J. Am. Chem. Soc., 2010, 132(22), 7568

5. Kang et al. ACS Nano, 2012, 6 (6), 5642

6. Kang et al. Angew. Chem. Int. Ed., 2011, 50(19), 4378

7. Kang et al. J. Am. Chem. Soc., 2013, 135 (4), 1499

8. Kang et al. J. Am. Chem. Soc., 2013, 135 (1), 42

9. Kang et al. Science, 2014, DOI: 10.1126/science.1249061

10. Kang et al. ACS Nano, 2012, 6 (3), 2818