475963 Towards the Computational Design of Heterogeneous Electrocatalysts

Sunday, November 13, 2016
Continental 4 & 5 (Hilton San Francisco Union Square)
Zhenhua Zeng, Chemical Engineering, Purdue University, West Lafayette, IN

Research Interests:

Design of cost-effective electrocatalysts with enhanced activity and stability is of paramount importance for the next generation of energy conversion systems, including fuel cells and electrolyzers. Ultrathin (hydroxy)oxide films on transition metal substrates possess impressive activities for these reactions. These hybrid systems with novel properties are often prepared and characterized under very specific conditions, such as ultrahigh vacuum. When subsequently used in various applications, however, significant structural evolution, which depends strongly on the reaction conditions in-situ, is expected but remains largely unexplored. In order to identify structure-property relationships and, ultimately design/screen new materials with improved performance, the development of such understanding is essential.

The complex interfaces have long been a challenge for modeling, because of the complications introduced by strongly-correlated transition metal oxides and mysterious interface structures. As a consequence, even many ancient questions are still unanswered, for example the origin of the strong metal support interaction (SMSI) that plays a central role on the stability of the interfaces.

Recently, I have resolved the major challenge toward self-consistent and highly accurate description of strongly-correlated transition metal oxides, hydroxides and oxyhydroxides, through synergistic use of a Hubbard U correction, a state-of-the-art dispersion correction, and a water-based bulk reference state for the calculations. The strong performance is illustrated on a series of bulk transition metal (Mn, Fe, Co and Ni) hydroxides, oxyhydroxides, binary, and ternary oxides, where the corresponding thermodynamics of redox and (de)hydration are described with standard errors of 0.04 eV per (reaction) formula unit.

I have applied these methodologies to investigate the growth of monolayer Ti- and Fe-(hydroxy)oxide films on VIIB group and IB group noble transitional metal surfaces, and have obtained mechanistic understanding of the origin of strong metal support interaction. This understanding has been extended to predict the structure evolution of the other monolayer transitional metal (Mn, Co, and Ni) (hydroxy)oxide films on a variety of substrates. It has been demonstrated that the structure and oxidation state of the films can be systematically tuned by changing the applied electrode potential, the nature of substrates and/or the size of film cluster, which is further confirmed by synchrotron experiment on the monolayer Ni-(hydroxy)oxide/Pt, a hydrogen evolution (HER) electrocatalyst with improved performance in comparison with traditional Pt catalyst in alkaline conditions.

The identified interface under hydrogen evolution HER condition, NiOH/Pt, is subsequently used for a detailed structural and electrocatalytic analysis at three-phase boundaries of NiOH films, Pt substrates, and the surrounding water. The kinetic analysis shows that water can be dissociated to adsorbed OH* and H* groups in a bifunctional mechanism that significantly lowers the activation barriers for alkaline HER and provides a clearly measureable acceleration of the catalyst process as compared to mono-functional Pt catalysts. The mechanistic understanding and kinetic analysis also suggest a new catalyst, NiOH/Pt3Ni, with improved performance in comparison with NiOH/Pt, which is further confirmed by electrochemical experiment.

The radical development on highly accurate description of strongly-correlated transition metal oxides, the mechanistic understanding of interface structure evolution and structure-properties relationships, and the successful prediction of new electrocatalyst has provided the foundation towards rational design of novel catalysts with increased activity, improved selectivity and/or under-controlled stability. In my future research, by leveraging and combining my experience, we will open up fundamentally new research thrusts, providing fundamental scientific insight into the interfacial properties of the relevant systems, and ultimately identifying new electrocatalysts of fuel cells, electrolyzers, CO oxidation and CO2 reduction, and heterogeneous catalyst of CO oxidation and water gas shift reactions.

Teaching Interests:

Albert Einstein once said “Most of the fundamental ideas of science are essentially simple, and may, as a rule, be expressed in a language comprehensible to everyone”. I would like to say it is also a rule for my teaching. I find that most courses are essentially made up of concepts, not simply equations or mathematics. The concepts may be expressed in many specific ways and used in different fields, but the essence does not change, which is exactly like the second law in thermodynamics. The essential simplicity and generality make the concepts can be retained in students' memories for a long time period, and can be smoothly used to different fields later in life. Thus, good teaching should focus on the concepts; this is what I have learned from my study and research experience. For example, there are so many equations and derivations I learned but never use in my research; however, I still clearly remember conceptual mechanisms behind and can use them correctly when necessary. I have found this strategy useful when I teach “Advanced Modeling for Catalysis Studies” class and when I supervised graduate and undergraduate students. It has been highly effective when I first introduced the essential concept, then explained the role of each individual equation, and left the details for the students themselves to deepen the understanding. Thus my future teaching will mainly focus on the ideas and concepts, and using mathematics as a tool to express them more simply.

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