Rational Catalyst Design Methodologies: Principles and Factors Affecting the Catalyst Design

Abstract

The main focus of this chapter is the rational design of efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) on hydrogen fuel cell cathodes. The prohibitively high cost of the so-far best platinum-based ORR catalysts is one of the major obstacles for the large-scale application of hydrogen fuel cells. In addition, such catalysts have relatively low activity toward ORR and low electrochemical stability. Therefore, designing cost-effective, highly active, and stable ORR catalysts is of great importance to address the pressing need of renewable and sustainable energy resources. At present, the main research direction in such regard focuses on core–shell structures in which the shell is a monolayer of a catalytically active element (AE) and the core is an inexpensive metal substrate (MS). Therefore, it is critical to pin down the key factors determining the thermodynamic/electrochemical stability and catalytic activity of tailored catalysts such as structural match or mismatch between AE and MS and the hybridization between the AE and MS electronic states. As such, the rational selection of the AE and MS elements requires understanding of the tight relation between four variables: composition, electronic structure, binding energies of the ORR intermediates, and activity toward ORR . From our point of view, the least known link in the above chained dependence is the relation between electronic structure and binding energy. A number of models rationalizing that link (d-band center model, linear scaling between binding energies of different ORR intermediates, and relation between surface strain and its reactivity) have been shown to be insufficiently accurate to predict surface reactivity. Moreover, the electrochemical and thermodynamic stability of the AE/MS structures calls for a strong hybridization between the AE and MS electronic states, for which designing catalysts for practical applications requires fulfilling such demand. In this chapter, based on the wealth of information available from two decades of research, we will discuss the paths to efficiently design better ORR catalysts. Regarding the compromise of electrochemical stability (namely, most elements dissolve in the reaction environment) and ORR activity, we will discuss ways (a) to increase the dissolution potential of AE by depositing it on the proper MS; (b) to activate toward ORR some electrochemically stable elements (e.g., Au) through the AEMS interaction. The thermodynamic stability will also be rationalized in terms of the ratio between the strength of the AEAE, AEMS, and MSMS bonds, cohesive energy of AE and MS elements, and surface energies. Our approach to rational design of the ORR catalysts consists of (a) using the existing knowledge on the above-mentioned relations to preselect promising candidates for the ORR catalysts (a dozen, not hundreds or thousands); (b) performing first principles calculations to confirm and quantify the properties in question (in particular, thermodynamic and electrochemical stability and activity toward ORR), and narrow down the selection; (c) testing experimentally the systems found to be most promising in steps (a) and (b). We will use our recent results to illustrate the efficiency of this approach.