(240e) Overcoming Site Heterogeneity in Search of Metal Nanocatalysts for Oxygen Reduction

Low temperature polymer electrolyte membrane (PEM) fuel cells have great potential as a clean and efficient electric energy generation system. An important obstacle to the commercialization of this technology is the significant voltage loss in converting chemical energy of fuels, e.g., H₂, to electricity, also known as overpotential, mainly due to the sluggish electrochemical kinetics of the oxygen reduction reaction (ORR) at the cathode, O₂ + 4(H⁺ + e^-) → 2H₂O (E₀: +1.23 V). The state-of-the-art elemental metal electrocatalyst, consisting of 2~5 nm Platinum (Pt) nanoparticles, sacrifices ~300 mV for appreciable current densities. Therefore, there has been a great deal of interest in identifying electrocatalysts that exhibit improved catalytic performance, possibly with a reduced amount of precious metals1.

Among many types of materials, multimetallic Pt monolayer electrocatalysts have emerged as a promising alternative. It has been demonstrated that the adsorption energies of oxygen-containing species (e.g., *O, *OH, and *OOH) at an active site are predictive ORR reactivity descriptors. The stability of those intermediates can be tuned by controlling the lattice strain (the bond distance of an active site with neighboring atoms) and the metal ligand (the nature of atoms surrounding a catalytic center). Since a perturbation of a metal site by alloying affects concurrently the ligand and strain, it is not known a priori which metals can be introduced in what geometric arrangements to attain desired catalytic properties. To accelerate catalyst discovery, it is of pivotal importance to develop an approach that efficiently maps catalytic activity onto geometry-based descriptors while considering the geometric strain and metal ligand of an active site2,3. We demonstrate that there exist linear correlations between orbitalwise coordination numbers and free formation energies of oxygen species (e.g., *OH and *OOH) at Pt sites. Kinetic analysis along with herein developed structure-activity relationships accurately predicts the activity trend of pure Pt nanoparticles (~1-7 nm) toward oxygen reduction. Application of the approach to an extensive search of Pt nanocatalysts leads to several Pt monolayer core-shell nanostructures with enhanced oxygen reduction activity and reduced cost.

(1) Xin, H.; Holewinski, A.; Linic, S. Predictive Structure–Reactivity Models for Rapid Screening of Pt-Based Multimetallic Electrocatalysts for the Oxygen Reduction Reaction. ACS Catal. 2012, 2 (1), 12–16.

(2) Ma, X.; Xin, H. Orbitalwise Coordination Number for Predicting Adsorption Properties of Metal Nanocatalysts. Phys. Rev. Lett. 2017, 118 (3), 036101.

(3) Wang, S.; Omidvar, N.; Marx, E.; Xin, H. Coordination Numbers for Unraveling Intrinsic Size Effects in Gold-Catalyzed CO Oxidation. Phys. Chem. Chem. Phys. 2018, 20 (9), 6055–6059.