Tailoring Shapes and Sizes of Pure Pt Electrocatalysts to Improve the Oxygen Reduction Activity
Authors: Rueck Marlon , Gagliardi Alessio 
Published: February 2020

Based on the Sabatier principle, it is widely accepted that the ORR activity is decisively controlled by the adsorption energies of the reaction intermediates. Weakening the *OH adsorption energy by ~0.1-0.15 eV with respect to Pt(111) improves the ORR activity, which has been shown by various experimental [1] and theoretical [2] studies. Recently, Calle-Vallejo et al. have extended the geometrical concept of coordination numbers to second nearest neighors, which reveals fundamental design principles on how to tailor active single sites in unstrained pure Pt nanostructured electrocatalysts [2]. The effects from the atomic coordination stress that the interplay between shapes and sizes tune the adsorption energies and, therefore, shapes and sizes rule the ORR activity.
Herein, we combine site-specific design principles from theory with experimental studies to develop a computational model [3], which rapidly predicts ORR mass activities of arbitrarily nanostructured pure Pt electrocatalysts for proton-exchange membrane fuel cells (PEMFCs) in precise agreement with experiments. For spherical nanoparticles, optimal activities are predicted for sizes near 1.1, 2.07, and 2.87 nm with very small size distributions around 0.1 nm (see figure below). Subsequently, the proposed 1.1 nm sized pure Pt nanoparticles with sufficiently small 0.17 nm size distribution have been synthesized experimentally in a metal-organic framework [4]. Their observed mass activities (0.87±0.14 A/mgPt) are close to the computational prediction (0.99 A/mgPt), which improves the mass activity of Tanaka commercial Pt/C (0.42 A/mgPt) by a factor of 2. Furthermore, the stability of our MOF-derived 1.1 nm sized nanoparticles is comparable to the Tanaka commercial Pt/C electrocatalysts.
The theoretical limit that pure Pt spherical nanocatalysts cannot exceed ~2 A/mgPt in mass activity [3] underlines the high demand for shape and size engineering to achieve electrocatalysts with further improved mass activities. Based on a screening of thousands of shapes, we tailor nanoparticle shapes and sizes toward highest mass activity, which simultaneously feature controllable size distribution and appropriate mechanical stability [5]. The predicted optimal shapes have high mass activities up to 4.28 A/mgPt, which corresponds to a 7.8-fold enhancement over (ionomer-free) Tanaka commercial Pt/C electrocatalysts. For the optimal electrocatalysts, we theoretically study single-atom-resolved size effects on the mass activity to provide experimentalists with practical synthesis guidelines.
[1] V. Čolić, A.S. Bandarenka, ACS Catalysis (2016), 6, 8.
[2] F. Calle-Vallejo, M.D. Pohl, D. Reinisch, D. Loffreda, P. Sautet, A.S. Bandarenka, Chem. Sci. (2017), 8, 3.
[3] M. Rück, A. Bandarenka, F. Calle-Vallejo, A. Gagliardi, J. Phys. Chem. Lett. (2018), 9, 15.
[4] B. Garlyyev, K. Kratzl, M. Rück, A.S. Bandarenka, A. Gagliardi, R.A. Fischer et al., (2019), in review.
[5] M. Rück, A. Bandarenka, F. Calle-Vallejo, A. Gagliardi, (2019), in review.

Figure 1