Unveilling the Degradation Pathway of Highly Defective Hollow PtNi/C in Operando Conditions

Abstract

Due to strain and ligand effects, the simultaneous presence of concave and convex surfaces and their highly-defective nanostructure (atomic vacancies, grain boundaries), highly defective hollow PtNi/C electrocatalysts have proven to enhance remarkably the oxygen reduction reaction (ORR) kinetics [1,2]. However, a technologically-relevant proton-exchange membrane fuel cell (PEMFC) cathode catalyst should be able to maintain its initial catalytic activity on the long-term. This is hardly feasible on nanocatalysts based on platinum alloyed with transition metals (PtM alloys, M being a transition metal) or M-rich core@Pt-rich shell nanoparticles due to the dissolution of the transition metal in the harsh operating conditions of a PEMFC cathode but the question remains open for nanocatalysts in which catalytic activity is not solely due to alloying effects. Herein, the stability of highly defective hollow PtNi/C nanoparticles was evaluated in X-Rays operando conditions under simulated PEMFC operating conditions (5,000 potential cycles between 0.6 and 1.0 V or 1.1 V vs. RHE at T = 80°C). The combined physical, chemical and electrochemical results show that the degradation pathway of highly defective hollow PtNi/C nanoparticles closely depends on the upper potential limit of the cycling protocol (Figure 1). Beyond 1V, the restructuring kinetics is accelerated compared to the 0.6 – 1.0 V protocol and the losses in ORR activity are related to both the weakening of strain and ligand effects associated with the dissolution of Ni atoms and to the decrease of the concentration of structural defects. [1] L. Dubau, T. Asset, C. Bonnaud, R. Chattot, V. van Peene, F. Maillard, ACS Catal., 5 (2015) 5333-5341. [2] L. Dubau, J. Nelayah, S. Moldovan, O. Ersen, P. Bordet, J. Drnec, T. Asset, R. Chattot, F. Maillard, ACS catal., 6 (2016) 4673-4684. Figure 1