Nano-Engineering of High Performance Pt-Alloy Intermetallics

Abstract

Pt-alloy (Pt-M) nanoparticles (NPs) with less expensive 3d transition metals (M = Ni, Cu, Co) supported on high surface area carbon supports, are currently the state-of-the-art (SoA) solution to reach the production phase in proton exchange membrane fuel cells (PEMFCs).1 However, while Pt-M electrocatalysts show promise in terms of increased activity for oxygen reduction reaction (ORR) and thus, cost reductions related with a significantly lower use of expensive and rare Pt, many key challenges remain at unlocking their true potential.This work systematically tackles several of these key challenges with a combination of electrocatalysts synthesis and characterization methodologies, namely thin-film rotating disc electrode (TF-RDE), electrochemical flow cell coupled to inductively coupled plasma mass spectrometer (EFC-ICP-MS) as well as the membrane electrode assembly (MEA). For instance, intermetallics as a sub-class of Pt-M electrocatalysts, holds promise at improving their intrinsic stability, however, usually at the sacrifice of the electrochemically active surface area (ECSA). In relation to this, we show a production pathway based on the proprietary double passivation with galvanic displacement (GD) method 2,3 as an intrinsically better methodology for deposition of Pt NPs, combining both the intermetallic structure and very high ECSA in the same electrocatalyst material. This is possible due to the intrinsically better mechanism of Pt NP deposition on carbon substrates. Whereas Pt NP synthesis and deposition is sequential in nature (2. step process) when using conventional deposition methods, in the case of double passivation with GD method Pt NPs crystallize directly out of the carbon support and combining these two crucial steps into a single one (Scheme 1). Secondly, we highlight the decisive importance of the chemical activation (de-alloying) step on the performance of Pt-M electrocatalysts in the MEA, namely at high current densities (HCDs). In addition, we provide the scientific community the necessary tools to properly evaluate their suitability of de-alloyed (chemically activated) Pt-M electrocatalysts using a much simpler and affordable TF-RDE methodology by using the well-known CO-electrooxidation.