In Situ Electrochemical STM Study of Potential-Induced Coarsening and Corrosion of Platinum Nanocrystals

Abstract

In situ electrochemical scanning tunneling microscopy was used to study the coarsening of platinum nanocrystals under potentiostating and the dissolution and coarsening of platinum nanocrystals through potential cycling. A rapid coarsening of platinum nanocrystals was observed for potentiostating in the double-layer region. No obvious crystal size changes were observed for potentiostating in the platinum oxidation-reduction region and the hydrogen-adsorption proton-oxidation region. The coarsening is proposed as a variant of Ostwald ripening, whereby the dominant process is potential-induced surface-diffusion transport. Additionally, potential cycling causes not only surface platinum dissolution as a result of the platinum redox reaction, but also platinum nanocrystal coarsening. Details of the platinum nanocrystal restructuring are described, and the possible influences of these phenomena on fuel cell operation are discussed.Fuel cells are expected to be key next-generation energy sources used for vehicles and homes, offering both high energy conversion efficiencies and minimal pollutant emissions. 1-3 The high energy efficiency arises from the direct conversion of chemical energy to electricity without the Carnot limitation that applies to thermal engines. For fuel cells operated at relatively low temperatures, such as polymer electrolyte fuel cells ͑PEFCs͒ and phosphoric acid fuel cells ͑PAFCs͒, the sluggish kinetics of hydrogen oxidation and oxygen reduction necessitate the use of platinum ͑Pt͒ and platinum alloy catalysts. However, these precious-metal-based catalysts, particularly when used at the cathode, lose electrochemical surface area ͑ECSA͒ during operation, resulting in severe performance degradation. Extensive studies have been performed in order to understand the mechanisms of ECSA loss, including Pt dissolution from and Pt nanocrystal growth on the cathode; 4-13 however, direct in situ observations of thermodynamically and electrochemically induced processes are still lacking.Electrochemical scanning tunneling microscopy ͑STM͒ is a powerful tool for in situ characterization of surfaces under electrochemical control at atomic-scale resolution. Electrochemical STM allows the determination of electrode nanostructure at the electrode/ electrolyte interface and additionally provides monitoring of electrochemical reactions on the electrode, including the dissolution and deposition of metals and semiconductors. 14-21 Therefore, electrochemical STM is an ideal technique to gain insight into catalyst surface restructuring under electrochemical control.Previously, we reported initial results of an in situ electrochemical STM study of the coarsening of platinum nanocrystals at potentials of 0.4, 0.5, and 0.6 V ͓vs reversible hydrogen electrode ͑RHE͔͒ in the double-layer region in 0.1 M perchloric acid. 22 Several interesting phenomena were observed, and surface-diffusion transport was identified as a dominant process of Pt nanocrystal coarsening. In this study, the investigation of potential-induced surface ...

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“…The only difference is in the initial grain size, which were smaller for Pt-Co (7 nm) than for pure Pt (8 nm). An increase of the particles size of pure Pt under potential cycling is in agreement with previously reported results [21,23].…”

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Experimental Methodologies to Understand Degradation of Nanostructured Electrocatalysts for PEM Fuel Cells: Advances and Opportunities

“…Xu et al applied EC-STM to track the coarsening occurring on a polycrystalline Pt electrode during potential holds at different potentials. [28] They found that the surface reconstruction, attributed to adatom diffusion processes, is most pronounced when the potential is held in the so-called double layer region, as at lower and higher potentials the surface of Pt is covered by protons and oxygenated species, respectively. The most stable surface orientation of the formed grains was Pt (111), in accord with ex situ observations.…”