Abstract
The present work reports novel soft X-ray Fresnel CDI ptychography results, demonstrating the potential of this method for dynamic in situ studies. Specifically, in situ ptychography experiments explored the electrochemical fabrication of Co-doped Mn-oxide/polypyrrole nanocomposites for sustainable and cost-effective fuel-cell air-electrodes. Oxygen-reduction catalysts based on Mn-oxides exhibit relatively high activity, but poor durability: doping with Co has been shown to improve both reduction rate and stability. In this study, we examine the chemical state distribution of the catalytically crucial Co dopant to elucidate details of the Co dopant incorporation into the Mn/polymer matrix. The measurements were performed using a custom-made three-electrode thin-layer microcell, developed at the TwinMic beamline of Elettra Synchrotron during a series of experiments that were continued at the SXRI beamline of the Australian Synchrotron. Our time-resolved ptychography-based investigation was carried out in situ after two representative growth steps, controlled by electrochemical bias. In addition to the observation of morphological changes, we retrieved the spectroscopic information, provided by multiple ptychographic energy scans across Co L3-edge, shedding light on the doping mechanism and demonstrating a general approach for the molecular-level investigation complex multimaterial electrodeposition processes.
Highlights
Research and development efforts developing battery-based high-energy storage technologies— grid-support systems and automotive, as well as aerospace electrical propulsion—are impeded by poorly understood, and inefficiently controllable electrochemical phase-formation processes involved both in the fabrication and operation of the devices (Larcher and Tarascon 2014; Kittner et al 2017)
In coherent diffractive imaging (CDI) the size of the object has to be smaller than the illumination spot, but extended objects can be studied by Fresnel CDI methods—such as Fresnel CDI ptychography (Phillips et al 2014; Kourousias et al 2016), in which the material under investigation is placed downstream of the focus of a Fresnel zone plate (FZP)
In a recent work on this material, we have investigated the chemical-state distribution of Mn—the main component of the catalyst—while in this study we shall attack the dynamic morphochemistry of the Co dopant
Summary
Research and development efforts developing battery-based high-energy storage technologies— grid-support systems and automotive, as well as aerospace electrical propulsion—are impeded by poorly understood, and inefficiently controllable electrochemical phase-formation processes involved both in the fabrication and operation of the devices (Larcher and Tarascon 2014; Kittner et al 2017). The recently proposed microspectroscopic approaches based on coherent diffractive imaging (CDI) have shown that nanometric resolution can be achieved quasi-in situ (Bozzini et al 2017a) as well as in situ (Kourousias et al 2016; Bozzini et al 2017b) and have shown dynamic capabilities that—with appropriate technological development—can be extended down to the millisecond range, opening up the possibility of following chemical kinetics on a scale suitable for dynamic mechanistic studies The uniqueness of this type of imaging is its capability of overcoming the resolution limits imposed by focusing optics, enabled by the replacement the image-forming lens with an iterative algorithm simultaneously recovering phase and absorption images (Mancuso et al 2010; Chapman and Nugent 2010). Ptychography can provide space-resolved chemical-state information through the phase and amplitude maps recovered by scanning the photon energy across elemental resonances (Hoppe et al 2013; Maiden et al 2013; Kourousias et al 2016; Bozzini et al 2017b; Pfeiffer 2018)
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