In-Situ Soft X-Ray Spectromicroscopy Characterization of Electrochemical Processes

Abstract

Soft X-ray Scanning Transmission X-ray microscopy (STXM) is a synchrotron-based technique which can provide both spectroscopic characterization (near edge X-ray absorption fine structure, NEXAFS) and chemically selective imaging with high spatial resolution (~30 nm). Recently, we have developed in situ flow electrochemical devices [1,2] which allow control of the electrochemical environment while conducting STXM measurements, thus providing a platform for in-situ studies of electrochemical oxidation and reduction processes. This presentation reports results of in situ flow electrochemical STXM studies on three different systems to demonstrate the present capabilities. First, the ability to rapidly exchange the electrolyte is demonstrated by a STXM Fe 2p and in situ electrochemical study of the ferro/ferricyanide solution redox system. Second Cu and Ag-doped Cu catalysts for CO2 electrochemical reduction (CO2R) were successfully prepared using in situ electrodeposition. Third, the electrolyte was changed from CuSO4 to NaHCO3 (as substrate for CO2R) and the cell was operated under electrochemical CO2 reduction conditions, while monitoring the changes to the Cu deposited layer at various potentials, including –0.5 V where CO2 reduction is expected [3]. The figure shows a cyclic voltammogram (CV) and color-coded Cu oxidation state maps which were derived from Cu 2p stacks measured under chronoamperometric conditions at the indicated potentials. These results demonstrate that in situ flow electrochemical STXM measurements can be performed in our device under varying electrochemical reaction conditions, enabling visualization of the morphology changes from selective energy imaging, and quantitative tracking of electrochemical transformations from spectromicroscopy. This system will be used for in situ studies of CO2 electrochemical reduction catalysis with the goal of obtaining mechanistic insights to guide the development of catalysts with improved efficiency and selectivity. In addition, the system will be used to study a variety of material science, chemistry and environmental science related questions associated with oxidation or reduction processes, such as mechanisms of extra-cellular electron transport in marine sediment microbial biofilms [4]. This research is supported by NSERC (Canada). STXM measurements were performed at the ambient STXM facility at the Canadian Light Source, which is funded by the Canadian Foundation for Innovation. [1] V. Prabu et al., Rev. Sci. Inst. 89 (2018) 063702. [2] P. Ingino, et al., in preparation [3] L. Wang, D.C. Higgins, et al., Proc. Nat. Acad. Sci. (2020) 01821683. DOI: 10.1073/pnas.1821683117 [4] M. Obst, et al., Microsc. Microanal. 24 (S-2) (2018) 502-504. Figure 1

Chemical Structure and Distribution in Nickel–Nitrogen–Carbon Catalysts for CO2 Electroreduction Identified by Scanning Transmission X-ray Microscopy

“…As an indication of the resolution gains that ptychography can provide, Figure S19 presents a ptychographic amplitude image at 852.7 eV of the A2 region of the Ni–N–C-high sample, along with an evaluation of its spatial resolution. In addition to ptychography, we are performing in situ STXM characterization of these electrocatalyst materials using a microfluidic flow-electrochemical system developed by our team. , This will allow determination of the chemical species present and their spatial distribution in catalyst materials under relevant operating conditions, thereby providing better mechanistic understanding and guiding new catalyst designs …”

“…In addition to ptychography, we are performing in situ STXM characterization of these electrocatalyst materials using a microfluidic flow-electrochemical system developed by our team. 43,76 This will allow determination of the chemical species present and their spatial distribution in catalyst materials under relevant operating conditions, thereby providing better mechanistic understanding and guiding new catalyst designs. 77 Overall, this work demonstrates that STXM is a very effective tool for studying catalysts such as Ni−N−C, where their highly heterogeneous nature leads to difficulty in characterization by nonspatially resolved methods.…”

Copper CO2 Reduction Electrocatalysts Studied by In-situ Soft X-ray Spectro-Ptychography

“…[28] In-situ STXM has been used to study charge/ discharge processes in battery materials [29][30][31], the electrodeposition of copper [32], and catalysis of oxygen evolution (OER) and CO2R reactions. [24,33,34] Such studies demonstrate the ability of STXM to provide spatially resolved spectroscopic and microscopic information on the studied materials at different time/potential points during electrochemical processes. However, a major challenge limiting the scientific impact of in-situ STXM is its limited spatial resolution, typically on the order of 40 -50 nm.…”

“…This has motivated the development and implementation of in-situ characterization methods that enable control of the local reaction conditions and applied electrochemical potentials to gain mechanistic insight into electrochemical CO2R processes and materials. [6,[22][23][24][25] Results from these efforts are crucial for establishing fundamental structure-propertyperformance relationships which can inform further improvements in Cu-based CO2R electrocatalyst designs.…”

“…In this work, we used in-situ spectro-ptychography in the soft X-ray region to characterize electrodeposited Cu electrocatalysts under electrochemical CO2R conditions. By leveraging our custom micro-fluidic reactor design that employs a three-electrode micro-chip electrochemical cell [24,32,33,46], in-situ soft X-ray spectro-ptychography measurements were conducted under applied electrode potentials in the presence of controlled electrolyte flow. Through direct comparison we demonstrate that the spatial resolution for in-situ spectro-microscopy measurements is improved three-fold by using spectro-ptychography rather than conventional STXM.…”