In Situ Studies of Cu Catalyzed CO2 Electro-Reduction by Soft X-ray Scanning Transmission X-ray Microscopy and Soft X-ray Spectro-Ptychography

Carbon dioxide electro-reduction (CO2R) is a promising route to generate valuable chemical feedstocks through the electrochemical conversion of CO2 to specific compounds (ethane, ethanol, etc). When CO2R is powered by electricity generated by renewable energy resources, it can reduce greenhouse gas emissions and ameliorate climate change. [1] One of the critical challenges for practical CO2R is optimizing selectivity and efficiency. Determining the structure and chemistry of active CO2R electro-catalysts through experimental in situ and operando studies can provide a framework for rational design and optimization of high-performance CO2R electrocatalysts. Copper and some of its alloys are the only known species to produce significant amounts of high value C2+ products. Thus they are the focus of an intense effort to understand and optimize their catalytic performance. Soft X-ray scanning transmission microscopy (STXM) is a powerful tool for nanoscale materials analysis. Ptychography (scanning coherent diffraction imaging), which can be measured using soft X-ray STXMs equipped with a suitable, post specimen X-ray camera, provides significant improvements in spatial resolution (∼10 nm, as opposed to ∼30 nm for conventional STXM). I will describe results of our recent soft X-ray STXM [2] and spectro-ptychography [3] studies of Cu catalyzed CO2 electro-reduction (CO2R) with in situ control of electrolyte composition, flow rate and electrochemical potential. Fig.1a is a sketch of the in situ device used for both STXM and spectro-ptychography studies, along with a side-on image of the cell, and results of Cu L-edge mapping of changes in the spatial distribution of Cu oxidation states as the applied potential is reduced into the CO2 range (< -0.3 VRHE). The microfluidic device features dual inlet and outlet ports which facilitate rapid change of electrolyte (∼10 m), a 3-electrode electrochemical cell with Au working, reference and counter electrodes, and electrolyte thicknesses in the few μm range, controllable through spacers. The Si / SiNx electrode and spacer chips are custom manufactured by Norcada (www.norcada.com). Fig. 1b plots current as f(VRHE) during the in situ experiment. Fig. 1c plots the quantitative fractions of Cu(0) and Cu(I) as a function of applied potential, indicating that the active species is Cu metal under CO2R conditions. Since the spatial resolution of STXM is ∼30 nm, it is typically insufficient to monitor morph-ological changes of the catalytic nanoparticles. Thus, the in situ experiment was also carried out using spectro-ptychography [3]. Fig. 2a is a schematic of the ptychography measurement. The single channel STXM detector is replaced with an X-ray camera (Tucsen Dhyana-92, sCMOS), which is used to record sets of diffraction images which are then processed using PYNX software to generate amplitude and phase images. A 1 μm defocused beam and >80% overlap was used. Amplitude images at a sequence of photon energies are higher-resolution equivalents to STXM stacks. These are converted to absorption and fit with Cu L-edge absorption spectra to produce color coded component maps of Cu(0) and Cu(I). Fig. 2b displays the evolution of a single nanoparticle as a function of applied potential. The initially electrodeposited Cu(0)/Cu(I) cubic particle is converted to metallic Cu at potentials below -0.2 VRHE. At more reducing electrode potentials where CO2R occurs, the particle undergoes morphological changes from a cubic structure to a dendritic-like structure. To our knowledge this is the first report of a spectro-ptychography study of an electrochemical reaction using an in situ flow electrochemical device. While still very challenging, we have also identified conditions where the electrolyte thickness can be reduced sufficiently to permit imaging and spectroscopy at the C K-edge, opening up the exciting possibility for operando identification of gaseous products of CO2R. Fig. 3A is a background subtracted CK spectrum measured at -0.8 VRHE. Signals from the electrolyte (CO2 saturated 0.1 M KHCO3) and from CO2R products (CO, ethylene) are observed and can be semi-quantitatively analyzed (Fig. 3B) [4]. in situ flow electrochemical STXM. A. Microfluidic device with controlled electrolyte flow/exchange, in a 3-electrode cell. Cu oxidation state mapped by Cu 2p stacks. B. current from CO2R reaction. C. quantitative Cu(I)/ Cu(0) maps as a function of potential. CO2R starts at -0.3 VRHE [2]. in situ flow electrochemical spectro-ptychography. A. Schematic of ptychography applied to in situ conditions. B. in situ Cu L-edge study of a single Cu nanoparticle as f (VRHE). A. C 1s spectrum of electrolyte at -0.8 VRHE. B. Semi- quantitative decomposition of the C K-edge spectrum.

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