Atomically dispersed Ni-N-C electrocatalysts, studied by Ni L-edge spectro-ptychography

Abstract

Soft X-ray spectro-ptychography of nickel-nitrogen-carbon electrocatalysts containing atomically dispersed Ni-based active sites were measured at the Ni L₃ edge. Samples prepared with two different loadings of Ni precursors were investigated and compared to the results of an earlier study using scanning transmission X-ray microscopy (STXM) [Zhang et al., ACS Catalysis 12 (2022) 8746]. The ptychography data sets were measured using a defocused probe (1–3 µm). The spatial resolution was improved from ∼60 nm (STXM) to ∼20 nm (ptychography). Spectro-ptychography stacks were measured at 4 component-specific energies (4-E stack) and at many energies across the full Ni L₃ edge (34-E stack). Maps of three key chemical components (Ni metal, Ni₃S_2, and atomically dispersed N-coordinated Ni catalyst sites) were derived by fits of suitable reference spectra to absorption signals derived from the amplitude images from ptychographic reconstruction. The spectro-ptychography 4-E and 34-E stacks gave chemical mapping similar to each other and to the earlier STXM results. The phase signals obtained from the same data set and reconstruction were also found to be analyzable using reference phase spectra extracted from the phase stack, which generated chemical maps similar to those based on ptychography amplitude data. By using a defocused probe, the radiation dose and acquisition times for spectro-ptychography are comparable to conventional STXM, but significantly improved spatial resolution was achieved. This study highlights the added value of spectro-ptychography relative to STXM for studies of electrocatalysts.