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

Abstract

Atomically dispersed metal–nitrogen–carbon (M–N–C) materials are a class of electrocatalysts for fuel cell and electrochemical CO2 reduction (CO2R) applications. However, it is challenging to characterize the identity and concentration of catalytically active species owing to the structural heterogeneity of M–N–C materials. We utilize scanning transmission X-ray microscopy (STXM) as a correlative spectromicroscopy approach for spatially resolved imaging, identification, and quantification of structures and chemical species in mesoscale regions of nickel–nitrogen–carbon (Ni–N–C) catalysts, thereby elucidating the relationship between Ni content/speciation and CO2R activity/selectivity. STXM results are correlated with conventional characterization approaches relying on either bulk average (X-ray absorption spectroscopy) or spatially localized (scanning transmission electron microscopy with electron energy loss spectroscopy) measurements. This comparison illustrates the advantages of soft X-ray STXM to provide spatially resolved identification and quantification of active structures in Ni–N–C catalysts. The active site structures in these catalysts are identified to be atomically dispersed NiN x /C sites distributed throughout entire catalyst particles. The NiN x /C sites were notably demonstrated by spectroscopy to possess a variety of chemical structures with a spectroscopic signature that most closely resembles nickel­(II) tetraphenylporphyrin molecules. The quantification and spatial distribution mapping of atomically dispersed Ni active sites achieved by STXM address a target that was elusive to the scientific community despite its importance in guiding advanced material designs.

In Situ/Operando Characterization Techniques: The Guiding Tool for the Development of Li–CO2 Battery

“…The electrochemical performance of the system can be correlated to the obtained physicochemical properties. [92] Further, with the need for in situ techniques for Li-CO 2 batteries, the TXM can also be operated in situ under the operating gas conditions needed for the system. For example, de Smit et al [93] used in situ TXM to study the products of the reduction reaction of Fe 3 O 4 by a Fischer-Tropsch-based catalyst under the operating gas atmosphere of H 2 at a pressure of 1 bar.…”

“…The electrochemical performance of the system can be correlated to the obtained physicochemical properties. [ 92 ] Further, with the need for in situ techniques for Li–CO 2 batteries, the TXM can also be operated in situ under the operating gas conditions needed for the system. For example, de Smit et al.…”

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

“…Scanning transmission X-ray microscopy (STXM) uses a focused and coherent X-ray beam from a synchrotron source to image the transmission of X-rays through the catalyst sample. − By tuning the X-ray energy to match the absorption edge of a specific electronic transition for an element, this technique enables nanoscale chemical mapping of the local oxidation state of that element in the catalyst. − The distribution of different elements in the sample can be obtained by imaging with multiple X-ray energies. Typical spatial resolutions for in situ STXM are 40 to 100 nm, although resolutions below 10 nm have been obtained for ex situ STXM .…”

Electrocatalysis with molecules and molecular assemblies within gas diffusion electrodes

“…Both transition metal-containing porphyrins/phthalocyanines and M-N 4 SACs feature exceptional activity for CO 2 reduction to CO, and recently, M-N 4 SACs have been integrated into GDE-based CO 2 R reactors for selective CO production at industrially relevant current densities. 39,67,68 One limitation to the aforementioned SACs is that the synthesis oen yields a mixture of active site congurations 69 and generally, the complexity and secondary coordination sphere of the active sites is more limited as compared to molecular catalysts. [70][71][72] While purely molecular catalysts are oen not as scalable and industrially translatable, we contend that they are an important model system for precise studies of structure-activity relationships to extract lessons that can subsequently be incorporated into industrially ready platforms.…”