Covalent Organic Framework (COF) Derived Ni‐N‐C Catalysts for Electrochemical CO₂ Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites

Abstract

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel‐nitrogen‐doped carbon (Ni‐N‐C) is an efficient catalyst for CO2 reduction to CO, and the single‐site Ni−Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)‐derived Ni‐N‐C catalysts, for which the Ni−Nx content could be adjusted by the pyrolysis temperature. The combination of high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure evidenced the presence of Ni single‐sites, and quantitative X‐ray photoemission addressed the relation between active site density and turnover frequency.

Investigation of the structure of atomically dispersed NiNx sites in Ni, N-doped carbon electrocatalysts by 61Ni Mössbauer Spectroscopy and Simulations

“…43 The derivative XANES spectra demonstrate that the 1s→4p feature (8340 eV) exists in the 61 NiPACN samples which supports the existence of a square planar-like geometry of 15 Ni atoms in agreement with prior reports (Figure 4B, 4E). 15,44 In addition, the size of this feature decreases and the metallic-like peak at 8334 eV grows for the 1 wt% sample which is consistent with increasing Ni aggregate character for that sample. The extended XAS fine structure (EXAFS), originating from the scattering of photoelectrons off neighboring atoms, gives direct information of the bonding environment in the samples (Figure 4C, 4F, k-space plot in SI-Figure 2).…”

Design and Diagnosis of high-performance CO2-to-CO electrolyzer cells

“…These two chemical Ni states are commonly found for the N-coordinated, catalytically active single Ni atom sites. 31,38 The overall elemental composition of either catalyst is very similar, with metallic Ni particles adding extra Ni at% for NiNC-IMI (Supplementary Table 4). The TEM micrographs in Supplementary Figure 3 revealed the catalyst microstructures.…”

“…It is well-documented that such Ni particles are chemically inert for the CO2 reduction electrocatalysis. 28,[30][31][32][33] The chemical state and elemental composition of the catalysts surface were obtained from the core level regions of C1s, N1s, and Ni2p(3/2) (Figure 2b-e in NiNx site motifs) were identified (Supplementary Table 3). [34][35][36][37] In both NiNC-IMI and NiNC-PANI, Ni prevails in the chemical states of Ni 2+ and Ni + .…”

Two-dimensional materials for electrocatalysis and energy storage applications

“…Interestingly, the Ni-N x content was affected by the pyrolysis temperature. 193 The HAADF-STEM image (Fig. 23c) provides evidence of the presence of isolated Ni single atoms, and the active site density is identified as a critical parameter to describe the catalytic activity.…”