Atomically Isolated Nickel–Nitrogen–Carbon Electrocatalysts Derived by the Utilization of Mg²⁺ ions as Spacers in Bimetallic Ni/Mg–Metal–Organic Framework Precursors for Boosting the Electroreduction of CO₂

Abstract

Electrochemical CO2 reduction (CO2R) is a promising avenue for the conversion of CO2 into fuels and beneficial chemicals. Significant efforts have been devoted to the development of active and selective electrocatalysts for CO2R. Atomically dispersed transition metal electrocatalysts have recently attracted consideration for CO2R due to their unique electronic and structural properties that can impart good catalytic performance and high active metal utilization. Among different precursors used for preparing these catalysts, metal–organic frameworks (MOFs) have been utilized as templates for generating atomically dispersed transition metal active sites as they provide well-defined structures that contain transition metals isolated from each other by organic ligands, along with high surface areas. However, maintaining the isolation of the transition metal species, achieving high surface concentrations of active sites, and imparting porosity during the high-temperature heat treatment of MOFs used during synthesis remain a challenge. In this report, Mg2+ ions have been employed as spacers in a bimetallic metal–organic framework (NiMg-MOF-74) to assist in preventing the coalescence of the Ni atoms into Ni-based particles during heat treatment, which combined with the use of urea as a nitrogen source resulted in the formation of isolated Ni–N x /C active sites. Our findings demonstrated that Mg2+ ions play a crucial role in extending the distance between the adjacent Ni sites in the precursor structure and generating atomically isolated Ni sites. On the contrary, the utilization of Mg-free Ni-MOF-74 led to the formation of metallic Ni particles embedded in a carbon nanotube-based structure. Furthermore, we investigated the impact of pyrolysis temperature on the produced catalyst morphology. The generation of isolated Ni–N x /C sites was promoted at higher pyrolysis temperatures (900 °C), while Ni-based particles were predominantly formed at a lower temperature (700 °C). The optimized atomically dispersed Ni–N–C catalyst exhibited excellent selectivity toward CO with a Faradaic efficiency of ∼90% and a current density of −4.2 mA/cm2 at −0.76 V vs a reversible hydrogen electrode.

Mesoporous Single Atom-Cluster Fe–N/C Oxygen Evolution Electrocatalysts Synthesized with Bottlebrush Block Copolymer-Templated Rapid Thermal Annealing

“…The C 1s spectra revealed two different peaks at 284.5 and 285.5 eV, which correspond to graphitic (CC) sp 2 and (C–C) sp 3 peaks, respectively (Figure S9). This is consistent with the Raman data (Figure a,b). The Fe 2p core-level spectra exhibit peaks at 709.3 eV for Fe 2+ 2p 3/2 and at 722.7 eV for Fe 2+ 2p 1/2, as well as a satellite peak at 711.9 eV for Fe 3+ 2p 3/2 respectively (Figure d) .…”

“…Based on the above-mentioned results, the high OER performance of porous nitrogen-doped Fe–N/C material can be attributed to the following factors: (a) The short intersite distance facilitates the electron transfer from the iron atomic clusters to the iron single atoms that leads to higher electrochemical activity for OER, indicating that the cluster mainly acts as an activity booster. According to the previously reported literature, a distance of <1.6 nm between the Fe atomic cluster and a single atom helps reduce the overpotential, resulting from the weakened binding energy of *OH. , (b) The high electrochemically active surface area partly due to the Fe–N coordination environment increases the intrinsic activity of the catalyst which has also been reported to improve the OER process. , (c) Iron oxyhydroxide could form as a reconstructed surface during the OER process and improve electrochemical water oxidation performance, and (d) there is the efficient charge transfer from the electrode to the electrolyte due to iron incorporation, mesoporous structure, and the beneficial synergy with nickel foam.…”

“…The high pyridinic nitrogen content (43.8%) indicates that most of the N atoms in the carbon matrix were preferred to occupy at carbon defect or the edge sites . Although XPS spectroscopy revealed the presence of nitrogen and iron species in the material, there was little direct evidence in the N 1s spectrum (binding energy ∼399.0 eV) for coordination between iron and nitrogen atoms on Fe–N/C (Figure c) . Due to similar binding energies, it is difficult to deconvolute the XPS spectrum to distinguish between pyrrolic- and Fe–N coordinations .…”

“…28,29 (b) The high electrochemically active surface area partly due to the Fe−N coordination environment increases the intrinsic activity of the catalyst which has also been reported to improve the OER process. 8,47 (c) Iron oxyhydroxide could form as a reconstructed surface during the OER process and improve electrochemical water oxidation performance, and (d) there is the efficient charge transfer from the electrode to the electrolyte due to iron incorporation, mesoporous structure, and the beneficial synergy with nickel foam.…”

“…For example, cobalt(II) in silica-templated porous carbon has an overpotential of 320 mV at 10 mA/ cm 2 current density for OER in 1 M KOH electrolyte, 10 while N-doped carbon materials with single-atom nickel sites showed good electrocatalytic activity for CO 2 reduction (current density of −4.2 mA/cm 2 at an overpotential of −0.76 V in 0.1 M KHCO 3 electrolyte). 8 A necessary characteristic for high performance was the large electrochemical surface area associated with the carbon materials. However, most of the reported porous carbon electrocatalysts involve a complicated synthesis process (such as a sol−gel hydrothermal process), following long pyrolysis (minimum of 10 h) to obtain a carbon skeleton for enhanced conductivity and electrocatalytic performance, 11,12 or employ expensive, low-yielding metal− organic frameworks (MOFs).…”