Atomically Dispersed CuN_x Sites from Thermal Activation of Boron Imidazolate Cages for Electrocatalytic Methane Generation

Abstract

Atomically dispersed metal sites (ADMSs) have been recognized as promising candidates for electrochemical conversion. Among a diverse range of molecular precursors for ADMS synthesis, framework materials are particularly interesting due to their high degree of tunability and control over the primary coordination sphere of the metal ions. Herein, we demonstrate that a copper boron imidazolate cage, BIF-29­(Cu), is a convenient precursor for a competent catalyst with isolated Cu sites coordinated by N donors for carbon dioxide electroreduction (CO2RR). Although BIF-29­(Cu) exhibited moderate methane selectivity over hydrogen evolution reaction (HER), the methane selectivity is significantly enhanced by 2 times (55% CH4 at −1.25 V vs RHE) after mild thermal activation. Extensive characterization methods indicate the transformation of crystalline BIF-29­(Cu) into an amorphous carbonaceous material comprising isolated CuN x sites. Moreover, in situ X-ray absorbance spectroscopy indicates stable CuN x sites that are reduced during CO2RR. This work encourages the discovery of single-site electrocatalytic systems through a rational selection of molecular precursor and calcination parameters for promoting product selectivity.

Efficient Electromethanation from CO2 on Monodisperse Cu-Based Catalysts

“…In comparison to the recently published electromethanation of CO 2 (Figure a), Cu–DCD 40 demonstrates high selectivity (FE) and activity (partial current density) of electromethanation. ,− To evaluate the stability of the as-prepared catalyst, “pause electrolysis” to suppress the carbonate issue is adopted in this work . The pause electrolysis is shown in Figure b.…”

Electrochemical C–N Bond Formation within Boron Imidazolate Cages Featuring Single Copper Sites

“…Infrared absorption spectroscopy (FTIR), Figure D, red trace) showed characteristic B–H (2426 cm –1 ), imidazole C–H (3132 cm –1 ), and axial water O–H (3435 cm –1 ) bands. Previous reports have illustrated the feasibility of dissolving the resulting framework material into colloidally stable solutions by disrupting the van der Waals interactions between cages. , This unique feature of the framework material lends itself to versatile processability and incorporation into electrochemical systems. The colloidal stability of a 1 mg mL –1 MeOH solution of BIF-29­(Cu) was confirmed by dynamic light scattering (DLS) measurements, which showed an average particle size of 300 nm (Figure S1).…”

“…Boron-imidazolate frameworks (BIFs) are a subset of MOFs that are composed of metal-imidazole-boron linkages. These materials are credited for having versatile structural topologies while also maintaining high chemical and thermal stability . Recently, our group has shown that BIF-29­(Cu), a copper-containing BIF, and its thermolyzed derivative produce methane from CO 2 RR with high selectivity (32 and 55%, respectively) .…”

“…15 Recently, our group has shown that BIF-29(Cu), a copper-containing BIF, and its thermolyzed derivative produce methane from CO 2 RR with high selectivity (32 and 55%, respectively). 15 We inferred that the propensity to form CH 4 in high yield suggests a relatively strong *CO intermediate that could be leveraged for electrochemical C−N bond formation. 16 Despite containing isolated metal sites, BIF-29(Cu) can catalyze the formation of C−N bonds from CO 2 and NO 3 − , exhibiting a high Faradaic efficiency for urea production of 68.5 ± 0.57% with a partial current density of 424 ± 16 μA cm −2 .…”

“…These materials are credited for having versatile structural topologies 13 while also maintaining high chemical 14 and thermal stability. 15 Recently, our group has shown that BIF-29(Cu), a copper-containing BIF, and its thermolyzed derivative produce methane from CO 2 RR with high selectivity (32 and 55%, respectively). 15 We inferred that the propensity to form CH 4 in high yield suggests a relatively strong *CO intermediate that could be leveraged for electrochemical C−N bond formation.…”