Surveying the Homogeneity of a Molecular Electrocatalyst Embedded in a Metal-Organic Framework Using Operando Characterization

Abstract

Homogeneous catalysis generally yields low catalytic current densities due to the small number of catalytic centers at the electrode surface. Incorporating molecular catalysts in metal-organic frameworks (MOFs) has been proposed as a viable approach to immobilize them on electrodes, increasing current densities. In addition, molecular catalysts do not always remain in their homogeneous state, sometimes partially taking on a more heterogeneous character, which challenges the clear identification of the active species. Despite the risk of homogeneity loss, most studies on molecular catalysts embedded in MOFs have so far overlooked the possibility of heterogeneous deposit formation during electrocatalysis. In this work, a more comprehensive study on the changes of homogeneity exhibited by an MOF-embedded molecular catalyst is presented. The Cu species formed in the NU1000|Cu-tmpaCOOH MOF before, during, and after the oxygen reduction reaction using operando X-ray absorption spectroscopy are investigated. The initial Cu²⁺ catalyst forms Cu⁰ clusters of diameter <2 nm upon application of a reductive potential. This work demonstrates that for Cu-based molecular catalysts embedded in MOFs, it is essential to account for the possible changes in a molecular catalyst's homogeneity, regardless of the catalytic benefits its supporting structure might grant.

Keywords: homogeneity; metal‐organic frameworks; molecular electrochemistry; operando X‐ray absorption spectroscopy; oxygen reduction reaction.

Figure 1

Spectroscopic investigation of NU1000|Cu‐tmpaCOOH. a) Schematic image of the NU1000|CutmpaCOOH structure and b) its CV at 50 mV s⁻¹ under Ar before (blue) and after electrocatalysis at 0.3 V vs. RHE for 6 h (gray dash). c) Schematic representation of the operando XAS experiment.

Figure 2

Cu K‐edge XANES of NU1000|Cu‐tmpaCOOH. a) Operando XAS measurements during CA under O₂ atmosphere at 0.3 V vs. RHE (light blue) and −0.1 V vs. RHE (dark blue). b) In situ measurements at OCP before (gray) and after (gray intermittent) CA. Insets present the zoom of the pre‐edge region. The offset spectra of Cu foil, Cu₂O, and Cu(OH)₂ standards (reds) are given for qualitative comparison. c) MCR fitting of the NU1000|Cu‐tmpaCOOH at −0.1 V vs. RHE under O₂ atmosphere yields three distinct components. d) The relative ratio of the three MCR fitting components as a function of time and the applied potential indicates the progressive change in the distribution of Cu states in the sample. An irreversible loss of the third MCR component (gray) is observed after carrying out electrolysis at −0.1 V vs. RHE for ≈3 h.

Figure 3

Cu K‐edge EXAFS of NU1000|Cu‐tmpaCOOH in R‐space. a) Phase‐uncorrected FT‐EXAFS spectra before (gray), after (gray intermittent), and during CA at 0.3 V vs. RHE (light blue) and −0.1 V vs. RHE (dark blue). Cu foil (red) is plotted to indicate the usual radial distance of Cu—Cu in bulk metallic Cu⁰. b) Comparison of the phase‐uncorrected FT‐EXAFS spectra from the sample before (gray) and after CA (gray intermittent) with the Cu₂O and Cu(OH)₂ standards (reds).

Figure 4

Schematic representation of the Cu species hypothesized to be formed before, during, and after ORR by NU1000|Cu‐tmpaCOOH.