Analysis of Electrocatalytic Metal-Organic Frameworks

Abstract

The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.

Keywords: electroactive thin films; electrocatalysis; electrochemistry; metal-organic frameworks.

Figure 1

Schematic representation of (A) through-bond and (B & C) through-space approaches to improving conductivity in MOFs. Panel (a) reprinted with permission from ref. [24]. Copyright (2016) Wiley. Panel (b) adapted with permission from ref. [24]. Copyright (2012) American Chemical Society. Panel (c) adapted from ref. [24] with permission from The Royal Society of Chemistry.

Figure 2

pH stability ranges of representative common MOFs. Reprinted with permission from ref. [51] (Springer Nature, Copyright 2016).

Figure 3

PXRD patterns of the Zr-btba (btba = 4,4′,4′′,4′′′-([1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoate) MOF following activation under various conditions. The pattern (i), corresponding to thermal activation at 70 °C, contains the halo indicative of a new amorphous phase [53]. (Reprinted with permission from John Wiley and Sons, Inc.).

Figure 4

Overview of methods for attaching MOFs to electrode surfaces.

Figure 5

Schematic of electrophoretic deposition in a) anodic and b) cathodic modes.

Figure 6

Increasing the electrophoretic deposition time for the redox active NU-1000 MOF (A) resulted in a higher percentage of the total deposited MOF being electroactive (B). It was hypothesized that the films at higher deposition times had increased particle-particle contacts resulting in improved inter-particle redox connectivity. Adapted with permission from ref. [121]. Copyright (2014) Wiley.

Figure 7

A) Structure of a MOF constructed using naphthalene diimide (NDI) redox-active linkers and B) cyclic voltammograms of the linker dissolved in solution with 0.1 M n-Bu₄NPF₆ in DMF (blue) and the corresponding MOF tested in 0.8 M KPF₆ in DMF (black). In this example, comparison of the CVs allowed quick assignment of the redox features seen in the MOF as arising from the NDI linker components. Panel A reprinted with reprinted with permission from ref. [140]. Copyright (2018) American Chemical Society. Cyclic voltammetry data from ref. [140].

Figure 8

Cyclic voltammograms in 0.1 M n-Bu₄NPF₆ in DMF of 1 mM Fe₂(dcbdt)(CO)₆ (blue) and a 2-5 μm UiO-66 film on FTO before (green) and after (red) post synthetic exchange of some of the linkers with Fe₂(dcbdt)(CO)₆ (dcbdt = 2,3-dithiolato-1,4-benzene dicarboxylate). Adapted with permission from Ref. [141]. Copyright (2015) The Royal Society of Chemistry.

Figure 9

Comparison of reported redox potentials for coordination polymer (CP, y-axis) modified electrodes with their respective redox-active molecular components (x-axis). Reprinted by permission from ref [142]. Copyright (2016) Springer.

Figure 10

Illustration of increased current density of an NDI-based thin film MOF while cycling from the first (purple) to the last (dark red) scan. Reprinted by permission from ref [140]. Copyright (2018) American Chemical Society.

Figure 11

(left) Comparison of films of the pyrene-based NU-1000 MOF with (red) and without (green) the solvent-assisted ligand incorporation (SALI) of ferrocene measured on FTO electrodes in a solution of 0.05 M TBAPF6 in acetonitrile at a scan rate of 50 mV/s. (right) CVs of Fc-NU-1000 recorded in different concentrations of supporting electrolyte TBAPF₆. At concentrations of PF₆⁻ higher than that of the fixed Fc⁺ sites the oxidation of the pyrene linkers is restored. Reprinted with permission from ref [146]. Copyright (2015) American Chemical Society.

Figure 12

Simulated electrochemically irreversible CVs of A) a homogeneous analyte and B) an electrode-attached electroactive MOF. In the case of the homogeneous analyte, stirring the solution between scans brings fresh analyte to the electrode surface and the same redox feature can be observed on the second scan. For the electrode-adsorbed MOF, however, the second scan reveals that the MOF was irreversibly changed on the first scan.

Figure 13

Example of gradual current loss for a hydrogen-evolving 2D MOF during a bulk electrolysis. This represents an in situ electrochemical method for detecting irreversible degradation of an electroactive MOF. Reprinted with permission from ref [159]. Copyright (2015) American Chemical Society.

Figure 14

PXRD patterns of an oxygen-evolving electrocatalytic MOF as-synthesized and after 24 and 96 hours of bulk electrolysis. Use of PXRD provided information on how long the bulk MOF structure was stable under electrocatalytic conditions. Reprinted with permission from ref [165]. Copyright (2017) American Chemical Society.

Figure 15

Effect of MOF film thickness (as controlled by the number of ALD cycles) on catalytic current density for a Al₂(OH)₂TCPP-Co (TCPP = 4,4′,4″,4′″-(porphyrin-5,10,15,20-tetrayl)tetrabenzoate) MOF capable of reducing CO₂ to CO. Reprinted with permission from ref. [131]. Copyright (2015) American Chemical Society.

Scheme 1. Strategies to construct stable MOFs as guided by HSAB theory.

Reprinted with permission from ref. [50]. BTP and BDP are 1,3,5-tris(1H-pyrazol-4-yl)benzene and 1,4-benzenedi(4′-pyrazolyl), respectively. Copyright (2016) Wiley.