Introduction
The structurally adaptive nature of metal-organic frameworks (MOFs) arises from complex interactions between the metal connectors (nodes) and the organic linkers. The hollow (extraordinarily porous) structure defines the characteristics of MOFs and can be described as a fractal-like network of central metal units, interconnected via covalently bound organic ligands (e.g. HHTP linkers, which are discussed later). Periodic atomic arrangement and highly-ordered structures allows the utilization of MOFs in important electrochemical reactions.
| Single cite | ||
|---|---|---|
| ||
Baumann, A.E., Burns, D.A., Liu, B. et al. Metal-organic framework functionalization and design strategies for advanced electrochemical energy storage devices. Commun Chem 2, 86 (2019). https://doi.org/10.1038/s42004-019-0184-6 |
. The hollow (extraordinarily porous) structure defines the characteristics of MOFs and can be described as a fractal-like network of central metal units, interconnected via covalently bound organic ligands (e.g. HHTP linkers, which are discussed later). Periodic atomic arrangement and highly-ordered structures allows the utilization of MOFs in important electrochemical reactions. Many of these reactions, such as the hydrogen evolution reaction (HER) and or the oxygen evolution reaction (OER), bear an essential role in sustainable energy conversion applications (e.g. fuel cells). For example, a novel 2-dimensional cobalt-based MOF (Co-MOF) electrocatalyst has shown remarkable OER activity, outpowering the widely used, noble-metal based RuO2.
| Single cite | ||
|---|---|---|
| ||
Joshi, A., G. Ashish, et al. One-Pot Crystallization of 2D and 3D Cobalt-Based Metal–Organic Frameworks and Their High-Performance Electrocatalytic Oxygen Evolution. Inorg. Chem., 2021, 60(17), 12685–12690 https://doi.org/10.1021/acs.inorgchem.1c01665 |
Figure 1.1 VESTA structure
| Single cite | ||
|---|---|---|
| ||
Joshi, A., G. Ashish, et al. CCDC 1985273: Experimental Crystal Structure Determination, 2021, DOI: 10.5517/ccdc.csd.cc24mv2q |
of a cobalt-based MOF [Co6(btc)2(DMF)6(HCOO)6]
from side and top views. Co atoms are blue (illustrated as pink coordination polyhedras), O red, N light blue, C brown, and H atoms are light pink. Abbreviation btc stands for 1,3,5-benzenetricarboxylic acid, and DMF for N,N-dimethylformamide. (Figure: Marianne Rahikka)Single cite short citeID Joshi2021
Figure 1.2 Chemical diagram of [Co6(btc)2(DMF)6(HCOO)6]
. Each Co2+ is coordinated by three oxygen atoms of three formate ions, one DMF-molecule and two of btc-linkers to form a distorted CoO6 octahedronSingle cite short citeID Joshi2021CCDC
. (Figure: Marianne Rahikka) Single cite short citeID Joshi2021
ORR and HER
Electrochemical reactions in fuel cells
As any electrochemical reaction, an electrocatalytic reaction is heterogenous and the electron transfer takes place on a solid surface (e.g. on the surface of a Pt-electrode). For example, in proton-exchange membrane (PEM) fuel cells, an important rate-limiting (i.e. slow and inefficient
| Single cite | ||
|---|---|---|
| ||
Quaino, P.; Juarez, F.; Santos, E.; Schmickler, W., Volcano plots in hydrogen electrocatalysis – uses and abuses, Beilstein J. Nanotechnol. 2014, 5, 846–854. doi:10.3762/bjnano.5.96 |
) step is the four-electron transfer in the oxygen reduction reaction (ORR).
| Single cite | ||
|---|---|---|
| ||
Yang, Y., Yang, Y., Liu, Y., Zhao, S. and Tang, Z. (2021), Metal–Organic Frameworks for Electrocatalysis: Beyond Their Derivatives. Small Sci., 1: 2100015. https://doi.org/10.1002/smsc.202100015 |
ORR takes place on the cathode, where oxygen (supplied to the system) reacts with hydrogen ions and electrons (species that are previously generated in the anodic hydrogen oxidation reaction) forming H2O as an end-product.
| Single cite | ||
|---|---|---|
| ||
Schumm, Brooke. "fuel cell". Encyclopedia Britannica, 9 Feb. 2023, https://www.britannica.com/technology/fuel-cell. Accessed 25 February 2023. |
These reactions require a catalyst to lower the activation energy of – for example – the water splitting (OER/HER) reaction in electrolyzers. HER is an important reaction as it is a direct path for producing the fuel (H2) used in a PEMFC illustrated in Fig. 2.
MOFs as electrocatalysts
Noble-metal based electrocatalysts are scarce and expensive
| Single cite short | ||
|---|---|---|
|
| Single cite | ||
|---|---|---|
| ||
Cheng, W., Zhang, H. et al, Exposing unsaturated Cu1-O2 sites in nanoscale Cu-MOF for efficient electrocatalytic hydrogen evolution, Sci. Adv. 2021; 7:eabg2580, https://doi.org/10.1126/sciadv.abg2580 |
, such as the aforementioned platinum-electrode. Consequently, there is an increasing demand for inexpensive, novel electrocatalyst materials such as MOFs, which can be synthesized from earth-abundant elements using rather facile methods such as solvothermal approach, or chemical vapor deposition (CVD).
| Single cite short | ||
|---|---|---|
|
| Single cite short | ||
|---|---|---|
|
Figure 3. Summary of pristine MOFs for electrocatalysis and their role in key reactions.
| Single cite short | ||
|---|---|---|
|
Conveniently, MOFs have huge internal surface areas, possessing multiple potential adsorption sites for the reduction or oxidation of a substance to occur, thus explaining the interest of their use as electrocatalytic materials. However, there is discussion whether MOFs fall to the category of pre-catalysts (i.e. producing active phases during the electrochemical operations) instead of catalysts. Another concern is related to the stability of MOF electrocatalysts. Although some MOFs can resist the cation-condensed electric double layer (EDL) during electrocatalysis, their stability during cyclic voltammetry studies can be poor, even if their electrochemical performance during such process is goodstable.
| Single cite | ||
|---|---|---|
| ||
Zheng, W. & Lee, L. Metal-organic frameworks for electrocatalysis: Catalyst or precatalyst? ACS Energy Lett. 2021, 6, 2838−2843, https://doi.org/10.1021/acsenergylett.1c01350 |
The post-OER characterization of the previously introduced, electrocatalytic Co-MOF (Fig. 1.1 and 1.2) has shown in situ formation of Co(OH)2 and CoHO2, indicating these hydroxides and hydr(oxy)oxides working as active sites during the oxygen evolution process, requiring 175 mVoverpotential to attain 10 mA cm-2 with a small Tafel slope of 80 mV dec-1 (in 1.0 M KOH solution)
| Single cite short | ||
|---|---|---|
|
Here, another interesting 2-dimensional, transition metal -based MOF showing promising electrocatalytic efficiency for hydrogen evolution is introduced.
Electrocatalytic Fe(OH)x@Cu-MOF nanoboxes
Using a facile template-engaged solvothermal synthesis followed by redox-etching, Cheng et al. (2021) synthesized a nanoscale, conductive ultrathin layer of copper-based MOF with 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) linkers (Fig. 4). This Cu-MOF do not only exhibit excellent electrocatalytic activity for hydrogen evolution (112 mV overpotential to attain 10mA cm-2 current density with a Tafel slope of 76 mV dec-1), but also the much-needed stability in alkaline (1.0 M KOH) solution.
| Single cite short | ||
|---|---|---|
|
Figure 4. Crystal structure of unsaturated Cu-MOF [Cu3(HHTP)2] along c-axis.
(Under license CC BY-NC 4.0) (Figure: Marianne Rahikka)Single cite short citeID Cheng2021
Figure 5. Schematics of the synthetic process for Fe(OH)x@Cu-MOF NB (NB = nanobox)
| Single cite short | ||
|---|---|---|
|
References
Cite summary local true