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 and properties (e.g. conductivity, stability, and flexibility) of MOFs. The structure can be described as a network of active central metal units coordinated by adjacent organic ligands^[1]. Periodic atomic arrangement and highly-ordered, yet tunable structures allows the utilization of MOFs in important electrochemical reactions. Many of these reactions, such as the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), bear an essential role in sustainable, next-generation energy conversion and storage applications (e.g. fuel cells and batteries).^[2] For example, a novel 2-dimensional cobalt-based MOF (Co-MOF) electrocatalyst has shown quite impressive OER activity, outpowering the widely used, noble-metal based RuO₂^[3]. The crystal structure of this functional material is illustrated in Fig. 1.1, and for clarification its chemical diagram is illustrated in Fig. 1.2. 

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. without catalyst a slow and inefficient^[5]) step is the four-electron transfer in the oxygen reduction reaction (ORR).^[1]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.^[6]A catalyst is needed 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. 

MOFs as electrocatalysts

Noble-metal based electrocatalysts are scarce and expensive^[1][7], 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).^[1][2] Because of their hollow structures, 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. An overview of the synthesis methods of pristine MOFs and their roles in electrocatalysis is illustrated in Fig. 3.

Figure 3. Summary of pristine MOFs for electrocatalysis and their role in key reactions.^[1](Under license: CC BY 4.0)

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 (structural) stability during cyclic voltammetry studies can be poor, even if their electrochemical performance during such process is stable. ^[8]

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 for the oxygen evolution process, requiring 175 mV overpotential to attain 10 mA cm^-2 current density with a small Tafel slope of 80 mV dec^-1 in 1.0 M KOH solution ^[3]. To clarify the terminology and to give some context, a "typical" cathodic Tafel slope is ~120 mV per current decade, and it is generally obtained from the intercept of a η vs. log₁₀ (j) plot where η is overpotential (V) and j the current density (A cm^-2 ). Taking a common logarithm from the latter quantity thus explains the unit dec^-1 used in Tafel analysis.^[9]

Saving the reader from further (and out-of-scope) mathematics, let us explore another interesting, 2-dimensional transition metal -based MOF that has shown promising electrocatalytic efficiency for hydrogen evolution.

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, fully supported by iron hydr(oxy)oxide Fe(OH)_x nanoboxes (NBs) on the surface. This Cu-MOF, having highly exposed active Cu centers, do not only exhibit excellent electrocatalytic activity for HER (112 mV overpotential to attain 10 mA cm^-2 current density with a Tafel slope of 76 mV dec^-1), but also the much-needed structural stability in alkaline (1.0 M KOH) solution. The local electronic polarization of unsaturated Cu₁-O₂ centers (Fig. 4, highlighted in red) in the outer layer of this Cu-MOF plays an essential role by promoting the formation of the H* intermediates in hydrogen evolution. Thus, these abundant unsaturated Cu₁-O₂ centers greatly enhance the kinetics of HER. On the other hand, the robust Fe(OH)_x inner shell eems to be the main factor behind the structural stability.  ^[7]

Figure 4. Crystal structure of unsaturated Cu-MOF [Cu₃(HHTP)₂] along c-axis.^[7](Under license CC BY-NC 4.0)

The Cu-MOF is synthesized using uniform Cu₂O nanocubes as a starting material. The surface growth of the conductive Cu-MOF layer over the nanocube is carried out via solvothermal reaction, where locally dissolved Cu cations react with organic ligands (HHTP), generating well-defined Cu₂O@Cu-MOF nanocubes. To convert this intermediate to the final product (Fe(OH)_x@Cu-MOF NB) the inner Cu₂O core of the obtained Cu₂O@Cu-MOF is selectively removed by oxidative etching in the presence of Fe(III) ions. During the redox-etching, a thin Fe(OH)_x shell will eventually precipitate beneath the surface of the Cu-MOF layer. The exact value for x in Fe(OH)_x is undetermined, and the inner Fe(OH)_x layer in Fe(OH)_x@Cu-MOF NB is suggested to have rather amorphous nature. ^[7] Schematics of the synthetic process is illustrated in Fig. 5.

Figure 5. Schematics of the synthetic process for Fe(OH)x@Cu-MOF NB (NB = nanobox)^[7]. (Under license CC BY-NC 4.0)

Summary

The increasing demand for greener energy storage and conversion techniques underlines the urgency of creating advanced functional materials using inexpensive, earth-abundant materials and facile synthesis methods. It is not surprising that MOFs have attracted a lot of interest as a future electrocatalysts, especially in fuel cells and modern electrolyzers. Here, two different examples of electrocatalytic transition metal -based MOFs were introduced: Co-MOF for OER and Cu-MOF for HER, both exhibiting remarkable catalytic efficiency that easily competes with traditional noble-metal based catalysts^[3]. However, the structural stability of MOFs is a valid concern, although there are encouraging results obtained by Cheng et al. (2021), for example. Nano-scale MOFs have more dislocations/defects on the surface than bulk MOFs, indicating that they have more active sites for a catalytic reaction to occur. Also, converting a 3-dimensional structure to 2-dimensional nanosheet improves the accessibility to the metal nodes, speeding up the mass transfer and diffusion during the electrocatalytic processes^[1]. 

References