A chromotropic PtIIPdIICoII coordination polymer with dual electrocatalytic activity for water reduction and oxidation

Here, we present a heterometallic coordination polymer that exhibits heterogeneous electrocatalytic activities for both water reduction and water oxidation. Treatment of the PtII2PdII2 tetranuclear complex [Pd₂{Pt(NH₃)₂(D-pen)₂}₂] ([1]; D-H₂pen = D-penicillamine) with CoX₂ (X = Cl, Br) provided (PtII2PdII2CoII2)_n coordination polymers [Co₂(H₂O)₆(1)]X₄ ([2]X₄), in which the PtII2PdII2 units of [1] are linked by [Co₂(μ-H₂O)(H₂O)₅]⁴⁺ moieties in a 3D network structure. [2]X₄ showed a colour change from orange to dark green upon dehydration, reflecting the geometrical conversion of the Co^II centres in [Co₂(μ-H₂O)(H₂O)₅]⁴⁺ from an octahedron to a tetrahedron by the removal of aqua ligands. While both [2]Cl₄ and [2]Br₄ electrochemically catalysed water reduction to H₂ in the solid state due to the presence of Pd^II active centres, water oxidation to O₂ was catalysed only by [2]Br₄, which is ascribed to the presence of Br^- ions that mediate the catalytic reactions that occurred at Co^II active centres.

Hydrogen Evolving Activity of Dithiolene-Based Metal-Organic Frameworks with Mixed Cobalt and Iron Centers

Electrocatalytic systems based on metal-organic frameworks (MOFs) have attracted great attention due to their potential application in commercially viable renewable energy-converting devices. We have recently shown that the cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) framework can catalyze the hydrogen evolution reaction (HER) in fully aqueous media with Tafel slopes as low as 71 mV/dec and near-unity Faradaic efficiency (FE). Taking advantage of the high synthetic tunability of MOFs, here, we synthesize a series of iron and mixed iron/cobalt THT-based MOFs. The incorporation of the iron and cobalt dithiolene moieties is verified by various spectroscopic techniques, and the integrity of the crystalline structure is maintained regardless of the stoichiometries of the two metals. The hydrogen evolving activity of the materials was explored in pH 1.3 aqueous electrolyte solutions. Unlike CoTHT, the FeTHT framework exhibits minimal activity due to a late catalytic onset [-0.440 V versus reversible hydrogen electrode (RHE)] and a large Tafel slope (210 mV/dec). The performance of the mixed-metal MOFs is adversely affected by the incorporation of Fe, where increasing Fe content results in MOFs with lower HER activity and diminished long-term stability and FE for H₂ production. It is proposed that the FeTHT domains undergo alternative Faradaic processes under catalytic conditions, which alter its local structure and electrochemical behavior, eventually resulting in a material with diminished HER performance.

Understanding the Evolution of Cobalt-Based Metal-Organic Frameworks in Electrocatalysis for the Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) have attracted increasing attention as a promising electrode material for the oxygen evolution reaction (OER). Comprehending catalytic mechanisms in the OER process is of key relevance for the design of efficient catalysts. In this study, two types of Co based MOF with different organic ligands (ZIF-67 and CoBDC; BDC=1,4-benzenedicarboxylate) are synthesized as OER electrocatalysts and their electrochemical behavior is studied in alkaline solution. Physical characterization indicates that ZIF-67, with tetrahedral Co sites, transforms into α-Co(OH)₂ on electrochemical activation, which provides continuous active sites in the following oxidation, whereas CoBDC, with octahedral sites, evolves into β-Co(OH)₂ through hydrolysis, which is inert for the OER. Electrochemical characterization reveals that Co sites coordinated by nitrogen from imidazole ligands (Co-N coordination) are more inclined to electrochemical activation than Co-O sites. The successive exposure and accumulation of real active sites is responsible for the gradual increase in activity of ZIF-67 in OER. This work not only indicates that CoMOFs are promising OER electrocatalysts but also provides a reference system to understand how metal coordination in MOFs affects the OER process.

Cu[Ni(2,3-pyrazinedithiolate)2] Metal-Organic Framework for Electrocatalytic Hydrogen Evolution

The application of metal-organic frameworks (MOFs) as electrocatalysts for small molecule activation has been an emerging topic of research. Previous studies have suggested that two-dimensional (2D) dithiolene-based MOFs are among the most active for the hydrogen evolution reaction (HER). Here, a three-dimensional (3D) dithiolene-based MOF, Cu[Ni(2,3-pyrazinedithiolate)₂] (1), is evaluated as an electrocatalyst for the HER. In pH 1.3 aqueous electrolyte solution, 1 exhibits a catalytic onset at -0.43 V vs the reversible hydrogen electrode (RHE), an overpotential (η_10 mA/cm²) of 0.53 V to reach a current density of 10 mA/cm², and a Tafel slope of 69.0 mV/dec. Interestingly, under controlled potential electrolysis, 1 undergoes an activation process that results in a more active catalyst with a 200 mV reduction in the catalytic onset and η_10 mA/cm². It is proposed that the activation process is a result of the cleavage of Cu-N bonds in the presence of protons and electrons. This hypothesis is supported by various experimental studies and density functional theory calculations.

Advanced Electrocatalysts Based on Metal-Organic Frameworks

In recent years, metal-organic frameworks (MOFs) have been wildly studied as heterogeneous catalysts due to their diversity of structures and outstanding physical and chemical properties. Meanwhile, MOFs have also been regarded as promising templates for the synthesis of conductive and electrochemically active catalysts. However, in most of the studies, high-temperature annealing is needed to transform nonconductive or low-conductive MOFs to conductive materials for electrocatalyis, during which the unique structures and intrinsic active sites in MOFs can be easily destroyed. Therefore, in recent years, different strategies have been developed for improving the catalytic performances of MOF-based composites for electrochemical reactions with no need of post-treatment. This mini-review highlights the recent advances on MOF-based structures with improved conductivities and electrochemical activities for the application in electrocatalysis. Overall, the advanced MOF-based electrocatalysts include the highly conductive and electrochemically active pristine MOFs, MOFs combined with conductive substrates, and MOFs hybridized with active materials. Finally, we propose the direction for future works on MOF-based electrocatalysts.

Coordination polymers as heterogeneous catalysts in hydrogen evolution and oxygen evolution reactions

This article highlights various strategies of designing coordination polymers for catalysing water splitting reactions.

Evolution of metal organic frameworks as electrocatalysts for water oxidation

The development of metal organic framework based water oxidation catalysts is discussed here in connection with various design strategies.

Pristine Metal-Organic Frameworks and their Composites for Energy Storage and Conversion

Metal-organic frameworks (MOFs), a new class of crystalline porous organic-inorganic hybrid materials, have recently attracted increasing interest in the field of energy storage and conversion. Herein, recent progress of MOFs and MOF composites for energy storage and conversion applications, including photochemical and electrochemical fuel production (hydrogen production and CO₂ reduction), water oxidation, supercapacitors, and Li-based batteries (Li-ion, Li-S, and Li-O₂ batteries), is summarized. Typical development strategies (e.g., incorporation of active components, design of smart morphologies, and judicious selection of organic linkers and metal nodes) of MOFs and MOF composites for particular energy storage and conversion applications are highlighted. A broad overview of recent progress is provided, which will hopefully promote the future development of MOFs and MOF composites for advanced energy storage and conversion applications.

Evaluation of two- and three-dimensional electrode platforms for the electrochemical characterization of organometallic catalysts incorporated in non-conducting metal-organic frameworks

The development of a reliable platform for the electrochemical characterization of a redox-active molecular diiron complex, [FeFe], immobilized in a non-conducting metal organic framework (MOF), UiO-66, based on glassy-carbon electrodes is reported. Voltammetric data with appreciable current responses can be obtained by the use of multiwalled carbon nanotubes (MWCNT) or mesoporous carbon (CB) additives that function as conductive scaffolds to interface the MOF crystals in "three-dimensional" electrodes. In the investigated UiO-66-[FeFe] sample, the low abundance of [FeFe] in the MOF and the intrinsic insulating properties of UiO-66 prevent charge transport through the framework, and consequently, only [FeFe] units that are in direct physical contact with the electrode material are electrochemically addressable.

The role of redox hopping in metal–organic framework electrocatalysis

A perspective on redox hopping charge transport through metal organic frameworks and its role in driving efficient electrocatalysis.

Electrochemical Water Oxidation by a Catalyst-Modified Metal-Organic Framework Thin Film

Water oxidation, a key component in artificial photosynthesis, requires high overpotentials and exhibits slow reaction kinetics that necessitates the use of stable and efficient heterogeneous water-oxidation catalysts (WOCs). Here, we report the synthesis of UiO-67 metal-organic framework (MOF) thin films doped with [Ru(tpy)(dcbpy)OH₂ ]²⁺ (tpy=2,2':6',2''-terpyridine, dcbpy=5,5'-dicarboxy-2,2'-bipyridine) on conducting surfaces and their propensity for electrochemical water oxidation. The electrocatalyst oxidized water with a turnover frequency (TOF) of (0.2±0.1) s^-1 at 1.71 V versus the normal hydrogen electrode (NHE) in buffered solution (pH∼7) and exhibited structural and electrochemical stability. The electroactive sites were distributed throughout the MOF thin film on the basis of scan-ratedependent voltammetry studies. This work demonstrates a promising way to immobilize large concentrations of electroactive WOCs into a highly robust MOF scaffold and paves the way for future photoelectrochemical water-splitting systems.

Perspectives on metal–organic frameworks with intrinsic electrocatalytic activity

This highlight article focuses on the rapidly emerging area of electrocatalytic metal–organic frameworks (MOFs) with a particular emphasis on those systems displaying intrinsic activity.

Metal-Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions To Enable Electrocatalytic Water Oxidation

Thin films of the metal-organic framework (MOF) NU-1000 were grown on conducting glass substrates. The films uniformly cover the conducting glass substrates and are composed of free-standing sub-micrometer rods. Subsequently, atomic layer deposition (ALD) was utilized to deposit Co(2+) ions throughout the entire MOF film via self-limiting surface-mediated reaction chemistry. The Co ions bind at aqua and hydroxo sites lining the channels of NU-1000, resulting in three-dimensional arrays of separated Co ions in the MOF thin film. The Co-modified MOF thin films demonstrate promising electrocatalytic activity for water oxidation.

Metal–organic frameworks based on rigid ligands as separator membranes in supercapacitor

Two MOFs are used as separator membranes in a supercapacitor, and after a charge–discharge experiment the separator membrane of the Co compound becomes more porous.

Metal(II) complexes synthesized based on quinoline-2,3-dicarboxylate as electrocatalysts for the degradation of methyl orange

Based on quinoline-2,3-dicarboxylic acid (H2L), two metal(II) complexes formulated as MnL(phen)(H2O)·H2O (phen = 1,10-phenanthroline) (1) and Co(HL)2(PPA)·4H2O (PPA = N(1),N(4)-di(pyridin-4-yl)terephthalamide) (2) were synthesized and structurally characterized by single-crystal X-ray diffraction. Both complexes 1 and 2 exhibit one-dimensional (1D) chain-like structures, in which stable five-membered rings are observed. Different chains are linked by strong π-π stacking interactions into a three-dimensional (3D) supramolecular architecture. Both complexes can increase the degradation rate of methyl orange (MO), which is expected to be associated with their electrocatalytic activities for the H2 evolution reaction from water.

Metal–organic frameworks based on 4-(4-carboxyphenyl)-2,2,4,4-terpyridine: structures, topologies and electrocatalytic behaviors in sodium laurylsulfonate aqueous solution

Two isostructural MOFs show better solubilities in SDS solution than water, and they show different electrocatalytic behaviors for the HER in SDS solution.

A polyoxometalate-based complex with visible-light photochromism as the electrocatalyst for generating hydrogen from water

A polyoxometalate-based complex can act as the electrocatalyst for the H₂ evolution reaction (HER) and the HER current is enhanced with visible-light irradiation.

Metal(ii) complexes based on 4-(2,6-di(pyridin-4-yl)pyridin-4-yl)benzonitrile: structures and electrocatalysis in hydrogen evolution reaction from water

Based onL, three metal(ii)-complexes shows different electrocatalytic activities in HER from water.