With the global energy demand expected to increase drastically over the next several decades, the development of a sustainable energy system to meet this increase is paramount. Renewable energy ...sources can be coupled with electrochemical conversion processes to store energy in chemical bonds. To promote these difficult transformations, electrocatalysts that operate at high conversion rates and efficiency are required. Metal–organic frameworks (MOFs) have emerged as a promising class of materials; however, the insulating nature of MOFs has limited their application as electrocatalysts. The recent development of conductive MOFs has led to several electrocatalytic MOFs that display activity comparable to that of the best‐performing heterogeneous catalysts. Although many electrocatalytic MOFs exhibit low activity and stability, the few successful examples highlight the possibility of MOF electrocatalysts as replacements for noble‐metal‐based catalysts in commercial energy‐converting devices. We review herein the use of pristine MOFs as electrocatalysts to facilitate important energy‐related reactions.
Framed: Metal–organic frameworks (MOFs) have emerged as a promising class of materials; however, their insulating nature has limited their application as electrocatalysts. The recent development of conductive MOFs has led to several electrocatalytic MOFs that display activity comparable to that of the best‐performing heterogeneous catalysts. Herein, we review the use of pristine MOFs as electrocatalysts to facilitate important energy‐related reactions.
Hydrogen production through the reduction of water has emerged as an important strategy for the storage of renewable energy in chemical bonds. One attractive scenario for the construction of ...efficient devices for electrochemical splitting of water requires the attachment of stable and active hydrogen evolving catalysts to electrode surfaces, which remains a significant challenge. We demonstrate here the successful integration of cobalt dithiolene catalysts into a metal–organic surface to generate very active electrocatalytic cathode materials for hydrogen generation from water. These surfaces display high catalyst loadings and remarkable stability even under very acidic aqueous solutions.
Solar-driven hydrogen evolution from water has emerged as an important methodology for the storage of renewable energy in chemical bonds. Efficient and practical clean-energy devices for ...electrochemical or photoelectrochemical splitting of water require the immobilization of stable and active hydrogen-evolving catalysts onto electrode or photocathode materials, which remains a significant challenge. Here we show that cobalt(II) reacts with benzene-1,2,4,5-tetrathiol in the presence of base to form a cobalt dithiolene polymer 1. The generated polymer is immobilized onto glassy carbon electrodes (GCE) to generate a metal–organic surface (MOS 1|GCE), which displays efficient H2-evolving activity and stability in acidic aqueous solutions. Moreover, the generated polymer is integrated with planar p-type Si to generate very efficient photocathode materials (MOS 1|Si) for solar-driven hydrogen production from water. Photocurrents up to 3.8 mA/cm2 at 0 V vs RHE were achieved under simulated 1 Sun illumination. MOS 1|Si photocathodes operate at potentials 550 mV more positive than MOS 1|GCE cathodes to reach the same activity for H2 evolution from water (1 mA/cm2).
We report here the efficient reduction of CO2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the ...azacalix4(2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the CoI/0 couple, therefore decreasing the overpotential for CO2 reduction.
Several cobalt complexes catalyze the evolution of hydrogen from acidic solutions, both homogeneously and at electrodes. The detailed molecular mechanisms of these transformations remain unresolved, ...largely owing to the fact that key reactive intermediates have eluded detection. One method of stabilizing reactive intermediates involves minimizing the overall reaction free-energy change. Here, we report a new cobalt(I) complex that reacts with tosylic acid to evolve hydrogen with a driving force of just 30 meV/Co. Protonation of CoI produces a transient CoIII-H complex that was characterized by nuclear magnetic resonance spectroscopy. The CoIII-H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics suggests that CoIII-H produces hydrogen by two competing pathways: a slower homolytic route involving two CoIII-H species and a dominant heterolytic channel in which a highly reactive CoII-H transient is generated by CoI reduction of CoIII-H.
The incorporation of homogeneous catalysts for CO2 reduction into extended frameworks has been a successful strategy for increasing catalyst lifetime and activity, but the effects of the linkers on ...catalysis are underexplored. In this work, a novel rhenium bipyridine complex was synthesized for the purpose of designing a covalent-organic framework (COF) with both metalloporphyrin and metal bipyridine moieties. Investigation of the rhenium complex as a homogeneous catalyst shows a faradaic efficiency of 81(8)% for the electrocatalytic conversion of CO2 to CO upon the addition of methanol as the proton source. Treatment of the rhenium complex with tetra(4-aminophenyl)porphyrin under Schiff base conditions produces the desired COF, as indicated by powder X-ray diffraction (PXRD) studies. Metalation of the porphyrins was accomplished through postsynthetic modification with CoCl2 and FeCl3 metal precursors. The retention of the PXRD peaks and appearance of new Co and Fe peaks in the corresponding X-ray photoelectron spectroscopy spectra suggest the successful incorporation of a secondary metal site into the framework. Cyclic voltammetry measurements display increases in current densities when the atmosphere is changed from N2 to CO2. Controlled potential electrolyses show that the cobalt-postmetalated COF has the highest activity toward CO2 reduction, reaching a faradaic efficiency of 18(2)%.
Two-dimensional (2D) metal–organic frameworks (MOFs) have received a great deal of attention due to their relatively high charge carrier mobility and low resistivity. Here we report on the ...temperature-dependent charge transport properties of a 2D cobalt 2,3,6,7,10,11-triphenylenehexathiolate framework. Variable temperature resistivity studies reveal a transition from a semiconducting to a metallic phase with decreasing temperature, which is unprecedented in MOFs. We find this transition to be highly dependent on the film thickness and the amount of solvent trapped in the pores, with density functional theory calculations of the electronic-structure supporting the complex metallic conductivity of the material. These results identify the first experimentally observed MOF that exhibits band-like metallic conductivity.
Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require ...active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.
Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power.
Metal–organic frameworks (MOFs) containing redox active linkers have led to hybrid compounds exhibiting high electrical conductivity, which enables their use in applications in electronics and ...electrocatalysis. While many computational studies predict two-dimensional (2D) MOFs to be metallic, the majority of experiments show decreasing conductivity on cooling, indicative of a gap in the electronic band structure. To date, only a handful of MOFs have been reported that exhibit increased electrical conductivity upon cooling indicative of a metallic character, which highlights the need for a better understanding of the origin of the conductivity. A 2D MOF containing iron bis(dithiolene) motifs was recently reported to exhibit semiconducting behavior with record carrier mobility. Herein, we report that high crystallinity and the elimination of guest species results in an iron 2,3,6,7,10,11-tripheylenehexathiolate (THT) MOF, FeTHT, exhibiting a complex transition from semiconducting to metallic upon cooling, similar to what was shown for the analogous CoTHT. Remarkably, exposing the FeTHT to air significantly influences the semiconducting-to-metallic transition temperature (100 to 300 K) and ultimately results in a material showing metallic-like character at, and above, room temperature. This study indicates these materials can tolerate a substantial degree of doping that ultimately results in charge delocalization and metallic-like conductivity, an important step toward enabling their use in chemiresistive sensing and optoelectronics.
The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO2 to higher energy products. However, the ...mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO2 substrate.
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