Molecular electrocatalysts for H
2
evolution are usually studied under various conditions (solvent and proton sources) that prevent direct comparison of their performances. We provide here a rational ...method for such a benchmark based on (i) the recent analysis of the current-potential response for two-electron-two-step mechanisms and (ii) the derivation of catalytic Tafel plots reflecting the interdependency of turnover frequency and overpotential based on the intrinsic properties of the catalyst, independent of contingent factors such as cell characteristics. Such a methodology is exemplified on a series of molecular catalysts among the most efficient in the recent literature.
How "catalytic Tafel plots" allow comparison of the performances of molecular H
2
-evolving catalysts: fundamentals and illustrating examples.
The active sites of hydrogenases have inspired the design of molecular catalysts for hydrogen evolution and oxidation. In this feature article, we showcase key elements of bio-inspiration before ...embarking on a tour of a representative series of molecular hydrogen evolving catalysts (HECs) and describing the toolbox available for benchmarking their performances. We then show how such catalysts can be immobilized on conducting substrates to prepare electrode materials active for hydrogen evolution and oxidation with a special emphasis on cobalt diimine-dioxime complexes and DuBois' nickel diphosphine compounds. We finally discuss the optimization required for implementing molecular-engineered materials into operational devices and illustrate how such molecular approaches can be expanded to other fuel-forming processes such as the electrochemical valorisation of carbon dioxide and the oxygen reduction or water oxidation reactions.
Surface functionalization allows the immobilization of molecular catalysts for hydrogen evolution and uptake onto conducting materials and yields electrodes based on earth-abundant elements as alternative to the use of platinum catalysts.
•New hydrogen evolving molecular cobalt–polypyridyl catalysts have recently emerged.•Their evaluation for electro- and photocatalytic hydrogen production is discussed.•These robust catalysts display ...high activities under fully aqueous conditions.•Mechanistic considerations and structure–activity relationships are highlighted.
The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)–polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure–catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.
Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. NiFe hydrogenases are organometallic enzymes containing nickel and iron centres ...that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of NiFe hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H
evolution (second-order rate constant of 2.5 × 10
M
s
; turnover frequency of 250 s
at 10 mM H
concentration) from mildly acidic solutions.
Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites ...have been well established for crystalline molybdenum disulfide (c-MoS2) but not for amorphous molybdenum sulfide (a-MoSx), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a-MoSx, prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete Mo3S13(2-) building blocks. Of the three terminal disulfide (S2(2-)) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a-MoSx catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum.
Electrolysis at Nickel
One drawback of solar and wind power is the need for an efficient storage system to release accumulated energy when neither source is readily available (during still nights, ...for example). Hydrogen derived from electrolysis of water is potentially a useful medium for this purpose, but catalyzing the interconversion efficiently at large scale would currently require a substantial amount of the scarce precious metal platinum. An alternative approach would be to mimic natural enzymatic reactions, which accomplish the interconversion using hydrogenases that incorporate the more abundant metals iron and nickel. In this vein,
Le Goff
et al.
(p.
1384
; see the Perspective by
Hambourger and Moore
) have lightly modified a hydrogenase-inspired nickel complex in order to append it to a conductive carbon nanotube support. The resulting hybrid material shows promising catalytic efficiency for reversible aqueous electrolysis in a standard apparatus.
A nickel electrocatalyst supported on carbon nanotubes shows promising activity for proton-hydrogen interconversion in water.
Interconversion of water and hydrogen in unitized regenerative fuel cells is a promising energy storage framework for smoothing out the temporal fluctuations of solar and wind power. However, replacement of presently available platinum catalysts by lower-cost and more abundant materials is a requisite for this technology to become economically viable. Here, we show that the covalent attachment of a nickel bisdiphosphine–based mimic of the active site of hydrogenase enzymes onto multiwalled carbon nanotubes results in a high–surface area cathode material with high catalytic activity under the strongly acidic conditions required in proton exchange membrane technology. Hydrogen evolves from aqueous sulfuric acid solution with very low overvoltages (20 millivolts), and the catalyst exhibits exceptional stability (more than 100,000 turnovers). The same catalyst is also very efficient for hydrogen oxidation in this environment, exhibiting current densities similar to those observed for hydrogenase-based materials.
Copper is currently extensively studied because it provides promising electrodes for carbon dioxide electroreduction. The original combination, reported here, of a nanostructured porous dendritic ...Cu-based material, characterized by electron microcopy (SEM, TEM) and X-ray diffraction methods, and a water/ionic liquid mixture as the solvent, contributing to CO2 solubilization and activation, results in a remarkably efficient (large current densities at low overpotentials), stable and selective (large faradic yields) electrocatalytic system for the conversion of CO2 into formic acid, a product with a variety of uses. These results provide new directions for the further improvement of Cu electrodes.
Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light‐to‐chemical energetic transduction. This field of ...research, called “artificial photosynthesis,” is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon‐free fuel hydrogen through the light‐driven water splitting. In this review, we highlight the recent achievements on light‐driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.
This review surveys the recent achievements in the field of artificial photosynthesis and light‐driven water splitting. Homogeneous photocatalytic H2‐ and O2‐evolving systems (bioconstructs as well as synthetic molecules) are presented, with a specific focus on those containing noble‐metal‐free catalytic centres. Openings regarding the development of a fully molecular‐based technological device for water splitting are discussed.
We report on the first systematic study of cobaloxime-based hydrogen photoproduction in mixed pH 7 aqueous/acetonitrile solutions and demonstrate that H2 evolution can be tuned through electronic ...modifications of the axial cobalt ligand or through introduction of TiO2 nanoparticles. The photocatalytic systems consist of various cobaloxime catalysts Co(dmgH)2(L)Cl (L = nitrogen-based axial ligands) and a water soluble porphyrin photosensitizer. They were assayed in the presence of triethanolamine as a sacrificial electron donor. Optimal turnover numbers related to the photosensitizer are obtained with electron-rich axial ligands such as imidazole derivatives (1131 TONs with N-methyl imidazole). Lower stabilities are observed with various pyridine axial ligands (443 TONs for para-methylpyridine), especially for those containing electron-acceptor substituents. Interestingly, when L is para-carboxylatopyridine the activity of the system is increased from 40 to 223 TONs in the presence of TiO2 nanoparticles.
We describe here a systematic, reliable, and fast screening method that allows the comparison of H2‐forming catalysts that work under aqueous conditions with two readily prepared chemical reductants ...and two commonly used photosensitizers. This method uses a Clark‐type microsensor for H2 detection and complements previous methods based on rotating disk electrode measurements. The efficiencies of a series of H2‐producing catalysts based on Co, Ni, Fe, and Pt were investigated in aqueous solutions under thermal conditions with europium(II) reductants and under photochemical conditions in the presence of two different photosensitizers {Ru(bipy)3Cl2 (bipy=2,2‐bipyridine) and eosin‐Y} and sacrificial electron donors (ascorbate and triethanolamine, respectively). The majority of catalysts tested were active only under specific conditions. However, our results also demonstrate the impressive versatility of a group of Co catalysts, which were able to produce H2 under different reducing conditions and at various pH values. In particular, a cobaloxime, Co(dmgH)2(H2O)2 (dmgH2=dimethylglyoxime), and a cobalt tetraazamacrocyclic complex, {Co(CR)Cl2}+ CR=2,12‐dimethyl‐3,7,11,17‐tetraazabicylo(11.3.1)heptadeca‐1(17),2,11,13,15‐pentaene, displayed excellent catalytic rates under the studied conditions, and the best rates were observed under thermal conditions.
How much hydrogen? The systematic screening of eight hydrogen‐producing transition‐metal complexes is performed in aqueous media under thermal and photochemical conditions with a Clark‐type H2 microsensor. This method allows the quick evaluation of the catalytic rates and turnover numbers through the quantification of hydrogen and, thus, complements the existing electrochemical methods based on rotating disk electrode measurements.
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