Electrocatalytic reduction of protons to hydrogen by a water-compatible cobalt polypyridyl platform

Bigi, Julian P.; Hanna, Tamara E.; Harman, W. Dean; Chang, Andrew S.; Chang, Christopher J.

doi:10.1039/b915846d

Cited by 200 publications

(164 citation statements)

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“…The prevalence of molecular cobalt-based proton reduction catalysts, 9,39,40,42−63 including contributions from our 11,16,36,37,41,64,65 has attracted many experimental and computational mechanistic investigations. Although the operative mechanism for these catalysts can vary widely, formation of a cobalt-hydride is commonly invoked.…”

Section: ■ Redox-active Ligands For Redox-leveling and Facilitating Mmentioning

confidence: 99%

“…Moreover, being aromatic and possessing strong bonds, pyridine ligands are 2+ (1, Figure 2). 16 The cyclic voltammogram (CV) of 1 in acetonitrile displays a reversible Co(II)/(I) redox couple at modest reduction potentials. Addition of trifluoroacetic acid (TFA) triggered proton reduction catalysis, with ∼99% Faradaic yield (Table 1).…”

Section: ■ Polypyridyl Ligands For Catalysis In Watermentioning

confidence: 99%

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Metal–Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen

et al. 2015

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CONSPECTUS: Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world's energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production−consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earthabundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal−polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH 3 CN) 2 ] 2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me 2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me 2 )MoO] 2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H 2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H 2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me 2 )MoS 2 ] 2+ , which bears a molecular MoS 2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me 2 )MoS 2 ] 2+ onsets at 400 mV of overpotential, whereas [(PY5Me 2 )MoO] 2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me 2 ligand with an appropriate Co(II) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me 2 )Co(H 2 O)] 2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF 3 SO 3 )] 1+ system, we showed that introducing a redox-active moiety can facilitate the electro-and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we...

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Section: ■ Redox-active Ligands For Redox-leveling and Facilitating Mmentioning

confidence: 99%

Section: ■ Polypyridyl Ligands For Catalysis In Watermentioning

confidence: 99%

Metal–Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen

et al. 2015

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“…To date, most of the well-studied electrocatalysts for proton reduction and H 2 generation operate in nonaqueous media with the proton source being added acid. In contrast, photochemical efforts on the reductive side of water splitting have generally been conducted in aqueous or aqueous/organic media (33)(34)(35)(36). Since CoðbdtÞ − 2 was previously reported to be an active electrocatalyst in aqueous/organic solutions (30), the other Co dithiolene complexes (2-4) were examined as electrocatalysts for H 2 generation in both dry CH 3 CN and aqueous organic solutions.…”

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confidence: 99%

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

McNamara

Han

Yin

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

276

267

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Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H 2 ). Harnessing solar energy to generate H 2 from H + is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 1∶1 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to . Subsequent reduction and reaction with H + lead to H 2 formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

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“…There also has been recent progress on molecular catalysts that are active in water -solvent mixtures [27,40,42] and even in neutral pH water [29,39]. However, a stable molecular catalyst that functions in water near neutral pH, displays a low overpotential, is not sensitive to oxygen and evolves hydrogen at a rapid rate has not yet been discovered.…”

Section: Molecular Components For Solar Hydrogen Generationmentioning

confidence: 99%

Multidisciplinary approaches to solar hydrogen

Bren

2015

Interface Focus.

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This review summarizes three different approaches to engineering systems for the solar-driven evolution of hydrogen fuel from water: molecular, nanomaterials and biomolecular. Molecular systems have the advantage of being highly amenable to modification and detailed study and have provided great insight into photophysics, electron transfer and catalytic mechanism. However, they tend to display poor stability. Systems based on nanomaterials are more robust but also are more difficult to synthesize in a controlled manner and to modify and study in detail. Biomolecular systems share many properties with molecular systems and have the advantage of displaying inherently high efficiencies for light absorption, electron–hole separation and catalysis. However, biological systems must be engineered to couple modules that capture and convert solar photons to modules that produce hydrogen fuel. Furthermore, biological systems are prone to degradation when employed in vitro . Advances that use combinations of these three tactics also are described. Multidisciplinary approaches to this problem allow scientists to take advantage of the best features of biological, molecular and nanomaterials systems provided that the components can be coupled for efficient function.

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Electrocatalytic reduction of protons to hydrogen by a water-compatible cobalt polypyridyl platform

Abstract: A cobalt(ii) complex supported by the new tetradentate polypyridyl ligand PY4 is an electrocatalyst for the reduction of protons to hydrogen and can operate in 50% aqueous media.

Cited by 200 publications

References 58 publications

Metal–Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen

Metal–Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Multidisciplinary approaches to solar hydrogen

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