Water Reduction Catalyst Research Articles

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Site‐Selective Real‐Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L‐Edge X‐Ray Absorption Spectroscopy**

Time‐resolved X‐ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X‐ray transitions with insights from high‐level ab initio calculations. The specific target has been a heteroleptic [IrIII(ppy)2(bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high‐resolution Ir L‐edge X‐ray absorption spectroscopy on the picosecond time scale and restricted active space self‐consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

High-Throughput Screening of Earth-Abundant Water Reduction Catalysts toward Photocatalytic Hydrogen Evolution.

Noble-metal photosensitizers and water reduction co-catalysts (WRCs) still present the highest activity in homogeneous photocatalytic hydrogen production. The search for earth-abundant alternatives is usually limited by the time required to screen new catalyst combinations; however, here, we utilize newly designed and developed high-throughput photoreactors for the parallel synthesis of novel WRCs and colorimetric screening of hydrogen evolution. This unique approach allowed rapid optimization of photocatalytic water reduction using the organic photosensitizer Eosin Y and the archetypal cobaloxime WRC [Co(GL1)2pyCl], where GL1 is dimethylglyoxime and py is pyridine. Subsequent combinatorial synthesis generated 646 unique cobalt complexes of the type [Co(LL)2pyCl], where LL is a bidentate ligand, that identified promising new WRC candidates for hydrogen production. Density functional theory (DFT) calculations performed on such cobaloxime derivative complexes demonstrated that reactivity depends on hydride affinity. Alkyl-substituted glyoximes were necessary for hydrogen production and showed increased activity when paired with ligands containing strong hydrogen-bond donors.

Ruthenium nanodendrites on reduced graphene oxide: an efficient water and 4-nitrophenol reduction catalyst

A green and efficient protocol is reported for the elegant design of reduced graphene oxide (rGO)-supported Ru nanodendrites for promotion of electrochemical water reduction in a wide pH range as well as for environmental remediation.

Polypyridyl Co complex-based water reduction catalysts: why replace a pyridine group with isoquinoline rather than quinoline?

The electronic effect of the substituent has been fully leveraged to improve the activity of molecular water reduction catalysts (WRCs). However, the steric effect of the substituents has received less attention. In this work, a steric hindrance effect was observed in a quinoline-involved polypyridyl Co complex-based water reduction catalyst (WRC), which impedes the formation of Co(iii)-H from Co(i), two pivotal intermediates for H2 evolution, leading to significantly impaired electrocatalytic and photocatalytic activity with respect to its parent complex, [Co(TPA)Cl]Cl (TPA = tris(2-pyridinylmethyl)-amine). In sharp contrast, two isoquinoline-involved polypyridyl Co complexes exhibited significantly improved H2 evolution efficiencies compared to [Co(TPA)Cl]Cl, benefitting mainly from the more basic and conjugated features of isoquinoline over pyridine. The dramatically different influences caused by the replacement of a pyridine group in the TPA ligand by quinoline and isoquinoline fully demonstrates the important roles of both the electronic and steric effects of a substituent. Our results may provide novel insights for designing more efficient WRCs.

Electronic effects on polypyridyl Co complex-based water reduction catalysts.

Three new isomeric cobalt complexes of TPA (tris(2-pyridylmethyl)amine) based on methoxy substitution at the ortho, meta and para positions, respectively, were constructed and their photocatalytic proton reduction efficiencies were compared. It was found that there are good linear correlations with the Hammett constants of the substituents for the computed Co–N bond lengths, redox potentials of CoII/I and CoI/0 events, and the photocatalytic activities of the complexes. The ortho-substituted Co complex distinguished itself from the others remarkably in all these comparisons, demonstrating the presence of a steric effect besides the electronic effect. For other examined complexes, a stronger electron-donating substituent may lead to a higher hydrogen evolution efficiency, suggesting that the formation of a Co(iii) hydride intermediate is the rate-limiting step.

Comparison of Penta and Tetra-pyridyl Cobalt-based Catalysts for Water Reduction: H2 Production Cycle, Solvent Response and Reduction Free Energy.

Understanding water reduction towards H2 generation is crucial to overcome today's renewable energy obstacles. Previous studies have shown the superior H2 production performances of Cobalt based penta-pyridyl (CoaPPy) and tetra-pyridyl (CoaTPy) complexes in solution. We investigate H2 production cycles of CoaPPy and CoaTPy complexes immersed in water solution by means of Ab-initio Molecular Dynamics and Density Functional Theory. We monitor dynamic properties of the systems, solvent response and structural changes occurring in the catalysts, by simulating all intermediate steps of the H2 production cycle. Reduction free energies and reorganization energies are calculated. Our results show that, following the first electron injection, H2 production proceeds with the singlet spin state. Following the first electron insertion, we observe a significant rearrangement of the hydrogen bonding network in the first solvation shell. The cobalt center turns out to be more accessible for the surrounding water molecules in the case of CoaTPy at all the intermediate steps, which explains its higher catalytic performance over CoaPPy. Following the first reduction reaction, a larger gain in reduction free energy is estimated for CoaTPy with respect to CoaPPy, with a difference of 0.14 eV, in line with the experiments. For the second reduction, instead, CoaPPy shows more negative reduction potential, by 0.41 eV.

Plasmon-Mediated Photochemical Reactions

The Cu2O-based photocathode is considered as one of the best performing photocathode materials for solar water reduction. However, the relatively negative onset potential for H2 production of these photocathodes impedes further optimization of the solar-to-fuel conversion efficiency. An important reason is the inability to achieve meaningful photovoltages with Cu2O. Here, the photovoltage barrier can be readily broken by replacing the semiconductor/water interface with a semiconductor/semiconductor one. A n-Cu2O layer was found to form high-quality buried junction with p-Cu2O to increase the photovoltage and thus shift the turn-on voltage positively. Mott−Schottky Measurements confirmed that the improvement was benefited from a thermodynamic shift of the flatband potentials. An extremely positive onset potential in n-Cu2O/AuAg/p-Cu2O was further obtained, which is comparable to what has been measured using water reduction catalysts. The alloy nanoparticles sandwiched between the homojunction of n-Cu2O and p-Cu2O play an important role in enhancing the photocatalytic performance. First, the alloy nanoparticles not only serve as an electron relay, but also promote electron-hole pair generation in nearby semiconductors, which facilitates the charge transfer between n-Cu2O and p-Cu2O since the sandwich structure is measured by X-ray absorption spectroscopy. Second, the alloy nanoparticles act as a plasmonic photosensitizer, which enables the solar-to-hydrogen conversion at wavelengths longer than the band edge of homojunction structure, extending the incident photo-to-current conversion efficiency wavelength. Finally, the plasmonic energy-transfer mechanism is identified as direct transfer of the plasmonic hot carriers, and the interfacial Schottky barrier height is shown to modulate the plasmonic hot electron transfer. This facile sandwich structure combines both the electrical and the optical functions of alloy nanoparticles into a single structure, which has implications for the design of efficient solar-energy-harvesting devices.

The Heteroleptic Cu(I) Photosensitizer‐Containing 3,8‐Disubstituted Phenanthroline: Synthesis, Photophysical Properties and Photocatalytic Hydrogen Evolution from Water

Four novel heteroleptic Cu(I) photosensitizers with 3,8‐diaryl‐2,9‐diisopropyl‐1,10‐phenanthroline derivatives as electron‐donating bidentate N‐ligands have been designed and synthesized. The metal copper complexes are studied in light‐driven water reduction system. And the substituent effect at the 3,8‐positions of N‐ligands impacting on photosensitive activity and photoelectric chemical properties are explored and discussed. Surprisingly, with the enhancement of electron‐donating capacity of substituent group at 3,8‐sites of 1,10‐phenanthroline, the photosensitive activity of photosensitizer linearly increases, and photocatalytic performance up to 817 turnover numbers (TON) was reached combining with water reduction catalyst (WRC), Fe3(CO)12. The photocatalytic performance of these photosensitizers dramatically increased compared to 2,9‐diispropyl‐1,10‐phenanthroline complex (TON = 89). Moreover, the introduction of methoxy substituent to copper complexes, which could increase the fluorescence lifetimes (τmax = 209 ns) and promote hydrogen evolution.

Versatile Production of Novel PNP Based Metal Complexes Applicable as Water Reduction Catalysts Showing CH/M as Well as CH/π Interactions

Several PNP‐type ligands of the form bis(dianisylphosphanylmethyl)alkylamine, where alkyl is methyl, ethyl, isopropyl, and benzyl (1–4), have been coordinated to Co(II), Ni(II), Pd(II), and Pt(II) (5–26). This series of water reduction catalysts (WRC) has been characterized by single‐crystal X‐ray structure analysis, multinuclear and 2D NMR spectroscopy, mass spectrometry and a computational study. Intramolecular contact approaches show differences depending on hetero‐ or homoleptic complexes. Both solid state and solution structures indicate an enhancement of steric pressure for the latter. As a consequence CH/M as well as CH/π interactions appear in the X‐ray structures and 1H NMR spectra. They can also be clearly identified by quantum mechanical calculations on a B3LYP level. Since these WRC contain proton relays due to the used PNP‐ligands, they are prone to proton coupled electron transfer (PCET) during photocatalysis. The different steric pressure influences their reorganisation energy. Obviously, the observed intramolecular contact approaches should be regarded as a tool for the design of future WRC.

Exfoliation-induced exposure of active sites for g-C3N4/N-doped carbon dots heterojunction to improve hydrogen evolution activity

Photocatalysis is a surface catalytic process in which photogenerated electrons are transferred to the surface of the catalyst for water reduction. In this paper, we described a novel two-step calcination strategy that not only realizes the integrating of N-doped carbon dots (NCDs) with g-C3N4 nanosheets, but also can the g-C3N4/NCDs heterojunction provide more catalytic active sites and shortens the distance of charges transport to the surface of materials. The first thermal treatment in the air lead to the formation of bulk g-C3N4 while NCDs was loaded in it. The subsequent secondary calcination was conducive to the exfoliation of bulk g-C3N4 into thinner nanosheets with thickness of 1 nm. Meanwhile, more NCDs insetted in the pristine g-C3N4 were exposed to the surface of materials and more catalytic active site were formed, thus promoting the hydrogen evolution. Electron paramagnetic resonance (EPR) tests certified that the g-C3N4 nanosheet/NCDs composites can generate more OH radical compared with bulk g-C3N4 and bulk g-C3N4/NCDs, which further validated that photogenerated electron-hole was effective separated due to more exposure of NCDs and shorter distance of electron transport to the surface. Impressively, results showed that modification of g-C3N4 with NCDs (1.0 wt % loading) exhibited the highest photocatalytic hydrogen production rate (3319.3 μmol g−1 h−1) and an apparent quantum yield of 29.8 % at 420 nm, which was 13 folds of bulk g-C3N4. Our work illuminates a new method for high activity g-C3N4/carbon dots photocatalyst in the field of photocatalysis.

A flexible homoleptic pentadentate Cu(II) molecular catalyst for effective proton and water reduction

A copper (II) based molecular catalyst [Cu(tpen)](PF6)2 has been synthesized, and employed for the catalytic activity towards hydrogen evolution both in organic and aqueous media (where tpen = N,N,N′,N’-Tetrakis(2-pyridylmethyl)ethylenediamine). The acid-base equilibria of the complex in phosphate buffer suggest that the initial protonation occurs at the free N atom of the pyridine ring. The pKa value for the [Cu(tpenH)]3+ ⇆ [Cu(tpen)]2+ was found to be 4.6. The electrocatalytic activity of the [Cu(tpen)](PF6)2 towards proton reduction in organic media was determined in presence of acetic acid as an external proton source. Controlled potential electrolysis was performed for the [Cu(tpen)](PF6)2 complex in the presence of acetic acid with varying overpotential ranging from 555 mV to 955 mV vs. SCE for 2 h, and a Faradaic efficiency of 89 ± 5% was observed. Electrochemical proton reduction by the complex follows first-order dependency with respect to complex concentration, and it is second order with respect to the concentration of acetic acid. The ic/ip value for the [Cu(tpen)](PF6)2 complex in the acid independent region was found to be 133 at a scan rate of 100 mV s−1 with observed rate constant (kobs) of 3415 s−1. The [Cu(tpen)](PF6)2 complex also undergoes the electrochemical proton reduction in aqueous media to evolve hydrogen. The catalyst generates hydrogen from an aqueous medium at pH 7.0 in phosphate buffer. The plausible mechanisms for pH higher than the pKa value has been derived from the spectroscopic evidence and supported by DFT studies, wherein an EECC process is involved in the catalytic cycle (E = electrochemical; C = chemical).

Effects of metal centers of complexes supported by S,S′‐bis(2‐pyridylmethyl)‐1,2‐thioethane on catalytic activities for electrochemical‐ and photochemical‐driven hydrogen production

S,S′‐bis(2‐pyridylmethyl)‐1,2‐thioethane (bpte) reacts with MCl2 (M = Co, Ni, and Fe) to give three complexes, namely, [CoII(bpte)Cl2] (1), [NiII(bpte)Cl2] (2), and [FeII(bpte)Cl2] (3), respectively. They all act as catalysts for proton or water reduction to dihydrogen via electrolysis or photolysis. Under an overpotential of 837.6 mV, the electrolysis of a neutral buffer with complex 1, 2, or 3 can provide 418 (±3), 555 (±3), and 243 (±3) moles of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h), respectively. Under blue light, together with a photosensitizer and ascorbic acid (H2A) as a sacrificial electron donor, the photolysis of an aqueous solution (pH 4.5) containing complex 1, 2, or 3 can afford 9060 (±5), 24,900 (±5), and 10,630 (±5) moles H2 per mole of catalyst (mol of H2 [mol of cat]−1) during 83‐h irradiation with an average apparent quantum yield of 7.1%, 24%, and 10%, respectively. The results show that the nickel complex [NiII(bpte)Cl2] exhibits a more efficient activity for hydrogen generation than the iron or cobalt species. These findings may offer a new chemical paradigm for the design of efficient catalysts.

Shedding Light on the Molecular Surface Assembly at the Nanoscale Level: Dynamics of a Re(I) Carbonyl Photosensitizer with a Coadsorbed Cobalt Tetrapyridyl Water Reduction Catalyst on ZrO2

We present systematic kinetic studies of the interaction of a rhenium-based photosensitizer with a cobalt(II) tetrapyridyl water reduction catalyst coadsorbed on ZrO2 by transient IR and visible spectroscopies. The study focuses on the competition between the reduction of the excited photosensitizer by an electron donor in solution and nonproductive quenching between the photosensitizer and the catalyst, either by Dexter energy transfer or by electron transfer followed by ultrafast geminate recombination. The implications of both interactions for the charge transfer reactions on the surface are investigated. We find that the kinetics of the system as a whole and the achievable yield of reduced photosensitizer are determined by the inhomogeneous distribution of next neighbor distances between photosensitizers and the water reduction catalysts at the nanoscale level. This provides insight for rational design of heterogeneous water splitting systems with coimmobilized photosensitizers and catalysts.

Understanding Surface Modulation to Improve the Photo/Electrocatalysts for Water Oxidation/Reduction.

Water oxidation and reduction reactions play vital roles in highly efficient hydrogen production conducted by an electrolyzer, in which the enhanced efficiency of the system is apparently accompanied by the development of active electrocatalysts. Solar energy, a sustainable and clean energy source, can supply the kinetic energy to increase the rates of catalytic reactions. In this regard, understanding of the underlying fundamental mechanisms of the photo/electrochemical process is critical for future development. Combining light-absorbing materials with catalysts has become essential to maximizing the efficiency of hydrogen production. To fabricate an efficient absorber-catalysts system, it is imperative to fully understand the vital role of surface/interface modulation for enhanced charge transfer/separation and catalytic activity for a specific reaction. The electronic and chemical structures at the interface are directly correlated to charge carrier movements and subsequent chemical adsorption and reaction of the reactants. Therefore, rational surface modulation can indeed enhance the catalytic efficiency by preventing charge recombination and prompting transfer, increasing the reactant concentration, and ultimately boosting the catalytic reaction. Herein, the authors review recent progress on the surface modification of nanomaterials as photo/electrochemical catalysts for water reduction and oxidation, considering two successive photogenerated charge transfer/separation and catalytic chemical reactions. It is expected that this review paper will be helpful for the future development of photo/electrocatalysts.

Establishing Stability in Organic Semiconductor Photocathodes for Solar Hydrogen Production.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm-2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS3, MoP, or RuO2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS3:MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm-2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO2 where initial photocurrent density (>8 mA cm-2) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.

Two new Cu-based borate catalysts with cubic supramolecular cages for efficient catalytic hydrogen evolution

Focusing on renewable energy, we are devoted to developping efficient, robust and low cost water reduction catalysts (WRCs). Two new Cu-based borate catalysts, namely H2Na2K2[(μ4-O)Cu4@B20O32(OH)8]·21H2O (1) and H2Rb1.6K2.4[(μ4-O)Cu4@B20O32 (OH)8]·15H2O (2), with cubic supramolecular cages were synthesized under a hydrothermal condition. Moreover, new copper complexes were applied as water reduction catalysts (WRCs) in the presence of [Ir(ppy)2(dtbbpy)][PF6] as photosensitizer and triethanolamine (TEOA) as the sacrificial electron donor. Nevertheless, the main active place is attributed to the centre of Borates [(μ4-O)Cu4@B20O32(OH)8], and the atomic radius of the counter cation would be the critical factor of the photocatalytic activity. Increasing the atomic radius from the Na atom to the Rb atom, causes the photocatalytic activity to decrease efficiently. The experimental results match well with the density functional theory (DFT) conclusion. It is noteworthy to mention that our research not only enriches the Cu-based borate chemistry, but also investigates the photocatalytic activity of Cu-based borates. This would guide us through the borate synthesis and to develop their applications toward energy and the environment.

Light-driven proton reduction with in situ supported copper nanoparticles

Heterogeneous copper particles have been widely investigated in photocatalytic reduction processes to produce hydrogen. In this contribution, copper nanoparticles were in situ generated and supported on different C- and SiO2-based additives in the presence of an iridium photosensitizer (PS), which itself was activated by visible light absorption. Thereby, the influence of the supports on the formation of the active species and their performance as water reduction catalysts were studied. The additives ensure heterogeneous nucleation leading to lower sizes of the formed copper particles and prevent their further growth during the reaction which was verified by STEM analysis. Consequently, the hydrogen productivity in the presence of the supports was enhanced and the best carbon material rGO(H2) revealed a 3 times higher activity compared to the non-supported system after 20 h. Photoluminescence measurements confirmed the proposed reductive quenching mechanism in this system and suggested that rGO(H2) mediates reoxidation of the PS. However, in the presence of the ligand 2,2′-bipyridine (bpy) the application of the silica MCM-41 showed a fourfold increase of hydrogen evolution compared to the non-supported system, which was still active after 160 h and could be successfully reused in recycling experiments.

Sandwich-Nanostructured n-Cu2O/AuAg/p-Cu2O Photocathode with Highly Positive Onset Potential for Improved Water Reduction.

An n-Cu2O layer formed a high-quality buried junction with p-Cu2O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott-Schottky measurements confirmed that the improvement benefited from a positive shift in flat-band potential. The obtained extremely positive onset potential, 0.8 VRHE in n-Cu2O/AuAg/p-Cu2O, is comparable with measurements from water reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu2O and p-Cu2O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu2O and p-Cu2O in the sandwich structure was measured with X-ray absorption spectra. The proposed sandwich structure can be considered as a new direction for the design of efficient solar-related devices.

Surface modification of a Co9S8 nanorods with Ni(OH)2 on nickel foam for high water splitting performance

Electrocatalytic water splitting to produce H2 and O2 has been considered as one of the most perfect solutions to resolve energy problems. Herein, we report the in situ deposit method on surface of Co9S8 by adjusting the number of sedimentary cycles to adjust Ni(OH)2 loading capacity, and then firstly using the materials as water oxidation and water reduction catalysts. The one-dimensional Co9S8 nanorods was acted as an perfect support to enhance the uniform dispersion of amorphous Ni(OH)2 nanoparticles, and the electrocatalytic deposited amorphous Ni(OH)2 nanoparticles wrapped in the surface of Co9S8 nanorods highly improve the interfacial effect and afford more effective active centers for water splitting reaction. When Co9S8@Ni(OH)2- amorphous/NF-2h-2 cycles, Co9S8@Ni(OH)2 - amorphous/NF-10h-2 cycles and Ni(OH)2- amorphous/NF is acted as a bifunctional catalyst, Co9S8@Ni(OH)2- amorphous/NF-2h-2 cycles presents a lower cell voltage (1.55 V@10 mAcm−2) than Co9S8@Ni(OH)2- amorphous/NF-10h-2 cycles (1.57 V@10 mAcm−2) and Ni(OH)2- amorphous/NF (NA@10 mAcm−2). The activity of Co9S8@Ni(OH)2- amorphous/NF-2h-2 cycles is compared to that of the ideal the benchmark 25 wt % RuO2/NF−30 wt % Pt/NF (1.56 V@10 mAcm−2). Density functional theory (DFT) calculations were calculated to expound more mechanism on the interface effect of the sulfide/hydroxide-based catalysts for HER. For the two kinds of composite catalysts, both catalysts show good activity and stability in this work. Experiments show that Ni(OH)2 does protect Co9S8 from corrosion under alkaline conditions. Our work paves the way for the development of environmentally friendly and relatively non-toxic water splitting electrocatalysts in terms of large-scale applications.