Visible-light-driven hydrogen evolution with polyoxometalate as electron relay

A highly efficient visible-light-induced hydrogen evolution system without an electron mediator such as methyl viologen, constructed with rose bengal as a photosensitizer, triethanolamine (TEA) as an electron donor, and Pt as a cocatalyst, has been reported. Under visible light irradiation (λ ≥ 420 nm), the highest rate of hydrogen evolution and apparent quantum efficiency are about 56.9 μmol h−1 and 11.3%, respectively. According to the effects of urea and ionic strength on the activity of hydrogen evolution, the dimers of rose bengal play a significant role in the photosensitized hydrogen evolution process. On the basis of activity tests and UV−vis absorbance and fluorescence measurements, the probable mechanism for photosensitized hydrogen evolution has been postulated.

A novel hybrid artificial photosynthesis system using MoS2 embedded in carbon nanofibers as electron relay and hydrogen evolution catalyst

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier. Both the efficiency and scalability of water- splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of solar energy [1]. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because such systems can be spread over wide areas by inexpensive processes potentially. The author’s group has studied various oxide and oxynitride semiconductors as photocatalysts for water splitting. In my talk, recent progress in photocatalytic materials and reactors will be presented with focus on perovskite-type materials.SrTiO3 loaded with proper cocatalysts has been known to be active in overall water splitting under ultraviolet light irradiation since 1980s. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically [2]. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [3]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY of overall water splitting has reached more than 90% at 365 nm [2], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultrviolet illumination even when the water depth is merely 1 mm [4]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [5]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A solar hydrogen production system with a greater size (100 m2) was recently built, and its performance and system characteristics are under investigation.SrTiO3:Al harvests only the ultraviolet light of the sunlight. It is essential to develop photocatalysts active under visible light irradiation. BaTaO2N is a visible light that can evolve hydrogen and oxygen from aqueous solutions containing sacrificial electron donors and acceptors, respectively. Recently, the AQYs of BaTaO2N in these sacrificial reactions have been improved significantly. Highly-crystalline BaTaO2N can be synthesized by directly nitriding a mixture of BaCO3 in the presence of RbCl flux [6]. The resultant BaTaO2N shows decent hydrogen evolution activity when a platinum cocatalyst is loaded by impregnation. By optimizing the cocatalyst loading conditions, the electron transfer from the BaTaO2N photocatalyst to the platinum cocatalyst, and thus the hydrogen evolution activity, was enhanced. The AQY of the resultant Pt-loaded BaTaO2N reached 6.8% at 420 nm in the sacrificial hydrogen evolution reaction. This is exceptionally high among nitride-type photocatalysts with visible light absorption of up to 650 nm.In conventional synthesis, BaTaO2N is nitrided from a mixture of BaCO3, Ta2O5, Ba- and Ta-containing complex oxides, often in the presence of flux. However, these starting materials have different crystal structures from the product BaTaO2N, and so the crystal structures have to be change during the nitridation. This is thought to induce defects in the product material. Therefore, a perovskite-type oxide with a Ba/Ta molar ratio of unity was designed, namely a complex oxide nominally denoted as Na1/4Ba3/4Zn1/4Ta3/4O3 [7]. This oxide can be nitrided into BaTaO2N because Na and Zn components are volatile at high temperature. The resultant BaTaO2N exhibited an AQY of 11.9% at 420 nm in the sacrificial oxygen evolution reaction after loading of a cobalt oxide cocatalyst. Hisatomi et al. Catal. 2019, 2, 387.Takata et al., Nature 2020, 581, 411.Ham et al., Mater. Chem. A 2016, 4, 3027.Goto et al., Joule 2018, 2, 509.Lyu et al. Sci. 2019, 10, 3196.Luo et al., Growth Des. 2020, 20, 255.Jadhav et al. J. Mater. Chem. A. 2020, 8, 1127.

Revealing high hydrogen evolution activity in zinc porphyrin sensitized hierarchical porous TiO2 photocatalysts

Donor-acceptor dyes are a well-established class of photosensitizers, used to enhance visible-light harvesting in solar cells and in direct photocatalytic reactions, such as H2 production by photoreforming of sacrificial electron donors (SEDs). Amines-typically triethanolamine (TEOA)-are commonly employed as SEDs in such reactions. Dye-sensitized photoreforming of more sustainable, biomass-derived alcohols, on the other hand, was only recently reported by using methanol as the electron donor. In this work, several rationally designed donor-acceptor dyes were used as sensitizers in H2 photocatalytic production, comparing the efficiency of TEOA and EtOH as SEDs. In particular, the effect of hydrophobic chains in the spacer and/or the donor unit of the dyes was systematically studied. The H2 production rates were higher when TEOA was used as SED, whereas the activity trends depended on the SED used. The best performance was obtained with TEOA by using a sensitizer with just one bulky hydrophobic moiety, propylenedioxythiophene, placed on the spacer unit. In the case of EtOH, the best-performing sensitizers were the ones featuring a thiazolo[5,4-d]thiazole internal unit, needed for enhancing light harvesting, and carrying alkyl chains on both the donor part and the spacer unit. The results are discussed in terms of reaction mechanism, interaction with the SED, and structural/electrochemical properties of the sensitizers.

A Cobalt Complex of Bis(Methylthioether)Pyridine Catalyst for Hydrogen Evolution from Water

Two nickel-based catalysts, TBA2[NiII(mnt)2] 1 and TBA[NiIII(mnt)2] 2 are formed by the reactions of NiCl2·6H2O, Na2(mnt), and tetra-n-butylammoniumbromide (TBABr) without or with iodine (I2) (mnt = 2,2-dicyanoethylene-1,1-dithiolate), respectively. Under an overpotential (OP) of 941.6 mV, complexes 1 and 2 can electrocatalyze hydrogen evolution from acetic acid with a turnover frequency (TOF) of 36.3 and 148.31 mol H2/mol catalyst/h, respectively. Under blue light (λ max = 469 nm), 1 or 2, together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, can produce hydrogen with a turnover number (TON) of 7760 and 17,570 moles of H2 per mole of catalyst during 35 h irradiation, respectively. The average value of apparent quantum yield (AQY) is ∼9.2% (complex 1) and 28% (complex 2), respectively. The results indicate that the nickel(III) complex has a better active activity for hydrogen production than the nickel(II) species.

Visible-light-driven hydrogen evolution from water using a noble-metal-free polyoxometalate catalyst

Copper-based complexes have been largely neglected as potential water reduction catalysts. This article reports the synthesis and characterization of a tetra-copper-containing polyoxotungstate, Na3K7[Cu4(H2O)2(B-α-PW9O34)2]·30H2O (Na3K7-Cu4P2). Cu4P2 is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate ([Ir(ppy)2(dtbbpy)][PF6]) as a light absorber and triethanolamine (TEOA) as sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of ∼1270 per Cu4P2 catalyst is obtained after 5 h of irradiation (light-emitting diode; λ = 455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9% is achieved. Both oxidative and reductive quenching pathways are observed by measuring the luminescence intensity of excited state [Ir(ppy)2(dtbbpy)](+*) in the presence of Cu4P2 or TEOA, respectively. Many stability studies (e.g., UV-vis absorption, FT-IR, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy) show that catalyst Cu4P2 undergoes slow decomposition under turnover conditions; however, both the starting Cu4P2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuOx particles. Considering the high abundance and low cost of copper, the present work provides considerations for the design and synthesis of efficient, molecular, water-compatible Cu-based water reduction catalysts.

A novel CoSeO3 photocatalyst assisting g-C3N4 in enhancing hydrogen evolution through Z scheme mode

A noble-metal-free artificial photosynthesis system with TiO2 as electron relay for efficient photocatalytic hydrogen evolution

The effect of central metal in phthalocyanine for photocatalytic hydrogen evolution via artificial photosynthesis

Mg-air battery has a theoretical energy density of 6800 Wh/kg, specific capacity of 2200 Ah/kg, and a theoretical cell voltage of 3.1 V. The values are comparable or superior to that of a conventional Li-ion battery[1]. A wide-spread use of Mg-air battery is plagued by several factors such as: (a) a low operating voltage (1.2 V) due to high polarizability of the electrodes (mixed potentials of Mg anode and air cathode), (b) low coulombic efficiency due to parasitic reactions such as hydrogen evolution, and chemical dissolution or surface layer formation, (c) sluggish oxygen reduction reaction kinetics at the air cathode. This presentation will focus on the corrosion issues of the magnesium with a particular reference to the increased hydrogen evolution with the increase in the anodic polarization, which is referred to as negative difference effect (NDE). The hydrogen evolution reaction consumes two electrons per mole of hydrogen (2H2O + 2e- → 2OH- + H2). Since the electrons required for the hydrogen evolution are supplied by the discharge reaction (Mg → Mg2+ + 2e-), the overall coulombic efficiency is drastically reduced. Therefore, the anode material for Mg-air battery should show very low catalytic activity for hydrogen evolution reaction in order to minimize the NDE. Cathodic polarization behavior of different Mg-rare earth (RE) alloys such as EV31A, WE43C, and ZE10A is being evaluated in different heat treated conditions, and different surface conditions (such as as-polished, and covered by a surface layer) along with volumetric determination of hydrogen gas evolved during cathodic and anodic polarization. The results indicate that presence of Mg(OH)2 layer helped increase the exchange current density for the hydrogen evolution reaction (HER). Enhanced hydrogen evolution on the hydroxide covered magnesium was reported by other researchers also [[2], [3]]. The Tafel slopes are steeper on the surface oxide covered specimens than the Tafel slopes of freshly polished specimens. This implies that the electrodes will be polarized significantly during the hydrogen evolution, and thus the Coulombic efficiency is reduced further. The hydrogen evolution reaction steps can be given as [[4], [5]]: Mg + H2O → Mg(OHads) (Hads) (1) which is similar to Volmer reaction. Mg(OHads) (Hads) → Mg(Oads) + H2 (2) Mg(OHads) (Hads) + H2O → Mg(OHads)2 (Hads)2 (3) Mg(OHads)2 (Hads)2 → Mg(OHads)2 + H2 (4) This presentation will focus on how the heat treatment conditions (in terms of microstructure, secondary phases, grain size), and the surface conditions influence the hydrogen evolution kinetics. The strategies to minimize the NDE and increase the performance of anode of the Mg-air battery will be discussed based on the experimental results and literature review. [1] T. Zhang, Z-.L. Tao, J. Chen, Mater. Horiz., 2014, 1, 196 [[2]] S.H. Salleh, S. Thomas, J.A. Yuwono, K. Venkatesan, N. Birbilis, Electrochim. Acta 161 (2015) 144 – 152. [[3]] G.L. Song, K.A. Unocic, Corros. Sci. 98 (2015) 758 – 765. [[4]] K.S. Williams, V. Rodriguez-Santiago, J.W. Andzelm, Electrochim. Acta 210 (2016) 261 - 270 [[5]] K.S. Williams, J.P. Labukas, V. Rodriguez-Santiago, J.W. Andzelm, Corrosion 71 (2015) 209 - 223

Evaluation of microstructure and photochromic behavior of polyvinyl alcohol nanocomposite films containing polyoxometalates

The light-driven reduction of dinitrogen (N2) to ammonia (NH3) catalyzed by a cadmium sulfide (CdS) nanocrystal‑nitrogenase MoFe protein biohybrid is dependent on a range of different factors, including an appropriate hole-scavenging sacrificial electron donor (SED). Here, the impact of different SEDs on the overall rate of N2 reduction catalyzed by a CdS quantum dot (QD)-MoFe protein system was determined. The selection of SED was guided by several goals: (i) molecules with standard reduction potentials sufficient to reduce the oxidized CdS QD, (ii) molecules that do not absorb the excitation wavelength of the CdS QD, and (iii) molecules that could be readily reduced by sustainable processes. Earlier studies utilized buffer molecules or ascorbic acid as the SED. The effectiveness of ascorbic acid as SED was compared to dithionite (DT), triethanolamine (TEOA), and hydroquinone (HQ) across a range of concentrations in supporting N2 reduction to NH3 in a CdS QD-MoFe protein photocatalytic system. It was found that TEOA supported N2 reduction rates comparable to those observed for dithionite and ascorbic acid. HQ was found to support significantly higher rates of N2 reduction compared to the other SEDs at a concentration of 50 mM. A comparison of the rates of N2 reduction by the biohybrid complex to the standard reduction potential (Eo) of the SEDs reveals that Eo is not the only factor impacting the efficiency of hole-scavenging. These findings reveal the importance of the SED properties for improving the efficiency of hole-scavenging in the light-driven N2 reduction reaction catalyzed by a CdS QD–MoFe protein hybrid.