S. David Tilley

Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts.

The power of a pair of perovskites In the past several years, perovskite solar cells have emerged as a low-cost experimental alternative to more traditional silicon devices. Luo et al. now show that a pair of perovskite cells connected in series can power the electrochemical breakdown of water into hydrogen and oxygen efficiently (see the Perspective by Hamann). Hydrogen generation from water is being actively studied as a supplement in solar power generation to smooth out the fluctuations due to variations in sunlight. Science, this issue p. 1593; see also p. 1566 A pair of perovskite solar cells can power efficient hydrogen generation from water. [Also see Perspective by Hamann] Although sunlight-driven water splitting is a promising route to sustainable hydrogen fuel production, widespread implementation is hampered by the expense of the necessary photovoltaic and photoelectrochemical apparatus. Here, we describe a highly efficient and low-cost water-splitting cell combining a state-of-the-art solution-processed perovskite tandem solar cell and a bifunctional Earth-abundant catalyst. The catalyst electrode, a NiFe layered double hydroxide, exhibits high activity toward both the oxygen and hydrogen evolution reactions in alkaline electrolyte. The combination of the two yields a water-splitting photocurrent density of around 10 milliamperes per square centimeter, corresponding to a solar-to-hydrogen efficiency of 12.3%. Currently, the perovskite instability limits the cell lifetime.

Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis

Revved-up rust! Light-induced water splitting over iron oxide (hematite) has been achieved by using a particle-assisted deposition technique and IrO2-based surface catalysis. Photocurrents in excess of 3 mA cm-2 were obtained at +1.23 V versus the reversible hydrogen electrode under AM 1.5 G 100 mW cm-2 simulated sunlight. These photocurrents are unmatched by any other oxide-based photoanode. FTO=fluorine-doped tin oxide. Copyright

Hydrogen evolution from a copper(I) oxide photocathode coated with an amorphous molybdenum sulphide catalyst

Concerns over climate change resulting from accumulation of anthropogenic carbon dioxide in the atmosphere and the uncertainty in the amount of recoverable fossil fuel reserves are driving forces for the development of renewable, carbon-neutral energy technologies. A promising clean solution is photoelectrochemical water splitting to produce hydrogen using abundant solar energy. Here we present a simple and scalable technique for the deposition of amorphous molybdenum sulphide films as hydrogen evolution catalyst onto protected copper(I) oxide films. The efficient extraction of excited electrons by the conformal catalyst film leads to photocurrents of up to -5.7 mA cm(-2) at 0 V versus the reversible hydrogen electrode (pH 1.0) under simulated AM 1.5 solar illumination. Furthermore, the photocathode exhibits enhanced stability under acidic environments, whereas photocathodes with platinum nanoparticles as catalyst deactivate more rapidly under identical conditions. The work demonstrates the potential of earth-abundant light-harvesting material and catalysts for solar hydrogen production.

Back Electron–Hole Recombination in Hematite Photoanodes for Water Splitting

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

An Optically Transparent Iron Nickel Oxide Catalyst for Solar Water Splitting

Sunlight-driven water splitting to produce hydrogen fuel is an attractive method for renewable energy conversion. Tandem photoelectrochemical water splitting devices utilize two photoabsorbers to harvest the sunlight and drive the water splitting reaction. The absorption of sunlight by electrocatalysts is a severe problem for tandem water splitting devices where light needs to be transmitted through the larger bandgap component to illuminate the smaller bandgap component. Herein, we describe a novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

Photoelectrochemical Hydrogen Production in Alkaline Solutions Using Cu2O Coated with Earth-Abundant Hydrogen Evolution Catalysts

The splitting of water into hydrogen and oxygen molecules using sunlight is an attractive method for solar energy storage. Until now, photoelectrochemical hydrogen evolution is mostly studied in acidic solutions, in which the hydrogen evolution is more facile than in alkaline solutions. Herein, we report photoelectrochemical hydrogen production in alkaline solutions, which are more favorable than acidic solutions for the complementary oxygen evolution half-reaction. We show for the first time that amorphous molybdenum sulfide is a highly active hydrogen evolution catalyst in basic medium. The amorphous molybdenum sulfide catalyst and a Ni-Mo catalyst are then deposited on surface-protected cuprous oxide photocathodes to catalyze sunlight-driven hydrogen production in 1 M KOH. The photocathodes give photocurrents of -6.3 mA cm(-2) at the reversible hydrogen evolution potential, the highest yet reported for a metal oxide photocathode using an earth-abundant hydrogen evolution reaction catalyst.

Optimization and Stabilization of Electrodeposited Cu2ZnSnS4 Photocathodes for Solar Water Reduction

Cu2ZnSnS4 (CZTS) is a promising p-type semiconductor that has not yet been extensively investigated for solar fuel production via water splitting. Here, we optimize and compare two different electrodeposition routes (simultaneous and sequential) for preparing CZTS electrodes. More consistent results are observed with the simultaneous route. In addition, the effect of etching and the presence of a CdS buffer layer on the photocurrent are investigated. Finally, we demonstrate for the first time the stabilization of these electrodes using protecting overlayers deposited by atomic layer deposition (ALD). Our best performing protected electrodes (Mo/CZTS/CdS/AZO/TiO2/Pt) exhibited a photocurrent of over 1 mA cm(-2) under standard one sun illumination conditions and a significant improvement in stability over unprotected electrodes.

Rate law analysis of water oxidation on a hematite surface.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias

Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps−μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.

Bond formations by intermolecular and intramolecular trappings of acylketenes and their applications in natural product synthesis

The reactive intermediates known as acylketenes exhibit a rich chemistry and have been extensively utilized for many types of inter- and intramolecular bond-forming reactions within the field of organic synthesis. Characteristic reactions of acylketenes include cycloadditions, carbon-carbon bond-forming reactions, and nucleophilic capture with alcohols or amines to give beta-keto acid derivatives. In particular, the intramolecular capture of acylketene intermediates with pendant nucleophiles represents a powerful method for forming both medium-sized rings and macrocycles, often in high yield. This tutorial review examines the history, generation, and reactivity of acylketenes with a special focus on their applications in the synthesis of natural products.