To achieve a sustainable society with an energy mix primarily based on solar energy, we need methods of storing energy from sunlight as chemical fuels. Photoelectrochemical (PEC) devices offer the ...promise of solar fuel production through artificial photosynthesis. Although the idea of a carbon-neutral energy economy powered by such ‘artificial leaves’ is intriguing, viable PEC energy conversion on a global scale requires the development of devices that are highly efficient, stable and simple in design. In this Review, recently developed semiconductor materials for the direct conversion of light into fuels are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance as photoelectrodes in PEC cells. The processes of light absorption, charge separation and transport, and suitable energetics for energy conversion in PEC devices are emphasized. Both the advantageous and unfavourable aspects of multinary oxides, oxynitrides, chalcogenides, classic semiconductors and carbon-based semiconductors are critically considered on the basis of their experimentally demonstrated performance and predicted properties.Photoelectrochemical (PEC) devices offer the promise of efficient artificial photosynthesis. In this Review, recently developed light-harvesting materials for PEC application are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance in PEC cells.
For nearly half a century, water oxidation has been extensively investigated as the electron source for solar-powered H2 fuel production from water. However, despite a thermodynamic potential of only ...1.23 V required at standard conditions, driving the oxygen evolution reaction (OER) typically requires 1.5–1.8 V resulting in a significant loss. Over the past decade, numerous researchers have begun to re-explore the idea of replacing water oxidation with more kinetically facile oxidation reactions in photoelectrochemical and photocatalytic solar H2 production systems. Alternate photo-oxidation reactions can be employed as a means of chemical valorization, in addition to providing electrons for H2 production from water while reducing the losses associated with the OER. In this Perspective, we discuss other possible oxidation reactions, and in particular, we highlight recent progress in the investigation of organic-based photo-oxidation reactions. We focus on oxidation reactions that have potential applications as a form of chemical valorization and that can take place in aqueous solutions to allow concurrent H2 production via water reduction at a (photo)cathode. A critical assessment and an outlook toward the prospective large-scale implementation of this technology is finally considered.
A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction ...of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.
Hematite (α-Fe2O3) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to ...date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 – 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe2O3 photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5–10 nm) and of the hole diffusion length (0.5–1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodesespecially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis.
In order to be economically competitive with simple “brute force” (i.e., PV + electrolyzer) strategies or the production of promising solar fuels, like H2, from fossil fuels, a practical ...photoelectrochemical device must optimize cost, longevity, and performance. A promising approach that meets these requirements is the combination of stable and inexpensive oxide semiconductor electrodes in a tandem photoelectrochemical device. In this article, we give an overview of the field including an examination of the potential solar-to-fuel conversion efficiency expected in a device with realistic losses. We next discuss recent advances with increasing the performance of promising semiconductor electrode materials and highlight how these advances have led to state-of-the-art solar-to-chemical efficiencies in the 2–3% range in real devices. Challenges for further optimization are further outlined.
We introduce a simple solution-based strategy to decouple morphological and functional effects of annealing nanostructured, porous electrodes by encapsulation with a SiO2 confinement scaffold before ...high temperature treatment. We demonstrate the effectiveness of this approach using porous hematite (α-Fe2O3) photoanodes applied for the storage of solar energy via water splitting and show that the feature size and electrode functionality due to dopant activation can be independently controlled. This allows a significant increase in water oxidation photocurrent from 1.57 mA cm−2 (in the control case) to 2.34 mA cm−2 under standard illumination conditions in 1 M NaOH electrolytethe highest reported for a solution-processed hematite photoanode. This increase is attributed to the improved quantum efficiency, especially with longer wavelength photons, due to a smaller particle size, which is afforded by our encapsulation strategy.
Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into ...molecular oxygen and hydrogen. Hematite (α‐Fe2O3) has emerged as a promising photo‐electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water‐oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.
Hematite (α‐Fe2O3) is a promising material for solar water splitting. However, its performance as a photoanode has been crucially limited by poor optoelectronic properties. Recent advances in nanostructuring and surface chemistry have catalyzed a rapid advance in the performance of this promising solar energy conversion material. Here the latest efforts to increase understanding and improve the performace are comprehensively examined.
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