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p-Type delafossite CuFeO2 has recently been reported as a promising candidate for direct photoelectrochemical solar water reduction in alkaline conditions. However, despite its favorable optical band gap energy and light absorption, this material suffers from poor electron–hole separation that limits its optimum thickness to a few hundred nanometers. This limitation can be addressed by a host–guest strategy, where a mesoporous p-type scaffold is used to support a thin-film of the light absorber. Here we demonstrate this host–guest approach for the first time with CuFeO2 using p-type transparent CuAlO2 as a scaffold. Optimizing the scaffold layer thickness at 2 μm resulted in a 2.4-fold increase in photocurrent in the presence of O2—a sacrificial electron scavenger—reaching 2.4 mA cm−2 at +0.4 V vs. RHE. Moreover, comparing the performance to host–guest electrodes prepared with an insulating SiO2 scaffold as a control suggested that the observed improvement with CuAlO2 was due to a decreased recombination stemming from improved charge separation in addition to improved charge transport through the scaffold compared to the CuFeO2 film alone.

Anodization of a Fe 2O 3 (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core level X ray photoelectron spectra (XPS) and near edge X ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra however are shifted towards the opposite direction that is towards the Fermi energy suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential. © 2012 Wiley VCH Verlag GmbH Co. KGaA Weinheim.

Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.

An overview of a collaborative experimental and theoretical effort toward efficient hydrogen production via photoelectrochemical splitting of water into di-hydrogen and di-oxygen is presented here. We present state-of-the-art experimental studies using hematite and TiO2 functionalized with gold nanoparticles as photoanode materials, and theoretical studies on electro and photo-catalysis of water on a range of metal oxide semiconductor materials, including recently developed implementation of self-interaction corrected energy functionals., We thank the EU for financial support of the Marie Curie Research Training Network ‘‘Hydrogen’’ (Contract No. MRTNCT-2006-032474). In addition, economic support from the Swiss Federal Office of Energy (Project Number 102326, PECHouse), the Foundation for Strategic Environmental Research (Mistra, Dnr 2004-118), the Icelandic Research Foundation, the Danish Center for scientific computing, the Center for Atomic Scale Material Design, the Catalysis for Sustainable Energy (CASE) initiative, the Danish Strategic Research Council’s HyCycle program, and the Danish Council for Technology and Innovation’s FTP program is also acknowledged.

Utilizing renewable sources of energy is very attractive to provide the growing population on earth in the future but demands the development of efficient storage to mitigate their intermittent nature. Chemical storage, with energy stored in the bonds of chemical compounds such as hydrogen or carbon-containing molecules, is promising as these energy vectors can be reserved and transported easily. In this review, we aim to present the advantages and drawbacks of the main water electrolysis technologies available today: alkaline and PEM electrolysis. The choice of electrode materials for utilization in very basic and very acid conditions is discussed, with specific focus on anodes for the oxygen evolution reaction, considered as the most demanding and energy consuming reaction in an electrolyzer. State-of-the-art performance of materials academically developed for two alternative technologies: electrolysis in neutral or seawater, and the direct electrochemical conversion from solar to hydrogen are also introduced.

Electrochemical oxidation of hematite (alpha-Fe2O3) nanoparticulate films at 600 mV vs Ag+/AgCl in KOH electrolyte forms a species at the hematite surface which causes a new transition in the upper Hubbard band between the Fe(3d)-O(2p) state region and the Fe(4sp)-O(2p) region, as evidenced by oxygen near-edge X-ray absorption fine structure (NEXAFS) spectra. The electrochemical origin of this transition suggests that it is related to a surface state. This transition, not previously observed for pristine alpha-Fe2O3, is at about the same X-ray energy as that of 196 Si-doped Si: Fe2O3. The occurrence of this state coincides with the onset of an oxidative dark current wave at around 535-mV a potential range where the tunneling exchange current has been previously reported to increase by 3 orders of magnitude with the valence band and the transfer coefficient by a factor of 10. Oxidation to only 200 mV does not form such an extra NE.XAFS feature, suggesting that a critical electrochemical potential between 200 and 600 mV is necessary to change the electronic structure of the iron oxide at the surface. A decrease of the surface roughness, as suggested by visual inspection, profilometry, and X-ray reflectivity, points to faceting as the potential structural origin of the surface state.

We study hematite (α-Fe2O3) photoelectrodes for water splitting by examining the fate of photogenerated holes. Using H2O2 as an efficient hole scavenger, we collect all holes that arrive at the electrode/electrolyte interface. This provides the ability to distinguish between and quantify bulk and surface recombination processes involved in the photoelectrochemical oxidation of water. Below 1.0 VRHE, electrolyte oxidation kinetics limits the performance but above 1.2 VRHE bulk recombination becomes the limiting factor.

Solution processable organic semiconductors are well-established as high-performance materials for inexpensive and scalable solar energy conversion in organic photovoltaic (OPV) devices, but their promise in the economic conversion of solar energy into chemical energy (solar fuels) has only recently been recognized. Herein, the main approaches employing organic semiconductor-based devices toward solar H-2 generation via water splitting are compared and performance demonstrations are reviewed. OPV-biased water electrolysis is seen to advance significantly with the development of the tandem OPV device and the optimization of operating potential and redox catalysts. This approach now exceeds 6% solar-to-hydrogen conversion efficiency while over 10% is reasonably feasible. By contrast, while the direct water splitting by an organic semiconductor in a photo-electrochemical cell has attractive advantages, increasing the performance remains a challenge. Photocathodes employing a bulk-heterojunction have been optimized to give 7-8 mA cm(-2) water reduction photocurrent under standard conditions, but photoanodes remain

Zinc spinel ferrite, ZnFe2 O4 (ZFO), is an emerging photoanode material for photoelectrochemical (PEC) solar fuel production. However, a lack of fundamental insight into the factors limiting the photocurrent has prevented substantial advance in its performance. Herein, it is found that ZFO nanorod array photoelectrodes with varying crystallinity exhibit vastly different PEC properties. Using a sacrificial hole scavenger (H2 O2 ), spatially defined carrier generation, and electrochemical impedance spectroscopy, it is shown that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) exhibits superior photogenerated charge separation efficiency and improved majority charge carrier transport compared to ZFO with higher crystallinity and a lower inversion degree. Conversely, the latter condition leads to better charge injection efficiency. Optimization of these factors, and the addition of a nickel-iron oxide cocatalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) and 1.7 mA cm-2 at 1.6 V versus RHE. Importantly, the observed correlation between the cation disorder and the PEC performance represents a new insight into the factors important to the PEC performance of the spinel ferrites and suggests a path to further improvement.

The layered semiconducting transition metal dichalcogenides can be exfoliated into atomically-thin 2D sheets offering promising opto-electronic characteristics for application in solar energy conversion. However, the challenges to fabricate high-quality thin films of these 2D sheets using scalable and cost-effective methods limit their practical application. Here we present novel solution-based approaches for large-area semiconducting films of liquid exfoliated WSe2 with controllable flake alignment and defect density, which is leveraged to advance the understanding of the morphological and structural factors on the optoelectronic performance in devices. Specifically, we develop a thin film self-assembly method employing spatial confinement of WSe2 nanoflakes which affords overlapping-free morphology and superior charge transfer character over films with aggregations.[1, 2] The critical roles of the flake edge density, flake lateral size and thickness on the photogenerated charge transport and transfer are established by both experimental (by solution-based flake size sorting) and theoretical (by anisotropic charge transport simulation) routes.[3] In addition, we recently develop approaches to reduce charge recombination at internal crystal defects and surface dangling bonds by applying pre-annealing and surfactant treatments, respectively, which affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe2. Solar photocurrents for H2 evolution up to 4.0 mA cm–2 (at 0V vs RHE, AM 1.5G illumination), and internal quantum efficiency over 60% are reported for 10 nm thick WSe2 photoelectrodes. [1] Yu X.; Prevot, M. S.; Guijarro, N.; Sivula, K. Nat. Commun. 2015, 6, 7596. [2] Yu X.; Prévot, M. S.; Sivula, K. Chem. Mater. 2014, 26, 5892. [3] Yu, X.; Sivula, K. Chem. Mater. 2017, 29, 6863.

A promising route to increase the performance of hematite (alpha-Fe2O3) photoelectrodes for solar hydrogen production through water-splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm(-2) at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiOx is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum-dot and extremely-thin-absorber (ETA)-type solar cells.

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode parts.

A low-cost, environmentally benign method was used to prepare nanostructured thin films of Co 5 (OH) 8 (NO 3 ) 2 ·2H 2 O, a layered double hydroxide p-type semiconductor. When infilled with poly(3-butylthiophene) (P3BT), an n-type semiconducting polymer, the resulting hybrid bulk heterojunction yields a photovoltaic device. The indium-doped tin oxide/Co 5 (OH) 8 (NO 3 ) 2 ·2H 2 O/P3BT/Al cell described here is an unprecedented example of an optoelectronic device fabricated by a low-cost biologically inspired pathway independent of organic structure-directing agents. Under illumination, this proof-of-principle device yields an open circuit voltage of 1.38 V, a short circuit current of 9 μA/cm 2 , a fill factor of 26% and a power efficiency of 3.2·10 − 3 %. While the open circuit voltage of this prototype cell is close to its theoretical maximum, potential sources of the observed low efficiency are identified, and a suggested path for improvement is discussed.

Solar hydrogen production via watersplitting with hematite (Fe2O3) has been limited by poor light absorption and a small hole diffusion length. These drawbacks can be overcome by using a high-surface-area host to support a thin layer of hematite—allowing photogenerated holes to be produced in high proximity to the semiconductor-liquid junction. Here we demonstrate the effectiveness of this concept using a nanostructured host scaffold (WO3) prepared by atmospheric pressure CVD to support a thin layer of Fe2O3 nanoparticles deposited by a similar method. A 20% increase in the photocurrent was observed in host−guest electrodes as compared to control films with the same amount of hematite (equivalent to a 60 nm film) deposited without the host scaffold. The improvement is attributed to an increase in the absorbed photon conversion efficiency (APCE), especially for longer wavelengths where the photon penetration depth is large in hematite. For light with a wavelength of 565 nm, the APCE improves to 8.0%, as co...

Templating nanocrystals into mesoporous electrodes using an organic porogen typically requires thermal oxidation at >400 degrees C to completely remove the organic material and afford good electronic properties. These oxidation conditions are incompatible with many NC materials. Here, we demonstrate that nitrocellulose can afford mesoporous CdS and CZTS nanocrystal thin film electrodes at only 250-300 degrees C in air or under argon. Remarkable control over the surface area and the average pore size (20-100 nm) in thin films are demonstrated by varying the ratio of preformed nanocrystals and nitrocellulose. Moreover the mesoporous electrodes exhibit excellent optoelectronic properties. Photoelectrochemical performance is enhanced due to the increased active surface area, showing an 8-fold photocurrent increase over compact nanocrystal films, and an IPCE over 70%.

Given the urgent need to develop a sustainable, carbon neutral energy storage system on a global scale, intense efforts are currently underway to advance the field of artificial photosynthesis: i.e. solar fuel engineering. In this review we give an overview of the field of artificial photosynthesis using a semiconductor–electrolyte interface employed in a photoelectrochemical device or as a heterogeneous photocatalyst. First we present a basic description of the operation principles of a semiconductor–liquid junction based device. The role of nanotechnology in the recent advances in the field is highlighted and common material systems under current study are briefly reviewed. The importance of the material surfaces are further scrutinized by presenting recent advances in interfacial engineering. Technical challenges and an outlook towards industrialization of the technology are given.

Photoelectrochemical (PEC) cells have emerged as promising devices that afford the direct conversion of solar energy into electric power and/or chemical fuels. Apart from the obvious importance of the bulk properties of semiconductor materials employed as photoelectrodes, the semiconductor–liquid interface has proven to strongly govern surface-related processes, i.e. the stability, charge separation/recombination and catalytic activity. Because of this, numerous surface treatments have been reported in an effort to tailor the physicochemical properties of the semiconductor–liquid interface, and in turn, the overall PEC response. In this Perspective article we provide a brief conceptual overview of these surface engineering treatments, connecting the particular effects on the interfacial energetics with the respective consequences on the performance. The beneficial effects that arise from surface treatment are categorized as (i) the protection of the surface against photocorrosion, (ii) the passivation of deleterious surface states, (iii) the modification of the band edge positions or band bending, and (iv) the selective extraction of carriers and improved catalytic activity. State-of-the-art surface treatments such as the adsorption of organic molecules or ions, the deposition of semiconductor overlayers and metal nanoparticles or etching procedures are exemplified and described with respect to the observed beneficial effects. A common emerging theme from recent work is that one single surface treatment can lead to multiple distinct effects. Overall, we suggest that surface engineering holds the key for effectively managing the intrinsic common defects of native semiconductor photoelectrodes regardless of their nature, leading to improved light harvesting efficiency.

The development of robust and inexpensive semiconducting materials that operate at high efficiency are needed to make the direct solar-to-fuel energy conversion by photoelectrochemical cells economically viable. In this presentation our laboratory’s progress in the development new light absorbing materials as photocathodes will be discussed along with the application toward overall solar water splitting tandem cells for H2 production. Specifically, this talk will highlight recent results with the ternary oxide CuFeO2, 2D transition metal dichalcogenides, and organic (π-conjugated) semiconductors as solution-processed photoelectrodes. With respect to CuFeO2, in our recent work [1] we demonstrate state-of-the-art sacrificial p-type photocurrent with optimized nanostructuring. Recent results addressing interfacial recombination by the electrochemical characterization of the surface states and attached co-catalysts will be presented along with approaches to overcome the limitations of this material. In addition, two-dimensional (2-D) transition metal dichalcogenides (TMDs) generally have intriguing electronic properties making them promising candidates for high-efficiency solar energy conversion. However, it is notoriously difficult to fabricate thin films of 2-D TMDs over the large areas required to convert solar energy on a practical scale. We recently developed a simple method to fabricate high-quality thin films of 2-D layered TMDs at low cost and with good efficiency towards solar-to-fuel energy conversion [2]. The challenges with charge transport, separation [3] and water redox catalysis in these systems will also be discussed with respect to the 2D flake size. Finally, with respect to π-conjugated organic semiconductors, in our recent work [4] we demonstrate a π-conjugated organic semiconductor for the sustained direct solar water oxidation reaction. Aspects of catalysis and charge-carrier separation/transport are discussed. [1] Prevot, M. S.; Li, Y.; Guijarro, N.; Sivula, K. J. Mater. Chem. A 2016, 4, 3018-3026. [2] Yu, X.; Prevot, M. S.; Guijarro, N.; Sivula, K., Nat. Commun. 2015, 6, 7596. [3] Yu, X.; Rahmanudin, A.; Jeanbourquin, X. A.; Tsokkou, D.; Guijarro, N.; Banerji, N.; Sivula, K. ACS Energy Lett. 2017, 2, 524. [4] Bornoz, P.; Prévot, M. S.; Yu, X.; Guijarro, N.; Sivula, K. J. Am. Chem. Soc. 2015, 137, 15338.

The direct energy conversion of sunlight into hydrogen by using photoelectrochemical (PEC) tandem cells has emerged as a leading technology to tackle the growing demand of hydrogen in an environmentally-friendly and potentially cost-effective way. Basically, the device comprises a photocathode and a photoanode wired together, which under illumination delivers an overall photovoltage above 1.23 V, enough to drive the reduction and oxidation of water on the respective surfaces of the electrodes with reasonable currents. Despite the encouraging progress in the development of a wide-variety of low-cost and high-performing photocathodes by solution-based approaches, the rather short family of available photoanodes for water oxidation (Fe2O3, WO3, BiVO4, TaON) and their low-performance is one major bottlenecks in the overall solar-to-hydrogen conversion of these devices. Exploring new materials for water photo-oxidation may help the development of promising photoanodes with favorable optical and electronic properties enlarging the family of photoanodes available to combine with the vast library of photocathodes. The quest for new materials for water photo-oxidation has recently found in the vast family of spinel ferrites (MFe2O4, M = Ba, Ca, Cu, Co, Mg, Mn, Ni, Zn) a promising candidate, leveraging a remarkable chemical stability together with a broad optoelectronic tunability (band gap between 1.4-2.7 eV by selecting the M cation) [1]. However, very little is known about the potential of these materials as photoelectrodes for solar energy conversion or on the main limiting factors on the performance. Herein, thin-film electrodes of three representative spinels, namely CuFe2O4, MgFe2O4 and ZnFe2O4, were fabricated by a solution-based approach and their PEC properties were extensively characterized. Annealing post-treatments together with the deposition of a well-known electrocatalyst (NiFeOx) demonstrated to effectively improve the native n-type response, although a strong bulk recombination (especially in MgFe2O4) limits the saturation photocurrents (below 0.4 mA cm-2 at 1.23 V vs RHE). Apart from the drawbacks related to bulk properties, a strong Fermi level pinning detected at around 0.9 V vs RHE in all the cases appears to dictate the low photovoltages delivered by the electrodes (~ 300 mV for ZnFe2O4), which evidences the presence of a high-density of surface states. Although recent studies support the role of NiFeOx as an effective passivating agent capable of mitigating the Fermi level pinning in Fe2O3 [2], the combination of photocurrent and open circuit potential experiments proved unambiguously that here NiFeOx is incapable to alleviate the pinning, but participates as an electrocatalyst to improve the overall performance. Detailed analysis of the surface energetics by fast-scan cyclic voltammetry, revealed hole accumulation at the potential where Fermi level pinning occurs and next to the turn-on voltage. This provides direct evidence for the location of surface states, inherent to the oxide electrodes, and/or for intermediates involved in the water oxidation reaction. In more general vein, these results draw a complete picture of the PEC behavior of the spinel ferrites evidencing the current intrinsic limitations, and establishes a roadmap for the design of the next generation of photoanodes based on spinel ferrites. [1] Dillert, R.; Taffa, D. H.; Wark, M.; Bredow, T.; Bahnemann, D. W. APL Mater. 2015, 3 (10), 104001. [2] Jang, J.-W.; Du, C.; Ye, Y.; Lin, Y.; Yao, X.; Thorne, J.; Liu, E.; McMahon, G.; Zhu, J.; Javey, A.; Guo, J.; Wang, D. Nat. Commun. 2015, 6, 7447.

Cu2ZnSnS4 (CZTS) colloidal inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. CdS, CdSe, and ZnSe buffer layers along with methylviologen (MV) adsorption, on the photoelectrochemical (PEC) performance using aqueous Eu(3+) redox electrolyte. PEC experiments revealed that ZnSe and CdSe overlayers outperform traditional CdS, and the additional surface modification with MV was found to further boost the charge extraction. By analyzing the photocurrent onset behavior and measuring the open circuit photopotentials, insights are gained into the nature of the observed improvements. While a more favorable conduction band offset rationalizes the improvement offered by CdSe, charge transfer through midgap states is invoked for ZnSe. Improvement offered by MV treatment is clearly caused by both the shifting of the flat-band potential and a charge-transfer mediation effect. Overall, this work suggests promising alternative surface treatments for CZTS photocathodes for PEC energy conversion.

The realization of photoelectrochemical tandem cells for efficient solar-to-hydrogen energy conversion is currently impeded by the lack of inexpensive, stable, and efficient photocathodes. The family of sulfide chalcopyrites (CuIn x Ga1−x S2) has recently demonstrated a remarkable stability and performance even when prepared by solution-based routes that potentially lower the cost of fabrication. However, the photovoltage delivered by the photocathodes is still well-below the attainable values, a classical limitation linked to a large density of surface states in these materials. In the present work, we show that the identity of halide present during the growth of the solution-processed CuIn0.3Ga0.7S2 (CIGS) thin-films governs the overall performance by directing the crystal growth and the passivation of surface states. Replacing chlorine by iodine leads to CIGS photocathodes that deliver photocurrents of 5 mA cm−2 (at 0 V versus RHE) and a turn-on voltage of 0.5 V versus RHE without charge extracting overlayer nor any sign of deterioration during stability test.

A novel class of soluble fullerenoids based on a dihydronaphthyl functionalization have been synthesized and characterized. Initial testing of these materials in photovoltaic devices identifies a new soluble fullerene derivative, which when blended with poly(3-hexylthiophene) is capable of 4.5% power conversion efficiencies under standard illumination conditions.

Reference EPFL-ARTICLE-208329doi:10.1021/acs.jpclett.5b00406View record in Web of Science Record created on 2015-05-29, modified on 2017-05-12

A fullerene-containing norbornene derivative is used for ring-opening metathesis polymerization to synthesize living polymers with high fullerene content. Incorporation of fullerene at every repeat of a small monomer allows high fullerene content and short inter-fullerene distances in a soluble polymer. The living nature of the polymerization also allows the synthesis of diblock copolymers, which exhibit micellar aggregation in solution and phase separation with interpenetrating C60-domains in a thin film upon TEM imaging.

In this chapter the motivation and theoretical framework for using tandem cells to achieve overall solar-to-hydrogen energy conversion are first examined. The many different systems that have been investigated using various combinations of photovoltaic cells and photoelectrodes are critically reviewed. In order to be economically competitive with simple “brute force” strategies or the production of H2 from fossil fuels, a practical water splitting tandem cell must optimize cost, longevity and performance. A promising approach that meets these requirements is the combination of a stable photoanode material (oxide semiconductor) in tandem with an inexpensive PV cell like the dye sensitized solar cell (DSSC). State-of-the-art solar-to-hydrogen efficiencies in the 2–3% range can be already obtained with this approach in real devices, but much more research is needed to realize a simple tandem device with an efficiency > 10%. Future research needs are finally outlined.

This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoO x ) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga 2 O 3 overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe 2 O 3 for a given applied bias.

Nb2O5 and TiO2 underlayers enhance the photoconversion efficiency of ultrathin hematite (α-Fe2O3) photoelectrodes for water splitting.

Visible light active photoelectrodes for hydrogen generation by solar photoelectrochemical water splitting have been under scrutiny for many decades. In particular the role of electron holes and charge transfer remains controversial. We have investigated the oxygen evolution of hematite in alkaline aqueous electrolyte under a bias potential during visible light illumination in a photoelectrochemical cell operando with soft X ray (O 1s) spectroscopy. Only under these conditions two new spectral signatures evolve in the valence band which we identify as an O 2p hole transition into the charge transfer band and an Fe 3d type hole into the upper Hubbard band. Quantitative analysis of their spectral weight and comparison with the photocurrent reveals that both types of holes contrary to earlier speculations and common perception contribute to the photocurrent. © 2012 American Chemical Society.

A 2-nm thick Nb(2)O(5) underlayer deposited by atomic layer deposition increases the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection. Absorbed photon-to-current efficiencies (APCE) as high as 40%, which are one of the highest ever reported with hematite photoanodes, are obtained at 400 nm at +1.43 V vs. RHE.

LaTiO(2)N photoanodes for solar water splitting were prepared by electrophoretic deposition and demonstrated the best photocurrents ever reported for this material. Further important enhancement of the performance was obtained by the use of a sputtered In(2)O(3) overlayer.

Due to its abundance, stability, and ability to absorb solar irradiation, Hematite, α-Fe2O3, has been investigated for its application in solar hydrogen production via water splitting for more than three decades. However, the recent application of nanostructuring techniques has provided significant advances in the performance of hematite photoanodes. Here, the basic material properties, the attractive aspects, and the challenges presented by hematite for photoelectrochemical (PEC) water splitting are reviewed. Various methods of enhancing performance by nanometer morphology control are detailed and the resulting PEC performances are compared. These techniques, including solution-based routes for porous thin films and nanowire arrays, potentiostatic anodization for nanotube arrays, electrodeposition, ultrasonic spray pyrolysis, and atmospheric pressure chemical vapor deposition, have increased the understanding of the material parameters critical to the performance of hematite, and resulted in an increase of quantum efficiency to over 20% with 450 nm light (compared to 6% with optimized single crystals under similar conditions) and an overall solar-to-hydrogen (STH) conversion efficiency of 3.3% when used in a tandem device. In addition, the remaining limitations of morphology, carrier recombination, slow oxidation kinetics, and flatband potential are presented with the recent advances and approaches in overcoming them and realizing the full potential of hematite for solar hydrogen production.

Competition between charge recombination and the forward reactions required for water splitting limits the efficiency of metal-oxide photocatalysts. A key requirement for the photochemical oxidation of water on both nanostructured α-Fe(2)O(3) and TiO(2) is the generation of photoholes with lifetimes on the order of milliseconds to seconds. Here we use transient absorption spectroscopy to directly probe the long-lived holes on both nc-TiO(2) and α-Fe(2)O(3) in complete PEC cells, and we investigate the factors controlling this slow hole decay, which can be described as the rate-limiting step in water oxidation. In both cases this rate-limiting step is tentatively assigned to the hole transfer from the metal oxide to a surface-bound water species. We demonstrate that one reason for the slow hole transfer on α-Fe(2)O(3) is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined. This is in contrast to nanocrystalline nc-TiO(2), where no distinct thermal barrier to hole transfer is observed.

Keywords: atomic layer deposition ; fuel cells ; photocatalysis ; titanium ; water splitting Reference EPFL-ARTICLE-202186doi:10.1002/cctc.201402532View record in Web of Science Record created on 2014-10-21, modified on 2017-05-12

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-Fe(2)O(3)) 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.

Transient absorption spectroscopy on the μs–s time scale is used to monitor the yield and decay dynamics of photogenerated holes in nanocrystalline hematite photoanodes. In the absence of a positive applied bias, these holes are observed to undergo rapid electron–hole recombination. The application of a positive bias results in the generation of long-lived (3 ± 1 s lifetime) photoholes.

We introduce a simple solution-based strategy to decouple morphological and functional effects of annealing nanostructured, porous electrodes by encapsulation with a SiO(2) confinement scaffold before high temperature treatment. We demonstrate the effectiveness of this approach using porous hematite (α-Fe(2)O(3)) 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 electrolyte-the 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.

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%.

Sustainable hydrogen production through photoelectrochemical water splitting using hematite (alpha-Fe(2)O(3)) is a promising approach for the chemical storage of solar energy, but is complicated by the material's nonoptimal optoelectronic properties. Nanostructuring approaches have been shown to increase the performance of hematite, but the ideal nanostructure giving high efficiencies for all absorbed light wavelengths remains elusive. Here, we report for the first time mesoporous hematite photoelectodes prepared by a solution-based colloidal method which yield water-splitting photocurrents of 0.56 mA cm(-2) under standard conditions (AM 1.5G 100 mW cm(-2), 1.23 V vs reversible hydrogen electrode, RHE) and over 1.0 mA cm(-2) before the dark current onset (1.55 V vs RHE). The sintering temperature is found to increase the average particle size, and have a drastic effect on the photoactivity. X-ray photoelectron spectroscopy and magnetic measurements using a SQUID magnetometer link this effect to the diffusion and incorporation of dopant atoms from the transparent conducting substrate. In addition, examining the optical properties of the films reveals a considerable change in the absorption coefficient and onset properties, critical aspects for hematite as a solar energy converter, as a function of the sintering temperature. A detailed investigation into hematite's crystal structure using powder X-ray diffraction with Rietveld refinement to account for these effects correlates an increase in a C(3v)-type crystal lattice distortion to the improved optical properties.

Metal sulfide (PbS and CdS) quantum dots (QDs) were prepared over mesoporous TiO2 films by improved successive ionic layer adsorption and reaction (SILAR) processes. The as-prepared QD-sensitized electrodes were combined with a cobalt complex redox couple [Co(o-phen)(3)](2+/3+) to make a regenerative liquid-type photovoltaic cell. The optimized PbS QD-sensitized solar cells exhibited promising incident photon-to-current conversion efficiency (IPCE) of over 50% and an overall conversion efficiency of 2% at 0.1 sun in a regenerative mode. The overall photovoltaic performance of the PbS QD-sensitized cells was observed to be dependent on the final turn of the SILA R process, giving a better result when the final deposition was Pb2+, not S2-. However, in the case of CdS QD-sensitized cells, S2- termination was better than that of Cd2+. The cobalt complex herein used as a regenerative redox couple was found to be more efficient in generating photocurrents from PbS QD cells than the typical hole scavenger Na2S in a three-electrode configuration. The CdS-sensitized cell with this redox mediator also showed better defined current-voltage curves and an IPCE reaching 40%.

High-efficiency solar-to-fuel energy conversion can be achieved using direct semiconductor-liquid junctions in a photoelectrochemical (PEC) device consisting of an n-type photoanode in tandem with a p-type photocathode. However, the development of high-performance, stable and inexpensive photoelectrodes are needed to make PEC devices economically viable. In this presentation an overview and the state-of-the-art of this technology including economic considerations is given. In addition, our group’s progress in the development of economically-prepared, high performance photoelectrodes will be discussed along with the application toward overall PEC water splitting tandem cells. Specifically, how the use of scalable solution-processing techniques (e.g. colloidal processing of nanoparticles or sol-gels) leads to limitations in charge transport and charge transfer in the resulting thin-film photoelectrode will be examined. Strategies to overcome these limitations such as using charge extraction buffer layers, catalysts, annealing/doping and nanoparticle self-assembly will be additionally presented. Materials of interest are delafossite CuFeO2, CZTS, CIGS, CdS, organic (conjugated) semiconductors and 2D-layered MoS2 and WSe2.

Regioregularity has been shown to be an important aspect for the properties of poly(3-hexylthiophene) both in the pristine material and in bulk heterojunction photovoltaics with the fullerene derivative PCBM. Here we present a straightforward method to adjust the effective regioregularity of poly(3-hexylthiophene) by introducing 3,4-dihexylthiophene into the polymer chain. By investigating bulk heterojunction morphology and photovoltaic performance upon annealing, we observe that polythiophene with 91% regioregularity maintains equivalent electronic properties but forms a more thermally stable interpenetrating network with PCBM than polythiophene with regioregularity >96%.

A poly(3-hexylthiophene) containing an interacting amino chain end enhances the performance of P3HT/CdSe solar cells by increasing the dispersion of CdSe nanocrystals and improving the morphology of the nanocomposite without introducing insulating surfactants.

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We present a new, easily scalable method for the deposition of nanocrystalline hematite photoelectrodes based on the spin-coating of a mixed solution containing tin(II) and iron(III) chlorides followed by thermal treatment. Our facile approach does not require any additional film-forming organic species and allows simple control of the photoelectrochemical performance of the electrode by adjusting the degree of tin doping. When annealed at 650 degrees C a strong increase in the water oxidation photocurrent is observed with increasing tin concentration. The maximum performance (0.45 mA cm(-2) at 1.43 V vs. RHE) was found at the highest possible tin loading (20 : 100, Sn : Fe). The contrasting performance of electrodes annealed at 650 degrees C and 800 degrees C suggests different activation processes for dopant diffusion and activation. The doping of tin into the crystal structure of hematite thin films is directly evidenced by X-ray photoelectron spectroscopy and indirectly by changes in the intrinsic magnetic parameters (Morin temperature, Neel temperature) of the hematite films. The magnetization measurements thus represent a potential technique to quantify doping amounts in hematite.

A photo-assisted electrodeposition approach was used to deposit a cobalt–phosphate water oxidation catalyst (“Co–Pi”) onto recently improved dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrodeposition of Co–Pi, and Co2+ wet impregnation showed that photo-assisted electrodeposition of Co–Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA cm−2 at 1.0 V and 2.8 mA cm−2 at 1.23 V vs. RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrodeposition provides a more uniform distribution of Co–Pi onto α-Fe2O3 than obtained by electrodeposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.