Solar Water Splitting Systems Research Articles

Elucidating the impact of A-site cation change on photocatalytic H2 and O2 evolution activities of perovskite-type LnTaON2 (Ln = La and Pr).

Transition metal (oxy)nitrides with perovskite-type structures have been regarded as one of the promising classes of inorganic semiconductor materials that can be used in solar water splitting systems for the production of hydrogen as a renewable and storable energy carrier. The performance of transition metal (oxy)nitrides in solar water splitting is strongly influenced by the crystal structure-related dynamics of photogenerated charge carriers. Here, we have systematically assessed the influence of A-site cation exchange on the visible-light-induced photocatalytic H2 and O2 evolution activities, photoanodic response, and dynamics of photogenerated charge carriers of perovskite-type LnTaON2 (Ln = La and Pr). The structural refinement results reveal the orthorhombic Imma and Pnma structures for LaTaON2 and PrTaON2, respectively; the latter has a more distorted crystal structure from the ideal cubic perovskite due to the smaller size of Pr3+ cations. Compared with LaTaON2, PrTaON2 exhibits lower photocatalytic H2 and O2 gas evolution activities and photoanodic response owing to an excessive amount of intrinsic defects associated with anionic vacancies and reduced tantalum species stemming from a long high-temperature nitridation process under reductive NH3 atmosphere. Transient absorption signals evidence the faster decay of photogenerated electrons (holes) in Pt (CoOx)-loaded LaTaON2 than that in Pt (CoOx)-loaded PrTaON2, consistent with the photocatalytic and photoelectrochemical performance of the two photocatalysts. This study suggests that in addition to selecting a suitable A-site cation, it is prerequisite to synthesize LnTaON2 (Ln = La and Pr) crystals with a low defect density to improve their photo-conversion efficiency for solar water splitting.

Solar Water Splitting and Nitrogen Fixation with Layered Bismuth Oxyhalides.

Hydrogen and ammonia are the chemical molecules that are vital to Earth's energy, environmental, and biological processes. Hydrogen with renewable, carbon-free, and high combustion-enthalpy hallmarks lays the foundation of next-generation energy source, while ammonia furnishes the building blocks of fertilizers and proteins to sustain the lives of plants and organisms. Such merits fascinate worldwide scientists in developing viable strategies to produce hydrogen and ammonia. Currently, at the forefronts of hydrogen and ammonia syntheses are solar water splitting and nitrogen fixation, because they go beyond the high temperature and pressure requirements of methane stream reforming and Haber-Bosch reaction, respectively, as the commercialized hydrogen and ammonia production routes, and inherit the natural photosynthesis virtues that are green and sustainable and operate at room temperature and atmospheric pressure. The key to propelling such photochemical reactions lies in searching photocatalysts that enable water splitting into hydrogen and nitrogen fixation to make ammonia efficiently. Although the past 40 years have witnessed significant breakthroughs using the most widely studied TiO2, SrTiO3, (Ga1-xZnx)(N1-xOx), CdS, and g-C3N4 for solar chemical synthesis, two crucial yet still unsolved issues challenge their further progress toward robust solar water splitting and nitrogen fixation, including the inefficient steering of electron transportation from the bulk to the surface and the difficulty of activating the N≡N triple bond of N2. This Account details our endeavors that leverage layered bismuth oxyhalides as photocatalysts for efficient solar water splitting and nitrogen fixation, with a focus on addressing the above two problems. We first demonstrate that the layered structures of bismuth oxyhalides can stimulate an internal electric field (IEF) that is capable of efficiently separating electrons and holes after their formation and of precisely channeling their migration from the bulk to the surface along the different directions, thus enabling more electrons to reach the surface for water splitting and nitrogen fixation. Simultaneously, their oxygen termination feature and the strain differences between interlayers and intralayers render the easy generation of surface oxygen vacancies (OVs) that afford Lewis-base and unsaturated-unsaturated sites for nitrogen activation. With these rationales as the guideline, we can obtain striking visible-light hydrogen- and ammonia-evolving rates without using any noble-metal cocatalysts. Then we show how to utilize IEF and OV based strategies to improve the solar water splitting and nitrogen fixation performances of bismuth oxyhalide photocatalysts. Finally, we highlight the challenges remaining in using bismuth oxyhalides for solar hydrogen and ammonia syntheses, and the prospect of further development of this research field. We believe that our mechanistic insights could serve as a blueprint for the design of more efficient solar water splitting and nitrogen fixation systems, and layered bismuth oxyhalides might open up new photocatalyst paradigm for such two solar chemical syntheses.

New Insight into the Angle Insensitivity of Ultrathin Planar Optical Absorbers for Broadband Solar Energy Harvesting.

Two challenging problems still remain for optical absorbers consisting of an ultrathin planar semiconductor film on top of an opaque metallic substrate. One is the angle-insensitive mechanism and the other is the system design needed for broadband solar energy harvesting. Here, first we theoretically demonstrates that the high refractive index, instead of the ultrathin feature as reported in previous studies, is the physical origin of the angle insensitivity for ultrathin planar optical absorbers. They exhibit omnidirectional resonance for TE polarization due to the high complex refractive index difference between the semiconductor and the air, while for TM polarization the angle insensitivity persists up to an incident angle related to the semiconductor refractive index. These findings were validated by fabricating and characterizing an 18 nm Ge/Ag absorber sample (representative of small band gap semiconductors for photovoltaic applications) and a 22 nm hematite/Ag sample (representative of large band gap semiconductors for photoelectrochemical applications). Then, we took advantage of angle insensitivity and designed a spectrum splitting configuration for broadband solar energy harvesting. The cascaded solar cell and unassisted solar water splitting systems have photovoltaic and photoelectrochemical cells that are also spectrum splitters, so an external spectrum splitting element is not needed.

Thermodynamic and efficiency analysis of solar thermochemical water splitting using Ce–Zr mixtures

We demonstrate the evaluation of the solar-to-fuel conversion efficiency of ceria-zirconia mixtures in a solar thermochemical water splitting system. Energy balance calculations were performed to study the effect of different process conditions on the thermodynamic efficiency of the system. Zirconium-doped ceria showed an enhanced efficiency compared to pure ceria when used in thermochemical water splitting system. A significant enhancement in solar-to-fuel efficiency was shown in case of isothermal redox cycles, at temperature approaches 1800K with 90% gas heat recovery efficiency, 5% Zr-doped ceria gives an efficiency of 0.032% compared to an efficiency of 0.005% given by pure ceria. However, negative effects were observed upon their use in two-temperature redox cycles at normal oxidation temperatures (900–1200K). Higher or lower oxidation temperatures resulted in significant enhancement in the conversion efficiency. At oxidation temperature approaching 1600K and reduction temperature approaching 1773K with 90% and 80% gas and solid heat recovery efficiency respectively, an efficiency of 0.762% is obtained upon using 5% Zr-doped ceria compared to an efficiency of 0.34% when using pure ceria. At oxidation temperature of 600K and at the same reduction temperature and heat recovery efficiencies to the previous case, an efficiency of 6.4% is obtained upon using 15% Zr-doped ceria compared to an efficiency of 5.2% when using pure ceria. The optimum conditions for operating a thermochemical water splitting reactor using 5%, 10%, 15% and 20% Zr-doped ceria were investigated and identified.

Solar-to-Hydrogen Efficiency: Shining Light on Photoelectrochemical Device Performance

Direct photoelectrochemical (PEC) hydrogen production aims to provide a clean and cost-effective solar fuel. Solar-to-hydrogen (STH) conversion efficiency is central to evaluating and comparing research results, and it largely establishes the prospect for successfully introducing commercial solar water-splitting systems. Present measurement practices do not follow well-defined standards, and common methods potentially impact research results and their implications. We demonstrate underestimated influence factors and experimental strategies for improved accuracy[1]. Our focus is tandem devices that have the prospect for greater STH efficiency[2], but increased complexity that requires more careful consideration of characterization practices. We perform measurements on an advanced version of the classical GaInP/GaAs design[3] while considering (i) calibration and adjustment of the illumination light-source; (ii) confirmation of the consistency of results by incident photon-to-current efficiency (IPCE), and (iii) definition and confinement of the active area of the device. We initially measured 21.8% STH efficiency (black dashed line) using a tungsten white-light source, a calibrated GaInP photovoltaic reference cell, and epoxy-encased photocathodes. In contrast, integrating experimental IPCE over the AM 1.5G solar irradiance showed that less than 10% STH conversion appeared conceivable. We then performed a set of on-sun measurements that gave 16.1% STH (black line), before eliminating indirect light coupled to the sample by using a collimating tube and 13.8% STH efficiency (blue line) thereafter. However, the value still vastly exceeded the current density expected according to the quantum efficiency measured via IPCE. Finally, suspecting that the illuminated area is poorly defined by epoxy, we use a compression cell for an epoxy-free area definition, resulting in 9.3% STH efficiency (red line) – a number also compatible with our IPCE results. We propose applying the following standards for future PEC performance reporting: (i) traceable disclosure of the illumination-source configuration (lamp, filters, optics, PEC configuration) and/or its measured spectral distribution; (ii) thorough device-area definition (including confinement of the illumination area and avoidance of indirect light paths); (iii) complementary IPCE confirmation of the solar-generation potential; and (iv) proper consideration of faradaic efficiency. [1] H. Döscher, J. L. Young, J. F. Geisz, J. A. Turner, and T. G. Deutsch, “Solar to hydrogen efficiency: Shining light on phoelectrochemical device performance,” Energy Environ. Sci.2015. [2] H. Döscher, J. F. Geisz, T. G. Deutsch, and J. A. Turner, “Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices,” Energy Environ. Sci.2014. [3] O. Khaselev and J. A. Turner, “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science. 1998. Figure 1

Impact of Electrocatalyst Activity and Ion Permeability on Water-Splitting Photoanodes.

Electrocatalyst (EC)-modified semiconductor (SC) photoelectrodes are key elements of solar water-splitting systems. The SC|EC interface affects the composite photoelectrode behavior but is poorly understood. We uncover the role of EC activity and SC|EC interface properties using a range of metal (Ni, Fe, Ni-Fe, Co, Ir) oxide or (oxy)hydroxide ECs deposited on model single-crystal n-TiO2 photoanodes. The impedance and photoelectrochemical response of the system was nearly independent of EC oxygen evolution activity if the catalyst was deposited electrochemically as an ion-permeable (oxy)hydroxide or hydrous oxide. When dense oxides (e.g., ion-impermeable) ECs were used, the response depended strongly on the EC. These data demonstrate that the EC and SC interface structures are more important than the EC activity in determining the composite photoanode response, confirming recent SC|EC interface simulations for ion-permeable ECs. These results thus inform the design of high-performance water-oxidizing photoanodes with direct SC|EC interfaces.

Self-assembly synthesis of precious-metal-free 3D ZnO nano/micro spheres with excellent photocatalytic hydrogen production from solar water splitting

A simple and straightforward solution growth routine is developed to prepare microporous 3D nano/micro ZnO microsphere with a large BET surface area of 288 m2 g−1 at room temperature. The formation mechanism of the hierarchical 3D nano/micro ZnO microsphere and its corresponding hydrogen evolution performance has been deeply discussed. In particular, this novel hierarchical 3D ZnO microspheres performs undiminished hydrogen evolution for at least 24 h under simulated solar light illumination, even under the condition of no precious metal as cocatalyst. Since the complex production process of photocatalysts and high cost of precious metal cocatalyst remains a major constraint that hinders the application of solar water splitting, this 3D nano/micro ZnO microspheres could be expected to be applicable in the precious-metal-free solar water splitting system due to its merits of low cost, simple procedure and high catalytic activity.

Experimental Demonstrations of Spontaneous, Solar-Driven Photoelectrochemical Water Splitting

There is significant recent interest in understanding the science and developing technologies associated with solar-driven photoelectrochemical water splitting with the goal of producing hydrogen as a potential carbon-neutral transportation fuel [1]. One specific implementation of this concept is an integrated or monolithic device that operates spontaneously, without added electrical bias [2]. Experimental demonstrations of such systems date back to the mid-1970s [3], and, since that time, there have been improvements in both the functional understanding and performance of the devices. This presentation will review laboratory demonstrations of spontaneous, solar water splitting. There are a number of design considerations which arise when considering the full solar to fuel conversion system. These include the type and number of semiconductor junctions used to generate the requisite photovoltage, the use of co-catalysts, the electrolyte conditions, a product separation method such as a membrane, and the geometric layout and size of components. To date, all approaches which have a solar to hydrogen (STH) conversion efficiency of >5% employ either 2 or 3 semiconductor junctions arranged in a voltage-additive tandem configuration. The degree of integration varies widely, as well as the definition of this term. Some demonstrations employ isolated photovoltaic elements connected by wires to hydrogen and oxygen evolution catalysts that are in contact the electrolyte. Others employ a more integrated strategy, with the photocathode and/or the photoanode in near or direct contact with the electrolyte. A small subset of devices employ a mechanism, such as a proton or anion conducting membrane, to separate products and yield a pure H2 stream. Reported STH conversion efficiencies range from <1% to 18%. In most cases, these are calculated by measuring current densities and assuming 100% Faradaic efficiency, though H2 production rates are measured directly in some reports. It will be emphasized that STH values are not validated by an independent test facility as is the case with solar photovoltaic efficiency “world records” [4]. Device longevities under operation will also be discussed. Again, while there is not an accepted protocol for reporting device lifetime, it will be shown that few reports demonstrate operation for >24 hours. Standard and accepted international guidelines and protocols for evaluating and reporting device metrics for complete and integrated solar water splitting systems would be of great value in quantifying technological progress in the field. 1. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, and N. S. Lewis, Chem. Rev. 110, 6446 (2010). 2. J. Rongé, T. Bosserez, D. Martel, C. Nervi, L. Boarino, F. Taulelle, G. Decher, S. Bordiga, and J. A. Martens, Chem. Soc. Rev. 43, 7963 (2014). 3. A. J. Nozik Applied Physics Letters 29, 150 (1976). 4. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Prog. Photovoltaics Res. Appl. 22, 701 (2014). Figure 1

All First Row Transition Metal Oxide Photoanode for Water Splitting Based on Cu3V2O8

Identification of viable photoanode candidates for use in a tandem photoelectrochemical water splitting system remains a significant challenge to the realization of efficient solar-driven hydrogen production. Herein, copper vanadate (Cu3V2O8) is introduced as a new, all first row transition metal oxide with a band gap of near 2 eV that makes it suitable as a photoanode candidate in such a solar water splitting system. In this work, many of the key physical and photoelectrochemical properties of Cu3V2O8 are established including band gap, doping type, ability to extrinsically dope, flat-band potential, band positions, electron diffusion length, chemical stability, and O2 evolution faradaic efficiency. This study provides a key initial step in identifying the features that can lead to a complete understanding of this new ternary metal oxide and motivate discovery of related photoanodes comprised of multicomponent oxides.

Photoelectrochemical Hydrogen Generation Using C-dot/ZnO Hierarchical Nanostructure as an Efficient Photoanode

In this study, we have developed a stable and environmental-friendly photoelectrode of carbon nanodots (C-dots) coupled with a 3 dimensional ZnO structure. Our C-dots are synthesized hydrothermally with a high yield of 40%, and they are successfully anchored onto the ZnO backbone via a facile solution procedure. The as-prepared C-dot/ZnO photoelectrodes have exhibited reasonable photocurrent density and remarkable photostability under the 1 sun irradiation condition without any sacrificial reagent. We have studied the chemistry beneath the C-dot/ZnO interface through various spectroscopic and electrochemical techniques. This work could shed light on future application of C-dots in efficient and toxin-free solar water-splitting systems.

Achieving solar overall water splitting with hybrid photosystems of photosystem II and artificial photocatalysts.

Solar overall water splitting is a promising sustainable approach for solar-to-chemical energy conversion, which harnesses solar irradiation to oxidize water to oxygen and reduce the protons to hydrogen. The water oxidation step is vital but difficult to achieve through inorganic photocatalysis. However, nature offers an efficient light-driven water-oxidizing enzyme, photosystem II (PSII). Here we report an overall water splitting natural-artificial hybrid system, in which the plant PSII and inorganic photocatalysts (for example, Ru/SrTiO3:Rh), coupled with an inorganic electron shuttle [Fe(CN)6(3-)/Fe(CN)6(4-)], are integrated and dispersed in aqueous solutions. The activity of this hybrid photosystem reaches to around 2,489 mol H2 (mol PSII)(-1) h(-1) under visible light irradiation, and solar overall water splitting is also achieved under solar irradiation outdoors. The optical imaging shows that the hybrid photosystems are constructed through the self-assembly of PSII adhered onto the inorganic photocatalyst surface. Our work may provide a prototype of natural-artificial hybrids for developing autonomous solar water splitting system.

Photoelectrochemical cell/dye-sensitized solar cell tandem water splitting systems with transparent and vertically aligned quantum dot sensitized TiO2 nanorod arrays

The present work reports fabrication of vertically aligned CdS sensitized TiO2 nanorod arrays grown on transparent conducting oxide substrate with high transparency as a photoanode in photoelectrochemical cell for water splitting. To realize an unassisted water splitting system, the photoanode and dye-sensitized solar cell tandem structures are tried and their electrochemical behaviors are also investigated. The hydrothermally grown TiO2 nanorod arrays followed by CdS nanoparticle decoration can improve the light absorption of long wavelength light resulting in increased photocurrent density. Two different techniques (electrodeposition and spray pyrolysis deposition) of CdS nanoparticle sensitization are carried out and their water splitting behaviors in the tandem cell are compared.

Efficiency Evaluation of a Photovoltaic System Simultaneously Generating Solar Electricity and Hydrogen for Energy Storage

The direct combination of a photovoltaic system with an energy storage component appears desirable since it produces and stores electrical energy simultaneously, enabling it to compensate power generation fluctuations and supply sufficient energy during low- or non-irradiation periods. A novel concept based on hydrogenated amorphous silicon (a-Si:H) triple-junction solar cells, as for example a-Si:H/a-SiGe:H/a-SiGe:H, and a solar water splitting system integrating a polymer electrolyte membrane (PEM) electrolyser is presented. The thin film layer-by-layer concept allows large-area module fabrication applicable to buildings, and exhibits strong cost-reduction potential as compared to similar concepts. The evaluation shows that it is possible to achieve a sufficient voltage of greater than 1.5 V for effective water splitting with the a-Si based solar cell. Nevertheless, in the case of grid-connection, the actual energy production cost for hydrogen storage by the proposed system is currently too high.

Synergetic effect of Cu and graphene as cocatalyst on TiO2 for enhanced photocatalytic hydrogen evolution from solar water splitting

In photocatalytic solar water splitting systems, non-noble and highly active cocatalysts have always been pursued with tremendous interest. Herein an active and cheap photocatalyst, using Cu and graphene synergetically as co-catalysts immobilized on TiO2, was studied. This synergetic photocatalyst displayed enhanced photocatalytic hydrogen generation from water splitting in the presence of methanol as a sacrificial reagent. The hydrogen generation efficiency from the Cu–graphene synergetic cocatalyst was about 5 times higher than that of a pure graphene cocatalyst, and can be compared with that of systems containing the well-known Pt cocatalyst. Therefore this Cu–graphene synergetic cocatalyst provides an inexpensive means of harnessing solar energy to achieve efficient hydrogen evolution from water splitting.

Enhancement of the O 2 or H 2 photoproduction rate in a Ce 3+/Ce 4+–TiO 2 system by the TiO 2 surface and structure modification

The effect of structural and surface changes in the suspended TiO 2 on the dioxygen and dihydrogen evolution rates was studied with the purpose to improve photogenerated charges utilization efficiency for future solar light water splitting systems which use reversible inorganic electron relays. Prepared rutile and anatase catalysts are characterized by N 2 adsorption–desorption measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was discovered that the dioxygen photoproduction rate from the Ce 4+-containing TiO 2 suspension increased linearly with the rutile specific surface area. A further increase in the O 2 generation rate provides a modification of TiO 2 surface with Pt. Photodeposition (PD) of Pt resulted in a higher O 2 evolution rate than Pt deposition by soft chemical reduction (SCR). The highest photocatalytic activity at pH equal to 0.4 was obtained for 1 wt.% Pt/TiO 2 Degussa P25 platinized by PD. In contrast to O 2, the H 2 photocatalytic production from the Ce 3+-containing TiO 2 suspension solution was faster over Pt/TiO 2 prepared by SCR. Uniform coverage of the TiO 2 surface with Pt particles turned out to be more important for the H 2 generation than rather for the O 2 one. A lower quantum efficiency of the H 2 generation associated with the low Ce 3+ surface coverage on Pt/TiO 2 was attempted to be improved by treating TiO 2 surface with inorganic acids. Sulfuric acid increased the rate by 20% while phosphoric and hydrofluoric acids did not. The best photocatalytic dihydrogen production catalyst at pH equal to 1.0 was found to be 1 wt.% Pt/Degussa P25 prepared by SCR and treated with H 2SO 4. The results demonstrate that the bulk structure (phase composition), morphology (surface area), as well as the surface morphology (Pt distribution) and composition (acid additives) are all important for the O 2 and H 2 photogeneration in the separate TiO 2—suspended based.