Solar-to-hydrogen Efficiency Research Articles

Crystal Facet-Controlled Efficient SnS Photocathodes for High Performance Bias-Free Solar Water Splitting.

To achieve a high solar‐to‐hydrogen (STH) conversion efficiency, delicate strategies toward high photocurrent together with sufficient onset potential should be developed. Herein, an SnS semiconductor is reported as a high‐performance photocathode. Use of proper sulfur precursor having weak dipole moment allows to obtain high‐quality dense SnS nanoplates with enlarged favorable crystallographic facet, while suppressing inevitable anisotropic growth. Furthermore, the introducing Ga2O3 layer between SnS and TiO2 in SnS photocathodes efficiently improves the charge transport kinetics without charge trapping. The SnS photocathode reveals the highest photocurrent density of 28 mA cm−2 at 0 V versus the reversible hydrogen electrode. Overall solar water splitting is demonstrated for the first time by combining the optimized SnS photocathode with a Mo:BiVO4 photoanode, achieving a STH efficiency of 1.7% and long‐term stability of 24 h. High performance and low‐cost SnS photocathode represent a promising new material in the field of photoelectrochemical solar water splitting.

Photocatalytic solar hydrogen production from water on a 100-m2 scale.

The unprecedented impact of human activity on Earth's climate and the ongoing increase in global energy demand have made the development of carbon-neutral energy sources ever more important. Hydrogen is an attractive and versatile energy carrier (and important and widely used chemical) obtainable from water through photocatalysis using sunlight, and through electrolysis driven by solar or wind energy1,2. The most efficient solar hydrogen production schemes, which couple solar cells to electrolysis systems, reach solar-to-hydrogen (STH) energy conversion efficiencies of 30% at a laboratory scale3. Photocatalytic water splitting reaches notably lower conversion efficiencies of only around 1%, but the system design is much simpler and cheaper and more amenable to scale-up1,2-provided the moist, stoichiometric hydrogen and oxygen product mixture can be handled safely in a field environment and the hydrogen recovered. Extending our earlier demonstration of a 1-m2 panel reactor system based on a modified, aluminium-doped strontium titanate particulate photocatalyst4, we here report safe operation of a 100-m2 array of panel reactors over several months with autonomous recovery of hydrogen from the moist gas product mixture using a commercial polyimide membrane5. The system, optimized for safety and durability, and remaining undamaged on intentional ignition of recovered hydrogen, reaches a maximum STH of 0.76%. While the hydrogen production is inefficient and energy negative overall, our findings demonstrate that safe, large-scale photocatalytic water splitting, and gas collection and separation are possible. To make the technology economically viable and practically useful, essential next steps are reactor and process optimization to substantially reduce costs and improve STH efficiency, photocatalyst stability and gas separation efficiency.

Counterbalancing light absorption and ionic transport losses in the electrolyte for integrated solar water splitting with III–V/Si dual-junctions

Recently, significant progress in the development of III–V/Si dual-junction solar cells has been achieved. This not only boosts the efficiency of Si-based photovoltaic solar cells but also offers the possibility of highly efficient green hydrogen production via solar water splitting. Using such dual-junction cells in a highly integrated photoelectrochemical approach and aiming for upscaled devices with solar-to-hydrogen (STH) efficiencies beyond 20%, however, the following frequently neglected contrary effects become relevant: (i) light absorption in the electrolyte layer in front of the top absorber and (ii) the impact of this layer on the Ohmic and transport losses. Here, we initially model the influence of the electrolyte layer thickness on the maximum achievable solar-to-hydrogen efficiency of a device with an Si bottom cell and show how the top absorber bandgap has to be adapted to minimize efficiency losses. Then, the contrary effects of increasing Ohmic and transport losses with the decreasing electrolyte layer thickness are evaluated. This allows us to estimate an optimum electrolyte layer thickness range that counterbalances the effects of parasitic absorption and Ohmic/transport losses. We show that fine-tuning of the top absorber bandgap and the water layer thickness can lead to an STH efficiency increase of up to 1% absolute. Our results allow us to propose important design rules for high-efficiency photoelectrochemical devices based on multi-junction photoabsorbers.

Direct Solar Hydrogen Generation at 20% Efficiency Using Low‐Cost Materials

AbstractWhile direct solar‐driven water splitting has been investigated as an important technology for low‐cost hydrogen production, the systems demonstrated so far either required expensive materials or presented low solar‐to‐hydrogen (STH) conversion efficiencies, both of which increase the levelized cost of hydrogen (LCOH). Here, a low‐cost material system is demonstrated, consisting of perovskite/Si tandem semiconductors and Ni‐based earth‐abundant catalysts for direct solar hydrogen generation. NiMo‐based hydrogen evolution reaction catalyst is reported, which has innovative “flower‐stem” morphology with enhanced reaction sites and presents very low reaction overpotential of 6 mV at 10 mA cm−2. A perovskite solar cell with an unprecedented high open circuit voltage (Voc) of 1.271 V is developed, which is enabled by an optimized perovskite composition and an improved surface passivation. When the NiMo hydrogen evolution catalyst is wire‐connected with an optimally designed NiFe‐based oxygen evolution catalyst and a high‐performance perovskite‐Si tandem cell, the resulting integrated water splitting cell achieves a record 20% STH efficiency. Detailed analysis of the integrated system reveals that STH efficiencies of 25% can be achieved with realistic improvements in the perovskite cell and an LCOH below ≈$3 kg−1 is feasible.

Photovoltaic Partner Selection for High-Efficiency Photovoltaic-Electrolytic Water Splitting Systems: Brief Review and Perspective

Photovoltaic-electrolysis water splitting (PV-EWS) is the most promising approach for high solar-to-hydrogen (STH) efficiency. The present PV-EWS systems achieve the highest STH performance by using a III-V triple-junction configuration, which, however, involves a complex and expensive manufacture process. Therefore, in this work, we demonstrate a III–V double junction device that can be used as an alternative to the III–V triple-junction device for high STH conversion efficiency of the PV-EWS systems. We estimate the STH performance via coupling world-recorded multi-junction photovoltaic (PV) and our experimented cell configurations with an EWS system. The results show that the III–V double junction, owing to the good trade-off between the efficiency loss and compensation, exhibits a higher STH efficiency than the III–V triple-junction. Furthermore, strategies for improving the efficiency of the III–V double junction device for low-cost PV-EWS system are discussed.

(Invited) A Stable Integrated Photoelectrochemical Reactor for Hydrogen Production from Water

Among the major challenges in solar water, splitting to molecular hydrogen and oxygen is making a stable and affordable system for largescale applications. In this work we present results of the design, fabrication, and testing a photoelectrochemical reactor composed of the following. 1) An integrated device to reduce the balance of the system cost. 2) A concentrated sunlight to reduce the photoabsorber cost. 3) An alkaline electrolyte to reduce catalyst cost and eliminate external thermal management needs. The system consists of an III-V-based photovoltaic cell integrated with Ni foil as catalyst for oxygen production that also protects the cell from corrosion. At low light concentration and without the use of optical lenses, the solar-to-hydrogen (STH) efficiency was found to be 18.3%, while at high light concentration (up to 207 suns) with the use of optical lenses, the STH efficiency was 13%. Catalytic tests conducted for over 100 hours at 100–200 suns showed no sign of degradation nor deviation from product stoichiometry (H2/O2=2). Further tests projected a system stability of over nine years. Figure 1

Recent Developments in Visible‐Light‐Absorbing Semitransparent Photoanodes for Tandem Cells Driving Solar Water Splitting

The development of an efficient conversion system to transform solar energy into chemical energy, such as renewable hydrogen, is a promising way to overcome energy problems. Photoelectrochemical (PEC) water splitting is a promising means of obtaining renewable hydrogen directly from water utilizing sunlight. Recent reports have demonstrated that a PEC cell with a tandem configuration (tandem cell) has the potential to realize a high solar‐to‐hydrogen (STH) energy conversion efficiency by solar water splitting. However, there are still many obstacles to the development of practical and cost‐effective tandem cells. In particular, development of efficient photoanodes for the oxygen evolution reaction (OER) is a prerequisite for improving the STH efficiency. Herein, recent progress in developing (semi)transparent photoanodes for the OER, such as Fe2O3, BiVO4, and Ta3N5, is described based on the topics of preparation methods, semiconductor properties, and PEC performance. In addition, the strategies for enhancing the STH efficiency of tandem cells consisting of (semi)transparent photoanodes conjugated with photovoltaic (PV)‐based cathodes are summarized. This Review is expected to provide guidelines for the future development of tandem cells capable of highly efficient and stable water splitting.

Photoelectrochemical water splitting by hybrid organic-inorganic systems: Setting the path from 2% to 20% solar-to-hydrogen conversion efficiency

SummaryPromoting solar fuels as a viable alternative to hydrocarbons calls for technologies that couple efficiency, durability, and low cost. In this work we elucidate how hybrid organic-inorganic systems employing hybrid photocathodes (HPC) and perovskite solar cells (PSC) could eventually match these needs, enabling sustainable and clean hydrogen production. First, we demonstrate a system comprising an HPC, a PSC, and a Ru-based oxygen evolution catalyst reaching a solar-to-hydrogen (STH) efficiency above 2%. Moving from this experimental result, we elaborate a perspective for this technology by adapting the existing models to the specific case of an HPC-PSC tandem. We found two very promising scenarios: one with a 10% STH efficiency, achievable using the currently available semiconducting polymers and the widely used methylammonium lead iodide (MAPI) PSC, and the other one with a 20% STH efficiency, requiring dedicated development for water-splitting applications of recently reported high-performing organic semiconductors and narrow band-gap perovskites.

Recent Developments in the Use of Heterogeneous Semiconductor Photocatalyst Based Materials for a Visible-Light-Induced Water-Splitting System—A Brief Review

Visible-light-driven photoelectrochemical (PEC) and photocatalytic water splitting systems featuring heterogeneous semiconductor photocatalysts (oxynitrides, oxysulfides, organophotocatalysts) signify an environmentally friendly and promising approach for the manufacturing of renewable hydrogen fuel. Semiconducting electrode materials as the main constituents in the PEC water splitting system have substantial effects on the device’s solar-to-hydrogen (STH) conversion efficiency. Given the complication of the photocatalysis and photoelectrolysis methods, it is indispensable to include the different electrocatalytic materials for advancing visible-light-driven water splitting, considered a difficult challenge. Heterogeneous semiconductor-based materials with narrower bandgaps (2.5 to 1.9 eV), equivalent to the theoretical STH efficiencies ranging from 9.3% to 20.9%, are recognized as new types of photoabsorbents to engage as photoelectrodes for PEC water oxidation and have fascinated much consideration. Herein, we spotlight mainly on heterogenous semiconductor-based photoanode materials for PEC water splitting. Different heterogeneous photocatalysts based materials are emphasized in different groups, such as oxynitrides, oxysulfides, and organic solids. Lastly, the design approach and future developments regarding heterogeneous photocatalysts oxide electrodes for PEC applications and photocatalytic applications are also discussed.

Quantifying and Comparing Fundamental Loss Mechanisms to Enable Solar‐to‐Hydrogen Conversion Efficiencies above 20% Using Perovskite–Silicon Tandem Absorbers

Photovoltaic (PV)‐based solar hydrogen generation is a promising pathway for the scalable production of renewable fuels. Understanding the limitations of solar‐to‐hydrogen (STH) conversion efficiencies is critical to identify performance limits and conceptualize practical device designs. Herein, the losses in PV‐based solar hydrogen generation systems are quantified and the potential of loss‐mitigation techniques to improve the STH efficiency is assessed. The analysis shows that the two largest losses in an ideal system are current and voltage mismatches due to suboptimal system configurations and energy lost as heat in the PV component. A temperature‐dependent model is developed to evaluate the relative potential of two techniques to mitigate these losses: decoupling the PV system to remove current and voltage matching requirements and thermal integration to use the heat losses from PV to increase the electrolyte temperature and improve the reaction dynamics for water splitting. It is shown that optimal system configuration strategies provide more than three times the STH efficiency increase of thermal integration at high operating temperatures. Combining both techniques results in predicted STH efficiencies approaching 20% for low‐cost perovskite–silicon tandem‐based systems with earth‐abundant catalysts at realistic working temperatures.

Design and on-Sun Testing of Tandem III-V Photoelectrochemical Water-Splitting Systems

Photoelectrochemical (PEC) water-splitting systems utilize sunlight to directly split water to hydrogen and oxygen, providing a storable chemical fuel.1 While a variety of semiconductor material systems have demonstrated unassisted solar hydrogen production, III-V semiconductors have shown the highest reported solar-to-hydrogen (STH) conversion efficiencies and best device longevities.2–4 However, to date, work in the field has focused mostly on laboratory conditions, with few PEC systems having been tested outdoors using natural sunlight.In this work, we design III-V semiconductor systems with a molybdenum disulfide catalyst layer and demonstrate unassisted PEC hydrogen generation on-sun. First, a window and capping layer in conjunction with the MoS2 catalytic protection layer are shown to improve the photovoltage and durability of a single-junction pn-GaInP2 photocathode over a MoS2/pn-GaInP2 photocathode. Translating this top cell architecture to a tandem absorber device allows for more meaningful outdoor measurements and improved unassisted water-splitting performance. Specifically, we employ a tandem, lattice-matched GaInP2/GaAs (1.8/1.4 eV bandgaps) system that has been developed with high quality fabrication and yields>10% STH efficiency.2,3 An on-sun photoreactor platform was designed for PEC benchmarking and material testing that incorporates a 2-axis solar tracker and gas collection system.5 Insolation and environmental instrumentation at the co-located NREL Solar Radiation Research Laboratory provide a full record of the solar irradiance and environmental conditions, allowing for more accurate benchmarking of on-sun performance. PEC device characteristics were probed via current-voltage and chronoamperometry measurements under both natural and simulated sunlight illumination, while material structure is probed by XPS and SEM.The MoS2-protected tandem absorbers exhibit >10% STH efficiency under both laboratory and on-sun conditions. During a day of PEC testing under natural sunlight, >11 standard mL of hydrogen is generated with no applied bias. We will discuss challenges in catalyst and semiconductor stability and in cell design that limit PEC durability and hydrogen yield.

Electrochromic solar water splitting using a cathodic WO3 electrocatalyst

Abstract Solar-driven water splitting is an emerging technology with high potential to generate fuel cleanly and sustainably. In this work, we show that WO3 can be used as a cathodic electrocatalyst in combination with (Ag,Cu)InGaSe2 solar cell modules to produce hydrogen and provide electrochromic functionality to water splitting devices. This electrochromic effect can be used to monitor the charge state or performance of the catalyst for process control or for controlling the temperature and absorbed heat due to tunable optical modulation of the electrocatalyst. WO3 films coated on Ni foam, using a wide range of different sputtering conditions, were investigated as cathodic electrocatalysts for the water splitting reaction. The solar-to-hydrogen ( STH ) efficiency of solar-driven water electrolysis was extracted using (Ag,Cu)InGaSe2 solar cell modules with a cell band gap varied in between 1.15 and 1.25 eV with WO3 on Ni foam-based electrolyzers and yielded up to 13% STH efficiency. Electrochromic properties during water electrolysis were characterized for the WO3 films on transparent substrate (indium tin oxide). Transmittance varied between 10% and 78% and the coloration efficiency at a wavelength of 528 nm and the overpotential of 400 mV was 40 cm2 C−1. Hydrogen ion consumption in ion intercalation for electrochromic and hydrogen gas production for water electrolysis processes was discussed.

A study on energy and fluid flows for a novel photoelectrochemical reactor developed for hydrogen production

A numerical study of a new photoelectrochemical (PEC) cell for hydrogen production is performed to investigate energy and fluid flows in particularly removing the adhering bubbles on the surface of the electrodes. The proposed ratio of the electrodes’ surface area is taken as 2.6:1 of the photocathode to the anode. The total surface area of the dome photoelectrode and the disc anode is designed to be 1680.2 cm2 and 645.4 cm2. The developed reactor primarily consists of two cylindrical compartments made of acrylic glass with good absorptivity to allow the maximum wavelengths to pass through. The sunlight is supplied from a solar simulator on the top cover of the PEC cell. The governing equations of the proposed model include Navier-Stokes equations, energy equation, and the radiative transfer equation (RTE). The COMSOL package is used to perform the fluid flow simulation for two different models. Two different COMSOL modules are utilized to solve the governing equations, including Laminar flow and heat transfer in solids and fluids. Also, all electrochemical equations are investigated by the Engineering Equation Solver (EES). The velocity contours, streamlines, temperature contours, and pressure lines are presented and discussed. The present results indicate that the electrolysis and solar to hydrogen (STH) efficiencies are found to be 26.70% and 5.17%, with a mass flow of hydrogen at 66.3 × 10−9 kg/s. A comprehensive parametric study is performed to find the best-operating conditions of the studied PEC reactor. Furthermore, the highest STH efficiency is obtained as 7.3% at a solar incident flux of 400 W/m2. The hydrogen mass production rate can be generated of 132.7 × 10−9 kg/s at a solar incident flux of 1200 W/m2.

Theoretical advances in two-dimensional materials for photocatalytic water splitting

<p indent=0mm>Currently, the energy and environment associated problems become more and more serious. Hydrogen is a kind of clean and renewable resource. Hydrogen generation through photocatalytic water splitting is a feasible and efficient route to resolve these problems. In general, the photocatalytic water splitting process is composed of three pivotal steps: Light harvest, carrier separation and transportation, and hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). Improving the efficiencies of these three steps are keys to enhance the solar-to-hydrogen (STH) efficiency, which stimulates the development of high-performance photocatalysts. Typically, for two-dimensional (2D) materials, the photo-generated electrons and holes produce directly from the surfaces of 2D materials with atomic thickness. In addition, compared with their bulk counterparts, 2D materials with an extremely large specific surface area, rather high carrier mobility, and controllable interfaces offer a large number of advantages for the photocatalysts exploration. Thus, search and design of 2D photocatalysts for water splitting has been a hot topic. Density functional theory (DFT) based first-principles calculations is a powerful tool for materials design and has been widely used in exploring 2D photocatalysts for water splitting. Here, recent advances in the design of 2D materials for photocatalytic water splitting are presented from a theoretical perspective. In the initial stage, a great of new 2D photocatalysts for water splitting are expected to be discovered. Mechanical and chemical assisted exfoliation of layered materials is straightforward approaches to get access to single-layer and few-layer 2D materials. By using DFT calculations, many potential 2D photocatalysts are screened out for water splitting. In addition, molecular designing and chemical regulating are alternative approaches for exploring new 2D photocatalysts for water splitting. The light harvest is the initial process of photocatalytic water splitting, and increasing the light adsorption is one of keys to enhance the STH efficiency. Generally, there are two ways for enhancing the light harvest of 2D materials. One strategy is designing and constructing new 2D materials for photocatalytic water splitting. The other one is improving the light harvest by regulating 2D materials that have been reported previously. Both of the two strategies are employed for developing 2D visible-light-driven covalent organic frameworks for photocatalytic water splitting. Improving the separation of the photo-generated electrons and holes is another key to enhance the STH efficiency. The development of (type-II and Z-scheme) heterojunctions provides a feasible way to design high performance photocatalysts for water splitting due to the benefit for inhibiting electron-hole recombination. That is because that the HER and OER occur on different materials. By using first-principles calculations, a variety of 2D materials based heterojunctions with an expectation of designing highly efficient photocatalytic 2D materials for water splitting have been developed. The STH efficiency is determined by the product of the efficiencies of all the steps for photocatalytic water splitting. Thus, the successful enhancement of energy conversion efficiency calls for the simultaneous improvement for the light harvest and electron-hole separation. A new mechanism for photocatalytic water splitting reveals that the vertical intrinsic electric field in 2D material not only accelerates the carrier separation, but also breaks the conventional limitation of <sc>1.23 eV</sc> for band gap of photocatalysts, leading to enlarged light absorption, even to infrared region. Referring to this new mechanism, some 2D materials with vertical electric field have been theoretically proved to be potential photocatalysts for water splitting. In additional, future opportunities and challenges in theoretical design of 2D photocatalysts toward water splitting are also included.

Perovskite Microcrystals with Intercalated Monolayer MoS2 Nanosheets as Advanced Photocatalyst for Solar-Powered Hydrogen Generation

Summary Photocatalytic water splitting holds a key to realizing the hydrogen economy but faces a grand challenge, namely, the severe charge recombination causes low solar-to-hydrogen (STH) efficiency. This work reports a new form of MAPbI3 microcrystal (MAPbI3-MC)/monolayer MoS2 nanosheets (ML-MoS2) composite photocatalyst with type II heterojunction to effectively suppress the charge recombination for efficient photocatalytic hydrogen generation via HI splitting. The ML-MoS2/MAPbI3-MC is made of a large-sized MAPbI3-MC light harvester anchored with electrocatalytically active ML-MoS2 nanosheets to form a type II heterojunction that possesses a strong built-in electric field aligned between the spatially well-defined MAPbI3-MC and ML-MoS2 nanosheets to empower the photocatalyst with superior capability to effectively promote the charge separation. The ML-MoS2/MAPbI3-MC brings the hydrogen generation performance of the perovskite-based photocatalysts to a new horizon with a champion STH efficiency of 1.09% and a record hydrogen generation activity of 13.6 mmol g−1 h−1 under visible light.

A Stable Integrated Photoelectrochemical Reactor for H2 Production from Water Attains a Solar-to-Hydrogen Efficiency of 18 % at 15 Suns and 13 % at 207 Suns.

The major challenge in solar water splitting to H2 and O2 is in making a stable and affordable system for large‐scale applications. We have designed, fabricated, and tested a photoelectrochemical reactor characterized as follows: 1) it comprises an integrated device to reduce the balance of the system cost, 2) it utilizes concentrated sunlight to reduce the photoabsorber cost, and 3) it employs and alkaline electrolyte to reduce catalyst cost and eliminate external thermal management needs. The system consists of an III‐V‐based photovoltaic cell integrated with Ni foil as an O2 evolution catalyst that also protects the cell from corrosion. At low light concentration, without the use of optical lenses, the solar‐to‐hydrogen (STH) efficiency was 18.3 %, while at high light concentration (up to 207 suns) with the use of optical lenses, the STH efficiency was 13 %. Catalytic tests conducted for over 100 hours at 100–200 suns showed no sign of degradation nor deviation from product stoichiometry (H2/O2=2). Further tests projected a system stability of years.

A Stable Integrated Photoelectrochemical Reactor for H2 Production from Water Attains a Solar‐to‐Hydrogen Efficiency of 18 % at 15 Suns and 13 % at 207 Suns

AbstractThe major challenge in solar water splitting to H2 and O2 is in making a stable and affordable system for large‐scale applications. We have designed, fabricated, and tested a photoelectrochemical reactor characterized as follows: 1) it comprises an integrated device to reduce the balance of the system cost, 2) it utilizes concentrated sunlight to reduce the photoabsorber cost, and 3) it employs and alkaline electrolyte to reduce catalyst cost and eliminate external thermal management needs. The system consists of an III‐V‐based photovoltaic cell integrated with Ni foil as an O2 evolution catalyst that also protects the cell from corrosion. At low light concentration, without the use of optical lenses, the solar‐to‐hydrogen (STH) efficiency was 18.3 %, while at high light concentration (up to 207 suns) with the use of optical lenses, the STH efficiency was 13 %. Catalytic tests conducted for over 100 hours at 100–200 suns showed no sign of degradation nor deviation from product stoichiometry (H2/O2=2). Further tests projected a system stability of years.

Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers

AbstractRealizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p+nn+ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

Halogen modified two-dimensional covalent triazine frameworks as visible-light driven photocatalysts for overall water splitting

The covalent triazine framework CTF-1 as a member of the two-dimensional covalent organic frameworks (COFs) is a category of novel metal-free photocatalysts for water splitting. The large band gap severely restricts its energy conversion efficiency. By means of the first-principles calculations, we proposed the decoration of CTF-1 by anchoring halogen atoms onto benzene moieties for improving the solar-to-hydrogen (STH) efficiency. The electronic structures reveal that the halogen substitution successfully decreases the band gap of CTF-1. Meanwhile, the calculated free energy changes along the reaction pathway indicate that all these COFs can spontaneously drive overall water splitting under light irradiation in a specific acid-base environment. The time-dependent ab initio non-adiabatic molecular dynamics simulations suggest that the electron-hole recombination periods of these COFs fall in a few to tens of nanoseconds. Excitingly, CTF-1 modified by linking six iodine atoms onto the benzene ring in the para-position (CTF-1-6I) shows a quite low band gap of 2.81 eV, indicating that it is a visible-light driven COF for overall photocatalytic water splitting. Correspondingly, CTF-1-6I also exhibits an extraordinarily promising STH efficiency of 3.70%, which is an order magnitude higher than that of the pristine CTF-1.

An efficient and stable photoelectrochemical system with 9% solar-to-hydrogen conversion efficiency via InGaP/GaAs double junction

Despite III-V semiconductors demonstrating extraordinary solar-to-hydrogen (STH) conversion efficiencies, high cost and poor stability greatly impede their practical implementation in photoelectrochemical (PEC) water splitting applications. Here, we present a simple and efficient strategy for III-V-based photoelectrodes that functionally and spatially decouples the light harvesting component of the device from the electrolysis part that eliminates parasitic light absorption, reduces the cost, and enhances the stability without any compromise in efficiency. The monolithically integrated PEC cell was fabricated by an epitaxial lift-off and transfer of inversely grown InGaP/GaAs to a robust Ni-substrate and the resultant photoanode exhibits an STH efficiency of ~9% with stability ~150 h. Moreover, with the ability to access both sides of the device, we constructed a fully-integrated, unassisted-wireless “artificial leaf” system with an STH efficiency of ~6%. The excellent efficiency and stability achieved herein are attributed to the light harvesting/catalysis decoupling scheme, which concurrently improves the optical, electrical, and electrocatalytic characteristics.