Unassisted Solar Water Splitting Research Articles

Ultrathin Hematite‐Hercynite Films for Future Unassisted Solar Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting requires stable, efficient, and cost‐effective photoelectrodes to enable future large‐scale solar hydrogen production. Ultrathin hematite‐hercynite photoanodes that meet all these criteria in an excellent way is presented here. Hematite‐hercynite photoelectrodes are synthesized in a self‐forming manner by thermal oxidation of iron–aluminum alloy films and characterized with regard to water splitting applications. Photoanodes fabricated from 17 wt.% Al at 493 °C for 8 h and 685 °C for 5 min exhibit, for instance, a photocurrent density of 1.24 and 1.53 mA cm−2 at 1.23 V versus RHE, respectively, as well as superior light absorption in the visible range of the solar spectrum. The PEC performance improvement in comparison to pure hematite thin film electrodes is first achieved by adjusting the aluminum concentration with an optimum range of 12–17 wt.% and second by optimizing the annealing conditions. The resulting photocurrent densities are about a factor of three higher than those obtained from electrodes synthesized from pure iron thin films using the same synthesis conditions. Finally, it is shown that ultrathin hematite‐hercynite photoelectrodes enable even unassisted solar water splitting in a NaOH (1 m) electrolyte with a maximum solar‐to‐hydrogen conversion efficiency of 0.78%.

Organometal Halide Perovskite-Based Photoelectrochemical Module Systems for Scalable Unassisted Solar Water Splitting.

Despite achievements in the remarkable photoelectrochemical (PEC) performance of photoelectrodes based on organometal halide perovskites (OHPs), the scaling up of small-scale OHP-based PEC systems to large-scale systems remains a great challenge for their practical application in solar water splitting. Significant resistive losses and intrinsic defects are major obstacles to the scaling up of OHP-based PEC systems, leading to the PEC performance degradation of large-scale OHP photoelectrodes. Herein, a scalable design of the OHP-based PEC systems by modularization of the optimized OHP photoelectrodes exhibiting a high solar-to-hydrogen conversion efficiency of 10.4% is suggested. As a proof-of-concept, the OHP-based PEC module achieves an optimal PEC performance by avoiding major obstacles in the scaling up of the OHP photoelectrodes. The constructed OHP module is composed of a total of 16 OHP photoelectrodes, and a photocurrent of 11.52mA is achieved under natural sunlight without external bias. The successful operation of unassisted solar water splitting using the OHP module without external bias can provide insights into the design of scalable OHP-based PEC systems for future practical application and commercialization.

Optical Properties and Photocatalytic Performance of Si/TiO2 Tandem Semiconductor Microwire Slurries

Semiconductor particle suspension reactors hold promise as a possible low-cost strategy for solar water-splitting, but they face several challenges that have inhibited their solar-to-hydrogen (STH) efficiencies. A tandem microparticle with a buried junction addresses some of these challenges and offers a pathway to high STH efficiency. As a slurry, the tandem microparticles need to be suspended and well dispersed with maximum light absorption for a minimal photoactive particle concentration. Herein, proof-of-concept Ni/np+-Si/FTO/TiO2 tandem microwire structures capable of unassisted solar water-splitting were investigated as a slurry using uplifting N2 carrier gas bubbles. Transmittance, reflectance, and absorptance of the slurry were characterized as a function of wavelength, bubble flowrate, and tandem microwire concentration using an integrating sphere. Notably, a slurry absorptance of 70–85% was achieved with only 1% of the solution volume filled with a photoactive material. Photochemical activity of the slurry was characterized with in situ monitoring of the photodegradation of methylene blue, including the effects of particle concentration, bubble flowrate, spectral mismatch, intermixed light scattering particles, and a back reflector.

All-Perovskite Tandem Photoelectrodes for Unassisted Solar Hydrogen Production

Unassisted solar water splitting by multijunction tandem photoelectrodes is a sustainable approach to generating green hydrogen fuels. Here, we report on the fabrication of monolithically integrated all-perovskite tandem photocathodes for efficient unassisted solar water splitting. All-perovskite tandem photocathodes wired to an iridium oxide anode deliver a high photovoltage of more than 2 V with an operating photocurrent density of 12.5 mA cm–2 at zero applied bias under simulated AM1.5G 1 sun illumination, yielding a solar-to-hydrogen (STH) conversion efficiency of 15%. The tandem photoelectrode demonstrates continuous operation in water for more than 120 h under simulated 1 sun illumination with less than 5% efficiency loss. A technoeconomic analysis shows the projected production cost of all-perovskite tandem photoelectrodes is $30 m–2, promising a levelized cost of hydrogen of less than $1/kg if sufficient device longevity is realized. This work provides a path toward achieving cost-effective unassisted solar hydrogen fuel production.

Unassisted solar water splitting by dual Cu2O–based tandem device with complementary wavelength–dependent quantum efficiency and antipodal conductivity

Unprecedented standalone tandem cell consisted of dual Cu2O, of which the complementary wavelength–dependent quantum efficiency is in favor of the full utilization of the broadband sunlight with the photon energy above their characteristic band gap and the striking difference in the Fermi level as a result of their distinct p–type and n–type conductivities turns into a strong photovoltage, is developed in the present contribution to carry out unassisted photoelectrochemical water splitting using energy exclusively from the sun. Further deposition of the n–type TiOx overlayer on p–type Cu2O engineered into a wire–like nanostructure, forming the core–shell p–n heterojunction to provide additional photovoltage to accelerate the hydrogen evolution reaction, and the circumvention of the severe charge accumulation at the Schottky Cu/n–Cu2O back contact well relax the associated kinetic overpotentials. The synergistic effect renders the solar–to–hydrogen efficiency of the tandem n–TiOx/p–Cu2O//n–Cu2O device amounting to 0.32%, which is among the highest performance reported to date for the Cu2O–based tandem PEC//PEC cells in the literature.

An integrated n-Si/BiVO4 photoelectrode with an interfacial bi-layer for unassisted solar water splitting.

Integrated n-Si/BiVO4 is one of the most promising candidates for unbiased photoelectrochemical water splitting. However, a direct connection between n-Si and BiVO4 will not attain overall water splitting due to the small band offset as well as the interfacial defects at the n-Si/BiVO4 interface that severely impede carrier separation and transport, limiting the photovoltage generation. This paper describes the design and fabrication of an integrated n-Si/BiVO4 device with enhanced photovoltage extracted from the interfacial bi-layer for unassisted water splitting. An Al2O3/indium tin oxide (ITO) interfacial bi-layer was inserted at the n-Si/BiVO4 interface, which promotes the interfacial carrier transport by enlarging the band offset while healing interfacial defects. When coupled to a separate cathode for hydrogen evolution, spontaneous water splitting could be realized with this n-Si/Al2O3/ITO/BiVO4 tandem anode, with an average solar-to-hydrogen (STH) efficiency of 0.62% for over 1000 hours.

Super-Hydrophilic Leaflike Sn4P3 on the Porous Seamless Graphene-Carbon Nanotube Heterostructure as an Efficient Electrocatalyst for Solar-Driven Overall Water Splitting.

Water splitting using renewable energy resources is an economic and green approach that is immensely enviable for the production of high-purity hydrogen fuel to resolve the currently alarming energy and environmental crisis. One of the effective routes to produce green fuel with the help of an integrated solar system is to develop a cost-effective, robust, and bifunctional electrocatalyst by complete water splitting. Herein, we report a superhydrophilic layered leaflike Sn4P3 on a graphene-carbon nanotube matrix which shows outstanding electrochemical performance in terms of low overpotential (hydrogen evolution reaction (HER), 62 mV@10 mA/cm2, and oxygen evolution reaction (OER), 169 mV@20 mA/cm2). The outstanding stability of HER at least for 15 days at a high applied current density of 400 mA/cm2 with a minimum loss of potential (1%) in acid medium infers its potential compatibility toward the industrial sector. Theoretical calculations indicate that the decoration of Sn4P3 on carbon nanotubes modulates the electronic structure by creating a higher density of state near Fermi energy. The catalyst also reveals an admirable overall water splitting performance by generating a low cell voltage of 1.482 V@10 mA/cm2 with a stability of at least 65 h without obvious degradation of potential in 1 M KOH. It exhibited unassisted solar energy-driven water splitting when coupled with a silicon solar cell by extracting a high stable photocurrent density of 8.89 mA/cm2 at least for 90 h with 100% retention that demonstrates a high solar-to-hydrogen conversion efficiency of ∼10.82%. The catalyst unveils a footprint for pure renewable fuel production toward carbon-free future green energy innovation.

Photoelectrochemical performance of a spin coated TiO2 protected BiVO4-Cu2O thin film tandem cell for unassisted solar water splitting.

A tandem cell consisting of a Mo-BiVO4/TiO2/FeOOH photoanode-Cu2O/TiO2/MoS2 photocathode was prepared for unassisted solar water splitting. The protective TiO2 layer was prepared by a cost-effective spin coating technique. The individual Mo-BiVO4/TiO2/FeOOH photoanode and the Cu2O/TiO2/MoS2 photocathode yielded a current density of ∼0.81 mA cm-2 at 1.23 V vs. RHE and ∼-1.88 mA cm-2 at 0 V vs. RHE, respectively under 100 mW cm-2 xenon lamp illumination. From the individual photoelectrochemical analysis, we identify the operating points of the tandem cell as 0.66 V vs. RHE and 0.124 mA cm-2. The positive current density from the operating points proves the possibility of non-zero operation of the tandem cell. Finally, a two-electrode Mo-BiVO4/TiO2/FeOOH-Cu2O/TiO2/MoS2 tandem cell was constructed and analysed for unassisted operation. The obtained unassisted current density of the tandem cell was ∼65.3 μA cm-2 with better stability compared to the bare BiVO4-Cu2O tandem cell. The results prove that the spin coated TiO2 protective layer can be a viable approach to protect the photoelectrodes from photocorrosion with better stability and enhanced photoelectrochemical (PEC) performance.

In-Situ Magnetic Alignment of Tandem Semiconductor Microwire Slurries for Solar Hydrogen Generation

Technoeconomic models have consistently shown that a photoactive slurry reactor for water-splitting could offer the cheapest route to storing solar energy as hydrogen fuel. However, abundant challenges with this approach have thus far limited such particle-based reactors to prohibitively low efficiencies. Many of the weaknesses of a particulate slurry reactor system can be addressed with monolithically integrated semiconductor tandem particles, which can achieve higher photovoltages while more efficiently utilizing the solar spectrum for water-splitting. However, subcell layers in a tandem must be current-matched and thus the orientation of such particles to the illumination may affect their performance. While ideally matched complementary bandgaps are ultimately sought, the unassisted solar water-splitting in the tandem particulate form factor was first demonstrated with a TiO2/Si microwire system. Nickel hydrogen evolution catalyst was selectively deposited on one end of the wire via a photodeposition process. A proof-of-concept was then demonstrated to use an electromagnet along with the paramagnetic Ni HER catalyst to align the microwire particles in a slurry under active agitation. A simple model is used to predict the conditions under which alignment could lead to more efficient tandem particle slurry performance.

A Review of Inorganic Photoelectrode Developments and Reactor Scale‐Up Challenges for Solar Hydrogen Production

AbstractGreen hydrogen, produced using solar energy, is a promising means of reducing greenhouse gas emissions. Photoelectrochemical (PEC) water splitting devices can produce hydrogen using sunlight and integrate the distinct functions of photovoltaics and electrolyzers in a single device. There is flexibility in the degree of integration between these electrical and chemical energy generating components, and so a plethora of archetypal PEC device designs has emerged. Although some materials have effectively been ruled out for use in commercial PEC devices, many principles of material design and synthesis have been learned. Here, the fundamental requirements of PEC materials, the top performances of the most widely studied inorganic photoelectrode materials, and reactor structures reported for unassisted solar water splitting are revisited. The main phenomena limiting the performance of up‐scaled PEC devices are discussed, showing that engineering must be considered in parallel with material development for the future piloting of PEC water splitting systems. To establish the future commercial viability of this technology, more accurate techno‐economic analyses should be carried out using data from larger scale demonstrations, and hence more durable and efficient PEC systems need to be developed that meet the challenges imposed from both material and engineering perspectives.

Monolithically-integrated BiVO4/p+-n GaAs1-xPx tandem photoanodes capable of unassisted solar water splitting

Monolithically-integrated tandem photoanodes were fabricated on substrates consisting of epitaxial n-GaAs1-xPx (x ≅ 0.32) grown on n+-GaAs wafers. A p+-n junction photovoltaic (PV) cell was first formed by zinc diffusion into the n-GaAs0.68P0.32 from a deposited ZnO coating. After diffusion the ZnO serves as a transparent electrical contact to the resulting p+-GaAs0.68P0.32 surface layer. Transparent, conducting SnO2:F provides chemical and mechanical protection for the ZnO and the underlying PV cell, and it electrically connects this cell to a top BiVO4 photocatalyst layer. In some photoanodes, a WO3 thin film was interposed between the SnO2:F and BiVO4. All oxide coatings were produced by ultrasonic spray pyrolysis except WO3, which was spin coated. Unassisted (unbiased) solar water splitting was achieved, with a solar-to-hydrogen efficiency approaching 2%, without addition of any co-catalyst to the BiVO4 surface. This work can provide insights to other researchers regarding scalable, low cost approaches for the planar monolithic integration of oxide photoanode materials with PV cells to create new tandem devices.

A highly transparent thin film hematite with multi-element dopability for an efficient unassisted water splitting system

A tandem system consisting of a combined hematite photoanode/perovskite solar cell is a novel configuration enabling unbiased solar-driven water splitting in a cost-efficient way. However, similar light absorption spectrums between hematite and perovskite, causing significant optical loss of the solar cell located behind the photoelectrochemical cell, have been a critical issue, resulting in a substantial reduction in the overall photoelectrochemical performance in the bias-free tandem system. Herein, we report an efficient thin film hematite/perovskite tandem cell optimized with consideration of both quantum efficiency of the hematite photoanode and optical loss of the solar cell. Ti,Si co-doping with controlled dopability and deposition of transparent NiFeOx co-catalyst on hematite (NiFeOx@Ti:Si–Fe2O3(t)) allowed for highly improved surface reaction kinetics and charge transport behaviors, endowing our thin film with state-of-the-art performance among thin film hematite photoanodes reported to date, with a photocurrent density of 2.62mAcm−2 at 1.23VRHE and an onset potential of 0.71V. The perovskite solar cell in the thin hematite/solar cell exhibited an anodic shift of Voc from 1.08V to 1.16V and a three-fold increased photocurrent density (15.5mAcm−2) compared to the solar cell of the conventional thick hematite/solar cell system. The assembled dual photoanodes (dual NiFeOx@Ti:Si–Fe2O3) reached a maximum photocurrent density of 4.0mAcm−2 at 1.23VRHE. Our tandem photoelectrochemical system comprising assembled dual thin film NiFeOx@Ti:Si–Fe2O3/solar cell showed outstanding performance of an unbiased solar-to-hydrogen (STH) conversion efficiency of 4.49%, which is the highest recorded value in the hematite/perovskite tandem cell systems. This work provides a great potential for further development of an unassisted solar water splitting system with the thin film hematite.

Noble-Metal-Free Perovskite–BiVO4 Tandem Device with Simple Preparation Method for Unassisted Solar Water Splitting

Unassisted solar water splitting can be realized by connecting a photovoltaic (PV) device in series with a photoelectrochemical cell (PEC). However, the complex fabrication methods and inevitable u...

Benchmark performance of low-cost Sb2Se3 photocathodes for unassisted solar overall water splitting

Determining cost-effective semiconductors exhibiting desirable properties for commercial photoelectrochemical water splitting remains a challenge. Herein, we report a Sb2Se3 semiconductor that satisfies most requirements for an ideal high-performance photoelectrode, including a small band gap and favourable cost, optoelectronic properties, processability, and photocorrosion stability. Strong anisotropy, a major issue for Sb2Se3, is resolved by suppressing growth kinetics via close space sublimation to obtain high-quality compact thin films with favourable crystallographic orientation. The Sb2Se3 photocathode exhibits a high photocurrent density of almost 30 mA cm−2 at 0 V against the reversible hydrogen electrode, the highest value so far. We demonstrate unassisted solar overall water splitting by combining the optimised Sb2Se3 photocathode with a BiVO4 photoanode, achieving a solar-to-hydrogen efficiency of 1.5% with stability over 10 h under simulated 1 sun conditions employing a broad range of solar fluxes. Low-cost Sb2Se3 can thus be an attractive breakthrough material for commercial solar fuel production.

An integrated photoanode based on non-critical raw materials for robust solar water splitting

An integrated photovoltaic–electrocatalytic device with a novel design is presented, achieving 7.7.% STH efficiency for unassisted solar water splitting.

An in situ combustion method for scale-up fabrication of BiVO4 photoanodes with enhanced long-term photostability for unassisted solar water splitting

In situ combustion method can be used for the fabrication of large-area, low-cost photoanodes for photoelectrochemical (PEC) water splitting.

Unassisted solar water splitting using a Cu2O/Ni(OH)2-ZnO/Au tandem photoelectrochemical cell

A tandem photoelectrochemical (PEC) cell consisting of a Cu2O/Ni(OH)2 photocathode and a ZnO/Au photoanode was investigated for unassisted solar water splitting. It was found that the two photoelectrodes were optically matched in UV and visible range, according to the absorbed luminous flux. Compared with the bare Cu2O photocathode, the Cu2O/Ni(OH)2 photocathode had larger photocurrent and smaller onset potential, because the Ni(OH)2 electrocatalysts reduced charge recombination and the overpotential for hydrogen evolution. The tandem PEC cell achieved a photoconversion efficiency of about 0.20% under 1 sun illumination. The results indicate that metal oxide semiconductors synthesized via facile and scalable solution methods are promising photoelectrode materials for unassisted solar water splitting.

(Invited) Nanostructured Photoelectrodes for Unassisted Solar Water Splitting with Enhanced Stability and Surface Catalysts

Nanostructured photoelectrodes with high surface area and tunable optical and electrical properties can potentially benefit a photoelectrochemical (PEC) water splitting system. PEC performance of a nanostructured photoelectrode is usually quantified in a standard three-electrode configuration under potential-assisted conditions because of the additional overpotentials for the two half-reactions of water splitting. However, it is a necessity to fully recognize their potential to split water under unassisted conditions by designing a tandem cell that can provide sufficient voltage to split water. Herein, we present a tandem cell comprised of a photocathode (e.g., carbon-modified cuprous oxide (C/Cu2O) nanoneedles and MoS2-coated Cu2O) and a photoanode (e.g., Co modified BiVO3 and oxygen-deficient titanium dioxide (TiO2−x ) nanorods) for unassisted solar water splitting. The synthesized photoelectrodes were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and electrochemical impedance spectroscopy (EIS) techniques. The tandem cell performance was analyzed by measuring the current-voltage responses in various photoelectrode configurations to validate the collective contributions of both photoelectrodes to unassisted solar water splitting. The PEC properties of these paired photoanodes and photocathodes in a tandem configuration exhibited reasonable photocurrent in the absence of any redox mediator and external bias. Surface active species of photoanode are studied with an ultra-micro electrode of a scanning electrochemical microscope (SECM) to reveal the relationship to substrate and photoexcitation.

A Single-Junction Cathodic Approach for Stable Unassisted Solar Water Splitting

Summary Unassisted solar water splitting is one key step of artificial photosynthesis converting solar energy to chemical fuels. To date, however, there has been no demonstration of efficient and stable semiconductor photoelectrodes without extra surface protection for unassisted solar water splitting. In this work, we show that a single-junction approach of p-type In0.25Ga0.75N nanowires monolithically integrated on n-type Si wafers through an n++/p++-InGaN tunnel junction can drive relatively efficient and stable unassisted water splitting. A photocurrent density of 2.8 mA cm−2 was measured at zero bias versus a platinum counter electrode in a two-electrode configuration, leading to a solar-to-hydrogen efficiency of 3.4%. No performance degradation was observed for ∼300 h of unassisted solar water splitting without using any extra surface protection layers. Such a single-junction photocathode can be further integrated with a narrow band-gap junction, e.g., Si or GaAs, to achieve further improved efficiency for long-term stable solar water splitting.

Three-dimensional Cu2O nanorods modified by hydrogen treated Ti3C2TX MXene with enriched oxygen vacancies as a photocathode and a tandem cell for unassisted solar water splitting

Cuprous oxide (Cu2O) has been envisioned as a promising photocathode material for photoelectrochemical (PEC) water reduction due to its abundance, scalability and nontoxicity. Herein, a three-dimensional (3D) Cu2O nanorods (NRs) photocathode is prepared by in-situ growing Cu2O NRs on a copper foam substrate. The bare 3D Cu2O NRs photocathode exhibits a high photocurrent density of −4.02 mA cm−2 at 0 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G simulated sunlight. The oxygen vacancies enriched Ti3C2TX flakes are obtained by reducing Ti3C2TX with H2/Ar gas (denoted as H:Ti3C2TX). The H:Ti3C2TX/Cu2O, Ti3C2TX/Cu2O as well as Au/Cu2O composite photocathodes are fabricated through a facile dip-coating method. The H:Ti3C2TX/Cu2O and Ti3C2TX/Cu2O yield a photocurrent density of −5.41 and −4.45 mA cm−2 respectively, which is higher than the bare Cu2O and Au/Cu2O (−4.31 mA cm−2). The results suggest that the Ti3C2TX is more effective than noble metal Au for improving the PEC performance of Cu2O photocathode. The superior performance of H:Ti3C2TX/Cu2O is attributed to enriched oxygen vacancies in the H:Ti3C2TX which can enhance the conductivity, light-harvesting efficiency and charge-transfer ability of fabricated photocathode. A PEC tandem cell with H:Ti3C2TX/Cu2O as the photocathode and BiVO4 as the photoanode is designed for unassisted solar water splitting. Due to the high porosity and optical transmittance of the 3D copper foam substrate, we illuminate the cell from the H:Ti3C2TX/Cu2O side rather than from the BiVO4 side in usual fashion. The present configuration exhibits a 1.6 times higher solar-to-hydrogen (STH) conversion efficiency of 0.55% than the conventional design with a STH conversion efficiency of 0.34%. The enhanced STH conversion efficiency in our configuration is due to the higher light-harvesting efficiency.