Solar-driven Water-splitting Systems Research Articles

Self-assembled monolayer mediated fast charge transport of Sb2(S,Se)3 photocathode enabling high-performance unbiased water splitting

The photoelectrochemical (PEC) water splitting system, capable of generating green hydrogen fuel, is crucial for achieving carbon net zero. However, the performance of the photocathode, responsible for the hydrogen evolution reaction, remains suboptimal. We propose bottom interface engineering using a self-assembled monolayer (SAM) to enhance the quality of the Sb2(S,Se)3 absorber layer. The SAM, composed of (3-aminopropyl)triethoxysilane (APTES), is introduced on the Au substrate to enhance adhesion between the substrate and the Sb2(S,Se)3 absorber. Notably, the APTES treatment facilitates the formation of strong electrophile functional groups on the substrate, which predominantly attract tartrate-Sb-Se ionic clusters in the precursor ink. This interaction between the precursor and the substrate induces preferential nucleation of Sb2Se3 near the substrate, followed by Sb2S3 formation at a later stage near the top surface. The APTES-treated Sb2(S,Se)3 photocathodes exhibit a steep band energy gradient, resulting in a photocurrent density of 16 mA cm–2, an onset potential of 0.24 V versus the standard hydrogen electrode, and stability of 24 h in neutral electrolyte (0.5 M K-Pi, pH 6.13). By combining the APTES-treated Sb2(S,Se)3 photocathode with a perovskite photoanode, the co-planar PEC–PEC water splitting device achieves a remarkable solar-to-hydrogen efficiency of 6.5 % under unbiased conditions, along with operating stability for over 120 min. This study suggests that SAM treatment-based interface engineering could be a potential breakthrough in improving the performance of chalcogenide photocathodes in solar-driven water splitting systems.

Photocatalytic Hydrogen Production with A Molecular Cobalt Complex in Alkaline Aqueous Solutions.

The thermodynamic favorability of an alkaline solution for the oxidation of water suggests the need for developing hydrogen evolution reaction (HER) catalysts that can function in basic aqueous solutions so that both of the half reactions in overall water splitting can occur in mutually compatible solutions. Although photocatalytic HERs have been reported mostly in acidic solutions and a few at basic pHs in mixed organic aqueous solutions, visible-light driven HER catalyzed by molecular metal complexes in purely alkaline aqueous solutions remains largely unexplored. Here, we report a new cobalt complex with a tetrapyridylamine ligand that catalyzes photolytic HER with turnover number up to 218 000 in purely aqueous solutions at pH 9.0. Density functional theory (DFT) calculations suggested a modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) pathway for hydrogen production from the protonation of CoII-H species. The remarkable catalytic activity resulting from subtle structural changes of the ligand scaffold highlights the importance of studying structure-function relationships in molecular catalyst design. Our present work significantly advances the development of a molecular metal catalyst for visible-light driven HER in more challenging alkaline aqueous solutions that holds substantial promise in solar-driven water-splitting systems.

Heterogeneous bimetallic oxides/phosphides nanorod with upshifted d band center for efficient overall water splitting

Electrocatalytic water splitting is the most directly available route to generate renewable and sustainable hydrogen. Here, we report the design of a composite material in which arrays of square pillar-like NiMoO4 nanorods coated with N, P-doped carbon layers are uniformly contained in numerous nested nanoparticle structures. The catalysts have superior catalytic activity, requiring only 59 mV and 187 mV for HER and OER to attain a current density of 10 mA/cm2, respectively. The assembled two-electrode electrolytic cell required a voltage of 1.48 V to reach 10 mA/cm2, along with excellent long-term stability. Theoretical calculations reveal that electrons aggregate and redistribute at the heterogeneous interface, with the d-band centers of the Ni and Fe atoms being positively shifted compared to the Fermi level, effectively optimizing the adsorption of intermediates and reducing the Gibbs free energy, thus accelerating the catalytic process. Meanwhile, an integrated solar-driven water-splitting system demonstrated a high and stable solar-to-hydrogen efficiency of 18.20%. This work provides new possibilities for developing non-precious metal-based bifunctional electrocatalysts for large-scale water splitting applications.

Triphasic Ni2P−Fe2P−CoP heterostructure interfaces for efficient overall water splitting powered by solar energy

In this study, we design a triphasic Ni2P − Fe2P − CoP heterostructure appearing under the form of a hierarchical 3D core–shell hybrid (CoP@Ni2P − Fe2P) for bifunctionally catalyzing both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. Density functional theory results indicate that the rich interfaces of Ni2P − Fe2P − CoP architecture exhibit the best electronic properties and enhanced multiple electroactive site number with optimum hydrogen adsorption energy, significantly surpassing the CoP, CoP−Ni2P, or CoP−Fe2P cases. Therefore, the electrolyzer cell of CoP@Ni2P − Fe2P(+,−) requires a small operating voltage of 1.51 V to reach 10 mA cm−2 during alkaline water splitting, outperforming the commercial catalyst couple of Pt/C(−)//RuO2(+) and numerous earlier reports. In addition, the solar-driven water splitting system demonstrates a high solar-to-hydrogen (STH) conversion of 15.33 %, along with excellent long-term stability. The results open a potential approach to design innovative high-performance electrocatalysts for practical green hydrogen production via electrochemical water splitting process.

Ternary cobalt–iron–manganese layered double hydroxides with 1D/2D hierarchical nanostructure for oxygen evolution reaction and urea oxidation reaction

Developing high-efficient and stable electrocatalysts for oxygen evolution reaction (OER) is crucial for water splitting and metal-air batteries. Herein, ternary CoFe1−XMnX (0 <X < 1) layered double hydroxides (LDHs) grown on nickel form (NF) with nanosheet-nanowires (1D/2D) hierarchical nanostructure are synthesized. The introduction of Mn causes the change of crystal phase, morphology, and electronic structure of CoFe-LDH through electron transfer from Co/Fe to Mn, which provides the whole merit structural features and synergistic effect. Further experimental results exhibit that CoFe0.75Mn0.25-LDH possesses superior OER performance, which only requires overpotentials of 243 and 298 mV to reach 20 and 100 mA cm−2. The remodeling of LDHs in the OER process is also discussed to further explore the OER catalytic mechanism. In addition, CoFe0.75Mn0.25-LDH can also be utilized to the urea oxidation reaction (UOR), which requests a low potential of 1.367 V to reach 10 mA cm−2. A solar driven water splitting system is assembled and acquires the current density of 10 mA cm−2 at the low cell voltage of 1.58 V. This work offers an effective approach to boost the OER and UOR performance of CoFe-LDH based bifunctional electrocatalysts.

Defect Healing in FAPb(I1‐xBrx)3 Perovskites: Multifunctional Fluorinated Sulfonate Surfactant Anchoring Enables &gt;21% Modules with Improved Operation Stability (Adv. Energy Mater. 20/2022)

Perovskite Solar Cells In article number 2200632, Gill Sang Han, Hyun Suk Jung and co-workers report a defect healing strategy that enables the fabrication of highly efficient and stable perovskite large-area solar cell modules. In addition, an exciting solar-driven water-splitting system is demonstrated using clean and renewable solar energy, which is expected to help to realize a carbon-neutral economy and bring the hydrogen economy closer to reality.

Graphene Quantum Dots Improved "Caterpillar"-like TiO2 for Highly Efficient Photocatalytic Hydrogen Production.

Photocatalytic water splitting for hydrogen production via heterojunction provides a convenient approach to solve the world crises of energy supply. Herein, graphene quantum dots modified TiO2 hybrids (TiO2-GQDs) with a “caterpillar”-like structure exhibit stronger light absorption in the visible region and an enhanced hydrogen production capacity of about 3.5-fold compared to the pristine TiO2 caterpillar. These results inferred that the addition of GQDs drastically promotes the interfacial electron transfer from GQDs to TiO2 through C–O–Ti bonds via the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional groups in GQDs. Using a “caterpillar”-like structure are expected to provide a new platform for the development of highly efficient solar-driven water splitting systems based on nanocomposite photocatalyst.

Highly efficient solar hydrogen production through the use of bifacial photovoltaics and membrane electrolysis

The large-scale implementation of solar hydrogen production requires an optimal combination of photovoltaic systems with suitably-designed electrochemical cells, possibly avoiding power electronics for DC-DC conversion, to decrease costs. Here, a stable, solar-driven water splitting system is presented, obtained through the direct connection of a state-of-the-art proton exchange membrane (PEM) electrolyzer to a bifacial silicon heterojunction (SHJ) solar module of three cells in series with total area of 730 cm2. The bifaciality of the solar module has been optimized through modeling in terms of the number of cells, module height and inclination. During outdoor operation in the standard monofacial configuration, the system is able to produce 3.7 gr of H2 h−1m−2 with an irradiation of 1000 W m−2 and a solar-to-hydrogen efficiency (STH) of 11.55%. The same system operating in bifacial mode gives rise to a higher H2 flux and STH efficiency, reaching values of 4.2 gr of H2 h−1m−2 and STH of 13.5%. Such a noticeable difference is achieved through the collection of albedo radiation from the ground by the bifacial PV system. The system has been tested outdoors for more than 55 h, exhibiting very good endurance, with no appreciable change in production and efficiency.

An Artificial Z-Scheme Constructed from Dye-Sensitized Metal Oxide Nanosheets for Visible Light-Driven Overall Water Splitting.

Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h-1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Se-Doping Activates FeOOH for Cost-Effective and Efficient Electrochemical Water Oxidation.

Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such a Se-doped FeOOH electrode via an FeSe precatalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm-2 is secured at a low overpotential of 348 mV. The application of such an Fe-rich OER electrode in a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.

A low-cost photoelectrochemical tandem cell for highly-stable and efficient solar water splitting

The stability and cost of solar-driven water splitting system are still the main bottlenecks for its large-scale implementation. Here, an ingeniously designed low-cost photoelectrochemical tandem cell possessing three performance-superior components, i.e., a front novel photoanode of WO3 nanoplates epitaxially coated with [001]-oriented TiO2 nanoprickles, and a rear Si photovoltaic cell, and a counter-cathode of interlayer-expanded MoS2 nanostructures, is reported for highly-stable and efficient solar water splitting. The tandem cell shows a close-to-unity Coulomb efficiency for water splitting, and a high and sustained-stable solar-to-hydrogen efficiency in a 35 days’ test. The result also reveals that it is versatile in solar-driven wastewater treatment. Most importantly, the composite photoanode that, for the first time, is proposed using epitaxial-growth based on double-match, i.e. lattice match and band match, shows dramatical improvement in stability and photoelectrochemical performance, in which the photocurrent of the WO3/ TiO2 photoanode is improved by 80% without any obvious decay after 100h of continuous testing compared with a 92% decay in photocurrent for the WO3 photoanode. The epitaxial TiO2 layer facilitates interfacial charge transportations and provides a complete sealing of WO3 surfaces at an atomic level. Such efficient interfacial design and system-level integration of high-performance nanomaterials shed a unique light on the potential large-scale implementation of solar water splitting for renewable hydrogen fuel production.

ChemInform Abstract: Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen‐Evolution Reaction

AbstractReview: 200 refs.

Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction

Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides ...

Modeling, simulation, and fabrication of a fully integrated, acid-stable, scalable solar-driven water-splitting system.

A fully integrated solar-driven water-splitting system comprised of WO3 /FTO/p(+) n Si as the photoanode, Pt/TiO2 /Ti/n(+) p Si as the photocathode, and Nafion as the membrane separator, was simulated, assembled, operated in 1.0 M HClO4 , and evaluated for performance and safety characteristics under dual side illumination. A multi-physics model that accounted for the performance of the photoabsorbers and electrocatalysts, ion transport in the solution electrolyte, and gaseous product crossover was first used to define the optimal geometric design space for the system. The photoelectrodes and the membrane separators were then interconnected in a louvered design system configuration, for which the light-absorbing area and the solution-transport pathways were simultaneously optimized. The performance of the photocathode and the photoanode were separately evaluated in a traditional three-electrode photoelectrochemical cell configuration. The photocathode and photoanode were then assembled back-to-back in a tandem configuration to provide sufficient photovoltage to sustain solar-driven unassisted water-splitting. The current-voltage characteristics of the photoelectrodes showed that the low photocurrent density of the photoanode limited the overall solar-to-hydrogen (STH) conversion efficiency due to the large band gap of WO3 . A hydrogen-production rate of 0.17 mL hr(-1) and a STH conversion efficiency of 0.24 % was observed in a full cell configuration for >20 h with minimal product crossover in the fully operational, intrinsically safe, solar-driven water-splitting system. The solar-to-hydrogen conversion efficiency, ηSTH , calculated using the multiphysics numerical simulation was in excellent agreement with the experimental behavior of the system. The value of ηSTH was entirely limited by the performance of the photoelectrochemical assemblies employed in this study. The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available.

An electrochemical engineering assessment of the operational conditions and constraints for solar-driven water-splitting systems at near-neutral pH

Identified operating conditions and constraints for efficient operation of solar-driven water-splitting systems at near-neutral pH.

A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III–V light absorbers protected by amorphous TiO2 films

A monolithically integrated device, protected by a TiO2 stabilization layer, splits water using sunlight.

Use of Bipolar Membranes for Maintaining Steady‐State pH Gradients in Membrane‐Supported, Solar‐Driven Water Splitting

A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.

Improved Stability of Polycrystalline Bismuth Vanadate Photoanodes by Use of Dual-Layer Thin TiO2/Ni Coatings

Ultrathin dual layers of TiO_2 and Ni have been used to stabilize polycrystalline BiVO_4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO_2 layers were deposited on BiVO_4 thin films by atomic-layer deposition, with Ni deposited onto the TiO_2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO_4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.

An experimental and modeling/simulation-based evaluation of the efficiency and operational performance characteristics of an integrated, membrane-free, neutral pH solar-driven water-splitting system

An experimental and modeling based evaluation of a solar-hydrogen device that operates in electrolytes buffered at near-neutral pH.