Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

Fullerene as a Photoelectron Transfer Promoter Enabling Stable TiO2‐Protected Sb2Se3 Photocathodes for Photo‐Electrochemical Water Splitting

Understanding the degradation mechanisms of photoelectrodes and improving their stability are essential for fully realizing solar‐to‐hydrogen conversion via photo‐electrochemical (PEC) devices. Although amorphous TiO₂ layers have been widely employed as a protective layer on top of p‐type semiconductors to implement durable photocathodes, gradual photocurrent degradation is still unavoidable. This study elucidates the photocurrent degradation mechanisms of TiO₂‐protected Sb₂Se₃ photocathodes and proposes a novel interface‐modification methodology in which fullerene (C₆₀) is introduced as a photoelectron transfer promoter for significantly enhancing long‐term stability. It is demonstrated that the accumulation of photogenerated electrons at the surface of the TiO₂ layer induces the reductive dissolution of TiO₂, accompanied by photocurrent degradation. In addition, the insertion of the C₆₀ photoelectron transfer promoter at the Pt/TiO₂ interface facilitates the rapid transfer of photogenerated electrons out of the TiO₂ layer, thereby yielding enhanced stability. The Pt/C₆₀/TiO₂/Sb₂Se₃ device exhibits a high photocurrent density of 17 mA cm^?2 and outstanding stability over 10 h of operation, representing the best PEC performance and long‐term stability compared with previously reported Sb₂Se₃‐based photocathodes. This research not only provides in‐depth understanding of the degradation mechanisms of TiO₂‐protected photocathodes, but also suggests a new direction to achieve durable photocathodes for photo‐electrochemical water splitting.     

High Efficiency Photocatalytic Water Splitting Using 2D α‐Fe2O3/g‐C3N4 Z‐Scheme Catalysts

Photocatalysis is the most promising method for achieving artificial photosynthesis, but a bottleneck is encountered in finding materials that could efficiently promote the water splitting reaction. The nontoxicity, low cost, and versatility of photocatalysts make them especially attractive for this application. This study demonstrates that small amounts of α‐Fe₂O₃ nanosheets can actively promote exfoliation of g‐C₃N₄, producing 2D hybrid that exhibits tight interfaces and an all‐solid‐state Z‐scheme junction. These nanostructured hybrids present a high H₂ evolution rate >3 × 10⁴ µmol g^‐1 h^‐1 and external quantum efficiency of 44.35% at λ = 420 nm, the highest value so far reported among the family of g‐C₃N₄ photocatalysts. Besides effectively suppressing the recombination of electron–hole pairs, this Z‐scheme junction also exhibits activity toward overall water splitting without any sacrificial donor. The proposed synthetic route for controlled production of 2D g‐C₃N₄‐based structures provides a scalable alternative toward the development of highly efficient and active photocatalysts.     

Poly(1,4‐di(2‐thienyl))benzene Facilitating Complete Light‐Driven Water Splitting under Visible Light at High pH

The recent discovery that metal‐free polyterthiophene (PTTh) prepared by iodine‐vapor‐assisted polymerization (IVP) can catalyze the hydrogen evolution reaction (HER) when illuminated, and this light‐enhanced electrolysis expresses a non‐Nernstian relation with pH, provides the foundation for further improvement of the photovoltage of the reaction by engineering the band structure of the light‐absorbing polymer. Deviating from an all‐thiophene backbone, using poly(1,4‐di(2‐thienyl))benzene (PDTB) lowers the highest occupied molecular orbital level by ≈0.3 eV compared with polythiophene, and PDTB simultaneously maintains the photoelectrocatalytic properties without an all‐thiophene backbone, resulting in very high conversion rate of 600 mmol(H₂) h^?1 g^?1 at 0 V versus the reversible hydrogen electrode (RHE) at pH 11. PDTB shows the same non‐Nernstian behavior as PTTh with increasing onset potential (versus RHE) at higher pH, and the open circuit potential on PDTB under visible light reaches 1.4 V versus RHE at pH 12. The PDTB photocathode thus produces a photovoltage above the theoretical potential for the complete water‐splitting (1.229 V) and is indeed able to produce hydrogen in a one‐photon‐per‐electron light‐driven water splitting setup with MnO_x as the anode at a rate of 6.4 mmol h^?1 g_PDTB^?1.     

2H‐Pyran‐2‐one and 2H‐Furan‐2‐one Derivatives from the Plant Endophytic Fungus Pestalotiopsis fici

Two new α‐pyrones (=2H‐pyran‐2‐ones), ficipyrones A and B ( 1 and 2 , resp.), and two new α‐furanones (=2H‐furan‐2‐ones), ficifuranones A and B ( 3 and 4 , resp.), together with three known metabolites, antibiotic F 0368 ( 5 ), hydroxyseiridin ( 6 ), and hydroxyisoseiridin ( 7 ), were isolated from solid cultures of the plant endophytic fungus Pestalotiopsis fici. Their structures were elucidated primarily by NMR spectroscopy, and the absolute configuration of 1 was deduced from the circular‐dichroism (CD) data. Compound 1 showed antifungal activity against the plant pathogen Gibberella zeae (CGMCC 3.2873) with an IC₅₀ value of 15.9 μM .     

Single‐Step Preparation of Large Area TiO2 Photoelectrodes for Water Splitting

The fast, single‐step and easily scalable production by plasma electrolytic oxidation (PEO) of large area TiO₂ electrodes with excellent photoactivity in water splitting under simulated solar light is systematically investigated here. In particular, the effects that the cell voltage (100–180 V) and the processing time (0.5–15 min) have on the electrode properties are studied. The PEO‐produced oxide layers are porous, the predominant crystalline structure shifting from anatase, to an anatase‐rutile mixture, and finally to rutile by rising the cell voltage. The electrodes show a double‐layered structure, with a more compact layer at the interface with the titanium substrate and a thick porous layer on the external surface. The photocurrent density versus wavelength reflects the phase composition, with a maximum incident photon‐to‐current efficiency of 90% at 320 nm. The highest H₂ production rate is attained with the mixed anatase‐rutile electrode prepared by 300 s‐long PEO at 150 V.     

Long‐Lived,Visible‐Light‐Excited Charge Carriers of TiO2/BiVO4 Nanocomposites and their Unexpected Photoactivity for Water Splitting

Different mole ratios of TiO₂/BiVO₄ nanocomposites with effective contacts have are fabricated by putting BiVO₄ nanoparticles into the TiO₂ sol, followed by thermal treatment at 450 °C. Based on the transient‐state surface photovoltage responses and the atmosphere‐controlled steady‐state surface photovoltage spectra, it is concluded that the photogenerated charge carriers in the TiO₂/BiVO₄ nanocomposite with a proper mole ratio (5%) display much longer lifetime and higher separation than those in the BiVO₄ alone. This is responsible for the unexpected activity for photoelectrochemical oxidation of water, for photocatalytic production of H₂, and for photocatalytic degradation of phenol as a model pollutant under visible irradiation. Moreover, it is suggested that the prolonged lifetime and increased separation of photogenerated charges in the fabricated TiO₂/BiVO₄ nanocomposite is attributed to the unusual spatial transfer of visible‐excited high‐energy electrons of BiVO₄ to TiO₂. This work will provide feasible routes to synthesize visible‐light responsive nanomaterials for efficient solar utilization.     

Defect‐Engineered Ultrathin δ‐MnO2 Nanosheet Arrays as Bifunctional Electrodes for Efficient Overall Water Splitting

Recently, defect engineering has been used to intruduce half‐metallicity into selected semiconductors, thereby significantly enhancing their electrical conductivity and catalytic/electrocatalytic performance. Taking inspiration from this, we developed a novel bifunctional electrode consisting of two monolayer thick manganese dioxide (δ‐MnO₂) nanosheet arrays on a nickel foam, using a novel in‐situ method. The bifunctional electrode exposes numerous active sites for electrocatalytic rections and displays excellent electrical conductivity, resulting in strong performance for both HER and OER. Based on detailed structure analysis and density functional theory (DFT) calculations, the remarkably OER and HER activity of the bifunctional electrode can be attributed to the ultrathin δ‐MnO₂ nanosheets containing abundant oxygen vacancies lead to the formation od Mn³⁺ active sites, which give rise to half‐metallicity properties and strong H₂O adsorption. This synthetic strategy introduced here represents a new method for the development of non‐precious metal Mn‐based electrocatalysts for eddicient energy conversion.     

Atomically Thin Mesoporous In2O3–x/In2S3 Lateral Heterostructures Enabling Robust Broadband‐Light Photo‐Electrochemical Water Splitting

Atomically thin 2D heterostructures have opened new realms in electronic and optoelectronic devices. Herein, 2D lateral heterostructures of mesoporous In₂O_3–x/In₂S₃ atomic layers are synthesized through the in situ oxidation of In₂S₃ atomic layers by an oxygen plasma‐induced strategy. Based on experimental observations and theoretical calculations, the prolonged charge carrier lifetime and increased electron density reveal the efficient photoexcited carrier transport and separation in the In₂O_3–x/In₂S₃ layers by interfacial bonding at the atomic level. As expected, the synergistic structural and electronic modulations of the In₂O_3–x/In₂S₃ layers generate a photocurrent of 1.28 mA cm^?2 at 1.23 V versus a reversible hydrogen electrode, nearly 21 and 79 times higher than those of the In₂S₃ atomic layers and bulk counterpart, respectively. Due to the large surface area, abundant active sites, broadband‐light harvesting ability, and effective charge transport pathways, the In₂O_3–x/In₂S₃ layers build efficient pathways for photoexcited charge in the 2D semiconductive channels, expediting charge transport and kinetic processes and enhancing the robust broadband‐light photo‐electrochemical water splitting performance. This work paves new avenues for the exploration and design of atomically thin 2D lateral heterostructures toward robust photo‐electrochemical applications and solar energy utilization.     

Rational Design of Core@shell Structured CoSx@Cu2MoS4 Hybridized MoS2/N,S‐Codoped Graphene as Advanced Electrocatalyst for Water Splitting and Zn‐Air Battery

A novel hybrid of small core@shell structured CoS_x@Cu₂MoS₄ uniformly hybridizing with a molybdenum dichalcogenide/N,S‐codoped graphene hetero‐network (CoS_x@Cu₂MoS₄‐MoS₂/NSG) is prepared by a facile route. It shows excellent performance toward the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in alkaline medium. The hybrid exhibits rapid kinetics for ORR with high electron transfer number of ≈3.97 and exciting durability superior to commercial Pt/C. It also demonstrates great potential with remarkable stability for HER and OER, requiring low overpotential of 118.1 and 351.4 mV, respectively, to reach a current density of 10 mA cm^?2. An electrolyzer based on CoS_x@Cu₂MoS₄‐MoS₂/NSG produces low cell voltage of 1.60 V and long‐term stability, surpassing a device of Pt/C + RuO₂/C. In addition, a Zn‐air battery using cathodic CoS_x@Cu₂MoS₄‐MoS₂/NSG catalyst delivers a high cell voltage of ≈1.44 V and a power density of 40 mW cm^?2 at 58 mA cm^?2, better than the state‐of‐the‐art Pt/C catalyst. These achievements are due to the rational combination of highly active core@shell CoS_x@Cu₂MoS₄ with large‐area and high‐porosity MoS₂/NSG to produce unique physicochemical properties with multi‐integrated active centers and synergistic effects. The outperformances of such catalyst suggest an advanced candidate for multielectrocatalysis applications in metal‐air batteries and hydrogen production.     

Low‐Overpotential Electrocatalytic Water Splitting with Noble‐Metal‐Free Nanoparticles Supported in a sp3 N‐Rich Flexible COF

Covalent organic frameworks (COFs) are crystalline organic polymers with tunable structures. Here, a COF is prepared using building units with highly flexible tetrahedral sp³ nitrogens. This flexibility gives rise to structural changes which generate mesopores capable of confining very small (<2 nm sized) non‐noble‐metal‐based nanoparticles (NPs). This nanocomposite shows exceptional activity toward the oxygen‐evolution reaction from alkaline water with an overpotential of 258 mV at a current density of 10 mA cm^?2. The overpotential observed in the COF‐nanoparticle system is the best in class, and is close to the current record of ≈200 mV for any noble‐metal‐free electrocatalytic water splitting system—the Fe–Co–Ni metal‐oxide‐film system. Also, it possesses outstanding kinetics (Tafel slope of 38.9 mV dec^?1) for the reaction. The COF is able to stabilize such small‐sized NP in the absence of any capping agent because of the COF–Ni(OH)₂ interactions arising from the N‐rich backbone of the COF. Density‐functional‐theory modeling of the interaction between the hexagonal Ni(OH)₂ nanosheets and the COF shows that in the most favorable configuration the Ni(OH)₂ nanosheets are sandwiched between the sp³ nitrogens of the adjacent COF layers and this can be crucial to maximizing their synergistic interactions.     

3D‐Printed Conical Arrays of TiO2 Electrodes for Enhanced Photoelectrochemical Water Splitting

Control over the topography of semiconducting materials can lead to enhanced performances in photoelectrochemical related applications. One means of implementing this is through direct patterning of metal‐based substrates, though this is inadequately developed. Conventional techniques for patterned fabrication commonly involve technologically demanding and tedious processes. 3D printing, a form of additive fabrication, enables creation of a 3D object by deposition of successive layers of material via computer control. In this work, the feasibility of fabricating metal‐based 3D printed photoelectrodes is explored. Electrodes comprised of conical arrays are fabricated and the performance for photoelectrochemical water splitting is further enhanced by the direct growth of TiO2 nanotubes on this platform. 3D metal printing provides a flexible and versatile approach for the design and fabrication of novel electrode structures.     

MOF‐Based Transparent Passivation Layer Modified ZnO Nanorod Arrays for Enhanced Photo‐Electrochemical Water Splitting

This study introduces zeolitic imidazolate framework‐8 (ZIF‐8) as the first metal‐organic framework based transparent surface passivation layer for photo‐electrochemical (PEC) water splitting. A significant enhancement for PEC water oxidation is demonstrated based on the in situ seamless coating of ZIF‐8 surface passivation layer on Ni foam (NF) supported ZnO nanorod arrays photoanode. The PEC performance is improved by optimizing the ZIF‐8 thickness and by grafting Ni(OH)₂ nanosheets as synergetic co‐catalyst. With respect to ZnO/NF, the optimized Ni(OH)₂/ZIF‐8/ZnO/NF photoanode exhibits a two times larger photocurrent density of 1.95 mA cm^?2 and also a two times larger incident photon to current conversion efficiency of 40.05% (350 nm) at 1.23 V versus RHE (V_RHE) under AM 1.5 G. The synergetic surface passivation and the co‐catalyst modification contribute to prolonging the charge lifetime, to promoting the charge transfer, and to decreasing the overpotential for water oxidation.     

Extended Light Harvesting with Dual Cu2O‐Based Photocathodes for High Efficiency Water Splitting

Cu₂O is one of the most promising light absorbing materials for solar energy conversion. Previous studies with Cu₂O for water splitting usually deliver high photocurrent or high photovoltage, but not both. Here, a Cu₂O/Ga₂O₃/TiO₂/RuO_x photocathode that benefits from a high quality thermally oxidized Cu₂O layer and good band alignment of the Ga₂O₃ buffer layer is reported, yielding a photocurrent of 6 mA cm^?2 at 0 V versus reversible hydrogen electrode (RHE), an onset potential of 0.9 V versus RHE, and 3.5 mA cm^?2 at 0.5 V versus RHE. The quantum efficiency spectrum (incident photon to current efficiency, IPCE) reveals a dramatically improved green/red response and a decreased blue response compared with electrodeposited Cu₂O films. Light intensity dependence and photocurrent transient studies enable the identification of the limitations in the performance. Due to the complementary IPCE curves of thermally oxidized and electrodeposited Cu₂O photocathodes, a dual photocathode is fabricated to maximize the absorption over the entire range of above band gap radiation. Photocurrents of 7 mA cm^?2 at 0 V versus RHE are obtained in the dual photocathodes, with an onset potential of 0.9 V versus RHE and a thermodynamically based energy conversion efficiency of 1.9%.