A Sandwich‐Like Organolead Halide Perovskite Photocathode for Efficient and Durable Photoelectrochemical Hydrogen Evolution in Water

Organolead halide perovskite materials have demonstrated great potential in the solar cells field owing to their excellent optoelectronic properties. However, the instability issue of the perovskites impedes the translation of their attractive features for the solar fuel production such as photoelectrochemical H₂ production from water splitting. Herein, CH₃NH₃PbI₃ a photocathode with a sandwich‐like structure is fabricated with a general and scalable approach toward addressing this issue. The photocathode exhibits an onset potential at 0.95 V versus reversible hydrogen electrode (RHE) and a photocurrent density of ?18 mA cm^?2 at 0 V versus RHE with an impressive ideal ratiometric power‐saved efficiency of 7.63%. More impressively, the photocathode retains good stability under 12 h continuous illumination in water at wide pH range. This performance is much superior to that of the best perovskite‐based photoelectrode ever reported.     

New Insights into Defect‐Mediated Heterostructures for Photoelectrochemical Water Splitting

Understanding the interfacial electronic structures of heterojunctions, a challenging undertaking, is extremely important to the design of photoelectrodes for efficient water splitting. The heterostructured interfaces in terms of crystal defects at the atomic‐level exemplified by TiO₂/BiVO₄ are studied. Results from both experimental observations and theoretical calculations clearly confirm the spontaneous formation of defective interfaces in the heterostructures. TiO₂/BiVO₄ junction with engineered interfacial defects can efficiently increase the carrier density and extend the lifetime of electrons. The inherent phenomenon of defective electronic structures in different heterostructures creates a significant impact on their photoelectrochemical performance. The synergetic effect between defect‐mediated mechanism and organic quantum dots sensitization yields significantly increased photoconversion efficiency, which is even superior to that of common metal sulfide sensitized ones. This result demonstrates an approach worthy for the design and fabrication of defect‐mediated heterostructures for water splitting, without utilizing harmful metal sulfides. Moreover, new insights into the influence of intrinsic defects on the interfacial charge transfer process between two different semiconductors for energy‐related applications have also been provided.     

The “Midas Touch” Transformation of TiO2 Nanowire Arrays during Visible Light Photoelectrochemical Performance by Carbon/Nitrogen Coimplantation

Titanium dioxide is a promising photoanode material for water oxidation, but it is substantially limited by its poor efficiency in the visible light range. Herein, an innovative carbon/nitrogen coimplantation method is utilized to realize the “Midas touch” transformation of TiO₂ nanowire (NW) arrays for photoelectrochemical (PEC) water splitting in visible light. These modified golden–yellow rutile TiO₂ NW arrays (C/N‐TiO₂) exhibit remarkably enhanced absorption in visible light regions and more efficient charge separation and transfer. As a result, the photocurrent density of carbon/nitrogen co‐implanted TiO₂ under visible light (>420 nm) can reach 0.76 mA cm^?2, which far exceeds the value of 3 µA cm^?2 seen for pristine TiO₂ nanowire arrays at 0.8 V versus Ag/AgCl. An incident photon to electron conversion efficiency of ≈14.8% is achieved at 450 nm on C/N‐TiO₂ without any other cocatalysts. The ion implantation doping approach, combined with codoping strategies, is proved to be an effective strategy for enhancing the photoelectrochemical conversion and can enable further improvement of the PEC water‐splitting performance of many other semiconductor photoelectrodes.     

Recent Advancement of p‐ and d‐Block Elements,Single Atoms,and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Solar‐assisted photoelectrochemical (PEC) water splitting to produce hydrogen energy is considered the most promising solution for clean, green, and renewable sources of energy. For scaled production of hydrogen and oxygen, highly active, robust, and cost‐effective PEC electrodes are required. However, most of the available semiconductors as a PEC electrodes have poor light absorption, material degradation, charge separation, and transportability, which result in very low efficiency for photo‐water splitting. Generally, a promising photoelectrode is obtained when the surface of the semiconductor is modified/decorated with a suitable co‐catalyst because it increases the light absorbance spectrum and prevents electron–hole recombination during photoelectrode reactions. In this regard, numerous p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes have been widely used as semiconductor/co‐catalyst junctions to boost the performances of PEC overall water splitting. This review enumerates the recent progress and applications of p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes for water splitting. The focus is placed on fundamental mechanism, efficiency, cells design, and various aspects that contribute to the large‐scale prototype device. Finally, future perspectives, summary, challenges, and outlook for improving the activity of PEC photoelectrodes toward whole‐cell water splitting are addressed.     

Highly Efficient Sunlight‐Driven Seawater Splitting in a Photoelectrochemical Cell with Chlorine Evolved at Nanostructured WO3 Photoanode and Hydrogen Stored as Hydride within Metallic Cathode

A seawater splitting photoelectrochemical cell featuring a nanostructured tungsten trioxide photoanode that exhibits very high and stable photocurrents producing chlorine with average 70% Faradaic efficiency is described. Fabrication of the WO₃ electrodes on fluorine‐doped tin oxide substrates involves a simple solution‐based method and sequential layer‐by‐layer deposition with a progressively adjusted amount of structure‐directing agent in the precursor and a two‐step annealing. Such a procedure allows tailoring of thick, highly porous, structurally stable WO₃ films with a large internal photoactive surface area optimizing utilization of visible light wavelengths by the photoanode. With the application of an anodic potential of 0.76 V versus Ag/AgCl reference electrode (0.4 V below the thermodynamic Cl₂/Cl^? potential) in synthetic seawater, the designed WO₃ photoanodes irradiated with simulated 1 sun AM 1.5G light reach currents exceeding 4.5 mA cm^?2. Photocurrents close to 5 mA cm^?2 are attained in the case of fresh water splitting using 1 m methane–sulfonic acid supporting electrolyte with oxygen evolved at the WO₃ photoanode. The amount of formed hydrogen is determined by discharging the palladium sheet electrode employed as a cathode. Collection of hydrogen in the form of a hydride opens, more generally, the prospect of subsequently using such materials as anodes in batteries employing oxygen reduction cathodes.     

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.     

A Review of Phosphide‐Based Materials for Electrocatalytic Hydrogen Evolution

Hydrogen evolution by means of electrocatalytic water‐splitting is pivotal for efficient and economical production of hydrogen, which relies on the development of inexpensive, highly active catalysts. In addition to sulfides, the search for non‐noble metal catalysts has been mainly directed at phosphides due to the superb activity of phosphides for hydrogen evolution reaction (HER) and their low‐cost considering the abundance of the non‐noble constituents of phosphides. Here, recent research focusing on phosphides is summarized based on their synthetic methodology. A comparative study of the catalytic activity of different phosphides towards HER is then conducted. The catalytic activity is evaluated by overpotentials at fixed current density, Tafel slope, turnover frequency, and the Gibbs free energy of hydrogen adsorption. Based on the methods discussed, perspectives for the various methods of phosphides synthesis are given, and the origins of the high activity and the role of phosphorus on the improved activity towards HER are discussed.     

A Monodisperse Rh2P‐Based Electrocatalyst for Highly Efficient and pH‐Universal Hydrogen Evolution Reaction

The search for Pt‐free electrocatalysts exceeding pH‐universal hydrogen evolution reaction (HER) activities when compared to the state‐of‐the‐art commercial Pt/C is highly desirable for the development of renewable energy conversion systems but still remains a huge challenge. Here a colloidal synthesis of monodisperse Rh₂P nanoparticles with an average size of 2.8 nm and their superior catalytic activities for pH‐universal HER are reported. Significantly, the Rh₂P catalyst displays remarkable HER performance with overpotentials of 14, 30, and 38 mV to achieve 10 mA cm^?2 in 0.5 m H₂SO₄, 1.0 m KOH, and 1.0 m phosphate‐buffered saline, respectively, exceeding almost all the documented electrocatalysts, including the commercial 20 wt% Pt/C. Density functional theory calculations reveal that the introduction of P into Rh can weaken the H adsorption strength of Rh₂P to nearly zero, beneficial for boosting HER performance.