Co3O4 Nanoparticles as Robust Water Oxidation Catalysts Towards Remarkably Enhanced Photostability of a Ta3N5 Photoanode

Despite the fact that Ta₃N₅ absorbs a major fraction of the visible spectrum, the rapid decrease of photocurrent encountered in water photoelectrolysis over time remains a serious hurdle for the practical application of Ta₃N₅ photoelectrodes. Here, by employing a Co₃O₄ nanoparticle water oxidation catalyst (WOC) as well as an alkaline electrolyte, the photostability of Ta₃N₅ electrode is significantly improved. Co₃O₄/Ta₃N₅ photoanode exhibits the best durability against photocorrosion to date, when compared with Co(OH)_x/Ta₃N₅ and IrO₂/Ta₃N₅ photoanodes. Specifically, about 75% of the initial stable photocurrent remains after 2 h irradiation at 1.2 V vs. RHE (reversible hydrogen electrode). Meanwhile, a photocurrent density of 3.1 mA cm^?2 has been achieved on Co₃O₄/Ta₃N₅ photoanode at 1.2 V vs. RHE with backside illumination under 1 sun AM 1.5 G simulated sunlight. The reason for the relatively high stability is discussed on the basis of electron microscopic observations and photoelectrochemical measurements, and the surface nitrogen content is monitored by X‐ray photoelectron spectroscopic analysis.     

Phase‐Modulated Band Alignment in CdS Nanorod/SnSx Nanosheet Hierarchical Heterojunctions toward Efficient Water Splitting

CdS is a promising visible light response photoanode of photoelectrochemical (PEC) water splitting, but it remains a great challenge for practical application, due to the photohole‐induced self‐corrosion, and sulfide/sulfite ions as hole scavengers are always necessary for stable solar hydrogen generation. Herein, a CdS/SnS_x nanorods/nanosheets hierarchical heterostructure with novel phase‐engineered band alignment is rationally designed via a two‐step solution reaction route for PEC water splitting. In the Na₂SO₄ aqueous electrolyte without any hole scavengers, compared with the pristine CdS, the CdS/SnS_x photoanode achieves a remarkably enhanced photocurrent density of 1.59 mA cm^?2 and a considerable stability at bias potential 1.23 V versus reversible hydrogen electrode (RHE) under simulated sunlight. It is proposed that the deposited SnS_x nanosheets not only act as protective layers to restrain the photocorrosion of CdS, but also facilitate the charge separation in CdS by the virtue of the Type II heterojunction formed between CdS and SnS_x.     

Metal Oxide/Graphene/Metal Sandwich Structure for Efficient Photoelectrochemical Water Oxidation

Single-layer graphene (SLG) has drawn considerable interest in photoelectrochemical (PEC) cells due to its atomically flat pinhole-free structure and remarkable in-plane carrier mobility. It is challenging, however, to obtain efficient SLG-modified photoelectrodes for PEC water splitting mainly due to the inefficient charge transfer interface. Here, a transition metal oxide/SLG/transition metal sandwich structure modified n-Si-based model photoanode is constructed to regulate the interfacial charge transfer behavior for enhanced PEC water oxidation performance. In this sandwich configuration, SLG tailors the morphology, structure, and work function properties of surface metal electrocatalysts to obtain both higher thermodynamic photovoltage and faster kinetical charge transfer at the semiconductor/electrolyte interface. In addition, SLG promotes the surface catalytic reaction as an effective charge trap and storage layer. This study provides a new structural design to engineer the SLG interfacial properties for high-performance energy conversion devices.     

Phosphorus‐Doped Perovskite Oxide as Highly Efficient Water Oxidation Electrocatalyst in Alkaline Solution

Developing cost‐effective and efficient electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the storage of renewable energies. Perovskite oxides serve as attractive candidates given their structural and compositional flexibility in addition to high intrinsic catalytic activity. In a departure from the conventional doping approach utilizing metal elements only, here it is shown that non‐metal element doping provides an another attractive avenue to optimize the structure stability and OER performance of perovskite oxides. This is exemplified by a novel tetragonal perovskite developed in this work, i.e., SrCo_0.95P_0.05O_{3– δ} (SCP) which features higher electrical conductivity and larger amount of O₂ ^2?/O^? species relative to the non‐doped parent SrCoO_{3– δ} (SC), and thus shows improved OER activity. Also, the performance of SCP compares favorably to that of well‐developed perovskite oxides reported. More importantly, an unusual activation process with enhanced activity during accelerated durability test (ADT) is observed for SCP, whereas SC delivers deactivation for the OER. Such an activation phenomenon for SCP may be primarily attributed to the in situ formation of active A‐site‐deficient structure on the surface and the increased electrochemical surface area during ADT. The concept presented here bolsters the prospect to develop a viable alternative to precious metal‐based catalysts.     

Carbon Nitride-Based Photoanode with Enhanced Photostability and Water Oxidation Kinetics

Carbon nitrides (CN) have emerged as promising photoanode materials for water-splitting photoelectrochemical cells (PECs). However, their poor charge separation and transfer properties, together with slow water-oxidation kinetics, have resulted in low PEC activity and instability, which strongly impede their further development. In this work, these limitations are addressed by optimizing the charge separation and transfer process. To this end, a nickel–iron based metal-organic framework, Ni/Fe-MIL-53, is deposited, that acts as an oxygen evolution pre-catalyst within the CN layer and incorporate reduced graphene oxide as an electron acceptor. Upon electrochemical activation, a uniform distribution of highly active oxygen evolution reaction (OER) catalysts is obtained on the porous CN surface. Detailed mechanistic studies reveal excellent hole extraction properties with high OER catalytic activity (83% faradaic efficiency) and long-term stability, up to 35 h. These results indicate that the decrease in performance is mainly due to the slow leaching of the catalyst from the CN layer. The CN photoanode exhibits a reproducible photocurrent density of 472 ± 20 µA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 m KOH, an exceptionally low onset potential of ≈0.034 V versus RHE, and high external quantum yield.     

Toward High Performance Photoelectrochemical Water Oxidation: Combined Effects of Ultrafine Cobalt Iron Oxide Nanoparticle

Photocleavage of H₂O into clean and storable H₂ fuel by photoelectrochemical (PEC) cell is a vital part of the sustainable hydrogen economy. However, thus far one of the limitations confronted by PEC cell to preferable performance is the insufficient behavior of photoanode for water oxidation half‐reaction. One of the strategies to elevate the photoanode performance is integrating with an oxygen evolution catalyst (OEC) to remove the bottleneck of the water oxidation kinetics. Herein, an ultrafine cobalt iron oxide (CIO) nanocrystalline is reported as a novel OEC for photoelectrochemical water splitting. The CIO evenly distributing on the surface of hematite nanorod arrays not only greatly facilitates the surface hole injection, but also promotes the charge separation along with passivating the surface states. Such combined effects of CIO finally lead to an impressive 1.71 fold enhancement on the photocurrent density at 1.23 V_RHE and ≈170 mV negative shift of onset potential, even overwhelms the commonly utilized Co‐Pi. Along with its excellent long‐term stability, the CIO possesses a great potential application in PEC water splitting devices.     

Heterogeneous Bimetallic Mo-NiPx/NiSy as a Highly Efficient Electrocatalyst for Robust Overall Water Splitting

Highly efficient electrocatalysts composed of earth-abundant elements are desired for water-splitting to produce clean and renewable chemical fuel. Herein, a heteroatomic-doped multi-phase Mo-doped nickel phosphide/nickel sulfide (Mo-NiP_x/NiS_y) nanowire electrocatalyst is designed by a successive phosphorization and sulfuration method for boosting overall water splitting (both oxygen and hydrogen evolution reactions (HER)) in alkaline solution. As expected, the Mo-NiP_x/NiS_y electrode possesses low overpotentials both at low and high current densities in HER, while the Mo-NiP_x/NiS_y heterostructure exhibits high active performance with ultra-low overpotentials of 137, 182, and 250 mV at the current density of 10, 100, and 400 mA cm⁻² in 1 m KOH solution, respectively, in oxygen evolution reaction. In particular, the as-prepared Mo-NiP_x/NiS_y electrodes exhibit remarkable full water splitting performance at both low and high current densities of 10, 100, and 400 mA cm⁻² with 1.42, 1.70, and 2.36 V, respectively, which is comparable to commercial electrolysis.     

Ultrasmall Dispersible Crystalline Nickel Oxide Nanoparticles as High‐Performance Catalysts for Electrochemical Water Splitting

Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert‐butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase‐pure stoichiometric nickel(ii ) oxide with a partially oxidized surface exhibiting Ni(iii ) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn‐over frequency of 0.29 s^–1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole‐conducting layers for organic solar cells.     

Interfacial Modulation with Aluminum Oxide for Efficient Plasmon-Induced Water Oxidation

Plasmon-induced photocatalysts hold great promise for solar energy conversion owing to their strong light-harvesting ability and tunable optical properties. However, the complex process of interfacial extraction of hot carriers and the roles of metal/semiconductor interfaces in plasmonic photocatalysts are still not clearly understood. Herein, the manipulation of the interface between a plasmon metal (Au) and a semiconductor (rutile TiO₂) by introducing an interfacial metal oxide (Al₂O₃) is reported. The resulting Au/Al₂O₃/TiO₂ exhibits remarkable enhancement in photocatalytic water oxidation activity compared with Au/TiO₂, giving an apparent quantum efficiency exceeding 1.3% at 520 nm for photocatalytic water oxidation. Such an interfacial modulation approach significantly prolongs the lifetime of hot carriers in the Au/TiO₂ system, which conclusively improves the utilization of hot carriers for plasmon-induced water oxidation reaction upon irradiation. This work emphasizes the essential role of the interfacial structure in plasmonic devices and provides an alternative method for designing efficient plasmonic photocatalysts for solar energy conversion.     

Ultra-Thin Protective Coatings for Sustained Photoelectrochemical Water Oxidation with Mo:BiVO4

As global warming caused by the greenhouse effect is becoming one of the major issues of the 21st century, hydrogen as an alternative to fossil-based fuels and other energy carriers has gained importance in current research. One promising approach to produce hydrogen is photoelectrochemical water splitting, which uses solar energy combined with suitable semiconducting photoabsorber electrodes to generate hydrogen and oxygen from water. However, most water splitting applications reported to date suffer from degradation of the photoabsorber, resulting in a loss of activity after just a few seconds or minutes. Here, a new approach using conformal ultra-thin and oxidation-stable protective layers is presented on Mo:BiVO₄ thin films combined with a thin Fe_0.1Ni_0.9O water oxidation co-catalyst, applied by electrochemical deposition, to achieve unprecedented photocurrent densities of up to 5.6 mA cm⁻² under simulated AM1.5G illumination and a neutral pH while providing more stable electrodes for water oxidation.     

Metal–Organic Framework Derived Co3O4/TiO2/Si Heterostructured Nanorod Array Photoanodes for Efficient Photoelectrochemical Water Oxidation

A novel hierarchical structured photoanode based on metal–organic frameworks (MOFs)‐derived porous Co₃O₄‐modified TiO₂ nanorod array grown on Si (MOFs‐derived Co₃O₄/TiO₂/Si) is developed as photoanode for efficiently photoelectrochemical (PEC) water oxidation. The ternary Co₃O₄/TiO₂/Si heterojunction displays enhanced carrier separation performance and electron injection efficiency. In the ternary system, an abnormal type‐II heterojunction between TiO₂ and Si is introduced, because the conduction band and valence band position of Si are higher than those of TiO₂, the photogenerated electrons from TiO₂ will rapidly recombine with the photogenerated holes from Si, thus leading to an efficient separation of photogenerated electrons from Si/holes from TiO₂ at the TiO₂/Si interface, greatly improving the separation efficiency of photogenerated hole within TiO₂ and enhances the photogenerated electron injection efficiency in Si. While the MOFs‐derived Co₃O₄ obviously improves the optical‐response performance and surface water oxidation kinetics due to the large specific surface area and porous channel structure. Compared with MOFs‐derived Co₃O₄/TiO₂/FTO photoanode, the synergistic function in the MOFs‐derived Co₃O₄/TiO₂/Si NR photoanode brings greatly enhanced photoconversion efficiency of 0.54% (1.04 V vs reversible hydrogen electrode) and photocurrent density of 2.71 mA cm^?2 in alkaline electrolyte. This work provides promising methods for constructing high‐performance PEC water splitting photoanode based on MOFs‐derived materials.     

Three Birds,One‐Stone Strategy for Hybrid Microwave Synthesis of Ta and Sn Codoped Fe2O3@FeTaO4 Nanorods for Photo‐Electrochemical Water Oxidation

A “three birds, one stone” strategy is proposed to enhance the performance of hematite photoanode for photoelectrochemical water splitting. One‐pot hybrid microwave synthesis of Ta and Sn codoped Fe₂O₃@FeTaO₄ core–shell nanorods on F:SnO₂ substrate achieves three synergetic effects simultaneously: i) core–shell heterojunction formation to alleviate the significant electron–hole recombination; ii) preserved morphology of small‐diameter nanorods to provide a short hole diffusion distance; and iii) Ta and Sn codoping to enhance the electrical conductivity. These effects are not possible with conventional high temperature thermal synthesis in a furnace. As a result, core–shell Fe₂O₃@FeTaO₄ electrode with FeOOH cocatalyst achieves a photocurrent density of 2.86 mA cm^?2 at 1.23 V_RHE under AM 1.5 G simulated sunlight (100 mW cm^?2), which is ≈2.4 times higher than that of bare hematite (1.17 mA cm^?2). In addition, the FeOOH/Fe₂O₃@FeTaO₄ electrode exhibits a high surface charge separation efficiency of ≈85% and a modest bulk charge separation efficiency of ≈24%.     

Uniform,Assembled 4 nm Mn3O4 Nanoparticles as Efficient Water Oxidation Electrocatalysts at Neutral pH

Electrochemical water splitting is one of the ways to produce environmentally‐friendly hydrogen energy. Transition‐metal (TM)‐based catalysts have been attracting attention due to their low cost and abundance, but their insufficient activity still remains a challenge. Here, 4 nm Mn₃O₄ nanoparticles (NPs) are successfully synthesized and their electrochemical behavior is investigated. Using electrokinetic analyses, an identical water oxidizing mechanism is demonstrated between the 4 and 8 nm Mn₃O₄ NPs. In addition, it is confirmed that the overall increase in the active surface area is strongly correlated with the superb catalytic activity of the 4 nm Mn₃O₄ NPs. To further enhance the oxygen evolution reaction (OER) performance, Ni foam substrate is introduced to maximize the entire number of the NPs participating in OER. The 4 nm Mn₃O₄/Ni foam electrode exhibits outstanding electrocatalytic activity for OER with overpotential of 395 mV at a current density of 10 mA cm^?2 under neutral conditions (0.5 m PBS, pH 7).     

LaTiOxNy Thin Film Model Systems for Photocatalytic Water Splitting: Physicochemical Evolution of the Solid–Liquid Interface and the Role of the Crystallographic Orientation

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. Intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention: One is the critical issue of compositional/structural surface modifications that occur during operation and how these affect photoelectrochemical performance. The second concerns the relation between electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance, since it is exactly at the surface where the visible‐light‐driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state‐of‐the‐art oxynitride thin film fabrication and characterization, before focusing on LaTiO₂N, selected as a representative photocatalyst. An investigation of the initial physicochemical evolution of the surface is reported. Then, it is shown that after stabilization the absorbed photon‐to‐current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations, and be up to 5 times larger than for polycrystalline samples.     

New Iron‐Cobalt Oxide Catalysts Promoting BiVO4 Films for Photoelectrochemical Water Splitting

Owing to the sluggish kinetics for water oxidation, severe surface charge recombination is a major energy loss that hinders efficient photoelectrochemical (PEC) water splitting. Herein, a simple process is developed for preparing a new type of low‐cost iron‐cobalt oxide (FeCoO_x) as an efficient co‐catalyst to suppress the surface charge recombination on bismuth vanadate (BiVO₄) photoanodes. The new FeCoO_x/BiVO₄ photoanode exhibits a high photocurrent density of 4.82 mA cm^?2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which corresponds to >100% increase compared to that of the pristine BiVO₄ photoanode. The photoanode also demonstrates a high charge separation efficiency of ≈90% with excellent stability of over 10 h, indicating the excellent catalytic performance of FeCoO_x in the PEC process. Density functional theory calculations and experimental studies reveal that the incorporation of Fe into CoO_x generates abundant oxygen vacancies and forms a p‐n heterojunction with BiVO₄, which effectively promotes the hole transport/trapping from the BiVO₄ photocatalyst and reduces the overpotential for oxygen evolution reaction (OER), resulting in remarkably increased photocurrent densities and durability. This work demonstrates a feasible process for depositing cheap FeCoO_x as an excellent OER cocatalyst on photoanodes for PEC water splitting.     

CuCo Hybrid Oxides as Bifunctional Electrocatalyst for Efficient Water Splitting

Solar‐driven water splitting is a promising approach for renewable energy, where the development of efficient and stable bifunctional electrocatalysts for simultaneously producing hydrogen and oxygen is still challenging. Herein, combined with the hydrogen evolution reaction (HER) activity of a copper(I) complex and oxygen evolution reaction (OER) activity of cobalt‐based oxides, a type of 1D copper‐cobalt hybrid oxide nanowires (CuCoO‐NWs) is developed via a facile two‐step growth‐conversion process toward a bifunctional water splitting catalyst. The CuCoO‐NWs exhibit excellent catalytic performances for both HER and OER in the same basic electrolyte, with optimized low onset overpotentials and high current densities. The efficient HER activity is ascribed to the formation of Cu₂O, while the activity for OER is primarily enabled by Co‐based oxides and abundant oxygen vacancies. The CuCoO‐NWs allow for the assembly of a water electrolyzer with strong alkaline media, with a current density of 10 mA cm^?2 at 1.61 V. Further combination with a commercial silicon photovoltaic allows the direct use of solar energy for spontaneous water splitting with excellent stability for over 72 h, suggesting the potential as a promising bifunctional electrocatalyst for efficient solar‐driven water splitting.     

Transition Metal-Based Catalysts for Urea Oxidation Reaction (UOR): Catalyst Design Strategies,Applications, and Future Perspectives

Urea oxidation reaction (UOR) has garnered significant attention in recent years as a promising and sustainable clean-energy technology. Urea-containing wastewater poses severe threats to the environment and human health. Numerous studies hence focus on developing UOR as a viable process for simultaneously remediating wastewater and converting it into energy. Moreover, UOR, which has a thermodynamic potential of 0.37 V (vs reversible hydrogen electrode, RHE), shows great promise in replacing the energy-intensive oxygen evolution reaction (OER; 1.23 V vs RHE). The versatility and stability of urea, particularly at ambient temperatures, make it an attractive alternative to hydrogen in fuel cells. Since UOR entails a complex intermediate adsorption/desorption process, many studies are devoted to designing cost-effective and efficient catalysts. Notably, transition metal-based materials with regulated d orbitals have demonstrated significant potential for the UOR process. However, comprehensive reviews focusing on transition metal-based catalysts remain scarce. In light of this, the review aims to bridge the gap by offering an in-depth and systematic overview of cutting-edge design strategies for transition metal-based catalysts and their diverse applications in UOR. Additionally, the review delves into the status quo and future directions, charting the course for further advancements in this exciting field.     

Reduced Graphene Oxide as a Catalyst Binder: Greatly Enhanced Photoelectrochemical Stability of Cu(In,Ga)Se2 Photocathode for Solar Water Splitting

The photoelectrochemical (PEC) properties of a Cu(In,Ga)Se₂ (CIGS) photocathode covered with reduced graphene oxide (rGO) as a catalyst binder for solar‐driven hydrogen evolution are reported. Chemically reduced rGO with various concentrations is deposited as an adhesive interlayer between CIGS/CdS and Pt. PEC characteristics of the CIGS/CdS/rGO/Pt are improved compared to the photocathode without rGO due to enhancement of charge transfer via efficient lateral distribution of photogenerated electrons by conductive rGO to the Pt. More importantly, the introduction of rGO to the CIGS photocathode significantly enhances the PEC stability; in the absence of rGO, a rapid loss of PEC stability is observed in 2.5 h, while the optimal rGO increases the PEC stability of the CIGS photocathode for more than 7 h. Chemical and structural characterizations show that the loss of the Pt catalyst is one of the main reasons for the lack of long‐term PEC stability; the introduction of rGO, which acts as a binder to the Pt catalysts by providing anchoring sites in the rGO, results in complete conservation of the Pt and hence much enhanced stability. Multiple functionality of rGO as an adhesive interlayer, an efficient charge transport layer, a diffusion barrier, and protection layer is demonstrated.     

Aging of a Vanadium Precursor Solution: Influencing Material Properties and Photoelectrochemical Water Oxidation Performance of Solution‐Processed BiVO4 Photoanodes

Metal–organic decomposition is an easy way to fabricate BiVO₄ (BVO) photoanodes; however, it often experiences a reproducibility issue. Here, the aging duration of a vanadium precursor solution, vanadyl acetylacetonate in methanol, is identified as a factor that profoundly affects reproducibility. Substantial changes in structural, optical, and electrical properties of BVO films are observed upon varying aging time of vanadium precursor solutions, which subsequently impacts photoelectrochemical (PEC) water oxidation and sulfite oxidation reactions. With the optimum number of aging days (3 d), some deficiency of oxygen is observed, which is accompanied by an increase in carrier concentration and a reduced charge transfer resistance in the PEC device, which produces the highest PEC performance that is comparable to the state‐of‐the‐art undoped BVO photoanodes. The findings point to the importance of understanding solution chemistry and demonstrate that utilization of the understanding of fine adjustment of the composition of BVO films can produce highly reproducible and efficient BiVO₄ photoanodes.