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Abstract Most electrocatalysts are known to experience structural change during the oxygen evolution reaction (OER) process. Considerable endeavors have been dedicated thus far to comprehending the catalytic process and uncovering the underlying mechanism. During the dynamic evolution of catalyst structure, component leaching of electrocatalysts is the most common phenomenon. This article offers a concise overview of recent findings and developments related to the leaching phenomena in the OER process in terms of fundamental understanding of leaching, advanced characterization techniques used to investigate leaching, leaching of inactive components, and leaching of active components. Leaching behaviors and the induced effects in various kinds of OER catalysts are discussed, progress in manipulating leaching amount/degree toward a tunable surface evolution is spotlighted, and finally, three representative types of structure transformations induced by leaching metastable species in OER condition are proposed. By understanding the process of component leaching in the OER, it will provide more guidance for the rational design of superior electrocatalysts.

The oxygen evolution reaction (OER) is a vital half-reaction in many applications, such as the electrochemical H2O splitting, CO2, and N2 conversion processes. The OER involves a four-electron transfer and is a kinetically sluggish reaction that requires additional potential to drive. To enhance the electrochemical performance of the above-mentioned applications, highly efficient, corrosion-resistant, earth-abundant, and eco-friendly electrocatalysts are required. Here, we report a highly porous, minimally Ru-doped copper phosphate electrocatalyst obtained through co-precipitation. The optimized electrocatalyst (5% Ru-doped copper phosphate) exhibits a low overpotential of 340 mV to achieve 10 mA cm−2 compared to copper-based materials, and it remains stable over 20 h. The high performance is attributed to a high electrochemically effective surface area (ECSA) of 30.25 cm2, facilitating effective ion transportation at the electrode/electrolyte interface and excellent electrical conductivity. This result is supported by density functional theory calculations, which demonstrate that ruthenium enhances the electrochemical properties by increasing electronic conductivity, reducing the theoretical overpotential, and influencing the rate-determining step of the oxygen evolution reaction. Herein, the electrocatalyst is attractive for commercialization due to its utilization of minimal ruthenium in earth-abundant electrocatalysts, which offer competitive performance.

Photoelectrochemical (PEC) water splitting is regarded as a potential technique for converting solar energy. However, the fast charge recombination and slow water oxidation kinetics significantly have hindered its practical application. It is found that an elevation in operation temperature can activate the charge transport in the photoanodes. Here, a strategy was performed that carbon nanoparticles were employed to TiO2 nanorods, acting as electron reservoirs as well as photothermal materials. More specifically, a record photocurrent density of 1.62 mA cm−2 at 1.23 V vs. RHE has been achieved, accompanied by a high charge separation efficiency of 96% and a long-term durability for 8 h. The detailed experimental results reveal that under NIR light irradiation, the synergistic effect between electron storage and temperature rise leads to accelerated charge transport in the bulk and water oxidation kinetics on the surface. This research offers a new perspective on how to boost the PEC performance of photoelectrodes.

Abstract Protein structure plays an essential role on their stability, functionality, and catalytic activity. In this work, the interplay between the β‐sheet structure and its catalytic implications to the design of enzyme‐inspired materials is investigated. Here, inspiration is drawn from the active sites and β‐sheet rich structure of the highly efficient multicopper oxidase (MCO) to engineer a bio‐inspired electrocatalyst for water oxidation utilizing the abundant metal, copper. Copper ions are coordinated to poly‐histidine (polyCuHis), as they are in MCO active sites. The resultant polyCuHis material effectively promotes water oxidation with low overpotentials (0.15 V) in alkaline systems. This activity is due to the 3D structure of the poly‐histidine backbone. By increasing the prevalence of β‐sheet structure and decreasing the random coil nature of the polyCuHis secondary structures, this study is able to modulates the electrocatalytic activity of this material is modulated, shifting it toward water oxidation. These results highlight the crucial role of the local environment at catalytic sites for efficient, energy‐relevant transformations. Moreover, this work highlights the importance of conformational structure in the design of scaffolds for high‐performance electrocatalysts.

Abstract BiVO4 is one of the most promising photoanode materials for photoelectrochemical (PEC) solar energy conversion, but it still suffers from poor photocurrent density due to insufficient light‐harvesting efficiency (LHE), weak photogenerated charge separation efficiency (ΦSep), and low water oxidation efficiency (ΦOX). Herein, we tackle these challenges of the BiVO4 photoanodes using systematic engineering, including catalysis engineering, bandgap engineering, and morphology engineering. In particular, we deposit a NiCoOx layer onto the BiVO4 photoanode as the oxygen evolution catalyst to enhance the ΦOX of Fe‐g‐C3N4/BiVO4 for PEC water oxidation, and incorporate Fe‐doped graphite‐phase C3N4 (Fe‐g‐C3N4) into the BiVO4 photoanode to optimize the bandgap and surface areas to subsequently expand the light absorption range of the photoanode from 530 to 690 nm, increase the LHE and ΦSep, and further improve the oxygen evolution reaction activity of the NiCoOx catalytic layer. Consequently, the maximum photocurrent density of the as‐prepared NiCoOx/Fe‐g‐C3N4/BiVO4 is remarkably boosted from 4.6 to 7.4 mA cm−2. This work suggests that the proposed systematic engineering strategy is exceptionally promising for improving LHE, ΦSep, and ΦOX of BiVO4‐based photoanodes, which will substantially benefit the design, preparation, and large‐scale application of next‐generation high‐performance photoanodes.

Abstract How to put together a comprehensive water oxidation design is the long‐term problem for the generation of sustainable fuels. In nature, plants are equipped with a complex mechanism (the Calvin cycle) in the stroma that, over the billions of years, efficiently takes CO2 and converts it to sugar. This reductive process needs electrons from an oxygen‐evolving complex (OEC) in the core of photosystem II, where a water oxidation reaction (WOR) releases molecular oxygen. The initial function of proton relays in natural WOR happens when Tyrosine Yz is oxidized by light harvested P680 through a proton‐coupled electron transfer reaction. The second and main one, is composed of water‐included polypeptide channels from and to CaMn4O5. These channels were created utilizing hydrogen‐bonded networks (HBN). HBN is crucial in providing water substrate and removing the released protons to prevent the accumulation of protons and preserve the stability of the catalytic center. This concept summarizes the challenges in employing biomimicry to create artificial leaves from the viewpoint of proton relays, with the main purpose being providing structural functionality to assist the quick proton hopping.

Abstract As the most desirable hydrogen production device, the highly efficient acidic proton exchange membrane water electrolyzers (PEMWE) are severely limited by the sluggish kinetics of oxygen evolution reaction (OER) at the anode. Rutile IrO2 is a commercial acid‐stable OER catalyst with poor activity and high cost, which has motivated the development of alternatives. However, hitherto most of the designed acidic OER catalysts have disadvantages of low activity or stability, which cannot meet the requirement of industrial applications. Thus, exploring suitable strategies to enhance the activity and stability of cost‐effective acidic OER catalysts is crucial for developing the PEMWE technique. In this review, the main OER mechanisms, different types of catalysts, and their activity and stability characteristics are summarized and discussed, and then possible strategies to improve activity and stability are proposed. Finally, the problems and prospects of such catalysts are generalized to shed some light on the future research of advanced catalysts for acidic OER.

Abstract A rational regulation of the solar water splitting reaction pathway by adjusting the surface composition and phase structure of catalysts is a substantial approach to ameliorate the sluggish reaction kinetics and improve the energy conversion efficiency. In this study, we demonstrate a nanocrystalline iron pyrophosphate (Fe4(P2O7)3, FePy)-regulated hybrid overlayer with amorphous iron phosphate (FePO4, FePi) on the surface of metal oxide nanostructure with boosted photoelectrochemical (PEC) water oxidation. By manipulating the facile electrochemical surface treatment followed by the phosphating process, nanocrystalline FePy is localized in the FePi amorphous overlayer to form a heterogeneous hybrid structure. The FePy-regulated hybrid overlayer (FePy@FePi) results in significantly enhanced PEC performance with long-term durability. Compared with the homogeneous FePi amorphous overlayer, FePy@FePi can improve the charge transfer efficiency more significantly, from 60% of FePi to 79% of FePy@FePi. Our density-functional theory calculations reveal that the coexistence of FePi and FePy phases on the surface of metal oxide results in much better oxygen evolution reaction kinetics, where the FePi was found to have a typical down-hill reaction for the conversion from OH* to O2, while FePy has a low free energy for the formation of OH*. Graphical abstract

Mn-oxidizing microorganisms oxidize environmental Mn(II), producing Mn(IV) oxides. Pseudomonas putida MnB1 is a widely studied organism for the oxidation of manganese(II) to manganese(IV) by a multi-copper oxidase. The biogenic manganese oxides (BMOs) produced by MnB1 and similar organisms have unique properties compared to non-biological manganese oxides. Along with an amorphous, poorly crystalline structure, previous studies have indicated that BMOs have high surface areas and high reactivities. It is also known that abiotic Mn oxides promote oxidation of organics and have been studied for their water oxidation catalytic function. MnB1 was grown and maintained and subsequently transferred to culturing media containing manganese(II) salts to observe the oxidation of manganese(II) to manganese(IV). The structures and compositions of these manganese(IV) oxides were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction, and their properties were assessed regarding catalytic functionality towards water oxidation in comparison to abiotic acid birnessite. Water oxidation was accomplished through the whole-cell catalysis of MnB1, the results for which compare favorably to the water-oxidizing ability of abiotic Mn(IV) oxides.

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32− (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s−1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s−1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32− bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

In the present work, nickel-iron layered double hydroxide was prepared by the hydrothermal method. The synthesized electrocatalyst was studied using X-ray diffraction pattern, infrared spectroscopy, nitrogen absorb-desorption, and scanning electron microscopy. The results confirmed the synthesis of a high surface area electrocatalyst. In the next step, the electrocatalytic activity of the synthesized sample was investigated by dropping the sample on the glassy carbon electrode at neutral media. The voltammetric results show a decrease in the onset potential and overpotential of hydrothermal nickel-iron layered double hydroxide in compared to the nickel-iron layered double hydroxide which was synthesized by the co-precipitation method. High surface area, low onset potential, high stability, and repeatability are the prominent features of this electrocatalyst.

Emerging as a new class of advanced functional materials with hierarchical architectures and redox characters, organic–inorganic hybrid materials (OIHs) have been well developed and widely applied in various energy conversion reactions recently. In this review, we focus on the applications and structure–performance relationship of OIHs for electrochemical water oxidation. The general principles of water oxidation will be presented first, followed by the progresses on the applications of OIHs that are classified as metal organic frameworks (MOFs) and their derivates, covalent organic framework (COF)-based hybrids and other OIHs. The roles of organic counterparts on catalytic active centers will be fully discussed and highlighted with typical examples. Finally, the challenges and perspectives assessing this promising hybrid material as an electrocatalyst will be provided.

5-hydroxymethylfurfural oxidation reaction (HMFOR) has been considered as promising anodic reaction alternating oxygen evolution reaction (OER). The introduction of Fe into layered nickel hydroxide has been effective strategy to enhance performance of OER as well as HMFOR. However, considering that HMFOR and OER are competitive with each other, it is hardly understandable how Fe simultaneously enhances both reactions. Herein, we provide an insight toward the role of Fe in the layered Ni(OH)2 in both oxidation reactions. While the OER activity of the layered Ni(OH)2 electrocatalyst has volcanic relation with Fe content, the HMFOR activity was highest in the Fe-free Ni(OH)2.

Water oxidation (WO) is the first step in the water-splitting process aiming at the production of hydrogen as a green renewable fuel. To successfully perform WO, potent strategies for overcoming the high energetic barrier and slow kinetics of this reaction are urgently required. One such strategy is the use of molecular catalysis. Specifically, Cu-based catalysts have been highlighted over the last decade due to their stability and fast kinetics. Among them, Cu-peptoids, where peptoids are peptidomimetics akin to peptides and are N-substituted glycine oligomers, can act as stable and active catalysts for oxidation transformations including electrocatalytic WO. Previously, we suggested that a benzyl group incorporated as a side chain near the catalytic site within a Cu-peptoid electrocatalyst for WO has a structural role in the activity of the electrocatalyst in phosphate buffer (PBS). Herein, we aimed to test this hypothesis and understand how an incorporated structural element side chain affects WO. To this aim, we prepared a set of peptoid trimers each with a different structural element replacing the benzyl group by either naphthyl, cyclohexyl, benzyl, propyl chloride, or propyl side chains as well as a peptoid lacking a structural element. We studied the structure of their Cu complexes and tested these complexes as electrocatalysts for WO. We discovered that while all the peptoids self-assemble to form dinuclear Cu-peptoid complexes, the duplex that has no structural side chain, Cu2(BE)2, is structurally different from the others in the solid state. Moreover, Cu2(BE)2 remains dinuclear in a PBS at pH 11, while all the other duplexes are mononuclear in the PBS. Finally, though most of the complexes showed low electrocatalytic activity for WO, the dinuclear complex Cu2(BE)2 performed with the highest turnover frequency of 484 s−1. Nevertheless, this dinuclear complex slowly decomposes to the corresponding mononuclear complex as a more stable species during WO, while the other mononuclear complexes retain their structure in solution but display much slower kinetics (ca. 5 to 8 s−1) under the same conditions. Overall, our results demonstrate that bulkier side chains hamper the stability of dinuclear Cu-peptoids in a PBS, and hence, their efficiency as WO electrocatalysts is also hampered.

Abstract Highly efficient photoelectrochemical (PEC) water oxidation under solar visible light is crucial for water splitting to produce hydrogen as a source of sustainable energy. Particularly, silver-based nanomaterials are important for PEC performance due to their surface plasmon resonance which can enhance the photoelectrochemical efficiency. However, the PEC of ZnO/Ag2WO4/AgBr with enhanced visible-light water oxidation has not been studied so far. Herein, we present a novel photoelectrodes based on ZnO/Ag2WO4/AgBr nanorods (NRs) for PEC application, which is prepared by the low-temperature chemical growth method and then by successive ionic layer adsorption and reaction (SILAR) method. The synthesized photoelectrodes were investigated by several characterization techniques, emphasizing a successful synthesis of the ZnO/Ag2WO4/AgBr heterostructure NRs with excellent photocatalysis performance compared to pure ZnO NRs photoelectrode. The significantly enhanced PEC was due to improved photogeneration and transportation of electrons in the heterojunction due to the synergistic effect of the heterostructure. This study is significant for basic understanding of the photocatalytic mechanism of the heterojunction which can prompt further development of novel efficient photoelectrochemical-catalytic materials. Graphic Abstract

Artificial water splitting is a promising technology that allows the storage of renewable energy in the form of energy-rich compounds. This mini-review showcases how theoretical studies contribute to the under-standing of existing water oxidation catalysts (WOCs) as well as inspiring the development of novel WOCs. In order to understand the chemical complexity of transition metal complexes and their interaction with the solvent environment, the use of sophisticated simulation protocols is necessary. As an illustration, a family of ruthe- nium-based WOCs is presented which were investigated employing a wide range of forefront computational methods with emphasis on ab initiomolecular dynamic based approaches. In those studies a base assisted oxygen–oxygen bond formation was identified as the energetically most favourable reaction mechanism. By examining the role of local environmental effects at ambient temperature and the effect of modifications in the ligand framework, a comprehensible picture of the WOCs can be given, where the latter can serve as a guideline for further experimental and computational studies. In this mini-review, we provide a description of the methods, and the findings of our previous computational studies in compacted form, aimed at scientists with a theoretical as well as experimental background.

In this work, NiO modified BiVO4 (BiVO4/NiO) nanocomposite was synthesized using hydrothermal and calcination method. The composite of BiVO4/NiO, further employed as a low-overpotential photoanode, was consisted of BiVO4 nanoparticles and NiO nanosheets, in which the BiVO4 nanoelectrode served as the matrix for the attachment of NiO nanosheets. Photoelectrochemical (PEC) tests show that BiVO4/NiO displayed improved PEC performance compared with pure BiVO4. The BiVO4/NiO photoanode delivers a photocurrent density of 1.2 mA/cm2 at 1.23 V vs. RHE in a Na2SO4 electrolyte under an AM 1.5G solar simulator, which is 0.3 mA/cm2 higher than pure BiVO4 photoanode. Meanwhile, the onset potential also generates a 350 mV cathodic shift. The enhanced performance of the BiVO4/NiO nanocomposite is attributed to NiO unique lamellar structure capable of providing a large number of active sites. Measurements of electrochemical impedance spectra (EIS) and the incident photon-to-current efficiency (IPCE) illustrate that the enhanced PEC activities are ascribed to the improved charge carrier separation/transport and the promoted water oxidation kinetics furnished by the decoration of NiO cocatalyst.

Hydrogen production by electrolyzing water is an important technique to store energy from renewables into chemical energy. Many efforts have been made to improve the energy conversion efficiency. In this review article, we mainly summarized the emerging ideas on water oxidation by multi-energy coupling. First, the physicochemical nature of electrolyzing water reaction is described. Then, we conceptually proposed the physical basis of energy coupling with a goal to maximize the energy conversion efficiency and showed the methods to achieve heat–electricity and magnetism–electricity coupling to drive water splitting. Finally, the material requirements for creating efficient energy coupling water splitting system were proposed. These new ideas unlock a big potential direction for developing multi-energy coupling hydrogen production devices to efficiently store the intermittent and fluctuating renewables.

Molecular hydrogen (H2) is considered an ideal energy vector and a clean fuel, due to its zero-carbon combustion. Nevertheless, despite hydrogen is the most and one of the most abundant elements in the universe and in earth crust, respectively, it is always combined with other elements in our planet and never appears in its elemental state. This means that H2 must be produced through, almost always, endergonic processes, whose sustainability depend not only on the starting material but also on the source of energy necessary for these processes to occur. Colors have been assigned to identify the level of sustainability of H2 production with the green one indicating H2 produced from water using a renewable source of energy, preferably sunlight. Redox water splitting (WS) into H2 (hydrogen evolution reaction, HER) and O2 (oxygen evolution reaction, OER) is, nevertheless, an extremely difficult process not only from the thermodynamic but also from the kinetic point of view. Relevant kinetic barriers are present in both sides of the redox process, especially in OER. For this reason, performing WS in an efficient manner requires the development of active and robust catalysts capable of offering alternative reaction pathways to WS, lowering down the unfavorable kinetic barriers and thus maximizing the energy conversion efficiency. Inspiration for developing efficient catalysts for HER and OER has traditionally derived from Nature, who, over the course of many billions of years, according to the evolutionary theory, has assembled two molecular catalytic pools, namely oxygen evolving complex and ferredoxin/ferredoxin NADP+ reductase, which offer viable kinetic pathways to both OER and reduction of NADP+ (the “biological form” of H2). In reality, after several attempts of mimicking natural catalysts, the efforts of the researchers have been addressed to different molecular systems, which exhibit best performances, unfortunately often based on noble-metal atoms, especially for OER. In this contribution we review the journey of the development of molecular catalysts for both HER and the OER, highlighting selected systems, which have brought us to the current level of knowledge.

Abstract Hematite nanoceramics is considered as one of the most promised materials for photoelectrochemical (PEC) water splitting due to its remarkable characteristics such as low band gap, earth abundant composition and stability. In general, the hematite design over fluorine‐doped tin oxide (FTO) glass substrate involve high thermal treatment to promote its surface activation. As consequence, Sn from FTO substrate is commonly found migrating through the hematite layer. This unintentional Sn species diffusion is often associated with improvements on the photoelectrochemical response. The role played by Sn migrating from FTO against intentionally incorporated during the synthesis process into hematite to boost its performance remain unclear. In this perspective, the complexity of these matters was discussed to reveal the contribution and limiting aspects of each effect on the hematite manufacture. Indeed, a deep comprehension and latest advances on Sn diffusion and incorporation role on hematite led us to find synergistic characteristics between them, indicating economically viable alternatives to design active hematite nanoceramics. Regardless of its morphology, the key direction is based on external modifications which reduce energetic barriers on interfaces to favor electron carrier dynamics.

Developing highly active electrocatalysts for oxygen evolution reaction (OER) is crucial for the scalable production of renewable hydrogen fuels by water electrolysis. Perovskite oxides are extensively studied as OER catalysts as they can have high activity and also offer considerable flexibility in composition and structure. Recently, there are increasingly numerous reports regarding dynamic surface reconstruction of perovskite oxides under OER conditions, with claims that the reconstruction‐derived species are the actual catalysts responsible for the measured OER activity. To enable rational design of perovskite oxides as precatalysts to generate actual active components in situ, gaining essential understanding of their reconstruction behaviors is crucial. This perspective discusses the roles of initial bulk chemistry in the surface evolution process of perovskite oxides during OER, including the dependency of surface stability on electronic structure of the precatalyst and the possibility of occurrence of lattice oxygen evolution reaction and cation leaching on the surface of a perovskite oxide precatalyst. It is reasonably argued that tailoring the bulk properties of perovskite precatalysts, such as electronic structure, crystallographic structure, and ion stoichiometry, can influence the occurrence of surface reconstruction and the formation of actual active surface species.

A highly efficient visible-light-driven photoanode, N2-intercalated tungsten trioxide (WO3) nanorod, has been controllably synthesized by using the dual role of hydrazine (N2H4), which functioned simultaneously as a structure directing agent and as a nitrogen source for N2 intercalation. The SEM results indicated that the controllable formation of WO3 nanorod by changing the amount of N2H4. The β values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the nW: nN2H4 ratio. This is consistent with the addition of N2H4 dependence of the N content, clarifying the intercalation of N2 in the WO3 lattice. The UV-visible diffuse reflectance spectra (DRS) of N2-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470–600 nm, which became more intense as the nW:nN2H4 ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of N2H4 dependence is consistent with the case of the N contents. This suggests that N2 intercalating into the WO3 lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470−600 nm owing to formation of an intra-bandgap above the VB edges and a dopant energy level below the CB of WO3. The N2 intercalated WO3 photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO3 doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO3-2.5 photoanode is due to efficient electron transport through the WO3 nanorod film.

Industrial water electrolysis requires highly-active and ampere-current-bearing oxygen evolution reaction (OER) catalysts, but achieving such a large operating current density at low overpotentials in available OER catalysts still remains a grand challenge. Herein, we report a facile and conventional strategy to grow vertically non-metallic modified nickel ferrite nanosheets on Fe foam (S-NiFe2O4/IF) as largecurrent-density OER catalysts. The vertically aligned nanosheet arrays can offer large electrochemical surface area, and the spacing between nanoarrays can allow for effective electrolyte access to the active sites. The introduction of S enhanced the adsorption and dissociation to water and optimized the adsorption of OER intermediates. As a result, S-NiFe2O4/IF exhibits superior OER activity with an overpotential of 287 and 326 mV to achieve 100 and 500 mA cm−2, respectively, and displays robust stability in alkaline media. In addition, the synthesis strategy developed here can be applied to other mixed transition metal oxides with similar selfsupported earth-abundant nanoarrays for advanced electrocatalysis in energy-related reactions.

One mononuclear Mn(III) complex [MnIIIL(H2O)(MeCN)](ClO4) (1) and one hetero-binuclear complex [(CuIILMnII(H2O)3)(CuIIL)2](ClO4)2·CH3OH (2) have been synthesized with the Schiff base ligand (H2L = N,N′-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO2 reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO2 to CO are 0.38 s−1 at −1.65 V and 15.97 s−1 at −1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO2 reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO2 reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area.

In this study, 1-μm-thick TiO2 nanotubes array was prepared through anodization in aqueous or organic electrolytes, and their photoelectrochemical properties were compared. TiO2 nanotubes array prepared in an aqueous electrolyte exhibited better intrinsic characteristics for water oxidation, including larger naked surface area, higher oxygen vacancies leading to improved electrical conductivity, higher donor density, and lower valence band edge position, compared to those prepared in an organic electrolyte. However, a higher photocurrent density was observed in the TiO2 nanotubes array prepared in an organic electrolyte because of facile mass transfer, resulting in effective interaction between the generated holes and reactant.

The attachment of molecular catalysts to conductive supports for the preparation of solid‐state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1‐pyrenyl(2‐pyridyl)ethanolate‐κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2‐pyridyl)methanolate‐κO,κN)Cl (2), and [Cp*Ir(4‐(1‐pyrenyl)‐2,2′‐bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π‐stacking to ordered mesoporous carbon (OMC). The resulting iridium‐based OMC materials (Ir‐1, Ir‐2, and Ir‐3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9–1.6 s−1 at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X‐ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

Properties of heterotructured semiconductors based on ZnO/CdS nanosheets are investigated for their possible application in photocatalytic and photoelectrochemical reactions. Semiconductor material is the main active coating of photoanodes, which triggers the half-reaction of water oxidation and reduction, which entails the purifying or splitting of water. This article explains nanocomposite assembly by convenient and simple methods. The study of the physicochemical properties of semiconductor layers is carried out using electron microscopy, X-ray diffractometry, and UV-visible spectroscopy. Studies of electrochemical properties are carried out by potential static methods in electrochemical cells.

An Sb2S3-sensitized TiO2 (Sb2S3/TiO2) photo-anode (PA) exhibiting a high photo-electrochemical (PEC) performance in water oxidation has been successfully prepared by a simple chemical bath deposition (CBD) technique. Herein, the Raman spectra and XPS spectrum of Sb2S3/TiO2 confirmed the formation of Sb2S3 on the TiO2 coatings. The Sb2S3/TiO2 photo-anode significantly shifted the absorption edge from 395 nm (3.10 eV) to 650 nm (1.90 eV). Furthermore, the Sb2S3/TiO2 photo-anode generated a photo-anodic current under visible light irradiation below 650 nm due to the photo-electrochemical action compared with the TiO2 photo-anode at 390 nm. The incident photon-to-current conversion efficiency (IPCE = 7.7%) at 400 nm and −0.3 V vs. Ag/AgCl was 37 times higher than that (0.21%) of the TiO2 photo-anodes due to the low recombination rate and acceleration of the carriers of Sb2S3/TiO2. Moreover, the photo-anodic current and photostability of the Sb2S3/TiO2 photo-anodes improved via adding the Co2+ ions to the electrolyte solution during photo-electrocatalysis.

Developing a cost-effective, efficient, and stable oxygen evolution reaction (OER) catalyst is of great importance for sustainable energy conversion and storage. In this study, we report a facile one-step fabrication of cationic surfactant-assisted Prussian blue analogues (PBAs) Mx[Fe(CN)5CH3C6H4NH2]∙yC19H34NBr abbreviated as SF[Fe-Tol-M] (where SF = N-tridecyl-3-methylpyridinium bromide and M = Mn, Co and Ni) as efficient heterogeneous OER electrocatalysts. The electrocatalysts have been characterized by Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). In the presence of cationic surfactant (SF), PBAs-based electrodes showed enhanced redox current, high surface area and robust stability compared to the recently reported PBAs. SF[Fe-Tol-Co] hybrid catalyst shows superior electrochemical OER activity with a much lower over-potential (610 mV) to attain the current density of 10 mA cm−2 with the Tafel slope value of 103 mV·dec−1 than that for SF[Fe-Tol-Ni] and SF[Fe-Tol-Mn]. Moreover, the electrochemical impedance spectroscopy (EIS) unveiled that SF[Fe-Tol-Co] exhibits smaller charge transfer resistance, which results in a faster kinetics towards OER. Furthermore, SF[Fe-Tol-Co] offered excellent stability for continues oxygen production over extended reaction time. This work provides a surface assisted facile electrode fabrication approach for developing binder-free OER electrocatalysts for efficient water oxidation.

Hydrogen peroxide (H2O2) has demonstrated applicability in a wide range of applications, spanning from a bleaching agent in the pulp industry, environmental remediation, and fuel cell technology. Industrial scale synthesis, either by the anthraquinone method or catalytic oxidation of hydrogen gas, has serious drawbacks which are related with energy demanding and multi-step processes. An alternative green strategy involves the photocatalytic synthesis of H2O2. All that is needed is the renewable energy of the sun, a semiconducting species absorbing in the visible region, water, and oxygen. In this minireview, we describe the evolution of research milestones that have been achieved within the recent decades regarding the development of functional photocatalytic systems. In the early studies, back in the 1980’s, TiO2-based systems were mostly investigated. However, due to the large band gap of titania (3.2 eV), alternative semiconductors were studied which strongly absorb in the visible region. Thus, a variety of semiconductor families have been investigated, such as doped titania systems, other metal oxides, metal sulfides, organic semiconductors, metal-organic frameworks, carbon nitride systems, etc. In parallel, the development of functional dopants onto the surface of the main semiconductor has lead to both the inhibition of electron-hole recombination and H2O2 degradation. The current minireview collectively provides the studies of the higher H2O2 production rates and offer some suggestions for the near future.

Layered double hydroxides (LDH) are being used as electrocatalysts for oxygen evolution reactions (OERs). However, low current densities limit their practical applications. Herein, we report a facile and economic synthesis of an iron-copper based LDH integrated with a cobalt-based metal-organic framework (ZIF-12) to form LDH-ZIF-12 composite (1) through a co-precipitation method. The as-synthesized composite 1 requires a low overpotential of 337 mV to achieve a catalytic current density of 10 mA cm−2 with a Tafel slope of 89 mV dec−1. Tafel analysis further demonstrates that 1 exhibits a slope of 89 mV dec−1 which is much lower than the slope of 284 mV dec−1 for LDH and 172 mV dec−1 for ZIF-12. The slope value of 1 is also lower than previously reported electrocatalysts, including Ni-Co LDH (113 mV dec−1) and Zn-Co LDH nanosheets (101 mV dec−1), under similar conditions. Controlled potential electrolysis and stability test experiments show the potential application of 1 as a heterogeneous electrocatalyst for water oxidation.

Abstract Nanocrystalline BaTiO3 photocatalysts were prepared by spray‐freeze/freeze‐drying procedure in presence of structure directing gelatin. The synthetic approach yields materials with particle sizes ranging between 20 and 60 nm conforming to cubic perovskite structure. Regardless of the structural differences, the materials show particle size independent bandgap energy of ca. 3.27 eV. All prepared materials are photo‐electrochemically active in water oxidation with intrinsic activity decreasing with decreasing particle size. The photo‐electrochemical activity of BaTiO3 in water oxidation is pH dependent with the hole charge transfer processes being significantly suppressed in alkaline media. Such a behavior can be ascribed to deprotonation of surface OH groups encountered in alkaline media that promotes surface state catalyzed electron transfer reactions at the illuminated BaTiO3 surface. The barium titanate shows the ability to oxidize water with formation of oxygen and ozone. The ozone formation is pronounced on large nanocrystals particularly in acid media. No ozone formation was observed in alkaline solutions.

Photocatalytic conversion of CO2 into solar fuels has gained increasing attention due to its great potential for alleviating the energy and environmental crisis at the same time. The low-cost TiO2 with suitable band structure and high resistibility to light corrosion has proven to be very promising for photoreduction of CO2 using water as the source of electrons and protons. However, the narrow spectral response range (ultraviolet region only) as well as the rapid recombination of photo-induced electron-hole pairs within pristine TiO2 results in the low utilization of solar energy and limited photocatalytic efficiency. Besides, its low selectivity toward photoreduction products of CO2 should also be improved. Combination of TiO2 with other photoelectric active materials, such as metal oxide/sulfide semiconductors, metal nanoparticles and carbon-based nanostructures, for the construction of well-defined heterostructures can enhance the quantum efficiency significantly by promoting visible light adsorption, facilitating charge transfer and suppressing the recombination of charge carriers, resulting in the enhanced photocatalytic performance of the composite photocatalytic system. In addition, the adsorption and activation of CO2 on these heterojunctions are also promoted, therefore enhancing the turnover frequency (TOF) of CO2 molecules, so as to the improved selectivity of photoreduction products. This review focus on the recent advances of photocatalytic CO2 reduction via TiO2-based heterojunctions with water oxidation. The rational design, fabrication, photocatalytic performance and CO2 photoreduction mechanisms of typical TiO2-based heterojunctions, including semiconductor-semiconductor (S-S), semiconductor-metal (S-M), semiconductor-carbon group (S-C) and multicomponent heterojunction are reviewed and discussed. Moreover, the TiO2-based phase heterojunction and facet heterojunction are also summarized and analyzed. In the end, the current challenges and future prospects of the TiO2-based heterostructures for photoreduction of CO2 with high efficiency, even for practical application are discussed.

We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system and use this model to interpret recently published data for Ca2+- and Sr2+-containing PSII in the S2 state, in which the g = 2.0 and g = 4.1 conformations coexist. The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics, allowing us to more accurately interpret the results. Based on this model and the previously reported data on the S2 state g = 2.0 conformation, we obtain the substrate-water exchange rates of the g = 4.1 conformation and the conformational change rates. Two conclusions are made from the analyses. First, contrary to previous reports, there is no significant effect of substituting Sr2+ for Ca2+ on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (Ws) in the S2 state g = 4.1 conformation is much faster than that in the S2 state g = 2.0 conformation. The second conclusion is consistent with the assignment of Ws to W1 or W2 bound as terminal ligands to Mn4; Mn4 has been proposed to undergo an oxidation state change from Mn(IV) in the g = 2.0 conformation to Mn(III) in the g = 4.1 conformation.

In this work, a highly efficient wide-visible-light-driven photoanode, namely, nitrogen and sulfur co-doped tungsten trioxide (S-N-WO3), was synthesized using tungstic acid (H2WO4) as W source and ammonium sulfide ((NH4)2S), which functioned simultaneously as a sulfur source and as a nitrogen source for the co-doping of nitrogen and sulfur. The EDS and XPS results indicated that the controllable formation of either N-doped WO3 (N-WO3) or S-N-WO3 by changing the nW:n(NH4)2S ratio below or above 1:5. Both N and S contents increased when increasing the nW:n(NH4)2S ratio from 1:0 to 1:15 and thereafter decreased up to 1:25. The UV-visible diffuse reflectance spectra (DRS) of S-N-WO3 exhibited a significant redshift of the absorption edge with new shoulders appearing at 470–650 nm, which became more intense as the nW:n(NH4)2S ratio increased from 1:5 and then decreased up to 1:25, with the maximum at 1:15. The values of nW:n(NH4)2S ratio dependence is consistent with the cases of the S and N contents. This suggests that S and N co-doped into the WO3 lattice are responsible for the considerable redshift in the absorption edge, with a new shoulder appearing at 470–650 nm owing to the intrabandgap formation above the valence band (VB) edge and a dopant energy level below the conduction band (CB) of WO3. Therefore, benefiting from the S and N co-doping, the S-N-WO3 photoanode generated a photoanodic current under visible light irradiation below 580 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO3 doing so below 470 nm.

Photocatalytic water splitting is one of the promising approaches to solving environmental problems and energy crises. However, the sluggish 4e− transfer kinetics in water oxidation half-reaction restricts the 2e− reduction efficiency in photocatalytic water splitting. Herein, cobalt vanadate-decorated polymeric carbon nitride (named CoVO/PCN) was constructed to mediate the carrier kinetic process in a photocatalytic water oxidation reaction (WOR). The photocatalysts were well-characterized by various physicochemical techniques such as XRD, FT-IR, TEM, and XPS. Under UV and visible light irradiation, the O2 evolution rate of optimized 3 wt% CoVO/PCN reached 467 and 200 μmol h−1 g−1, which were about 6.5 and 5.9 times higher than that of PCN, respectively. Electrochemical tests and PL results reveal that the recombination of photogenerated carriers on PCN is effectively suppressed and the kinetics of WOR is significantly enhanced after CoVO introduction. This work highlights key features of the tuning carrier kinetics of PCN using charge-conducting materials, which should be the basis for the further development of photocatalytic O2 reactions.

Here, we provide a brief report on the 2019 Gordon Research Conference (GRC) and Gordon Research Seminar (GRS) on photosynthesis. Both the GRC (July 21–26, 2019) and GRS (July 20–21, 2019) were held at Sunday River Resort, Maine, USA, bringing together the beginners as well as the established scientists to discuss recent advances in photosynthesis research.

Water oxidation is the bottleneck reaction for overall water splitting as a direct and promising strategy toward clean fuels. However, the development of robust and affordable heterogeneous water oxidation catalysts remains challenging, especially with respect to the wide parameter space of synthesis and resulting material properties. Oxide catalysts performance in particular has been shown to depend on both synthetic routes and applied catalytic test methods. We here focus on spinel-type Co3O4 as a representative case for an in-depth study of the influence of rather subtle synthetic parameter variations on the catalytic performance. To this end, a series of Co3O4 samples was prepared via time-saving and tunable microwave-hydrothermal synthesis, while systematically varying a single parameter at a time. The resulting spinel-type catalysts were characterized with respect to key materials properties, including crystallinity, oxidation state and surface area using a wide range of analytical methods, such as PXRD, Raman/IR, XAS and XPS spectroscopy. Their water oxidation activity in electrocatalytic and chemical oxidation setups was then compared and correlated with the obtained catalyst properties. Both water oxidation methods displayed related trends concerning favorable synthetic parameters, namely higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction times, respectively. In addition to the surface area, structural features such as disorder were found to be influential for the water oxidation activity. The results prove that synthetic parameter screening is essential for optimal catalytic performance, given the complexity of the underlying performance-properties relationships.

The development of efficient, robust and earth-abundant catalysts for photocatalytic conversions has been the Achilles’ heel of solar energy utilization. Here, we report on a chemical approach based on ligand designed architectures to fabricate unique structural molecular catalysts coupled with appropriate light harvesters (e.g., carbon nitride and Ru(bpy)32+) for photoredox reactions. The “Co4O4” cubane complex Co4O4(CO2Me)4(RNC5H4)4 (R = CN, Br, H, Me, OMe), serves as a molecular catalyst for the efficient and stable photocatalytic water oxidation and CO2 reduction. A comprehensive structure–function analysis emerged herein, highlights the regulation of electronic characteristics for a molecular catalyst by selective ligand modification. This work demonstrates a modulation method for fabricating effective, stable and earth-abundant molecular catalysts, which might facilitate further innovation in the function-led design and synthesis of cubane clusters for photoredox reactions.

The utilization of solar energy into electrochemical reduction systems has received considerable attention. Most of these attempts have been conducted in a single electrolyte without a membrane. Here, we report the system combined by the electrochemical CO2 reduction on the Au dendrite electrode and the water oxidation on the Co-Pi electrode with a Nafion membrane. An efficient reduction of CO2 to CO in the cathode using the proton from water oxidation in the anode is conducted using perovskite solar cells under 1 sun condition. The sustainable reaction condition is secured by balancing each reaction rate based on products analysis. Through this system, we collect reduction products such as CO and H2 and oxidation product, O2, separately. Employing separation of each electrode system and series-connected perovskite solar cells, we achieve 8% of solar to fuel efficiency with 85% of CO selectivity under 1 sun illumination.

Abstract The synthesis of polymer-based assemblies for light harvesting has been motivated by the multi-chromophore antennas that play a role in natural photosynthesis for the potential use in solar conversion technologies. This review describes a general strategy for using polymer-based chromophore–catalyst assemblies for solar-driven water oxidation at a photoanode in a dye-sensitized photoelectrochemical cell (DSPEC). This report begins with a summary of the synthetic methods and fundamental photophysical studies of light harvesting polychormophores in solution which show these materials can transport excited state energy to an acceptor where charge-separation can occur. In addition, studies describing light harvesting polychromophores containing an anchoring moiety (ionic carboxylate) for covalent bounding to wide band gap mesoporous semiconductor surfaces are summarized to understand the photophysical mechanisms of directional energy flow at the interface. Finally, the performance of polychromophore/catalyst assembly-based photoanodes capable of light-driven water splitting to oxygen and hydrogen in a DSPEC are summarized.

Abstract To study the effect of partial Ba2+-to-Sr2+ and/or Ta5+-to-W4–6+ substitution on various properties of SrW(O,N)3, cubic perovskite-type W- and/or Ta-based oxynitrides Sr1−x Ba x W1−y Ta y (O,N)3, where x,y = 0,0; 0.25,0; 0.125,0.125; 0,0.25; and 1.1, were synthesized by ammonolyzing their corresponding oxide precursors under an NH3 flow. The synthesized oxynitrides have highly porous structures and consist of small crystallites in the range of 53–630 nm and with specific surface areas in the range of 5.4–14.7 m2·g−1. Interestingly, the Ta5+-to-W4~6+ substitution in SrW(O,N)3 can suppress the formation of reduced tungsten species during thermal ammonolysis. The weaker absorptions beyond 560 and 580 nm in the UV–Vis diffuse reflectance spectra, which correspond to reduced tungsten species, are observed in SrW0.75Ta0.25(O,N)3 and Sr0.875Ba0.125W0.875Ta0.125(O,N)3 compared to SrW(O,N)3 and Sr0.75Ba0.25W(O,N)3. The XPS results reveal that low-valent transition metal oxides, nitrides, and oxynitrides and/or tungsten metal are present on the surfaces of the as-synthesized oxynitrides. After 5 h of the photocatalytic oxygen evolution reaction, CoO x -loaded SrW0.75Ta0.25(O,N)3 exhibited the highest amount of evolved O2 gas due to its higher specific surface area and lower concentration of intrinsic defects. During the photocatalytic reaction, the N2 gas is also evolved because of the self-oxidation of oxynitrides consuming photo-generated holes. The estimated TONs of the oxynitride samples exceeded one, evidencing that the observed O2 gas evolution reactions were catalytic. Accordingly, the photostability enhancement of oxynitrides reduces the loss of photo-generated charge carriers and increases their photocatalytic activity. Graphical abstract

A photoelectrochemical device was achieved by interfacial self-assembly of macrocyclic π-conjugated copper phthalocyanine (CuPc) on surface of TiO2 nanorod arrays (NRs). The photocurrent density of the elegant TiO2@CuPc NRs photoanode reaches 2.40 mA/cm2 at 1.23 V vs. RHE under the illumination of 100 mW/cm2 from AM 1.5G sun simulator, which is 2.4 times higher than that of the pure TiO2. At the same time, the photoelectrochemical device constructed through this strategy has good stability and the photocurrent density remain almost no decline after 8 h of continuous operation. The Mott-Schottky and LSV curves demonstrate that CuPc act as a co-catalyst for water oxidation and a possible mechanism is proposed for water oxidation based on careful analysis of the detailed results. The holes from VB of TiO2 photogenerated by electrons exciting are consumed by a process in which Cu2+ is oxidized to Cu3+ and Cu4+, and then oxidize water to produce oxygen. CuPc species is considered to be a fast redox mediator to reduce the activation energy of water oxidation in and effectively promote charge separation.

Sam Granick opened his seminal 1957 paper titled ‘Speculations on the origins and evolution of photosynthesis’ with the assertion that there is a constant urge in human beings to seek beginnings (I concur). This urge has led to an incessant stream of speculative ideas and debates on the evolution of photosynthesis that started in the first half of the twentieth century and shows no signs of abating. Some of these speculative ideas have become commonplace, are taken as fact, but find little support. Here, I review and scrutinize three widely accepted ideas that underpin the current study of the evolution of photosynthesis: first, that the photochemical reaction centres used in anoxygenic photosynthesis are more primitive than those in oxygenic photosynthesis; second, that the probability of acquiring photosynthesis via horizontal gene transfer is greater than the probability of losing photosynthesis; and third, and most important, that the origin of anoxygenic photosynthesis pre-dates the origin of oxygenic photosynthesis. I shall attempt to demonstrate that these three ideas are often grounded in incorrect assumptions built on more assumptions with no experimental or observational support. I hope that this brief review will not only serve as a cautionary tale but also that it will open new avenues of research aimed at disentangling the complex evolution of photosynthesis and its impact on the early history of life and the planet.

Photoelectrochemical (PEC) water splitting is a promising strategy to improve the efficiency of oxygen evolution reactions (OERs). However, the efficient adsorption of visible light as well as long-term stability of light-harvesting electrocatalysis is the crucial issue in PEC cells. Metal–organic framework (MOF)-derived bimetallic electrocatalysis with its superior performance has wide application prospects in OER and PEC applications. Herein, we have fabricated a nickel and iron bimetallic organic framework (FeNi-MOF) deposited on top of anodized TiO2 nanotube arrays (TNTA) for PEC and OER applications. The FeNi-MOF/TNTA was incorporated through the electrochemical deposition of Ni2+ and Fe3+ onto the surface of TNTA and then connected with organic ligands by the hydrothermal transformation. Therefore, FeNi-MOF/TNTA demonstrates abundant photoelectrocatalytic active sites that can enhance the photocurrent up to 1.91 mA/cm2 under 100 mW/cm2 and a negligible loss in activity after 180 min of photoreaction. The FeNi-MOF-doped photoanode shows predominant photoelectrochemical performance due to the boosted excellent light-harvesting ability, rapid photoresponse, and stimulated interfacial energy of charge separation under the UV-visible light irradiation conditions. The results of this study give deep insight into MOF-derived bimetallic nanomaterial synthesis for photoelectrochemical OER and provide guidance on future electrocatalysis design.

Water oxidation and reduction reactions play vital roles in highly efficient hydrogen production conducted by an electrolyzer, in which the enhanced efficiency of the system is apparently accompanied by the development of active electrocatalysts. Solar energy, a sustainable and clean energy source, can supply the kinetic energy to increase the rates of catalytic reactions. In this regard, understanding of the underlying fundamental mechanisms of the photo/electrochemical process is critical for future development. Combining light-absorbing materials with catalysts has become essential to maximizing the efficiency of hydrogen production. To fabricate an efficient absorber-catalysts system, it is imperative to fully understand the vital role of surface/interface modulation for enhanced charge transfer/separation and catalytic activity for a specific reaction. The electronic and chemical structures at the interface are directly correlated to charge carrier movements and subsequent chemical adsorption and reaction of the reactants. Therefore, rational surface modulation can indeed enhance the catalytic efficiency by preventing charge recombination and prompting transfer, increasing the reactant concentration, and ultimately boosting the catalytic reaction. Herein, the authors review recent progress on the surface modification of nanomaterials as photo/electrochemical catalysts for water reduction and oxidation, considering two successive photogenerated charge transfer/separation and catalytic chemical reactions. It is expected that this review paper will be helpful for the future development of photo/electrocatalysts.

Photoelectrochemical water splitting is considered as a long-term solution for the ever-increasing energy demands. Various strategies have been employed to improve the traditional TiO2 photoanode. In this study, TiO2 nanorods were decorated by graphitic carbon nitride (C3N4) derived from different precursors such as thiourea, melamine, and a mixture of thiourea and melamine. Photoelectrochemical activity of TiO2/C3N4 photoanode can be modified by tuning the number of precursors used to synthesize C3N4. C3N4 derived from the mixture of melamine and thiourea in TiO2/C3N4 photoanode showed photocurrent density as high as 2.74 mA/cm2 at 1.23 V vs. RHE. C3N4 synthesized by thiourea showed particle-like morphology, while melamine and melamine with thiourea derived C3N4 yielded two dimensional (2D) nanosheets. Nanosheet-like C3N4 showed higher photoelectrochemical performance than that of particle-like nanostructures as specific surface area, and the redox ability of nanosheets are believed to be superior to particle-like nanostructures. TiO2/C3N4 displayed excellent photostability up to 20 h under continuous illumination. Thiourea plays an important role in enhancing the photoelectrochemical performance of TiO2/C3N4. This study emphasizes the fact that the improved photoelectrochemical performance can be achieved by varying the precursors of C3N4 in TiO2/C3N4 heterojunction. This is the first report to show the influence of C3N4 precursors on photoelectrochemical performance in TiO2/C3N4 systems. This would pave the way to explore different precursors influence on C3N4 with respect to the photoelectrochemical response of TiO2/C3N4 heterojunction photoanode.

The lithium salt of the binuclear nitrido complex of ruthenium (IV) Li3(Ru2NCl8·2H2O) was synthesized. Using UV spectroscopy and voltammetry, we studied complex behavior in aqueous solutions. It was found that in dilute solutions of this compound, Cl− ions are replaced by H2O molecules, and the intra-sphere redox reaction between Ru (IV) and H2O, as well as the oxidation of water with the formation of oxygen and the acidic dissociation of coordinated water molecules also have been taking place. It was established by IR spectroscopy and ESI mass spectrometric analysis that not only the binuclear structure of the complex is preserved in acidic solutions, but also its dimerization product into the tetra-ruthenium dinitrido cluster Ru4N2O5+, which is a catalyst for the water oxidation reaction. The activity of the catalyst was TOF = 0.33 s−1, TON = 304.

Photocatalytic CO2 reduction in water is one of the most attractive research pursuits of our time. In this article we report a giant polyoxometalate {Mo368} based homogeneous catalytic system, which efficiently reduces CO2 to formic acid with a maximum turnover number (TON) of 27,666, turnover frequency (TOF) of 4,611 h−1 and external quantum efficiency of the reaction is 0.6%. The catalytic system oxidizes water and releases electrons, and these electrons are further utilized for the reduction of CO2 to formic acid. A maximum of 8.3 mmol of formic acid was observed with the loading of 0.3 μmol of the catalyst. Our catalyst material is also stable throughout the reaction. The starting materials for this experiment are CO2 and H2O and the end products are HCOOH and O2. The formic acid formed in this reaction is an important H2 gas carrier and thus significant in renewable energy research.