O2 Evolution Reaction Research Articles

(Invited) Semipermeable Oxide Coatings for Selective Z-Scheme Photocatalytic Water Splitting

Similar to natural photosynthesis, Z-scheme photocatalytic water splitting relies on two different light absorbing components that are coupled by a redox active mediator that shuttles charge between them. Such a two-absorber system possesses several advantages over single absorber photocatalytic system, including higher theoretical solar-to-hydrogen conversion efficiency, relaxed band alignment requirements, and the potential for inherently safe operation whereby H2 and O2 evolution occur in separated compartments. However, a major disadvantage and challenge for Z-scheme photocatalysis is that the presence of a redox mediator introduces two undesirable back-reactions on top of parasitic H2 oxidation and O2 reduction reactions that can occur in a single absorber photocatalytic system. Previous research efforts have identified the use of semi-permeable oxide coatings as an attractive approach to suppress these thermodynamically favored redox reactions while still permitting the desired water splitting and mediator redox reactions to occur. Here, we present a combined experimental and computational approach based on model thin films that is used to (i) probe the performance limits of oxide-encapsulated photocatalysts, (ii) quantify the effects of coating defects on performance, and (iii) guide the rational design of coatings aimed at maximizing the solar-to-hydrogen conversion efficiency of a target photocatalytic system. This work specifically focusses on the development of silicon and titanium oxide coatings for Z-scheme water splitting based on a Fe(II)/Fe(III) mediator, showing that the best coatings can achieve selectivities > 90 % towards the H2 and O2 evolution reactions over undesired Fe(II)/Fe(III) back reactions. Another key finding from this work is that coating defects can have a significant influence on the performance of encapsulated electrodes, as revealed by scanning electrochemical microscopy (SECM) measurements that were used to locally quantify the parasitic back reaction rates around individual defects to determine their impact on the global selectivity of an encapsulated electrode.

LaNbON2 mesoporous single crystals with expedited photocarrier separation for efficient visible-light-driven water redox reactions

Combining structural coherence and large porosity, semiconducting porous single crystals (PSCs) warrant a great potential as photocatalysts. High photocarrier mobility can be realized in PSCs because of a deficiency in grain boundaries, which in turn greatly alleviates photocarriers’ recombination. The large porosity of PSCs not only provides plenty of accessible surfaces to accommodate catalytic reaction sites but also fortifies photon-matter interplay by reflecting and scattering photons. In this work, taking LaNbON2 PSCs as an example, we demonstrate the importance of PSCs in activating otherwise almost inactive photocatalyst. Compared to conventional inert LaNbON2, LaNbON2 PSCs exhibit exceptional activity for photocatalytic water redox reactions, delivering a high apparent quantum efficiency, i.e. 6.3% and 10.2% at 420 ± 20 nm for H2- and O2-evolution reactions, respectively. A Z-scheme system containing LaNbON2 PSCs has succeeded in overall water splitting (H2/O2 molar ratio 2:1) with high stability under visible light illumination (λ ≥ 420 nm).

Kinetic of the Oxygen and Chlorine Evolution Reaction on Platinum Electrodes at Neutral pH

The platinum anode modified by metal oxides electrodes degrades Abidjan wastewater which contains a high concentration of Cl-. During this degradation process, the organic polluants are oxidized, O2 and Cl2 are produced. The purpose of this study is to contribute to the understanding of these reaction mechanisms by studying the kinetics of O2 and Cl2 evolution at neutral pH on Pt. The study was performed by interpreting the voltammograms and Tafel slopes obtained. The voltammetric measurements were carried out using an Autolab Potentiostat from ECHOCHEMIE (PGSTAT 20) connected by interface to a computer. Pt electrode was prepared on titanium (Ti) substrate by thermal decomposition techniques at 400°C. The characterization of the surface of the prepared electrode by scanning electron microscopy and X-ray photoelectron spectrometry showed the presence of platinum on its surface. The results obtained show that the OH· are adsorbed on the active sites of Pt. Then they react to form PtO. Then by reaction between the surface oxygen and PtO, O2 is produced and the active sites are regenerated. In the presence of low Cl- concentration, there is a competition between the Cl2 and O2 evolution reactions. However, Cl2 only is produced for high Cl- concentrations. The kinetics of the evolution reaction of chlorine increases with the concentration of Cl- and remains constant for concentrations greater than 0.5 M. This study also showed that the chlorine reduction reaction produced in solution is a diffusion-controlled reaction for low scan rates.

Breaking the Scaling Relationship Limit: From Single-Atom to Dual-Atom Catalysts

ConspectusRecent decades have witnessed the rapid development of catalytic science, especially after Taylor and Armstrong proposed the notion of the “active site” in 1925. By optimizing reaction paths and reducing the activation energies of reactions, catalysts appear in more than 90% of chemical production reactions, involving homogeneous catalysis, heterogeneous catalysis, and enzyme catalysis. Because of the 100% efficiency of active atom utilization and the adjustable microenvironment of metal centers, single-atom catalysts (SACs) shine in various catalytic fields for enhancing the rate, conversion, and selectivity of chemical reactions. Nevertheless, a solo active site determines a fixed adsorption mode, and the adsorption energies of intermediates from multistep reactions linking with a solo metal site are related to each other. For a specific multistep reaction, it is almost impossible to optimally adjust the adsorption of every intermediate on the solo site simultaneously. This phenomenon is termed the scaling relationship limit (SRL) and is an unavoidable obstacle in the development of pure SACs.Dual-atom catalysts (DACs), perfectly inheriting the advantages of SACs, can exhibit better catalytic performance than simple SACs and thus have gradually gained researchers’ attention. Depending on the dual-metal structure, dual-metal sites (DMSs) in DACs can be divided into two separated heterometal sites, two linked homometal sites, and two linked heterometal sites. Two separated heterometal sites prescribe a distance limit between two metal sites for electron interaction. Currently, the active origins of DACs can be summarized in the following three points: (1) electronic effect, in which only one metal center serves as active site and the other plays an electronic regulatory role; (2) synergistic effect, in which two metal centers separately catalyze different core steps to improve catalytic performance together; (3) adsorption effect, in which offering additional sites changes the adsorption structures to break the SRL based on SACs. Among the three active origins, optimizing the adsorption structure of intermediates upon DMSs is one of the most effective technologies to boost the catalytic property of DACs on the basis of SACs. To date, few contributions have focused on the development of DACs in various heterogeneous catalysis environments, including O2 reduction reaction, O2 evolution reaction, H2 evolution reaction, CO2 reduction reaction, N2 reduction reaction, and other conversion reactions.In this Account, a summary of recent progress regarding DACs in heterogeneous catalysis will be presented. First, the evolution of DACs from an unpopular discovery to research hot spot is illustrated through a timeline. In the next section, the DACs are divided into three categories, and the potential active origins of DACs are revealed by comparison with SACs. In addition, the techniques for constructing DACs are systematically summarized, including preparation of carbonous, pyrolysis-free, noncarbon-supported, and complex-type DACs. Furthermore, the underlying active origins of DACs in specific energy- and environment-related reactions are introduced in detail with assistance of theoretical calculations. Finally, we affirm the contribution of DACs to catalysis, particularly heterogeneous electrocatalysis, and provide an outlook regarding the development direction for DACs by discussing the major challenges. It is anticipated that this Account can inspire researchers to propel the advance of DACs.

Nickel Nanoparticles Encapsulated in Nitrogen‐Doped Carbon Nanofibers as Excellent Bifunctional Catalyst for Hydrogen and Oxygen Evolution Processes

AbstractThe potential of nitrogen‐doped carbon nanofibers with encapsulated Ni nanoparticles as a bifunctional catalyst in H2 and O2 evolution reactions (HER and OER, respectively) was studied. The proposed heterostructure is fabricated via a facile chemical vapor deposition on Ni supported on graphitic carbon nitride (GCN) as a precursor. HER was an effect of photocatalytic water splitting and catalytic dehydrogenation of lactic acid used as sacrificial electron donor. However, photocatalysis played the dominant role in the acceleration of the reaction rate which was 17 times and 330 times higher in respect to the sample prior the CVD and to pristine GCN, respectively. The catalyst exhibited boosted electrocatalytic activity in OER and outstanding stability in respect to RuO2 in alkaline medium. The overpotentials calculated from linear sweep voltammetry at 10 mA cm−2 is ∼383 mV for the fabricated electrocatalyst in respect to 490 mV for the RuO2 and Tafel slope is calculated to be 106 mV dec−1 in comparison to 136 mV dec−1 for the reference which is above the state‐of the‐art bifunctional catalysts reported to date. The results reveal that the catalytic processes were boosted due to several key effects such as enhanced visible light absorption, better charge carriers transfer and separation, increased free charge carrier concentration and higher reducing ability of electrons.

Metal-free pristine halloysite nanotubes: Electrochemically active and stable oxygen evolution reaction

Halloysite nanotubes (HNT) are naturally occurring one-dimensional porous clay minerals and have a unique chemical structure of kaolinite. The presence of hydroxyl functional groups on the surface of HNT is more suitable for O2 evolution reaction (OER). The HNT based electrocatalyst coated on Ni foam substrate shows excellent water oxidation activity with an overpotential of 287 mV to reach a current density of 10 mA cm−2 and have excellent stability in alkaline electrolyte solution over 32 h without any decay. The adsorption of OH− species occurs on the surface of HNT from the alkaline electrolyte via one-electron transfer, the metal cation underwent repeated oxidation and reduction reactions during the OER. This result outperforms many of the transition metal-based electrocatalysts. This work provides a new platform to use naturally abundant clay minerals as an anode material in fuel cell applications.

Self-adaptive evolution of nickel silicide nanowires for the enhancement of bifunctional electrocatalytic activities

We demonstrated the electrochemical tuning (ECT) and subsequent self-adaptive evolution (SAE) of NiSi nanowires, which imparted bifunctional electrocatalytic activities comparable to those of noble-metal catalysts. Through the ECT process with lithiation and subsequent rapid delithiation in water, the NiSi nanowires developed amorphous shells dispersed with nanoparticles around intact cores, which preserved their electrical conductivity. Moreover, the amorphous shells of the NiSi nanowires exhibited structural and chemical reconstructions, which further regulated the surface states to become favorable for the O2 evolution reaction (OER) and H2 evolution reaction (HER) by enabling the lattice oxide mechanism and hydrogen adsorption energy, respectively. Because of these SAEs, the NiSi nanowires exhibited excellent electrochemical activities for both the OER and HER with overpotentials of 296 and 156 mV, respectively, at a current density of 100 mA/cm2. This study provides new opportunities for the unveiling and optimizing non-noble-metal electrocatalysts with simple compositions and earth-abundant elements.

Unfolding the Role of B Site-Selective Doping of Aliovalent Cations on Enhancing Sacrificial Visible Light-Induced Photocatalytic H2 and O2 Evolution over BaTaO2N

The doping of foreign cations and anions is one of the effective strategies for engineering defects and modulating the optical, electronic, and surface properties that directly govern the photocatalytic O2 and H2 evolution reactions. BaTaO2N (BTON) is a promising 600 nm-class photocatalyst because of its absorption of visible light up to 660 nm, small band gap (Eg = 1.9 eV), appropriate valence band-edge position for oxygen evolution, good stability under light irradiation in concentrated alkaline solutions, and nontoxicity. Although the photocatalytic and photoelectrochemical water-splitting efficiencies of BaTaO2N have been progressively improved, it is still far from the requirements set for practical applications. Here, we employ a 5% B site-selective doping of aliovalent metal cations (Al3+, Ga3+, Mg2+, Sc3+, and Zr4+) to enhance sacrificial visible light-induced photocatalytic H2 and O2 evolution over BaTaO2N. The results of physicochemical characterizations reveal that no significant change in crystal structure, crystal morphology, and optical absorption edge is observed upon cation doping. Therefore, the difference observed in O2 and H2 evolution during the photocatalytic reactions over pristine and doped BaTaO2N photocatalysts is explained by examining optical, electronic, and surface properties. Also, molecular dynamics (MD) is used to gain insights into the respective effect of cation doping on adsorption energy of water molecules and formed intermediates (H* for H2 evolution and HO*, O*, and HOO* for O2 evolution) on the BaTaO2N surfaces terminated with TaO6, TaN6, and TaO4N2 octahedra. Finally, the experimental reaction rates for H2 and O2 evolution are correlated well using a linear energy–performance relationship, elucidating the doping and surface-termination trends observed in the BaTaO2N photocatalysts.

Effects of coexisting oxoanions on SO<sub>3</sub> decomposition activity of molten-phase potassium metavanadate catalysts

Molten-state potassium metavanadate (KVO3) supported on mesoporous SiO2 materials have emerged as active catalysts for SO3 decomposition over a moderate temperature range (≤650 °C), which is a potential O2 evolution reaction useful for solar thermochemical water splitting. The molten phase formed at ≥520 °C contained tetrahedral VO42−, which plays a vital role in accelerating the SO3 uptake and conversion to SO2/O2. The present study aimed to reveal how the SO3 decomposition activity is affected by adding other oxoanions such as borate (BO33−), carbonate (CO32−), and phosphate (PO43−) into the melt. Although borate showed a deteriorating effect, phosphate tended to improve the catalytic activity when the P/V molar ratio was equal to or less than 0.5. The addition of phosphate produced a mixed phosphate vanadate with a composition of KV2PO8, which consists of infinite tetrahedral PO4 and pyramidal VO5 linked by vertex sharing. Because of the congruent melting at temperature as low as ∼530 °C, KV2PO8 may be expected as another candidate of active molten phase catalyst for SO3 decomposition.

Surface modification of carbon nitride with single Co sites via a solvent-driven strategy promoting high‐efficiency photocatalytic overall water splitting

Photocatalytic overall water splitting is one of the most challenging reactions in modern chemistry. In addition, the O2 evolution reaction (OER) is considered to be the rate-determining step. Here we develop a surface modification strategy for carbon nitride (CN) to improve its photocatalytic OER activity, thus making the overall reaction possible. We take CN nanosheets (CNNS) as an example to demonstrate that the single Co sites with high density (3.0 wt%) can be obtained on the surface of CNNS via a simple organic-solvent driven deposition method. The photocatalytic O2 evolution rate of the as-synthesized Co/CNNS is significantly increased, up to about 46 times that of pristine CNNS. In the presence of Pt as the reduction cocatalyst, the Co/CNNS sample can induce overall reactions with H2 and O2 evolution rates of 14 and 7.2 μmol h−1, respectively, about five times those of CNNS. The abundant exposed Co atoms not only act as active sites to facilitate the surface water oxidation reaction, but also quickly trap the holes to promote the charge separation and transfer. This research reports a universal strategy of surface modification CNs to promote photocatalytic overall water splitting.

SnO2 as thin conformal layer over BiVO4 surface for enhanced charge carrier separation towards O2 evolution from water oxidation

Recently, bismuth vanadate (BiVO4) has attracted substantial attention for photocatalytic applications owing to its many advantages as semiconductor to drive water oxidation. Despite possessing promising characteristics to be applied in photocatalysis, the fast electron-hole pairs recombination is still one of the drawbacks involving this semiconductor. The use of conformal layers to minimize such limitation and to improve charge separation is the aim of this work. Herein, we report on the use of SnO2 as a conformal coating layer over the BiVO4 surface for O2 evolution reaction. The incorporation of SnO2 over the BiVO4 surface showed the beneficial effect in O2 evolution compared to the pristine material, resulting in 490 μmol after 6 h of irradiation. Transient absorption and surface photovoltage spectroscopies indicate enhanced charge carrier separation and transport by passivating BiVO4 with SnO2 thin layer. Even though the O2 evolution tests in this work were carried out without the presence of a co-catalyst, the use of a thin conformal coating of SnO2 over BiVO4 showed promising results for further studies.

Noble metals doped tin dioxide for sodium hypochlorite synthesis from low concentrated NaCl solutions

AbstractBACKGROUNDCreation of anode materials with high selectivity to the target process is the main problem in low concentration sodium chloride (NaCl) solutions. A specificity of such solutions is the achievement of high potentials at which the anodes traditionally used for the synthesis of chlorine [rutile oxide/titania (RuO2‐TiO2)] are destroyed. The main aim of the work concerns the eveluation of the effect of platinum (Pt) group metals in a tin oxide (SnO2) matrix on the electrocatalytic activity and selectivity in reactions of the synthesis of sodium hypochlorite and chlorate (NaClO and NaClO3) and oxygen (O2) evolution.RESULTSIt was revealed that palladium (Pd) in oxide Ti/SnO2‐Pd and Ti/SnO2‐Pt‐Pd coatings existed in the PdO form, which has a high affinity for hydroxylation. Electrocatalytic activity of coatings in anode processes in low concentrated chloride‐containing solutions would be determined by the bond strength of chemisorbed with the surface O‐containing particles. Participation in the process of Cl‐ oxidation of labile O‐containing particles increased the rate of formation of NaClO and led to inhibition of adverse reactions of O2 evolution and synthesis of chlorates and chlorites.CONCLUSIONA correlation between the selectivity of formation of NaClO and NaClO3, and the activity of the catalyst in the O2 evolution reaction is proposed. Significant formation of chlorates occurred only at the anodes, where in 1 mol L–1 HClO4 the O2 evolution potential was >1.58 V. Parameters influencing the electrocatalytic activity and selectivity of electrode materials in the target process were proposed based on this dependence. © 2021 Society of Chemical Industry (SCI).

Bifunctional Nickel and Copper Electrocatalysts for CO2 Reduction and the Oxygen Evolution Reaction

In this study, a bifunctional electrocatalyst for CO2 reduction and the O2 evolution reaction (OER) was constructed from the electrodeposition of cuprous oxide (Cu2O) and Ni on a carbon substrate. Different Ni thicknesses on Cu2O were achieved by varying the time of chronopotentiometric deposition of Ni. Electrochemical CO2 reduction was carried out at -0.89 V and -1.89 V vs. RHE, and it was found that formate and CO were the two major products. Cu2O modified with a Ni overlayer with a thickness of ~700 nm resulted in the highest formate Faradaic efficiency of 18%, and Cu2O resulted in highest CO Faradaic efficiency of 7.9%. The enhanced Faradaic efficiency for formate is attributed to the synergistic effect between Ni and Cu2O due to maximized amounts of exposed bimetallic sites that facilitate CO2 reduction. The electrocatalyst also produces ~9 times more current density than previous studies using Ni-Cu2O electrocatalysts for the OER. The ability of the Ni-Cu2O thin films to catalyze both the OER and CO2 reduction allows them to be incorporated in the first demonstration of a two-electrode CO2 conversion device with a bifunctional catalyst.

Fabrication of Polyoxometalate Anchored Zinc Cobalt Sulfide Nanowires as a Remarkable Bifunctional Electrocatalyst for Overall Water Splitting

AbstractThe advancement of a naturally rich and effective bifunctional substance for hydrogen and oxygen evolution reaction is crucial to enhance hydrogen fuel production efficiency via the electrolysis process. Herein, facile and scalable hydrothermal synthesis of bifunctional electrocatalyst of polyoxometalate anchored zinc cobalt sulfide nanowire on Ni‐foam (NF) for overall water splitting is reported for the first time. The electrochemical analysis of POM@ZnCoS/NF displays significantly low HER and OER overpotentials of 170/337 and 200/300 mV to attain a current density of 10/40 and 20/50 mA cm−2, respectively, demonstrating the notable performance of POM@ZnCoS/NF toward H2 and O2 evolution reaction in alkaline medium. Additionally, the electrolyzer consisting of the POM@ZnCoS/NF anode and cathode shows an appealing potential of 1.56 V to deliver 10 mA cm−2 current density for overall water splitting. The high electrocatalytic activity of the POM@ZnCoS/NF is attributed to modulation of the electronic and chemical properties, increment of the electroactive sites and electrochemically active surface area of the zinc cobalt sulfide nanowires due to the anchorage of polyoxometalate nanoparticles. These results demonstrate the advantage of the polyoxometalate incorporation strategy for the design of cost‐effective and highly competent bifunctional catalysts for complete water splitting.

Nanoplasmonic zirconium nitride photocatalyst for direct overall water splitting

The ability of plasmonic nanostructures to efficiently harvest light energy and generate energetic hot carriers makes them promising materials for utilization in photocatalytic water spitting. Apart from the traditional Au and Ag based plasmonic photocatalysts, more recently the noble–metal–free alternative plasmonic materials have attracted ever–increasing interest. Here we report the first use of plasmonic zirconium nitride (ZrN) nanoparticles as a promising photocatalyst for water splitting. Highly crystalline ZrN nanoparticles with sizes dominating at 30–50 nm were synthesized that exhibit intense visible and near–infrared absorption due to localized surface plasmon resonance (LSPR). Without utilizing any noble metal cocatalysts such as Pt, the plasmonic ZrN nanoparticles alone showed stable photocatalytic activity for H2 evolution in aqueous solution with methanol as sacrificial electron donor. The addition of a cobalt oxide (CoOx) cocatalyst can facilitate the separation of photogenerated charge carriers and further improve the photocatalytic activity. The optimized CoOx modified ZrN photocatalyst was observed not only to activate the O2 evolution reaction with presence of electron acceptor, but also to drive overall water splitting for the simultaneous H2 and O2 evolution in the absence of any sacrificial agents.

Fabrication of Three-Dimensional Porous Materials with NiO Nanowalls for Electrocatalytic Oxygen Evolution

Nanosheets are two-dimensional (2D) nanocrystals that exhibit superior electrochemical performance. However, nanosheets are typically stacked horizontally toward the current collector, leading to poor utilization of individual nanosheets. Although most studies have fabricated three-dimensional (3D) structures using nanosheets, the wall thickness was a few hundred nanometers, suggesting that hundreds of nanosheets were restacked along the wall. In this study, we demonstrated the fabrication of a 3D structure constructed by NiO nanowalls with a thickness of 2–3 nm (3D-NiO). The specific surface area of 3D-NiO was found to be 42 m2 g–1, which is 2.6 times larger than that of NiO nanoparticles. The specific capacity of 3D-NiO was 2.5 times higher than that of NiO nanoparticles and could be attributed to the large specific surface area because of the nature of the thin nanowalls. The O2-evolution reaction (OER) activity of 3D-NiO at 1.65 V vs RHE was sixfold greater than that of NiO nanoparticles. The enhanced OER activity of 3D-NiO could be attributed to both the large specific surface area and the fabrication of diffusion paths within the 3D structure. The OER activity of 3D-NiO after 500 cycles was four times higher than that of NiO nanoparticles. Our results show that the 3D structure fabricated with thin nanowalls is a promising porous material for capacitor electrodes and electrocatalysts for applications in electrolysis and fuel cells.

Stable α-MoO3 Electrode with a Widened Electrochemical Potential Window for Aqueous Electrochemical Capacitors

α-MoO3 is a promising electrode material for electrocapacitive energy storage. However, it has poor stability in aqueous electrolytes due to dissolution in water. Here, we demonstrate that an α-MoO3 electrode prepared using the electrochemical deposition method is stable over a wide potential window of 2.2 V in a neutral aqueous electrolyte. The improved stability of the α-MoO3 electrode in water is attributable to enhanced electron conductivity, which minimizes the polarization-induced structural collapse, and the dense microstructure of the α-MoO3 film electrode. The widened potential window is related to the electrocatalytic inactivity of the α-MoO3 toward water electrolysis and the high overpotentials of the neutral electrolyte toward H2 and O2 evolution reactions. A symmetric supercapacitor cell fabricated with the electrochemically deposited α-MoO3 electrode delivers a specific energy as high as ∼22 Wh/kg at a specific power of ∼301 W/kg. This work demonstrates that the electrochemical deposition method is a viable approach to fabricating α-MoO3 electrodes for aqueous electrochemical energy storage.

Recent advances in vacancy engineering of metal‐organic frameworks and their derivatives for electrocatalysis

AbstractThe efficient electrocatalysis plays the key role in the development of electrochemical energy conversion technologies to alleviate energy crisis. Given their multiple active sites and large specific surface areas as electrocatalysts, metal‐organic frameworks (MOFs) and their derivatives have attracted considerable interests in recent years. Specially, exploring the roles of the enhanced active sites in MOFs and their derivatives is significant for understanding and developing new effective electrocatalysts. Recently, the vital role of vacancies has been proven to promote electrocatalytic processes (such as H2 or O2 evolution reactions, O2 reduction reactions, and N2 reduction reactions). In order to in‐depth exploring the effect of vacancies in electrocatalysts, the vacancies classification, synthetic strategy, and the recent development of various vacancies in MOFs and their derivatives for electrocatalysis are reviewed. Also, the perspectives on the challenges and opportunities of vacancies in MOFs and their derivatives for electrocatalysis are presented.

The construction of a dual direct Z-scheme NiAl LDH/g-C3N4/Ag3PO4 nanocomposite for enhanced photocatalytic oxygen and hydrogen evolution.

Dual direct Z-scheme photocatalysts for overall water decomposition have demonstrated strong redox abilities and the efficient separation of photogenerated electron–hole pairs. Overall water splitting utilizing NiAl-LDH-based binary and ternary nanocomposites has been extensively investigated. The synthesized binary and ternary nanocomposites were characterized via XRD, FTIR, SEM, HRTEM, XPS, UV-DRS, and photoelectrochemical measurements. The surface wettability properties of the prepared nanocomposites were measured via contact angle measurements. The application of the NiAl-LDH/g-C3N4/Ag3PO4 ternary nanocomposite was investigated for photocatalytic overall water splitting under light irradiation. In this work, we found that in the presence of Ag3PO4, the evolution of H2 and O2 is high over LCN30, and 2.8- fold (O2) and 1.4-fold (H2) activity increases can be obtained compared with the use of LCN30 alone. It is proposed that Ag3PO4 is involved in the O2 evolution reaction during water oxidation and g-C3N4 is involved in overall water splitting. Our work not only reports overall water splitting using NiAl-LDH-based nanocomposites but it also provides experimental evidence for understanding the possible reaction process and the mechanism of photocatalytic water splitting. Photoelectrochemical measurements confirmed the better H2 and O2 evolution abilities of NiAl-LDH/g-C3N4/Ag3PO4 in comparison with NiAl LDH, g-C3N4, Ag3PO4, and LCN30. The observed improvement in the gas evolution properties of NiAl LDH in the presence of Ag3PO4 is due to the formation of a dual direct Z-scheme, which allows for the easier and faster separation of charge carriers. More importantly, the LCNAP5 heterostructure shows high levels of H2 and O2 evolution, which are significantly enhanced compared with LCN30 and pure NiAl LDH.

Stable and efficient solar-driven photoelectrochemical water splitting into H2 and O2 based on a BaTaO2N photoanode decorated with CoO microflowers.

Stable and efficient photoelectrochemical water splitting has been achieved using a BaTaO2N photoanode decorated with CoO microflowers. The CoO microflowers effectively collect holes from BaTaO2N which kinetically protects BaTaO2N against photocorrosion. A Faraday efficiency of almost unity (99.2%) has been recorded for O2 evolution reactions. The tips of the CoO microflowers are the most active sites for water oxidation reactions.