ZnO As an Active and Selective Catalyst for Electrochemical Water Oxidation to Hydrogen Peroxide

Stable production of hydrogen peroxide over zinc oxide @ zeolitic imidazolate Framework-8 composite catalysts

A promising method of producing hydrogen peroxide (H2O2) is the electrochemical two-electron water oxidation reaction (2e- WOR). In this process, it is important to design electrocatalysts that are both earth abundant and environmentally friendly, as well as offering high stability and production rates. The research of WOR catalysts, such as the extensively used transition metal oxides, is mainly focused on the modification of transition metal elements. Few studies pay attention to the protective heterostructure of metal oxides. Here, we demonstrate for the first time an organometallic skeleton protection strategy to develop highly stable WOR catalysts for H2O2 generation. Unlike the pure ZnO and zeolite imidazole framework-8 (ZIF-8) catalysts, ZnO@ZIF-8 enabled the production of hydrogen peroxide at high voltages. The experimental results demonstrate that the ZnO@ZIF-8 catalyst stably generates H2O2 even under a high voltage of 3.0 V vs. RHE, with a yield reaching 2845.819 μmolmin-1 g-1. ZnO@ZIF-8 shows a relatively low overpotential, with a current density of 10 mA cm-2 and an overpotential of 110 mV. The ZnO@ZIF-8 catalyst's maximal FE value was 4.72 %. Moreover, the ZnO@ZIF-8 catalyst exhibits remarkable durability even after an extended 60-hour stability test. Operando Raman and theoretic calculation analyses reveal that the metal-organic skeleton being encapsulated on the metal oxide surface synergizes with each other, not only expanding the electrochemical surface area, but also adjusting the catalyst metal sites' adsorption capacity. A novel approach to the modification of 2e- WOR metal oxide catalyst is presented in this work.

Bioinspired Tetranuclear Manganese Cubane Complex as an Efficient Molecular Electrocatalyst for Two-Electron Water Oxidation Towards Hydrogen Peroxide

Stable homogeneous two-electron water oxidation electrocatalysts are highly demanded to understand the precise mechanism and reaction intermediates of electrochemical H2O2 production. Here we report a tetranuclear manganese complex with a cubane structure which can electrocatalyze water oxidation to hydrogen peroxide under alkaline and neutral conditions. Such a complex demonstrates an optimal Faradaic efficiency (FE) of 87 %, which is amongst (if not) the highest FE(H2O2) of reported homogeneous and heterogeneous electrocatalysts. In addition, active species were identified and co-catalysts were excluded through ESI-MS characterization. Furthermore, we identified water binding sites and isolated one-electron oxidation intermediate by chemical oxidation of the catalyst in the presence of water substrates. It is evident that efficient proton-accepting electrolytes avoid rapid proton building-up at electrode and substantially improve reaction rate and selectivity. Accordingly, we propose a two-electron catalytic cycle model for water oxidation to hydrogen peroxide with the bioinspired molecular electrocatalyst. The present work is expected to provide an ideal platform to elucidate the two-electron WOR mechanism at the atomic level.

Oxygen vacancy based WO3/SnO2-x promote electrochemical H2O2 accumulation by two-electron water oxidation reaction and toxic uniform dimethylhydrazine degradation

The key to constructing an anodic electro-Fenton system hinges on two pivotal criteria: enhancing the catalyst activity and selectivity in water oxidation reaction (WOR), while simultaneously inhibiting the decomposition of hydrogen peroxide (H2O2) which is on-site electrosynthesized at the anode. To address the issues, we synthesized novel WO3/SnO2-x electrocatalysts, enriched with oxygen vacancies, capitalize on the combined activity and selectivity advantages of both WO3 and SnO2-x for the two-electron pathway electrocatalytic production of H2O2. Moreover, the introduction of oxygen vacancies plays a critical role in impeding the decomposition of H2O2. This innovative design ensures that the Faraday efficiency and yield of H2O2 are maintained at over 80 %, with a noteworthy production rate of 0.2 mmol h-1 cm-2. We constructed a novel electro-Fenton system that operates using only H2O as its feedstock and applied it to treat highly toxic uniform dimethylhydrazine (UDMH) from rocket launch effluent. Our experiments revealed a substantial total organic carbon (TOC) removal, achieving approximately 90 % after 120 mins of treatment. Additionally, the toxicity of N-nitrosodimethylamine (NDMA), a byproduct of great concern, was shown to be effectively mitigated, as evidenced by acute toxicity evaluations using zebrafish embryos. The degradation mechanism of UDMH is predominantly characterized by the advanced oxidative action of H2O2 and hydroxyl radicals, as well as by complex electron transfer processes that warrant further investigation.

The Nexus of Innovation: Electrochemically Synthesizing H2O2 and Its Integration with Downstream Reactions

Hydrogen peroxide (H2O2) represents a chemically significant oxidant that is prized for its diverse applicability across various industrial domains. Recent innovations have shed light on the electrosynthesis of H2O2 through two-electron oxygen reduction reactions (2e- ORR) or two-electron water oxidation reactions (2e- WOR), processes that underscore the attractive possibility for the on-site production of this indispensable oxidizing agent. However, the translation of these methods into practical utilization within chemical manufacturing industries remains an aspiration rather than a realized goal. This Perspective intends to furnish a comprehensive overview of the latest advancements in the domain of coupled chemical reactions with H2O2, critically examining emergent strategies that may pave the way for the development of new reaction pathways. These pathways could enable applications that hinge on the availability and reactivity of H2O2, including, but not limited to the chemical synthesis coupled with H2O2 and waste water treatment byFenton-like reactions. Concurrently, the Perspective acknowledges and elucidates some of the salient challenges and opportunities inherent in the coupling of electrochemically generated H2O2, thereby providing a scholarly analysis that might guide future research.

Controlled crystal facet of tungsten trioxide photoanode to improve on-demand hydrogen peroxide production for in-situ tetracycline degradation

Advanced oxidation processes utilizing hydrogen peroxide (H2O2) are widely employed for the treatment of organic pollutions. However, the conventional anthraquinone method for H2O2 synthesis is unsuitable for this application owing to its hazardous and costly nature. Alternative approaches involve a photoelectrochemical method. Herein, tungsten trioxide (WO3) photoanode has been used for the conversion of H2O into H2O2 through oxidation reaction from a PEC system, simultaneously utilizing in-situ generated hydroxyl (OH•) radicals for tetracycline degradation. By manipulating the ratio of crystal facets between (020) and (200) of the WO3 photoanode, a significant improvement in H2O2 production has been achieved by increasing the proportion of (020) facet. The production rate of WO3 photoanode enriched with the (020) facet is approximately 1.9 times higher than that enriched with (200) facet. This enhanced H2O2 production performance can be attributed to the improved formation of OH• radicals and the accelerated desorption of H2O2 on the (020) facet. Simultaneously, the in-situ generated OH• radicals are applied for tetracycline degradation. Under illumination of sunlight stimulator for 180 min, the optimal photoanode achieves a degradation rate of 86.7% for tetracycline. Furthermore, the resulting chemicals have been analyzed, revealing that C8H10O and C7H8O were formed as the primary products. Notably, these products exhibit significantly lower toxicity compared to tetracycline. This study presents a promising approach for the rational design of WO3 based photoanodes for oxidation reaction, including not only H2O2 production but also the efficient degradation of organic pollutants.

O2-Independent H2O2 Production via Water-Polymer Contact Electrification

Hydrogen peroxide (H2O2), as a critical green chemical, has received immense attention in energy and environmental fields. The ability to produce H2O2 in earth-abundant water without relying on low solubility oxygen would be a sustainable and potentially economic process, applicable even to anaerobic microenvironments, such as groundwater treatment. However, the direct water to H2O2 process is currently hindered by low selectivity and low production rates. Herein, we report that poly(tetrafluoroethylene) (PTFE), a commonly used inert polymer, can act as an efficient triboelectric catalyst for H2O2 generation. For example, a high H2O2 production rate of 24.8 mmol gcat-1 h-1 at a dosage of 0.01 g/L PTFE was achieved under the condition of pure water, ambient atmosphere, and no sacrificial agents, which exceeds the performance of state-of-the-art aqueous H2O2 powder catalysts. Electron spin resonance and isotope experiments provide strong evidence that water-PTFE tribocatalysis can directly oxidize water to produce H2O2 under both anaerobic and aerobic conditions, albeit with different synthetic pathways. This study demonstrates a potential strategy for green and effective tribocatalytic H2O2 production that may be particularly useful toward environmental applications.

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2 O2

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 μmol/h/cm2 H2 O2 production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.

Concurrent oxygen reduction and water oxidation at high ionic strength for scalable electrosynthesis of hydrogen peroxide

Electrosynthesis of hydrogen peroxide via selective two-electron transfer oxygen reduction or water oxidation reactions offers a cleaner, cost-effective alternative to anthraquinone processes. However, it remains a challenge to achieve high Faradaic efficiencies at elevated current densities. Herein, we report that oxygen-deficient Pr1.0Sr1.0Fe0.75Zn0.25O4-δ perovskite oxides rich of oxygen vacancies can favorably bind the reaction intermediates to facilitate selective and efficient two-electron transfer pathways. These oxides exhibited superior Faradic efficiencies (~99%) for oxygen reduction over a wide potential range (0.05 to 0.45 V versus reversible hydrogen electrode) and current densities surpassing 50 mA cm-2 under high ionic strengths. We further found that the oxides perform a high selectivity (~80%) for two-electron transfer water oxidation reaction at a low overpotential (0.39 V). Lastly, we devised a membrane-free electrolyser employing bifunctional electrocatalysts, achieving a record-high Faradaic efficiency of 163.0% at 2.10 V and 50 mA cm-2. This marks the first report of the concurrent oxygen reduction and water oxidation catalysed by efficient bifunctional oxides in a novel membrane-free electrolyser for scalable hydrogen peroxide electrosynthesis.

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H2 O2 production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H2 O2 to O2 . Here, ZnO passivator-coated BiVO4 photoanode is reported for selective PEC H2 O2 production. Both the H2 O2 selectivity and production rate increase in the range of 1.0-2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO4 upon ZnO coating, facilitating H2 O2 generation and suppressing the competitive reaction of O2 evolution. The ZnO overlayer also inhibits H2 O2 decomposition, accelerates charge extraction from BiVO4 , and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H2 O2 synthesis from PEC water oxidation.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Tailored BiVO4 Photoanode Hydrophobic Microenvironment Enables Water Oxidative H2 O2 Accumulation

Direct photoelectrochemical 2-electron water oxidation to renewable H₂ O₂ production on an anode increases the value of solar water splitting. BiVO₄ has a theoretical thermodynamic activity trend toward highly selective water oxidation H₂ O₂ formation, but the challenges of competing 4-electron O₂ evolution and H₂ O₂ decomposition reaction need to overcome. The influence of surface microenvironment has never been considered as a possible activity loss factor in the BiVO₄ -based system. Herein, it is theoretically and experimentally demonstrated that the situ confined O₂ , where coating BiVO₄ with hydrophobic polymers, can regulate the thermodynamic activity aiming for water oxidation H₂ O₂ . Also, the hydrophobicity is responsible for the H₂ O₂ production and decomposition process kinetically. Therefore, after the addition of hydrophobic polytetrafluoroethylene on BiVO₄ surface, it achieves an average Faradaic efficiency (FE) of 81.6% in a wide applied bias region (0.6-2.1 V vs RHE) with the best FE of 85%, which is 4-time higher than BiVO₄ photoanode. The accumulated H₂ O₂ concentration can reach 150 µm at 1.23 V versus RHE under AM 1.5 illumination in 2 h. This concept of modifying the catalyst surface microenvironment via stable polymers provides a new approach to tune the multiple-electrons competitive reactions in aqueous solution.

Electrochemical generation of hydrogen peroxide from a zinc gallium oxide anode with dual active sites

Electrochemical water oxidation enables the conversion of H₂O to H₂O₂. It holds distinct advantages to the O₂ reduction reaction, which is restricted by the inefficient mass transfer and limited solubility of O₂ in aqueous media. Nonetheless, most reported anodes suffer from high overpotentials (usually >1000 mV) and low selectivity. Electrolysis at high overpotentials often causes serious decomposition of peroxides and leads to declined selectivity. Herein, we report a ZnGa₂O₄ anode with dual active sites to improve the selectivity and resist the decomposition of peroxides. Its faradaic efficiency reaches 82% at 2.3 V versus RHE for H₂O₂ generation through both direct (via OH^-) and indirect (via HCO₃^-) pathways. The percarbonate is the critical species generated through the conversion of bicarbonate at Ga-Ga dual sites. The peroxy bond is stable on the surface of the ZnGa₂O₄ anode, significantly improving faradaic efficiency.

Photocatalytic and Electrocatalytic Generation of Hydrogen Peroxide: Principles, Catalyst Design and Performance

Hydrogen peroxide (H2O2) is a high-demand organic chemical reagent and has been widely used in various modern industrial applications. Currently, the prominent method for the preparation of H2O2 is the anthraquinone oxidation. Unfortunately, it is not conducive to economic and sustainable development since it is a complex process and involves unfriendly environment and potential hazards. In this context, numerous approaches have been developed to synthesize H2O2. Among them, photo/electro-catalytic ones are considered as two of the most promising manners for on-site synthesis of H2O2. These alternatives are sustainable in that only water or O2 is required. Namely, water oxidation (WOR) or oxygen reduction (ORR) reactions can be further coupled with clean and sustainable energy. For photo/electro-catalytic reactions for H2O2 generation, the design of the catalysts is extremely important and has been extensively conducted with an aim to obtain ultimate catalytic performance. This article overviews the basic principles of WOR and ORR, followed by the summary of recent progresses and achievements on the design and performance of various photo/electro-catalysts for H2O2 generation. The related mechanisms for these approaches are highlighted from theoretical and experimental aspects. Scientific challenges and opportunities of engineering photo/electro-catalysts for H2O2 generation are also outlined and discussed.

Single atomic Ru in TiO2 boost efficient electrocatalytic water oxidation to hydrogen peroxide

Electrocatalytic two-electron water oxidation affords a promising approach for distributed production of H₂O₂ using electricity. However, it suffers from the trade-off between the selectivity and high production rate of H₂O₂ due to the lack of suitable electrocatalysts. In this study, single atoms of Ru were controllably introduced into titanium dioxide to produce H₂O₂ through an electrocatalytic two-electron water oxidation reaction. The adsorption energy values of OH intermediates could be tuned by introducing Ru single atoms, offering superior H₂O₂ production under high current density. Notably, a Faradaic efficiency of 62.8% with an H₂O₂ production rate of 24.2 μmol min^-1 cm^-2 (>400 ppm within 10 min) was achieved at a current density of 120 mA cm^-2. Consequently, herein, the possibility of high-yield H₂O₂ production under high current density was demonstrated and the importance of regulating intermediate adsorption during electrocatalysis was evidenced.

The development of cobalt phosphide co-catalysts on BiVO4 photoanodes to improve H2O2 production

Photoanodic hydrogen peroxide (H₂O₂) production via water oxidation is limited by low yields and poor selectivity. Herein, four variations of cobalt phosphides, including pristine CoP and Co₂P crystals, and two mixed-phase cobalt phosphides (CoP/Co₂P) with different ratios, were applied as co-catalysts on the BiVO₄ (BVO) photoanode to improve H₂O₂ production. The optimal yield and selectivity were approximately 9.6 µmol‧h^-1‧cm^-2 and 25.2 % at a voltage bias of 1.7 V vs reversible hydrogen electrode (V_RHE) under sunlight illumination, respectively. This performance is approximately 1.8 times that of pristine BVO photoanode. The roles of the Co and P sites were investigated. In particular, the Co site promotes the breaking of one HO bond in water to form OH^• radicals, which is the rate-determining step in H₂O₂ production. The P site plays an important role in the desorption of H₂O₂ formed from the catalyst, which is responsible for the recovery of fresh catalytic sites. Among the four samples, Co₂P exhibited the best performance for H₂O₂ production because it had the highest rate of OH^• formation owing to its improved accumulation property. This study offers a rational design strategy for co-catalysts for photoanodic H₂O₂ production.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH₄ ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH₄ into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH₄ at low temperatures. In this review, the efficient catalysts designed to reduce the CH₄ oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH₄ in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH₄ at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH₄ at low temperatures.

Increasing the Efficiency of Photocatalytic Water Splitting via Introducing Intermediate Bands

Photocatalytic water splitting is a potential way to utilize solar energy. To be practically useful, it is important to have a high solar-to-hydrogen (STH) efficiency. In this study, we propose a conceptually new photocatalytic water splitting model based on intermediate bands (IBs). In this new model, introducing IBs within the band gap can significantly increase the STH efficiency limit (from 30.7% to 48.1% without an overpotential and from 13.4% to 36.2% with overpotentials) compared to that in conventional single-band gap photocatalytic water splitting. First-principles calculations indicate that N-doped TiO₂, Bi-doped TiO₂, and P-doped ZnO have suitable IBs that can be used to construct IB photocatalytic water splitting systems. The STH efficiency limits of these three doped systems are 10.0%, 12.0%, and 19.0%, respectively, while those of pristine TiO₂ and ZnO without IB are only 0.9% and 1.6%, respectively. The IB photocatalytic water splitting model proposed in this study opens a new avenue for photocatalytic water splitting design.

Materials Screening by the Descriptor G max(η): The Free-Energy Span Model in Electrocatalysis

To move from fossil-based energy resources to a society based on renewables, electrode materials free of precious noble metals are required to efficiently catalyze electrochemical processes in fuel cells, batteries, or electrolyzers. Materials screening operating at minimal computational cost is a powerful method to assess the performance of potential electrode compositions based on heuristic concepts. While the thermodynamic overpotential in combination with the volcano concept refers to the most popular descriptor-based analysis in the literature, this notion cannot reproduce experimental trends reasonably well. About two years ago, the concept of G _max(η), based on the idea of the free-energy span model, has been proposed as a universal approach for the screening of electrocatalysts. In contrast to other available descriptor-based methods, G _max(η) factors overpotential and kinetic effects by a dedicated evacuation scheme of adsorption free energies into an analysis of trends. In the present perspective, we discuss the application of G _max(η) to different electrocatalytic processes, including the oxygen evolution and reduction reactions, the nitrogen reduction reaction, and the selectivity problem of the competing oxygen evolution and peroxide formation reactions, and we outline the advantages of this screening approach over previous investigations.

Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide

Electrochemical two-electron water oxidation reaction (2e-WOR) has drawn significant attention as a promising process to achieve the continuous on-site production of hydrogen peroxide (H₂O₂). However, compared to the cathodic H₂O₂ generation, the anodic 2e-WOR is more challenging to establish catalysts due to the severe oxidizing environment. In this study, we combine density functional theory (DFT) calculations with experiments to discover a stable and efficient perovskite catalyst for the anodic 2e-WOR. Our theoretical screening efforts identify LaAlO₃ perovskite as a stable, active, and selective candidate for catalyzing 2e-WOR. Our experimental results verify that LaAlO₃ achieves an overpotential of 510 mV at 10 mA cm^-2 in 4 M K₂CO₃/KHCO₃, lower than those of many reported metal oxide catalysts. In addition, LaAlO₃ maintains a stable H₂O₂ Faradaic efficiency with only a 3% decrease after 3 h at 2.7 V vs. RHE. This computation-experiment synergistic approach introduces another effective direction to discover promising catalysts for the harsh anodic 2e-WOR towards H₂O₂.

A novel three-dimensional flow-through graphite felt-matrix cathode for in-situ hydrogen peroxide generation in multi-environment systems-Multiphysics modeling for in-situ hydrogen peroxide generation

In order to achieve in-situ H₂O₂ generation in multi-environment systems, polyvinylpyrrolidone (PVP) and modified carbon nitride (t-g-C₃N₄) are co-doped onto graphite felt-matrix (GF-matrix) by electrodeposition to develop a novel cathode electrode. By means of 3D-X-ray CT, High-Resolution Transmission Electron Microscope (HRTEM), X-ray Photoelectron Spectrometer (XPS), Raman and Electrochemical Workstation, microscopic physical-chemical properties of materials are researched to optimize the electrode structure. Results show that the optimal electrode presents over H₂O₂ production rate of 2000 mgL^-1·h^-1, and as high as current efficiency of 93% to 98% in simulated freshwater (50 mM Na₂SO₄, pH = 1-12) at 20 mAcm^-2. Furthermore, we built an original three-dimensional (3D) flow-through GF-matrix cathode model on H₂O₂ generation in simulated freshwater, explaining solution pH change reasons from solution inlet to outlet.

Novel Synthesis of Sensitive Cu-ZnO Nanorod-Based Sensor for Hydrogen Peroxide Sensing

We aimed to synthesize sensitive electrochemical sensors for hydrogen peroxide sensing by using zinc oxide nanorods grown on a fluorine-doped tin oxide electrode by using the facial hydrothermal method. It was essential to keep the surface morphology of the material (nanorods structure); due to its large surface area, the concerned material has enhanced detection ability toward the analyte. The work presents a non-enzymatic H₂O₂ sensor using vertically grown zinc oxide nanorods on the electrode (FTO) surfaces with Cu nanoparticles deposited on zinc oxide nanorods to enhance the activity. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-Ray (EDX), X-ray diffraction (XRD), and electrochemical methods were used to characterize copper–zinc oxide nanorods. In addition to the high surface area, the hexagonal Cu-ZnO nanorods exhibited enhanced electrochemical features of H₂O₂ oxidation. Nanorods made from Cu-ZnO exhibit highly efficient sensitivity of 3415 μAmM⁻¹cm⁻² low detection limits (LODs) of 0.16 μM and extremely wide linear ranges (0.001–11 mM). In addition, copper-zinc oxide nanorods demonstrated decent reproducibility, repeatability, stability, and selectivity after being used for H₂O₂ sensing in water samples with an RSD value of 3.83%. Cu nanoparticles decorated on ZnO nanorods demonstrate excellent potential for the detection of hydrogen peroxide, providing a new way to prepare hydrogen peroxide detecting devices.

Enhancement of electrocatalytic oxygen evolution by chiral molecular functionalization of hybrid 2D electrodes

A sustainable future requires highly efficient energy conversion and storage processes, where electrocatalysis plays a crucial role. The activity of an electrocatalyst is governed by the binding energy towards the reaction intermediates, while the scaling relationships prevent the improvement of a catalytic system over its volcano-plot limits. To overcome these limitations, unconventional methods that are not fully determined by the surface binding energy can be helpful. Here, we use organic chiral molecules, i.e., hetero-helicenes such as thiadiazole-[7]helicene and bis(thiadiazole)-[8]helicene, to boost the oxygen evolution reaction (OER) by up to ca. 130 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D Ni- and NiFe-based catalysts via a spin-polarization mechanism. Our results show that chiral molecule-functionalization is able to increase the OER activity of catalysts beyond the volcano limits. A guideline for optimizing the catalytic activity via chiral molecular functionalization of hybrid 2D electrodes is given.

CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide

Electrochemical water oxidation reaction (WOR) to hydrogen peroxide (H₂O₂) via a 2e⁻ pathway provides a sustainable H₂O₂ synthetic route, but is challenged by the traditional 4e⁻ counterpart of oxygen evolution. Here we report a CO₂/carbonate mediation approach to steering the WOR pathway from 4e⁻ to 2e⁻. Using fluorine-doped tin oxide electrode in carbonate solutions, we achieved high H₂O₂ selectivity of up to 87%, and delivered unprecedented H₂O₂ partial currents of up to 1.3 A cm⁻², which represents orders of magnitude improvement compared to literature. Molecular dynamics simulations, coupled with electron paramagnetic resonance and isotope labeling experiments, suggested that carbonate mediates the WOR pathway to H₂O₂ through the formation of carbonate radical and percarbonate intermediates. The high selectivity, industrial-relevant activity, and good durability open up practical opportunities for delocalized H₂O₂ production.

Electrochemical H₂O oxidation to H₂O₂ is challenged by the competitive O₂ evolution reaction. Here, the authors report a CO₂/carbonate mediation approach to steering the H₂O oxidation pathway from O₂ evolution to H₂O₂ generation.

Exchange Functionals and Basis Sets for Density Functional Theory Studies of Water Splitting on Selected ZnO Nanocluster Catalysts

In this communication, we use density functional theory (DFT) to study the structural (geometry) and electronic properties (vertical detachment energy and electron affinity) of ZnO monomers and dimers that can be used to form ZnO clusters of different sizes, with a view to adapting one or more of them as catalysts or photocatalysts, standing alone or on suitable substrates like graphene, to split water. We also investigate different pairs of exchange functionals and basis sets to optimize their choice in our DFT calculations and to compare the singlet-triplet energy gaps of small ZnO clusters of different sizes to select an optimal cluster size for water splitting. We find that the B3LYP/DGDZVP2 exchange functional/basis set is a reliable combination for use with DFT to calculate the geometry and electronic properties of small ZnO nanoclusters from among several other combinations of exchange functionals and basis sets. Comparisons of the singlet-triplet energy gaps show that the trimer (ZnO)₃ has an energy gap of 58.66 k cal/mol. which is approximately equal to the energy of a visible photon at a wavelength of 500 nm, and a HOMO-LUMO gap of 4.4 eV, making it a suitable choice of photocatalyst for the oxidation of water from among six (ZnO) _n nanoclusters of monomers, with n ranging from 1 to 6. We used this exchange functional/basis set to study the structural and energetic details of hydration and hydrolysis of water absorbed on the (ZnO)₃ nanocatalyst and calculated the corresponding potential energy profiles to identify three sets of singlet-triplet pathways for water splitting. Detailed study of a pathway showed that oxygen is produced after hydrogen, and the rate-determining step is the formation of hydrogen.

ZnO nanoparticles embedded in hollow carbon fiber membrane for electrochemical H2O2 production by two-electron water oxidation reaction

Electrochemical two-electron water oxidation reaction (2e-WOR) provides a promising alternative route for hydrogen peroxide (H₂O₂) production, where the design of earth abundant and environmentally friendly electrocatalysts with both high selectivity and production rate is crucial. Here we report the synthesis of ZnO nanoparticles embedded in hollow carbon fiber membrane as efficient 2e-WOR electrocatalyst by a metal-organic framework engaged electrospinning-pyrolysis method. The resultant ZnO@carbon composite fiber exhibits a foam-like hierarchical structure composed of interconnected hollow carbon nanocubes encapsulated with oxygen vacancy rich ZnO nanocrystals. Owing to the improved selectivity of ZnO, excellent conductivity of carbon fiber, promoted active site exposure and mass transfer of hollow structure, the free-standing membrane electrode shows superior 2e-WOR performances with high selectivity (83.8% at 2.8 V vs. RHE), H₂O₂ generation rate (19.7 μmol cm^-2 min^-1) and robust stability.

Electrochemical Oxidation of Sodium Metabisulfite for Sensing Zinc Oxide Nanoparticles Deposited on Graphite Electrode

A novel concept was successfully evaluated for the electrochemical quantitative analysis of zinc oxide nanoparticles originally in aqueous suspension. An aliquot of the suspension was first placed on the working area of a graphite screen-printed electrode and the water was evaporated to form a dry deposit of ZnO nanoparticles. Deposition of ZnO nanoparticles on the electrode was confirmed by energy-dispersive X-ray spectroscopy. A probe solution containing KCl and sodium metabisulfite was added on top of the deposit for electrochemical analysis by cyclic voltammetry. The anodic peak current (Ipa) for metabisulfite, measured at +1.2 V vs. Ag/AgCl, afforded a lower detection limit of 3 µg and exhibited a linear dependence on the mass of deposited ZnO nanoparticles up to 15 μg. Further, the current increased nonlinearly until it reached a saturation level beyond 60 μg of ZnO nanoparticles. The diffusion coefficient of metabisulfite anions through the electrical double layer was determined to be 4.16 × 10−5 cm2/s. Apparently the surface reactivity of ZnO originated from the oxide anion rather than the superoxide anion or the hydroxyl radical. Enhancement of the metabisulfite oxidation peak current can be developed into a sensitive method for the quantitation of ZnO nanoparticles.

Carbonate-Induced Electrosynthesis of Hydrogen Peroxide via Two-Electron Water Oxidation

Electrochemical synthesis of hydrogen peroxide (H₂ O₂ ), via the two-electron water oxidation reaction (2e^- WOR), is an attractive method for the sustainable production of valuable chemicals in place of oxygen during water electrolysis. While the majority of 2e^- WOR studies have focussed on electrocatalyst design, little research has been carried out on the selection of the supporting electrolyte. In this work, we investigate the impact of potassium carbonate (K₂ CO₃ ) electrolytes, and their key properties, on H₂ O₂ production. We found that at electrolyte pH values (>9.5) where the carbonate anion (CO₃ ^2- ) was prevalent in the mixture, a 26.5 % increase in the Faraday efficiency (%FE) for H₂ O₂ production was achieved, compared to bicarbonate (HCO₃ ^- ) dominant solutions. Utilising boron-doped diamond (BDD) in highly concentrated K₂ CO₃ solutions, current densities of up to 511 mA cm^-2 (in 4 m) and %FEs of 91.5 % (in 5 m) could be attained. The results presented in this work highlight the influence of CO₃ ^2- on electrochemical H₂ O₂ generation via the 2e^- WOR and provide novel pathways to produce desirable commodities at the anode during electrochemical water splitting.

Unveiling the Atomic Structure and Growth Dynamics of One-Dimensional Water on ZnO(10-10)

The adsorption and organization state of water on the metal oxide surface is of critical importance for wide fields where interface chemistry dominates. On the technically important ZnO(10-10) surface, we found water assembles into an one-dimensional (1D) chain structure at submonolayer coverage instead of the well-known half-dissociated two-dimensional (2D) island. With a combination of high resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we clearly distinguished the single and double water chains, which are composed of dissociated monomers and half-dissociated dimers, respectively. Moreover, we unambiguously determined that single water molecules dissociate spontaneously before agglomerating into ordered phase, which is contrary to the proposition of previous studies. These results have deepened our understandings of the adsorbed water species on the ZnO surface, which may bring new insights into the mechanisms of water-stimulated surface reactions.

Nanostructure Shape-Effects in ZnO heterogeneous photocatalysis

Selective oxidation of alcohols is an essential reaction for fine chemical production. Here, the photocatalytic oxidation of benzyl alcohol by zinc oxide (ZnO) nanocrystals was investigated to clarify the mechanism of selective oxidation with this process. Reactivity when in contact with three distinct ZnO nanocrystal shapes: nanocones, nanorods and nanoplates, was studied in order to compare crystal facet-specific effects in the reaction system. The same non-hydrothermal and non-hydrolytic aminolysis method was used to synthesise all three nanocrystal shapes. The ZnO catalysts were characterized using by a range of techniques to establish the key properties of the prominent ZnO crystal facets exposed to the reaction medium. The ZnO nanocrystals photocatalysed the benzyl alcohol oxidation reaction when irradiated by a 370 - 375 nm LED output and each ZnO crystal morphology exhibited different reaction kinetics for the oxidation reaction. ZnO nanocones displayed the highest benzyl alcohol conversion rate while nanorods gave the lowest. This established a facet-dependent kinetic activity for the benzyl alcohol reaction of (101¯1) > (0001) > (101¯0). Experimental and density functional theory computation results confirm that the {101¯1} facet is a surface that exposes undercoordinated O atoms to the reaction medium, which explains why the reactant benzyl alcohol adsorption on this facet is the highest. Light irradiation can excite valence band electrons to the conduction band, which are then captured by O₂ molecules to yield superoxide (O₂^•-). In a non-aqueous solvent, the photogenerated holes oxidise benzyl alcohol to form a radical species, which reacts with O₂^•- to yield benzaldehyde. This results in 100% product selectivity for benzaldehyde, rather than the carboxylic acid derivative.

Electronic configuration modulation of tin dioxide by phosphorus dopant for pathway change in electrocatalytic water oxidation

Electronic configuration modulation of SnO2 nanoparticles was realized using a phosphorus dopant, contributing to the reaction pathway change of water oxidation.

Development of an alkaline electro-Fenton process based on the synthesis of H2O2 in bicarbonate electrolytes

An alkaline electro-Fenton process be proposed based on H2O2 synthesis in a bicarbonate electrolyte and H2O2 activation into ˙OH. This alkaline electro-Fenton process can achieve the degradation of organic pollutants in alkaline aqueous solution.

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth's atmosphere such as H₂O, O₂, N₂ and CO₂, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

The Significance of Properly Reporting Turnover Frequency in Electrocatalysis Research

For decades, turnover frequency (TOF) has served as an accurate descriptor of the intrinsic activity of a catalyst, including those in electrocatalytic reactions involving both fuel generation and fuel consumption. Unfortunately, in most of the recent reports in this area, TOF is often not properly reported or not reported at all, in contrast to the overpotentials at a benchmarking current density. The current density is significant in determining the apparent activity, but it is affected by catalyst-centric parasitic reactions, electrolyte-centric competing reactions, and capacitance. Luckily, a properly calculated TOF can precisely give the intrinsic activity free from these phenomena in electrocatalysis. In this Viewpoint we ask: 1) What makes the commonly used activity markers unsuitable for intrinsic activity determination? 2) How can TOF reflect the intrinsic activity? 3) Why is TOF still underused in electrocatalysis? 4) What methods are used in TOF determination? and 5) What is essential in the more accurate calculation of TOF? Finally, the significance of normalizing TOF by Faradaic efficiency (FE) is stressed and we give our views on the development of universal analytical tools to determine the exact number of active sites and real surface area for all kinds of materials.

Microwave-assisted assembly of Ag2O-ZnO composite nanocones for electrochemical detection of 4-Nitrophenol and assessment of their photocatalytic activity towards degradation of 4-Nitrophenol and Methylene blue dye

4-Nitrophenol (4-NP) is an extensively utilized industrial chemical and one of major toxic water pollutant. Therefore, there is an urgent need to monitor the levels of 4-NP from environmental samples as well as its eradication are extremely important. Keeping this as a motivation, this research for the first-time reports microwave-assisted cost-effective synthesis of silver oxide (Ag₂O)-zinc oxide (ZnO) composite nanocones (CNCs, 80-100 nm) for simultaneous electrochemical detection and photodegradation of 4-NP from aqueous solutions. The Ag₂O-ZnO CNCs modified gold electrode was fabricated for electrochemical detection of 4-NP. Such fabricated sensor exhibited a sensitivity of 1.6 µA µM^-1cm^-2, wide linear detection range of 0.4-26 µM & 28-326 µM, and a low limit of detection of 23 nM. The sensor also exhibited good selectivity in real water samples. Also, an outstanding photocatalytic performance of Ag₂O-ZnO CNCs was evaluated towards UV-assisted degradation of 4-NP and organic water pollutant dye, methylene blue. The Ag₂O-ZnO CNCs exhibited excellent electro- and photocatalytic activities due to the formation of p-n nano-heterojunction comprising of p-type Ag₂O and n-type ZnO semiconductor nanoparticles within the composite. Therefore, herein reported smart CNCs can be projected as applied nano-system for cost-effective and rapid simultaneous detection and removal of 4-NP from aqueous solutions. Such nano-system can be useful for industrial application where detection and removal of 4-NP is a key issue to resolve.

Influence of calcination temperature on photocatalytic H2O2productivity of hierarchical porous ZnO microspheres

Photocatalytic production of H₂O₂from water and atmospheric oxygen has been recognized as a green and sustainable chemical process, due to the abundance of raw materials and sustainable solar energy. Herein, flower-like hierarchical ZnO microspheres were prepared by hydrothermal method followed by calcination at different temperatures, and their photocatalytic performance in H₂O₂production was examined under simulated sunlight irradiation. The calcination temperature plays a vital role in the structure, morphology, and surface area of the final ZnO products as well as their optical and electrochemical properties, which are determining factors in their photocatalytic activity. The ZnO calcined at 300 °C (Zn-300) exhibits the highest activity and optimal stability, showing productivity of 2793μmol l^-1within 60 min of irradiation, which was 6.5 times higher than that of uncalcined ZnO precursor. The remarkable photocatalytic activity is attributed to enhanced light utilization, large surface area, abundant exposed active sites, improved separation efficiency, and prolonged carrier lifespan. Moreover, the results from cycling experiments indicate the as-prepared ZnO samples exhibit good stability and long-time performance. This work provides useful information for the preparation of hierarchical ZnO photocatalysts.