Artificial photosynthesis for production of hydrogen peroxide and its fuel cells

Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety

A cobalt phthalocyanine having an electron-poor CoN4 (+δ) in its phthalocyanine moiety was presented as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR). We suggested that hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allows CoN4 to have the highly HPOR-active monovalent oxidation state and facilitates HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co─OOH- encourages an axially coordinated cobalt oxo complex (O═CoN4 ) to form, the O═CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction guaranteeing low-voltage hydrogen production is successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4 . Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V is obtained. Also, the techno-economic benefit of hydrogen peroxide as a hydrogen carrier is evaluated by comparing hydrogen peroxide with other hydrogen carriers such as ammonia and liquid organic hydrogen carriers.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Contribution of the Fenton reaction and ligninolytic enzymes to soil organic matter mineralisation under anoxic conditions

Mechanisms of carbon dioxide (CO₂) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H₂O₂ with Fe(II) yielding a hydroxyl radical (OH), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO₂ release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro)environments under high precipitation and explain intensive CO₂ release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO₂ released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled H₂¹⁸O₂ and Fe(II) to induce the FR and measured CO¹⁸O, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO₂ amount was released when the FR was induced in combination with LiP addition. The CO₂ efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of ¹⁸O from peroxide to CO₂ release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the CO¹⁸O derived from soil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils.

Are Polyaniline and Polypyrrole Electrocatalysts for Oxygen (O2) Reduction to Hydrogen Peroxide (H2O2)?

In this report, we present results on the electrocatalytic activity of conducting polymers [polyaniline (PANI) and polypyrrole (PPy)] toward the electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H₂O₂). The electropolymerization of the polymers and electrolysis conditions were optimized for H₂O₂ production. On flat glassy carbon (GC) electrodes, the faradaic efficiency (FE) for H₂O₂ production was significantly improved by the polymers. Rotating disc electrode (RDE) studies revealed that this is mainly a result of blocking further H₂O₂ to the water reduction pathway by the polymers. PPy on carbon paper (CP) significantly increased the molar production of H₂O₂ by over 250% at an average FE of above 95% compared to bare CP with a FE of 25%. Thus, the polymers are acting as catalysts on the electrode for the ORR, although their catalytic mechanisms differ from other electrocatalysts.

Analysis of Photosynthetic Systems and Their Applications with Mathematical and Computational Models

In biological and life science applications, photosynthesis is an important process that involves the absorption and transformation of sunlight into chemical energy. During the photosynthesis process, the light photons are captured by the green chlorophyll pigments in their photosynthetic antennae and further funneled to the reaction center. One of the most important light harvesting complexes that are highly important in the study of photosynthesis is the membrane-attached Fenna–Matthews–Olson (FMO) complex found in the green sulfur bacteria. In this review, we discuss the mathematical formulations and computational modeling of some of the light harvesting complexes including FMO. The most recent research developments in the photosynthetic light harvesting complexes are thoroughly discussed. The theoretical background related to the spectral density, quantum coherence and density functional theory has been elaborated. Furthermore, details about the transfer and excitation of energy in different sites of the FMO complex along with other vital photosynthetic light harvesting complexes have also been provided. Finally, we conclude this review by providing the current and potential applications in environmental science, energy, health and medicine, where such mathematical and computational studies of the photosynthesis and the light harvesting complexes can be readily integrated.

Mechanically Interlocked Carbon Nanotubes as a Stable Electrocatalytic Platform for Oxygen Reduction

Mechanically interlocking redox-active anthraquinone onto single-walled carbon nanotubes (AQ-MINT) gives a new and advanced example of a noncovalent architecture for an electrochemical platform. Electrochemical studies of AQ-MINT as an electrode reveal enhanced electrochemical stability in both aqueous and organic solvents compared to physisorbed AQ-based electrodes. While maintaining the electrochemical properties of the parent anthraquinone molecules, we observe a stable oxygen reduction reaction to hydrogen peroxide (H₂O₂). Using such AQ-MINT electrodes, 7 and 2 μmol of H₂O₂ are produced over 8 h under basic and neutral conditions, while the control system of SWCNTs produces 2.2 and 0.5 μmol, respectively. These results reveal the potential of this rotaxane-type immobilization approach for heterogenized electrocatalysis.

Novel homogeneous photocatalyst for oxygen to hydrogen peroxide reduction in aqueous media

An isoquinolinium-pyrrole donor-acceptor dyad was found to exhibit photocatalytic activity in oxygen-to-peroxide photoreduction with oxalate as a sacrificial electron donor. The concentration of hydrogen peroxide was shown to reach a plateau of 0.57 mM. The screening of related pyridinium-pyrrole dyads showed the importance of the isoquinoline moiety in securing the photocatalytic activity.

Removal of contaminants of emerging concern from urban wastewater in novel algal-bacterial photobioreactors

This work evaluates the removal of five pharmaceuticals and personal care products, i.e., ibuprofen, naproxen, salicylic acid, triclosan and propylparaben, from urban wastewater under two novel algal-bacterial photobioreactor settings. The first configuration (phase A) consisted of an anoxic-aerobic photobioreactor operating at a hydraulic retention time (HRT) of 2d at different concentrations of total organic carbon (TOC) (90mgL^-1-200mgL^-1). In the second configuration (phase B) an anaerobic step was introduced before the anoxic tank to set a photosynthetic A₂O process. In this phase, the HRT varied between 3 and 4d and the TOC was kept constant at 200mgL^-1. In addition, the impact of external aeration in the aerobic photobioreactor was assessed. The maximum removals for ibuprofen, naproxen, salicylic acid, triclosan and propylparaben (94±1%, 52±43%, 98±2%, 100±0%, 100±0%, respectively) were recorded during phase B. In phase A, low TOC concentrations triggered higher ibuprofen and naproxen removals likely due to the high contribution of biological oxidation on their removal. In phase B, total or very high removal efficiencies were observed for ibuprofen, propylparaben and triclosan independently on the operating conditions. In contrast, the removal efficiency of naproxen and salicylic acid decreased when the HRT dropped from 4 to 3d in the absence of external aeration, which suggests that biodegradation played a key role in their removal. In addition, sorption might have contributed to the elimination of triclosan and propylparaben from the wastewater.

General Observation of Photocatalytic Oxygen Reduction to Hydrogen Peroxide by Organic Semiconductor Thin Films and Colloidal Crystals

Low-cost semiconductor photocatalysts offer unique possibilities for industrial chemical transformations and energy conversion applications. We report that a range of organic semiconductors are capable of efficient photocatalytic oxygen reduction to H₂O₂ in aqueous conditions. These semiconductors, in the form of thin films, support a 2-electron/2-proton redox cycle involving photoreduction of dissolved O₂ to H₂O₂, with the concurrent photooxidation of organic substrates: formate, oxalate, and phenol. Photochemical oxygen reduction is observed in a pH range from 2 to 12. In cases where valence band energy of the semiconductor is energetically high, autoxidation competes with oxidation of the donors, and thus turnover numbers are low. Materials with deeper valence band energies afford higher stability and also oxidation of H₂O to O₂. We found increased H₂O₂ evolution rate for surfactant-stabilized nanoparticles versus planar thin films. These results evidence that photochemical O₂ reduction may be a widespread feature of organic semiconductors, and open potential avenues for organic semiconductors for catalytic applications.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes

We report on a self-assembled system comprising a molecular copper-porphyrin photoelectrocatalyst, 5-(4-carboxy-phenyl)-10,15,20-triphenylporphyrinatocopper(II) (CuTPP-COOH), covalently bound to self-organized, anodic titania nanotube arrays (TiO₂ NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin-covered TiO₂ NTs under cathodic polarization up to -0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H₂O₂ in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H₂O₂ upon irradiation is proven and quantified by direct colorimetric detection using 4-nitrophenyl boronic acid (p-NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO₂ NTs may ultimately allow for the preparation of a low-cost H₂O₂ evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Single-compartment hydrogen peroxide fuel cells with poly(3,4-ethylenedioxythiophene) cathodes

Efficient single-compartment hydrogen peroxide fuel cells can be fabricated using the conducting polymer poly(3,4-ethylenedioxythiophene), PEDOT as a cathodic electrocatalyst.

Tuning the reactivity of copper complexes supported by tridentate ligands leading to two-electron reduction of dioxygen

Dinuclear copper complex with tridentate ligand and anthracene linkage catalyses 2-electron reduction of O₂.

Fuel Production from Seawater and Fuel Cells Using Seawater

Seawater is the most abundant resource on our planet and fuel production from seawater has the notable advantage that it would not compete with growing demands for pure water. This Review focuses on the production of fuels from seawater and their direct use in fuel cells. Electrolysis of seawater under appropriate conditions affords hydrogen and dioxygen with 100 % faradaic efficiency without oxidation of chloride. Photoelectrocatalytic production of hydrogen from seawater provides a promising way to produce hydrogen with low cost and high efficiency. Microbial solar cells (MSCs) that use biofilms produced in seawater can generate electricity from sunlight without additional fuel because the products of photosynthesis can be utilized as electrode reactants, whereas the electrode products can be utilized as photosynthetic reactants. Another important source for hydrogen is hydrogen sulfide, which is abundantly found in Black Sea deep water. Hydrogen produced by electrolysis of Black Sea deep water can also be used in hydrogen fuel cells. Production of a fuel and its direct use in a fuel cell has been made possible for the first time by a combination of photocatalytic production of hydrogen peroxide from seawater and dioxygen in the air and its direct use in one-compartment hydrogen peroxide fuel cells to obtain electric power.

High Performance Reduction of H2O2 with an Electron Transport Decaheme Cytochrome on a Porous ITO Electrode

The decaheme cytochrome MtrC from Shewanella oneidensis MR-1 immobilized on an ITO electrode displays unprecedented H2O2 reduction activity. Although MtrC showed lower peroxidase activity in solution compared to horseradish peroxidase, the ten heme cofactors enable excellent electronic communication and a superior activity on the electrode surface. A hierarchical ITO electrode enabled optimal immobilization of MtrC and a high current density of 1 mA cm-2 at 0.4 V vs SHE could be obtained at pH 6.5 (Eonset = 0.72 V). UV-visible and Resonance Raman spectroelectrochemical studies suggest the formation of a high valent iron-oxo species as the catalytic intermediate. Our findings demonstrate the potential of multiheme cytochromes to catalyze technologically relevant reactions and establish MtrC as a new benchmark in biotechnological H2O2 reduction with scope for applications in fuel cells and biosensors.

Dual function photocatalysis of cyano-bridged heteronuclear metal complexes for water oxidation and two-electron reduction of dioxygen to produce hydrogen peroxide as a solar fuel

Hydrogen peroxide was produced from oxygen and water via visible-light irradiation of polymeric heteronuclear cyano-bridged complexes with the 2,2′-bipyridine ligand (bpy) (M^II[Ru^II(CN)₄(bpy)]; M^II = Ni^II, Fe^II and Mn^II).

Catalytic Formation of Hydrogen Peroxide from Coenzyme NADH and Dioxygen with a Water-Soluble Iridium Complex and a Ubiquinone Coenzyme Analogue

A ubiquinone coenzyme analogue (Q0: 2,3-dimethoxy-5-methyl-1,4-benzoquinone) was reduced by coenzyme NADH to yield the corresponding reduced form of Q0 (Q0H2) in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex (1: [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H2O)]2SO4) in water at ambient temperature as observed in the respiratory chain complex I (Complex I). In the catalytic cycle, the reduction of 1 by NADH produces the corresponding iridium hydride complex that in turn reduces Q0 to produce Q0H2. Q0H2 reduced dioxygen to yield hydrogen peroxide (H2O2) under slightly basic conditions. Catalytic generation of H2O2 was made possible in the reaction of O2 with NADH as the functional expression of NADH oxidase in white blood cells utilizing the redox cycle of Q0 as well as 1 for the first time in a nonenzymatic homogeneous reaction system.

Production of hydrogen peroxide by combination of semiconductor-photocatalysed oxidation of water and photocatalytic two-electron reduction of dioxygen

Hydrogen peroxide (H₂O₂) was produced from H₂O and O₂via semiconductor-photocatalysed oxidation of H₂O in combination with photocatalytic two-electron reduction of O₂ with [Ru^II((MeO)₂bpy)₃]²⁺ ((MeO)₂bpy = 4,4′-dimethoxy-2,2′-bipyridine) in water.

Two-phase oxidation of toluene derivatives by dioxygen using the 3-cyano-1-decylquinolinium ion as a photocatalyst

The two-phase photocatalytic oxidation of toluene by O₂ occurred efficiently using the 3-cyano-1-decylquinolinium ion in toluene with H₂O to produce the oxygenated products in the organic phase and H₂O₂ in the aqueous phase.