Copper(II) Catalysis of Water Oxidation

Iron-Catalyzed Water Oxidation: O-O Bond Formation via Intramolecular Oxo-Oxo Interaction

Herein, we report the importance of structure regulation on the O-O bond formation process in binuclear iron catalysts. Three complexes, [Fe₂ (μ-O)(OH₂ )₂ (TPA)₂ ]⁴⁺ (1), [Fe₂ (μ-O)(OH₂ )₂ (6-HPA)]⁴⁺ (2) and [Fe₂ (μ-O)(OH₂ )₂ (BPMAN)]⁴⁺ (3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO₃ solution (pH 8.4). We found that 1 and 2 are molecular catalysts and that O-O bond formation proceeds via oxo-oxo coupling rather than by the water nucleophilic attack (WNA) pathway. In contrast, complex 3 displays negligible catalytic activity. DFT calculations suggested that the anti to syn isomerization of the two high-valent Fe=O moieties in these catalysts takes place via the axial rotation of one Fe=O unit around the Fe-O-Fe center. This is followed by the O-O bond formation via an oxo-oxo coupling pathway at the Fe^IV Fe^IV state or via oxo-oxyl coupling pathway at the Fe^IV Fe^V state. Importantly, the rigid BPMAN ligand in complex 3 limits the anti to syn isomerization and axial rotation of the Fe=O moiety, which accounts for the negligible catalytic activity.

Water Oxidation at Neutral pH using a Highly Active Copper-Based Electrocatalyst

The sluggish kinetics of the oxygen evolution reaction (OER) at the anode severely limit hydrogen production at the cathode in water splitting systems. Although electrocatalytic systems based on cheap and earth-abundant copper catalysts have shown promise for water oxidation under basic conditions, only very few examples with high overpotential can be operated under acidic or neutral conditions, even though hydrogen evolution in the latter case is much easier. This work presents an efficient and robust Cu-based molecular catalyst, which self-assembles as a periodic film from its precursors under aqueous conditions on the surface of a glassy carbon electrode. This film catalyzes the OER under neutral conditions with impressively low overpotential. In controlled potential electrolysis, a stable catalytic current of 1.0 mA cm^-2 can be achieved at only 2.0 V (vs. RHE) and no significant decrease in the catalytic current is observed even after prolonged bulk electrolysis. The catalyst displays first-order kinetics and a single site mechanism for water oxidation with a TOF (k_cat ) of 0.6 s^-1 . DFT calculations on of the periodic Cu(TCA)₂ (HTCA=1-mesityl-1H-1,2,3-triazole-4-carboxylic acid) film reveal that TCA defects within the film create Cu^I active sites that provide a low overpotential route for OER, which involves Cu^I , Cu^II -OH, Cu^III =O and Cu^II -OOH intermediates and is enabled at a potential of 1.54 V (vs. RHE), requiring an overpotential of 0.31 V. This corresponds well with an overpotential of approximately 0.29 V obtained experimentally for the grown catalytic film after 100 CV cycles at pH 6. However, to reach a higher current density of 1 mA cm^-2 , an overpotential of 0.72 V is required.

Electronic Modulation of Nickel Disulfide toward Efficient Water Electrolysis

Developing highly efficient earth-abundant nickel-based compounds is an important step to realize hydrogen generation from water. Herein, the electronic modulation of the semiconducting NiS₂ by cation doping for advanced water electrolysis is reported. Both theoretical calculations and temperature-dependent resistivity measurements indicate the semiconductor-to-conductor transition of NiS₂ after Cu incorporation. Further calculations also suggest the advantages of Cu dopant to cathodic water electrolysis by bringing Gibbs free energy of H adsorption at both Ni sites and S sites much closer to zero. It is noteworthy that water dissociation on Cu-doped NiS₂ (Cu-NiS₂ ) surface is even more favorable than those on NiS₂ and Pt(111). Thus, the prepared Cu-NiS₂ shows noticeably improved performance toward alkaline hydrogen and oxygen evolution reactions (HER and OER). Specifically, it requires merely 232 mV OER overpotential to drive 10 mA cm^-2 ; in parallel with Tafel slopes of 46 mV dec^-1 . Regarding HER, an onset overpotential of only 68 mV is achieved. When integrated as both electrodes for water electrolysis, Cu-NiS₂ needs only 1.64 V to drive 10 mA cm^-2 , surpassing the state-of-the-art Ir/C-Pt/C couple (1.71 V). This work opens up an avenue to engineer low-cost and earth-abundant catalysts performing on par with the noble-metal-based one for water splitting.

Redox-Active Ligand Assisted Catalytic Water Oxidation by a RuIV =O Intermediate

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru^V =O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with Ru^IV =(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of Ru^IV =O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Co^II salts in the presence of HCO₃ ^- /CO₃ ^2- in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at E_pa ≈0.71 V (vs. Ag/AgCl) owing to the Co^III/II redox couple. (ii) A second wave is observed at E_pa ≈1.10 V with a considerably larger current. In which the Co^III undergoes oxidation to form a Co^IV species. The large current is attributed to catalytic oxidation of HCO₃ ^- /CO₃ ^2- to HCO₄ ^- . (iii) A process with very large currents at >1.2 V owing to the formation of Co^V (CO₃ )₃ ^- , which oxidizes both water and HCO₃ ^- /CO₃ ^2- . These processes depend on [Co^II ], [NaHCO₃ ], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Coⁿ (CO₃ )₃ ^n-6 , n=4, 5 are attainable at potentials similar to those experimentally observed.

Sodalite-type Cu-based Three-dimensional Metal-Organic Framework for Efficient Oxygen Reduction Reaction

Inspired by copper-based oxygen reduction biocatalysts, we have studied the electrocatalytic behavior of a Cu-based MOF (Cu-BTT) for oxygen reduction reaction (ORR) in alkaline medium. This catalyst reduces the oxygen at the onset (E_onset ) and half-wave potential (E^1/2 ) of 0. 940 V and 0.778 V, respectively. The high halfway potential supports the good activity of Cu-BTT MOF. The high ORR catalytic activity can be interpreted by the presence of nitrogen-rich ligand (tetrazole) and the generation of nascent copper(I) during the reaction. In addition to the excellent activity, Cu-BTT MOF showed exceptional stability too, which was confirmed through chronoamperometry study, where current was unchanged up to 12 h. Further, the 4-electrons transfer of ORR kinetics was confirmed by hydrodynamic voltammetry. The oxygen active center namely copper(I) generation during ORR has been understood by the reduction peak in cyclic voltammetry as well in the XPS analysis.

Controlled Synthesis of CuCo2 S4 @Ni(OH)2 Hybrid Nanorod Arrays for Water Splitting at an Ultralow Cell Voltage of 1.47 V

Developing environmentally friendly and highly active water splitting catalysts would be of great significance for clean energy conversion and utilization processes. Heterogeneous CuCo₂ S₄ @Ni(OH)₂ nanorod arrays with abundant oxygen vacancy firstly have been designed through a controllable hydrothermal and electrodeposition method. The synergies and open structures of the particular hierarchical structure together with the abundant oxygen vacancies offer more surface reactive centers, which can promote the electron transfer rate and reduce the activation energy of intermediate species. The CuCo₂ S₄ @Ni(OH)₂ -20 min nanorod arrays are considered as an excellent and robust electrocatalyst for the proton reduction under an alkaline condition with an extraordinary low overpotential of 117 mV at 10 mA cm^-2 . The CuCo₂ S₄ @Ni(OH)₂ -20 min heterostructures electrode is also stable and robust for the water oxidation reaction, needing an overpotential of only 250 mV to obtain 100 mA cm^-2 . Therefore, an alkaline electrolyzer was designed using CuCo₂ S₄ @Ni(OH)₂ -20 min nanorod arrays as bifunctional electrocatalyst, which can complete overall water splitting at a cell voltage of 1.47 V with 10 mA cm^-2 , suggesting a promising combination of the same material for efficient overall water splitting device. The cell voltage of 1.47 V, to our knowledge, is among the lowest values of the published support catalysts for electrocatalytic water splitting up to now.

Promotive Effect of Bicarbonate Ion on Two-Electron Water Oxidation to Form H2 O2 Catalyzed by Aluminum Porphyrins

Two-electron water oxidation initiated by one-electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four-electron pathway and could help to avoid the bottleneck subject of photon-flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two-electron water oxidation to form H₂ O₂ is reported. An addition of sodium carbonate (Na₂ CO₃ ) controlled both catalytic current and product selectivity of the two-electron water oxidation to enhance the activity of AlTMPyP at pH≈10-11. Controlled potential electrolysis experiments at different concentrations of Na₂ CO₃ (10-100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na₂ CO₃ ] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H₂ O₂ from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H₂ O₂ from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na₂ CO₃ on the catalytic reactivity was observed.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Homogeneous photochemical water oxidation with metal salophen complexes in neutral media

The development of water oxidation catalysts based on Earth-abundant metals that can function at neutral pH remains a basic chemical challenge.

Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron Complex with a Square Polypyridine Ligand: A DFT Study

The mononuclear [Cl-Fe^III(dpa)-Cl]⁺ (1_Cl) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1_Cl has to exchange with a water molecule to generate 1, [Cl-Fe^III(dpa)-OH₂]²⁺, as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl-Fe^IV(dpa)═O]⁺ (2). Another one-electron transfer from 2 leads to the formation of an Fe^V═O complex [Cl-Fe^V(dpa)═O]²⁺ (3), which triggers the critical O-O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a π-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O-O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol^-1. In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol^-1. In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O-O bond formation, while the aromatic carbon hydroxylation has a higher barrier.

Water oxidation by a copper(ii) complex: new findings, questions, challenges and a new hypothesis

We propose that an uncomplexed Cu(ii) ion or oxide is a candidate as a contributor to the observed catalysis in the presence of a Cu(ii) complex under water oxidation.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

The design of efficient and robust water oxidation catalysts has proven challenging in the development of artificial photosynthetic systems for solar energy harnessing and storage. Tremendous progress has been made in the development of homogeneous transition-metal complexes capable of mediating water oxidation. To improve the efficiency of the catalyst and to design new catalysts, a detailed mechanistic understanding is necessary. Quantum chemical modeling calculations have been successfully used to complement the experimental techniques to suggest a catalytic mechanism and identify all stationary points, including transition states for both O-O bond formation and O₂ release. In this review, recent progress in the applications of quantum chemical methods for the modeling of homogeneous water oxidation catalysis, covering various transition metals, including manganese, iron, cobalt, nickel, copper, ruthenium, and iridium, is discussed.

Mechanistic Studies on the Role of [CuII (CO3 )n ]2-2n as a Water Oxidation Catalyst: Carbonate as a Non-Innocent Ligand

Recently it was reported that copper bicarbonate/carbonate complexes are good electro-catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a Cu^III or a Cu^IV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that Cu^III (CO₃ )_n^3-2n complexes are the active intermediates in the electrolysis of Cu^II (CO₃ )_n^2-2n solution. The results enable the evaluation of E°[(Cu^III/II (CO₃ )_n )_aq ]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single-electron transfer from Cu^III (CO₃ )_n^3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to Cu^III results in the formation of C₂ O₆^2- by means of a second-order reaction of Cu^III (CO₃ )_n^3-2n . The results point out that carbonate stabilizes transition-metal cations at high oxidation states, not only as a good sigma donor, but also as a non-innocent ligand.

A Robust Molecular Catalyst Generated In Situ for Photo- and Electrochemical Water Oxidation

Water splitting is the key step towards artificial photosystems for solar energy conversion and storage in the form of chemical bonding. The oxidation of water is the bottle-neck of this process that hampers its practical utility; hence, efficient, robust, and easy to make catalytic systems based on cheap and earth-abundant materials are of exceptional importance. Herein, an in situ generated cobalt catalyst, [Co^II (TCA)₂ (H₂ O)₂ ] (TCA=1-mesityl-1,2,3-1H-triazole-4-carboxylate), that efficiently conducts photochemical water oxidation under near-neutral conditions is presented. The catalyst showed high stability under photolytic conditions for more than 3 h of photoirradiation. During electrochemical water oxidation, the catalytic system assembled a catalyst film, which proved not to be cobalt oxide/hydroxide as normally expected, but instead, and for the first time, generated a molecular cobalt complex that incorporated the organic ligand bound to cobalt ions. The catalyst film exhibited a low overpotential for electrocatalytic water oxidation (360 mV) and high oxygen evolution peak current densities of 9 and 2.7 mA cm^-2 on glassy carbon and indium-doped tin oxide electrodes, respectively, at only 1.49 and 1.39 V (versus a normal hydrogen electrode), respectively, under neutral conditions. This finding, exemplified on the in situ generated cobalt complex, might be applicable to other molecular systems and suggests that the formation of a catalytic film in electrochemical water oxidation experiments is not always an indication of catalyst decomposition and the formation of nanoparticles.

Carbonate Ions Induce Highly Efficient Electrocatalytic Water Oxidation by Cobalt Oxyhydroxide Nanoparticles

Synthetic models of oxygen evolving complex (OEC) are used not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also as water oxidation catalysts to realize artificial photosynthesis, which is anticipated as a promising solar fuel production system. However, although much attention has been paid to the composition and structure of active sites for development of heterogeneous OEC models, the cofactors, which are essential for water oxidation by the photosynthetic OEC, remain little studied. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO₃^2- cofactor. The possibility of CO₃^2- ions acting as proton acceptors for O-O bond formation based on the proton-concerted oxygen atom transfer mechanism is proposed. The O-O bond formation is supposed to be accelerated due to effective proton acceptance by adjacent CO₃^2- ions coordinated on the Co^IV center in the intermediate, which is consistent with Michaelis-Menten-type kinetics and the significant H/D isotope effect observed in electrocatalysis.

Electrochemical Formation of FeV (O) and Mechanism of Its Reaction with Water During O-O Bond Formation

A detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles Fe^III -bTAML), including the first electrochemical generation of Fe^V (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated Fe^V (O) as the active oxidant, formed due to two redox transitions, which were assigned as Fe^IV (O)/Fe^III (OH₂ ) and Fe^V (O)/Fe^IV (O). The spectral properties of both of these high-valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O-O bond-formation step occurs by nucleophilic attack of H₂ O on Fe^V (O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised Fe^V (O) in CH₃ CN and water was directly probed by electrochemistry and was found to be first-order in water. The pK_a value of the buffer base plays a critical role in the rate-determining step by increasing the reaction rate several-fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron-withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO₂ resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.

Crystalline Copper Phosphide Nanosheets as an Efficient Janus Catalyst for Overall Water Splitting

Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu₃P phosphide nanosheets on conductive nickel foam (Cu₃P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu₃P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm² requires an overpotential of only ∼320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ∼105 mV to achieve a catalytic current density of 10 mA cm^-2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu₃P@NF electrode, which showed a catalytic current density of 10 mA/cm² under an applied voltage of ∼1.67 V. The same current density can also be obtained using a silicon solar cell under ∼1.70 V for both the HER and the OER. This new Janus Cu₃P@NF electrode is made of inexpensive and nonprecious metal-based materials, which opens new possibilities based on copper to exploit overall water splitting for hydrogen production. To the best of our knowledge, such high performance of a copper-based water oxidation and overall water splitting catalyst has not been reported to date.

Self-Supported Cu-Based Nanowire Arrays as Noble-Metal-Free Electrocatalysts for Oxygen Evolution

Crystalline Cu-based nanowire arrays (NWAs) including Cu(OH)2 , CuO, Cu2 O, and CuOx are facilely grown on Cu foil and are found to act as highly efficient, low-cost, and robust electrocatalysts for the oxygen evolution reaction (OER). Impressively, this noble-metal-free 3 D Cu(OH)2 -NWAs/Cu foil electrode shows the highest catalytic activity with a Tafel slope of 86 mV dec(-1) , an overpotential (η) of about 530 mV at ∼10 mA cm(-2) (controlled-potential electrolysis method without iR correction) and almost 100 % Faradic efficiency, paralleling the performance of the state-of-the-art RuO2 OER catalyst in 0.1 m NaOH solution (pH 12.8). To the best of our knowledge, this work represents one of the best results ever reported on Cu-based OER systems.

Kagóme Cobalt(II)-Organic Layers as Robust Scaffolds for Highly Efficient Photocatalytic Oxygen Evolution

Two Kagóme cobalt(II)-organic layers of [Co3 (μ3 -OH)2 (bdc)2 ]n (1) and [Co3 (μ3 -OH)2 (chdc)2 ]n (2) (bdc=o-benzenedicarboxylate and chdc=1,2-cyclohexanedicarboxylate) that bear bridging OH(-) ligands were explored as water oxidation catalysts (WOCs) for photocatalytic O2 production. The activities of 1 and 2 towards H2 O oxidation were assessed by monitoring the in situ O2 concentration versus time in the reaction medium by utilizing a Clark-type oxygen electrode under photochemical conditions. The oxygen evolution rate (RO2 ) was 24.3 μmol s(-1)  g(-1) for 1 and 48.8 μmol s(-1)  g(-1) for 2 at pH 8.0. Photocatalytic reaction studies show that 1 and 2 exhibit enhanced activities toward the oxidation of water compared to commercial nanosized Co3 O4 . In scaled-up photoreactions, the pH value of the reaction medium decreased from 8.0 to around 7.0 after 20 min and the O2 production ceased. Based on the amounts of the sacrificial oxidant (K2 S2 O8 ) used, the yield of O2 produced is 49.6 % for 2 and 29.8 % for 1. However, the catalyst can be recycled without a significant loss of catalytic activity. Spectroscopic studies suggest that the structure and composition of recycled 1 and 2 are maintained. In isotope-labeling H2 (18) O (97 % enriched) experiments, the distribution of (16) O(16) O/(16) O(18) O/(18) O(18) O detected was 0:7.55:92.45, which is comparable to the theoretical values of 0.09:5.82:94.09. This work not only provides new catalysts that resemble ligand-protected cobalt oxide materials but also establishes the significance of the existence of OH(-) (or H2 O) binding sites at the metal center in WOCs.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H(+)/4 e(-) process, while oxygen can be fully reduced to water by a 4 e(-)/4 H(+) process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2(-). We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^- process, while oxygen can be fully reduced to water by a 4 e^-/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^-. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

CuO-Functionalized Silicon Photoanodes for Photoelectrochemical Water Splitting Devices

One main difficulty for the technological development of photoelectrochemical (PEC) water splitting (WS) devices is the fabrication of active, stable and cost-effective photoelectrodes that ensure high performance. Here, we report the development of a CuO/Silicon based photoanode, which shows an onset potential for the water oxidation of 0.53 V vs SCE at pH 9, that is, an overpotential of 75 mV, and high stability above 10 h. These values account for a photovoltage of 420 mV due to the absorbed photons by silicon, as proven by comparing with analogous CuO/FTO electrodes that are not photoactive. The photoanodes have been fabricated by sputtering a thin film of Cu(0) on commercially available n-type Si wafers, followed by a photoelectrochemical treatment in basic pH conditions. The resulting CuO/Cu layer acts as (1) protective layer to avoid the corrosion of nSi, (2) p-type hole conducting layer for efficient charge separation and transportation, and (3) electrocatalyst to reduce the overpotential of the water oxidation reaction. The low cost, low toxicity, and good performance of CuO-based coatings can be an attractive solution to functionalize unstable materials for solar energy conversion.

Copper as a robust and transparent electrocatalyst for water oxidation

Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu(III) and/or Cu(IV) , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu(II) in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu(II) and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.

Effects of morphology and exposed facets of α-Fe2O3 nanocrystals on photocatalytic water oxidation

The catalytic activity of α-Fe₂O₃ nanocubes, nanoplates, nanoflakes and nanoparticles for visible light-driven water oxidation is strongly morphology-dependent; α-Fe₂O₃ nanocubes with exposed {012} facets exhibit far higher activity than nanosheets with exposed {001} facets.