A Half-Reaction Alternative to Water Oxidation: Chloride Oxidation to Chlorine Catalyzed by Silver Ion

Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as a solar fuels half-reaction. Ag­(I) is potentially an oxidative catalyst but is inhibited by the high potentials for accessing the Ag­(II/I) and Ag­(III/II) couples. We report here that the complex ions AgCl₂^– and AgCl₃^2– form in concentrated Cl^– solutions, avoiding AgCl precipitation and providing access to the higher oxidation states by delocalizing the oxidative charge over the Cl^– ligands. Catalysis is homogeneous and occurs at high rates and low overpotentials (10 mV at the onset) with μM Ag­(I). Catalysis is enhanced in D₂O as solvent, with a significant H₂O/D₂O inverse kinetic isotope effect of 0.25. The results of computational studies suggest that Cl^– oxidation occurs by 1e^– oxidation of AgCl₃^2– to AgCl₃^– at a decreased potential, followed by Cl^– coordination, presumably to form AgCl₄^2– as an intermediate. Adding a second Cl^– results in “redox potential leveling”, with further oxidation to {AgCl₂(Cl₂)}⁻ followed by Cl₂ release

Hierarchically Structured CuCo₂S₄ Nanowire Arrays as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Hydrogen produced from water splitting offers a green alternative to conventional energy such as fossil fuels. Herein, CuCo₂S₄ nanowire arrays were synthesized on a nickel foam substrate by a two-step hydrothermal approach and utilized as highly efficient bifunctional electrocatalyst for overall water splitting. The CuCo₂S₄ nanowire arrays were identified as an exceptionally active catalyst for the hydrogen evolution reaction (HER) in a basic solution with an extremely low overpotential of 65 mV to reach a current density of 10 mA/cm². The hierarchically structured CuCo₂S₄ electrode was also highly active toward the oxygen evolution reaction (OER), achieving a high current density of 100 mA/cm² at an overpotential of only 310 mV. Consequently, an alkaline electrolyzer constructed using CuCo₂S₄ nanowire arrays as both anode and cathode can realize overall water splitting with a current density of 100 mA/cm² at a cell voltage of 1.65 V, suggesting a promising bifunctional electrocatalyst for efficient overall water splitting

In Situ Preparation of Pt Nanoparticles Supported on N‑Doped Carbon as Highly Efficient Electrocatalysts for Hydrogen Production

We describe here that the electrode materials toward the hydrogen evolution reaction (HER) can be cathodically activated by anodic dissolution of Pt counter electrode, dependent on the nature of substrate materials and solution pH. It leads to a direct approach for in situ fabrication of a highly dispersed and active HER electrocatalyst with minimal Pt loading that requires only a piece of Pt (instead of Pt salt, such as K₂PtCl₆) as Pt source combined with judicious choices of substrate materials and electrolyte solution. For a typical sample obtained by pyrolyzing poly­(2,6-diaminopyridine) (PDAP) under ammonia atmosphere followed by successive cyclic voltammetry scans in 0.5 M H₂SO₄, a current density of 60 mA cm^–2 was obtained at an overpotential of only 50 mV. Although the Pt loading is only 1.5 wt % in the sample, this performance is even better than that of the commercial 20 wt % Pt/C. The experimental results indicate that the deposited Pt nanoparticles are highly dispersed on the electrode substrate with a size of 2–4 nm. Further experimental results suggest that the combination of three factors, including the slow release of Pt into solution, high specific surface area of the substrate materials, and homogeneously doped N atoms acting as Pt anchor sites, is the key for formation of the highly active Pt nanoparticles. This study thus also raises an alarm regarding the use of Pt counter electrode in HER catalysis, especially by N-doped carbon in an acidic solution

Cs(I) Cation Enhanced Cu(II) Catalysis of Water Oxidation

We report here a new catalytic water oxidation system based on Cu­(II) ions and a remarkable countercation effect on the catalysis. In a concentrated fluoride solution at neutral to weakly basic pHs, simple Cu­(II) salts are highly active and robust in catalyzing water oxidation homogeneously. F^– in solution acts as a proton acceptor and an oxidatively robust ligand. F^– coordination prevents precipitation of Cu­(II) as CuF₂/Cu­(OH)₂ and lowers potentials for accessing high-oxidation-state Cu by delocalizing the oxidative charge over F^– ligands. Significantly, the catalytic current is greatly enhanced in a solution of CsF compared to those of KF and NaF. Although countercations are not directly involved in the catalytic redox cycle, UV–vis and ¹⁹F nuclear magnetic resonance measurements reveal that coordination of F^– to Cu­(II) is dependent on countercations by Coulombic interaction. A less intense interaction between F^– and well-solvated Cs⁺ as compared with Na⁺ and K⁺ leads to a more intense coordination of F^– to Cu­(II), which accounts for the improved catalytic performance

Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex

A self-assembly-formed tri­glycyl­glycine macro­cyclic ligand (TGG^4–) complex of Cu­(II), [(TGG^4–)­Cu^II–OH₂]^2–, efficiently catalyzes water oxidation in a phosphate buffer at pH 11 at room temperature by a well-defined mechanism. In the mechanism, initial oxidation to Cu­(III) is followed by further oxidation to a formal “Cu­(IV)” with formation of a peroxide intermediate, which undergoes further oxidation to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidation under these conditions with a high turnover frequency of 33 s^–1

Nonaqueous Electrocatalytic Oxidation of the Alkylaromatic Ethylbenzene by a Surface Bound Ru^V(O) Catalyst

The catalyst [Ru­(Mebimpy)­(4,4′-((HO)₂OPCH₂)₂bpy­(OH₂)]²⁺, where Mebimpy is 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine and 4,4′-((HO)₂OPCH₂)₂bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine, attached to nanocrystalline Sn­(IV)-doped In₂O₃ (nanoITO) electrodes (nanoITO|Ru^II–OH₂²⁺) has been utilized for the electrocatalytic oxidation of the alkylaromatics ethylbenzene, toluene, and cumene in propylene carbonate/water mixtures. Oxidative activation of the surface site to nanoITO|Ru^V(O)³⁺ is followed by hydrocarbon oxidation at the surface with a rate constant of 2.5 ± 0.2 M^–1 s^–1 (<i>I</i> = 0.1 M LiClO₄, <i>T</i> = 23 ± 2 °C) for the oxidation of ethylbenzene. Electrocatalytic oxidation of ethylbenzene to acetophenone occurs with a faradic efficiency of 95%. H/D kinetic isotope effects determined for oxidation of ethylbenzene point to a mechanism involving oxygen atom insertion into a C–H bond of ethylbenzene followed by further 2e^–/2H⁺ oxidation to acetophenone

Hierarchically Structured Cu-Based Electrocatalysts with Nanowires Array for Water Splitting

We report here the fabrication of CuO nanowires and their use as efficient electrocatalyst for the oxygen evolution reaction (OER) or as precursor for preparation of Cu₃P nanowires for the hydrogen evolution reaction (HER). The surface-bound Cu­(OH)₂ nanowires are <i>in situ</i> grown on a three-dimensional copper foam (CF) by anodic treatment, which are then converted to CuO nanowires by calcination in air. The direct growth of nanowires from the underlying conductive substrate can eliminate the use of any conductive agents and binders, which ensures good electrical contact between the electrocatalyst and the conductive substrate. The hierarchically nanostructured Cu-based electrode exhibits excellent catalytic performance toward OER in 1 M KOH solution. Phosphorization of the CuO/CF electrode generates the Cu₃P/CF electrode, which can act as an excellent electrocatalyst for HER in 1 M KOH. An alkaline electrolyzer is constructed using CuO and Cu₃P nanowires coated copper foams as anode and cathode, which can realize overall water splitting with a current density of 102 mA/cm² at an applied cell voltage of 2.2 V

Hierarchically Structured Ni Nanotube Array-Based Integrated Electrodes for Water Splitting

The development of high-performance nonprecious electrocatalysts for overall water splitting has attracted increasing attention but remains a vital challenge. Herein, we report a ZnO-based template method to fabricate Ni nanotube arrays (NTAs) anchored on nickel foil for applications in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). On the basis of this precursor electrode, the three-dimensional NiSe₂ NTAs of unique sandwich-like coaxial structure have been fabricated by electrodeposition of NiSe₂ on Ni NTAs, which exhibits high performance toward the HER in both acidic and alkaline media. The method based on Ni NTAs can be readily extended to fabricate Ni₂P NTAs by gas–solid phosphorization for the HER, and NiFeO_<i>x</i> NTAs by anodic codeposition of Ni and Fe for the OER. Consequently, an alkaline electrolyzer has been constructed using NiFeO_<i>x</i> NTAs and NiSe₂ NTAs as anode and cathode, respectively, which can realize overall water splitting with a current density of 100 mA cm^–2 at an overpotential of 510 mV

Real-Time Visualization of Diffusion-Controlled Nanowire Growth in Solution

This Letter shows that copper nanowires grow through the diffusion-controlled reduction of dihydroxycopper­(I), Cu­(OH)₂^–. A combination of potentiostatic coulometry, UV–visible spectroscopy, and thermodynamic calculations was used to determine the species adding to growing Cu nanowires is Cu­(OH)₂^–. Cyclic voltammetry was then used to measure the diffusion coefficient of Cu­(OH)₂^– in the reaction solution. Given the diameter of a Cu nanowire and the diffusion coefficient of Cu­(OH)₂^–, we calculated the dependence of the diffusion-limited growth rate on the concentration of copper ions to be 26 nm s^–1 mM^–1. Independent measurements of the nanowire growth rate with dark-field optical microscopy yielded 24 nm s^–1 mM^–1 for the growth rate dependence on the concentration of copper. Dependence of the nanowire growth rate on temperature yielded a low activation energy of 11.5 kJ mol^–1, consistent with diffusion-limited growth

High-Yield and Selective Photoelectrocatalytic Reduction of CO₂ to Formate by Metallic Copper Decorated Co₃O₄ Nanotube Arrays

Carbon dioxide (CO₂) reduction to useful chemicals is of great significance to global climate and energy supply. In this study, CO₂ has been photoelectrocatalytically reduced to formate at metallic Cu nanoparticles (Cu NPs) decorated Co₃O₄ nanotube arrays (NTs) with high yield and high selectivity of nearly 100%. Noticeably, up to 6.75 mmol·L^–1·cm^–2 of formate was produced in an 8 h photoelectrochemical process, representing one of the highest yields among those in the literature. The results of scanning electron microscopy, transmission electron microscopy and photoelectrochemical characterization demonstrated that the enhanced production of formate was attributable to the self-supported Co₃O₄ NTs/Co structure and the interface band structure of Co₃O₄ NTs and metallic Cu NPs. Furthermore, a possible two-electron reduction mechanism on the selective PEC CO₂ reduction to formate at the Cu–Co₃O₄ NTs was explored. The first electron reduction intermediate, CO_2 ads^•–, was adsorbed on Cu in the form of Cu–O. With the carbon atom suspended in solution, CO_2 ads^•– is readily protonated to form the HCOO^– radical. And HCOO^– as a product rapidly desorbs from the copper surface with a second electron transfer to the adsorbed species