Defining the Role of Cr³⁺ as a Reductant in the Hydrothermal Synthesis of CuCrO₂ Delafossite

The synthesis of nanocrystalline, p-type delafossite metal oxides (CuMO2) via hydrothermal methods has been explored for a variety of energy conversion and storage applications. However, isolation of the pure phase ternary product is challenging due to the facile growth of unwanted, binary byproducts (CuO, Cu2O, and M2O3) which could ultimately influence the optoelectronic properties of the resulting nanocrystals. Here, we report on the optimized hydrothermal synthesis of CuCrO2 nanocrystals to limit the production of such byproducts. This material possesses a wide band gap and high reported conductivity, making it attractive for applications as the hole transport layer in a variety of heterojunction solar cells. An important aspect of this work is the consideration of Cr3+ as the reductant used to reduce Cu2+ to Cu+. This was confirmed by detection and quantification of CrO42– as a product of hydrothermal synthesis in addition to the fact that CuCrO2 purity was maximized at a ratio of 4:3 Cr/Cu, consistent with the proposed stoichiometric reaction: 4Cr3+ + 3Cu2+ + 20 OH– → 3CuCrO2 + CrO42– + 10 H2O. Using a 4:3 ratio of Cr/Cu starting materials and allowing the synthesis to proceed for 60 h eliminates the presence of CuO beyond detection by powder X-ray diffraction (pXRD). Furthermore, washing the solid product in 0.5 M NH4OH removes Cu2O and Cr2O3 impurities, leaving behind the isolated CuCrO2 product as confirmed using pXRD and inductively coupled plasma mass spectrometry

Co(II) Complex with a Covalently Attached Pendent Quinol Selectively Reduces O₂ to H₂O

A Co(II) complex with the polydentate quinol-containing ligand H2qp1 acts as an efficient electrocatalyst for oxygen reduction. Without any additional electron–proton transfer mediators, the electrocatalysis is selective for H2O; a related complex that substitutes a phenol for the quinol, conversely, instead produces mostly H2O2 under the same conditions. We propose that the ability of the redox-active quinol to donate two electrons impacts the product-determining step

Nickel-Based Two-Electron Redox Shuttle for Dye-Sensitized Solar Cells in Low Light Applications

Dye-sensitized solar cells (DSCs) are important to indoor solar powered devices and energy sustainable buildings because of their remarkable performance under indoor/ambient light conditions. Triiodide/iodide (I3–/I–) has been used as the most common redox mediator in DSCs because of its desirable kinetic properties and multielectron redox cycle. However, the low redox potential, corrosiveness, competitive visible light absorption, and lack of tunability of this redox mediator limit its performance in many DSC devices. Here we report a class of transition metal complex redox shuttles which operate on a similar multielectron redox cycle as I3–/I– while maintaining desirable kinetics and improving on its limitations. These complexes, nickel dithiocarbamates, were evaluated as redox shuttles in DSCs, which exhibited excellent performance under low light conditions. The recombination behavior of the redox shuttles with electrons in TiO2, dye regeneration behavior, and counter electrode electron transfer resistance were studied via chronoamperometry and electrochemical impedance spectroscopy (EIS). Further, DSC devices were studied with the Ni-based redox shuttles via incident photon-to-current conversion efficiencies (IPCEs) and current–voltage (J–V) curves under varied light intensities. The Ni-based redox shuttles showed up to 20.4% power conversion efficiency under fluorescent illumination, which was higher than I3–/I–-based devices (13%) at similar electrolyte concentrations. Taken together, these results show that nickel dithiocarbamate redox shuttles have faster rates of dye regeneration than the I3–/I– shuttle but suffer from faster recombination of photoinjected electrons with oxidized Ni­(IV) species, which decrease photovoltages

Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Herein, we investigated the redox cycle of nickel(II) bis(diethyldithiocarbamate), NiII(dtc)2, for potential use as a multielectron storage catholyte in nonaqueous redox flow batteries (RFBs). Previous studies have shown that the unique redox cycle of NiII(dtc)2 offers 2e– chemistry upon oxidation from NiII → NiIV but 1e– chemistry upon reduction from NiIV → NiIII → NiII. Electrochemical experiments presented here show that the addition of as little as 10 mol % ZnII(ClO4)2 to the electrolyte consolidates the two 1e– reduction peaks into a single 2e– reduction where [NiIV(dtc)3]+ is reduced directly to NiII(dtc)2. This catalytic enhancement is believed to be due to ZnII removal of a dtc– ligand from a NiIII(dtc)3 intermediate, resulting in more facile reduction to NiII(dtc)2. The addition of ZnII also improves the 2e– oxidation, shifting the anodic peak negative and decreasing the 2e– peak separation. H-cell cycling experiments showed that 97% Coulombic efficiency and 98% charge storage efficiency was maintained for 50 cycles over 25 h using 0.1 M ZnII(ClO4)2 as the supporting electrolyte. If ZnII(ClO4)2 was replaced with TBAPF6 in the electrolyte, the Coulombic efficiency fell to 78%. The use of ZnII to increase the reversibility of 2e– transfer is a promising result that points to the ability to use nickel dithiocarbonates for multielectron storage in RFBs

Chloride Ion-Pairing with Ru(II) Polypyridyl Compounds in Dichloromethane

Chloride ion-pairing with a series of four dicationic Ru­(II) polypyridyl compounds of the general form [Ru­(bpy)_3–<i>x</i>(deeb)_<i>x</i>]­(PF₆)₂, where bpy is 2,2′-bipyridine and deeb is 4,4′-diethylester-2,2′-bipyridine, was observed in dichloromethane solution. The heteroleptic compounds [Ru­(bpy)₂(deeb)]²⁺ and [Ru­(bpy)­(deeb)₂]²⁺ were found to be far less sensitive to ligand loss photochemistry than were the homoleptic compounds [Ru­(bpy)₃]²⁺ and [Ru­(deeb)₃]²⁺ and were thus quantified in most detail. X-ray crystal structure and ¹H NMR analysis showed that, when present, the C-3/C-3′ position of bpy was the preferred site for adduct formation with chloride. Ion-pairing was manifest in UV–visible absorption spectral changes observed during titrations with TBACl, where TBA is tetrabutyl ammonium. A modified Benesi–Hildebrand analysis yielded equilibrium constants for ion-pairing that ranged from 13 700 to 64 000 M^–1 and increased with the number of deeb ligands present. A Job plot indicated a 2:1 chloride-to-ruthenium complex ratio in the ion-paired state. The chloride ion was found to decrease both the excited state lifetime and the quantum yield for photoluminescence. Nonlinear Stern–Volmer plots were observed that plateaued at high chloride concentrations. The radiative rate constants decreased and the nonradiative rate constants increased with chloride concentration in a manner consistent with theory for radiative rate constants and the energy gap law. Equilibrium constants for excited state ion-pairing abstracted from such data were found to be significantly larger than that measured for the ground state. Photophysical studies of hydroxide and bromide ion-pairing with [Ru­(bpy)₂(deeb)]²⁺ are also reported

Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes

We report on the synthesis and photophysical properties of a photocathode consisting of a molecular bilayer structure self-assembled on p-type NiO nanostructured films. The resulting photocathode and its nanostructured indium–tin oxide analog absorb visible light and convert it into injected holes with injection yields of ∼30%, measured at the first observation time by nanosecond transient absorption spectroscopy, and long-lived reducing equivalents that last for several milliseconds without applied bias. An initial quantum yield of 15% was achieved for photogeneration of the reduced dye on the p-NiO electrode. Nanosecond transient absorption experiments and detailed analyses of the underlying electron transfer steps demonstrate that the overall efficiency of the cell is limited by hole injection and charge recombination processes. Compared with the highly doped indium–tin oxide photocathode, the NiO photocathode shows superior photoconversion efficiencies for generating reducing equivalents and longer lifetimes of surface-bound redox-separated states due to an inhibition toward charge recombination with the external assembly

Evidence and Influence of Copper Vacancies in p‑Type CuGaO₂ Mesoporous Films

Delafossite CuGaO2 nanocrystals were hydrothermally synthesized and characterized spectroscopically and electrochemically as mesoporous thin films. The nanocrystals demonstrate a preferred orientation within the film structure, as shown by enhancement of the (00l) peaks via two-dimensional powder X-ray diffraction. Annealing conditions of low and high temperature (i.e., 100–300 °C), with oxygen and/or argon atmospheres, were investigated, and the resulting effect on the thin film electrochemistry was measured. Cyclic voltammetry showed an increase in non-faradaic current with higher annealing temperatures and demonstrated a quasi-reversible redox feature (E1/2 = 0.1 V vs Fc+1/0). This feature is assigned to a CuII/CuI redox couple associated with surface defects. X-ray photoelectron and energy dispersive spectroscopies provide evidence for CuII surface defects and copper vacancies. Electrochemical impedance spectroscopy revealed that CuGaO2 films were highly conductive with σ ∼ 10–5 Ω–1 cm–1, consistent with a large density of hole carriers induced by copper vacancies. The significance of synthesis, film preparation, and annealing conditions on the presence of surface defects and large hole densities is discussed. The prevalence of such defects in delafossite CuGaO2 is expected to have a large impact on the use of this material as a hole transport layer in solar cell architectures

Inner Layer Control of Performance in a Dye-Sensitized Photoelectrosynthesis Cell

Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for water splitting into H2 and O2. An important element in the design of the photoanode in these devices is a core/shell structure which controls local electron transfer dynamics. Here, we introduce a new element, an internal layer of Al2O3 lying between the Sb:SnO2/TiO2 layers in a core/shell electrode which can improve photocurrents by up to 300%. In these structures, the results of photocurrent, transient absorption, and linear scan voltammetry measurements point to an important role for the Al2O3 layer in controlling internal electron transfer within the core/shell structure

Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate

The unique redox cycle of NiII(dtc)2, where dtc– is N,N-diethyldithiocarbamate, in acetonitrile displays 2e– redox chemistry upon oxidation from NiII(dtc)2 → [NiIV(dtc)3]+ but 1e– redox chemistry upon reduction from [NiIV(dtc)3]+ → NiIII(dtc)3 → NiII(dtc)2. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate NiII(dtc)2 and six-coordinate [NiIV(dtc)3]+. Cyclic voltammetry (CV) experiments show that these 1e– and 2e– pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1e– ligand-coupled electron transfer (LCET) redox wave, which produced a mixture of pyridine-bound Ni­(III) complexes, [NiIII(dtc)2(L)]+, and [NiIII(dtc)2(L)2]+. Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation of trans-[NiIII(dtc)2(L)2]+. Density functional theory calculations were also used to support the formation of pyridine coordinated Ni­(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g., 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [NiIII(dtc)2(L)x]+ to be reduced directly back to NiII(dtc)2 + xL. For weakly basic pyridines (e.g., 3-bromopyridine) and/or slow scan rates, [NiIII(dtc)2(L)x]+ decayed irreversibly to form [NiIV(dtc)3]+. Detailed kinetics studies using CV reveal that [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([NiIII(dtc)2(L)2]+ → [NiIII(dtc)2(L)]+ + L) along with decomposition of [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ species were found to increase with the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate-limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1e– intermediates along a 2e– oxidation path