Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru­(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru­(N^∧C^∧N)­(N^∧N^∧N) (Ru­[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N^∧C^∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N^∧N^∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH₃CN solution, electron transfer to form the Ru­(III)–NDI^– occurs with a rate constant <i>k</i>_f = 1.1 × 10¹⁰ s^–1 (τ_f = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant <i>k</i>_b = 4.1 × 10⁹ s^–1 (τ_b = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the NDI to produce the interfacial charge-separated state [NiO­(h⁺)–Ru­(II)–NDI^–] with a rate constant <i>k</i>_f = 9.1 × 10¹¹ s^–1 (τ_f = 1.1 ps). The recovery of the ground state occurs with a multiexponential rate constant <i>k</i>_b = 2.3 × 10⁹ s^–1 (τ_b = 426 ps). The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τ_b = 371 ps). The fast formation of interfacial charge separated state is a result of ultrafast hole injection resulting in the reduced form of sensitizer, which provides a larger driving force for NDI reduction. The kinetic study suggests that Ru­(II) chromophore–acceptor dyads are promising sensitizers for the NiO photocathode devices

Cyclometalated Ruthenium Sensitizers Bearing a Triphenylamino Group for p‑Type NiO Dye-Sensitized Solar Cells

We report the synthesis, photophysical, and electrochemical studies of a series of cyclometalated ruthenium sensitizers carrying triphenylamino linkers for p-type NiO dye-sensitized solar cells (DSSCs). The general structure of these ruthenium sensitizers is Ru­[N^∧N]₂[N^∧C], where [N^∧N] is a diimine ligand and [N^∧C] is a cyclometalated ligand. The triphenylamino group is attached to the <i>-para</i> position of the ruthenium–carbon bond of the [N^∧C] ligand as a linker to bridge the ruthenium chromophore and the NiO surface and to enhance the electronic coupling for hole injection. As a result, cells made with these sensitizers generate higher short-circuit currents (<i>J</i>_sc) than cells sensitized with our prior sensitizers with phenylene linkers. Morever the N^∧N ligands are systematically tuned from 2,2′-bipyridine (<b>O3</b>), to 1,10-phenanthroline (<b>O13</b>), and to bathophenanthroline (<b>O17</b>). Following the series, the conjugation of the N^̂N ligand is increased, which results in the enhancement of extinction coefficient and the red shift of light absorption. However the solar cell sensitized with <b>O3</b> still gives the largest <i>J</i>_sc of 3.04 mA/cm². The large <i>J</i>_sc highlights the promising potential of using these cyclometalated ruthenium sensitizers for NiO DSSCs. In addition, the carrier dynamics of these solar cells has been systematically studied by intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS). The results suggest that the <b>O3</b> solar cell giving the largest <i>J</i>_sc is likely caused by the slow geminate charge recombination and efficient dye regeneration

Reversible Dendrite-Free Potassium Plating and Stripping Electrochemistry for Potassium Secondary Batteries

Rechargeable potassium metal batteries have recently emerged as alternative energy storage devices beyond lithium-ion batteries. However, potassium metal anodes suffer from poor reversibility during plating and stripping processes due to their high reactivity and unstable solid electrolyte interphase (SEI). Herein, it is reported for the first time that a potassium bis­(fluoro­slufonyl)­imide (KFSI)-dimethoxy­ethane (DME) electrolyte forms a uniform SEI on the surface of potassium enabling reversible potassium plating/stripping electrochemistry with high efficiency (∼99%) at ambient temperature. Furthermore, the superconcentrated KFSI-DME electrolyte shows excellent electrochemical stability up to 5 V (vs K/K⁺) which enables good compatibility with high-voltage cathodes. Full cells with potassium Prussian blue cathodes are demonstrated. Our work contributes toward the understanding of potassium plating/stripping electrochemistry and paves the way for the development of potassium metal battery technologies

Understanding the Crystallization Mechanism of Delafossite CuGaO₂ for Controlled Hydrothermal Synthesis of Nanoparticles and Nanoplates

The delafossite CuGaO₂ is an important p-type transparent conducting oxide for both fundamental science and industrial applications. An emerging application is for p-type dye-sensitized solar cells. Obtaining delafossite CuGaO₂ nanoparticles is challenging but desirable for efficient dye loading. In this work, the phase formation and crystal growth mechanism of delafossite CuGaO₂ under low-temperature (<250 °C) hydrothermal conditions are systematically studied. The stabilization of Cu^I cations in aqueous solution and the controlling of the hydrolysis of Ga^III species are two crucial factors that determine the phase formation. The oriented attachment (OA) growth is proposed as the crystal growth mechanism to explain the formation of large CuGaO₂ nanoplates. Importantly, by suppressing this OA process, delafossite CuGaO₂ nanoparticles that are 20 nm in size were successfully synthesized for the first time. Moreover, considering the structural and chemical similarities between the Cu-based delafossite series compounds, the understanding of the hydrothermal chemistry and crystallization mechanism of CuGaO₂ should also benefit syntheses of other similar delafossites such as CuAlO₂ and CuScO₂

2H-CuScO₂ Prepared by Low-Temperature Hydrothermal Methods and Post-Annealing Effects on Optical and Photoelectrochemical Properties

The delafossite structured CuScO₂ is a p-type, wide band gap oxide that has been shown to support significant oxygen intercalation, leading to darkened color and increased conductivity. Control of this oxidation proves difficult by the conventional high-temperature solid-state syntheses. In addition, a pure hexagonal (2H) or rhombohedral (3R) polytype of CuScO₂ requires careful control of synthetic parameters or intentional doping. Lower-temperature hydrothermal syntheses have thus far led to only a mixed 2H/3R product. Herein, control of hydrothermal conditions with the consideration of copper and scandium hydrolysis led to the synthesis of light beige, hierarchically structured particles of 2H-CuScO₂. Absorption of the particles in the visible range was found to increase upon annealing of the sample in air, most likely due to the Cu^II formation from oxygen interstitials. X-ray photoelectron spectroscopy confirmed purely Cu^I in the as-synthesized 2H-CuScO₂ and increased Cu^II amounts upon annealing. Oxidation of the samples also led to shifts of the Fermi level toward the valence band as observed by increases in the measured flat band potentials versus normal hydrogen electrode, confirming increased hole carrier densities

pH-Tuning a Solar Redox Flow Battery for Integrated Energy Conversion and Storage

The intermittent nature of renewable energy sources such as solar and wind requires an energy storage method for future viability. Integrated solar energy conversion and storage devices such as solar redox flow batteries offer an innovative approach to this problem. Herein, we demonstrate electrolyte pH to be a valuable and tunable parameter for optimization of aqueous solar redox flow batteries. This can be accomplished by utilizing a pH-dependent redox anolyte and pH-independent catholyte to effectively tune the cell voltage by varying the operating pH, which allows direct integration of a dye-sensitized photoelectrode. A quinone–iodine redox flow battery can achieve high columbic efficiency over ∼90% for 50 cycles under mild pH conditions (pH ∼ 2–8). Furthermore, a pH-tunable solar redox flow battery can be charged using only solar illumination, thus allowing for integrated energy conversion and storage within a single devic

p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO₂ Nanoplates with Saturation Photovoltages Exceeding 460 mV

Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (<i>V</i>_oc). Here we report the first application of delafossite CuGaO₂ nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO₂ nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of <i>V</i>_oc on the illumination intensity to estimate the maximum obtainable <i>V</i>_oc from the CuGaO₂-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co^3+/2+(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a <i>V</i>_oc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs

p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO₂ Nanoplates with Saturation Photovoltages Exceeding 460 mV

Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (<i>V</i>_oc). Here we report the first application of delafossite CuGaO₂ nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO₂ nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of <i>V</i>_oc on the illumination intensity to estimate the maximum obtainable <i>V</i>_oc from the CuGaO₂-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co^3+/2+(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a <i>V</i>_oc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs

Photostable p‑Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction

A photostable p-type NiO photocathode based on a bifunctional cyclometalated ruthenium sensitizer and a cobaloxime catalyst has been created for visible-light-driven water reduction to produce H₂. The sensitizer is anchored firmly on the surface of NiO, and the binding is resistant to the hydrolytic cleavage. The bifunctional sensitizer can also immobilize the water reduction catalyst. The resultant photoelectrode exhibits superior stability in aqueous solutions. Stable photocurrents have been observed over a period of hours. This finding is useful for addressing the degradation issue in dye-sensitized photoelectrochemical cells caused by desorption of dyes and catalysts. The high stability of our photocathodes should be important for the practical application of these devices for solar fuel production

Probing Mechanisms for Inverse Correlation between Rate Performance and Capacity in K–O₂ Batteries

Owing to the formation of potassium superoxide (K⁺ + O₂ + <i>e</i>^– = KO₂), K–O₂ batteries exhibit superior round-trip efficiency and considerable energy density in the absence of any electrocatalysts. For further improving the practical performance of K–O₂ batteries, it is important to carry out a systematic study on parameters that control rate performance and capacity to comprehensively understand the limiting factors in superoxide-based metal–oxygen batteries. Herein, we investigate the influence of current density and oxygen diffusion on the nucleation, growth, and distribution of potassium superoxide (KO₂) during the discharge process. It is observed that higher current results in smaller average sizes of KO₂ crystals but a larger surface coverage on the carbon fiber electrode. As KO₂ grows and covers the cathode surface, the discharge will eventually end due to depletion of the oxygen-approachable electrode surface. Additionally, higher current also induces a greater gradient of oxygen concentration in the porous carbon electrode, resulting in less efficient loading of the discharge product. These two factors explain the observed inverse correlation between current and capacity of K–O₂ batteries. Lastly, we demonstrate a reduced graphene oxide-based K–O₂ battery with a large specific capacity (up to 8400 mAh/g_carbon at a discharge rate of 1000 mA/g_carbon) and a long cycle life (over 200 cycles)