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

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

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

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)

Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production

Tandem dye-sensitized photoelectrochemical cells (DSPECs) for water splitting are a promising method for sustainable energy conversion but so far have been limited by their lack of aqueous stability and photocurrent mismatch between the cathode and anode. In nature, membrane-enabled subcellular compartmentation is a general approach to control local chemical environments in the cell. The hydrophobic tails of the lipid make the bilayer impermeable to ions and hydrophilic molecules. Herein we report the use of an organic donor–acceptor dye that prevents both dye desorption and semiconductor degradation by mimicking the hydrophobic/hydrophilic properties of lipid bilayer membranes. The dual-functional photosensitizer (denoted as BH4) allows for efficient light harvesting while also protecting the semiconductor surface from protons and water via its hydrophobic π linker. The protection afforded by this membrane-mimicking dye gives this system excellent stability in extremely acidic (pH 0) conditions. The acidic stability also allows for the use of cubane molybdenum-sulfide cluster as the hydrogen evolution reaction (HER) catalyst. This system produces a proton-reducing current of 183 ± 36 μA/cm² (0 V vs NHE with 300 W Xe lamp) for an unprecedented 16 h with no degradation. These results introduce a method for developing high-current, low-pH DSPECs and are a significant move toward practical dye-sensitized solar fuel production

Potassium-Ion Oxygen Battery Based on a High Capacity Antimony Anode

Recent investigations into the application of potassium in the form of potassium–oxygen, potassium–sulfur, and potassium-ion batteries represent a new approach to moving beyond current lithium-ion technology. Herein, we report on a high capacity anode material for use in potassium–oxygen and potassium-ion batteries. An antimony-based electrode exhibits a reversible storage capacity of 650 mAh/g (98% of theoretical capacity, 660 mAh/g) corresponding to the formation of a cubic K₃Sb alloy. The Sb electrode can cycle for over 50 cycles at a capacity of 250 mAh/g, which is one of the highest reported capacities for a potassium-ion anode material. X-ray diffraction and galvanostatic techniques were used to study the alloy structure and cycling performance, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy were used to provide insight into the thermodynamics and kinetics of the K–Sb alloying reaction. Finally, we explore the application of this anode material in the form of a K₃Sb–O₂ cell which displays relatively high operating voltages, low overpotentials, increased safety, and interfacial stability, effectively demonstrating its applicability to the field of metal oxygen batteries