25 research outputs found
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 RuÂ(II) chromophore–acceptor dyad sensitized NiO photocathode.
The dyad (O26) is based on a cyclometalated RuÂ(N<sup>∧</sup>C<sup>∧</sup>N)Â(N<sup>∧</sup>N<sup>∧</sup>N)
(RuÂ[II]) chromophore and a naphthalenediimide (NDI) acceptor, where
N<sup>∧</sup>C<sup>∧</sup>N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine
and N<sup>∧</sup>N<sup>∧</sup>N represents 2,2′,6′,6″-terpyridine
ligand. When the dyad is dissolved in a CH<sub>3</sub>CN solution,
electron transfer to form the RuÂ(III)–NDI<sup>–</sup> occurs with a rate constant <i>k</i><sub>f</sub> = 1.1
× 10<sup>10</sup> s<sup>–1</sup> (τ<sub>f</sub> =
91 ps), and electron–hole pair recombines to regenerate ground
state with a rate constant <i>k</i><sub>b</sub> = 4.1 ×
10<sup>9</sup> s<sup>–1</sup> (τ<sub>b</sub> = 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 [NiOÂ(h<sup>+</sup>)–RuÂ(II)–NDI<sup>–</sup>] with a rate constant <i>k</i><sub>f</sub> = 9.1 ×
10<sup>11</sup> s<sup>–1</sup> (τ<sub>f</sub> = 1.1 ps).
The recovery of the ground state occurs with a multiexponential rate
constant <i>k</i><sub>b</sub> = 2.3 × 10<sup>9</sup> s<sup>–1</sup> (τ<sub>b</sub> = 426 ps). The charge
recombination rate constant is slightly slower than a reference cyclometalated
ruthenium compound (O25) with no NDI group (Ï„<sub>b</sub> =
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 RuÂ(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 RuÂ[N<sup>∧</sup>N]<sub>2</sub>[N<sup>∧</sup>C], where
[N<sup>∧</sup>N] is a diimine ligand and [N<sup>∧</sup>C] is a cyclometalated ligand. The triphenylamino group is attached
to the <i>-para</i> position of the ruthenium–carbon
bond of the [N<sup>∧</sup>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><sub>sc</sub>) than cells sensitized with our prior sensitizers with phenylene
linkers. Morever the N<sup>∧</sup>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<sup>Ì‚</sup>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><sub>sc</sub> of 3.04 mA/cm<sup>2</sup>. The large <i>J</i><sub>sc</sub> 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><sub>sc</sub> 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 bisÂ(fluoroÂslufonyl)Âimide (KFSI)-dimethoxyÂethane
(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<sup>+</sup>) 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 CuGaO<sub>2</sub> for Controlled Hydrothermal Synthesis of Nanoparticles and Nanoplates
The
delafossite CuGaO<sub>2</sub> 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 CuGaO<sub>2</sub> nanoparticles is challenging
but desirable for efficient dye loading. In this work, the phase formation
and crystal growth mechanism of delafossite CuGaO<sub>2</sub> under
low-temperature (<250 °C) hydrothermal conditions are systematically
studied. The stabilization of Cu<sup>I</sup> cations in aqueous solution
and the controlling of the hydrolysis of Ga<sup>III</sup> 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 CuGaO<sub>2</sub> nanoplates. Importantly,
by suppressing this OA process, delafossite CuGaO<sub>2</sub> 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 CuGaO<sub>2</sub> should also benefit syntheses of other similar delafossites
such as CuAlO<sub>2</sub> and CuScO<sub>2</sub>
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
2H-CuScO<sub>2</sub> Prepared by Low-Temperature Hydrothermal Methods and Post-Annealing Effects on Optical and Photoelectrochemical Properties
The delafossite structured CuScO<sub>2</sub> 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 CuScO<sub>2</sub> 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-CuScO<sub>2</sub>. 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<sup>II</sup> formation
from oxygen interstitials. X-ray photoelectron spectroscopy confirmed
purely Cu<sup>I</sup> in the as-synthesized 2H-CuScO<sub>2</sub> and
increased Cu<sup>II</sup> 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
p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO<sub>2</sub> 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><sub>oc</sub>). Here we report the first application of delafossite
CuGaO<sub>2</sub> nanoplates for p-DSSCs with high photovoltages.
In contrast to the dark color of NiO, our CuGaO<sub>2</sub> 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><sub>oc</sub> on the illumination intensity to estimate the maximum obtainable <i>V</i><sub>oc</sub> from the CuGaO<sub>2</sub>-based p-DSSCs.
Excitingly, a saturation photovoltage of 464 mV has been observed
when a polypyridyl Co<sup>3+/2+</sup>(dtb-bpy) electrolyte was used.
Under 1 Sun AM 1.5 illumination, a <i>V</i><sub>oc</sub> 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 CuGaO<sub>2</sub> 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><sub>oc</sub>). Here we report the first application of delafossite
CuGaO<sub>2</sub> nanoplates for p-DSSCs with high photovoltages.
In contrast to the dark color of NiO, our CuGaO<sub>2</sub> 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><sub>oc</sub> on the illumination intensity to estimate the maximum obtainable <i>V</i><sub>oc</sub> from the CuGaO<sub>2</sub>-based p-DSSCs.
Excitingly, a saturation photovoltage of 464 mV has been observed
when a polypyridyl Co<sup>3+/2+</sup>(dtb-bpy) electrolyte was used.
Under 1 Sun AM 1.5 illumination, a <i>V</i><sub>oc</sub> 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 H<sub>2</sub>. 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–O<sub>2</sub> Batteries
Owing to the formation of potassium
superoxide (K<sup>+</sup> + O<sub>2</sub> + <i>e</i><sup>–</sup> = KO<sub>2</sub>), K–O<sub>2</sub> batteries
exhibit superior round-trip efficiency and considerable energy density
in the absence of any electrocatalysts. For further improving the
practical performance of K–O<sub>2</sub> 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 (KO<sub>2</sub>)
during the discharge process. It is observed that higher current results
in smaller average sizes of KO<sub>2</sub> crystals but a larger surface
coverage on the carbon fiber electrode. As KO<sub>2</sub> 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–O<sub>2</sub> batteries. Lastly, we demonstrate a reduced graphene oxide-based
K–O<sub>2</sub> battery with a large specific capacity (up
to 8400 mAh/g<sub>carbon</sub> at a discharge rate of 1000 mA/g<sub>carbon</sub>) and a long cycle life (over 200 cycles)