Yuqin Zou

Oxygen Reduction Reaction in a Droplet on Graphite: Direct Evidence that the Edge Is More Active than the Basal Plane†

Carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium have been extensively investigated with the aim of replacing the commercially available, but precious platinum-based catalysts. For the proper design of carbon-based metal-free electrocatalysts for the ORR, it would be interesting to identify the active sites of the electrocatalyst. The ORR was now studied with an air-saturated electrolyte solution droplet (diameter ca. 15 μm), which was deposited at a specified position either on the edge or on the basal plane of highly oriented pyrolytic graphite. Electrochemical measurements suggest that the edge carbon atoms are more active than the basal-plane ones for the ORR. This provides a direct way to identify the active sites of carbon materials for the ORR. Ball-milled graphite and carbon nanotubes with more exposed edges were also prepared and showed significantly enhanced ORR activity. DFT calculations elucidated the mechanism by which the charged edge carbon atoms result in the higher ORR activity.

Mesoporous Vertical Co3O4 Nanosheet Arrays on Nitrogen-Doped Graphene Foam with Enhanced Charge-Storage Performance

A hierarchical electrode structure, consisting of cobalt oxide and nitrogen-doped graphene foam (NGF), has been fabricated with the aim of achieving enhanced charge-storage performance. Characterization of the material via electron microscopy and Raman spectroscopy demonstrates that the Co3O4 nanosheets grow vertically on NGF and the nanosheets are mesoporous with pore diameters between 3 and 8 nm. The Co3O4/NGF electrode shows an enhanced charge-storage performance, attributed to the 3D hierarchical structure and the synergistic effect of Co3O4 and NGF. The present study shows that specific capacitances as high as 451 F g(-1) can be obtained, indicating that high-performance electrochemical capacitors can be made using electrode materials with advanced structures. The present electrode design can be readily extended to other electroactive materials and their composites.

Nitrogen-doped and crumpled graphene sheets with improved supercapacitance

Nitrogen-doped thermally expanded graphene oxide (NtGO) was prepared by a facile thermal expansion and hydrothermal doping process. The thermal expansion process plays a vital role in improving the electrochemical performance of N-doped graphene by preventing its aggregation and improving its conductivity. The specific capacitance of NtGO is 270 F g−1 at a discharge current density of 1 A g−1 and the capacitance retention is 97% after 2000 cycles at this density. The strategy developed here provides an efficient and facile way to prepare nitrogen-doped graphene.

Enhanced Photoelectrochemical Performance of Cuprous Oxide/Graphene Nanohybrids

Combination of an oxide semiconductor with a highly conductive nanocarbon framework (such as graphene or carbon nanotubes) is an attractive avenue to assemble efficient photoelectrodes for solar fuel generation. To fully exploit the possible synergies of the hybrid formation, however, precise knowledge of these systems is required to allow rational design and morphological engineering. In this paper, we present the controlled electrochemical deposition of nanocrystalline p-Cu2O on the surface of different graphene substrates. The developed synthetic protocol allowed tuning of the morphological features of the hybrids as deduced from electron microscopy. (Photo)electrochemical measurements (including photovoltammetry, electrochemical impedance spectroscopy, photocurrent transient analysis) demonstrated better performance for the 2D graphene containing photoelectrodes, compared to the bare Cu2O films, the enhanced performance being rooted in suppressed charge carrier recombination. To elucidate the precise role of graphene, comparative studies were performed with carbon nanotube (CNT) films and 3D graphene foams. These studies revealed, after allowing for the effect of increased surface area, that the 3D graphene substrate outperformed the other two nanocarbons. Its interconnected structure facilitated effective charge separation and transport, leading to better harvesting of the generated photoelectrons. These hybrid assemblies are shown to be potentially attractive candidates in photoelectrochemical energy conversion schemes, namely CO2 reduction.

Ultra-low voltage electrowetting using graphite surfaces

The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10-100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.

Engineering the coordination geometry of metal–organic complex electrocatalysts for highly enhanced oxygen evolution reaction

Designing highly efficient oxygen evolution reaction (OER) electrocatalysts is very important for various electrochemical devices. In this work, for the first time, we have successfully generated coordinatively unsaturated metal sites (CUMSs) in phytic acid–Co2+ (Phy–Co2+) based metal–organic complexes by engineering the coordination geometry with room-temperature plasma technology. The CUMSs can serve as active centers to catalyze the OER. The electron spin resonance and X-ray absorption spectra provide direct evidence that the coordination geometry is obviously modified with many CUMSs by the plasma treatment. The plasma treated Phy–Co2+ (P-Phy–Co2+) only requires an overpotential of 306 mV to reach 10 mA cm−2 on glassy carbon electrodes. When we expand this strategy to a CoFe bimetallic system, it only needs an overpotential of 265 mV to achieve 10 mA cm−2 with a small Tafel slope of 36.51 mV dec−1. P-Phy–Co2+ is superior to the state-of-the-art. Our findings not only provide alternative excellent OER electrocatalysts, but also introduce a promising principle to design advanced electrocatalysts by creating more CUMSs.

Rapidly engineering the electronic properties and morphological structure of NiSe nanowires for the oxygen evolution reaction

The oxygen evolution reaction (OER) is one of the most important reactions in a wide range of renewable energy technologies. It is important to develop highly efficient electrocatalysts for the OER due to its sluggish kinetics. The electronic properties and morphological structure of electrocatalysts can significantly affect their OER performance. Electrocatalysts with the morphology of nanosheets can expose more active sites which would enhance the OER activity. Here, we report an extremely simple and fast method to synthesize a NixFe1−xSe@Ni(Fe)OOH core–shell nanostructure with a nanosheet shell by a facile solvothermal selenization and ion exchange reaction. The NixFe1−xSe@Ni(Fe)OOH core–shell nanostructure gives an excellent catalytic activity toward the OER with an overpotential as low as 260 mV to reach a current density of 100 mA cm−2 and excellent electrochemical long-term stability in 1 M KOH solution. The enhanced OER activity can be attributed to the dual modulation of electronic properties and the morphological structure by Fe doping.

Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting

Abstract Layered double hydroxide (LDH)‐based materials have attracted widespread attention in various applications due to their unique layered structure with high specific surface area and unique electron distribution, resulting in a good electrocatalytic performance. Moreover, the existence of multiple metal cations invests a flexible tunability in the host layers; the unique intercalation characteristics lead to flexible ion exchange and exfoliation. Thus, their electrocatalytic performance can be tuned by regulating the morphology, composition, intercalation ion, and exfoliation. However, the poor conductivity limits their electrocatalytic performance, which therefore has motivated researchers to combine them with conductive materials to improve their electrocatalytic performance. Another factor hampering their electrocatalytic activity is their large lateral size and the bulk thickness of LDHs. Introducing defects and tuning electronic structure in LDH‐based materials are considered to be effective strategies to increase the number of active sites and enhance their intrinsic activity. Given the unique advantages of LDH‐based materials, their derivatives have been also used as advanced electrocatalysts for water splitting. Here, recent progress on LDHs and their derivatives as advanced electrocatalysts for water splitting is summarized, current strategies for their designing are proposed, and significant challenges and perspectives of LDHs are discussed.

Recent Advances on Black Phosphorus for Energy Storage, Catalysis, and Sensor Applications

As a new type of 2D semiconductor, black phosphorus (BP) possesses high charge-carrier mobility and theoretical capacity, thickness-dependent bandgap, and anisotropic structure, which has attracted tremendous attention since early 2014. To explore its full application in all aspects, studies based on BP nanostructures are swiftly expanding from the electronic field to energy storage and even biochemistry. The mechanism and application of BP in Li-/Na-ion battery anodes, oxygen evolution reaction/hydrogen evolution reaction catalysis, photocatalytic hydrogen production, and selective sensors are summarized. Based on the solid research on this topic, feasible improvements and constructive suggestions regarding these four fields are put forward.