Long Song

Tailored graphene systems for unconventional applications in energy conversion and storage devices

Graphene-based materials have shown great potential in various fields across physics, chemistry, biology, and electronics, due to their unique electronic properties, facile synthesis, and ease of functionalization. In this review, we summarize the significant advances in tailored graphene systems for the recently developed unconventional energy conversion and storage devices reported by our group and others, namely focused on their tunable and controllable preparation and remarkable applications in new types of supercapacitors, lithium ion batteries, photovoltaic cells, and other emerging generators. This featured article also highlights the working principles and outlines the problems hindering the practical applications of graphene-based materials in these energy-related devices. Future research trends towards new methodologies in the design and synthesis of graphene-based systems with unique properties for emerging energy storage and energy conversion devices are also proposed.

Functional graphene nanomesh foam

Rationally designed graphene nanomesh assembled foam (GMF) with hierarchical pore arrangement has been successfully fabricated for the first time by a site-localized nanoparticle-induced etching strategy on the basis of hydrothermally self-assembled graphene architecture. The newly developed GMF provides a new material platform for developing high-performance functional devices. Specially, the N- and S-codoped GMF electrode exhibits excellent electrocatalytic activities for oxygen reduction reaction (ORR), better than most of the graphene-based ORR catalysts reported previously.

Stimulus-responsive graphene systems towards actuator applications

The rise of graphene has triggered the fast increasing research upsurge in both controllable synthesis of graphene and unique applications associated with its miraculous properties. In particular, graphene-based smart devices that can automatically respond to external stimulations are among those attracting our most attention. In this featured article, we summarize some of the recent advances in stimulus-responsive graphene actuation systems contributed by us and others, and discuss the different roles that graphene plays in various actuation circumstances such as under electrical, chemical, photonic, thermal and other stimuli. Impressive progress including graphene-based robots is also presented, demonstrating the great prospects of graphene actuation systems in a wide range of applications including sensors, switches, artificial muscles, nano/micro electromechanical devices, etc.

Functionalized Graphitic Carbon Nitride for Metal-free, Flexible and Rewritable Nonvolatile Memory Device via Direct Laser-Writing

Graphitic carbon nitride nanosheet (g-C3N4-NS) has layered structure similar with graphene nanosheet and presents unusual physicochemical properties due to the s-triazine fragments. But their electronic and electrochemical applications are limited by the relatively poor conductivity. The current work provides the first example that atomically thick g-C3N4-NSs are the ideal candidate as the active insulator layer with tunable conductivity for achieving the high performance memory devices with electrical bistability. Unlike in conventional memory diodes, the g-C3N4-NSs based devices combined with graphene layer electrodes are flexible, metal-free and low cost. The functionalized g-C3N4-NSs exhibit desirable dispersibility and dielectricity which support the all-solution fabrication and high performance of the memory diodes. Moreover, the flexible memory diodes are conveniently fabricated through the fast laser writing process on graphene oxide/g-C3N4-NSs/graphene oxide thin film. The obtained devices not only have the nonvolatile electrical bistability with great retention and endurance, but also show the rewritable memory effect with a reliable ON/OFF ratio of up to 105, which is the highest among all the metal-free flexible memory diodes reported so far, and even higher than those of metal-containing devices.

Environmentally responsive graphene systems.

Graphene materials have been attracting significant research interest in the past few years, with the recent focuses on graphene-based electronic devices and smart stimulus-responsive systems that have a certain degree of automatism. Owing to its huge specific surface area, large room-temperature electron mobility, excellent mechanical flexibility, exceptionally high thermal conductivity and environmental stability, graphene is identified as a beneficial additive or an effective responding component by itself to improve the conductivity, flexibility, mechanical strength and/or the overall responsive performance of smart systems. In this review article, we aim to present the recent advances in graphene systems that are of spontaneous responses to external stimulations, such as environmental variation in pH, temperature, electric current, light, moisture and even gas ambient. These smart stimulus-responsive graphene systems are believed to have great theoretical and practical interests to a wide range of device applications including actuators, switches, robots, sensors, drug/gene deliveries, etc.

Enhanced Photocarrier Separation in Hierarchical Graphitic-C3N4-Supported CuInS2 for Noble-Metal-Free Z-Scheme Photocatalytic Water Splitting

The effective separation of photogenerated electrons and holes in photocatalysts is a prerequisite for efficient photocatalytic water splitting. CuInS2 (CIS) is a widely used light absorber that works properly in photovoltaics but only shows limited performance in solar-driven hydrogen evolution due to its intrinsically severe charge recombination. Here, we prepare hierarchical graphitic C3N4-supported CuInS2 (denoted as GsC) by an in situ growth of CIS directly on exfoliated thin graphitic C3N4 nanosheets (g-C3N4 NS) and demonstrate efficient separation of photoinduced charge carriers in the GsC by forming the Z-scheme system for the first time in CIS-catalyzed water splitting. Under visible light illumination, the GsC features an enhanced hydrogen evolution rate up to 1290 μmol g-1 h-1, which is 3.3 and 6.1 times higher than that of g-C3N4 NS and bare-CIS, respectively, thus setting a new performance benchmark for CIS-based water-splitting photocatalysts.

Substrate‐Assisted Deposition of Metal Oxides on Three‐Dimensional Porous Reduced Graphene Oxide Networks as Bifunctional Hybrid Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of great importance for energy conversion processes, such as water electrolysis, metal–air batteries, and fuel cells. Herein, we report a substrate‐assisted in situ reduction method to prepare three‐dimensional reduced graphene oxide (3DRGO) networks decorated with a range of mixed cobalt oxide and nickel oxide nanosheets as a new type of bifunctional catalyst for oxygen evolution and oxygen reduction. Among the different composite catalysts, Co3O4@3DRGO showed highest catalytic activities in OER and ORR with an excellent stability. This catalyst is used to catalyze the oxygen reactions in a Zn–air battery as a proof‐of‐concept. The current results render new insights into developing low‐cost, next‐generation bifunctional catalysts for photoelectrochemical cells for energy storage and conversion.

Electric Power Generation through the Direct Interaction of Pristine Graphene‐Oxide with Water Molecules

Converting ubiquitous environmental energy into electric power holds tremendous social and financial interests. Traditional energy harvesters and converters are limited by the specific materials and complex configuration of devices. Herein, it is presented that electric power can be directly produced from pristine graphene oxide (GO) without any pretreatment or additives once encountering the water vapor, which will generate an open-circuit-voltage of up to 0.4-0.7 V and a short-circuit-current-density of 2-25 µA cm-2 on a single piece of GO film. This phenomenon results from the directional movement of charged hydrogen ions through the GO film. The present work demonstrates and provides an extremely simple method for electric energy generation, which offers more applications of graphene-based materials in green energy converting field.

Solution-processed MoS2 nanotubes/reduced graphene oxide nanocomposite as an active electrocatalyst toward the hydrogen evolution reaction

Advanced materials for electrocatalytic water splitting are essential to the area of renewable energy. In this work, we report a hierarchical 3D nanostructure of MoS2 nanotubes supported on a reduced graphene oxide network (MNTs@rGO) through a surfactant-assisted lyophilization process. The resulting MNTs@rGO exhibited good electrocatalytic activity in the hydrogen evolution reaction (HER) relative to other representative MoS2-based electrocatalysts, with an onset overpotential of 180 mV, a small Tafel slope of 69 mV dec−1 as well as a large cathodic current density (38.91 mA cm−2 at an overpotential of 300 mV). Linear sweep voltammetry (LSV) tests and electrochemical impedance spectroscopic (EIS) measurements reveal that the MNT building blocks with high exposure of surface atoms function as the active catalytic sites in the HER and the rGO support serves as the conductive footstone connecting all catalytic sites with fast electron transport. The EIS characterization also demonstrates that the electron transfer at the catalyst/electrolyte interface is the rate-limiting step in the catalyzed HER and assembling MoS2 nanosheets into nanotubes can significantly facilitate the HER. The current results are deemed to provide new insights into next-generation HER catalysts with high activity and low cost.