Graphene and graphene oxide have attracted tremendous interest over the past decade due to their unique and excellent electronic, optical, mechanical, and chemical properties. This review focuses on the functional modification of graphene and graphene oxide. First, the basic structure, preparation methods and properties of graphene and graphene oxide are briefly described. Subsequently, the methods for the reduction of graphene oxide are introduced. Next, the functionalization of graphene and graphene oxide is mainly divided into covalent binding modification, non-covalent binding modification and elemental doping. Then, the properties and application prospects of the modified products are summarized. Finally, the current challenges and future research directions are presented in terms of surface functional modification for graphene and graphene oxide.

https://pubs.rsc.org/en/content/articlepdf/2020/ra/d0ra01068e

Progress in the functional modification of graphene/graphene oxide: a review

In contemporary years, the increasing demand for high-capacity and safe energy storage has spurred wide attentions in zinc batteries featuring either high voltage, high capacity or both. Despite the extensive research progress, achieving high-energy-density zinc batteries is still challenging and requires a synergy of the multiple factors including reaction mechanisms, electrodes, and electrolytes. In this Review, we comprehensively summarize the rational design strategies, and critically analyze the positive effects and potential issues in optimizing the electrochemistry, cathode materials, electrolytes and device architecture. At the end, the challenges and prospects for the further development of high-energy-density zinc batteries are outlined to guide the research towards new-generation batteries for household appliance, low-speed electric vehicles and large-scale energy storage system.

Design Strategies for High-Energy-Density Aqueous Zinc Batteries.

The exploration of the utilization of sustainable, green energy represents one way in which it is possible to ameliorate the growing threat of the global environmental issues and the crisis in energy. Moisture, which is ubiquitous on Earth, contains a vast reservoir of low-grade energy in the form of gaseous water molecules and water droplets. It has now been found that a number of functionalized materials can generate electricity directly from their interaction with moisture. This suggests that electrical energy can be harvested from atmospheric moisture and enables the creation of a new range of self-powered devices. Herein, the basic mechanisms of moisture-induced electricity generation are discussed, the recent advances in materials (including carbon nanoparticles, graphene materials, metal oxide nanomaterials, biofibers, and polymers) for harvesting electrical energy from moisture are summarized, and some strategies for improving energy conversion efficiency and output power in these devices are provided. The potential applications of moisture electrical generators in self-powered electronics, healthcare, security, information storage, artificial intelligence, and Internet-of-things are also discussed. Some remaining challenges are also considered, together with a number of suggestions for potential new developments of this emerging technology.

Moisture-Enabled Electricity Generation: From Physics and Materials to Self-Powered Applications.

When external pressure drives an electrolyte solution in a capillary tube with a charged inner surface, we obtain a streaming potential/current. This effect is also manifested when water flows through the microchannels of a tree, which is driven by capillary pressure and natural evaporation. Thus, by making use of natural evaporation, we took advantage of the anisotropic three-dimensional wood structures to fabricate nanogenerators drawing electricity from the streaming potential/current. As a result, direct current can be harvested continuously, simply through a piece of wood. A 300 mV open-circuit voltage and a 10 μA short-circuit current (ISC) were recorded from a single device, which surpassed the ISC values of most previous works by an order. By connecting five wood nanogenerators in series, a calculator can be completely functional, as a demonstration for practical application.

Harvesting Electricity from Water Evaporation through Microchannels of Natural Wood.

Environmentally adaptive power generation is attractive for the development of next-generation energy sources. Here we develop a heterogeneous moisture-enabled electric generator (HMEG) based on a bilayer of polyelectrolyte films. Through the spontaneous adsorption of water molecules in air and induced diffusion of oppositely charged ions, one single HMEG unit can produce a high voltage of ~0.95 V at low (25%) relative humidity (RH), and even jump to 1.38 V at 85% RH. A sequentially aligned stacking strategy is created for large-scale integration of HMEG units, to offer a voltage of more than 1,000 V under ambient conditions (25% RH, 25 °C). Using origami assembly, a small section of folded HMEGs renders an output of up to 43 V cm−3. Such integration devices supply sufficient power to illuminate a lamp bulb of 10 W, to drive a dynamic electronic ink screen and to control the gate voltage for a self-powered field effect transistor. A power generator exhibits enhanced output due to a dual-charge-carrier design. The voltage produced is constant yet competitive even under low relative humidity.

Bilayer of polyelectrolyte films for spontaneous power generation in air up to an integrated 1,000 V output.

At relatively low temperature (e.g., −30 °C), most flexible supercapacitors that work well at room temperature will lose their stretchability due to the poor cold intolerance of conventional electrolytes and the absence of intrinsically stretchable electrodes. Herein, an anti-freezing and highly stretchable supercapacitor (AF-SSC) was fabricated for the first time by one-step in situ growth of polyaniline onto an organohydrogel polyelectrolyte from cross-linked polyacrylamide networks soaked by ethylene glycol/water/H2SO4. As no extra stretchable substrates (e.g. elastic fiber, polydimethylsiloxane, rubber, etc.) or predefining stretchable structures (e.g. helical, spring, wrinkle, honeycombed structures etc.) are needed in such a design, the prepared AF-SSC shows a high mechanical stretchability of 200% at −30 °C and can be repeatedly stretched for 100 cycles without significant capacitance loss. Besides, an impressive capacitance retention of 73.1% at −30 °C is achieved when the current density increases by 20 times, which is much higher than those of stretchable supercapacitors based on pseudocapacitive materials that operate at room temperature. Furthermore, the device shows an ultralong lifespan with 91.7% capacitance retention after 100 000 cycles at −30 °C, outperforming all stretchable supercapacitors reported previously. With other merits of high compressibility, strong adhesion, good processability and excellent anti-drying ability, the current device is an ideal power source to drive multi-functional electronic components in real-life scenarios.

/pdf/stretchable-supercapacitor-at-30-c-4hqm17yjq7.pdf

Stretchable supercapacitor at −30 °C

As a new kind of zero-dimensional (0D) material, graphene quantum dots (GQDs) have broad prospects in energy storage and conversion due to their unique physical and chemical properties. In addition to the excellent properties of graphene, GQDs also have quantum confinement effects and edge effects. The size of GQDs and types of their edges determine their excellent properties. In this paper, the progress in the synthesis, doping and modification methods of GQDs in recent years is reviewed, and the significant advances of GQDs in energy conversion devices such as supercapacitors/microsupercapacitors, solar cells, batteries and LEDs are summarized. In addition, we rationally analysed the shortcomings of GQDs for energy storage and conversion, and predicted the future development trend of GQD research and its challenges and opportunities.

/pdf/graphene-quantum-dots-for-energy-storage-and-conversion-from-tc0k6qskag.pdf

Graphene quantum dots for energy storage and conversion: from fabrication to applications

Advanced intelligent electronic systems are usually integrated with a power supply, sensors and many other components, which can lead to their bulky sizes and complex structures. Here, we introduce a smart self-powered device concept that exploits a type of Jaboticaba-like carbon nanosphere@TiO2 nanowire (C@T) as a highly efficient, light-sensitive liquid (including but not limited to water, methanol, acetone, and ethanol) evaporation power generation (EPG) platform. Typically, the separate-piece EPG device could generate a flow-induced voltage (FIV) of 1.6 V under the evaporation of water, which was comparable to that of a commercial dry battery. When methanol was used, FIV was as high as 2.0 V. The FIV value of the EPG device using water could be easily scaled up to nearly 20 V and used for a stable power source in electronic devices. The photoresponse performance is an exclusive feature of EPG, which was successfully applied to a self-powered light-control switch. The significance of this work is that it not only provides a satisfactory and practical candidate for cheap and efficient evaporation power generation, but also opens up a green way for building advanced intelligent self-powered electronics.

Qianwen Liu

Papers

Stretchable supercapacitor at −30 °C

Graphene quantum dots for energy storage and conversion: from fabrication to applications

Intelligent multiple-liquid evaporation power generation platform using distinctive Jaboticaba-like carbon nanosphere@TiO2 nanowires

Maximizing Energy Storage of Flexible Aqueous Batteries through Decoupling Charge Carriers

The First Flexible Dual-Ion Microbattery Demonstrates Superior Capacity and Ultrahigh Energy Density: Small and Powerful