The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

Helpful elements: A facile bottom-up method using citric acid and L-cysteine as a precursor has been developed to prepare graphene quantum dots (GQDs) co-doped with nitrogen and sulfur. A new type and high density of surface state of GQDs arises, leading to high yields (more than 70 %) and excitation-independent emission. FLQY = fluorescence quantum yield.

http://www1.spms.ntu.edu.sg/~yuting/Publications/Publication%202013/Carbon-Based%20Dots%20Co-doped%20with%20Nitrogen%20and%20Sulfur%20for%20High%20Quantum%20Yield%20and%20Excitation-Independent%20Emission.pdf

Carbon-Based Dots Co-doped with Nitrogen and Sulfur for High Quantum Yield and Excitation-Independent Emission†

The emerging graphene quantum dots (GQDs) and carbon dots (C-dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications.

Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications

Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid

A facile, low cost and high yield method has been developed to prepare single- and multi-layer graphene quantum dots (GQDs) from XC-72 carbon black by chemical oxidation. The single-layer GQDs are demonstrated to be excellent probes for cellular imaging, while the multi-layer GQDs may offer great potential applications in optoelectronic devices.

One-step and high yield simultaneous preparation of single- and multi-layer graphene quantum dots from CX-72 carbon black

Porous carbons produced by the pyrolysisof green onion leaves and their capacitive behavior

Coaxial yarn electrodes composed of activated carbon fibers tows and MoS2 nanosheets (ACFTs/MoS2) are synthesized by a facile hydrothermal approach. The active material, hierarchical MoS2 nanosheets, delivers a high specific capacitance of 308.5 F g−1 at scan rate of 5 mV s−1. Solid-state supercapacitors (SCs) in the planar format based on the ACFTs/MoS2 yarn electrode in PVA/H3PO4 gel electrolyte show an excellent energy density of 3.76 mW h g−1 at constant current density of 0.21 A g−1, a high power density of 474.49 mW g−1 at 3.16 A g−1 and a long cycle life with a capacitance retention as high as 97.38% even after 6000 times of charging–discharging under 2.21 A g−1. Furthermore, the SCs in a planar format represent superior combination feasibility when combined in parallel or series, approaching the theoretical value. Additionally, twisting SCs made from ACFTs/MoS2 yarn electrodes show good flexibility and electrochemical stability, which maintain 97.6% of their initial capacitance after being bent to different angles. These outstanding performances can be ascribed to the high accessible surface area for the electrolyte, the short path for ion diffusion provided by the MoS2 nanosheets (NSs), as well as the effective transport channels for electrons provided by the carbon fibers. The rough surface and oxygen containing groups on the activated carbon fiber offer robust anchor sites for MoS2 NSs, which not only facilitate electron transport but also provide strong adhesion between MoS2 NSs and carbon fibers.

Lili Gao

Papers

One-step and high yield simultaneous preparation of single- and multi-layer graphene quantum dots from CX-72 carbon black

Porous carbons produced by the pyrolysisof green onion leaves and their capacitive behavior

A coaxial yarn electrode based on hierarchical MoS2 nanosheets/carbon fiber tows for flexible solid-state supercapacitors