Jiacheng Wang

KOH activation of carbon-based materials for energy storage

Because of their availability, adjustable microstructure, varieties of forms, and large specific surface area, porous carbon materials are of increasing interest for use in hydrogen storage adsorbents and electrode materials in supercapacitors and lithium–sulfur cells from the viewpoint of social sustainability and environmental friendliness. Therefore, much effort has been made to synthesize and tailor the microstructures of porous carbon materials via various activation procedures (physical and chemical activation). In particular, the chemical activation of various carbon sources using KOH as the activating reagent is very promising because of its lower activation temperature and higher yields, and well-defined micropore size distribution and ultrahigh specific surface area up to 3000 m2 g−1 of the resulting porous carbons. In this feature article, we will cover recent research progress since 2007 on the synthesis of KOH-activated carbons for hydrogen and electrical energy storage (supercapacitors and lithium–sulfur batteries). The textural properties and surface chemistry of KOH-activated carbons depend on not only the synthesis parameters, but also different carbon sources employed including fossil/biomass-derived materials, synthetic organic polymers, and various nanostructured carbons (e.g. carbon nanotubes, carbon nanofibers, carbon aerogels, carbide-derived carbons, graphene, etc.). Following the introduction to KOH activation mechanisms and processing technologies, the characteristics and performance of KOH-activated carbons as well as their relationships are summarized and discussed through the extensive analysis of the literature based on different energy storage systems.

Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst

The replacement of platinum with non-precious-metal electrocatalysts with high efficiency and superior stability for the hydrogen-evolution reaction (HER) remains a great challenge. Herein, we report the one-step synthesis of uniform, ultrafine molybdenum carbide (Mo2C) nanoparticles (NPs) within a carbon matrix from inexpensive starting materials (dicyanamide and ammonium molybdate). The optimized catalyst consisting of Mo2C NPs with sizes lower than 3 nm encapsulated by ultrathin graphene shells (ca. 1-3 layers) showed superior HER activity in acidic media, with a very low onset potential of -6 mV, a small Tafel slope of 41 mV dec(-1), and a large exchange current density of 0.179 mA cm(-2), as well as good stability during operation for 12 h. These excellent properties are similar to those of state-of-the-art 20% Pt/C and make the catalyst one of the most active acid-stable electrocatalysts ever reported for HER.

Imine-linked polymer-derived nitrogen-doped microporous carbons with excellent CO2 capture properties.

A series of nitrogen-doped microporous carbons (NCs) was successfully prepared by direct pyrolysis of high-surface-area microporous imine-linked polymer (ILP, 744 m(2)/g) which was formed using commercial starting materials based on the Schiff base condensation under catalyst-free conditions. These NCs have moderate specific surface areas of up to 366 m(2)/g, pore volumes of 0.43 cm(3)/g, narrow micropore size distributions, and a high density of nitrogen functional groups (5.58-8.74%). The resulting NCs are highly suitable for CO2 capture adsorbents because of their microporous textural properties and large amount of Lewis basic sites. At 1 bar, NC-800 prepared by the pyrolysis of ILP at 800 °C showed the highest CO2 uptakes of 1.95 and 2.65 mmol/g at 25 and 0 °C, respectively. The calculated adsorption capacity for CO2 per m(2) (μmol of CO2/m(2)) of NC-800 is 7.41 μmol of CO2/m(2) at 1 bar and 25 °C, the highest ever reported for porous carbon adsorbents. The isosteric heats of CO2 adsorption (Qst) for these NCs are as high as 49 kJ/mol at low CO2 surface coverage, and still ~25 kJ/mol even at high CO2 uptake (2.0 mmol/g), respectively. Furthermore, these NCs also exhibit high stability, excellent adsorption selectivity for CO2 over N2, and easy regeneration and reuse without any evident loss of CO2 adsorption capacity.

Highly porous nitrogen-doped polyimine-based carbons with adjustable microstructures for CO2 capture

A series of highly porous nitrogen doped porous carbons (NPCs) have been successfully prepared using a novel porous polyimine as the precursor. The resulting NPCs have a high specific surface area of up to 3195 m2 g−1, high pore volume and micropore volume (up to 1.58 and 1.38 cm3 g−1, respectively), narrow micropore size distributions, and adjustable nitrogen (1.52–5.05 wt%) depending on the activation temperatures (600–750 °C). The CO2 uptakes of the NPCs prepared at higher temperatures (700–750 °C) are lower than those prepared at milder conditions (600–650 °C). At 1 bar, NPC-650 demonstrates the best CO2 capture performance and could efficiently adsorb CO2 molecules of 3.10 mmol g−1 (136 mg g−1) and 5.26 mmol g−1 (231.3 mg g−1), at 25 and 0 °C, respectively. The NPCs also show good a initial CO2/N2 adsorption selectivity of up to 23.4 and an adsorption ratio of CO2/N2 (6.6) at 1 bar. Meanwhile, these NPCs exhibit a high stability and facile regeneration/recyclability without evident loss of the CO2 capture capacities.

Magnesiothermic synthesis of sulfur-doped graphene as an efficient metal-free electrocatalyst for oxygen reduction

Efficient metal-free electrocatalysts for oxygen reduction reaction (ORR) are highly expected in future low-cost energy systems. We have successfully prepared crumpled, sheet-like, sulfur-doped graphene by magnesiothermic reduction of easily available, low-cost, nontoxic CO2 (in the form of Na2CO3) and Na2SO4 as the carbon and sulfur sources, respectively. At high temperature, Mg can reduce not only carbon in the oxidation state of +4 in CO32− to form graphene, but also sulfur in SO42− from its highest (+6) to lowest valence which was hybridized into the carbon sp2 framework. Various characterization results show that sulfur-doped graphene with only few layers has an appropriate sulfur content, hierarchically robust porous structure, large surface area/pore volume, and highly graphitized textures. The S-doped graphene samples exhibit not only a high activity for ORR with a four-electron pathway, but also superior durability and tolerance to MeOH crossover to 40% Pt/C. This is mainly ascribed to the combination of sulfur-related active sites and hierarchical porous textures, facilitating fast diffusion of oxygen molecules and electrolyte to catalytic sites and release of products from the sites.

In situ growth of spinel CoFe2O4 nanoparticles on rod-like ordered mesoporous carbon for bifunctional electrocatalysis of both oxygen reduction and oxygen evolution

The lack of efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a fatal issue for the development of metal–air batteries in large-scale commercialization. In this paper, spinel CoFe2O4 (CFO) nanoparticles were successfully in situ grown onto rod-like ordered mesoporous carbon (RC) by a facile, scalable hydrothermal method, followed by annealing at different temperatures. The as-acquired CFO/RC nanohybrid pyrolyzed at 400 °C (CFO/RC-400) has a high specific surface area (150.3 m2 g−1) and two sets of uniform mesopore systems (3.38 and 19.1 nm), all of which are favorable for the improvement of the electrocatalytic activity. The hybridization of CFO nanoparticles and the RC matrix results in increased ORR and OER electrocatalytic activity of the CFO/RC nanohybrids, which is significantly superior to that of unsupported CFO nanoparticles and pure RC. CFO/RC-400 shows better catalytic activity for the ORR with a direct four-electron reaction pathway than those prepared at other temperatures in terms of the onset potential and limiting current density. Furthermore, the CFO/RC-400 nanohybrid exhibits outstanding durability for both the ORR and OER, and can outperform commercial Pt/C. The excellent bifunctional electrocatalytic activities of the CFO/RC nanohybrids are mainly owing to the hierarchical mesoporous structures of the nanohybrids and strong coupling between the CFO nanoparticles and the RC matrix.

Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction

Nitrogen-doped graphene (NG) was successfully synthesized by a novel, facile, and scalable bottom-up method. The annealed NG (NG-A) possessed high specific surface area and a hierarchical porous texture, and exhibited remarkably improved electrocatalytic activity in the oxygen reduction reaction in both alkaline and acidic media. Ab initio molecular dynamic simulations indicated that rapid H transfer and the thermodynamic stability of six-membered N structures promoted the transformation of N-containing species from pyrrolic to pyridinic at 600 °C. In O2-staturated 0.1 M KOH solution, the half-wave potential (E1/2) of NG-A was only 62 mV lower than that of a commercial Pt/C catalyst, and the limiting current density of NG-A was 0.5 mA·cm–2 larger than that of Pt/C. Koutecky–Levich (K–L) plots and rotating ring-disk electrode measurement indicated a four-electron-transfer pathway in NG-A, which could be ascribed to its high content of pyridinic N.

Influence of spatial configurations on electromagnetic interference shielding of ordered mesoporous carbon/ordered mesoporous silica/silica composites

Ordered mesoporous carbons (OMCs), obtained by nanocasting using ordered mesoporous silicas (OMSs) as hard templates, exhibit unique arrangements of ordered regular nanopore/nanowire mesostructures. Here, we used nanocasting combined with hot-pressing to prepare 10 wt% OMC/OMS/SiO2 ternary composites possessing various carbon mesostructure configurations of different dimensionalities (1D isolated CS41 carbon nanowires, 2D hexagonal CMK-3 carbon, and 3D cubic CMK-1 carbon). The electric/dielectric properties and electromagnetic interference (EMI) shielding efficiency (SE) of the composites were influenced by spatial configurations of carbon networks. The complex permittivity and the EMI SE of the composites in the X-band frequency range decreased for the carbon mesostructures in the following order: CMK-3-filled > CMK-1-filled > CS41-filled. Our study provides technical directions for designing and preparing high-performance EMI shielding materials. Our OMC-based silica composites can be used for EMI shielding, especially in high-temperature or corrosive environments, owing to the high stability of the OMC/OMS fillers and the SiO2 matrix. Related shielding mechanisms are also discussed.

Controlled synthesis of europium-doped lutetium compounds: nanoflakes, nanoquadrels, and nanorods

In this contribution, we report the successful preparation of large-scale and uniform europium-doped lutetium compounds with the varied morphologies of nanoflakes, nanoquadrels and nanorods, using colloidal Lu(OH)3–Eu at different concentrations as a precursor for the hydrothermal method (170 °C and pH = 13 for 24 h). The as-made products have different crystalline structures as well as different morphologies. The as-synthesized nanoflakes have a “perfect” square shape with side lengths of 220–400 nm and thicknesses of 50 nm or below. On the basis of the observed “non-perfect” nanoflakes, we have deduced a mechanism of “side wrapping” to describe the growth of the “perfect” square nanoflakes. The as-made nanoquadrels possess a sandwich-like nanostructure and they are all composed of several square nanoflakes via face-to-face “self-reorganization”. The formation of nanorods is due to the high chemical potential resulting from the high precursor concentration. At the same time, the pH value of the precursor and the method of mixing of the lanthanide nitrate solution with the NaOH solution have a great effect on the morphology of the as-synthesized products. The shapes of the as-made products were sustained after thermal decomposition to europium-doped Lu2O3. The crystalline structure, morphology and thermal behavior of as-synthesized products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TG) and differential scanning calorimetric analysis (DSC), respectively.

Fungi-derived hierarchically porous carbons for high-performance supercapacitors

Hierarchical porous activated carbons (ACs) were prepared via a chemical activation procedure with sustainable, renewable biomass fungi as carbon precursor and KOH as activating reagent. The as-produced porous ACs present not only a hierarchical porous structure containing macroporous frameworks and microporous textures, but also a high specific surface area of up to 2264 m2 g−1, a large pore volume of up to 1.02 cm3 g−1, and adjustable heteroatom doping (nitrogen: 2.15–4.75 wt%; oxygen: 8.53–14.48 wt%). The microstructural features can be easily controlled by adjusting the mass ratio of KOH/carbon precursor. The porous ACs possess a specific capacitance of up to 158 F g−1 in organic electrolyte, which significantly outperforms the commercially available ACs. The fungi-based ACs electrode also retains 93% of the specific capacitance as the current density increases from 0.1 to 5 A g−1, and has superior cycling performance (92% retention after 10 000 cycles).