Denisa Hulicova-Jurcakova

Facile Oxygen Reduction on a Three‐Dimensionally Ordered Macroporous Graphitic C3N4/Carbon Composite Electrocatalyst

Honeycomb catalysis: a facile oxygen reduction reaction has been observed on a graphitic C(3)N(4)/carbon catalyst with three-dimensional interconnected macropores (see picture with SiO(2) template). This material not only shows catalytic activity that is comparable to that of commercial Pt/C, but also has much higher organic-fuel tolerance and long-term stability.

Highly stable performance of supercapacitors from phosphorus-enriched carbons

Phosphorus-rich microporous carbons (P-carbons) prepared by a simple H(3)PO(4) activation of three different carbon precursors exhibit enhanced supercapacitive performance in 1 M H(2)SO(4) when highly stable performance can be achieved at potentials larger than the theoretical decomposition potential of water. This ability of P-carbons greatly enhances the energy density of supercapacitors that are capable of delivering 16 Wh/kg compared to 5 Wh/kg for the commercial carbon. An intercept-free multiple linear regression model confirms the strongest influence of phosphorus on capacitance together with micropores 0.65-0.83 nm in width that are the most effective in forming the electric double layer.

Synthesis of Phosphorus-Doped Graphene and its Wide Potential Window in Aqueous Supercapacitors

Phosphorus-doped (P-doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X-ray photoelectron spectroscopy, while the morphology of P-doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P-doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H2 SO4. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P-doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g(-1), providing a high energy density of 11.64 Wh kg(-1) and a high power density of 831 W kg(-1).

Wide Electrochemical Window of Supercapacitors from Coffee Bean‐Derived Phosphorus‐Rich Carbons

Phosphorus-rich carbons (PCs) were prepared by phosphoric acid activation of waste coffee grounds in different impregnation ratios. PCs were characterized by nitrogen and carbon dioxide adsorption and X-ray photoelectron spectroscopy. The results indicate that the activation step not only creates a porous structure, but also introduces various phosphorus and oxygen functional groups to the surface of carbons. As evidenced by cyclic voltammetry, galvanostatic charge/discharge, and wide potential window tests, a supercapacitor constructed from PC-2 (impregnation ratio of 2), with the highest phosphorus content, can operate very stably in 1 M H2 SO4 at 1.5 V with only 18 % degradation after 10 000 cycles at a current density of 5 A g(-1) . Due to the wide electrochemical window, a supercapacitor assembled with PC-2 has a high energy density of 15 Wh kg(-1) at a power density of 75 W kg(-1) . The possibility of widening the potential window above the theoretical potential for the decomposition of water is attributed to reversible electrochemical hydrogen storage in narrow micropores and the positive effect of phosphorus-rich functional groups, particularly the polyphosphates on the carbon surface.

Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries

Well-confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium-sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as-prepared composite material consists of graphene-wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g(-1) at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano-network for use in lithium-sulfur rechargeable batteries.

Effect of the Incorporation of Nitrogen to a Carbon Matrix on the Selectivity and Capacity for Adsorption of Dibenzothiophenes from Model Diesel Fuel

Two synthetic, polymer-derived carbons were modified with urea to incorporate nitrogen surface functional groups. Then they were investigated as adsorbents of dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene (DMDBT) from simulated diesel fuel under dynamic conditions with the total concentration of sulfur being 20 ppmw. The materials before and after adsorption were characterized using elemental analysis, XPS, adsorption of nitrogen, potentiometric titration, and thermal analysis. The incorporation of nitrogen species caused a visible increase in the adsorption capacity. However, selectivity evaluated on the basis of the adsorption of naphthalene decreased. Whereas at low surface coverage the volume of pores smaller than 10 A is important, with the progress of adsorption the surface chemistry gradually starts to play a more important role via either polar or acid/base interactions. The latter are important for the selectivity of adsorption when the aromatic hydrocarbons are present. Although polar interactions are weaker than the acid-base ones, the centers that they represent seem to be more favorable to attract DBT and DMDBT than arenes. There is an indication that nitrogen-containing groups contribute to chemical transformations of DBT and DMDBT/oxidation and promote the involvement of oxygen from the surface groups in the reactive adsorption.

A comparative study of chemical treatment by FeCl3, MgCl2, and ZnCl2 on microstructure, surface chemistry, and double-layer capacitance of carbons from waste biomass

The effect of chemical treatment on the capacitance of carbon electrodes prepared from waste coffee grounds was investigated. Coffee grounds were impregnated with FeCl3 and MgCl2 and then treated at 900 degrees C. The resultant carbons were compared with activated coffee ground carbons prepared by ZnCl2 treatment. The carbon treatment processes of FeCl3 and MgCl2 were studied using thermal gravimetric analysis. Raman spectroscopy, x-ray photoelectron spectroscopy, and N-2 and CO2 adsorption were used to characterize the activated carbons. Activation with ZnCl2 and FeCl3 produced carbons with higher surface areas (977 and 846 m(2)/g, respectively) than treatment with MgCl2 (123 m(2)/g). Electrochemical double-layer capacitances of the carbons were evaluated in 1 M H2SO4 using two-electrode cells. The system with FeCl3-treated carbon electrodes provided a specific cell capacitance of 57 F/g.

Understanding the stepwise capacity increase of high energy low-Co Li-rich cathode materials for lithium ion batteries

Li-rich layered materials as promising high-energy cathode candidates have attracted much attention in recent years for next generation lithium ion batteries. However, the fundamental mechanism of high specific capacity in these cathode materials has not been fully revealed so far. In this work, we report a new class of Li-rich cathode materials Li[CoxLi1/3−x/3Mn2/3−2x/3]O2 (x = 0.087, 0.1, and 0.118) with a very low level of Co doping, which exhibit impressive stepwise capacity increase over dozens of cycles from less than 50 mA h g−1 to around 250 mA h g−1. A systematic study on their composition, crystal structure and electrochemical performance revealed that the small change of Co content has negligible effect on the crystal structure and morphology, but plays an important role in enhancing the activation rate of the Li2MnO3 phase. In addition, the optimized cycling potential window and current rate were proven to be critically important for effective Li2MnO3 activation and better long-term cycling stability.

Nitrogen and phosphorous co-doped graphene monolith for supercapacitors

The co-doping of heteroatoms has been regarded as a promising approach to improve the energy-storage performance of graphene-based materials because of the synergetic effect of the heteroatom dopants. In this work, a single precursor melamine phosphate was used for the first time to synthesise nitrogen/phosphorus co-doped graphene (N/P-G) monoliths by a facile hydrothermal method. The nitrogen contents of 4.27-6.58 at% and phosphorus levels of 1.03-3.00 at% could be controlled by tuning the mass ratio of melamine phosphate to graphene oxide in the precursors. The N/P-G monoliths exhibited excellent electrochemical performances as electrodes for supercapacitors with a high specific capacitance of 183 F g(-1) at a current density of 0.05 A g(-1), good rate performance and excellent cycling performance. Additionally, the N/P-G electrode was stable at 1.6 V in 1 m H2 SO4 aqueous electrolyte and delivered a high energy density of 11.33 Wh kg(-1) at 1.6 V.

Changes in surface chemistry of carbon materials upon electrochemical measurements and their effects on capacitance in acidic and neutral electrolytes

Four porous carbon samples with very similar porosities but visible differences in their surface chemistry are investigated as supercapacitor electrodes in 1 M H₂SO₄ and 3 M NaCl. The key objective is to monitor the changes to the oxygen- and nitrogen-containing functionalities in oxygen- and nitrogen+oxygen-rich carbons upon a three-electrode test and the effect of these changes on the energy storage capacity in a real two-electrode supercapacitor setup. The carbon samples are thoroughly characterized by nitrogen sorption measurements, Raman spectroscopy, potentiometric titrations, elemental analysis, and synchrotron XPS. The findings presented in this work imply that the pretreatment of the oxygen- and nitrogen+oxygen-rich carbons under the conditions of the three-electrode test in an acidic electrolyte are beneficial to the overall energy storage capacity as the pores become more accessible to the electrolyte ions and the contribution of pseudocapacitive oxygen-containing groups increases in the oxygen-rich carbons, whereas favorable changes to the electronic structure take place in the nitrogen+oxygen-rich carbons. Thus, the total capacitance increases as a result of the improved double-layer capacitance as well as pseudocapacitance. Greater capacitance after the three-electrode test is also measured in a neutral electrolyte for both sets of samples, which is a result of improved double-layer capacitance upon the removal of some oxygen-containing functional groups that leads to better accessibility of the pores.