Kathrin Preuss

Fe–N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions

High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen reduction reaction in both alkaline and acidic media. In alkaline conditions, they exhibit more positive onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than commercial Pt/C, while in acidic media the onset potential is comparable to that of commercial Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Additionally, the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to commercial Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chemical functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large number of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alkaline anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a commercial Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.

The influence of pore size distribution on the oxygen reduction reaction performance in nitrogen doped carbon microspheres

Nitrogen-doped carbon microspheres with tunable porosity are investigated as electrocatalysts for the oxygen reduction reaction (ORR). The materials were synthesized by “nanocasting” involving the use of pyrrole as the carbon source and N-dopant, and porous silica microspheres as template. The engineered nitrogen-doped carbon particles combine several indispensable characteristics for a highly active metal-free carbon electrocatalyst: (i) a high content of nitrogen functionalities (∼8 wt%) mainly distributed in quaternary and pyridinic groups, which are highly active catalytic centers for the ORR reaction, and (ii) a high specific surface area (1200–1300 m2 g−1). Furthermore, the porosity of the N-doped microspheres can be modulated from a micro- to a mesoporous structure, i.e. from a micropore size distribution centered at ∼1 nm to a widely accessible mesoporosity with two mesopores systems (∼3 nm and ∼14 nm). The electrocatalytic activity of the N-doped carbon microspheres in the oxygen reduction reaction (ORR) was studied in both basic and acid media. Both types of materials catalyze the ORR via the efficient 4-electron process. However, the mesoporous carbon exhibits a more positive onset potential and a higher kinetic current density than the microporous microspheres and noteworthy the values are comparable to those of commercial Pt/C under basic conditions. Moreover, the mesoporous microspheres also show a better electrocatalytic activity than the microporous ones in acid medium, and a similar onset potential to that of Pt/C with a peroxide yield lower than 10%. A detailed comparison between the N-doped micro- and mesoporous microspheres reveals that the mesoporous material outperforms the microporous one not only in catalytic activity but also in durability in both electrolytes, which proves that the bimodal mesoporous structure acts as interconnected highways providing quick and full transport towards/from the catalytic sites for both reactant and products. This leads in turn to an effective metal-free carbon catalyst that can match the commercial Pt/C catalyst.

Nanoporous Materials for the Onboard Storage of Natural Gas

Climate change, global warming, urban air pollution, energy supply uncertainty and depletion, and rising costs of conventional energy sources are, among others, potential socioeconomic threats that our community faces today. Transportation is one of the primary sectors contributing to oil consumption and global warming, and natural gas (NG) is considered to be a relatively clean transportation fuel that can significantly improve local air quality, reduce greenhouse-gas emissions, and decrease the energy dependency on oil sources. Internal combustion engines (ignited or compression) require only slight modifications for use with natural gas; rather, the main problem is the relatively short driving distance of natural-gas-powered vehicles due to the lack of an appropriate storage method for the gas, which has a low energy density. The U.S. Department of Energy (DOE) has set some targets for NG storage capacity to obtain a reasonable driving range in automotive applications, ruling out the option of storing methane at cryogenic temperatures. In recent years, both academia and industry have foreseen the storage of natural gas by adsorption (ANG) in porous materials, at relatively low pressures and ambient temperatures, as a solution to this difficult problem. This review presents recent developments in the search for novel porous materials with high methane storage capacities. Within this scenario, both carbon-based materials and metal-organic frameworks are considered to be the most promising materials for natural gas storage, as they exhibit properties such as large surface areas and micropore volumes, that favor a high adsorption capacity for natural gas. Recent advancements, technological issues, advantages, and drawbacks involved in natural gas storage in these two classes of materials are also summarized. Further, an overview of the recent developments and technical challenges in storing natural gas as hydrates in wetted porous carbon materials is also included. Finally, an analysis of design factors and technical issues that need to be considered before adapting vehicles to ANG technology is also presented.

Salt Templating with Pore Padding: Hierarchical Pore Tailoring towards Functionalised Porous Carbons

We propose a new synthetic route towards nanoporous functional carbon materials based on salt templating with pore-padding approach (STPP). STPP relies on the use of a pore-padding agent that undergoes an initial polymerisation/ condensation process prior to the formation of a solid carbon framework. The pore-padding agent allows tailoring hierarchically the pore-size distribution and controlling the amount of heteroatom (nitrogen in this case) functionalities as well as the type of nitrogen (graphitic, pyridinic, oxides of nitrogen) incorporated within the carbon framework in a single-step-process. Our newly developed STPP method offers a unique pathway and new design principle to create simultaneously high surface area, microporosity, functionality and pore hierarchy. The functional carbon materials produced by STPP showed a remarkable CO2 /N2 selectivity. At 273 K, a carbon with only micropores offered an exceptionally high CO2 adsorption capacity whereas a carbon with only mesopores showed promising CO2 -philicity with high CO2 /N2 selectivity in the range of 46-60 %, making them excellent candidates for CO2 capture from flue gas or for CO2 storage.

Sustainable metal-free carbogels as oxygen reduction electrocatalysts

Tuneable nitrogen doped carbogels have been synthesised by a simple one-pot hydrothermal carbonisation, followed by pyrolysis at 1000 °C, using highly available and low cost precursors such as glucose and ovalbumin. Different physical activation ratios of nitrogen/oxygen were used to demonstrate a sustainable and easy method for changing surface area, pore size and elemental composition in order to investigate their effect on the oxygen reduction reaction when used as electrocatalysts. A ratio of nitrogen mixed with 2% of oxygen was found to be most beneficial for enhancing the catalytic activity by creating a high surface area of 874 m2 g−1 as well as a favourable ratio of pyridinic to graphitic nitrogen. The influence of sulphur doping and/or boron on the carbogel structure was investigated. Incorporation of sulphur does not interfere with the structure formation, but decreases the surface area and nitrogen content resulting in diminished ORR performance. However, boron doping with boric acid results in a different carbogel structure by acting as a catalyst, creating an altered morphology, surface area, pore properties and higher nitrogen content by fully utilising ovalbumin as a nitrogen source instead of as a structure directing/surface stabilising agent. Nitrogen content is found to determine the limiting current, while the oxygen content has a small influence on the onset potential. An assumed synergistic effect between nitrogen and boron generates higher electron transfer numbers and lower hydrogen peroxide yields in boron nitrogen co-doped carbogels than those observed in purely nitrogen doped systems.

Biomass-Derived Nitrogen-Doped Carbon Aerogel Counter Electrodes for Dye Sensitized Solar Cells

Dye sensitized solar cells have emerged as an attractive alternative to conventional solar cells due to their easy processing and the abundance and low cost of their materials. However, the counter electrode in these cells employs platinum which significantly impacts their cost. Here, we report biomass-derived, nitrogen-doped carbon aerogel as an effective alternative to conventional platinum-based counter electrodes for dye sensitized solar cells. A stable suspension of biomass-derived, nitrogen-doped carbon aerogel was prepared in DMF by using oleylamine as a binder. The nitrogen-doped carbon aerogel electrode was annealed at different temperatures, and its impact on photovoltaic performance is investigated. I-V measurements confirm that the annealing temperature substantially enhances the photovoltaic parameters of these devices; these enhancements are linked to the removal of the organic binders. Electrochemical impedance spectra of the counter electrodes confirm that removal of oleylamine in nitrogen-doped carbon aerogels reduces the series resistance of the resulting electrodes. The power conversion efficiency of the solar cells from optimized nitrogen-doped carbon aerogel exhibited comparable efficiency to that of a cell fabricated using a platinum-based counter electrode. This study demonstrates the potential of biomass-derived carbon aerogels as a cheap and sustainable replacement of platinum in DSSCs.