Doped-carbon electrocatalysts with trimodal porosity from a homogeneous polypeptide gel

One of the biggest challenges for materials science is to design facile routes to structurally complex materials, which is particularly important for global applications such as fuel cells. Doped nanostructured carbons are targeted as noble metal-free electrocatalysts for this purpose. Their intended widespread use, however, necessitates simple and robust preparation methods that do not compromise on material performance. Here, we demonstrate a versatile one-pot synthesis of nitrogen-doped carbons that exploits the templating ability of biological polymers. Starting with just metal nitrates and gelatin, multiphase C/Fe3C/MgO nanomaterials are formed, which are then etched to produce active carbon electrocatalysts with accessible trimodal porosity. These show remarkable performance in the oxygen reduction reaction – a key process in proton exchange membrane fuel cells. The activity is comparable to commercial platinum catalysts and shows improved stability with reduced crossover effects. This simple method offers a new route to widely applicable porous multicomponent nanocomposites

Molecular-Level Insights into Oxygen Reduction Catalysis by Graphite-Conjugated Active Sites

Using a combination of experimental and computational investigations, we assemble a consistent mechanistic model for the oxygen reduction reaction (ORR) at molecularly well-defined graphite-conjugated catalyst (GCC) active sites featuring aryl-pyridinium moieties $(N^+-GCC)$. ORR catalysis at glassy carbon surfaces modified with $N^+-GCC$ fragments displays near-first-order dependence in $O_2$ partial pressure and near-zero-order dependence on electrolyte pH. Tafel analysis suggests an equilibrium one-electron transfer process followed by a rate-limiting chemical step at modest overpotentials that transitions to a rate-limiting electron transfer sequence at higher overpotentials. Finite-cluster computational modeling of the $N^+-GCC$ active site reveals preferential $O_2$ adsorption at electrophilic carbons alpha to the pyridinium moiety. Together, the experimental and computational data indicate that ORR proceeds via a proton-decoupled $O_2$ activation sequence involving either concerted or stepwise electron transfer and adsorption of $O_2$, which is then followed by a series of electron/proton transfer steps to generate water and turn over the catalytic cycle. The proposed mechanistic model serves as a roadmap for the bottom-up synthesis of highly active N-doped carbon ORR catalysts

Computational chemistry for graphene-based energy applications: progress and challenges

YesResearch in graphene-based energy materials is a rapidly growing area. Many graphene-based energy applications involve interfacial processes. To enable advances in the design of these energy materials, such that their operation, economy, efficiency and durability is at least comparable with fossil-fuel based alternatives, connections between the molecular-scale structure and function of these interfaces are needed. While it is experimentally challenging to resolve this interfacial structure, molecular simulation and computational chemistry can help bridge these gaps. In this Review, we summarise recent progress in the application of computational chemistry to graphene-based materials for fuel cells, batteries, photovoltaics and supercapacitors. We also outline both the bright prospects and emerging challenges these techniques face for application to graphene-based energy materials in future.vesk

Nanostructured materials for water purification: synthesis, insights and performance evaluation

Membrane filtration and Advanced Oxidative Processes (AOPs) are among the most efficient and cost-effective methods employed in water purification. A system to integrate the two methods using photoactive colloidal particles was studied in this thesis, with the final purpose of overcoming membrane fouling, one of the main issues occurring in filtration processes. The production of nanostructured TiO2 microparticles through a simple and extremely rapid synthesis and an easy method to assemble a multifunctional coating, integrating inorganic particles on filtration membranes, were targeted as the most promising solutions from the technological and environmental point of view. The control of microwave-assisted heating applied to hydrothermal treatments, a relatively recent synthetic method, allowed the production of nanostructured mesoporous spherical TiO2 particles, bringing the synthesis to the minute scale, extremely rapid compared with conventional heating, and achieving products otherwise difficult to obtain without the help of surfactants or templating agent. The as-synthesised particles showed photoactivity under visible light, with rate of specific reactions (selective de-ethylation) 4 times higher compared with commercial photocatalysts. Furthermore, the particles were modified to extend the limited intrinsic absorbance of TiO2 in the visible light, with promising results given by formation of stoichiometric defects (in particular oxygen vacancies) through annealing under vacuum. This treatment allowed the achievement of comparable or even higher performance in photocatalytic degradation of rhodamine B with respect to commercial TiO2 photocatalysts, including Aeroxide P25, with degradation rate towards organic molecules (rhodamine B) of even 60-70% after 1 hours, compared to the 25% of P25. The production of a multifunctional coating for water treatment by integration of colloidal and nanometric TiO2 particles has been also studied. A simple technique to integrate TiO2 nanoparticles onto different substrate, in particular filtration membranes, was developed by simple electrostatic interactions involving the use of polyelectrolytes, water-soluble charged polymer forming organised layers when assembled in a macromolecular structure defined as polyelectrolyte multilayers (PEMs). Electrostatic assembly was applied as an environmentally friendly technique to anchor nanoparticles (P25) on different surfaces, transferring their properties to these. In particular, the application of TiO2 particles conferred hydrophilic and superhydrophilic to a relatively hydrophobic surface (Mylar) by controlling the multilayer assembly conditions, in particular the ionic strength of the polyelectrolyte solutions. The achievement of superhydrophilic behaviour on the treated surfaces, with contact angles below 15° on Mylar surfaces, and the possibility of removing fouled active layer from a membrane replacing it with a newly generated one can be both implemented as potential antifouling strategies in water treatment

Surface-mediated selective photocatalytic aerobic oxidation reactions on TiO2 nanofibres

N-doped TiO2 nanofibres were observed to possess lower aerobic oxidation activity than undoped TiO2 nanofibres in the selective photocatalytic aerobic oxidation of enzylamine and 4-methoxybenzyl alcohol. This was attributed to the reduction free energy of O2 adsorption in the vicinity of nitrogen dopant sites, as indicated by density functional theory (DFT) calculations when three-coordinated oxygen atoms are substituted by nitrogen atoms. It was found that the activity recovered following a controlled calcination of the N-doped NFs in air. The dependence of the conversion of benzylamine and 4-methoxybenzyl alcohol on the intensity of light irradiation confirmed that these reactions were driven by light. Action spectra showed that the two oxidation reactions are responsive to light from the UV region through to the visible light irradiation range. The extended light absorption wavelength range in these systems compared to pure TiO2 materials was found to result from the formation of surface complex species following adsorption of reactants onto the catalysts' surface, evidenced by the in situ IR experiment. Both catalytic and in situ IR results reveal that benzaldehyde is the intermediate in the aerobic oxidation of benzylamine to N-benzylidenebenzylamine process

Electronic and Chemical Properties of Donor, Acceptor Centers in Graphene

Chemical doping is one of the most suitable ways of tuning the electronic properties of graphene and a promising candidate for a band gap opening. In this work we report a reliable and tunable method for preparation of high-quality boron and nitrogen co-doped graphene on silicon carbide substrate. We combine experimental (dAFM, STM, XPS, NEXAFS) and theoretical (total energy DFT and simulated STM) studies to analyze the structural, chemical, and electronic properties of the single-atom substitutional dopants in graphene. We show that chemical identification of boron and nitrogen substitutional defects can be achieved in the STM channel due to the quantum interference effect, arising due to the specific electronic structure of nitrogen dopant sites. Chemical reactivity of single boron and nitrogen dopants is analyzed using force–distance spectroscopy by means of dAFM

Controlling Hydrogen-Transfer Rate in Molecules on Graphene by Tunable Molecular Orbital Levels

International audienceMolecular switches are building blocks of potential interest to store binary information, especially when they can be organized in periodic lattices. Among the variety of possible systems, switches based on hydrogen transfer are of special importance because they allow the switching operation to occur without severe conformational change that may interfere with neighboring molecular units. We have studied the excitation process of hydrogen transfer inside porphyrin molecules assembled on a graphene surface, using a low-temperature scanning tunneling microscope. We show that this hydrogen transfer is induced by an electronic resonant tunneling process through the molecular orbitals. Using nitrogen doping of graphene, we tune the rate of hydrogen transfer by shifting the molecular orbital energies owing to the charge transfer at nitrogen dopant sites in the graphene lattice. The control of the switching process allows the storage of information inside a molecular lattice, which is demonstrated by writing an artificial pattern inside a molecular island

Role of Nitrogen-Doped Graphene for Improved High-Capacity Potassium Ion Battery Anodes

Potassium is an earth abundant alternative to lithium for rechargeable batteries, but a critical limitation in potassium ion battery anodes is the low capacity of KC₈ graphite intercalation compounds in comparison to conventional LiC₆. Here we demonstrate that nitrogen doping of few-layered graphene can increase the storage capacity of potassium from a theoretical maximum of 278 mAh/g in graphite to over 350 mAh/g, competitive with anode capacity in commercial lithium ion batteries and the highest reported anode capacity so far for potassium ion batteries. Control studies distinguish the importance of nitrogen dopant sites as opposed to sp³ carbon defect sites to achieve the improved performance, which also enables >6× increase in rate performance of doped <i>vs</i> undoped materials. Finally, <i>in situ</i> Raman spectroscopy studies elucidate the staging sequence for doped and undoped materials and demonstrate the mechanism of the observed capacity enhancement to be correlated with distributed storage at local nitrogen sites in a staged KC₈ compound. This study demonstrates a pathway to overcome the limitations of graphitic carbons for anodes in potassium ion batteries by atomically precise engineering of nanomaterials