Efficient Metal-Free Electrocatalysts for Oxygen Reduction: Polyaniline-Derived N- and O‑Doped Mesoporous Carbons

The oxygen reduction reaction (ORR)one of the two half-reactions in fuel cellsis one of the bottlenecks that has prevented fuel cells from finding a wide range of applications today. This is because ORR is inherently a sluggish reaction; it is also because inexpensive and sustainable ORR electrocatalysts that are not only efficient but also are based on earth-abundant elements are hard to come by. Herein we report the synthesis of novel carbon-based materials that can contribute to solving these challenges associated with ORR. Mesoporous oxygen- and nitrogen-doped carbons were synthesized from <i>in situ</i> polymerized mesoporous silica-supported polyaniline (PANI) by carbonization of the latter, followed by etching away the mesoporous silica template from it. The synthetic method also allowed the immobilization of different metals such as Fe and Co easily into the system. While all the resulting materials showed outstanding electrocatalytic activity toward ORR, the metal-free, PANI-derived mesoporous carbon (dubbed PDMC), in particular, exhibited the highest activity, challenging conventional paradigms. This unprecedented activity by the metal-free PDMC toward ORR was attributed to the synergetic activities of nitrogen and oxygen (or hydroxyl) species that were implanted in it by PANI/mesoporous silica during pyrolysis

Mesoporous TiO₂ Comprising Small, Highly Crystalline Nanoparticles for Efficient CO₂ Reduction by H₂O

The conversion of CO₂ into hydrocarbon fuels with H₂O using low-cost photocatalysts can offer a sustainable route to meet some of our energy needs, besides being able to contribute to the solutions of global warming. In this work, a series of highly crystalline mesoporous titanium dioxide (TiO₂) photocatalysts are synthesized via a simple template-free synthetic method. The synthesis involves preparation of titanium glycolate microspheres (TGMs), then hydrolysis of the TGMs in boiling water under ambient pressure, and finally calcination of the products in air. The hydrolysis step is found to play a crucial role in the formation of TiO₂ microspheres comprising a network of small anatase grains. The hydrolysis of the TGMs is also found to considerably inhibit the possible phase transformation of anatase to rutile during the subsequent high-temperature crystallization process. The resulting materials have good crystallinity and efficient charge carrier separation capabilities, as well as large specific surface areas, and thus large density of accessible catalytically active sites. These unique structural features enable these materials to exhibit high photocatalytic activities for the conversion of CO₂ with H₂O into hydrocarbon fuels (CH₄) and to show much better catalytic activities than that of the commercial photocatalyst Degussa P25 TiO₂

N‑, O‑, and S‑Tridoped Nanoporous Carbons as Selective Catalysts for Oxygen Reduction and Alcohol Oxidation Reactions

Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein we report the synthesis of N-, O-, and S-tridoped, polypyrrole-derived nanoporous carbons (NOSCs) that can serve as metal-free, selective electrocatalysts and catalysts for oxygen reduction reaction (ORR) and alcohol oxidation reaction (AOR), respectively. The NOSCs are synthesized via polymerization of pyrrole using (NH₄)₂S₂O₈ as oxidant and colloidal silica nanoparticles as templates, followed by carbonization of the resulting S-containing polypyrrole/silica composite materials and then removal of the silica templates. The NOSCs exhibit good catalytic activity toward ORR with low onset potential and low Tafel slope, along with different electron-transfer numbers, or in other words, different ratios H₂O/H₂O₂ as products, depending on the relative amount of colloidal silica used as templates. The NOSCs also effectively catalyze AOR at relatively low temperature, giving good conversions and high selectivity

Magnetic Activated Carbon Derived from Biomass Waste by Concurrent Synthesis: Efficient Adsorbent for Toxic Dyes

The development of advanced carbon nanomaterials that can efficiently extract pollutants from solutions is of great interest for environmental remediation and human safety. Herein we report the synthesis of magnetic activated carbons via simultaneous activation and magnetization processes using carbonized biomass waste from coconut shells (Cb’s) and FeCl₃·6H₂O as precursor. We also show the ability of the materials to efficiently extract toxic organic dyes from solutions and their ease of separation and recovery from the solutions using a simple bar magnet. Textural characterization shows that the materials are microporous. Further analyses of the deconvoluted XPS spectra and X-ray diffraction patterns reveal that the materials possess magnetite, maghemite and hematite. SEM and TEM images show that an increase in the ratio of FeCl₃·6H₂O:Cb leads to an increase in the material’s magnetic properties. The point of zero charge (pH_pzc) indicates that the materials have acidic characteristics. Adsorption kinetic studies carried out onto MAC1 indicates that the Elovich model can satisfactorily describe the experimental data at low initial concentrations and the pseudo-second order model can best fit the data at higher initial concentrations. Moreover, adsorption equilibrium studies reveal that the Langmuir model adequately allows the determination of the materials’ adsorption capacity. Our adsorption and equilibrium fit of the data include nonlinear models and are thus more informative compared with those in other recent, related works, in which only linear fits have been presented. Extensive mechanistic studies for the adsorption processes are also included in the work

Dendritic Silica Nanomaterials (KCC-1) with Fibrous Pore Structure Possess High DNA Adsorption Capacity and Effectively Deliver Genes In Vitro

The pore size and pore structure of nanoporous materials can affect the materials’ physical properties, as well as potential applications in different areas, including catalysis, drug delivery, and biomolecular therapeutics. KCC-1, one of the newest members of silica nanomaterials, possesses fibrous, large pore, dendritic pore networks with wide pore entrances, large pore size distribution, spacious pore volume and large surface areastructural features that are conducive for adsorption and release of large guest molecules and biomacromolecules (e.g., proteins and DNAs). Here, we report the results of our comparative studies of adsorption of salmon DNA in a series of KCC-1-based nanomaterials that are functionalized with different organoamine groups on different parts of their surfaces (channel walls, external surfaces or both). For comparison the results of our studies of adsorption of salmon DNA in similarly functionalized, MCM-41 mesoporous silica nanomaterials with cylindrical pores, some of the most studied silica nanomaterials for drug/gene delivery, are also included. Our results indicate that, despite their relatively lower specific surface area, the KCC-1-based nanomaterials show high adsorption capacity for DNA than the corresponding MCM-41-based nanomaterials, most likely because of KCC-1’s large pores, wide pore mouths, fibrous pore network, and thereby more accessible and amenable structure for DNA molecules to diffuse through. Conversely, the MCM-41-based nanomaterials adsorb much less DNA, presumably because their outer surfaces/cylindrical channel pore entrances can get blocked by the DNA molecules, making the inner parts of the materials inaccessible. Moreover, experiments involving fluorescent dye-tagged DNAs suggest that the amine-grafted KCC-1 materials are better suited for delivering the DNAs adsorbed on their surfaces into cellular environments than their MCM-41 counterparts. Finally, cellular toxicity tests show that the KCC-1-based materials are biocompatible. On the basis of these results, the fibrous and porous KCC-1-based nanomaterials can be said to be more suitable to carry, transport, and deliver DNAs and genes than cylindrical porous nanomaterials such as MCM-41

Yeast Cells-Derived Hollow Core/Shell Heteroatom-Doped Carbon Microparticles for Sustainable Electrocatalysis

The use of renewable resources to make various synthetic materials is increasing in order to meet some of our sustainability challenges. Yeast is one of the most common household ingredients, which is cheap and easy to reproduce. Herein we report that yeast cells can be thermally transformed into hollow, core–shell heteroatom-doped carbon microparticles that can effectively electrocatalyze the oxygen reduction and hydrazine oxidation reactions, reactions that are highly pertinent to fuel cells or renewable energy applications. We also show that yeast cell walls, which can easily be separated from the cells, can produce carbon materials with electrocatalytic activity for both reactions, albeit with lower activity compared with the ones obtained from intact yeast cells. The results reveal that the intracellular components of the yeast cells such as proteins, phospholipids, DNAs and RNAs are indirectly responsible for the latter’s higher electrocatalytic activity, by providing it with more heteroatom dopants. The synthetic method we report here can serve as a general route for the synthesis of (electro)­catalysts using microorganisms as raw materials

Conducting MoS₂ Nanosheets as Catalysts for Hydrogen Evolution Reaction

We report chemically exfoliated MoS₂ nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS₂ nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS₂, we found that the activity of 2H MoS₂ is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS₂ remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS₂

High-Index Faceted Ni₃S₂ Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting

Elaborate design of highly active and stable catalysts from Earth-abundant elements has great potential to produce materials that can replace the noble-metal-based catalysts commonly used in a range of useful (electro)­chemical processes. Here we report, for the first time, a synthetic method that leads to <i>in situ</i> growth of {2̅10} high-index faceted Ni₃S₂ nanosheet arrays on nickel foam (NF). We show that the resulting material, denoted Ni₃S₂/NF, can serve as a highly active, binder-free, bifunctional electro­catalyst for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Ni₃S₂/NF is found to give ∼100% Faradaic yield toward both HER and OER and to show remarkable catalytic stability (for >200 h). Experimental results and theoretical calculations indicate that Ni₃S₂/NF’s excellent catalytic activity is mainly due to the synergistic catalytic effects produced in it by its nanosheet arrays and exposed {2̅10} high-index facets

Highly Active, Nonprecious Electrocatalyst Comprising Borophene Subunits for the Hydrogen Evolution Reaction

Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm²: a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of α-phase molybdenum diboride (α-MoB₂) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS₂-based catalysts, those of α-MoB₂ can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm α-MoB₂ can deliver large current densities in the order of 1000 mA/cm², and also has excellent catalytic stability during HER. The theoretical and experimental results show α-MoB₂’s catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property

Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst

Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H₂ from formic acid