Turning Polyethylene Waste to Hydrocarbons Using a Sustainable Acidic Carbocatalyst

Careless release of plastic waste is a pressing problem for marine and other eco-environments, and materials recycling of this stream is an open problem. For this purpose, a new metal-free acidic carbocatalyst with 8 wt % sulfur is constructed from a side product of the paper industry namely Na-lignosulfonate. The catalyst shows an extraordinary performance for the fragmentation of polymer waste which smoothly occurs above the ceiling temperature of the polymers. The reaction is run without hydrogen and at ambient pressure with commercially available high-density polyethylene (HDPE) as well as a real polymer waste mixture of high and low-density polyethylene (HDPE, LDPE). In all cases, a homologous series of n-alkanes and n-alkenes are obtained. The unique sulfur-rich carbonaceous structure (transfer hydrogenation functionality) and the metal-free character of the acidic carbocatalyst makes it inert against many typical catalyst poisons, among them water, salt, polar functionalities, and sulfur species. The described performance in plastic recycling, as well as the low cost and large-scale availability of lignosulfonate from the pulp industry, makes this metal-free acidic carbocatalyst promising for real-life environmental applications.

How to Make a Cocktail of Palladium Catalysts with Cola and Alcohol: Heteroatom Doping vs. Nanoscale Morphology of Carbon Supports

Sparkling drinks such as cola can be considered an affordable and inexpensive starting material consisting of carbohydrates and sulfur- and nitrogen-containing organic substances in phosphoric acid, which makes them an excellent precursor for the production of heteroatom-doped carbon materials. In this study, heteroatom-doped carbon materials were successfully prepared in a quick and simple manner using direct carbonization of regular cola and diet cola. The low content of carbon in diet cola allowed reaching a higher level of phosphorus in the prepared carbon material, as well as obtaining additional doping with nitrogen and sulfur due to the presence of sweeteners and caffeine. Effects of carbon support doping with phosphorus, nitrogen and sulfur, as well as of changes in textural properties by ball milling, on the catalytic activity of palladium catalysts were investigated in the Suzuki–Miyaura and Mizoroki–Heck reactions. Contributions of the heteroatom doping and specific surface area of the carbon supports to the increased activity of supported catalysts were discussed. Additionally, the possibility of these reactions to proceed in 40% potable ethanol was studied. Moreover, transformation of various palladium particles (complexes and nanoparticles) in the reaction medium was detected by mass spectrometry and transmission electron microscopy, which evidenced the formation of a cocktail of catalysts in a commercial 40% ethanol/water solution.

Pyrolysis of Porous Organic Polymers under a Chlorine Atmosphere to Produce Heteroatom-Doped Microporous Carbons

Three types of cross-linked porous organic polymers (either oxygen-, nitrogen-, or sulfur-doped) were carbonized under a chlorine atmosphere to obtain chars in the form of microporous heteroatom-doped carbons. The studied organic polymers constitute thermosetting resins obtained via sol-gel polycondensation of resorcinol and five-membered heterocyclic aldehydes (either furan, pyrrole, or thiophene). Carbonization under highly oxidative chlorine (concentrated and diluted Cl2 atmosphere) was compared with pyrolysis under an inert helium atmosphere. All pyrolyzed samples were additionally annealed under NH3. The influence of pyrolysis and additional annealing conditions on the carbon materials' porosity and chemical composition was elucidated.

Interactions of Fe-N-S Co-Doped Porous Carbons with Bacteria: Sorption Effect and Enzyme-Like Properties

Carbon-based (nano)materials doped with transition metals, nitrogen and other heteroatoms are considered active heterogeneous catalysts in a wide range of chemical processes. Recently they have been scrutinized as artificial enzymes since they can catalyze proton-coupled electron transfer reactions vital for living organisms. Herein, interactions between Gram-positive and Gram-negative bacteria and either metal-free N and/or S doped or metal containing Fe-N-S co-doped porous carbons are studied. The Fe- and N-co-doped porous carbons (Fe-N-C) exhibit enhanced affinity toward bacteria as they show the highest adsorption capacity. Fe-N-C materials also show the strongest influence on the bacteria viability with visible toxic effect. Both types of bacteria studied reacted to the presence of Fe-doped carbons in a similar manner, showing a decrease in dehydrogenases activity in comparison to controls. The N-coordinated iron-doped carbons (Fe-N-C) may exhibit oxidase/peroxidase-like activity and activate O2 dissolved in the solution and/or oxygen-containing species released by the bacteria (e.g., H2O2) to yield highly bactericidal reactive oxygen species. As Fe/N/ and/or S-doped carbon materials efficiently adsorb bacteria exhibiting simultaneously antibacterial properties, they can be applied, inter alia, as microbiological filters with enhanced biofouling resistance.

Heteroatom-Doped Carbon Materials for Hydrazine Oxidation

The key in designing efficient direct liquid fuel cells (DLFCs), which can offer some solutions to society's grand challenges associated with sustainability and energy future, currently lies in the development of cost-effective electrocatalysts. Among the many types of fuel cells, direct hydrazine fuel cells (DHFCs) are of particular interest, especially due to their high theoretical cell voltages and clean emission. However, DHFCs currently use noble-metal-based electrocatalysts, and the scarcity and high cost of noble metals are hindering these fuel cells from finding large-scale practical applications. In order to replace noble-metal-based electrocatalysts with sustainable ones and help DHFCs become widely usable, great efforts are being made to develop stable heteroatom (e.g., B, N, O, P and S)-doped carbon electrocatalysts, the activities of which are comparable to, or better than, those of noble metals. Here, the recent research progress and the advancements made on the development of heteroatom-doped carbon materials, their general properties, their electrocatalytic activities toward the HzOR, and their dopant- and structure-related electrocatalytic properties for the HzOR are summarized. Perspectives on the different directions that the research endeavors in this field need to take in the future and the challenges associated with DHFCs are included.

Three-Dimensional Biocarbon Framework Coupled with Uniformly Distributed FeSe Nanoparticles Derived from Pollen as Bifunctional Electrocatalysts for Oxygen Electrode Reactions

The development of carbon-based catalysts with satisfactory performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical but challenging to achieve sustainable energy conversion and storage. Herein, a pollen biomass-derived carbon electrocatalyst with a three-dimensional porous framework, coupled with uniform distribution of FeSe nanoparticles, has been prepared by pyrolysis of the pollen precursor, followed by selenylation. The optimal catalyst FeSe/NC-PoFeSe exhibits a superb ORR activity with a half-wave potential of 0.86 V versus a reversible hydrogen electrode and OER activity with a low overpotential (330 mV at 10 mA cm^-2) in alkaline media, compared with commercial Pt/C and IrO₂/C catalysts, respectively. On the basis of the characterization results, we ascribe the enhanced ORR performance to sufficient Fe-N _x, pyridinic N, and graphitic N and the excellent OER performance in the presence of FeSe nanoparticles uniformly mounted on the N-doped carbon materials. In addition, we believe that the coupling effect between the FeSe nanoparticles and biocarbon led to a further improvement in the electrochemical performance. Significantly, the prominent ORR and OER stability of FeSe/NC-PoFeSe shows great promise in renewable energy devices.

Template Conversion of Covalent Organic Frameworks into 2D Conducting Nanocarbons for Catalyzing Oxygen Reduction Reaction

Progress over the past decades in porous materials has exerted great effect on the design of metal-free carbon electrochemical catalysts in fuel cells. The carbon material must combine three functions, i.e., electrical conductivity for electron transport, optimal pores for ion motion, and abundant heteroatom sites for catalysis. Here, an ideal carbon catalyst is achieved by combining two strategies-the use of a 2D covalent organic framework (COF) and the development of a suitable template to guide the pyrolysis. The COF produces nanosized carbon sheets that combine high conductivity, hierarchical porosity, and abundant heteroatom catalytic edges. The catalysts achieve superior performance to authentic Pt/C with exceptional onset potential (0 V vs -0.03 V), half-wave potentials (-0.11 V vs -0.16 V), high limit current density (7.2 mA cm^-2 vs 6.0 mA cm^-2 ), low Tafel slope (110 mV decade^-1 vs 121 mV decade^-1 ), long-time stability, and methanol tolerance. These results reveal a novel material platform based on 2D COFs for designing novel 2D carbon materials.

Cobalt Phosphide Coupled with Heteroatom-Doped Nanocarbon Hybrid Electroctalysts for Efficient, Long-Life Rechargeable Zinc-Air Batteries

Metal phosphides and heteroatom-doped carbons have been regarded as promising candidates as bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, both have suffered from stability issues during repeated ORR and OER operations in zinc-air batteries (ZABs). Herein, this study reports a versatile cobalt-based hybrid catalyst with a 1D structure by integrating the metal-organic framework-derived conversion approach and an in situ crosslinking method. Among them, the 1D hybrid catalyst composed of ultrasmall cobalt phosphide nanoparticles supported by nitrogen-, sulfur-, phosphorus-doped carbon matrix shows remarkable bifunctional activity close to that of the benchmark precious-metal catalysts along with an excellent durability in the full potential range covering both the OER and ORR. The overall overpotential of the rechargeable ZABs can be greatly reduced with this bifunctional hybrid catalyst as an air-electrode, and the cycling stability outperforms the commercial Pt/C catalyst. It is revealed that the cobalt phosphide nanoparticles are in situ converted to cobalt oxide under the accelerated conditions during OER (and/or ORR) of the ZABs and reduces the anodic current applied to the carbon. This contributes to the stability of the carbon material and in maintaining the high initial catalytic properties of the hybrid catalyst.

2D Porous Carbons prepared from Layered Organic-Inorganic Hybrids and their Use as Oxygen-Reduction Electrocatalysts

2D porous carbon nanomaterials have attracted tremendous attention in different disciplines especially for electrochemical catalysis. The significant advantage of such 2D materials is that nearly all their surfaces are exposed to the electrolyte and can take part in the electrochemical reaction. Here, a versatile active-salt-templating strategy to efficiently synthesize 2D porous carbon nanosheets from layered organic-inorganic hybrids is presented. The resulting heteroatom-doped carbon nanosheets (NFe/CNs) exhibit exceptional performance for the oxygen-reduction reaction and in Zn-air battery electrodes. The activity of the best catalyst within a series of NFe/CNs exceeds the performance of conventional carbon-supported Pt catalysts in terms of onset potential (0.930 vs 0.915 V of Pt/C), half-wave potential (0.859 vs 0.816 V of Pt/C), long-time stability, and methanol tolerance. Also, when applied as a cathode catalyst in a zinc-air battery the NFe/CNs presented here outperform commercial Pt/C catalysts.

Heteroatom-Doped Carbon Materials for Electrocatalysis

Fuel cells, water electrolyzers, and metal-air batteries are important energy systems that have started to play some roles in our renewable energy landscapes. However, despite much research works carried out on them, they have not yet found large-scale applications, mainly due to the unavailability of sustainable catalysts that can catalyze the reactions employed in them. Currently, noble metal-based materials are the ones that are commonly used as catalysts in most commercial fuel cells, electrolyzers, and metal-air batteries. Hence, there has been considerable research efforts worldwide to find alternative noble metal-free and metal-free catalysts composed of inexpensive, earth-abundant elements for use in the catalytic reactions employed in these energy systems. In this concept paper, a brief introduction on catalysis in renewable energy systems, followed by the recent efforts to develop sustainable, heteroatom-doped carbon and non-noble metal-based electrocatalysts, the challenges to unravel their structure-catalytic activity relationships, and the authors' perspectives on these topics and materials, are discussed.

Heteroatom-Doped Carbon Nanostructures Derived from Conjugated Polymers for Energy Applications

Heteroatom-doped carbon materials have been one of the most remarkable families of materials with promising applications in fuel cells, supercapacitors, and batteries. Among them, conjugated polymer (CP)-derived heteroatom-doped carbon materials exhibit remarkable electrochemical performances because the heteroatoms can be preserved at a relatively high content and keep stable under harsh working conditions. In this review, we summarized recent advances in the rational design and various applications of CP-derived heteroatom-doped carbon materials, including polyaniline (PANI), polypyrrole (PPy), and their ramification-derived carbons, as well as transition metal-carbon nanocomposites. The key point of considering CP-derived heteroatom-doped carbon materials as important candidates of electrode materials is that CPs contain only nonmetallic elements and some key heteroatoms in their backbones which provide great chances for the synthesis of metal-free heteroatom-doped carbon nanostructures. The presented examples in this review will provide new insights in designing and optimizing heteroatom-doped carbon materials for the development of anode and cathode materials for electrochemical device applications.

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.