Recent Progress on Carbon‐Based Electrocatalysts for Oxygen Reduction Reaction: Insights on the Type of Synthesis Protocols, Performances and Outlook Mechanisms

Posted on 7 September 2023 by

This review explores carbon-based catalysts for oxygen reduction (ORR) in acidic and alkaline electrolytes, focusing on their mechanism, performance modulation strategies such as functionalization engineering, and doping strategies. Carbon-based materials are cost-effective, highly conductive, and have a wide range of allotropes. However, no specific review distinguishes between ORR activity, mechanism, and fuel cell performance in acidic and alkaline media for carbon functionalized and doped nanomaterials. That aspect is outlined in this review.

Abstract

Due to their low cost, accessibility of resources, and improved stability and durability, carbon-based nanomaterials have attracted significant attention as cathode materials for oxygen reduction reactions. These materials also exhibit intrinsic physical and electrochemical features. However, their potential for use in fuel cells is constrained by low ORR activity and slow kinetics. Carbon nanomaterials can be functionalized and doped with heteroatoms to change their morphologies and generate a large number of oxygen reduction active sites to lessen the problems. Doping the carbon lattice with heteroatoms like N, S, and P and functionalizing the carbon structure with −OCH₃, −F, −COO⁻, −O⁻ are two of these modifications that can change specific properties of the carbon nanomaterials like expanding interlayer distance, producing a large number of active sites, and enhancing oxygen reduction activity. When compared to pristine carbon-based nanomaterials, these doped and functionalized carbon nanomaterials, including their composites, exhibit accelerated rate performance, outstanding stability, and higher methanol tolerance. This article summarizes the most recent developments in heteroatom-doped and functionalized carbon-based nanomaterials, covering different synthesis approaches, characterization methods, electrochemical performance, and oxygen reduction reaction mechanisms. As cathode materials for fuel cell technologies, the significance of heteroatom co-doping and transition metal heteroatom co-doping is also underlined.

High‐temperature platinum‐catalyzed hydrosilylation and dehydrocoupling cross‐linking of silicones

Posted on 7 September 2023 by

We propose to use the platinum(II) C,N-cyclometalated complex (PCC) to catalyze the hydrosilylation and dehydrocoupling reactions of high molecular weight polysiloxanes at elevated temperatures (above 100°C). PCC was prepared via a three-step procedure consisting of the 2-hydrazinopyridine synthesis, its treatment with 3-methoxy-4-(prop-2-yn-1-yloxy)benzaldehyde, and coupling of the obtained product with cis-[PtCl₂(CNXyl)₂]. This complex exhibits thermal stability up to 150°C even at heating in air. PCC allows carrying out the cross-linking of vinyl-terminated polydimethylsiloxane (V-PDMS) and polymethylhydrosiloxane (PMHS) by hydrosilylation, as well as PMHS dehydrocoupling cross-linking at 150 and 120°C, respectively. The coupling (cross-linking) patterns were successfully confirmed by ¹H, ¹³C, and ²⁹Si solid-state NMR spectroscopy. The thermal and swelling characteristics and the transparency of the obtained silicone materials indicate the absence of aggregation of platinum particles.

Use of Hydrothermal Carbonization to Improve the Performance of Biowaste‐Derived Hard Carbons in Sodium Ion‐Batteries

Posted on 7 September 2023 by

From Waste to Anode Material: Hard carbons are produced from waste biomass (spent coffee grounds, sunflower seed shells and rose stems) by two methods: direct pyrolysis and by combined hydrothermal carbonization and pyrolysis. Electrochemical performance of as-obtained hard carbons using hydrothermal carbonization combined with pyrolysis is improved with up to 76 % ICE and 280 mAh g⁻¹ at C/5.

Abstract

Over the last years, hard carbon (HC) has been the most promising anode material for sodium-ion batteries due to its low voltage plateau, low cost and sustainability. In this study, biomass waste (spent coffee grounds, sunflower seed shells and rose stems) was investigated as potential material for hard carbon preparation combining a two-step method consisting of on hydrothermal carbonization (HTC), to remove the inorganic impurities and increase the carbon content, and a subsequent pyrolysis process. The use of HTC as pretreatment prior to pyrolysis improves the specific capacity in all the materials compared to the ones directly pyrolyzed by more than 100 % at high C-rates. The obtained capacity ranging between 210 and 280 mAh g⁻¹ at C/15 is similar to the values reported in literature for biomass-based hard carbons. Overall, HC obtained from sunflower seed shell performs better than that obtained from the other precursors with an initial Coulombic efficiency (ICE) of 76 % and capacities of 120 mAh g⁻¹ during 1000 cycles at C with a high capacity retention of 86–93 %.

Kraft Lignin: A Valuable, Sustainable Resource, Opportunities and Challenges

Posted on 7 September 2023 by

To be or not to be burnt: that's the question. Read more about kraft lignin: the potential, the chemistry of how it is formed, and stateof-the-art applications in both fuels and materials. A technoeconomic discussions discloses two important economic incentives to recover lignin from pulp production.

Abstract

Kraft lignin, a by-product from the production of pulp, is currently incinerated in the recovery boiler during the chemical recovery cycle, generating valuable bioenergy and recycling inorganic chemicals to the pulping process operation. Removing lignin from the black liquor or its gasification lowers the recovery boiler load enabling increased pulp production. During the past ten years, lignin separation technologies have emerged and the interest of the research community to valorize this underutilized resource has been invigorated. The aim of this Review is to give (1) a dedicated overview of the kraft process with a focus on the lignin, (2) an overview of applications that are being developed, and (3) a techno-economic and life cycle asseeements of value chains from black liquor to different products. Overall, it is anticipated that this effort will inspire further work for developing and using kraft lignin as a commodity raw material for new applications undeniably promoting pivotal global sustainability concerns.

Post‐Synthetic Modification of Zr‐based Metal‐Organic Frameworks with Imidazole: Variable Optical Behavior and Sensing

Posted on 7 September 2023 by

Post-synthetic modification (PSM) with imidazole makes UiO-66-NH₂ metal-organic framework (MOF) luminescent. This enables it to detect health-hazardous pollutants such as acetone, aq. Fe³⁺, and aq. CO₃ ²⁻ by luminescence ON/OFF. This PSM MOF exhibits the highest sensitivity for pollutants among other no rare-earth element MOFs reported thus far in the literature.

Abstract

UiO-66-NH₂-IM, a fluorescent metal-organic framework (MOF), was synthesized by post-synthetic modification of UiO-66-NH₂ with 2-imidazole carboxaldehyde via a Schiff base reaction. It was examined using various characterization techniques (PXRD, FTIR, NMR, SEM, TGA, UV-Vis DRS, and photoluminescence spectroscopy). The emissive feature of UiO-66-NH₂-IM was utilized to detect volatile organic compounds (VOCs), metal ions, and anions, such as acetone, Fe³⁺, and carbonate (CO₃ ²⁻). Acetone turns off the high luminescence of UiO-66-NH₂-IM in DMSO, with the limit of detection (LOD) being 3.6 ppm. Similarly, Fe³⁺ in an aqueous medium is detected at LOD=0.67 μM (0.04 ppm) via quenching. On the contrary, CO₃ ²⁻ in an aqueous medium significantly enhances the luminescence of UiO-66-NH₂-IM, which is detected with extremely high sensitivity (LOD=1.16 μM, i. e., 0.07 ppm). Large Stern-Volmer constant, K_sv, and low LOD values indicate excellent sensitivity of the post-synthetic MOF. Experimental data supported by density functional theory (DFT) calculations discern photo-induced electron transfer (PET), resonance energy transfer (RET), inner filter effect (IFE), or proton abstraction as putative sensing mechanisms. NMR and computational studies propose a proton abstraction mechanism for luminescence enhancement with CO₃ ²⁻. Moreover, the optical behavior of the post-synthetic material toward analytes is recyclable.

A General Enantioselective C−H Arylation Using an Immobilized Recoverable Palladium Catalyst

Posted on 7 September 2023 by

The enantioselective C−H arylation of aryl bromides herein developed afforded 30 enantioenriched products with high yields and enantioselectivities. By exploiting the “release and catch” mechanism of recoverable SP-NHC-Pd^II catalyst, in combination with BozPhos as a broadly applicable chiral ligand, good performances have been obtained across different substrates containing methyl, cyclopropyl and aryl C−H bonds.

Abstract

We herein report a general and efficient enantioselective C−H arylation of aryl bromides based on the use of BozPhos as the bisphosphine ligand and SP-NHC-Pd^II as recoverable heterogeneous catalyst. By exploiting the “release and catch” mechanism of action of the catalytic system, we used BozPhos as a broadly applicable chiral ligand, furnishing high enantioselectivities across all types of examined substrates containing methyl, cyclopropyl and aryl C−H bonds. For each reaction, the reaction scope was investigated, giving rise to 30 enantioenriched products, obtained with high yields and enantioselectivities, and minimal palladium leaching. The developed catalytic system provides a more sustainable solution compared to homogeneous systems for the synthesis of high added-value chiral products through recycling of the precious metal.

Outstanding Compatibility of Hard‐Carbon Anodes for Sodium‐Ion Batteries in Ionic Liquid Electrolytes

Posted on 7 September 2023 by

The 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMI][FSI]) and, especially, N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide ([N₁₁₁₄][FSI]) have shown very good compatibility towards hard carbon electrode with excellent cycling behavior, which represents one of the best results obtained for hard carbon electrodes in ionic liquid electrolytes, exceeding even that exhibited in organic electrolytes, making them rather appealing for the realization of safe, reliable and highly performing Na-ion cells.

Abstract

Hard carbons (HC) from natural biowaste have been investigated as anodes for sodium-ion batteries in electrolytes based on 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMI][FSI]) and N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide ([N1114][FSI]) ionic liquids. The Na⁺ intercalation process has been analyzed by cyclic voltammetry tests, performed at different scan rates for hundreds of cycles, in combination with impedance spectroscopy measurements to decouple bulk and interfacial resistances of the cells. The Na⁺ diffusion coefficient in the HC host has been also evaluated via the Randles-Sevcik equation. Battery performance of HC anodes in the ionic liquid electrolytes has been evaluated in galvanostatic charge/discharge cycles at room temperature. The evolution of the SEI (solid electrochemical interface) layer grown on the HC surface has been carried out by Raman spectroscopy. Overall the sodiation process of the HC host is highly reversible and reproducible. In particular, a capacity retention exceeding 98 % of the initial value has been recorded in[N1114][FSI] electrolytes after more than 1500 cycles with a coulombic efficiency above 99 %, largely beyond standard carbonate-based electrolytes. Raman, transport properties and impedance confirms that ILs disclose the formation of SEI layers with superior ability to support the reversible Na+ intercalation with the possible minor contributions from the EMI+cation.

Controlled Potential Electrolysis: Transition from Fast to Slow Regimes in Homogeneous Molecular Catalysis. Application to the Electroreduction of CO2 Catalyzed by Iron Porphyrin

Posted on 7 September 2023 by

The resting state of a homogeneous molecular catalyst during a controlled potential electrolysis depends on operational parameters (catalytic rate constant, cell dimensions and stirring rate). A formal description is given and illustrated through the electroreduction of CO₂ catalyzed by Iron porphyrin switching from fast (confined catalysis) to slow (bulk catalysis) regimes.

Abstract

Molecular catalysis of electrochemical reactions is a field of intense activity because of the current interest in electrifying chemical transformations, including both electrosynthesis of organic molecules and production of fuels via small molecule activation. Controlled potential electrolysis (CPE) is often coupled with in situ in operando spectroscopic methods with the aim to gather mechanistic information regarding the catalytic species involved. Herein, considering a simple mechanism for a homogeneous molecular catalysis of an electrochemical reaction, we establish the concentration profile of the catalyst in the electrolysis cell enabling to envision the information that can be obtained from the coupling of this CPE with a spectroscopic probe in the cell compartment. We show how the characteristic parameters of the system (catalytic rate constant, cell dimensions and stirring rate) affect the response with particular emphasis on the transition between two limiting cases, namely a ‘fast’ catalysis regime where catalysis only takes place in a small layer adjacent to the electrode surface and a ‘slow’ catalysis regime where catalysis takes place in the bulk of the solution. These formal concepts are then illustrated with an experimental example, the electroreduction of CO₂ in dimethylformamide homogeneously catalyzed by iron tetraphenylporphyrin and followed by UV-vis spectroscopy.

Water‐Abundant Electrolytes: Towards Safer and Greener Aqueous Zinc‐Metal Batteries

Posted on 7 September 2023 by nLong Qian, nRui Yao, nCheng Yangn

This concept article aims to emphasize how to fabricate green and safe water-abundant Zn metal batteries. Several typical and advanced strategies towards water-abundant electrolyte systems are reviewed. We hope to arouse the attention of researchers for safer and greener aqueous Zn metal batteries when a large amount of toxic or expensive non-aqueous components are added into electrolytes.

Abstract

Aqueous Zn metal batteries have been regarded as promising candidates as an alternative to Li-ion batteries in large-scale energy storage systems due to their low-cost, safe and environmentally benign advantages. However, because of the introduction of solvent water, several problems, for example dendrites, parasite reactions, hydrogen evolution, and so on, are brought into aqueous Zn metal batteries. Regrettably, when trying to solve these problems, most efforts have taken the form of adding a large amount of non-aqueous components, which are usually harmful to the environment and not conducive to greener and safer aqueous batteries. In this Concept, we will introduce several electrolyte systems and mainly focus on how to build a water-abundant electrolyte with fewer non-aqueous components. This work will review the literature and offer instructive guidance for environmentally benign Zn metal batteries.

Diverse Reactivity of a Ca(I) Synthon

Posted on 7 September 2023 by

Although complexes with Ca−Ca bonds are still elusive, a complex with a bridging C₆H₆ ²⁻ dianion reacts like a Ca^I synthon. However, depending on the reagent, different modes of reactivity have been observed.

Abstract

Low-valent Mg^I complexes like (BDI)Mg−Mg(BDI) have found wide-spread application as specialty reducing agents (BDI=β-diketiminate). Also their redox reactivity was extensively investigated. In contrast, attempts to isolate similar Ca^I complexes led to reduction of the aromatic solvents or N₂. Complex (^DIPePBDI)Ca(μ ₆,μ ₆-C₆H₆)Ca(^DIPePBDI) (VIII) should be regarded a Ca^II complex with a bridging C₆H₆ ²⁻ dianion (^DIPePBDI=HC[C(Me)N-DIPeP]₂, DIPeP=2,6-C(H)Et₂-phenyl). It can react as a Ca^I synthon by releasing benzene and two electrons. Herein we describe the reactivity of VIII with benzene, biphenyl, naphthalene, anthracene, COT, Ph₃SiCl, PhSiH₃, a (BDI)AlI₂ complex, H₂, PhX (X=F, Cl, Br, I), tBuOH and tBuCH₂I. The C₆H₆ ²⁻ dianion in VIII can react as a 2e ⁻ source, a nucleophile or a Brønsted base. In some cases radical reactivity cannot be excluded. Crystal structures of (^DIPePBDI)Ca(μ ₈,μ ₈-COT)Ca(^DIPePBDI) (1) and [(^DIPePBDI)CaX ⋅ (THF)]₂ (X=F, Cl, Br, I) (2–5) are described.