Highly fluorescent carbon quantum dots-Nafion as proton selective hybrid membrane for direct methanol fuel cells

Recent Developments in Nanocomposite Membranes Based on Carbon Dots

Carbon dots (CDs) have aroused colossal attention in the fabrication of nanocomposite membranes ascribed to their ultra-small size, good dispersibility, biocompatibility, excellent fluorescence, facile synthesis, and ease of functionalization. Their unique properties could significantly improve membrane performance, including permeance, selectivity, and antifouling ability. In this review, we summarized the recent development of CDs-based nanocomposite membranes in many application areas. Specifically, we paid attention to the structural regulation and functionalization of CDs-based nanocomposite membranes by CDs. Thus, a detailed discussion about the relationship between the CDs' properties and microstructures and the separation performance of the prepared membranes was presented, highlighting the advantages of CDs in designing high-performance separation membranes. In addition, the excellent optical and electric properties of CDs enable the nanocomposite membranes with multiple functions, which was also presented in this review.

Anchoring of Fe-MIL-101-NH2 to the Polymer Membrane Matrix through the Hinsberg Reaction to Promote Conductivity of SPEEK Membranes

SCPEEK@MOF proton exchange membranes, where SCPEEK is sulfinyl chloride polyether ether ketone and MOF is a metal-organic framework, were prepared by doping Fe-MIL-101-NH2 into polymers. The amino group in the MOF and the -SOCl2 group in thionyl chloride polyether ether ketone cross-link to form a covalent bond through the Hinsberg reaction, and the prepared composite membrane has stronger stability than other electrostatic interactions and simple physical doping composite membranes. The formation of covalent bonds improves the water absorption of the composite membrane, which makes it easy for water molecules to form hydrogen bonds. Moreover, SPEEK as a proton conductive polymer and the synergy of MOFs improve the proton conductivity of composite membranes. The composite membranes were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The swelling rate, water absorption, mechanical stability, ion exchange capacity, and proton conductivity of the pure sulfonated polyether ether ketone (SPEEK) membrane were compared with those of the mechanically doped SPEEK/MOF membrane and the composite membrane SCPEEK@MOF doped with different ratios of Fe-MIL-101-NH2, and all of the SCPEEK@MOF showed superior performance. When the Fe-MIL-101-NH2 loading rate of the composite membrane is 2%, the proton conductivity of the composite membrane can reach 0.202 S cm-1 at 363 K and a 98% relative humidity, which is much higher than that of the SPEEK/MOF membrane obtained by simple physical doping under the same conditions.

An effective method for fabricating crosslinked multilayer biomembranes with excellent stability and methanol resistance

To develop the proton exchange membranes with excellent comprehensive performance, especially high methanol resistance, the multilayer biomembranes were first prepared by alternately depositing chitosan and self-made functionalized organosilane on the surface of crosslinked chitosan via layer-by-layer self-assembly and were further treated via immersing in sulfuric acid and then heating at high temperature. SEM and FTIR spectra confirmed the presence of thin self-assemble layers with good adhesion on the substrate due to the interactions and the condensation reaction. The introduction of self-assemble layers and crosslinked structure significantly improved the stability and methanol resistance of biomembrane. The methanol diffusion coefficient of 15 bilayers modified biomembrane was only 2.6 × 10-8 cm2 S-1 in 12 M methanol, which was very favorable for its application in direct methanol fuel cell with high methanol concentration. Furthermore, the crosslinked multilayer biomembranes exhibited enhanced stability and the functionalized organosilane with high conductive groups ensured the biomembranes with better proton conductivity. The biomembrane with 15 bilayers showed extremely high selectivity value (1.05 × 106 Sscm-3), indicating its attractive potential as proton exchange membrane in direct methanol fuel cell.

Acidic Groups Functionalized Carbon Dots Capping Channels of a Proton Conductive Metal-Organic Framework by Coordination Bonds to Improve the Water-Retention Capacity and Boost Proton Conduction

Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been demonstrated to be versatile material platforms for the development of solid proton conductors. However, most crystalline porous proton conductors suffer from decreasing proton conductivity with increasing temperature due to releasing water molecules, and this disadvantage severely restricts their practical application in electrochemical devices. In this work, for the first time, hydrophilic carbon dots (CDs) were utilized to hybridize with high proton conductivity MOF-802, which is a model of MOF proton conductors, aiming to improve its water-retention capacity and thus enhance proton conduction. The resultant CDs@MOF-802 exhibits impregnable proton conduction with increasing temperature, and the proton conductivity reaches 10^-1 S cm^-1, much superior to that of MOF-802, making CDs@MOF-802 one of the most efficient MOF proton conductors reported so far. This study provides a new strategy to improve the water-retention capacity of porous proton conductors and further realize excellent proton conduction.

Modifying Ionic Membranes with Carbon Dots Enables Direct Production of High-Purity Hydrogen through Water Electrolysis

Traditional electrolysis of water produces hydrogen with a purity of ∼98%. Using ion transport membranes to prevent the migration of oxygen (O₂) from the anode to the cathode, the purity of H₂ can be increased to ∼99.8%, but it still cannot fulfill the requirement for use in fuel cells (>99.97%). Here, we report that modifying a commercial ionic membrane (Nafion) with carbon dots (CDs) can further inhibit O₂ permeation across the membrane, while effectively improving its ion transportability. The key to achieve these effects is the rational design and proper loading of the CDs, which narrows the pore size of the membrane from ∼4 nm to <2 nm and alters its surface functionality. Using the CDs-modified Nafion membrane in an H-type electrolysis cell, we demonstrate that H₂ with a purity of up to 99.98%, which meets the standards of hydrogen fuel cells, can be directly produced without any additional purification process. Our study provides a new route for the low-cost electrolysis of water to produce high-purity hydrogen.

Potential carbon nanomaterials as additives for state-of-the-art Nafion electrolyte in proton-exchange membrane fuel cells: a concise review

Proton-exchange membrane fuel cells (PEMFCs) have received great attention as a potential alternative energy device for internal combustion engines due to their high conversion efficiency compared to other fuel cells. The main hindrance for the wide commercial adoption of PEMFCs is the high cost, low proton conductivity, and high fuel permeability of the state-of-the-art Nafion membrane. Typically, to improve the Nafion membrane, a wide range of strategies have been developed, in which efforts on the incorporation of carbon nanomaterial (CN)-based fillers are highly imperative. Even though many research endeavors have been achieved in relation to CN-based fillers applicable for Nafion, still their collective summary has rarely been reported. This review aims to outline the mechanisms involved in proton conduction in proton-exchange membranes (PEMs) and the significant requirements of PEMs for PEMFCs. This review also emphasizes the improvements achieved in the proton conductivity, fuel barrier properties, and PEMFC performance of Nafion membranes by incorporating carbon nanotubes, graphene oxide, and fullerene as additives.

Proton Conduction of Nafion Hybrid Membranes Promoted by NH3-Modified Zn-MOF with Host-Guest Collaborative Hydrogen Bonds for H2/O2 Fuel Cell Applications

It is of great significance to develop creative proton exchange membrane materials for proton exchange membrane fuel cells (PEMFCs). The strategy of doping metal-organic frameworks (MOFs) with guest molecules into the Nafion matrix is adopted to improve the electrochemical performance of Nafion hybrid membranes. Various and abundant hydrogen bonds can make a tremendous contribution to the proton conduction of hybrid membranes. In this work, we used high proton-conducting Zn-MOFs with the characteristics of host-guest collaborative hydrogen bonds as the filler to prepare Zn-MOF/Nafion hybrid membranes. Alternating current (AC) impedance tests show that when the doping amount of Zn-MOF is 5%, the proton conductivity reaches 7.29 × 10^-3 S·cm^-1, being 1.87 times that of the pure Nafion membrane at 58% relative humidity (RH) and 80 °C. In an attempt to prove the promotion effect of guest NH₃ on proton conductivity of Nafion hybrid membranes, Zn-MOF-NH₃ was filled into the Nafion matrix. Under the same conditions, its proton conductivity reaches the maximum value of 2.13 × 10^-2 S·cm^-1, which is 5.47 times that of the pure Nafion membrane. Zn-MOF-NH₃/Nafion-5 was used to fabricate a proton exchange membrane for application in H₂/O₂ fuel cells. The maximum power density of 212 mW cm^-2 and a current density of 630 mA cm^-2 reveal a respectable single cell performance. This study provides a promising method for optimizing the structure of MOF proton conductors and inspires the preparation of high-performance Nafion hybrid membranes.

A review of carbon quantum dots and their applications in wastewater treatment

Carbon quantum dots (CQDs) are a fascinating class of carbon nanoparticles with sizes around 10 nm. The unique properties of CQDs are low toxicity, chemical inertness, excellent biocompatibility, photo-induced electron transfer and highly tunable photoluminescence behaviour. Sustainable raw materials are commonly used for the fabrication of CQDs because they are cost-effective, eco-friendly and effective to minimise waste production. CQDs can be fabricated using laser ablation, microwave irradiation, hydrothermal reaction, electrochemical oxidation, reflux method and ultrasonication. These methods undergo several chemical reactions such as oxidation, carbonisation, pyrolysis and polymerisation processes to produce CQDs. Due to small particle sizes of CQDs, they possess strong tunable fluorescent properties and highly photo-luminescent emissions. It also contains oxygen-based functional groups and highly desired properties as semiconductor nanoparticles. Therefore, CQDs are promising nanomaterials for photo-catalysis, ions sensing, biological imaging, heavy metal detection, adsorption treatment, supercapacitor, membrane fabrication and water pollution treatment. This review paper will discuss the physical and chemical properties of CQDs, raw materials and methods used in the fabrication of CQDs, the stability of CQDs as well as their potential applications in wastewater treatment and biomedical field.

High-performance thermoplastic polyurethane elastomer/carbon dots bulk nanocomposites with strong luminescence

In this work, high-performance thermoplastic polyurethane elastomer/carbon dots (TPU/CDs) bulk nanocomposites with strong luminescence were fabricated via in situ polymerization. The CDs were synthesized from citric acid and 2-aminothiophenol. Transmission electron microscope, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and systematic characterization indicated the formation of the CDs and the covalent conjugation of the CDs with TPU. The optical properties of the TPU/CDs nanocomposites were characterized by ultraviolet–visible and fluorescence spectroscopy. Compared to the initial solid-state CDs (the absolute photoluminescence quantum yields (QY): 20%), all the composites exhibited stronger luminescence behavior. When the CDs content was 0.5 wt%, the QY was as high as 68%. Furthermore, the rheological, mechanical, and thermal properties of the nanocomposites were investigated. The rheological properties established the structure–property relationships of the composites. The incorporation of the CDs enhanced the elastic response in viscoelasticity of the nanocomposites. The tensile strength of 1.0 wt% CDs loaded TPU increased from 18.2 MPa to 28.6 MPa, nearly 57% higher than that of the neat TPU. Given the excellent Ag+ detection performance of the CDs, the high QY and the processability of the nanocomposites, Ag+ detection experiments for the composite film were performed. The study will facilitate the applications of luminescent nanocomposites in potential fields.

Performance enhancement of direct methanol fuel cells using a methanol barrier boron nitride–Nafion hybrid membrane

Sulfonated hexagonal boron nitride is explored as a potential filler to prepare Nafion hybrid membranes for direct methanol fuel cell (DMFC) applications.