Metal–nitrogen–carbon (M–N–C) molecular catalysts with NiN4 active structure have been extensively studied as selective and active catalysts toward electrochemical reduction of CO2 to CO. The key challenge for a practical M–N–C catalyst is to increase the density of atomic metal active sites that achieves the partial current density of CO (j CO) relevant to the industrial level at lower overpotentials. Here, we revealed the effect of physical and chemical properties of carbon substrates and synthetic processes on the tuning of the density of atomic metal active sites as well as the role of reaction chemistry in enhancing the j CO and reducing the overpotential. The achievable loading of NiN4 active site in the Ni–N–C is determined by the combined content of pyridinic and pyrrolic N functionalities and Ni–N coordination efficiency derived from the pyrolytic step rather than the uptake capability of Ni2+ in the adsorption step in the case of carbon black with high specific surface area (>1000 m2/g). The N dopant content can be improved by modifying oxygen functional groups on the surface of carbon black, optimizing the pyrolytic temperature, and iterating the doping step. Through a combination of all optimum factors, the resultant Ni–N–C catalyst has a maximum loading of ∼4.4 wt % for atomic Ni. This Ni–N–C catalyst exhibited Faradaic efficiency (FE) of CO of 97% and j CO of −152 mA cm–2 at −0.93 V vs RHE in a flow cell using 0.5 M KHCO3 electrolyte while showing 93% FE of CO and j CO of −67 mA cm–2 at −0.61 V vs RHE at 1 M KOH. Adding KI to the base electrolyte significantly magnified the j CO to larger than −200 mA cm–2 at a potential of −0.51 V vs RHE while maintaining the almost unity FE of CO. The Ni–N–C is compatible with the membrane-electrode-assembly-based electrolyzer in which the j CO also achieved >200 mA cm–2 at a cell voltage of around 2.7 V.

In this study, we give an insight into the char formation mechanism for the addition of CNTs and MoS2 into halogen-free flame retarding thermoplastic poly(ether ester) elastomers (TPEE).

Carbon-based polymer can catch fire when used as cathode material in batteries and supercapacitors, due to short circuiting. Polyimide is known to exhibit flame retardancy by forming char layer in condensed phase. The high char yield of polyimide is attributed to its aromatic nature and the existence of a donor–acceptor complex in its backbone. Fabrication of hybrid polyimide material can provide better protection against fire based on multiple fire-retardancy mechanisms. Nanocomposites generally show a significant enhancement in mechanical, electrical, and thermal properties. Nanoparticles, such as graphene and carbon nanotubes, can enhance flame retardancy in condensed phase by forming a dense char layer. Silicone-based materials can also provide fire retardancy in condensed phase by a similar mechanism as polyimide. However, some inorganic fire retardants, such as phosphazene, can enhance flame retardancy in gaseous phase by releasing flame inhibiting radicals. The flame inhibiting radicals generated by phosphazene are released into the gaseous phase during combustion. A hybrid system constituted of polyimide, silicone-based additives, and phosphazene would provide significant improvement in flame retardancy in both the condensed phase and gas phase. In this review, several flame-retardant polyimide-based systems are described. This review which focuses on the various combinations of polyimide and other candidate fire-retardant materials would shed light on the nature of an effective multifunctional flame-retardant hybrid materials.

Polyimide-block-poly(dimethyl siloxane) copolymer was synthesized by a two-step process, initiated by coupling anhydride terminated poly(amic acid), AT-PAA with amino terminated poly(dimethyl siloxane), (NH2)2-PDMS to form poly(amic acid)-block-poly(dimethyl siloxane). The resulting copolymer is then thermally treated to produce polyimide-block-poly(dimethyl siloxane), PI-PDMS. Because of the high glass transition temperature, Tg of polyimide, it is usually cured at a high temperature of about 300°C for over 2.5 h. Copolymerization of polyimide with polysiloxane, reduces the imidization temperature while maintaining high thermomechanical properties. A series of instruments were used to monitor the progress of copolymerization. The time-based analysis of the product of copolymerization enables the optimization of the structure and properties of the copolymers. The chemical structure and composition of the copolymer were studied by Fourier Transform Infrared Spectroscopy, (FT-IR). The incorporation of PDMS blocks into the copolymer and the degree of imidization of the polyimide block increased with increasing reaction time. The change in the viscosity of the copolymerizing solution was monitored by simple shear viscometry conducted with the Brookfield Viscometer. The reported increase in solution viscosity with increasing copolymerization time is associated with increasing molecular weight of the copolymer. The intrinsic viscosity of the copolymer solution was measured as a function of copolymerization time and it was found that the intrinsic viscosity of the copolymer solution increased with increasing reaction time. The glass transition temperature (Tg) and the thermal stability of the copolymer were determined by differential scanning calorimetry, DSC and thermogravimetric analysis, and TGA, respectively. Between 25°C and 420°C, the copolymers synthesized in this study show two glass transition temperatures due to the polyimide, PI block at around 380°C and another peak associated with PDMS plasticized polyimide at about 290–300°C. The two Tg peaks observed in the DSC thermogram are believed to be indicative of the structure of a block copolymer. TGA analysis shows that the thermoxidative stability of the copolymers increased with increasing reaction time, due to the incorporation of increased amount of PDMS unit into the copolymer. The combination of increasing molecular weight of copolymer, higher degree of imidization of polyimide blocks and enhanced thermoxidative stability may translate into improved flame retardancy of copolymers. This suggested enhancement in flame retardancy in air atmosphere, is believed to be due the incorporated PDMS blocks, which can be converted into silica, SiO2, a recognized thermally stable material.

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