摘要: The electroreduction of carbon dioxide (CO2) driven by renewable energy is an important route for CO2 conversion and utilization. Formic acid (HCOOH), as an important chemical and safe hydrogen storage material, is one of the main and promising materials for CO2 electroreduction. The physical and chemical properties of CO2 and the reaction mechanisms for its electroreduction to HCOOH are outlined and the recent development of carbon-based catalysts, including metal-free carbon catalysts and carbon-supported catalysts, for CO2 electroreduction to HCOOH is reviewed. The design of reactors for HCOOH production and strategies for their optimization are summarized and discussed. Hybrid CO2 electrolysis technology is analyzed, such as electroreduction coupled with the methanol electrooxidation reaction. Lastly, key challenges and development trends for CO2 electroreduction to HCOOH are presented, which are expected to provide guidance for the development of this technique.
摘要: Various types of energy conversion and storage devices have been developed in recent years to tackle with the problems of the over-consumption of fossil fuels and the environmental pollution they cause. The oxygen evolution reaction (OER) is the key half-cell reaction of many energy conversion and storage devices. The influences of the heteroatom doping of metal-free carbon-based electrocatalysts with N, P, S or B and co-doping with N/P or N/S on their performance as OER electrocatalysts are reviewed. Doping methods to prepare metal-free carbon-based electrocatalysts are summarized, and problems that need to be solved are discussed and challenges for future research are proposed.
摘要: In recent years, metal-free carbon materials have been the subject of much research concerning their potential use in replacing high-cost Pt-based oxygen reduction reaction (ORR) electrocatalysts. Myriads of research papers in this field have been dedicated to the preparation and characterization of various metal-free nanocarbon materials, as well as to their practical applications. Non-metal heteroatom doping and the introduction of edge defects are typical nanocarbon modification methods, which can significantly reduce the overpotential of the ORR in alkaline and acidic electrolytes. In order to have good activity in actual devices such as fuel cells, it is necessary to increase the ORR intrinsic activity of nanocarbons. Despite many studies of the subject, the intrinsic relationship between nanocarbon composition, structure regulation and catalytic activity is still not clear and needs further exploration. This review details the various nanocarbons used for the ORR as well as their reaction mechanisms in an attempt to propose scientific and specific structural modification strategies. The development of carbon-based metal-free electrocatalysts in the field of oxygen reduction catalysis in recent years is summarized, with a view to providing relevant knowledge for the future design, synthesis and applications of these carbon-based non-metallic catalysts for the ORR.
摘要: Graphite has proved to be inactive for Na+ storage in ester-based electrolytes when used as the anode material. Recent studies have shown the feasibility of a graphite anode for Na+ storage with a large capacity and a high initial Coulombic efficiency (ICE) in linear ether-based electrolytes. Understanding such solvent-dependent electrochemical behavior at the nanometer scale is essential but has remained elusive, especially the direct visualization of the graphite/electrolyte interface. We report the in-situ observation by atomic force microscopy of a working battery that allowed us to monitor and visualize the changes of the graphite/electrolyte interface in both linear ether and ester-based electrolytes. Results indicate that there is no solid electrolyte interphase (SEI) formation in the linear ether-based electrolytes and the co-intercalation is reversible and stable in the following cycles, which are responsible for the relatively high ICE, large capacity and excellent stability. In the ester-based electrolytes, SEI deposition is obvious during the sodiation process, but not in the desodiation process, leading to a serious consumption of the electrolyte, and thus a low ICE and irreversible Na+ storage. Our findings provide insights into the dynamics of changes in the graphite/electrolyte interface and reveal the solvent-dependent Na+ storage at the nanometer scale, paving the way to develop high-performance Na+ batteries.
摘要: The electrochemical properties of graphene-polyaniline (PANI) hybrids are largely determined by their microstructures and the distribution of PANI on the graphene network. Uniform hybridization of each component is critical to avoid the re-stacking of the graphene sheets and the agglomeration of PANI nanoassemblies during the use of the hybrids. Conventional strategies, such as layer-by-layer assembly or electrochemical in-situ polymerization, involve intricate procedures, making it difficult to achieve the large-scale production of the hybrids. We report a completely miscible cosolvent consisting of N, N-dimethylformamide and water that solves this problem and was used to produce graphene-PANI hybrid flexible fibers. It was found that the composite fiber had a homogeneous microstructure with PANI nanoassemblies uniformly distributed on the graphene sheets, and had outstanding electrochemical properties, much better than the counterpart fabricated using only water as the solvent. The work proposes a universal but simple strategy to achieve the mass production of graphene-PANI hybrids or similar materials with uniform hybridization of the two components.
摘要: Because of its high concentration of in-plane elemental nitrogen, superior chemical/thermal stability, tunable electronic band structure and environmentally friendly nature, graphite-like carbon nitride (g-C3N4) is a new promising metal-free material that has drawn much attention in photo-/electric catalysis. Compared with the regulation of the band structure in photocatalysis, the deliberate synthesis of g-C3N4 electrocatalysts is mainly focused on the construction of catalytic sites and the modulation of the charge transfer kinetics. This work reports a rapid method for synthesizing ultrafine g-C3N4 quantum dots (QDs) by electrochemical exfoliation using Al3+ ions as an intercalation agent. Uniform g-C3N4 QDs with small lateral size and thickness were collected more easily due to the higher charge density and stronger electrostatic force of Al3+ ions in the lattice of the host material, compared to conventional univalent alkali cations. The QDs had an average lateral dimension and thickness of 3.5 nm and 1.0 nm, respectively, as determined by TEM and AFM measurements. The presence of a large number of C/N defects was verified by the UV-vis spectra. The ultrafine g-C3N4 QDs had a superior hydrogen evolution reaction performance with an ultra-low onset-potential approaching 0 V, and a low overpotential of 208 mV at 10 mA cm−2, as well as a remarkably low Tafel slope (52 mV·dec−1) in an acidic electrolyte.
摘要: Graphite is the most widely used anode material for lithium ion batteries (LIBs). Increasing the sphericity and tap density of the graphite particles is important for improving their volumetric energy density. We report a simple approach to prepare high tap-density graphite granules by high-shear wet granulation. Graphitic onion-like carbon (GOC) and artificial graphite (AG) were densified into granules by wet-granulation to obtain WG-GOC and WG-AG, respectively. Results indicate that, compared with the original graphite before granulation, the tap densities of WG-GOC and WG-AG increased by 34% and 44%, respectively. The respective volumetric energy densities of WG-GOC and WG-AG increased by 35% and 55% at a current density of 50 mA g−1. The rate performance of WG-GOC was also significantly improved. The volumetric capacity of WG-GOC at a current density of 2 000 mA g−1 was 169.1% of the original GOC. The significant improvement of electrochemical performance is ascribed to the increased tap density of the graphite granules.
摘要: Porous carbons are widely used in supercapacitors, owing to their long cycle life and natural abundance. However, most of these electrode materials give a low capacitance, which leads to low energy density. Cu-doped biomass-derived activated carbons (Cu-ACs) were synthesized using a simple, low-cost carbonization and KOH activation method. The copper nanoparticles had mixed valence states (CuO, Cu2O, Cu0) and were uniformly dispersed on the surface of the AC. Due to the fast electron/ion transfer paths provided by the pore structure, and an accelerated redox reaction between the three Cu species, the Cu-ACs achieved an excellent capacitive performance. In a three-electrode system, the Cu-AC sample prepared by KOH activation with a KOH/ （Cu+char）mass ratio of 2 had a high specific capacitance of 360 F g−1 at 0.5 A g−1, 1.21 times that of AC (163 F g−1). When it was fabricated into a symmetric capacitor, the device had a good electrochemical performance with a specific capacitance of 143.44 F g−1 at 0.5 A g−1 and a good cyclic stability with an 81.8% capacitance retention after 6000 cycles.
Novel hybrid aerogels were prepared by adding ZnCl2, NiCl2·6H2O, FeCl2·4H2O and FeCl3·6H2O to a suspension of equal weights of graphene oxide and oxidized carbon nanotubes, followed by co-precipitation under basic conditions. The aerogels were then crosslinked with polyvinyl alcohol in water and freeze-dried. They consisted of magnetic Ni0.5Zn0.5Fe2O4 nanoparticles, graphene oxide, carbon nanotubes and polyvinyl alcohol, which have active sites that attract dye molecules and could be extracted from water by applying a magnetic field. Using optimum mass ratios of ZnCl2/NiCl2·6H2O/FeCl2·4H2O/FeCl3·6H2O/(graphene oxide+oxidized carbon nanotube) at 6∶6∶12∶12∶1, the hybrid aerogel has a high adsorption capacity of 71.03 mg g−1 for methylene blue and a moderate magnetic strength of MS = 3.519 emu g−1. Its removal efficiencies for methylene blue, methyl orange, crystal violet and a mixture of equal masses of the three were 70.1%, 4.2%, 8.9% and 11.1%, respectively for the same dye concentration of 0.025 mg mL−1. It could be used for 3 regeneration cycles with a regeneration efficiency of over 82%. It was also not toxic to the living organisms, suggesting that it is a promising adsorbent for treating industrial wastewater.