Effect of the graphitization degree for electrospun carbon nanofibers on their electrochemical activity towards VO2+/VO2+ redox couple

Carbon Structure Regulation Strategy for the Electrode of Vanadium Redox Flow Battery

Vanadium redox flow battery (VRFB) is a type of energy storage device known for its large-scale capacity, long-term durability, and high-level safety. It serves as an effective solution to address the instability and intermittency of renewable energy sources. Carbon-based materials are widely used as VRFB electrodes due to cost-effectiveness and well-stability. However, pristine electrodes need proper modification to overcome original poor hydrophilicity and fewer reaction active sites. Adjusting the carbon structure is recognized as a viable method to boost the electrochemical activity of electrodes. This review delves into the advancements in research related to ordered and disordered carbon structure electrodes including the adjusting methods, structural characteristics, and catalytic properties. Ordered carbon structures are categorized into nanoscale and macroscale orderliness based on size, leading to improved conductivity and overall performance of the electrode. Disordered carbon structures encompass methods such as doping atoms, grafting functional groups, and creating engineered holes to enhance active sites and hydrophilicity. Based on the current research findings on carbon electrode structures, this work puts forth some promising prospects for future feasibility.

Selective synthesis of core-shell structured catalyst χ-Fe5C2 surrounded by nanosized Fe3O4 for conversion of syngas to liquid fuels

Enhancing liquid hydrocarbons selectivity and simultaneously suppressing CO2 formation are highly desirable yet challenges in iron-based Fischer-Tropsch synthesis. Herein, we report an in-situ oxidation method for the fabrication of a...

Enhanced Catalysis of P-doped SnO2 for the V2+/V3+ Redox Reaction in Vanadium Redox Flow Battery

In this work, nanosized P-doped SnO₂ (SnO₂-P) was prepared by a sol–gel method as a catalyst for the V³⁺/V²⁺ redox reaction in vanadium redox flow battery. Compared with SnO₂, the electrochemical performance of SnO₂-P is significantly improved. This is because P doping provides more active sites and shows greatly improved electrical conductivity, thereby increasing the electron transfer rate. As a result, SnO₂-P shows better catalytic performance than SnO₂. The SnO₂-P modified cell is designed, and it exhibits an increase of 47.2 mA h in discharge capacity and 8.7% in energy efficiency compared with the pristine cell at 150 mA cm⁻². These increases indicate that the modified cell has a higher electrolyte utilization rate. This study shows that SnO₂-P is a new and efficient catalyst for vanadium redox flow battery.

Cobalt and nitrogen co-doped mesoporous carbon for electrochemical hydrogen peroxide sensing: the effect of graphitization

In this study, a facile strategy for the scalable synthesis of cobalt and nitrogen co-doped mesoporous carbon (Co-N/C) is reported. Structural characterization demonstrated that Co and N were successfully co-doped in the highly porous carbon. Graphitization of porous carbon was achieved by the introduction of cobalt species. The degree of graphitization of Co-N/C could be further promoted by increasing the calcination temperature. By taking advantage of the excellent mass and electron transfer kinetics attributed to the high specific surface area, high porosity and high graphitization, the obtained Co-N/C exhibited good electrochemical activity towards H₂O₂ reduction and excellent sensing performance for the electrochemical detection of H₂O₂. The Co-N/C-950 catalyst obtained at 950 °C showed good electrochemical sensing performance with a detection limit of 2 μM and a wide linear response over the concentration range from 0.03 mM to 13 mM. Meanwhile, Co-N/C exhibited high selectivity toward the detection of H₂O₂ in the presence of possible interferences during the applications such as NaCl, glucose, ascorbic acid and so on. The results confirm that Co-N/C could be used as an efficient electrocatalyst to fabricate electrochemical sensing devices.

Resolving charge-transfer and mass-transfer processes of VO2+/VO2 + redox species across the electrode/electrolyte interface using electrochemical impedance spectroscopy for vanadium redox flow battery

Electrochemical impedance spectroscopy is used to investigate the charge-transfer and mass-transfer processes of VO²⁺/VO₂ ⁺ (V⁴⁺/V⁵⁺) redox species across the carbon-modified glassy carbon disk electrode/electrolyte interface. The features of the EIS patterns depend on the potential, concentrations of the redox species and mass-transport conditions at the electrode/electrolyte interface. With the starting electrolyte containing either only V⁴⁺ or V⁵⁺ redox species, EIS shows a straight line capacitor feature, as no oxidation or reduction reaction take place at the measured open circuit potential (OCP). With the electrolyte containing equimolar concentration of V⁴⁺ and V⁵⁺, EIS pattern has both charge-transfer and mass-transfer features at the equilibrium potential. The features of the charge-transfer process are observed to be influenced by the mass-transfer process. Optimum concentrations of the V⁴⁺/V⁵⁺ redox species and supporting H₂SO₄ electrolyte are required to resolve the EIS features corresponding to the underlying physical processes. The semi-infinite linear diffusion characteristics of the V⁴⁺/V⁵⁺ redox species observed with a static condition of the electrode converges to that of a finite diffusion under hydrodynamic condition.

Carbon Nanofibers Production via the Electrospinning Process

Electrospun fibers with different concentrations of polyacrylonitrile (PAN) were synthesized and the results are reported in this study. The aim was to obtain carbon nanofibers for manufacturing gas diffusion layers for proton exchange membrane (PEM) fuel cells. The electrospun fibers obtained were carbonized at 1200 °C, 1300 °C, and 1400 °C, in order to have nanofibers with more than 96% of carbon atoms. The scanning electron microscopy (SEM) results revealed an increase in the diameter from 400–700 nm at 1200 °C to 1000–1400 nm at 1300 °C and 1400 °C. The Raman measurements disclose a higher degree of crystallinity for the sample carbonized at elevated temperatures. The surface area was estimated from the Brunauer–Emmett–Teller (BET) method and the results revealed an increase from 40.69 m2g−1 to 66.89 m2g−1 and 89.92 m2g−1 as the carbonization temperature increased. Simultaneously, the pore volume increased with increasing carbonization temperature. The Fourier-transform infrared spectroscopy (FTIR) spectra reveal that during carbonization treatment, C≡N triple bonds are destroyed with the appearance of C=N double bonds. Decreasing the ID/IG intensities’ ratio from ~1.07 to ~1.00 denotes the defects reduction in carbonaceous materials due to the graphitization process. Therefore, the carbon fibers developed in optimum conditions are appropriate to be further used to produce gas diffusion layers for Proton-exchange membrane fuel cells (PEMFC).