Electrospun Nitrogen-Doped Carbon Nanofibers Encapsulating Cobalt Nanoparticles as Efficient Oxygen Reduction Reaction Catalysts

Fibrous-Structured Freestanding Electrodes for Oxygen Electrocatalysis

Electrocatalysts used for oxygen reduction and oxygen evolution reactions are critical materials in many renewable-energy devices, such as rechargeable metal-air batteries, regenerative fuel cells, and water-splitting systems. Compared with conventional electrodes made from catalyst powders, oxygen electrodes with a freestanding architecture are highly desirable because of their binder-free fabrication and effective elimination of catalyst agglomeration. Among all freestanding electrode structures that have been investigated so far, fibrous materials exhibit many unique advantages, such as a wide range of available fibers, low material and material-processing costs, large specific surface area, highly porous structure, and simplicity of fiber functionalization. Recent advances in the use of fibrous structures for freestanding electrocatalytic oxygen electrodes are summarized, including electrospun nanofibers, bacterial cellulose, cellulose fibrous structures, carbon clothes/papers, metal nanowires, and metal meshes. After detailed discussion of common techniques for oxygen electrode evaluation, freestanding electrode fabrication, and their electrocatalytic performance, current challenges and future prospects are also presented for future development.

Single copper sites dispersed on defective TiO2-x as a synergistic oxygen reduction reaction catalyst

Catalysts containing isolated single atoms have attracted much interest due to their good catalytic behavior, bridging the gap between homogeneous and heterogeneous catalysts. Here, we report an efficient oxygen reduction reaction (ORR) catalyst that consists of atomically dispersed single copper sites confined by defective mixed-phased TiO_2-x. This synergistic catalyst was produced by introducing Cu²⁺ to a metal organic framework (MOF) using the Mannich reaction, occurring between the carbonyl group in Cu(acac)₂ and the amino group on the skeleton of the MOF. The embedding of single copper atoms was confirmed by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and x-ray absorption fine structure spectroscopy. Electronic structure modulation of the single copper sites coupling with oxygen vacancies was further established by electron paramagnetic resonance spectroscopy and first-principles calculations. Significantly enhanced ORR activity and stability were achieved on this special Cu single site. The promising application of this novel electrocatalyst was demonstrated in a prototype Zn-air battery. This strategy of the stabilization of single-atom active sites by optimization of the atomic and electronic structure on a mixed matrix support sheds light on the development of highly efficient electrocatalysts.

Preparation of Palladium Nanoparticles Decorated Polyethyleneimine/Polycaprolactone Composite Fibers Constructed by Electrospinning with Highly Efficient and Recyclable Catalytic Performances

Nano-sized palladium nanoparticles showed high catalytic activity with severe limitations in catalytic field due to the tendency to aggregate. A solid substrate with large specific surface area is an ideal carrier for palladium nanoparticles. In present work, polyethyleneimine/polycaprolactone/Pd nanoparticles (PEI/PCL@PdNPs) composite catalysts were successfully designed and prepared by electrospinning and reduction methods using PEI/PCL elexctrospun fiber as carrier. The added PEI component effectively regulated the microscopic morphology of the PEI/PCL fibers, following a large number of pit structures which increased the specific surface area of the electrospun fibers and provided active sites for loading of the palladium particles. The obtained PEI/PCL@PdNPs catalysts for reductions of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) exhibited extremely efficient, stable, and reusable catalytic performance. It was worth mentioning that the reaction rate constant of catalytic reduction of 4-NP was as high as 0.16597 s−1. Therefore, we have developed a highly efficient catalyst with potential applications in the field of catalysis and water treatment.

Cu/Cu2O/rGO nanocomposites: solid-state self-reduction synthesis and catalytic activity for p-nitrophenol reduction

Cu/Cu₂O/rGO nanocomposites were produced via a solid-state self-reduction route with high catalytic performance for 4-NP reduction.

Advanced electrospun nanomaterials for highly efficient electrocatalysis

We highlight the recent developments of electrospun nanomaterials with controlled morphology, composition and architecture for highly efficient electrocatalysis.

Electrocatalytic N-Doped Graphitic Nanofiber - Metal/Metal Oxide Nanoparticle Composites

Carbon-based nanocomposites have shown promising results in replacing commercial Pt/C as high-performance, low cost, nonprecious metal-based oxygen reduction reaction (ORR) catalysts. Developing unique nanostructures of active components (e.g., metal oxides) and carbon materials is essential for their application in next generation electrode materials for fuel cells and metal-air batteries. Herein, a general approach for the production of 1D porous nitrogen-doped graphitic carbon fibers embedded with active ORR components, (M/MO_x , i.e., metal or metal oxide nanoparticles) using a facile two-step electrospinning and annealing process is reported. Metal nanoparticles/nanoclusters nucleate within the polymer nanofibers and subsequently catalyze graphitization of the surrounding polymer matrix and following oxidation, create an interconnected graphite-metal oxide framework with large pore channels, considerable active sites, and high specific surface area. The metal/metal oxide@N-doped graphitic carbon fibers, especially Co₃ O₄ , exhibit comparable ORR catalytic activity but superior stability and methanol tolerance versus Pt in alkaline solutions, which can be ascribed to the synergistic chemical coupling effects between Co₃ O₄ and robust 1D porous structures composed of interconnected N-doped graphitic nanocarbon rings. This finding provides a novel insight into the design of functional electrocatalysts using electrospun carbon nanomaterials for their application in energy storage and conversion fields.

Coherent nanoscale cobalt/cobalt oxide heterostructures embedded in porous carbon for the oxygen reduction reaction

Cost-effective and efficient electrocatalysts for the oxygen reduction reaction (ORR) are crucial for fuel cells and metal–air batteries.

Creation of Triple Hierarchical Micro-Meso-Macroporous N-doped Carbon Shells with Hollow Cores Toward the Electrocatalytic Oxygen Reduction Reaction

A series of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores have been successfully prepared via etching N-doped hollow carbon spheres with CO₂ at high temperatures. The surface areas, total pore volumes and micropore percentages of the CO₂-activated samples evidently increase with increasing activation temperature from 800 to 950 °C, while the N contents show a contrary trend from 7.6 to 3.8 at%. The pyridinic and graphitic nitrogen groups are dominant among various N-containing groups in the samples. The 950 °C-activated sample (CANHCS-950) has the largest surface area (2072 m² g^-1), pore volume (1.96 cm³ g^-1), hierarchical micro-mesopore distributions (1.2, 2.6 and 6.2 nm), hollow macropore cores (~91 nm) and highest relative content of pyridinic and graphitic N groups. This triple micro-meso-macropore system could synergistically enhance the activity because macropores could store up the reactant, mesopores could reduce the transport resistance of the reactants to the active sites, and micropores could be in favor of the accumulation of ions. Therefore, the CANHCS-950 with optimized structure shows the optimal and comparable oxygen reduction reaction (ORR) activity but superior methanol tolerance and long-term durability to commercial Pt/C with a 4e^--dominant transfer pathway in alkaline media. These excellent properties in combination with good stability and recyclability make CANHCSs among the most promising metal-free ORR electrocatalysts reported so far in practical applications.

Facile Synthesis of N-Doped Graphene-Like Carbon Nanoflakes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction

A series of N-doped carbon materials (NCs) were synthesized by using biomass citric acid and dicyandiamide as renewable raw materials via a facile one-step pyrolysis method. The characterization of microstructural features shows that the NCs samples are composed of few-layered graphene-like nanoflakes with controlled in situ N doping, which is attributed to the confined pyrolysis of citric acid within the interlayers of the dicyandiamide-derived g-C₃N₄ with high nitrogen contents. Evidently, the pore volumes of the NCs increased with the increasing content of dicyandiamide in the precursor. Among these samples, the NCs nanoflakes prepared with the citric acid/dicyandiamide mass ratio of 1:6, NC-6, show the highest N content of ~6.2 at%, in which pyridinic and graphitic N groups are predominant. Compared to the commercial Pt/C catalyst, the as-prepared NC-6 exhibits a small negative shift of ~66 mV at the half-wave potential, demonstrating excellent electrocatalytic activity in the oxygen reduction reaction. Moreover, NC-6 also shows better long-term stability and resistance to methanol crossover compared to Pt/C. The efficient and stable performance are attributed to the graphene-like microstructure and high content of pyridinic and graphitic doped nitrogen in the sample, which creates more active sites as well as facilitating charge transfer due to the close four-electron reaction pathway. The superior electrocatalytic activity coupled with the facile synthetic method presents a new pathway to cost-effective electrocatalysts for practical fuel cells or metal-air batteries.

Mussel-inspired facile synthesis of Fe/Co-polydopamine complex nanospheres: complexation mechanism and application of the carbonized hybrid nanospheres as an efficient bifunctional electrocatalyst

A one-pot synthesis of Co(ii)–Fe(iii)–PDA complex nanospheres as a facile approach to obtain binary metal-doped carbon nanostructures.

Ultrafine N-doped carbon nanoparticles with controllable size to enhance electrocatalytic activity for oxygen reduction reaction

The ORR electrocatalytic activity of ultrafine nanoparticles prepared from carbonization ofSterculia scaphigerais significantly improved through a simple hydrothermal treatment.