Nanowrinkled Carbon Aerogels Embedded with FeN x Sites as Effective Oxygen Electrodes for Rechargeable Zinc-Air Battery

Nitrogen-Doped Carbon Flowers with Fe and Ni Dual Metal Centers for Effective Electroreduction of Oxygen

Carbon-based nanocomposites have been attracting extensive attention as high-performance catalysts in alkaline media towards the electrochemical reduction of oxygen. Herein, polyacrylonitrile nanoflowers are synthesized via a free-radical polymerization route and used as a structural scaffold and precursor, whereby controlled pyrolysis leads to the ready preparation of carbon nanocomposites (FeNi-NCF) doped with both metal (Fe and Ni) and nonmetal (N) elements. Transmission electron microscopy studies show that the FeNi-NCF composites retain the flower-like morphology, with the metal species atomically dispersed into the flaky carbon petals. Remarkably, despite a similar structure, elemental composition, and total metal content, the FeNi-NCF sample with a high Fe:Ni ratio exhibits an electrocatalytic performance towards oxygen reduction reaction (ORR) in alkaline media that is similar to that by commercial Pt/C, likely due to the Ni to Fe electron transfer that promotes the adsorption and eventual reduction of oxygen, as evidenced in X-ray photoelectron spectroscopic measurements. Results from this study underline the importance of the electronic properties of metal dopants in the manipulation of the ORR activity of carbon nanocomposites.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

Iron, Nitrogen-Doped Carbon Aerogels for Fluorescent and Electrochemical Dual-Mode Detection of Glucose

Due to their effective catalytic activity and maximum atom utilization, single metal atoms dispersed in carbon matrices have found diverse applications in electrocatalysis, photocatalysis, organic catalysis, and biosensing. Herein, iron is atomically dispersed into nitrogen-doped porous carbon aerogel by a facile pyrolysis procedure, and the resulting nanocomposite behaves both as a peroxidase mimic for the sensitive detection of glucose by fluorescence spectroscopy and as an effective catalyst for the electrochemical oxidation of glucose. The glucose concentration can be quantified within the millimolar to micromolar range with a limit of detection of 3.1 and 0.5 μM, respectively. Such a dual-functional detection platform also shows excellent reproducibility, stability, and selectivity, and the performance in glucose detection of clinical and artificial human body fluids is highly comparable to that of leading assays in recent studies and results from commercial sensors. Results from this study suggest that carbon aerogel-supported single atoms can be used as a dual-functional nanozyme for the construction of low-cost, high-performance dual-signal readout platforms for glucose detection.

Boosting oxygen evolution activity of nickel iron hydroxide by iron hydroxide colloidal particles

Nickel iron hydroxides (NiFeOH) have been drawing enormous attention as effective catalysts for oxygen evolution reaction (OER), a key process in water splitting. Herein, we report that negatively charged iron(III) hydroxide colloidal particles, can significantly enhance the OER activity of NiFeOH in alkaline media. NiFeOH is grown on nickel foam in a supersaturated iron(III) salt solution, which also contains a high content of Fe(OH)₃ colloidal nanoparticles, forming free-standing NiFeOH@C_x electrodes (with x being the Fe(OH)₃ concentration). The interface between NiFeOH and Fe(OH)₃ colloidal particles, as manifested by the unique volcano-like holes on the NiFeOH@C_x surface, is likely the OER active sites. In comparison to Fe(OH)₃-free NiFeOH, NiFeOH@C₁₀₀₀ exhibits a 40-fold enhancement of the OER activity, confirming the significant effect of Fe(OH)₃ colloidal nanoparticles in boosting the OER activity, likely as a result of enhanced charge transfer from Ni²⁺ to Fe³⁺ that facilitates the adsorption of key reaction intermediates. Furthermore, by coupling the free-standing NiFeOH@C₁₀₀₀ electrode with commercial Pt/C, full water splitting can occur and reach a current density of 10 mA cm^-2 under a cell voltage of 1.51 V, which is lower than that (1.59 V) based on noble metal catalysts of RuO₂ + Pt/C.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.