Fe/Fe 3 C Nanoparticles Encapsulated in N‐Doped Hollow Carbon Spheres as Efficient Electrocatalysts for the Oxygen Reduction Reaction over a Wide pH Range

In Situ Construction of Zinc-Mediated Fe, N-Codoped Hollow Carbon Nanocages with Boosted Oxygen Reduction for Zn-Air Batteries

The rational design of bifunctional oxygen electrocatalysts with unique morphology and luxuriant porous structure is significant but challenging for accelerating the reaction kinetics of rechargeable Zn-air batteries (ZABs). Herein, zinc-mediated Fe, N-codoped carbon nanocages (Zn-FeNCNs) are synthesized by pyrolyzing the polymerized iron-doped polydopamine on the surface of the ZIF-8 crystal polyhedron. The formation of the chelate between polydopamine and Fe serves as the covering layer to prevent the porous carbon nanocages from collapsing and boosts enough exposure and utilization of metal-based active species during carbonization. Furthermore, both the theoretical calculation and experimental results show that the strong interaction between polyhedron and polydopamine facilitates the evolution of high-activity zinc-modulated FeNx sites and electron transportation and then stimulates the excellent bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). As expected, the Zn-air battery with Zn-FeNCNs as an air cathode displays a superior power density (256 mW cm-2) and a high specific capacity (813.3 mA h gZn-1), as well as long-term stability over 1000 h. Besides, when this catalyst is applied to the solid-state battery, the device exhibited outstanding mechanical stability and a high round-trip efficiency under different bending angles.

A Portable Self-Powered Electrochemical Sensor Based on Zinc-Air Battery for Detection of Hydrogen Sulfide

The self-powered electrochemical sensor (SPES), an analytical sensing device without external power supply, is integrated with the dual function of power supply and detection performance, which lay the foundation for the development of intelligent and portable electrochemical sensing devices. Herein, a novel SPES based on a zinc-air battery was constructed for the detection of hydrogen sulfide (H2S) in the lysate of colon cancer cells. Typically, an Fe/Fe3C@graphene foam with oxygen reduction performance was used to construct SPES based on a zinc-air battery (ZAB-SPES), which brings the open-circuit voltage to 1.30 V. Among them, the poisoning effect of H2S causes the catalytic performance of the oxygen reduction catalyst to decrease, causing a significant decrease in the discharge voltage of ZAB. Based on this principle, ZAB-SPES was constructed for the detection of H2S using a digital multimeter. The proposed ZAB-SPES demonstrated good selectivity and reproducibility for detecting H2S compared to the results of the H2S-specific fluorescence probe. This strategy enriches the idea of constructing a self-powered sensor and a digital multimeter as detection devices, providing technical support for the portability of SPESs.

ZIF-67-derived N-doped double layer carbon cage as efficient catalyst for oxygen reduction reaction

The slow kinetic of oxygen reduction reaction (ORR) hampers the practical application of energy conversion devices, such as fuel cells, metal-air batteries. Here, an efficient ORR electrocatalyst consists of Co, Ni co-decorated nitrogen-doped double shell hollow carbon cage (Ni-Co@NHC) was fabricated by pyrolyzing Ni-doped polydopamine wrapped ZIF-67. During the preparation, polydopamine served as a protective layer can effectively prevent the aggregation of Co and Ni nanoparticles during the pyrolysis process, and at the same time forming a carbon layer to grow a double layer carbon cage. This unique hollow structure endows the catalyst with a high specific surface area as well as more exposed active sites. Also benefited from the synergistic effect between Ni and Co nanoparticles, the Ni-Co@NHC catalyst leads to an outstanding ORR performance of half-wave potential (E_1/2, 0.862 V), outperforms that of commercial Pt/C catalyst. Additionally, when Ni-Co@NHC was used in the cathode for the zinc-air battery, the cell exhibits high power density (108 mW cm^-2) and high specific capacity (806 mAh g^-1) at 20 mA cm^-2outperforming Pt/C. This work offers a promising design strategy for the development of high-performance ORR electrocatalysts.

Mesoporous N-P Codoped Carbon Nanosheets as Superior Cathodic Catalysts of Neutral Metal-Air Batteries

Development of high-efficiency oxygen reduction reaction (ORR) catalysts under neutral conditions has made little research progress. In this work, we synthesized a three-dimensional porous N/P codoped carbon nanosheet composites (CNP@PNS) by high-temperature thermal treatment of dicyandiamide, starch, and triphenylphosphine and subsequent porous structure-making treatment using the NaCl molten salt template. In the neutral solution, the electrocatalytic performance of the CNP@PNS-4 catalyst exhibits an onset potential of 0.98 V (vs reversible hydrogen electrode) and a half-wave potential of 0.91 V for ORR, which greatly surpasses commercial Pt/C (40%). Three kinds of neutral metal-air batteries (Zn-air, Al-air, and Fe-air) using the prepared samples as cathodic catalysts were constructed, corresponding to the maximum power density of 120.2, 78.3, and 18.9 mW·cm^-2, respectively. Also, they reveal outstanding discharge stability under different current densities. The density functional theory calculation depicts the reduction of the free energy of the determining step and subsequent decline of the overpotential for ORR.

Enhanced electrocatalytic oxygen reduction reaction for Fe-N4-C by the incorporation of Co nanoparticles

Oxygen reduction reaction (ORR) catalytic activity can be improved by means of enhancing the synergy between transition metals. In this work, a novel porous Fe-N₄-C nanostructure containing uniformly dispersed Co nanoparticles (CoNPs) is prepared by an assisted thermal loading method. The as-prepared Co@Fe-N-C catalyst shows enhanced ORR activity with a half-wave potential (E_1/2) of 0.92 V vs. RHE, which is much higher than those of the direct pyrolysis CoNP-free sample Fe-N-C (E_1/2 = 0.85 V) and Pt/C (E_1/2 = 0.90 V) in alkaline media. It exhibits remarkable stability with only a 10 mV decrease in E_1/2 after 10 000 cycles and an outstanding long-term durability with 85% current remaining after 60 000 s. In acidic media, this catalyst exhibits catalytic activity with an E_1/2 of 0.79 V, comparable to Pt/C (E_1/2 = 0.82 V). X-ray absorption fine spectroscopy analysis revealed the presence of active centres of Fe-N₄. Density functional theory calculations confirmed the strong synergy between CoNPs and Fe-N₄ sites, providing a lower overpotential and beneficial electronic structure and a local coordination environment for the ORR. The incorporation of CoNPs on the surface of Fe-N₄-C nanomaterials plays a key role in enhancing the ORR catalytic activity and stability, providing a new route to prepare efficient Pt-free ORR catalysts.

Ultrafine NiFe clusters anchored on N-doped carbon as bifunctional electrocatalysts for efficient water and urea oxidation

Hydrogen production through electrocatalysis is crucial in renewable energy technologies but significantly impeded by sluggish anodic reactions. Developing bifunctional anode noble-metal-free electrocatalysts towards oxygen evolution reaction (OER) and urea oxidation reaction (UOR) to boost cathodic hydrogen evolution reaction (HER) is promising but challenging to meet different reaction media and multiple applications for simultaneous clean energy production and pollution treatment. Herein, a facile one-pot thermal treatment strategy is presented to anchor NiFe nanoclusters (with a size of about 2 nm) on N-doped carbon as bifunctional electrocatalysts for both OER and UOR. Such an electrocatalyst can deliver a current density of 20 mA cm-2 with a low overpotential of 260 mV and a small Tafel slope of 42 mV dec-1 for OER, superior to the state-of-the-art Ru-based materials. Besides, this electrocatalyst also shows excellent activity for UOR with the need for just 1.37 V (vs. RHE) to attain a current density of 100 mA cm-2. In a two-electrode electrolyzer for both cathodic HER and anodic UOR, only a cell voltage of 1.50 V is required to drive a current density of 10 mA cm-2, which is 140 mV lower than that of overall water splitting electrolysis (1.64 V). The excellent electrooxidative performance can be attributed to the improved conductivity, abundant active sites and fast charge transfer and transport benefiting from the ultrafine structure of NiFe clusters and their synergistic effect with N-doped carbon.

Nitrogen and Oxygen Co-Doping Assisted Synthesis of Highly Dispersed Pd Nanoparticles on Hollow Carbon Spheres as Efficient Electrocatalysts for Oxygen Reduction Reaction

Electrochemical reduction of O₂ (oxygen reduction reaction; ORR) provides an opportunity to achieve the commercial application of clean energy, but it remains challenging, so the rational design of inexpensive and efficient electrocatalysts is required. Palladium-based electrocatalysts have emerged as a class of the most promising candidates for the ORR, which could accelerate O₂ adsorption, dissociation, and electron transfer. However, the metal Pd atoms tend to aggregate into nanoparticles, driven by the tendency of the metal surface free energy to decrease, which significantly reduces the atom utilization efficiency and the catalytic performance. Herein, a facile double solvent impregnation method is developed for the synthesis of highly dispersed Pd nanoparticles supported on hollow carbon spheres (Pd-HCS), which could act as efficient electrocatalysts for the ORR in basic solution. Systematic investigation reveals that the nitrogen-containing and oxygen-containing functional groups (especially -COOH groups) are essential for achieving the homogenous dispersion of Pd nanoparticles. Significantly, the optimized Pd-HCS electrocatalyst with homogeneously dispersed Pd nanoparticles and Pd-N sites delivers high electrocatalytic activity for the ORR and excellent stability, without significant decay in onset potential and half-potential and good resistance to methanol crossover. This work offers a new route for the rational design of efficient ORR electrocatalysts toward advanced materials and emerging applications.

Nitrogen-doped iron for selective catalytic reduction of nitrate to dinitrogen

Nitrate is the leading cause of eutrophication worldwide and is one of the most challenging pollutants for restoration of polluted surface waters such as lakes, rivers and reservoirs. We report herein a new architecture of iron nanoparticles for high-efficiency denitrification by selective reduction of nitrate (NO₃^-) to dinitrogen (N₂). The iron nanoparticles are doped with nitrogen (FeN) and encapsulated within a thin layer of nitride-carbon (NC). The nanoparticles have high pyrrolic N content (17.4 at.%) and large specific surface area (2040 m²/g). Laboratory experiments demonstrated high N₂ selectivity (91%) and nitrate removal capacity (6004 mg N/g Fe) for treatment of nitrate-containing water. This iron-based nanomaterial overcomes shortcomings of conventional catalysts by eliminating the use of precious and toxic heavy metals (e.g., Pd, Pt, Cu, Ni) and minimizing the generation of undesirable byproducts (e.g., ammonia) from the reactions with nanoscale zero-valent iron (nZVI). The multiple electron transfers process from NO₃^- to N₂ can be fine-tuned by adjusting the NC shell thickness. Superior electrocatalytic performance, low cost and minimal environmental impact of the iron-derived nanocatalyst offer promising prospects for water purification, waste treatment and environmental remediation.

Rational Synthesis of Iron/Nitrogen-Doped Carbon Catalyst through a Spatial Isolation Strategy for Efficient Oxygen Reduction in Acidic and Alkaline Media

It remains challenging to rationally synthesize iron/nitrogen-doped carbon (Fe/N-C) catalysts with rich Fe-N_x atomic active sites for improved oxygen reduction reaction (ORR) electrocatalysis. A highly efficient Fe/N-C catalyst, which has been synthesized through a spatial isolation strategy, is reported. Derived from bioinspired polydopamine (PDA)-based hybrid microsphere precursors, it is a multifunctional carrier that loads atomically dispersed Fe³⁺ /Zn²⁺ ions through coordination interactions and N-rich melamine through electrostatic attraction and covalent bonding. The Zn²⁺ ions and melamine in the precursor efficiently isolate Fe³⁺ atoms upon pyrolysis to form rich Fe-N_x atomic active sites, and generate abundant micropores during high-temperature treatment; as a consequence, the resultant Fe-N/C catalyst contains rich catalytically active Fe-N_x sites and a hierarchical porous structure. The catalyst exhibits improved ORR activity that is superior to and close to that of Pt/C in alkaline and acidic solutions, respectively.