Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction

Surface Engineered Single-atom Systems for Energy Conversion

Single-atom catalysts (SACs) are demonstrated to show exceptional reactivity and selectivity in catalytic reactions by effectively utilizing metal species, making them a favorable choice among the different active materials for energy conversion. However, SACs are still in the early stages of energy conversion, and problems like agglomeration and low energy conversion efficiency are hampering their practical applications. Substantial research focus on support modifications, which are vital for SAC reactivity and stability due to the intimate relationship between metal atoms and support. In this review, a category of supports and a variety of surface engineering strategies employed in SA systems are summarized, including surface site engineering (heteroatom doping, vacancy introducing, surface groups grafting, and coordination tunning) and surface structure engineering (size/morphology control, cocatalyst deposition, facet engineering, and crystallinity control). Also, the merits of support surface engineering in single-atom systems are systematically introduced. Highlights are the comprehensive summary and discussions on the utilization of surface-engineered SACs in diversified energy conversion applications including photocatalysis, electrocatalysis, thermocatalysis, and energy conversion devices. At the end of this review, the potential and obstacles of using surface-engineered SACs in the field of energy conversion are discussed. This review aims to guide the rational design and manipulation of SACs for target-specific applications by capitalizing on the characteristic benefits of support surface engineering.

Catalytic degradation of benzene at room temperature over FeN4O2 sites embedded in porous carbon

Benzene and its aromatic derivatives are typical volatile organic compounds for indoor and outdoor air pollution, harmful to human health and the environment. It has been considered extremely difficult to break down benzene rings at ambient conditions without external energy input, due to the extraordinary stability of the aromatic structure. Here, we show one such solution that can thoroughly degrade benzene to basically water and carbon dioxide at 25 °C in air using atomically dispersed Fe in N-doped porous carbon, with almost 100% benzene conversion. Further experimental studies combined with molecular simulations reveal the mechanism of this catalytic reaction. Hydroxyl radicals (·OH) evolved on the atomically dispersed FeN4O2 catalytic centers were found responsible for initiating and completing the oxidation of benzene. This work provides a new chemistry to degrade aromatics at ambient conditions and also a pathway to generate active ·OH oxidant for generic remediation of organic pollutants.

High Density Single Fe Atoms on Mesoporous N-Doped Carbons: Noble Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Acidic and Alkaline Media

It remains a challenge to develop efficient noble metal-free electrocatalysts for the oxygen reduction reaction (ORR) in various renewable energy systems. Single atom catalysts have recently drawn great attention as promising candidates both due to their high activity and their utmost atom utilization for electrocatalytic ORR. Herein, the synthesis of an efficient ORR electrocatalyst that is composed of N-doped mesoporous carbon and a high density (4.05 wt%) of single Fe atoms via pyrolysis Fe-conjugated polymer is reported. Benefiting from the abundant atomic Fe-N4 sites on its conductive, mesoporous carbon structures, this material exhibits an excellent electrocatalytic activity for ORR, with positive onset potentials of 0.93 and 0.98 V in acidic and alkaline media, respectively. Its electrocatalytic performance for ORR is also comparable to that of Pt/C (20 wt%) in both media. Furthermore, it electrocatalyzes the reaction almost fully to H2 O (or barely to H2 O2 ). Additionally, it is durable and tolerates the methanol crossover reaction well. Furthermore, a proton exchange membrane fuel cell and a zinc-air battery assembled using it on their cathode deliver high maximum power densities (320 and 91 mW cm-2 , respectively). Density functional theory calculation reveals that the material's decent electrocatalytic performance for ORR is due to its atomically dispersed Fe-N4 sites.

Giving New Life to Waste Cigarette Butts: Transformation into Platinum Group Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Acid, Neutral and Alkaline Environment

Following the core theme of a circular economy, a novel strategy to upcycle cigarette butt waste into platinum group metal (PGM)-free metal nitrogen carbon (M-N-C) electrocatalysts for oxygen reduction reaction (ORR) is presented. The experimental route was composed of (i) the transformation of the powdered cigarette butts into carbonaceous char via pyrolysis at 450 °C, 600 °C, 750 °C and 900 °C, (ii) the porosity activation with KOH and (iii) the functionalization of the activated chars with iron (II) phthalocyanine (FePc). The electrochemical outcomes obtained by the rotating disk electrode (RRDE) technique revealed that the sample pyrolyzed at 450 °C (i.e., cig_450) outperformed the other counterparts with its highest onset (Eon) and half-wave potentials (E1/2) and demonstrated nearly tetra-electronic ORR in acidic, neutral and alkaline electrolytes, all resulting from the optimal surface chemistry and textural properties.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Nano-mediated uniform ternary Cu–Co–Ni-based nitrogen-doped carbon nanotubes with synergistic reactivity for high-performance oxygen reduction

Here, we report nano-mediated Cu–Co–Ni-based nitrogen-doped carbon nanotubes (N-CNTs/T-CCN) by hydrothermal and procedural calcination strategy. The nitrogen-doped carbon nanotubes (N-CNTs) show more average diameter and the N-CNTs are uniformly modified with ternary Cu–Co–Ni-based nanoparticles (T-CCN). The hybrid exhibits excellent ORR catalytic activity. The onset potential (Eonset) and half-wave potential (E1/2) are 0.96 V and 0.87 V (versus reversible hydrogen electrode, RHE) in 0.1 M KOH. Most importantly, compared to 20% Pt/C, N-CNTs/T-CCN catalyst displays better methanol tolerance and higher stability. The H2O2 yield of the N-CNTs/T-CCN is less than 7.5% and the electron-transfer number (n) is about 3.9. High ORR performance may be related to the synergistic enhancement effect. The N-CNTs supply good electrical conductivity and allow large numbers of active sites to efficiently participate; the T-CCN can improve the local work function of the N-CNTs by synergistic electronic interaction and promote O2 adsorption; the stability of embedded T-CCN can be greatly improved, mainly due to the weakness of Ostwald effect. All these advantages make the hybrid a promising ORR catalyst.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Current trends and perspectives on emerging Fe-derived noble-metal-free oxygen electrocatalysts

This article discusses recent progress in the development of Fe-derived noble metal-free electrocatalysts, including the strategies used for design, synthesis, and assessment of their performance in alkaline conditions.