Ultrathin atomic Mn-decorated formamide-converted N-doped carbon for efficient oxygen reduction reaction

P-doped binary Ni/Fe-N-C for enhanced oxygen electrocatalysis performance

Adjusting the micro-environment of highly dispersive metals on carbon supports has been proved to be effective for achieving enhanced electrocatalysis performance. Herein, we delicately design a phosphorus-doped binary NiFe-nitrogen-carbon material (denoted as P-NiFe-NC), taking advantage of the coupling reaction between phenylphosphonamide (P dopant) and formamide (the carbon and nitrogen sources). The XPS N 1s fine scan reveals the strong interplay of N and P by the positively shifted binding energy of pyridinic N species after P incorporation, and the chemical state of Fe species is influenced accordingly. In addition, the P doping can enlarge the specific surface area and increase the meso/macroporosity of NiFe-NC, thus contributing to the enhancement of mass transfer inside the pores. The P-NiFe-NC sample exhibits favorable bifunctional oxygen electrocatalysis performance, rendering an ORR half-wave potential of 0.85 V and an OER potential of 1.69 V@10.0 mA cm-2, superior to those of P-free NiFe-NC. Assembled into Zn-air batteries, P-NiFe-NC delivers a high specific power of 161.36 mW cm-2 and stable charge/discharge for over 100 h (corresponding to 300 cycles).

Mn-Doped Zn Metal-Organic Framework-Derived Porous N-Doped Carbon Composite as a High-Performance Nonprecious Electrocatalyst for Oxygen Reduction and Aqueous/Flexible Zinc-Air Batteries

Developing low-cost, efficient, and stable oxygen reduction reaction (ORR) electrocatalysts is crucial for the commercialization of energy conversion devices such as metal-air batteries. In this study, we report a Mn-doped Zn metal-organic framework-derived porous N-doped carbon composite (30-ZnMn-NC) as a high-performance ORR catalyst. 30-ZnMn-NC exhibits excellent electrocatalytic activity, demonstrating a kinetic current density of 9.58 mA cm-2 (0.8 V) and a half-wave potential of 0.83 V, surpassing the benchmark Pt/C and most of the recently reported non-noble metal-based catalysts. Moreover, the assembled zinc-air battery with 30-ZnMn-NC demonstrates high peak power densities of 207 and 66.3 mW cm-2 in liquid and flexible batteries, respectively, highlighting its potential for practical applications. The excellent electrocatalytic activity of 30-ZnMn-NC is attributed to its unique porous structure, the strong electronic interaction between metal Zn/Mn and adjacent N-doped carbon, as well as the bimetallic Mn/Zn-N active sites, which synergistically promote faster reaction kinetics. This work offers a controllable design strategy for efficient electrocatalysts with porous structures and bimetallic active sites, which can significantly enhance the performance of energy conversion devices.

Constructing atomically-dispersed Mn on ZIF-derived nitrogen-doped carbon for boosting oxygen reduction

Exploring durable and highly-active non-noble-metal nanomaterials to supersede Pt-based nanomaterials is an effective way, which can reduce the cost and boost the catalytic efficiency of oxygen reduction reaction (ORR). Herein, we constructed atomically-dispersed Mn atoms on the ZIF-derived nitrogen-doped carbon frameworks (Mn-N_x/NC) by stepwise pyrolysis. The Mn-N_x/NC relative to pure nitrogen-doped carbon (NC) exhibited superior electrocatalytic activity with a higher half-wave potential (E_1/2 = 0.88 V) and a modest Tafel slope (90 mV dec⁻¹) toward ORR. The enhanced ORR performance of Mn-N_x/NC may be attributed to the existence of Mn-N_x active sites, which can more easily adsorb intermediates, promoting the efficiency of ORR. This work provides a facile route to synthesize single-atom catalysts for ORR.

Rational design of M-N4-Gr/V2C heterostructures as highly active ORR catalysts: a density functional theory study

Inspired by the composites of N-doped graphene and transition metal-based materials as well as MXene-based materials, heterostructures (M-N₄-Gr/V₂C) of eight different transition metals (M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped with nitrogen-coordinated graphene and V₂C as potential catalysts for the oxygen reduction reaction (ORR) using density functional theory (DFT) were designed and are described herein. The calculations showed that the heterostructure catalysts (except for Zn-N₄-Gr/V₂C) were thermodynamically stable. Ni-N₄-Gr/V₂C and Co-N₄-Gr/V₂C showed higher activities towards the ORR, with overpotentials as low as 0.32 and 0.45 V, respectively. Excellent catalytic performance results were observed from the change in electronic structure caused by the strong interaction between V₂C and the graphene layers as well as the synergistic effect between the MN₄ groups and the graphene layers. This study further provides insights into the practical application of ORR catalysts for MXene systems through the modulation of the electronic structure of two-dimensional materials.

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.

Alkaline Metal Oxide Assisting the Ionothermal Method for Efficient Fe-NX/C Catalyst Preparation

Owing to low cost and high efficiency, nonprecious metal catalysts have been widely used in various types of fuel cells. To obtain a high-activity electrocatalyst, a simple method for the synthesis of iron-modified covalent triazine frameworks by the direct heating of a mixture of FeCl3, ZnCl2, ZnO, and m-phthalodinitrile is reported. The role and a possible evolution pathway of the oxygen of metallic oxides are well discussed. To further verify our assumption, the Fe3O4 microspherical nanomaterials were synthesized and the relative Fe-based catalyst (Fe-NX/C) was successfully obtained by the ionothermal polymerization method. Fe-NX/C exhibits an extraordinary oxygen reduction reaction (ORR) performance in acidic solution, with a half-wave potential of only 25 mV negative shifts compared with Pt/C, while the power density is approximately 56% of that of Pt/C catalysts under the proton exchange membrane fuel cell testing condition. This work represents a new strategy to synthesize high-performance Fe-based catalysts toward ORR.

FeCo Nanoparticles Encapsulated in N-Doped Carbon Nanotubes Coupled with Layered Double (Co, Fe) Hydroxide as an Efficient Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Low-cost bifunctional nonprecious metal catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for the commercialization of rechargeable zinc-air batteries (ZABs). However, the preparation of highly active and durable bifunctional catalysts is still challenging. Herein, an efficient catalyst is reported consisting of FeCo nanoparticles embedded in N-doped carbon nanotubes (FeCo NPs-N-CNTs) by an in situ catalytic strategy. Due to the encapsulation and porous structure of N-doped carbon nanotubes, the catalyst shows high activity toward ORR and excellent durability. Furthermore, to enhance the OER activity, CoFe-layer double hydroxide (CoFe-LDH) is coupled with FeCo NPs-N-CNTs by in situ reaction approach. As the air electrode for rechargeable ZABs, the cell with CoFe-LDH@FeCo NPs-N-CNTs catalyst exhibits high open-circuit potential (OCP) of 1.51 V, high power density of 116 mW cm^-2 , and remarkable durability up to 100 h, demonstrating its great promise for the practical application of the rechargeable ZABs.

Single Mn Atom Anchored on Nitrogen-Doped Graphene as a Highly Efficient Electrocatalyst for Oxygen Reduction Reaction

Single Mn atom on nitrogen-doped graphene (MnN₄ -G) has exhibited good structural stability and high activity for the adsorption and dissociation of an O₂ molecule, becoming a promising single-atom catalyst (SAC) candidate for oxygen reduction reaction (ORR). However, the catalytic activity of MnN₄ -G for the ORR and the optimal reaction pathway remain obscure. In this work, density-functional theory calculations were employed to comprehensively investigate all the possible pathways and intermediate reactions of the ORR on MnN₄ -G. The feasible active sites and the most stable adsorption configurations of the intermediates and transition states during the ORR were identified. Screened from all the possibilities, three optimal four-electron O₂ hydrogenation pathways with an ultralow energy barrier of 0.13 eV were discovered that are energetically more favorable than direct O₂ dissociation pathways. Analysis of the free energy diagram further verified the thermodynamical feasibility of the three pathways. Thus, MnN₄ -G possesses superior ORR activity. This study provides a fundamental understanding of the design of highly efficient SACs for the ORR.

Oxygen-Reconstituted Active Species of Single-Atom Cu Catalysts for Oxygen Reduction Reaction

Identification of an active center of catalysts under realistic working conditions of oxygen reduction reaction (ORR) still remains a great challenge and unclear. Herein, we synthesize the Cu single atom embedded on nitrogen-doped graphene-like matrix electrocatalyst (abbreviated as SA-Cu/NG). The results show that SA-Cu/NG possesses a higher ORR capability than 20% Pt/C at alkaline solution while the inferior activity to 20% Pt/C at acidic medium. Based on the experiment and simulation calculation, we identify the atomic structure of Cu-N₂C₂ in SA-Cu/NG and for the first time unravels that the oxygen-reconstituted Cu-N₂C₂-O structure is really the active species of alkaline ORR, while the oxygen reconstitution does not happen at acidic medium. The finding of oxygen-reconstituted active species of SA-Cu/NG at alkaline media successfully unveils the bottleneck puzzle of why the performance of ORR catalysts at alkaline solution is better than that at acidic media, which provides new physical insight into the development of new ORR catalysts.

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

The review provides a comprehensive understanding of the atomically dispersed metal–nitrogen–carbon cathode catalysts for proton-exchange membrane fuel cell applications.

A theoretical study of atomically dispersed MN4/C (M = Fe or Mn) as a high-activity catalyst for the oxygen reduction reaction

Carbon-based, non-noble metal catalysts for the oxygen reduction reaction (ORR) are crucial for the large-scale application of metal-air batteries and fuel cells. Density functional theory calculations were performed to explore the potential of atomically dispersed MN4/C (M = Fe or Mn) as an ORR catalyst in an acidic electrolyte and the ORR mechanism on MN4/C was systematically studied. The results indicated MN4 as the active site of MN4/C and a four-electron OOH transformation pathway as the preferred ORR mechanism on the MN4/C surface. The Gibbs free energy diagram showed that the rate-determining step of the FeN4/C and MnN4/C catalysts is the formation of the second H2O molecule and OOH*, respectively. FeN4/C exhibited higher thermodynamic limiting potential (0.79 V) and, thus, higher ORR activity than MnN4/C (0.52 V) in an acidic environment; its excellent catalytic performance is due to the nice electron structure and adsorption properties of the FeN4 site. Therefore, this work demonstrates that atomically dispersed MN4/C is a promising catalyst for the ORR.