High-performance Oxygen Reduction Reaction Research Articles

Study the single-atom Mn-doped catalysts on boron nitride sheet surface as cathode for oxygen reduction reaction in proton‐exchange membrane fuel cells

An important subject in improvement of polymer electrolyte membrane fuel cells (FCs) is sluggish kinetics of cathodic oxygen reduction reaction (ORR). In present work, Mn-doped vacancy boron nitride nanosheet has been offered as a noble-metal-free and efficient electrocatalyst for ORR process in fuel cells employing DFT computation. It is discovered that adsorption energy (Eads) values on Mn–N active site of selected catalysts enhance in order of: H2O > H2O2 > O2 > OOH > OH > O. For all intermediates containing oxygen, there is almost a consistent trend in Eads alteration. A slight thermodynamic force drives H2O2 formation and considerable ones for reduction of OOH into O* (or to 2OH*) indicate that 4e− pathway is more useful compared to 2e− pathway. Besides, from thermodynamic point of view, final stage of reduction (OH* + H+ + e− → H2O + *) within the highest value of ΔG for Mn-doped vacancy BN catalyst is rate-determining step (RDS). A larger HOMO-LUMO gap or a manganese d-band center remote from Fermi level indicates that catalyst doesn't favor adsorption of oxygen-containing species, which in turn results in higher ORR performance.

P-Block-metal bismuth-based electrocatalysts featuring tunable selectivity for high-performance oxygen reduction reaction

•The discovery of p-block-metal bismuth-based ORR electrocatalysts is reported •The metallic Bi nanoparticles exhibit outstanding activity for H2O2 production •The single-atomic-site Bi catalysts give excellent performance for 4e− ORR •DFT calculations explain the origin of the high performance of Bi-based ORR catalysts For oxygen reduction reaction (ORR), replacing the conventional electrocatalysts based on platinum-group metals (PGMs) with alternatives based on non-noble metals is vital for the large-scale applications of green energy conversion and chemical synthesis. Here, we report the discovery of ORR catalysts based on the p-block-metal bismuth (Bi); the selectivity in ORR pathways could be controlled by tailoring the size of Bi. Specifically, the metallic Bi nanoparticles gave a high selectivity (>96%) for 2e− ORR, an ultra-high kinetic current density (3.8 mA·cm−2 at 0.65 V), and an excellent stability; in contrast, the single-atomic-site Bi catalysts gave a good selectivity for 4e− ORR and a corresponding half-wave potential of 0.875 V, approaching that of the best-known Fe/NC catalysts. This work presents new high-performance ORR catalysts based on the p-block-metal Bi, in contrast to the intensively studied d-block transition metals, and thus would provide new perspectives for developing PGM-free ORR catalysts. For oxygen reduction reaction (ORR), replacing the conventional electrocatalysts based on platinum-group metals (PGMs) with alternatives based on non-noble metals is vital for the large-scale applications of green energy conversion and chemical synthesis. Here, we report the discovery of ORR catalysts based on the p-block-metal bismuth (Bi); the selectivity in ORR pathways could be controlled by tailoring the size of Bi. Specifically, the metallic Bi nanoparticles gave a high selectivity (>96%) for 2e− ORR, an ultra-high kinetic current density (3.8 mA·cm−2 at 0.65 V), and an excellent stability; in contrast, the single-atomic-site Bi catalysts gave a good selectivity for 4e− ORR and a corresponding half-wave potential of 0.875 V, approaching that of the best-known Fe/NC catalysts. This work presents new high-performance ORR catalysts based on the p-block-metal Bi, in contrast to the intensively studied d-block transition metals, and thus would provide new perspectives for developing PGM-free ORR catalysts.

1,2,4-triazole-assisted metal-organic framework-derived nitrogen-doped carbon nanotubes with encapsulated Co4N particles as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

The design of high-performance oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) dual-functional catalysts is not only important for the further applications of zinc-air batteries (ZABs) but also a major challenge in the field of energy conversion. The cheap 1,2,4-triazole (1,2,4-TZ) can be decomposed easily by heat, making it a high research value in carbon catalysts derived from metal-organic frameworks (MOFs). Here, Co4N particles encapsulated at the top of N-doped carbon nanotubes (Co4N@NCNTs) were conveniently prepared by 1,2,4-TZ-assisted pyrolysis of Co-MOF-74 for the first time. Owing to the excellent activity of Co4N particles and the highly graphitized N-doped carbon nanotubes (NCNTs), Co4N@NCNTs obtained at 900 °C (Co4N@NCNT-900) exhibited astonishing catalytic performance in both ORR and OER, and high reversible oxygen bifunctional activity (ΔE = 0.685 V). Moreover, Co4N@NCNT-900 displayed a larger discharge power density (122 mW cm−2), a better specific capacity (811.8 mAh g−1), and more excellent durability during the ZAB test, implying that Co4N@NCNT-900 can act as a bifunctional high active catalyst in ZABs.

Ammonia Tolerance of Atomically Dispersed Single Metal Site Catalysts: Mechanistic Understanding and High‐Performance Oxygen Reduction Electrocatalysis

AbstractThe anion‐exchange membrane direct ammonia fuel cell, as a carbon‐free fuel cell type, has recently received increasing attention albeit suffering from high cost of using the platinum‐group metal oxygen reduction reaction (ORR) catalysts. To pave the development of this promising power source, the atomically dispersed transition metal‐nitrogen‐carbon (M‐N‐C) materials with low cost and high ORR performance have allured to investigate their ammonia tolerance during the ORR. Herein, it is initially deconvoluted how compositional and structural elements of FeN4 sites modulate catalyst's performance. Furthermore, ORR catalytic activities of the M‐N‐C (M = Fe, Co or Mn) and Pt/C catalysts are investigated in ammonia‐containing electrolytes, showing that M‐N‐C catalysts have better ammonia tolerance than Pt/C. Among others, the Fe‐N‐C exhibits the best ammonia tolerance with only 4 mV negative shifts of half‐wave potential, 2.7% decrease of current, and negligibly irreversible activity loss. The superior ammonia tolerance of MN4 sites to Pt (111) surface is further confirmed by density functional theory calculations. The adsorption capacity of MN4 for O2 is higher than NH3 and the bonding force between MN4 and O2 is stronger than NH3, whereas opposite adsorption capacity and bonding force trends are observed on Pt (111) surface.

Amorphous low-coordinated cobalt sulphide nanosheet electrode for electrochemically synthesizing hydrogen peroxide in acid media

This study investigates in-depth the local atomic environment-property relationship of amorphous low-coordination catalysts toward the 2e− oxygen reduction reaction (ORR). Here, an ethanol-assisted slow nucleation strategy was adopted to flexibly regulate the intrinsic activity of CoSx nanostructures for efficient 2e− ORR by oriented control the coordination number of Co. When Co and S form Co-S4 coordination structures, the CuNW@CoS4 exhibits an ultra-low overpotential (0.018 V) with 93 % H2O2 selectivity, and 91 % Faraday efficiency in acidic media at 0.1 VRHE. Furthermore, thus produced H2O2 satisfies in-situ organic pollutants degradation by electro-Fenton. XAS and DFT calculations unveil the S-induced forming of abundant low-coordinated Co atoms as active sites in Co-S4, validating that a favorable d-band energy level of Co center can weaken the OOH* adsorption for accelerating the H2O2 production kinetics. This work is anticipated to enlighten the rational design of high-performance ORR catalysts for practically feasible H2O2 electrosynthesis toward wastewater remediation.

Progress and Perspective for In Situ Studies of Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Proton exchange membrane fuel cell (PEMFC) is one of the most promising energy conversion devices with high efficiency and zero emission. However, oxygen reduction reaction (ORR) at the cathode is still the dominant limiting factor for the practical development of PEMFC due to its sluggish kinetics and the vulnerability of ORR catalysts under harsh operating conditions. Thus, the development of high-performance ORR catalysts is essential and requires a better understanding of the underlying ORR mechanism and the failure mechanisms of ORR catalysts with in situ characterization techniques. This review starts with the introduction of in situ techniques that have been used in the research of the ORR processes, including the principle of the techniques, the design of the in situ cells, and the application of the techniques. Then the in situ studies of the ORR mechanism as well as the failure mechanisms of ORR catalysts in terms of Pt nanoparticle degradation, Pt oxidation, and poisoning by air contaminants are elaborated. Furthermore, the development of high-performance ORR catalysts with high activity, anti-oxidation ability, and toxic-resistance guided by the aforementioned mechanisms and other in situ studies are outlined. Finally, the prospects and challenges for in situ studies of ORR in the future are proposed.

Rare-metal-free Zn–air batteries with ultrahigh voltage and high power density achieved by iron azaphthalocyanine unimolecular layer (AZUL) electrocatalysts and acid/alkaline tandem aqueous electrolyte cells

Zn–air batteries have only been used in limited applications, such as hearing aid batteries, due to their low power density and standard voltage of around 1.4 V. Therefore, to use Zn–air batteries as a drive power source in cutting-edge devices such as drones, it is essential to improve the drive voltage and output power density. Here, we propose Zn–air batteries with a high potential (∼2.25 V) and high power density (∼318 mW/cm2) by using the newly designed iron azaphthalocyanine unimolecular layer (AZUL) electrocatalyst and a tandem Zn–air battery cell. The AZUL electrocatalyst in this new type of cell had a high electrochemical stability and high oxygen reduction reaction performance in the ultralow pH region, in which Pt and other metallic and inorganic electrocatalysts cannot be used. Furthermore, the tandem-electrolyte cells had a cell voltage of over 1.0 V at a high discharge current density of 200 mA/cm2, and the output power density was 1139 mWh/g(Zn) at 100 mA/cm2 discharge.

Iron and cobalt phthalocyanine embedded electrospun carbon nanofiber-based catalysts for anion exchange membrane fuel cell cathode

The successful implementation of renewable energy transition assumes the introduction and widespread use of zero-emission and sustainable energy conversion devices. Anion-exchange membrane fuel cell (AEMFC) is one of the appliances that meets the prescribed criteria. In this study, Fe(II) phthalocyanine and Co(II) phthalocyanine co-doped polyacrylonitrile-based electrospun carbon nanofibre (CNF) catalysts were prepared via pyrolysis. The influence of carbonisation temperature, acid leaching conditions, presence of ionic liquid (IL) as porogen, and the metal phthalocyanine content were investigated. Physical characterisation revealed the nanofibrous structure, high specific surface area, presence of Co-Nx, Fe-Nx sites, and different N-species with high oxygen reduction reaction (ORR) activity. The catalyst material with the highest ORR performance, D-MN4-CNF-IL-A, showed a half-wave potential of 857 ± 6 mV vs. reversible hydrogen electrode in 0.1 M KOH and the maximum power density value of 231 mW cm−2 when applied as a cathode catalyst in the AEMFC.

Density functional theory studies of Pt2Ga and Pd2Ga monolayers as multifunctional electrocatalytic materials

Developing multifunctional electrocatalysts with high performance for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is still a challenging task. In this work, the electrocatalytic activities of Pt2Ga and Pd2Ga monolayers are systematically investigated based on first-principles calculations. The high dynamic and thermal stabilities of Pt2Ga and Pd2Ga monolayers are confirmed by phonon dispersion calculations and ab initio molecular dynamics (AIMD) simulations, respectively. In addition, the Pd2Ga free energy for H adsorption is predicted to be −0.03 eV, implying its extremely high activity for HER. Furthermore, the Pt2Ga overpotential for OER and ORR are estimated to be 0.48 and 0.42 V, correspondingly, indicating its high catalytic performance. This work provides adequate guidance for designing multifunctional electrocatalysts with high performance for OER, ORR and HER.

Three-dimensional urchin-like N/S co-doped Mn-based metal-organic frameworks for efficiently enabling oxygen reduction reaction

The exploration of economical and effective non-noble metal catalysts is essential for oxygen reduction reaction (ORR) in energy devices. Recently, heteroatom-doped metal-organic frameworks (MOFs) have shown great potential in ORR due to their high efficiencies and low costs. Herein, three-dimensional (3D) urchin-like N/S co-doped Mn-based MOFs (U–N/S–Mn-MOFs) as an effective ORR catalyst have been successfully prepared by a facile one-step solvothermal methodology. The unique 3D urchin-like structure with a spherical interior and dendritic exterior may provide more catalytic sites and transport channels for ORR. Simultaneously, the doped N and S combined with Mn can promote oxygen adsorption and reduce the reaction energy barrier. These characteristics endow U–N/S–Mn-MOFs with high ORR performance. The present work provides a new opportunity for multiple heteroatom-doped MOFs to achieve high electrocatalytic performance.

N-doping carbon-coated Cu@C(N) nanocapsules synthesized by arc plasma toward high-performance ORR electrocatalyst

The development of efficient and cost-effective methods for the synthesis of non-precious metal as the catalytic material for oxygen reduction reaction (ORR) is addressing a key barrier to fuel cell deployment in a wide range of applications. In this study, core-shell carbon-coated copper nanoparticles (Cu@C) were prepared using arc discharge plasma under argon and hydrogen atmospheres. And nitrogen doping was achieved during the subsequent heat treatment using dicyandiamide as the nitrogen source. The results showed that the catalysts had better ORR catalytic activity, better stability and better methanol tolerance than Pt/C, and the ORR reaction process was nearly four-electron pathway. The successful doping of nitrogen increased the reaction sites. And the addition of copper core improved the conductivity of the catalytic material. Meanwhile, the N-C shell layer protected the Cu nanoparticles from dissolving in aqueous solutions or being chemically oxidized, facilitating the electronic interaction of the core Cu nanoparticles with the N-C shell layer. This study provides an insight into the preparation of high-efficiency non-precious metal-based ORR electrocatalysts by DC arc plasma, a beneficial method.

Dispersed cobalt nanoparticles in the nitrogen doped carbon black as efficient catalysts for oxygen reduction reaction and Zinc-Air batteries

Developing an effective and durable Pt-free oxygen reduction reaction (ORR) catalyst for Zinc-Air batteries (ZABs) is propitious to easing the energy and pollution crisis. Herein, we have prepared Co nanoparticles/N-doped carbon (Co/N/C) with tunable Co contents by the pyrolysis of polydopamine-coated cobalt phthalocyanine/carbon black precursors. Benefiting from the well-dispersed Co nanoparticles and the formation of various active sites (i.e. pyridinic, graphitic N and Co-Nx), the resulting Co/N/C-2.86 wt% catalysts possess a half-wave potential of 0.84 V and a mass activity of 5.18 A/g, illustrating the superior ORR performance. Noteworthy, the ZABs with the cathode of Co/N/C-2.86 wt% display an outstanding cell performance with a large open-circuit voltage (1.542 V), a high specific discharge capacity (744 mAh g−1) and an excellent maximum power density (236 mW cm−2). This work offers a promising approach to rationally design and fabricate high-performance ORR electrocatalysts for advanced ZABs.

Boosted Oxygen Reduction Reaction Activity by Ordering Cations in the A-Site of a Perovskite Catalyst

As concerns related to climate change continue to grow, there has been an increasing number of studies on materials that can enhance the efficiency of energy conversion/storage systems. Since the oxygen reduction reaction (ORR) is considered as a rate-determining reaction for the O2 conversion system, it is essential to develop an efficient electrocatalyst that enables greater ORR activity. Perovskite has been explored as a promising electrocatalyst due to its excellent ORR activity at high and room temperatures. Recently, unique perovskite structures that can contain high oxygen vacancy ratios, such as brownmillerite, Ruddlesden–Popper, and layered perovskite, have been studied for exhibiting high ORR performance. However, few studies have dealt with the electrochemical properties of perovskite, which has different structures but the same cation stoichiometry, to know the exact effect of a specific structure. In this study, we synthesized two materials with different structures (cubic structure perovskite and layered structure perovskite) but with the same cation stoichiometry. The layered structure rather than the cubic structure showed cation ordering in the A-site of the perovskite structure with an enhanced oxygen vacancy ratio. As a result, the layered structure exhibits superior electrochemical performance than the cubic structure at high and room temperatures by a factor of ca. 2.7 at 500 °C in fuel cell mode, and ca. 5.5% decrease in overpotential (η) at −3 mA cm–2 for ORR catalysis. Moreover, perovskite hybridized with nitrogen doped reduced graphene oxide (N-rGO) could compensate for the intrinsic shortcoming of perovskite oxide and exert a catalytic synergy (27% decrease in η).

Modulating the electronic structure of hollow Cu/Cu3P hetero‐nanoparticles to boost the oxygen reduction performance in long‐lasting Zn‐air battery

AbstractDeveloping cost–benefit and high‐performance non‐noble metal oxygen reduction reaction (ORR) electrocatalysts is highly imperative for wide applications of renewable energy conversion devices. Herein, a one stone two birds phosphorization strategy has been proposed to synthesize hollow structured Cu/Cu3P heterogeneous nanoparticles supported on N, P co‐doped carbon (Cu/Cu3P@NP‐Cs). The optimized Cu/Cu3P@NP‐C‐900 features high ORR performance under both alkaline and acidic conditions. Moreover, the Cu/Cu3P@NP‐C‐900‐drivened Zn‐air battery exhibits a substantially higher power density output (148.2 mW cm−2) and stronger charge–discharge stability (300 h, 1805 cycles) than those of Pt/C‐equipped counterpart. The cross‐interface electron transfer from Cu3P to Cu effectively regulates the d‐band center of Cu/Cu3P, thereby leading to the balanced adsorption/desorption energy of oxygen species. Meanwhile, the hollow structure maximizes the exposure of accessible active centers, resulting in much accelerated ORR kinetics. This work proposes an innovative insight for developing hollow hetero‐structured catalysts to improve ORR performance.image

Self-Sacrificing Template Synthesis of Carbon Nanosheets Assembled Hollow Spheres with Abundant Active Fe-N4 O1 Moieties for Electrocatalytic Oxygen Reduction.

Single-atom Fe-N-C (Fe1 -N-C) materials represent the benchmarked electrocatalysts for oxygen reduction reaction (ORR). However, single Fe atoms in the carbon skeletons cannot be fully utilized due to the mass transfer limitation, severely restricting their intrinsic ORR properties. Herein, a self-sacrificing template strategy is developed to fabricate ultrathin nanosheets assembled Fe1 -N-C hollow microspheres (denoted as Fe1 /N-HCMs) by rational carbonization of Fe3+ chelating polydopamine coated melamine cyanuric acid complex. The shell of Fe1 /N-HCMs is constructed by ultrathin nanosheets with thickness of only 2nm, which is supposed to be an ideal platform to isolate and fully expose single metal atoms. Benefiting from unique hierarchical hollow architecture with highly open porous structure, 2nm-thick ultrathin nanosheet subunits and abundant Fe-N4 O1 active sites revealed by X-ray absorption fine structure analysis, the Fe1 /N-HCMs exhibit high ORR performance with a positive half-wave potential of 0.88V versus the reversible hydrogen electrode and robust stability. When served as air-cathode catalysts with ultralow loading mass of 0.25mg cm-2 , Fe1 /N-HCMs based Zn-air batteries present a maximum power density of 187mW cm-2 and discharge specific capacity of 806mA h gZn -1 in primary Zn-air batteries, all exceeding those of commercial Pt/C.

Co supported on N and S dual-doped reduced graphene oxide as highly active oxygen-reduction catalyst for direct ethanol fuel cells

Oxygen reduction reaction (ORR) is one of the key features for the efficient functioning of several energy conversion devices such as fuel cells, appearing the necessity of development of new low-cost catalyst materials. Heteroatom-doped carbon materials have attracted attention in this field due to its physicochemical and electronic properties. In this work, a nitrogen and sulfur doped material with anchored Co3O4 nanoparticles (Co/SN-rGO) is proposed as cathode catalyst for direct ethanol fuel cells (DEFCs) and results are compared with different doped graphene nanomaterials (GMs). The effect of the heteroatoms and cobalt oxide nanoparticles in the final efficiency was studied. Synthesized materials were characterized and the activity of Co/SN-rGO and GMs for the ORR was studied. Co/SN-rGO presents high ORR performance in terms of onset potential (Eonset), 0.86 V (vs RHE) and half-wave potential (E1/2) 0.72 V (vs RHE). Tafel analysis shows 60 mV dec-1 at low overpotential for potential dependent ORR mechanism. Besides, when Co/SN-rGO performance is evaluated in a DEFC using a fuel cell test station, main results indicate higher catalytic activity, stability, and ethanol tolerance of Co/SN-rGO in comparison to a carbon-supported Pt catalyst.

Plasma-engineered cobalt nanoparticle encapsulated N-doped graphene nanoplatelets as high-performance oxygen reduction reaction electrocatalysts for aluminum–air batteries

The increasing demand for continuous and eco-friendly alternative energy has prompted interest in aluminum–air (Al–air) batteries owing to their high energy density and safety. On the other hand, the sluggish oxygen reduction reaction (ORR) rate in alkaline electrolytes, which adversely affects the efficiency of the Al–air battery, is still a challenge. A transition metal electrocatalyst consisting of cobalt nanoparticles encapsulated with N-doped graphene nanoplatelets (Co@N/GNP) was synthesized by plasma engineering. In this design, the active Co nanoparticle acted as the ORR active site and the core-shell structure of dominant pyridinic-N graphene led to outstanding catalytic performance (E1/2 = 0.87 V, Eonset = 0.98 V) in an alkaline electrolyte. Furthermore, when applied as a cathode catalyst in practical Al–air batteries, Co@N/GNP showed excellent performance with a peak power density of 143 mW cm-2.

Ternary PtZrNi nanorods for efficient multifunctional electrocatalysis towards oxygen reduction and alcohol oxidation

Pt-based alloys with precise structure and composition design have been considered to be effective and robust novel electrocatalysts for fuel cells. Whereas, the sluggish kinetics of oxygen reduction reaction (ORR) and low intrinsic activity of Pt limited their real application on a large scale. Herein, a novel ternary PtZrNi nanorods (PtZrNi NRs) was synthesized via a facile wet-chemical method to achieve high electrocatalytic performance for both ORR and alcohol oxidation reaction owing to the synergism of chosen three elements and prominent one-dimensional morphology. Specifically, the PtZrNi NRs show enhanced mass and specific activities of 0.755 A mgPt-1 and of 0.97 mA/cm2 at 0.9 VRHE towards ORR in acidic media, which are 4.7 and 4.4 times of those of commercial Pt/C, respectively. Additionally, in alkaline media, the PtZrNi NRs also exhibit superior ORR mass and specific activities of 3.216 A mgPt-1and 4.13 mA/cm2, enhanced by 34.6 and 31.3 times compared with those of commercial Pt/C, respectively. The PtZrNi NRs retain the nanorod shape well without agglomeration after an accelerated durability test (20000 cycles). This work may offer a new perspective for engineering high-performance Pt-based electrocatalysts for commercial fuel cells.

Synthesis and characterization of nickel doped LSM as possible cathode materials for LT-SOFC application

Doping lanthanum strontium manganite with Ni2+ at Mn site for 10–30 mol% was investigated as cathode material for LT-SOFC application. Nanopowders were prepared using a simple solution combustion technique and only after calcination at 1200 °C phase purity was obtained. While TEM analysis confirmed particle size around 30 nm, the DLS study showed average most probable size as 13 nm. The SEM-EDAX analysis revealed porous nanopowder of all the studied compositions. It also confirmed composition of the cathode material. In symmetric cell configuration, using lanthanum gallate as electrolyte, oxygen reduction reaction (ORR) and hence polarization resistance (RP) was studied in detail in the temperature range 500–800 °C. The composition La0.7Sr0.3Mn0.8Ni0.2O3 (LSMN7382) showed lowest RP value of 0.117–0.039 Ω-cm2 in the temperature range 500–800 °C. In contrast, La0.7Sr0.3MnO3 (LSM731) showed much higher RP value. Among the cathodes with Sr2+doping variation in La site, 30 mol% showed the best performance. The electrical conductivity of ∼80% dense electrolyte pellets were studied and highest conductivity of 70 S/cm at 800 °C was observed for LSMN7382 among the all compositions. Generation of hole takes place due to the charge compensation in Mn3+ to Mn4+ conversion while doping Sr2+ as well as Ni2+. At 30 mol% Ni2+ doping (LSMN7373), the decrease of conductivity may be attributed to the secondary phase formation. A detailed electrical circuit analysis of the impedance spectrum indicates that charge transfer polarization resistance is the key to control the interface performance. Such high ORR performance by nickel doping can be explained due to Mn4+ converted to Mn3+ at high temperature and thus creating oxygen vacancy to enhance ORR ionic path on the cathode surface.

Ordered mesoporous carbon fiber bundles with high-density and accessible Fe-NX active sites as efficient ORR catalysts for Zn-air batteries

Fe-NX/C electrocatalysts have aroused extensive interest in accelerating sluggish oxygen reduction reaction (ORR) kinetics as potential alternatives to platinum catalysts in rechargeable Zn-air batteries (ZABs). However, the low density and poor accessibility of Fe-NX sites have severely restricted the electrocatalytic performance of Fe-NX/C. Herein, Fe, N co-doped ordered mesoporous carbon fiber bundles are prepared through a ligand-assisted strategy with nitrogen-rich 1,10-phenanthroline as space isolation agent. 1,10-Phenanthroline reveals a six-membered heterocyclic structure containing abundant nitrogen species to tightly coordinate with Fe ions, which is conducive to achieving high-density Fe-NX sites. Meanwhile, the adoption of SBA-15 as hard-templates enables the catalysts with highly ordered channels and large specific surface areas, improving the accessibility of Fe-NX sites. The optimal catalyst (PDA-Fe-900) demonstrates a positive half-wave potential of 0.84 V (vs. RHE) in alkaline solution, outperforming the commercial Pt/C (0.83 V). In addition, PDA-Fe-900 delivers comparable ORR performance to commercial Pt/C in acidic electrolyte. Impressively, when PDA-Fe-900 is employed as an air cathode, it achieves large power densities of 163.0 mW/cm2 in liquid-state ZAB and 116.6 mW/cm2 in the flexible solid-state ZAB. This work provides an efficient ligand-assisted pathway for fabricating catalysts with dense and accessible Fe-NX sites as high-performance ORR electrocatalysts for ZABs.