Efficient Oxygen Reduction Research Articles

Synergistic coupling of defective SOx-containing carbon and iron phthalocyanine for efficient oxygen reduction

In this paper, we dispersed FePc on an SOx-riched carbon substrate through a facile ultrasound method. The additional introduced SOx functional groups leading to optimized intermediate adsorption behavior, resulting in optimized ORR activity.

Precisely constructing charge-asymmetric dual-atom Fe sites supported on hollow porous carbon spheres for efficient oxygen reduction

Transition group metal catalysts showing atomic dispersion are on the rise as affordable electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell batteries, but their activity in acidic media remains constrained.

Nanowires-based MnO2-Ru/rGO: An efficient oxygen reduction reaction electrocatalyst

Oxygen Reduction Reaction (ORR) for clean energy is hindered by expensive Pt-based electrocatalysts, prompting efforts to replace it with alternative electrocatalysts. Thus, we started by synthesizing MnO2 nanowires through a hydrothermal approach, followed by the growth of ruthenium nanoparticles (Ru NPs) without surface modification, using just 2.0 wt% of the noble metal (MnO2-Ru). However, to further enhance the electrocatalyst's performance and reduce costs, we combined different ratios of reduced graphene oxide (rGO) with the electrocatalyst. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy were employed to characterize the chemical composition and morphological properties of MnO2-Ru. These analyses identified the presence of the compounds during synthesis and confirmed the deposition of Ru NPs on the surface of MnO2 nanowires. The optimized MnO2-Ru/rGO demonstrated superior ORR activity than rGO, MnO2, and MnO2-Ru individually, with more positive onset potential (−0.054 V) and half-wave potential of −0.173 V. Notably, MnO2-Ru/rGO reduced oxygen via the four-electron transfer pathway. Furthermore, the higher stability and excellent methanol tolerance of MnO2-Ru/rGO compared to the commercial 20 wt% Pt/C indicates its suitability for fuel cells, maintaining approximately 70 % of its initial current after 8000 s.

Dipole-Dipole Tuned Electronic Reconfiguration of Defective Carbon Sites for Efficient Oxygen Reduction into H2O2.

Metal-free carbon-based materials are one of the most promising electrocatalysts toward 2-electron oxygen reduction reaction (2e-ORR) for on-site production of hydrogen peroxide (H2O2), which however suffer from uncontrollable carbonizations and inferior 2e-ORR selectivity. To this end, a polydopamine (PDA)-modified carbon catalyst with a dipole-dipole enhancement is developed via a calcination-free method. The H2O2 yield rate outstandingly reaches 1.8mol gcat -1 h-1 with high faradaic efficiency of above 95% under a wide potential range of 0.4-0.7 VRHE, overwhelming most of carbon electrocatalysts. Meanwhile, within a lab-made flow cell, the synthesized ORR electrode features an exceptional stability for over 250h, achieved a pure H2O2 production efficacy of 306g kWh-1. By virtue of its industrial-level capabilities, the established flow cell manages to perform a rapid pulp bleaching within 30min. The superior performance and enhanced selectivity of 2e-ORR is experimentally revealed and attributed to the electronic reconfiguration on defective carbon sites induced by non-covalent dipole-dipole influence between PDA and carbon, thereby prohibiting the cleavage of O-O in OOH intermediates. This proposed strategy of dipole-dipole effects is universally applicable over 1D carbon nanotubes and 2D graphene, providing a practical route to design 2e-ORR catalysts.

Ceria nanoclusters coupled with Ce-Nx sites for efficient oxygen reduction in Zn-air batteries

Rational construction of efficient carbon-supported rare earth cerium nanoclusters as oxygen reduction reaction (ORR) is of great significance to promote the practical application of zinc-air batteries (ZABs). Herein, N doped conductive carbon black anchored CeO2 nanoclusters (CeO2 Clusters/NC) for the ORR is reported. The volatile cerium species vaporized by CeO2 nanoclusters at high temperatures are captured by nitrogen-rich carbon carriers to form highly dispersed Ce-Nx active sites. Benefiting from the coupling effect between oxygen vacancies-enriched CeO2 nanoclusters and highly dispersed Ce-Nx sites, the prepared 2CeO2 Clusters/NC catalyst possesses an ORR half-wave potential of 0.88 V, superior electrochemical stability, and better methanol tolerance compared to commercial Pt/C catalysts. Moreover, the 2CeO2 Clusters/NC involved liquid ZABs show excellent energy efficiency, superior stability, and a high energy density of 982 Wh kg−1 at 10 mA cm−2.

Activated Vacuum Residue for Efficient Oxygen Reduction Reaction in Alkaline Media

As a potential replacement for Pt-based materials, the heteroatom-doped and metal-free porous carbons have always been a crucial challenge in modern electrocatalysis. On the other hand, petroleum waste, particularly, vacuum residue presents as a carbon-rich precursor for the development of porous carbons for electrocatalysis. Herein, the new catalysts were prepared from vacuum residue gained from the local petroleum industry. The porous carbons (PCs) as the catalysts were prepared via a simple single route activation at 800 °C and 2 hours in the presence of phosphoric acid. PC possesses a high surface area at ~2200 m2/g with mixed micro- and meso porosities as well as high graphitization content. Spectroscopic analyses reveal up to ~6 % of phosphorous doping content. PCs were tested against oxygen reduction reaction (ORR) as an important challenge in electrochemistry. The structural features and the differences in the activating agents for the development of the PCs were compared to decipher the importance of the doped heteroatoms’ environment in ORR. The ORR efficiency of the electrocatalysts was compared against the commercial 5% Pt/C under alkaline conditions to show the potential of the vacuum residue-derived PCs as the sustainable utilization of petroleum waste.

Construction of core‐shell heterostructured MnO@Co/NC for enhancing the oxygen electrocatalytic performance

AbstractThe development of non‐precious metal electrocatalysts for efficient oxygen reduction reaction (ORR) is a significant research field vital for the progress of zinc−air battery technology. Here, we purposely constructed a core‐shell heterostructured electrocatalyst consisting of MnO nanotubes covered by Co nanoparticles anchored on N‐doped carbon substrate. MnO@Co/NC displayed superior ORR capability with a half‐wave potential of 0.85 V and almost no potential decay after 10,000 cycles CV test, outform that of commercial Pt/C catalysts. Employing MnO@Co/NC as the air electrode catalyst in primary Zn−air batteries leads to a top power density of 145 mW cm−2 and an enhanced specific capacity of 869 mAh g−1 as compared to commercial noble metal catalysts. Such superb ORR performance outcomes primarily stem from the synergistic effect between Co/NC shell and MnO core since the coupling structure not only regulates the inherent electronic structure but also accelerates electron transfer speed and boosts the active sites. Besides, the Co/NC outer layer can also help to improve the corrosion resistance of MnO@Co/NC in the electrolyte. This work demonstrated that the core‐shell heterostructured non‐precious catalysts open up a new avenue for obtaining sufficiently effective ORR catalysts in Zn−air batteries and other related applications.

Highly Efficient Oxygen Reduction N-Doped Carbon Nanosheets Were Prepared by Hydrothermal Carbonization.

A metal-free carbon catalyst is a kind of oxygen reduction catalyst with great prospects. It is an important material with potential to replace the traditional Pt catalyst. In this paper, a kind of irregular and ultra-thin carbon nanosheet (K180M-300-900) with high catalytic activity was synthesized by hydrothermal calcination using okra as a biomass and NH4Cl as an N source. The prepared nitrogen-doped metal-free catalyst with high pyridine-N and graphitic-N provides an extremely large number of active sites and has certain lattice defects. Ultra-thin carbon nanosheets promote sufficient contact between the catalyst and electrolyte, promote the diffusion of oxygen, and result in a faster transfer rate of electrons. The initial potential and half-slope potential of K180M-300-900 are 0.99 V and 0.82 V, respectively, which are comparable to those of 20% Pt/C. In addition, the stability and methanol tolerance of this catalyst (K180M-300-900) are better than 20% Pt/C, so it has great development potential and application value. This result provides a new method to prepare metal-free carbon materials that will take the place of traditional Pt catalysts.

Engineering FeCo Dual Sites on Tube-on-Plate Hollow Structure for Efficient Oxygen Electroreduction.

Atomically dispersed single-atom catalysts are intriguing catalysts in the field of electrocatalysis for nearly 100% exploitation of metal atoms. However, they are still far from practical usage due to the scaling relationship limit and metal loading limit. Generation of a diatomic complex would offer superior catalytic performance through the cooperation of two neighboring atoms as active sites. Herein, Fe/Co dual atomic sites embedded in a tube-on-plate hollow structure are designed and fabricated for an efficient electrochemical oxygen reduction reaction (ORR). The unique structure composed of ultrathin nanotube building blocks dramatically maximizes the surface area for copious active site exposure. Thanks to the synergetic interaction between Fe/Co pairs, the obtained FeCo/NC exhibits outstanding ORR activity and stability in alkaline media. Furthermore, density functional theory calculations have revealed that the remarkable activity is attributed to the electron-deficient Fe sites in FeCoN6. This work may pave the way for the innovative design of highly dispersed dual-site catalysts for broader applications in the realm of electrochemical catalysis.

Tuning ORR selectivity of π-conjugated cobalt corroles from 2e- to 4e-

Efficient oxygen reduction reaction (ORR) is crucial for the performance of fuel cells and other electrochemical devices. Seeking for stable and electrochemically selective 2e- and 4e- ORR electrocatalyst is yet a challenge, making the design extremely important and outmost of interest. In this study, we demonstrate a method to tune ORR selectivity by adjusting the local cobalt density through oligo-/polymerization of π-conjugated cobalt(III) A2B and A3-corroles to linear and two-dimensional matrix structures. These heterogeneous catalysts exhibit remarkable physicochemical properties, fast charge transfer kinetics, electrochemical reversibility, and high durability. Unselective three-electron transfer kinetics with n = 2.5–2.9 in between −0.20 V and +0.40 V vs. RHE are detected with the π-conjugated cobalt(III) A2B- corrole polymers. Highly selective four-electron kinetics with n = 3.7 in between −0.20 V and +0.40 V vs. RHE are detected by employing the cobalt A3-corrole oligomers during the ORR catalysis.

Novel Perovskite Structured Nd0.5Ba0.5Co1/3Ni1/3Mn1/3O3-δ as Highly Efficient Catalyst for Oxygen Electrode in Solid Oxide Electrochemical Cells.

Developing catalytic materials with highly efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for lower-temperature solid oxide fuel cell (SOFC) and electrolysis cell (SOEC) technologies. In this work, a novel triple perovskite material, Nd0.5Ba0.5Co1/3Ni1/3Mn1/3O3-δ, has been developed and employed as a catalyst for both ORR and OER in SOFC and SOEC operations at relatively lower temperatures, showing a low polarization resistance of 0.327 Ω cm2, high-power output of SOFC up to 773 mW cm-2 at 650 °C, and a high current density of 1.57 A cm-2 from SOEC operation at 1.5 V at 600 °C. The relaxation time distribution reveals that Nd0.5Ba0.5Co1/3Ni1/3Mn1/3O3-δ could maintain a slow polarization process at the relatively low operating temperature, offering a significant antipolarization advantage over other perovskite electrode materials. The Nd0.5Ba0.5Co1/3Ni1/3Mn1/3O3-δ electrode provides a low energy barrier of about 0.36 eV in oxygen ion mobility, which is beneficent for oxygen reduction/evolution reaction processes.

Modulating Electronic Structure and Atomic Insights into the Novel Hierarchically Porous PdCuFe Trimetallic Alloy Aerogel for Efficient Oxygen Reduction.

The high cost of noble Pd/Pt required for the oxygen reduction reaction (ORR) in the cathode restricts the wide applications of fuel cells. In this study, the synthesis of a novel Pd3CuFe0.5 aerogel electrocatalyst is successfully demonstrated using self-assembly and lyophilization techniques, employing a mild reducing agent. The resulting aerogel electrocatalyst exhibits a distinctive 3D network structure, possessing a substantial BET-specific surface area of 75.19 m2g-1. It is worth noting that the optimized Pd3CuFe0.5 aerogel demonstrates exceptional ORR performance with a high half-wave potential of 0.92V versus RHE, a significant limiting current density of 7.6mAcm-2, and the excellent electrocatalytic stability, superior to the reported noble metal electrocatalysts, with the ORR activity decays only 4.9% after 16000 s. In addition, the Pd3CuFe0.5 aerogel electrocatalyst shows superior cycling stability for ≈120h at a charge/discharge current density of 10mAcm-2, indicating its promising application in fuel cells. Furthermore, the resulting composite aerogel possesses excellent hydrogen evolution reaction and ethanol oxidation reaction activity. The density functional theory calculations show that the partial oxidation of Pd3CuFe0.5 aerogel leads to a negative shift of the d-band center, which energetically optimizes the binding strength of *O intermediates, therefore accelerating the ORR activity.

Tailoring the Chemisorption Manner of Fe d‐Band Center with La2O3 for Enhanced Oxygen Reduction in Anion Exchange Membrane Fuel Cells

AbstractEngineering the electronic configuration and intermediates adsorption behaviors of high‐performance non‐noble‐metal‐based catalysts for the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is highly imperative for the development of anion exchange membrane fuel cells (AEMFCs), yet remains an enormous challenge. Herein, a rare‐earth metal oxide engineering tactic through the formation of Fe3O4/La2O3 heterostructures in N,O‐doped carbon nanospheres (Fe3O4/La2O3@N,O‐CNSs) for efficient oxygen reduction electrocatalysis is reported. The theoretical calculations reveal that the interfacial bonds formed by the La─O─Fe heterogeneous interface effectively optimize the electronic structure of the Fe d‐band center relative to the Fermi level, which results in a significant reduction of the reaction barriers of rate‐limiting steps during the ORR. The modulation in intermediates chemisorption enables Fe3O4/La2O3@N,O‐CNSs an outstanding ORR performance and improved stability, with a significantly higher half‐wave potential value (0.88 V). More impressively, this integrated catalyst delivers a remarkable power density of 148.7 mW cm−2 in practical AEMFC operating conditions, along with negligible performance degradation over 100 h using an H2‐air atmosphere, which is higher than commercial Pt/C‐coupled electrodes. The results presented here are believed to provide guidelines for fabricating high‐performance AEMFCs electrocatalysts in terms of heterointerface engineering and strong electronic coupling effect induced by rare‐earth oxides.

Cobalt-Free BaFe0.6Zr0.1Y0.3O3−δ Oxygen Electrode for Reversible Protonic Ceramic Electrochemical Cells

Reversible protonic ceramic electrochemical cells (R-PCECs) are ideal, high-efficiency devices that are environmentally friendly and have a modular design. This paper studies BaFe0.6Zr0.1Y0.3O3−δ (BFZY3) as a cobalt-free perovskite oxygen electrode for high-performance R-PCECs where Y ions doping can increase the concentration of oxygen vacancies with a remarkable increase in catalytic performance. The cell with configuration of Ni-BZCYYb/BZCYYb/BFZY3 demonstrated promising performance in dual modes of fuel cells (FCs) and electrolysis cells (ECs) at 650 °C with low polarization resistance of 0.13 Ω cm2, peak power density of 546.59 mW/cm2 in FC mode, and current density of − 1.03 A/cm2 at 1.3 V in EC mode. The alternative operation between FC and EC modes for up to eight cycles with a total of 80 h suggests that the cell with BFZY3 is exceptionally stable and reversible over the long term. The results indicated that BFZY3 has considerable potential as an air electrode material for R-PCECs, permitting efficient oxygen reduction and water splitting.

Design and synthesis of self-supporting FeCoNi- and N-doped carbon fibers/nanotubes as oxygen bifunctional catalysts for solid-state flexible Zn-air batteries

Developing effective bifunctional electrocatalysts with efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) capabilities is critical to advancing rechargeable zinc-air batteries. This study harnessed the properties of organic salts in polymer solutions via electrospinning to synthesize self-supporting flexible bifunctional electrocatalysts, termed FeCoNi flexible carbon nanofibers (FeCoNi FCNFs). These catalysts benefitted from the secondary growth of FeCoNi- and N-doped carbon nanotubes on fiber surfaces, generating numerous metal-nitrogen-carbon (M−N−C) ORR active sites and FeCoNi alloy-based OER active sites. At 10 mA/cm2, FeCoNi FCNFs exhibited a half-wave potential of 0.92 V for ORR and 1.64 V for OER. Density functional theory calculations based on first principles demonstrated that FeCoNi FCNFs possessed superior bifunctional activity. Moreover, rechargeable liquid Zn-air batteries constructed with this catalyst achieved a high peak power density and exceptional cycling stability lasting over 630 h. Additionally, a solid-state wearable battery using this catalyst displayed outstanding cycle stability, continuously cycling for over 27 h. This research offers both insight and a fresh approach for designing efficient, cost-effective bifunctional solid-state or liquid zinc-air batteries.

Facile synthesis of the multidimensional oxygen redox electrocatalysts by anion-mediated self-template one-step in situ assembly

The exploration of a non-precious metal bifunctional electrocatalyst with novel structures for the efficient oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) remains a great challenge. Multidimensional assembled nanostructures with nitrogen-doped carbon matrix inlaid with transition-metal (M-N-C) can be used as efficient electrocatalysts due to its large surface area and highly revealed active sites. Herein, we report an anion-mediated self-template one-step in situ transformation into multidimensional hierarchical nano-assembled M-N-C catalysts, which is assembled from 0D cobalt nanoparticles terminally inlaid in 1D carbon nanotubes growing on a honeycomb-like ultrathin 3D carbon network (denoted as Co@N-CNTs/CS). By further synthetic process characterization, the nitrate-mediated structural evolution during rising thermal stages is disclosed. Thanks to the fully exposed surfaces and highly accessible active sites in the unique multidimensional structure, our developed catalyst exhibits a large surface area and enhanced performance in bifunctional oxygen redox in alkaline media, which further endows potentials in rechargeable Zn-air batteries, surpassing the typical benchmark catalysts Pt/C + Ir/C. Our study affords a fresh path to facile fabrication of M-N-C electrocatalysts.

Guest molecule-directed conversion of covalent organic framework into carbon with synergistic high content N dopants and defects for efficient oxygen reduction

Creating intrinsic defects and N dopants in metal-free carbon matrix has proven to be an effective approach for enhancing their electroactivity toward oxygen reduction (ORR), though it is still a big challenge. Herein, metal-free carbon materials with tunable contents of defects and N dopants were prepared through the guest molecule-directed conversion of covalent organic framework (COFs). Due to the corrosion of guest molecules and subsequent substitution by ammonia (NH3) during the pyrolysis, the optimal carbon catalyst is provided with high content of N dopants (8.22 at%) and carbon defects (ID/IG, 1.41). It delivers remarkable ORR activity, with a half-wave potential (E1/2) of 0.863 V versus reversible hydrogen electrode (vs. RHE) and impressive durability, with a 10.9 % current loss over 40 h. Moreover, the optimal carbon catalyst-assembled Zn-air battery achieves a maximum power density of 107.1 mW cm−2, demonstrating its potential in practical devices. Theoretical calculations reveal that the synergistic effect between carbon defects and heteroatom N dopants is responsible for the enhancement of ORR activity. This work might inspire the development of highly efficient metal-free carbon catalysts for various energy conversion applications.

Metal-organic framework derived porous N, P co-doping carbon microcubes toward aqueous and flexible quasi-solid-state rechargeable Zn-air batteries

The sluggish kinetics of oxygen reduction and/or evolution reactions in air cathodes of Zn-air batteries (ZABs) limits their practical applications. Herein, a facile strategy of phosphating metal-organic framework is developed to fabricate porous defect-enriched nitrogen and phosphorus co-doing carbon microcubes (NPC) both toward efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as a cost-efficiency bi-functional electrocatalyst. With detailed regulation in annealing temperatures, the optimized NPC sample (NPC-10) with nitrogen (N, 1.0 at.%) and phosphorus (P, 1.6 at.%) co-doping possesses bi-model micro-/meso-pores, and large specific surface area of 1627.1 m2 g−1. Thanks to unique structural merits and N/P synergetic contribution, the NPC-10 is endowed with abundant active sur-/interfaces and rapid kinetics for efficient ORR (limited ORR current density of 5.4 mA cm−2, half-wave potential of 0.89 V at 10 mA cm−2, Tafel slope of 141.6 mV dec−1 and a four-electron selectivity of >94.7 %) and OER (overpotential of 220 mV at 10 mA cm−2 and Tafel slope of 57.3 mV dec−1). More encouragingly, the NPC-10 assembled aqueous and flexible quasi-solid-state (QSS) rechargeable ZABs both deliver high power density and long-cycle stability. The work provides highly efficient, durable, and low-cost ORR/OER electrocatalysts toward rechargeable aqueous and/or QSS ZABs.

Water hyacinth root derived hybrid metal oxides/nitrogen doped porous carbon as an efficient non-precious metal oxygen reduction reaction electrocatalyst in alkaline media

A biomass derived hybrid metal oxides/nitrogen doped porous carbon (NC) electrocatalyst is fabricated by direct pyrolysis of the water hyacinth root (WHR) mixed ZnCl2 raw materials at 600 °C (WHR600), 700 °C (WHR700) and 800 °C (WHR800) under autogenic pressure conditions. XRD analysis presents a novel Zn(Al1.8Fe0.2)O4 compound combined with hematite quart and muscovite phases for WHR700 and WHR800 samples. In addition, the WHR700 sample accounts for the maximum pyridinic N content incorporated into carbon framework (NC). Electrochemically, the WHR700 catalyst displays the highest oxygen reduction reaction (ORR) performance in alkaline media. The high ORR performance for WHR700 catalyst is significantly a result of the presence of the Zn(Al1.8Fe0.2)O4 phase and the pyridinic N species. Furthermore, an accelerate durability test for WHR700 sample over 5000 cycles exhibits only 8 mV half-wave potential reduction. The WHR700 catalyst is also extremely stable to the methanol poisoning during ORR operation.

Customized Synthesis of MOF Nanoplates via Molecular Scalpel Strategy for Efficient Oxygen Reduction in Zn-Air Batteries.

The production of metal-organic framework (MOF) nanoplates with well-defined geometric morphology is remarkable for expanding their applications. Herein, the cobalt-based MOF nanoplates with hexagonal channels from a layer-pillared MOF are accomplished, via a molecular scalpel strategy, utilizing monodentate pyridine to replace the bidentate 4,4'-bipyridine. The morphology can be modified from nanorods to nanoplates with controllable thickness tuned by the amounts of pyridine. Succeeding carbonization treatment transforms the MOF nanoplates into Co particles homogeneously encapsulated in the nitrogen-doped carbon layers. The prepared catalyst with a unique platelike morphology displays a high half-wave potential of 0.88 V in oxygen reduction reaction. When used in primary Zn-air batteries, it delivers a high peak power density of 280 mW cm-2 . This work clarifies the structure-morphology-reactivity connection of MOF nanoplates.