Oxygen Reduction Reaction Mechanism Research Articles

Catalase‐Based Air‐Cathode for Biocompatible Zinc–Air Battery

AbstractThe use of catalase (CAT) from bovine liver as an electrocatalyst in the air electrode of a primary zinc–air battery (ZAB) with neutral electrolyte (pH 7.4) is reported. First, the use of Nafion for immobilizing CAT onto electrode surface allowed to obtain long‐term stability of the catalytic activity of CAT. Besides, analysis of the CAT and CAT‐Nafion as catalysts for oxygen reduction reaction (ORR) was carried out by linear sweep voltammetry (LSV) using a rotating ring‐disk electrode (RRDE), demonstrating a four‐electron ORR mechanism. Finally, CAT and CAT‐Nafion were tested as air electrodes in flooded ZAB and compared against human hemoglobin‐based ZAB. Our results demonstrated the significant influence of the protein coating of the heme groups on the ZAB performance, affecting greatly the discharge capacity and the power density.

Synergistic effects of metal single atoms and clusters on graphene-supported electrocatalysts: insights into the mechanism of oxygen reduction reaction

Synergistic effects of metal single atoms and clusters on graphene-supported electrocatalysts: insights into the mechanism of oxygen reduction reaction

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory.

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.

Enhanced Oxygen Reduction Reaction Via Oxygen Vacancy‐Rich Silica‐Supported Ag/Pd Nanoshells

AbstractHerein, we demonstrated the fine‐tuning of catalysts’ active phase by employing AgPd nanoshells with distinct Ag/Pd ratios synthesized via a galvanic replacement method for the oxygen reduction reaction (ORR). However, more interestingly, the subsequent immobilization of such Ag/Pd ratios onto silica further influenced the support characteristics, creating an increasing concentration of oxygen vacancies in this typically inert support — a surprising and unparalleled outcome attested by electrochemical impedance spectroscopy, electron paramagnetic resonance, and theoretical calculation. Such a phenomenon promoted obtaining an optimized electro/photocatalyst with exceptional activity, facilitating not just the ORR but also the photochemical water‐splitting reaction. Curiously, adjusting the Ag/Pd ratio also affected the ORR mechanism, which was switched from a 2‐electron to a 4‐electron after optimization. Finally, Ag38Pd62/SiO2, the catalyst with the best proportion, exhibited a remarkable hydrogen production rate of 1039.8 μmol/gcatalyst during 300 minutes of water splitting, surpassing the performance of the conventional Degussa TiO2 P25 catalyst.

DFT modeling of the oxygen electroreduction reaction on SiN3-doped carbon nanotubes

The thermodynamic features and mechanism of the electrocatalytic oxygen reduction reaction were studied using the revPBE0-D3(BJ)/Def2-TZVP method on the example of (6,6)-armchair carbon nanotube doped with a tricoordinated silicon atom and nitrogen atoms of pyridinic and graphitic nature. Irreversible oxidation of the silicon center as a result of the formation of stable oxygen-containing adsorbates was shown. It was found that Si-poisoned structures are capable of participating in the catalysis of the target reaction along two- and four-electron routes at high overpotentials. For a nanotube doped simultaneously with pyridinic and graphitic nitrogens the potential possibility of eliminating the silicon atom from the catalyst composition in the form of orthosilicic acid and the participation of a silicon-free nitrogen-doped framework in the oxygen electroreduction reaction, for which the stage of tautomerization of pyridin-2(1H)-one to pyridin-2-ol is the limiting step was shown.

Bifunctional catalytic activity of anion-doped LaSrCoO4 for oxygen reduction and evolution reactions.

Here, we synthesized Co-based, anion-incorporated‍ ‌R‌u‌d‌d‌l‌e‌s-d‌e‌n‌-‌Popper perovskite electrocatalysts (LaSrCoO4-x X y ) and compared their catalytic performances in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The ORR mechanism with the newly synthesized F-doped LaSrCoO4 catalyst was dominated by a four-electron process, and the number of electrons involved in the reaction increased compared with that for LaSrCoO4. The OER activity of the hydride-doped LaSrCoO4 catalyst was the highest among the LaSrCoO4 system catalysts. Density functional theory calculations revealed that there is a correlation between the Co 3d unoccupied orbital band centre and the OER activity. The addition of anions and substitution of metal sites improved the ORR and OER activities of the catalysts. Our findings confirmed that the addition of heteroatom anions can improve the activity of perovskite-type electrocatalysts, promoting their application in various fields.

Effect of mixed metal ions and incorporation of activated carbon on the physiochemical and catalytic properties of spinel based (Ni,Fe)Co2O4@C nanoparticles

The challenge of producing cost-effective oxygen electrocatalysts for the oxygen reduction reaction (ORR) continues to impede the advancement of fuel cell technology. Herein, we report the effect of Ni and activated carbon on mixed metal (Ni,Fe)Co2O4 and (Ni,Fe)Co2O4@C spinel oxide nanoparticles synthesized by facile and versatile co-precipitation method followed by the physio-chemical, morphological properties and electrochemical analysis towards oxygen reduction reaction. The formation of FeCo2O4, (Ni,Fe)Co2O4 and (Ni,Fe)Co2O4@C nanoparticles was confirmed with X-ray diffraction and Raman analysis while D and G band revealed the presence of carbon in the (Ni,Fe)Co2O4@C nanoparticles. The Brunauer–Emmett–Teller analysis reveals the mesoporous behaviour of the synthesized multi-metal oxide nanoparticles. X-ray photoelectron spectroscopy analysis reveals the presence of Co3+/Co2+, Fe3+/Fe2+, Ni3+/Ni2+, O1s and C1s confirms the formation of complex (Fe3+Co2+)(Ni2+Ni3+Fe2+Fe3+Co3+)2O4 spinel structure. The ionic radii of the Co, Fe and Ni ions plays a vital role in the formation of complex spinel structures. The ORR analysis was studied by electrochemical analysis and observed (Ni,Fe)Co2O4 outperformed FeCo2O4 in terms of onset potential and current densities for the ORR. (Ni,Fe)Co2O4@C exhibited the highest catalytic activity with −2.18 mAcm−2 at 0.1 V vs RHE, which is relatively comparable with that of commercial Pt/C. Furthermore, the electrocatalytic analysis revealed the ORR mechanism adheres to the direct "4e−" process. The intermetallic interactions between the Co, Fe and Ni ions along with high crystallinity, significant surface area, high porosity, and the influence of carbon incorporation that are all contribute to the improved electrocatalytic activity of the cobalite-based multi-metal spinel oxides.

Mechanistic Study on Oxygen Reduction Reaction in High-Concentrated Electrolytes for Aprotic Lithium-Oxygen Batteries.

Highly concentrated electrolytes (HCEs) have energized the development of high-energy-density lithium metal batteries by facilitating the formation of robust inorganic-derived solid electrolyte interfaces on the lithium anode. However, the oxygen reduction reaction (ORR) occurring on the cathode side remains ambiguous in HCE-based lithium-oxygen (Li-O2) batteries. Herein, we investigate the ORR mechanism in a highly concentrated LiTFSI-CH3CN electrolyte using ultra-microelectrode voltammetry coupled with in situ spectroscopies. It is found that, compared to the dilute electrolyte, the HCE prolongs the lifespan of superoxide intermediates and decelerates their migration rate to the bulk solution, resulting in a change in growth mode for the discharge product of Li2O2 from traditional two-dimensional film growth to surface three-dimensional expansion growth. This alteration reduces the cathode passivation and thus delivers the enhanced discharge capacity. Additionally, the HCE also increases the reaction energy barrier between superoxide and solvent molecules, thereby minimizing parasitic reactions and improving the cycle performance of Li-O2 batteries. Our study reveals the intricate interplay between electrolytes and oxygen intermediates and provides important insights into electrolyte chemistries for better Li-O2 batteries.

Piezo‐catalysis Mechanism Elucidation by Tracking Oxygen Reduction to Hydrogen Peroxide with In situ EPR Spectroscopy

AbstractFor piezoelectric catalysis, the catalytic mechanism is a topic of great controversy, with debates centered around whether it belongs to the energy band theory or the screening charge effect which are similar to mechanisms of photocatalysis and electrochemical catalysis, respectively. Due to the formation of different intermediate active‐species during two‐electron oxygen reduction reaction (ORR) via electro‐ and photo‐catalysis, the key to solving this problem is precisely monitoring the active species involved in ORR during electro‐, photo‐, and piezo‐catalysis under identical condition. Here, a semiconductor material, BiOBr with abundant oxygen vacancies (BOB‐OV) was found remarkable catalytic activity in H2O2 production by all three catalytic methods. By employing in situ electron paramagnetic resonance (EPR) spectroscopy, the H2O2 evolution pathway through piezo‐catalysis over BOB‐OV was monitored, which showed a similar reaction pathway to that observed in photo‐catalytic process. This finding represents solid evidence supporting the notion that piezo‐catalytic mechanism of ORR is more inclined towards photo‐catalysis rather than electro‐catalysis. Significantly, this exploratory conclusion provides insight to deepen our understanding of piezo‐catalysis.

Piezo-catalysis Mechanism Elucidation by Tracking Oxygen Reduction to Hydrogen Peroxide with In-situ EPR Spectroscopy.

For piezoelectric catalysis, the catalytic mechanism is a topic of great controversy, with debates centered around whether it belongs to the energy band theory of photocatalysis or the screening charge effect of electrochemical catalysis. Due to the formation of different intermediate active-species during two-electron oxygen reduction reaction (ORR) via electro- and photo-catalysis, the key to solving this problem is precisely monitoring the active species involved in ORR during electro-, photo-, and piezo-catalysis under identical condition. Here, a semiconductor material, BiOBr with abundant oxygen vacancies (BOB-OV) was found remarkable catalytic activity in H2O2 production by all three catalytic methods. By employing in-situ electron paramagnetic resonance (EPR) spectroscopy, the H2O2 evolution pathway through piezo-catalysis over BOB-OV was monitored, which showed a similar reaction pathway to that observed in photo-catalytic process. This finding represents solid evidence supporting the notion that piezo-catalytic mechanism of ORR is more inclined towards photo-catalysis rather than electro-catalysis. Significantly, this exploratory conclusion provides insight to deepen our understanding of piezo-catalysis.

Catalysis of oxygen reduction reaction by iron porphyrin and its sensing application

Hemin, or iron porphyrin, serves as a non-noble metal catalyst for the oxygen reduction reaction (ORR). The stability of hemin is maintained in high salt concentrations, functioning as an ORR redox catalyst in the aqueous phase. Hemin facilitates the development of an oxygen sensor that transitions the ORR mechanism from a 2-electron to a more efficient 4-electron process. This sensor demonstrates excellent reproducibility (RSD<5%), with a wide linear range of 38.9–613 μM, a sensitivity of 0.136 ± 0.003 μA μM−1, and a limit of detection (3SB/m) of 11.8 μM.

Computational Engineering of Non-van der Waals 2D Magnetene for Enhanced Oxygen Evolution and Reduction Reactions.

Non-van der Waals two-dimensional materials containing exposed transition metal atoms are promising catalysts for green energy storage and conversion. For instance, hematene and ilmenene have been successfully applied as catalysts. Building on these reports, this work is the first investigation of recently synthesized magnetene towards the Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR). Using Density Functional Theory (DFT) calculations, we unveil the mechanism, performance and ideal conditions for OER and ORR on magnetene. With overpotentials of ηOER = 0.50 V and ηORR = 0.41 V, the material is not only a bifunctional catalyst, but also superior to state-of-the-art systems such as Pt and IrO2. Additionally, its catalytic properties can be further enhanced through engineering strategies such as point defects and in-plane compression. It reaches ηORR = 0.28 V at a compressive strain of only 2%, while the presence of Ni boosts it to ηOER = 0.39 V and ηORR = 0.31 V, comparable to many reported single-atom catalysts. Overall, this work demonstrates that magnetene is a promising bifunctional catalyst for applications such as regenerative fuel cells and metal-air batteries.

A first principles study of the oxygen reduction reaction mechanism on porous Ag and Ag-TM nanostructure

Porous Ag has good electron conductivity and is one of the typical oxygen reduction reaction(ORR) catalysts. In order to investigate the mechanism of porous Ag for ORR, the relaxed structure and detailed partial density of states are determined using density-functional theory. Among multiple possible active sites, the overpotential of porous Ag is 0.50 V, which is better than that of Ag (111) at 0.62 V. After doping Pt and Pd, the overpotentials are 0.47 V and 0.49 V, respectively. Furthermore, the introduction of a transition metal has led to changes in the charge distribution on the catalyst surface, which has resulted in improved catalytic performance. By investigating the synergistic effects between doped transition metals and ORR intermediates, this can facilitate the development of catalysts with higher activity and better stability.

Investigating Alloy-Induced Modifications in the Oxygen Reduction Reaction Mechanism on PtPd Single Crystals

Investigating Alloy-Induced Modifications in the Oxygen Reduction Reaction Mechanism on PtPd Single Crystals

Design and Impact: Navigating the Electrochemical Characterization Methods for Supported Catalysts.

This review will investigate the impact of electrochemical characterization method design choices on intrinsic catalyst activity measurements by predominantly using the oxygen reduction reaction (ORR) on supported catalysts as a model reaction. The wider use of hydrogen for transportation or electrical grid stabilization requires improvements in proton exchange membrane fuel cell (PEMFC) performance. One of the areas for improvement is the (ORR) catalyst efficiency and durability. Research and development of the traditional platinum-based catalysts have commonly been performed using rotating disk electrodes (RDE), rotating ring disk electrodes (RRDE), and membrane electrode assemblies (MEAs). However, the mass transport conditions of RDE and RRDE limit their usefulness in characterizing supported catalysts at high current densities, and MEA characterizations can be complex, lengthy, and costly. Ultramicroelectrode with a catalyst-filled cavity addresses some of these problems, but with limited success. Due to the properties discussed in this review, the recent floating electrode (FE) and the gas diffusion electrode (GDE) methods offer additional capabilities in the electrochemical characterization process. With the FE technique, the intrinsic activity of catalysts for ORR can be investigated, leading to a better understanding of the ORR mechanism through more reliable experimental data from application-relevant high-mass transport conditions. The GDEs are helpful bridging tools between RDE and MEA experiments, simplifying the fuel cell and electrolyzer manufacturing and operating optimization process.

Construction of Fe–In Alloy to Enable High Activity and Durability of Fe–N–C Catalysts

AbstractThe strategic regulation of the electronic properties and coordination environment of single‐atom sites through the integration of metal nanoclusters emerges as a promising route to enhance the oxygen reduction reaction (ORR) performance of Fe–N–C materials. Here, a catalyst (FeIn–NC) is successfully developed in which Fe–N–C materials encapsulate Fe–In alloy nanoclusters, and it shows excellent ORR activity and durability under alkaline conditions, with a high half‐wave potential of 0.924 V (vs RHE) and a zinc–air battery power density of 202.1 mW cm−2, superior to commercial Pt/C catalysts. Theoretical calculations unravel that the synergistic interaction between the Fe–In alloy and the FeN4 single‐atom site modifies the electronic structure and charge distribution at the FeN4 site, thereby enhancing the electrocatalytic activity and durability of the ORR. Potential‐dependent microkinetic modeling (MKM) further discloses the ORR mechanisms on the identified FeN4 sites. This work provides a viable strategy for the ORR improvement of Fe–N–C materials via p‐block metal‐based alloy nanoclusters.

Breaking through the State-of-the-Art Frontier in Oxygen Electrocatalysis: Geometry Adaptive Catalysts (GACs)

The grand challenge in oxygen electrocatalysis lies in overcoming scaling relations. The theoretical activity volcano positions the ideal catalyst at the apex. However, the OH–OOH scaling relation determines a linear path on the volcano surface that limits the activity. The current state-of-the-art (SOTA) in modeling confirms this scaling relation, at the same time it hinders the experimental development of novel oxygen reduction reaction (ORR) catalysts, impeding progress in sustainable energy solutions. To address this challenge, we propose to use the geometry effect and introduce the concept of Geometry Adaptive Catalysts (GACs).[1]The geometry effect, based on preliminary research, involves changing catalytic activity by adjusting the adsorption energy of intermediates through the control of catalyst geometry. Our goal is to break through the SOTA frontier for ORR by synthesizing GACs and supporting our efforts with theoretical and computational modeling of the geometric effect. In this work we aimed to utilize the geometry effect by synthesizing GACs with various controlled structures, including specific intersite distances, and variable curvature. This enables precise control of catalyst geometry, allowing manipulation of catalytic activity and overcoming scaling relations. Key geometric parameters, namely inter-site distance, curvature, and specific stabilizations of ORR intermediates, are explored to test our hypothesis: Controlling catalyst geometry adjusts the ORR mechanism and adsorption energies of intermediates, enabling the switching, pushing, and even bypassing of scaling relations.This approach holds promise for substantial improvements in environmental sustainability and human life quality, with broad applicability to various electrocatalytic reactions beyond ORR.

(Invited) Engendering Non-Noble Metal Catalysts for Electrolyzers: From Fundamentals to Devices

In memory of Prof. Sri Narayanan, this presentation will showcase the electrocatalysis of hydrogen evolution reaction (HER) and oxygen reduction in the context of oxygen depolarization cathode (ODC) for electrolyzer applications. Fundamentals of HER/HOR in alkaline pH will be presented in the context of various theories proposed to rationalize pH, cations, and structure effects. It will be shown that none of them can effectively explain all observations. In this presentation, four schools of thought for HER/HOR will be discussed, focusing on the strengths and shortcomings of each hypothesis and highlighting the magnitude of electrochemical interface structure in hydrogen electrocatalysis. Translation of these fundamental concepts into a durable electrocatalyst will be presented as a functionalized mono-metallic Ni catalyst concept. Durability results will be presented from both steady-state operation and uncontrolled shutdown conditions. These will be presented in conjunction with anion exchange membranes such as VersogenÒ, OrionÒ. Another cathodic process in the context of electrolysis is the oxygen-depolarized cathode, typically paired with anodic processes, such as chlorine generation. ODC cathode using coordinated Fe-N-C catalysts will be presented in the context of hydrochloric acid recovery, where activity and durability in the context of both fundamentals of oxygen reduction reaction mechanism and effects of uncontrolled shutdown will be presented.

In situ Regulating the phase structure of Ce-based catalytic sites to boost the performance of zinc-air batteries

We report an iodide-induced phase regulation strategy to in-situ synthesize a cerium-based catalyst (Ce-N4O2 SACs) with high Ce loading (15.9 wt%), radicals scavenging ability and high-density Ce-N4O2 sites on the surface of porous nanocarbon frameworks. The oxygen reduction reaction (ORR) mechanisms on the Ce-N4O2 SACs are recorded by in-situ Raman spectra. Moreover, the density functional theory (DFT) calculations show that the Ce-N4O2 structure can optimize the electronic configuration of Ce and positively promote the activation of adsorbates. As expected, this catalyst delivers excellent ORR electrocatalytic activity and stability (half-wave potential decay ∼11 mV after cycling for 30k cycles). The assembled primary zinc-air battery demonstrates a remarkable energy density (ED) of 946 Wh kgZn−1 and exhibits predominant long-term durability (ED reduction ∼3.5 % after for 130 h). This work shows that rare-earth single-atom catalysts, subjected to regulation encompassing structure-density-configuration-mass transfer, hold great promise in realizing high ED metal-air batteries.

The Tetrapyrollic Motif in Nitrogen Doped Carbons and M‐N‐C Electrocatalysts as Active Site in the Outer‐Sphere Mechanism of the Alkaline Oxygen Reduction Reaction

The Tetrapyrollic Motif in Nitrogen Doped Carbons and M‐N‐C Electrocatalysts as Active Site in the Outer‐Sphere Mechanism of the Alkaline Oxygen Reduction Reaction