Oxygen Reduction Reaction Mechanism Research Articles

Dendritic hollow nitrogen-doped carbon nanospheres for oxygen reduction at primary zinc–air batteries

Role of pore size in the N-doped porous carbon in the mechanism of oxygen reduction reaction at electrochemical energy storage and conversion systems.

Effects of functional groups in iron porphyrin on the mechanism and activity of oxygen reduction reaction.

The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising single-atomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (-COOH), methyl (-CH3), and amino (-NH2) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O2 dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin-(CH3)4 < porphyrin-(NH2)4 < original porphyrin < porphyrin-(COOH)4 for the associative mechanism and porphyrin-(NH2)4 < porphyrin-(CH3)4 < porphyrin-(COOH)4 < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH3)4 and porphyrin-(NH2)4 should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d z 2 , d xz , and d yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.

Research advances in earth-abundant-element-based electrocatalysts for oxygen evolution reaction and oxygen reduction reaction

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are crucial half-reactions of green electrochemical energy storage and conversion technologies, such as electrochemical water-splitting devices and regenerative fuel cells. Researchers always committed to synthesizing earth-abundant-element-based nanomaterials as high-efficiency electrocatalysts for realizing their industrial applications. In this review, we briefly elaborate on the underlying mechanisms of OER and ORR during the electrochemical process. Then, we systematically sum up the recent research progress in representative metal-free carbon (C)-based electrocatalysts; metal-nitrogen-C electrocatalysts; and nonprecious-metal OER/ORR electrocatalysts, including transition-metal oxides, phosphides, nitrides/oxynitrides, chalcogenides, and carbides. Among these, some representative bifunctional electrocatalysts for the OER/ORR are mentioned. In particular, we discuss the effects of physicochemical properties-morphology, phases, crystallinity, composition, defects, heteroatom doping, and strain engineering-on the comprehensive performance of the abovementioned electrocatalysts, with the aim of establishing the nanostructure-function relationships of the electrocatalysts. In addition, the development directions of OER and ORR electrocatalysts are determined and highlighted. The generic approach in this review expands the frontiers of and provides inspiration for developing high-efficiency OER/ORR electrocatalysts.

Reaction mechanism and kinetics of oxygen reduction reaction on the iron–nickel dual atom catalyst

Dual-atom catalysts (DACs) have recently emerged as promising and high-activity catalysts for the oxygen reduction reaction (ORR), a key process in many electrochemical energy conversion devices.

Nanostructure engineering of Pt/Pd-based oxygen reduction reaction electrocatalysts.

Increasing the atomic utilization of Pt and Pd elements is the key to the advancement and broad dissemination of fuel cells. Central to this task is the design and fabrication of highly active and stable Pt- or Pd-based electrocatalysts for the oxygen reduction reaction (ORR), which requires a comprehensive understanding of the ORR pathways and mechanism. Past endeavors have accumulated a wealth of knowledge about the Pt/Pd-based ORR electrocatalysts based on structure engineering, while a systematic review of the nanostructure engineering of Pt/Pd-based ORR electrocatalysts has been rarely reported. In this review, we provide a systematic discussion about the current status of Pt/Pd-based ORR electrocatalysts from the perspective of nanostructure engineering, and we highlight the ORR pathways, mechanisms and theories in order to understand the ORR in a more complex nanocatalyst. Particularly, the underlying structure-function relationship of Pt/Pd-based ORR electrocatalysts is specifically highlighted, which will guide the future synthesis of more efficient ORR electrocatalysts.

Origin of the superior oxygen reduction activity of zirconium nitride in alkaline media.

The anion exchange membrane fuel cell (AEMFC), which can operate in alkaline media, paves a promising avenue for the broad application of earth-abundant element based catalysts. Recent pioneering studies found that zirconium nitride (ZrN) with low upfront capital cost can exhibit high activity, even surpassing that of Pt in alkaline oxygen reduction reaction (ORR). However, the origin of its superior ORR activity was not well understood. Herein, we propose a new theoretical framework to uncover the ORR mechanism of ZrN by integrating surface state analysis, electric field effect simulations, and pH-dependent microkinetic modelling. The ZrN surface was found to be covered by ∼1 monolayer (ML) HO* under ORR operating conditions, which can accommodate the adsorbates in a bridge-site configuration for the ORR. Electric field effect simulations demonstrate that O* adsorption on a 1 ML HO* covered surface only induces a consistently small dipole moment change, resulting in a moderate bonding strength that can account for the superior activity. Based on the identified surface state of ZrN and electric field simulations, pH-dependent microkinetic modelling found that ZrN reaches the Sabatier optimum of the kinetic ORR volcano model in alkaline media, with the simulated polarization curves being in excellent agreement with the experimental data of ZrN and Pt/C. Finally, we show that this theoretical framework can lead to a good explanation for the alkaline oxygen electrocatalysis of other transition metal nitrites such as Fe3N, TiN, and HfN. In summary, this study proposes a new framework to rationalize and design transition metal nitrides for alkaline ORR.

Mixture of an ionic liquid and organic solvent at graphene: interface structure and ORR mechanism.

Aprotic lithium-oxygen batteries are attracting the attention of the scientific community due to their outstanding theoretical performance, which, however, still has not been achieved in practice. One of the promising directions for improving the stability of Li-O2 batteries is electrolyte design, which would provide good cyclability, inhibition of parasitic processes, and high energy density. In recent years, there has been progress in the use of ionic liquids in the composition of the electrolyte. The present work reveals possible explanations of the influence of the ionic liquid on the mechanism of the oxygen reduction reaction by the example of a combined electrolyte from an organic solvent DME and an ionic liquid Pyr14TFSI. Molecular dynamics modelling of the interface between a graphene electrode and an organic solvent DME with a variable volume fraction of the ionic liquid shows the influence of the electrolyte structure at the interface to the kinetics on the adsorption and desorption stages of oxygen reduction reaction reactants. The obtained results suggest the promotion of a two-electron oxygen reduction reaction mechanism through the formation of solvated O22-, which may explain the reduced recharge overpotential reported in experiments.

Oxygen Reduction Reaction Mechanism on the Square Paddle-Wheel Cage Site of TM-BTC (TM=Mn, Fe, Cu) Metal-Organic Framework

Our study examined the mechanism of oxygen reduction reactions (ORR) at the square paddle-wheel cage active site of TM-BTC metal-organic frameworks (MOFs), where TM is a transition metal, Mn, Fe, or Cu. We used a combination of density functional theory (DFT) and microkinetic calculations to investigate this mechanism. By using a small cluster for modeling the TM-BTC active site structure, we successfully reproduced the experimental trend of ORR activity in the TM-BTC systems: Mn-BTC &gt; Fe-BTC &gt; Cu-BTC. We also found that the unusual ORR activity trend in experiments for Mn and Fe systems originates from the strength of OH adsorption on these systems. The Mn-BTC system exhibits higher ORR activity than the Fe-BTC system due to its weaker adsorption of OH groups. A very strong OH adsorption makes the final OH reduction step sluggish, hence hindering the ORR process.

Mechanism of Pt3Co nanocatalysts to improve the performance for oxygen reduction reactions: DFT study on oxygen adsorption and durability of different facets

Pt3Co nanocatalyst shows better oxygen reduction reaction (ORR) activity and durability toward Pt. In this paper, the oxygen adsorption, durability and the ORR mechanism of Pt3Co are investigated based on density functional theory (DFT) calculations by CASTEP code. The adsorption energy of O atom at each site on (100), (110), and (111) surfaces indicated that the adsorption strength of O atom on Pt3Co surface follows the order (100) > (110) > (111). In addition, the ORR mechanism is explored through the electron density difference, d-band center, and the ORR elementary reactions. The results are consistent with the tendency for adsorption energy and prove that the O hydrogenation step is the rate-determining step on Pt3Co (111). Furthermore, the calculation of surface Pt vacancy formation energy reveals that the Pt3Co (111) catalyst has the best durability. The results could guide the design of surface crystal orientation of Pt3Co nanocatalyst for ORR.

N-doped coralline Co9S8−xNx for inducing Amitriptyline decontamination in Electro-Fenton Process: Degradation scheme Elucidation, nitrogen activating catalyst delocalized electron and enhancing 2-Electron oxygen reduction reaction mechanism investigation

Improving the recalcitrant emergent contaminant removal in electron-Fenton (EF) has been one of the focal problems. Here, a N-doped coralline Co9S8−xNx catalyst was synthesized with high electronegativity and strong spin polarization for promoting the H2O2 generation efficiency and Amitriptyline (AMT) removal. After doping nitrogen, catalytic performance of Co9S8−xNx has increased ∼10 % on AMT decay and the treatment time was shortened ∼40 min. Co9S8−xNx induced 2-electron oxygen reduction reaction (ORR) mechanism has also been investigated. According to electronic structure analysis, electrons surrounding the cobalt atoms were redistributed and the π* orbitals of *O2 were occupied asymmetrically forcing *O2 to be of radical nature active for hydrogenation, which significantly reduced required absorption energy in the rate-determining step (RDS). This work clarifies the mediate role of N-doped activating delocalized electron of adjacent atoms on the surface of Co9S8−xNx and also gives new insights into the rational design of catalyst for refractory pollutant degradation in wastewater.

Electrochemical Properties of Low Bi Doped La0.60Dy0.10Sr0.30Mn(1-x)BixO3 Perovskite Magneticaloric Materials

Developing low-cost, high-durability and high-performance electrocatalytic materials are needed for hydrogen/oxygen evolution (HER/OER) and oxygen reduction reactions (ORR) which are crucial steps for regenerative fuel cells. Although Pt-based, RuO2 and IrO2 materials are widely used for these purposes, other alternative materials are required with desired properties. In this regard, we studied the electrochemical properties of lanthanum-based perovskites which were synthesized by sol-gel method. We investigated the addition of 10% Dy and 30% Sr into the A site and a small amount of Bi (x: 0, 0.03 and 0.1) added into the B site of La0.60Dy0.10Sr0.30Mn(1-x)BixO3 perovskites on HER, OER and ORR in 1 M KOH alkaline media at room temperature. While the addition of 0% and 3% Bi exhibited poor HER activities, the effect of 10% Bi in the structure enhanced HER activities by lowering onset potential from -1.389 V to -1.036 V (vs. Ag/AgCl) and increasing specific current density at -1.4 V from -13.3 mA cm-2 to -121.8 mA cm-2. Similarly, OER activity was also improved due to 10% Bi and onset potential and specific current density was found to be 0.758 V and 88.3 mA cm-2, respectively. The large Tafel slopes indicate that the ORR mechanism is possible in the structure but at a slow rate. The addition of 10% Bi in the structure resulted in a very high resistance of 19.3 kΩ and it reduced to a desired value of 3.5 kΩ due to ion conducting paths developed in the catalysts.

Axial Coordination Effect on the Oxygen Reduction Reaction of FeN4 Electrocatalysts Based on Grand Canonical Density Functional Theory

The Fe–N–C materials are promising noble-free alternatives to Pt-based oxygen reduction reaction (ORR) electrocatalysts. Identifying the actual active structure of FeN4-based moieties is helpful for the regulation and design of high-performance electrocatalysts. However, the current theoretical researches were mostly based on the charge-neutral model (CNM), which cannot accurately describe the electrochemical interface. Herein, we employed the constant potential based grand canonical density functional theory (GC-DFT) computations to study the ORR mechanism of axially decorated FeN4 electrocatalysts. We studied around ten types of axial ligands, and the constant potential energetics and microkinetic modeling demonstrated that the Fe center can exhibit excellent activity for boosting the four-electron ORR via covalently linked −NH2 ligand. The lowering of the antibonding dz2–pz orbital is responsible for weakening the adsorption of oxygen in FeN4–Ls to promote ORR activity, and the −NH2 decoration led to the lowest antibonding orbital energy. In particular, the adsorption free energy (ΔG*O2) and O–O bond length (LO–O) of the adsorbed O2 reactant can be used as the effective energetic and geometric descriptors to describe the ORR activity of FeN4–Ls electrocatalysts. Our results not only elucidate the axial coordination effect on the ORR performance of FeN4 SACs but also demonstrate the importance of electrode potential in computational electrochemistry.

Challenges in Elucidating the Free Energy Scheme of the Laccase Catalyzed Reduction of Oxygen.

Artificial redox catalysts are typically limited by unfavorable scaling relations of reaction intermediates leading to a significant overpotential in multi-electron redox reactions such as for example the oxygen reduction reaction (ORR). The multicopper oxidase laccase is able to catalyze the ORR in nature. In particular the high-potential variants show a remarkably low overpotential for the ORR and apparently do not suffer from such unfavorable scaling relations. Although laccases are intensively studied, it is presently unknown why the overpotential for ORR is so low and a clear description regarding the thermodynamics of the catalytic cycle and the underlying design principles is lacking. In order to understand the laccase catalyzed ORR from an electrochemical perspective, elucidation of the free energy scheme would be of high value. This article reviews the energetics of the proposed laccase catalyzed ORR mechanisms based on experimental and computational studies. However, there are still remaining challenges to overcome to elucidate the free energy scheme of laccase. Obtaining thermodynamic data on intermediates is hard or even impossible with analytical techniques. On the other hand, several computational studies have been performed with significantly different parameters and conditions, thus making a direct comparison difficult. For these reasons, a consensus on a clear free energy scheme is still lacking. We anticipate that ultimately conquering these challenges will result in a better understanding of laccase catalyzed ORR and will allow for the design of low overpotential redox catalysts.

Coal-Derived N-Doped Porous Carbon Accompanied with Fe2O3 for Oxygen Reduction Reaction

In this study, a class of non-precious metal catalyst for oxygen reduction reaction (ORR), namely N-doped porous carbon accompanied with Fe2O3 (herein named as AC-4), is synthesized from one-step pyrolyzing the precursors of KOH, coal, Fe salt and melamine, followed by acid-leaching. Several catalysts, AC-1 from KOH + coal, AC-2 from KOH + coal + melamine and AC-3 from KOH + coal + Fe salt, are also shown for comparison. As a result, AC-4 provides the best ORR catalytic activity with the ORR peak potential (Ep) of 818 mV, followed by the order of AC-2 (783 mV) > AC-3 (775 mV) > AC-1(771 mV), meaning that the N-doping strategy plays more effect than the Fe2O3-doping method, and AC-4 has the synergetic effect of these two ways. Furthermore, the ORR mechanism on AC-4 is near a four-electron transfer pathway with the electron transfer number around 3.7–3.9. It also has a good stability and methanol-resistance. Structure measurements display that introduction of N heteroatom and/or metal can increase the disorder degree of AC, and AC-4 has a structure of N-doped porous carbon (the special surface area of 1193 m2 g−1) accompanied with Fe2O3.

High performance La2NiO4+δ impregnated La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes for solid oxide fuel cells

La2NiO4+δ (LNO) impregnated La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes were prepared and the effects of the impregnation on the electrical conductivity and oxygen surface exchange property were studied to reveal the enhancement mechanism of oxygen reduction reaction (ORR) catalytic activities. Results of electrochemical impedance spectroscopy (EIS) with distribution of relaxation times (DRT) deconvoluting and electrical conductivity relaxation (ECR) indicate that LNO impregnation into LSCF cathodes enhances the oxygen diffusion process and oxygen surface exchange property. The optimal ORR catalytic activity is achieved after LNO impregnation for two times on the LSCF scaffold (LNO2-LSCF). LNO2-LSCF cathodes also have outstanding performance for their compromise thermal expansion coefficients (TEC) and conductivities.

Degradation Mechanism and Enhancing Strategies of Oxygen Reduction Reaction Catalyzed by Carbon‐Based Metal Free Catalysts in Acidic Solution

AbstractCarbon‐based metal free catalysts (CMFCs) are far away from commercial availability mainly attributed to their poor oxygen reduction reaction (ORR) performance in acidic environment with the causes remaining obscure. By investigating the heteroatoms (N, B, P, S, Se, and Te)‐doped reduced graphene oxides, the degradation mechanism of acidic ORR performance of CMFCs is found to be correlated with the oxygen‐baring defects in the carbon sp2 lattice, which exhibit overpotentials as low as 0.44 V but weak trapping capabilities for oxygen molecules. These findings not only revise the previously reported strategy of modeling the active sites in the basal plane of CMFCs but also highlight the connections between those active sites and the triple‐coordinated VIA group elements (XC3). Further calculations demonstrate that the XC3 dimer can efficiently enhance the acidic ORR performance and the 2D trigonal carbon‐chalcogenides C6X (X = S, Se, and Te) are accordingly designed toward acidic ORR, which contain homogeneously distributed basal plane active sites and exhibit low overpotentials but strong trapping capabilities for oxygen molecules. This work will help to cease the debates on the active sites in CMFCs for ORR in both acidic and alkaline solutions and to open a new avenue to design CMFCs independent on doping strategy.

The synergistic effect of the adjacent dual NiNx sites to boost the oxygen reduction reaction activity for proton exchange membrane fuel cell

The investigation on the application of dual metallic sites in electro-catalysis for oxygen reduction reaction (ORR) has been given a lot of consideration. In this study, the reaction mechanisms and corresponding thermodynamic energy evolution in the dual Ni saturated coordinate with N atoms embedded in graphene have been investigated using first-principles density functional theory (DFT) calculations. The results show that the dual NiNx active sites are more stable than that of the NiN4 in view of the comparison of their formation energies. The reduction reaction of O2∗ on NiN4/G obviously promotes the two-electron reduction process due to its loosely bonded with O2. While the oxygen reduction reaction process on these dual NiNx active sites is thermally downhill, all of which promote a single site with four electron ORR process. The dual Ni atoms directly bonded catalysts (Ni2N6-ST2/G and Ni2N6-ST3/G) are in favor of the ORR mechanism through the reaction pathway of O2→O2∗→OOH∗→O∗→OH∗→H2O. However, the dual NiNx catalysts of others (Ni2N6-ST1/G and Ni2N7-ST4/G) prefer to undergo an unconventional 2OH∗ mechanism (O2→O2∗→OOH∗→2HO∗→OH∗→H2O). Moreover, it was found that the first reduction step (O2∗→OOH∗) is the thermodynamic rate-determining step (RDS) for the ORR on these catalysts. Both of Ni2N6-ST2/G and Ni2N6-ST3/G exhibit outstanding ORR activity with the on-set potentials of 0.65 V and 0.68 V, respectively. In particular, the two catalysts are able to adsorb the H2O2 and further reduce it into H2O through a two-electron pathway. Furthermore, it was found that the most stable intermediates in the oxygen reduction pathway depend on their lowest free energy at a given set of pH and applied potential. The free energy for these O-contained intermediates is linearly decreasing with the increment of applied potential. The critical potential for these O-contained intermediates is also linearly decreased with the increment of pH value.

Front Cover: Application of AgPt Nanoshells in Direct Methanol Fuel Cells: Experimental and Theoretical Insights of Design Electrocatalysts over Methanol Crossover Effect (ChemCatChem 23/2022)

The Front Cover shows that nanoengineering is a breakthrough way to look at the electrocatalysis field under a different perspective. In their Research Article, M. Aurélio Suller Garcia, T. Silva Rodrigues and co-workers provided the design of a hollow AgPt-based electrocatalyst with improved resistance to methanol for Direct Methanol Fuel Cells applications. After immobilization onto SiO2, the nanostructures presented remarkable stability, with a four-electron oxygen reduction reaction mechanism as the Pt/C commercial catalyst, with 20 times less Pt content. More information can be found in the Research Article by M. Aurélio Suller Garcia, T. Silva Rodrigues and co-workers.

Further optimization of the properties of Pr0.6Sr0.4Fe0.9W0.1O3-δ by Cu-doping as symmetric electrodes for solid oxide fuel cells

Copper-doped Pr0.6Sr0.4Fe0.8Cu0.1W0.1O3-δ (PSFCuW) as a symmetric electrode material is investigated for La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM)-based symmetric solid oxide fuel cells (SSOFCs). The effect of Cu-doping on stability and mechanism of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) is investigated and analyzed. Fe partially replaced by Cu on the B-site could also keep the phase structure stable after treatment under reducing conditions and improve the electrical conductivity, which reaches 30.3 and 0.78-1 under oxidizing and reducing conditions at 800 oC, respectively. The distribution of relaxation times (DRT) indicates that the rate-limiting step processes under oxidation conditions are oxygen adsorption and diffusion, while hydrogen adsorption and charge transfer have important influences on the rate-limiting step under reduced atmospheres, respectively. Finally, the LSGM-supported SSOFC (PSFCuW|LSGM|PSFCuW) shows excellent stability and reversibility. These results imply that PSFCuW is an up-to-coming symmetric electrode material for application in SSOFCs.

Theoretical Study of Oxygen Reduction Reaction Mechanism in Metal-Free Carbon Materials: Defects, Structural Flexibility, and Chemical Reaction.

Metal-free carbon materials are attractive Pt-based catalyst alternatives. However, despite efforts, the reaction mechanism remains elusive. Thus, we investigated the role of defects (dopant nitrogen and carbon vacancy) on the catalytic oxygen reduction reaction in a metal-free carbon material focusing on the effect of structural flexibility. Crucially, defects lower the energy barrier for the sp2/sp3 transition of the carbon-centered O2-adsorption sites by releasing structural strain during the reaction. In particular, low-coordinated pyridinic-N displaces from the carbon plane to release the strain, whereas weak C-C bonds around the carbon vacancy change the bond lengths to release the strain. Defects indirectly promote the adsorption of oxygen by enhancing structural flexibility. Thus, the nonlocal structural environment is as critical as the direct interaction between adsorption sites and adsorbate in the chemical reaction. Molecular dynamics simulations reveal that pyridinic-N doping is a facile route to introduce stable catalytic active sites. Overall, our results provide a deeper understanding of chemical processes on defective carbon materials.