Oxygen Reduction Reaction Mechanism Research Articles

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve.

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5eV. Above 0.7V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Assessing the potential of functionalized cobalt-doped graphyne-like boron nitride as an efficient and accessible catalyst for oxygen reduction reaction in fuel cells

Due to its exorbitant expense and scarcity on Earth, there is a need to anticipate a cost-effective and widely accessible catalyst that can effectively replace platinum, which is known for its efficiency in fuel cells. In this context, catalysts such as OH, H, and COOH group-functionalized cobalt-doped Graphyne-like boron nitride (referred to as BNyen) are being examined to enhance the mechanism of oxygen reduction reaction (ORR) via direct 4e− reduction. According to findings derived from DFT, it has been observed that reactions predominantly take place above Co metal center rather than edge site of BNyen. Additionally, the thermochemical characteristics point to the fact that these reactions release a significant amount of heat, suggesting their feasibility. By examining reversible potential of products and intermediates on phthalocyanines as well as adsorption energy (Eads), it has been determined that Co-BNyen(OH) serves as a superior catalyst for ORR process. In conclusion, this research affirms that BNyen materials hold significant promise as catalysts for ORR, a crucial process in fuel cell applications.

Theoretical investigation of electrocatalytic activity of Pt-free dual atom-doped graphene for O2 reduction in an alkaline solution

Non-precious electrocatalysts as the alternative to Pt have become a hot research area in the last decade due to the suitable catalytic activity in Oxygen reduction reaction (ORR) in electrochemical systems. In this work, the density functional theory calculations were investigated to explore the activity of Fe, Cu, and Fe-Cu atoms supported by N-doped graphene as the ORR electrocatalyst for Oxygen-depolarized cathodes (ODCs). To this end, the ORR mechanism was surveyed in detail in the gas and solvent phases. The results show that the solvent phase leads to a higher overpotential and thermodynamic limiting potential. According to the density of states curves, there are strong interactions between metal atom and substrate that can effectively tune the electronics of catalysts. Bader's analysis confirms that, in addition to the single metal atoms, nitrogen atoms have also played a critical role in charge transfer between substrates and oxygen molecules in ORR. It is also predicted that Fe-Cu@NC SAC exhibits the highest catalytic activity which is consistent with thermodynamic limiting potential and theoretical overpotential of − 0.26 and 0.66 (V vs. SHE), respectively, indicating that this type of catalyst may be a suitable candidate instead of precious metals in oxygen-depolarized cathodes in electrochemical devices.

Iron-based graphene composite for oxygen reduction reaction in alkaline media: Electrocatalytic activity and lifetime evaluation

The development of electrocatalysts with high catalytic performance and low costs for oxygen reduction reaction (ORR) is still challenging. Herein, an overall solution for ORR in alkaline media, from the catalyst synthesis to catalyst regeneration and to the development of a rapid, reliable and easy approach for electrode surface evaluation, is presented. We focused on the development and characterization of an efficient material for ORR in alkaline media, α-Fe2O3 N-doped graphene (Fe-N-Gr). The associative pathway for the four electron transfer ORR mechanism is sustained by the Raman spectra recorded from the electrode surface. We highlighted the possibility of catalyst regeneration by a simple electrochemical method. After two regeneration rounds and 1500 cycles in O2-saturated 1 M NaOH, the catalyst still retains 40.8 % catalytic activity. Finally, as a part of the overall solution, we demonstrated that a methodology based on Raman spectroscopic measurements and machine learning algorithms can be applied for graphene-based catalysts lifetime investigation.

In situ exploration of oxygen electrocatalysis using core-shell nanostructure-enhanced Raman spectroscopy

Advancements in fuel cells and water electrolyzers have significantly bolstered the utilization of hydrogen energy. Notably, the oxidation and reduction processes of oxygen at the electrode—termed oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)—manifest sluggish reaction kinetics, thus requiring noble metals as catalysts, which considerably impedes system efficiency and cost. The imperative for enhancing reaction rates and diminishing overpotential necessitates the development of effective catalysts, which strongly depends on the mechanistic understanding of these reactions at the molecular level. Therefore, this review summarizes our recent efforts in utilizing in situ enhanced Raman spectroscopy, especially the borrowing surface-enhanced Raman spectroscopy (SERS) strategy, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and the SHINERS-satellite strategy, to capture oxygen intermediate species as a bridge to investigate the molecular mechanisms of OER and ORR. Combining in situ SERS with other characterization techniques and theoretical simulation, the structural evolution of active sites and intermediates, including ∗OOH, ∗OH, ∗OO, etc., during OER/ORR has been monitored under reaction conditions, and the reaction mechanisms together with structure-activity correlations have been identified at the molecular level. These findings may provide a pivotal scientific foundation towards the discovery of better materials for electrochemical hydrogen energy.

Investigation into electrochemical catalytic properties and electronic structure of Mn doped SrCoO3 perovskite catalysts

High activity, long lifetime and low-cost cathode catalyst plays an irreplaceable role in oxygen reduction reactions process. In the present work, SrCoO3 and SrCo0.875Mn0.125O3 perovskite are prepared by sol–gel method and first principles calculation to study the role of Mn doping. The characterizations of phase, microstructure and valence state of elements demonstrate that Mn doping stabilizes the cubic structure of SrCo0.875Mn0.125O3 perovskite and forms a porous structure with large specific surface area. The high-valence and multivalent states of Co and Mn are produced, which provide abundant active sites for oxygen reduction reaction. The charge transfer resistance (1.820 Ω·cm2), Tafel slope (88 mV dec−1), discharge capacity (4.173 mWh/cm2), and positive shift of onset potential and half-wave potentials reveal that the SrCo0.875Mn0.125O3 perovskite is a promising cathode catalyst. Mn doping helps the O p-band center move up relative to the Fermi level, and adsorption energy of O2 and formation energy of oxygen vacancies increase. The improvement of these parameters has a positive effect on predicting the catalytic activity of SrCo0.875Mn0.125O3 perovskite. The possible oxygen reduction reaction mechanism of SrCo0.875Mn0.125O3 perovskite is discussed. Mn doping accelerates the transport process of SrCo0.875Mn0.125O3 perovskite.

Bifunctional mechanism of oxygen evolution reaction and oxygen reduction reaction induced by surface reconstruction in Perovskite electrocatalysts

Elucidation of the relationship among crystal structure, material compositions, and electrocatalytic performance is beneficial to the design of electrocatalysts with high activity. Specially, for bifunctional electrocatalysts, there is a lack of the unified understanding of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) mechanisms. Based on the first-principles calculations, LaCoO3 surface reconstructions with different OH coverage under OER/ORR condition are determined. Thereby, the different reaction pathways for OER and ORR with the cooperation of Co and O sites are revealed, under which Co and lattice O sites are activated dynamically, and an overpotential of 0.35 V for the OER and 0.44 V for the ORR are obtained. Furthermore, the octahedra lattice distortion-related bifunctional mechanism (DRBM) is proposed from electron structure analysis, and the bifunctional activity of different B-site doped LaCoO3 (LaCoMO3) are studied. Based on this mechanism, we find that OER, ORR, and their bifunctional activity of LaCoMO3 all exhibit a volcano trend as a function of the O-B-O bond angle formed by Co and lattice oxygen. Consequently, we predict that Mn, Cr, Fe, and Ti doped LaCoO3 electrocatalysts exhibit remarkable bifunctional activity, which is corroborated by the reported experimental results.

Effects of electrolyte pH on oxygen reduction properties using a cobalt porphyrin-coated carbon composite

Understanding the pH-dependent mechanism of metalloporphyrin-based composite electrocatalystic oxygen reduction reactions (ORRs) can provide the basis for rational porphyrin molecular design. Tunable ORR properties of a cobalt porphyrin-based composite catalyst aFP-TCoP/C is accomplished by adjusting the electrolyte pH values spanning from 0.7 to 13.7. The composite performs the best ORR reactivity in the pH13.7 electrolyte, exhibiting the most positive characterized potentials (the ORR reduction potential EORR of 0.76 V and the half-wave potential E1/2 of 0.80 V) and the smallest Tafel slope of 43.1 mV dec−1. Whilst the sample with the pH3.7 electrolyte displays quite negative reduction potentials and a large Tafel slope, it exhibits an electron transfer number of 3.84, which indicates a nearly 4-electron transfer selectivity. The findings provide a practical protocol for regulating the catalytic reactivity and selectivity of ORRs by adjusting the components of electrolytes.

Oxygen Electroreduction Reaction on Iron, Nitrogen-doped Nanocarbons: Structure – Reactivity Relationship

Theoretical calculations at the revPBE0-D3(PCM)/def2-TZVP level were performed to investigate the mechanism of the oxygen reduction reaction (ORR) on the FeN4-doped NCMs (graphene, single-walled (6,6)-armchair and (12,0)-zigzag carbon nanotubes) as catalysts. The effect of the support and relative orientation of FeN4 fragment in the catalyst structure on the ORR activity and stability of the differently adsorbed successive intermediates (O2*, HO2*, O*, HO*, H2O2*, H2O*) is discussed. Special attention is given to the possible poisoning effect of CO. The relative catalytic activity of the metal center and the adjacent C2 site of the carbon support are analyzed depending on the structure of the support. The metal catalytic center on the armchair nanotube and graphene supports is prone to deactivation by poisoning with CO, whereas in the zigzag nanotube supported catalyst the metal center is tolerant to CO. The four-electron mechanism of ORR is shown to be preferable over the two-electron mechanism in both acidic and alkaline media, both on the metal and C2 centers.

Regulation Strategies for Fe-N-C and Co-N-C Catalysts for the Oxygen Reduction Reaction.

Proton exchange membrane fuel cells (PEMFCs) and alkaline membrane fuel cells (AEMFCs) have received great attention as energy devices of the next generation. Accelerating oxygen reduction reaction (ORR) kinetics is the key to improve PEMFC and AEMFC performance. Platinum-based catalysts are the most widely used catalysts for the ORR, but their high price and low abundance limit the commercialization of fuel cells. Non-noble metal-nitrogen-carbon (M-N-C) is considered to be the most likely material class to replace Pt-based catalysts, among which Fe-N-C and Co-N-C have been widely studied due to their excellent intrinsic ORR performance and have made great progress in the past decades. With the improvement of synthesis technology and a deeper understanding of the ORR mechanism, some reported Fe-N-C and Co-N-C catalysts have shown excellent ORR activity close to that of commercial Pt/C catalysts. Inspired by the progress, regulation strategies for Fe-N-C and Co-N-C catalysts are summarized in this Review from 5 perspectives: (1) coordinated atoms, (2) environmental heteroatoms and defects, (3) dual-metal active sites, (4) metal-based particle promoters, and (5) curved carbon layers. We also make suggestions on some challenges facing Fe-N-C and Co-N-C research.

Exploring the impact of S-doped Fe-N-C materials on the ORR mechanism within a constant potential solvation model

The widespread application of S doping in Fe-N-C materials aims to enhance the catalytic activity for oxygen reduction reactions (ORR). However, the current doping strategies are primarily centered on the first coordination layer encompassing Fe atoms. Consequently, this study employs first-principles calculations to compute the formation and binding energies of FeN4 doped with S atom. It further delves into the influence of diverse doping sites on catalyst stability and meticulously analyzes the configurations and corresponding ORR activities of four catalysts, namely FeN4S-N (Number=0∼3), utilizing a constant-potential implicit solvent model to simulate authentic electrocatalytic environments. The findings reveal that under vacuum conditions, FeN4S1 exhibits the utmost ORR activity, boasting a potential of 0.53 V. Conversely, within the constant-potential implicit solvation model, FeN4S2 emerges as the catalyst with the highest ORR activity, achieving a potential of 0.39 V. Therefore, calculations leveraging the constant-potential solvation model offer more realistic insights into catalyst design, thereby guiding the development of superior catalysts.

Pivotal role of Ce3+ polarons on promoting oxygen reduction reaction activity of Pt1/CeO2 catalysts

The Pt1/CeO2 single-atom catalyst excels across catalytic fields, promising applications in the oxygen reduction reactions (ORRs). Here, we employ first-principles calculations to systematically investigate the ORR mechanism catalyzed by Pt1/CeO2 systems and the activity regulated by Ce3+ polarons. We determine the reaction pathways of different Pt1/CeO2 systems by evaluating four critical factors: O2 adsorption energy, O2 dissociation Gibbs barrier, Gibbs free energy change for O2→OOH*, and H2O2 adsorption structure. Notably, most Pt1/CeO2 systems tend to react through the 4e− associative path. Theoretical activity calculations reveal that PtOH/CeO2(110) system show low overpotential of 0.47 V comparable to pure Pt. The excellent ORR activity is attributed to the synergistic mechanism of Pt and surrounding Ce3+ polarons, where the polaron regulation mechanism dominates the reaction. The quantity of Ce3+ polarons not only affects the reaction active sites but also regulates the radical adsorption state. With an elevated Ce3+ polarons concentration, the ORR overpotential in Pt1/CeO2 system can decrease from 0.47 to 0.16 V, leading to a substantial enhancement in activity. This work first underscores the crucial involvement of Ce3+ polarons in catalyzing the ORR, reducing the significance of Pt in catalysis, inspiring a new way to design active non-platinum electrocatalysts for ORR in experiments.

First-Principles Insights into Tungsten Semicarbide-Based Single-Atom Catalysts: Single-Atom Migration and Mechanisms in Oxygen Reduction.

Understanding the structural evolution of single-atom catalysts (SACs) in catalytic reactions is crucial for unraveling their catalytic mechanisms. In this study, we utilize density functional theory calculations to delve into the active phase evolution and the oxygen reduction reaction (ORR) mechanism of tungsten semicarbide-based transition metal SACs (TM1/W2C). The stable crystal phases and optimal surface exposures of W2C are identified by using ab initio atomistic thermodynamics simulations. Focusing on the W-terminated (001) surface, we screen 13 stable TM1/W2C variants, ultimately selecting Pt1/W2C(001) as our primary model. The surface Pourbaix diagram, mapped for this model under ORR conditions, reveals dynamic Pt1 migration on the surface, triggered by surface oxidation. This discovery suggests a novel single-atom evolution pathway. Remarkably, this single-atom migration behavior is also discerned in seven other group VIII SACs, enhancing both their catalytic activity and their stability. Our findings offer insights into the evolution of active phases in SACs, considering substrate structural arrangement, single-atom incorporation, and self-optimization of catalysts under various conditions.

Density functional theory study on the mechanism of oxygen reduction reaction on nitrogen-doped graphene with adjacent Mn and Ni sites

The catalysts for oxygen reduction reaction (ORR) are of great significance to the development of electrochemical energy systems. In this paper, the ORR catalytic performance of Mn–Ni dual-metal doped N-coordinated graphene is studied by density functional theory (DFT). The results show that the catalysts with dual-metal active sites are more stable than that with single metal site, and all catalysts show certain catalytic activity. The reaction pathway analyses of catalysts with two different adsorption configurations of O2 show that when O2 is adsorbed on two metal sites, OOH will automatically split into OH and O. When O2 is adsorbed on Mn site alone, the other site does not participate in the adsorption of oxygen-containing intermediates. The thermodynamic calculation results show that the catalytic performance is mainly influenced by the adsorption strength of O, which is controlled by the number of active sites involved in the adsorption, bond length and the number of transferred electrons at the active sites. The G-MnNiN6-3 shows the best catalytic performance with an overpotential of 0.453 V and a reaction energy barrier of 0.19 eV. The results of this study highlight the potential application of Mn–Ni dual-metal doped N-coordinated graphene as a highly efficient non-precious catalyst.

Porphyrin-based frameworks and derivatives for the oxygen reduction reaction

The global energy crisis and environmental pollution issues caused by the continuous consumption of fossil fuels are increasingly becoming severe. Developing clean and sustainable hydrogen (H2) energy and corresponding storage and conversion devices have attracted increasing attention. The utilization of H2 energy highly depends on the fuel cell (FC). However, the slow kinetics of oxygen reduction reaction (ORR) on the cathode of FC cannot be improved without efficient catalysts. Currently, precious metal platinum (Pt) and its alloys are commonly used catalysts for ORR process. However, the scarcity and high cost of Pt hinder their large-scale practical application. To replace typical Pt-based catalysts, non-precious metal electrocatalysts have been investigated for ORR. Inspired by the catalytic ORR active center Fe porphyrin of cytochrome c oxidase in nature, metalloporphyrin-based frameworks and their derivatives have become promising electrocatalysts due to their abundant conjugated electronic structure, tunable functional groups, and large specific surface area. First, this review briefly introduces porphyrins and catalytic ORR mechanisms. Second, recent progress on porphyrin-based ORR catalysts including porphyrin-based frameworks, framework@substrate composites, and framework-based derivatives was summarized. Porphyrin-based frameworks mainly include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polymers constructed with porphyrin building blocks. Porphyrin-based framework@substrate composites were usually assembled and constructed to improve the electron transfer rate of frameworks. To further improve the conductivity, framework-based derivatives, usually named metal, nitrogen-doped carbon (M-N-C) materials, were synthesized through pyrolyzing frameworks at high temperatures. Finally, research challenges and directions of porphyrin-based electrocatalysts for ORR were discussed. This review is meaningful and enlightening for designing and developing other porphyrin-based electrocatalysts for ORR.

Atomically Dispersed Iron Active Sites on Covalent Organic Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

AbstractArtificial photosynthesis has been regarded as a promising solution toward solar energy conversion, generating storable and transportable chemical fuels such as hydrogen (H2) and hydrogen peroxide (H2O2). However, the design of robust catalytic sites not only affects the activity, but also identify the atomic‐level correlation between active sites and natural photosynthesis performance. Herein, a synthesis method of single‐atomic Iron (Fe) active sites anchored on novel covalent organic framework (COF) for the production of H2O2 under visible light irradiation. When benzyl alcohol is the most sacrificial agent, the state‐of‐the‐art Fe‐based COF exhibits an excellent H2O2 generation rate of 4130 µmol g−1 h−1, over 5.3 times higher than that of pristine COF, achieving an apparent quantum yield of 6.4% at 420 nm. The enhanced photocatalytic performance is ascribed to the synergistic effect of atomically dispersed Fe sites and COF hosts, reducing the reaction energy barrier for the formation of *OOH intermediates and optimizing the adsorption of O2 and thus promoting two‐electron oxygen reduction reaction (ORR). This work establishes an atomic‐level engineering approach to build atomically dispersed Fe active sites on COF photocatalysts and provides in‐depth insight upon the ORR mechanism for promising artificial photosynthesis of H2O2.

Influence of the Pt/ionomer/water interface on the oxygen reduction reaction: insights into the micro-three-phase interface.

Understanding the Pt/ionomer/water interface structure and its impact on the oxygen reduction reaction (ORR) activity is essential for enhancing catalyst utilization and performance of fuel cells. This study aimed to investigate the influence of sulfonic acid groups on the Pt/ionomer/water interface and the ORR mechanism. By using a combination of DFT, AIMD, and microkinetic simulations, the results showed that when the sulfonic acid group is located at the edge of the Helmholtz plane, it creates an optimal three-phase interface, providing more available active sites, a stronger interfacial electric field, and a more continuous H-bond network. This configuration results in the *OOH dissociation becoming the rate-determining step, demonstrating significantly higher intrinsic ORR activity with a much lower theoretical overpotential of 0.11 V. Conversely, when the sulfonic acid group is in contact with the Pt surface, it causes the Pt surface's d-band center to shift down, weakens the interfacial electric field, and disrupts the H-bond network, resulting in a blocking effect on the ORR with an overpotential of 0.23 V. These insights shed light on the role of solid-solid-liquid interfaces in the ORR performance and provide valuable information for the rational design of catalyst interfaces.

Boosting the oxygen reduction reaction activity of dual-atom catalysts on N-doped graphene by regulating the N coordination environment.

Development of low-cost and high-efficiency oxygen reduction reaction (ORR) catalysts is of significance for fuel cells and metal-air batteries. Here, by regulating the N environment, a series of dual-atom embedded N5-coordinated graphene catalysts, namely M1M2N5 (M1, M2 = Fe, Co, and Ni), were constructed and systematically investigated by DFT calculations. The results reveal that all M1M2N5 configurations are structurally and thermodynamically stable. The strong adsorption of *OH hinders the proceeding of ORR on the surface of M1M2N5, but M1M2N5(OH2) complexes are formed to improve their catalytic activity. In particular, FeNiN5(OH2) and CoNiN5(OH2) with the overpotentials of 0.33 and 0.41 V, respectively, possess superior ORR catalytic activity. This superiority should be attributed to the reduced occupation of d-orbitals of Fe and Co atoms in the Fermi level and the apparent shift of dyz and dz2 orbitals of Ni atoms towards the Fermi level after adsorbing *OH, thus regulating the active sites and exhibiting appropriate adsorption strength for reaction intermediates. This work provides significant insight into the ORR mechanism and theoretical guidance for the discovery and design of low-cost and high-efficiency graphene-based dual-atom ORR catalysts.

Concluding remarks: a summary of the Faraday Discussion on rechargeable non-aqueous metal-oxygen batteries.

The Faraday Discussion on rechargeable non-aqueous metal-oxygen batteries is summarised. The remarks paper highlights the specific science contributions made in the vital areas of oxygen reduction and evolution reaction mechanisms in non-aqueous electrolytes; material developments for stable metal-oxygen battery cathodes; achieving stable metal anodes and protected interfaces; and, finally, contributions concerning the progress towards practical metal-oxygen batteries. Key conclusions associated with these papers will be highlighted in an order such that readers can identify papers that they would like to explore in more detail.

Investigating the Role of Fe‐Pyrrolic N4 Configuration in the Oxygen Reduction Reaction via Covalently Bound Porphyrin Functionalized Carbon Nanotubes

AbstractAtomically dispersed iron–nitrogen–carbon catalysts are promised, low‐cost, and high‐performance electrocatalysts for the Oxygen Reduction Reaction (ORR) in fuel cells. However, most Fe–N–C materials are produced via pyrolysis at a high temperature and it is difficult to characterise the precise Fe–N configurations. This can lead to confusion surrounding the best chemical and coordination environment for Fe and understanding the subsequent ORR mechanisms. In this work, Fe porphyrin is used to produce a specific Fe–N environment, therefore allowing the role and activity of this environment to be studied. Carbon nanotubes (CNTs) are covalently functionalized with iron 5,10,15,20‐triphenylporphyrin (FeTPP) motifs via aryl diazonium methodology, enabling the exact role of only the Fe‐Pyrrolic N4 configuration of FeTPP in ORR to be studied and better understood. Upon covalent functionalization, a high electrochemical active site density of 1.12 × 1015 sites cm−2, approximately six‐fold more than that of noncovalently functionalized samples with 12.7% electrochemical active site. The heightened active site density and superior electrochemical active site utilization (12.7%) lead to the more favorable 4‐electron pathway for the ORR. Furthermore, a preliminary discussion regarding the selectivity of the ORR pathway is initiated.