Orr Mechanism Research Articles

Regenerative Orr mechanism yielding large non-modal perturbation energy growth in a viscosity stratified plane shear flow

Transiently growing non-modal perturbations can play a crucial role in the transition of plane shear flows in modally stable regimes. In terms of the extent of transient amplification, three-dimensional perturbations are typically more prominent due to the lift-up effect. In contrast, two-dimensional (2D) spanwise-independent perturbations are often considered less important as they typically undergo modest levels of transient growth and are short-lived. The Orr mechanism is key to the amplification of energy for 2D perturbations. In this work, we discuss 2D non-modal perturbations of three-layer viscosity stratified flows with the mean shear rates of the outer layers being equal. Strikingly, a novel regenerative Orr mechanism is found that allows for significant amount of energy amplification despite the 2D nature of the perturbations. Moreover, these perturbations survive for considerably long times. The perturbation structure shows symmetry about the middle layer and evolves such that the Orr mechanism can repeatedly occur in a regenerative manner resulting in the perturbation energy evolving in a markedly non-monotonic fashion. When these same perturbations are introduced in a uniform plane shear flow, the corresponding non-modal transient amplifications are shown to be much smaller.

Potential of V-Si72, V-C72, and V-Al36N36 as catalysts for oxygen reduction reaction.

The capacities of V-Si72, V-C72, and V-Al36N36 nanocages to catalyze the ORR processes have been investigated. The acceptable pathways of ORR processes on V-Si72, V-C72, and V-Al36N36 nanocages have been examined by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water. The ΔGreaction values of reaction steps of ORR pathways on V-Si72, V-C72, and V-Al36N36 nanocages are calculated. The Eadoption and Eformation of V-Si72, V-C72, and V-Al36N36 nanocages are negative values and these nanostructures are stable materials. The H2O has the lowest Eadsorption on V-Si72, V-C72, and V-Al36N36 nanocages. The *OH formation, creation of *OH-OH*, and formation of O* are rate-determining steps of ORR mechanisms. The overpotential values of ORR processes on V-Si72, V-C72, and V-Al36N36 nanocages are 0.41, 0.37, and 0.30V, respectively. The V-Al36N36 nanocage have lower overpotential for ORR processes than V-Si72 and V-C72 nanocages by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water. The V-Al36N36 nanocage have more negative ∆Greaction for reaction steps of ORR than V-Si72 and V-C72 nanocages. The V-Al36N36 nanocage with lower overpotential is proposed as an effective catalyst for ORR processes via studied pathways. The DSD-PBEPBE-D3/aug-cc-pVDZ method has been used to optimize and calculate the frequencies of V-Si72, V-C72, and V-Al36N36 nanocages in GAMESS software. The complexes of O, OH, OOH, and H2O with V-Si72, V-C72, and V-Al36N36 nanocages are optimized and frequencies are determined by the DSD-PBEPBE-D3/aug-cc-pVDZ method. The Gactivation and ∆Greaction of ORR pathways on V-Si72, V-C72, and V-Al36N36 nanocages are calculated by DSD-PBEPBE-D3/aug-cc-pVDZ, PW91PW91/aug-cc-pVDZ, and COSMO model in the gas phase and water.

On the generation of near-wall dilatational motions in hypersonic turbulent boundary layers

Dilatational motions in the shape of travelling wave packets have been identified recently to be dynamically significant in hypersonic turbulent boundary layers. The present study investigates the mechanisms of their generation and their association with the solenoidal motions, especially the well-recognized near-wall self-sustaining process of the regeneration cycle between the velocity streaks and quasi-streamwise vortices. By exploiting the direct numerical simulation databases and orchestrating numerical experiments, we explore systematically the near-wall flow dynamics in the processes of the formation and transient growth of low-speed streaks. We conclude via theoretical ansatz that the nonlinearity related to the parallel density and pressure gradients close to the wall due to the restriction of the isothermal boundary condition is the primary cause of the generation of the dilatational structures at small scales. In fully developed turbulence, the formation and the existence of healthy dilatational travelling wave packets require the participation of the turbulence at scales larger than those of the near-wall regeneration cycles, especially the occurrence of the bursting events that generate vortex clusters. This is proven by the less intensified dilatational motions in the numerical experiments in which the Orr mechanism is alleviated and the vortical structures and turbulent bursts are weakened.

The evolution of second harmonic waves in oblique transition

The evolution of second harmonic waves in oblique transition

Investigation of ORR and OER Mechanisms by Co- and Fe-doped Silicon Nanocages (Si48 and Si60) and Co- and Fe-doped Silicon Nanotubes (SiNT(5, 0) and SiNT(6, 0)) as Acceptable Catalysts

Investigation of ORR and OER Mechanisms by Co- and Fe-doped Silicon Nanocages (Si48 and Si60) and Co- and Fe-doped Silicon Nanotubes (SiNT(5, 0) and SiNT(6, 0)) as Acceptable Catalysts

Identifying the two-electron oxygen reduction mechanism on BC3 site in Cl−-containing electrolytes

The electrochemical synthesis of H2O2 has great potential for applications in the treatment of environmental pollution and energy, but the toxic effect of Cl− on the active sites of catalysts has severely limited the application of this technology in these fields. Developing efficient non-metal-doped catalysts represents an effective strategy to address this issue. Although B-doped carbon materials have demonstrated excellent ORR activity, the understanding of the ORR mechanism on B surface sites remains highly controversial, particularly in seawater electrolytes. The influence of Cl− adsorption behavior on the catalytic reaction center mechanism is still unknown. In this paper, catalysts containing the BC3 structure were successfully prepared with an H2O2 yield of 3.55 mol/ (gcat h) in simulated seawater. The DFT results demonstrated that BC3 functions as a 2e− ORR active site, and the presence of Cl− does not poison it. Instead, it promotes the hydrogenation process of *OOH on its surface, further enhancing its ORR performance. The results provide clarity on how the Cl− of the catalytic environment affects the ORR performance and pathway selectivity of non-metallic B-sites. Additionally, the results offer valuable insights for designing electrocatalysts in complex environments.

Alluaudites: From Sulfates to Tungstates

Alluaudites are essentially naturally occurring mixed metal (Mn and Fe) phosphate-based minerals [1] consisting of an open framework structure to support fast alkali migration. The general formula for alluaudite can be expressed by A(1)A(2)M(1)M(2)2(XO4)3 where A and M sites are alkali ion and transition metals respectively and X can be S, P, As, Mo, W, V and so on [2]. The presence of polyanionic moieties (XO4) assures structural stability and high-voltage operation arising from the inductive effect [3]. Till date, a variety of phosphate and sulfate-based alluaudites have been reported as cathodes for Li-ion and Na-ion batteries [4,5]. In 2014, Yamada group reported a Fe-based sulfate alluaudite, Na2Fe2(SO4)3, acting as a 3.8 V SIB cathode material with excellent rate capability and energy density. It ushered the discovery of various high-voltage sulfate alluaudites materials by optimizing different synthesis routes. While they offer high-voltage performance, the presence of SO4 species makes them susceptible to thermal decomposition and moisture-sensitive. We have observed a clear phase transition from alluaudite to hydrated bloedite with exposure to the humid atmosphere [6]. As compared to sulfates, phosphate alluaudites are moisture stable but suffer from lower voltage operation due to the lower inductive effect of P. On the other hand, they had shown excellent electrocatalytic activity for both ORR and OER mechanism [7]. Recently, Gao et al. reported the first molybdate-based alluaudite Na2.67Mn1.67(MoO4)3 working as a 3.45 V Mn-based insertion cathode for SIBs [8]. Inspired by this work, we have extended the molybdate alluaudite chemistry towards its Co, Ni and Cu- analogue, Na3.36Co1.32(MoO4)3, Na2.4Ni0.8(MoO4)2 and Na4Cu(MoO4)3. The Co- molybdate was found to act as 4.0 V cathode material against Na/Na+. All three molybdates have been found to show excellent activity while cycling within the lower voltage region by exploiting its Mo- redox which has been probed by different ex-situ post-mortem analysis. Moreover, both Co- and Ni- analogues had shown excellent activity in ORR and OER reaction as an add-on to its battery application. We have detected the mechanism by using in-situ Raman spectroscopy, TEM and XPS analysis. Finally, for the first time, we have synthesized Na4Mn(WO4)3, a tungstate alluaudite whose structure and electrochemistry are not reported anywhere to the best of our knowledge. Using inductive effect principle, various alluaudite systems will be discussed.

Nonlinear optimal perturbation of turbulent channel flow as a precursor of extreme events

This work aims at studying the mechanisms behind the occurrence of extreme dissipation events in a channel flow, identifying nonlinear optimal perturbations as potential precursors of these events. Nonlinear optimal perturbations with respect to a generic turbulent instantaneous snapshot are computed for the first time using a direct-adjoint algorithm in the channel flow at $Re_{\tau }\approx 180$ . The resulting initial perturbation displays the upstream tilting characteristic of Orr's mechanism and is positioned along the interfaces between two opposite-sign velocity streaks of the pre-existing turbulent field. Such a perturbation induces a sudden breakdown of the pre-existing structures and a heavier tail in the dissipation probability density function distribution. Different mechanisms are at play during this process: the high shear present at the interface between coherent low- and high-momentum regions is exploited to break down the larger structures and drive energy to small scales. This energy cascade is fed by an enhanced lift-up effect that produces intense streaks near the wall. It is found that the optimal perturbation grows exponentially during the first phase of its evolution reflecting the existence of a secondary modal instability of the streaks. To corroborate the results, the conditional spatiotemporal proper orthogonal decomposition (POD) analysis of Hack & Schimdt (J. Fluid Mech., vol. 907, 2021, A9) is performed both in the perturbed and in the unperturbed flow, showing a clear agreement between the two cases and with the reference study. Thus, the optimal perturbation at initial time can be considered as a precursor of extreme events.

Bifunctional Oxygen Electrocatalysis on Manganese Oxide

Limited supply of precious metals impedes the scale up of electrolyzer production. To enable 100s of GWs of PEM electrolyzer capacity by 2030, iridium supply must grow by more than 10x.[1] Alternatively, catalysts made of cheaper, more abundant manganese could relieve these constraints, if engineered to have comparable activity and stability. The unique bifunctional OER/ORR activity of Mn oxide could also enable electrolyzers that both produce and consume fuel with a single catalyst. Yet rational design of Mn oxide catalysts requires a better understanding of the structural and chemical motifs that lead to high bifunctional performance.In this talk, we will discuss our investigation of the bifunctional OER & ORR activity of Mn oxide and its unusual pH dependence. Our model system, 𝛼-KxMnO2, is among the best reported Mn oxide catalysts for both the OER and the ORR, with ORR activity matching that of Pt.[2] While the OER activity of Mn oxide has been shown to increase at higher pH,[3] we will show that its ORR activity improves as well. We will discuss our attempts to understand this phenomenon with operando scanning transmission x-ray microscopy (STXM), which enables spectroscopic investigation of single catalyst particles during reaction with spatial resolution down to 25 nm. Complementary surface DFT calculations provide further evidence for identification of the active crystal facets and insights into the pH-dependency of the OER & ORR mechanisms. We will discuss the implications of this work on the design of bidirectional electrolyzers with low-cost, non-precious metal catalysts.[1] IRENA. Green Hydrogen Cost Reduction. 2020 [2] Meng, Y. et al. J. Am. Chem. Soc. 2014, 136 (32), 11452–11464.[3] Takashima, T. et al. J. Am. Chem. Soc. 2012, 134 (3), 1519–1527.

Mechanisms of the Higher Catalytic Activity of Polymer Forms over Complexes for Non-pyrolytic Mono-1,10-phenanthroline-Coordinated Cu2+ (Cu-N2 Type) in an Oxygen Reduction Reaction.

In this paper, we have thoroughly investigated the ORR mechanism of non-pyrolytic mono-1,10-phenanthroline-coordinated Cu2+ (Cu-N2 type) complexes and polymers by molecular dynamics and quantum mechanics calculation. In contrast to the complex-catalyzed ORR, which follows a direct four-electron pathway along intermediates of Cu(I)-Phen, the polymer-catalyzed ORR follows an indirect four-electron pathway by intermediates of Cu(II)-Phen. By analyzing the structure, spin population, electrostatic potential (ESP), and density of states, we confirmed that the higher ORR catalytic activity of the polymer is due to the conjugation effect of coplanar phenanthroline and Cu(II) in the planar reactants or at the base of the square-pyramidal intermediates. The conjugation effect allows the highest ESP to be located near the active center Cu(II), while the lower ESPs are distributed on the phenanthroline, which is very favorable for the reduction current. This will serve as a theoretical foundation for the development of new highly efficient ORR non-pyrolytic CuN2 polymer catalysts.

Enhanced Electron Confinement of p-Block Indium Site in Extended Macrocyclic Conjugation Boosting Oxygen Reduction to Hydrogen Peroxide.

The electrocatalytic oxygen reduction reaction via a two-electron pathway (2e- ORR) is a promising route for hydrogen peroxide (H2O2) production. However, the strong electron interaction between the metal site and oxygen-containing intermediates usually generates 4-electron ORR, limiting H2O2 selectivity. Here, combining theoretical and experimental studies, we propose to enhance the electron confinement of the indium (In) center in an extended macrocyclic conjugation system toward high-efficiency H2O2 production. The extended macrocyclic conjugation in indium polyphthalocyanine (InPPc) evokes the attenuated transfer electron ability of the In center and weakens the interaction between the s orbital of In and the p obital of OOH*, favoring protonation of OOH* to H2O2. Experimentally, the prepared InPPc catalyst exhibits a noticeable H2O2 selectivity above 90% in 0.1-0.6 V vs RHE, outperforming the counterpart InPc. Importantly, the InPPc displays a high average H2O2 production rate of 23.77 mg/cm2/h in a flow cell. This study proposes a novel strategy to engineer molecular catalysts and provides new insights into the ORR mechanism.

Nonlinear optimal perturbations and formation mechanism of localized wave packet in channel flow

The subcritical transition in a two-dimensional channel flow is characterized by the emergence of localized wave packet (LWP). In this paper, the nonlinear nonmodal optimization method is used, and when the target time is long enough the minimal seed, the minimal-energy perturbation triggering the transition is found to be the localized wave packet tilted upstream with energy scaling as Re−3.8 for Re>2500. The formation of LWP from a minimal seed includes three stages. First, the disturbance kinetic energy of the minimal seed increases to a local maximum through a transient growth governed by the Orr mechanism, and second, large scale vortex structures are generated due to the nonlinear forcing mainly contributed by the wave components and causes a streamwise symmetry breaking, i.e., a long but weak upstream tail and a strong downstream front are formed. As a result, the perturbation structure approaches the unstable lower branch solution and is attracted and captured at the last stage by the stable upper branch solution of LWP in the phase space.

Optimal white-noise stochastic forcing for linear models of turbulent channel flow

In the present study an optimisation problem is formulated to determine the forcing of an eddy-viscosity-based linearised Navier–Stokes model in channel flow at$Re_\tau \approx 5200$($Re_\tau$is the friction Reynolds number), where the forcing is white-in-time and spatially decorrelated. The objective functional is prescribed such that the forcing drives a response to best match a set of velocity spectra from direct numerical simulation (DNS), as well as remaining sufficiently smooth. Strong quantitative agreement is obtained between the velocity spectra from the linear model with optimal forcing and from DNS, but only qualitative agreement between the Reynolds shear stress co-spectra from the model and DNS. The forcing spectra exhibit a level of self-similarity, associated with the primary peak in the velocity spectra, but they also reveal a non-negligible amount of energy spent in phenomenologically mimicking the non-self-similar part of the velocity spectra associated with energy cascade. By exploiting linearity, the effect of the individual forcing components is assessed and the contributions from the Orr mechanism and the lift-up effect are also identified. Finally, the effect of the strength of the eddy viscosity on the optimisation performance is investigated. The inclusion of the eddy viscosity diffusion operator is shown to be essential in modelling of the near-wall features, while still allowing the forcing of the self-similar primary peak. In particular, reducing the strength of the eddy viscosity results in a considerable increase in the near-wall forcing of wall-parallel components.

Attenuation of Tollmien–Schlichting waves using resonating surface-embedded phononic crystals

A novel method for control of convective boundary layer instabilities using metamaterial concepts is investigated. Attenuation of Tollmien–Schlichting (TS) waves with surface-embedded one-dimensional phononic crystals (PCs) is theoretically and numerically modeled, capitalizing on the inherent frequency band stop of PCs. The PC is tuned to the targeted TS wave characteristics through the use of analytical models derived from transfer matrix and interface response theories, verified using a finite elements analysis. The interaction between TS waves and a single PC is investigated using coupled two-dimensional fluid structure interaction simulations in the frequency domain. It is shown that TS waves are either amplified or attenuated depending on whether the PC free-face surface displacement and unsteady perturbation pressure at the wall are in-phase or out-of-phase, respectively. The perturbation pressure acts solely as the driver for the mechanical oscillation of the PC. The emerging hydrodynamic coupling between TS waves and the PC is found to be governed by a combination of the Orr mechanism and wall-normal velocity linear superposition near the wall. Finally, a metasurface comprised of an array of streamwise-distributed PCs is evaluated, resulting in an amplitude growth delay of 11.3% of the TS wavelength along the metasurface extent.

Ir-trimer anchored on the Co-supported Pd nanocrystals Opens the Ultra-efficient Channel on oxygen reduction reaction

A novel type of Pt-free electrocatalysts composed of Co-supported Pd nanocrystals hierarchical structure with Ir-ensemble superficial doping, denoted as Co@Pd-Irn, is investigated using ab-initio method. The results elucidate the clinal distribution of the adsorption energy on the Co@Pd-Ir3 system predicts an Eads-oriented ultra-efficient loop from the Ir-trimer to its edge-area and the extended Pd atoms. Simulated kinetics verifies the abnormal ORR behavior with substantially reduced reaction barriers in the three key ORR subpaths, which wipes out the toughest barrier of the first “O2 dissociation”, outdistancing the benchmarking Pd and Pt. The scenarios of stress–strain distribution and physical charge relocation reveal the charge-regulating role of the superficially-doped Ir-trimer, which generates an unbalanced charge redistribution on the operated surface and the differentiated catalytic selectivity concerning the four crucial absorbates with different charge preference. A multi-level linkage of synergistic effect has been traced out that triggers a unique ORR mechanism of regional cooperative division, meanwhile manage to the cost control via dramatically slashing the expensive Pt up to the non-Pt level. Overall, our results provide novel insights into the design of commercial non-Pt metal nanocatalysts, offering a process window guidance with atomic-level accuracy on regulating the microstructure and composition of multi-metallic nanocatalysts.

Recent Developments of Atomically Dispersed Metal Electrocatalysts for Oxygen Reduction Reaction†

Comprehensive SummaryOxygen reduction reaction (ORR) is the pivotal portion in many electrochemical energy conversion and storage technologies. However, the complex mechanisms and sluggish kinetics of ORR have also become one of the key issues hindering the development and application of these technologies. Recently, single‐atom catalysts (SACs) with well‐defined atomic active centers and theoretical 100% atomic utilization have attracted broad interests in the area of electrocatalytic ORR. The electrocatalytic ORR performance of SACs is fundamentally determined by the intrinsic activity of the single‐atom active site and increasing the number of active sites involved in the ORR can further enhance the ORR activity of SACs. In this review, advances in atomically dispersed metal electrocatalysts for ORR in the last three years are summarized. Three main regulation strategies for achieving SACs with excellent intrinsic activity in the context of the ORR mechanism are involved including modulation of coordination environments, the construction of intrinsic defects, and the introduction of dual active sites. Moreover, discussions on improving the loading and utilization of active sites are given. Finally, the current challenges and opportunities for the development of SACs for ORR are presented.

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.

Rapid Screening of Mechanistic Pathways for Oxygen‐Reduction Catalysts

AbstractOxygen reduction reaction (ORR) corresponds to the limiting process in fuel cells to convert gaseous hydrogen and oxygen from the air into electricity. Most commonly, electronic structure calculations in the density functional theory (DFT) approximation are applied to comprehend the elementary reaction steps on the atomic scale, including the identification of limiting reaction steps and approximation of electrocatalytic activity. A major challenge in the modeling of the ORR refers to the complexity of the mechanistic pathways given that a plethora of different descriptions, ranging from the mononuclear to OOH dissociation mechanisms, dissociative and oxide pathways, have been reported in the literature. In the present work, the adsorption free energy of the *OH intermediate, ΔG1, is introduced as an effective descriptor for rapid screening of ORR mechanisms. By applying a rigorous thermodynamic analysis in conjunction with a descriptor‐based approach, it is demonstrated how the calculation of a single binding energy in terms of ΔG1 can be used to identify the energetically favored mechanistic description for any ORR catalyst by analyzing the location of the material in the volcano plot. The introduced procedure may support future DFT‐based studies in that the preferred pathway is not missed when modeling the four‐electron ORR by a binding‐energy approach.

S-doped M-N-C catalysts for the oxygen reduction reaction: Synthetic strategies, characterization, and mechanism

The systematic summarization of the synthesis-structure-performance-mechanism correlations provides guidance for the rational design of S-doped M-N-C catalysts for ORR. • Recent review on preparation of S-doped M-N-C catalysts for ORR. • Effects of doping S atoms on the ORR performance of M-N-C catalysts. • The advanced characterization methods and computational methods for S-doped M-N-C catalysts are introduced. • The future development direction of M-N-C catalysts is described. Due to the strong electronegativity of the N atom, the adsorption energy of the transition metal –nitrogen active site for the ORR intermediate is too high to facilitate the subsequent elementary reactions. It is worth noting that introducing sulfur atoms in transition metal–nitrogen–carbon catalysts can optimize the charge distribution of nitrogen atoms around metal sites to improve the electronic structure and structural defects of the catalysis, so as to boost the adsorption and transfer of oxygen-containing intermediates. Meanwhile, sulfur-doping can also inhibit metal nanoparticle formation and promote hierarchical porous carbon formation. This review will afford the S-doped M-N-C catalysts rational design principle and strategies and introduce the corresponding ORR electrochemical properties. We will combine advanced characterization techniques and DFT calculation to discuss the coordination environment of active sites and the ORR mechanism of S-doped M-N-C catalysts. Finally, a general perspective on the challenges and opportunities for S-doped M-N-C catalysts will be proposed to facilitate further development.

Weak nonlinearity for strong non-normality

We propose a theoretical approach to derive amplitude equations governing the weakly nonlinear evolution of non-normal dynamical systems, when they experience transient growth or respond to harmonic forcing. This approach reconciles the non-modal nature of these growth mechanisms and the need for a centre manifold to project the leading-order dynamics. Under the hypothesis of strong non-normality, we take advantage of the fact that small operator perturbations suffice to make the inverse resolvent and the inverse propagator singular, which we encompass in a multiple-scale asymptotic expansion. The methodology is outlined for a generic nonlinear dynamical system, and four application cases highlight common non-normal mechanisms in hydrodynamics: the streamwise convective non-normal amplification in the flow past a backward-facing step, and the Orr and lift-up mechanisms in the plane Poiseuille flow.