Four-electron Reduction Of Oxygen Research Articles

In situ modulating coordination fields of single-atom cobalt catalyst for enhanced oxygen reduction reaction

Single-atom catalysts, especially those with metal−N4 moieties, hold great promise for facilitating the oxygen reduction reaction. However, the symmetrical distribution of electrons within the metal−N4 moiety results in unsatisfactory adsorption strength of intermediates, thereby limiting their performance improvements. Herein, we present atomically coordination-regulated Co single-atom catalysts that comprise a symmetry-broken Cl−Co−N4 moiety, which serves to break the symmetrical electron distribution. In situ characterizations reveal the dynamic evolution of the symmetry-broken Cl−Co−N4 moiety into a coordination-reduced Cl−Co−N2 structure, effectively optimizing the 3d electron filling of Co sites toward a reduced d-band electron occupancy (d5.8 → d5.28) under reaction conditions for a fast four-electron oxygen reduction reaction process. As a result, the coordination-regulated Co single-atom catalysts deliver a large half-potential of 0.93 V and a mass activity of 5480 A gmetal−1. Importantly, a Zn-air battery using the coordination-regulated Co single-atom catalysts as the cathode also exhibits a large power density and excellent stability.

Bimetal-bridging Nitrogen Coordination in Carbon-based Electrocatalysts for pH-universal Oxygen Reduction.

Electrocatalysts with atomically dispersed metal sites (e.g., metal-nitrogen-carbon) have been deemed as promising alternatives for noble-metal catalysts in couples of electrocatalytic reactions. However, the modulation of such atomic sites and the understanding of their interactions are still highly challenging. Herein, we propose a unique supermolecule assembly-profile coating strategy to prepare a series of diatomic electrocatalysts by profile coating of eight Prussian blue analogues (PBAs) on supramolecular supports respectively as bimetallic sources. The detailed microstructure analysis revealed that the metal-nitrogen-carbon sites with four- (Zn-N4 ) and five-coordination (Fe-N5 ) via the nitrogen coordination are similar to the cytochrome c oxidases. For promising electrocatalysis, such unique microstructure is able to activate oxygen molecules due to nitrogen-bonding coordination with bimetal sites, thus leading to efficient four-electron oxygen reduction in alkaline, neutral, and acid electrolytes. Especially, zinc group elements (e.g., Zn and Cd) with d10 electron configuration would significantly boost the nitrogen-bonding coordination with bimetal sites to enhance electrocatalytic activity. The proof-of-concept for the general synthesis of advanced electrocatalysts with controllable bimetal active sites and the mechanistic understanding will promote the promising electrocatalysis by applying the similar principles.

Iron phthalocyanine integrated with boron-doped reduced graphene oxide for highly selective four-electron oxygen reduction: an experimental study

Iron phthalocyanine (FePc) has been integrated on boron-doped reduced graphene oxide (B-RGO) resulting in the composite, FePc@B-RGO which shows a highly selective four-electron oxygen reduction reaction.

(Invited) Enhancement of Activity of Low-Pt-Content-Catalysts Toward Oxygen Reduction through Admixing with Metal Oxide Cocatalysts

The proton-exchange membrane fuel cell (PEMFC) technology is one of the most promising approaches for energy conversion in automotive applications. Despite the use of expensive noble metal electrocatalysts, the sluggish kinetics of the oxygen reduction reaction (ORR) in acid media, and formation of the undesirable hydrogen peroxide intermediate are still the main drawbacks, on top of the stability problems, when it comes to the widespread commercialization of PEMFC devices. The progress in this subject is greatly hindered by the high cost and scarcity of the state-of-the-art platinum-based materials which are regarded as the most effective cathode catalysts. Therefore, most of the current research studies have been devoted to the optimization of active centers and maximization of their utilization, which should allow the lowering of the cathode Pt loadings without loss of performance and durability. Unfortunately, the problem of electrochemical stability and the danger of generation of higher quantities of the undesirable hydrogen peroxide intermediate may become even more serious in the case of systems utilizing lower amounts of the Pt catalystAmong important strategies is hybridization, activation, and stabilization of carbon-supported Pt catalysts by functionalization through admixing with certain nanostructured and typically substoichiometric metal oxides. Special attention is paid to application of the bi- or multi-metallic Pt-based alloys in their various forms and structures. Additionally, doping of carbon carriers with heteroatoms would produce surface functional groups and active centers capable of not only improving the catalysts’ performance, but also affecting their stability. Among other important issues are such features as porosity, hydrophilicity, and degree of graphitization of carbon components, in addition to the existence of metal–support interactions, high electrochemical active surface area, electronic structure of interfacial Pt, and the feasibility of adsorptive or activating interactions with oxygen molecules.Platinum is a main catalyst for the electroreduction of oxygen, the reaction of primary importance to the technology of low-temperature fuel cells. Because of high cost of platinum. there is a need to significantly lower its loadings at interfaces. But then O2-reduction proceeds often at less positive potentials and produces higher amounts of undesirable H2O2-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, Ta2O5, Nb2O5, ZrO2), stabilize Pt and carbon nanostructures, diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction of oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. The catalytic efficiency depends on geometric (decrease of Pt–Pt bond distances) and electronic (increase of d-electron vacancy in Pt) factors, in addition to possible metal-support interactions and interfacial structural changes affecting adsorption and activation of O2-molecule.

Electrochemistry of and Electrocatalysis by Low-Symmetry N4 Macrocycles

The introduction of efficient and affordable catalysts for three seemingly simple electrochemical reactions could improve very much the sustainability of our energy supply: reduction of protons to hydrogen gas (the hydrogen evolution reaction, HER), water oxidation, and selective reduction of oxygen to water (the oxygen reduction reaction, ORR).1–6 Platinum group metals (PGM) are outstanding catalysts for all these reactions, but being rare and expensive prevents their utility for large scale operations.7 Within the search for catalysts based on earth abundant 1st row transition metals, we now introduce two different molecular catalysts: Co(III) complexes of corroles, with very different in size meso-C substituents (C6F5 >> CF3 >> H), as well by newly synthesized and fully characterized Co(II), Cu(II) and Ni(II) monoazaporphyrins (MAzP) with meso-CF3 substituents. All the catalyst were adsorbed on carbon supports with very different porosity (Vulcan << BP2000). The corrole-modified electrodes were studied as ORR catalysts, revealing that the onset potential of the best performing cobalt corrole is approaching that of Pt and with low percentage formation of undesired hydrogen peroxide. Testing that complex as a cathode for an anion exchange membrane fuel cell (AEMFC) revealed high power density relative to other molecular catalysts and excellent stability over 12 hours.Metal complexes of a novel MAzP with CF3 on the three meso-C positions were studied as catalysts for HER in PBS buffer (pH=7.4), as well as for water oxidation and ORR at pH 13. The cobalt complex performed best in all processes. Compared to cobalt porphyrins with either three or four CF3 groups on the meso-C positions, Co-MAzP performed better for water oxidation. For HER catalysis, it has the same onset potential and current as the tetra-CF3 cobalt porphyrin and more current than the tris-CF3 analogue. In ORR catalysis, Co-MAzP performs with the lowest onset potential, but less H2O2 is formed via catalysis by the other porphyrins.(1) Meng, J.; Lei, H.; Li, X.; Qi, J.; Zhang, W.; Cao, R. Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency. ACS catalysis. 2019, pp 4551–4560.(2) Beyene, B. B.; Hung, C.-H. Recent Progress on Metalloporphyrin-Based Hydrogen Evolution Catalysis. Coord. Chem. Rev. 2020, 410, 213234.(3) Mondal, B.; Chattopadhyay, S.; Dey, S.; Mahammed, A.; Mittra, K.; Rana, A.; Gross, Z.; Dey, A. Elucidation of Factors That Govern the 2e–/2H+ vs 4e–/4H+ Selectivity of Water Oxidation by a Cobalt Corrole. J. Am. Chem. Soc. 2020, 142 (50), 21040–21049.(4) Cui, X.; Cui, Y.; Chen, M.; Xiong, R.; Huang, Y.; Liu, X. Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO4-Based Microrod Arrays in Neutral Media through Alkaline Activation. ACS Appl. Mater. Interfaces 2020, 12 (27), 30905–30914.(5) Fornaciari, J. C.; Weng, L.-C.; Alia, S. M.; Zhan, C.; Pham, T. A.; Bell, A. T.; Ogitsu, T.; Danilovic, N.; Weber, A. Z. Mechanistic Understanding of PH Effects on the Oxygen Evolution Reaction. Electrochim. Acta 2022, 405, 139810.(6) Yao, B.; He, Y.; Wang, S.; Sun, H.; Liu, X. Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction. International journal of molecular sciences. 2022, p 6036.(7) Mehta, V.; Cooper, J. S. Review and Analysis of PEM Fuel Cell Design and Manufacturing. Journal of power sources. 2003, pp 32–53.

The millisecond fabrication of medium-entropy alloy as a high-performance bifunctional electrocatalyst for ultralong-term rechargeable zinc–air batteries

Zinc air battery (ZAB) has widely recognized advantages such as high theoretical energy density, environmental friendliness, and safety. However, its cathodic reactions involve sluggish four-electron process oxygen reduction and oxygen evolution reactions (ORR and OER). Therefore, developing a cheap, efficient and durable dual functional oxygen electrocatalyst has important scientific significance and practical value for the development of ZAB. Herein, we applied noble metal free medium-entropy alloy (CoFeNi) nanoparticles which were in embedded nitrogen doped carbon framework (N-PCF) as a highly active and ultra-stable bifunctional electrocatalyst. The CoFeNi@N-PCF was prepared by high temperature thermal shock approach from ion-exchanged ZIF percussor to ensure the uniform dispersion of medium-entropy alloy, hierarchical porosity and strong metal-support interaction. Due to the medium-entropy induced synergistic effects, the as-prepared bifunctional electrocatalyst has a large ORR onset potential (Eonset) of 0.92 V and half-wave potential (E1/2) of 0.85 V (vs. RHE) in 0.1 M KOH. And the overpotential of OER in 1.0 M KOH is only 320 mV at 10 mA cm−2. When used in ZAB, the as-assembled rechargeable ZAB with the self-supported air electrode provides an extremely high-power density of 203 mW cm−2, a superior specific capacity of 812 mAh g−1, and amazing cycled steadily for 600 h. The assembled flexible ZAB with CoFeNi@N-PCF exhibits an excellent stability (over 45 h) and the excellent bending ability under different angles. This work can provide some new insights into the design and preparation of advanced bifunctional oxygen electrocatalysts for next-generation metal-air batteries.

One-step synthesis of thin-carbon-shell-encapsulated binary cobalt chromium nitrides for oxygen reduction reaction

Hydrogen fuel cells are expected to be widely used in transportation and portable power generation. Unfortunately, sluggish cathodic oxygen reduction reaction (ORR) kinetics hamper the commercial application of the fuel cells. The quest for cost effective and highly efficient ORR catalysts is of great importance. Transition metal nitride (TMN) has received widespread attention mainly for its Pt-like characteristics. Here, we propose a new one-step “NH3-free” synthesis of thin-carbon-shell-encapsulated binary cobalt chromium nitrides (Co-CrN@C) using 1,2,4-1H-triazole as the nitrogen/carbon source. The Co-CrN@C exhibits almost ideal four-electron reduction of oxygen and achieves remarkable long-term durability and better methanol tolerance due to high-degree graphitization of the carbon shell. The X-ray photoelectron spectra and the density functional theory calculations unveil the origin of the intrinsic activity of the catalyst and the reaction mechanism. Furthermore, Co-CrN@C exhibits an impressive peak power density (PPD) of 488 mW cm−2, surpassing the 423 mW cm−2 for the Pt/C-driven anion exchange membrane fuel cells. These findings offer an indispensable strategy for rational design of high-efficient and durable non-noble catalysts.

Revealing the Orbital Interactions between Dissimilar Metal Sites during Oxygen Reduction Process.

A FeCo/DA@NC catalyst with the well-defined FeCoN6 moiety is customized through a novel and ultrafast Joule heating technique. This catalyst demonstrates superior oxygen reduction reaction activity and stability in an alkaline environment. The power density and charge-discharge cycling of znic-air batteries driven by FeCo/DA@NC also surpass those of Pt/C catalyst. The source of the excellent oxygen reduction reaction activity of FeCo/DA@NC originates from the significantly changed charge environment and 3d orbital spin state. These not only improve the bonding strength between active sites and oxygen-containing intermediates, but also provide spare reaction sites for oxygen-containing intermediates. Moreover, various in situ detection techniques reveal that the rate-determining step in the four-electron oxygen reduction reaction is *O2 protonation. This work provides strong support for the precise design and rapid preparation of bimetallic catalysts and opens up new ideas for understanding orbital interactions during oxygen reduction reactions.

Recycling sulfidic spent caustic streams using a sulfide/air fuel cell

It is urgent to develop a green and cost-efficient process to recycle the resources and energy from spent caustic streams. We develop an energy-free strategy to recover sulfur, caustic, and energy from spent caustic streams in the meantime. Two-electron route sulfide oxidation reaction and four-electron route oxygen reduction reaction are combined to run as an abiotic sulfide/air fuel cell. The feasibility of the sulfide/air fuel cell is proven. The lab-scale prototype can self-driven run at >6.5 mA cm−2 for 10 h, with a current efficiency of about 14 %. A sulfidic spent caustic stream is adopted to evaluate the real stream treatment performances. The cell can discharge at >2 mA cm−2 for 60 h, where the sulfide concentration of the sulfidic spent caustic stream is continuously decreased from 0.56 mol/L to 0.31 mol/L. Cost-efficient carbon-supported silver catalysts are synthesized as the cathode catalysts, which can be prepared from silver electroplating effluent.

Structural insights into functional properties of the oxidized form of cytochrome c oxidase

Cytochrome c oxidase (CcO) is an essential enzyme in mitochondrial and bacterial respiration. It catalyzes the four-electron reduction of molecular oxygen to water and harnesses the chemical energy to translocate four protons across biological membranes. The turnover of the CcO reaction involves an oxidative phase, in which the reduced enzyme (R) is oxidized to the metastable OH state, and a reductive phase, in which OH is reduced back to the R state. During each phase, two protons are translocated across the membrane. However, if OH is allowed to relax to the resting oxidized state (O), a redox equivalent to OH, its subsequent reduction to R is incapable of driving proton translocation. Here, with resonance Raman spectroscopy and serial femtosecond X-ray crystallography (SFX), we show that the heme a3 iron and CuB in the active site of the O state, like those in the OH state, are coordinated by a hydroxide ion and a water molecule, respectively. However, Y244, critical for the oxygen reduction chemistry, is in the neutral protonated form, which distinguishes O from OH, where Y244 is in the deprotonated tyrosinate form. These structural characteristics of O provide insights into the proton translocation mechanism of CcO.

Regulation Mechanism of Electron Transfer Reaction from Cytochrome C to Cytochrome C Oxidase

The respiratory chain in mitochondria is quite essential for the energy generation in cells, effectively producing “the energy currency in living organisms”, ATP. The driving force for this system is a series of electron transfers form Complex I (NADH:quinone oxidoreductase) to Complex IV (Cytochrome c Oxidase; CcO) in inner mitochondria membrane, and these electron transfer reactions promote the proton pumping from the matrix to the intermembrane space in mitochondria, and, by use of the resulting proton concentration gradient across the membrane, Complex V (ATP synthase) produces ATP. These series of electron transfer reactions terminate at CcO to reduce molecular oxygen to water molecules. The electrons required for this four-electron reduction of molecular oxygen in CcO are donated from a small soluble electron transfer hemoprotein, Cytochrome c (Cyt c), and this electron transfer is also associated with the proton pumping for the ATP generation in the respiratory chain. To examine the molecular regulation mechanism of the electron transfer reaction to terminate the electron transport chain for four-electron reduction of molecular oxygen and proton pumping for the ATP generation, we kinetically followed the electron transfer reaction and analyzed the electron transfer reaction by the Michaelis-Menten equation. By utilizing the Cyt c mutants, we identified the amino acid residues to regulate the electron transfer reactions (Biochem. J., 2020, 477,1565). The kinetic analysis of the electron transfer reaction also revealed that the apparent rate for the electron transfer reaction determined by the steady-state kinetics is much slower than the intramolecular electron transfer rate in the Cyt c – CcO complex. On the other hand, we successfully determined the interaction site between Cyt c and CcO by using NMR (Proc. Natl. Acad. Sci., 2011, 108, 12271) and protein-protein docking simulation (J. Biol. Chem., 2016, 291, 15320). The Cyt c – CcO complex structure estimated from the NMR and protein-protein docking simulation clearly showed that the two redox centers, heme iron in Cyt c and CuA in subunit II of CcO, were located more than 20 Å apart, and the complexes exhibiting shorter redox center distances were energetically unstable. Combined with the results from kinetic and structural studies, we proposed a “conformationally gated electron transfer mechanism”, where the thermal fluctuation of the protein structure controls the electron transfer rate. However, direct experimental evidence that protein conformational fluctuations regulate electron transfer reactions has not been obtained. In this study, we focused on the viscosity of the protein solution as a factor to regulate the conformational fluctuation of the proteins, based on the assumption that high viscosity of the solution suppresses the conformational fluctuation. The electron transfer reaction from Cyt c to CcO was examined in the high viscosity solution and NMR relaxation measurements under high viscosity were also performed to identify the local structural fluctuation responsible for the regulation of the electron transfer rate. Together with the transient complex structures between Cyt c and CcO estimated by the MD simulation, the regulation mechanism of the electron transfer from Cyt c to CcO based on the “conformational gating” will be discussed.

(Invited) High-Rate and High-Efficiency Molten Salt Li-O2 and Na-O2 Batteries Enabled by Nitrate Redox

The decarbonization of the electrical grid requires advancements in affordable, long-term energy storage to address the intermittency of energy sources like solar and wind, while the continued decarbonization of the transportation sector necessitates advancements in energy dense storage devices such as batteries, with low cost and made from abundant materials. Alkali metal-oxygen batteries can provide greater specific energy than Li-ion batteries and can potentially lower cost by reducing reliance on late transition metals like nickel and cobalt. In this talk, we share our recent work on molten-salt Li-O2 1 and Na-O2 2 batteries operating at 150-170 °C which can deliver high areal energy (33 mWh/cm2 geo) and power densities (19 mW/cm2 geo), with high energy efficiency (~90% at 5 mA/cm2 geo). We show that nitrate redox plays a critical role in these batteries, where the apparent four-electron oxygen reduction to form Li2O in Li-O2 batteries is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen. In Na-O2 batteries, on the other hand, Na2O is further oxidized by O2 to Na2O2 to give an apparent two-electron oxygen reduction rection. We demonstrate the role of transition metal oxide catalysts for reducing the overpotential of nitrate redox in the positive electrode, as well as strategies for enhancing the power density and specific energy of these molten-salt metal-oxygen batteries. References (1) Zhu, Y. G.; Leverick, G.; Giordano, L.; Feng, S.; Zhang, Y.; Yu, Y.; Tatara, R.; Lunger, J. R.; Shao-Horn, Y. Nitrate-Mediated Four-Electron Oxygen Reduction on Metal Oxides for Lithium-Oxygen Batteries. Joule 2022, 6 (8), 1887–1903.(2) Zhu, Y. G.; Leverick, G.; Accogli, A.; Gordiz, K.; Zhang, Y.; Shao-Horn, Y. A High-Rate and High-Efficiency Molten-Salt Sodium–Oxygen Battery. Energy Env. Sci 2022, 15, 4636–4646.

The terminal oxidase cytochrome bd-I confers carbon monoxide resistance to Escherichia coli cells

Carbon monoxide (CO) plays a multifaceted role in the physiology of organisms, from poison to signaling molecule. Heme proteins, including terminal oxidases, are plausible CO targets. Three quinol oxidases terminate the branched aerobic respiratory chain of Escherichia coli. These are the heme‑copper cytochrome bo3 and two copper-lacking bd-type cytochromes, bd-I and bd-II. All three enzymes generate a proton motive force during the four-electron oxygen reduction reaction that is used for ATP production. The bd-type oxidases also contribute to mechanisms of bacterial defense against various types of stresses. Here we report that in E. coli cells, at the enzyme concentrations tested, cytochrome bd-I is much more resistant to inhibition by CO than cytochrome bd-II and cytochrome bo3. The apparent half-maximal inhibitory concentration values, IC50, for inhibition of O2 consumption of the membrane-bound bd-II and bo3 oxidases by CO at ~150 μM O2 were estimated to be 187.1 ± 11.1 and 183.3 ± 13.5 μM CO, respectively. Under the same conditions, the maximum inhibition observed with the membrane-bound cytochrome bd-I was 20 ± 10% at ~200 μM CO.

Facet effect of MnCo2O4 nanocrystals for enhanced oxygen reduction reaction in alkaline medium

Understanding the mechanism of the four-electron transfer oxygen reduction reaction (ORR) and the structure–activity relationship of spinel oxide remains a challenge. Two bimetallic manganese-cobalt spinel oxide supported on carbon nanotube (MnCo2O4/CNT) composite materials with different crystal planes exposed are synthesized in this paper by adding unequal amounts of ammonia during the hydrothermal process. Physical characterization results indicate that the exposed surfaces of the two materials (MnCo2O4/CNT-100, MnCo2O4/CNT-800) are (001) and (112), respectively. Electrochemical tests demonstrate that the MnCo2O4/CNT-800 composite material exposed to the (112) crystal plane exhibit higher ORR catalytic activity in alkaline electrolyte, with a half-wave potential of 0.79 V and a limiting current density of 5.15 mA cm−2. The outstanding activity is attributed to the greater specific surface area, the exposure of high-index crystal plane and the high-valence metal sites on the surface. Density functional theory (DFT) calculations reveal that the active sites on the (112) crystal plane have improved adsorption and ORR thermodynamics. This work provides a strategy for developing the advanced bimetallic spinel electrocatalysts by adjusting the exposed plane.

The oxygen reduction reaction activity and selectivity of porous-carbon supported transition metals (M-C: M Mn, Fe, Co, Ni, Cu) electrocatalysts

In general, both transition metal addition and nitrogen doping are important to the four-electron (4e−) oxygen reduction reaction (ORR) activity of carbon-based catalysts, in which nitrogen-containing species, such as M-Nx, pyridinic N and graphitic N, are believed to be the active sites. On the other hand, nitrogen-free carbon supported transition metals often show their excellent electroactivity toward two-electron (2e−) ORR. Herein, carbon supported transition metal catalysts (M-C: MMn, Fe, Co, Ni, Cu) with high specific surface areas were prepared by a simple zinc-mediated pyrolysis method and their ORR activity and selectivity were systematically studied. It is found that various transition metals result in a large difference in the content and type of oxygen-containing functional groups of the M-C catalysts. The results show that CuC and CoC exhibit promising ORR activity close to the 4e− transfer mechanism in 0.1 M KOH electrolyte. FeC and MnC show a mixture of 2e− and 4e− transfer pathways. NiC is an efficient catalyst for efficient H2O2 generation through a primary 2e− ORR. It is found that the ORR selectivity of M-C is correlated to the content and type of oxygen-containing functional groups in the catalysts, and C-O-C is likely an effective active site for 4e− ORR based on both experimental results and theoretical calculations.

Sulfonic Acid Functionalization-Boosted Ultrafast, Durable, and Selective Four-Electron Oxygen Reduction Reaction: Evidenced by EC-SHINERS and DFT Studies

Sulfonic Acid Functionalization-Boosted Ultrafast, Durable, and Selective Four-Electron Oxygen Reduction Reaction: Evidenced by EC-SHINERS and DFT Studies

Steering Selectivity in the Four-Electron and Two-Electron Oxygen Reduction Reactions: On the Importance of the Volcano Slope.

In the last decade, trends for competing electrocatalytic processes have been largely captured by volcano plots, which can be constructed by the analysis of adsorption free energies as derived from electronic structure theory in the density functional theory approximation. One prototypical example refers to the four-electron and two-electron oxygen reduction reactions (ORRs), resulting in the formation of water and hydrogen peroxide, respectively. The conventional thermodynamic volcano curve illustrates that the four-electron and two-electron ORRs reveal the same slopes at the volcano legs. This finding is related to two facts, namely, that only a single mechanistic description is considered in the model, and electrocatalytic activity is assessed by the concept of the limiting potential, a simple thermodynamic descriptor evaluated at the equilibrium potential. In the present contribution, the selectivity challenge of the four-electron and two-electron ORRs is analyzed, thereby accounting for two major expansions. First, different reaction mechanisms are included into the analysis, and second, G max(U), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, is applied for approximation of electrocatalytic activity. It is illustrated that the slope of the four-electron ORR is not constant at the volcano legs but rather is prone to change as soon as another mechanistic pathway is energetically preferred or another elementary step becomes the limiting one. Due to the varying slope of the four-electron ORR volcano, a trade-off between activity and selectivity for hydrogen peroxide formation is observed. It is demonstrated that the two-electron ORR is energetically preferred at the left and right volcano legs, thus opening a new strategy for the selective formation of H2O2 by an environmentally benign route.

Non-bonding interaction of dual atom catalysts for enhanced oxygen reduction reaction

We demonstrate the design of graphene-supported dual atom catalysts (DACs) for the four-electron oxygen reduction reaction (ORR), by utilizing the non-bonding interaction of counterpart metals (M) that synergistically tune the electronic properties and catalytic activity of the Fe active site in FeMN6-DAC and FeMN8-DAC systems, where M stands for Fe, Co, Ni, Cu, and Zn. More specifically, for Fe-M distances below 15 Å, the non-bonding interaction is significant, making the system act as the DAC. We predicted that FeNiN6-DAC and FeNiN8-DAC exhibit a low ORR overpotential (ηORR) of 0.28 V and 0.47 V, respectively, which are at the summits of volcano plots. This low ηORR originates from the high Bader charge transfer coupled with high spin density at the Fe site in both the FeNiN6-DAC and FeNiN8-DAC systems, which weakens the adsorption of OH* intermediate while enhancing its desorption to H2O. Guided by these density functional theory (DFT) computational results, we synthesized FeCoN8-DAC and FeNiN8-DAC along with N-doped graphene and confirmed their structures with scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR). We verify experimentally the catalytic activities and find that FeNiN8-DAC has the low experimental overpotential of 0.39 V with a Tafel slope of 47 mVdec−1. Based on these results, we propose a DFT-guided strategy to tune the charge transfer and spin population of the active site toward designing DACs for electrochemical ORR.

Metal-Free Triazine-Based Porous Organic Polymer-Derived N-Doped Porous Carbons as Effective Electrocatalysts for Oxygen Reduction Reaction

In recent years, porous heteroatom-doped carbon materials have been very promising for energy conversion. A newly designed porous organic polymer (POPQ) has been synthesized using two organic monomers, i.e., 2,6-diaminoanthraquinone and cyanuric chloride, under reflux conditions for 72 h in an inert atmosphere. The triazine-containing porous organic polymers undergo pyrolysis, which produces two nitrogen-doped porous carbon materials, N/POPQ600 and N/POPQ800, at 600 and 800 °C temperatures, respectively. Since the resultant N-doped porous materials have a higher surface area than the parent porous organic polymer and the materials have a synergistic effect due to the enriched nitrogen content throughout the matrix, the metal-free N/POPQ600 and N/POPQ800 materials exhibit good electrocatalytic activity toward oxygen reduction reaction (ORR). Among these, the N/POPQ800 material shows excellent ORR activity with a nearly four-electron oxygen reduction pathway where the half-wave potential is estimated to be 0.728 V vs reversible hydrogen electrode (RHE), comparable with the commercially available Pt/C catalyst. Most interestingly, the N/POPQ800 catalyst displays outstanding long-lasting stability. It shows a better methanol tolerance capability than Pt/C, which can be attributed to the high specific surface area and N-doped well-defined crystalline porous structure. Also, the homogeneously distributed active sites throughout the carbon framework are the most precious for the electrochemical oxygen reduction reaction.

Inner-pore reduction nucleation of palladium nanoparticles in highly conductive wurster-type covalent organic frameworks for efficient oxygen reduction electrocatalysis

Covalent organic frameworks (COFs) have emerged as a class of promising supports for electrocatalysis because of their advantages including good crystallinity, highly ordered pores, and structural diversity. However, their poor conductivity represents the main obstruction to their practical application. Here, we reported a novel synthesis strategy for synergistically endowing a triphenylamine-based COFs with improved electrical conductivity and excellent catalytic activity for oxygen reduction, via the in-situ redox deposition and confined growth of palladium nanoparticles inside the porous structure of COFs using reductive triphenylamine frameworks as reducing agent; meanwhile, the triphenylamine unit was oxidized to radical cation structure and affords radical cation COFs with conductivity as high as 3.2*10−1 S m−1. Such a uniform confine palladium nanoparticle on highly conductive COFs makes it an efficient electrocatalyst for four-electron oxygen reduction reaction (4e-ORR), showing excellent activities and fast kinetics with a remarkable half-wave potential (E1/2) of 0.865 V and an ultralow Tafel slope of 39.7 mV dec−1 in alkaline media even in the absence of extra commercial conductive fillers. The generality of this strategy was proved by preparing the different metal and metal alloy nanoparticles supported on COFs (Au@COF, Pt@COF, AuPd@COF, AgPd@COF, and PtPd@COF) using reductive triphenylamine frameworks as reducing agent. This work not only provides a facile strategy for the fabrication of highly conductive COF supported ORR electrocatalysts, but also sheds new light on the practical application of Zn-air battery.