Oxygen Reduction Reaction Pathway Research Articles

Implications of boron doping on electrocatalytic activities of graphyne and graphdiyne families: a first principles study.

Dispersive force corrected density functional theory is used to map the oxygen reduction reaction (ORR) kinetics of six kinds of graphyne (Gy) and graphdiyne (Gdy) systems (namely αGy, βGy, γGy, δGy, 6,6,12Gy, RGy and Gdy) with substitutional boron (B) atom doping. To this end, the most favorable sites for B doping of each structures are determined by comparing their formation energies and then the best configuration for di-oxygen (O2) adsorption is computed by analyzing the corresponding adsorption energies. Even though oxygen adsorption is found to be energetically favorable on all of these and all Gys and Gdy are found to distinctly favor the four electron pathways for ORR, a reaction scheme with monotonically exothermic ΔG is observed only for B doped RGy. Further computations performed by varying electrode potential indicated this monotonically exothermic nature of the ΔG of B doped RGy to persist in the range 0-0.22 V and also indicated the first (H(+) + e) transfer step to be the rate limiting step.

Nitrogen-doped Co/Co9S8/partly-graphitized carbon as durable catalysts for oxygen reduction in microbial fuel cells

Durability of catalysts for oxygen reduction reaction (ORR) is the key factor for governing the performance of microbial fuel cells (MFCs). The cobalt (Co) chelated polyaniline (PANI) is used as the nitrogen and carbon sources to prepare the N-doped Co/Co9S8/partly-graphitized carbon (Co/Co9S8/NPGC) catalysts. Structure-activity correlations for Co/Co9S8/NPGC are explored by tuning the heating temperature (600–1000 °C) to investigate how the active components (Co/Co9S8) and N-doped functionalities (N-species) influence the ORR activity. As temperature increases, the gradual crystallization of Co originating from the reduction of Co9S8 is conducted to form the Co/Co9S8 heterojunction. MFCs with Co/Co9S8/NPGC (800 °C) cathode obtain the highest power density (1156 mW m−2) and the lowest charge transfer resistance (11.1 Ω) after 75 d running, which are better than commercial Pt/C (10 wt.%). Although the sole Co9S8 plays a limited role in ORR, the resulting Co/Co9S8 is found to be indispensable to achieve high activity and durability in MFCs cathodes. The dominant ORR pathways of Co/Co9S8/NPGC (800 and 900 °C) are the four-electron O2 reduction, which are attributed to the co-existence of pyridinic N, graphitic N and Co-Nx species. These new N-doped metal sulfide/PGC composites show promise for applications in MFCs.

Catalytic Activity for Oxygen Reduction Reaction on CoN2-Graphene: A Density Functional Theory Study

The detailed catalytic mechanism of oxygen reduction reaction (ORR) on CoN2 embedded graphene (CoN2-G) has been investigated based on a density functional theory calculation. Pathways of four- and two-electron reductions are investigated for ORR. The most feasible four-electron reduction pathways are arranged in the following order: the reductions of O2(ads) into OOH(ads), OOH(ads) into O(ads) and H2O, then O(ads) into OH(ads), and finally OH(ads) into H2O. Free energy diagrams show that the elementary steps of ORR along the four-electron pathway are downhill at low electrode potential (up to 0.62 V vs. standard hydrogen electrode). The first reduction step, O2(ads)-to-OOH(ads) reaction has the highest reaction barrier (0.53 eV), acting as the kinetic rate-determining step. However, at a higher potential, the last reduction step, OH(ads)-to-H2O(ads) becomes uphill, and is the thermodynamic rate-determining step. Therefore, the first and the last reduction steps restrict the ORR performance together at high potential on CoN2-G catalysts. The two-electron reduction product H2O2 can be chemisorbed on the surface. However, the H2O2(ads) is unstable and easy to dissociate into OH species. Therefore, the two-electron ORR pathway is unfavorable on CoN2-G.

Oxygen Reduction at Carbon‐Supported Lanthanides: The Role of the B‐Site

AbstractThe kinetics of the oxygen reduction reaction (ORR) at carbon‐supported transition‐metal oxides in alkaline solutions is systematically investigated as a function of the nature of the B‐site. The study is focused on LaBO3 (B=Cr, Co, Fe, Mn and Ni) nanoparticles synthesized by using an ionic‐liquid route, offering fine control over phase purity and composition. Activity towards the ORR was compared with the commercial Pt/Etek catalyst. Detailed electrochemical analysis employing a rotating ring‐disk electrode provides conclusive evidence that the carbon support plays an important contribution in the faradaic responses. Decoupling the contribution of the carbon support uncovers that the reactivity of LaMnO3 towards the four‐electron ORR pathway is orders of magnitude higher than that for the other lanthanides. We rationalize these observations in terms of changes in the redox state at the B‐site close to the formal oxygen reduction potential.

Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal–Nitrogen–Carbon Electrocatalysts

Development and optimization of non-platinum group metal (non-PGM) electrocatalysts for oxygen reduction reaction (ORR), consisting of transition metal–nitrogen–carbon (M–N–C) framework, is hindered by the partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. In this study, we have analyzed more than 45 M–N–C electrocatalysts synthesized from three different families of precursors, such as polymer-based, macrocycles, and small organic molecules. Catalysts were electrochemically tested and analyzed structurally using exactly the same protocol for deriving structure-to-property relationships. We have identified possible active sites participating in different ORR pathways: (1) metal-free electrocatalysts support partial reduction of O2 to H2O2; (2) pyrrolic nitrogen acts as a site for partial O2 reduction to H2O2; (3) pyridinic nitrogen displays catalytic activity in reducing H2O2 to H2O; (4) Fe coordinated to N (Fe–Nx) serves as an active site for four-electron...

Metal-organic framework derived hierarchically porous nitrogen-doped carbon nanostructures as novel electrocatalyst for oxygen reduction reaction

The hierarchically porous nitrogen-doped carbon materials, derived from nitrogen-containing isoreticular metal-organic framework-3 (IRMOF-3) through direct carbonization, exhibited excellent electrocatalytic activity in alkaline solution for oxygen reduction reaction (ORR). This high activity is attributed to the presence of high percentage of quaternary and pyridinic nitrogen, the high surface area as well as good conductivity. When IRMOF-3 was carbonized at 950°C (CIRMOF-3-950), it showed four-electron reduction pathway for ORR and exhibited better stability (about 78.5% current density was maintained) than platinum/carbon (Pt/C) in the current durability test. In addition, CIRMOF-3-950 presented high selectivity to cathode reactions compared to commercial Pt/C.

Kinetic Approach to Investigate the Mechanistic Pathways of Oxygen Reduction Reaction on Fe-Containing N-Doped Carbon Catalysts

The mechanism of the oxygen reduction reaction (ORR) on polyimide-based N-doped carbon catalysts with and without Fe (i.e., Fe–N–C and N–C) in acidic media was studied, using a rotating ring-disk electrode voltammetry, based on the estimation of the rate constants of the ORR, i.e., k1, k2, and k3 corresponding to the 4-electron direct reduction of O2 to H2O, the 2-electron reduction of O2 to H2O2, and the 2-electron reduction of H2O2 to H2O, respectively as functions of the loading density of Fe–N–C or N–C and the electrode potential, in which two different kinetic models for the ORR, i.e., Damjanovic and Wroblowa models were used to estimate the values of the k1, k2, and k3. The H2O2 disproportionation reaction rate constants (k4) of the Fe–N–C and N–C catalysts were also estimated. For both catalysts, the ORR follows, as a whole, a 4-electron pathway (k1 ≫ k2) when the loading density is high (>ca. 200 μg/cm2), whereas at the lower loading density the 2-electron reduction of O2 becomes more dominant (k1...

Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts

The oxygen reduction reaction (ORR) is an important electrode reaction for energy storage and conversion devices based on oxygen electrocatalysis. This paper introduces the thermodynamics, reaction kinetics, reaction mechanisms, and reaction pathways of ORR in aqueous alkaline media. Recent advances of the catalysts for ORR were extensively reviewed, including precious metals, nonmetal-doped carbon, carbon–transition metal hybrids, transition metal oxides with spinel and perovskite structures, and so forth. The applications of those ORR catalysts to zinc–air batteries and alkaline fuel cells were briefly introduced. A concluding remark summarizes the current status of the reaction pathways, advanced catalysts, and the future challenges of the research and development of ORR.

Mathematical Modelling and Experimental Analysis of Thin, Low-Loading Fuel Cell Electrodes

The use of expensive platinum in PEFC remains a major barrier to achieve a low cost fuel cell stack. To reduce stack cost, thin, low loading electrodes have been studied at various laboratories [1]. Experimental data showed that the cell performance is severely limited for cathode electrode loadings below 0.1 mg/cm2. This decreased performance has been attributed to several physical processes [2]. First, the oxygen reduction reaction (ORR) is a multi-step reaction that does not follow Tafel kinetics. Low loading electrodes might operate following a different ORR pathway. Second, due to the higher local mass flux to the Pt sites, mass transport resistances in the ionomer thin film, presumably covering the catalytic sites, might limit oxygen transport to the reaction sites. Finally, water management issues are exacerbated due to the reduced thickness and thereby increased water production rate per unit volume. Experimental and theoretical investigations in thin, low loading fuel cell electrodes are being performed in our laboratory. Thin, low loading fuel cell electrodes of various ionomer and Pt loadings, and Pt/C content were fabricated and tested under a variety of operating conditions. In order to accurately control catalyst/ionomer loading and layer uniformity, the electrodes were deposited on a membrane using a material inkjet printer. Results show that cells with cathode Pt loadings of 0.025 mg/cm2 are severely limited in the kinetic region with cell voltages as low as 0.6V at 200 mA/cm2 under hydrogen/air conditions at ambient pressure. At cell voltages of 0.2 V however, mass activities as high as 50 A/mgPt and current densities above 1 A/cm2were observed [3]. Macro-scale mathematical modelling studies of low loading electrodes have either been based on simplified one-dimensional models, or have relied on simple Tafel kinetics for the ORR. In this work, meso- and macro-scale simulations of thin, low loading electrodes are performed using OpenFCST, an open-source fuel cell simulation software developed by our research group [4]. A multi-dimensional, non-isothermal, two-phase membrane electrode assembly (MEA) model was developed. The Butler-Volmer/Tafel representations of the HOR and ORR were replaced by multi-step reaction kinetic models, i.e., the dual path and double trap models proposed by Wang et al. and modified by our group [5]. To account for thin film mass transport resistances, a pseudo-agglomerate model that accounts only for the ionomer thin film was implemented. The model is justified by microscopy observations and our previous mathematical modelling results that suggest that, based on the expected agglomerate size, the ionomer thin film is the most important micro-scale parameter [6]. The use of multi-step kinetic and ionomer thin film models improves fuel cell predictions for both conventional and low loading electrodes in the kinetic, ohmic and mass transport regions. Water build-up in thin, low loading electrodes is likely to be exacerbated. Several researchers have proposed a variety of pore-size distribution (PSD) based models to account for the layers' micro-structures [7]. These models however have not yet been implemented in a multi-dimensional MEA model. A dual wettability PSD based model was implemented in OpenFCST. The PSD based model has been compared to the common saturation-based two-phase flow model in porous media, e.g., [8]. Preliminary results show that the PSD model is able to identify discontinuities in the saturation profiles inside the MEA as a result of the MPL hydrophobic nature and small PSD. The saturation-based model on the other hand, provides a continuous saturation profile. Focused ion beam-scanning electron microscopy (FIB-SEM) and nano-computed tomography (nanoCT) have been performed on our low loading electrodes. A statistical analysis tool was developed and used to compute multiple correlation functions to assess porosity, active area and pore connectivity. Using the statistical correlation functions and a novel different phase neighbors based simulated annealing strategy, meso-scale reconstructions of the electrode have been created [9]. The electrode reconstructions can be used to estimate the effective transport properties in each phase of the electrode using OpenFCST. [1] J. Wee et al., J. Power Sources, 2007, 165(2) 667. [2] J. Owejan et al., JES, 2013, 160(8), F824. [3] S. Shukla et al., Electrochimica Acta, 2015, 156:289. [4] M. Secanell et al., ECSTrans., 2014, 64(3): 655. [5] J. Wang et al., JES., 2006, 153(9), A1732; J. Wang et al., J. Phys. Chem. A, 2007, 111, 12702; Moore et al., JES, 2013, 160(6), F670. [6] M. Moore et al., JES., 2014, 161(8):E3125. [7] A. Weber. JES., 2004, 151(10), A1715. [8] D. Natarajan, JES, 2001, 148(12), A1324. [9] L. Pant et al., Phys.Rev.E, 2014, 90, 023306. Figure 1

Long Term Effects of Airborne Contaminants in PEMFC Cathode

Air pollution is a challenge for the commercialization of proton exchange membrane fuel cells (PEMFCs) [1]. Recently, twenty-one species were selected from a list of more than 200 airborne pollutants (US Environmental Protection Agency database) for single cell accelerated tests [2]. Results showed that most contaminants differently affected cell performance. Seven of these twenty-one species were down selected for detailed studies based on quantitative criteria [2]. The seven contaminants are acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene and propene [2]. All seven contaminants can adsorb on the Pt surface, reduce the effective electrochemical active area for the oxygen reduction reaction (ORR), and even impact the ORR pathway and increase the formation of hydrogen peroxide (H2O2) [3] that promotes ionomer decomposition [4]. Acetonitrile also decreases the proton conductivity of the membrane. The larger H2O2 production rate determined with a rotating ring/disc electrode maintained at 30 °C is not expected to be representative of a PEMFC. Therefore, it is currently unclear if long term operation of a PEMFC exposed to a contaminant will lead to faster catalyst layer ionomer and membrane degradation.A long term test with acetonitrile injected in the cathode was conducted. Acetonitrile was selected because it has several impacts on cell performance and promotes the formation of H2O2. Electrochemical impedance spectroscopy (EIS) was intermittently used to obtain information on cell processes with minimal disturbance to the test. Cyclic voltammetry (CV) was applied before and after the long term test to measure the decrease in electrochemically active Pt sites. Transmission electron microscopy (TEM) was employed to analyze Pt particles agglomeration and Pt deposition in the electrolyte membrane. Inductively coupled plasma mass spectroscopy (ICP-MS) was utilized to detect Pt dissolution in exhaust water. Ion chromatography (IC) was exploited to measure the F– emission rate in the exhaust water and identify other ions including NH4 +. Scanning electron microscopy (SEM) was used to investigate thickness changes of catalyst layers and membrane.Figure 1 shows the cell voltage and high frequency resistance responses during the long term acetonitrile contamination experiment. Initially, the cell voltage at 0.4 A cm–2 was 0.667 V and the high frequency resistance (HFR) was ~0.1 Ω cm2. Within the 300 h pre-poisoning period, the cell voltage decreased with a degradation rate of 1.18 x 10–4 V h–1 and the HFR stabilized at 0.088 Ω cm2 at 220 h. During the first 20 h of acetonitrile exposure, the cell voltage dropped by 0.13 V and the HFR increased by 0.021 Ω cm2. Interestingly, the cell performance was stable during the following ~600 h even though the HFR continued its slow increase. The cell voltage eventually decreased by ~0.02 V during the last 400 h of exposure to acetonitrile. For the recovery phase, the cell voltage and HFR were respectively restored to 0.551 V and 0.109 Ω cm2 during the first 20 h. After that period, an accidental interruption occurred leading to a 4 min OCV event that subsequently increased the voltage and decreased the HFR. After that event, the cell voltage resumed its decay with a degradation rate of 1.40 x 10–4 V h–1. At around 425 h of the recovery period, a second interruption occurred resulting in a cell voltage jump of ~0.02 V which was followed by degradation with a similar rate after the first interruption. The long term exposure to acetonitrile appears to mitigate the regular degradation of cell performance; despite the initial performance loss due to acetonitrile adsorption on Pt and the continuing HFR increase attributed to the effect of acetonitrile reaction products on ionomer electrolyte. The OCV operation may also accelerate the cell performance recovery.The presence of long term contaminant effects was ascertained by focusing on Pt catalysts and ionomer electrolyte degradation. The Pt catalysts’ degradation was probed by comparing EIS, CV, ICP-MS and TEM results with control samples. The ionomer degradation was explored by comparing EIS, IC and SEM results with control samples.ACKNOWLEDGMENTSThis work is supported by the Department of Energy (DE-EE0000467). Authors are also grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.REFERENCES1. J. St-Pierre, in Polymer Electrolyte Fuel Cell Durability, F. N. Büchi, M. Inaba, and T. J. Schmidt, Editors, p. 289, Springer (2009).2. J. St-Pierre, M. S. Angelo, and Y. Zhai, J. Electrochem. Soc., 161, F280 (2014).3. J. Ge, J. St-Pierre, and Y. Zhai, Electrochim. Acta, 134, 272 (2014).4. A. Bosnjakovic and S. Schlick, J. Phys. Chem. B, 108, 4332 (2004). Figure 1

A Computational Investigation Of Oxygen Reduction Reaction Mechanisms On Si- And Al-doped Graphene: A Comparative Study

In this letter, the mechanisms of the oxygen reduction reaction (ORR) on Si- and Al-doped graphene have been investigated to understand the effect of doped graphene on the ORR and predict details of ORR pathways. Density functional theory (DFT) calculations were used to achieve the true mechanism pathways of ORR on the surfaces. Also, free energy diagrams for the ORR were constructed to provide the stability of possible intermediates in the electrochemical reaction pathways. At first stage, the adsorption of O2 molecule on both surfaces was studied with two possible configurations: atop (most stable) and bridge with the Eads of -60.6 and -72.4 kcal/mol, while for bridge site they were about -48.9 and -60.4 kcal/mol, respectively. Then, the most stable configuration (atop) was selected and the pathways formed after the adsorption of four atomic hydrogen to O2 molecule for both surfaces. These mechanisms were similar in both Si- and Al- doped graphene but there was a little difference in the obtained intermediates formed in each surface. In each pathway, the O2 dissociation reaction was neglected because it was unlikely to occur due to the high activation energy (> 45 kcal/mol). The results of this study show an easy and economic way to obtain Si- and Al-doped graphene as a non-metal catalyst for ORR at the cathode electrode in fuel cells. Copyright © 2015 VBRI Press.

Biomimetic Oxygen Reduction Reaction Catalyzed by Microperoxidase-11 at Liquid/Liquid Interfaces

An investigation of oxygen reduction reaction (ORR) catalyzed by microperoxidase-11 (MP-11) at the polarized water/1,2-dichloroethane (DCE) interface is reported. MP-11 contains a heme group covalently bonded to an undecapeptide chain via two thioether bonds of cysteine residues, as in cytochrome c oxidases (CcOs), and has been widely studied as a biomimetic model of CcOs. Herein we demonstrated that MP-11 can adsorb at the water/DCE interface and catalyze the O2 reduction by lipophilic electron donors, namely tetrathiafulvalene (TTF) and 1,1′-dimethylferrocene (DFc). The overall catalytic ORR corresponds to a proton coupled electron transfer (PCET) reaction and is kinetically controlled by the heterogeneous conversion of MP-11 from ferric (FeIII-MP-11) to ferrous state (FeII-MP-11). Given that a significant amount of H2O2 was produced for both electron donors, it indicates that MP-11 has a remarkable impact on the ORR pathway and that MP-11, similar to other mononuclear macrocyclic compounds, cannot sele...

Fe/N/C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel Cells Using Nitrogen-Rich Ligand as Precursor

High temperature pyrolysis can significantly improve the activity and stability of Fe-based catalysts. However, unwanted iron nanoparticles, which are proven inactive to oxygen reduction reaction (ORR), will form under this procedure. Herein, a nitrogen-rich and hindrance multifunctional 6,7-di(pyridin-2-yl)pteridine-2,4-diamine (DPPD) monomer was deliberately designed and synthesized. High content of thermally stable nitrogen in DPPD can increase the degree of coordination with iron and provide a high content of active nitrogen after pyrolysis. Distorted nitrogen-rich ferrous complex polymers were successfully prepared to keep iron ions well separated and prevent them from aggregating during the heat treatment. Carbon-supported Fe-based catalysts with different initial iron loadings from 0.2 to 4.0 wt % were obtained. Transmission electron microscopy (TEM) revealed that there were no obvious nanocrystals observed, even the initial iron loading was up to 2.0 wt %. The electrochemical performance of the Fe-based catalysts was evaluated via cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The result shows that an Fe-based catalyst with 2.0 wt % initial iron loading is the best ORR catalyst in acid media among all the iron loadings. Typically, in basic media, the catalyst with 2.0 wt % initial iron loading exhibits comparable electrocatalytic activity to commercial Pt/C material via an efficient four-electron-dominant ORR pathway coupled with better methanol tolerance as well as durability. XPS measurements confirmed that the outstanding activity of the catalyst with 2.0 wt % initial iron loading was likely attributed to higher content of pyridinic nitrogen, providing the highest density of active site structures.

Effect of the TiO2 phase and loading on oxygen reduction reaction activity of PtCo/C catalysts in proton exchange membrane fuel cells

We investigated the effect of the TiO2 phase, as either pure rutile (TiO2(R)) or a 4 : 1 (w/w) anatase: rutile ratio (TiO2(AR)), and the loading on the activity of PtCo/C catalyst in the oxygen reduction reaction (ORR) in a proton exchange membrane (PEM) fuel cell. The incorporation of the different phases and loading of TiO2 on the PtCo/C catalyst did not affect the alloy properties or the crystalline size of the PtCo/C catalyst, but affected importantly the electrochemical surface area (ESA), conductivity of catalyst layer and the water management ability. The presence of TiO2(AR) at appropriate quantity can decrease the mass transport limitation as well as the ohmic resistance of catalyst layer. As a result, the optimum loading of TiO2(AR) used to incorporated in the layer of PtCo/C catalyst was 0.06mg/cm2. At this content, the TiO2(AR)-PtCo/C catalyst provided the highest current density of 438 mA/cm2 at 0.6V at atmospheric pressure in PEM fuel cell and provided the kinetic current in acid solution of 20.53 mA/cm2. In addition, the presence of TiO2(AR) did not alter the ORR electron pathway of PtCo/C catalyst. The electron pathway of ORR of TiO2(AR)-PtCo/C was still the four-electron pathway.

Elucidation of oxygen reduction reaction pathway on carbon-supported manganese oxides

The oxygen reduction reaction (ORR) is a complex process. This is particularly the case for carbon-supported electrocatalysts in alkaline electrolytes, because carbon can catalyze the ORR via a two-electron transfer to generate hydroperoxide (HO2−), which subsequently undergoes either chemical decomposition to generate O2 and OH− (HODR) or electrochemical reduction to OH− (HORR). In this study, we elucidated the ORR pathway on a series of carbon-supported manganese oxides, which have been extensively studied as electrocatalysts in alkaline electrolytes. A comparison of the turnover frequencies of the HODR and HORR showed that although an apparent four-electron transfer process was identified when the HO2− yield was measured using the rotating ring disk electrode technique, the real ORR pathway involved a two-electron transfer process to generate HO2−, with subsequent chemical decomposition of HO2−. These results will help us to understand the intrinsic catalytic behavior of carbon-supported transition-metal oxides for the ORR in alkaline electrolytes.

Optimizing the oxygen evolution reaction for electrochemical water oxidation by tuning solvent properties.

Electrochemical water-based energy cycles provide a most promising alternative to fossil-fuel sources of energy. However, current electrocatalysts are not adequate (high overpotential, lack of selectivity toward O2 production, catalyst degradation). We propose here mechanistic guidelines for experimental examination of modified catalysts based on the dependence of kinetic rates on the solvent dielectric constant. To illustrate the procedure we consider the fcc(111) platinum surface and show that the individual steps for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) change systematically with the polarizability of the medium. Thus changing this environmental variable can be used to tune the rate determining steps and the barriers, providing a means for screening and validating new systems to optimize the rate determining steps for the ORR and OER reaction pathways.

A Density Functional Theory Study on Mechanism of Electrochemical Oxygen Reduction on FeN4-Graphene

Metal-coordinated nitrogen-doped carbons are catalytically active for oxygen reduction reaction (ORR). The present study describes in detail the ORR on Fe-coordinated, pyrrolic nitrogen-doped graphene (FeN4-G) based on a density functional theory calculation. On this model, pathways of four- and two-electron reductions are investigated for ORR. The most feasible four-electron reduction pathways are arranged in the following order: O2(ads)→OOH(ads)→O(ads)+H2O(ads) (or 2OH(ads))→OH(ads)+H2O(ads) →2H2O(ads). Free energy diagrams show that the elementary steps of the ORR along the four-electron pathway are downhill at a low electrode potential (up to 0.41 V vs. standard hydrogen electrode). The rate-determining step appears at the OH(ads)-to-H2O(ads)reduction, with a reaction barrier of 1.02 eV. The two-electron reduction product H2O2 can chemisorb on the surface. However, the free energy diagrams show that the reduction of OOH into H2O2 remains uphill for all positive electrode potential vs. the normal hydrogen electrode. The high endothermic ΔG value of H2O2 formation indicates that the two-electron ORR pathway is unfavorable on FeN4-G catalysts.

Graphyne nanotubes as electrocatalysts for oxygen reduction reaction: the effect of doping elements on the catalytic mechanisms.

The detailed density functional theory (DFT) calculations have been carried out to study the oxygen reduction reaction (ORR) activity and the mechanism on doped single-walled γ-graphyne nanotubes (GNTs). Four of the most commonly used doping elements, N, B, P, and S atoms, were used to investigate the doping effects. The calculated results indicate that doping GNTs with different elements could lead to different ORR catalytic mechanisms. The ORR catalyzed by both N- and B-GNTs is initiated by the end-on adsorption of O2 and is completed via an O-H2O dissociation mechanism, while the ORR on the P-GNT is initiated by the bridge adsorption of O2 and is completed via an O2 dissociation mechanism. The predicted catalytic performance of the doped GNT would decrease in the sequence N-GNT > B-GNT > P-GNT > S-GNT, based on the analysis of adsorption energies of ORR species and landscapes of ORR pathways. Electronic structure analysis shows that the high ORR activity of doped GNTs is directly related to the suitable HOMO levels and HOMO-LUMO energy gaps.

Nanocrystal Bismuth Telluride Electrocatalysts for Highly Efficient Oxygen Reduction

Non-precious metal-based bismuth telluride (Bi2Te3) catalysts were synthesized using a solvothermal method at different temperatures. The morphology, composition and structure of the resulting catalysts were subsequently characterized by SEM, TEM, XRD and XPS analysis. The oxygen reduction reaction (ORR) properties of the catalysts were studied by electrochemical testing. The 150°C Bi2Te3 sample exhibited excellent electrocatalytic ORR activity, almost four-electron (n = 3.8) ORR pathway selectivity in an alkaline aqueous solution, and excellent methanol tolerance. This is due to the exposure of the (0 0 6) planes combined with the van der Waals force (the force between (0 0 6) planes). Novel Bi2Te3 nanocrystals with highly efficient ORR activity could be applied to next-generation methanol alkaline fuel cells.

The role of trace Fe in Fe–N-doped amorphous carbon with excellent electrocatalytic performance for oxygen reduction reaction

Synthesized by dripping iron acetate into the N-doped carbon film enriched with pyridinic N and followed by annealing at 800°C, Fe–N-doped amorphous carbon (dFe–N-C) with an Fe content of 0.2at.% showed excellent electrocatalytic activity, stability and methanol tolerance via a four-electron pathway for oxygen reduction reaction (ORR), which outperformed commercial Pt/C catalyst. More importantly, by tuning the Fe content and annealing temperature, the trace Fe in dFe–N-C was supposed to form high active FeN4 sites with pyridinic N and played an important role in the excellent electrocatalytic performance for ORR.