Oxygen Reduction Reaction Current Density Research Articles

Carbon Supported Platinum-Molybdenum Alloy Nanoparticles for Oxygen Reduction Reaction

Fuel cells are gaining importance in the emerging area of power generation. However, sluggishness of the cathodic oxygen reduction reaction (ORR) and usage of expensive electrocatalysts are hindering its widespread application. Hence, an effort has been made in the present study to synthesize efficient electrocatalysts based on Pt-Mo alloys with varying atomic ratios (0-100 at. %) by thermal decomposition method. The synthesized samples were characterized using XRD, SEM, TEM and XPS techniques. The electrocatalytic activity for ORR was measured using cyclic voltammetry and rotating disk electrode for all the samples and Pt-Mo (1:1) electrocatalyst performed better among the synthesized electrocatalysts with ORR current density of 63 mA/cm2 at an applied potential of 0.6 V vs. Hg/HgSO4. The present study suggests that Pt-Mo studied are proven to be a superior catalyst than a costly Pt catalyst with high ORR activity.

Factors Affecting ORR Activity of Nb-Added TiOx Catalyst Using Carbon Support for PEFC

1. IntroductionCarbon-supported platinum catalysts used for the cathode of present polymer electrolyte fuel cells (PEFCs) have some problems such as a large amount of platinum use and low durability under operation condition. Therefore, we have attempted to develop substitute of platinum based on group 4 and 5 transition metal oxides. We found that stable oxygen vacancies on the surface and lattice distortion of these metal oxides could act as active sites of oxygen reduction reaction (ORR)1). On the other hand, cup-stacked carbon nanotubes (CSCNTs) coated with TiO2 have been investigated as durable supports for a platinum-based catalysts2). We already revealed that Nb-added titanium oxide with lattice distortion showed some ORR activity1). Then, we thought that CSCNTs coated with Nb-added titanium oxide might show definite ORR activity. In this study, we prepared CSCNTs coated with Nb-added TiOx and evaluated their behavior to investigate the factors which affect the ORR activity.2. ExperimentalThe CSCNTs (GSI Creos) without amorphous carbon on the surface was added to 2-propanol and dispersed ultrasonically, then a desired amount of Ti isopropoxide and Nb ethoxide were added with stirring. A sufficient amount of water mixed with 2-propanol were added to proceed hydrolysis and stirred overnight. Suction filtration was performed to obtain a precursor. In this experiment, Nb was added into TiO2 with an atomic% of 10 against Ti, and a loading of oxide was adjusted to 30 wt% of total catalyst. The precursor was heat-treated under an Ar gas containing 4% hydrogen at desired temperature and time to obtain Nb-added TiOx catalysts, designated as Nb-added TiOx/CSCNTs.The Nb-added TiOx/CSCNT was supported on a glassy carbon rod to make a working electrode. Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. ORR current based on oxides was obtained by subtracting the reduction current of carbon from the reduction current of the entire catalyst. The oxygen reduction current density, iORR, was normalized by the mass of the oxides.3. Results and discussionFigure 1 shows the dependence of iORR at 0.6 V of Nb-added TiOX/CSCNTs on heat treatment temperature. The results of Nb-added TiOx powders prepared without carbon were also plotted. Because the electro-conductivities of the Nb-added TiOx powders were very low, Ketjenblack was mixed as an electro-conductive material to evaluate the ORR activity. The current value of Nb-added TiOx/CSCNT is significantly improved compared to oxide powder. This is because the electron supply increased due to the deposition of oxides on the CSCNT. The highest ORR current density was observed at 800 oC.Figure 2 shows the SEM images of Nb-added TiOX/CSCNTs. Amorphous TiO2 covers with the surface of CSCNTs in the unheated catalyst. At 700 ° C and 800 ° C, the oxide particles of around 10 nanometers were supported with high dispersion. However, at 900 ° C, the particles aggregated to become larger.Figure 3 shows the Ti 2p XPS spectra of Nb-added TiOx/CSCNTs heat-treated at 600-1000 °C. Ti2p peak shifted to the lower binding energy from 600 ° C to 800 ° C and to the higher energy from 800 ° C to 1000 ° C. The proportion of low-valence state Ti3+ increased as the temperature increased from 600 to 800 ° C. The reason why the shift to the higher binding energy occurred above 800 ° C was that the oxide surface prepared at higher heat-treatment temperature was oxidized by exposure of the air.Figure 4 shows the relationship between ratio of Ti3+ / (Ti3+ + Ti4+) and ORR current density at 0.6 V. The ratio of Ti3+ was calculated from the Ti 2p XPS spectra. The iORR at 0.6 V increases as ratio of Ti3+ increases. The ratio of Ti3+ indicates the presence of oxygen vacancies on the surface of the oxide. Therefore, it is considered that the increase of oxygen vacancies on the oxide surface strongly affected the ORR activity.AcknowledgementThe authors wish to thank the supply of the CSCNT from GSI Creos and the support of the New Energy and Industrial Technology Development Organization (NEDO), JSPS grants-in-aid for scientific research, Suzuki Foundation, and Tonen General Sekiyu Research / Development Encouragement & Scholarship FoundationReferences1) A. Ishihara, et. al., Electrochim. Acta, 283,1779 (2018).2) F. Ando, et. al., Electrochim. Acta, 404, 232 (2010) Figure 1

Modeling Temperature- and Humidity-Dependent Kinetics of Oxygen Reduction Reaction

Designing active catalysts for oxygen reduction reaction (ORR) is one of the major challenges for polymer electrolyte fuel cells. Density functional theory (DFT) calculations have made significant contributions on understanding ORR pathways, rate-limiting step and catalytic activity on various transition metal surfaces 1-5. However, all these theoretical studies are for reactions at room temperature in aqueous liquid electrolytes. The condition is significantly different from actual operation conditions of fuel cells [60 to 80 °C and 30 to 100 % relative humidity (RH)]. Although the DFT models reasonably explain experimental results obtained by the rotating disk electrode (RDE) at room temperature in aqueous liquid electrolytes, the reaction mechanism and catalytic activity at the operating conditions can be significantly different. Here, we examine effects of temperature and RH on the ORR by using a micro-kinetic model. The examination indicates that both the rate-limiting step and material-dependence of the catalytic activity are strongly influenced by both temperature and RH.The theoretical analyses were carried out by extending a micro-kinetic model proposed by Hansen et al5 to higher temperature and lower RH conditions. For the extension, following assumptions were adopted. Free energies for reaction intermediates (adsorbed O, OH and HO2) are independent of temperature and relative humidity (RH) and fixed at the values in the original model.An additional reaction barrier due to the solvent reorganization, which was introduced to explain the experimentally observed prefactor in the original model, is entirely attributed to be enthalpic. The assumption (1) can be partially validated by relatively small entropies of the adsorbed species comparing with gaseous species. Because the assumption (2) is not verified, we also carried out the same simulation assuming that the solvent reorganization barrier is fully entropic [we define this assumption as (3)].Figures 1 (a) and (c) show the calculated ORR current density at 0.9V vs. reversible hydrogen electrode (RHE) under 1 atm O2 under the assumption (2) as a function of the binding energy of OH adsorbate vs. that of Pt (111). Figures 1 (b) and (d) show the calculated ORR current density in the same way as Figures 1 (a) and (c) under the assumption (3). In all conditions, in the right leg of the volcano-plot, where the OH binding energy is larger than the optimal point, the reaction is limited by the formation of the reaction intermediates. On the other hand, in the left leg of the volcano, the reaction is limited by the removal of the OH adsorbate. Regardless the type of the assumption [(2) or (3)], both the optimal catalytic activity and the optimal OH binding energy decrease with the rise in the temperature from 25 to 80 °C [see Figs. 1 (a) and (b)]. The low catalytic activity at high temperature is attributed to the drop in the reversible potential of ORR at high temperature. The shift in the optimal OH binding energy is attributed to the destabilization of OH adsorbate relative to the non-adsorbed species, which hold larger entropies than the adsorbates. The drop in RH enhances the optimal catalytic activity, while the opposite shift is observed for the optimal OH binding energy [see Figs. 1 (c) and (d)]. The both trends stem from the lowering of the water activity, which promotes the OH removal from the surface. All in all, our theoretical analyses indicate that the optimal OH binding energy strongly depends on both temperature and RH, indicating that the optimal catalyst at low temperature and in aqueous liquid electrolytes may not be optimal anymore at high temperature and low RH. Although our theoretical analyses strongly rely on non-verified assumptions (1)-(3) and the DFT model adopting the ice-like water,1 which is unlikely a proper model of the electrolyte at high temperature and low RH, the observation indicates the significance of more precise understanding of temperature-, RH- and material-dependence of ORR.In the presentation, we will present experimental data on temperature- and RH-dependent ORR activity and discuss the validity of the theoretical model. References J. K. Nørskov et al., J. Phys. Chem. B, 108, 17886 (2004).J. Greeley et al., Nat. Chem., 1, 552 (2009).J. X. Wang et al., J. Phs. Chem. A, 111, 12702 (2007).R. Jinnouch et al., Phys. Chem. Chem. Phys., 13, 21070 (2011).H. A. Hansen et al., J. Phys. Chem. C, 118, 6706 (2014). Figure 1

(Invited) Benchmarking Carbon-Supported Pt-Based Oxygen Reduction Reaction Electrocatalysts for PEMFC Cathodes

The present performance of state-of-the-art proton exchange membrane fuel cells (PEMFC) enable their commercialization, as demonstrated by the recent release of fuel-cell powered vehicles (forklifts by Plug Power, FCEV Miraï by Toyota, etc.). Now, all PEMFC manufacturers need to reduce the PEMFC systems initial cost and to enhance their durability and reliability in operation to reach the market requirements and industrial sustainability. To this goal, one must develop highly-efficient oxygen reduction reaction (ORR) electrocatalysts, which was claimed to be obtained by numerous groups in a recent past [1-6]. Unfortunately, the scientific community still struggles to obtain the best performance of these materials in operating PEFMCs: their practical performance in membrane electrode assembly (MEA) do not approach those expected from an analytical approach performed using the rotating disk electrode setup, RDE (the most widely-used setup to assess a material’s ORR performance).There are several reasons that may explain why RDE data are not relevant to account for MEA data. Firstly, in RDE, the O2 reactant is fed as gas dissolved in the electrolyte (slow diffusivity and solubility), hence the mass-transfer rate and related limiting current density are ca. 3 orders of magnitude lower than in real PEMFC conditions. Secondly, the activity data is only extracted at high potential values in RDE (E > 0.85 V vs RHE), although PEMFC optimal performance is reached in the 0.6 – 0.8 V vs RHE cathode potential range. Thirdly, the proton conduction can be limiting in the thin layer of ionomer covering the electrocatalyst nanoparticles in MEA, which could limit their operation, especially at the high current densities at stake in MEA. In essence, one is not sure that the electrocatalysts are used in optimized conditions in MEA, as emphasized mass-transport of reactants (both H+ and O2) to the catalytic sites is not granted for present ionomers and MEA structure [7, 8]. Recent results obtained in newly-developed electrochemical characterization setups like the floating electrode (FE) [9], the gas-diffusion electrode (GDE) [10] or differential cell (DC), could enable to reach much higher ORR current densities at the lab scale and therefore better match MEA data, a prerequisite for relevant benchmarking.In the present contribution, these setups will be used to characterize three state-of-the-art ORR electrocatalysts (50 wt% Pt/Vulcan XC72, 50 wt% Pt3Co/Vulcan XC72 and 30 wt% Pt/graphitized carbon black), and comparison will be made with large single-cell PEMFC data in automotive conditions. We will show whether the “RDE promises” of alloyed electrocatalysts (measured in RDE configuration) do maintain in FE, GDE and DC, and whether the “advanced electrocatalysts” still exhibit their plain potential in real PEMFC catalytic layer configuration.

Factors Affecting ORR Activity of Nb-Added TiOx Catalyst Using Carbon Support for PEFC

Cup-stacked carbon nanotubes (CSCNTs) were covered with niobium-added titanium oxide by hydrolysis method to increase effective surface area of the oxides. In addition, reductive heat-treatment was performed to produce the active sites for the oxygen reduction reaction (ORR). The use of CSCNTs as supports was found to be effective to form the electron conductive path. The ORR currents of the Nb-added titanium oxide supported by CSCNTs significantly larger than those of Nb-added titanium oxide powder mixed with Ketjenblack. On the other hand, the ORR current at 0.6 V had maximum at the heat-treatment temperature of 800°C. We focused on the ratio of Ti3+ of the catalysts and found that the positive correlation between the ratio of Ti3+ and the ORR current density at 0.6 V. Therefore, it was found that the formation of the oxygen vacancies on the oxide surface strongly affected the ORR activity.

Waste engine oil derived porous carbon/ZnS Nanocomposite as Bi-functional electrocatalyst for supercapacitor and oxygen reduction

Abstract Herein, we report the effective conversion of waste automobile engine oil into porous activated carbon/ZnS nanocomposite. Initially, the waste engine oil was thermally decomposed under an argon atmosphere at 700 °C and followed by acid treatment with a 10% piranha solution to form activated carbon/ZnS (ACZS) nanocomposites. The ACZS was used as electrode active material for symmetric supercapacitor and electrocatalyst for oxygen reduction reaction (ORR) studies. It is interesting to note that, the ACZS electrode performed well for supercapacitor delivered a high specific capacitance of 241 F/g at 1 A/g current density. The symmetric supercapacitor cell fabricated using ACZS electrodes delivered a high specific energy and specific power of 13.8 Wh/kg and 4500 W/kg with an exceptional electrochemical stability of 95% for 10,000 cycles, respectively. The electrochemical RRDE studies shows higher ORR current density of ACZS electro-catalyst (4.46 mA/cm2) than that for the HiSPEC Pt/C catalyst (3.91 mA/cm2).

Cobalt‑nitrogen‑carbon nanotube co-implanted activated carbon as efficient cathodic oxygen reduction catalyst in microbial fuel cells

The low catalytic activity of activated carbon (AC) toward oxygen reduction reaction (ORR) in air cathode severely limits the electricity production of microbial fuel cells (MFCs). In this work, a novel cobalt‑nitrogen‑carbon nanotube co-implanted activated carbon catalysts ([email protected]) were prepared by growth of Co/Zn-based metal organic frameworks on AC followed by thermal pyrolysis. The [email protected] catalyzed ORR via an efficient four-electron pathway in pH-neutral medium, and exhibited superb ORR catalytic activity comparable to commercial Pt/C. As compared with pristine AC, implantation of Co, N and carbon nanotubes into [email protected] improved the ORR current density and exchange current density of air cathodes by 120% and 73% respectively. The maximum power density of MFCs achieved 2045 ± 43 mW m−2 by using the optimized [email protected] pyrolyzed at 850 °C, which was 65% higher than that of pristine AC. The superb ORR performance of [email protected] originated from the synergistic effect of abundant Co-Nx, Pyridinic-N and Graphitic-N active sites, high graphitization degree, well dispersed Co nanoparticles and improved micro-mesoporous coexistence structure, which enabled high ORR catalytic activity and rapid mass transfer at the three-phase boundary. This work provides a new routine for large-scale synthesis of highly-efficient ORR electrocatalysts based on AC and metal organic frameworks, which has promising potential for the application in practical MFCs.

Development of Porous Pt Electrocatalysts for Oxygen Reduction and Evolution Reactions.

Porous Pt electrocatalysts have been developed as an example of carbon-free porous metal catalysts in anticipation of polymer electrolyte membrane (PEM) fuel cells and PEM water electrolyzers through the assembly of the metal precursor and surfactant. In this study, porous Pt was structurally evaluated and found to have a porous structure composed of connected Pt particles. The resulting specific electrochemical surface area (ECSA) of porous Pt was 12.4 m2 g−1, which was higher than that of commercially available Pt black. Accordingly, porous Pt showed higher oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity than Pt black. When the activity was compared to that of a common carbon-supported electrocatalyst, Pt/ketjen black (KB), porous Pt showed a comparable ORR current density (2.5 mA cm−2 at 0.9 V for Pt/KB and 2.1 mA cm−2 at 0.9 V for porous Pt), and OER current density (6.8 mA cm−2 at 1.8 V for Pt/KB and 7.0 mA cm−1 at 1.8 V), even though the ECSA of porous Pt was only one-sixth that of Pt/KB. Moreover, it exhibited a higher durability against 1.8 V. In addition, when catalyst layers were spray-printed on the Nafion® membrane, porous Pt displayed more uniform layers in comparison to Pt black, showing an advantage in its usage as a thin layer.

An RDE Approach to Investigate the Influence of Chromate on the Cathodic Kinetics on 7XXX Series Al Alloys under Simulated Thin Film Electrolytes

The effect of sodium chromate on the cathodic current availability pertinent to the micro-galvanic-induced corrosion of high-strength AA7XXX alloys under simulated thin electrolyte films representative of atmospheric conditions was investigated utilizing a combination of electrochemical and surface characterization techniques. The rotating disk electrode technique provided a means to simulate the effects of water layer thickness to differentiate thin film conditions from full immersion conditions, and enabled the study of the mass-transport-limited oxygen reduction reaction (ORR) on AA7XXX alloys as a function of chromate concentration. The ORR current density decreased by up to two orders of magnitude upon addition of 10 mM chromate, however, the degree of inhibition was observed to depend on the Cu content of the alloy. Chromate was reduced irreversibly to form a Cr3+-rich film on the alloy surface that blocked cathodic sites and hindered ORR. This film was confirmed by X-ray photoelectron spectroscopic characterization of the chemistry and thickness of the chromate-induced layer formed on the specimens after exposure to chromate. The layer was approximately 13 nm in thickness and consisted of mixed Cr3+/Cr6+ oxides with some metallic Cr. Studies on a Pt electrode demonstrated the intrinsic ability of chromate as an effective inhibitor for ORR.

Catalyst Particle Scale Analysis By Transport and Reaction Simulation on 3D Nano-Scale Images for PEMFC

Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3]. Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations. AcknowledgementThis work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Preparation and properties of PMMA-modified Sr2Ni0.4Co1.6O6 composites

Nano-microsphere polymathy methacrylate (PMMA)–modified nanoparticulate double perovskite Sr2Ni0.4Co1.6O6 was prepared. It is found that the modification can reduce the Sr2Ni0.4Co1.6O6 particle size and make the distribution more uniform. The electrochemical performance tests show that the maximum current density of oxygen reduction reaction (ORR) of single Sr2Ni0.4Co1.6O6 is 0.1830 A cm−2 (− 0.6 V vs. Hg/HgO) and the maximum current density of oxygen evolution reaction (OER) is 0.2677 A cm−2 (1 V vs. Hg/HgO), while the maximum current density of ORR of PMMA-modified Sr2Ni0.4Co1.6O6 (Sr2Ni0.4Co1.6O6 to PMMA mass ratio of 1:3) is 0.2687 A cm−2 (− 0.6 V vs. Hg/HgO) and the maximum current density of OER is 0.3579 A cm−2 (1 V vs. Hg/HgO). AC impedance analysis also shows that PMMA-modified Sr2Ni0.4Co1.6O6 has the lowest charge transfer resistance. This work certifies that PMMA-modified Sr2Ni0.4Co1.6O6 catalyst is better than Sr2Ni0.4Co1.6O6 catalyst.

Atomically Dispersed Bimetallic FeNi Catalysts as Highly Efficient Bifunctional Catalysts for Reversible Oxygen Evolution and Oxygen Reduction Reactions

AbstractBimetallic atomically dispersed FeNi catalysts anchored on N‐doped carbon nanotube with catalyst loading of 2–7 wt % with different Fe : Ni ratio have been developed as highly active and stable bifunctional catalyst for reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for metal air batteries via a modified one‐pot synthesis method. Compared with atomically dispersed single Fe and Ni catalysts, the bimetallic FeNi catalysts exhibit outstanding performance for reversible OER and ORR, achieving a low potential gap (ΔE) of 0.81 V to deliver an OER current density of 10 mA cm−2 and an ORR current density of 3 mA cm−2. The FeNi electrodes also show a much better stability in the cyclic tests, compared to that of the state‐of‐the‐art Pt/C and Pt/C+Ir/C electrodes for reversible OER and ORR. The high performance is likely due to the significantly enhanced OER activity contributed by the introduction of Ni atoms, forming bridged FeNi bimetallic dual atom active sites for OER. This study provides a new platform for the development of highly active bimetallic atomic catalysts based bifunctional electrocatalysts for metal‐air batteries. The modified one‐pot synthesis methods demonstrated in this study can also be applicable to other atomically dispersed catalysts on CNTs or graphenes.

Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core-Shell Co3O4@NiFe LDH Catalysts for Zn-Air Batteries: Effects of Interfacial Cation Valences.

The rational design of excellent electrocatalysts is significant for triggering the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable metal-air batteries. Hereby, we report a bifunctional catalytic material with core-shell structure constructed by Co3O4 nanowire arrays as cores and ultrathin NiFe-layered double hydroxides (NiFe LDHs) as shells (Co3O4@NiFe LDHs). The introduction of Co3O4 nanowires could provide abundant active sites for NiFe LDH nanosheets. Most importantly, the deposition of NiFe LDHs on the surface of Co3O4 can modulate the surface chemical valences of Co, Ni, and Fe species via changing the electron donor and/or electron absorption effects, finally achieving the balance and optimization of ORR and OER properties. By this core-shell design, the maximum ORR current densities of Co3O4@NiFe LDHs increase to 3-7 mA cm-2, almost an order of magnitude increases compared to pure NiFe LDH (0.45 mA cm-2). Significantly, an OER overpotential as low as 226 mV (35 mA cm-2) is achieved in the designed core-shell catalyst, which is comparable to and/or even better than those of commercial Ir/C. Hence, the primary zinc-air battery employing Co3O4@NiFe LDH as an air electrode achieves a high specific capacity (667.5 mA h g-1) and first-class energy density (797.6 W h kg-1); the rechargeable battery can show superior reversibility, excellent stability, and voltage gaps of ∼0.8 V (∼60% of round-trip efficiency) in >1200 continuous cycles. Furthermore, the flexible quasi-solid-state zinc-air battery with bendable ability holds practical potential in portable and wearable electronic devices.

Electrospun Carbon Nanofiber Sprinkled with Co3O4 as an Efficient Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium

AbstractDesign and development of an efficient and earth abundant electrocatalyst for improving the kinetics of oxygen reduction reaction (ORR) is considered as important task especially in renewable energy sector and energy‐intensive appliances. In particular, developing a low cost, environmental benign, robust and stable ORR catalyst still remains a challenging task due to its sluggish kinetics. Here in this work we developed a simple and environmentally green electro‐spinning technique for decoration of cobalt oxides (Co3O4) on polyacrylonitrile (PAN) fibers to generate a hybrid nanostructure of carbon nanofiber‐ Co3O4 (CNF‐ Co3O4) during calcinations and employed resultant electrocatalyst towards ORR. The CNF provided active surface to seed as well an anchoring spot for nanocrystals and act as a conducting network. The resulting CNF‐Co3O4 hybrid displayed a supeior ORR current density that surpassed bare CNF and occurred via a 4 electron direct reduction pathway. It may due to the as formed heterogeneous interface during the decoration of Co3O4 on CNF leads to enhanced conductivity and thereby decreases ORR overpotential as well as enhanced reaction rate at CNF‐Co3O4 surface.

CoNi-doped C–N/CNT nanocomposites as cathodic catalysts of neutral Zn–air battery

A neutral zinc–air battery has unique characteristics. One of the key issues for its normal operation is how to ensure that the cathode catalyst can effectively electrocatalyze oxygen reduction reaction (ORR) in a neutral solution. Here, we report a simple preparation of CoNi-doped C–N/CNT nanocomposite catalysts (CoNi/C–N/CNT) by pyrolysis of a mixture of nickel/cobalt salt, carbon nanotube (CNT), dicyandiamide (DCD), and ethylenediamine. The prepared CoNi/C–N/CNT nanocatalysts are composed of CoNi nanoparticles, hollow tubular C–N nanocomposites obtained from CoNi salts and DCD, and CNT added. Among the synthesized catalysts, the Co3Ni2/C–N/CNT45 has a high and stable ORR current density of 6.2 mA cm−2 in 0.5 mol L−1 KNO3 solution. Neutral Zn–air battery with the carbon paper coated with the CoNi/C–N/CNT nanocomposite as an air electrode and metal Zn as an anode was assembled. For the catalyst Co3Ni2/C–N/CNT45, the neutral Zn–air battery in 0.5 mol L−1 KNO3 solution presents an open-circuit voltage of 1.16 V and a maximum power density of 50.1 mW cm−2. Moreover, the constant discharging current density of 100 and 150 mA cm−2 can last respectively 88 and 28 h until complete consumption of the Zn foil anode.

Nitrogen/Cobalt Co‐doped Mesoporous Carbon Microspheres Derived from Amorphous Metal‐Organic Frameworks as a Catalyst for the Oxygen Reduction Reaction in Both Alkaline and Acidic Electrolytes

AbstractNitrogen/cobalt co‐doped carbon (Co/N−C) catalysts are the best candidates to replace their Fe‐based analogues for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs), owing to the absence of radical‐induced PEMFC membrane/ionomer degradations that are commonly encountered in the Fe/N−C system. Herein, we present an amorphous Co‐metal organic framework (MOF) route for the fabrication of N/Co co‐doped mesoporous carbon spheres by directly pyrolyzing amorphous Co‐MOF microspheres. The as‐synthesized Co/N−C‐800 catalyst (heat‐treated at 800 °C) has a unique spherical architecture with short nanotubes on its surface and simultaneously possesses a relatively large Brunauer‐Emmett‐Teller surface area (381 m2 g−1) and a high N content (4.95 at%). Owing to these benefits, the Co/N−C‐800 catalyst displays good ORR performance in both alkaline and acidic electrolytes, that is, the ORR onset potential (Eo), half‐wave potential (E1/2), and current density at 0.4 V (vs. RHE, J@0.4) are 0.981 V (vs. RHE), 0.872 V (vs. RHE), and 5.47 mA cm−2, respectively, in alkaline media, and 0.863 V (vs. RHE), 0.707 V (vs. RHE), and 5.29 mA cm−2, respectively, in acidic electrolyte. More importantly, the Co/N−C‐800 catalyst also exhibits excellent poison tolerance in acidic electrolytes.

Structure-Function Relationships of PGM-Free ORR Electrocatalysts from Density Functional Theory

Pyrolyzed platinum group metal free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts are a promising class of earth-abundant materials for low-temperature polymer electrolyte fuel cell (PEFC) cathodes. Understanding of the atomic scale structure of PGM-free ORR active sites remains a key focus of research efforts with the aim of improving performance of these materials. The pyrolysis process leads to highly heterogeneous catalyst systems which complicates direct active site study. In particular, (i) understanding how these active sites give rise to ORR activity, (ii) how they interact with the environment during material degradation, (iii) their interactions with probe molecules for the purposes of active site quantification, and (iv) their spectroscopic signatures are all important aspects governed by atomic scale structure. Through the use of density functional theory (DFT), we investigate these four structure-function relationships. ORR activity, one of the greatest challenges faced by PEFCs, is explored via several binding energy parameterized descriptor models. These models include the computational hydrogen electrode (CHE) model which yields thermodynamic limiting potentials, and the linearized Gibbs energy relationship (LGER) model which yields reversible potentials for reaction steps. Additionally, kinetics of OOH bond dissociation via nudged elastic band (NEB) DFT calculations are also considered as this has been proposed by some groups to be rate determining for pathways that include binding to local C sites. Combined, these models give insight into how local arrangement of atoms affects reaction pathways and enables exploration of varied reaction pathways on and local to the active site. Durability of PGM-free electrocatalysts remains a key challenge in these materials. While a variety of degradation mechanisms have been proposed, the relation between environment and degradation mechanism has not been firmly established, complicating any mitigation strategies that might be applied. Through the use of an automated ab initio molecular dynamics (AIMD)-based model, the kinetics of bond breaking local to the active site is explored. Resulting degraded structures can then be studied via the activity models previously mentioned to show how such degradation affects calculated ORR activity descriptors. Additionally, the use of reaction rate models applied to activity loss curves can also enable some discrimination of degradation mechanism indicating either a single autocatalytic process or two mechanisms with different temporal behavior. Another issue faced by PGM-free electrocatalysts is the lack of a method for quantifying the number of ORR active sites. Unlike Pt-based systems where integration of charge transferred in the H adsorption/desorption region can give information about density of active sites, no such method has been fully established for determining the number of PGM-free active sites, a value required to deconvolute turn over frequency from ORR current densities. Promising approaches include use of probe molecules that, if bound to ORR active sites (and only ORR active sites) could provide insight into how many active sites are in a given sample. This is highly dependent on the specificity of binding for these molecules which can be explored via binding energy calculations with DFT. Finally, DFT can also be a valuable tool for understanding spectroscopic signatures of highly ORR active materials. The main issue faced in such studies is focusing on just the active sites which generally requires considering changes in the Fe states, particularly for so called atomically dispersed catalysts that exhibit the highest ORR activities to date. X-ray adsorption spectroscopy (XAS, in particular XANES and EXAFS) can give information about local structure and the state of Fe with and without probe molecules. DFT and related theoretical approaches can simulate these spectra as a function of local environment. Vibrational calculations with and without probe molecules give insight into Fe-specific nuclear resonance vibrational spectroscopy (NRVS). Calculation of energy density at the Fe nucleus and electric field gradient gives input regarding isomer shift and quadrupole splitting, giving much needed interpretation of Mössbauer spectroscopy, especially in instances where no standard exists or changes in measured parameters with ligands/probes are required. Combined, these coupled experimental/theoretical approaches give insight into the nature of active sites in PGM-free systems. In particular, in this contribution we will focus on the presence of spontaneously evolved ligands that, using DFT, are shown to be likely contributors to electrocatalyst activity in-situ and experimental signatures thereof.

Recent Mechanistic Understanding of Oxygen Reduction Reaction over Pt-Supported 3D Carbon Nanofiber Catalysts

For both proton exchange membrane and direct methanol fuel cells, oxygen reduction reaction (ORR) is a performance-determining step. While Pt has been established as a standard bearer in ORR catalyst development, practical long-term applications require continuing effort to pursue active, stable, as well as affordable materials as alternatives to precious metals. In this talk, carbon-based vertically aligned carbon nanofibers (VACNFs), essentially a stack of conical graphitic structures, will be discussed as highly promising ORR catalysts. In an alkaline medium, at -0.4 V applied potential, the current density measured on VACNF cathode is twice more than that measured with commercial Pt catalyst (Pt/C). Molecular insights have been gained through the use of Density functional theory (DFT) calculations to: (1) reveal interactions of ORR intermediate species at the catalytically active sites; and (2) elucidate the origin of catalytic activities of VACNF under reaction conditions. Specifically, semi-periodic fishbone-like stacked graphene sheets, bare or terminated with H and OH, have been employed to understand the impact on ORR mechanism and corresponding reactivities. Bindings of O2, O, and OH at various configurations obtained from DFT are then used as descriptors to assess catalyst performance against simple Pt (111) and Pt (211) surfaces. In addition, Pt nanocatalysts supported on VACNF (Pt/VACNF), which can push the ORR current density even higher, will be discussed as well. Again, with DFT, a model with Pt anchored at the VACNF edges was proposed to investigate the functionalities of Pt/VACNF for alkaline ORR reactions. In this talk, both associative and dissociative ORR pathways will be analyzed based on the established models. In particular, the overall computed ORR thermodynamics are shown to be favored toward Pt/VACNF than Pt (111), consistent with experimental results. Nevertheless, the unique interactions of ORR intermediates such as O*, OOH*, and OH* with catalytic active VACNF edge sites, revealed by DFT calculations, suggest intriguing but also complex electrochemistries.

Ionic Conductivity over Metal/Water Interfaces in Ionomer‐Free Fuel Cell Electrodes

AbstractIon conduction and oxygen reduction reaction (ORR) kinetics at metal/water interfaces are important in determining the performance of proton exchange membrane fuel cells (PEMFCs). However, the coupled ion transport and electrochemical reactions in ionomer‐free domains of catalyst layers are still unclear. In this work, we deconvolute the ion transport and oxygen reduction reaction (ORR) by studying metal/water interfaces of platinum (Pt) and gold (Au) ionomer‐free porous cathodes. In our experiments, a conventional membrane electrode assembly (MEA) was used for electrochemical characterization of ionomer‐free electrodes. Cyclic voltammetry (CV) and polarization curve measurements at different relative humidity (RH) conditions show that active surface area and ORR current density increases with RH. We also use microstructured electrode scaffold (MES) apparatus for measurement of the ionic conductivity in the cathode. The results show that the ionic conductivity increases with the RH and water coverage. The potential dependence of ionic conductivity for Au approximately follows the classical Guoy‐Chapman electrical double layer (EDL) behavior, while the ionic conductivity of the Pt electrode did not show a strong potential dependence, indicating that it could also be affected by other mechanisms, such as formation of surface oxide and diffusion of adsorbed hydrogen on its surface.

Pure nitrogen-doped graphene aerogel with rich micropores yields high ORR performance

How good can the oxygen reduction reaction (ORR) performance of metal-free 3D carbon material be, if the ratio of micropores to all the micropores and mesopores in it can be greatly increased? To answer this question, a nitrogen doped graphene aerogel (NGA) with rich micropores is prepared by a facial one-pot assembly. Pores especially rich micropores introduced by the dense packing of graphene oxide (GO) precursor with small size facilitated by the crosslinking agent dopamine (DA) in walls of NGA with a large specific surface area (1631 m2/g) shorten the diffusion time and decrease the transport distance of O2 inside the pores; a high N content (7.21 at%) increases the adsorption and diffusion of O2 in NGA; consequently increase the ORR speed, rendering so far the largest ORR current density of NGA among reported metal-free carbon catalysts and Pt-based catalysts, promising for applications in fuel cells.