Oxygen Reduction In Alkaline Media Research Articles

Catalytic Evaluation of Nanoflower Structured Manganese Oxide Electrocatalyst for Oxygen Reduction in Alkaline Media

An electrochemical nanoflowers manganese oxide (MnO2) catalyst has gained much interest due to its high stability and high specific surface area. However, there are a lack of insightful studies of electrocatalyst performance in nanoflower MnO2. This study assesses the electrocatalytic performances of nanoflower structure MnO2 for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in a zinc–air battery as a bifunctional electrocatalyst. The prepared catalyst was characterized in term of morphology, crystallinity, and total surface area. Cyclic voltammetry and linear sweep voltammetry were used to evaluate the electrochemical behaviors of the as-prepared nanoflower-like MnO2. The discharge performance test for zinc–air battery with a MnO2 catalyst was also conducted. The results show that the MnO2 prepared at dwell times of 2, 4 and 6 h were nanoflowers, nanoflower mixed with nanowires, and nanowires with corresponding specific surface areas of 52.4, 34.9 and 32.4 g/cm2, respectively. The nanoflower-like MnO2 catalyst exhibits a better electrocatalytic performance towards both ORR and OER compared to the nanowires. The number of electrons transferred for the MnO2 with nanoflower, nanoflower mixed with nanowires, and nanowire structures is 3.68, 3.31 and 3.00, respectively. The as-prepared MnO2 nanoflower-like structure exhibits the best discharge performance of 31% higher than the nanowires and reaches up to 30% of the theoretical discharge capacity of the zinc–air battery.

Plasmon-enhanced electrocatalytic oxygen reduction in alkaline media on gold nanohole electrodes

Plasmon-driven chemical transformation has become a promising approach for enhancing sluggish electrocatalytic reactions.

Oxygen Reduction at Palladium Decorated Copper-Molybdenum Oxide Modified Glassy Carbon Electrode

In this study, metal nanoparticles decorated metal oxide electrode was fabricated via electrochemical technique. Firstly, molybdenum/copper oxide was electrochemically deposited on the bare glassy carbon electrode surface. Then palladium nanoparticles were modified on this oxide functionalized surface by consecutive potential cycles. Resulting composite electrode was characterized with scanning electron microscopy. The proposed composite electrode was employed for electrocatalytic oxygen reduction in alkaline media. As compared to bare electrode, the resulting composite surface has tremendous effect on oxygen reduction in terms of accelerated peak current values.

Recent Advances in the Design of 3D PGM-Free Molecular Catalysts for ORR

In the past couple of decades great advancements have been made in the development of PGM-free catalysts based on earth-abundant elements, nitrogen, carbon and transition metals (usually Fe or Co), inspired by biological systems such as porphyrins and phthalocyanines.1-6 In order to overcome the poor stability and low catalytic activity of transition-metal complexes, a new class of high temperature-treated (HT-treated) catalysts, composed of the same elements, i.e., a transition metal, carbon and nitrogen, was developed. Although HT-treated PGM-free catalysts exhibit improved activity and stability, their performance remains inferior to PGM catalysts, calling for further improvements to make them a viable alternative to the state-of-the-art materials. In this work, we designed, synthesized and characterized ORR catalysts based on iron, carbon and nitrogen in a well-defined, high surface-area covalent framework (COF) of aerogels. Aerogels are ultralight, porous materials, with ultra-low density and high void volume (> 97%), also known for their unique physicochemical properties such as high porosity, controllable pore size and surface area, low thermal conductivity, just to name a few.7 The variety of precursors used for aerogel synthesis makes them promising candidates for a wide range of applications in catalysts, capacitors, insulators, absorbents, and many more.8-9 In the context of electrocatalysis of fuel cell reactions, carbon-aerogels have been mostly used so far as catalyst supports for PGM and PGM-free catalysts.10-11 In their in form, aerogels can have ultra-high catalytic site density, high surface area, and tunable physical and chemical structures - all very important features for a heterogeneous catalyst. In this talk, we will discuss the synthesis and electrocatalytic properties of an iron-porphyrin aerogel. 5,10,15,20-(tetra-4-aminophenyl)porphyrin (H2TAPP) and Fe(II) were used as the building blocks of the aerogel, which was later heat-treated at 600 °C to enhance electronic conductivity and catalytic activity while preserving its macro-structure. The resulting material has a very high concentration of atomically dispersed catalytic sites (4.01∙1019 sites cm-3), capable of catalyzing the oxygen reduction reaction in alkaline solution (Eonset = 0.93 V vs. RHE, TOF = 0.2 e- site-1 s-1 at 0.8 V vs. RHE). Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J., A review on non-precious metal electrocatalysts for PEM fuel cells. Energy & Environmental Science 2011, 4 (9), 3167-3192.Zion, N.; Friedman, A.; Levy, N.; Elbaz, L., Bioinspired Electrocatalysis of Oxygen Reduction Reaction in Fuel Cells Using Molecular Catalysts. Advanced Materials 2018, 1800406.Jasinski, R., A new fuel cell cathode catalyst. Nature 1964, 201 (4925), 1212-1213.Levy, N.; Mahammed, A.; Friedman, A.; Gavriel, B.; Gross, Z.; Elbaz, L., Metallocorroles as Non-Precious Metal Electrocatalysts for Highly Efficient Oxygen Reduction in Alkaline Media. ChemCatChem 2016, 8 (17), 2832-2837.Levy, N.; Mahammed, A.; Kosa, M.; Major, D. T.; Gross, Z.; Elbaz, L., Metallocorroles as Nonprecious-Metal Catalysts for Oxygen Reduction. Angew. Chem.-Int. Edit. 2015, 54 (47), 14080-14084.Snitkoff, R. Z.; Levy, N.; Ozery, I.; Ruthstein, S.; Elbaz, L., Imidazole decorated reduced graphene oxide: A biomimetic ligand for selective oxygen reduction electrocatalysis with Metalloporphyrins. Carbon 2019, 143, 223-229.Hrubesh, L. W., Aerogel applications. Journal of Non-Crystalline Solids 1998, 225, 335-342.Mayer, S.; Pekala, R.; Kaschmitter, J., The aerocapacitor: An electrochemical double‐layer energy‐storage device. Journal of the Electrochemical Society 1993, 140 (2), 446-451.Aegerter, M. A.; Leventis, N.; Koebel, M. M., Aerogels handbook. Springer Science & Business Media: 2011.Marie, J.; Berthon-Fabry, S.; Achard, P.; Chatenet, M.; Pradourat, A.; Chainet, E., Highly dispersed platinum on carbon aerogels as supported catalysts for PEM fuel cell-electrodes: comparison of two different synthesis paths. Journal of Non-Crystalline Solids 2004, 350, 88-96.Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y., Highly efficient nonprecious metal catalysts towards oxygen reduction reaction based on three-dimensional porous carbon nanostructures. Chem Soc Rev 2016, 45 (3), 517-531.

Gold/silver bimetal nanoparticles incorporated graphitic carbon nitride nanohybrid materials for oxygen reduction reaction

A facile method for the preparation of bimetal AuAg (gold/silver) alloy nanoparticles (NPs) incorporated graphitic carbon nitride (g-C3N4) nanohybrids (NHs) materials embedded in Nafion matrix (Nf/g-C3N4/AuAg NHs) and its application towards electrocatalytic reduction of oxygen in alkaline medium are reported. The prepared Nf/g-C3N4/AuAg NHs were characterized by using X-ray diffraction (XRD), absorption spectroscopy, high resolution transmission electron spectroscopy (HRTEM), scanning electron microscopy-energy dispersive X-ray spectrophotometer (SEM-EDX), SEM-EDX elemental mapping, SEM-EDX lines scan spectra and cyclic voltammetry (CV) techniques. The absorption spectra of Nf/g-C3N4/AuAg NHs showed the characteristic surface plasmon resonance (SPR) absorption band for AuAg alloy NPs and was found in between the wavelength regions of the monometal NPs (Au and Ag NPs). The bimetal alloy formation was confirmed from the observation of single SPR absorption band and further a blue shifting in the SPR band was observed with increasing Ag content in the g-C3N4/AuAg NHs. The HRTEM images explicitly demonstrated the formation of Au, Ag and alloy AuAg NPs on g-C3N4 surface where the size of the bimetal AuAg NPs are found to be smaller than the corresponding monometals (Au and Ag NPs). The electrochemical characteristics of bimetal AuAg NPs at the Nf/g-C3N4/Au, Nf/g-C3N4/Ag and Nf/g-C3N4/AuAg NHs modified electrodes were studied using cyclic voltammetry. The combination of Nf/g-C3N4 and bimetal AuAg alloy NPs enhanced the electrocatalytic oxygen reduction reaction at the modified electrode. The GC/Nf/g-C3N4/Au75Ag25 modified electrode showed the highest catalytic current among the other controlled NHs materials towards the electrocatalytic oxygen reduction in alkaline medium. The Koutecky-Levich (K-L) plots obtained from the modified RDE studies showed four-electron transfer of oxygen reduction reaction (ORR) at the GC/Nf/g-C3N4/Au75Ag25 modified electrode.

Sequential hydrothermal synthesized Co–Mn-oxide/C electrocatalysts for oxygen reduction in alkaline media

In order to design and synthesize oxygen reduction reaction catalysts with high activity and low cost, a series of Co–Mn-oxide/C catalysts with different Co:Mn ratios have been prepared using a hydrothermal method applied in sequential steps. The monotonically systematic trends of the catalysts’ phases, morphologies and particle sizes have been verified, and the trending of Mn ions and Co ions in different valence states follows the increasing Co:Mn ratio. Electrochemical performance of the catalysts in oxygen reduction reaction results in a volcano-type trend with an optimal Co:Mn ratio of 3 giving the best performance, which is comparable to that of commercial Pt/C. Lastly, a Koutecky-Levich approach has been employed to deduce the electron transfer values, in an attempt to rationalize their selectivity towards the varying 2 and 4 electron pathways. The systematic research is significant for understanding and designing a new generation of non-noble metal oxygen reduction reaction catalysts.

Fe3C nanoparticles-loaded 3D nanoporous N-doped carbon: A highly efficient electrocatalyst for oxygen reduction in alkaline media

High-performance, non-precious metal electrocatalysts have been widely considered among the most prospective candidates to replace Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Herein, we report a synthetic method, involving templating, polymerization and pyrolysis, that produces catalytically active iron carbide (Fe3C) nanoparticles-loaded porous N-doped carbon materials from polyaniline- and Fe(III)-modified mesoporous graphitic carbon nitride (g-C3N4). We also show that the resulting noble metal-free materials exhibit good electrocatalytic activity for ORR, with good onset and half-wave potentials, in O2-saturated alkaline solution. The structure, composition, crystallinity, and electrocatalytic activity of these materials are found to depend on the pyrolysis temperature and the specific components in the precursor. In particular, the material obtained by pyrolysis at 1000 °C, named Fe3C/NC-1000, shows excellent electrocatalytic activity and better performance, in terms of both onset and half-wave potentials, than Pt/C (20 wt% Pt). The material also tolerates the methanol crossover reaction better than Pt/C and shows negligible shift in onset and half-wave potentials to negative values even after use in 3000 cycles of electrocatalysis. This robust, non-noble metal-based carbon material can potentially become a viable alternative to precious metal electrocatalysts for ORR.

Beyond dealloying: development of nanoporous gold via metal-induced crystallization and its electrochemical properties

Nanoporous metals (NPMs) possess a number of intriguing properties that result in NPMs being an important family of nanomaterials for many advanced applications. However, the methods of preparing NPMs are relatively complicated and have many limitations, which have hindered the commercial application of NPMs thus far. By introducing metal-induced crystallization, a solid-phase reaction method for preparing NPMs was developed in this study, which is highly efficient and environmentally friendly. The microstructure of the prepared nanoporous gold (NPG) was characterized on an atomic scale by scanning electron microscopy and high-resolution transmission electron microscopy. The results confirmed that the solid-phase reaction method is an effective alternative means of preparing highly pure NPG. The results of electrochemical tests demonstrated that thus-prepared NPG possesses higher electrocatalytic activity than other types of gold electrodes toward oxygen reduction in alkaline media. The combination of a simple preparation process and higher activity suggests that the developed method may promote the future use of NPG in new energy applications, such as fuel cells and metal-air batteries.

Biomass derivative-based fibrous perovskite electrocatalysts with a hierarchical porous structure for oxygen reduction in alkaline media

The CaMnO3 perovskites have been proposed as promising electrocatalysts for the oxygen reduction reaction (ORR) as substitutes for the noble metal. However, their ORR catalytic activities still need to be further improved. Herein, fibrous CaMnO3 catalysts with hierarchical mesoporous/macroporous structures for the ORR were prepared through a simple ion exchange process with biomass derivative calcium alginate as a precursor. Optimization of the synthesis conditions, in particular, the La doping content, is conducted. The as-prepared CaMnO3 shows superior ORR performance in alkaline media with a limited diffusion density of 6.13 mA cm−2. Furthermore, the ORR performance was further promoted by La-doping. The optimal La-doped CaMnO3 sample exhibits the most positive onset potential of −0.01 V, the most positive half-wave potential of −0.28 V (VS. Ag/AgCl), and a greatly enhanced limited diffusion density of 6.76 mA cm−2. The improvement of catalytic activity for ORR can be attributed to the porous structure, the rich surface chemisorbed oxygen and the redox couple Mn3+/Mn4+. The as-prepared perovskite fibers have potential as low cost and efficient ORR catalysts.

Carbon-based lanthanum nickelate material La2−x−yNdxPryNiO4+δ (x = 0, 0.3, and 0.5; y = 0 and 0.2) as a bifunctional electrocatalyst for oxygen reduction in alkaline media

The kinetics and mechanism of oxygen reduction reaction (ORR) in alkaline medium are studied on lanthanum nickelate materials La2−x−yNdxPryNiO4±δ (x = 0, 0.3 and 0.5; y = 0 and 0.2) using the electrochemical technique of the rotating disk electrode in a 0.5-M solution of NaOH. The oxide powders are synthesized by the citrate–nitrate method. Structural and surface characterizations are performed by X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS), while the morphology is studied by scanning electron microscopy (SEM). Electrochemical studies are carried out by linear voltamperometry, cyclic voltamperometry, and impedance spectroscopy. The doped and undoped electrocatalyst composites (La2−x−yNdxPryNiO4±δ/C), made of the rare earth nickel oxides mixed with carbon black (Vulcan XC-72(C)), are deposited as a thin layer on a glassy carbon substrate. At room temperature, the undoped electrocatalyst La2NiO4±δ material shows single-step kinetics unlike the doped materials. The doping by the rare earths Nd or/and Pr significantly enhances the electrical conductivity of the electrode under air and the diffusion of oxygen. On the other hand, the steric hindrance between the atomic oxygen orbital (π-orbital (O2)–π-orbital (O2)) and the dz2–orbital (Ni)–π-orbital (O2) influences the training model of the liaison (dz2(Ni)–π (O2)). The structure, oxygen adsorption, and oxidation states of the catalyst elements have a large influence on the mechanism and kinetics of the ORR. The LNNO3/C and LNPNO5/C electrocatalysts have better electrocatalytic performances, which allow them to be used as a bifunctional electrocatalyst for the reduction of oxygen in alkaline media.

Patterning Cu nanostructures tailored for CO2 reduction to electrooxidizable fuels and oxygen reduction in alkaline media.

Due to the limited availability of noble metal catalysts, such as platinum, palladium, or gold, their substitution by more abundant elements is highly advisable. Considerably challenging is the controlled and reproducible synthesis of stable non-noble metallic nanostructures with accessible active sites. Here, we report a method of preparation of bare (ligand-free) Cu nanostructures from polycrystalline metal in a controlled manner. This procedure relies on heterogeneous localized electrorefining of polycrystalline Cu on indium tin oxide (ITO) and glassy carbon as model supports using scanning electrochemical microscopy (SECM). The morphology of nanostructures and thus their catalytic properties are tunable by adjusting the electrorefining parameters, i.e., the electrodeposition voltage, the translation rate of the metal source and the composition of the supporting electrolyte. The activity of the obtained materials towards the carbon dioxide reduction reaction (CO2RR), oxygen reduction reaction (ORR) in alkaline media and hydrogen evolution reaction (HER), is studied by feedback mode SECM. Spiky Cu nanostructures obtained at a high concentration of chloride ions exhibit enhanced electrocatalytic activity. Nanostructures deposited under high cathodic overpotentials possess a high surface-to-volume ratio with a large number of catalytic sites active towards the reversible CO2RR and ORR. The CO2RR yields easily electrooxidizable compounds – formic acid and carbon monoxide. The HER seems to occur efficiently at the crystallographic facets of Cu nanostructures electrodeposited under mild polarization.

Microwave-Assisted Biogenic Synthesis of Metal-Decorated Reduced Graphene Oxide and their Electrochemical Properties.

Herein we report a microwave assisted, fungal (Ganoderma lucidum) extract mediated synthesis of noble metal decorated reduced graphene oxide(r-GO). The carbon to oxygen ratio increased from 1.46 in GO to 2.72 in r-GO. The electron rate transfer capabilities of Pt, Pd, Ru, Pt-Pd and Pt- Ru decorated r- GO were tested in ferri-ferro coupling reaction. Excellent electrochemical behaviour were observed in their ability to oxidize hydrazine, reduce H2O2, as well as oxygen reduction in alkaline medium and hydrogen evolution in acidic medium. These reactions represent diverse applications in energy production, storage, electrochemical sensing and electro catalysis, thus the method presented here can be quite useful in diverse applications.

Electrocatalytic Oxygen Reduction in Alkaline Medium at\xa0Graphene-Supported Silver-Iron Carbon Nitride Sites\xa0Generated During\xa0Thermal Decomposition of Silver Hexacyanoferrate

Silver-iron carbon nitride, which has been prepared by pyrolysis (under inert atmosphere) of silver hexacyanoferrate(II) deposited on graphene nanoplatelets, is considered here as electrocatalyst for oxygen reduction in alkaline medium (0.1 M potassium hydroxide electrolyte) in comparison to simple silver nanoparticles and iron carbon nitride (prepared separately in a similar manner on graphene nanoplatelets). The performance of catalytic materials has been examined using such electrochemical diagnostic techniques as cyclic voltammetry and rotating ring-disk electrode voltammetry. Upon application of the graphene nanoplatelet-supported mixed silver-iron carbon nitride catalyst, the reduction of oxygen proceeds at more positive potentials, as well as the amounts of hydrogen peroxide (generated during reduction of oxygen at potentials more positive than 0.3 V) are lower relative to those determined at pristine silver nanoparticles and iron carbon nitride (supported on graphene nanoplatelets), when they have been examined separately. The enhancement effect shall be attributed to high activity of silver toward the reduction/decomposition of H2O2 in basic medium. Additionally, it has been observed that the systems based on carbon nitrides show considerable stability due to strong fixation of metal complexes to CN shells.Graphical ᅟ

MOF-derived Co nanoparticles embedded in N,S-codoped carbon layer/MWCNTs for efficient oxygen reduction in alkaline media

The hydrothermal reaction of cobalt salt in the presence of 4-pyridyl-tetrathiafulvalene-4-pyridyl (4-py-TTF-4-py) and terephthalic acid (PTA) has been employed for the preparation of a novel metal–organic framework (MOF), i.e., (4-py-TTF-4-py)2M2(PTA)4 (M = Co2+). The obtained MOF is then used as a starting material for the synthesis of Co nanoparticles embedded in N,S-codoped carbon layer and supported on multi-walled carbon nanotubes (Co@NSC/MWCNTs) through the high-temperature calcination. Specifically, the calcination leads to the formation of N,S-codoped carbon-coated Co nanoparticles with simultaneous growth on the MWCNTs due to decomposition of the MOF. When used as the electrocatalyst, the Co@NSC/MWCNTs are found to have a higher activity for the oxygen reduction reaction (ORR) and follow a four-electron pathway. The catalytic activity of the Co@NSC/MWCNTs is much higher than that of the pure MWCNTs and the MOFs/MWCNTs. Although the Co@NSC/MWCNTs still exhibit slightly higher overpotential for the ORR, it is indeed more kinetically facile than the commercial Pt/C catalyst, as demonstrated by its higher limiting current density and lower Tafel slope. Additionally, the Co@NSC/MWCNTs also show superior stability and better tolerance to methanol crossover and CO poisoning, compared with those of the commercial Pt/C catalyst. These results strongly suggest that the Co@NSC/MWCNTs could be used as one of the most promising ORR electrocatalysts for the ORR with great potential to replace the Pt/C. The work present here opens up a new route for the design of carbon-integrated ORR electrocatalysts with high performance from a great number of available and yet rapidly growing MOFs.

Seed-mediated atomic-scale reconstruction of silver manganate nanoplates for oxygen reduction towards high-energy aluminum-air flow batteries

Aluminum–air batteries are promising candidates for next-generation high-energy-density storage, but the inherent limitations hinder their practical use. Here, we show that silver nanoparticle-mediated silver manganate nanoplates are a highly active and chemically stable catalyst for oxygen reduction in alkaline media. By means of atomic-resolved transmission electron microscopy, we find that the formation of stripe patterns on the surface of a silver manganate nanoplate originates from the zigzag atomic arrangement of silver and manganese, creating a high concentration of dislocations in the crystal lattice. This structure can provide high electrical conductivity with low electrode resistance and abundant active sites for ion adsorption. The catalyst exhibits outstanding performance in a flow-based aluminum–air battery, demonstrating high gravimetric and volumetric energy densities of ~2552 Wh kgAl−1 and ~6890 Wh lAl−1 at 100 mA cm−2, as well as high stability during a mechanical recharging process.

A family of platinum group metal-free catalysts for oxygen reduction in alkaline media

In this work a series of platinum group metal-free catalysts based on iron, cobalt, copper, nickel, silver and their bimetallic combinations are synthesized and their behavior for oxygen reduction in alkaline media is tested. A phthalocyanine-like ligand supported on carbon black serves as the nitrogen source and carbon support, respectively, for the catalysts. The as-prepared materials are thermally activated by pyrolysis at temperatures ranging from 600 °C to 1000 °C. Oxygen reduction reaction (ORR) activity and stability in 0.1 M KOH are tested by rotating ring disk electrode (RRDE) experiments. Several of the catalysts show ORR activities in the RRDE experiment that are superior to Pt, with Co, Co/Fe and Ag/Co being the best. All catalysts show remarkable stability, even when they are tested under air for 5000 RRDE cycles. RRDE experiments are also used to study the effect of catalyst loading and to determine reaction order. A thorough study of the catalyst performance in a single cell fuel cell test is conducted; exploring different methods to prepare the electrodes and the effects of catalyst loading, the catalyst-to-binder ratio and the removal of excess metal on the cell performance. Different methods of preparation of membrane electrode assemblies are also studied.

Functionalization of multi-walled carbon nanotubes with iron phthalocyanine via a liquid chemical reaction for oxygen reduction in alkaline media

Iron single-atom catalyst in form of iron-nitrogen-carbon structure possesses the excellent catalytic activity in various chemical reactions. However, exploring a sustainable and stable single-atom metal catalyst still faces a great challenge due to low yield and complicated synthesis. Here, we report a functional multi-wall carbon nanotubes modified with iron phthalocyanine molecules via a liquid chemical reaction and realize the performance of similar single-atom catalysis for oxygen reduction reaction. A serial of characterizations strongly imply the structure change of iron phthalocyanine molecule and its close recombination with multi-wall carbon nanotubes, which are in favor of ORR catalysis. Compared to commercial platinum-carbon catalyst, composites exhibit superior activity for oxygen reduction reaction with higher half-wave potential (0.86 V), lower Tafel slope (38 mV dec−1), higher limiting current density and excellent electrochemical stability. The corresponding Zinc-air battery also presents higher maximum power density and discharge stability. Therefore, these findings provide a facile route to synthesize a highly efficient non-precious metal carbon-based catalyst.

Cobalt-Doped Ba2In2O5 Brownmillerites: An Efficient Electrocatalyst for Oxygen Reduction in Alkaline Medium.

A series of compounds with cobalt doping in the indium site of Ba2In2O5 brownmillerites exhibited excellent oxygen reduction activity under alkaline conditions. Doping (25%) retains the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. Further substitution of cobalt in the indium site leads to the loss of a brownmillerite structure, and the compound attains a perovskite structure. Cobalt-doped samples exhibited far better oxygen reduction reaction (ORR) activity when compared to the parent Ba2In2O5 brownmillerite. Among the series of compounds, BaIn0.25Co0.75O3−δ with the highest Co doping and oxygen vacancies randomly distributed in the lattice exhibited the best ORR activity. BaIn0.25Co0.75O3−δ showed a 40 mV positive shift in the onset potential with better limiting current density and a nearly four-electron-transfer reduction pathway when compared to the parent Ba2In2O5 brownmillerite.

Facile Preparation and High Activity of TiO2 Nanotube Arrays toward Oxygen Reduction in Alkaline Media

Four TiO2 electrodes have been investigated for the oxygen reduction reaction (ORR) in 2 M NaOH. Anodic oxidation of Ti-foil was used as the final or the first step in the preparation of four electrodes based on TiO2: amorphous (Ti/AM-TiO2), two anatase forms (Ti/A400-TiO2 and Ti/A550-TiO2), and, finally, lithium-anatase form (Ti/A-Li0.25TiO2), all of them obtained as TiO2 nanotube arrays. The electrodes’ structure was examined by Raman spectroscopy. Cyclic voltammetry and chronoamperometry were employed for the study of kinetics of ORR. Ti/A-Li0.25TiO2 and Ti/A400-TiO2 electrodes showed the highest catalytic activity for ORR with the highest cathodic peak current density and the lowest Tafel slope values. Number of electrons exchanged during ORR at TiO2 electrodes was in the range from 2.3 to 2.8. Additionally, TiO2 electrodes showed good stability as well as acceptable selectivity in the presence of borohydride, methanol or ethanol as typical fuels for fuel cells. Thus this study introduces a simple, low-cost preparation route of TiO2-based electrodes with high efficiency for ORR for energy conversion and storage applications.

A Novel Metal–Organic Framework Route to Embed Co Nanoparticles into Multi-Walled Carbon Nanotubes for Effective Oxygen Reduction in Alkaline Media

Metal–organic framework (MOF) materials can be used as precursors to prepare non-precious metal catalysts (NPMCs) for oxygen reduction reaction (ORR). Herein, we prepared a novel MOF material (denoted as Co-bpdc) and then combined it with multi-walled carbon nanotubes (MWCNTs) to form Co-bpdc/MWCNTs composites. After calcination, the cobalt ions from Co-bpdc were converted into Co nanoparticles, which were distributed in the graphite carbon layers and MWCNTs to form Co-bpdc/MWCNTs. The prepared catalysts were characterized by TEM (Transmission electron microscopy), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), BET (Brunauer–Emmett–Teller), and Raman spectroscopy. The electrocatalytic activity was measured by using rotating disk electrode (RDE) voltammetry. The catalysts showed higher ORR catalytic activity than the commercial Pt/C catalyst in alkaline solution. Co-bpdc/MWCNTs-100 showed the highest ORR catalytic activity, with an initial reduction potential and half-wave potential reaching 0.99 V and 0.92 V, respectively. The prepared catalysts also showed superior stability and followed the 4-electron pathway ORR process in alkaline solution.