Fast Oxygen Reduction Reaction Kinetics Research Articles

Single-atom Al precisely modulate the strain of Pt3Co intermetallic for superior oxygen catalytic performance

Precisely controlling the strain of Pt-based intermetallic to ensure high performance and stability is a massive task for sustainable proton-exchange-membrane-fuel-cells. Herein, the Al single-atom was trapped into the L12-Pt3Co intermetallic core to precisely tailor the strain state on the Pt-skin for fast oxygen reduction reaction kinetics. Theoretical calculations firstly predicted that only tailored tension can accelerate the protonation of O2 on L12-Pt3Co@Pt without creating an additional energy barrier for subsequent oxygen-containing intermediates desorption. Experimentally, Al single-atom confined in the L12-Pt3Co lattice by substituting partial Co occupancy, imposing tailored ∼ 0.2 % tension on Pt-skin compared to the L12-Pt3Co. L12-Al-Pt3Co@Pt/C exhibits enhanced mass activity which is ten-time improvement over commercial Pt/C. More significantly, XAS and DFT results reveal that the Al single-atom can strengthen the Pt-Co bonding, enhancing the stability of L12-Al-Pt3Co@Pt/C in oxygen reduction. This work provides an avenue to design the strain by single-atom for sustainable energy conversion technologies.

Accelerated Confined Mass Transfer of MoS2 1D Nanotube in Photo-Assisted Metal-Air Batteries.

Applying solar energy into energy storage battery systems is challenging in achieving green and sustainable development, however, the efficient progress of photo-assisted metal-air batteries is restricted by the rapid recombination of photogenerated electrons and holes upon the photocathode. Herein, a 1D-ordered MoS2 nanotube (MoS2-ONT) with confined mass transfer can be used to extend the lifetime of photogenerated carriers, which is capable of overcoming the challenge of rapid recombination of electron and holes. The tubular confined space cannot only promote the orderly separation and migration of charge carriers but also realize the accumulation of charge and the rapid activation of oxygen molecules. The concave surface of MoS2-ONT can improve the carrier separation ability and prolong the carrier lifetime. Meanwhile, the ordered tubular confined space can effectively realize the rapid transfer of charge, ion, and oxygen. Under light irradiation, a fast oxygen reduction reaction kinetics of 70mWcm-2 for photo-assisted Zn-air battery is achieved, which is the highest value reported for photo-assisted Zn-air batteries. Significantly, the photo-assisted Li-O2 battery based on MoS2-ONT also shows superior rate capability and other exciting battery performance. This work shows the universality of the confined carrier separation strategy in photo-assisted metal-air batteries.

Highly efficient electrosynthesis of H2O2 in acidic electrolyte on metal-free heteroatoms co-doped carbon nanosheets and simultaneously promoting Fenton process

Recently electrochemical synthesis of H2O2 through oxygen reduction reaction (ORR) via 2e− pathway is considered as a green and on-site route. However, it still remains a big challenge for fabricating novel metal-free catalysts under acidic solutions, since it suffers from high overpotential due to the intrinsically week *OOH adsorption. Herein, a co-doped carbon nanosheet (O/NC) catalyst toward regulating O and N content was synthesized for improving the selectivity and activity of H2O2 electrosynthesis process. The O/NC exhibits outstanding 2e− ORR performance with low onset potential of 0.4 V (vs. RHE) and a selectivity of 92.4% in 0.1 mol/L HClO4 solutions. The in situ electrochemical impedance spectroscopy (EIS) tests reveals that the N incorporation contributes to the fast ORR kinetics. The density functional theory (DFT) calculations demonstrate that the binding strength of *OOH was optimized by the co-doping of oxygen and nitrogen at certain content, and the O/NCCOOH site exhibits a lower theoretical overpotential for H2O2 formation than OCCOOH site. Furthermore, the promoted kinetics for typical organic dye degradation in simultaneous electron-Fenton process on O/NC catalyst was demonstrated particularly for broadening its environmental application.

Cascade Anchoring Strategy for Fabricating High-Loading Pt Single Atoms as Bifunctional Catalysts for Electrocatalytic Hydrogen Evolution and Oxygen Reduction Reactions.

Carbon supports containing single-atomically dispersed metal-Nx (denoted as MSAC-NxCy, x, y: coordination number) have attracted increasing attention due to their superb performance in heterogeneous catalysis. However, large-scale controllable preparation of single-atom catalysts (SACs) with high concentration of supported metal-Nx is still a big challenge because of the metal atom agglomeration during synthesis at high density and temperatures. Herein, we report a stepwise anchoring strategy from a 1,10-o-phenanthroline Pt chelate to an Nx-doped carbon (NxCy) with isolated Pt single-atom catalysts (PtSAC-NxCy) containing Pt loadings up to 5.31 wt % measured via energy-dispersive X-ray spectroscopy (EDS). The results show that 1,10-o-phenanthroline Pt chelate predominantly contributes to the formation of chelate single metal sites that bind tightly to platinum ions to prevent metal atoms from aggregating, resulting in high metal loading. The high-loading PtSAC-NxCy exhibits a low hydrogen evolution (HER) overpotential of 24 mV at 0.010 A cm-2 current density with a relatively small Tafel gradient of 60.25 mV dec-1 and excellent stable performance. In addition, the PtSAC-NxCy catalyst shows excellent oxygen reduction reaction (ORR) catalytic activity with good stability, represented by the fast ORR kinetics under high-potential conditions. Theoretical calculations show that PtSAC-NC3 (x = 1, y = 3) offers a lower H2O activation energy barrier than Pt nanoparticles. The adsorption free energy of a H atom on a Pt single-atom site is lower than that on a Pt cluster, which is easier for H2 desorption. This study provides a potentially powerful cascade anchoring strategy in the design of other stable MSAC-NxCy catalysts with high-density metal-Nx sites for the HER and ORR.

Preparation and alkaline stability of polyethylene composite hydroxide exchange membranes with different cations

Hydroxide exchange membrane fuel cells (HEMFCs) are a cost-effective alternative to proton exchange membrane fuel cells due to the advantages on fast oxygen reduction reaction kinetics, low cost and weak corrosion. Hydroxide exchange membrane (HEM) is the core component for HEMFC. At present, lacking highly robust and chemically stable HEMs is the main challenge to realizing durable HEMFCs. Polyethylene composite HEMs are considered as the-state-of-the-art HEM candidates due to their excellent mechanical properties, thus allowing for the use of ultrathin HEMs in HEMFCs to achieve a high cell and stack performance. Yet, the chemical stability of polyethylene composite HEM still needs to be improved for more durable HEMFC. In this study, we report that the preparation and chemical stability understanding of three polyethylene composite HEMs (named AEH-TMA, AEH-DMP and AEH-TMI) functionalized with trimethylamine (TMA), N-methylpiperidine (DMP) and 1, 2, 4, 5-tetramethylimidazole (TMI), respectively. We find that AEH-TMI exhibits a low conductivity and inferior chemical stability in comparison to AEH-TMA and AEH-DMP. To gain insight into chemical stability behavior, we have also prepared three organic chemicals (named Bn-TMA, Bn-DMP and Bn-TMI) as cations model by reacting benzyl chloride with TMA, DMP and TMI cations and elaborately studied their chemical stability in various water content systems. The results show that the overall chemical stability of cations can be strongly improved with increasing the surrounding water, and Bn-TMI suggests a lower chemical stability than Bn-TMA and Bn-DMP at high water content condition.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N 5 Site for Enhanced Electrocatalytic Reaction

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N ₅ Site for Enhanced Electrocatalytic Reaction

Maximizing Fe-N exposure by tuning surface composition via twice acid treatment based on an ultrathin hollow nanocarbon structure for highly efficient oxygen reduction reaction

• A newly developed hollow structural Fe-N-C catalyst is synthesized by a template-free method. • Fe-N sites are embedded in an ultrasmall, ultrathin, and interconnected porous carbon structure. • The exposure and utilization of Fe-N sites was maximized by twice acid treatment. • The optimal catalyst (HNP-16) exhibited higher E 1/2 and faster ORR kinetics than Pt/C. • HNP-16 delivered a P max of 37.6 mW cm −2 in a membraneless DFFC, 140% higher than Pt/C. Fe-N-C oxygen reduction reaction (ORR) catalysts are still limited in poor ORR activity due to low Fe-N exposure and utilization. Here, we report a newly developed Fe-N-C catalyst with Fe-N sites embedded in a micro/mesopore-interconnected, ultrasmall (5–45 nm), ultrathin (1.3 nm), and hollow nanocarbon structure, which is synthesized by a facile, scalable, and template-free method by using cheap and safe reagents. Moreover, the exposure of Fe-N is maximized through twice acid treatment to tune the surface composition of the catalysts. The mass sites density of the optimal catalyst HNP-16 reached 45.0 μmol g −1 , much higher than that of untreated HNP-0 (7.4 μmol g −1 ) and common treated HNP-1 (12.2 μmol g −1 ). Benefiting from a collective contribution of abundant surface Fe-N active sites and structural advantages, HNP-16 exhibits higher half-wave potential and faster ORR kinetics than commercial Pt/C. Detailed electrochemical analysis revealed the relationships between coordination and ORR activity. The higher content of Fe-N results in faster ORR kinetics while O-related species lead to an undesired 2-electron ORR pathway. When HNP-16 was employed as the cathode catalyst in a membraneless direct formate fuel cell, it delivered an accelerated ORR current response and achieved an unprecedented maximum power density of 37.6 mW cm −2 , which was 1.4 times higher than that of Pt/C. The superior performance of HNP-16 is attributed not only to the highly exposed Fe-N sites but also to the rapid mass transfer provided by the favorable hollow structure.

CoN4 active sites in locally distorted carbon structure for efficient oxygen reduction reaction via regulating coordination environment

Adjusting the coordination environment of active sites for oxygen reduction reaction (ORR) is a desired method to realize efficient energy conversion. Herein, we synthesized a single atom Co catalyst (Co-SA/NC) with CoN4 active sites in locally distorted carbon configuration via a coordination-intercalation assisted strategy. The distinctive existing form of CoN4 structure has been confirmed according to X-ray absorption fine spectroscopy. The Co-SA/NC displays satisfactory ORR performance with higher half-wave potential of 0.89 V and kinetic current density (Jk) of 15.4 mA cm−2 at 0.85 V in alkaline solution, than those of Pt/C (0.87 V and 8.7 mA cm−2). Density functional theory calculations demonstrate that locally distorted CoN4 sites are instrumental in adsorption/desorption of oxygen species, promoting the reaction kinetics of ORR. Furthermore, the fast ORR kinetics is proved by small Tafel slope of 57.9 mV/dec. Applying Co-SA/NC as air electrode in Al-air battery, it presents high specific capacity (1992.7 mAh gAl−1) and energy density (2466.7 Wh kgAl−1) at 200 mA cm−2. The electrocatalyst with unique coordination configuration for M-N4 active sites at atomic level will have wide applications in regulating its intrinsic activity.

Heterostructured simple perovskite nanorod-decorated double perovskite cathode for solid oxide fuel cells: Highly catalytic activity, stability and CO2-durability for oxygen reduction reaction

Apart from conventional composite materials, in situ exsolution for constructing the heterostructure is one of the most effective strategies to design the high-performance electro-catalysts. Herein we report a novel heterostructured simple perovskite nanorod-decorated A site-deficient double perovskite PrBa0.94Co2O5+δ (SPN-A-PBC) cathode, synthesized by an in situ exsolving process from A site-deficient double perovskite PrBa0.94Co2O5+δ (A-PBC). The results demonstrate a highly electro-catalytic activity of the SPN-A-PBC cathode toward oxygen reduction reaction (ORR) for intermediate-temperature solid oxide fuel cells (IT-SOFCs), achieving a very low and stable polarization resistance of ˜0.025 Ω cm2 at 700 °C in air. The anode-supported single cell with this heterostructured cathode delivers a maximum power density of 1.1 W cm−2 at 700 °C and a superior steady operation over 120 h at a loading voltage of 0.6 V. Furthermore, the SPN-A-PBC electrode exhibits a good tolerance to CO2. When tested in air with 6 vol% CO2 at 700 °C, the SPN-A-PBC electrode still maintains a stable polarization resistance of ˜0.078 Ω cm2. The unique catalytic activity of the SPN-A-PBC cathode for ORR may be attributed to extended active sites, abundant interface defects, enhanced redox property and oxygen mobility. The fast ORR kinetics and excellent durability in air with CO2 highlight the potential of SPN-A-PBC as a potential functional material in the energy conversion devices.

Fe3Se4/FeSe heterojunctions in cornstalk-derived N-doped carbon framework enhance charge transfer and cathodic oxygen reduction reaction to boost bio-electricity generation

Sluggish kinetics of oxygen reduction reaction (ORR) on air-cathode of microbial fuel cells (AC-MFCs) is one of the main obstacles for energy loss. In this study, nitrogen-doped Fe3Se4/FeSe/partially-graphitized carbon (Fe3Se4/FeSe/NPGC) composites as non-precious-metal air-cathode (ORR) catalysts are obtained using waste biomass (cornstalk cores) as raw material. As carbonization temperature increases (800–950 °C), the crystalline phase transition between Fe3Se4 and FeSe is strengthened to form the Fe3Se4/FeSe heterojunctions. The highest power density (1003 mW m−2) and durability (decline of 7.8% after 105 d operation) are obtained by Fe3Se4/FeSe/NPGC (850 °C) cathode in AC-MFCs, which are higher than those of Pt/C (840 mW m−2, 52.4%). The high ORR activity of Fe3Se4/FeSe/NPGC (850 °C) is partly attributed to the large specific surface area (356.68 m2 g−1) and porous structure. Doped N atoms (pyridinic N, pyrrolic N and graphitic N) in carbon skeleton enhance the charge delocalization of C atoms to reduce the electron loss to enhance the electron utilization via a four-electron (4e−) ORR pathway. Fe3Se4/FeSe heterojunctions should greatly promote the charge transfer and oxygen dissociation efficiencies. The highly-conductive NPGC skeleton also contributes to the efficient charge transfer. The good long-term durability of AC-MFCs with Fe3Se4/FeSe/NPGC (850 °C) cathode is mainly ascribed to its fast ORR kinetics, which still generates a small amount (below 10.0%) of H2O2 (•OH and •O2−) intermediate to inhibit the electrogenic microbe growth on cathode surface. This work not only provides the fundamental studies on carbon-supported transition-metal selenides for ORR, but also provides a new kind of promising alternatives for precious metal-based electrodes for AC-MFCs.

Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4 were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T' and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr1.6Sr0.4CuO4 is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La0.6Sr0.4Co0.8Fe0.2O3-δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance.

Synthesis and electrochemical characterization of binary carbon supported Pd3Co nanocatalyst for oxygen reduction reaction in direct methanol fuel cells

Binary carbon supports are applied to make the membrane electrode assembly (MEA) of passive direct methanol fuel cells (PDMFCs) and their comprehensive electrochemical characterization is considered. Electrocatalytic properties for the oxygen reduction reaction (ORR) are evaluated using polarization curves and electrochemical impedance spectroscopy (EIS) in PDMFC. The best efficiency is obtained when the mass ratio of multi-walled carbon nanotubes (MWCNTs) and Vulcan XC-72R (VC) is 25:75. The results of the electrode kinetic parameters indicate that introduction of MWCNTs as a secondary support delivers high available surface area, good electronic conductivity, and fast ORR kinetics. In addition to the significant role of Pd3Co active surface area on kinetic of the system, the improvement of kinetic parameters is also associated with the decrease of activation energy for ORR on binary-support electrode resulting in a notable enhancement of kinetic parameters. An 8-h lifetime test is performed for the single cell to evaluate the durability in which the MEA is made using MWCNT/VC with the mass ratio of 25:75. The test results show that the mentioned ratio of MWCNT and VC support results in highest stability and durability.

La2O2CO3 Encapsulated La2O3 Nanoparticles Supported on Carbon as Superior Electrocatalysts for Oxygen Reduction Reaction.

Constructing nanoscale hybrid materials with unique interfacial structures by using various metal oxides and carbon supports as building blocks are of great importance to develop highly active, economical hybrid catalysts for oxygen reduction reaction (ORR). In this work, La2O2CO3 encapsulated La2O3 nanoparticles on a carbon black (La2O2CO3@La2O3/C) were fabricated via chemical precipitation in an aqueous solution containing different concentrations of cetyltrimethyl ammonium bromide (CTAB), followed by calcination at 750 °C. At a given CTAB concentration 24.8 mmol/L, the obtained lanthanum compound nanoparticles reach the smallest particle size (7.1 nm) and are well-dispersed on the carbon surface. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the formation of La2O2CO3 located on the surface of La2O3 nanoparticles in the hybrid. The synthesized La2O2CO3@La2O3/C hybrid exhibits a significantly enhanced electrocatalytic activity in electrocatalysis experiments relative to pure La2O3, La2O2CO3, and carbon in an alkaline environment, by using the R(R)DE technique. Moreover, its long-term stability also outperforms that obtained by commercial Pt/C catalysts (E-TEK). The exact origin of the fast ORR kinetics is mainly ascribed to the La2O2CO3 layer sandwiched at the interface of carbon and La2O3, which contributes favorable surface-adsorbed hydroxide (-OH(-)(ad)) substitution and promotes active oxygen adsorption at the interfaces. The unique covalent -C-O-C(═O)-O-La-O- bonds, formed at the interfaces between La2O2CO3 and carbon, can act as active sites for the improved ORR kinetics over this hybrid catalyst. Therefore, the fabrication of lanthanum compound-based hybrid material with an unique interfacial structure maybe open a new way to develop carbon-supported metal oxides as next-generation of ORR catalysts.

Doped and Decorated Graphene Foam Electrocatalysts

Graphene is an ideal material for electrochemical applications due to high electrical conductivity, large surface area, and stability over a wide electrochemical potential window. However graphene is expensive; it is difficult to control porosity (and thereby mass diffusion); and it is difficult to dope effectively. Simple carbon foams with large micron-scale pores, very thin walls, and very large surface area (e.g. 2500 m2/g) can be synthesized at low cost by thermal decomposition of sodium ethoxide.[1,2] Crucially, by decomposing nitrogen-containing alkoxides, nitrogen-doped graphene powders are made.[3] These do not contain transition metal contamination, as is the case with carbon nanotubes and carbon black. Therefore they can be used to probe the fundamental oxygen reduction reaction (ORR) activity of nitrogen-doped carbon in acid – a relatively under-represented research topic, plagued by contamination issues.[4,5] We record surprisingly high mass activity and onset potential for the ORR, with high electron transfer number (3.6). The negligible metal content was confirmed by ICP-AES. This work shows that 4-electron ORR is possible even in the absence of transition metals, most likely at (or adjacent to) tertiary nitrogen sites.[6] The activity in alkaline medium is also of interest, due to the development of new alkaline ion exchange membranes and the faster ORR kinetics. The nitrogen-doped graphene foams presented here display negligible degradation in cyclic voltammograms over 60,000 load potential cycles, demonstrating the applicability of such metal-free non-precious electrocatalysts in application that require long lifetimes.[7] [1] S. M. Lyth, H. Shao, J. Liu, K. Sasaki, Int. J. Hydrogen Energy, 2014, 39, 376[2] J. Liu, D. Takeshi, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F838[3] S. M. Lyth, Y. Nabae, N. M. Islam, et al., eJournal of Surface Science and Technology, 2012, 10, 29-32.[4] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Nanosci. Nanotechnol. 2012, 12, 4887[5] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Electrochem. Soc. 2011, 158, B194[6] J. Liu, D. Takeshi, D. Orejon, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F544[7] J. Liu, K. Sasaki, S. M. Lyth, ECS Trans. 2014, 64, 1161 Figure 1

Highly ionic-conductive crosslinked cardo poly(arylene ether sulfone)s as anion exchange membranes for alkaline fuel cells

Anion exchange membrane fuel cells (AEMFCs) have become one of the most promising energy-conversion devices due to the potential of adopting non-noble metal catalysts and faster oxygen reduction reaction kinetics in an alkaline operating environment. Here, a series of anion exchange membranes with high ionic-conductivity and low swelling ratio were prepared from imidazolium-functionalized crosslinked cardo poly(arylene ether sulfone)s for AEMFCs. Two oligomers, tertiary amine-containing poly(arylene ether sulfone) (PES-DA) and bromomethyl-containing poly(arylene ether sulfone) (BPES), are synthesized and then mixed at −40°C. They are spontaneously inter-crosslinked at room temperature to form dense anion exchange membranes (AEMs), in which the tertiary amine groups of PES-DA rapidly react with the bromomethyl groups of BPES to form the crosslinked bonds. This avoids the use of crosslinking agents and the expense of functional groups. The resulting membranes show a high hydroxide conductivity up to 82.4mScm−1 at 80°C and a slight swelling low to traditional AEMs due to its crosslinked network structure. An open circuit voltage (OCV) of 0.82V and a maximum power density of 92.1mWcm−2 are achieved in a H2/O2 single cell. The as-prepared membranes have a great potential for the application in AEMFCs.

Accelerated oxygen exchange kinetics on Nd2NiO(4+δ) thin films with tensile strain along c-axis.

The influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was investigated at the surface of Nd2NiO4+δ (NNO). Nanoscale dense NNO thin films with tensile, compressive and no strain along the c-axis were fabricated by pulsed laser deposition on single-crystalline Y0.08Zr0.92O2 substrates. The ORR kinetics on the NNO thin film cathodes was investigated by electrochemical impedance spectroscopy at 360-420 °C in air. The oxygen exchange kinetics on the NNO films with tensile strain along the c-axis was found to be 2-10 times faster than that on the films with compressive strain along the c-axis. A larger concentration of oxygen interstitials (δ) is found in the tensile NNO films compared to the films with no strain or compressive strain, deduced from the measured chemical capacitance. This is consistent with the increase in the distance between the NdO rock-salt layers observed by transmission electron microscopy. The surface structure of the nonstrained and tensile strained films remained stable upon annealing in air at 500 °C, while a significant morphology change accompanied by the enrichment of Nd was found at the surface of the films with compressive strain. The faster ORR kinetics on the tensile strained NNO films was attributed to the ability of these films to incorporate oxygen interstitials more easily, and to the better stability of the surface chemistry in comparison to the nonstrained or compressively strained films.

Reducibility of Co at the La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4hetero-interface at elevated temperatures

The fast kinetics of oxygen reduction reaction (ORR) at oxide hetero-structures made of La0.8Sr0.2CoO3 and (La0.5Sr0.5)2CoO4 (LSC113/214) attracted great interest to enable high performance cathodes for solid oxide fuel cells. The aim of this work is to uncover the underlying mechanism of fast ORR kinetics at the LSC113/214 system from a defect chemistry and electronic structure perspective. X-ray photoelectron spectroscopy with depth profiling was used to compare the reducibility of the Co cation and the valence band offset in the LSC113/214 multilayer (ML) and in single phase LSC113 and LSC214 films. At 250 °C, the Co 2p core-level photoelectron spectra showed the presence of Co2+ across the ML interfaces in both the LSC113 and LSC214 layers. While this is similar to the Co valence state of LSC113 single phase films, it is contrary to the single phase LSC214 films which had only the higher oxidation state of cobalt, Co3+. The greater reducibility of Co in LSC214 in the ML structure compared to that of Co in the LSC214 single phase film was attributed to electron donation and transfer of oxygen vacancies from LSC113 across the interfaces, and it is one mechanism to enhance the oxygen reduction activity at the LSC113/214 hetero-structure.

Electrocatalytic oxygen reduction on single-walled carbon nanotubes supported Pt alloys nanoparticles in acidic and alkaline conditions

The objective of this study is to improve the catalytic activity of platinum by alloying with transition metal (Pd) in gas diffusion electrodes (GDEs) by oxygen reduction reaction (ORR) at cathode site and comparison of the acidic and alkaline electrolytes. The high porosity of single-walled carbon nanotubes (SWCNTs) facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is also evident that SWCNTs enhance the stability of the electrocatalyst. Functionalized SWCNTs are used as a means to facilitate the uniform deposition of Pt on the SWCNT surface. The structure of SWCNTs is nearly perfect, even after functionalization, while other types of CNTs contain a significant concentration of structural defects in their walls. So catalysts supported on SWCNTs are studied in this research. The electrocatalytic properties of ORR were evaluated by cyclic voltammetry, polarization experiments, and chronoamperometry. The morphology and elemental composition of Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The catalytic activities of the bimetallic catalysts in GDEs have been shown to be not only dependent on the composition, but also on the nature of the electrolytes. The GDEs have shown a transition from the slow ORR kinetics in alkaline electrolyte to the fast ORR kinetics in the acidic electrolyte. The results also show that introduction of Pd as transition metal in the Pt alloys provides fast ORR kinetics in both acidic and alkaline electrolytes. The performance of GDEs with Pt–Pd alloy surfaces towards the ORR as a function of the alloy’s overall composition and their behavior in acidic electrolyte was also studied. These results show that the alloy’s overall composition and also the nature of the electrolytes have a large effect on the performance of GDEs for ORR.

Enhancement of electrocatalytic O2 reduction on carbon nanotube-supported Pt alloys nanoparticles in gas diffusion electrodes

The use of Pt binary and ternary alloys prepared by alloying of Pt with transition metals, as catalysts for fabricating of gas diffusion electrodes (GDEs) is reported. Electrocatalytic properties of oxygen reduction reaction (ORR) were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. The morphology of the GDEs and elemental compositions of the Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The results indicate that the introduction of Pd and Cd as transition metals in Pt alloys provides fast ORR kinetics. The performance of GDEs with Pt–Pd alloy surfaces for ORR was also studied as a function of overall composition and surface atomic distribution of Pt alloys. The results also show that alloying of Pt with transition metals and various amounts of Pt and Pd in the binary catalysts has a large effect on the performance of GDEs for ORR.

Carbon nanotubes as a secondary support of a catalyst layer in a gas diffusion electrode for metal air batteries

In this paper, we report the use of binary carbon supports (carbon nanotubes (CNTs) and active carbon) as a catalyst layer for fabricating gas diffusion electrodes. The electrocatalytic properties for the oxygen reduction reaction (ORR) were evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) in an alkaline electrolyte. The binary-support electrode exhibits better performance than the single-support electrode, and the best performance is obtained when the mass ratio of carbon nanotubes and active carbon is 50:50. The results from the electrode kinetic parameters indicate that the introduction of carbon nanotubes as a secondary support provides high accessible surface area, good electronic conductivity, and fast ORR kinetics. Furthermore, the effect of CNT support on the electrocatalytic properties of Pt nanoparticles for binary-support electrodes was also investigated by different loading-reduction methods. The electrocatalytic activity of the binary-support electrodes is improved dramatically by Pt loading on CNT carbon support, even at very low Pt loading. Additionally, the EIS analysis results indicate that the process of ORR may be controlled by diffusion of oxygen in the electrode thin film for binary-support electrodes with or without Pt catalyst.