Intrinsic Oxygen Reduction Reaction Research Articles

Nitrogen-doped graphene anchored with mixed growth patterns of CuPt alloy nanoparticles as a highly efficient and durable electrocatalyst for the oxygen reduction reaction in an alkaline medium.

A highly active and durable CuPt alloy catalyst with trigonal bipyramidal and truncated cube-type mixed morphologies, anchored on the nitrogen-doped graphene (NGr) surface (CuPt-TBTC/NGr), was prepared by a simple and fast method. The obtained CuPt alloy showed improved oxygen reduction reaction (ORR) activity, with a 30 mV positive shift in the half-wave potential value, as compared to the state-of-the-art Pt/C catalyst in a 0.1 M KOH solution. The CuPt alloy with the trigonal bipyramidal morphology possesses porous type inter-connected sides, which help to achieve improved mass transport of oxygen during the ORR. The exposure of the (111) plane of the CuPt alloy further improved the catalytic activity towards the dioxygen reduction in alkaline media. The ORR activity of the NGr-supported CuPt alloy was found to be dependent on the reaction time, and improved activity was obtained on the material derived at a reaction time of 90 min (CuPt-TBTC/NGr-90). The material synthesized at a lower or higher reaction time than 90 min resulted in a partially formed trigonal bipyramidal morphology with more truncated cubes or agglomerated trigonal bipyramidal and truncated cubes with closed type structures, respectively. Along with the high intrinsic ORR activity, CuPt-TBTC/NGr-90 displayed excellent electrochemical stability. Even after repeated 1000 potential cycling in a window ranging from 0.10 to 1.0 V (vs. RHE), the system clearly outperformed the state-of-the-art Pt/C catalyst with 15 and 60 mV positive shifts in the onset and half-wave potentials, respectively. CuPt-TBTC/NGr-90 also exhibited 2.1 times higher mass activity and 2.2 times higher specific activity, compared to Pt/C at 0.90 V (vs. RHE). Finally, a zinc-air battery fabricated with the alloy catalyst as the air electrode displayed a peak power density of 300 mW cm-2, which is much higher than the peak power density of 253 mW cm-2 obtained for the state-of-the-art Pt/C catalyst as the air electrode.

Developing Cobalt Doped Pr0.5Ba0.5MnO3-δ Electrospun Nanofiber Bifunctional Catalyst for Oxygen Reduction Reaction and Oxygen Evolution Reaction

The development of energy storage and conversion devices provides a beneficial approach for the renewable energy application, which can help relieve the severe reliance on fossil fuels and also address problems related to global climate change. Currently, the efficiencies of energy conversion devices such as the metal–air batteries and fuel cells are mainly limited by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. Developing catalytically active and cost effective catalyst for the ORR and OER is of prime importance. So far, noble metals and their alloys, such as Pt and Pt-Pd, have been exclusively used as electrochemical bifunctional catalyst in ORR and OER due to their superior catalytic activity. However, the high cost, limited availability, poor durability and sluggish electron transfer kinetics of noble metal based bifunctional catalysts have impeded the practical application of metal-air batteries. Therefore, the discovery of cost effective, catalytically active alternative bifunctional catalyst such as non-precious metals, carbonaceous materials, and transition metal oxides is highly desirable. Perovskite oxide (ABO3) possesses a unique electronic structure and chemical defect properties, and has been demonstrated to be a promising non-precious metal catalyst among the transition metal oxides in the application of metal air batteries and alkaline fuel cells. It is known that the intrinsic electrochemical catalytic activity is mainly determined by the B site cation in perovskite oxide. Extensive research conducted by Yang et al. demonstrated that the ORR and OER catalytic activities of perovskite oxides follow a volcanic relationship with the filling of electrons in antibonding orbitals [1]. Among the typical LaMO3 (M= Mn, Co, Fe, Ni, Cr), LaMnO3 and LaCoO3 have exhibited highest ORR and OER catalytic activities, respectively. In addition, due to the desirable electronic properties of the perovskite oxides, strategies such as cation partial substitution and oxygen non-stoichiometry formation could, therefore, be utilized as the effective approaches to fine-tune the catalytic properties and to achieve a better bifunctional activity [2,3]. To enhance the bifunctional catalytic performance, improved specific surface area as well as enhanced oxygen channel are desired. Researchers have found that electrochemical catalysts with porous nanofiber structures could favor the ORR and OER pathways by providing uniform O2 electrolyte distribution, and beneficial oxygen diffusion channels. Zhao et al. have synthesized mesoporous La0.5Sr0.5CoO2.91 nanowires through the multistep microemulsion method, showing significant enhanced catalytic performance [4]. Xu et al. have demonstrated that the porous La0.75Sr0.25MnO3 nanotube catalyst fabricated through facile electrospinning technologies provides favorable advantages to the availability of the catalytic active sites in the organic solvent electrolyte in Li-O2 batteries [5].Herein, we developed a bifunctional perovskite catalyst towards both ORR and OER in alkaline solution. Cobalt cations were doped into Pr0.5Ba0.5MnO3-δ perovskite to achieve the higher intrinsic ORR and OER activities by engineering the structure symmetry, electronic and the oxygen vacancy defects of the material. The bifunctional catalyst with a unique porous nanofiber structure was fabricated by electrospinning technology (Figure.1). A single phase perovskite oxide Co doped Pr0.5Ba0.5MnO3-δ was obtained after sintering the electrospun precursor at high temperature. The ORR and OER activities of the composite catalysts consisting 50 wt% of the as-synthesized perovskite oxides and 50 wt% carbon black were investigated in 0.1 KOH with rotating disk electrode (RDE). Significant enhancement in ORR and OER performance was achieved via using the composite catalyst with Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black with respect to the reduced overpotential and improved current density. In particular, the Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black composite demonstrated enhanced electron transfer number in the ORR process, indicating preferable dominance of four electron transfer pathway. Moreover, the Co doped Pr0.5Ba0.5MnO3-δ composite exhibited high stability during the cycling tests, indicating its promising applications in metal-air batteries.

Dimensionless Model Analysis of PEFC Cathode

INTRODUCTION Although extensive studies have been conducted on PEFC behavior and a lot of knowledge has been published, understanding the PEFC is still not easy since the system is considerably complicated. By investigating a dimensionless form of equations describing the cathode catalyst layer (CL), the cathode model was simplified and a few intrinsic moduli which control the dimensionless rate profile were proposed (1). The impact of convection on the oxygen reduction reaction (ORR) rate and some case studies are demonstrated. DIMENSIONLESS MODEL The ORR rate is proportional to the oxygen partial pressure at fixed cathode emf. The emf dependency follows a Tafel equation. Oxygen balance in cathode catalyst layer is expressed as follows: d(N g y O – C g D eO dy O/dz) / dz = –r vc[1] where N g is the total gas flux, y O is the oxygen mole fraction, and D eOis the effective oxygen diffusivity (2). Proton flux can be calculated by the reaction stoichiometry. Proton potential follows the Ohm’s law. With the 1D model of cathode CL, 12 variables including boundary conditions have to be specified so as to determine the profiles of partial pressure, emf, reaction rate, etc. Dimensionless forms of model equations were derived, which include the following dimensionless moduli we propose: M O (C) m = δ (C) (k vc m RT / D eO)1/2[2] M p (C) m = δ (C) {4F k vc m p Oc / (σ ep b c)}1/2 [3] where σ ep is the effective proton conductivity, b c is the Tafel slope, δ (C) is the CL thickness, k vcm is the ORR rate constant per unit volume of cathode CL at the PEM–CL boundary and p Oc is oxygen partial pressure at the CL–GDL boundary. k vcm is the greatest rate constant and p Oc is the highest partial pressure in the CL. Therefore, k vcm p Oc represents the intrinsic ORR rate without transport resistance. M O (C) m and M p (C) m respectively represent the ratio of reaction performance to oxygen transport performance and the ratio of reaction to proton transport. A Peclet number which represents the ratio of convection to diffusion of oxygen is also required for formulation: P O (C) m = δ (C) N A(M) /(C g D eO) [4] where N A (M)is the net water flux through the PEM, which equals the total gas flux at the PEM–CL boundary. The dimensionless ORR rate profile is determined by only 4 dimensionless parameters, M O (C) m , M p (C) m , P O (C) m , and y Oc . The dimensionless system greatly reduces the complication of system. RESULTS AND DISCUSSION Define the cathode effectiveness factor F ec as a ratio of the ORR rate to the intrinsic ORR rate without transport resistance. Figure 1 shows that increase in transport resistance reduces the effectiveness factor. Typical values of M O (C) m and M p (C) m in usual cells are around 1. When M p (C) m is high, proton transport is slow, which increases the cathode emf and reduces the ORR rate. If M O (C) m >> M p (C) m , oxygen transport is suppressed and ORR takes place near the oxygen inlet, the GDL side. As the convective flow from PEM to GDL increases, oxygen distribution is shifted towards the GDL, reducing the effectiveness factor as shown in Fig. 2. Even at quite low M O (C) m and M p (C) m of 0.01, the convection can reduce the effectiveness factor. When M O (C) m and M p (C) m are high, the impact of convection is not remarkable since the reaction takes place only near the boundaries. When back diffusion of water in the PEM is dominant, P O (C) m can be negative. F ec reaches 1 under reaction control. Figure 3 shows a case study in which the CL thickness δ (C) is varied with a fixed Pt loading per volume. As δ (C) increases, the Pt amount increases proportionally but the transport resistance increases M O (C) m and M p (C) m and the effectiveness factor decreases as shown in Fig. 1. As a result, the increase in the current density is limited. Since the increase in δ (C)raises both moduli, it overtakes the increase of Pt; a maximum current appears in Fig. 3. Employing the proposed dimensionless moduli, the effects of dimensions and operating conditions can be estimated quantitatively without expensive simulation. Acknowledgement This work is a part of the project by the New Energy and Industrial Technology Development Organization (NEDO), Japan. REFERENCES 1. M. Kawase et al., 24th Int. Symp. Chem. React Eng. (Minneapolis, June 2016). 2. M. Kawase et al., ECS Trans. 16(2), 563–573 (2008). Figure 1

Investigations of Performance and Durability of PtNi-Alloy Based MEAs Utilizing In-Cell Diagnostics

The successful commercialization of Polymer Electrolyte Fuel Cells (PEFCs) system requires highly active oxygen-reduction reaction (ORR) electrocatalysts to meet the cost, performance, and durability targets for automotive applications. Although significant progress has been made in the past decade, Pt/C catalyst has the limitation of high material cost and insufficient ORR activity. Platinum-nickel alloy catalysts with core-shell structure have been demonstrated by multiple research groups to have higher intrinsic ORR activity, which can exceed the U.S. DOE 2015 ORR activity target, and also have promising stability in PEFC conditions [1, 2]. Additionally, Membrane Electrode Assemblies (MEAs) with low loadings of highly active Pt-group metal (PGM) catalysts have already been demonstrated to exceed the high-efficiency target of > 0.3 A/cm2 at 0.8 V [3]; however, the rated-power target of 1 W/cm2 cannot be met with these MEAs due to transport losses that are unique to MEAs with ultra-low catalyst loadings [4, 5]. United Technologies Research Center (UTRC) is a part of a DOE-supported research project, led by Argonne National Laboratory (ANL), focused on improving the understanding, performance, and durability of advanced Pt-alloy catalysts operating in complete PEFCs. In the current study, advanced MEAs with high ORR activity Pt-Ni alloy catalysts and ultra-low Pt loading (total ~ 0.13 mg-Pt/cm2) are prepared by Johnson Matthey Fuel Cells (JMFC) and tested by UTRC. The focus of this talk will be on using a variety of in-cell diagnostics and analysis methods to investigate both the initial performance and durability of these state-of-the-art MEAs. One example diagnostic that will be utilized are Polarization Change Curves (PCCs), which were originally developed for durability studies [6], but can also be used to investigate changes in performance due to operating conditions or MEA composition. For example, Fig 1a is a PCC comparison of the initial performance of the PtNi MEA and a Pt-only MEA, which have similar average catalyst particle size and the same carbon support, metal/carbon ratio, and ionomer/carbon ratio. Clearly, in addition to the differences in catalyst activity, there are also differences in the transport losses between these two different electrocatalysts. The durability of ultra-low Pt loading MEAs will also be discussed. Fig. 1b is a durability example of a PCC result, which is after applying a recent DOE-suggested AST protocol (0.6V-0.95V trapezoid potential cycle, with 700mV/s ramp rate and 6s/cycle). Other diagnostics and analysis beyond PCCs will also be included. The results obtained from these in-cell diagnostics will be used to hypothesize the potential mechanisms responsible for the differences in polarization curves, as well as project possible pathways to future performance and durability improvements. Acknowledgements This work is partially funded by the U. S. Department of Energy (DOE) under contract number DE-AC02-06CH11357. The authors would like to thank their DOE-project colleagues, especially those at ANL and JMFC.

Hierarchical Mesoporous/Macroporous Perovskite La0.5Sr0.5CoO3-x Nanotubes: A Bifunctional Catalyst with Enhanced Activity and Cycle Stability for Rechargeable Lithium Oxygen Batteries.

Perovskites show excellent specific catalytic activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline solutions; however, small surface areas of the perovskites synthesized by traditional sol-gel methods lead to low utilization of catalytic sites, which gives rise to poor Li-O2 batteries performance and restricts their application. Herein, a hierarchical mesporous/macroporous perovskite La0.5Sr0.5CoO3-x (HPN-LSC) nanotube is developed to promote its application in Li-O2 batteries. The HPN-LSC nanotubes were synthesized via electrospinning technique followed by postannealing. The as-prepared HPN-LSC catalyst exhibits outstanding intrinsic ORR and OER catalytic activity. The HPN-LSC/KB electrode displays excellent performance toward both discharge and charge processes for Li-O2 batteries, which enhances the reversibility, the round-trip efficiency, and the capacity of resultant batteries. The synergy of high catalytic activity and hierarchical mesoporous/macroporous nanotubular structure results in the Li-O2 batteries with good rate capability and excellent cycle stability of sustaining 50 cycles at a current density of 0.1 mA cm(-2) with an upper-limit capacity of 500 mAh g(-1). The results will benefit for the future development of high-performance Li-O2 batteries using hierarchical mesoporous/macroporous nanostructured perovskite-type catalysts.

Nitrogen-Doped Large-Sized Graphene Tubes As an Active Support for a Hybrid Pt Electrocatalyst Towards Oxygen-Reduction

Currently, Pt remains the most effective catalysts to catalyze the oxygen reduction reaction (ORR) in both acidic and alkaline solutions for polymer electrolye fuel cells and meal-air batteries.(1) In order to significantly increase Pt utilization in the catalysts, Pt nanoparticles are supported on high-surface-area carbon blacks such as Vulcan VX-72 (Pt/C); however, a fast and significant loss of electrochemically active surface area (ECSA), and thus performance degradation, are often observed with traditional Pt/C catalysts over long periods of operation due to corrosion of the carbon supports and the Ostwald ripening and agglomeration of Pt nanoparticles.(2) To overcome activity and stability limitations, use of advanced carbon materials with ordered graphitic structures including carbon nanotubes (CNTs),(3) onion-like carbon,(4, 5) and graphene(6) is able to provide a new opportunity for Pt electrocatalysts with improved activity and durability. In exploring carbon-based ORR transition metal-nitrogen-carbon (M-N-C) catalysts (M: Fe or Co),(7-11) in situ formation of various carbon nanostructures with nitrogen doping can be realized by catalyzing the decomposition of the nitrogen/carbon precursor at high temperatures (800-1000°C),(12) which was found to result in catalysts with superior activity for the ORR is comparison to other types of nonprecious metal catalysts (NPMCs). Importantly, we can control the formation of different nanostructures (e.g., CNT, onion-like carbon, graphene) during the catalyst synthesis by optimizing the transition metals, nitrogen/carbon precursors and templates.(12) The highly graphitized carbon nanostructures present in the Fe-N-C materials may serve as a matrix for hosting catalytically active nitrogen-metal moieties.(12) The presence of graphitized carbon appears to enhance the stability of the ORR catalysts.(8) In exploring such carbon nanostructure-rich catalysts for ORR, although substantial progress has been achieved in the synthesis and performance improvement, practical resolution regarding its long-term durability and high activity in fuel cells or other energy application is still far from effective. Herein, we conceive a new method to prepare highly active and stable ORR catalysts by innovatively coupling Pt nanoparticles and highly active Fe-N-C materials in a unique hybrid configuration (Figure 1). In doing so, large-diameter nitrogen-doped graphene tubes (N-GTs) derived from dicyandiamide (DCDA), iron acetate, and metal organic frameworks (MOF), MIL-100(Fe), viaa high temperature method were prepared as novel supports for the development of Pt catalysts. The large size of N-GTs provides a better platform than common carbon nanotubes (with diameters usually less than 30 nm), thus favorably anchoring the deposited metal nanoparticles. While the as-prepared N-GTs were found to show good intrinsic ORR activity and stability in acidic electrolytes, by modification with well-dispersed Pt nanoparticles, the unique Pt/N-GT hybrid materials were found to provide excellent performance that is superior to commercial Pt/C catalyst. The observed enhancements are likely due to the complementary ORR active sites on N-GTs, their highly graphitized structure formed during the high-temeprature heat treatment process, and favourable interactions between the nitrogen dopant species and Pt nanoparticles. 1. Y. H. Bing, H. S. Liu, L. Zhang, D. Ghosh and J. J. Zhang, Chem Soc Rev, 39, 2184 (2010). 2. H. Nakano, W. Z. Li, L. B. Xu, Z. W. Chen, M. Waje, S. Kuwabata and Y. S. Yan, Electrochemistry, 75, 705 (2007). 3. G. Wu, Y.-S. Chen and B.-Q. Xu, Electrochem. Commun., 7, 1237 (2005). 4. G. Wu, D. Li, C. Dai, D. Wang and N. Li, Langmuir, 24, 3566 (2008). 5. G. Wu, C. Dai, D. Wang, D. Li and N. Li, Journal of Materials Chemistry, 20, 3059 (2010). 6. C. Z. Zhu and S. J. Dong, Nanoscale, 5, 1753 (2013). 7. G. Wu, N. H. Mack, W. Gao, S. Ma, R. Zhong, J. Han, J. K. Baldwin and P. Zelenay, ACS Nano, 6, 9764 (2012). 8. G. Wu, K. L. More, P. Xu, H.-L. Wang, M. Ferrandon, A. J. Kropf, D. J. Myers, S. Ma, C. M. Johnston and P. Zelenay, Chem. Commun., 49, 3291 (2013). 9. G. Wu, M. Nelson, S. Ma, H. Meng, G. Cui and P. K. Shen, Carbon, 49, 3972 (2011). 10. G. Wu, M. A. Nelson, N. H. Mack, S. Ma, P. Sekhar, F. H. Garzon and P. Zelenay, Chem. Commun., 46, 7489 (2010). 11. Q. Li, G. Wu, D. A. Cullen, K. L. More, N. H. Mack, H. T. Chung and P. Zelenay, ACS Catalysis, 4, 3193 (2014). 12. G. Wu and P. Zelenay, Acc. Chem. Res., 46, 1878 (2013). Figure 1

Electrochemical Analysis and Density Functional Theory (DFT) Simulation to Investigate the Phosphoric Acid Adsorption on Pt3m (M = Fe, Co, Ni) Nanoparticles

High temperature-polymer electrolyte membrane fuel cell (HT-PEMFC) utilizing phosphoric acid doped polybenzimidazol membrane is a promising power supply for a number of stationary applications. At high operating temperature, significant enhancement in oxygen reduction reaction (ORR) kinetics is expected. However, typically the cell performance of HT-PEMFC is lower than that of conventional Nafion-based PEMFCs. One of important reason is the strong adsorption of phosphoric acid or anions, so called ‘phosphoric acid poisoning’, which blocks the active sites on Pt/C.Alloying with transition metals have been suggested for an efficient way to improve the performance of HT-PEMFC as the alloying may provide enhanced intrinsic ORR activity and improved tolerance against the phosphoric acid poisoning. Experimental observation and computational simulations revealed that lowered adsorption energy of OH on alloy of Pt and transition metal nanoparticle catalysts (PtM/C) provided the enhanced ORR activity1.However, there were few reports investigating effect of transition metal alloying on the phosphoric acid tolerance. Recently, He et al. demonstrated improved tolerance against the phosphoric acid poisoning in bulk PtSn2 and PtNi nanoparticle3 alloy surfaces. They postulated that ligand effect or geometric effect may be responsible for the tolerance of alloy surfaces. As a theoretical investigation of the alloying effect, Savizi and Janik calculated potential dependence of phosphoric acid adsorption free energy4 on Pt(111) surfaces, using density functional theory (DFT) simulations. However, there has been no further study, including PtM/C electrocatalysts.In this research, we investigated the effect of transition metal alloying on the phosphoric acid poisoning in Pt3M (where, M = Fe, Co, Ni) nanoparticle catalyst (Pt3M/C) surfaces. Using the cyclic voltammetry, adsorption strength of phosphoric acid on Pt3M/C was analyzed. Also, the effect of transition metal alloying on the adsorption strength of phosphoric acid in Pt3M/C was analyzed based on the DFT simulations. Combination of electrochemical analysis and DFT simulations allowed investigating systematically the effect of alloying on the phosphoric acid adsorption in Pt3M/C surfaces.

Electrocatalytic performances of LaNi1−xMgxO3 perovskite oxides as bi-functional catalysts for lithium air batteries

Mg-doped perovskite oxides LaNi1−xMgxO3 (x = 0, 0.08, 0.15) electrocatalysts are synthesized by a sol–gel method using citric acid as complex agent and ethylene glycol as thickening agent. The intrinsic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity of as-prepared perovskite oxides in aqueous electrolyte are examined on a rotating disk electrode (RDE) set up. Li–air primary batteries on the basis of Mg-doped perovskite oxides LaNi1−xMgxO3 (x = 0, 0.08, 0.15) and nonaqueous electrolyte are also fabricated and tested. In terms of the ORR current densities and OER current densities, the performance is enhanced in the order of LaNiO3, LaNi0.92Mg0.08O3 and LaNi0.85Mg0.15O3. Most notably, partially substituting nickel with magnesium suppresses formation of Ni2+ and ensures high concentration of both OER and ORR reaction energy favorable Ni3+ (eg = 1) on the surface of perovskite catalysts. Nonaqueous Li–air primary battery using LaNi0.92Mg0.08O3 and LaNi0.85Mg0.15O3 as the cathode catalysts exhibit improved performances compared with LaNiO3 catalyst, which are consistent with the ORR current densities.

La0.8Sr0.2MnO3−δ Decorated with Ba0.5Sr0.5Co0.8Fe0.2O3−δ: A Bifunctional Surface for Oxygen Electrocatalysis with Enhanced Stability and Activity

Developing highly active and stable catalysts based on earth-abundant elements for oxygen electrocatalysis is critical to enable efficient energy storage and conversion. In this work, we took advantage of the high intrinsic oxygen reduction reaction (ORR) activity of La0.8Sr0.2MnO3−δ (LSMO) and the high intrinsic oxygen evolution reaction (OER) activity of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) to develop a novel bifunctional catalyst. We used pulsed laser deposition to fabricate well-defined surfaces composed of BSCF on thin-film LSMO grown on (001)-oriented Nb-doped SrTiO3. These surfaces exhibit bifunctionality for oxygen electrocatalysis with enhanced activities and stability for both the ORR and OER that rival the state-of-the-art single- and multicomponent catalysts in the literature.

Effects of pore structure in nitrogen functionalized mesoporous carbon on oxygen reduction reaction activity of platinum nanoparticles

Nitrogen-doped mesoporous carbons (NMCs) with ordered to disordered pore structures were fabricated on SBA-15 modified with different concentrations of tetraethyl orthosilicate using pyrrole as a carbon source. The carbonization temperature of NMCs was maintained at 800°C so that the amount and type of nitrogen functionalities were constant. Pt nanoparticles (NPs) were deposited onto NMCs using a modified polyol process. N2 adsorption isotherms, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize NMCs and Pt NPs. A significant shift in binding energy was found in Pt NPs deposited on NMC with disordered pore structure compared to Pt NPs deposited on NMC with ordered pore structure. Pt NPs deposited on NMC with the disordered pore structure had the highest intrinsic oxygen reduction reaction (ORR) activity among the Pt/NMC catalysts. It showed that the interaction between Pt NPs and NMCs could be modulated for enhancement of the ORR activity of Pt NPs by changing the pore structure of NMCs.

Hierarchical mesoporous perovskite La 0 .5 Sr 0.5 CoO 2.91 nanowires with ultrahigh capacity for Li-air batteries

Lithium-air batteries have captured worldwide attention due to their highest energy density among the chemical batteries. To provide continuous oxygen channels, here, we synthesized hierarchical mesoporous perovskite La(0.5)Sr(0.5)CoO(2.91) (LSCO) nanowires. We tested the intrinsic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity in both aqueous electrolytes and nonaqueous electrolytes via rotating disk electrode (RDE) measurements and demonstrated that the hierarchical mesoporous LSCO nanowires are high-performance catalysts for the ORR with low peak-up potential and high limiting diffusion current. Furthermore, we fabricated Li-air batteries on the basis of hierarchical mesoporous LSCO nanowires and nonaqueous electrolytes, which exhibited ultrahigh capacity, ca. over 11,000 mAh⋅g(-1), one order of magnitude higher than that of LSCO nanoparticles. Besides, the possible reaction mechanism is proposed to explain the catalytic activity of the LSCO mesoporous nanowire.

Catalytic Activity Trends of Oxygen Reduction Reaction for Nonaqueous Li-Air Batteries

We report the intrinsic oxygen reduction reaction (ORR) activity of polycrystalline palladium, platinum, ruthenium, gold, and glassy carbon surfaces in 0.1 M LiClO(4) 1,2-dimethoxyethane via rotating disk electrode measurements. The nonaqueous Li(+)-ORR activity of these surfaces primarily correlates to oxygen adsorption energy, forming a "volcano-type" trend. The activity trend found on the polycrystalline surfaces was in good agreement with the trend in the discharge voltage of Li-O(2) cells catalyzed by nanoparticle catalysts. Our findings provide insights into Li(+)-ORR mechanisms in nonaqueous media and design of efficient air electrodes for Li-air battery applications.

Thermal, electrical and electrochemical characteristics of Ba 1− xSr xCo 0.8Fe 0.2O 3− δ cathode material for intermediate temperature solid oxide fuel cells

Ba 1− x Sr x Co 0.8Fe 0.2O 3− δ (x = 0.3–0.9) perovskite oxides have been studied as cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The structural characteristics, temperature dependent weight loss, thermal expansion, electrical conductivity, and electrochemical properties in combination with YSZ electrolyte together with an SDC buffer layer were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), dilatometry, DC four probe conductivity measurement and electrochemical impedance spectroscopy (EIS) techniques respectively. XRD study revealed the lattice parameter and unit cell volume decrease with increase in Sr +2 content at the A-site. TEC and electrical conductivity were found to increase with increasing Sr +2 content. Electrical conductivity was found to be dependent on the thermal history of the samples. Polarization resistance of the samples with SDC buffered YSZ electrolyte decreased with increasing Sr +2 content which was ascribed to the higher conductivity with improved oxygen adsorption/desorption and oxygen ions diffusion processes. The intrinsic oxygen reduction reaction rate also increased with Sr +2 content at the A-site. The exchange current for intrinsic oxygen reduction reaction at 700 °C was found to be 50.0 mA cm −2 for Ba 0.3Sr 0.7Co 0.8Fe 0.2O 3− δ ; a value which is about 50% higher than that for Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ , a widely studied cathode material. Therefore, the present composition may be a potential cathode material for IT-SOFC application.

Pt-Covered Multiwall Carbon Nanotubes for Oxygen Reduction in Fuel Cell Applications.

Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm(2) Pt at 0.9 ViR-corrected versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst -46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO4 at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt at 0.9 ViR-corrected vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs.

Electrocatalytic Activity Studies of Select Metal Surfaces and Implications in Li-Air Batteries

Rechargeable lithium-air batteries have the potential to provide ≈3 times higher specific energy of fully packaged batteries than conventional lithium rechargeable batteries. However, very little is known about the oxygen reduction reaction (ORR) and oxygen evolution in the presence of lithium ions in aprotic electrolytes, which hinders the improvement of low round-trip efficiencies of current lithium-air batteries. We report the intrinsic ORR activity on glassy carbon (GC) as well as polycrystalline Au and Pt electrodes, where Au is the most active with an activity trend of . Rotating disk electrode (RDE) measurements were used to obtain the kinetic current of the ORR and the reaction order with respect to oxygen partial pressure in 1 M propylene carbonate:1,2-dimethoxyethane (1:2 v/v). In addition, air electrodes with Vulcan carbon or Au or Pt nanoparticles supported on Vulcan were examined in single cells, where the observed discharge cell voltages follow the catalytic trend established by RDE measurements. The ORR mechanism and the rate-determining steps were discussed and contrasted with the ORR activity trend in acid and alkaline solutions.

Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part II. O 2 reduction

In order to mimic the operation of the air-cathode in a direct borohydride alkaline fuel cell, we studied the oxygen reduction reaction (ORR) in sodium hydroxide solution containing traces of borohydride. The activity of several ORR electrocatalysts, namely carbon-supported platinum, gold, silver and manganese oxide, has been investigated using slow-scan linear voltammetry. Whereas platinum is one of the best electrocatalyst in pure sodium hydroxide, none of the classical electrocatalysts: gold, silver and platinum, exhibit sufficient selectivity towards the ORR. When BH 4 − is present in solution, the potential taken by electrodes using such materials is a mixed potential, following the competition between the ORR and the NaBH 4 hydrolysis and/or oxidation. Conversely, manganese oxide-based electrocatalysts exhibit very interesting behaviour towards the ORR in alkaline medium; while their intrinsic ORR activity in pure sodium hydroxide is quite as good as that for platinum, they still display a remarkable selectivity for this reaction when the electrolyte contains traces of sodium borohydride. As a result, carbon-supported manganese oxide-based nanoparticles seem very interesting materials to be used in direct borohydride fuel cell.

Effect of Relative Humidity on Oxygen Reduction Kinetics in a PEMFC

The theoretical contributions to overall cell voltage in a proton exchange membrane fuel cell (PEMFC) were calculated, accounted for, and compared to measured IR free cell voltage as relative humidity was varied from 30-110%. The intrinsic oxygen reduction reaction (ORR) kinetics in a PEMFC, with regards to water and proton activity, were shown to be independent of above 50-60%, but significant losses in ORR kinetics were observed at lower values. The losses amounted to ca. 20 mV at , corresponding to an approximate two-fold decrease in the ORR exchange current density. The dependence of ORR kinetics on water and proton activity is consistent with reduced activity at low .