Oxygen Reduction Reaction Specific Activity Research Articles

Effects of transition metal doping in Pt/M-TiO2 (M = V, Cr, and Nb) on oxygen reduction reaction activity

High cost and low durability are unresolved issues that impede the commercialization of proton exchange membrane fuel cells (PEMFCs). To overcome these limitations, Pt/TiO2 is reported as an alternative electrocatalyst for enhancing the oxygen reduction reaction (ORR) activity and/or durability of the system. However, the low electrical conductivity of TiO2 is a drawback that may be addressed by doping. To date, most reports related to Pt/doped-TiO2 focus on changes in the catalyst activity caused by the Pt-TiO2 interaction (metal-support interaction), instead of the effect of doping itself; doping is merely considered to enhance the electrical conductivity of TiO2. In this study, we discuss the variation in the electronic fine structure of Pt caused by the dopant, and its correlation with the ORR activity. More extensive contraction of the Pt lattice in Pt/M-TiO2 (M = V, Cr, and Nb) relative to Pt/TiO2 and Pt/C leads to outstanding ORR specific activity of Pt/M-TiO2. Notably, a fourfold increase of the specific activity is achieved with Pt/V-TiO2 relative to Pt/C. Furthermore, an accelerated durability test (ADT) of Pt/V-TiO2 demonstrates that this system is three times more durable than conventional Pt/C due to the metal-support interaction.

Enhancing the Specific Activity of Metal Catalysts Toward Oxygen Reduction by Introducing Proton Conductor

Enhancing oxygen reduction reaction (ORR) activity and simultaneously reducing usage of noble metal catalysts are significantly important both in fundamental and applied science communities for polymer electrolyte fuel cells (PEFCs). In this work, we confirm the proton conductor (perfluorosulfonic acid, containing [Formula: see text]SO3H) can promote the specific activity [Formula: see text] of metal catalysts toward ORR. Herein, Pt nanoparticles (NPs) with a small and narrow size distribution are encapsulated with perfluorosulfonic acid through a simple colloidal route. The resulting catalyst obtains about two times [Formula: see text] towards ORR than that of the pristine Pt/C. Significantly, the amount of [Formula: see text]SO3H groups is controlled by a heat-treatment method to investigate the influence of [Formula: see text]SO3H groups on [Formula: see text]. The results evidence the contribution of [Formula: see text]SO3H groups to elevating the ORR specific activity. The mechanism can be ascribed to the [Formula: see text]SO3H groups which effectively promote the transfer process of reaction species (e.g., H[Formula: see text], H2O), improving the triple-phase boundary (TPB).

Pd@Pt Core-Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability.

We report a facile synthesis of multiply twinned Pd@Pt core-shell concave decahedra by controlling the deposition of Pt on preformed Pd decahedral seeds. The Pt atoms are initially deposited on the vertices of a decahedral seed, followed by surface diffusion to other regions along the edges/ridges and then across the faces. Different from the coating of a Pd icosahedral seed, the Pt atoms prefer to stay at the vertices and edges/ridges of a decahedral seed even when the deposition is conducted at 200 °C, naturally generating a core-shell structure covered by concave facets. The nonuniformity in the Pt coating can be attributed to the presence of twin boundaries at the vertices, as well as the {100} facets and twin defects along the edges/ridges of a decahedron, effectively trapping the Pt adatoms at these high-energy sites. As compared to a commercial Pt/C catalyst, the Pd@Pt concave decahedra show substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). For the concave decahedra with 29.6% Pt by weight, their specific (1.66 mA/cm(2)Pt) and mass (1.60 A/mgPt) ORR activities are enhanced by 4.4 and 6.6 times relative to those of the Pt/C catalyst (0.36 mA/cm(2)Pt and 0.32 A/mgPt, respectively). After 10,000 cycles of accelerated durability test, the concave decahedra still exhibit a mass activity of 0.69 A/mgPt, more than twice that of the pristine Pt/C catalyst.

Methanol Oxidation and Oxygen Reduction Activity of PtIrCo-Alloy Nanocatalysts Supercritically Deposited within 3D Carbon Aerogel Matrix

The comprehensive investigation of ternary PtIrCo-alloy nanoparticles supported on 3D carbon aerogel matrix toward Methanol Oxidation Reaction (MOR) and Oxygen Reduction Reaction (ORR) is focused on CO tolerance in MOR, methanol vs. oxygen selectivity in ORR, and the structural changes within the metal alloy nanoparticles caused by the heat treatment and the electrochemical conditioning. Amorphous tri-metallic Pt59Ir34Co7-nanoparticles with mean size of 1.47±0.21nm were formed within 3D CA matrix in supercritical CO2 (scCO2) from the Pt, Ir, and Co organometallic precursors. Depending on the heat-treatment temperature (600°C or 900°C), the amorphous organometallic phase of Pt, Ir and Co was transformed into a single-phase fcc PtIrCo-alloy nanoparticles with a mean particle size of 1.68±0.33nm and 2.68±0.61nm, respectively. The fcc lattice parameters of the PtIrCo-alloy (0.389nm at 600°C and 0.386nm at 900°C) were smaller than that of the Pt fcc lattice (0.392nm) and the lattice parameters reduced proportionally to the heat-treatment temperatures. After 50 electrochemical cycles, PtIrCo/CA catalysts display improved MOR specific activity (∼2.5 X Pt/C), mass activity (∼3 X Pt/C) at 0.7V vs. SHE, higher CO-tolerance, and better electrochemical stability than Pt/C, but less CO-tolerance and stability than PtRu/C. Regarding ORR, the PtIrCo/CA catalysts demonstrate enhanced ORR specific activity (∼5.5 X Pt/C) and mass activity (∼6.3 X Pt/C) at 0.7V vs. SHE along with superior methanol-tolerance in comparison to Pt/C. The structural changes associated with PtIrCo/CA fcc-lattice parameter with respect to Pt-lattice have contributed to better MOR and ORR activities. Faster surface diffusion associated with weak adsorption of hydroxide species and higher selective adsorption of oxygen in comparison to methanol onto PtIrCo-alloy surface could have contributed to their better CO-tolerant MOR and methanol-resistant ORR activity compared to Pt-catalyst.

Electrocatalytic Activity and Stability of Platinum Nanoparticles Supported on Carbon–Molybdenum Oxides for the Oxygen Reduction Reaction

AbstractThe degradation of Pt‐based electrocatalysts used in proton‐exchange membrane fuel cell (PEMFC) cathodes is one of the main issues restricting the widespread application of PEMFCs as energy converters. This work aims to contribute to the improvement of the stability of platinum nanoparticles (Pt NPs) by modifying the support to which they are anchored. Thus, syntheses of catalyst supports based on molybdenum oxides and carbon are carried out, followed by impregnation of the supports with Pt NPs. The Pt/MoO3–C catalyst shows the highest specific activity in the oxygen reduction reaction (ORR), and this must be because of synergistic metal–support effects. Regarding the electrochemical stability of the materials, it is observed that, in principle, none of the Mo oxides decrease the extent of Pt degradation. However, after comparing the specific ORR activities before and after electrochemical ageing, it is concluded that Pt/MoO2*–C is a more stable material compared to Pt/C and Pt/MoO3–C.

Oxygen reduction reaction electrocatalytic activity of tilted pt nanorod arrays fabricated by glancing angle deposition

Sputter glancing angle deposition (GLAD) technique was used to fabricate 50 and 200 nm long tilted Pt nanorods on glassy carbon electrodes with weight loadings of 0.042 and 0.117 mg cm−2, respectively. GLAD-tilted Pt nanorod electrodes were investigated as potential electrocatalysts for proton exchange membrane fuel cells utilizing cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques in aqueous perchloric acid (HClO4) to determine their oxygen reduction reaction (ORR) activities and stabilities. The CV results demonstrate that the tilted Pt nanorods electrocatalyst exhibits more positive oxide reduction peak potential compared to conventional carbon supported Pt particles (Pt/C), indicating that tilted GLAD Pt nanorods are less oxophilic than Pt/C. In addition, the multiple CV cycles in acidic electrolyte showed that tilted Pt nanorods are significantly more stable than Pt/C. Furthermore, specific ORR activities of tilted Pt nanorods determined by RDE technique were analyzed and compared to literature values of Pt/C and vertically aligned Pt nanorod electrodes. The room temperature RDE results revealed that tilted Pt nanorods demonstrate higher area-specific activity than that of Pt/C catalyst. The enhanced ORR activity of tilted Pt nanorods is due to their large crystallite size and possible dominance of the preferred crystal orientations for ORR. However, the tilted Pt nanorods showed lower mass-specific activity than Pt/C electrocatalyst caused by the large diameters of the tilted Pt nanorods. Overall ORR activity and stability of tilted Pt nanorods were found to be comparable to those of vertical ones.

Preparation and Performance Evaluation of Pt/SnO2/KB As Cathode Catalyst of PEMFC

Several technical issues must be addressed to allow widespread commercialization of proton exchange membrane fuel cells (PEMFCs). In particular, the electrode catalyst that has high durability and an acceptable low cost (reducing Pt amount) is required. For this purpose, we prepared novel electrode supports which are modified Ketjen black with tin dioxide nanoparticles (SnO2/KB) and reported oxygen reduction reaction (ORR) activity as well as the durability of the resultant catalyst (after Pt loading) [1]. In particular, the ORR activity and the durability were strongly depended on the loading amounts of SnO2 as well as the heat treatment temperature [1]. In this presentation, the relation between the preparation condition of SnO2 / KB and the catalytic performance of the resultant catalyst are reported. The preparation detail of SnO2/KB can be found in our previous reports [1]. In brief, KB (Ketjen Black EC-300j) was dispersed in 0.01 mol dm-3 SnF2 aqueous solution for 24 h or 72 h. It was then dispersed in 0.01 mol dm-3 H3BO3 aqueous solution for 24 h. Subsequent heat treatment was carried out at 500 oC under Ar atmosphere. Loading of Pt nanoparticles on SnO2/KB was carried out by an impregnation method following drying under N2 atmosphere at 120 oC [1]. The catalysts were characterized by XRD, TGA under air, N2 adsorption and desorption measurements, FE-SEM observation, ICP measurements and CO pulse tests. The prepared catalysts were dispersed ultrasonically in 2-propanol and were dropped on a glassy carbon disk. A thin Nafion film with a calculated thickness of 80 nm was then formed. A glass half-cell equipped with an RHE (reversible hydrogen electrode) and a Pt mesh counter electrode was used in this study. 0.1 mol dm-3 HClO4 aqueous solution was used as the electrolyte. ORR activity was evaluated by Koutecky-Levich plot analysis based on the rotating disk electrode measurements under O2 atmosphere. The potential pulse durability tests were conducted between 1.0 V and 0.6 V at 60 oC. Electrochemical surface area (ECA) of Pt was estimated by cyclic voltammetry. From the TGA results, the weight ratios of SnO2 of SnO2/KB were 39wt% after 24 h in SnF2 solution and 35wt% after 2 h, respectively. Then, we hereafter called the catalyst prepared by 24 h-dispersing in SnF2 solution as Pt/39-SnO2/KB-24 and another as Pt/35-SnO2/KB-72. In FE-SEM images of Pt/39-SnO2/KB-24 and Pt/35-SnO2/KB-72, SnO2 particles of ca. 10 nm in the diameter were found on the surface of KB. On the other hand, Pt particle size could be estimated as 2 - 3 nm, which corresponding to the results of CO pulse tests. In addition, the particle size is estimated as 1.8 nm for Pt/KB. Table 1 shows the Pt particle size, the values of specific and mass activity for ORR and the ECA remaining after the durability test. The ORR activity is clearly increased by using SnO2/KB as the support; especially, the mass activity was high although the Pt particle size was large. Furthermore, the ORR activity is likely to be improved by increasing the time for dispersing in SnF2 aqueous solution. The durability is also improved by using SnO2/KB as the support. Then, we attempted to investigate the reasons for higher durability of the catalysts using SnO2/KB based on the degradation mechanisms. The stability of the supports (SnO2/KB) was elucidated the increasing rate of pseudo-capacitance during the potential pulse durability tests. The increasing rates of KB and Pt/35-SnO2/KB-72 were 12.5% and 8.5%, respectively, which shows that the durability of Pt/35-SnO2/KB-72 is higher than that of KB. In addition, to elucidate chemical stability of Pt, the stability tests were also conducted in O2 saturated HClO4 aqueous solution at 60 oC for 72 h. As the results, the dissolution ratio of Pt/35-SnO2/KB-72 was ca. 0.8%, which was almost half for the catalyst without SnO2. The migration of Pt particles was also one degradation factor. Then, we carried out identical location FE-SEM observation of the catalysts before and after heating at 300 oC under Ar atmosphere for 12 h [2]. The frequency for migration of Pt particles was found to be ca. 58% for Pt/KB. On the other hand, that of Pt/35-SnO2/KB-72 was 42%, which suggests that migration of Pt particles were depressed by using SnO2/KB as the support. [1] T. Kinumoto, et al, ECS Trans., 64(3), 199 (2014), 58(1), 1259 (2013), 50(2), 1701 (2012) and Electrochemistry, 79, 334 (2011). [2] T. Kinumoto, et al., Electrochemistry, 83(1), 12 (2015). Figure 1

Synchrotron Based Structural Investigations of Mass-Selected PtxGd Nanoparticles and a Gd/Pt(111) Single Crystal for Electrochemical Oxygen Reduction

The sluggish kinetics of the oxygen reduction reaction (ORR) hinders the commercialization of proton exchange membrane fuel cells (PEMFC). The ORR activity is enhanced by alloying Pt with late transition 3d metals (i.e. Fe, Co, Ni, and Cu)1. However, these compounds tend to degrade in a fuel cell by dealloying. An alternative approach is to alloy Pt with rare-earth elements. Their highly negative alloying energy may provide them with kinetic stability against dealloying under reaction conditions. A recent publication from our group reported the high ORR activity and stability of polycrystalline Pt5Gd2. In this work, we present the experimental results of mass-selected PtxGd nanoparticles synthesized by gas aggregation after sputtering of an alloy target in an ultrahigh vacuum (UHV)3. PtxGd nanoparticles with nominal sizes of 3, 5, 7, and 9 nm were selected using time-of-flight mass filtering and deposited on glassy carbon (GC) disk electrodes. Rotating ring disk electrode (RRDE) measurements in 0.1 M HClO4 were used to measure the activity in comparison to pure Pt 4. The ORR specific activity increases with the nanoparticle size; a maximum mass activity is achieved with the 7 nm sample, ~3.6 A/mg Pt at 0.9 V. X-ray absorption spectroscopy measurements suggest that the high ORR activity is due to a compressive strain exerted by the alloy core onto the Pt overlayer at the surface. The structure formed on these types of alloys2 is further elucidated using a Gd/Pt(111) single crystal. The alloy was prepared in UHV by depositing 150 Å of Gd followed by annealing, thus simulating a bulk single crystal. It was characterized in vacuo using low energy electron diffraction, ion scattering spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption of CO. Subsequently, the crystal was transferred to an electrochemical cell, where a 1 nm thick Pt overlayer was formed; this constitutes the active phase for oxygen reduction. Using synchrotron based grazing X-ray diffraction, we determine the structure of the alloy and the Pt overlayer. The diffraction contributions from the Pt overlayer is separated from the Pt5Gd alloy, and the analysis of both diffraction patterns are presented. By investigating such well-defined structures, we gain valuable scientific insight into the relationship between their structure and functionality. On the basis of this insight, we can develop even better catalysts for oxygen electroreduction.

Ptfe Porous Hollow Nanocapsule Electrocatalysts with Remarkably Enhanced Oxygen Reduction Activity

Expansion of the commercial applications of polymer electrolyte fuel cells (PEFCs) strongly requires great improvements in cost, performance, and durability. At a cathode where oxygen reduction reaction (ORR) occurs, platinum (Pt) catalysts show sluggish ORR leading to high cost and poor performance of PEFCs. In addition, carbon-supports for Pt catalyst degrade due to carbon-corrosion during start-up/shut-down operations; it results in a severe deterioration of PEFC performance. Here we develop platinum-iron (PtFe) porous hollow nanocapsules (Fig. 1) as carbon-free electrocatalysts for ORR. The nanocapsules are composed of fused PtFe nanoparticles with a crystallite size of 6-7 nm and a chemically-ordered (face-centered tetragonal; fct) structure. The fused PtFe network possesses an electron conductivity, thus any carbon-support is not required at the cathode made of PtFe nanocapsules. We demonstrated the MEA using the carbon-free cathode catalyst-layer of PtFe nanocapsules exhibited sufficiently high IV performance, and excellent durability against start-up/shut-down operations at 80°C, by intrinsically eliminating the problem of carbon-corrosion.[1] The exclusion of carbon-supports from the cathode catalyst layer also enabled the formation of the ultrathin catalyst-layer that would provide great advantages to mass transport of oxygen. Moreover, it is noteworthy that the specific ORR activity of the PtFe nanocapsule is approximately 8.5 times higher than that of the commercial Pt catalyst supported on carbon black (Pt/C). Interestingly, the specific activity of the nanocapsule is also 2~3 times enhanced, compared with that of fct-PtFe nanoparticles (without any fused networks) supported on carbon black. Thus, it implies that the unique structures of the nanocapsule, such as the connected network structure of fused PtFe metal nanoparticles, and the support-free structure, would allow to exhibit higher ORR activities. We have also investigated the structure and electronic structure via in-situx-ray absorption spectroscopy combined with electron diffraction and Rietveld structure refinement with powder x-ray diffraction data. In this presentation, the relationship of the surface structures on the PtFe nanocapsules to their ORR activities will be discussed in details. The above results indicate that the newly-developed nanocapsule is a quite promising catalyst-layer material because it potentially can solve all the current problems, regarding ORR activity, durability, and mass-transport of oxygen, in the conventional cathode catalyst layers using Pt-based catalysts on carbon supports. [1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, “Pt-Fe Porous Hollow Nanocapsule as Carbon-free Electrocatalyst for Oxygen Reduction”, Submitted. Fig. 1. Schematic illustration of PtFe porous hollow nanocapsules as carbon-free electrocatalysts. Figure 1

Effect of pretreatment atmosphere on the particle size and oxygen reduction activity of low-loading platinum impregnated titanium carbide powder electrocatalysts

Low-loading Pt supported on TiC powder catalysts were synthesized by an impregnation method. After the Pt(NH3)4(NO3)2 precursor was impregnated onto the TiC support, different pretreatment atmospheres were used to study the influence on Pt dispersion, surface composition, and catalytic activity towards oxygen reduction reaction (ORR). Direct reduction of the Pt precursor in hydrogen led to small Pt particles with an average size of ∼2.2 nm and superior ORR activity at low overpotential compared to commercial Pt/C. However, calcination of the Pt precursor in air resulted in larger Pt particles with an average size of ∼6.7 nm and lower ORR specific activity. The decrease in ORR activity was primarily attributed to the surface oxidation of the TiC support during calcination. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) confirmed that the TiC powder was oxidized when the catalyst was calcined in air. The finding reported here demonstrates the importance of pretreatment atmosphere for synthesizing Pt-modified transition metal carbides as highly active electrocatalysts.

Role of LiCoO2 Surface Terminations in Oxygen Reduction and Evolution Kinetics.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of LiCoO2 nanorods with sizes in the range from 9 to 40 nm were studied in alkaline solution. The sides of these nanorods were terminated with low-index surfaces such as (003), while the tips were terminated largely with high-index surfaces such as (104), as revealed by high-resolution transmission electron microscopy. Electron energy loss spectroscopy demonstrated that low-spin Co(3+) prevailed on the sides, while the tips exhibited predominantly high- or intermediate-spin Co(3+). We correlated the electronic and atomic structure to higher specific ORR and OER activities at the tips as compared to the sides, which was accompanied by more facile redox of Co(2+/3+) and higher charge transferred per unit area. These findings highlight the critical role of surface terminations and electronic structures of transition-metal oxides on the ORR and OER activity.

Mixed-phase Pd–Pt bimetallic alloy on graphene oxide with high activity for electrocatalytic applications

Bimetallic PdPt alloy nanoparticles on graphene oxide (GO) have been prepared by a simple and facile chemical route, in which the reduction of metal precursors is carried out using CO as a reductant. Structural and morphological characterizations of GO/PdPt composites are performed using X-ray diffraction, X-ray photoelectron spectroscopy analysis and transmission electron microscopy. It is found that PdPt bimetallic nanoparticles are successfully synthesized and uniformly attached on the graphene sheets. The electrocatalytic and electrochemical properties of GO/PdPt composites including methanol oxidation reaction (MOR), oxygen reduction reaction (ORR) and methanol tolerant oxygen reduction reaction (MTORR) are studied in HClO4 aqueous solution. A significant improvement in the electrocatalytic activities is observed by increasing the atomic ratio of Pt in PdPt bimetallic alloys compared to the freestanding Pd nanoparticles on GO. The prepared GO/PdPt composites with an (Pd:Pt) atomic ratio of 40:60 exhibits higher methanol oxidation activity, higher specific ORR activity and better tolerance to CO poisoning. The results can be attributed to the collective effects of the PdPt nanoparticles and the enhanced electron transfer of graphene.

Metallated porphyrin based porous organic polymers as efficient electrocatalysts.

Developing efficient, stable and low-cost catalysts for Oxygen Reduction Reaction (ORR) is of great significance to many emerging technologies including fuel cells and metal-air batteries. Herein, we report the development of a cobalt(II) porphyrin based porous organic polymer (CoPOP) and its pyrolyzed derivatives as highly active ORR catalysts. The as-synthesized CoPOP exhibits high porosity and excellent catalytic performance stability, retaining ∼100% constant ORR current over 50,000 s in both alkaline and acidic media. Pyrolysis of CoPOP at various temperatures (600 °C, 800 °C, and 1000 °C) yields the materials consisting of graphitic carbon layers and cobalt nanoparticles, which show greatly enhanced catalytic activity compared to the as-synthesized CoPOP. Among them, CoPOP-800/C pyrolyzed at 800 °C shows the highest specific surface area and ORR activity, displaying the most positive half-wave potential (0.825 V vs. RHE) and the largest limited diffusion current density (5.35 mA cm(-2)) in an alkaline medium, which are comparable to those of commercial Pt/C (20 wt%) (half-wave potential 0.829 V vs. RHE, limited diffusion current density 5.10 mA cm(-2)). RDE and RRDE experiments indicate that CoPOP-800/C directly reduces molecular oxygen to water through a 4-e(-) pathway in both alkaline and acidic media. More importantly, CoPOP-800/C exhibits excellent durability and methanol-tolerance under acidic and alkaline conditions, which surpass the Pt/C (20 wt%) system.

Oxygen reduction reaction activity and structural stability of Pt-Au nanoparticles prepared by arc-plasma deposition.

The oxygen reduction reaction (ORR) activity and durability of various Au(x)/Pt100 nanoparticles (where x is the atomic ratio of Au against Pt) are evaluated herein. The samples were fabricated on a highly-oriented pyrolytic graphite substrate at 773 K through sequential arc-plasma depositions of Pt and Au. The electrochemical hydrogen adsorption charges (electrochemical surface area), particularly the characteristic currents caused by the corner and edge sites of the Pt nanoparticles, decrease with increasing Au atomic ratio (x). In contrast, the specific ORR activities of the Au(x)/Pt100 samples were dependent on the atomic ratios of Pt and Au: the Au28/Pt100 sample showed the highest specific activity among all the investigated samples (x = 0-42). As for ORR durability evaluated by applying potential cycles between 0.6 and 1.0 V in oxygen-saturated 0.1 M HClO4, Au28/Pt100 was the most durable sample against the electrochemical potential cycles. The results clearly showed that the Au atoms located at coordinatively-unsaturated sites, e.g. at the corners or edges of the Pt nanoparticles, can improve the ORR durability by suppressing unsaturated-site-induced degradation of the Pt nanoparticles.

Effects of Oxygen Vacancies and Reaction Conditions on Oxygen Reduction Reaction on Pyrochlore-Type Lead-Ruthenium Oxide

A series of pyrochlore-type lead-ruthenium oxide (Pb2Ru2O7-δ) containing various amounts of oxygen vacancies was prepared by varying the heat-treatment temperature, and the oxygen reduction reaction (ORR) activity as well as the electrochemical redox property in acid and alkaline solution was evaluated using rotating disk electrode. The specific ORR activity was increased with increasing the amount of oxygen vacancies, and linearly correlated to the charge amount of electrochemical redox of Pb2Ru2O7-δ both in acid and alkaline solution. We concluded that the surface oxygen vacancy participates in the electrochemical redox of Pb2Ru2O7-δ, and at the same time, behaves as the active site for ORR. The ORR activity in alkaline solution was much higher than that in acid solution, possibly because of the difference of the facility of electrochemical desorption of adsorbed oxygen species in the ORR process. Additionally, the effect of humidity on the ORR activity was investigated by single cell test. It was revealed that Pb2Ru2O7-δ shows absolutely high ORR activity in dry condition.

Synthesis and Characterization of Pd@Pt–Ni Core–Shell Octahedra with High Activity toward Oxygen Reduction

The oxygen reduction reaction (ORR) on the cathode of a polymer electrolyte fuel cell requires the use of a catalyst based on Pt, one of the most expensive metals on the earth. A number of strategies, including optimization of shape or facet, formation of alloys with other metals, and incorporation of a different metal into the core, have been investigated to enhance the activity of a Pt-based catalyst and thus reduce the loading of Pt. This article reports the synthesis and characterization of Pd@Pt-Ni core-shell octahedra with high activity toward ORR. The octahedra with an edge length of 8 nm were obtained by directly depositing thin, conformal shells of a Pt-Ni alloy on Pd octahedra of 6 nm in edge length. The key to the success of this synthesis is the use of an amphiphilic solvent to ensure good compatibility between the solvents typically used for the syntheses of Pd and Pt-Ni nanocrystals. The core-shell structure was confirmed by a number of techniques, including scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy mapping, in situ X-ray diffraction under H2 and He, and electrochemical measurements. Relative to the state-of-the-art Pt/C catalyst, the Pd@Pt-Ni/C catalyst showed mass and specific ORR activities enhanced by 12.5- and 14-fold, respectively. The formation of a core-shell structure helped increase the electroactive surface area in terms of Pt and thus the mass activity. During an accelerated durability test, the mass activity of the Pd@Pt-Ni/C catalyst only dropped by 1.7% after 10,000 cycles.

Pt Monolayer Shell on Hollow Pd Core Electrocatalysts: Scale up Synthesis, Structure, and Activity for the Oxygen Reduction Reaction

The rapidly increasing demand for fossil fuels used in transportation and power generation, and their detrimental environmental effects, have resulted in a wide-spread challenge for the development of renewable energy technologies. Fuel cells are one of the most promising clean energy technologies, particularly attractive for automobile applications, due to their high efficiency, high energy density, and low or zero emissions. The most challenging problem of their application is the slow kinetics of oxygen reduction reaction (ORR) at fuel cell cathodes, even on Pt - the best single element electrocatalyst, which causes a large loss of the cell voltage, resulting in significant efficiency loss. In addition, Pt dissolves under certain operating condition of fuel cells. To mitigate these two drawbacks, catalysts containing large amounts of expensive Pt are required. This is one of the main reasons for the slow commercialization of fuel cells.To address these drawbacks, the use of Pt monolayer (PtML) electrocatalysts was proposed to reduce the cost of Pt while attempting to enhance their ORR activity and stability [1, 2]. Such catalysts, consisting of a monolayer of Pt on a substrate of another material, minimize the amount of Pt while ensuring that all Pt atoms are available at the surface for catalytic activity [1, 2]. Additionally, through geometric and electronic interactions with the substrate [3, 4], a PtMLcan change its electronic properties and be more active and durable than pure Pt electrocatalysts.Several single crystal surfaces, including Pd (111), Ru (0001), Ir (111), Rh (111), and Au (111) have been investigated as supports for PtML catalysts for the ORR [5]. Variations in ORR activity could be accounted for by the oxygen binding energy, which must be tuned to intermediate strength. This leads to a classic catalysis ‘volcano plot’ of ORR activity vs. oxygen binding energy with PtMLon Pd(111) on the top of the volcano plot having higher ORR activity than Pt(111) [5].Since oxygen binding energy appears to be the major descriptor of the ORR kinetics, DFT calculations suggests that further enhancement in ORR activity of PtML on Pd could be accomplished by weakening the O and OH binding energies [5]. This could be achieved by additional contraction of the PtML shell. A hollow core is an interesting structure to investigate geometric interaction between the PtML shell and the core because a hollow core may induce a desirable lattice contraction of a PtML, leading to improved ORR activity by reducing the oxygen binding energy.In this contribution, the synthesis, characterization and kinetics of the ORR of a PtMLshell on Pd(hollow), or Pd-Au(hollow) core electrocatalysts are reported. Comparisons between the ORR catalytic activity of the electrocatalysts with hollow cores and those of Pt solid and Pt hollow nanoparticles were obtained using the rotating disk electrode technique. Hollow nanoparticles were made using Ni or Cu nanoparticles as sacrificial templates. The Pt ORR specific and mass activities of the electrocatalysts with hollow cores were found to be considerably higher than those of the electrocatalysts with solid cores, Fig. 1. This enhanced Pt activity is attributed to the smooth surface morphology and hollow-induced lattice contraction. In addition, the hollow particles have a mass-saving geometry.AcknowledgmentWork at Brookhaven National Laboratory is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886. The work at BNL was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering, under Contracts No. DE-AC02-98CH10886. DB thanks the MICINN, Spain (MAT2010-20442 and MAT2011-28673-C02-01) for FPU grant.

Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-stripping voltammetry

This study intends to provide some insight in the up-to-date elusive assessment of a correct choice of method for estimating the active surface area of Pt alloy nanoparticle catalysts. Taking PtNi3 nanoparticles as an example, we have compared three types of electrochemically active surface area (ECSA) data, CO-ECSA, Hupd-ECSA, and Hupd/CO-ECSA, which were evaluated from CO stripping and underpotentially deposited hydrogen stripping steps applied at different times along a reference catalyst activity test protocol. Considering a total of six different detailed voltammetric test protocols, we address Pt alloy particle size effects, analyze the effect of the time of application of CO and hydrogen stripping, and study their effect on the Pt mass and Pt surface-specific activities for the oxygen reduction reaction (ORR). In a discussion of the ratio of CO charge to hydrogen charge, it is shown that this quantity is more complex than previously thought and not associated with a specific surface structure. The Hupd/CO-ECSA data are found to be a reasonable balance for the estimate of surface area normalized, so-called specific catalytic ORR activities.

Synthesis of PtCu nanowires in nonaqueous solvent with enhanced activity and stability for oxygen reduction reaction

Pt-based core–shell electrocatalysts with one-dimensional (1D) nanostructure show a great opportunity to improve the catalytic activity and durability of pure Pt catalyst for oxygen reduction reaction (ORR). Here, we synthesize Cu@CuPt core@shell nanowires (NWs) with 1D nanostructure by using Cu NWs as templates in organic solvent medium. The ORR mass activity and specific activity of PtCu NWs are 0.216 A mgpt−1 and 0.404 mA cm−2 at 0.9 V, respectively, which are 3.1 and 3.7 times larger than that of the commercial Pt/C catalyst (0.07 A mgpt−1 and 0.110 mA cm−2, respectively). Theoretical studies suggest that the electronic effect of the Cu substrate on the Pt monolayer could be the main reason for the higher activity of PtCu NWs than that of the commercial Pt/C catalyst. In addition, the PtCu NWs show much better durability than the commercial Pt/C catalyst after stability test. It is expected that the as-synthesized PtCu NWs in organic solvent medium could be excellent candidates as high performance catalysts for ORR.

Atomic layer-by-layer deposition of Pt on Pd nanocubes for catalysts with enhanced activity and durability toward oxygen reduction.

An effective strategy for reducing the Pt content while retaining the activity of a Pt-based catalyst is to deposit the Pt atoms as ultrathin skins of only a few atomic layers thick on nanoscale substrates made of another metal. During deposition, however, the Pt atoms often take an island growth mode because of a strong bonding between Pt atoms. Here we report a versatile route to the conformal deposition of Pt as uniform, ultrathin shells on Pd nanocubes in a solution phase. The introduction of the Pt precursor at a relatively slow rate and high temperature allowed the deposited Pt atoms to spread across the entire surface of a Pd nanocube to generate a uniform shell. The thickness of the Pt shell could be controlled from one to six atomic layers by varying the amount of Pt precursor added into the system. Compared to a commercial Pt/C catalyst, the Pd@PtnL (n = 1-6) core-shell nanocubes showed enhancements in specific activity and durability toward the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations on model (100) surfaces suggest that the enhancement in specific activity can be attributed to the weakening of OH binding through ligand and strain effects, which, in turn, increases the rate of OH hydrogenation. A volcano-type relationship between the ORR specific activity and the number of Pt atomic layers was derived, in good agreement with the experimental results. Both theoretical and experimental studies indicate that the ORR specific activity was maximized for the catalysts based on Pd@Pt2-3L nanocubes. Because of the reduction in Pt content used and the enhancement in specific activity, the Pd@Pt1L nanocubes showed a Pt mass activity with almost three-fold enhancement relative to the Pt/C catalyst.