Area-specific Oxygen Reduction Reaction Research Articles

Effects of support particle size and Pt content on catalytic activity and durability of Pt/TiO2 catalyst for oxygen reduction reaction in proton exchange membrane fuel cells environment

For proton exchange membrane fuel cells (PEMFCs), instability of Pt-based catalysts due to carbon support corrosion is a critical issue that limits their wide application. Alternative corrosion resistant supports, such as metal oxides, have been investigated in conductive carbon containing matrix and demonstrated improved stability. This study probes systematically the effects of TiO2 support particle size (5, 18, and 30 nm) and Pt content (6, 12, and 37 wt%) on the oxygen reduction reaction (ORR) activity and stability of the Pt/TiO2 catalyst in the absence of corrosive carbon. The average Pt nanoparticle size increases with increased TiO2 support size and Pt content. For 12 wt% Pt loading, with increasing support size the ECSA increases from 4.91 to 7.23 m2gPt−1, 6.5%–10.6% of the predicted electrochemical surface area, respectively. This result suggests that there is an increased support contribution with increased TiO2 support size to improve Pt accessibility. The mass and area specific ORR activities are the highest for the 12 wt% Pt on 30 nm TiO2, 0.083 AmgPt−1 and 1134.56 μAcmPt−2, respectively. Accelerated stress testing results in only a 5.9% loss in ECSA for the 12 wt% Pt/TiO2 (30 nm) catalyst.

Control of the composition of Pt-Ni electrocatalysts in surfactant-free synthesis using neat N-formylpiperidine.

This paper describes the facile and surfactant-free synthesis of faceted Pt-Ni alloy nanoparticle electrocatalysts using neat N-formylpiperidine as a new type of solvent. Unlike the widely-used colloidal synthesis based on long-carbon chain surfactants, nanoparticles made in neat N-formylpiperidine possess a directly accessible surface for electrocatalytic reactions, making it a very attractive alternative solvent. The area-specific oxygen reduction reaction (ORR) activity is much higher than the commercial Pt/C catalyst reference and reaches a maximum of 1.12 mA cm(-2) for the Pt-Ni alloy nanoparticles. We observed that the freshly formed Pt-Ni alloy could have controllable bulk and near surface compositions under the same initial reaction conditions and precursor ratio. The change in the composition could be attributed to the effect of CO on the formation of uniform nuclei at the initial stage, and a different deposition rate between Pt and Ni metals during the growth. The well-defined Pt-Ni nanoparticle catalysts show strong composition-dependent catalytic behavior in ORR, highlighting the important role of controlling the growth kinetics in the preparation of active Pt-Ni ORR catalysts.

Fabrication and Performance of Membrane Electrode Assembly Using a Hydrophilic Pt/[TaOPO4/VC] Electrocatalyst

Oxide- and phosphate-supported catalysts have been shown to promote the oxygen reduction reaction (ORR) in acid and alkaline electrolyte. We previously reported a high mass- and area-specific ORR activity for an electrocatalyst with 18 wt% loading of 2.4 nm Pt nanoparticles supported on a 3 wt% tantalum oxyphosphate-treated VC (Pt/[TaOPO4/VC]). The electrocatalyst was heat treated at 660°C in a reducing atmosphere and characterized using thin-film rotating disc electrode (RDE) in an acidic electrolyte and alkaline electrolyte. In this study, we focused on Proton exchange membrane fuel cell (PEMFC) catalyst layers (CLs) fabricated by by direct deposition of the Pt/[TaOPO4/VC] electrocatalyst as the active cathode material onto the HP Nafion® membranes using an in-house built ultrasonic spray system. We focused our attention on the addition of a surfactant (i.e. Triton X100) to the catalyst ink formulation, as it has been shown that a surfactant can improve the wetting characteristics of the ink. Performance evaluation and cross-sectional visualization of the CCMs were performed. The performance of the CCMs prepared using the Triton ink formulation is superior to the performance of the CCMs prepared using the Standard ink formulation for both for the Pt/[TaOPO4/VC] and the Pt/C electrocatalyst, especially in the high current density region.

Fabrication and Performance of Membrane Electrode Assembly Using a Hydrophilic Pt/[TaOPO4/VC] Electrocatalyst

Oxide- and phosphate-supported catalysts promote the oxygen reduction reaction (ORR) in acid and alkaline electrolyte. We previously reported a high mass- and area-specific ORR activity for an electrocatalyst with 18 wt% loading of 2.4 nm Pt nanoparticles supported on a 3 wt% tantalum oxyphosphate-treated VC (Pt/[TaOPO4/VC]). The electrocatalyst was heat treated at 660◦C in a reducing atmosphere and characterized using thin-film rotating disc electrode (RDE) in an acidic electrolyte and alkaline electrolyte. However preliminary experiments show that the performance of the Pt-TaOPO4 catalysts as measured by RDE is not representative of its performance in a proton exchange membrane fuel cell (PEMFC), possibly because of the hydrophilicity of the oxyphosphate. We reconcile the difference between RDE and PEMFC performance by developing a range of catalyst layers (CLs) fabricated by decal method and by direct deposition of the Pt/[TaOPO4/VC] electrocatalyst onto the Nafion® membranes using an in-house built ultrasonic spray coating system. Nitrogen adsorption, mercury porosimetry, and scanning electron microscopy (SEM) are used to investigate the effects of ionomer/carbon ratio (I/C) on the surface area, pore structure and morphology of the CLs; cyclic voltammetry and polarization curves are used to determine the electrochemically active area (ECSA) and PEMFC performance.

Glad-SAD Pt-Ni Alloy/Ni Nanorods As Highly Active Oxygen Reduction Reaction Electrocatalysts

Polymer electrolyte membrane fuel cells (PEMFCs) have drawn significant attention as alternatives to traditional power sources, especially in automotive applications, due to their high energy conversion efficiency, zero emissions, and system robustness.1 However, the sluggish kinetics of oxygen reduction reaction (ORR) on the cathode demand a high loading of active platinum (Pt) or Pt-rich alloy catalysts,2 which increases the cost of PEMFC systems. Cost reductions can be achieved by increasing the ORR kinetics of platinum-based catalysts and by more efficient utilization of the platinum component of the catalyst. In addition, the limited durability of standard and advanced ORR electrocatalysts, associated with corrosion of the catalyst particles and support,3 ,4 remains a major obstacle. The development of carbon-free catalysts could address degradation issues associated with the carbon support. Relying on the high area-specific ORR activity of Pt thin films,4 electrocatalysts comprised of thin continuous Pt or Pt-alloy layers on patterned non-precious metal nanorod substrate supports could result in reductions in Pt loading due to both higher Pt utilization and higher ORR activity while also mitigating the stability issues associated with carbon supports. In this work, well-adherent and continuous Pt-Ni thin films, with several different molar ratios of Pt to Ni were deposited on Ni nanorods (NRs) by combining the glancing angle deposition (GLAD) technique5 for the deposition of Ni-NRs and small angle deposition (SAD) technique6 for the deposition of a thin conformal coating of Pt-Ni on the Ni-NRs (designated Pt-Ni@Ni-NRs). The Pt-Ni@Ni-NRs structures were supported on glassy carbon for evaluation of ORR activity in aqueous acidic electrolyte using the rotating disk electrode technique. These Pt-Ni@Ni-NRs catalysts showed superior area-specific and mass activities for ORR compared to PtNi nanorod catalysts7 that were prepared using the GLAD technique.

Pt@Nb-TiO2 Catalyst Membranes Fabricated by Electrospinning and Atomic Layer Deposition

A facile method was developed to fabricate fibrous membranes of niobium-doped titania-supported platinum catalysts (Pt@Nb-TiO2) by a two-step approach. The process started with generating niobium-doped titania (Nb-TiO2) fibrous membranes by electrospinning, followed by the deposition of Pt nanoparticles (NPs) using an atomic layer deposition (ALD) technique. The area-specific oxygen reduction reaction (ORR) activity of Pt@TiO2 catalyst membrane was increased by ∼20 folds if 10 at.% of Nb was incorporated into the ceramic fibers. The area-specific activity also increased with the number of ALD cycles, because of the increase of the Pt loading in the catalysts. After post-treatment of the catalyst membrane at high temperature in H2-containing atmosphere, the ORR activity became 0.28 mA/cm2Pt at 0.9 V (vs RHE), because of the improvement in conductivity of Nb-TiO2 fibers and better crystalinity of Pt NPs. The results of accelerated-stability test showed that the Pt@Nb-TiO2 catalyst membrane was highly stable...

Oxygen Electroreduction on Nanoscale Pt/[TaOPO4/VC] and Pt/[Ta2O5/VC] in Alkaline Electrolyte

Oxide- and phosphate-supported catalysts have been shown to promote the oxygen reduction reaction (ORR) in acid [see references in 1 ], and our question herein is whether similar enhancement of the ORR occurs in alkaline condition. We previously reported a high mass- and area-specific ORR activity for an electrocatalyst with 18% loading of 2.4 nm Pt nanoparticles supported on a 3 wt% tantalumoxyphosphate-treated VC (Pt/[TaOPO4/VC]). The electrocatalyst was heat treated at 660 ◦ C in a reducing atmosphere and characterized using thin-film RDE in an acidic electrolyte. 2,3 A two-fold increase in ORR activity was observed for the Pt/[TaOPO4/VC] compared with a commercial Pt/VC electrocatalyst based on RDE results: at 0.90 V vs. RHE, 1600 rpm, and a potential sweep rate of 20 mV s −1 ,am ass and area-specific ORR activity of 0.46 A mgPt −1 and 625 μ Ac mPt −2 , respectively,weremeasuredfortheheattreatedPt/[TaOPO4/VC]electrocatalyst, compared to mass and area-specific ORR activities of 0.24AmgPt −1 and 333 μ Ac mPt −2 , respectively, for a standard Pt/VC electrocatalyst. The Pt/[TaOPO4/VC] electrocatalyst also proved highly durable to extensive voltage cycling in the acid electrolyte. Meanwhile, cyclic voltammetry (CV) of the Pt/[TaOPO4/VC] and Pt/VC showed no difference in onset potentials for oxygen-species adsorption on the Pt, suggesting there was a lack of an electronic effect to change the oxygen coverage of the Pt via a change in bond strength. Further investigation of the Pt/[TaOPO4/VC] electrocatalyst using an in situ X-ray absorption spectroscopy (XAS) difference technique, � μ, showed differences in the surface species adsorbed on the active Pt/[TaOPO4/VC] electrocatalyst compared to Pt/VC. 3 This combination of characterization results led to the conclusion that the higher ORR activity of the 660 ◦ C-heated Pt/[TaOPO4/VC] electrocatalyst was due to the presence of phosphate groups on the surfaces of the Pt nanoparticles, creating a higher proton concentration or acidity at the electrocatalyst surface. Encouraged by the high activity and durability results previously reported, the work presented here examines the ORR performance of Pt/[TaOPO4/VC] and Pt/[Ta2O5/VC] electrocatalysts, heat treated at 660 ◦ C in an Ar atmosphere, in an alkaline electrolyte. The catalyst particle dispersion, size distribution, and structural information were obtained using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The two supported materials, Pt/[TaOPO4/VC] and Pt/[Ta2O5/VC], and Pt/VC standards were evaluatedinanalkalineelectrolytebyCVandRDEevaluationoftheORR activity.

Shape and Composition-Controlled Platinum Alloy Nanocrystals Using Carbon Monoxide as Reducing Agent

The shape of metal alloy nanocrystals plays an important role in catalytic performances. Many methods developed so far in controlling the morphologies of nanocrystals are however limited by the synthesis that is often material and shape specific. Here we show using a gas reducing agent in liquid solution (GRAILS) method, different Pt alloy (Pt-M, M = Co, Fe, Ni, Pd) nanocrystals with cubic and octahedral morphologies can be prepared under the same kind of reducing reaction condition. A broad range of compositions can also be obtained for these Pt alloy nanocrystals. Thus, this GRAILS method is a general approach to the preparation of uniform shape and composition-controlled Pt alloy nanocrystals. The area-specific oxygen reduction reaction (ORR) activities of Pt(3)Ni catalysts at 0.9 V are 0.85 mA/cm(2)(Pt) for the nanocubes, and 1.26 mA/cm(2)(Pt) for the nanooctahedra. The ORR mass activity of the octahedral Pt(3)Ni catalyst reaches 0.44 A/mg(Pt).

Glancing Angle Deposited Platinum Nanorod Arrays for Oxygen Reduction Reaction

ABSTRACTIn this work, we investigated the electrocatalytic oxygen reduction reaction (ORR) activity of vertically aligned, single-layer, carbon-free, and single crystal Pt nanorod arrays utilizing cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques. A glancing angle deposition (GLAD) technique was used to fabricate 200 nm long Pt nanorods, which corresponds to Pt loading of 0.16 mg/cm2, on glassy carbon (GC) electrode at a glancing angle of 85° as measured from the substrate normal. An electrode comprised of conventional carbon-supported Pt nanoparticles (Pt/C) was also prepared for comparison with the electrocatalytic ORR activity and stability of Pt nanorods. CV results showed that the Pt nanorod electrocatalyst exhibits a more positive oxide reduction peak potential compared to Pt/C, indicating that GLAD Pt nanorods are less oxophilic. In addition, a series of CV cycles in acidic electrolyte revealed that Pt nanorods are significantly more stable against electrochemically-active surface area loss than Pt/C. Moreover, room temperature RDE results demonstrated that GLAD Pt nanorods exhibit higher area-specific ORR activity than Pt/C. The enhanced electrocatalytic ORR activity of Pt nanorods is attributed to their larger crystallite size, single-crystal property, and the dominance of (110) crystal planes on the large surface area nanorods sidewalls, which has been found to be the most active plane for ORR. However, the Pt nanorods showed lower mass specific activity than the Pt/C electrocatalyst due to the large diameter of the Pt nanorods.

Activity of dealloyed PtCo 3 and PtCu 3 nanoparticle electrocatalyst for oxygen reduction reaction in polymer electrolyte membrane fuel cell

We report a comparative study of the alloy formation and electrochemical activity of dealloyed PtCo 3 and PtCu 3 nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). For the Pt–Co system the maximum annealing temperatures were 650 °C, 800 °C and 900 °C for 7 h to drive the Pt–Co alloy formation and the particle growth. EDS and XRD were employed for the characterization of catalyst powders. The RDE and RRDE experiments were conducted in 0.1 M HClO 4 at room temperature. We demonstrate that the mass and surface area specific ORR activities of Pt–Co and Pt–Cu alloys after voltammetric activation exhibit a considerable improvement compared to those of pure Pt/C. The dealloyed PtCo 3 (800 °C/7 h) electrocatalyst performs 3 times higher in terms of Pt-based mass activity and 4–5 times higher in terms of ECSA-based specific activity than a 28.2 wt.% Pt/C. Dealloyed Pt–Co catalysts (800 °C/7 h) show the most favorable balance between mass and specific ORR activity with a particle size of 2.2 ± 0.1 nm. We hypothesize that geometric strain effects of the dealloyed Pt–Co nanoparticles, similar to those found in dealloyed PtCu 3 nanoparticles, are responsible for the improvement in ORR activity [1].

Preparation and Structural Analysis of Carbon-Supported Co Core/Pt Shell Electrocatalysts Using Electroless Deposition Methods

Cobalt core/platinum shell nanoparticles were prepared by the electroless deposition (ED) of Pt on carbon-supported cobalt catalyst (Co/C) and verified by HRTEM images. For a 2.0 wt % Co/C core, the ED technique permitted the Pt loading to be adjusted to obtain a series of bimetallic compositions with varying numbers of monolayers (ML). The tendency for corrosion of Co and the electrochemical (i.e., oxygen reduction reaction (ORR)) activity of the structures were measured. The results from temperature-programmed reduction (TPR) analysis suggest that a single Pt ML coverage is formed at a Pt weight loading between 0.5 and 0.7% on the 2.0% Co/C. HRTEM analysis indicates that the continuity of the Pt shell on the Co core depends on the precursor Co particle size, where "large" Co particles (>10 nm) favor noncontinuous, three-dimensional Pt structures and "small" Co particles (<6 nm) favor layer-by-layer growth. For these larger core-shell particles, Co was observed to quickly corrode in 0.3 M H(2)SO(4). Surface area specific ORR activity, measured by chemisorption techniques, revealed that the Pt-Co/C catalysts performed better than a commercial Pt/C catalyst; however, on a Pt mass basis, only the lower Pt:Co atomic ratio Pt-Co/C catalysts outperformed the Pt/C catalyst.

Electrochemical Observation of Ligand Effects on Oxygen Reduction at Ligand-Stabilized Pt Nanoparticle Electrocatalysts

We investigate the effect of triphenylphosphine triphosphonate, TPPTP, on the electrocatalytic activity for the oxygen reduction reaction (ORR) at ligand-stabilized Pt nanoparticles (Pt 2.1 nm - TPPTP). The Pt 2.1 nm - TPPTP catalyst supported on Vulcan carbon (VC) (Pt 2.1 mm - TPPTP/VC) adsorbs and desorbs oxygen +40 mV vs the potential for bare Pt nanoparticles supported on VC (Pt 2.4 nm /VC), as measured by cyclic voltammetry in Ar-saturated 0.1 M HClO 4 . The area-specific ORR activity (i s ) of Pt in the Pt 2.1 - TPPTP/VC catalyst is -22% higher than that of Pt 2.4 nm /VC, as measured by rotating disk electrode voltammetry. These results suggest that the TPPTP ligand weakens the Pt-O bond, resulting in the observed kinetic enhancement of the ORR.

Oxygen Reduction Activity of Carbon-Supported Pt−M (M = V, Ni, Cr, Co, and Fe) Alloys Prepared by Nanocapsule Method

Monodispersed Pt and Pt-M (M = V, Cr, Fe, Co, and Ni) alloy nanoparticles supported on carbon black (denoted as Pt/CB and Pt-M/CB) were prepared by the simultaneous reduction of platinum acetylacetonate and the second metal acetylacetonate within nanocapsules formed in diphenyl ether in the presence of carbon black. For the Pt/CBs, the average Pt diameters measured by scanning transmission electron microscopy (STEM) or X-ray diffraction (XRD) ranged from 2.0 to 2.5 nm, regardless of the catalyst-loading level from 10 to 55 wt % on CB. The alloy composition was found to be well-controlled to the projected value among the supported particles. The activities for the oxygen reduction reaction (ORR) at Nafion-coated catalysts in O2-saturated 0.1 M HClO4 solution were evaluated by using a channel flow electrode (CFE) cell at 30 degrees C. The area-specific ORR activities at Pt-M/CB were found to be 1.3 to 1.8 times higher than that at Pt/CB. The ORR activity increased in the order Pt/CB < Pt-Ni/CB < Pt-Fe/CB < Pt-Co/CB < Pt-V/CB < Pt-Cr/CB.