Hongsen Wang

Structurally ordered intermetallic platinum–cobalt core–shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts

To enhance and optimize nanocatalyst performance and durability for the oxygen reduction reaction in fuel-cell applications, we look beyond Pt-metal disordered alloys and describe a new class of Pt-Co nanocatalysts composed of ordered Pt(3)Co intermetallic cores with a 2-3 atomic-layer-thick platinum shell. These nanocatalysts exhibited over 200% increase in mass activity and over 300% increase in specific activity when compared with the disordered Pt(3)Co alloy nanoparticles as well as Pt/C. So far, this mass activity for the oxygen reduction reaction is the highest among the Pt-Co systems reported in the literature under similar testing conditions. Stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core-shell structure was maintained virtually intact, as established by atomic-scale elemental mapping. The high activity and stability are attributed to the Pt-rich shell and the stable intermetallic Pt(3)Co core arrangement. These ordered nanoparticles provide a new direction for catalyst performance optimization for next-generation fuel cells.

Pt-Decorated PdCo@Pd/C Core−Shell Nanoparticles with Enhanced Stability and Electrocatalytic Activity for the Oxygen Reduction Reaction

A simple method for the preparation of PdCo@Pd core-shell nanoparticles supported on carbon based on an adsorbate-induced surface segregation effect has been developed. The stability of these PdCo@Pd nanoparticles and their electrocatalytic activity for the oxygen reduction reaction (ORR) were enhanced by decoration with a small amount of Pt deposited via a spontaneous displacement reaction. The facile method described herein is suitable for large-scale, lower-cost production and significantly lowers the Pt loading and thus the cost. The as-prepared PdCo@Pd and Pd-decorated PdCo@Pd nanocatalysts have a higher methanol tolerance than Pt/C in the ORR and are promising cathode catalysts for fuel cell applications.

Effect of biaxial strain on the electrical and magnetic properties of (001) La0.7Sr0.3MnO3 thin films

We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive molecular-beam epitaxy on different single crystalline substrates, varying the substrate-induced biaxial strain from −2.3% to +3.2%. Magnetization and electrical transport measurements reveal that the dependence of the Curie temperature on biaxial strain is in very good agreement with the theoretical predictions of Millis et al. [J. Appl. Phys. 83, 1588 (1998)].

Highly Stable and CO-Tolerant Pt/Ti0.7W0.3O2 Electrocatalyst for Proton-Exchange Membrane Fuel Cells

The current materials used in proton-exchange membrane fuel cells (PEMFCs) are not sufficiently durable for commercial deployment. One of the major challenges lies in the development of an inexpensive, efficient, and CO-tolerant anode catalyst. Here we report the unique CO-tolerant property of Pt nanoparticles supported on Ti(0.7)W(0.3)O(2). The Ti(0.7)W(0.3)O(2) nanoparticles (50 nm) were synthesized via a sol-gel process and platinized using an impregnation-reduction technique. Electrochemical studies of Pt/Ti(0.7)W(0.3)O(2) show unique CO-tolerant electrocatalytic activity for hydrogen oxidation compared to commercial E-TEK PtRu/C catalysts. Differential electrochemical mass spectrometry measurements show the onset potential for CO oxidation on Pt/Ti(0.7)W(0.3)O(2) to be below 0.1 V (vs RHE). Pt/Ti(0.7)W(0.3)O(2) is a promising new anode catalyst for PEMFC applications.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)[subscript 4]O[subscript 4] Cubanes

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Methanol electrooxidation on PtRu bulk alloys and carbon-supported PtRu nanoparticle catalysts: a quantitative DEMS study.

Methanol electrooxidation on smooth Pt and PtRu bulk alloys and carbon-supported Pt and PtRu nanoparticle catalysts has been studied using cyclic voltammetry and potential step chronoamperometry combined with differential electrochemical mass spectrometry (DEMS). The current efficiencies for generated CO2 and methyl formate were calculated from Faradaic current (coulometric charge) and mass spectrometric currents (charges) at m/z=44 and m/z=60. The effects of Ru content in PtRu catalysts, catalyst loading/roughness, and the concentration of sulfuric acid as supporting electrolyte on the reaction kinetics and product distribution during methanol electrooxidation have been investigated. The results indicate that Pt-rich PtRu alloys and carbon-supported PtRu catalysts with ca. 20 atom % Ru content exhibit the highest catalytic activity for methanol electrooxidation, that is, the highest Faradaic current and the highest current efficiency for CO2 generation at low applied potentials. As the catalyst loading/roughness increases, the current efficiency for CO2 formation increases due to the further oxidation of soluble intermediates (formaldehyde and formic acid). At high concentrations of sulfuric acid, the electrooxidation of methanol was suppressed; both the oxidative current and the current efficiency of CO2 decreased, likely due to sulfate/bisulfate adsorption.

A rechargeable Na–CO2/O2 battery enabled by stable nanoparticle hybrid electrolytes

We report on rechargeable batteries that use metallic sodium as the anode, a mixture of CO2 and O2 as the active material in the cathode, and an organic–inorganic hybrid liquid as electrolyte. The batteries are attractive among energy storage technologies because they provide a mechanism for simultaneously capturing CO2 emissions while generating electrical energy. Through in and ex situ chemical analysis of the cathode we show that NaHCO3 is the principal discharge product, and that its relative instability permits cell recharging. By means of differential electrochemical mass spectrometry (DEMS) based on 12C and 13C we further show that addition of as little as 10% of 1-methyl-3-propylimidazolium bis(trifluoromethanesulfone)imide ionic liquid tethered to SiO2 nanoparticles extends the high-voltage stability of the electrolyte by at least 1 V, allowing recharge of the Na–CO2/O2 cells.

CO2 and O2 Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study

A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Li-ion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO2 or LiMn2O4 was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF6 in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO2, LiMn2O4, and LiNi(0.5)Mn(1.5)O4 were studied. We found that 1 M LiPF6 + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi(0.5)Mn(1.5)O4 is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF6 + EC/DEC electrolyte on a LiCoO2 cathode, as indicated by CO2 and O2 evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described.

New Double-Band-Electrode Channel Flow Differential Electrochemical Mass Spectrometry Cell: Application for Detecting Product Formation during Methanol Electrooxidation

We present a new double-band-electrode channel flow DEMS (differential electrochemical mass spectrometry) cell and demonstrate its application in mechanistic studies with particular relevance to fuel cells. The cell is composed of two band electrodes, which serve as working and detecting electrodes, respectively, separated by a porous Teflon membrane. The Teflon membrane serves as the interface between the aqueous solution and vacuum, through which gases and volatile species can be transported. The hydrodynamic electrochemical characteristics and mass spectrometric behavior have been characterized. With this DEMS cell, gaseous and volatile electrochemical products formed at the working electrode are monitored by mass spectrometry, while nonvolatile products can be selectively detected at the detecting (downstream) electrode. Thus, this system can be considered as the DEMS analogue of a rotating ring/disk electrode. As test cases, the electrooxidation of formaldehyde and methanol on carbon supported Pt nanoparticle catalysts have been studied using this new channel flow DEMS cell.