Mehtap Oezaslan

Noble Metal Aerogels—Synthesis, Characterization, and Application as Electrocatalysts

Conspectus Metallic and catalytically active materials with high surface area and large porosity are a long-desired goal in both industry and academia. In this Account, we summarize the strategies for making a variety of self-supported noble metal aerogels consisting of extended metal backbone nanonetworks. We discuss their outstanding physical and chemical properties, including their three-dimensional network structure, the simple control over their composition, their large specific surface area, and their hierarchical porosity. Additionally, we show some initial results on their excellent performance as electrocatalysts combining both high catalytic activity and high durability for fuel cell reactions such as ethanol oxidation and the oxygen reduction reaction (ORR). Finally, we give some hints on the future challenges in the research area of metal aerogels. We believe that metal aerogels are a new, promising class of electrocatalysts for polymer electrolyte fuel cells (PEFCs) and will also open great opportunities for other electrochemical energy systems, catalysis, and sensors. The commercialization of PEFCs encounters three critical obstacles, viz., high cost, insufficient activity, and inadequate long-term durability. Besides others, the sluggish kinetics of the ORR and alcohol oxidation and insufficient catalyst stability are important reasons for these obstacles. Various approaches have been taken to overcome these obstacles, e.g., by controlling the catalyst particle size in an optimized range, forming multimetallic catalysts, controlling the surface compositions, shaping the catalysts into nanocrystals, and designing supportless catalysts with extended surfaces such as nanostructured thin films, nanotubes, and porous nanostructures. These efforts have produced plenty of excellent electrocatalysts, but the development of multisynergetic functional catalysts exhibiting low cost, high activity, and high durability still faces great challenges. In this Account, we demonstrate that the sol–gel process represents a powerful “bottom-up” strategy for creating nanostructured materials that tackles the problems mentioned above. Aerogels are unique solid materials with ultralow densities, large open pores, and ultimately high inner surface areas. They magnify the specific properties of nanomaterials to the macroscale via self-assembly, which endow them with superior properties. Despite numerous investigations of metal oxide aerogels, the investigation of metal aerogels is in the early stage. Recently, aerogels including Fe, Co, Ni, Sn, and Cu have been obtained by nanosmelting of hybrid polymer–metal oxide aerogels. We report here exclusively on mono-, bi- and multimetallic noble metal aerogels consisting of Ag, Au, Pt, and Pd and their application as electrocatalysts.

Activity, Stability, and Degradation Mechanisms of Dealloyed PtCu3 and PtCo3 Nanoparticle Fuel Cell Catalysts

A key challenge in today’s fuel cell research is the understanding and maintaining the durability of the structure and performance of initially highly active Pt fuel cell electrocatalysts, such as dealloyed Pt or Pt monolayer catalysts. Here, we present a comparative long‐term stability and activity study of supported dealloyed PtCu3 and PtCo3 nanoparticle fuel cell catalysts for the oxygen reduction reaction (ORR) and benchmark them to a commercial Pt catalyst. PtCu3 and PtCo3 were subjected to two distinctly different voltage cycling tests: the “lifetime” regime [10 000 cycles, 0.5–1.0 V vs. RHE (reversible hydrogen electrode), 50 mV s−1] and the corrosive “start‐up” regime (2000 cycles, 0.5–1.5 V vs. RHE, 50 mV s−1). Our results highlight significant activity and stability benefits of dealloyed PtCu3 and PtCo3 for the ORR compared with those of pure Pt. In particular, after testing in the “lifetime” regime, the Pt‐surface‐area‐based activity of the Pt alloy catalysts is still two times higher than that of pure Pt. From our electrochemical, morphological, and compositional results, we provide a general picture of the temporal sequence of dominant degradation mechanisms of a Pt alloy catalyst during its life cycle.

Mesoporous Nitrogen Doped Carbon Supported Platinum PEM Fuel Cell Electrocatalyst Made From Ionic Liquids

A multitude of new and improved catalyst materials and concepts for membrane fuel cells were developed over the last decade. The requirements of these catalysts are low cost, high activity and durability. For example, platinum based catalyst concepts such as Pt monolayer catalysts, 2] Pt skin catalysts, Pt multimetallic catalysts, and dealloyed bimetallic Pt core-shell nanoparticle catalysts show promising activities based on Pt mass and Pt surface area for the oxygen reduction reaction (ORR). Furthermore, non-noble metal catalyst concepts could reduce the costs, but they currently still do not meet the activity targets for commercial fuel cell electrocatalysts. To improve the durability of fuel cell catalysts, also the support material is becoming more important. Oxidation resistance of the support material is one point of concern. Alternatives to pure carbon blacks (e.g. Vulcan XC 72R) were evaluated for the oxygen reduction, such as carbon nanotubes, 24] silicon carbide derived carbons, hollow spherical carbons, nitrogen modified carbons, or titanium-based materials. Especially nitrogen doped carbons show interesting properties like high conductivity, mesoporosity and the opportunity to adjust the nitrogen content in the support material. In this communication, we report the synthesis of a mesoporous nitrogen doped carbon supported platinum catalyst (Pt/ meso-BMP) based on an ionic liquid as nitrogen/carbon precursor and the evaluation of the catalytic system for ORR. Further, we analyzed the long-term behavior of this new catalyst and compared it with commercial high surface area carbon (HSAC) supported platinum catalyst. The mesoporous nitrogen doped carbon supported platinum nanoparticle fuel cell electrocatalyst (Pt/meso-BMP) was prepared by a two-step synthesis, as shown in Figure 1. In the first step, the mesoporous nitrogen doped carbon material (meso-BMP) was synthesized corresponding to the reference 35] by using N-butyl-3-methylpyridinedicyanamide (BMPdca) as ionic liquid compound. As evaluated by X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA) the nitrogen content of 14.2 wt. % (XPS)/17.2 wt. % (EA) is very high. The variation of the values can be explained by the surface specificity of XPS measurements. In the second step, platinum nanoparticles were deposited on the meso-BMP substrate. The deposition of Pt occurred by a wet impregnation–freeze-drying method and followed by thermal annealing in a reductive atmosphere. Shown in Figure 2 are the XRD profiles for meso-BMP and Pt/meso-BMP. The as synthesized meso-BMP support material exhibits broad XRD reflections at 2 q= 26.1 and 42.98 corresponding to the inter (002) and intra (101) lattice planes of graphitized carbon. The reference powder diffraction patterns of (111), (200), and (220) lattice planes for pure face centered Figure 1. Synthesis route for mesoporous nitrogen doped carbon supported platinum nanoparticle catalyst.

Structure-Activity Relationship of Dealloyed PtCo3 and PtCu3 Nanoparticle Electrocatalyst for Oxygen Reduction Reaction in PEMFC

We report a synthesis and study on carbon supported PtCo3 and PtCu3 alloy nanoparticle catalyst for ORR. The chemical composition of alloys was carried out with EDS. The electrochemical measurements were conducted using a thin-film RDE method. Recently, we have demonstrated that dealloyed PtCu3 nanoparticle exhibits 3-4 times higher mass activity and 4 times higher specific activity for ORR than Pt. Here, the dealloyed PtCo3 also shows about 4 fold increase in specific activity, but 2-3 fold in mass activity than Pt/C. The in-situ generated Pt rich surface of Co rich Pt alloy nanoparticle catalyst tested for ORR activity. Geometric effects based on Pt surface constitution were assumed for the high activity for ORR. PtCo3 nanoparticle catalyst seems to be an interesting opportunity for further studies for ORR. The thermodynamic instable deposition of Co and the robust Pt ECSA are probably the large advantages to other Pt alloys.

Activity and Structure of Dealloyed PtNi3 Nanoparticle Electrocatalyst for Oxygen Reduction Reaction in PEMFC

Here, we report a synthesis and activity study of the dealloyed, highly active PtNi3 alloy nanoparticle catalyst for the oxygen reduction reaction (ORR). The dealloyed PtNi3 exhibits 7 – 8 times higher Pt mass based activity and 6 – 7 times higher Pt surface area specific based activity for ORR than pure Pt by similar mean particle size. Further, we have tested the long-term durability of the dealloyed PtNi3 for the typical and corrosive operating fuel cell conditions. After the voltage testing with 10000 voltage cycles between 0.5 – 1.0 V vs. RHE and a scan rate of 50 mV s in deaerated 0.1 M HClO4 the activated PtNi3 catalyst still shows 4 – 5 fold increase in Pt surface area specific based activity compared with that for pure Pt.

In Situ Observation of the Thermally Induced Growth of Platinum‐Nanoparticle Catalysts Using High‐Temperature X‐ray Diffraction

Fundamental understanding about the thermal stability of nanoparticles and deliberate control of structural and morphological changes under reactive conditions is of general importance for a wide range of reaction processes in heterogeneous and electrochemical catalysis. Herein, we present a parametric study of the thermal stability of carbon-supported Pt nanoparticles at 80 °C and 160 °C, with an initial particle size below 3 nm, using in situ high-temperature X-ray diffraction (HT-XRD). The effects on the thermal stability of carbon-supported Pt nanoparticles are investigated with control parameters such as Brunauer-Emmet-Teller (BET) surface area, metal loading, temperature, and gas environment. We demonstrate that the growth rate exhibits a complex, nonlinear behavior and is largely controlled by the temperature, the initial particle size, and the interparticle distance. In addition, an ex situ transmission electron microscopy study was performed to verify our results obtained from the in situ HT-XRD study.

Alloying Behavior of Self-Assembled Noble Metal Nanoparticles

The atomic redistribution processes occurring in multiparticle nanostructures are hardly understood. To obtain a more detailed insight, we applied high-resolution microscopic, diffraction and spectroscopic characterization techniques to investigate the fine structure and elemental distribution of various bimetallic aerogels with 1:1 compositions, prepared by self-assembly of single monometallic nanoparticles. The system Au-Ag exhibited a complete alloy formation, whereas Pt-Pd aerogels formed a Pd-based network with embedded Pt particles. The assembly of Au and Pd nanoparticles resulted in a Pd-shell formation around the Au particles. This work confirms that bimetallic aerogels are subject to reorganization processes during their gel formation.

Oxygen Electroreduction on PtxCo1-x and PtxCu1-x Alloy Nanoparticles for Basic and Acidic PEM Fuel Cell

Our study presents the electrochemical characterization of PtxCo1-x and PtxCu1-x nanoparticle electrocatalysts after the voltammetric treatment in basic and acidic electrolytes at room temperature. The chemical composition and mean particle size of the Pt alloy nanoparticles were determined before and after the voltage cycling using TEM and EDS. The electrochemical experiments were conducted with the RDE technique. We show that the electrochemical conditioning is a critical step for the formation of highly active Pt alloy nanoparticle electrocatalysts for ORR. The voltage cycling in acid leads to the leaching of less noble metal to generate a reactive Pt enriched particle surface, while in basic stable metal hydroxide/oxide are primarily formed on the surface of the Pt alloy particles. In particular, in basic voltammetric pretreated PtM3 shows the lowest Pt mass based activity for ORR. In contrast, in acid dealloyed PtCu3 and PtCo3 exhibit 3 4 fold increase in jmass compared with Pt/HSAC.

Electroless plating of ultrathin palladium films: self-initiated deposition and application in microreactor fabrication

We present new electroless palladium plating reactions, which can be applied to complex-shaped substrates and lead to homogeneous, dense and conformal palladium films consisting of small nanoparticles. Notably, autocatalytic and surface-selective metal deposition could be achieved on a wide range of materials without sensitization and activation pretreatments. This provides a facile and competitive route to directly deposit well-defined palladium nanofilms on e.g. carbon, paper, polymers or glass substrates. The reactions proceed at mild conditions and are based on easily accessible chemicals (reducing agent: hydrazine; metal source: PdCl2; ligands: ethylenediaminetetraacetic acid (EDTA), acetylacetone). Additionally, the water-soluble capping agent 4-dimethylaminopyridine (DMAP) is employed to increase the bath stability, to ensure the formation of small particles and to improve the film conformity. The great potential of the outlined reactions for micro- and nanofabrication is demonstrated by coating an ion-track etched polycarbonate membrane with a uniform Pd film of approximately 20 nm thickness. The as-prepared membrane is then employed as a highly miniaturized flow reactor, using the reduction of 4-nitrophenol with NaBH4 as a model reaction.

Nanoparticles in a box: a concept to isolate, store and re-use colloidal surfactant-free precious metal nanoparticles

A concept is introduced that allows for the isolation, storage and re-use of surfactant-free precious metal nanoparticles (NPs) of catalytic relevance (Pt and Ru). “Surfactant-free NPs” well-defined in size (1–2 nm) are prepared in alkaline ethylene glycol. After synthesis these NPs are stabilized by surface bound CO, formed during synthesis by solvent oxidation, and OH−, added to the reaction mixture. We present a protocol that allows switching reversibly the stabilization between a “CO-protected” and “OH−-protected state”. Most importantly, “OH−-protected” Pt and Ru NPs exhibit remarkable resistance against sintering. These NPs can be isolated as solids, stored and “put into boxes” to be shipped. Thereafter they can be redispersed without changes in particle size or loss in catalytic activity. These results are expected to be of scientific and industrial relevance, as a methodology is introduced to handle “surfactant-free” catalytic nanoparticles like a normal solid chemical.