Article pubs.acs.org/JPCC
In Situ Liquid Cell TEM Study of Morphological Evolution and Degradation of Pt−Fe Nanocatalysts During Potential Cycling Guo-Zhen Zhu,†,⊥ Sagar Prabhudev,‡,⊥ Jie Yang,§ Christine M. Gabardo,§ Gianluigi A. Botton,‡ and Leyla Soleymani*,§,∥ †
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, People’s Republic of China ‡ Department of Materials Science and Engineering and Canadian Center for Electron Microscopy, §School of Biomedical Engineering, and ∥Department of Engineering Physics, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada S Supporting Information *
ABSTRACT: Nanocatalyst degradation is a serious limiting factor for the commercialization of proton exchange membrane fuel cells. Although the degradation has been extensively studied in the past through various ex situ electrochemical methods, employing an in situ technique can greatly improve our understanding of the mechanisms involved during the electrochemical cycling. In this work, we have employed an in situ liquid cell inside a TEM for a simultaneous investigation of the structural evolution and electrochemical response of Pt−Fe nanocatalysts. We demonstrate that the coarsening processes of these nanocatalyst particles, including the nucleation and growth, are not uniform, both in space and in time scale. The growth rate is found to be both site- and potential-dependent. Furthermore, these particles were found to exhibit considerably different behaviors when attached to an electrode as opposed to when isolated in the electrolyte. With Pt−Fe nanoalloy system as a candidate material, this work demonstrates that the in situ structural characterization of nanocatalysts under electrochemical bias and inside the native electrolyte environment provides much deeper insight into the catalyst degradation mechanisms as compared to the routine ex situ electrochemical studies. platinum noble metal with early transition metals such as Co,4,5 Fe,4−7 Ni,4,5 V,4 Cr,4 Sc,8 Y8 and Mn.9,10 Core−shell structured catalysts11,12 have been extensively studied in the recent years, which are known to demonstrate high catalytic performance at a low cost due to the presence of a high surface area Pt shell combined with inexpensive transition metal cores. In addition to compositional tuning, the structural modifications in terms of ordered alloy lattices13−15 have been demonstrated to enhance the catalytic activity and durability. Several attempts have been made to link the variability in catalytic performance to structural changes of the catalysts. However, most of these studies have been limited by ex situ characterization techniques,
1. INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFC) are promising clean energy sources for facing the pressure of global energy challenge and environmental issues. However, this technology has not yet reached out, commercially, due largely to slow cathodic reaction, termed the oxygen reduction reaction (ORR). The ORR is long known to be sluggish and hence needs to be accelerated by employing suitable catalysts, thereby introducing an associated prohibitive cost from the noble metals in use.1 One of the long trending challenges has been the performance degradation of these catalysts due to harsh operating environment within the fuel cell chamber, primarily involving the coarsening of nanocatalysts and the corrosion of support materials during the operation.2 With an intent to enhance the performance while reducing the cost, various novel catalyst designs3 have been explored by alloying © 2014 American Chemical Society
Received: July 9, 2014 Revised: August 30, 2014 Published: September 3, 2014 22111
dx.doi.org/10.1021/jp506857b | J. Phys. Chem. C 2014, 118, 22111−22119
The Journal of Physical Chemistry C
Article
Figure 1. Illustration of the in situ electrochemical liquid cell TEM holder. (a) Schematic representation of the liquid cell featuring two silicon chips. Note that the figure is not set to scale. Relevant dimensions are made available in the Supporting Information, section 1. (b) Schematic representation of the top electrochemistry silicon chip. The inset represents the TEM image of the carbon working electrode modified with platinum nanoparticles on the top electrochemistry chip. (c) Cyclic voltammagram measured using the TEM liquid cell holder of (b) with 0.1 M H2SO4 as the electrolyte at a scan rate of 100 mV/s with the electron beam turned on.
observed.20,21 Although general processes have been proposed on the basis of ex situ techniques followed by statistical studies, detailed mechanistic information regarding the coarsening processes occurring in native liquid environments and under electrical bias at the microscopic level is not well understood due to the lack of spatially resolved characterization techniques with an in situ capability. The difficulty in studying the catalyst structure evolution in real-time and during electrochemical cycling lies in the fact that conventional spatially resolved characterization techniques require dry catalyst samples extracted from the working electrode. Transmission electron microscopy (TEM) is a method combining nanoscale spatial resolution with subsecond temporal resolution, which is ideal for structural evolution studies of catalysts. In fact, the morphology evolution of nanocatalysts on a TEM grid has been tracked ex situ through what is known as “identical-location TEM” on-site observation,22,23 which allows the grid to be taken from the electrolyte and the same observation regions to be repeatedly recorded after different cycles. However, conventional ex situ TEM studies can cause the reprecipitation of soluble metals species and the detachment of particles when grids are taken out of
where investigation of the catalyst structure was limited to its initial and final states. While the identical location TEM (ILTEM)-based studies emerged as an improvement in this regard, a dynamic study was not able to be achieved as the modes of analyses were still confined to characterization, before and after cycling.16 Correlation between the structural evolution, during working condition in the native environment, and catalytic performance therefore remains poorly understood. In case of pure Pt/C nanocatalysts, comparing the initial structures and those after thousands of potential cycles has suggested several mechanisms regarding catalyst coarsening.17,18 The Pt dissolution and precipitation mechanisms state that the Pt nanocatalysts are dissolved into an ionomer phase, and then can be transferred and redeposited to form larger particles. Another mechanism contributing to the catalyst coarsening involves the movement of detached catalyst particles from the supports and the particle coalescence. In the case of Pt−alloy nanocatalysts, additional dealloying processes occur because early transition metals have a stronger tendency to be dissolved and stabilized in dilute acidic electrolytes.19 Thus, the formation of core−shell structures, the appearance of Pt-rich particles, and the increase in the particle size have been 22112
dx.doi.org/10.1021/jp506857b | J. Phys. Chem. C 2014, 118, 22111−22119
The Journal of Physical Chemistry C
Article
Figure 2. Numerical modeling of the current density within the electrochemical cell. (a) Plot of current density norm (A/m2) within acid electrolyte of electrochemical cell; red arrows indicate the current density vector. Inner ellipse represents the working electrode, and outer ring represents the counter electrode. (b) Magnified plot of current density norm around working electrode with platinum microparticles contacting or near the working electrode.
voltammogram (CV) curves were recorded simultaneously to analyze the leaching process of transition metals, the coarsening mechanism of these catalysts, as well as the reprecipitation behaviors of soluble metal species over the course of potential cycling.
their native liquid environments and placed under a vacuum. On the other hand, the catalytic structure can be investigated in a native wet environment, such as in the electrolyte, through in situ X-ray, synchrotron, and other spectroscopic techniques.10 Recently, the new “in-operando” techniques have demonstrated the capability of simultaneously observing structural changes during the operation while monitoring related electrochemical responses,24 making these techniques powerful tools for investigating the structural origins of the catalytic activity and durability. In this Article, we applied the recently developed in situ electrochemical “liquid cell” technique to observe the structural evolution of individual Pt−Fe nanocatalysts in real-time, under electrochemical bias, and inside their native liquid electrolyte environment using a TEM. Pt−M nanocatalysts (M = Co, Cu, Fe, Ni, Cr) have been under intense investigation over the past few years due to their potential in offering higher durability and activity at a lower cost as compared to Pt/C nanoparticles. Among PtM systems, PtFe nanocatalysts have gathered great interest toward the ORR14,25−29 because, in addition to high catalytic activity, they exhibit magnetic properties useful in high density information storage. Furthermore, the fact that Ptx and Fey composition can form a variety of alloy structures makes them an interesting test system. Many attempts are currently ongoing to fine-tune and optimize their alloy composition by controlling preferential segregation, atomic-ordering, and local strain distributions so as to maximize the ORR activities.14,25,26,29 For instance, a new class of electrocatalysts with an ordered intermetallic (Pt−Fe) core and bilayer thick Pt-shell was recently found to exhibit exceptional catalytic properties.14 With the aid of detailed atomic-resolution imaging, the authors attributed the enhancement in activity and durability to the strained nanoparticle lattice and persistent atomic order, respectively. Following this, Zhang et al.25 further optimized this structure to bear a ternary alloy core (FeCuPt) encapsulated within a Pt-rich shell and reported a 10 times higher ORR activity as compared to commercial Pt catalysts. As a result, we used Pt−Fe nanoparticles (supported on vulcan carbon, Pt−Fe/C) with alloy compositions illustrated in Supporting Information Figure 1 for this study. During the in situ liquid cell experiment, TEM micrographs and cyclic
2. RESULTS AND DISCUSSION The schematic of the electrochemical liquid cell used in this Article is presented in Figure 1 (note that the schematics are not set to scale; relevant dimensions are made available under section 1 of the Supporting Information). The liquid cell consists of two microfabricated chips (Protochips Inc., Poseidon 510) separated by a 500 nm silicon dioxide spacer (Figure 1a). Both chips contain a 50 nm silicon nitride electron transparent window; however, the top chip also features a miniaturized three electrode electrochemical cell. For this experiment, we used electrochemistry chips with carbon working and platinum counter and reference electrodes (Figure 1b). Using carbon working electrodes in these experiments served a dual purpose: its weak electron scattering enabled imaging through the working electrode with minimal loss in spatial resolution, and its wide electrochemical potential window allowed electrochemical measurements to be performed with minimal interference from background reactions. The electrochemical liquid cell was positioned in the tip of a Poseidon 510 in situ electrochemistry flow cell holder for electron microscopy (Protochips Inc.). The Poseidon holder features microfluidic and electrical circuitry for delivering fluid and applying electrical bias from external apparatus to the holder tip where the liquid cell is positioned. To study the effectiveness of the electrochemical liquid cell in TEM imaging and electrochemistry measurements, we performed an experiment where platinum nanoparticles were deposited in the electrochemical cell. Using this setup, the carbon working electrode was imaged (Figure 1b, inset), while its cyclic voltammogram (CV) was measured in an H2SO4 electrolyte (Figure 1c). The TEM image shows nanoparticles to be deposited on the silicon nitride window and the carbon working electrode. The Pt nanoparticles deposited on the working electrode are expected to give rise to the standard redox signature of Pt in acidic solutions30 with features showing 22113
dx.doi.org/10.1021/jp506857b | J. Phys. Chem. C 2014, 118, 22111−22119
The Journal of Physical Chemistry C
Article
Figure 3. Structural evolution of disordered Pt−Fe nanocatalysts during electrochemical cycling under 0.1 M HClO4 electrolyte. Parts (a), (c), and (d) are TEM images, showing the morphology of nanocatalysts (appearing with darker intensities) and their carbon supports in gray, before cycling (a), after 50 cycles (c), and after 100 cycles (d). Some of these nanocatalysts are identified with red arrows. Corresponding CV curves of the 1st, 50th, and 100th cycles are shown in (b).
electrode, which is reduced as we move toward the counter electrode (Figure 2 a). It should be noted that in the length scale of Figure 2a, we do not observe any spatial current density variation due to the deposited microparticles. However, if we look at a zoomed-in view of the working electrode (Figure 2b), we observe an increase in current density in the electrode regions with immobilized microparticles. In addition, we see current density hot spots, between particle aggregates. We expect higher electrochemical reaction rates to occur at areas with increased current density. However, due to the highly localized nature of this effect, we do not expect it to influence the electrochemical reaction rates in regions beyond a few micrometers away from the particles. Following the characterization of the electrochemical in situ TEM liquid cell, we sought to design an experiment for studying the degradation of Pt−Fe alloy nanocatalysts in realtime, under potential bias and inside a widely used native electrolyte. Pt−Fe alloy nanocatalysts supported on Vulcan carbon (refer to the Experimental Section for the synthesis procedure) were deposited on an electrochemical chip (as described above), and the carbon support materials were entangled using Nafion solution. A solution of 0.1 M HClO4 was used as the supporting electrolyte. Figure 3 shows the morphology of Pt−Fe nanocatalysts, shown to be disordered from Supporting Information Figure 1 and previous work,32 close to the carbon working electrode and the corresponding cyclic voltammogram (CV) curves for the first 130 potential cycles at a scanning rate of 100 mV/s. It should be noted that the presence of the silicon nitride window (100 nm), along with the liquid layer (500−1000 nm), deteriorates the resolution of the system, which is consistent with previous studies demonstrating a 4 nm resolution using a similar in situ liquid TEM system. The CV curves in Figure 3b, recorded at first, 50th, 100th cycles, clearly show dramatic changes in the current voltage characteristics with an increase in cycle number. Because the recorded CV curves represent the behavior of the entire liquid cell beyond the observation area,
platinum oxidation, platinum oxide reduction, hydrogen adsorption/absorption, and hydrogen desorption. Comparing the CV signal measured inside the miniaturized liquid cell and under the influence of the beam (Figure 1c) with those previously measured in conventional cells,30 we observe a similar redox signature, indicating that this system can be used reliably for electrochemical measurement. Two main differences were noted between this system and conventional systems: (1) the potential range was shifted as compared to systems with conventional reference electrodes; and (2) there is an Ohmic current superimposed on the Faradaic component resulting in a baseline with a positive slope. The first observation is expected as a platinum pseudoreference is used here, and the second point is also expected due to the large solution resistance and the resultant large IR drop implied by the thin (
