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Controlling Nanoparticle Interconnectivity in Thin-Film Pt Catalyst Layers Isaac Martens, Blaise A. Pinaud, Laurie Baxter, David Pentreath Wilkinson, and Dan Bizzotto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04952 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016
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The Journal of Physical Chemistry
Controlling Nanoparticle Interconnectivity in Thin-Film Pt Catalyst Layers Isaac Martens,†,‡ Blaise A. Pinaud,‡ Laurie Baxter,† David P. Wilkinson,∗,‡ and Dan Bizzotto∗,† †AMPEL, Department of Chemistry, University of British Columbia, Vancouver, Canada ‡Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, Canada E-mail:
[email protected];
[email protected] Abstract The optimization of conventional hydrogen fuel cell catalyst layers suffers from a poor understanding of their composite nanostructure during both initial preparation and its evolution during use. We demonstrate how highly active, ultralow loading Pt catalyst layers can be fabricated in a single, solution-processible step using electroless deposition. Growing Pt nanoparticles directly in the surface of a polyelectrolyte Nafion membrane yields a mechanically robust film with tunable optical reflectance and electronic conductivity. Small changes in the polymer hydration and Pt film thickness critically modulate nanoparticle interconnectivity near the percolation threshold. Conductive AFM and electron microscopy reveal how the film’s dynamic nanoscale morphology allows control over bulk electrochemical and optical properties. Well-defined composition and structure makes these layers an experimentally-accessible model system for studying thin film electrocatalyst architectures.
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Introduction The emerging commercial use of hydrogen fuel cells requires catalyst layer designs with a high degree of electrocatalytic activity and stability. Platinum based catalysts have endured as the highest performing systems, but their equally high cost limits the practical loading for consumer automotive applications to less than a few hundred μg/cm2 of electrode 1 . Nanostructured catalyst layers have appeared as a viable alternative to traditional catalyst coated membranes based on featureless 2-5nm Pt nanoparticles supported by carbon. Thin-film and extended-film Pt architectures in particular have demonstrated both dramatically enhanced durability, and excellent specific activity, although the precise origins of this performance remains incompletely resolved. 2–4 Most chemical or physical thin film deposition techniques being investigated, for example ALD or magnetron sputtering, require processing under vacuum or high temperature. Hydrogen fuel cell membranes are structurally dynamic and possess a delicate intrinsically hydrated nanostructure. This mismatch of environmental compatibility means that nanostructured catalyst films must be fabricated separately, then carefully transferred onto membranes, creating a need for the development of deposition techniques that are natively compatible with ambient conditions and the electrolyte chemistry. Additionally, embedding Pt catalysts directly into the polymer electrolyte has been previously shown to improve the cell lifetime, probably by scavenging reactive oxygen species. 5 A distinguishing feature of many thin film catalysts is reliance on electrical connectivity directly between noble metal particles, instead of through attachment to a conductive carbon support that corrodes under power cycling. 6,7 It is then essential that Pt grains form dense networks, since electrically isolated catalyst particles are inactive in terms of cell current. Once loaded into an electrode assembly, the ionomer electrolyte can distort (swell, stretch and bend) under the heat, hydration, and pressure of an active fuel cell. A useful thin-film catalyst layer must accommodate these mechanical stresses and maintain good contact with both the ionomer and the electrode. 8,9 A better understanding of how catalyst particles form 2
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The Journal of Physical Chemistry
electrical interconnections is useful in extracting robust and reliable performance from thin film systems inside electrochemical cells. In this work, we utilize an electrolessly deposited thin film of nanostructured Pt nanoparticles on Nafion membrane similar to our previously described process. 10 Briefly, a concentration gradient of cationic Pt species is established inside a divided cell separated with a Nafion membrane. This Pt species is reduced with borohydride, and Pt nanoparticles are formed at the solution interface of the membrane. While the raw catalytic activity of this model system falls short of fuel cell performance targets, their structure is appealing for several reasons. It is possible to assemble nanoparticle layers in the submonolayer, near monolayer, and multilayer regime by carefully controlling the conditions. This ultrathin, ultralow Pt loading catalyst is an ideal model system for studying interparticle electrical connectivity problems and their structural influence on performance. By growing the Pt thin film directly on the polymer electrolyte, good interfacial contact can be maintained with both the ionomer and the electrode independent of the dynamic hydrated morphology. Electrolessly deposited catalyst layers have been previously reported by a number of groups over the last several decades 11–13 . Unfortunately, electrocatalytic performance was typically accompanied by limited structural information, resulting in a poor understanding of the film morphology and structure-activity relationship. In the present study we show that the local and bulk electronic or optical behavior of the film can be directly linked to the nanostructure, and that this nanostructure is dynamic under environmental conditions relevant to electrochemical cells.
Experimental Methods Electroless Deposition The electroless deposition followed the procedure detailed by us previously, 10 with several small modifications. A piece of Nafion is held clamped between two chambers, allowing 3
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different solutions to access each side. One side is filled with dilute sulfuric acid, while the other is filled with a cationic Pt salt in dilute KOH. The Pt cations diffuse into the Nafion, and the Pt solution is then replaced by a solution of sodium borohydride. The clamped cell reactor used a 9x9cm opening over which Pt was deposited. The concentration of Pt(NH3 )4 (NO3 )2 was 2mM, while the NaBH4 solution was 0.2M and the water bath was held at 318K. The reactions were quenched by pouring the solutions out of the reactor followed by rinsing the Pt layer with dilute nitric acid. The sub-monolayer, near-monolayer, and multilayer films were reacted with borohydride solution for 4, 8, and 25 minutes respectively. Nafion 115 (Ionpower, Inc.) was cleaned by boiling in 1M H2 SO4 (1hr), and then water (1hr), then dehydrated inside a vacuum oven at 350K for several hours before use. The Pt layer is more heterogeneous near the edges of the film, so samples were taken from near the middle of the membrane for all measurements. The precise locations on each Pt film sampled for each analytical technique are not identical, but are representative. After deposition, the Pt films were soaked in 0.5M HClO4 to exchange salts and fully hydrate the Nafion.
Microscopy Scanning electron microscopy (SEM) images were collected on a FEI Helios 650 microscope in backscatter immersion mode. Using a 1kV accelerating voltage, 50pA emission current, and short dwell times minimizes Nafion damage. Image processing included linear histogram contrast adjustment, despeckling, and FFT bandpass filtering (40-3px) in ImageJ. Fracturing membranes under liquid nitrogen produces a clean exposed edge. Samples were dried under dynamic vacuum at 350K for 1-2 hours before transfer to the SEM chamber. Atomic force microscopy (AFM) images were collected on an Agilent 5500 microscope equipped with a 90μm scanner using AC and CSAFM nosecones. SiN 30kHz cantilevers with
