Article pubs.acs.org/ac
Electrochemical Quantifying, Counting, and Sizing Supported Pt Nanoparticles in Real Time Jing-Fang Huang* and Hui-Wen Yang Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, R.O.C S Supporting Information *
ABSTRACT: Knowledge about controlling the activity and catalyst degradation mechanisms of platinum-based catalysts has been limited by technical impediments. Here we show a facile in situ electrochemical procedure for the simultaneous assessment of the mean size and number of Pt nanoparticles (Ptnano) from an evaluation of the electrochemically surface area (ECSA) and the breakthrough in electrochemical quantification of the Pt content. The electrochemical procedure enables in situ characterization of the factors related to the catalytic activity and monitoring of the changes in Pt content during an accelerated durability test. Surprisingly, the ECSA loss was observed only from the growth of Ptnano mean size even without any Pt loss over the potential range, 0.6−1.0 V vs RHE, at room temperature. These results strongly support the long-standing debate that if the coarsening of Ptnano from crystal migration and coalescence can occur in low temperature fuel cells.
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(TEM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and in situ anomalous small-angle X-ray scattering (ASAXS).17,20−27 Although these techniques provide valuable information on catalyst degradation, they are generally limited to the instantaneous tracking of variations of the catalyst, e.g., the particle size distribution and number of particles, on account of their inefficiency in terms of cost and time. In addition, with some image analysis techniques, e.g., TEM and SEM, it is difficult to represent the average properties of entire samples based on the data obtained from these characterization techniques. Most techniques cannot monitor the loss of Pt and micromorphological variations simultaneously. Previous studies reported that a combination of ECSA of active metallic nanoparticles and their amount can provide a simple means of evaluating the mean diameter and the total number of active metallic nanoparticles.28−31 However, quantification of the amount of Ptnano on the electrocatalysts surface is a complicated process with the first step requiring the dissolution of Pt into the highly corrosive agent, aqua regia. Further indirect spectroscopic methods, e.g., inductively coupled plasma mass spectrometry (ICPMS),23,32−35 are used to detect Pt ions in solution. Unfortunately, they involve expensive instrumental methodologies and severe sample pretreatments. Although electrochemical methods provide accurate measurements of the trace determination of metals in solution or on the electrode surface with rapid analysis times and low cost instrumentation,28,36−38 Pt is considered to be one of the “noblest” of the noble metals and is limited in traditional electrochemical quantification by the ultralow current densities
xtensive research into fuel cells has focused on reducing the Pt content to lower the cost of the fuel cell catalyst materials while maintaining the activity of the catalysts during the long-term operation of fuel cells.1−5 Catalyst durability remains a key challenge in the development of fuel cells or other catalyst-requiring applications.6−12 Understanding the mechanisms and factors affecting catalyst degradation (loss of catalyst activity or loss of electrochemically active surface area (ECSA)) can assist in the design of more durable and economically viable catalysts.8,13−19 Three major mechanisms for ECSA loss of Pt-based catalysts have been proposed.14,15 The mechanism termed “electrochemical Ostwald ripening” involves the loss of ECSA due to the dissolution of Pt from smaller particles and redeposition of the soluble Pt ions onto larger particles on the carbon support. The mechanism is driven by the difference in the thermodynamic stability of Pt species of different diameters. Another mechanism is the diffusion of Pt ions into the proton conducting membrane, which is reduced further by crossover hydrogen from the anode to form large crystallites in the membrane. A third mechanism of ECSA loss is from the coalescence of Pt nanoparticles (Ptnano) via nanocrystallite migration and agglomeration on the carbon support surface. These proposed mechanisms mainly concern the ECSA loss caused by the loss of Pt from Pt dissolution and Ptnano growth from redeposition of the dissolved Pt ions on a larger Pt particle or direct Pt particle sintering. The first two mechanisms cause more significant Pt content loss, but the third mainly concerns ECSA loss from the growth of Ptnano size. In addition to the requisite in situ electrochemical evaluation of ECSA,15 the typical techniques employed to understand mechanisms and factors related to catalyst degradation (ECSA loss) include ex situ, post-mortem analysis by scanning electron microscopy (SEM), transmission electron microscopy © XXXX American Chemical Society
Received: March 11, 2016 Accepted: May 23, 2016
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DOI: 10.1021/acs.analchem.6b00966 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Figure 1. (A) MSCV of the Pt/C@GCE in a 0.5 M NaClO4 aqueous solution containing 0.5 M HCl recorded at a scan rate 0.01 Vs−1 at room temperature; (B) CVs of the Pt/C@GCE in 0.5 M H2SO4 were recorded before and after the de-Pt treatment at a scan rate of 0.2 Vs−1; (C) and (D) show TEM images of Pt/C with 20 wt% Pt before and after the anodic MSCV treatment. The inset is a high resolution TEM image of Pt/C with 20 wt% Pt in (C).
isopropyl alcohol, 30 μL of a 5% NF solution, and 20 min ultrasonication to obtain the Pt/C suspension. A GCE (BAS, 3.0 mm diameter, 0.07 cm2) served as the substrate electrode for the Pt/C electrocatalyst suspension. The Pt/C@GCE was fabricated using a drop coating procedure. Briefly, a GCE was polished successively with 1.0, 0.3, and 0.05 μm alumina powder cloth (Buchler) followed by sonication in deionized water and drying prior to use. One microliter of the Pt/C suspension was pipetted onto the surface of GCE as a circle with a geometric area of 0.07 cm2. A Pt/C@GCE was obtained after drying under an Ar flow at room temperature for solvent evaporation. Electrodissolution of Ptnano. A Pt/C@GCE was used as the working electrode for the electrodissolution of Ptnano. The Pt/C@GCE was dipped in a stirred 0.5 M NaClO4 aqueous solution containing 0.5 M HCl. The anodic multiple-scan cyclic voltammetry (MSCV) potential was scanned in the anodic direction from 0.6 to 1.25 V (vs RHE). The scan rate was 0.01 Vs−1 at room temperature. The anodic charge (QPt) used for the anodic stripping of Ptnano from Pt/C@GCE with a known Pt loading was evaluated during the anodic MSCV treatment. The oxidation state of the Pt ions was determined according to Faraday’s laws of electrolysis. The Ptnano content was evaluated directly from QPt with the value of 4 electrons transferred per Pt atom or confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). In Situ Electrochemical Characterization of Ptnano. The as-prepared or postaccelerating durability tested Pt/C@GCEs as the working electrodes were cleaned electrochemically by potential cycling 10 times between 0.0 and 1.0 V vs RHE in an Ar-purged 0.5 M H2SO4 aqueous solution electrolyte (scan rate, 0.2 Vs−1). The ECSA were determined by measuring the areas (charges) under the hydrogen adsorption/desorption peaks (Hupd) of the cyclic voltammograms. A conversion factor of 0.21 mC cm−2 was used to determine the ECSA.42 The charges (QPt) corresponding to the anodic stripping of Ptnano from the Pt/C@GCE in a 0.5 M NaClO4 aqueous solution containing 0.5 M HCl were used to determine the Pt loading. The combination of ECSA and QPt was used to evaluate the
(slow anodic kinetics) and electrode passivation. In our previous studies, the accelerated electrodissolution of Pt was realized in a Cl−-containing nonaqueous solvent system (ionic liquids) and acidic Cl−-containing aqueous solution, respectively.39,40 Especially, in aqueous solutions, the Cl−-complexing ability of Pt ions facilitates anodic electrodissolution of the active supported Pt metal with the requisite “nano-size” (particle diameter
