Design criteria for stable Pt/C fuel cell catalysts

Summary Platinum and Pt alloy nanoparticles supported on carbon are the state of the art electrocatalysts in proton exchange membrane fuel cells. To develop a better understanding on how material design can influence the degradation processes on the nanoscale, three specific Pt/C catalysts with different structural characteristics were investigated in depth: a conventional Pt/Vulcan catalyst with a particle size of 3–4 nm and two Pt@HGS catalysts with different particle size, 1–2 nm and 3–4 nm. Specifically, Pt@HGS corresponds to platinum nanoparticles incorporated and confined within the pore structure of the nanostructured carbon support, i.e., hollow graphitic spheres (HGS). All three materials are characterized by the same platinum loading, so that the differences in their performance can be correlated to the structural characteristics of each material. The comparison of the activity and stability behavior of the three catalysts, as obtained from thin film rotating disk electrode measurements and identical location electron microscopy, is also extended to commercial materials and used as a basis for a discussion of general fuel cell catalyst design principles. Namely, the effects of particle size, inter-particle distance, certain support characteristics and thermal treatment on the catalyst performance and in particular the catalyst stability are evaluated. Based on our results, a set of design criteria for more stable and active Pt/C and Pt-alloy/C materials is suggested.

S3 composite material together with the silica exotemplate (II.). The subsequent carbonization of the polymer in an inert atmosphere at 1000 °C results in a carbon composite material (III.). It is during this step that graphitic domains are formed assisted by iron acting as graphitization catalyst. Finally dissolving of the silica template with HF 10 vol % in water (alternatively also with NaOH) and a removal of iron with concentrated HCl followed by some cleaning steps in ultrapure water and ethanol provides the hollow graphitic sphere support (IV.).
The incorporation of platinum into the mesoporous network of the spheres is achieved via ultrasound assisted incipient wetness impregnation of a solution of hexachloroplatinic acid, H 2 PtCl 6 ·xH 2 O, and successive reduction with H 2 at 250 °C (V.). This method has the advantage, that after reduction there is only a small population of particles at the external surface of the support, while the majority of particles is incorporated into the mesoporous shell. The catalyst material, which was synthesized also for electrochemical characterization, has a platinum loading of 20 wt % and platinum particles predominantly in the range of 1-2 nm. It is hereafter denoted as Pt@HGS 1-2 nm.
If the Pt@HGS 1-2 nm material is furthermore subjected to a thermal treatment under argon atmosphere (annealing rate: 5 °C·min −1 ) up to ca. 900 °C (for several hours) a catalyst with an average platinum particle size of 3-4 nm is obtained (VI.). This thermally treated material is hereafter abbreviated as Pt@HGS 3-4 nm and is characterized by a platinum content of 20 wt %.
Pt 3 100 ρ 10 3 3  With l being the "average inter-particle distance" or short AID (nm), ρ Pt the density of platinum (21.45 g·cm −3 ), L Pt is the platinum content (wt %), A s is the specific surface area of the support (m 2 ·g −1 ), and d is the platinum particle diameter (nm). The average inter-particle distance equation is derived as described in the following on the basis of simple geometric considerations. For the calculation all platinum particles are assumed to have the same platinum particle diameter d and the same inter-particle distance l, which is referring to the distance of the surfaces of the platinum particles ( Figure S5). Figure S5: Two neighboring platinum particles with particle diameter d and inter-particle distance l.
The carbon support surface area A Support is approximated by assuming a 2D surface area onto which the platinum particles are perfectly distributed, i.e., with equal distance to each other ( Figure S6). Figure S6: The platinum particles are assumed to be equidistant to each other on the carbon support that which is assumed to be two-dimensional. The surface of the carbon support, A Support , is the area in the black rectangle, while the area of the red rectangle is the area, A Particle , available for a single particle.

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The area enclosed by the black rectangle corresponds to the area of the carbon support A Support while the red rectangle reflects the fraction of the carbon support surface area for a single platinum particle and is denoted in the following with A Particle . A Particle can be expressed as a function of the particle diameter d and the inter-particle distance l. The geometric correlations can be deduced from Figure S7, which visualizes the dependence of the width x and the height y of the red rectangle on the particle diameter and the interparticle distance. S10 Figure S7: The fraction of the carbon support surface area that belongs to a single platinum particle A Particle can be expressed as a function of particle diameter d and inter-particle l distance by using simple geometric relationships.
The area that belongs to a single platinum particle is thus given by: The area of the carbon support A Support equals the area per platinum particle A Particle multiplied by the total number of platinum particles Z: The total number of particles is given by the total mass of platinum divided by the mass of platinum of a single particle: The mass of a single platinum particle can also be expressed by the product of density of platinum Pt ρ and the volume of the single particle V Particle : Assuming spherical geometry the mass of a single platinum particle thus corresponds to: The total mass of platinum in the catalyst corresponds to the content of platinum L Pt in wt % times the mass of the Pt/C catalyst, m Catalyst : Combining the equations for the total mass of platinum and the mass of a single platinum particle the total number of platinum particles Z is:

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By using the term from above for the area per single platinum particle gives: Solving the equation provides the average inter-particle distance l: The support surface area A Support is given by the product of specific carbon support surface area and the mass of the carbon support m Support : For a more comfortable use concerning common units to obtain the average inter-particle distance in nm (entering Pt ρ = 21.45 g·cm −3 , the platinum loading L Pt in percentage, the specific carbon surface area in m 2 ·g −1 and the particle diameter in nm) the equation transforms to: Pt Pt 3 100 ρ 10 3 3 π The equation directly reflects the dependence of the average inter-particle distance on the three parameters specific carbon support surface area, platinum loading, and the parameter with the strongest impact, the platinum particle diameter.