Abstract
Platinum electrocatalysts supported on high surface area and Vulcan carbon blacks (Pt/HSC, Pt/V) were characterized in rotating disk electrode (RDE) setups for electrochemical area (ECA) and oxygen reduction reaction (ORR) area specific activity (SA) and mass specific activity (MA) at 0.9 V. Films fabricated using several ink formulations and film-drying techniques were characterized for a statistically significant number of independent samples. The highest quality Pt/HSC films exhibited MA 870 ± 91 mA/mgPt and SA 864 ± 56 μA/cm2Pt while Pt/V had MA 706 ± 42 mA/mgPt and SA 1120 ± 70 μA/cm2Pt when measured in 0.1 M HClO4, 20 mV/s, 100 kPa O2 and 23 ± 2°C. An enhancement factor of 2.8 in the measured SA was observable on eliminating Nafion ionomer and employing extremely thin, uniform films (∼4.5 μg/cm2Pt) of Pt/HSC. The ECA for Pt/HSC (99 ± 7 m2/gPt) and Pt/V (65 ± 5 m2/gPt) were statistically invariant and insensitive to film uniformity/thickness/fabrication technique; accordingly, enhancements in MA are wholly attributable to increases in SA. Impedance measurements coupled with scanning electron microscopy were used to de-convolute the losses within the catalyst layer and ascribed to the catalyst layer resistance, oxygen diffusion, and sulfonate anion adsorption/blocking. The ramifications of these results for proton exchange membrane fuel cells have also been examined.
Highlights
With the initiation of commercialization of automotive proton exchange membrane fuel cells (PEMFCs) rapidly approaching, a reduction in the cathode platinum electrocatalyst loading by a factor of ∼4, while maintaining the performance, has become imperative to meet the cost targets (∼10 gPt/100 kW stack; ∼$50/gPt).[1,2,3] A technique to rapidly screen novel advanced electrocatalysts that are typically synthesized in mg batches is indispensable to researchers pursuing this objective
The modified thin film rotating disk electrode (RDE) (TF-RDE) technique is well suited for the screening of oxygen reduction reaction (ORR) catalyst candidates as a first step to limit the time and expense invested in an expensive scale-up of catalyst synthesis and a necessarily timeconsuming and elaborate evaluation in a practical subscale PEMFC platform
We draw the following conclusions from the key experimental findings and analysis discussed in the paper: 1. The ORR activity for Pt/V catalyst layers show no discernible trend (Pt/HSC) films fabricated using the NFSIPAD technique was determined to be: MA 870 ± 91 mA/mgPt and SA 864 ± 56 μA/cm2Pt; and corresponding values for Pt/V were: MA 706 ± 42 mA/mgPt and SA 1120 ± 70 μA/cm2Pt measured in 0.1 M HClO4 at 20 mV/s, 100 kPa O2 and 23◦C
Summary
With the initiation of commercialization of automotive proton exchange membrane fuel cells (PEMFCs) rapidly approaching, a reduction in the cathode platinum electrocatalyst loading by a factor of ∼4, while maintaining the performance, has become imperative to meet the cost targets (∼10 gPt/100 kW stack; ∼$50/gPt).[1,2,3] A technique to rapidly screen novel advanced electrocatalysts that are typically synthesized in mg batches is indispensable to researchers pursuing this objective. For early works starting from the initiating work to obtain kinetic information for high surface area catalysts using rotating electrode system reported by Stonehart and Ross in 1976,4 Gasteiger and Schmidt provide an excellent detailed review.[5] The TF-RDE technique reported 22 years after the initiating work owes its inception to the seminal work of Gloaguen et al in 19946 who elucidated a method for fabricating electrodes using Nafion-based Pt/V inks to obtain the SA and MA They analyzed Pt/V catalyst layers having 1.1 μm and 5.6 μm thicknesses (calculated for a mixed catalyst/ionomer layer) using ORR Tafel analysis coupled with a macro-homogeneous model (uniformly distributed catalytic sites and electrolyte) to account for O2 diffusion within the catalyst layer. These assumptions are unrealistic and only applicable when present over a smooth non-porous bulk material like poly-Pt; for a porous Pt/C catalyst layer, the aliquot of Nafion used to form a cap penetrates and distributes itself over the depth of the catalyst layer
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