Re-Examination of the Particle Size Effect on the ORR for Nafion-Free Ultrathin Uniform Pt/C Catalyst Layers

The so-called ‘particle size effect’ for the oxygen reduction reaction on platinum based catalysts has been debated since the 1960’s and extensively studied in phosphoric, sulfuric and perchloric acid electrolytes.  Most literature report an increase in specific activity (SA) corresponding to an increase in particle size, giving rise to a peak mass activity (MA) at 2­–4 nm,^1–4 while some groups have not found evidence supporting the particle size effect.^5,6  Researchers have attempted to correlate the particle size effect with the following properties: i) oxide coverage—smaller particles are more oxophilic,⁷ ii) geometric effect—fraction of terrace/step sites that have different sensitivity to anion adsorption from electrolyte and oxide species,⁸ iii) surface electronic effects—potential of total zero charge (pztc),⁴ and, iv) electronic effects—d-band vacancy/position of d-band center.^9,10  Nesselberger et al.¹¹ reported a linear dependence of MA for particle sizes >~2 nm that departed from the typical peak observed by others.  They attributed the discrepancy to the inconsistent application of iR and background corrections to the raw I-V data in the literature.

   Our previous studies^12,13 have identified several artifacts that can influence the measured ORR activity and observed particle size effect in 0.1 M HClO₄ in TF-RDE.  They include: i) anion adsorption/blocking by Nafion ionomer incorporated in Pt/C catalyst films (but not for poly-Pt), ii) film thickness and uniformity—O₂ diffusion within the catalyst film, iii) ORR activity measurement protocols, and, iv) impurity levels in the electrochemical system.  We reported SA and MA for Pt/HSC and Pt/V using Nafion-free ultrathin uniform films fabricated using advanced techniques to be ~2.8x higher than that reported in the literature using conventional methods.  The SA (0.9–1.1­ mA/cm²_Pt) for 2–4 nm Pt/C approaches 30–40% the SA of poly-Pt; these high activity values would certainly be expected to alter the particle size effect profile and provided the impetus for this work.

   We conducted activity measurements for various catalysts having average Pt particle size in the range 2 nm to bulk poly-Pt.  Our results indicate a much steeper increase in SA with particle size in the range 2–6 nm followed by a shallow increase for larger particles.  We observe a MA peak around 3 nm and a profile that is in agreement with the theoretical model reported by Tritsaris et al.¹⁴ who attribute the effect to the proportion of terrace/step sites on particles with varying size.

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