Proton exchange membrane fuel cells (PEMFCs) are an attractive energy conversion technology, this due to their high theoretical fuel utilization eciencies compared to Carnot engines. However, due to potential losses, the operational eciencies achieved in state-of-the-art PEMFCs are only between 45% and 55%. The slow kinetics of the oxygen reduction reaction (ORR) over a platinum based electrode accounts for ca. 70% of the potential losses. As a result of the sluggish ORR kinetics, high platinum loadings are required. The high cost of platinum has made it crucial to improve the ORR activity and hence reduce platinum loading. The surface-area-specic ORR activity has been reported to decrease with platinum particle size. This places a limitation to the degree to which platinum loading can be reduced by increasing metal dispersion. To understand the origin of this behaviour, experimental studies have measured the ORR activity over dierent single crystalline surfaces and used model nanoparticle shapes to elucidate the overall ORR activity. Theoretical studies use density functional theory (DFT) to investigate the ORR activity on various site-types present on assumed model particle shapes. Thermodynamically, the exposed surface terminations aught to be predominantly Ptf111g and Ptf100g separated by edges and corners. It has been postulated that the overall ORR activity can be calculated as a weighted average of the activity of exposed surface terminations. Using DFT calculations and nanorod models the above postulations are tested for the edge sites between a Pt(111) and Pt(100) surface. A rhombic nanorod model is used due to its computational eciency compared to model nanoparticle clusters which are generally large and computationally expensive models. Furthermore, the use of rhombic nanorod model enables the investigation of the connection and communication between the Pt(111) and Pt(100) facets, this is dicult to investigate with stepped-surface models. It is argued that if, (i) the edge has insubstantial eect on the adsorption strength of adsorbed ORR intermediates as a function of distance from the edge and (ii) the diusion of ORR intermediates between adjacent surface planes is limited, then the above postulation does hold. Using atomic O and O2 it was observed that the edge eect is a local phenomenon with adsorption involving only edge atoms being stronger than on extended surfaces. The adsorption of both atomic O and O2 was weaker on the Pt(111) nanorod terrace sites compared to the adsorption on equivalent sites on extended Pt(111) slabs. These eects were not observed for adsorption on Pt(100) nanorod terrace sites. The diusion of atomic O from the Pt(100) nanorod terrace bridge sites towards the Pt(111) nanorod terrace fcc sites (across the edge) was investigated. It was observed that whilst at lower coverage the diusion of atomic O is limited with an overall hopping frequency of 1.8810􀀀2 s􀀀1 (at T = 85 °C and P = 1 bar), at a higher coverage, the diusion of atomic O across the edge is facile (overall hopping frequency of 1.12105 s􀀀1). At a higher coverage limit and current density of 100 mA/cm2 (an arbitrary current density in the activation polarization region), the hopping frequency of atomic O was determined to be three orders of magnitude higher than the turnover frequency for H2O production. Hence at high coverage, the rst condition (i.e. edge eect is localised) is satised, but the second condition (i.e. limited diusion of atomic O from one facet to the other facet) can not be ruled out. Therefore, the assumption necessary for the evaluation of the overall ORR activity as a weighted average of individual activity, can be questioned.

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