The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.

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We report on the influence of glass corrosion on establishing the electrocatalytic activity of fuel cell catalysts. A Teflon electrochemical cell was designed for measurements in alkaline electrolyte. The cell performance was tested and compared to a standard electrochemical glass cell by measuring the oxygen reduction reaction and the hydrogen oxidation reaction on polycrystalline platinum in 0.1 M KOH. It is demonstrated that in the Teflon cell the shape of the cyclic voltammogram as well as the activity for the reactions are reproducible and do not alter over a long period of time. By comparison, using the standard electrochemical cell made out of Duran glass, it is found that the experiments on polycrystalline Pt electrodes in alkaline electrolyte are insufficiently reproducible. The cyclic voltammograms alter over time, and the activities of the hydrogen oxidation as well as the oxygen reduction depend on the applied potential scan limits. This is due to the contamination of the electrolyte because of the etching of glass by KOH, which is further supported with an analysis of the alkaline electrolyte after usage in the respective cell types by inductively coupled plasma-optical emission spectroscopy. The oxygen reduction reaction ͑ORR͒ and the hydrogen oxidation reaction ͑HOR͒ are the key reactions in electrochemical energy conversion devices, such as fuel cells and batteries. Tremendous efforts have been directed especially into improving the electrocatalysts for the ORR in acid electrolytes over the last decades, in order to render proton exchange membrane fuel cells ͑PEMFCs͒ commercially viable.

Water and oxygen electrochemistry lies at the heart of interfacial processes controlling energy transformations in fuel cells, electrolyzers, and batteries. Here, by comparing results for the ORR obtained in alkaline aqueous media to those obtained in ultra-dry organic electrolytes with known amounts of H 2 O added intentionally, we propose a new rationale in which water itself plays an important role in determining the reaction kinetics. This effect derives from the formation of HO ad ···H 2 O (aqueous solutions) and LiO 2 ···H 2 O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to weakly adsorbed intermediates. We also find that even at low concentrations (<10 ppm), water acts simultaneously as a promoter and as a catalyst in the production of Li 2 O 2 , regenerating itself through a sequence of steps that include the formation and recombination of H + and OH -. We conclude that although the binding energy between metal surfaces and oxygen intermediates is an important descriptor in electrocatalysis, understanding the role of water as a proton-donor reactant may explain many anomalous features in electrocatalysis at metal-liquid interfaces.

Hydrogen production from renewable resources and its reconversion into electricity are two important pillars toward a more sustainable energy use. The efficiency and viability of these technologies heavily rely on active and stable electrocatalysts. Basic research to develop superior electrocatalysts is commonly performed in conventional electrochemical setups such as a rotating disk electrode (RDE) configuration or H-type electrochemical cells. These experiments are easy to set up; however, there is a large gap to real electrochemical conversion devices such as fuel cells or electrolyzers. To close this gap, gas diffusion electrode (GDE) setups were recently presented as a straightforward technique for testing fuel cell catalysts under more realistic conditions. Here, we demonstrate for the first time a GDE setup for measuring the oxygen evolution reaction (OER) of catalysts for proton exchange membrane water electrolyzers (PEMWEs). Using a commercially available benchmark IrO 2 catalyst deposited on a carbon gas diffusion layer (GDL), it is shown that key parameters such as the OER mass activity, the activation energy, and even reasonable estimates of the exchange current density can be extracted in a realistic range of catalyst loadings for PEMWEs. It is furthermore shown that the carbon-based GDL is not only suitable for activity determination but also short-term stability testing. Alternatively, the GDL can be replaced by Ti-based porous transport layers (PTLs) typically used in commercial PEMWEs. Here a simple preparation is shown involving the hot-pressing of a Nafion membrane onto a drop-cast glycerol-based ink on a Ti-PTL.

In our previous paper ͓Journal of the Electrochemical Society, 155, P1 ͑2008͔͒ we reported on the impact of glass corrosion on establishing the electrocatalytic activity of fuel cell catalysts. It was shown that the leaching of glass constituents into the electrolyte is responsible for insufficiently reproducible measurements of the oxygen reduction reaction as well as the hydrogen oxidation reaction on polycrystalline platinum. In the present report we elucidate which glass constituents are leached into the electrolyte through the analysis of alkaline electrolytes in contact with Duran glass by inductively coupled plasma optical emission spectroscopy. By adding these constituents, i.e., silicates, borates, aluminates, and lead, separately to the electrolyte, we evaluate their individual impact on electrocatalytic measurements. The results presented in this study help to explain the effects seen in measurements in alkaline electrolyte with glass cells.

In this study the oxygen reduction reaction (ORR) is investigated on a nanoparticulate silver electrocatalyst in alkaline solution. The catalytic activity of the catalyst is determined both in terms of mass activity as well as specific activity and turn over frequency, respectively. It is demonstrated that the established mass activities are independent of the applied catalyst loading, an essential requirement for a reasonable analysis. The determination of the electrochemically active surface area (ECA) or the number of electrochemically accessible sites (NECAS), respectively, is performed by the underpotential deposition of lead. The obtained value of the activity is compared to activities of polycrystalline silver and platinum measured in the same electrolyte, as well as to literature data.