Fuel cell technology is currently shifting very fast from fundamental research to real development. In addition to other aspects, this transition is possible because of the important improvements achieved in the field of electrocatalysis in the past decade. This perspective will give a focused overview summarizing the most outstanding contributions in the last 10 years in terms of activity and durability of the catalyst materials for ethanol oxidation and oxygen reduction reaction, respectively. In addition, it provides an outlook about new catalyst support materials with improved performance/stability, advanced characterization techniques, and fundamental studies of reaction mechanisms and degradation processes. All the studies referred to in this perspective significantly contribute to reaching the technical targets for PEFC commercialization.

The catalytic activity and selectivity of Pt nanoparticles towards the oxygen reduction reaction (ORR) were investigated as a function of the Pt catalyst distribution. By means of the sputtering deposition technique, it was possible to fabricate Pt catalysts with different loadings that consisted of dispersed 2–3 nm particles, nanoparticle agglomerates and extended particulate layers. The transition from dispersed nanoparticles to extended layers led to a decrease in the electrochemical surface area (ECSA, m2Pt gPt−1) and to a shift of the platinum oxide reduction peak to more positive potentials, which indicates a decrease in the adsorption energy for oxygenated species. The latter finding was correlated to the observed decrease in specific activity with the increasing ECSA, that is, in the case of isolated nanoparticles, the higher adsorption energy for oxygenated species causes a reduction in the specific activity towards the ORR as larger amounts of active sites are blocked compared to extended surfaces. The presented data of specific and mass activity versus ECSA were found to follow a “master curve” obtained by comparing normalised Pt activities from different studies. The transition from dispersed Pt nanoparticles to extended layers also influences the Pt selectivity. At a decreased interparticle distance, a significant increase in the H2O2 production was observed below 0.6 V versus the reversible hydrogen electrode, which indicates the important role of a H2O2 desorption–readsorption reaction mechanism during the ORR on Pt nanoparticles.

In this work, high surface area antimony doped tin oxide (Sb-SnO2) has been synthesized using a modified sol-gel synthesis method. The bulk and surface properties of the metal oxide support have been investigated as a function of the processing conditions. A change in the Sb-SnO2 processing conditions, while preserving an overall invariant bulk composition, led to substantial modification of the surface stoichiometry. Accelerated stability test protocols have shown that the surface composition represents a crucial parameter for the electrochemical stability of Sb-SnO2. Model Pt/Sb-SnO2 electrodes have been developed depositing Pt nanoparticles by magnetron sputtering on the optimized Sb-SnO2 porous surface. A significant enhancement in the corrosion stability upon 1000 potential cycles between 0.5 and 1.5 V (RHE) at 50 mV s(-1) has been observed for the Pt/Sb-SnO2 system compared to Pt/carbon.

The level of Pt loadings in polymer electrolyte fuel cells (PEFC) is still one of the main hindrances for implementation of PEFCs into the market. Therefore, new catalyst and electrode preparation methods such as sputtering are of current interest, because they allow thin film production and have many cost saving advantages for electrode preparation. This paper summarises some of the most important studies done for sputtered PEFCs, including non carbon supported electrodes. Furthermore, it will be shown that an understanding of the main morphological differences between sputtered and ink-based electrodes is crucial for a better understanding of the resulting fuel cell performance. Especially, the electrochemical surface area (ECSA) plays a key role for a further increase in PEFC performance of sputtered electrodes. The higher surface specific activities i(k,spec) of sputtered compared to ink-based electrodes will be discussed as advantage of the thin film formation. The so- called particle size effect, known in literature for several years, will be discussed as reason for the higher i(k,spec) of sputtered electrodes. Therefore, a model system on a rotating disc electrode (RDE) was studied. For sputtered PEFC cathodes Pt loadings were lowered to 100 μg(Pt)/cm(2), yet with severe performance losses compared to ink-based electrodes. Still, for Pt sputtered electrodes on a carbon support structure remarkably high current densities of 0.46 A/cm(2) at 0.6 V could be achieved.

SnO 2 thin films are studied experimentally and from firstprinciples as model supports for Pt nanoparticle catalysts in an acidic environment. SnO 2 thin film supports are attractive model systems because composition, microstructure, and surface termination can be tailored by varying the deposition parameters. SnO 2 films are synthesized by reactive dc magnetron sputtering, and the effects of the deposition conditions on the physicochemical and electrochemical properties are investigated experimentally and theoretically. Variation of the deposition conditions results in limited longrange order SnO or SnO 2 films. Annealing in either case leads to wellcrystallized SnO 2 films, but with different growth directions. Films deposited as SnO 2 show only growth along the [110] direction, while SnO 2 films formed from deposited SnO show no preferred orientations. Hybrid density functional theory (DFT) suggests that growth along the [110] direction is driven by ( 110) being the lowest energy surface, while the loss of orientation in the SnO derived films originates from an almost degenerate set of surface energies at the SnO|SnO 2 equilibrium. The oxygen reduction reaction activity of Pt nanoparticles depends on the SnO 2 film orientation. A 2-fold higher catalytic activity is observed for Pt nanoparticles on the SnO 2 film without preferential orientation compared to Pt on SnO 2 grown along the [110] direction, pointing to the presence of strong surface-dependent metal−support interaction.

Helically coiled tubes offer improved residence and thermal time distributions due to the formation of Dean vortices via centrifugal forces. Design and fabrication of several milli/microstructured helically coiled tube reactors are described for processes requiring a narrow residence time distribution (RTD) and efficient heat transfer at laminar flow regime. The performance of microstructured reactor capillaries, which provide a high specific surface area, is combined with a type of helically coiled tube, namely, a coiled flow inverter allowing for the narrowest RTD in laminar flow regimes. Axial dispersion is characterized by obtaining the RTD curves from different reactor setups. Overall heat transfer coefficients of a new reactor setup are measured in order to determine the heat transfer efficiency.

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The interaction between Pt catalysts for the electrochemical oxygen reduction reaction (ORR) and corrosion-resistant SnO 2 supports has been studied by varying the Pt morphology and the SnO 2 physicochemical properties in a model electrode study. Different Pt morphologies ranging from isolated particles to thin films have been deposited by magnetron sputtering on oxidized and reduced SnO 2 model film electrodes as well as on glassy carbon (GC). Furthermore, three different surface probe reactions, namely the hydrogen underpotential deposition (H upd ), the CO oxidation, and the ORR have been studied to investigate the support influence on the Pt electrocatalytic properties. A marked effect of the type of the support, that is, tin oxide versus carbon, on the Pt electrochemically active surface area calculated from the H upd charge was observed. Furthermore, a pronounced CO oxidation activity of Pt deposited on SnO 2 supports was observed compared to that of Pt supported on GC. The intrinsic ORR activities of Pt/SnO 2 and Pt/GC catalysts varied with both the Pt morphology and the SnO 2 stoichiometry. While very similar ORR activities of all catalysts were found at high Pt loadings where an extended surface Pt morphology is expected, a strong support dependence was observed for isolated Pt particles at low Pt loadings. Pt nanoparticles supported on reduced SnO 2 and on GC showed very comparable ORR activities, about five times higher than that of Pt nanoparticles on oxidized SnO 2 . Postmortem X-ray photoelectron spectroscopy investigations revealed that the reduced ORR activity of the latter catalysts can be explained with a stronger oxidation of Pt nanoparticles when supported on oxidized SnO 2 . This finding highlights the fundamental importance of tailoring the oxide support properties to maximize the Pt electrocatalyst performance.