In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development.

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The cathode catalyst layer ͑CCL͒ is the major competitive ground for electrochemical reaction, reactant transport, and water and heat exchange in a polymer electrolyte fuel cell ͑PEFC͒. Nevertheless, it is often treated as a thin interface. Its pivotal role in the fuel cell water balance is unexplored. Here, the structural picture of CCLs forms the basis for a novel model that links spatial distributions of processes with water handling capabilities and current voltage performance. In the first step, the statistical theory of random composite media is used to relate composition, porous structure, wetting properties, and partial saturation to effective properties. In the second step, these effective properties are used in a macrohomogeneous model of CCL performance. A set of reasonable simplifications leads to a full analytical solution. Results demonstrate that the CCL acts like a watershed in the fuel cell, regulating the balance of opposite water fluxes toward membrane and cathode outlet. Due to a benign porous structure, the CCL represents the prime component for the conversion of liquid to vapor fluxes in PEFCs. Furthermore, the CCL is highlighted as a critical component in view of excessive flooding that could give rise to limiting current behavior.

We provide a phenomenological description of proton conductance in polymer electrolyte membranes, based on contemporary views of proton transfer processes in condensed media and a model for heterogeneous polymer electrolyte membrane structure. The description combines the proton transfer events in a single pore with the total pore-network performance and, thereby, relates structural and kinetic characteristics of the membrane. The theory addresses specific experimentally studied issues such as the effect of the density of proton localization sites (equivalent weight) of the membrane material and the water content of the pores. The effect of the average distance between the sulfonate groups, which changes during membrane swelling, is analyzed in particular, and the factors which determine the temperature dependence of the macroscopic membrane conductance are disclosed. Numerical estimates of the specific membrane conductivity obtained from the theory agree very well with typical experimental data, thereby confirming the appropriateness of the theoretical concepts. Moreover, the versatility of the models offers a useful and transparent frame for combining the analysis of both experimental data and the results of molecular dynamics simulations.

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In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. These effects are of vital importance for electrochemical reactions in general. Historically, the potential of zero charge (pzc) of the metal has been employed to parametrize the surface charging relation. The structural model of the electrified interface presented in this article goes beyond the oversimplified pzc concept by accounting for the formation of surface oxide and the orientational ordering of interfacial water molecules. The analytical solution of the model reveals a peculiar non-monotonic charging behavior. The Pt surface exhibits a negative effective charge in a low potential region, a positive charge in an intermediate potential region, and a negative charge in a high potential region due to surface oxide dipoles. This non-monotonic behavior is in agreement with a seminal experimental work of Frumkin and Petrii [Electrochim. Acta197520347359] that had remained hitherto unexplained.

Partial dehydration of the proton-conducting membrane under working conditions is one of the major problems in low-temperature fuel cell technology In this paper a model, which accounts for the electro-osmotically induced drag of water from anode to cathode and the counterfiow in a hydraulic pressure gradient is proposed. A balance of these flows determines a gradient of water content across the membrane, which causes a decline of the current-voltage performance. Phenomenological transport equations coupled with the capillary pressure isotherm are used, involving the conductivity permeability and electro-osmotic drag coefficients dependent on the local water content. The effects of membrane parameters on current-voltage performance are investigated. A universal feature of the obtained current-voltage plots is the existence of a critical current at which the potential drop across the membrane increases dramatically due to the dehydration of membrane layers close to the anode. For a membrane with zero residual conductivity in its dry parts, the critical current is a limiting current. Well below the critical current the effect of dehydration is negligible and the currentvoltage plot obeys Ohm's law. The shape of the capillary pressure isotherm determines the nonohmic corrections. A comparison of the results of this study to those of the pertinent diffusion-type models reveals qualitatively different features, the convection model is found to be closer to experimental observations.