Surface diffusion electrocatalysts

The mechanisms for electrocatalyst surface area loss are by a) crystallite migration or b) atom or ion dissolution and reprecipitation, either to the electrolyte or over the carbon surface. It is well known that surface diffusion of atoms on the individual crystallites can provide for mobility (much like the treads on a military tank). In either case, small crystallite become annihilated and fewer but larger crystallites are produced.18 In either event, these processes lead to demetallization of the less noble components in the alloy. 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

EC-NMR has made considerable progress during the past few years. It is now possible to investigate in detail metal-liquid interfaces under potential control, to deduce electronic properties of electrodes (platinum) and of adsorbates (CO), and to study the surface diffusion of adsorbates. The method can also provide information on the dispersion of commercial carbon-supported platinum fuel cell electrocatalysts and on electrochem-ically generated sintering effects. Such progress has opened up many new research opportunities since we are now in the position to harness the wealth of electronic, Sp-LDOS as well as dynamic and thermodynamic information that can be obtained from NMR experiments. As such, it is to be expected that EC-NMR will continue to thrive and may eventually become a major characterization technique in the field of interfacial electrochemistry. 

Babu et al. carried out Pt and C NMR and electrochemical experiments on commercial Pt—Ru alloy nanoparticles and compared the results with those on Pt-black samples having similar particle sizes, and concluded that alloying with Ru reduces the total density of states at the Pt sites, in accord with conclusions drawn from synchrotron X-ray absorption studies ofPt-Ru electrocatalysts [199]. The COj,d diffusion studied by C electrochemical NMR spectroscopy in the temperature range 253—293 K revealed that CO surface diffusion is too fast to be considered as the rate-Hmiting factor in methanol oxidation. The NMR experiments also demonstrated that the addition of Ru to Pt increases the surface diffusion rates of CO, and a... 

The fraction of the maximum available surface of electrocatalyst that participates in electrochemical reactions under conditions of gas diffusion will depend upon numerous parameters like conductivity of electrolyte, temperature, gas pressure, pore size distribution, hydro-phobicity, rate-determining step of the electrochemical reaction, electrode potential, etc. An order of magnitude may be obtained [12] for this fraction by the ratio of the values of the double layer capacity under diffusion conditions and flooded conditions. 

An alternative mechanism in which the gas is adsorbed on the dry metal and moves towards the interface electrocatalyst/electrolyte by surface diffusion was also treated [66, 67] in the model of a cylindrical pore with flat meniscus. However, such a mechanism is not compatible [1, 24, 58] with the order of magnitude of the currents observed. 

Surface diffusion of adsorbed oxygen atoms to a three phase boundary (TPB) between the electrocatalyst (e.g. LSM) — electrolyte (e.g. YSZ) — gas phase,... 

To model a porous electrocatalyst we may consider a second type of mass transport (in addition to diffusion) locally within the electrode, i.e., a mass transport resistance between the electrode surface and the solution. This situation may arise, for example, when the electrode surface is covered by a thin layer of polymer electrolyte or as in a fuel cell electrode in which the electrocatalyst is also covered by a thin water layer. 

Whereas the rate-determining step for hydrogen molecule oxidation now is recognized69,70 to be the dissociative chemisorption of the hydrogen molecule on dual sites at the platinum surface, the rate of this step is so high that in most electrochemical environments platinum electrocatalysts are almost always operating under diffusion control. 

The time-dependent decline of reaction rate (and potential) of a porous electrocatalyst could be modeled in analogy to gas phase deactivation (267-268. Figure 19 shows a partly poisoned pore of a gas diffusion electrode. If Wp is the surface concentration of the poison in the poisoned part of the pore (moles per unit area of the catalyst) and Cp(x) is the local poison concentration in the pore, the one-dimensional continuity equation for the poison yields... 

The determination of the real surface area of the electrocatalysts is an important factor for the calculation of the important parameters in the electrochemical reactors. It has been noticed that the real surface area determined by the electrochemical methods depends on the method used and on the experimental conditions. The STM and similar techniques are quite expensive for this single purpose. It is possible to determine the real surface area by means of different electrochemical methods in the aqueous and non-aqueous solutions in the presence of a non-adsorbing electrolyte. The values of the roughness factor using the methods based on the Gouy-Chapman theory are dependent on the diffuse layer thickness via the electrolyte concentration or the solvent dielectric constant. In general, the methods for the determination of the real area are based on either the mass transfer processes under diffusion control, or the adsorption processes at the surface or the measurements of the differential capacitance in the double layer region [56],... 

This is the simplest model of an electrocatalyst system where the single energy dissipation is caused by the ohmic drop of the electrolyte, with no influence of the charge transfer in the electrochemical reaction. Thus, fast electrochemical reactions occur at current densities that are far from the limiting current density. The partial differential equation governing the potential distribution in the solution can be derived from the Laplace Equation 13.5. This equation also governs the conduction of heat in solids, steady-state diffusion, and electrostatic fields. The electric potential immediately adjacent to the electrocatalyst is modeled as a constant potential surface, and the current density is proportional to its gradient ... 

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