Water-filled nanopore layers

As for effective kinetic parameters of the ORR that should be used in macroscopic models, it seems reasonable to assume that the reaction order for oxygen concentration will be yo2 = 1 for conditions of interest. The effective transfer coefficient of the ORR, ac, will transition through a sequence of discrete values between 1 and 0.5, as a function of electrode potential. The reaction order for proton concentration, yh+, depends strongly on the adsorption regime and, therefore, a prediction of the value is not trivial. The difference ac - yh+ is a key determinant of electrostatic effects in water-filled nanopores inside of catalyst layers, as discussed in the section ORR in Water-Filled Nanopores Electrostatic Effects. ... 

The transport properties of water-filled nanopores inside of agglomerates and the properties of the ionomer film at the agglomerate surface define local reaction conditions at the mesoscopic scale. These local conditions, which involve distributions of electrolyte phase potential, proton density (or pH), and oxygen concentration, determine the kinetic regime, under which interfacial electrocatalytic processes must be considered. Combining this information, a local reaction current can be found, which represents the source term to be used in performance modeling of the cathode catalyst layer. 

Since UTCLs contain no added electrolyte, the mode of proton transport in such layers remains a debated question. It was postulated in Chan and Eikerling (2011) that protons in water-filled UTCL pores undergo bulk-water-like transport, similar to ion transport in charged nanofluidic channels (Daiguji, 2010 Stein et al., 2004) and gold nanoporous membranes (Nishizawa et al., 1995). The proton conductivity of the pore is then determined by the electrostatic interaction of protons with the surface charge of pore walls. 

Basically, the nanoporous water-filled medium with chargeable metal walls works like a tunable proton conductor. It could be thought of as a nanoprotonic transistor. In such a device, a nanoporous metal foam is sandwiched between two PEM slabs, acting as proton source (emitter) or sink (collector). The bias potential applied to the metal phase controls proton concentration and proton transmissive properties of the nanoporous medium. The value of cp needed to create a certain proton flux depends on surface charging properties and porous structure of the medium. Moreover, coating pore walls with an electroactive material, for example, Pt, would transform it from a tunable proton conductor into a catalytic layer with proton sinks at the interface. Owing to the intrinsically small reaction rate of the ORR, it would not significantly affect the proton transport properties. 

Carbonization of organics used in this study was carried out at a sufficiently low temperature ( 870 K). This condition leads to formation of the carbon layers whose graphene clusters have sizes that do not exceed 2 nm (Fenelonov 1995). Consequently, formation of partly graphitized areas on the carbon surface is hardly probable, and the up-field shifts of the H NMR signal for the adsorbed water may be due to location of water molecules in narrow mesopores partially filled by carbon deposits because this displacement does not exceed 5 ppm (in slit-shaped nanopores this shift could be larger see Chapter 3). 

Type II electrodes are two-phase composite media that consist of a nanoporous and electronically conductive medium filled with liquid electrolyte or ionic liquid. The electrochemically active interface forms at the boundary of the two phases. The electrolyte phase must provide pathways for diffusion and permeation of protons, water, and reactants. Flooded two-phase CLs could work well when they are made extremely thin, not significantly exceeding a thickness of Icl — 200 nm. Rates of diffusion of reactant molecules and protons in liquid water are then sufficient to provide uniform reaction rate distributions over the thickness of the layer. 

---