Water-Filled Nanopore with Charged Metal Walls

Model OF A Water-Filled Nanopore with Charged Metal Walls 

Without an embedded ionomer phase in the UTCL, the surface charge at the pore walls is the driving force for proton migration into the pore. Since the focus is on the bulk UTCL response, effects of the electric double layer at the PEM/UTCL interface are ignored. These effects could become significant only if the bulk of the UTCL is essentially inactive. 

It is worth mentioning that proton transport phenomena in UTCL pores resemble those in water-filled pores of PEM, discussed in the section Proton Transport in 

Water-filled nanopores with heterogeneous metal-solution interfaces 

Water-filled cylindrical pore with a smooth metalsolution interface 

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The importance of proton distribution and transport in water-filled nanopores with charged metal walls is most pronounced in ionomer-free UTCLs (type II electrodes), cf. the main case considered in this section. In either type of CLs, proton and potential distribution at the nanoscale are governed by electrostatic phenomena. 

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. 

A model of the ORR in a single water-filled pore with charged walls of Pt will be presented. It affords the definition of an effectiveness factor, by which the performance of any nanoporous CL material could be evaluated. Furthermore, a remarkable conclusion is drawn in view of the coupling of ORR kinetics and metal corrosive dissolution. 

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