Nanoprotonic Fuel Cells A New Design Paradigm

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. 

For simple metals, the metal charging behavior can be described by the potential of zero charge pP. If (p - pP 0 for such a medium, proton transport will be suppressed. For (p — pP 0, it should transmit protons with a protonic resistance that decreases upon decreasing (p. This tunability of proton conductivity could be applied as a method for determining (pP of porous metallic materials, for example, by using the linear relation of Equation 3.74. This setup would allow for systematic studies of effects of materials eomposition, surface roughness, and surface heterogeneity on (pP.  

In conclusion, the main challenge in the design of nanoporous materials for UTCLs is to fine-tune the proton concentration in order to optimize the interplay of ORR and Pt dissolution. This fundamental principle has not been widely realized, as most efforts in fuel cell electrocatalysis compare candidate catalyst materials at identical and normally high proton concentration. Proton concentration is usually not considered a parameter to tinker with although it is the key card in the game. 

As a further device modification, a thin porous metal foam with tunable proton concentration and vanishing electrocatalytic activity could be inserted between PEM and UTCLs. This layer could reduce proton concentration in the UTCL to a level that optimizes the interplay of ORR rate and Pt dissolution rate. 

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