Free Ultrathin Catalyst Layers

In recent years, advanced designs of UTCLs have shown great promise in view of achieving a tremendous Pt loading reduction in CCLs, from 0.4 mg cm in conventional CCLs, down to 0.1 mg cm in UTCLs. Enabled by a combination of 

The break-in process of 3M NSTF films involves voltage cycling to create a smooth polycrystalline Pt surface on the whiskers. The surface area enhancement factors of resulting structures are from 10 to 25, determined from cyclic voltammo-grams in the potential region of hydrogen underpotential deposition. The surface area enhancement factor (or roughness factor or real-to-apparent surface area ratio) of a 

FIGURES.22 An illustration of design and key properties of ionomer-free ultrathin catalyst layers with insulating or electronically conductive support materials. The typical thickness is in the range of 200 nm. 

It is also widely known that MEAs utilizing 3M NSTF technology face severe water management challenges. They show poorer performance than conventional CCLs at low RH, presumably caused by poor proton transport in insufficiently hydrated layers. Moreover, NSTF MEAs exhibit an increased propensity for flooding 

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. 

---

Moving up the scale to the level of flooded nanoporous electrodes, Michael s group has developed the first theoretical model of ionomer-free ultrathin catalyst layers—a type of layer that promises drastic savings in catalyst loading. Based on the Poisson-Nernst-Planck theory, the model rationalized the impact of interfacial charging effects at pore walls and nanoporosity on electrochemical performance. In the end, this model links fundamental material properties, kinetic parameters, and transport properties with current generation in nanoporous electrodes. 

---