Rethinking the Gas Diffusible Catalyst Layer with d-Band Tuned Pt Skeleton and Controlled Pore and Bone Size

Proton exchange membrane fuel cells (PEMFC) lag behind the theoretical efficiency because of kinetic and mass transfer problems of the catalyst layer, especially the oxygen reduction catalyst layer and the methanol oxidation catalyst layer. In these fields almost all research in catalysts and catalyst layers are done on Pt nanoparticles and the perfect composition of Nafion to Pt catalyst on the corresponding substrate, for example carbon nanoparticles. The typical catalyst layer for the oxygen reduction reaction has a thickness between 5 µm and 25 µm. For the methanol oxidation reaction the thickness is even higher. The kinetic barriers of the oxygen reduction and the methanol oxidation, oxygen and methanol diffusion, proton conductivity barriers and potential distribution across the catalyst layer hinder the Pt efficiency which is, at the end, only 10-20%.[1]

Pt usage has to be minimized since it will be responsible for over 50% of the system costs in PEMFC[2].

These problems can be circumvented using mesoporous Pt skeleton layers where H⁺ probably migrate across the surface and/or diffuse in H₂O, like in the nanostructured thin films (NSTF) [3]. Additionally in our approach no carbon support and Nafion is needed – it is supportless and ionomerless.

Pt skeleton was prepared by a dual-magnetron plasma-synthesis. The pore and bone scaling can be controlled by system parameters as pressure, deposition time, temperature and power.

For fundamental understanding of Pt skeleton it is necessary to investigate the influence of the solid to pore ratio on the performance.

In this work a Pt skeleton was synthesized by alternated sputtering of Pt and Co on gas diffusion layers and glassy carbon. The base metal was subsequently dissolved by an electrochemical treatment under acidic conditions, with the Pt skeleton remaining on the substrate. The remaining catalyst layer has a thickness of approximately 200 nm.

Cyclic voltammetry and oxygen reduction activity tests gave information on the global behaviour of the electrodes. Scanning Electron Microscopy was employed to investigate pore and bone morphology. It was possible to produce a Pt skeleton with different pore sizes and bone diameter. Furthermore Pt can be tuned by the remaining Co in the catalyst. The Cobalt remains in the catalyst layer which was tested also for different system parameters. The remaining Co was investigated by the d-band shift of the catalysts by XPS and the corresponding EDX atomic ratios before and after electrochemical testing.

The new approach of mesoporous Pt skeleton synthesized by a new plasma process is a revolution in catalyst layer architecture. The typical catalyst layer thickness can be reduced by a factor of 100 and there is no need for a corroding support. Furthermore Nafion as proton carrier can be excluded and therefore limiting anion adsorption can be avoided.

Our new approach could also be used for air electrodes or electrolyzers as the Power to Gas technology is going forward.

Figure 1: Scheme of the proposed reaction mechanism in the oxygen reduction reaction of Pt skeleton (left) and a SEM picture of Pt skeleton (right)

[1]        M. Eikerling, A.S. Ioselevich, A. a. Kornyshev, How good are the Electrodes we use in PEFC?, Fuel Cells. 4 (2004) 131–140. doi:10.1002/fuce.200400029.

[2]        U. Eberle, B. Müller, R. von Helmolt, Fuel cell electric vehicles and hydrogen infrastructure: status 2012, Energy Environ. Sci. 5 (2012) 8780. doi:10.1039/c2ee22596d.

[3]        M.K. Debe, Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts, J. Electrochem. Soc. 160 (2013) F522–F534. doi:10.1149/2.049306jes.