During the lifetime of a polymer electrolyte fuel cell, the pore structure of the Pt/C catalyst layer may change as a result of carbon corrosion. Three-dimensional visualization of porosity changes is important to understand the origin of fuel cell performance deterioration. A focused ion beam/scanning electron microscopy (FIB/SEM) approach was adopted together with electron tomographic studies to visualize the three-dimensional pore structure of a Pt/C catalyst. In the case of pristine catalyst layers, the pores form an interconnected network. After 1000 start-up/shut-down cycles, severe carbon corrosion leads to a collapse of the support structure. The porosity of the degraded catalyst layer shrinks drastically, resulting in a structure of predominantly isolated pores. These porosity changes hinder the mass transport in the catalyst layer, consequently leading to a substantial loss of fuel cell performance. FIB/SEM serial sectioning and electron tomography allows three-dimensional imaging of the catalyst pore structure, which is a prerequisite for modeling and optimizing mass transport in catalyst layers.

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Indium tin oxide (ITO, In 2 O 3 :SnO 2 ) thin films exhibit a high conductance along with brilliant transparency in the optical range. [1][2][3] In spite of the existence of some other transparent conducting oxide materials and the high material costs, ITO is widely used as transparent electrode material in stateof-the-art optoelectronic devices such as TFT-LCD-s, [4][5] plasma TV-s, and touchscreens. It is also utilized in novel, smart optoelectronic applications including organic solar cells, [6][7] electrochromic devices, [8] and organic light emitting diodes (OLEDs) for displays [9][10][11] or for lighting. [12] Usually ITO thin films are fabricated by physical vapor deposition (PVD) methods including RF-sputtering [13,14] and magnetron sputtering [2,[15][16][17] as well as pulsed laser deposition [18,19] and several other evaporation techniques. [20][21][22][23][24] PVD derived ITO layers are characterized by high conductances of partly more than 10 000 V À1 cm À1 , [19,22] but also by high production costs. Cost drivers are the inevitable vacuum process which hinders continuous production and the requirement of substractively wet chemical patterning steps. [4] Wet deposition techniques are of emerging interest due to their high cost reduction potential based on missing vacuum demand, printing possibility comprising large area fabrication potential, and the feasibility of direct patterning. Beside the several deposition methods, there exist two different principles of wet chemical processing: The sol-gel-technique, characterized by deposition of precursors with subsequent transformation into ITO [25][26][27][28][29][30] and the deposition of a dispersion containing ITO nanoparticles. [30][31][32][33][34] Sol-gel ITO films can possess adequate conductances of >350 V À1 cm À1 , [25,26,35] which can be increased to >1600 V À1 cm À1 by temperature treatment in nitrogen or forming gas. [26,30] Due to the required low precursor to solvent ratio, only thicknesses of about 10-20 nm can be achieved for the sol-gel single layers, [26,30] which leads to high sheet resistances. To avoid this, costly repetitions of the deposition and transformation sequence are necessary.In contrast to sol-gel ITO layers, nanoparticle dispersion derived thin films enable the production of almost every thickness. They can also be produced on flexible polymer substrates, [34] but exhibit comparatively low conductances. The latter is related to a poor electrical contact between the primary particle aggregates and high inter-grain porosity. [30,32,33] Typical conductance after layer deposition is in the order of 10 À2 V À1 cm À1 and rises to several 10 0 V À1 cm À1 after annealing in air at 550 8C. [33,36] In order to overcome this conductance bottleneck, we investigated the impact of various conductance improving techniques on the electrical, optical, and morphological properties of nanoparticulate printed ITO layers. ExperimentalNanoparticulate ITO layers for electrical measurements were made by spin-coating a commercial ethanolic dispersi...

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