  Characterization of transport properties of porous media is increasingly
relying on computational methods that require reconstruction of the media
structure. We present a simple method of constructing the 3D surface of fibrous
porous media - the gas diffusion layer (GDL) used as the porous electrode in
PEM fuel cells. The method is based on extending the depth-of-field on the
whole attainable thickness of the GDL. A series of images of the GDL sample is
recorded by the sequential movement of the sample with respect to the
microscope focus. Different layers of the surface of the sample appear in focus
in the different images in the series. The indexed series of the in-focus
portions of the sample surface is combined into one sharp 2D image and
interpolated into the 3D surface representing the surface of an original GDL
sample. The method uses a conventional upright stage microscope that is
operated manually, the inexpensive Helicon Focus software, and the open source
MeshLab software. The accuracy of the reconstruction of the image features was
found to be ~ 1% for probing depth of ~ 100 um with the corresponding image
acquisition time being ~ 10 min. Applying this method we found that: i) this
technique is able to reproduce 3D-projection of the GDL structure for a depth
of ~ 50-100 um, ii) the depth of the deepest channel in the GDL can be as much
as ~ 100-160 um, for GDL samples with a thickness ~ 200 um.
