This is a rather old technique which I picked up from a 3D Max guy.
This got interesting as with Physical Renderer there is no Cel Renderer
or Sketch&Toon and it's rather easy to use. The core concept is to take
a copy of your object and make frontfacing Polygons transparent so we
are left with the backfacing ones. By displacing this copied geometry a
couple of cm, it gives your object an outline, letting the copy be visible
through the edges of your geometry. This technique doesn't work every time,
but it's worth giving a try if you need outlines with PhyRen. This is one
of many techniques, the Fresnel or Art Shader are among the others.