Nizar, did you try Aspen Electrolytes system. I am not sure how much detail you want to include in your model, but if you haven't looked at it, you may want to consider doing so.
1: Simulate anode and cathode separately and connect them via a stream in main Aspen Plus environment. Thus, each reactor could be an available black including CSTR and/or PFR reactor depending on your modelling specifications and framework.
2: Do FORTRAN coding for your model and incorporate it in Aspen
3: Go with Aspen Custom Modeller to develop your own block for SOFC by solving PDEs, ODEs and ADEs.
How to define design spec. for steam to carbon ratio as variable(dependant) in SOFC simulation by Aspen plus?! In fact,I want to know What's STCR reference type?!