I'm facing the same challenge.
I'm starting to learn how to make a rhodium plating bath starting with pure rhodium and then to procede with these flakes.
I still didn't analyse the flakes but certainly they aren't pure enough.
To purify there's 2 routes that I found in Brauer (Handbook of Preparative Inorganic Chemistry Volume 2) one of them is in Ammen's book (Recovery and Refining of Precious Metals):
Impure Rh solution then is made K3[Rh(NO2)6] or (NH4)3[Rh(NO2)6] that is a solid and must be filtered, this solid can be dissolved and reprecipitated.
or
Impure Rh solution then is made [RhCl(NH3)5]Cl2 that is a solid and must be filtered, this solid can be dissolved and reprecipitated.
These procedures are oversimplified (more information can be found in these books), and I do not garante that it will work. I'm searching that myself.
To dissolve rhodium you have 2 main routes, dissolve in boiling, refluxing, sulfuric acid or fuse with potassium/sodium hydrogen sulfate or potassium pyrosulfate.