(NH4)2HPO4 (DAP) (ChemSolute, 99%), NiSO4•6H2O (ChemSolute, 99%) and CoSO4•7H2O (Alfa Aesar, 98%) were used to synthesize M-phosphates (M = Ni, Co, NixCo1-x). At first, 1 mol/l stock solutions of DAP and the metal salts were prepared. The detailed synthesis procedure can be found elsewhere . The transition metal and DAP concentrations in the aqueous solution were varied from 0.004 mol/l to 1 mol/l resulting in a metal-to-phosphate ratio between 0.2 ≤ MP ≤ 1. By using DAP with a fixed nitrogen-to-phosphate ratio of 2 the occurring precipitation reaction of M-struvite was typically occurring according to the following mass-balance equation: