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In the last decade transition metal phosphates (TMPs) captured major interest due to their high electrochemical activity useful for electrode materials, supercapacitors or batteries. Importantly, TMPs are known to exhibit high proton conductivity of the order of >10-2 -10-5 S/cm from 25°C to temperatures as high as 400°C. The crucial milestone in the research on the applicational use of TMPs in all their varieties is an ability to obtain, explore and optimize different compositions and structures, both crystalline and amorphous. Thus, we elucidate the structures of amorphous and crystalline Ni- and Co phosphate phases, as they develop upon heating. This method relies on the thermal decomposition of a crystalline M-struvite precursor, i.e. NH4MPO4•6H2O (M = Mg, Ni, Co, NixCo1-x etc. Here, coincidently volatile components such as H2O or NH3 degas out of the compound resulting in a phase transformation to amorphous or crystalline metal phosphate phases depending on the reaction conditions.
For this we used a suite of advanced methods such as FTIR, FT-RS and synchrotron-based XAS. Ni-struvite transforms to amorphous phases over a broad range of temperatures (90°C < T < 600°C) in which it remains in an octahedral coordination environment. On the other hand, Co-struvite treatment leads to multiple crystalline phases with only small amounts of short-lived amorphous phases.
Importantly, the occurring amorphous phases exhibit uniform mesoporous frameworks (2–5 nm wide pore channels, specific surface area of 100 m2g−1 and a pore volume of 0.13 cm3g−1) at low temperatures (<200°C) while the crystalline compounds can be used as proton conductors demonstrating values of >10-4 S/cm at 25°C. Consequently, we investigated the amorphization/evolution of mesoporosity, the proton-conducting properties and the complex local structure with in- and ex-situ approaches during thermal treatment. Additionally, to this low cost, environmentally friendly and simple one pot synthesis, the precursor M-struvites could grow as a recovery product from industrial waste waters. In such a way, they would to a recycling economy of sought-after commodities like phosphorus or transition metals.