Phase stability studies on transition metal phosphates aided by an automated synthesis

Abstract

Transition metal phosphates (TMPs) have attracted interest as materials for (electro-) catalysis, and electrochemistry due to their low-cost, stability, and tunability. In this work, an automated synthesis platform was used for the preparation of transition metal phosphate crystals to efficiently explore the multidimensional parameter space, determining the phase selection, crystal sizes, shapes. By using X-ray diffraction and spectroscopy-based methods and electron microscopy imaging, a complete characterization of the phase stability fields, phase transitions, and crystal morphology/sizes was achieved. In an automated three-reactant synthesis, the individual effect of each reactant species NH₄⁺, M²⁺, and PO₄³⁻ on the formation of transition metal phosphate phases: M-struvite NH₄MPO₄·6H₂O, M–phosphate octahydrate M₃(PO₄)₂·8H₂O with M = Ni, Co and an amorphous phase, was investigated. The NH₄⁺ concentration dictates the phase composition, morphology, and particle size in the Ni-system (crystalline Ni-struvite versus amorphous Ni–PO₄ phase), whereas in the Co-system all reactant species – NH₄⁺, Co²⁺, and PO₄³⁻ – influence the reaction outcome equivalently (Co-struvite vs. Co–phosphate octahydrate). The coordination environment for all crystalline compounds and of the amorphous Ni–PO₄ phase was resolved by X-ray absorption spectroscopy, revealing matching characteristics to its crystalline analogue, Ni₃(PO₄)₂·8H₂O. The automated synthesis turned out to be significantly advantageous for the exploration of phase diagrams due to its simple modularity, facile traceability, and enhanced reproducibility compared to a typical manual synthesis.