Thermally processed Ni-and Co-struvites as functional materials for proton conductivity

Here, we describe how to synthesise proton-conductive transition metal phosphates (TMPs) by direct thermal processing of precursor M-struvites, NH ₄MPO ₄·6H ₂O, with M = Ni ²⁺, Co ²⁺. In the as-derived TMP phases their thermal history and bulk proton conductivity were linked with the structural information about the metal coordination, phosphate groups, and volatile compounds. These aspects were investigated with vibrational and synchrotron-based spectroscopic methods (FT-IR, FT-RS, XAS). We elucidated the structures of amorphous and crystalline Ni- and Co phosphate phases in association with different coordination changes and distortion degrees of the metal polyhedra as they developed upon heating. Ni-struvite transformed to a stable amorphous phase over a broad range of temperatures (90 °C < T < 600 °C), in which it remained in an octahedral coordination environment, but the degree of distortion changed with T. In contrast, heating of Co-struvite led to several successive crystalline phases with only unstable transitional and short-lived amorphous components. Among the as-occurring phases, a highly functional layered M-dittmarite NH ₄MPO ₄·H ₂O obtained at low temperatures (T < 200 °C) demonstrated high proton conductivity values of 4.2 × 10 ⁻⁵ S cm ⁻¹ for Ni-dittmarite and Co-dittmarite > 10 ⁻⁴ S cm ⁻¹ at room temperature. Even at low humidity, these values are comparable with those found for Nafion, MOFs, some perovskites or composite materials. Coprecipitation of phosphates and transition metal cations in the form of struvite is potentially a viable method to extract these elements from wastewater. Thus, we propose that recycled M-struvites could be potentially further directly upcycled into crystalline and amorphous TMPs useful for electrochemical applications.