The precursor struvites are converted into mesoporous and amorphous materials by a simple thermal treatment at low temperature (< 200°C).
Source: BAM
In the last decades, mesoporous transition metal phosphates (TMPs) gained attention as potential inexpensive functional materials for electrochemistry and -catalysis. Mesoporous materials exhibit a characteristic pore size range of 2 -50 nm. The mesoporosity is beneficial in electrochemical applications because it allows for an improved mass transfer of ions through the material and simultaneously enhances reactivity due to an increased surface area. The synthesis of mesoporous TMPs is highly challenging, and is typically carried out using hard and soft templates (e.g. rigid silicates or soft lipids). In both cases, the removal of the template must be performed by chemical etching or heating. Such removal of the template is highly problematic, as the pore structure or the material itself is likely to be deteriorated in the process. Moreover, the choice of the templating method affects the extent of the possible chemical compositions of the TMPs. In our work, we synthesized mesoporous TMPs by a template-free method to avoid this serious issue. In our approach, the evolution of mesoporous frameworks relied on the thermal decomposition of a precursor metal phosphate compound called struvite, which includes water and ammonia in its crystal structure.
Importantly for our approach, struvite can contain various metal cations in its crystal structure, such as Ni or Co. By a simple, yet controlled, thermal treatment of struvite crystals, water and ammonia are removed slowly out of the compounds, resulting in the formation of (meso-)porous and amorphous frameworks. Such frameworks are fundamentally composed of phosphate groups linked to metal cations which were originally isolated in the precursor struvites. This evolution of mesopores was characterized by us in detail. We propose that the low-cost, environmentally friendly transition metal struvites could be obtained as recycling products from industrial and agricultural wastewaters and upcycled into mesoporous TMPs. In this form, these materials could directly be used in electrochemistry due to their enhanced surface area/reactivity. Especially in the generation of green hydrogen, these compounds could accelerate the water splitting reaction towards the product side of oxygen and hydrogen. In such a way, metal phosphates can be recycled and upcycled e.g. for hydrogen applications and contribute to an environmental friendly circular economy.
Template-free synthesis of mesoporous and amorphous transition metal phosphate materials
Stephanos Karafiludis, Ana Guilherme Buzanich, Christian Heinekamp, Glen Jacob Smales, Vasile-Dan Hodoroaba, J. E. ten Elshof, Franziska Emmerling, Tomasz Maciej Stawski
Published in Nanoscale, 2023, 15, 3952–3966, Open Access
BAM Department Materials Chemistry
BAM Division Surface Analysis and Interfacial Chemistry
BAM Division Structure Analysis
BAM Division Synthesis and Scattering of Nanostructured Materials