A study of the dopant effect in metal pyrophosphates and their potential for use in supercapacitors

Abstract

This dissertation focuses on the comprehensive characterization of two sample sets of LaxHf1-xP2O7, denoted as Group 1 and 2, synthesised through two variations of the sol-gel method, shedding light on their structural and functional characteristics. Employing a range of characterisation techniques such as Powder X-ray Diffraction (PXRD), 31P Magic Angle Spinning Solid-State Nuclear Magnetic Resonance (31P MAS ssNMR) spectroscopy, Pair Distribution Function (PDF), Fourier Transform Infrared (FTIR) Spectroscopy, Transmission Electron Microscopy (TEM), Simultaneous Thermal Analysis (STA), and Potentiometric Electrochemical Impedance Spectroscopy (PEIS), this study achieved a thorough understanding of the materials' composition, structure, and physical properties. The PXRD analysis suggested that the materials from Group 1 were amorphous, contrasting with the crystalline behaviour of Group 2's materials. A pre-existing structure model of HfP2O7 was fitted to Group 2's PXRD data using the Rietveld refinement method with satisfactory fits, confirming the composition and structure of the materials, as well as emphasizing their crystalline nature. The 31P MAS ssNMR analysis revealed distinct phosphorous signals for both groups, elucidating their differing phosphorous environments. PDF data provided insights into short-range order for Group 1 and refined structural models for Group 2, enabling qualitative comparisons between the two groups. FTIR spectra confirmed the presence of pyrophosphate functional groups in both groups. TEM images substantiated the amorphous and crystalline nature of Group 1 and Group 2 materials, respectively, with dominant particle sizes in the 20-30 nm range. STA data highlighted excellent thermal stability in both groups, crucial for high temperature applications. PEIS data indicated enhanced ionic conductivity upon La3+ doping, albeit lacking a clear trend with varying dopant concentrations. Additionally, distinct synthetic procedures for Group 1 and Group 2 resulted in materials with different morphologies. Group 1's method led to amorphous materials, attributed to solvent elimination before calcination, while Group 2's approach yielded crystalline materials due to evaporation in air before calcination. These findings underscore the impact of subtle synthetic variations on material properties, emphasizing the need for precise control in synthesis parameters. Overall, this research provides valuable insights into LaxHf1-xP2O7 materials for energy applications, paving the way for further advancements in metal pyrophosphates and materials science as a whole.