Multifunctional Metal Phosphonates for Electrochemical Applications.

Abstract

This thesis explores the synthesis, characterisation and electrochemical properties of multifunctional metal phosphonates (MPs), focusing on their potential as proton conductors and as precursors of electrocatalysts for energy conversion applications. A series of divalent transition MPs have been prepared from multifunctional mono- and poly-phosphonic and phosphinic acids, yielding a wide variety of metal coordination environments and structural arrangements. Their proton conductivity properties were modulated by controlling parameters such as the number of phosphonic groups, the use of secondary auxiliary ligands or by functionalisation with a carboxylic or sulphonic group. Among the obtained compounds, layered Zn(II) sulfophosphonates showed the highest proton conductivities (~3·10-2 S-cm-1, at 80 ºC and 95% RH) and low activation energies (< 0.5 eV), characteristics of a water-mediated Grotthuss-type proton transfer mechanism, which involves the presence of protonated phosphonate and sulphonate groups, high water content and, in certain cases, charge-compensating ammonium ions. In addition, MPs have been investigated as precursors for electrocatalysts by pyrolysis under controlled atmospheres. The resulting materials, including metal polyphosphates and phosphides, showed remarkable catalytic activity for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Among them, Co- and Ni-based phosphides showed remarkable superior performance, especially when doped with Fe (OER) or heteroatom enrichment (P for OER/HER and N for ORR). As revealed by differential PDF analysis, OER Co-based catalysts are essentially sub-nanometric CoO(OH) polymorphs (size ~20 Å), being the most active those derived from Fe-doped Co phosphides (η10 ~ 275 mV), outperforming RuO₂ benchmark in terms of activity and stability in alkaline water electrolysis (AWE) systems.