Synthesis, structural characterization and electrocatalytic properties of cobalt phosphides and pyrophosphates derived from glyphosine

Abstract

Metal phosphonates (MPs), a subclass of coordination polymers, result from the bonding of phosphonic acids (RPO32-) with metal ions, giving rise to an uniformly dispersion of the metal sites at the atomic scale. This characteristic allows the preparation of metal-phosphorous-based nano-carbon composites by a simple one-step pyrolysis, what makes them very attractive precursors of Non-Precious Metal Catalysts (NPMCs). Herein, we report the synthesis, characterization and electrochemical properties of three cobalt(II) coordination polymers derived from the glycine-N,N-bis(methylenenphosphonic acid) (BPMGLY), with formula [Co(C4H9O8NP2(H2O)2]·nH2O (n=0, 2). In contrast with the zero-dimensional structure of [Co(C4H9O8NP2(H2O)2]·2H2O (Co-BPMGLY-I), the two new polymorphs synthesized, Co(C4H9O8NP2(H2O)2 (Co-BPMGLY-II and Co-BPMGLY-III), whose structures have been solved from powder diffraction data, show two-dimensional frameworks with different connectivity between the Co2+ ions and the ligand within the sheets. These MPs were used as precursors of NPMCs by pyrolyzing them under 5%-H2/Ar at different temperatures. The electrochemical behavior of the resulting compounds, mainly crystalline cobalt pyrophosphates and/or phosphides, is fully studied regarding to the Oxygen Evolution and Reduction Reactions (OER and ORR, respectively) as well as Hydrogen Evolution Reaction (HER). Cobalt phosphide (CoP) derived from Co-BPMGLY-I displayed the best results, showing an overpotential of 156 mV for HER.