AlP compound and P-doping for promotion of electrocatalytic activity of N-doped carbon derived from metal-organic framework

Water splitting plays a key role in future fuels, where two processes occur - the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Nitrogen-doped carbon derived from carbonized metal-organic frameworks (MOF) are promising materials due to their unique structures and high porosity. However, they usually show poor intrinsic conductivity and poor durability. Therefore, we demonstrate how the dual functionalization of N-doped carbon derived from MOF, induced by phosphorous doping and alumina phosphide (AlP), boosts OER activity. Further, robustness was promoted via aluminium (present from parent MOF) causing resistance decrease and structure stabilization. The aluminium-based MOF carbonized at 750°C and functionalized with phosphorus (750+Al+P) showed an overpotential of 353 mV at 10 mA·cm-2 and high durability during chronopotentiometry at 10, 20 and 50 mA·cm-2 in contrast to the material without P-functionalization, which reached 471 mV. We utilize X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) to demonstrate structure properties and to identify active species – N,P-doped carbon. The phosphorus content in 750+Al+P was determined to be 4.89%, for 750-Al+P - 1.65% indicating successful P-doping. Moreover, to reveal why the electrocatalyst with the presence of both elements: aluminium and phosphorus outperforms other studied materials their chemical and structural changes after OER were monitored in ex situ XPS and TEM. The data indicated the formation of an AlP layer on the surface of aluminium nanoparticles which reacts with electrolyte to form Al(OH)3 releasing PH3 and during the OER process reacts further into Al2O3 in the form of flat 2D structures on the carbon surface stabilizing the catalyst. Based on the collected data the potential mechanism of OER in the presence of 750+Al+P has been proposed.