Enhanced oxygen evolution reaction of 2-dimensional metal-organic frameworks with tunable nitrogen functionalities by ion beam sputtering

Modification of metal-organic frameworks (MOFs) is recently under the spotlight due to their versatile properties and potential applications in electrochemical catalysis. Here, we successfully demonstrate nitrogen doping into the MOF electrocatalyst without noble metals using a facile, tunable ion beam sputtering (IBS) process for the first time and evaluate the role of the incorporated heteroatom. Two-dimensional Ni-naphthalene-2,6-dicarboxylic acid MOF (NiNDC) with large surface area was subjected to nitrogen IBS (NIBS) and exhibited significantly improved performance for oxygen evolution reaction (OER) with a low overpotential of 222 mV at 10 mA cm-2; a Tafel slope of 88 mV dec-1; and over 120 h of stability at 100 mA cm-2. The relationship between the nitrogen functionalities and catalytic activity was elucidated by spectroscopic analysis and electrochemical measurements, i.e., (i) pyridinic N as an electron-withdrawing group that directly enhances the reaction kinetics, (ii) pyrrolic N to stabilize the catalyst and (iii) graphitic N to enhance the electrical conductivity. We found that the electrocatalytic performance was affected by the ratio of the three nitrogen species, which was controllable by the accelerating potential (AP) of NIBS. This study provides insights into the influence of the chemical state of MOF surfaces on catalytic reactions and presents a novel method for effective nitrogen doping.