Noble-Metal-Free MOF-74-Derived Nanocarbons: Insights on Metal Composition and Doping Effects on the Electrocatalytic Activity Toward Oxygen Reactions

Víctor K. Abdelkader-Fernández, Diana M. Fernandes, Salete S. Balula,Luís Cunha-Silva, Manuel José Pérez-Mendoza, F. Javier López-Garzón, M. Fernando Pereira and Cristina Freire

Abstract: The continuous and dramatic increase of global demand for energy resources makes it urgent to develop affordable nanostructured materials to act as efficient electrocatalysts (ECs) in the energy-related reactions. Metal−organic framework (MOF) template pyrolysis for the production of nanostructured carbon-based materials is a very promising methodology to produce carbon-based ECs. Herein, we report the preparation, characterization (XPS, CHNS analysis, ICP-OES, XRD, TEM, and SEM/EDS), and application as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) ECs of a plethora of nanostructured carbon materials derived from RT-synthesized (RT, room temperature) MOF-74 with different metal compositions, Co, Ni and Co/Ni, and dopant heteroatoms, N−, S− and N/S-dual-doping. This has allowed the study of the aforementioned parameters’ influence on the OER and ORR electrocatalytic activity in alkaline medium. Highly synergetic effects have been detected in two cases: (1) when N/S-dual-doped carbon is produced from a monometallic Co-MOF-74 template, N,S−Co@C, and (2) when an undoped carbon is derived from bimetallic Co/Ni-MOF-74, Co/Ni@C. These two samples achieve OER performances with η10 = 0.41 and 0.44 V, respectively, along with Tafel slopes of 101 and 93 mV dec−1, being close to the state-of-art OER catalyst performance. In addition, ORR tests showed that the effect of heteroatom doping on ORR activity is always positive, regardless of metal composition.