Mg2+ incorporated Co-based MOF precursors for hierarchical CNT-containing porous carbons with ORR activity

Abstract

In this paper, we introduced light and abundant metal magnesium into a cobalt-based metal organic framework (Co-MOF, [(CH₃)₂NH₂]₂[Co₃(bpdc)₄]·5DMF·4CH₃OH) (1, H₂bpdc = 4,4′-biphenyldicarboxylic acid, DMF = N,N-dimethylformamide) as a heteroatom to synthesize Mg–Co bimetallic MOFs, namely [(CH₃)₂NH₂]₂[MgCo₂(bpdc)₄]·4DMF·5CH₃OH (2) and [(CH₃)₂NH₂]₂[Mg_1.2Co_1.8(bpdc)₄] 4DMF·4CH₃OH·6H₂O (3). Based on the formation of a rather low density framework after the introduction of the light Mg²⁺, such bimetallic MOFs exhibited higher gas adsorption abilities than the isostructural Co-based MOF 1. N₂ adsorption measurements demonstrate that the BET surface area of 3 is 305.4 m² g⁻¹, exhibiting three times that of 1 (104.4 m² g⁻¹). Significantly, due to the introduction of the low-melting Mg²⁺, the Mg–Co MOFs could be further utilized as precursors for porous carbons only by calcination at a mild temperature of 600 °C which could exhibit a BET surface as high as 712.78 m² g⁻¹. Furthermore, after post-synthetic modification with a N/S heteroatom at 900 °C, the obtained hierarchical carbons exhibit superior activity in the oxygen reduction reaction (ORR) that is comparative to the commercial Pt/C catalyst. TEM results indicate that Co-embedded carbon nanotube (CNT)-containing hierarchically nanoporous carbons have been obtained. This study may offer a new avenue to prepare porous carbons utilizing Mg-containing bimetallic MOFs as sacrificial templates.