Trifunctional Catalysts for Overall Water Splitting and Oxygen Reduction Reaction Derived from Co,Ni MOFs

Abstract

In order to obtain platinum-group-metal-free catalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline electrolyzers, nitrogen-doped mesoporous carbons were prepared from pyrolysis of two cobalt metal organic frameworks (MOFs), one linear coordination polymer and one complex. The catalyst derived from cobalt 2,3-pyrazinedicarboxylate polymer (700 °C) had a Tafel slope of 90 mV dec−1 and 130 mV dec−1 for the HER and OER respectively, a current density of 10 mA cm−2 was reached at − 0.23 V and 1.56 V vs RHE for the HER and OER respectively. In order to prove the bifunctional catalytic activity towards the overall water splitting, a two electrode electrolyzer was constructed depositing the catalyst on carbon paper. H2 and O2 evolved volumes followed the Faraday law, showing efficiency very close to 100%. The same materials also showed catalytic activity towards the oxygen reduction reaction (ORR), reaching an electron transfer number close to 4 and low H2O2 yields. The acid leached catalyst derived from Cobalt 2,3-pyrazinedicarboxylate polymer (700 °C) reached a ∆E = Ej = 10(OER) −E½(ORR) = 0.80 V, making it an useful oxygen catalyst for metal-air batteries.