Highly efficient and robust nickel-iron bifunctional catalyst coupling selective methanol oxidation and freshwater/seawater hydrogen evolution via CO-free pathway

Abstract

The production of green hydrogen by water electrolysis is often kinetically limited by the sluggish oxygen evolution reaction (OER) at the anode. Here, we prepared a bifunctional nickel foam supported NiFe2O4 spinel catalyst (i.e. NiFe2O4/NF) that is capable of facilitating the coupling of hydrogen evolution reaction (HER) with selective methanol oxidation reaction (SMOR) in seawater to produce formate via a CO-free pathway. At a cell potential of 2.0 V, the NiFe2O4/NF||NiFe2O4/NF catalyzed HER-SMOR system produces remarkably high current density (>800 mA cm−2) with high Faradaic efficiencies (FE) at both electrodes (>96 % for HER and >95 % for SMOR to formate). The NiFe2O4/NF||NiFe2O4/NF HER-SMOR system also exhibits excellent stability over 48 h of continuous operation. With H2 being the only gaseous product, the HER-SMOR electrolysis system could operate in the absence of a membrane. Furthermore, the NiFe2O4/NF||NiFe2O4/NF electrodes are sufficiently robust for continuously catalyzing HER-SMOR in seawater electrolysis, showing no sign of deactivation or chlorine oxidation reactions over 6 h of continuous operation at a high current density of 700 mA cm−2. Mechanistic investigation and DFT calculations reveal that SMOR proceeds via a CO-free pathway, with Ni and Fe as the active sites for methanol and OH activation, respectively.