Iron carbonate hydroxide-iron oxide hetero-phase catalyst: A bifunctional electrocatalyst for urea boosted overall water splitting

Iron carbonate hydroxide-iron oxide hetero-phase bifunctional electrocatalyst was directly fabricated on nickel foam (NF), which exhibited strong urea oxidation reaction (UOR) and overall water splitting including sea water splitting. Hydrothermal method was adopted for fabrication of iron carbonate hydroxide-iron oxide catalyst (1-3) and deposition time was varied (3 h (1), 6 h (2) and 12 h (3)) to optimize the electrocatalytic activity. FTIR and X-ray photoelectron spectroscopic (XPS) analysis suggested the formation of iron carbonate hydroxide-iron oxide. OER studies revealed relatively strong activity for 1 compared to 2 and 3 in alkaline condition. 1 required the overpotential of 238 mV for producing the current density of 50 mA/cm² whereas 2 and 3 needed 248 and 280 mV, respectively. In contrast, 3 showed strong HER activity that required overpotential of 301 mV to achieve 50 mA/cm² whereas 2 and 1 needed 303 and 343 mV, respectively. Hence, 2 was chosen to fabricate overall water splitting and UOR. In presence of urea, 2 required low overpotential (130 mV) to produce 50 mA/cm² current density. For overall water splitting, 2 needed 1.71 V to produce 10 mA/cm² current density. The UOR combined cell required only 1.56 V to achieve 10 mA/cm². Tafel slope, impedance and electrochemical active surface area (ECSA) calculation suggests improved kinetics with reduced charge transfer resistance and more active sites for 2. The stability studies indicated good stability of 2 in OER, HER and overall water splitting. After catalysis analysis indicated the formation of FeOOH and FeO active species during OER and HER, respectively. Thus, the present work developed a low-cost transition metal based bifunctional electrocatalyst for overall water splitting.