FeCo/FeCoPxOy(OH)z as Bifunctional Electrodeposited-Film Electrodes for Overall Water Splitting

Development of efficient/robust/economical electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is emerging as a grand challenge. In consideration of the OER activity of iron/cobalt oxides and the superior electroconductivity/propitious H atom binding energy of metallic iron/cobalt, bifunctional electrodeposited-film electrode CFeCoP composed of CoFe and CoFePxOy(OH)z was prepared from the irreversible cathodic deposition of FeSO4 and CoSO4 on the surface of a graphite plate in 1 M phosphate buffer (pH = 7). The as-prepared CFeCoP electrode exhibits excellent HER activity (specific activity (js) = 0.169 mA/cm2) with low charge transfer resistance (4.5 Ω) and an overpotential of 57 mV, achieving a current density of 10 mA/cm2, and it also shows OER activity (js = 1.316 mA/cm2) with low charge-transfer resistance (7.8 Ω) and 282 mV of overpotential, approaching a current density of 10 mA/cm2 in a 1 M NaOH aqueous solution. In addition, the CFeCoP electrode displays long-term stability (139 h) for both HER and OER activity with a stable current density. In the whole cell, the CFeCoP–CFeCoP electrode-pair setting achieves a current density of 10 mA/cm2 at a voltage of 1.56 V (Tafel slope of 51 mV/dec), close to those (10 mA/cm2 at a voltage of 1.58 V (Tafel slope of 68 mV/dec)) of the Pt–IrO2 electrode-pair device in a 1 M NaOH aqueous solution. On the basis of XPS, SEM-EDX, and pXRD, the fabric of metallic iron/cobalt buried in a metal-oxide matrix and iron/cobalt (oxy)­hydroxides embedded in a metal-oxide matrix as films on the graphite surface for the CFeCoP electrode implies that the electrodeposition-derived film mainly made up of FeCo and FeCoPxOy(OH)z may mediate the kinetics that occurs at the catalyst–electrolyte interface and displays stability/intrinsic catalytic activity for overall water splitting.