Efficient Surface-Modified Steel Electrodes for Oxygen Evolution in Alkaline Media

Abstract

Inexpensive, efficient and durable electrocatalysts for oxygen evolution reaction (OER) will reduce the cost of alkaline water electrolyzers. The slow kinetics of the four-electron transfer process during oxygen evolution results in increased voltage loss and reduced energy efficiency.1 Although ruthenium and iridium-based electrocatalysts are particularly active towards OER, the stability of these catalysts is poor in alkaline media. Moreover, the high cost of noble metals is a barrier for the large-scale utilization of such electrocatalysts.2 Therefore, transition metal oxide based electrocatalysts supported on nickel substrates have been the choice for water electrolysis in alkaline systems. However, the cost added by nickel-based electrodes in these systems can also be quite significant, and its replacement by less expensive materials is much desired. Iron is at least 40 times less expensive than nickel. Thus, we were urged to consider low-carbon steel as an alternate material for oxygen evolution electrodes. Under anodic conditions, a steel electrode produces oxidized products of iron, disintegrating rapidly at oxygen evolution potentials. Surface modification by nickel was found to render iron electrodes stable to oxygen evolution.3 We have applied this surface-modification approach to steel and we report highly efficient, inexpensive and durable nickel modified (NS) and cobalt modified (CS) steel electrodes for oxygen evolution reaction. Such electrodes were prepared through a two-step process: (1) application of nickel/cobalt coating on steel mesh, and (2) calcination of the coating at different temperatures. NS and CS electrodes synthesized at 200 oC exhibited the highest oxygen evolution activity (Fig. 1 a, overpotential of 257 mV for NS electrodes and 337 mV for CS electrodes at 10 mA/cm2). At this temperature of preparation, the surface of the electrodes were found to be covered with an amorphous oxide layer. With increasing temperature of preparation a steady decrease in activity was observed in both the types of electrodes (Fig. 1 a). This change in activity is accompanied by the formation of a crystalline spinel phase and non-conductive iron (III) oxide as the temperature of preparation is increased from 200 to 800 oC (Fig. 1 b). The NS electrodes prepared at 200 and 400 oC exhibited higher activity compared to CS electrodes prepared under similar conditions. Physical characterization methods such as XRD and SEM have been used to probe the crystallinity and surface morphology of the electrodes. Electrochemical characterization by polarization studies and electrochemical impedance spectroscopy offer insights into the effect of temperature and composition on oxygen evolution activity of the catalysts.