An Efficient and Robust Surface-Modified Iron Electrode for Oxygen Evolution in Alkaline Media

Abstract

The efficiency and cost of water electrolysis systems and rechargeable metal-air batteries can be improved by reducing the overpotential losses at the oxygen evolution electrode in alkaline medium. [1] Therefore, we seek inexpensive, efficient and durable electrocatalysts for the oxygen evolution reaction in alkaline medium. Transition metal oxides supported on nickel substrates have been the work-horse catalysts of industrial alkaline water electrolysis. But the cost added by nickel-based substrates can also be quite significant, and its replacement by less expensive materials is much desired. In our earlier study we had showed for the first-time that porous iron-sintered substrates modified by surface treatment using nickel can act as remarkably stable electrode when anodically polarized and also present a highly active electrocatalytic surface for the oxygen evolution reaction. [2] On these nickel modified sintered iron electrodes (NSI) we found an overpotential of 230 mV at 10 mA/cm2 (Figure 1 a).In our quest for further improvements we have studied the effect of additive materials that can enhance the activity of NSI electrodes. Herein, we report a remarkably efficient and highly durable nickel modified iron electrode that has an iron sulfide additive (NSI-FeS) for oxygen evolution reaction. Such an electrode was prepared through a four-step process: (1) sintering of iron substrate along with 1 wt% of iron sulfide, (2) electrochemical oxidation of as-sintered electrode from open circuit potential to -0.6 V vs mercury-mercuric oxide electrode in 30 w/v % potassium hydroxide aqueous solution, (3) application of nickel coating and (4) calcination of nickel coating at 200o C. As-prepared NSI-FeS electrode exhibits an overpotential of ~200 mV to achieve 10 mA/cm2 current density with a Tafel slope of 46 mV/decade (Figure 1 a). The high activity for oxygen evolution may result from increased surface area obtained through electrochemical oxidation of iron substrate in presence of iron sulfide. The same electrode exhibits a highly stable overpotential for 1500 hours of testing at 10 mA/cm2 with an average change as small as 1 mV/1000 hour (Figure 1 b). Physical characterization methods such as XRD and XPS have been used to probe surface composition of the electrodes, and measurements for electrochemically active surface area were conducted through electrochemical impedance spectroscopy.