Electrochemical Stability of Stainless Steels-Made Alkaline Water Electrolysis Anodes Under Potential Cycle Loadings

Abstract

Introduction Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) have been needed for efficient hydrogen production by alkaline water electrolysis (AWE). Austenitic stainless steels (SS) have attracted attentions as the alternative anode materials to Ni-based electrodes (1, 2). We recently demonstrated that NiFe hydroxide/oxide hetero nanostructures that synthesized through the constant current density electrolysis of 316SS (NiFe-HyOx/SS) show high OER activity and stability under constant current operation conditions (3). However, the electrochemical stability and OER overpotentials of the surface catalyst layers generated on the stainless steel under potential fluctuation is still not clear. In this study, we investigated changes in OER overpotentials of the NiFe-HyOx/SS anode during applying potential cycles (PCs) of 0.5 and 1.8 V vs. reversible hydrogen electrode (RHE) and discussed the structural changes. Experimental A 316SS plate was used as a substrate for preparing NiFe-HyOx/SS anode. The SS plate surface was polished using emery paper and alumina paste and then, washed with ultrapure water and acetone. The pre-processing SS plate surface was subsequently subjected to constant current density electrolysis of 30 mA/cm2 in 1 M KOH solution at 75 ℃ for 5 h to generate NiFe-HyOx catalyst layers (3). After that, the OER properties were examined in 7 M KOH at 20 ℃. The OER polarization curves were measured at a sweep rate of 5 mV/s. The cyclic voltammograms were collected at 50 mV/s between 0.93 and 1.49 V vs. RHE. The electrochemical stability of the samples was evaluated by applying PCs between 0.5 and 1.8 V at 1 V/s (4). Nanostructures and compositions of the surface oxide layers generated after the potential cycle loadings were investigated by scanning transmission electron microscopy (STEM) with electron dispersive spectroscopy (EDS). Results and Discussions Figs.1 (a) shows cross-sectional STEM image of the pristine NiFe-HyOx/SS sample. Fiber-like nanostructure with ca. 50 nm thickness is formed on the SS substrate, accompanied with a relatively dense, 30nm-thick buffer layer. Figs.1 (b) shows changes in OER overpotentials during the 20000 PCs of the NiFe-HyOx/SS and pure Ni samples that estimated at current density of 100 mA/cm2. The initial overpotential of the NiFe-HyOx/SS (ca. 270 mV) was lower by 110 mV than Ni (ca. 380 mV), indicating that the NiFe-HyOx/SS shows much higher initial OER activity. Polarization curves recorded before and after the 20000 PCs are shown in Figs.1 (c) and (d). The OER current densities of the NiFe-HyOx/SS are remained nearly unchanged after 20000 PCs. On the other hand, the polarization curve of the Ni shifted to positive potential side by applying the PCs, accompanying decrease in current density. The OER overpotential of the Ni sample estimated after the 20000 PCs is 480 mV, the value of which is ca. 100 mV higher compared with pristine state, indicating that the NiFe-HyOx/SS is highly stable against the PCs, relative to the Ni.To clarify the origin of high stability of the NiFe-HyOx/SS, cross-sectional STEM observations were conducted. Cross-sectional STEM images of the NiFe-HyOx/SS collected after the 20000 PCs are summarized in Figs.1 (e) and (f). As can be seen in (e), the total thickness of the surface catalyst and PCs-generated interlayers was estimated to be ca. 900 nm in thickness. The magnified image of the topmost surface regions (f) shows the 50nm-thick catalyst layer at pristine state (Figs. 1(a)) keeps its thickness and nanofiber-like structure even after the 20000 PCs. On the other hand, the buffer layer between the surface catalyst layer and SS substrate grown by the PCs loading, resulting in ca. 850nm-thick interlayer. The XPS and STEM-EDS analysis results (not shown) showed initial surface chemical compositions (Ni:Fe = 1:1) of the catalyst layer almost retained after the 20000 PCs, indicating that the surface catalyst nanofiber-like structures which is electrochemically stable against the PCs loading should contribute to the stable OER property of the NiFe-HyOx/SS anode (5).AcknowledgmentsThis study was partly supported by JSPS KAKENHI Grant Number 18H01741 and 21H01661, Toyota Mobility Foundation Hydrogen Initiative, and Advanced Research and Education Center for Steel (ARECS). References H. Schäfer and M. Chatenet, ACS Energy Lett., 3, 574 (2018). F. Moureaux, P. Stevens, G. Toussaint and M. Chatenet, Appl. Catal., B, 258, 117963 (2019). N. Todoroki and T. Wadayama, ACS Appl. Mater., 11, 44161 (2019). S. Fujita, I. Nagashima, Y. Nishiki, C. Canaff, T. W. Napporn and S. Mitsushima, Electrocatalysis, 9, 162 (2018). N. Todoroki and T. Wadayama, Electrochem. Commun., 122, 106902 (2021). Figure 1