Ni-Fe Hydroxide/Oxide Nanostructures Generated on Stainless Steels for Efficient Alkaline Water Electrolysis
Authors: Todoroki Naoto , Wadayama Toshimasa 
Published: February 2020

Introduction

Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) have been needed for efficient hydrogen generation by electrochemical water splitting. Although various transition metal-based oxides, hydroxides, selenides etc. have been extensively explored as OER catalysts for alkaline water electrolyzer (AWE) (1, 2), detachments of the effective materials for the OER are serious issues from the electrode surface by vigorous oxygen gas evolution under the practical condition (3). Stainless steels (SS) have much attractive attentions as low-cost, highly active and stable electrode materials at high current density (4). However, relation between nano-level structures of the surface oxide layers on the SS substrate and OER properties should be clarified. In this study, we investigated changes in OER properties and nanostructures of the surface oxide layers generated on the 316SS electrode during the electrolysis.

Experimental

316SS (ø = 5mm, t = 4mm) substrate was used as a working electrode after the surface polishing by Emery papers (#800, #1200, #2000) and alumina paste (1 μm, 0.3 μm, 0.05 μm). Polarization curves for the OER with a disk rotation rate of 1600 rpm were measured in N­2-purged 1 M KOH solution at 75 ℃ at 5 mV/sec. Cyclic voltammograms were collected at 50 mV/sec without disk rotation. After that, constant current density electrolysis (CCE) was conducted at 30 mA/cm2 for 10 h at 1600 rpm. Nanostructures and compositions of the surface oxide layers generated after the electrolysis were investigated by scanning transmission electron microscopy (STEM) with electron dispersive spectroscopy (EDS).

Results and Discussions

Fig.1 (a) shows changes in the OER overpotentials estimated by the polarization curves at 10 (red) and 100 (blue) mA/cm2 as a function of the CCE times. The overpotentials decreased with increasing the CCE times. In addition, as shown in (b), the cyclic voltammograms indicate the Ni-oxidation peaks of the CCE-5h sample (red) shift to lower potential compared with the pristine substrate(CCE-0h), suggesting the electrochemical property of the SS substrate surface changed during the electrolysis. STEM images of the CCE-5h sample with different magnifications are presented in (c). The surface oxide layers are composed of 20nm-thick dense nanoporous-like (substrate side)and 60nm-thick nanofiber-like layers. The selected area electron diffraction (SAED) patterns suggests that crystal structures of each oxide layers are rock-salt NiO and β-Ni(OH)2, respectively. Furthermore, corresponding STEM-EDS mappings (d) reveals that Fe and Ni are only metal elements and Cr is almost absent in both the oxide layers. Therefore, increase in OER activity during the CCE probably stems from generation of the specific Ni-Fe hydroxide nanostructures that acting as highly-OER-active sites (5). The results in this study clearly show that electrochemical synthesis of such the hetero-structured hydroxide/oxide layers on the SS substrate is an effective approach for developments of the practical anode for AWE.
Acknowledgments
This study was partly supported by JSPS KAKENHI Grant Number 18H01741, Toyota Mobility Foundation Hydrogen Initiative, and Yazaki Memorial Foundation for Science and Technology (N.T.).

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Figure 1