Degradation of Ni-Co Coated Ni Oxygen Evolution Electrodes in Alkaline Water Electrolysis Using Accelerated Durability Test Based on Reverse Current Phenomenon

Abstract

INTRODUCTION Power to Gas (P2G) technology is expected to allow installation of a lot of renewable electricity without imbalance issue of electric power grid. Here, water electrolysis is a key device in the P2G. One of the conventional methods in water electrolysis is alkaline water electrolysis (AWE), which has the advantages of a low-cost material electrolyzer. On the other hand, there is an issue of electrode degradation caused by the reverse current generated by frequent start-stop of renewable energy power source. Therefore, further understanding of degradation phenomenon and development of durable electrodes are needed.In this study, degradation of NiCo spinel coated Ni anodes, which is a conventional anode for oxygen evolution electrode in alkaline medium has been investigated using accelerated durability test based on reverse current phenomenon [1]. The effect of stop condition of E min and short open circuit potential (OCP) holding before reverse current mode on degradation have been discussed with cyclic voltammetry (CV) as electrochemical measurement. EXPERIMENTAL The accelerated durability test (ADT) were performed in a three-electrode cell with 7 M KOH aqueous as electrolyte at 25℃. RHE, Ni coil and 1 cm2 electrode of NiCo spinel catalyst supported on Ni mesh substrate at 500 °C are used as reference, counter and working electrodes, respectively.Constant current electrolysis at 1 A cm-2 for 2 hours was conducted as pretreatment at 80℃ before ADT. The ADT protocol consisted of three steps: (1) constant current electrolysis at 600 mA cm-2 for 1 min, (2) potential scanning from open-circuit potential (OCP) to anodic potential E min (=0.3, 0.5, 0.7 V vs. RHE) at scanning rate of -500 mV s-1, and (3) holding at constant potential of E min for 1 min as shown in Figure 1.To evaluate the anode activity, CV (potential range: 0.5-2.0 V, scanning speed: 5, 50 mV s-1, 3 cycles) and AC impedance measurements (bias potential: 1.6, 1.7 V, frequency range: 106-10-1 Hz, amplitude: 10 mV) were performed at every 200 ADT cycles in 2,000 cycles of ADT. Furthermore, the same degradation evaluation was performed by the protocol inserting 10 s holding at OCP between (1) and (2) at Emin = 0.5 V vs. RHE to evaluate relaxation effect. RESULTS AND DISCUSSION Figure 2 shows the iR-free anode potential of CV at i =100 mA cm-2 as a function of the ADT cycles for various E min. The anode potential increased from ca. 1.6 to 1.8 V vs. RHE during 2,000 cycles of the ADT. There was no significant difference for degradation behavior and potential among various E mins after ADT. This was probably due to the significant consumption of the catalyst layer with detachment in earlier ADT cycles, because detached electrocatalyst powder precipitated at the bottom of the cell.Figure 3 also shows the iR-free anode potential as a function of the ADT cycles with and without 10 s of OCP at E min = 0.5 V. Anode potentials with and without OCP after ADT were almost same. However, potential with OCP was significantly smaller than that without OCP in the early cycles of the ADT. Figure 4 shows the anodic peak potential of the CV as a function of the ADT cycles. As inset of Fig. 4, the CV had two anodic peaks of (1)β-Ni(OH)2 /β-NiOOH at E 0 =1.33 V vs. RHE, and (2) NiOOH / NiO2 at E 0 =1.43 V vs. RHE. The peaks (1) and (2) correspond to active and inert site, respectively [2]. Therefore, the increase of the (2) and the decrease of the (1) with the ADT cycles would correspond to the degradation. In the early cycles, the peak currents with OCV were more active side than that without OCV. The insertion of OCP to suppress the potential increase in Fig. 3 would be affected by not only coherency of catalyst layer to suppress detachment but also the activation of catalyst itself. Acknowledgements This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Electrodes were provided by De Nora Permelec Ltd. We are grateful to all people concerned.