Systematic and Catalytic Approaches against a Reverse-Current in Alkaline Water Electrolysis

Abstract

Increasing the stability, effectiveness, and economic worth of water-splitting technology, which relies on a variable power supply from renewable energy sources, is necessary to expand the hydrogen economy. Alkaline water electrolysis is the most widely used water-splitting technique for producing hydrogen energy on an industrial scale, namely due to its scalability and affordability. However, it is susceptible to power fluctuations caused by intermittent renewable energy supplies. Alkaline water electrolysis systems are faced with a significant problem when it comes to stabilizing hydrogen production during intermittent startup and shut-down operation. In this thesis, I present two straightforward but effective solutions (strategy 1 and strategy 2) for the electrode degradation problem induced by the reverse-current under transient power condition based on a fundamental understanding of the degradation mechanism of nickel (Ni). In order to show how the reverse-current flow after shut-down causes the Ni cathode to degrade, I first develop a simulation model for reverse current. The reverse-current flow that occurred after shut-down caused the Ni cathode to irreversibly oxidize to either the β-Ni(OH)2 or NiO phases which led to significant electrode deterioration. In order to maintain a reversible nickel phase and activity for the hydrogen evolution reaction, the potential of the Ni electrode should be kept below 0.6 VRHE under transient conditions. The thesis includes two objectives and scopes. At first, the example of strategy 1 suggests a cathodic protection approach in which the potential of the Ni electrode is maintained below 0.6 VRHE by the dissolution of a sacrificial metal to satisfy the above requirement: irreversible oxidization of the cathode is prevented by connecting a sacrificial anode to the Ni cathode. Also, the example of strategy 2 is a metal oxide-based hydrogen evolution reaction electrocatalyst which is highly active and durable under reverse-current condition. The tolerance enhancement of Ni electrode to reverse-current is derived by the effect of co-catalyst on the electrode. Considered together, the thesis has eventually presented two strategies towards the electrode degradation problem in alkaline water electrolysis under fluctuating power supply. Also, a newly defined metrics, reverse-current stability factor and reverse-current activity factor, highlight that our system and electrocatalyst for protecting the cathode against the reverse-current is an efficient strategy for stable and cost effective alkaline hydrogen production. It can trigger a number of follow-up studies and shed a light on the indicator of the electrode design for alkaline water electrolysis by introducing the metrics for the transient stability of the catalyst under the shut-down operating condition. I firmly believe that this thesis clearly offers a mechanistic understanding of reverse-current in alkaline water electrolysis and its systematic and catalytic design principles under transient start-up/shut-down operation.