Electrochemical Characterization of Oxygen Evolution Electrocatalysts Beyond Rde: A Study on Nickel

Abstract

The rotating disc electrode (RDE) is a widespread and useful technique in alkaline oxygen evolution (OER) research.[1] However, the RDE technique is restricted to circular-geometry and certain techniques to prepare catalyst samples on electrode tips, usually made of Pt, Au or glassy carbon. Therefore, evaluation of electrocatalysts prepared on different deposition substrates, with diverse synthesis techniques, under various conditions, requires a more flexible alternative. An electrochemical flow cell (EFC) introduces new possibilities compared to the RDE, where instead of rotating the electrode, the electrolyte flows over the electrode. Different flow cell configurations are in use in the state-of-the-art electrocatalysis research, including high throughput analysis and spectro-electrochemistry. However, an EFC is a non-standard setup, developed in individual labs for desired reactions with different configurations and sizes.[2] In this study, we try to understand the relationships between the microscopic structure of the electrode, the mass transport and measurement conditions in the analytical technique, and the intrinsic OER activity and stability of nickel-based electrocatalysts. We used nickel-based electrodes as models since nickel has shown high activity and stability towards OER, including in situ activation.[3] First, we investigated OER electrocatalysis of nickel oxide (NiOx) on RDE with thin film electrodes (TF-RDE) fabricated using three different thin film preparation techniques that produce different microscopic structures in the electrode. The results show predictable, reproducible OER performance from a sufficiently thin electrode layer with compact microscopic structure. We show that conventional ink coated thin film electrode (ink TF-RDE) is a poor electrode example for RDE characterization. Second, we compared the RDE technique against a channel flow EFC design from our group[2c], which is designed to study electrodes in TF-RDE configuration. Comparison of ink TF-RDE in both techniques, together with purified and unpurified electrolyte, shows that electrode activity and stability response is enhanced in RDE than in EFC, and that this is influenced by the mass transport to the electrode.Thirdly a modified EFC, capable to characterize electrodes produced with diverse thin film techniques on various flat conductive substrates, was used to characterize nickel-based electrodes for OER. We validated the EFC with two different NiOx electrodes and applied this EFC as an alternative technique to RDE to screen electrocatalysts, with the ability to reach currents > 200 mA cm-2 geom at high flow rates without interference by gas bubbles. By coupling this EFC to an online Inductive Coupled Plasma – Optical Emission Spectrometry (ICP-OES) setup and a Clark-O2 sensor (Figure 1), we investigated the stability and degradation of the electrodes layer, activity of nickel-based materials and the electrode faradaic efficiency. We demonstrate the usefulness of this EFC technique to investigate electrocatalysts in the laboratory with ex situ analysis and scalability in mind.