Development of operando ORP-EIS for monitoring ongoing electrochemical processes

Electrochemistry plays an important role in the technology-driven society. It continues its expansion into a wide range of applications such as energy conversion and storage systems, corrosion, and corrosion protection systems. To fully understand electrochemical processes and materials behavior, an operando electrochemical characterization method is needed to investigate these processes under operation. Electrochemical Impedance Spectroscopy (EIS) is a potential candidate for this purpose. However, in its standard conditions, it is not applicable in fast-evolving electrochemical systems since to study an electrochemical system or process with the classical impedance, the system under investigation should satisfy causality, stationarity, and linearity constraints. These limitations hinder the application of the classical EIS. This project aims to investigate the development of operando Odd Random Phase EIS (ORP-EIS) as a monitoring tool for electrochemical processes under real operational conditions. It means that the goal is to measure the impedance, by applying ORP-EIS, when the electrochemical process like anodizing is ongoing. This new technique can be a breakthrough in providing relevant quantitative information and new insight into the occurring physical phenomena in the system under real operational
conditions. This can help to optimize or design new processes. To show the proof-of-concept of studying the electrochemical processes by operando ORP-EIS, this thesis examines three case studies that include both spontaneous and non-spontaneous processes, highlighting their diverse nature. These processes encompass key aspects of electrochemistry and hold substantial industrial relevance. Case studies are as follows: anodizing (nonspontaneous process), conversion (spontaneous process), and the solid electrolyte interface (SEI) formation during Li plating (charging) in anode-free Li metal batteries in which both spontaneous and non-spontaneous processes can occur, are selected. These case studies can contribute to the advancement of transferable knowledge and foster the maturation of the technique, enabling its application to other aspects of electrochemistry. In the operando ORP-EIS methodology, two signals (multisine AC+ DC) for non-spontaneous reactions and a multisine AC signal for spontaneous reactions are applied to the electrochemical cell under investigation. So it can run the electrochemical process and measure the impedance simultaneously. As a first research question, it is important to study the impact of the AC signal on the process to assess whether it impacts the process. The next step is to investigate the proof-of-concept of the technique for selected case studies. Moreover, what novel information can be obtained by operando ORP-EIS? As the last milestone, the application of this technique is examined as a monitoring tool for ongoing electrochemical processes in an industrial pilot plant. It is observed that the AC signal does not disturb any of the three selected case studies. It is investigated by comparing the results of the experiments performed with and without applying the multisine AC signal for every case study. Proof-of-the-concept of
operando ORP-EIS is shown for all case studies and it is examined and proven that the technique can monitor the processes and reveal the correct information. Regarding the novel information revealed by operando ORP-EIS, it can monitor the formation of the barrier oxide layer and investigate the acid anion incorporation- as a function of the anodizing potential, anodizing bath type, and concentration- into the oxide layer during anodizing. In the conversion treatment, the technique could monitor the formation of the conversion film and give information related to the morphology of the oxide film and the combination of charge transfer resistance and oxide resistivity. This technique facilitates the investigation of new advancements e.g. new generations of surface treatment solutions for surface treatment processes. Moreover, it holds great potential, after addressing the challenges associated with scale-up, to be applied in R&D pilot plants as a direct monitoring tool for anodizing and conversion treatments. In anode-free Li metal batteries, the formation of the SEI layer is studied. By fitting the obtained operando ORP-EIS data with an equivalent electrical circuit, it is possible to deconvolute the charge transfer and SEI resistances successfully before and during the Li platting (charging of anode-free Li metal batteries). By
studying the SEI layer under real operational conditions and obtaining novel insights into the complicated battery system, this operando technique can pave the path to developing safer and higher energy-density batteries. Moreover, it accelerates artificial SEI and electrolyte engineering by providing a fast and real-time investigation tool. Considering the advantages mentioned, operando ORP-EIS can play a critical role in optimizing and improving the efficiency of electrochemical processes/devices by providing real insight into occurring reactions under operational conditions.