In situ UV-Vis spectroscopy monitors redox processes in energy storage

On April 6, 2023, Beijing time, Xuehang Wang (currently an assistant professor at Delft University of Technology in the Netherlands) and Yury Gogotsi’s team published a new research result entitled “In situ monitoring redox processes in energy storage using UV–Vis spectroscopy” in the journal Nature Energy. The research group demonstrated the potential of in situ ultraviolet-visible absorption spectroscopy (UV-Vis) for electrochemical energy storage mechanisms. The first authors of the paper are Zhang Danzhen and Wang Ruocun.

In order to meet the diverse needs of modern society for the power and energy density of energy storage devices, it is important to develop electrochemical energy storage devices with diversified charge storage mechanisms. Distinguishing and understanding different electrochemical energy storage mechanisms is an important part of this. Although advances in recent years have blurred the boundaries between these charge storage mechanisms, they can be broadly divided into three main types: battery-type redox reactions, pseudocapacitors, and electric double-layer capacitors. To distinguish these three energy storage mechanisms, the traditional common method is to use electrochemical characterizations such as cyclic voltammetry (CVs), in addition to many electrochemical in situ characterizations such as in situ Raman, in situ X-ray absorption spectroscopy (XAS), in situ X-ray diffraction spectroscopy (XRD), in situ electron energy loss spectroscopy (EELS) and so on. However, these methods are limited by many factors, such as the need to use synchrotron radiation sources, the quantification accuracy is difficult to meet the standard, etc., and it is difficult to distinguish the specific energy storage mechanism of some emerging non-traditional energy storage systems, such as the paper (ACS Nano 15, 15274-15284, 2021) proposed a system with a battery-like cyclic voltammetry, but almost no redox reaction occurs. Based on this, the research team proposed a method using in situ ultraviolet-visible absorption spectroscopy (UV-Vis) to distinguish the electrochemical energy storage mechanism and quantitatively analyzed the number of electrons transferred by the pseudocapacitance behavior of Ti3C2Tx in acidic electrolyte, a system of their choice, and found that it was very similar to the in-situ XAS results.

In this work, the research group first selected three electrochemical systems, lithium titanate in 1M lithium perchlorate/acetonitrile (battery energy storage), Ti3C2Tx in 1M sulfuric acid electrolyte (pseudocapacitor energy storage), and Ti3C2Tx in 1M lithium sulfate solution (electric double-layer capacitor energy storage). The three systems tested cyclic voltammetry of in situ UV-Vis devices and cyclic voltammetry reconstructed using the MUCA method, as shown in Figure 1. The cyclic voltammetry of the device is very consistent with the three systems reported in the literature, which justifies its electrochemical testing.

Figure 1: Electrochemical cyclic voltammetry, reconstructed MUSCA cyclic voltammetry, and in situ UV-Vis absorption spectra

Further analysis of the obtained in situ UV-Vis spectra (e.g., differential processing of the peak of the absorption peak at 780 nm of Ti3C2Tx) and plotting it in combination with the applied voltage or the amount of charge stored in the system shows that the curves obtained by the three electrochemical energy storage mechanisms are very different (as shown in Figure 2). Among them, the electric double-layer capacitor basically maintains a linear absorption value and voltage/charge change relationship throughout the whole process; Pseudocapacitors increase in slope where surface redox occurs; The battery will increase sharply at the redox slope. Based on this phenomenon analysis, in situ UV-Vis can be preliminarily used to distinguish three common electrochemical energy storage mechanisms.

Figure 2: In situ UV-Vis absorption spectroscopy in relation to electrochemistry

By further comparing the change of absorption value at the in-situ UV-Vis spectral characteristic value of each system (as shown in Figure 3), and introducing two new electrochemical systems together, it can be seen that the three electrochemical mechanisms are clearly distinguished. At the same time, it is also proved that the energy storage mechanism of Ti3C2Tx in saturated salt solution is very close to that of Ti3C2Tx in 1M lithium sulfate solution (electric double-layer capacitor energy storage).

Figure 3: Comparison of five electrochemical systems

At the same time, combining Bill Lambaud’s law and Faraday’s law of electrolysis, the research group proposed a method for quantifying the number of transferred electrons in electrochemical reactions using in situ UV-Vis absorption spectroscopy.

Figure 4: Optical properties studies and applications

Substituting the data shows that the selected Ti3C2Tx will undergo 0.147 electron transfer per Ti atom in 1M sulfuric acid electrolyte, which is very close to the 0.134 electrons reported in situ XAS (ACS Energy Lett. 5, 2873-2880, 2020).

This study proves the potential of in situ UV-Vis in the application of electrochemical energy storage mechanism, which establishes a basis for the subsequent characterization of UV-Vis in situ/electrochemical mechanism, and also provides a possibility for its widespread application in the study of SEI formation, electrolyte decomposition, and electrocatalysis. (Source: Science Network)

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