Nonlinear Electrochemical Analysis: Worth the Effort to Reveal New Insights into Energy Materials

Abstract

and the effectiveness and durability of the materials applied, researchers always call for reliable and detailed knowledge of the mechanism(s) of both, main and side reactions, and especially for their kinetics. In order to gain the relevant information, a large number of methods have been proposed to characterize battery materials, battery electrodes and their interfaces, and battery cells. But when samples are taken out of the cells and analyzed later, there is a high risk of both, contamination and changes due to possible relaxation processes, and the challenge is to draw sound conclusions in respect to a "living and breathing" battery cell, i.e., to a cell under current flow (cf. Figure 1). Therefore, the so-called in situ and operando (and often online) methods are preferred. Sometimes they are the only option to collect relevant data to understand and improve materials for energy systems. [5] In many cases, results from different techniques need to be combined to obtain sound conclusions. [6,7] The terminology for the aforementioned methods shifted during the last decade and is not always consistent throughout the literature. Nowadays, the general understanding is as follows: An operando experiment means a characterization experiment in an electrochemical cell (e.g., battery, fuel cell, electrolyzer) under current flow, with either the potential or the current controlled. One of the many examples is the operando neutron powder diffraction, a technique analyzing the changes in the structure of battery materials in "real" battery cells under current flow with the help of neutron diffraction. [9] Another example is the operando X-ray photoelectron spectroscopy experiment (normally limited to vacuum-compatible cells with nonvolatile electrolytes) providing information about the changes of the chemical composition of the interfacial species with the potential. [10,11] An in situ experiment is also an experiment in a battery cell, however, the current flow is interrupted when the data are collected. An example here is the in situ X-ray absorption spectroscopy (XAS) experiment in which the cell has to be disconnected for technical reasons during the operation of the detector [7] or the tomography of energy materials by means of neutron or X-ray source. [12,13] Online techniques are understood here as either continuously or periodically analyzed samples that are taken out of the electrochemical cell. A typical example is the online (differential) electrochemical mass spectrometry (often shortened as online electrochemical mass spectrometry (OEMS)/differential electrochemical mass spectrometry (DEMS)) analyzing the volatile products of electrochemical reactions with the help of a mass spectrometer by

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

“…For example, despite significant advancements in the operando X-ray diffraction (XRD) technique, detecting the surface amorphous structure of materials and designing the operando cell remains a challenging task. [23] Additionally, although operando X-ray absorption spectroscopy (XAS) is sensitive to the local coordination structure and oxidation state of elements in materials, it provides only averaged information of bulk materials and tends to lack specificity in reflecting the information regarding formation or catalytic processes taking place at the surface region, and the limited availability and high cost of beam time have restricted its widespread usage. [21,23] Moreover, in situ transmission elec-tron microscopy (TEM) characterization provides a direct means of observing the structural changes in materials.…”

“…[23] Additionally, although operando X-ray absorption spectroscopy (XAS) is sensitive to the local coordination structure and oxidation state of elements in materials, it provides only averaged information of bulk materials and tends to lack specificity in reflecting the information regarding formation or catalytic processes taking place at the surface region, and the limited availability and high cost of beam time have restricted its widespread usage. [21,23] Moreover, in situ transmission elec-tron microscopy (TEM) characterization provides a direct means of observing the structural changes in materials. However, it faces challenges in terms of fabricating the in situ cell and potential electron beam damage.…”

An experimental guide to in operando electrochemical Raman spectroscopy

“…Besides advantages like high temporal and spectral resolutions and sensitivities [3], the weak water signals make Raman spectroscopy an ideal tool for the in situ/in operando investigation of aqueous electrochemical reactions. Several Raman spectroelectrochemical studies have improved the understanding of electroorganic synthesis [7][8][9][10][11], electrocatalysis [12,13], batteries [14], electrode interface behavior [15], and material science [16]. Also, molecule/surface interactions without applied potentials can be examined [17].…”

Lithium Insertion into Graphitic Carbon Observed via Operando Kerr-Gated Raman Spectroscopy Enables High State of Charge Diagnostics

“…The increase in the background fluorescence emission arising from these components makes it more challenging to analyze Raman spectroscopic data on these cycled materials due to the spectroscopic overlap of Raman scattering and fluorescence emission. 16 One methodology that can overcome the challenges of Raman/emission signal overlap is Kerr-gated Raman spectroscopy. 17 , 18 It has previously been demonstrated that Kerr-gated Raman spectroscopy can be exploited to effectively remove the fluorescence background on uncycled, cycled, and aged battery and electrolyte materials and reveal the Raman scattering signals.…”

“…During the electrochemical cycling of Li-ion batteries with conventional Li­[PF 6 ]/carbonate-based electrolytes, fluorescent species are formed by parasitic decomposition reactions, primarily due to the instability of the non-aqueous electrolytes and side reactions that occur at the electrode surface. The increase in the background fluorescence emission arising from these components makes it more challenging to analyze Raman spectroscopic data on these cycled materials due to the spectroscopic overlap of Raman scattering and fluorescence emission . One methodology that can overcome the challenges of Raman/emission signal overlap is Kerr-gated Raman spectroscopy. , It has previously been demonstrated that Kerr-gated Raman spectroscopy can be exploited to effectively remove the fluorescence background on uncycled, cycled, and aged battery and electrolyte materials and reveal the Raman scattering signals .…”