2024 Volume 92 Issue 7 Pages 074004
A detailed understanding of the energy storage mechanism is essential to enhance the energy density of electrochemical capacitors. This necessity has led to the widespread use of in situ and operando measurements of electrochemical devices with laboratory X-ray scattering equipment and synchrotron X-rays. For electrochemical capacitors, it is crucial to obtain comprehensive information on the behavior of the electrolytes within the nanopores, the formation of the electric double-layer, and the structural changes in both the electrode and the porous carbon used as the electrode material. This review will present recent studies on electric double-layer capacitors, highlighting the insights gained from X-ray scattering measurements.
While the analysis of electrochemical capacitors (ECs)—including electric double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors—shares specific methodologies and objectives with battery characterization, it also diverges significantly in several key areas.1,2 Like a typical battery analysis, EC evaluation involves an experimental charge-discharge test of the fabricated devices and detailed studies of the components of the EC, such as the electrolyte, separator, current collector, and electrode.1,3,4 As mentioned in other Reviews in this special issue, since the electrochemical capacitance exhibited by capacitor electrodes is strongly dependent on the specific surface area of the active material in the electrode and the charging voltage, much research has been conducted on active materials composed of carbon, metal oxides, and metal-organic frameworks.5–9 In particular, biomass-derived carbon materials for EC electrodes have been actively investigated recently.10–12 Except for some specific raw materials, their volume-specific capacities are not very high, but in the future, such materials may lead to breakthroughs in ECs. In addition, durability is an essential factor for all electrochemical devices, and it is significant for ECs designed for maintenance-free use.13–18 In other words, since ECs have high cycling characteristics as a fundamental property, durability tests are required under conditions that accelerate electrode and electrolyte degradation. In actual durability tests, a comprehensive evaluation of the degradation mechanism is performed by gas analysis, pressure measurement, and post-test electrode analysis,19–23 as well as by confirming the decrease in capacitance through charge-discharge tests.
The above efforts are primarily aimed at improving existing electrodes, enhancing their durability, and developing novel electrodes, with practical and straightforward research being carried out. Meanwhile, EC researchers share a common understanding that significantly increasing capacity beyond existing levels is challenging due to limitations in enhancing the specific surface area and voltage endurance of electrodes. To solve this problem, the analysis of ECs extends into unique territories, such as the fundamental mechanisms of charge storage in EDLCs, the dynamics of the electric double layer, and the peculiarities of area-specific capacitance. For example, tools like the electrochemical quartz crystal microbalance (EQCM) offer direct insights into ion adsorption/desorption processes on electrodes.24–28 Since ion adsorption/desorption itself serves as a mechanism for energy storage, such analytical methods are vital. The continued development of methods for in situ and operando analysis of ion adsorption/desorption will undoubtedly be crucial in extending the capacitance limits at EC electrodes. Especially in EDLCs, analyzing the adsorption/desorption of ions inside pores and the accompanying phenomena is crucial. Currently, using X-ray and neutron scattering as analytical methods to approach these issues seems appropriate.
Along with advances in electrochemical analysis techniques, the use of X-ray scattering, particularly with hard X-ray synchrotron radiation, for structural analysis has expanded, enabling analyses that were previously impossible. Especially in scattering experiments using synchrotron radiation in the X-ray region, sufficient scattering intensity can be obtained quickly. These rapid measurements enable time-resolved scattering analyses during charge and discharge cycles, thereby facilitating in situ and operando measurements. In EC electrodes, obtaining structural information related to the adsorption and desorption of ions under conditions that closely resemble the actual operating state of the device is desirable. From this perspective, the utility of X-ray scattering experiments is likely to be high. Of course, in the future, it will be possible to combine these experiments with other measurements, such as analyzing changes in the thickness of electrodes, further enhancing the potential for more advanced analyses. In today’s world, where energy efficiency is paramount across all fields,29,30 there is no doubt that using these advanced methods for the fundamental analysis of energy storage mechanisms leads to a better understanding and more efficient utilization of ECs.
This review focuses on the analysis of X-ray scattering in the field of EDLCs, which can be divided into wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS), depending on the angular region of the X-ray scattering. It is interesting to note that different angles of measurement change the size of the area that can be analyzed and thus change the object of analysis. In other words, WAXS provides information on the structural changes associated with ion adsorption/desorption, while SAXS offers insights into electron density variations within the pores and pore structure itself. Ideally, it would be desirable to perform WAXS measurements over a wide angular range and SAXS measurements with high small-angle resolution to make complementary discussions. The design of the electrochemical cell, including its structure, will also be important and is explained further in this text, particularly in the context of performing operando measurements under actual device operating conditions. We hope that this review will stimulate further development of EC technology.
The behaviors of electrolytes confined in nanopores of carbon electrodes affect on EDLC performances since the static and dynamic structure of nano-electrolytes is directly related to charging mechanism of EDLCs. Therefore, the understanding of confined electrolyte structures in carbon nanopores is very important to optimize EDLC properties.
For example, Ohkubo et al. reported the dehydration of ions in less than 1 nm size of carbon nanopores with extended X-ray absorption fine structure (EXAFS) measurements.31 After this work, Chmiola et al. showed the anomalous increase of EDLC capacitance with carbon electrodes whose pore width is less than 1 nm, and the increase could be related to the desolvation phenomena.32 Furthermore, Kondrat and Kornyshev theoretically proposed the formation of a superionic state, in which co-ions can be closer to each other, of monolayer-sized ionic liquids confined between the electrically conductive slabs because of the electrical screening effect for the electrostatic interaction between ions from the conductive pore walls.33,34 Futamura et al. reported the experimental evidence of the superionic state formation of ionic liquids in monolayer sized carbon micropores with hybrid reverse Monte Carlo simulation (HRMC)-aided X-ray scattering measurement.35
Here, we focus on the WAXS method for the structure analysis of nano-electrolytes confined in carbon micropores. We can obtain electron radial distribution functions (ERDFs), which are generally used for the structure analysis of disordered materials, of nano-electrolytes by Fourier transformation of WAXS profiles. The derivation of ERDFs is briefly shown as follows and the detailed explanation can be seen in references.36–39
The confined system is regarded as a three-phase system corresponding to the adsorbed molecules (admolecules), the solid carbon, and the vacant pore space. The experimental X-ray scattering intensity (Iobs) is the sum of the self-scattering terms of solid (Iscs) and adsorbed molecules (Isca), SAXS due to the presence of pores (Isaxs), and the interference terms between admolecules (Iifa-a), solid atoms (Iifs-s), and the cross-term between admolecules and solid atoms (Iifs-a), multiplied by several correction factors. Therefore, the Iobs is given by the following Eq. 1:
| \begin{equation} I_{\text{obs}} = kPGA\{I_{\text{sc}}^{\text{s}} + I_{\text{sc}}^{\text{a}} + I_{\text{if}}^{\text{s-s}} + I_{\text{if}}^{\text{a-a}} + I_{\text{if}}^{\text{s-a}} + I_{\text{SAXS}}\} \end{equation} | (1) |
where k is the coefficient converting the theoretical scattering intensity from e. u. (electron unit) to c. p. s., and P, G, and A are the correction factors corresponding to the polarization, the X-ray irradiating volume, and the X-ray absorption, respectively, depending on the experimental setups.36 Both Iscs and Iifs-s can be obtained from the scattering measurement of the carbon sample under a vacuum, assuming that no structural change occurs by adsorption. The Isca of admolecules is defined as a following Eq. 2:
| \begin{equation} I_{\text{sc}}^{\text{a}} = \sum_{m}^{n}f_{m}^{2} + \sum_{m}^{n}i_{m}^{\text{inc}} \end{equation} | (2) |
where fm and iinc are the atomic scattering factor and incoherent scattering intensity of mth atom in an admolecule of the system (m ≤ n).
The ERDF (g(r)) can be driven from the Iif (= $I_{\text{if}}^{\text{a-a}} + I_{\text{if}}^{\text{s-a}}$) by Fourier transformation as following Eq. 3:
| \begin{align} g(r) &= 4\pi r^{2}(\rho - \rho_{0}) \notag\\ &= \frac{2r}{\pi \sum Z_{i}}\int_{0}^{s_{\text{max}}}s(I_{\text{if}}^{\text{a-a}} + I_{\text{if}}^{\text{s-a}})\exp(- Bs^{2})\sin sr\,\text{d}s \end{align} | (3) |
where $\sum Z_{i}$ term expresses the normalization by the sum of electron numbers of all atoms in an admolecule of the system and exp(−Bs2) is convergence factor. Here, s (= 4π sin θ/λ) is scattering parameter. Since an X-ray scattering cross section of atom become larger as the electron number increases, we can mainly discuss the information of coordination structures around the elements with large atomic numbers from ERDFs. As an example for the investigation from WAXS measurements, we show the anomalous pair wise structure formation of co-ions of an ionic liquid in monolayer-sized carbon nanopores reported by Futamura et al.35
Figure 1a shows WAXS profiles of an ionic liquid of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) in bulk and in 0.7 nm sized carbon nanopores. The pore size of nanoporous carbon is an effective pore width which can be accessed by adsorbed molecules, considered as subtraction of Van der Waals diameter of carbon atom from the internuclear distance between the graphite pore walls. Here, one can mainly determine the anionic structure of EMI-TFSI with the WAXS analysis because of the higher X-ray scattering cross section of the anion. The WAXS profile of bulk EMI-TFSI shows two peaks located at 9 nm−1 and 14 nm−1, which are corresponding to periodic structure of co-ions and counter ions, respectively. For the confined EMI-TFSI, the relationship between the intensity of the first and the second peaks is reversed to that of bulk, indicating the different coordination structure of ions in the carbon nanopores. Figure 1b shows the ERDFs of the bulk and the confined EMI-TFSI. The most important difference between these ERDFs is the intensity increase of 1st coordination structure of ions locating at 0.5 nm for confined EMI-TFSI compared with that of bulk. Figure 1b also shows the simulated anion-anion ERDFs obtained from HRMC simulation, with which we can determine a 3D structure optimized from the view point of intermolecular interaction and of experimental scattering information,35 with red lines. The intensity increasing of ERDF at 0.5 nm for the confined EMI-TFSI from that of bulk ERDF is attributed to the large increase of the intensity of at 0.45 nm in the anion-anion ERDFs; this evidences the intrusion of anions into the cationic coordination shell surrounding a central anion. This result of anion-anion pairwise structure in monolayer-sized carbon pores (see Fig. 1c) is in very good agreement with the picture of superionic state which was proposed by Kondrat and Kornyshev as mentioned above.33
(a) WAXS profiles of EMI-TFSI in bulk (top) and in 0.7 nm sized carbon nanopores (bottom). The inset shows the three-dimensional sizes of TFSI anion. (b) ERDFs of EMI-TFSI in bulk (top) and in 0.7 nm sized carbon nanopores (bottom). Anion-anion ERDFs (red), which were calculated from HRMC simulation, are also shown. (c) Snapshots of co-ion pairs of anions of EMI-TFSI in bulk (top) and in 0.7 nm sized carbon nanopores (bottom) from HRMC simulation. The materials are reproduced with permission from Springer Nature.35
Furthermore, we can detect structural changing of nano-electrolyte by electric fields with operando X-ray scattering measurements. Figure 2a shows pictures and a schematic of an operando X-ray scattering cell for EDLC electrodes with EMI-TFSI as the electrolyte. The carbon electrodes with monolayer-sized pores (CDC : carbon black : PTFE = 8 : 1 : 1 in weight percent) were inserted in two capillaries with Pt wires, and then the capillaries were filled with EMI-TFSI and connected within 0.2 mm distances with glass microfiber filter as a separator. The WAXS profiles of the operando cell show that the 2nd peak (i.e. 14 nm−1 peak) intensity of EMI-TFSI increased when the electrode was positively charged (Fig. 2b). Since the peak at 14 nm−1 is due to the first neighbor intermolecular scattering, the increasing intensity indicates the enrichment of anions in the nearest coordination shell around an anion on the positively charged electrode. Moreover, the opposite trend was observed in the negatively polarized electrode. HRMC simulation for the confined EMI-TFSI under charged condition shows that the formation of pairs of anions (Fig. 2c) is largely facilitated in positively charged pores and the number of paired anions decreases in negatively charged pores, oppositely. These results indicates that the electrostatic screening effects from carbon pore walls play very important role for the charging mechanism of EDLC composed from carbon electrodes with monolayer-sized pores and ionic liquids.40
(a) Schematic and pictures of operando X-ray scattering cell for EDLC. (b) WAXS profiles of EMI-TFSI in the monolayer confinement of carbon nanopores under electric potentials of 0 V (black) and ±2 V (+: red and −: blue). Here, we express application of the electric potentials of 2 V for positive and negative electrodes as +2 V and −2 V, respectively. (c) Snapshots of co-ion pairs of anions of EMI-TFSI in the monolayer size of pores under +2 V, 0 V and −2 V calculated from HRMC simulation. The materials are reproduced with permission from Springer Nature.35
Small-angle X-ray scattering (SAXS) refers to the diffuse scattering patterns or diffraction phenomena that occur within the small-angle region relative to the incident X-ray when it interacts with a sample.41,42 This non-destructive analytical method is predominantly utilized to analyze structures within the 1–100 nm size range. Introduced in the 1930s and further developed in the 1940s, SAXS has established several fundamental principles that continue to be employed today.43 This technique has been applied to a wide variety of materials, including polymers, biomacromolecules, nanoparticles, and porous materials. Notably, in porous carbon materials often used as electrodes for electric double-layer EDLCs, this size range is critical for determining the practical surface area through micropores, facilitating substance diffusion via mesopores, and accommodating macropores as electrode components or interstitial spaces.44 By discussing the nanoscale structural information obtained from SAXS, along with the structural changes revealed by WAXS and EXAFS, and the changes in large size regions detected through electrode thickness measurements and image analysis, it will be possible to understand the electrode structural changes across multiple scales.
Presently, lab-scale SAXS instruments employ concentrated micro-focused X-rays, and advancements in sensitive and rapid 2D detectors have enabled high-quality data acquisition. The use of synchrotron radiation allows measurements to be made in extremely short times, enabling various in situ and time-resolved operando analyses of electrochemical devices. In other words, the structures of electrode materials and electrolytes can be monitored in real time. A potential challenge, however, might be the necessity for developing specialized electrochemical cells, which could present a relatively high barrier for adoption as a common measurement technique. This section aims to overview the applications of SAXS, particularly in relation to EDLCs, and to explore future challenges.
This section describes operando SAXS measurements for EDLCs. As previously mentioned, operando SAXS measurements necessitate an electrochemical cell capable of facilitating such measurements. This requirement is the same as for WAXS and EXAFS; however, with careful consideration of the scattering angles in WAXS, a cell with a similar structure could be utilized for all three types of measurements. In the battery field, there have been more reports on the development of such cells compared to ECs, which could be informative.45,46 Time-resolved SAXS measurements typically employ synchrotron radiation, but devices equipped with high-brightness generators are also commercially available, suggesting that new electrochemical cells capable of more complex measurements will be reported in the future. In addition, to accurately discuss the data obtained, it is necessary to measure the intensity of the incident and transmitted X-rays and correct for background and thickness. While measuring the intensity of incident and transmitted light has become commonplace, accurately determining changes in thickness remains challenging. Although changes in the thickness of EC electrodes have been extensively measured and discussed,47,48 simultaneous measurement in SAXS, which analyzes sizes in a similar range, would be desirable. Performing operando SAXS measurements while tracking changes in electrode thickness is experimentally quite challenging. However, even without determining thickness, SAXS can provide several pieces of information. In this regard, the reports by Paris and Prehal et al. are pioneering and highly informative. They used aqueous electrolytes and, by comprehensively discussing changes in transmittance and scattering intensity, demonstrated that new insights into the behavior of ions and concentration-dependent changes within nanopores are possible.49 Subsequent research has developed models with hierarchical ordered structures from the nanoscale to the macroscale, using model carbon electrodes,50,51 and studies employing KOH-activated carbon with a high specific surface area for more practical applications.52,53 Very recently, Small-Angle Neutron Scattering (SANS) has also been used to closely examine the energy storage mechanisms, making this research group highly notable.54
Hatakeyama and Shiraishi et al. are currently reworking an electrochemical cell developed for operando measurements of lithium-oxygen batteries for EDLC shown in Fig. 3 and performing operando time-resolved SAXS measurements on organic electrolyte-type EDLC electrodes.55,56 In this study, a single plate activated carbon with a stronger structure than a single plate and controlled macropores was used as the electrode,15,18 and PC containing 1 mol dm−3 concentration of 5-Azoniaspiro[4.4]nonane Tetrafluoroborate (SBPBF4) was used as the electrolyte to construct the EDLC.18,57 The SAXS experiments were performed using the apparatus at the BL-6A station of the Photon Factory (PF) operated at 2.5 GeV and 450 mA in the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan.58,59 The changes in SAXS intensity during the charge and discharge of the electrode, conducted at a current density of 80 mA g−1 relative to the weight of the electrode with a central hole and with the temperature of the test cell maintained at 40 °C, are shown in Figs. 4 and 5.56 As Prehal et al. already demonstrated in 2015 for aqueous EDLCs,49 changes in scattering intensity can also be observed at both the positive and negative electrodes in organic electrolyte-type EDLCs. Focusing on the changes in scattering intensity, it is evident that intensity changes occur across the entire pattern, from micropores important for capacitance to meso and macro-pores important for material diffusion. Furthermore, the trends of these changes were found to be opposite for the positive and negative electrodes. These changes in scattering intensity are, of course, caused by the adsorption and desorption of ions. For the positive electrode, the adsorption of the high electron density BF4 anion during charging is thought to increase the scattering intensity by increasing the electron density difference with the carbon matrix. On the other hand, for the negative electrode, the adsorption of the SBP cation, which is bulky and has a low electron density, may decrease the electron density difference, resulting in a decrease in the scattering intensity. It is also interesting to note that the change in scattering intensity at about 2.5 V is smaller for the anode than for the cathode. As with the previously reported change in electrode thickness,47 we believe that this is related to the bulkiness of the ions.
Structure of the EDLC test cell for operando measurement. This test cell was designed with reference to our Li–O2 test cells. (a) X-ray path, (b) X-ray windows, (c) spring, (d) weight, (e) current collector, (f) electrode, (g) guide ring, (h) separator, and (i) electrode to be measured.
(a) Charge/discharge curve during operando SAXS measurement of the EDLC negative electrode. (b) 3D SAXS pattern in the region where the scattering parameter q (= 4π sin θ/λ) is less than 1.5 nm−1. (c) 2D SAXS pattern with scattering parameter q greater than 1.5 nm−1.
(a) Charge/discharge curve during operando SAXS measurement of the EDLC positive electrode. (b) 3D SAXS pattern in the region where the scattering parameter q is less than 1.5 nm−1. (c) 2D SAXS pattern with scattering parameter q greater than 1.5 nm−1.
Thus, SAXS can also provide new insights in organic electrolyte-type EDLCs, and it is expected to be applied to changes under high voltage conditions important for the evaluation of EDLC durability in the future. While SAXS is limited to a specific nanoscale range, it is highly sensitive to the adsorption and desorption of ions, which are fundamental to the energy storage function, along with the X-ray transmittance measured simultaneously. Prehal et al. pointed out that the number of analytical methods under realistic operando conditions is limited in this size range, and SAXS and SANS will continue to be powerful techniques.49 A more detailed depiction of the energy storage mechanisms can be achieved by incorporating insights obtained from SAXS and SANS into existing measurement methodologies. This enhancement allows for a deeper understanding of the complex interplay between the structural properties of electrodes and their electrochemical performance in organic electrolyte-type EDLCs. Furthermore, the application of these insights is expected to be invaluable in elucidating the detailed mechanisms of energy storage in systems with high area-specific capacitance,32,60 particularly under high voltage conditions critical for assessing EDLC durability.61 SAXS, despite its limitation to a specific nanoscale range, offers exceptional sensitivity to the adsorption and desorption of ions, which are essential for energy storage, and is complemented by simultaneous X-ray transmission measurements. As highlighted by Prehal et al., in this size range, the repertoire of analytical methods capable of operando conditions is scarce, positioning SAXS and SANS as indispensable techniques for advancing our understanding of energy storage mechanisms, especially in future explorations of high specific area capacitance systems.
We have reviewed studies using WAXS and SAXS measurements on EDLCs fabricated with carbon electrodes. Although the X-ray scattering measurement principle is simple, it is well understood that the formation of the electric double-layer significantly changes the WAXS pattern of the electrolyte and the SAXS pattern, which initially showed the pore structure itself. Due to the complexity of the discussion, these studies have focused their analysis on relatively simple systems consisting of an electrolyte and electrodes. The discussion may be extended to electrodes with large area-specific capacitance or in combination with durability evaluation by high-voltage charging.
Further analysis of systems that are closer to practical ECs or exhibit unusual area-specific capacitances will contribute to developing ECs with higher performance. ECs with porous carbon electrodes will undoubtedly continue to play an important role in the EC industry and research.
The WAXS measurements were performed at BL02B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2012B1438, No. 2013B1243, No. 2014A1167 and No. 2014B1196) and at BL5S2 beamline of Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Approval No. 2016D4005 and No. 201606124). R.F. thanks to Prof. Katsumi Kaneko, Prof. Taku Iiyama, Prof. Mathieu Salanne, Prof. Mark J. Biggs, Prof. Patrice Simon, and Prof. Yury Gogotsi. The SAXS measurements were performed under the approval of the PF Program Advisory Committee (Proposal No. 2022G101). Y.H. thanks the PF Program Advisory Committee and the beamline staff. Y.H. also thanks Prof. Soshi Shiraishi and Mr. Hidehiko Tsukada for insightful discussions.
Yoshikiyo Hatakeyama: Conceptualization (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)
Ryusuke Futamura: Writing – original draft (Equal), Writing – review & editing (Equal)
The authors declare no conflict of interest in the manuscript.
Y. Hatakeyama: ECSJ Active Member
Yoshikiyo Hatakeyama (Assistant Professor, Graduate School of Science and Technology, Gunma University)
Yoshikiyo Hatakeyama earned his Ph.D. at Chiba University in 2009 under Prof. Keiko Nishikawa, specializing in gold nanoparticles with small-angle X-ray scattering and X-ray absorption spectroscopy. He was a postdoctoral researcher in the same lab, then an assistant professor in Prof. Ken Judai’s laboratory at Nihon University from 2012 to 2015, focusing on nanomaterials and electrocatalysts. After returning to Prof. Nishikawa’s lab briefly, he joined Gunma University in 2016. Currently, in Prof. Soshi Shiraishi’s group, he focuses on carbon materials for electrodes and operando measurements of electrode structure during charge/discharge, to improve energy storage performance.
Ryusuke Futamura (Assistant Professor, Faculty of Science, Shinshu University)
Ryusuke Futamura received his Ph.D. in Science from Shinshu University under the direction of Prof. Taku Iiyama in 2012. He has studied in situ X-ray scattering technique and the analysis methods. He has joined Katsumi Kaneko research group for the research of nanoporous carbon materials as a post-Doc. researcher and a project Assistant Professor (2012–2017). He joined in adsorption science at Shinshu University as an Assistant Professor (2017–). One of his recent interests is synthesis of a new kind of functional materials with ionic liquids.
