It has been found that electrical bilayer (EDL) plays a key role not only in electric bilayer capacitors (EDLC), but also in batteries and electrocatalysts. Although the effects of EDL on the electrochemical behavior of electrolytes have been extensively studied, their effects on the electrodes themselves, especially the energy storage mechanisms, remain unclear.
Scholars from Jilin University used the popular α-Fe2O3 to study the interaction between the electrode and the EDL, and found that the energy storage mechanism of the electrode can be adjusted by adjusting the EDL. Through the specific adsorption of ions on the Fe2O3 surface, EDL is primarily influenced by the inner Helmholtz plane (IHP), which determines which ions are present at the interface and how they react with the electrode. Ultimately, the composition and properties of the EDL determine the energy storage mechanism of the Fe2O3 electrode, including conversion reactions, ion insertion, surface redox reactions, and false capacitance.
The results of this study not only provide a new understanding of the nature of EDL, but also demonstrate that EDL should be promoted as a functional "component" that can be designed to achieve optimal synergy at the electrode-electrolyte interface. The work was published in Acta Materialia in a research article titled "Nature of the electric double layer to modulate the electrochemical behaviors of Fe2O3 electrode".
Paper Links:
Hatps://doi.org/10.1016/j.aktamat.2023.119500
In this study, we investigated the effect of EDL on the electrochemical behavior of Fe2O3 electrodes. The results show that the energy storage mechanism of the Fe2O3 electrode can basically be adjusted by adjusting the EDL. Specifically, in 1 M NaOH, IHP is filled with OH- ions, and as a result, the prepared α-Fe2O3 nanorods (NR-Fe2O3) undergo a conversion reaction: Fe2O3→ Fe(OH)2 (charge), → FeOOH (discharge). Interestingly, in 0.5 M Na2SO4, a neutral electrolyte, OH- ions also constitute IHP, resulting in the same transformation reaction observed in NR-Fe2O3. However, in 0.5 M Na2SO4, due to the depletion of OH- ions and the coulombic attraction between the negatively charged electrode and the Na+ ions, the Na+ ions in the outer Helmholtz plane (OHP) instead of the OH- ions in the IHP react directly with the electrode and attempt to enter the electrode lattice, resulting in spalling of the active material. Similarly, when IHP consists of NH4+ in 0.5 M (NH4)2SO4, no transformation reaction is observed, but NH4+ ions attempt to infiltrate NR-Fe2O3, resulting in immediate exfoliation of the active material. In contrast, in 0.5 M Al2(SO4)3 (acidic electrolyte), neither the conversion reaction nor the material peeling was observed, as charge storage was carried out by the (de)insertion of H+ ions. In 0.5 M Na2HPO4, IHP is composed of HPO42- ions, and as a result, NR-Fe2O3 stores charge through surface redox reactions, rather than through conversion or insertion processes. In 1 M NaNO3, IHP is composed of NO3- ions, and the NR-Fe2O3 electrode exhibits the electrochemical behavior of EDL. Different electrochemical behaviors, including conversion reactions, ion insertion, surface redox reactions, and false capacitance, all originate from EDL, which means that EDL plays a crucial role in determining the energy storage mechanism of the electrode.
Figure 1. The Critical Role of EDL in Electrochemical Energy Storage a) The role of EDL can be discussed from two aspects. The first aspect is the impact on the electrolyte, including the extension of the potential window, the facilitation of ion transport, and the facilitation of the formation of SEI/CEI. The second aspect is the effect of EDL on the electrode, which explores how EDL affects the electrochemical behavior of the electrode, which is the focus of this study. b) IHP acts as a barrier, allowing ion transmission when turned on and blocking the passage of unwanted ions when closed.
Figure 2. Structural and spectral characteristics of prepared NR-Fe2O3. a) Crystal structure of α-Fe2O3 (from the ICSD database); brown and blue spheres represent Fe and O, respectively. e-g) STEM images and corresponding EDS element diagrams, h-i) XPS data for Fe2p and O1 s, j) Fe L-edge EELS line scan (inset with HAADF-STEM images for EELS measurements) and Tauc plots of Fe2O3 electrodes and their UPS(k-l); m) UV-Vis data; n) corresponding energy level plots.
Figure 3. DFT calculations reveal the type of ion that forms IHP on the surface of the Fe2O3 electrode. a) Schematic diagram of the gate layer structure and the IHP formed by specific adsorption. b) Adsorption energy of selected ions adsorbed in the Fe2O3(110) plane. c-h) Corresponding EDL structures in NaOH, Na2HPO4, Na2SO4, (NH4)2SO4, NaCl, and NaNO3. i-p)Optimized adsorption models for Na+ads (110), H+ads (110), NH4+ads (110), SO42-ads (110), OH-ads (110), Cl-ads (110), NO3-ads (110), and HPO42-ads (110).
Figure 4. Energy storage mechanism of NR-Fe2O3 in 0.5 M Na2SO4. a) SXRD diagram. b) Enlarged SXRD plot showing diffraction peaks between 32.5° and 41.5°. e-f) XPS spectra of Fe 2p and O 1 s g-i) EELS spectra in O, D, and F states j) Line scan EELS spectra k-l) Schematic diagram of the dynamic EDL that may occur during charging of the NR-Fe2O3 electrode in 1 M NaOH and 0.5 M Na2SO4, respectively.
Figure 5. Schematic diagram showing the electrochemical behavior of a Fe2O3 electrode that can be adjusted by EDL.
Understanding the interaction between electrolytes/electrodes and EDLs is extremely important in the field of energy storage and conversion. The results of this study provide new insights into this interaction and the nature of EDL, suggesting that EDL can effectively modulate the electrochemical behavior of electrodes. This finding suggests that EDL should be promoted as a functional "component" and designed to achieve optimal performance in electrochemical energy storage systems. Specifically, optimal functionalization of the electrode can be achieved by fabricating IHPs containing specific target ions and actively controlling the dynamic adaptability of the EDL. In addition, this study also questioned the common view that the energy storage mechanism of the electrode is mainly determined by the acidity and alkalinity of the electrolyte. This study demonstrates that the key factor influencing the electrochemical behavior of the electrodes is the potential adaptability of the ions and EDLs that make up the IHP during charging and discharging. Future studies should investigate the interaction between electrodes and EDLs by considering factors such as the hydration layer around the electrolyte ions and electrode surface, the Coulomb force between the electrode and electrolyte ions, and the interaction between electrolyte ions. (Text: SSC)
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