Operando leaching of pre-incorporated Al and mechanism in transition-metal hybrids on carbon substrates for enhanced charge storage

•A "nano-tailoring" strategy via electrochemical leaching of Al species is proposed•Defective and highly active materials were formed via the "nano-tailoring" strategy•The integrated oxygen vacancies can boost charge storage of potassium-birnessite•The reducibility of M2+ is identified as the key descriptor for the reconstruction rate From the viewpoint of optimizing charge-storage dynamics, we for the first time propose a universal "nano-tailoring" strategy to fabricate low-crystalline and highly active materials via electrochemically dynamic leaching of pre-incorporated Al. With M2+Al hydroxides as the precursor, potassium-birnessite MnO2 with abundant defective sites and enriched edge-active sites is configured, resulting in the enhanced charge-storage capability. Soft XAS and Raman mapping techniques are adopted to finely track the dynamic reconstruction principles and intrinsic active sites. It is found that the reducibility of M2+ serves as the key descriptor for the reconstruction rate, which correspondingly is divided into two branches: oxidization-boosting type and non-oxidization type. This work can provide a novel avenue and spark new ideas to electrochemically modulate and ameliorate electrode materials for advanced energy storage and conversion applications in the future. Insufficient exposure and utilization of active sites often induces an inferior reactivity for transition-metal-based two-dimensional (2D) materials. In response, we for the first time propose a universal "nano-tailoring" strategy to incorporate abundant defects and active sites into low-crystallinity nanosheets by electrochemically leaching of Al species. With MnAl layered double hydroxides (LDHs) as a representative example, potassium-birnessite MnO2 (AK-MnO2) with oxygen vacancies and abundant edge sites is successfully produced. The oxygen vacancies are shown to help optimize the electron-transfer and ion-adsorption capability. These integrated advantages endow the AK-MnO2 with a high capacitance value of 239 F g−1 at 100 A g−1. By further combining with soft X-ray absorption spectroscopy techniques, we unravel that the reducibility of M2+ in M2+Al-LDH serves as the key descriptor for the reconstruction rate. This "nano-tailoring" strategy can provide some important implications and clues to manipulating 2D materials for efficient energy storage and conversion. Insufficient exposure and utilization of active sites often induces an inferior reactivity for transition-metal-based two-dimensional (2D) materials. In response, we for the first time propose a universal "nano-tailoring" strategy to incorporate abundant defects and active sites into low-crystallinity nanosheets by electrochemically leaching of Al species. With MnAl layered double hydroxides (LDHs) as a representative example, potassium-birnessite MnO2 (AK-MnO2) with oxygen vacancies and abundant edge sites is successfully produced. The oxygen vacancies are shown to help optimize the electron-transfer and ion-adsorption capability. These integrated advantages endow the AK-MnO2 with a high capacitance value of 239 F g−1 at 100 A g−1. By further combining with soft X-ray absorption spectroscopy techniques, we unravel that the reducibility of M2+ in M2+Al-LDH serves as the key descriptor for the reconstruction rate. This "nano-tailoring" strategy can provide some important implications and clues to manipulating 2D materials for efficient energy storage and conversion. 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To the best of our knowledge, aluminum, as a typical amphoteric metal element, displays a tendency to spontaneously dissolve in alkaline electrolytes, while this is thermodynamically unfavorable for transition-metal elements. In addition, Al-based transition-metal hybrids have a big family, including double/triple/multiple oxides, hydroxides, sulfides, and so on. Accordingly, it is expected that pre-incorporating Al into the lattice of transition-metal hybrids, followed by its in situ leaching, can generate a series of advanced materials with promoted reactivity, which remain less concerning and require more attention and detailed explorations. Herein, a universal "nano-tailoring" strategy, via the operando leaching of pre-incorporated Al driven by electrochemical effects, is first proposed for realizing the holistic activation of transition-metal hybrids. Using MnAl layered double hydroxides (LDHs) as a demonstration, we show that low-crystallinity potassium-birnessite MnO2 (AK-MnO2) can be produced quickly after the "nano-tailoring" process. With the help of soft X-ray absorption spectroscopy (sXAS), the structural evolution is finely tracked and decoupled. Interestingly, due to the irreversible leaching of Al and the intercalation of K+, the microstructure may undergo the emergence and release of internal strain, resulting in a positively tailored microstructure with enriched edge sites and oxygen vacancies. Also, density functional theory (DFT) calculation demonstrates that the introduced oxygen vacancies would be capable of promoting charge transfer and facilitating the adsorption of electrolyte ions, leading to optimized reaction kinetics for charge storage. As such, the as-formed AK-MnO2 hybrids can deliver a high capacitance value of 356 F g−1 at 1 A g−1, with a high retention rate of up to 67% at a super-large current density of 100 A g−1, indicative of a superior rate performance. More importantly, the universality of the "nano-tailoring" process to generate low-crystallinity nanosheets with enriched active sites is deeply verified and decoupled. We reveal that the leaching transformation is highly dependent on the reducibility of the adopted M2+, which acts as a prominent factor for the reconstruction rate. It is believed that the unique and universal "nano-tailoring" strategy can stimulate broad interest and inspiration of researchers to configure novel and highly efficient materials with positively tuned properties. As illustrated in Figure 1A, the "nano-tailoring" strategy involves M2+Al-based binary hydroxides as the precursor, where the molecular breakage and reconstruction happens during the electrochemical scanning process in an alkaline medium. As a typical example, during the "nano-tailoring" process, the MnAl-LDH will undergo a transformation into MnAl-based hydroxides, accompanied by partial leaching of Al, when soaked in an alkaline electrolyte for a short time (2 min), yielding Mn(Al)-OH as the intermediate. Subsequently, it will be reconstructed into low-crystallinity birnessite-type MnO2 with K+ in the interlayer after cyclic voltammetry (CV) cycling treatment, denoted as AK-MnO2. The dynamic reconstruction is ex situ tracked by scanning electron microscopy (SEM) images and the corresponding size distributions. As shown in Figures 1B–1D, the large LDH nanosheets are broken and reconstructed to much smaller ones with a higher stacking density. The average size decreases from 726 nm for the MnAl-LDH to 202 nm for the Mn(Al)-OH hybrids, and finally to 55 nm for the AK-MnO2 hybrids (about 13 times decrease in size). Notably, the small and dense nanosheets would deliver more abundant active sites and feature numerous short-distance channels for fast charge transfer, and thus are beneficial for highly efficient charge storage. The microstructure is further uncovered by transmission electron microscopy (TEM) images. As depicted in Figure S1A, the MnAl-LDH precursor displays a large size, and a lattice spacing of 0.26 nm corresponds to the (012) plane of LDH. For the Mn(Al)-OH, a broken structure with numerous in situ-formed holes can be observed, and the lattice spacing of 0.33 nm corresponds to the (100) plane of Mn(OH)2 (Figure S1B). Impressively, the well-tailored AK-MnO2 with densely stacked and ultrasmall nanosheet microstructure (Figure 1E) displays a low crystallinity, as shown in the high-resolution (HR)-TEM image (Figure 1F). It is further identified by the result of selected area electron diffraction (SAED), where no lattice patterns can be detected (Figure 1F), indicative of the generation of lattice distortion and randomly stacked internal construction. Furthermore, as presented by high-angle annular dark-field scanning TEM (HAADF-STEM) images and the corresponding elemental maps, the Mn, O, and K elements are uniformly distributed on the surface of the AK-MnO2 (Figure 1E). Also, we carried out atomic force microscopy (AFM) characterization to better study the morphology of the AK-MnO2. In Figure S2, the ultrathin and wrinkled nanosheet morphology with a thickness of about 7.7 nm is presented. In addition, the AK-MnO2 demonstrates a highly increased surface area (157 m2 g−1) as well as more abundant pore structure (Figure S3), compared with the MnAl-LDH (58 m2 g−1). The enlarged surface area, enabled by the novel "nano-tailoring" process, can help give access to more electrolyte ions at a given time and shorten the transport distance, thus accounting for the enhanced charge storage. The "nano-tailoring" process of the MnAl-LDH is further decoupled by X-ray diffraction (XRD) patterns (Figure S4). Notably, the intense peaks located at 26.4°, 43.1°, 54.6°, and 77.6° can be indexed to the characteristic peaks of carbon paper (CP). For the MnAl-LDH hybrids, the peaks at 10.1°, 20.2°, 33.1°, 36.4°, and 58.8° can be indexed to the (003), (006), (012), (015), and (110) planes of the LDH phase, respectively. Explicitly, for the Mn(Al)-OH hybrids, the peaks at 18.8°, 31.6°, 36.8°, 49.9°, and 59.5° can be indexed to the (001), (100), (101), (102), and (111) diffraction planes of Mn(OH)2 (JCPDS card no. 18-0787), respectively, indicative of the phase transformation from the MnAl-LDH to the Al-doped Mn(OH)2 phase. However, as for the AK-MnO2, it displays only one significant peak at about 12.7°, as well as an indiscernible wide band in the range of 35° to 40°. This implies the low-crystallinity/amorphous features of the [MnO6] octahedra laminate.36Lin B. Zhu X. Fang L. Liu X. Li S. Zhai T. Xue L. Guo Q. Xu J. Xia H. Birnessite nanosheet arrays with high K content as a high-capacity and ultrastable cathode for K-ion batteries.Adv. Mater. 2019; 31: 1900060Crossref PubMed Scopus (132) Google Scholar,37Dong Z.H. Lin F. Yao Y.H. Jiao L.F. Crystalline Ni(OH)2/amorphous NiMoOx mixed-catalyst with Pt-like performance for hydrogen production.Adv. Energy Mater. 2019; 9: 1902703Crossref Scopus (78) Google Scholar Notably, an increase of interlayer spacing from Mn(Al)-OH (d spacing: 4.7 Å) to AK-MnO2 (d spacing: 7.0 Å) can be attributed to a certain amount of K+ intercalated into the interlayer. To further confirm the existing species, Raman measurements were carried out. For the AK-MnO2 hybrids, the characteristic peaks at 265, 317, 396, 469, 578, and 654 cm−1 are matched with those of birnessite-type MnO2 with a little shift (Figure 1G), attributed to the varied ion radius between K+ and Na+.38Julien C. Raman spectra of birnessite manganese dioxides.Solid State Ionics. 2003; 159: 345-356Crossref Scopus (570) Google Scholar And the intense peaks at 1,350 and 1,580 cm−1 could be indexed to the D and G bands of CP. Moreover, Raman maps were recorded in the green, orange, and pink regions (as marked in Figure 1G), where the uniform distribution for the signals of the CP (orange and pink) and the AK-MnO2 (green) are presented in the maps (Figure 1H), indicative of the intimate interaction between them. The sXAS technique was used to decouple the fine structure of the as-formed hybrids and understand the dynamic phase reconstruction process. The mechanism of sXAS is illustrated in Figure 2A (refer to Yang and Devereaux and Lin et al.).39Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (132) Google Scholar,40Lin F. Nordlund D. Markus I.M. Weng T.-C. Xin H.L. Doeff M.M. Profiling the nanoscale gradient in stoichiometric layered cathode particles for lithium-ion batteries.Energy Environ. 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Energy Mater. 2017; 7: 1602010Crossref Scopus (34) Google Scholar The Mn L-edge soft XAS spectra are shown in Figure 2B. It is worth noting that the typical peaks at 638.8, 640.2, and 641.6 eV can be assigned to Mn2+, Mn3+, and Mn4+, respectively, while the peak intensity is generally used to identify the relative content.42Henderson C.M.B. Cressey G. Redfern S.A.T. Geological applications of synchrotron radiation.Radiat. Phys. Chem. 1995; 45: 459-481Crossref Scopus (66) Google Scholar Clearly, for the MnAl-LDH, the peak at 638.8 eV is much more intensive than the others, implying that the Mn primarily exists in the +2 status. Compared with the MnAl-LDH, the Mn(Al)-OH hybrids are also dominated by Mn2+, despite the slight increase in peak intensity of Mn3+. Interestingly, the Mn4+ ions are dominant for the as-formed AK-MnO2, where a certain amount of Mn3+ remained. It is noteworthy that the charge compensation can be realized by the intercalation of K+, which is described below in detail. Moreover, compared with the 30-cycled sample (AK-MnO2), no further obvious changes in Mn L-edge are found after 50 cycles, implying that a stable status can be attained after 30 cycles. These results correspond finely to the Mn 2p and Mn 3s X-ray photoelectron spectroscopy (XPS) results (Figure S5). To gain a more complete picture of the reconstruction process, the O K-edge characterization was conducted (Figure 2C). The pre-edge region (below 533 eV) and broad-band region (over 533 eV) can be attributed to the transition of O 1s to Mn 3d and O 2p hybrid-state orbital, and O 1s to Mn 4sp and O 2p hybrid-state orbital, respectively. Generally speaking, the positive shift of the broad band can indicate an increase in Mn valence.43Oishi M. Yamanaka K. Watanabe I. Shimoda K. Matsunaga T. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Ohta T. Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy.J. Mater. Chem. A. 2016; 4: 9293-9302Crossref Google Scholar Compared with the MnAl-LDH, there is no detectable shift of the broad band for the Mn(Al)-OH, indicative of the unobvious oxidation of Mn species. Nevertheless, the significantly positive shift can be detected for the 30-cycled sample (AK-MnO2), indicative of the oxidation of Mn species to a great degree. The 50-cycled sample demonstrates no more shift compared with the AK-MnO2, indicative of complete transformation after 30 electrochemical cycles, very consistent with the results we mentioned above. For the C K-edge spectra (Figure S6), the two new peaks at 299 and 302 eV, which correspond to the characteristic peaks of K L3- and L2-edge,44Dey A. Krishnamurthy S. Bowen J. Nordlund D. Meyyappan M. Gandhiraman R.P. Plasma jet printing and in situ reduction of highly acidic graphene oxide.ACS Nano. 2018; 12: 5473-5481Crossref PubMed Scopus (23) Google Scholar emerge for 30- and 50-cycled electrodes, indicative of the intercalation of K+. This is further proved by the presence of the intense K 2p peak signals for AK-MnO2 (Figure S7A). On top of that, the drastic reconstruction process enables the formation of internal defects to a great degree, which is studied and analyzed by the depth spectra of O 1s (Figure 2D). The peaks located at 530, 531.7, and 533 eV can be finely detected, corresponding to the bonding of metal and oxygen, oxygen vacancy, and physically/chemically adsorbed O, respectively.45Bao J. Zhang X.D. Fan B. Zhang J.J. Zhou M. Yang W.L. Hu X. Wang H. Pan B.C. Xie Y. Ultrathin spinel-structured nanosheets rich in oxygen deficiencies for enhanced electrocatalytic water oxidation.Angew. Chem. Int. Ed. 2015; 54: 7399-7404Crossref PubMed Scopus (923) Google Scholar,46Fang G.Z. Zhu C.Y. 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