Lead metal organic framework materials (MOFs) are complexes formed by coordination of organic ligands with metal ions or clusters
Because of its controllable morphology, high specific surface area and porous structure, it is widely used in the field of energy storage
The metal components in MOFs materials have a great influence on the performance of electrode materials
The activation of metal ions can effectively improve the performance of the materials
However, this activation is often accompanied by serious structural damage and can not maximize the electrochemical performance of the materials
Recently, Xia Baoyu research group of Huazhong University of science and technology adjusted the metal components in bimetal Co Ni MOF by a method of oxidation-reduction to obtain stable and low-cost Co Ni MOF derivatives with mixed valence, and the materials obtained have excellent electrochemical performance
Relevant results were published online in adv
mater
(DOI: 10.1002/adma.201905744)
Professor Xia Baoyu's research group is mainly engaged in the research of structural functional materials and energy conversion and storage
Focusing on the service and failure of new materials in new energy technology, guided by the electrode materials with high activity, long life and low cost, the team explored the corrosion phenomena and laws of materials, developed new and stable materials and devices by using traditional corrosion science and technology to achieve the purpose of high level and long life service, and realized the deep cross fusion of traditional corrosion science and new energy field All right
Prof
Xia Baoyu, Professor / doctoral supervisor, graduated from Shanghai Jiaotong University in 2010 with a doctor's degree
From 2011 to 2016, he worked as a postdoctoral researcher at Nanyang University of technology in Singapore
In 2016, he joined Huazhong University of science and technology and was selected into the 12th batch of "plans" of the central organization department
In recent years, science, nature energy, J
am
Chem
SOC., angel
Chem
Int
ed, Adv
material
And other international journals have published more than 70 academic papers (including 1 ESI hot paper and 21 ESI highly cited papers), published research work has been cited more than 8000 times by domestic and foreign peers, participated in the preparation of 3 Monographs (three chapters), applied for 6 patents (authorized 3), h index 29, research results have been reported by chemistry views, materials views and other media
Professor Xia Baoyu was selected as the global high cited scientist of Kerui Weian in 2018 and 2019 and the Journal of materials chemistry a emerging investigators in 2019
Cutting edge research achievements: redox regulates the crystallinity and electronic structure of bimetallic organic framework derivatives to enhance the material Faraday capacitor metal organic frameworks, MOFs), which is obtained by self-assembly of metal ions and organic ligands, is a kind of porous coordination polymer material with rapid development in recent ten years
It has the characteristics of high specific surface area, controllable pore structure and rich and diverse topological structure
MOFs and its derivatives have become one of the research hotspots in the field of energy storage, with both opportunities and challenges
In view of the poor conductivity of MOFs, it is a general strategy to composite MOFs with conductive polymer or carbon material to improve the conductivity and stability
However, the charge storage behavior of these MOFs materials is mainly based on the specific surface area and the illegal second layer capacitance (EDLC), while the utilization of its rich active metal ion sites is low
Therefore, this paper aims to improve the conductivity of MOFs by treating MOFs with sodium borohydride solution and sodium sulfide solution, activating metal components in MOFs through oxidation-reduction, and promoting fast and reversible Faraday reaction (Figure 1) to enhance the specific capacity of materials while maintaining the original structural integrity of MOFs
This activation strategy can achieve excellent charge storage performance of co-ni-b-s, including high specific capacitance, rate performance and long cycle life
Figure 1
Activation strategy of bimetallic Co Ni MOF (source: adv
mater.) first, the author obtained the Co Ni MOF with the best electrochemical performance by optimizing the composition ratio of CO / Ni, and then activated it
Even though the co-ni-b-s was boronized and vulcanized, the stacking morphology of ~ 5 μ m nano chips remained intact (Fig
2a-b), and a large number of smaller nano chips were formed on the surface in situ (Fig
2C)
Even after the drastic redox reaction, ultra-thin folded nanostructures can be obtained (Fig
2D)
Open nanostructures will be beneficial to expose more redox active sites on the surface
The SAED of the selected region in Fig
2E shows that the obtained co-ni-b-s is amorphous, but at the same time, the locally visible lattice fringes (Fig
2f) show that there are some crystalline and amorphous states in the fold part, which is helpful to improve the electrochemical performance of the material
Fig
2
SEM (A-C), TEM (D-E) and HRTEM of co-ni-b-s
the embedded diagram is the corresponding saed diagram (source: adv
mater)
In order to further explore the formation mechanism of co-ni-b-s, NaBH4 treatment (co-ni-b) and Na2S treatment (co-ni-s) were prepared, respectively
Na2S treatment was used first and then NaBH4 treatment（ Co-Ni – S-B), the three reference samples
The original Co Ni MOF is a ~ 5 μ m microsphere, which is composed of a stack of nanoflakes with a thickness of 3-5 nm (Fig
3a, e, I)
After boronization, co-ni-b shows smaller folded nano sheet size and polycrystalline lattice stripes with short-range order and long-range disorder (Fig
3b, F, J)
However, after the direct sulfurization of Co Ni MOF, the samples co-ni-s and co-ni-s-b showed the tendency of agglomeration, and the nanoflakes disappeared with serious agglomeration (Fig
3C, G, K and Fig
3D, h, l)
The results show that the nano structure of Co Ni MOF will be destroyed by Na 2S treatment, which is not conducive to the penetration of electrolyte and affects the electrochemical performance of the material
F I g
3
FESEM, TEM and HRTEM o f c o Ni MOF (a, e, I), CO Ni-B (B, F, J), Co Ni-S (C, G, K), Co ni-s-b (D, h, l) (source: adv
mater.) compared with the initial MOF, the diffraction peaks at ~ 35o indicate that the metal co / Ni (FIG
4A) was partially formed after the boration process, which is consistent with the HRTEM image
No obvious diffraction peak was observed after vulcanization, which proved the irreversibility of boration and vulcanization sequence during the evolution of amorphous structure
After sodium borohydride treatment, the valence of redox active Co / Ni decreased
In the subsequent sulfurization process, the oxidized boron is reduced to boron atoms, and at the same time, part of S2 - is oxidized to a high valence state
In combination with the S-O substance in the solution during the reaction, the low valence CO and Ni are oxidized to a stable low valence mixed valence state, while the other S2 - enters into the defects formed by the boration reaction through bonding with metal
The stable low mixed valence of cobalt and nickel is the metal component activated by redox, which plays an important role in improving the capacitance of the material
XPS analysis provides detailed evidence for the formation of co-ni-b-s with excellent electrochemical properties
Fig
4
XRD (a) and XPS high resolution (B-F) of Co Ni MOF and its derivatives (source: adv
mater.) can be found in the electrochemical test of these materials, The current density of the redox peak of co-ni-b-s increases with the increase of the scanning rate, indicating that the resistance of the electrode material is relatively low and the redox reaction at the electrode / electrolyte interface is rapid (Fig
5a)
At the same time, the co-ni-b-s electrode also shows a near rectangular shape with two pairs of redox peaks, which shows the fast and reversible charge discharge characteristics
Compared with the contribution curve of co-ni-mof, the diffusion and capacitance contribution of co-ni-b-s increased, which indicated that the electronic regulation during redox treatment could enhance the ion diffusion ability and capacitance at the same time
As shown in Figure 5D, a series of symmetrical GCD curves show the good electrochemical reversibility of co-ni-b-s
from 1ag-1 (1281.2fg-1) to 10ag-1 (1196.4fg-1), 93.4% capacity retention can be achieved
Co-ni-b-s shows excellent cycle performance
At the current density of 15A g-1, after 10000 cycles, the capacitance retention rate is 92.1% (Fig
5F)
Figure 5
(a) CV Curve of co-ni-b-s at different scanning rates, (b) CV Curve of Co-Ni MOF and its derivatives at 20 mV S-1 scanning rate, (c) capacitance and diffusion contribution of co-ni-b-s, (d) GCD curve of co-ni-b-s at different magnification, (E) GCD curve of Co-Ni MOF and its derivatives at 1 a g-1, (f) cycle life of co-ni-b-s at 15 a g-1 (source: adv
mater ）Then, the author uses the commercial active carbon (AC) as the negative electrode material and co-ni-b-s as the positive electrode material to construct the asymmetric supercapacitor, and tests the device with two electrodes (Fig
6a)
It is found that the voltage of this device can reach 1.7 V in aqueous electrolyte, which is higher than that of most aqueous capacitor devices (Fig
6b)
At the same time, the unsymmetrical super capacitor has the mixed energy storage behavior (Fig
6C, d) of battery type and double electric layer capacitance characteristics, and its electrochemical performance is very excellent
The asymmetric supercapacitor can provide 45.0 wh kg-1 energy density at 857.7 w kg-1 power density
Even when the power density increases to 13.6kw kg-1, the energy density of the device can reach 36.8wh kg-1, which is better than most asymmetric supercapacitors (Fig
6e)
In addition, at a current density of 12 a g-1, the capacity retention rate is 87.7% after 5000 cycles (Fig
6F)
Figure 6
(a) schematic diagram of asymmetric co-ni-b s / NF / / a C / NF device, (b) CV Curve under different potential windows of 10 MV s-1, (c) CV Curve under different scanning rates of 5-50 MV s-1, (d) GCD curve under different current densities of 0-1.7 V, (E) co-ni-bs / NF// The energy density and power density of AC / NF are compared with the Ragone diagram in the latest literature, (f) 5000 cycles life diagram under 12a g-1 (source: adv
mater.) to sum up, this work mainly proposes an effective oxidation-reduction method to improve the Faraday capacitance and enhance its electrochemical performance by adjusting the crystallinity and electronic structure of co-ni-b-s
The activated co-ni-b-s shows high specific capacitance (1281fg-1 at 1ag-1), excellent rate performance (802.9f-1 at 20a-1) and excellent cycle stability (92.1% retention after 10000 cycles)
The asymmetric energy storage device constructed by co-ni-b-s shows an energy density of 50.0 wh kg-1 at a power density of 857.7 w kg-1 and a capacity retention rate of 87.7% (at 12 a g-1)
This work can be extended to other related energy conversion technologies, providing new ideas for regulating the electronic structure of MOFs and building efficient electrode materials
This work was published in adv
mater
(DOI: 10.1002 / ADMA
201905744) with the title of "redox tuning in crystal and electronic structure of biomedical – organic frameworks derived cobalt / nickel bridge / sulfide for boosted faradic capture"
The first author is Wang Qingyong, a doctoral candidate of Huazhong University of science and technology
The corresponding authors are Professor Xia Baoyu of Huazhong University of science and technology and Professor Ho Seok Park of chengjunguan University of South Korea
Author: Qingyong Wang, Yumei Luo,
