Electronic Doping of Metal‐Organic Frameworks for High‐Performance Flexible Micro‐Supercapacitors

The combination of high specific surface areas, well‐defined porous structures, and redox‐active sites renders the organic frameworks as promising electrode materials for next‐generation energy storage devices. Despite the recent advancements in the fabrication of conductive metal‐organic frameworks (MOFs), they generally require tedious synthesis procedures, which hinder their energy‐related applications. Herein, a doping strategy using electron acceptor molecules is demonstrated to tune the ohmic electrical conductivity of MOF thin‐film electrodes. For instance, the conductivity of MOF Cu3(BTC)2 film is enhanced over 40 times after doping with 7,7,8,8‐tetracyanoquinododimethane (TCNQ). Thereby, asymmetric in‐plane micro‐supercapacitors (MSCs) are constructed utilizing in situ‐grown TCNQ@Cu3(BTC)2 as the cathode and activated carbon as the anode, which delivers remarkable areal capacitance of 95.1 mF cm−2 at a scan rate of 5 mV s−1, superior to those of the reported MSCs (0.1–50 mF cm−2). Moreover, the fabricated devices show long‐term stability with 94.1% capacitance retention up to 5000 charge‐discharge cycles at 10 mA cm−2. The molecular doping engineering of organic framework materials with excellent electronic properties for energy storage and conversion applications is inspired.


Introduction
Metal-organic frameworks (MOFs) are constructed by coordinating metal ions or clusters to organic ligands. [1] They are a new class of coordination polymers that exhibit high porosity and crystallinity. Importantly, they feature flexible structural designability as well as densely accessible redox-active sites, making MOFs promising materials toward high-performance electrochemical energy storage applications. [2] Nonetheless, traditional MOFs suffer from low electrical conductivity due to the insulating organic ligands and undesirable orbital overlap between π orbital of organic ligands and d orbital of metal ions. [3] In addition, the unique porous structure with redox-active sites cannot be fully utilized, hindering their practical applications in energy storage.
To date, significant efforts have been devoted to improving the electrical conductivity of MOFs. [3,4] On the one hand, pristine MOFs are generally mixed with conductive additives and binders to improve the overall conductivity in energy storage devices. However, this strategy does not enhance the inherent electrical properties of MOFs but rather decrease the accessible surface areas. [5] On the other hand, conductive MOFs with high electrical conductivity and permanent porosity have emerged in the past few years, which exhibited improved electrochemical performance in supercapacitors. [6] Despite the recent achievements, the synthesis of conductive MOFs and their thin-film fabrication remain tedious. Therefore, developing an efficient and straightforward approach toward conductive and redox-active MOF thin films is highly attractive for on-chip energy storage devices.
In this work, we demonstrate an efficient strategy for the in situ fabrication and doping of Cu 3 (BTC) 2 (BTC ¼ benzene-1,3,5-tricarboxylate) thin-film electrodes. Doping of Cu 3 (BTC) 2 thin films with electron acceptor molecules (such as 7,7,8,8tetracyanoquinododimethane [TCNQ], benzoquinone [BQ], and pyromellitic diimide [PMDI]) is found to largely improve the ohmic electrical conductivity up to over 40 times; and the achieved maximum conductivity is 0.46 mS m À1 . Thereby, the high conductivity, the large specific surface area of 655 m 2 g À1 together with excellent flexibility of TCNQ@Cu 3 (BTC) 2 thin film facilitate the fabrication of on-chip energy storage devices. As a result, flexible asymmetric micro-supercapacitors (MSCs) based on TCNQ@Cu 3 (BTC) 2 cathode and activated carbon (AC) anode deliver a remarkable areal capacitance of 95.1 mF cm À2 at a scan rate of 5 mV s À1 , 3.4 times higher than MSCs based on undoped MOF thin films. This value is also superior to those of state-of-DOI: 10.1002/sstr.202000095 The combination of high specific surface areas, well-defined porous structures, and redox-active sites renders the organic frameworks as promising electrode materials for next-generation energy storage devices. Despite the recent advancements in the fabrication of conductive metal-organic frameworks (MOFs), they generally require tedious synthesis procedures, which hinder their energy-related applications. Herein, a doping strategy using electron acceptor molecules is demonstrated to tune the ohmic electrical conductivity of MOF thin-film electrodes. For instance, the conductivity of MOF Cu 3 (BTC) 2 film is enhanced over 40 times after doping with 7,7,8,8-tetracyanoquinododimethane (TCNQ). Thereby, asymmetric in-plane micro-supercapacitors (MSCs) are constructed utilizing in situ-grown TCNQ@Cu 3 (BTC) 2 as the cathode and activated carbon as the anode, which delivers remarkable areal capacitance of 95.1 mF cm À2 at a scan rate of 5 mV s À1 , superior to those of the reported MSCs (0.1-50 mF cm À2 ). Moreover, the fabricated devices show long-term stability with 94.1% capacitance retention up to 5000 charge-discharge cycles at 10 mA cm À2 . The molecular doping engineering of organic framework materials with excellent electronic properties for energy storage and conversion applications is inspired.
the-art MSCs based on various types of hybrid material systems (0.1-50 mF cm À2 ). Moreover, the fabricated device displays superb electrochemical stability and great promise in flexible electronics.

Results and Discussion
The in situ synthesis process of Cu 3 (BTC) 2 thin film on Cu foil is schematically shown in Figure 1a. First, the pretreated Cu foil was placed onto the home-made spin coating holder and heated to 120 C. Then, the solution containing H 3 BTC (benzene-1,3,5tricarboxylic acid) and AgNO 3 was spin coated with a speed of 300 rpm on the Cu foil for 5 min. During this process, Ag þ reacted with Cu foil and generated Cu 2þ and Ag (2Ag þ þ Cu ¼ 2Ag þ Cu 2þ ). Ag deposited onto Cu foil and act as collector. Afterward, the generated Cu 2þ coordinated with H 3 BTC and formed the Cu 3 (BTC) 2 thin film, which was adhered by Ag layer on the Cu foil. The achieved Cu 3 (BTC) 2 thin film was dried in vacuum at 180 C to remove the coordinated water molecules. The thin film was then immersed into the solution containing electron acceptors (e.g., TCNQ, BQ, and PMDI) for 1 week. Because TCNQ, BQ, and PMDI have the specific functional groups that can coordinate with metal atom, showing the possibility to react with centered metal atom of Cu 3 (BTC) 2 . Moreover, the pore size of Cu 3 (BTC) 2 (1.17 nm) is large enough concerning the molecular diameters of electron acceptors (TCNQ: 0.92 nm, BQ: 0.54 nm, PMDI: 0.87 nm), the latter can be hosted into the pores of Cu 3 (BTC) 2 for the efficient doping. Thereby, the as-prepared doped Cu 3 (BTC) 2 thin films are denoted as TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , and PMDI@Cu 3 (BTC) 2 , respectively. The details about the synthesis process are shown in the Experimental Section in Supporting Information. As shown in Figure 1b,c, the obvious color changes from blue to green visually confirm the strong electronic interaction between Cu 3 (BTC) 2 and acceptor molecules.
The morphologies and structures of the fabricated Cu 3 (BTC) 2 , TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , and PMDI@Cu 3 (BTC) 2 thin films on Cu foils are characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) method, and thermal gravimetric analyzer (TGA). Figure 2a,b show that Cu 3 (BTC) 2 thin film consists of densely stacked nanoparticles with diameters ranging from 80 to 120 nm. After doping, the side-view SEM images of TCNQ@Cu 3 (BTC) 2 thin film indicate that the thickness is about 3 μm, and no apparent changes in the morphology were noticed (Figure 2c and Figure S1-S3, Supporting Information). The corresponding energy-dispersive spectroscopy (EDS) elemental mapping analysis implies the distinct elemental distribution. Furthermore, the homogeneous distribution of the N element suggests that electron acceptor molecules are well incorporated into Cu 3 (BTC) 2 thin film.
The XRD patterns in Figure 2d show the polycrystalline structure of doped Cu 3 (BTC) 2 in thin films. [7] Especially, the positions of (333) peak in TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , and PMDI@Cu 3 (BTC) 2 thin films all shifted to the lower degree compared with that of undoped Cu 3 (BTC) 2 thin film. This shift corresponds to the gap distance increase of (333) crystal facet, revealing the incorporation of acceptor molecules into the pores of Cu 3 (BTC) 2 . [3] Raman spectrum of TCNQ@Cu 3 (BTC) 2 discloses the peak at 2205 cm À1 , which is attributed to the nitrile stretch (C≡N) vibration of TCNQ (Figure 2e). In addition, the C¼C stretching frequency of TCNQ@Cu 3 (BTC) 2 shifts from 1447 cm À1 (TCNQ) to 1438 cm À1 and the peaks at 1392 and 1249 cm À1 indicate the strong electronic interaction between TCNQ (guest) and the available coordination sites of the Cu 2þ ions in the Cu 3 (BTC) 2 framework (host). [8] IRRAS spectra ( Figure S4, Supporting Information) also present that the C≡N stretch of TCNQ is influenced by the infiltration into the Cu 3 (BTC) 2 framework, with a shift from 2223 to 2179 cm À1  accompanied by peak broadening. This result supports the conclusion based on Raman spectroscopic studies. The Raman and IRRAS spectra of BQ@Cu 3 (BTC) 2 and PMDI@Cu 3 (BTC) 2 are presented in Figure S5 and S6, Supporting Information. The electronic interaction between TCNQ and Cu 3 (BTC) 2 was further investigated by XPS spectroscopy. As shown in Figure 2f and Figure S7, Supporting Information, the C═O peak in C 1s and -C≡N peak in N 1s were observed in TCNQ@Cu 3 (BTC) 2 , demonstrating the presence of TCNQ in the doped thin film. Moreover, the shift Cu 2p 3/2 from 933.8 to 934.7 eV was ascribed to the change of surface electronic state after the incorporation of TCNQ in the Cu 3 (BTC) 2 framework. Subsequently, the TGA curves revealed that the weight ratio of TCNQ, BQ, and PMDI in the doped Cu 3 (BTC) 2 thin films was estimated to be 17.9, 10.6, and 12.5 wt%, respectively ( Figure S8, Supporting Information). As indicated by the BET measurement (Figure 2g and Figure S9, Supporting Information), the specific surface area of Cu 3 (BTC) 2 , TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , and PMDI@Cu 3 (BTC) 2 was 1295, 655, 795, and 650 m 2 g À1 , respectively. This result demonstrates small molecules (TCNQ, BQ, and PMDI) could occupy some pores of Cu 3 (BTC) 2 and slightly decrease its specific surface area. Although the specific surface areas of doped MOFs were reduced, the utilization rate of specific surface area can be improved by enhancing the conductivity.
Encouraged by the enhanced electrical conductivity, large specific surface area, rich redox-active sites, and excellent flexibility of the doped Cu 3 (BTC) 2 thin films, [9] flexible asymmetric MSCs were assembled using TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , or PMDI@Cu 3 (BTC) 2 as the positive electrode and AC as the negative electrode in polyvinyl alcohol (PVA)/LiCl gel electrolyte ( Figure S16, Supporting Information). The corresponding devices are denoted as TCNQ-MOF-MSC, BQ-MOF-MSC, and PMDI-MOF-MSC, respectively. A benchmark MSC using the same AC anode but undoped Cu 3 (BTC) 2 cathode was also fabricated for comparison and named as MOF-MSC. The electrochemical behavior of the AC anode and the Cu 3 (BTC) 2 (TCNQ@Cu 3 (BTC) 2 , BQ@Cu 3 (BTC) 2 , or PMDI@Cu 3 (BTC) 2 ) cathode were investigated by CV in the three-electrode cell ( Figure S17, Supporting Information). Figure 4a clearly shows that the integral area of CV curve for TCNQ-MOF-MSC presents the maximum value compared with other MSCs. Furthermore, at the same current density of 2 mA cm À2 in galvanostatic chargedischarge (GCD) measurement ( Figure S18-S21, Supporting Information), TCNQ-MOF-MSC requires the most charge/ discharge duration, indicating the highest delivered capacitance. In addition, the CV curves of these devices display redox peaks associated with their pseudocapacitance from ligandbased redox activity. [6b] As shown in Figure 4b,c the areal Apparently, the improved conductivities of doped Cu 3 (BTC) 2 thin films benefited to the significant increase in the capacitances of the corresponding devices. Remarkably enough, the capacitance of TCNQ-MOF-MSC is 3.4 times higher than that of MOF-MSC and is also superior to those of the recently reported MSCs based on coordination polymer framework, [10] carbon nanotube/g-C 34 N 6 -COF, [11] graphene/conductive MOF, [12] graphene/carbon nanotube, [13] graphene/V 2 O 5 , [14] graphene/ thiophene, [15] and graphene/phosphorene [16] (0.1-50 mF cm À2 , Table S1, Supporting Information).
To further reveal the excellent performance of our fabricated devices, the electrochemical impedance spectra (EIS) were analyzed ( Figure S22, Supporting Information). At low frequency, the straight line of TCNQ-MOF-MSC with much larger slope suggested the enhanced mass transfer inside electrode material, resulting from the improved electrical conductivity. At high frequency, the much smaller semicircle of TCNQ-MOF-MSC indicated the decreased charge transfer resistance, due to the fully accessible electrode surface in the electrolyte. Moreover, TCNQ-MOF-MSC exhibited excellent cycling stability, with 94.1% of capacitance retention after 5000 charge/discharge cycles ( Figure S23, Supporting Information).
Concerning the limited mass loading of active materials, it is rational to calculate the energy and power densities based on area and volume of the electrode, other than its weight. To determine the overall performance of our devices, Ragone plots of volumetric energy density and power density are shown in Figure 4d. Notably, our TCNQ-MOF-MSC delivers an ultrahigh energy density of up to 46 mWh cm À3 at 1.67 W cm À3 , which considerably exceeds those of the state-of-the-art in-plane MSCs (0.1-10 mWh cm À3 ,  Table S1), e.g., MOF-based MSCs, [11,12] graphene-based MSCs, [15,16] conducting polymer-based MSCs, [17] Ti 3 C 2 T x MSCs, [18] and carbon-based MSCs. [19] In addition, the obtained high power density of 33.3 W cm À3 at 20 mWh cm À3 is superior to those of state-of-the-art MSCs (0.01-10 W cm À3 ) based on various electrode materials such as laser-scribed graphene, [20] MXene/graphene, [21] poly(3,4-ethylenedioxythiophene), [17a] and coordination polymer frameworks. [10] Furthermore, the high areal energy density (E A, max ¼ 13.8 μWh cm À2 ) and power density (P A, max ¼ 10 mW cm À2 ) are achieved for TCNQ-MOF-MSC, which is much better than other reported MSCs ( Figure S24, Supporting Information). On the other hand, good flexibility and variable working window are crucial for portable and wearable energy storage devices. To demonstrate the excellent flexibility of the fabricated devices, we evaluated the CV at different bending angles (Figure 4e and Figure S25, Supporting Information). No obvious changes in CV curves were observed, highlighting the great electrochemical stability. Moreover, the connection of single MSCs in series and parallel could achieve higher voltage and capacitance. For example, three serially connected TCNQ-MOF-MSCs could reach 3.0 V and light up a red LED ( Figure S26 and S27, Supporting Information). www.advancedsciencenews.com www.small-structures.com

Conclusion
In summary, we demonstrate a novel strategy for the in situ growth and doping of Cu 3 (BTC) 2 thin-film electrodes. The doping of Cu 3 (BTC) 2 thin films with electron acceptor molecules significantly improves the electrical conductivity. The mechanism behind is elucidated by the electrochemical investigations and supported by DFT calculations. As a result, flexible MSC assembled with TCNQ@Cu 3 (BTC) 2 as the cathode and AC as the anode delivers a remarkable areal capacitance of 95.1 mF cm À2 , superior to those of the reported MSCs (0.1-50 mF cm À2 ). Our work sheds light on the molecular doping strategy as an efficient approach for tailoring the electronic properties of organic framework materials for energy storage and conversion applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.