(Digital Presentation) Conductive Phthalocyanine-Based Metal-Organic Frameworks for Flexible Energy Storage Application
Authors: Jia Hongxing , Lu Shun , Gu Zhengrong 
Published: September 2023

Recently, two-dimensional conductive MOFs (2D c-MOFs) with improved intrinsic electrical conductivity have been synthesized and employed as electrocatalysts, gas sensors, supercapacitors, and so on. In previous literature, 2D c-MOFs were usually synthesized initially as the powder/film through solvothermal approach, after which a transfer and/or reshaping process is necessary for the electrode fabrication [1-3]. In most cases, the electrodes of 2D c-MOFs were prepared with conductive additives and binders through the slurry coating method [2]. In doing so, their superior intrinsic conductivity over traditional MOFs was obscured. Moreover, the use of additives might reduce the effective surface area and negatively affect long-term cycling performance. To avoid the use of additives, 2D c-MOFs could be pressed into self-supported pellets. Unfortunately, due to dense packing under high pressure, many MOF particles inside pellets are not able to capture ions, leading to the decrease in capacitance. Until now, restrained by flawed electrode fabrication approaches, the potential of 2D c-MOFs in supercapacitors was still far from fully exploited.
The electrochemical deposition technique is a promising approach to fabricating 2D c-MOFs as electrodes, with significant advantages over above methods [3]. On the one hand, electrochemical deposition allows in situ growth of 2D c-MOFs on substrates, thus reducing the cost and simplifying the process. On the other hand, binders, conductive additives, and compacting processes are no longer necessary, which will help improve the capacitor performance [4]. Moreover, the electrochemical deposition process could be conducted at mild conditions and all parameters could be precisely controlled, which makes it a mild, facile method with good reproducibility. To date, there is still no study to prepare 2D c-MOFs through direct electrochemical deposition.
In 2018, we reported the first synthesis of phthalocyanine-based 2D c-MOF nanosheets (NiPc-MOF) and its outstanding performance toward water oxidation [3]. Owing to the high electrical conductivity (~0.2 S cm-1) and large surface area (~593 m2 g-1), NiPc-MOF is also considered as a promising electrode material for supercapacitors. In this work, NiPc-MOF was grown in situ on nickel foam (NF) via the anodic electrodeposition (AED) approach (abbreviated as NiPc-MOFAED or NiPc-MOFAED@NF, Fig. 1a-d). As far as we know, the AED approach has never been used for the synthesis of 2D c-MOFs. Remarkably, the as-prepared NiPc-MOFAED@NF can be directly utilized as electrodes for flexible supercapacitors, which has also been well explored in this work.
The simplified electrode fabrication process, that does not involve binders and conductive additives, would significantly reduce the cost and will have enormous potential for the applications of NiPc-MOF in energy storage. The outstanding performance of the NiPc-MOFAED@NF supercapacitor (Fig. 1e-f), including high specific areal capacitances (11.5 mF cm-2 in aqueous electrolyte and 22.1 mF cm-2 in organic electrolyte), preeminent areal power density (1.35 mW cm-2 at 1 mA cm-2, organic system) and energy density (22.4 μWh cm-1 at 0.1 mA cm-2, organic system), robust cycling stability as well as prominent mechanical ﬂexibility, were further disclosed by electrochemical measurements. This present work not only reported an advanced phthalocyanine-based MOF material for a high-performance supercapacitor, but also opens up a novel avenue for the in situ growth of the 2D c-MOF.

[Figure insert]

Figure 1. (a) Schematic illustration of NiPc-NiN4-MOF; (b) XRD pattern, (c) High-resolution TEM image, and (d) EDX mappings of NiPc-NiN4-MOF.
NiPc-MOFAED@NF-based supercapacitor in aqueous system (PVA/LiClO4). (a) GCD curves at various current densities of 0.04-0.4 mA cm−2. (b) CV curves of supercapacitor at bending angles of 0°, 30°, 60°, and 90°. Scan rate: 20 mV s−1. (c) Photograph of a red LED powered by the three series-connected supercapacitors; NiPc-MOFAED@NF-based supercapacitor in organic system (TEABF4/Acetonitrile). (d) GCD curves at various current densities of 0.1-1 mA cm−2. (e) CV curves of supercapacitor at bending angles of 0°, 30°, 60°, and 90°. Scan rate: 100 mV s−1. (f) Photograph of a green LED powered by one supercapacitor. (Ref: Journal of Power Sources, 2022, 526: 231163) Copyright 2022 Elsevier.

Reference:

[1] Jia, Hongxing, et al. "In situ anodic electrodeposition of two-dimensional conductive metal-organic framework@nickel foam for high-performance flexible supercapacitor." Journal of Power Sources 526 (2022): 231163.
[2] Lu, Shun, et al. "Two-dimensional conductive phthalocyanine-based metal–organic frameworks for electrochemical nitrite sensing." RSC Advances 11.8 (2021): 4472-4477.
[3] Jia, Hongxing, et al. "A novel two-dimensional nickel phthalocyanine-based metal–organic framework for highly efficient water oxidation catalysis." Journal of Materials Chemistry A 6.3 (2018): 1188-1195.
[4] Yan, Caihong, et al. "Hydrothermal synthesis of vanadium doped nickel sulfide nanoflower for high-performance supercapacitor." Journal of Alloys and Compounds 928 (2022): 167189.

Figure 1