Metal-Organic Frameworks Derived Electrochemical Devices
The past decades have witnessed the ever-growing research and development efforts in the field of electrochemical devices. Metal compounds and composite materials play a critical role in these devices. In order to increase the electrochemical active sites in these devices, nano-scale materials are widely explored. However, the electronic conductivity, aggregation and degradation issues of these materials impede their wide applications. Among these materials, metal -carbon composite materials are one of the most promising materials for next-generation electrochemical devices.
Metal-organic frameworks (MOFs), constructed from metal ions or clusters bridged by organic ligands, have been considered as ideal self-sacrificial templates for metal-carbon nanomaterials due to their highly ordered cavities, open channels and rational composition and structure These unique characteristics of MOFs allow them to serve as both template and precursor materials (metal and carbon). The performance of MOF derived electrochemical devices are determined by two aspects: the properties of MOFs (tuned by metal ions and linkers) and the preparation process of electrode assemblies (controlled by the synthesis method and coating techniques). On the one hand, in-situ of synthesis MOF-derived materials is highly recommended for electrochemical (or photoelectric) devices not only because of the enhancement of reactivity and recycling but also to avoid binder issues. On the other hand, the huge number of different MOFs offers extensive possibilities for MOF-derived nanomaterials but at the expense of endless lab work. For the sake of minimizing unnecessary lab work and to avoid both the consumption of time and resources, the critical problem is to gain rational and comprehensive insight into the influence of MOF units (i.e. the organic ligands, metal units and interactions between these two parts) on key parameters of the electrochemical devices.
Herein, we proposed a novel method for in situ synthesis of metal oxide- carbon materials on 3D foam electrodes. For the first case, 2D-layered Ni-Co mixed metal-organic frameworks (MOFs) were deposited directly on nickel foams by anodic electrodeposition. Subsequent pyrolysis and activation procedure lead to the formation of carbon–metal oxide composite electrodes. After pyrolysis and activation, the layer-by-layer microstructures of 2D Co-Ni mixed MOFs can be replicated. The resulting porous 2D-layered carbon–metal oxide (2D-CMO) composite electrode was used as the supercapacitor electrode. This electrode showed superior performance with low resistance, high capacitance (2098 mF cm−2 at a current density of 1 mA cm−2), good long-term electrochemical stability and excellent rate performance (93% retention of the capacitance from 1 to 20 mA cm−2) with a high mass loading (13 mg cm−2). In order to demonstrate the versatility of the proposed method for the preparation of binder-free carbon–metal oxide composite electrodes for electrochemical devices, HKUST-1 was synthesized on different kinds of copper substrates by anodic electrodeposition using the same method. After pyrolysis, carbon-Cu composite materials can be grown on the Cu substrate. The resulting Cu@porous carbon matrix electrode was tested as a non-enzymatic electrochemical glucose sensor electrode. The Cu@porous carbon matrix on Cu foam electrode displayed much better electrochemical catalytic performance towards glucose than flat copper plate, Cu foam and anodizing Cu foam electrodes. The Cu@porous carbon matrix electrode displayed ultrahigh sensitivity (10 mA cm−2 mM−1), low detection limit (0.6 μM), short response time (less than 2 s) and good stability. All these results indicate that this strategy is promising way for the preparation of MOF-derived electrodes.
Furthermore, in order to change the properties of MOF-derived electrochemical devices, the simplest way is to change the type of MOFs. The unclear influence of MOF units on the resulting MOF derived materials hampers the scientific and industrial progress of MOF-derived nanomaterials. Hence, in order to minimize lab work and the consumption of experimental resources, it is important to gain rational and comprehensive insight into the influence of MOF units (i.e. organic ligands, metal units and the interactions between these two parts) on key parameters of certain devices via experiments and computations. In this thesis, a series of benzene-1,3,5-tricarboxylate linker based metal organic frameworks (MOFs) were used as self-sacrificial templates and tunable platform for the preparation of M-N-C catalysts. Changing the pillars between the 2D layers and the nature of the metal ions in the pristine MOFs significantly influenced the structure, chemical composition and catalytic activity of the resulting M-N-C catalysts for oxygen reduction reaction (ORR). Furthermore, the influence of the MOF units on the catalyst performance, the role of the metals in the M-N-C catalysts and the primary catalytically active sites for ORR were explored by a combination of density functional theory (DFT), in-depth structural and chemical/elemental characterizations, and electrochemical studies. Among the prepared catalysts, Co-BTC-bipy-700 exhibited the highest electrocatalytic activity for ORR, which showed a larger limiting current density and similar half-wave potentials with less catalyst degradation and much higher methanol tolerance than commercial Pt/C catalyst. Meanwhile, as a bifunctional electrocatalyst, Co-BTC-bipy-700 catalyst was also employed for oxygen evolution reaction (OER) and demonstrated a lower overpotential (lowered by 140 mV at a current density of 10 mA cm−2 than IrO2) and better durability than IrO2. Furthermore, in terms of device performance, the Zn-air battery enabled by Co-BTC-bipy-700 catalyst reached a maximum specific energy as high as 1010 Wh kg−1, which is 76.5% of the theoretical value (1320 Wh kg−1), and demonstrated higher discharging potential and lower charging potential than that based on the Pt/C catalyst. Importantly, the presented strategy for tailor-made M-N-C catalysts by controlling the synthesis of the pristine MOFs could offer guide lines for the future design of M-N-C catalysts family.
Based on the above MOFs, a new series of bimetallic MOFs were successfully synthesized. These bimetallic MOFs retain the same crystal structure when the mole fraction (based on metal) of the two metals changes from 0 to 1 and both metal ions occupy uniform random nodal positions. The incorporation of a second metal cation has a large influence on the intrinsic properties (e.g. thermal stabilities and band gap) of the MOFs. Furthermore, these bimetallic MOFs were used as a self-sacriﬁcial template to prepare bimetal oxide catalysts for OER. After pyrolysis, a porous and hierarchical honeycomb-like structure with a carbon network covering the (bi)metal oxides is formed. Among all the bimetallic MOF-derived catalysts, CoNi@C-1 showed the best performance for OER with the lowest Tafel slopes (55.6 mV dec−1) and overpotential (335 mV on GCE and 276 mV on Ni foam) at a current density of 10 mA cm−2, which is higher than state-of-the-art Co-Ni mixed oxide catalysts derived from MOFs for OER. Our results indicate that the incorporation of a second metal ion is a promising strategy to tailor the properties of MOFs.
In conclusion, in order to fully exploit the potentials of MOFs in electrochemical devices, it is critical to optimize properties of pristine MOFs and film forming technology. Both of these two aspects are considered in this thesis.