Why conductivity is not always king - physical properties governing the capacitance of 2D metal-organic framework-based EDLC supercapacitor electrodes: a Ni3(HITP)2 case study

From 0D to 2D: microwave-assisted synthesis of electrically conductive metal-organic frameworks with controlled morphologies

Morphology control of electrically conductive metal-organic frameworks (EC-MOFs) can be a powerful means to tune their surface area and carrier transport pathways, particularly beneficial for energy conversion and storage. However, controlling EC-MOFs' morphology is underexplored due to the uncontrollable crystal nucleation and rapid growth kinetics. This work introduces a microwave-assisted strategy to readily synthesize Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with controlled morphologies. We controlled solvent compositions to facilitate particles' directional growth to 1D and 2D crystals. Meanwhile, we found that ultrasonication can manipulate crystal seeding, yielding 0D spherical Cu-HHTP crystals. Electronic conductivity measurements suggest that the isotropic nature of the 0D crystals allows a conductivity of 7.34 × 10-1 S cm-1, much higher than 1D and 2D counterparts. Additionally, the controlled 0D morphology enhanced the material's capacitance and effective surface area and significantly improved its photocurrent response. These findings underscore the pivotal impact of controlled morphology in optimizing EC-MOFs' physicochemical properties.

Controlling the Spatiotemporal Self-Organization of Stimuli-Responsive Nanocrystals under Out-of-Equilibrium Conditions

Self-organization under out-of-equilibrium conditions is ubiquitous in natural systems for the generation of hierarchical solid-state patterns of complex structures with intricate properties. Efforts in applying this strategy to synthetic materials that mimic biological function have resulted in remarkable demonstrations of programmable self-healing and adaptive materials. However, the extension of these efforts to multifunctional stimuli-responsive solid-state materials across defined spatial distributions remains an unrealized technological opportunity. This paper describes the use of a nonequilibrium reaction-diffusion process to achieve the synthesis of a multifunctional stimuli-responsive electrically conductive metal-organic framework (cMOF) in a gelled medium with control over particle size and spatial periodicity on a macroscopic scale. Upon integration into chemiresistive devices, the resulting cMOF particles exhibit a size-dependent response toward hydrogen sulfide gas, as determined by their distinct surface-to-volume ratio, porosity, unique synthesis methodology, and unusual microcrystallite morphology compared to their counterparts obtained through bulk solution phase synthesis. Taken altogether, these achievements pave the way toward gaining access to functional nanomaterials with well-defined chemical composition, dimensions, and precisely tailored functions using far-from-equilibrium approaches.

Mechanistic Insight into the Formation and Deposition of Conductive, Layered Metal-Organic Framework Nanocrystals

This paper describes the use of the layered conductive metal-organic framework (MOF) (nickel)3-(hexahydroxytriphenylene)2 [Ni3(HHTP)2] as a model system for understanding the process of self-assembly within this class of materials. We confirm and quantify experimentally the role of the oxidant in the synthetic process. Monitoring the deposition of Ni3(HHTP)2 with in situ infrared spectroscopy revealed that MOF formation is characterized by an initial induction period, followed by linear growth with respect to time. The presence and identity of oxidizing agents is critical for the coordination-driven self-assembly of these materials and impacts both the length of the induction period and the observed rate of MOF growth. A large excess of hydrogen peroxide results in a 2× increase in the observed deposition rate (9.6 ± 6.8 × 10-4 vs 5.0 ± 2.8 × 10-4 min-1) over standard reaction conditions, but leads to the formation of large, irregularly shaped particles. Slower deposition rates in the presence of oxygen favor the formation of uniformly sized nanorods (98 ± 38 × 25 ± 6 nm). These quantitative insights into the mechanism of HHTP-based MOF formation provide valuable information about the fundamental aspects of coordination and polymerization that are critical for nanoscale crystal engineering of structure-property relationships in this class of materials.

Revealing Ion Adsorption and Charging Mechanisms in Layered Metal-Organic Framework Supercapacitors with Solid-State Nuclear Magnetic Resonance

Conductive layered metal-organic frameworks (MOFs) have demonstrated promising electrochemical performances as supercapacitor electrode materials. The well-defined chemical structures of these crystalline porous electrodes facilitate structure-performance studies; however, there is a fundamental lack in the molecular-level understanding of charge storage mechanisms in conductive layered MOFs. To address this, we employ solid-state nuclear magnetic resonance (NMR) spectroscopy to study ion adsorption in nickel 2,3,6,7,10,11-hexaiminotriphenylene, Ni3(HITP)2. In this system, we find that separate resonances can be observed for the MOF's in-pore and ex-pore ions. The chemical shift of in-pore electrolyte is found to be dominated by specific chemical interactions with the MOF functional groups, with this result supported by quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT) calculations. Quantification of the electrolyte environments by NMR was also found to provide a proxy for electrochemical performance, which could facilitate the rapid screening of synthesized MOF samples. Finally, the charge storage mechanism was explored using a combination of ex-situ NMR and operando electrochemical quartz crystal microbalance (EQCM) experiments. These measurements revealed that cations are the dominant contributors to charge storage in Ni3(HITP)2, with anions contributing only a minor contribution to the charge storage. Overall, this work establishes the methods for studying MOF-electrolyte interactions via NMR spectroscopy. Understanding how these interactions influence the charging storage mechanism will aid the design of MOF-electrolyte combinations to optimize the performance of supercapacitors, as well as other electrochemical devices including electrocatalysts and sensors.

Understanding Electrolyte Ion Size Effects on the Performance of Conducting Metal-Organic Framework Supercapacitors

Layered metal-organic frameworks (MOFs) have emerged as promising materials for next-generation supercapacitors. Understanding how and why electrolyte ion size impacts electrochemical performance is crucial for developing improved MOF-based devices. To address this, we investigate the energy storage performance of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with a series of 1 M tetraalkylammonium tetrafluoroborate (TAABF4) electrolytes with different cation sizes. Three-electrode experiments show that Cu3(HHTP)2 exhibits an asymmetric charging response with all ion sizes, with higher energy storage upon positive charging and a greater charging asymmetry with larger TAA+ cations. The results further show that smaller TAA+ cations demonstrate superior capacitive performances upon both positive and negative charging compared to larger TAA+ cations. To gain further insights, electrochemical quartz crystal microbalance measurements were performed to probe ion electrosorption during charging and discharging. These reveal that Cu3(HHTP)2 has a cation-dominated charging mechanism, but interestingly indicate that the solvent also participates in the charging process with larger cations. Overall, the results of this study suggest that larger TAA+ cations saturate the pores of the Cu3(HHTP)2-based electrodes. This leads to more asymmetric charging behavior and forces solvent molecules to play a role in the charge storage mechanism. These findings significantly enhance our understanding of ion electrosorption in layered MOFs, and they will guide the design of improved MOF-based supercapacitors.

A redox active rod coordination polymer from tetrakis(4-carboxylic acid biphenyl)tetrathiafulvalene

An enlarged version of the ubiquitous tetrathiafulvalene-tetrabenzoic acid is described, with 4,4'-biphenyl moieties as spacers between the coordination moieties and the electroactive core. The obtained rectangular ligand has a 14 × 22 Å2 size and is combined with Zn(II) under solvothermal conditions to yield a coordination polymer endowed with large cavities of ca. 15 × 11 Å2/10 × 10 Å2. The topology of the material is discussed in detail using the Points of Extension and Metals (PE&M) or the Straight-rod (STR) representation, and the sqc1121 or tfo topological type of the structure is observed, respectively. Its stability towards solvent removal and electrical properties are discussed. The material does not present any permanent porosity upon desolvation according to nitrogen sorption measurements at 77 K. Nevertheless, a significant increase in conductivity is observed on compressed pellets of the material upon post-synthetic oxidation with iodine. Raman spectroscopy combined with density functional theory (DFT) calculations has been used to characterize the oxidation state of tetrakis(4-carboxylic acid biphenyl)tetrathiafulvalene for coordination polymers.

Current perspectives, challenges, and future directions in the electrochemical detection of microplastics

Microplastics (5 μm) are a developing threat that contaminate every environmental compartment. The detection of these contaminants is undoubtedly an important topic of study because of their high potential to cause harm to ecosystems. For many years, scientists have been assiduously striving to surmount the obstacle of detection restrictions and minimize the likelihood of receiving results that are either false positives or false negatives. This study covers the current state of electrochemical sensing technology as well as its application as a low-cost analytical platform for the detection and characterization of novel contaminants. Examples of detection mechanisms, electrode modification procedures, device configuration, and performance are given to show how successful these approaches are for monitoring microplastics in the environment. Additionally included are the recent developments in nanoimpact techniques. Compared to electrochemical methods for microplastic remediation, the use of electrochemical sensors for microplastic detection has received very little attention. With an overview of microplastic electrochemical sensors, this review emphasizes the promise of existing electrochemical remediation platforms toward sensor design and development. In order to enhance the monitoring of these substances, a critical assessment of the requirements for future research, challenges associated with detection, and opportunities is provided. In addition to-or instead of-the now-in-use laboratory-based analytical equipment, these technologies can be utilized to support extensive research and manage issues pertaining to microplastics in the environment and other matrices.

Experimental manifestation of redox-conductivity in metal-organic frameworks and its implication for semiconductor/insulator switching

Electric conductivity in metal-organic frameworks (MOFs) follows either a band-like or a redox-hopping charge transport mechanism. While conductivity by the band-like mechanism is theoretically and experimentally well established, the field has struggled to experimentally demonstrate redox conductivity that is promoted by the electron hopping mechanism. Such redox conductivity is predicted to maximize at the mid-point potential of the redox-active units in the MOF, and decline rapidly when deviating from this situation. Herein, we present direct experimental evidence for redox conductivity in fluorine-doped tin oxide surface-grown thin films of Zn(pyrazol-NDI) (pyrazol-NDI = 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]naphthalenediimide). Following Nernstian behavior, the proportion of reduced and oxidized NDI linkers can be adjusted by the applied potential. Through a series of conductivity measurements, it is demonstrated that the MOF exhibits minimal electric resistance at the mid-point potentials of the NDI linker, and conductivity is enhanced by more than 10000-fold compared to that of either the neutral or completely reduced films. The generality of redox conductivity is demonstrated in MOFs with different linkers and secondary building units, and its implication for applications that require switching between insulating and semiconducting regimes is discussed.

Enhancing the energy storage performances of metal-organic frameworks by controlling microstructure

Metal-organic frameworks (MOFs) are among the most promising materials for next-generation energy storage systems. However, the impact of particle morphology on the energy storage performances of these frameworks is poorly understood. To address this, here we use coordination modulation to synthesise three samples of the conductive MOF Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with distinct microstructures. Supercapacitors assembled with these samples conclusively demonstrate that sample microstructure and particle morphology have a significant impact on the energy storage performances of MOFs. Samples with 'flake-like' particles, with a pore network comprised of many short pores, display superior capacitive performances than samples with either 'rod-like' or strongly agglomerated particles. The results of this study provide a target microstructure for conductive MOFs for energy storage applications.

Thousand-fold increase in O2 electroreduction rates with conductive MOFs

Molecular materials must deliver high current densities to be competitive with traditional heterogeneous catalysts. Despite their high density of active sites, it has been unclear why the reported O₂ reduction reaction (ORR) activity of molecularly defined conductive metal-organic frameworks (MOFs) have been very low: ca. -1 mA cm^-2. Here, we use a combination of gas diffusion electrolyses and nanoelectrochemical measurements to lift multiscale O₂ transport limitations and show that the intrinsic electrocatalytic ORR activity of a model 2D conductive MOF, Ni₃(HITP)₂, has been underestimated by at least 3 orders of magnitude. When it is supported on a gas diffusion electrode (GDE), Ni₃(HITP)₂ can deliver ORR activities >-150 mA cm^-2 and gravimetric H₂O₂ electrosynthesis rates exceeding or on par with those of prior heterogeneous electrocatalysts. Enforcing the fastest accessible mass transport rates using scanning electrochemical cell microscopy revealed that Ni₃(HITP)₂ is capable of ORR current densities exceeding -1200 mA cm^-2 and at least another 130-fold higher ORR mass activity than has been observed in GDEs. Our results directly implicate precise control over multiscale mass transport to achieve high-current-density electrocatalysis in molecular materials.

Oxidative control over the morphology of Cu3(HHTP)2, a 2D conductive metal–organic framework

The morphology of electrically conductive metal–organic frameworks strongly impacts their performance in applications such as energy storage and electrochemical sensing. However, identifying the appropriate conditions needed to achieve a specific nanocrystal size and shape can be a time-consuming, empirical process. Here we show how partial ligand oxidation dictates the morphology of Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), a prototypical 2D conductive metal–organic framework. Using organic quinones as the chemical oxidant, we demonstrate that partial oxidation of the ligand prior to metal binding alters the nanocrystal aspect ratio by over 60-fold. Systematically varying the extent of initial ligand oxidation leads to distinct rod, block, and flake-like morphologies. These results represent an important advance in the rational control of Cu₃(HHTP)₂ morphology and motivate future studies into how ligand oxidation impacts the nucleation and growth of 2D conductive metal–organic frameworks.

The morphology of a copper-based 2D conductive metal–organic framework can be tuned via controlled ligand oxidation. Using quinone oxidants, we show how partial ligand oxidation prior to metal binding alters the nanocrystal aspect ratio by >60-fold.