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Abstract:Owing to their broad range of redox potential, quinones/hydroquinones can be utilized for energy storage in redox flow batteries. In terms of stability, organic catholytes are more challenging than anolytes. The two-electron transfer feature adds value when building all-quinone flow battery systems. However, the dimerization of quinones/hydroquinones usually makes it difficult to achieve a full two-electron transfer in practical redox flow battery applications. In this work, we designed and synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring to probe molecular stability upon battery cycling. The redox potential of the four molecules were investigated, followed by long-term stability tests using different supporting electrolytes and cell cycling methods in a symmetric flow cell. The derivative with two unoccupied ortho positions was found highly unstable, the cell of which exhibited a capacity decay rate of 50% per day. Fully substituted hydroquinones turned out to be more stable. In particular, 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol (asym-O-5) displayed a capacity decay of only 0.45%/day with four-week potentiostatic cycling at 0.1 M in 1 M H3PO4. In addition, the three fully substituted hydroquinones displayed good accessible capacity of over 82%, much higher than those of conventional quinone derivatives.Keywords: hydroquinone; catholyte; stability; symmetric cell study; redox flow batteries; energy storage
acs organic chemistry exam study guide.26
Highly complex nucleation and crystallization in hard tissue involves the coordinated action of ions and/or molecules that can produce different organic and inorganic composite biomaterials. In addition, the healing of bone defects is also affected by the dynamic conditions of ions and nutrients in the bone regeneration process. Inorganics in the human body, especially calcium- and/or phosphorus-based materials, play an important role in hard tissues. Inorganic crystal growth is important for treating or remodeling the bone matrix. Biomaterials used in bone tissue regeneration require expertise in various fields of the scientific community. Chemical knowledge is indispensable for interpreting the relationship between biological factors and their formation. In addition, sources of energy for the nucleation and crystallization processes of such chemical bonds and minerals that make up the bone tissue must be considered. However, the exact mechanism for this process has not yet been elucidated. Therefore, a convergence of broader scientific fields such as chemistry, materials, and biology is urgently needed to induce a distinct bone tissue regeneration mechanism.
From the perspective of bone tissue research, scholars working in different fields with a single target have different approaches to experimentation and analysis. The control of biological and material functions of hard tissues such as bones and teeth, through a combination of cells, materials, and biological factors has been considered a promising approach. These technologies regenerate and repair hard tissues. Although there are numerous research papers and reviews of these studies, there are only few examples of scientific interpretations that include both clinical translation and practical application. Their practical application in the field is not easy [3]. For this reason, it is difficult to conclude form the experimental results obtained through the analysis of different fields. For example, in the study of mineralization, some scientists have analyzed the crystal structure at each stage of nucleation, the physical and chemical environment of the human body, and the materials required for the mineralization process.
In the early days, studies focused on the chemical properties of synthesized HAp. In recent years, studies have been conducted on the actual HAp growth mechanism that constitutes the human body. As we shall see later, the inorganic CaPs in the actual bone tissue are indeed placed in association with organic matter in a unique crystalline state, not in a bulk state; therefore, the study of the size and shape of the CaP crystals during growth is essential. Over the past few decades, much effort has been made to control the size and shape of CaP crystals, including developing new strategies and modifying existing methods [26, 34].
Figures 6b and c schematically show the mechanism,consisting of two steps of crystal formation and a path to the free energychange ΔG according to twofeasible models of the two-phase nucleation mechanism [82]. Inorganics in the human body are composed of specific ions; therefore, it isvery useful to deduce the mechanism based on existing theory. Vekilov et al. explained nuclei formation in a dense liquid phase, asshown in Fig. 6b [82, 90, 91]. This mechanism involves theformation of dense liquid clusters, and crystal nuclei can form inside theseclusters. Figure 6c shows the top curve when the dense liquid was unstable and ΔGDLº> GSS (ΔGDLº: standard free energy of theformation of the dense liquid. Gss: free energy of the supersaturated solution). If thedense liquid is stabilized by introducing an external interfaceΔGDLºΔG1* is a barrier to forming dense liquid clusters and ΔG2* isa barrier to crystalline nucleation inside the dense liquid. Wolf et al. demonstratedthe mechanism of crystal nucleation from a dense liquid precursor in aCaCO3 system [92]. They showed floating CaCO3solution droplets between a piezoelectric vibrator that produced an acousticwave and a concentrically adjusted sonic reflector. Calcium carbonate ishomogeneously formed inan amorphous liquid-like state at neutral pH without any stabilizing polymer oradditive. This stability in an amorphousliquid-like state at neutral pH can be closely related to the various carbonategroups present in the process,such as carbonates, bicarbonates, and non-dissociated carbonates. The formation of an amorphous liquid-phase mineral precursor wasconfirmed to be a hallmark of the true homogeneous formation of CaCO3itself, and the resulting primary particles also confirmed that it actsin the second stage as a template for the crystallization of calcite. Ultrasonic trapping is an excellent methodfor the real-time analysis of nucleation, crystal growth, and phase separationprocesses with minimal disruption and artifacts caused by solid-phaseboundaries. These resultsconcluded that acoustic levitation provides a reliable condition for studyinguniform precipitation reactions.
Another example of successful biomimetic mineralization of analogs of materials is the FAp-gelatin nanocomposite system obtained by double diffusion [120]. This study was performed based on atomistic simulations of the design of crystallized FAp structures by binding the corresponding ions to the triple helix of collagen molecules [121, 122]. Ca3F-triangles are preferably oriented in a plane perpendicular to the long axis of the triple-helical protein [121]. Fluorapatite nanoplatelets covered collagen fibrils in a mosaic arrangement, where the crystallographic c-axis of FAp was parallel to the long axis of the fibrils (Fig. 9c), similar to the bone structure.
Polyaspartic acid, which has amino acid domains, and is particularly negatively charged, has been applied in many other studies with objective confidence in the generation of precursors, which is the first step in the stable growth of inorganic crystals on the collagen matrix. Niu et al. established a new model for mineralizing collagen fibers in the presence of pAsp and other polyelectrolytes. Electrostatic attraction influences polyelectrolyte-directed intrafibrillar mineralization using polycation- and polyanion-directed intrafibrillar mineralization (Fig. 10b) [132]. Xu et al. conducted a theoretical calculation and simulation to improve the clarity reality of the sub-nanoscale nucleation mechanism of CaP in the collagen matrix in the mineralization of skeletal tissue [133]. These studies also analyzed mineral deposition and mineralization using pAsp and Glu in the computational process, providing atomic-level insight into the nucleation mechanism of inorganic crystals in the collagen matrix. Shao et al. experimentally demonstrated that citric acid molecules significantly reduced the interfacial energy between the collagen matrix and the CaP precursor and enhanced the wetting effect in the initial mineralization step, sequentially promoting the formation of CaP in Col-I fibrils (Fig. 10c) [134]. This study also provided results using pAsp as a basic additive in the CaP precursor formation process, which is the initial stage of mineralization after Col-I self-assembly, and then using citric acid as a variable.
The hierarchical assembly of organics and inorganics in the bone tissue is implemented bottom-up through interactions between cells and the ECM during growth, development, and maintenance [19]. However, such a hierarchical bone structure has been approached substantially top-down manner [168]. As expected, this approach can be attributed to the scale of the analytical methodology. Thus, with the development of analytical techniques, it is possible to verify up to the atomic level; simultaneously, it shows an excellent ability to analyze the crystalline direction and shape. The radical development of this analytical technique has led to surprising results for bone structure, allowing researchers to study bone growth mechanisms more objectively. Although the mechanisms of organics (collagen cross-linking into a continuous framework) and inorganics (crystallite aggregation) are different, the results of these studies converge to provide continuity of organic and inorganic components of bone tissue. Bone morphology has been previously demonstrated by deproteinization or demineralization in the hydrated state on several scales [169]. 2ff7e9595c
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