<p>  </p> <p>Nowadays, clean and renewable energy sources like wind and solar power have been rapidly growing for the goal of phasing out traditional fossil fuels, achieving carbon neutrality, and realizing sustainable development. Long-duration and large-scale energy storage is needed to address the intermittent nature of these sources. Especially, redox flow battery (RFB) is an attractive energy storage device for large scale applications because of its high scalability, design flexibility, and intrinsic safety. The all vanadium redox flow battery stands for the state-of-the-art system, but the high vanadium cost and limited energy density are among the limiting factors for wide commercialization. Therefore, it is necessary to develop new RFB materials that are cost-effective and highly soluble. Organic redox-active molecules (redoxmers) hold great potential to satisfy these requirements due to structural diversity, tunable chemical and electrochemical properties, and earth-abundant sources. With rational structural design, organic redoxmers can show favorable properties such as high solubility, suitable redox potential, and good chemical stability. However, current efforts are mainly on the development of anolyte redoxmers, e.g. phenazine, anthraquinone and viologen. Only limited types of catholyte candidates have been reported such as ferrocene and TEMPO. The major reason for such slow-paced progress is the limited chemical stability of these catholyte redoxmers. To bridge this critical gap, my efforts are focused mainly on the design and development of promising catholyte redoxmers for both aqueous organic (AORFBs) and non-aqueous organic redox flow batteries (NRFBs).</p> <p>Phenoxazine functionalized with a hydrophilic tetraalkylammonium group demonstrates good water solubility and suitable redox potential. Cyclic voltammograms (CV) and flow cell testing were used to evaluate the electrochemical properties and battery performance, respectively. Besides, the battery fading mechanism was systematically investigated by CV, liquid chromatography mass spectra (LC-MS) and electron paramagnetic resonance (EPR) spectroscopy. The redoxmer decomposition mechanism analysis will benefit future redoxmer development by guiding the molecular design of more stable structure candidates. </p> <p>A structural design strategy for the development of novel TMPD-based (tetramethyl-<em>p</em>-phenylenediamine) catholyte redoxmers for NORFBs is presented. Two categories of functional groups, including oligo(ethylene glycol) (EG) either chains and phenyl rings, were incorporated into the TMPD core to improve solubility and stability in non-aqueous electrolytes, respectively. EPR characterization and bulk electrolyte (BE) analysis were carried out to evaluate the redoxmers stability. In addition, DFT studies were conducted to understand the impacts of functional groups on redox potential and chemical stability. The present work demonstrates the feasibility of constructing promising redoxmers from TMPD and provides insights into molecular designing of catholytes to achieve high solubility and excellent stability for non-aqueous redox flow batteries.</p>

<p dir="ltr">As the demand for energy rises and the threat of climate change looms, the need for clean, reliable, and affordable energy solutions like renewable energies has been more crucial. Energy storage systems (ESSs) are indispensable in addressing the intermittent nature of renewable energies and optimizing grid efficiency. Redox flow batteries (RFBs), thanks to their scalability, independent energy and power, swift response time, and minimal environmental impact, are a particularly promising ESS technology for long-duration storage applications. Despite the technological maturity of aqueous RFBs, nonaqueous organic RFBs (NAORFBs) are a prospective solution due to their wider operational voltage, potentially higher energy density, and larger pool of redox-active materials. However, the current state-of-the-art NAORFBs face challenges due to the lack of suitable organic redox-active materials (ORMs).</p><p dir="ltr">Despite the development of new materials, how their variables influence the total system cost of RFBs remains an unsolved challenge. With this regard, we established a techno-economic (TE) model to calculate the capital cost of nonaqueous hybrid RFBs (NAHRFBs). Prior to this work, NAHRFBs, which employs lithium metal as the anode, were regarded as an RFB system with the highest energy density. However, the correlation between their features and the system cost remained unclear, leaving a research gap for new ORMs. In our model, we selected a state-of-the-art NAHRFB system where 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) serves as the catholyte and lithium metal functions as the anode. Thereafter, sensitivity analyses identified several key factors that determine the system cost, including operational current density, area-specific resistance, cell voltage, electrolyte composition, and both the price and equivalent molecular weight of the ORM. To enhance the cost-competitiveness of current NAHRFBs, it is advised to increase the current density by 10 times and modulate the ORM-related characteristics. The virtually optimized condition manifests that the system cost of NAHRFB can meet the long-term cost target set by the U. S. Department of Energy.</p><p dir="ltr">Informed by the TE model, we discovered that elevating the oxidation potential of catholyte ORMs is instrumental in reducing the system cost of RFBs. To explore this possibility, we incorporated fluorine atoms, a potent electron-withdrawing group (EWG), into a dimethoxybenzene (DMB) derivative, yielding a new ORM (ANL-C46) with an oxidation potential enhanced by ~0.41 V. Surprisingly, ANL-C46 demonstrated superior kinetic and electrochemical stability compared to its parent molecule, as indicated by electron paramagnetic resonance (EPR) study and bulk electrolysis. In particular, the cycling performance of ANL-46 during the bulk electrolysis outperformed most reported high-potential (> 1 V vs. Ag/Ag⁺) ORMs. Density functional theory (DFT) calculations reveals that the introduced fluorine substituents suppress the typical side reaction pathways of the DMB series. These findings offer valuable insights into molecular engineering strategies that concurrently improve multiple desired ORM properties.</p><p dir="ltr">The stability of ORMs is critical for ensuring the extended lifetime of RFBs. We conducted a systematic exploration of the conjugation effect, which potentially stabilizes the ORMs by facilitating a more homogeneous distribution of delocalized charges. This was applied to tailor the electrochemical and physical properties of several DMB derivatives with varying aromatic ring counts. As we extended the aromatic core from 1,4-dimethoxybenzene (1,4-DMB) to 1,4-dimethoxynaphthalene (1,4-DMN), we noted a decrease in oxidation potential, enhanced kinetic stability, and an extended cycling life. However, further extending the aromatic core to 2-ethyl-9,10-dimethyanthracene (EDMA) results in rapid dealkylation of the radical cation due to increased strain in the methoxy substituents. Additionally, 1,4-DMN shows cross-reactions between radical cations, likely via disproportionation. This study demonstrates that extending the π-conjugation changes reactivity in multiple ways. Therefore, attempts to lower oxidation potential and improve ORMs stability through π-conjugation should be pursued with caution.</p>