Redox Materials Research Articles

NiZrSe3/rGO modulated porous architecture for hybrid featured asymmetric supercapacitors

Designing transition metallic chalcogenides with rGO as a potential porous platform for growth are vital to upsurge the electrochemical energy storage capabilities of supercapacitors' electrodes. This work reports hydrothermally prepared agglomerated NiZrSe3 nanoparticles and a porous NiZrSe3/rGO, where the latter exhibits superior charge storage properties. In comparison to NiZrSe3 with specific capacitance (Cs) of 1415.7 F/g, the porous modulated material NiZrSe3/rGO delivers an exceptional Cs of 2750.8 F/g at current density of 2 A/g in the 3-electrode setup. Due to its the outstanding electrochemical performance, NiZrSe3/rGO is employed as a redox featured electrode material, and further integrated with activated carbon (ActC) as a capacitive featured electrode material to construct a hybrid asymmetric supercapacitor with widened potential window and enhanced energy density. The constructed asymmetric supercapacitor unveils a specific capacitance of 140.77 F/g at current density of 1.2 A/g and rate capability of 53.6 % at 7 A/g while functioning stably under the potential range of 0–1.65 V. Moreover, it achieves extremely high energy density of 53.22 Wh/kg at power density of 990 W/kg with its long-life span in regard to cyclic stability of 80.31 % after 10,000 charging-discharging cycles. In general, this work illustrates the significance of NiZrSe3/rGO as a highly redox active cathode material in hybrid featured asymmetric supercapacitors for future renewable energy storage systems.

A submillimeter bundled microtubular flow battery cell with ultrahigh volumetric power density

Flow batteries are a promising energy storage solution. However, the footprint and capital cost need further reduction for flow batteries to be commercially viable. The flow cell, where electron exchange takes place, is a central component of flow batteries. Improving the volumetric power density of the flow cell (W/Lcell) can reduce the size and cost of flow batteries. While significant progress has been made on flow battery redox, electrode, and membrane materials to improve energy density and durability, conventional flow batteries based on the planar cell configuration exhibit a large cell size with multiple bulky accessories such as flow distributors, resulting in low volumetric power density. Here, we introduce a submillimeter bundled microtubular (SBMT) flow battery cell configuration that significantly improves volumetric power density by reducing the membrane-to-membrane distance by almost 100 times and eliminating the bulky flow distributors completely. Using zinc-iodide chemistry as a demonstration, our SBMT cell shows peak charge and discharge power densities of 1,322 W/Lcell and 306.1 W/Lcell, respectively, compared with average charge and discharge power densities of <60 W/Lcell and 45 W/Lcell, respectively, of conventional planar flow battery cells. The battery cycled for more than 220 h corresponding to >2,500 cycles at off-peak conditions. Furthermore, the SBMT cell has been demonstrated to be compatible with zinc-bromide, quinone-bromide, and all-vanadium chemistries. The SBMT flow cell represents a device-level innovation to enhance the volumetric power of flow batteries and potentially reduce the size and cost of the cells and the entire flow battery.

Compositing redox-rich Co–Co@Ni–Fe PBA nanocubes into cauliflower-like conducting polypyrrole as an electrode material in supercapacitors

Supercapacitors (SCs) coupled with redox materials have been extensively investigated to maximize the energy density of SCs.

Recycling expired pharmaceutical drugs as redox materials for efficient and sustainable flexible supercapacitors

We report for the first time the use of pharmaceutical drugs as redox materials to enhance the performance of supercapacitors.

New Avenues for Organic Redox Materials as Sustainable Lithium-ion Battery Cathodes

As the demand for electrification of means of transportation and storage of electrical energy for later use is skyrocketing, rechargeable Li-ion batteries (LIBs) are at the heart of this revolution. Acknowledging the carbon footprints, environmental concerns and cost of the commercial cathode materials, this is the high time to advocate sustainable alternatives. This review aims at establishing the potential of organic redox-active molecules as a burgeoning class of sustainable solid cathode materials for LIBs. The materials are classified according to their structural features (molecules, metallo-organic complexes, and organic/metal–organic frameworks) and electrochemical performance to lay emphasis on practical applications and bottlenecks in commercialization. However, these materials are still in early stages of development, and new frontiers have been explored in the last five years.

Investigation of CO2 Splitting on Ceria-Based Redox Materials for Low-Temperature Solar Thermochemical Cycling with Oxygen Isotope Exchange Experiments

The surface exchange and bulk transport of oxygen are highly relevant to ceria-based redox materials, which are envisaged for the solar thermochemical splitting of carbon dioxide in the future. Experimental investigations of oxygen isotope exchange on CeO2-δ, Ce0.9M3+0.1O1.95-δ (with M3+ = Y, Sm) and Ce0.9M4+0.1O2-δ (with M4+ = Zr) samples were carried out for the first time utilizing oxygen-isotope-enriched C18O2 gas atmospheres as the tracer source, followed by Secondary Ion Mass Spectrometry (SIMS), at the temperature range 300 ≤ T ≤ 800 °C. The experimental K˜O and D˜O data reveal promising results in terms of CO2 splitting when trivalent (especially Sm)-doped ceria is employed. The reaction temperatures are lower than previously proposed/reported due to the weak temperature dependency of the parameters K˜O and D˜O. The majority of isotope exchange experiments show higher values of K˜O and D˜O for Sm-doped cerium dioxide in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. The apparent activation energies for both K˜O and D˜O are lowest for Sm-doped ceria. Using Zr-doped cerium oxide exhibits various negative aspects. The Zr-doping of ceria enhances the reducibility, but the possible Zr-based surface alteration effects and dopant-induced migration barrier enhancement in Zr-doped ceria are detrimental to surface exchange and oxygen diffusion at lower temperatures of T ≤ 800 °C.

Cation-Deficient Ce-Substituted Perovskite Oxides with Dual-Redox Active Sites for Thermochemical Applications.

Identifying thermodynamically favorable and stable non-stoichiometric metal oxides is of crucial importance for solar thermochemical (STC) fuel production via two-step redox cycles. The performance of a non-stoichiometric metal oxide depends on its thermodynamic properties, oxygen exchange capacity, and its phase stability under high-temperature redox cycling conditions. Perovskite oxides (ABO3-δ) are being considered as attractive alternatives to the state-of-the-art ceria (CeO2-δ) due to their high thermodynamic and structural tunability. However, perovskite oxides often exhibit low entropy change compared to ceria, as they generally have one only redox active site, leading to lower mass-specific fuel yields. Herein, we investigate cation-deficient Ce-substituted perovskite oxides as a new class of potential redox materials combining the advantages of perovskites and ceria. We newly synthesized the (CexSr1-x)0.95Ti0.5Mn0.5O3-δ (x = 0, 0.10, 0.15, and 0.20; CSTM) series, with dual-redox active sites comprising Ce (at the A-site) and Mn (at the B-site). By introducing a cation deficiency (∼5%), CSTM perovskite oxides with both phase purity (x ≤ 0.15) and high-temperature structural stability under STC redox cycling conditions are obtained. Thermodynamic properties are evaluated by measuring oxygen non-stoichiometry in the temperature range T = 700-1400 °C and the oxygen partial pressure range pO2 = 1-10-4 bar. The results demonstrate that CSTM perovskite oxides exhibit a composition-dependent simultaneous increase of enthalpy and entropy change with increasing Ce-substitution. (Ce0.20Sr0.80)0.95Ti0.5Mn0.5O3-δ (CSTM20) showed a combination of large entropy change of ∼141 J (mol-O)-1 K-1 and moderate enthalpy change of ∼238 kJ (mol-O)-1, thereby creating favorable conditions for thermochemical H2O splitting. Furthermore, the oxidation states and local coordination environment around Mn, Ce, and Ti sites in the pristine and reduced CSTM samples were extensively studied using X-ray absorption spectroscopy. The results confirmed that both Ce (at the A-site) and Mn (at the B-site) centers undergo simultaneous reduction during thermochemical redox cycling.

Highly hydrophobic TEMPO-functionalized conducting copolymers for solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SCISEs) emerged as the best electrode embodiment for miniaturized, wearable and disposable sensors for ion/electrolyte measurements in body fluids. The commercialization of inexpensive single-use “calibration-free” electrodes requires large scale manufacturing of electrodes with reproducible calibration parameters, e.g. E0. This is perhaps the most important shortcoming of SCISEs, beside the many advantages over their conventional liquid-contact counterparts. However, adjusting the E0 value for optimal potential stability is challenging for all state-of-the-art solid-contact materials, which may combine several types of transducing mechanism (e.g. capacitive and redox materials or their combination) for enhanced potential stability and analytical performance. Therefore, here we introduce for the first time the galvanostatic intermittent titration technique (GITT) to determine the best preadjusment potential. The proof of concept is shown for a novel type of solid-contact based on the copolymerization of 3,4-ethylenedioxythiophene with perfluorinated alkyl side chain (EDOTF) and (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl modified 3,4-ethylenedioxythiophene (EDOT-TEMPO). Such materials that are compliant with local electrodeposition and provide multiple functionalities, i.e. high hydrophobicity by the perfluorinated alkyl side chain, electron-to-ion transduction by the conducting polymer (EDOT) backbone and the confinement of well-defined redox couple (TEMPO), are expected to prevail as solid-contacts.

Potential of solar thermochemical water-splitting cycles: A review

Hydrogen fuel is a valuable tool to achieve the energy transition process, and according to the 2050 net zero emissions scenario its demand is expected to increase by more than 530 Mt H2. This article discusses several routes available to produce hydrogen fuel, with a special focus on solar thermochemical cycles for Water Splitting (WS). Solar thermochemical WS cycles are a potential technology to produce green hydrogen and CO in the future due to their great potential to become a commercial scale process. This technology is still under development and some challenges related to components and sub-processes are being addressed by several research groups in order to make green hydrogen production by this route technically and economically feasible. Specific technological aspects such as particle heating methods, suitability and properties of redox materials for two-step cycles, methods to achieve low oxygen pressure in the reduction chamber, as well as the importance of implementing heat recovery are analysed in detail, as their performance have a significant effect on the overall efficiency and economic of the whole process. Current progress indicates that the realisation of a sufficiently efficient thermochemical cycle is possible within the few years, if the mentioned limitations are overcome.

Reversible phosphorus-based five-electron transfer reaction for aluminium–phosphorus batteries

The use of multi-electron redox materials has been proved as an effective strategy to increase the energy density of batteries. Herein, we report a new reversible phosphorus-based five-electron transfer reaction (P(0) ⇆ P(+5)) in chloroaluminate ionic liquids (CAM-ILs), which represents a new reaction mechanism offering one of the theoretically highest specific capacities of 4325 mAh g−1 among all secondary batteries. Theoretical calculations further confirm the phosphorus electrochemical oxidation pathway with a potential oxidation voltage of ∼2.0 V. Detailed characterisation reveals fresh insight of the role of CAM-ILs, which can catalyze the electrochemical oxidation process of phosphorus by forming electron-enriched state of phosphorus. As a result, a conceptual aluminium-phosphorus battery was assembled, which deliver a high performance of 1512 mAh g−1 and 1176 Wh kg−1, outperforming many similar metal ion battery chemistry. This study sheds light on phosphorus oxidation process in CAM-ILs and presents a new pathway using non-metallic element based multi-electron oxidation to achieve high performance batteries.

Preparation and solar thermochemical properties analysis of NiFe2O4@SiC/ @Si3N4 for high-performance CO2-splitting

The solar-driven thermochemical CO2-to-CO conversion is an effective way to achieve the mission of carbon peaking and carbon neutrality. However, synthesizing porous reacting materials with excellent thermal stability, hardness and long-term cyclic stability, oxygen exchange capacity, and higher CO2-to-CO conversion are the most important challenges associated with the thermochemical CO2-splitting approach and technological upscaling to large-scale applications. This study presented the development of NiFe2O4 oxygen carriers, the synthesis method of SiC and Si3N4 supports, and solar-to-fuel processing of the newly prepared materials through CO2-splitting under a highly concentrated solar radiative heat flux. The newly synthesized NiFe2O4@SiC porous redox material resulted in higher solar energy absorption and CO2 conversion capability with an instantaneous CO production of 410 μmol/g and direct CO2-to-CO conversion rate of 18.1 % at 1073–1273 K reaction temperature. The media composite of NiFe2O4@SiC exhibited high-temperature thermal changes, good thermochemical reaction stability, and a higher CO production rate through six redox cycles compared to NiFe2O4@Si3N4 porous reacting material. The high oxidation potential and remarkably solar radiative heat flux absorption and thermochemical CO2-splitting capacities of the newly developed materials were demonstrated through experimental analysis. The synergistic effect of the oxygen carriers (NiFe2O4) and substrate materials including SiC and Si3N4 skeletons for CO2-splitting is highlighted. This study provided comprehensive and novel experimental insights that can be used as guidance for theoretical research and application in CO2 conversion into high-value-added energy products.

Clean Syngas and Hydrogen Co-Production by Gasification and Chemical Looping Hydrogen Process Using MgO-Doped Fe2O3 as Redox Material

Gasification converts biomass into syngas; however, severe cleaning processes are necessary due to the presence of tars, particulates and contaminants. The aim of this work is to propose a cleaning method system based on tar physical adsorption coupled with the production of pure H2 via a chemical looping process. Three fixed-bed reactors with a double-layer bed (NiO/Al2O3 and Fe-based particles) working in three different steps were used. First, NiO/Al2O3 is used to adsorb tar from syngas (300 °C); then, the adsorbed tar undergoes partial oxidization by NiO/Al2O3 to produce CO and H2 used for iron oxide reduction. In the third step, the reduced iron is oxidized with steam to produce pure H2 and to restore iron oxides. A double-layer fixed-bed reactor was fed alternatively by guaiacol and as tar model compounds, air and water were used. High-thermal-stability particles 60 wt% Fe2O3/40 wt% MgO synthetized by the coprecipitation method were used as Fe-based particles in six cycle tests. The adsorption efficiency of the NiO/Al2O3 bed is 98% and the gas phase formed is able to partially reduce iron, favoring the reduction kinetics. The efficiency of the process related to the H2 production after the first cycle is 35% and the amount of CO is less than 10 ppm.

Microemulsions As Emerging Electrolytes: The Correlation of Structure to Electrochemical Response

Microemulsions are a breakthrough electrolyte for energy storage (BEES) that is emerging as an alternative to traditional RFB electrolytes because of their ability to produce thermodynamically stable, homogeneous solutions of polar and non-polar molecules. The stabilization of oil and water solutions allows microemulsion electrolytes to accommodate organic redox species with fast electron transfer rates that are not soluble in the aqueous phase while still offering the high conductivity of an aqueous salt electrolyte. Microemulsion electrolytes consist of two immiscible phases stabilized by a surfactant, a non-polar phase, solubilizing the redox material—or the redox material can be the entire oil phase—and an aqueous brine phase. Moreover, each component can be environmentally and biologically safe. For example, the aqueous phase only consists of salt and water, the surfactant used in this study, polysorbate-20, is found in many cosmetics and food products, and some of the redox materials investigated in our lab are vitamins such as menadione (vitamin K3) and tocopherol (vitamin E).Microemulsions form many self-assembled bulk nanoscale structures, such as oil-in-water or water-in-oil droplets and bicontinuous. Furthermore, microemulsions exhibit ordering near surfaces that can be different from the bulk structure. Understanding the bulk and surface structures is critical to understanding a material's properties, such as conductivity and diffusion to an electrode surface. Understanding structural changes with composition variation and surface hydrophobicity provide a path to optimizing the balance between acceptable conductivity levels in the electrolyte and the high volume of an oil phase bearing the redox-active species in a µE. This optimization is critical to achieving high-performing electrochemical systems using µEs.This talk describes structural studies of microemulsions prepared from water, toluene, butanol, and polysorbate-20. Small-angle neutron scattering was used to monitor the development of the bicontinuous system as a function of the water-to-surfactant mass ratio on a constant oil-to-surfactant dilution line, revealing how the domain size, correlation length, amphiphilicity factor, and bending moduli change with composition. Kratky and Porod analyses are also employed, providing further structural detail of the scattering domains. We demonstrate that controlling the water-to-surfactant ratio with a constant oil-to-surfactant dilution affects the bicontinuous phase, reveals a sizeable compositional region with structural similarities, and provides insight into the correlation of structure to physical properties. Additionally, neutron reflectivity was used to probe the surface ordering of the microemulsions with hydrophilic and amphiphilic surfaces. Figure 1

Redox-Mediated Electrodialysis for Resource Recovery and Energy-Efficient Desalination

In the chemical and biochemical manufacturing industry, separation processes are amongst the most energy-demanding processes. Electrochemical separations have been growing in interest as promising candidates for both downstream processing for resource recovery and producing clean water due to their energy-efficient, environmentally benign, and modular features.1 Leveraging a sustainable redox reaction, we developed a redox-mediated electrodialysis system for energy-efficient desalination coupled with resource recovery of valuable resources such as lithium, and even biomolecules.2, 3 Our proposed redox-mediated electrodialysis system utilizes a reversible redox reaction as a driving force of salt removal at a much lower voltage than a water-splitting reaction (>1.2 V). As the redox materials are reduced or oxidized, all charged species migrate across the membranes to balance the charge in the electrolyte compartment. As the redox materials circulate in the cathode and anode compartments, they sustainably regenerated without additional regeneration steps, one of the major bottlenecks for industrializing the electrosorption system. We demonstrated the redox-mediated electrochemical system for resource recoveries such as simultaneous lithium recovery from brine3 and valorization of whey proteins and lactose from whey waste.2 Especially for the valorization of whey waste, our system treated 99% of salt from the whey solution within a single step and recovered > 98% of various whey proteins (e.g., valuable protein contents such as beta-lactoglobulin, alpha-lactalbumin, and lactose). The preliminary techno-economic analysis confirmed the economic potential of redox-mediated ED with 51−73% lower energy consumption compared to conventional ED. Overall. we envision the system can become a breakthrough in tackling the environmental challenges coupled with the limited resources. Kim, N.; Jeon, J.; Chen, R.; Su, X., Electrochemical separation of organic acids and proteins for food and biomanufacturing. Chemical Engineering Research and Design 2022, 178, 267-288.Kim, N.; Jeon, J.; Elbert, J.; Kim, C.; Su, X., Redox-mediated electrochemical desalination for waste valorization in dairy production. Chemical Engineering Journal 2022, 428, 131082.Kim, N.; Su, X.; Kim, C., Electrochemical lithium recovery system through the simultaneous lithium enrichment via sustainable redox reaction. Chemical Engineering Journal 2021, 420, 127715.

A Co-Axial Microtubular Flow Battery Cell with Ultra-High Volumetric Power Density

Flow batteries are a promising energy storage solution. However, the footprint and capital cost need to be further reduced for flow batteries to be commercially viable. The flow cell, where electron exchange takes place, is a central component in flow batteries. Improving the volumetric power density of the flow cell (W/Lcell) can reduce the size and cost of flow batteries. While significant progress has been made on flow battery redox, electrode and membrane materials to improve energy density and durability, conventional flow batteries based on planar cell configuration exhibit a large cell size with multiple bulky accessories such as flow distributors, resulting in low volumetric power density. In this talk, I will introduce a new co-axial microtubular (CAMT) flow battery cell configuration that significantly improves volumetric power density by reducing the membrane-to-membrane distance by almost 100 times and eliminating completely the bulky flow distributors. Using zinc-iodide chemistry as a demonstration, our CAMT cell shows peak charge and discharge power densities of 1322 W/Lcell and 306.1 W/Lcell compared to average charge and discharge power densities of < 60 W/Lcell and 45 W/Lcell of conventional planar flow battery cells. The battery cycled for more than 220 hours, corresponding to > 2,500 cycles at off-peak conditions. Furthermore, the CAMT cell has been demonstrated to be compatible with zinc-bromide, quinone-bromide, and all-vanadium chemistries. The CAMT flow cell represents a device-level revolution to enhance the volumetric power of flow batteries and potentially reduce the size and cost of the cells and the entire flow battery.Publication:https://chemrxiv.org/engage/chemrxiv/article-details/62070e98bd05a04ae103fa7e Figure 1

Optimization of Z-Scheme Photocatalytic Reactors for Solar Water Splitting

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, like those we recently invented. Systems using low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently operates at low voltages through use of an ensemble of optically thin photoabsorbers each operating at a low rate. Collectively the ensemble exhibits larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 and BiVO4 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively, our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

Exploring the Impact of Temperature on Non-Aqueous Redox Flow Cells

Nonaqueous redox flow batteries (NAqRFBs) remain an intriguing storage concept, but further improvements in performance and durability are needed to determine viability. An understudied but potentially-enabling feature of NAqRFBs is the ability to operate over a wide range of temperatures, facilitated by the breadth of solvent, supporting salt, and active species whose physicochemical properties differ from aqueous electrolytes. Elevated temperatures can increase reaction kinetics and transport rates, which are anticipated to improve performance, but may also promote decomposition processes, which are expected to lower durability. Elevated temperatures also increase system complexity, requiring energy input through additional balance-of-plant equipment, consequently necessitating energy balance and heat infrastructure considerations. While there are promising initial reports on molecular stability and electrolyte properties at elevated temperatures,1,2 there remains a need to better understand how temperature impacts performance-defining processes within NAqRFBs.In this presentation, we will discuss how molecular stability, electrolyte properties, and cell performance change as a function of temperature using 10-[2-(2-methoxyethoxy)ethyl]-10H-phenothiazine (MEEPT) as a model redox material.3 With increasing temperature, we observe that, as expected, the kinetic and species transport properties of the electrolyte improve (i.e., electrochemical reversibility, ionic conductivity, viscosity, active species diffusion). We connect these ex-situ findings to cell performance using a symmetric flow cell to quantify resistive losses and cycling stability, observing a marked decrease in cell resistance albeit at the expense of active species stability above a certain temperature threshold. Finally, we assess variable temperature operation of a NAqRFB full cell with MEEPT and (bis(2-(2-methoxyethoxy)ethyl)viologen bis(bis(trifluoromethanesulfonyl)imide) (MEEV-TFSI2) as the positive and negative electrolyte materials, respectively, observing enhancements in power performance but constraints on operating lifetime. These findings highlight that while temperature increases may be used as a means of increasing cell performance, care must be taken not to compromise component durability. While the exact set of conditions that balance this tradeoff are likely to be chemistry-dependent, the methods that will be described in the presentation are suitable for assessing these conditions in a general fashion. Acknowledgements This work was partially supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy. A.H.Q., N.J.M., and B.J.N. gratefully acknowledge the National Science Foundation Graduate Research Fellowship Program under Grant Number 1745302. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. A.H.Q acknowledges the Alfred P. Sloan Foundation’s Minority Ph.D. (MPHD) Program. The authors thank Dr. Aman Kaur of the Odom Research Group in the Department of Chemistry at the University of Kentucky for insightful discussions and for synthesis of MEEPT-BF4, MEEPT-TFSI, and MEEV-TFSI2 species used in this study. References (1) Odom, S. A.; et al. Meet. Abstr. 2021, MA2021-02 (1), 110.(2) Shkrob, I. A.; et al. Journal of Molecular Liquids 2021, 334, 116533.(3) Kaur, A. P.; et al. Chem. Mater. 2020, 32 (7), 3007–3017. Figure 1

Aqueous organic flow batteries for sustainable energy storage

Aqueous Organic Redox Flow Batteries (RFBs) have the potential to address the large-scale need for storing electrical energy from intermittent sources like solar- and wind-based generation. Unlike metal-based redox systems, small organic molecules present the prospect of achieving sustainability, by being synthesizable from abundantly available carbon dioxide, water, nitrogen, sulfur, and renewable energy. This mini-review focuses on the progress and challenges in designing water-based RFBs based on small organic molecules that can address the requirements of large-scale energy storage. Much of the recent research in this area involves discovering new molecular architectures via computational screening, understanding degradation, and cell configurations to address the techno-economic challenges of extraordinary electrochemical durability, recyclability, low cost of precursor materials, and good solubility. Redox materials for the positive side of the cell are notably few. Future research must continue to focus on these techno-economic challenges through rapid materials discovery. • Redox molecules must satisfy competing requirements of durability, low cost, and good solubility. • Specific degradation pathways are avoidable by tuning of electronic and steric factors. • Symmetric cells with mixed electrolyte is a practical solution for mitigating effects of crossover. • The use of sustainable biomaterials and low-cost waste products is an exciting prospect.

An ultra-high power density supercapacitor: Cu(II) phthalocyanine tetrasulfonic acid tetrasodium salt doped polyaniline

In this study, an excellent electrode and supercapacitor performance with aqueous electrolyte exhibiting high operating voltage was obtained. Cu(II) phthalocyanine tetrasulfonic acid tetrasodium salt (CuPcTs) was used as a dopant in the polymerization of aniline on carbon felt electrode (CFt) by one-step hydrothermal method. Scanning electron microscopy (SEM) was used to analyze the morphological structure of PANI and CuPcTs doped PANI electrodes. Chemical analysis of the electrodes was performed using X-Ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FT-IR); thermal properties of the electrodes were investigated using thermogravimetric analysis (TG/DTA). The surface area and pore volume of the electrodes were investigated using Brunauer-Emmett-Teller (BET) analysis. The electrochemical properties of the electrodes were investigated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) methods. With the addition of CuPcTs and the use of graphite plate (GP) as the counter electrode, an asymmetrical supercapacitor was created for pseudocapacitive supercapacitors, in which the properties of redox materials were improved. CuPcTs doped PANI electrode showed a high specific capacitance of 982.2 F g −1 at 5 mV s −1 and showed excellent rate capability at different current densities of charge-discharge tests. Furthermore, wide operating-voltage of 3.2 V exhibited an ultra-high energy density value of 12800 W kg −1 and had an energy density of 764.4 Wh kg −1 . The electrode and supercapacitor displayed retention of capacitance performances of 91.4% and 79.7% after 1000 and 5000 cycles in aqueous electrolyte, respectively. • CuPcTs doped PANI carbon felt electrode was developed for supercapacitor applications. • 3.2 V wide operating voltage supercapacitor with 3.0 M KCI electrolyte. • A high specific capacitance of 982.2 F g −1 at 5 mV s −1 and excellent rate capability at charge-discharge tests. • A -maximum energy density of 764.4 Wh kg −1 and ultra-high power density of 12800 W kg −1 can be obtained for device.

Thermochemical Properties of High Entropy Oxides Used as Redox-Active Materials in Two-Step Solar Fuel Production Cycles

The main challenges and obstacles to the development of hydrogen/carbon monoxide production from the splitting of water/carbon dioxide through two-step solar thermochemical cycles are strongly related to material concerns. Ineed, ceria is the main benchmark redox material used in such processes because it provides very good oxidation reaction kinetics, reactions reversibility and thermal cycling stability. This is at the expense of a low reduction yield (non-stoichiometry δ in CeO2-δ) at relatively high temperatures (≥1400 °C), which requires operation at low oxygen partial pressures during the reduction step. Hence, the specific fuel output per mass of redox material, i.e., the amount of H2/CO produced per cycle, remains low, thereby limiting the overall solar-to-fuel conversion efficiency. Perovskites offer larger amounts of fuel produced per cycle but the reaction kinetics are slow. This study addresses the thermochemical investigation of a new class of metal oxides, namely high entropy oxides (HEOs), with the aim of improving the specific amount of fuel generated per cycle with good kinetic rates. Different formulations of high entropy oxides were investigated and compared using thermogravimetric analysis to evaluate their redox activity and ability to split CO2 during thermochemical cycles. Among the different formulations tested, five HEOs yielded CO with a maximum specific fuel output of 154 µmol/g per cycle. These materials’ performances exceeded the production yields of ceria under similar conditions but are still far from the production yields reached with lanthanum–manganese perovskites. This new class of materials, however, opens a wide path for research into new formulations of redox-active catalysts comparing favorably with the ceria redox performance for solar thermochemical synthetic fuel production.