Fast Redox Reactions Research Articles

Controlling Hydrogen Evolution and CO2 Reduction at Transition Metal Hydrides.

ConspectusFuel-forming reactions such as the hydrogen evolution reaction (HER) and CO2 reduction (CO2R) are vital to transitioning to a carbon-neutral economy. The equivalent oxidation reactions are also important for efficient utilization in fuel cells. Metal hydride intermediates are common in these catalytic and electrocatalytic processes. Guiding metal hydride reactivity is important for achieving selective, kinetically fast, and low overpotential redox reactions. Our work has focused on understanding kinetic and thermodynamic aspects for controlling these reactive hydride species in an effort to design more selective electrocatalysts that operate at low overpotentials. Key to our research approach is understanding the free energy changes and rate of discrete steps of catalysis through the synthesis of proposed intermediates to independently investigate catalytic steps. Hydricity, the free energy of hydride dissociation, and how these values change with metal and ligand environment have informed catalyst design in the past few decades. We describe here how we have advanced upon these earlier studies.In our early studies we sought to understand solvent-dependent changes in hydricity for transition metal hydrides and how they impact the free energy for reduction of CO2 to formate (HCO2-). Additionally, we described how hydricity values can be applied to optimize HER and CO2R catalysis. This framework provides general guidelines for achieving selective CO2 reduction to formate without concomitant generation of H2. Kinetic information on steps in the proposed catalytic cycle of HER and CO2R catalysts were evaluated to identify potential rate-determining steps. As a second approach to achieve selective reduction for CO2, we explored two catalyst design strategies to kinetically inhibit HER using electrostatic (charged) and steric interactions. Hydricity values and other considerations for minimizing the free energy of proposed catalytic steps were also used to design an electrocatalyst for the interconversion between CO2 and HCO2- at low overpotentials. Further, we discuss our efforts to translate the CO2 hydrogenation activity of homogeneous catalysts to electrocatalysis.All of these catalytic systems operate with classical metal hydrides, where the electrons and proton are colocated on the metal center. However, classical metal hydrides all require very reducing potentials to generate sufficiently strong hydride donors for CO2 reduction. An analysis of metal hydride hydricity and reduction potentials shows that the strong correlation between reduction potential and hydricity is a general trend because the former is also highly correlated to pKa. However, formate dehydrogenase (FDH) generates a competent hydride donor at more mild potentials through bidirectional hydride transfer, where the proton and electrons of the hydride are not colocated. This bioinspired approach points to a promising new strategy for generating strong hydride donors at milder potentials and will surely open new avenues for using hydricity as a guide for addressing new and existing problems in catalysis.

Chemical spray pyrolysis deposited ZnO/ZIF-8 and cobalt doped ZnO/ZIF-8 composite thin films for highly sensitive and selective ammonia sensing at room temperature

ZnO is a widely studied metal oxide for gas sensor applications concerning its biocompatibility and physio-chemical properties. However, the high operating temperature and low response are drawbacks. ZIF-8, the metal-organic framework of Zinc and imidazolate, is renowned for its gas adsorption property. The poor stability of ZIF-8 also limits its application as an effective gas sensor. Thus, a composite thin film of ZnO and ZIF-8 is deposited on a glass substrate using the chemical spray pyrolysis technique to detect ammonia at room temperature (300 K). The composite thin film is stable, has a good response, and is repeatable. Further, to enhance the sensor response at room temperature, Co is doped to the ZnO/ZIF-8 composite thin films. The sensor response for 1 ppm of ammonia for the Co doped ZnO/ZIF-8 is 301 at room temperature. Along with the gas adsorption capability of ZIF-8 providing more active sites, the change in morphology from flakes to spherical grains on doping with Co provided higher active surface area and free electrons and all together instigated fast redox reaction and conduction mechanism in ZnO. Hence, the Co doped ZnO/ZIF-8 composite thin films have an enhanced response of 117,285 to 50 ppm of ammonia.

Shape controlled synthesis and crystal facet dependent gas sensitivity of tungsten oxide

Gas sensors can achieve remarkable functionality through the optimization of particle size, shapes, crystal facets, and oxygen defects, resulting in unsaturated coordination atoms, high charge densities, and enhanced bond energies. In this study, WO2.72 nanourchins, WO3-x nanowires, and WO3 nanorods/nanocube with (010), (001), (200), and (002) dominant crystal facets were synthesized and used as gas sensing materials. It was found that WO2.72 nanourchins exhibit an excellent response of Ra/Rg=21 to 100 ppm acetone with good selectivity among other sensors as-fabricated. First-principle calculations of Density Functional Theory were performed on acetone's adsorption on different crystal planes of tungsten oxide. The results verify spontaneous and fast adsorption on the (010) crystal plane than on the (001) and (200) planes. Further characterizations indicate that WO2.72 nanourchins with (010) crystal facets contain more reactive sites with high surface energy, facilitating charge separation, increasing charge carrier mobility, and enabling the redox reactions to occur independently at different rates. Moreover, their richer electron-donor surface oxygen defects, smaller feature sizes, and higher surface area (SBET) with hierarchical porous structures, all contribute to the enhanced acetone sensing. Our work provides a strategy for improved acetone sensing based on optimizing the particle shape with different crystal facets and oxygen vacancy density. We further expect our strategy to be applicable in designing other sensor materials with efficient charge separation and fast surface redox reactions to detect toxic gases.

Cobalt-doped molybdenum sulfide as an interlayer facilitates polysulfide conversion to obtain high-performance lithium−sulfur batteries

Lithium–sulfur (Li–S) batteries are considered to be a promising energy storage system due to their theoretical specific capacity. However, the “shuttle effect” of polysulfides (LiPSs) and the slow redox kinetics of LiPSs in the conversion reaction have hindered their large−scale application. In this study, the Co-MoS2 is prepared and coated on the polypropylene (PP) separator of Li–S batteries. The Co elements are successful addition and uniform distribution in the Co-MoS2. This coating is designed to provide chemical adsorption and catalysis for LiPSs. Consequently, Li–S batteries with a Co-MoS2 interlayer exhibit outstanding long-cycle and high-rate performance. After 500 cycles at 0.5C, the reversible capacity is found to be 561 mAh g−1, with only a low-capacity reduction of 0.097 % per cycle. Furthermore, 602 mAh g−1 is delivered even at 4C. The results of electrochemical measurements and first principles calculations demonstrate that the high performance of Li–S batteries is attributed to the chemical adsorption and electrocatalysis roles of the Co-MoS2 interlayer, which inhibit the “shuttle effect” and enable fast redox reaction kinetics. In addition, the introduction of cobalt doping results in the formation of the 1T phase of MoS2, which enhances the conductivity of the Co-MoS2. The presence of plentiful cobalt-doped MoS2 defects provides abundant catalytic sites for the electrochemical reaction of LiPSs.

PEDOT:PSS-based electronic “paper” with high surface-interface and mechanical strength and ultra-long wet-resistant capacity

Organic electrochromic conducting polymers (CPs) are at the forefront of flexible and ultra-thin display electronics. Solution processable poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) exhibits excellent electrical and electrochemical behavior in line with great processability, stability, and other promising properties. However, their uses are limited by the weak mechanical properties, wet instability, as well as inevitable conflicts among different functionalities and structural characteristics, etc. In this study, we designed and fabricated a high-mechanical-strength electronic “paper”, PEDOT:PSS/PEG-UPy film, employed a thermoplastic polyurethane poly(ethylene glycol)-2-amino-5-(2-hydroxyethyl)-6-methylpyrimidin-4-ol (PEG-UPy) using diverse processing (drop coating, spin coating, blade coating and 3D printing) of its “electric ink (E-ink)” from the mechanical mixture of its isopropanol (IPA) solution with aqueous PEDOT:PSS dispersion. Compared to PEDOT:PSS film, the incorporation of supramolecular chain-expanding unit 2-amino-4-hydroxy-6-methylpyrimidine (UPy) into the PEG-UPy molecular backbone resulted into optimized self-supporting (substrate-free) film of PEDOT:PSS/PEG-UPy(10 wt%) which showed great flexibility, enhanced mechanical strength (elongation at break 47.08 %, tensile strength of 19.16 MPa, toughness 7.973 MJ m−3), considerable electrical properties (0.509 S cm−1), desirable physical properties including resistance to abrasion and solvent-wiping, and excellent surface-interface properties (e.g., hardness of 3H, adhesion of level 0, glossiness of 91). Its electrochromic capabilities include an optical contrast (ΔT) value of 67 %, a fast redox reaction (4 s and 3 s, respectively), and a coloring efficiency (722.19 cm2 C−1). Similar to paper, they can be easily tailored or folded into a variety of forms, but have greater advances on dense morphology, flat surface, considerable optical transparency (79.5 %), as well as robust resistance to mechanical damage (in the case of bending, stretching, and folding, etc.), water (immersing > 150 days), and dirt. The combined effect of molecular design of PEG-UPy, abundant hydrogen-bonded cross-linking interaction and “good” solvent effect of IPA to PEDOT:PSS play important roles in this system. This is the first work on solution processable PEDOT:PSS-based electrochromic “paper” being able to meet much comprehensive performance requirements for the application in practical complex working environment.

(Invited) Diverse Strategies for Pseudocapacitance in 2D Materials and Beyond

As electronic technology advances, the need in safe and long-lasting energy storage devices that occupy minimum volume arises. Short charging times of several seconds to minutes, with energy densities comparable to those of batteries, can be achieved in pseudocapacitors. These are sub-class of supercapacitors, where capacitance is mediated by fast redox reactions and can enable at least an order of magnitude more energy to be stored than in typical electrical double layer capacitors. Transition metal oxides (e.g. RuO2, MnO2) and conducting polymers (e.g. polyaniline) serve as typical examples. However, these materials are often high in cost and/or suffer from low cycling stability. As a result, the search for new pseudocapacitive materials constitutes an important direction today.In my talk, I will discuss various strategies to improve the performance metrics of pseudocapacitors, specifically focusing on enhancing capacitance and charging rates. I will primarily discuss the electrochemical behavior of layered materials, such as 2D transition metal carbides (MXenes) and 2D pi-conjugated conductive metal-organic frameworks, with a strong emphasis on understanding the mechanisms of charge storage and the various factors influencing electrochemical performance. Additionally, I will explore the role of cation intercalation in layered MXenes as a means to finely tune their electrochemical responses.

Understanding the Reaction Processes in Li-Ion Batteries By Voltage Relaxation Analysis

The open circuit voltage (OCV) transient observed after current interruption is a very useful electrochemical signal providing crucial information about a Li-ion battery (LIB), including its state of charge and health, and kinetics of internal processes. Generally, OCV transient is divided into immediate and long-term relaxations, which are probably related to current-dependent kinetic processes and changes in the instantaneous thermodynamic state, respectively. Previous studies have attempted to analyze the pulse and relaxation voltages of LIBs under Li concentration gradients in both the electrolyte and electrode, with much success. Nevertheless, they have been limited in understanding what is really happening inside the cell in detail. This was mainly due to the fact that the experiments were performed on full cells with overlapping anode and cathode signals, which hindered the interpretation of detailed internal reactions, and for the same reason, only unsystematic phenomenological interpretations were possible. So it is necessary to separate relaxation signals from the anode and cathode for more accurate analysis. Even in a half-cell, where the signal contribution of the Li counter electrode (CE) to the overall cell signal is believed to be very small, the degree of potential relaxation attributable to the CE might be non-negligible due to the concentration change of active ions around the electrode. Therefore, in order to fully understand the electrochemical characteristics of the working electrode (WE), a precise analysis of the voltage relaxation of a single electrode is required.In this study, various test cells, including three-electrode half-cells and full cell, and a voltage-controllable symmetrical cell, were fabricated to analyze the pulse and relaxation process in order to further understand the reaction kinetics of the battery. In particular, we aimed to distinguish the detailed processes and to understand the overvoltage evolution during battery operation. For this purpose, Ni-rich layered oxide and graphite were used as the cathode and anode, respectively. Li metal was adopted as the CE and RE in both the half-cell and three-electrode cells. In the two- and three-electrode cathode half-cell experiments, it was confirmed that the overpotential contribution of Li CE in half-cells was one-third to half of total overpotential in specific operating conditions. Considering the relatively fast redox reaction rate of Li, this result strongly implies that a concentration overpotential due to the CE can play a critical role in OCV relaxation. In other words, a typical test cell structure where the opposing areas of WE and CE are similar, total cell overpotential is greatly affected by CE. Supporting this argument, a cell designed with a large CE area and abundant electrolyte showed the very small contribution of CE. In experiments with two- and three-electrode anode half-cells, the contribution of Li CE was similar to that of the cathode half-cell. Next, to investigate the change in the effect of Li CE on the overall overpotential as the cell degrades under repeated cycles, the OCV relaxation with cycles was analyzed for the test cells used above. The analysis revealed an obvious cycle dependence of the long-term OCV relaxation signal, which was confirmed to be largely an effect of Li CE. To investigate the unexpected voltage behavior of Li, Li symmetric cells were configured to obtain voltage curves over time with cycling. From the analysis results, it is proposed that the voltage curve variation is mainly due to different local reaction resistances and highly tortuous surface morphology of the Li surface.In conclusion, the overpotential of Li CE had a large impact on total overpotential in the commonly used two-electrode half-cells, which may cause errors in voltage curve interpretation. Under these circumstances, we further investigated whether the systematic and thorough analysis of the OCV relaxation could reliably separate the cathode and anode overpotential signals in the full cell, as well as the detailed reaction signals at each electrode. To this end, we performed an overpotential analysis using two- and three-electrode cells, as well as a voltage-controllable symmetrical cell, cross-validating the results with those obtained in the half-cell experiments above and electrochemical impedance spectroscopy. In this presentation, we will show how to derive the overpotential contribution of the cathode and anode, and the detailed reaction processes from the short- and long-term OCV relaxation curves obtained from a full-cell. Furthermore, we will discuss the reliability of the separated signals and possible future work to improve their accuracy.

A sulfur-infiltrated mesoporous silica/CNT composite-based functional interlayer for enhanced Li–S battery performance

The design and construction of functional interlayers for lithium–sulfur (Li–S) batteries has attracted much attention and was demonstrated to be effective to alleviate the notorious “shuttle effect.” An often neglected issue is that the introduction of interlayer will reduce the overall energy density of the battery. In this work, we report a sulfur-infiltrated mesoporous silica/carbon nanotube (CNT) composite as an interlayer for Li–S batteries. The mesoporous silica with large surface area (842 m2 g−1) and pore volume (0.85 cm3 g−1) can not only ensure abundant exposed sites for polysulfide capture but also accommodate a large amount of sulfur inside the pore structure. CNT was composited with silica to enhance the electronic conductivity of the interlayer, which is beneficial for fast sulfur redox reaction kinetics and improved utilization of sulfur. Compared to the pristine and CNT-modified separator, the mesoporous silica/CNT composite-modified separator enables better cycling stability and rate performance. More importantly, it was demonstrated that separately incorporating sulfur into a cathode and interlayer enables better battery performance than locating all the sulfur in the cathode. At a total sulfur loading of 4 mg cm−2 (3 mg cm−2 sulfur on the cathode and 1 mg cm−2 on the mesoporous silica/CNT interlayer), a high initial discharge capacity of 1410 mAh g−1 and a retained capacity of 952 mAh g−1 after 100 cycles were exhibited. This work provides important guidance for future design of functional interlayers for practical Li–S batteries.

High-Energy–Density Fiber Supercapacitors Based on Transition Metal Oxide Nanoribbon Yarns for Comprehensive Wearable Electronics

Fiber supercapacitors (FSs) based on transition metal oxides (TMOs) have garnered considerable attention as energy storage solutions for wearable electronics owing to their exceptional characteristics, including superior comfortability and low weights. These materials are known to exhibit high energy densities, high specific capacitances, and fast redox reactions. However, current fabrication methods for these structures primarily rely on chemical deposition, often resulting in undesirable material structures and necessitating the use of additives, which can degrade the electrochemical performance of such structures. Herein, physically deposited TMO nanoribbon yarns generated via delamination engineering of nanopatterned TMO/metal/TMO trilayer arrays are proposed as potential high-performance FSs. To prepare these arrays, the target materials were initially deposited using a nanoline mold, and subsequently, the nanoribbon was suspended through selective plasma etching to obtain the desired twisted yarn structures. Because of the direct formation of TMOs on Ni electrodes, a high energy/power density and excellent electrochemical stability were achieved in asymmetric FS devices incorporating CoNixOy nanoribbon yarns and graphene fibers. Furthermore, a triboelectric nanogenerator, pressure sensor, and flexible light-emitting diode were synergistically combined with the FS. The integration of wearable electronic components, encompassing energy harvesting, energy storage, and powering sensing/display devices, is promising for the development of future smart textiles.Graphical

Synergistically promoting the catalytic conversion of polysulfides via in-situ construction of Ni-MnO2 heterostructure on N-doped hollow carbon spheres towards high-performance sodium-sulfur batteries

The room-temperature sodium-sulfur (RT Na-S) battery is considered to be a highly promising electrochemical energy storage device, attributed to its high energy density, rich sulfur reserve, and nontoxicity. However, it is constrained by the polysulfide shuttling and the low intrinsic conductivity of active sulfur. A sacrificial template method has been used to develop hetero-structured Ni-MnO2 embedded in micro/mesoporous hollow carbon spheres (HCS@Ni-MnO2) to overcome these challenges. As evidenced by electrochemical properties and computational results, the construction of hetero-structured Ni-MnO2 provides a strong affinity to polysulfides. Meanwhile, the highly conductive in-situ constructing Ni-MnO2 structure has strong adsorption to soluble sodium polysulfides and effectively improves the catalytic conversion. The micro- and mesoporous hollow carbon sphere improves the reactivity of the sulfur cathodes and physically suppresses polysulfides shuttling. Befitting from the physical and chemical confinement and the fast redox reaction of polysulfides, the as-prepared S/HCS@Ni-MnO2 cathode exhibits a high capacity of 1031.7 mAh g−1 at 0.2 A g−1 after 100 cycles, surpassing the capacities of S/HCS@Ni (820.3 mAh g−1), S/HCS@MnO2 (942.4 mAh g−1) and S/HCS (866.7 mAh g−1). Moreover, the battery with S/HCS@Ni-MnO2 cathode also exhibits excellent cycle life (586.8 mAh g−1 at 5 A g−1 after 1000 cycles) and a high rate performance (553.8 mAh g−1 at 10 A g−1). This study provides an efficient strategy for synthesizing multiple compound hetero-structured materials to facilitate the development of energy storage applications.

Enhanced Redox Activity of Anthraquinone on N‐Doped Porous Carbon Electrode

AbstractAnthraquinone is one of the organic molecules that can potentially replace metal‐based electrodes due to its fast and reversible redox reaction as well as the abundance in nature. Here, we reported the redox activity of anthraquinone on N‐doped porous carbon electrodes derived from chemical activation of tea leaves with potassium carbonate. Electrochemical behavior was evaluated by cyclic voltammetry and galvanostatic charge/discharge in sulfuric acid, with X‐ray photoelectron spectroscopy providing insights into the surface chemistry of the electrodes. Anthraquinone on the composite electrode was able to achieve 97 % of its theoretical storage capacity. We found that nitrogen‐containing functional groups, especially graphitic and pyrrolic groups, had a major impact on the enhanced redox activity of anthraquinone.

Molybdenum Sulfide Nanoflowers as Electrodes for Efficient and Scalable Lithium-Ion Capacitors.

Hybrid supercapacitors (HSCs) bridge the unique advantages of batteries and capacitors and are considered promising energy storage devices for hybrid vehicles and other electronic gadgets. Lithium-ion capacitors (LICs) have attained particular interest due to their higher energy and power density than traditional supercapacitor devices. The limited voltage window and the deterioration of anode materials upsurged the demand for efficient and stable electrode materials. Two-dimensional (2D) molybdenum sulfide (MoS2) is a promising candidate for developing efficient and durable LICs due to its wide lithiation potential and unique layer structure, enhancing charge storage efficiency. Modifying the extrinsic features, such as the dimensions and shape at the nanoscale, serves as a potential path to overcome the sluggish kinetics observed in the LICs. Herein, the MoS2 nanoflowers have been synthesized through a hydrothermal route. The developed LIC exhibited a specific capacitance of 202.4 F g-1 at 0.25 A g-1 and capacitance retention of >90 % over 5,000 cycles. Using an ether electrolyte improved the voltage window (2.0 V) and enhanced the stability performance. The ex-situ material characterization after the stability test reveals that the storage mechanism in MoS2-LICs is not diffusion-controlled. Instead, the fast surface redox reactions, especially intercalation/deintercalation of ions, are more prominent for charge storage.

Universal, minute-scale synthesis of transition metal compound nanocatalysts via graphene-microwave system for enhancing sulfur kinetics in lithium-sulfur batteries

The application of Li-S batteries on large scale is held back by the sluggish sulfur kinetics and low synthesis efficiency of sulfur host. In addition, the preparation of catalysts that promote polysulfide redox kinetics is complex and time-consuming, reducing the cost of raw materials in Li-S. Here, a universal synthetic strategy for rapid fabrication of sulfur cathode and metal compounds nanocatalysts is reported based on microwave heating of graphene. Heat-sensitive materials can achieve rapid heating due to graphene reaching 500 ℃ within 4 s via microwave irradiation. The MoP-MoS2/rGO catalyst demonstrated in this work was synthesized within 60 s. When used for catalysts for Li-S batteries whose graphene/sulfur cathodes were also synthesized by microwave heating, enhanced catalytic effect for sulfur redox reaction was verified via experimental and DFT theoretical results. Benefiting from fast redox reaction (MoP), smooth Li+ diffusion pathways (MoS2), and large conductive network (rGO), the assembled Li-S battery with MoP-MoS2/rGO-Add@CS displays a remarkable initial specific capacity, stable lithium anode and good cycle stability (in pouch cells) using this two-pronged strategy. The work provides a practical strategy for advanced Li-S batteries toward a wide range of applications.

Highly Exothermic and Fast Mechanochemical Redox and Intercalation Reactions of V2O5 with Sodium Hydride.

Vanadium oxides exhibit promising characteristics for electrochemical energy storage, owing to their capability to switch between different oxidation states, in combination with the incorporation of alkali metals. Here, we report on a systematic investigation of the mechanochemical reduction of V2O5 with NaH. In contrast to conventional high-temperature synthesis methods, the mechanochemical reaction occurs already after a few minutes. We observed a mixture of different (sodium) vanadium oxides with vanadium oxidation states ranging from +III to +V. Remarkably, these highly exothermic self-propagating reactions occur even within a rudimentary pistil-mortar setup. Hereby, the hydride concentration has a greater effect on the final sample composition than the milling time. In general, higher percentages of sodium vanadates are formed instead of vanadium oxides, and the lower oxidation states of vanadium are accessible with increasing amounts of NaH. Theoretical calculations confirm these experimental observations and emphasize the central role of sodium vanadates, especially with vanadium in the +V oxidation state, in carrying out the observed exothermic reactions. This comprehensive study sheds light on the mechanochemical reduction of vanadium oxides and underlines their potential for further development of electrochemical energy storage systems.

Ion/Electron Co-Conductive Triple-Phase Interface Enabling Fast Redox Reaction Kinetics in Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) are promising next-generation energy storage systems because of their high energy densities and high theoretical specific capacities. However, most catalysts in the LSBs are based on carbon materials, which can only improve the conductivity and are unable to accelerate lithium-ion transport. Therefore, it would be worthwhile to develop a catalytic electrode exhibiting both ion and electron conductivity. Herein, a triple-phase interface using lithium lanthanum titanate/carbon (LLTO/C) nanofibers to construct ion/electron co-conductive materials was used to afford enhanced adsorption of lithium polysulfides (LiPSs), high conductivity, and fast ion transport in working LSBs. The triple-phase interface accelerates the kinetics of the soluble LiPSs and promotes uniform Li2S precipitation/dissolution. Additionally, the LLTO/C nanofibers decrease the reaction barrier of the LiPSs, significantly improving the conversion of LiPSs to Li2S and promoting rapid conversion. Specifically, the LLTO promotes ion transport owing to its high ionic conductivity, and the carbon enhances the conductivity to improve the utilization rate of sulfur. Therefore, the LSBs with LLTO/C functional separators deliver stable life cycles, high rates, and good electrocatalytic activities. This strategy is greatly important for designing ion/electron conductivity and interface engineering, providing novel insight for the development of the LSBs.

Multifunctional radical polymers-enabled rapid charge/discharge and high capacity for flexible self-standing LiFePO4/PETM/SWNT hybrid electrodes

The combination of high output and high energy storage performance at the same time is a long-standing and formidable challenge in electrode technology. Herein, a multifunctional radical polymer binder, poly (ethylene-alt-TEMPO maleate) (PETM) is first introduced to construct flexible self-standing organic/inorganic hybrid electrodes, PETM/lithium iron phosphate (LFP)/single-walled carbon nanotubes (SWNT). Notably, PETM plays multifunctional roles in the PETM/LFP/SWNT electrodes as a polymer binder, electrochemical mediator, and auxiliary active material. First, PETM provides strong adhesive strength to generate robust structural integrity, allowing a three-dimensional hierarchical consecutive structure. Second, together with the electrochemical mediation of PETM, the hybrid structure greatly expedites electron/ion transportation, thus bringing fast redox reaction kinetics and high output. Third, PETM offer additional energy storage owing to the electro-activeness of radical groups. Coupled with the self-supporting property of the electrode without any redundant electro-inactive components, the electrode energy density can be substantially improved. Profiting from those advantages, the formed electrode achieves exceptional rate performance (97.5 mA h/g at 20 C), extraordinary specific/areal capacity (173 mA h g−1/8.2 mA h cm−2 at 0.1 C), and ultralong-life cycle under ultrahigh rate (over 2000 cycles at 20 C). More importantly, the electrode can be utilized as a flexible cathode in flexible batteries.

Electron-delocalization catalyzers for high performance, low-temperature Li-S batteries.

The extremely depressive conversion kinetics of polysulfides due to sluggish Li+ diffusion kinetics remain to be resolved for low-temperature Li-S batteries (LT-LSB). Herein, the strategy for electron-delocalization of nanocatalysts has been designed through introducing oxygen defects on vanadium trioxide that was anchored on a porous carbon network (ODVO@PCN). The reconstructed active sites of the V2+ state tend to interact with sulfur species more easily due to the stronger hybridization between V2+ sites and S sites in sulfur species, allowing enhanced Li+ transformation kinetics across the electrolyte/electrode interface for a fast redox reaction in the low-temperature surrounding. Consequently, at a low temperature of 0 °C, the cell with the ODVO@PCN kinetic promotor exhibits 501 mA h g-1 at 1C and a long life time of up to 400 cycles at 0.5C. Reduced to ultralow -10 °C, the cell still provides a capacity of 706 mA h g-1 and stabilizes a remarkable capacity retention of 85% after 100 cycles.

Tungstosilicic Acid: A Promising Electrolyte for Redox Flow Battery

Large-scale energy-storage systems are vital for the integration of renewable energy technologies. Vanadium redox flow battery (VRFB) is one of the most developed and mature energy storage technologies available today. However, VRFB suffers from low energy density and non-availability of active materials. [1]. Recently, polyoxometalates are emerging as promising electrolyte materials with high energy density owing to their capability of multi-electron-transfer per molecule and faster redox reactions [2-3]. From the number of polyoxometalates which are available, tungstosilicic acid (TSA) is a good candidate as a negative electrolyte and is investigated in RFB. The equilibrium redox potential and the corresponding electron-transfer number of all the three redox peaks of TSA are determined. One electron-transfer in the first two redox peaks (0.008 V vs. RHE and –0.248 V vs. RHE) and approximately 14 electrons in the third redox reaction (-0.374 V vs. RHE) are determined using chronoamperometry. The equilibrium redox potential of the third redox peak is comparable to that of the V2+/3+ redox couple (−0.295 V vs. RHE), which is commonly used as a negative electrolyte in VRFB. There is a significant potential difference between the third redox peak of TSA (−0.374 V vs. RHE) and VO2 +/ VO2+ redox couple (1.118 V vs. RHE). The current loss due to parasitic hydrogen evolution at the third redox peak potential is very less in comparison to TSA reduction current and the same is confirmed by chronoamperometry and linear sweep voltammetry. Thus, multi-electron-transfer, optimum redox potential and negligible HER current is the motivation to fabricate a TSA-VO2+ RFB with vanadyl sulfate (VOSO4) as positive electrolyte. The high-electron transfer per TSA molecule leads to higher power density at relatively low concentrations of TSA. Power density of 100 mM TSA – 1 M VO2+ flow battery is higher than that of 1 M all vanadium RFB and the impact of concentration ratio between the electrolytes on the power density is studied. Higher concentration of vanadium electrolyte for a given TSA concentration gives better power density and charge capacity of the flow battery. On over-charging of the battery, TSA changes to irreversible state and further may precipitate [4]; thus, charging potential of RFB is optimized. The improvement in performance is also validated by the two-electrode cell electrochemical impedance spectroscopy results. Figure: Cyclic Voltammograms of 100 mM TSA in 0.5 M H2SO4 (black) over bare glassy carbon electrode, equimolar (1 M each) VO2 +/VO2+ (blue) and equimolar V3+/V2+ (red) in 3M H2SO4 over Vulcan-C coated GC electrode recorded at scan rate of 20 mV s-1.

(Invited) Revealing Charge Storage Processes at the Interfaces of Supercapacitor Electrodes

The fast development of electrochemical energy storage devices has revolutionized almost every aspect of modern life by enabling portable electronic devices, electric vehicles, and grid storage of renewable energy. To satisfy the growing industrial and consumer needs for energy storage systems with high output power and short recharge time, the electrode materials should have excellent high-rate performance.1 Electrical double-layer (EDL) capacitors exhibit high-rate capability and long-cycling life as they store charges electrostatically. Using ionic liquid (IL) as the electrolyte leads to a large voltage window but low capacitance. In this first session of the presentation, I will introduce a mathematical model to simulate the IL ion packing structure in nanopores. This model is capable of estimating the EDLC capacitance based on pore size distribution. Then, we further revealed that mixture IL ions can be selectively driven into the pores of different confinement levels, which was confirmed by solid-state NMR, and further explained by DFT simulation2.In the second part of the presentation, I will shift to pseudocapacitors, which are expected to have a much higher charge storage capacity than EDL capacitors and a much higher rate than batteries as they store energy through fast surface redox reactions.3 The emerging 2D material family, 2D transition metal carbides/nitrides MXenes, shows outstanding pseudocapacitive performance due to their ionophilicity, metallic conductivity, and highly reactive surfaces. The proper coupling between electrolyte and electrode is critical to increase energy and power density. In the organic electrolyte, we found that the solvent has a strong impact on the ion/solvent arrangement in 2D MXene material and hence the charge storage capability.4 In the aqueous electrolytes, we successfully introduced surface redox reactions to MXene electrodes by using the water-in-salt electrolytes and by adjusting the initial valence of Ti in Ti3C2 MXenes, which dramatically increases the capacitance of MXene in the neutral aqueous electrolytes5.References Simon, P.; Gogotsi, Y., Nat. Mater. 2020, 19 (11), 1151-1163. Wang, X.; Mehandzhiyski, A. Y.; et al., J. Am. Chem. Soc. 2017, 139 (51), 18681-18687. Fleischmann, S.; Mitchell, J. B.; et al., Chem. Rev. 2020, 120 (14), 6738-6782. Wang, X.; Mathis, T. S.; et al., Nat. Energy 2019, 4 (3), 241-248. Wang, X.; Mathis, T. S.; et al., ACS Nano 2021, 15 (9), 15274-15284.

An Optical-Electronic-Catalytic Coupled Multi-Physical Simulation for Sunlight-Driven CO2 Reduction Device Based on Light Absorbers Patterned with Island Electrocatalysts

Photoelectrochemical (PEC) reduction of CO2 to value-added chemicals is a promising approach to convert solar energy but it is often suffered from sluggish kinetics and photocorrosion at the semiconductor-electrolyte interface.1, 2 In general, there are two requirements for a good performing solar-driven CO2 reduction: i) effective catalysts for a fast surface redox reaction and ii) an effective semiconductor/catalyst interface for rapid charge transfer across interfaces.3 The deposit of a continuous electrocatalyst layer on the photoabsorber surface can reduce kinetic overpotentials for CO evolution reaction (COER) and can enhance the photoelectrode stability. However, the optical obscuration significantly attenuates the magnitude of the incident illumination. Reducing the coverage (f c) of the catalyst on the photoabsorber surface can reduce the optical obscuration but worsen the reaction kinetics. Moreover, the pinch-off effect occurs as the size of the catalyst is small enough to provide a facile path for carriers to transport across interfaces. The PEC CO2 reduction with the utilization of the pinch-off effects could potentially be advantageous to balance the kinetics and the optical absorption. Understanding the trade-offs between the reduction in photocurrent density due to optical obscuration and the decrease in kinetic overpotentials due to improved active area is important in optimizing PEC CO2 reduction devices.To elucidate the effects of the optical performance, interfacial carrier transport, and electrochemical activity for the PEC CO2 reduction, three possible interfacial configurations in PEC were considered in this study: i) Bare p-Si photocathode case. The bare p-Si photocathode immersed in aqueous electrolyte and CO2R occurs at the semiconductor-electrolyte (SC/E) interface; ii) Layered catalyst case. The continuous layer catalyst is deposited on the surface of p-Si photocathode and forms a semiconductor-metal (SC/M) interface; iii) Patterned catalyst case. Producing a patterned catalyst with a low geometric coverage fraction on the surface of p-Si photocathode. The Schottky junctions with a low barrier height formed by the SC/M are surrounded by the Schottky junctions with a high barrier height formed by the SC/E. We developed a multiphysics model based on the finite element method, including electromagnetic wave propagation, charge transport in bulk semiconductor, species transport in the electrolyte, and charge-transfer kinetics of surface redox reactions.The optical performance and electrochemical performance of CO2R reduction were compared in the three cases. Under the bare p-Si case, the saturation photocurrent densities (J sa) and the onset potential (V on) were -13.40 mA/cm2 and -0.80 V vs. RHE at -1 mA/cm2, respectively. Under the layered catalyst case, the J sa and the V on were -0.84 mA/cm2 and -0.50 V vs. RHE at -0.10 mA/cm2, respectively. Under the patterned catalysts case (f c is 0.1 in this case), the J sa is -12.30 mA/cm2 and V on is -0.69 V vs. RHE at -1 mA/cm2. The magnitude of J sa is influenced by the extent of light obscuration and the magnitude of V on is influenced by the photoelectrode activity and the pinch-off effects. The V on for the patterned case is higher than that of the bare p-Si case due to the improved electrode activity combined with the pinch-off effects. In the bare p-Si case and the patterned catalyst case, the CO2 mass transfer limits the CO partial current density (J CO) to further increase due to high J sa while CO2 mass transfer is not the limitation for COER at high overpotentials for the layered catalyst case due to significant light obscuration. The pinch-off effect also increased the applied voltage corresponding to the maximum J CO for the patterned case (-1.21 V vs. RHE) compared to that for the bare p-Si case (-1.35 V vs. RHE) due to the deceased transport resistance of electrons.The influence of different interfacial configurations, solar concentration ratio, different coverage of catalysts, and different sizes of catalysts for optical loss, pinch-off effect, CO2 mass transfer, current distribution at interfaces, and electrochemical performance are discussed in detail for the optimal design photoelectrode. This framework allows us to provide design guidelines for PEC CO2 reduction devices.