Solid-state Redox Research Articles

Insights into kinetic and transfer mechanisms for alkaline decoupled water electrolysis based on distribution of relaxation times

Electrochemical water splitting is crucial for the decarbonization of industrial processes and the coupling of renewable energy sources. Evaluation of individual HER and OER contributions to the transfer process is essential for optimizing electrolysis system performance. This study uses nickel-cobalt layered double hydroxide (NiCo-LDH) as a solid-state redox mediator (SRM), achieving efficient H2 and O2 separation. This study provides an in-depth analysis of kinetic and ion/charge/mass transfer mechanisms linked to various characteristic impedances in alkaline decoupled water electrolysis (DWE) system by integrating Electrochemical Impedance Spectroscopy (EIS) and Distribution of Relaxation Times (DRT) methods. The characteristic frequency evolution is consistent with the charge-discharge state of SRM and mass transfer frequency is related to the bubble size and gas diffusion rate of electrocatalysts at different current densities. The charge transfer Rct dominates the total polarization impedance in decoupled HER, and mass transfer Rm decreases remarkably in decoupled OER. OER shows lower Rct with Fe species existence and lower total polarization impedance than HER. Rm in decoupled HER increases by 38.24% and OER by 29.82% during prolonged decoupled test. This study provides valuable insights into the impedance contributions and kinetic mechanisms through novel electrolysis approaches, guiding further optimization of decoupled electrolyzer and electrode.

(Invited) Electrochemical Random Access Memory for Neuromorphic Computing

Electrochemical random access memory (ECRAM) is a three terminal device that functions by tuning electronic conductance in functional materials through solid-state electrochemical redox reactions, and is one of several emerging device concepts that are currently being investigated in academic and industrial laboratories for enabling energy efficient brain inspired computing. Early demonstrations of nearly ideal analog switching and the realization that electrochemical ion insertion can be used to tune the electronic properties of many types of materials including transition metal oxides, layered 2D material, organic and coordination polymers, has sparked tremendous interest in ECRAM. Depending on the specific material, the resulting changes in conduction can range from gradual increments needed for analog elements, to large, abrupt changes for dynamically reconfigurable adaptive architectures. At its core, ECRAM shares many fundamental aspects with rechargeable batteries, where ion insertion materials are used extensively for their ability to reversibly store charge and energy. Computing applications, however, present drastically different requirements: systems will require many millions of devices, scaled down to less than 100 nanometers, all while achieving reliable electronic-state tuning at scaled-up rates (~109 Hz) and endurances (>1012 cycles), and with minimal energy dissipation and noise. In my presentation, I will briefly cover the history of ECRAM, the recent progress in devices spanning various types of materials, circuits, architectures, and discuss the rich portfolio of challenging, fundamental questions and how we can harness ECRAM to realize a new paradigm for low power neuromorphic computing.

Exploring transition metal hydroxides performance in membrane-free electrolyzer based decoupled water splitting for step-wise production of hydrogen and oxygen

Alkaline decoupled electrolysis is an emerging safest approach for efficient green hydrogen production. Bifunctional electrocatalysts subjected to reverse polarity can be effectively used as the gas-evolving electrodes (GEE) in decoupled water electrolysis for generating both hydrogen and oxygen at different rates and at different times. Designing a bifunctional electrocatalyst with excellent activity and durability will successfully pave the way towards non-precious metal-based decoupled electrolysis systems. Here, for the first time, ternary NiFeMn layered hydroxide is presented as the bifunctional electrocatalyst associated with the β-Ni(OH)2 solid-state redox mediator (SRM) for independently generating hydrogen and oxygen in an alkaline solution without membranes. This electrocatalyst had shown low overpotentials in HER (230 mV at 10 mA/cm2) and OER (270 mV at 50 mA/cm2). In this work, a chemically synthesized hexagonal β-Ni(OH)2 nanosheet was employed as the solid-state redox mediator for fabricating the membrane-free electrolysis setup. Accompanying a bifunctional electrocatalyst in decoupled electrolysis, this device displayed a 1.91 V cell voltage at 10 mA/cm2. When considered together, the complexity and expense of the electrolyzer were decreased by using NiFeMn LH/NF as a bifunctional electrocatalyst. This approach can be considered for constructing a cost-effective and step-wise green hydrogen harvesting system of membrane-free water electrolyzer.

Toward Cost-Effective High-Energy Lithium-Ion Battery Cathodes: Covalent Bond Formation Empowers Solid-State Oxygen Redox in Antifluorite-Type Lithium-Rich Iron Oxide

Toward Cost-Effective High-Energy Lithium-Ion Battery Cathodes: Covalent Bond Formation Empowers Solid-State Oxygen Redox in Antifluorite-Type Lithium-Rich Iron Oxide

Solid-State Electrochemistry on Supramolecular Assemblies with Strong Isotropic Property

It has been mathematically proven that only honeycomb, diamond, and K4 lattices have a special symmetry called "strong isotropy". Note that crystallographic symmetry is determined only by the position of atoms, while strong isotropy is governed by both the position of atoms and bonds. The K4 lattice, called by various names such as gyroid lattice and srs network, is characterized by the fact that it exhibits chirality. It is recognized that their mathematically-defined “line graphs” correspond to kagome, hyper-kagome, and pyrochlore lattices, respectively, which are well known as spin frustration lattices. This relation suggests that the materials with the strong isotropic lattices possess “hidden” frustration. It is also noteworthy that the band structure of the three lattices contains exotic band dispersions such as Dirac cones due to their lattice symmetry, and that they possess porous structures. From this perspective, we proposed to form the supramolecular assemblies with the with strong isotropic property, and to carry out electrochemical valence control for realizing exotic physical properties such as Dirac electron, spin frustration, etc.In this presentation, we will explain our previous works on the solid-state electrochemistry of LiPc as an introduction. Then, we will report the rational synthesis of the molecule-based honeycomb and K4 structures, using MOF/COF- and supramolecular-chemistry, the nanohybridization using their porous structures, the physical properties such as spin frustration and circularly polarized luminescence, and the solid-state electrochemical redox control on them without destroying the original frameworks.

Understanding the Degradation Mechanisms in Lithium Manganese Oxyfluoride Cathodes

The development of next-generation nickel and cobalt-free Li-ion batteries with significantly greater energy density for electric vehicles is largely contingent on the discovery of new cathode materials. A number of novel candidates with a disordered rocksalt crystal structure have recently been reported to exhibit high capacities, offering a highly promising new avenue for cathode research.1–5 However, disordered rocksalt cathode materials generally suffer from voltage and capacity fade over cycling. For example, the Mn-based archetypal oxyfluoride Li2MnO2F shows 254 mAh g-1 discharge capacity in the first cycle but fades to 104 mAh g-1 after 100 cycles. For improving these materials leading to practical devices, it is vital to develop an understanding of the fading mechanisms.Various explanations have been proposed for the gradual deterioration in performance of disordered rocksalt oxide and oxyfluoride cathodes. These include O-loss and surface densification4,6, growth of a CEI layer7, phase segregation6,8. In this study, we examine the main degradation mechanisms in Li2MnO2F over cycling and demonstrate that the capacity and voltage retention can be improved through compositional control, figure below. This understanding points the way to manganese oxyfluorides with better cycling performance. House, R. A. et al. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ. Sci. 11, 926–932 (2018).Lee, J. et al. Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials. Nature 556, 185–190 (2018).Yabuuchi, N. et al. Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries. Nat. Commun. 7, 1–10 (2016).Li, L. et al. Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries. Adv. Funct. Mater. 2101888, 2101888 (2021).Chen, R. et al. Disordered lithium-rich oxyfluoride as a stable host for enhanced Li+ intercalation storage. Adv. Energy Mater. 5, (2015).Chen, D., Kan, W. H. & Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Adv. Energy Mater. 9, 1–15 (2019).Källquist, I. et al. Degradation Mechanisms in Li2VO2F Li-Rich Disordered Rock-Salt Cathodes. Chem. Mater. 31, 6084–6096 (2019).Chen, D., Ahn, J., Self, E., Nanda, J. & Chen, G. Understanding cation-disordered rocksalt oxyfluoride cathodes. J. Mater. Chem. A 2, 7826–7837 (2021). Figure 1

Exploring nucleation modeling of the voltammetry of solid-to-solid-state redox processes with phase changes

An operational description of the linear potential scan voltammetry of solids experiencing a solid-state redox transformation with phase changes is described. The modeling is based on the application of nucleation equations of solid-state reaction kinetics to express the transferred charge/applied potential relationships. The flexible use of Prout-Tompkins and Avrami-Erofe’ev kinetics permits a satisfactory description of the voltammetry of solid-to-solid redox transformations with phase segregation. The model satisfactorily applies to reproduce linear potential scan curves recorded for graphite electrodes modified with several lead compounds in contact with aqueous electrolyte solutions.

Enabling efficient decoupled alkaline water electrolysis using a low-cost sodium manganate solid-state redox mediator

The environmental pollution and energy crisis caused by the excessive consumption of fossil fuels have prompted a more urgent demand for the development of clean energy. As a carbon-free clean energy carrier, the sustainable production of hydrogen (H2) is a key step in the future decarbonization of the planet. Among many methods of hydrogen production, water electrolysis using renewable energy sources (e.g., solar, wind) can enable the generation of high-purity H2 in a low-carbon economy, which favors the alleviation of environmental and energy problems. In the past decades, water electrolysis has been dominated by alkaline water electrolysis, however, the oxidation and reduction reactions of water in conventional water electrolysis are closely coupled in space and time, which introduces challenges for the separation of gas products and the utilization of renewable energy. Herein, low-cost Na0.44MnO2 was chosen as solid-state redox mediator to decouple the evolution of H2 and O2 in alkaline electrolyte and obtain high-purity H2 without using any separator (membrane) or purification step. As an intermediate carrier for charge storage, the energy efficiency and decoupling efficiency can reach 98.7% and 97%, respectively, at constant current of 1.3 mA/cm2. Furthermore, we achieved flexible production of high-purity H2 by directly driving decoupled water electrolysis using intermittent renewable energy sources, confirming the potentially practical application of Na0.44MnO2.

Phenazine-based Compound Realizing Separate Hydrogen and Oxygen Production in Electrolytic Water Splitting.

Electrocatalytic water splitting powered by renewable energy is a sustainable approach for hydrogen production. However, conventional water electrolysis may suffer from gas mixing, and the different kinetics between hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) will limit the direct use of unstable renewable energies, leading to increased cost of H2 production. Herein, a novel phenazine-based compound is synthesized to develop the solid-state redox mediator associated water splititng process, and thus decoupling the H2 and O2 production in acid solution without the use of membrane. Excitingly, this organic redox mediator exhibits high specific capacity (290 mAh g-1 at 0.5 A g-1 ), excellent rate performance (186 mAh g-1 at 30 A g-1 ) and long cycle life (3000 cycles) due to its π-conjugated aromatic structure and the fast kinetics of H+ storage/release process. Furthermore, a membrane-free decoupled water electrolysis architecture driven by solar energy is achieved, demonstrating high-purity H2 production at different times.

Improving the rate capability of LiNi0.5Mn1.3Ti0.2O4 by modifying the lithium insertion mechanism

Since lithium-ion batteries are becoming increasingly popular, we aimed to increase the power density of lithium-ion batteries by improving the rate capability of lithium insertion electrodes by controlling the reaction mechanism of lithium insertion materials. LiNi0.5Mn1.3Ti0.2O4 (Ti-LNMO), in which a part of Mn4+ in LiNi0.5Mn1.5O4 (LNMO) was replaced by Ti4+, was synthesized and proceeded in a single-phase reaction over the entire region. This resulted in two types of LNMOs with the same spinel structures and solid-state redox reactions of Ni2+/Ni4+, each with different reaction mechanisms. According to rate-capability tests of these two types of LNMOs, the Ti-LNMO exhibited superior rate capability than the LNMO without Ti-substitution, which demonstrated that high-power capabilities could be achieved by controlling the reaction mechanism. Furthermore, the solid-state Li-ion diffusion coefficient in the LNMOs was determined from the rate-capability using the diluted electrode method, and that of Ti-LNMO was more than three times greater than that of the LNMO. These results reveal that the lithium insertion mechanism is important for the kinetics of lithium insertion reaction.

Quinoid-Based Three-Dimensional Metal–Organic Framework Fe2(dhbq)3: Porosity, Electrical Conductivity, and Solid-State Redox Properties

We report the synthesis, characterization, and electronic properties of the quinoid-based three-dimensional metal-organic framework [Fe2(dhbq)3]. The MOF was synthesized without using cations as a template, unlike other reported X2dhbq3-based coordination polymers, and the crystal structure was determined by using single-crystal X-ray diffraction. The crystal structure was entirely different from the other reported [Fe2(X2dhbq3)]2-; three independent 3D polymers were interpenetrated to give the overall structure. The absence of cations led to a microporous structure, investigated by N2 adsorption isotherms. Temperature dependence of electrical conductivity data revealed that it exhibited a relatively high electrical conductivity of 1.2 × 10-2 S cm-1 (Ea = 212 meV) due to extended d-π conjugation in a three-dimensional network. Thermoelectromotive force measurement revealed that it is an n-type semiconductor with electrons as the majority of charge carriers. Structural characterization and spectroscopic analyses, including SXRD, Mössbauer, UV-vis-NIR, IR, and XANES measurements, evidenced the occurrence of no mixed valency based on the metal and the ligand. [Fe2(dhbq)3] upon incorporating as a cathode material for lithium-ion batteries engendered an initial discharge capacity of 322 mAh/g.

Zero carbon solid-state rechargeable redox fuel for long duration and seasonal storage

<h2>Summary</h2> This work presents a unique thermochemical process for charging magnesium-manganese-oxide-based solid-state rechargeable redox fuel. High-temperature heating of the processing furnace can be driven by either renewable electricity or concentrated solar power. The simple recyclable fuel charging concept is based on a tubular falling bed reactor with countercurrent oxygen-depleted gas flow for complete heat recuperation. The main focus of this work is achieving solid flowability at high temperatures (1,450°C) and extracting chemically charged solid at ambient temperature with minimal energy loss. The operation strategies described in this work have enabled consistent magnesium-manganese-oxide particle flow up to 1,450°C. The measured extent of the thermal reduction reaction after cooling the particles is more than 90% of the fully reduced state at equilibrium. The thermal-to-chemical efficiency and overall system efficiency are 96% and 35% respectively, which are the highest reported for thermochemical fuels to date.

Supercapacitors as redox mediators for decoupled water splitting

With the help of the redox mediator, decoupled water-splitting allows O2 and H2 to be produced at different times, at different rates, and even in different cells, which promotes both the operation safety and the utilization of renewable power sources. However, the current densities and stabilities of these redox mediators are commonly low, which require further improvements for practical applications. Here, we propose to use supercapacitors as solid state redox mediators for decoupled water splitting. For demonstration, Na0.5MnO2 (pseudocapacitor) and active carbon (double layer capacitor), are both used as the redox mediator. These supercapacitors show superior current density (1 A/cm2) and ultralong cycle-life (8000 cycles) compared with commonly investigated battery-based mediators (NiOOH/Ni(OH)2). Our research proves supercapacitors can be used as redox relay with high current density and stability, which may bring new insights in the design of decoupled water splitting systems.

Suppressed shuttle effect and Self-Discharge of redox electrochemical capacitors using biphasic electrolyte with a Liquid-Liquid interface

Redox-enhanced electrochemical capacitors (redox ECs) can boost the energy density of conventional ECs by introducing soluble redox species in the electrolyte to gain additional capacity via faradaic reactions. The shuttle effect of the added redox species such as polyiodides, however, leads to rapid self-discharge of redox ECs and severely limits their capability for long-term energy storage. Herein, we propose an ionic liquid (IL)-based aqueous biphasic system (ABS) as the electrolyte of iodide-based redox ECs to suppress the self-discharge. Based on molecular dynamics (MD) simulation and mechanistic analysis, we reveal that the liquid–liquid interface in the ABS blocks the diffusion of polyiodides and leads to reduced self-discharge rate. Specifically, EC cells with ABS electrolyte deliver much lower open circuit voltage (OCV) drop (0.7 V) and better energy retention (>32%) in 24 h than that of cells with conventional iodide-based electrolyte (OCV drop: 1.6 V, energy retention: 0%). This strategy is also successfully extended to the fabrication of ABS-based hydrogel electrolyte for solid-state redox ECs with low self-discharge rate. The results of this work not only provide a mechanistic approach to elucidating the effect of redox couples on the self-discharge of redox ECs, but also demonstrate an effective way of mitigating the shuttle effect via liquid–liquid interface in a biphasic electrolyte to reduce self-discharge.

A High-Energy and Safe Lithium Battery Enabled by Solid-State Redox Chemistry in a Fireproof Gel Electrolyte.

Recent years have witnessed thriving efforts in pursuing high-energy batteries at an unaffordable cost of safety. Herein, a high-energy and safe quasi-solid-state lithium battery is proposed by solid-state redox chemistry of polymer-based molecular Li2 S cathode in a fireproof gel electrolyte. This chemistry fully eliminates not only the negative effect of extremely reactive Li metal and oxygen species on cell safety but also the damage of electrode reversibility by soluble redox intermediates. The molecular Li2 S cathode exhibits an exceptional lifetime of 2000 cycles, 100% Coulombic efficiency, high capacity of 830 mAh g-1 with ultralow capacity loss of 0.005-0.01% per cycle and superior rate capability up to 10 C. Meanwhile, it shows high stability in the carbonate-involving electrolyte for maximizing the compatibility with carbonate-efficient Si anode. The optimized cell chemistry exerts high energy over 750Wh kg-1 for 500 cycles with fast rate response, high-temperature adaptability, and no self-discharge. A fire-retardant composite gel electrolyte is developed to further strengthen the intrinsic safe redox between the Li2 S cathode and the Si anode, which secures remarkable safety against extreme abuse of overheating, short circuits, and mechanical damage in air/water or even when on fire.

Photovoltaic Performance of Dye-Sensitized Solar Cells with a Solid-State Redox Mediator Based on an Ionic Liquid and Hole-Transporting Triphenylamine Compound

An ionic liquid, 1-methyl-3-propylimidazolium iodide (MPII), was solidified with an organic hole-transporting material, 4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (m-MTDATA), and the resulting solid-state redox mediator (RM) (m-MTDATA-solidified MPII) was employed in solar devices to realize solid-state dye-sensitized solar cells (sDSSCs). Solar devices with only MPII or m-MTDATA as an RM showed almost 0 mA/cm2 of short-circuit current (Jsc) and thus 0% power conversion efficiency (PCE). However, an sDSSC with the m-MTDATA-solidified MPII exhibited 4.61 mA/cm2 of Jsc and 1.80% PCE. It was found that the increased Jsc and PCE were due to the formation of I3−, which resulted from a reaction between the iodie (I−) of MPII and m-MTDATA cation. Further enhancement in both Jsc (9.43 mA/cm2) and PCE (4.20%) was observed in an sDSSC with 4-tert butylpyridine (TBP) as well as with m-MTDATA-solidified MPII. We attributed the significant increase (about 230%) in PCE to the lowered diffusion resistance of I−/I3− ions in the solid-state RM composed of the m-MTDATA-solidified MPII and TBP, arising from TBP’s role as a plasticizer.

Screening and Understanding Lattice Silicon-Controlled Catalytically Active Site Motifs from a Library of Transition Metal-Silicon Intermetallics.

A joint theoretical and experimental study is reported to systematically explore over a library of transition metal-silicon intermetallics for understanding silicon-controlled active site motifs and discovering hydrogen-evolving electrocatalysts. On the one hand, every low-index surface termination of 115 transition metal (M)-silicon (Si) intermetallics is enumerated, followed by cataloging of stable adsorption sites and prediction of catalytic activities on the main exposed facets. It is theoretically found that silicon atoms in silicon-rich structures (especially MSi2 and MSi) show a strong site-isolating effect, which can eliminate M-M-M hollow and M-M bridge sites with too strong hydrogen-binding ability and thereby provide great opportunities for the exposure of novel highly active sites (e.g., M-top and Si-related sites). On the other hand, solid-state redox reactions are developed to synthesize a set of 24 silicides containing 5 MSi, 13 MSi2 , and 6 others, most of which are phase-pure samples. The experimental studies demonstrate that too rich silicon content in silicides (e.g., MSi2 ) leads to adverse effects, such as the formation of amorphous SiOx layers on the silicide surface, masking the presence of active sites during electrocatalysis. Finally, 5 MSi (M = Rh, Pd, Pt, Ru, Ir) as highly active hydrogen-evolving electrocatalysts are identified.

Plasma treated M1 MoVNbTeOx–CeO2 composite catalyst for improved performance of oxidative dehydrogenation of ethane

High activity and productivity of MoVNbTeOx catalyst are challenging tasks in oxidative dehydrogenation of ethane (ODHE) for industrial application. In this work, phase-pure M1 with 30 wt% CeO2 composite catalyst was treated by oxygen plasma to further enhance catalyst performance. The results show that the oxygen vacancies generated by the solid-state redox reaction between M1 and CeO2 capture active oxygen species in gas and transform V4+ to V5+ without damage to M1 structure. The space-time yield of ethylene of the plasma-treated catalyst was significantly increased, in which the catalyst shows an enhancement near ∼100% than that of phase-pure M1 at 400 °C for ODHE process. Plasma treatment for catalysts demonstrates an effective way to convert electrical energy into chemical energy in catalyst materials. Energy conversion is achieved by using the catalyst as a medium.

A novel strategy to adjust the oxygen vacancy of CuO/MnO2 catalysts toward the catalytic oxidation of toluene

A facile and efficient solid-state redox strategy with low consumption was applied to modify the surface defect concentration of α-MnO2. The surficial reduction of α-MnO2 by Cu2O tremendously increased the concentration of oxygen vacancy of the as-synthesized CuO/MnO2-R catalyst, which significantly enhanced its catalytic performance for the combustion of toluene. The T90 of CuO/MnO2-R-10 catalyst obtained by strengthening surface defects was 36 °C and 32 °C lower than α-MnO2 and CuO/MnO2-10 catalyst, respectively, which could be contributed to the increase of structural defects. In addition, the reaction mechanism of toluene over the surface of CuO/MnO2-R-10 was also revealed by in-situ DRIFTS. Meanwhile, the CuO/MnO2-R-10 also exhibited excellent stability and resistance to water vapor and showed great potential in practical application.

Nanosized and metastable molybdenum oxides as negative electrode materials for durable high-energy aqueous Li-ion batteries

Significance This study describes a high-energy and durable aqueous battery system with metastable and nanosized Mo-based oxides used as high-capacity negative electrodes. A wider electrochemical window is achieved with concentrated aqueous electrolytes through which highly reversible Li storage without the decomposition of water molecules is achieved for the Mo-based oxides. A full cell with an Mn-based oxide shows good capacity retention over 2,000 cycles. X-ray absorption spectroscopy reveals that the solid-state redox reaction of Mo ions reversibly proceeds in aqueous electrolytes for the metastable Mo oxide. This study opens a way to develop high-energy, durable, and safe batteries on the basis of metastable and nanosized oxides with aqueous electrolyte solutions.