Discovery of fast and stable proton storage in bulk hexagonal molybdenum oxide

MoO3 nanobelts cathode promotes Al3+ insertion in aqueous aluminum-ion batteries

Aqueous aluminium ion batteries (AAIBs) have attracted much attention due to their high theoretical capacity, safety, and environmental friendliness. However, the Research and Development (R&D) of cathode materials has limited its development and application. MoO3 has been proven to be a reliable and stable cathode material, nevertheless, it faces the dilemma of poor cycling performance and low specific capacity in AAIBs due to the irreversible phase transition in its structure. In this paper, MoO3 synthesized by a hydrothermal method has a unique nanobelt structure, which significantly enhances the structural stability of MoO3 and reduces its structural damage during charging/discharging. In addition, the nanobelt structure also gives MoO3 a rougher surface, which provides a large number of active sites and spaces for the insertion and extraction of Al3+ and improves the diffusion rate of Al3+ to a large extent. Experimental results demonstrate that this MoO3 nanobelt cathode exhibits significantly improved cycling stability and high specific capacity in AAIBs. This paper provides a practical solution to the existing challenges of AAIBs and further promotes the development and application of molybdenum-based materials in AAIBs.

Tailoring Acid-Salt Hybrid Electrolyte Structure for Stable Proton Storage at Ultralow Temperature

The critical challenges in developing ultralow-temperature proton-based energy storage systems are enhancing the diffusion kinetics of charge carriers and inhibiting water-triggered interfacial side reactions between electrolytes and electrodes. Here an acid-salt hybrid electrolyte with a stable anion-cation-H2O solvation structure that realizes unconventional proton transport at ultralow temperature is shown, which is crucial for electrodes and devices to achieve high rate-capacity and stable interface compatibility with electrodes. Through multiscale simulations and experimental investigations in the electrolyte employing ZnCl2 introduced into 0.2 M H2SO4 solution, it is discovered that unique anion-cation-H2O solvation structure endows the electrolyte with low-temperature-adaptive feature and favorable water network channels for rapid proton transport. In situ XRD and multiple spectroscopic techniques further reveal that the stable 3D network structure inhibits free water-triggered deleterious electrode structure distortion by immobilizing free water molecules to achieve outstanding cycling stability. Hence, VHCF//α-MoO3 hybrid proton capacitors deliver an unexpected capacity of 39.8 mAh g-1 at a high current density of 1 A g-1 (-80 °C) and steady power supply under ultralow temperatures (96% capacity retention after 1500 cycles at -80 °C). The anti-freezing hybrid electrolyte design provides an effective strategy to improve the application of energy storage devices in ultralow temperatures.

The Preparation of High-Performance MoO3 Nanorods for 2.1 V Aqueous Asymmetric Supercapacitor

In order to broaden the working voltage (1.23 V) of aqueous supercapacitors, a high-performance asymmetric supercapacitor with a working voltage window reaching up to 2.1 V is assembled using a nanorod-shaped molybdenum trioxide (MoO3) negative electrode and an activated carbon (AC) positive electrode, as well as a sodium sulfate-ethylene glycol ((Na2SO4-EG) electrolyte. MoO3 electrode materials are fabricated by adjusting the hydrothermal temperature, hydrothermal time and solution's pH value. The specific capacity of the optimal MoO3 electrode material can reach as high as 244.35 F g-1 at a current density of 0.5 A g-1. For the assembled MoO3//AC asymmetric supercapacitor with a voltage window of 2.1 V, its specific capacity, the energy density, and the power density are 13.52 F g-1, 8.28 Wh kg-1, and 382.15 W kg-1 at 0.5 A g-1, respectively. Moreover, after 5000 charge-discharge cycles, the capacity retention rate of the device still reaches 109.2%. This is mainly attributed to the small particle size of MoO3 nanorods, which can expose more electrochemically active sites, thus greatly facilitating the transport of electrolyte ions, immersion at the electrolyte/electrolyte interface and the occurrence of electrochemical reactions.

Advanced characterization of confined electrochemical interfaces in electrochemical capacitors

The advancement of high-performance fast-charging materials has significantly propelled progress in electrochemical capacitors (ECs). Electrochemical capacitors store charges at the nanoscale electrode material-electrolyte interface, where the charge storage and transport mechanisms are mediated by factors such as nanoconfinement, local electrode structure, surface properties and non-electrostatic ion-electrode interactions. This Review offers a comprehensive exploration of probing the confined electrochemical interface using advanced characterization techniques. Unlike classical two-dimensional (2D) planar interfaces, partial desolvation and image charges play crucial roles in effective charge storage under nanoconfinement in porous materials. This Review also highlights the potential of zero charge as a key design principle driving nanoscale ion fluxes and carbon-electrolyte interactions in materials such as 2D and three-dimensional (3D) porous carbons. These considerations are crucial for developing efficient and rapid energy storage solutions for a wide range of applications.

Galvanic hydrogenation reaction in metal oxide

Rational reforming of metal oxide has a potential importance to modulate their inherent properties toward appealing characteristics for various applications. Here, we present a detailed fundamental study of the proton migration phenomena between mediums and propose the methodology for controllable metal oxide hydrogenation through galvanic reactions with metallic cation under ambient atmosphere. As a proof of concept for hydrogenation, we study the role of proton adoption on the structural properties of molybdenum trioxide, as a representative, and its impact on redox characteristics in Li-ion battery (LiB) systems using electrochemical experiments and first-principles calculation. The proton adoption contributes to a lattice rearrangement facilitating the faster Li-ion diffusion along the selected layered and mediates the diffusion pathway that promote the enhancements of high-rate performance and cyclic stability. Our work provides physicochemical insights of hydrogenations and underscores the viable approach for improving the redox characteristics of layered oxide materials.

Emerging Two-Dimensional Materials for Proton-Based Energy Storage

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Tailoring d-p Orbital Hybridization to Decipher the Essential Effects of Heteroatom Substitution on Redox Kinetics

The heteroatom substitution is considered as a promising strategy for boosting the redox kinetics of transition metal compounds in hybrid supercapacitors (HSCs) although the dissimilar metal identification and essential mechanism that dominate the kinetics remain unclear. It is presented that d-p orbital hybridization between the metal and electrolyte ions can be utilized as a descriptor for understanding the redox kinetics. Herein, a series of Co, Fe and Cu heteroatoms are respectively introduced into Ni3Se4 cathodes, among them, only the moderate Co-substituted Ni3Se4 can hold the optimal d-p orbital hybridization resulted from the formed more unoccupied antibonding states π*. It inevitably enhances the interfacial charge transfer and ensures the balanced OH- adsorption-desorption to accelerate the redox kinetics validated by the lowest reaction barrier (0.59 eV, matching well with the theoretical calculations). Coupling with the lower OH- diffusion energy barrier, the prepared cathode delivers ultrahigh rate capability (~68.7 % capacity retention even the current density increases by 200 times), and an assembled HSC also presents high energy/power density. This work establishes the principles for determining heteroatoms and deciphers the underlying effects of the heteroatom substitution on improving redox kinetics and the rate performance of battery-type electrodes from a novel perspective of orbital-scale manipulation.

Imaging Anisotropic Proton Intercalation in Photochromic MoO3

Protonation represents a fundamental chemical process with promising applications in the fields of energy, environment, and memory devices. Probing the protonation mechanism, however, presents a formidable challenge owing to the elusiveness of intercalated protons. In this work, we utilize the atomic and electronic structure changes associated with protonation to directly image the proton intercalation pathways in α-MoO3 induced by UV illumination. We reveal the anisotropic intercalation behavior which is initiated by photocatalyzed water dissociation preferentially at the (001) edges and then propagates along the c axis, transforming α-MoO3 into HxMoO3 to realize photochromism. This photochromic process can be reversed via heating in air, leading to anisotropic proton deintercalation, also preferentially along the c axis. The observed anisotropic behavior can be attributed to the intrinsically low energy barriers for both proton migration along the c axis and water dissociation/formation at (001) edges.

Synchronous Redox Reactions in Copper Oxalate Enable High-Capacity Anode for Proton Battery

Proton batteries are competitive due to their merits such as high safety, low cost, and fast kinetics. However, it is generally difficult for current studies of proton batteries to combine high capacity and high stability, while the research on proton storage mechanism and redox behavior is still in its infancy. Herein, the polyanionic layered copper oxalate is proposed as the anode for a high-capacity proton battery for the first time. The copper oxalate allows for reversible proton insertion/extraction through the layered space but also achieves high capacity through synchronous redox reactions of Cu2+ and C2O42-. During the discharge process, the bivalent Cu-ion is reduced, whereas the C═O of the oxalate group is partially converted to C-O. This synchronous behavior presents two units of charge transfer, enabling the embedding of two units of protons in the (110) crystal face. As a result, the copper oxalate anode demonstrates a high specific capacity of 226 mAh g-1 and maintains stable operation over 1000 cycles with a retention of 98%. This work offers new insights into the development of dual-redox electrode materials for high-capacity proton batteries.