Hydrous Nickel-Iron Turnbull's Blue as a High-Rate and Low-Temperature Proton Electrode

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.

An Emerging Chemistry Revives Proton Batteries

Developing new energy techniques that simultaneously integrate the fast rate capabilities of supercapacitors and high capacities of batteries represents an ultimate goal in the field of electrochemical energy storage. A new possibility arises with an emerging battery chemistry that relies on proton-ions as the ion-charge-carrier and benefits from the fast transportation kinetics. Proton-based battery chemistry starts with the recent discoveries of materials for proton redox reactions and leads to a renaissance of proton batteries. In this article, the historical developments of proton batteries are outlined and key aspects of battery chemistry are reviewed. First, the fundamental knowledge of proton-ions and their transportation characteristics is introduced; second, Faradaic electrodes for proton storage are categorized and highlighted in detail; then, reported electrolytes and different designs of proton batteries are summarized; last, perspectives of developments for proton batteries are proposed. It is hoped that this review will provide guidance on the rational designs of proton batteries and benefit future developments.

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

Ionic and electronic transport in electrodes is crucial for electrochemical energy storage technology. To optimize the transport pathway of ions and electrons, electrode materials are minimized to nanometer-sized dimensions, leading to problems of volumetric performance, stability, cost, and pollution. Here we find that a bulk hexagonal molybdenum oxide with unconventional ion channels can store large amounts of protons at a high rate even if its particle size is tens of micrometers. The diffusion-free proton transport kinetics based on hydrogen bonding topochemistry is demonstrated in hexagonal molybdenum oxide whose proton conductivity is several orders of magnitude higher than traditional orthorhombic molybdenum oxide. In situ X-ray diffraction and theoretical calculation reveal that the structural self-optimization in the first discharge effectively promotes the reversible intercalation/de-intercalation of subsequent protons. The open crystal structure, suitable proton channels, and negligible volume strain enable rapid and stable proton transport and storage, resulting in extremely high volumetric capacitance (~1750 F cm-3), excellent rate performance, and ultralong cycle life (>10,000 cycles). The discovery of unconventional materials and mechanisms that enable proton storage of micrometer-sized particles in seconds boosts the development of fast-charging energy storage systems and high-power practical applications.

Electrochemical cells from water ice? Preliminary methods and results

Electrochemical cells from ice will be an important seasonal addition to power generation in cold regions. We demonstrate power generation on the order of 0.1 mW at 0.3 V and 0.13 m2 surface area using an electrochemical cell with 2% HCl providing a pH gradient in ice, and suggest a solar add-on effect due to temperature changes under direct sunlight. Different models are discussed, and data are presented related to different additives: (1) solutes such as NaCl and monopotassium phosphate; (2) pH modifying agents such as acids and bases; (3) particulate suspensions with kaolinite and other substances. The results are positive and suggest viable use of electrochemical cells from ice with low fabrication costs and safe environmental impact for ephemeral power generation, especially with future material improvements and refinement of technique. Current research in this nascent field is also briefly introduced. The model presented has implications both for power systems and for biology: an icy-worlds hypothesis for the origin of life suggests a protometabolism with an ice-based pH gradient.

An Internal Real-Time Microscopic Diagnosis of a Proton Battery Stack during Charging and Discharging

The proton battery has facilitated a new research direction for technologies related to fuel cells and energy storage. Our R&D team has developed a prototype of a proton battery stack, but there are still problems to be solved, such as leakage and unstable power generation. Moreover, it is unlikely that the multiple important physical parameters inside the proton battery stack can be measured accurately and simultaneously. At present, external or single measurements represent the bottleneck, yet the multiple important physical parameters (oxygen, hydrogen, voltage, current, temperature, flow, and humidity) are interrelated and have a significant impact on the performance, life, and safety of the proton battery stack. This research uses micro-electro-mechanical systems (MEMS) technology to develop a micro oxygen sensor and integrates the six-in-one microsensor that our R&D team previously developed in order to improve sensor output and facilitate overall operation by redesigning the incremental mask and having this co-operate with a flexible board for sensor back-end integration, completing the development of a flexible seven-in-one (oxygen, hydrogen, voltage, current, temperature, flow, and humidity) microsensor.

Rational Design of Electrode-Electrolyte Interphase and Electrolytes for Rechargeable Proton Batteries

Rechargeable proton batteries have been regarded as a promising technology for next-generation energy storage devices, due to the smallest size, lightest weight, ultrafast diffusion kinetics and negligible cost of proton as charge carriers. Nevertheless, a proton battery possessing both high energy and power density is yet achieved. In addition, poor cycling stability is another major challenge making the lifespan of proton batteries unsatisfactory. These issues have motivated extensive research into electrode materials. Nonetheless, the design of electrode-electrolyte interphase and electrolytes is underdeveloped for solving the challenges. In this review, we summarize the development of interphase and electrolytes for proton batteries and elaborate on their importance in enhancing the energy density, power density and battery lifespan. The fundamental understanding of interphase is reviewed with respect to the desolvation process, interfacial reaction kinetics, solvent-electrode interactions, and analysis techniques. We categorize the currently used electrolytes according to their physicochemical properties and analyze their electrochemical potential window, solvent (e.g., water) activities, ionic conductivity, thermal stability, and safety. Finally, we offer our views on the challenges and opportunities toward the future research for both interphase and electrolytes for achieving high-performance proton batteries for energy storage.

Pairing of Aqueous and Nonaqueous Electrosynthetic Reactions Enabled by a Redox Reservoir Electrode

Paired electrolysis methods are appealing for chemical synthesis because they generate valuable products at both electrodes; however, development of such reactions is complicated by the need for both half-reactions to proceed under mutually compatible conditions. Here, a modular electrochemical synthesis (ModES) strategy bypasses these constraints using a "redox reservoir" (RR) to pair electrochemical half-reactions across aqueous and nonaqueous solvents. Electrochemical oxidation reactions in organic solvents, the conversion of 4-t-butyltoluene to benzylic dimethyl acetal and aldehyde in methanol or the oxidative C-H amination of naphthalene in acetonitrile, and the reduction of oxygen to hydrogen peroxide in water were paired using nickel hexacyanoferrate as an RR that can selectively store and release protons (and electrons) while serving as the counter electrode for these reactions. Selective proton transport through the RR is optimized and confirmed to enable the ion balance, and thus the successful pairing, between redox half-reactions that proceed with different rates, on different scales, and in different solvents (methanol, acetonitrile, and water).

Molecular Crowding Electrolytes for Stable Proton Batteries

Proton electrochemistry is promising for developing post-lithium energy storage devices with high capacity and rate capability. However, some electrode materials are vulnerable because of the co-intercalation of free water molecules in traditional acid electrolytes, resulting in rapid capacity fading. Here, the authors report a molecular crowding electrolyte with the usage of poly(ethylene glycol) (PEG) as a crowding agent, achieving fast and stable electrochemical proton storage and expanded working potential window (3.2 V). Spectroscopic characterisations reveal the formation of hydrogen bonds between water and PEG molecules, which is beneficial for confining the activity of water molecules. Molecular dynamics simulations confirm a significant decrease of free water fraction in the molecular crowding electrolyte. Dynamic structural evolution of the MoO₃ anode is studied by in-situ synchrotron X-ray diffraction (XRD), revealing a reversible multi-step naked proton (de)intercalation mechanism. Surficial adsorption of PEG molecules on MoO₃ anode works in synergy to alleviate the destructive effect of concurrent water desolvation, thereby achieving enhanced cycling stability. This strategy offers possibilities of practical applications of proton electrochemistry thanks to the low-cost and eco-friendly nature of PEG additives.

Solvent-free protic liquid enabling batteries operation at an ultra-wide temperature range

Nowadays, electrolytes for commercial batteries are mostly liquid solutions composed of solvent and salt to migrate the ions. However, solvents of the electrolyte bring several inherent limitations, either the electrochemical window, working temperature, volatility or flammability. Herein, we report polyphosphoric acid as a solvent-free protic liquid electrolyte, which excludes the demerits of solvent and exhibits unprecedented superiorities, including nonflammability, wider electrochemical stability window (>2.5 V) than aqueous electrolyte, low volatility and wide working temperature range (>400 °C). The proton conductive electrolyte enables MoO₃/LiVPO₄F rocking-chair battery to operate well in a wide temperature range from 0 °C to 250 °C and deliver a high power density of 4975 W kg⁻¹ at a high temperature of 100 °C. The solvent-free electrolyte could provide a viable route for the stable and safe batteries working under harsh conditions, opening up a route towards designing wide-temperature electrolytes.

It is challenging to prepare electrolyte that could achieve wide electrochemical window, broad working temperature, non-inflammability, and fast ion transport simultaneously. Here the authors report a rocking-chair proton battery utilizing a solvent-free protic liquid electrolyte, which could operate in a broad temperature range from 0 to 250 celsius degree.

Electrochemical Proton Storage: From Fundamental Understanding to Materials to Devices

Simultaneously improving the energy density and power density of electrochemical energy storage systems is the ultimate goal of electrochemical energy storage technology. An effective strategy to achieve this goal is to take advantage of the high capacity and rapid kinetics of electrochemical proton storage to break through the power limit of batteries and the energy limit of capacitors. This article aims to review the research progress on the physicochemical properties, electrochemical performance, and reaction mechanisms of electrode materials for electrochemical proton storage. According to the different charge storage mechanisms, the surface redox, intercalation, and conversion materials are classified and introduced in detail, where the influence of crystal water and other nanostructures on the migration kinetics of protons is clarified. Several reported advanced full cell devices are summarized to promote the commercialization of electrochemical proton storage. Finally, this review provides a framework for research directions of charge storage mechanism, basic principles of material structure design, construction strategies of full cell device, and goals of practical application for electrochemical proton storage.

Boosted H+ Intercalation Enables Ultrahigh Rate Performance of the δ-MnO2 Cathode for Aqueous Zinc Batteries

H+ intercalation, as a critical battery chemistry, enables electrodes' high rate performance due to the fast diffusion kinetics of H+. In this work, more water molecules are introduced into δ-MnO2 by the protonation of δ-MnO2 with abundant oxygen vacancies. Benefiting from the structure with a close arrangement of water molecules in interlayers, the Grotthuss transport of proton is achieved in the energy storage of the δ-MnO2 cathode. As a result, the δ-MnO2 cathode exhibits an ultrahigh rate performance with a capacity of 368.1 mAh g-1 at 0.5 A g-1 and 83.4 mAh g-1 at 50 A g-1, which has a capacity retention of 73% after 1100 cycles at 10 A g-1. The study of the storage mechanism reveals that the Grotthuss intercalation of proton predominates the storage process, which empowers the cathode with high rate performance.

Optimization of hydrogen-ion storage performance of tungsten trioxide nanowires by niobium doping

The transport and storage of ions within solid state structures is a fundamental limitation for fabricate more advanced electrochemical energy storage, memristor, and electrochromic devices. Crystallographic shear structure can be induced in the tungsten bronze structures composed of corner-sharing WO₆octahedra by the addition of edge-sharing NbO₆octahedra, which might provide more storage sites and more convenient transport channels for external ions such as hydrogen ions and alkali metal ions. Here, we show that Nb₂O₅·15WO₃nanowires (Nb/W = 0.008) with long length-diameter ratio, smooth surface, and uniform diameter have been successfully synthesized by a simple hydrothermal method. The Nb₂O₅·15WO₃nanowires do exhibit more advantages over h-WO₃nanowires in electrochemical hydrogen ion storage such as smaller polarization, larger capacity (71 mAh g^-1, at 10C, 1C = 100 mA g^-1), better cycle performance (remain at 99% of the initial capacity after 200 cycles at 100C) and faster H⁺ions diffusion kinetics. It might be the crystallographic shear structure induced by Nb doping that does result in the marked improvement in the hydrogen-ion storage performance of WO₃. Therefore, complex niobium tungsten oxide nanowires might offer great promise for the next generation of electrochemical energy and information storage devices.

An Ultrafast and Ultra-Low-Temperature Hydrogen Gas-Proton Battery

Aqueous proton batteries are regarded as one of the most promising energy technologies for next-generation grid storage due to the distinctive merits of H⁺ charge carriers with small ionic radius and light weight. Various materials have been explored for aqueous proton batteries; however, their full batteries show undesirable electrochemical performance with limited rate capability and cycling stability. Here we introduce a novel aqueous proton full battery that shows remarkable rate capability, cycling stability, and ultralow temperature performance, which is driven by a hydrogen gas anode and a Prussian blue analogue cathode in a concentrated phosphoric acid electrolyte. Its operation involves hydrogen evolution/oxidation redox reactions on the anode and H⁺ insertion/extraction reactions on the cathode, in parallel with the ideal transfer of only H⁺ between these two electrodes. The fabricated aqueous hydrogen gas-proton battery exhibits an unprecedented charge/discharge capability of up to 960 C with a superior power density of 36.5 kW kg^-1, along with an ultralong cycle life of over 0.35 million cycles. Furthermore, this hydrogen gas-proton battery is able to work well at an ultralow temperature of -80 °C with 54% of its room-temperature capacity and under -60 °C with a stable cycle life of 1150 cycles. This work provides new opportunities to construct aqueous proton batteries with high performance in extreme conditions for large-scale energy storage.

Electrolyte Design for Lithium Metal Anode-Based Batteries Toward Extreme Temperature Application

Lithium anode-based batteries (LBs) are highly demanded in society owing to the high theoretical capacity and low reduction potential of metallic lithium. They are expected to see increasing deployment in performance critical areas including electric vehicles, grid storage, space, and sea vehicle operations. Unfortunately, competitive performance cannot be achieved when LBs operating under extreme temperature conditions where the lithium-ion chemistry fail to perform optimally. In this review, a brief overview of the challenges in developing LBs for low temperature (<0 °C) and high temperature (>60 °C) operation are provided followed by electrolyte design strategies involving Li salt modification, solvation structure optimization, additive introduction, and solid-state electrolyte utilization for LBs are introduced. Specifically, the prospects of using lithium metal batteries (LMBs), lithium sulfur (Li-S) batteries, and lithium oxygen (Li-O2 ) batteries for performance under low and high temperature applications are evaluated. These three chemistries are presented as prototypical examples of how the conventional low temperature charge transfer resistances and high temperature side reactions can be overcome. This review also points out the research direction of extreme temperature electrolyte design toward practical applications.

Prussian Blue Analogues in Aqueous Batteries and Desalination Batteries

In the applications of large-scale energy storage, aqueous batteries are considered as rivals for organic batteries due to their environmentally friendly and low-cost nature. However, carrier ions always exhibit huge hydrated radius in aqueous electrolyte, which brings difficulty to find suitable host materials that can achieve highly reversible insertion and extraction of cations. Owing to open three-dimensional rigid framework and facile synthesis, Prussian blue analogues (PBAs) receive the most extensive attention among various host candidates in aqueous system. Herein, a comprehensive review on recent progresses of PBAs in aqueous batteries is presented. Based on the application in different aqueous systems, the relationship between electrochemical behaviors (redox potential, capacity, cycling stability and rate performance) and structural characteristics (preparation method, structure type, particle size, morphology, crystallinity, defect, metal atom in high-spin state and chemical composition) is analyzed and summarized thoroughly. It can be concluded that the required type of PBAs is different for various carrier ions. In particular, the desalination batteries worked with the same mechanism as aqueous batteries are also discussed in detail to introduce the application of PBAs in aqueous systems comprehensively. This report can help the readers to understand the relationship between physical/chemical characteristics and electrochemical properties for PBAs and find a way to fabricate high-performance PBAs in aqueous batteries and desalination batteries.

Proton-conductive coordination polymer glass for solid-state anhydrous proton batteries

Designing solid-state electrolytes for proton batteries at moderate temperatures is challenging as most solid-state proton conductors suffer from poor moldability and thermal stability. Crystal-glass transformation of coordination polymers (CPs) and metal-organic frameworks (MOFs) via melt-quenching offers diverse accessibility to unique properties as well as processing abilities. Here, we synthesized a glassy-state CP, [Zn3(H2PO4)6(H2O)3](1,2,3-benzotriazole), that exhibited a low melting temperature (114 °C) and a high anhydrous single-ion proton conductivity (8.0 × 10-3 S cm-1 at 120 °C). Converting crystalline CPs to their glassy-state counterparts via melt-quenching not only initiated an isotropic disordered domain that enhanced H+ dynamics, but also generated an immersive interface that was beneficial for solid electrolyte applications. Finally, we demonstrated the first example of a rechargeable all-solid-state H+ battery utilizing the new glassy-state CP, which exhibited a wide operating-temperature range of 25 to 110 °C.

Vanadium hexacyanoferrate as high-capacity cathode for fast proton storage

Proton electrochemistry is promising for developing future energy storage devices with both high capacity and good rate capability. However, the development of this technology is now hindered by the limited choice of accessible electrodes, especially for cathodes. Herein, we report vanadium hexacyanoferrate (VHCF) as a candidate cathode for proton batteries. Exploiting dual redox-centers of vanadium and iron, VHCF delivers a high specific capacity of 108 mA h g^-1. Furthermore, an outstanding rate capability (∼60% of initial value at 100C) and stable cycling for tens of thousands of cycles are also demonstrated. These results are anticipated to inspire searches for more accessible materials and accelerate advances in energy storage devices.