Chemical looping combustion (CLC) has emerged as a cost-effective technology for carbon capture at the combustion source. The reactor, being central to the implementation of CLC, primarily adheres to two technological pathways: the dual fluidized bed reactor and the packed bed reactor. However, the intricate interaction between gas-solid reaction flow and heat/mass transfer processes in these reactors gives rise to diverse operational principles at both macroscopic and microscopic levels across various reactor forms and scales, making performance prediction challenging. Consequently, the rational design of CLC reactors poses a significant challenge in advancing this technology to commercial viability. This article offers an extensive review of the prevailing reactor designs in CLC, delving into reactor characteristics, pivotal aspects of the design process, methodologies, and representative studies in the field. The predominant reactor design approaches are categorized into engineering and numerical methods. The former encompasses phenomenological and similarity analysis methods, whereas the latter consists of macroscopic and computational fluid dynamics simulation methods. Each method possesses its theoretical framework, distinctive characteristics, appropriate applications, and respective advantages and limitations. In practical applications, integrating these aspects is essential. For instance, the engineering design, which is less costly but also less precise, is effective for quickly screening numerous potential design scenarios. In contrast, the numerical design, despite its higher computational demand and greater model complexity, offers improved predictive accuracy and is optimal for validating and refining engineering design solutions.

With the widespread of wearable electronics in healthcare, military and entertainment sectors, flexible power sources have attracted great attention, among which flexible fuel cells are relatively young compared with flexible batteries, supercapacitors and energy harvesters. Fuel cell is well known for its uninterrupted operation, high energy density and instant refueling ability, which is especially advantageous for long-term and outdoor missions. To date, existing flexible fuel cell studies can be classified into three major types based on their electrolyte and catalyst material, namely the flexible polymer electrolyte membrane fuel cell (PEMFC), membraneless fuel cell (MFC) and biofuel cell (BFC). The flexible PEMFC generally employs hydrogen as fuel so that a power density of hundreds of mW cm−2 can be achieved. Relevant research efforts are mainly paid to the replacement of conventional rigid cell components with flexible substitutes. Moreover, novel cell structures such as ultrathin cell and tubular cell have also been proposed. However, the flexible hydrogen storage is still a research gap. The flexible MFC has a much wider choice of fuel such as methanol, ethanol and formate, but the power output is limited to dozens of mW cm−2 due to more sluggish fuel oxidation. To circumvent the demand of pumping, porous materials with capillary action are preferred as cell substrate, such as cellulose paper and cotton thread, which can absorb electrolyte solution passively. Nevertheless, the capillary flow rate is not controllable at the moment. As for the flexible BFC, it is primarily targeted for epidermal applications in order to utilize natural organic materials in human body fluid. Benefited from this, the flexible BFC can have the simplest cell structure of two bioelectrodes only, which can be integrated onto contact lenses, tattoos, clothes, etc. However, the complex organic fuel oxidation as well as the mild electrolyte pH have greatly restricted its power density to μW cm−2 level. In this work, a comprehensive review on existing flexible fuel cell studies is provided, including cell structure, material, performance together with their advantages and disadvantages. Based on this, solid conclusions are made on their development trend and future perspectives are presented as well.

Hydrogen, primarily produced from steam methane reforming, plays a crucial role in oil refining, and provides a solution for the oil and gas industry's long-term energy transition by reducing CO2 emissions. This paper examines hydrogen’s role in this transition. Firstly, experiences from oil and gas exploration, including in-situ gasification, can be leveraged for hydrogen production from subsurface natural hydrogen reservoirs. The produced hydrogen can serve as fuel for generating steam and heat for thermal oil recovery. Secondly, hydrogen can be blended into gas for pipeline transportation and used as an alternative fuel for oil and gas hauling trucks. Additionally, hydrogen can be stored underground in depleted gas fields. Lastly, oilfield water can be utilized for hydrogen production using geothermal energy from subsurface oil and gas fields. Scaling up hydrogen production faces challenges, such as shared use of oil and gas infrastructures, increased carbon tax for promoting blue hydrogen, and the introduction of financial incentives for hydrogen production and consumption, hydrogen leakage prevention and detection.

Perovskite oxides with high oxygen ionic conductivity have played major roles in five important devices of significance for clean energy future and automated manufacturing, i.e., solid oxide fuel cell, solid oxide electrolysis cell, oxygen permeable membranes, gas sensors, and oxygen pumps, all of which exhibit different performance requirements and challenges. Although improving the oxygen ion transport within the perovskite lattice serves as the key to increasing the efficiencies of perovskites’ components in these devices, the different criteria, structures, and/or physicochemical properties often become the complicating factors. It is understood that the oxygen ion transport performance of perovskite is mainly determined by the crystal structure, the A/B-site cations and their content, and the oxygen vacancies. This perspective overviews these factors, which can be manipulated to adapt the needs for different devices. Brief discussions are then made on the concepts, status, and outlook of each device together with the strategies for improving the device performance. The summaries and insights provided in this review are anticipated to promote more strategic research and development directions and activities in these important applications.

CO2 capture and sequestration via the hydrate method has attracted much attention because of its high sequestration density and energy density. However, the influencing mechanism of the promoters on the formation of CO2 hydrate was not yet clarified. Also, there was no comprehensive review of the effect of promoters on CO2 hydrate formation from laboratory-scale studies. Therefore, this paper reviewed the effect of promoters on CO2 capture and sequestration via the hydrate method from the laboratory research scale. Typical CO2 hydrate formation processes were summarized in detail. And the characteristic parameters of CO2 hydrate were analyzed. Then, the different types of promoters were carefully overviewed. In particular, the influencing mechanisms of promoters on CO2 hydrates were highlighted and compared. Thermodynamic promoters mainly affected the conditions of hydrate formation, while kinetic promoters mainly acted at the gas-liquid contact interface. However, the promotional mechanism of CO2 hydrate promoters was controversial and the solution formulation units were not consistent. In addition, the effects of different promoters on CO2 capture, separation, and sequestration were also summarized. The existing studies ignored the CO2 hydrate formation experiments under in-situ conditions in the presence of promoters. The influence of different promoters on the sequestration effect was critically evaluated through parameters such as phase equilibrium curves, induction time, gas consumption, and sequestration density. A uniform evaluation standard for CO2 hydrate promoters was lacking. Finally, future development recommendations should focus on large-scale, low-energy, commercial, in-situ, environmentally friendly, and intelligent research directions. The review may provide some theoretical guidance for the commercial application of CO2 hydrate.

In the development of new electrochemical concepts for the fabrication of high-energy-density batteries, fluoride-ion batteries (FIBs) have emerged as one of the valid candidates for the next generation electrochemical energy storage technologies, showing the potential to match or even surpass the current lithium-ion batteries (LIBs) in terms of energy density, safety without dendritic grains, and elimination of dependence on scarce lithium and cobalt resources. However, the development of FIBs is still in its infancy and their performance is far from satisfactory, with issues such as the lower fluoride-ion conductivity of the electrolytes and the reversibility of the electrodes hindering their commercialization. Previous reviews have mainly focused on inorganic solid electrolytes with a brief emphasis on the development of various fluoride-ion conductors and their ion-conducting properties. Therefore, this review summarizes the current developments in various electrolytes, a systematic overview of the current progress for various fluoride-ion electrolytes is presented by beginning with the history, structure and classification of FIBs, ion-transport mechanisms are briefly discussed. Recent advances in different classes of fluoride-ion electrolytes are described. The methods for optimizing the ionic conductivity characteristics of the fluoride-ion electrolytes are highlighted. Finally, an outlook on the future research direction of FIBs is given by highlighting some critical issues, challenges and prospects of fluoride-ion electrolytes.

The explosive development of the Internet of Things (IoT) that connects the physical world with digitally intelligent systems by receiving signals from sensors and actuators embedded in physical objects, urgently requires high-performance power supplies for the sensors and actuators located at the restricted spaces. Among various power supply systems, flexible Zinc-ion energy storage devices have attracted extensive interest due to their cost-effectiveness, high theoretical capacity, superior safety, and environmental friendliness. To achieve flexibility in Zn-ion-based energy storage devices, the design and fabrication of flexible and/or freestanding active electrodes is one of the key issues. MXene as a novel family of two-dimensional (2D) materials shows outstanding integration of metal-like conductivity, abundant active sites, and excellent mechanical properties, which can be promising flexible electrodes for Zn ion storage. This review comprehensively expounds on the progress of reported flexible freestanding MXene-based composite films for different Zn-ion-based energy storage systems including Zn-ion capacitors (ZICs), Zinc-ion batteries (ZIBs), and Zinc-air batteries (ZABs). We also propose the challenges and perspective of improving the electrochemical performance of flexible freestanding MXene-based Zn-ion-based energy storage. This review serves as a guideline to inspire more future endeavors in synthesizing advanced flexible MXene-based electrodes for efficient Zinc ion storage.

A hydrogen compressed air energy storage power plant with an integrated electrolyzer is ideal for large-scale, long-term energy storage because of the emission-free operation and the possibility to offer multiple ancillary services on the German energy market. This paper defines analyzes such a storage concept and conducts an extensive comparison with four additional storage concepts based on various criteria. The results show that the combination of storing compressed air and hydrogen offers a higher efficiency than storing only hydrogen and lower specific investment costs than storing only compressed air. This result is confirmed with analysis of the optimal sizing of each power plant component for simultaneous participation on multiple energy markets with a linear optimization dispatch mode. The hydrogen compressed air energy storage (HCAES) power plant can utilize more revenue possibilities than a hydrogen energy storage because of the higher round-trip efficiency and the combination of the air compressor and the integrated electrolyzer during charging mode. The integration of the electrolyzer, however, offers a couple of challenges itself because of the highly flexible operation mode. A new concept for the controllable 24-pulse diode-thyristor rectifier of the electrolyzer is presented, that uses mostly common components while offering little to no grid harmonics and a long lifetime. The flexible integrated electrolyzer allows for the 4-quadrant operation of the storage power plant.

Rechargeable zinc-air batteries (ZABs) have gained extensive research attention as a promising sustainable energy technology due to their considerable theoretical specific energy density, low toxicity, abundant availability, and robust safety features. However, the practical implementation of ZABs still faces challenges, primarily attributed to the sluggish kinetics of oxygen-involved reactions, including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharge and charge process. Therefore, searching for efficient bifunctional oxygen electrocatalysts is crucial to address these challenges. Dual-atom catalysts (DACs), an extension of single-atom catalysts (SACs), exhibit flexible architectures that allow for the combination of homogeneous and/or heterogeneous active sites, making them highly attractive for improving bifunctional activity. In this review, we first introduce the basic framework of ZABs and the structural characteristics of DACs. Subsequently, we organize the research progress on applying DACs in liquid and solid-state ZABs and elaborate on their unique catalytic mechanism. Finally, we highlight the challenges and future research directions for further innovation of DACs in ZABs. In summary, this review highlights the advantages of DACs compared with SACs used as bifunctional oxygen electrocatalysts and provides a reference for the broad applications of DACs in energy conversion and storage.