Hybrid battery cells combining liquid electrolytes (LEs) with inorganic solid electrolyte (SE) separators or different SEs and polymer electrolytes (PEs), respectively, are developed to solve the issues of single-electrolyte cells. Among the issues that can be solved are detrimental shuttle effects, decomposition reactions between the electrolyte and the electrodes, and dendrite propagation. However, the introduction of new interfaces by contacting different ionic conductors leads to other problems, which cannot be neglected before commercialization is possible. The interfaces between the different types of ionic conductors (LE/SE and PE/SE) often result in significant charge-transfer resistances, which increase the internal resistance considerably. This review highlights studies evaluating the interfacial resistances and activation barriers in such systems to present an overview of the issues still hampering hybrid battery systems. The interfaces between different SEs in hybrid all-solid-state batteries (SSBs) are considered as well. In addition, a short summary of physicochemical models describing heteroionic interfaces—interfaces between two different ion conductors—is given in an attempt to explain high interface resistances. In doing so, we hope to inspire future work on the crucial topic of interface optimization toward better SSBs.

The electrochemical synthesis of ammonia under mild conditions has attracted significant interest in recent years because it can allow for the direct conversion of renewable electricity to chemical energy in the form of ammonia, which is an ideal medium for energy storage and transportation. And in contradistinction to the Haber–Bosch process, the electrochemical synthesis of ammonia is a much more environmentally friendly process that can operate under mild conditions with zero carbon dioxide (CO₂) emission. However, this process is severely hindered by poor ammonia formation rates and Faradaic efficiency due to the competing hydrogen evolution reaction. Based on this, a review focused on the current status and challenges of the electrochemical synthesis of ammonia is imperative to promulgate this key process and promote future research. And therefore, this review will systematically survey the recent progress of the electrochemical synthesis of ammonia; and different from previous reviews, this review will include not only advances in electrocatalysts, but also in reactors, electrolytes and reaction mechanisms. In addition, future research directions and strategies to improve the performance of ammonia electrochemical synthesis systems are proposed with the aim of shedding light on the future direction of ammonia synthesis through nitrogen electrochemical reduction.

The development of novel electrochemical energy storage (EES) technologies to enhance the performance of EES devices in terms of energy capacity, power capability and cycling life is urgently needed. To address this need, supercapatteries are being developed as innovative hybrid EES devices that can combine the merits of rechargeable batteries with the merits of supercapacitors into one device. Based on these developments, this review will present various aspects of supercapatteries ranging from charge storage mechanisms to material selection including electrode and electrolyte materials. In addition, strategies to pair different types of electrode materials will be discussed and proposed, including the bipolar stacking of multiple supercapattery cells internally connected in series to enhance the energy density of stacks by reducing the number of bipolar plates. Furthermore, challenges for this stack design will also be discussed together with recent progress on bipolar plates.

Supercapattery is an innovated hybrid electrochemical energy storage (EES) device that combines the merit of rechargeable battery and supercapacitor characteristics into one device. This article reviews supercapatteries from the charge storage mechanisms to the selection of materials including the materials of electrodes and electrolytes. Strategies for pairing different kinds of electrode materials and device engineering are discussed.

Defect engineering involves the manipulation of the type, concentration, mobility or spatial distribution of defects within crystalline structures and can play a pivotal role in transition metal oxides in terms of optimizing electronic structure, conductivity, surface properties and mass ion transport behaviors. And of the various transition metal oxides, titanium-based oxides have been keenly investigated due to their extensive application in electrochemical storage devices in which the atomic-scale modification of titanium-based oxides involving defect engineering has become increasingly sophisticated in recent years through the manipulation of the type, concentration, spatial distribution and mobility of defects. As a result, this review will present recent advancements in defect-engineered titanium-based oxides, including defect formation mechanisms, fabrication strategies, characterization techniques, density functional theory calculations and applications in energy conversion and storage devices. In addition, this review will highlight trends and challenges to guide the future research into more efficient electrochemical storage devices.

This work reviews the recent advances in defect-engineered Ti-based oxides, including the mechanism of defect formation, fabrication strategies, the characterization techniques, density functional theory calculations and the applications in energy conversion and storage.

Environmental concerns such as climate change due to rapid population growth are becoming increasingly serious and require amelioration. One solution is to create large capacity batteries that can be applied in electricity-based applications to lessen dependence on petroleum. Here, aluminum–air batteries are considered to be promising for next-generation energy storage applications due to a high theoretical energy density of 8.1 kWh kg⁻¹ that is significantly larger than that of the current lithium-ion batteries. Based on this, this review will present the fundamentals and challenges involved in the fabrication of aluminum–air batteries in terms of individual components, including aluminum anodes, electrolytes and air cathodes. In addition, this review will discuss the possibility of creating rechargeable aluminum–air batteries.

Graphene-based nanomaterials are promising bifunctional electrocatalysts for overall water splitting (OWS) to produce hydrogen and oxygen as sustainable fuel sources because graphene-based bifunctional electrocatalysts can provide distinct features such as large surface areas, more active sites and facile synthesis of multiple co-doped nanomaterials. Based on this, this review will present recent advancements in the development of various bifunctional graphene-based electrocatalysts for OWS reactions and discuss advancements in the tuning of electronic surface-active sites for the electrolytic splitting of water. In addition, this review will evaluate perspectives and challenges to provide a deep understanding of this emerging field.