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The increased demand of energy due to the recent technological advances in diverse fields such as portable electronics and electric vehicles is often hindered by the poor capability of energy-storage systems. Although supercapacitors (SCs) exhibit higher power density than state-of-the art batteries, their insufficient energy density remains a major challenge. An emerging concept of hybrid supercapacitors (HSCs) with the combination of one capacitive and one battery electrode in a single cell holds a great promise to deliver high energy density without sacrificing power density and cycling stability. This Minireview elaborates the recent advances of use of nickel cobaltite (NiCo2O4) as a potential positive electrode (battery-like) for HSCs. A brief introduction on the structural benefits and charge storage mechanisms of NiCo2O4 was provided. It further shed a light on composites of NiCo2O4 with different materials like carbon, polymers, metal oxides, and others, which altogether helps in increasing the electrochemical performance of HSCs. Finally, the key scientific challenges and perspectives on building high-performance HSCs for future-generation applications were reviewed.

Curiosity about the mysteries of outer space has driven humans to organize space exploration. Space missions and polar-region expeditions require the use of new energy storage technologies with excellent tolerance to extremely low temperatures and the capability to operate efficiently for extended periods of time. In this context, there is an urgent need to explore new approaches and concepts that address the critical related challenges, gaining a mechanistic understanding of new electrochemical reactions and phenomena in diverse scenarios. This paper presents an overview of how electrode and electrolyte materials in supercapacitor (SC) cells operate at extremely low temperatures. The critical challenges related to engineering such SCs and the major breakthroughs that have overcome these problems are presented. In addition, the effects of different electrolytes (e.g., aqueous, organic, and ionic liquid (IL) examples) on ion accessibility, interfacial charge transfer, and transport kinetics in SC cells under cold conditions are discussed. This review is expected to contribute to the development of high-performance SC cells that can function at extreme temperatures via providing insight into key challenges and guidelines that could facilitate future work.

The structural instability and sluggish kinetics of conventional positive electrodes with the lower capacitance of carbon-based negative electrodes result in an inferior performance for state-of-art supercapacitors (SCs). A general yet sustainable approach is proposed here to overcome this hitch by assembling hybrid SC cells utilising porous and stable 2D-on-2D core–shell and carbon/pseudocapacitive composite electrodes. Porous Co3(PO4)2 transparent stacked micropetals (TSMs) were synthesised and decorated with Co2Mo3O8 nanosheets (NSs) (Co3(PO4)2@Co2Mo3O8) forming a 2D-on-2D core–shell positive electrode, which was combined with a 2D carbon nanotube/MXene (CNT–Ti3C2TX) composite negative electrode. The core–shell electrode achieved a specific capacity of 184.7 mA h g−1 (738 mF cm−2) and cycling stability of 95.6% over 15 000 charge/discharge cycles. The CNT–Ti3C2TX electrode exhibited a remarkable areal capacitance of 187.5 mF cm−2 and cycling stability of 93.1%. Consequently, the assembled unique hybrid solid-state SCs delivered an exceptional volumetric capacitance of 7.9 F cm−3 and a specific energy of 74.06 W h kg−1 (2.47 mW h cm−3) at a specific power and cycling stability of 1.13 kW kg−1 and 93.2%, respectively. Overall, the techniques and electrode materials presented in this study can serve as a reference to produce a range of electrode materials for next-generation energy storage devices.

The conversion of nitrogen to ammonia offers a sustainable and environmentally friendly approach for producing precursors for fertilizers and efficient energy carriers. Owing to the large energy density and significant gravimetric hydrogen content, NH3 is considered an apt next-generation energy carrier and liquid fuel. However, the low conversion efficiency and slow production of ammonia through the nitrogen reduction reaction (NRR) are currently bottlenecks, making it an unviable alternative to the traditional Haber-Bosch process for ammonia production. The rational design and engineering of catalysts (both photo- and electro-) represent a crucial challenge for improving the efficiency and exploiting the full capability of the NRR. In the present review, we highlight recent progress in the development of graphene-based systems and graphene derivatives as catalysts for the NRR. Initially, the history, fundamental mechanism, and importance of the NRR to produce ammonia are briefly discussed. We also outline how surface functionalization, defects, and hybrid structures (single-atom/multiatom as well as composites) affect the N2 conversion efficiency. The potential of graphene and graphene derivatives as NRR catalysts is highlighted using pertinent examples from theoretical simulations as well as machine learning based performance predictive methods. The review is concluded by identifying the crucial advantages, drawbacks, and challenges associated with principal scientific and technological breakthroughs in ambient catalytic NRR.

Polyoxometalates (POMs) represent a class of nanomaterials, which hold enormous promise for a range of energy-related applications. Their promise is owing to their "special"structure that gives POMs a truly unique ability to control redox reactions in energy conversion and storage. One such amazing capability is their large number of redox active sites that arises from the complex three-dimensional cluster of metal-oxide ions linked together by oxygen atoms. Here, a critical review on how POMs emerged from being molecular clusters for fundamental studies, to next-generation materials for energy applications is provided. We highlight how exploiting the versatility and activity of these molecules can lead to improved performance in energy devices such as supercapacitors and batteries, and in energy catalyst applications. The potential of POMs across numerous fields is systematically outlined by investigating structure-property-performance relationships and the determinant factors for energy systems. Finally, the challenges and opportunities for this class of materials with respect to addressing our pressing energy-related concerns are identified. This journal is

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish “pseudocapacitive” and “battery” materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish batterytype and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

Due to their high capacity, nickel-cobalt-based cathode materials have attracted significant attention as potential components of hybrid solid-state supercapacitors (HSSCs). However, their poor cycling stability and low rate capability have impeded their implementation. In the present study, a single-step, binder-free potentiodynamic polarization approach is presented for the preparation of battery-Type phosphorus-containing amorphous trimetal nickel-ruthenium-cobalt hydroxide (P@NRC-OH) nanofibers on Ni foam for use in high-energy, stable HSSCs. The phosphate dopant and the trimetal-rich electrode surface increase the intrinsic electron conductivity and redox activity and generate a large number of active defects. As a consequence, a P@NRC-OH electrode exhibited enhanced energy storage properties in terms of specific capacity (541.66 mA h g-1 at 3 mA cm-2), cycling durability (90.35% over 20 000 cycles), and rate capability (308.64 mA h g-1 at 20 mA cm-2). An assembled full-cell HSSC with P@NRC-OH nanofibers as the cathode material and porous activated carbon as the anode material produced a maximum specific energy of 90.02 W h kg-1 at a specific power of 1363 W kg-1 which remained as high as 37.87 W h kg-1 at a power density of 6818.18 W kg-1, with remarkable cycling stability over 15 000 charge-discharge cycles. The proposed approach thus represents a scalable and efficient strategy for the design of electrodes and devices with superior electrochemical performance.

In present work, we have developed 2.0 V symmetric supercapacitor with rationally prepared iron tungstate (FeWO4) nanoparticles as electrodes and redox-active electrolyte. It is revealed that the electrochemical performances of FeWO4-system were significantly improved due to the addition of potassium iodide (KI) redox additive in conventional KOH electrolyte in terms of the specific capacitance and energy density. Notably, FeWO4-based symmetric cell with KI-additive shown two-fold enhancement in specific energy (113 Wh/kg) compared with the cell with pristine KOH electrolyte (41.62 Wh/kg). Such an excellent enhancement is attributed to the improvement in the stability of existing KOH electrolyte by KI which influences the strength of OH bond in aqueous media and prevents the breakdown of electrolyte without adversely affecting the redox behavior and on contrary supporting the interactions at the higher potential to produce better results.

Lithium ion capacitor (LIC) is a promising energy storage system that can simultaneously provide high energy with high rate (high power). Generally, LIC is fabricated using capacitive cathode (activated carbon, AC) and insertion‐type anode (graphite) with Li‐ion based organic electrolyte. However, the limited specific capacities of both anode and cathode materials limit the performance of LIC, in particular energy density. In this context, we have developed “two in one” synthetic approach to engineer both cathode and anode from single precursor for high performance LIC. Firstly, we have engineered a low cost 1D polypyrrole nanopipes (PPy‐NPipes), which was utilized as cathode material and delivered a maximum specific capacity of 126 mAh/g, far higher than that of conventional AC cathodes (35 mAh/g). Later, N doped carbon nanopipes (N‐CNPipes) was derived from direct carbonization of PPy‐NPipes and successfully applied as anode material in LIC. Thus, a full LIC was fabricated using both pseudo‐capacitive cathode (PPy‐NPipes) and anode (N‐CNPipes) materials, respectively. The cell delivered a remarkable specific energy of 107 Wh/kg with maximum specific power of 10 kW/kg and good capacity retention of 93 % over 2000 cycles. Thus, this work provide a new approach of utilization of nanostructured conducting polymers as a promising pseudocapacitive cathode for high performance energy storage systems.