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The structural deformation during the embedding/de-embedding of K+ with a large ionic size is an important factor limiting the development of potassium-ion batteries (PIBs). The iron-based mixed polyanionic material Na4Fe3(PO4)2(P2O7) (NFPP) makes an excellent cathode material for PIBs due to its low cost, non-toxicity, suitable theoretical capacity, and stable structure. Here, a 3D reduced graphene oxide network wrapped NFPP (NFPP@rGO) was synthesized using a freeze-drying and subsequent annealing treatment. The NFPP@rGO material as a PIB cathode achieves a high discharge specific capacity of 119.1 mA h g−1 at 0.1C and 80.3 mA h g−1 even at 2C current density, and retains 82.1% of its initial capacity after 500 cycles. Ex situ XPS and XRD are utilized to understand the K+ storage mechanism of NFPP@rGO in PIBs. The superior electrochemical properties are due to the 3D reduced graphene oxide network, which improves the material's electrical conductivity and K+ diffusion rate, and stabilizes structural deformation during charge and discharge. These results indicate the feasibility of the NFPP@rGO cathode material for PIBs. [ABSTRACT FROM AUTHOR]

The scarcity and weak durability of precious metal catalysts are among the biggest obstacles to achieving cost-effective electrocatalysts in fuel cells and metal–air batteries. Hence, it is imperative to develop low-cost non-precious metal catalysts with comparable oxygen reduction reaction (ORR) activity to precious metal catalysts. Herein, we report a highly effective strategy for the facile synthesis of Fe/N-functionalized 3D porous carbon networks. A major advantage of the newly designed catalyst is that ultrafine Fe4N nanoparticles are grown and uniformly mounted on the carbon framework upon pyrolysis treatment at 800 °C, and co-exist with numerous in situ formed Fe–N4 moieties in the carbon matrix, being evidenced by using X-ray absorption and photoelectron spectroscopy. The new electrocatalysts exhibit high ORR activity, comparable/superior to that of the state-of-the-art Fe/N–carbon based catalysts reported to date. Specifically, the catalysts show a half-wave potential of 0.890 V (vs. RHE) and a limited current density of 6.18 mA cm−2. By resorting to experimental measurements and density-functional theory (DFT) calculations, the synergistic effects between Fe–N4 moieties and the Fe4N support are identified for the first time, which play a key role in boosting the catalytic performance of the Fe/N-functionalized porous carbon networks. [ABSTRACT FROM AUTHOR]

Designing low-cost and highly efficient bifunctional electrocatalysts for compatible integration with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for overall water splitting is critical but challenging. Herein, mutually beneficial Co3O4@MoS2 heterostructures were adopted to efficiently balance both HER and OER performance by improving the sluggish kinetics. These heterostructures synergistically favoured the reduction of the energy barrier of the initial water dissociation step and optimization of the subsequent H adsorption/desorption for MoS2 in alkaline HER. Moreover, the adsorption of oxygen intermediates was enhanced for Co3O4 in the OER process. As a result, the Co3O4@MoS2 heterostructures showed excellent overall water splitting performance with a low overpotential and Tafel slope. [ABSTRACT FROM AUTHOR]

Integration of Ni compound-based nanomaterials into energy storage devices, especially electrochemical capacitors, offers opportunities to timely meet the ever increasing demands of fast charging/discharging processes, longer cycle life, and higher power density and energy density. Besides that, Ni compound-based nanomaterials are also found to bring such merits as high electrochemical activity, low price, and being environmentally friendly. Therefore, it is crucial to design nanostructured Ni compound-based nanomaterials towards high-performance electrochemical capacitors and understand the role of morphology in enhancing the electrochemical performance at the atomic or molecular scale. In this review, we focus on the rational design and synthesis of different structured Ni compound-based nanomaterials, such as nickel oxides, ternary Ni-based metal oxides, nickel sulfides, ternary Ni-based metal sulfides, Ni-based metal organic frameworks, and their composites. More importantly, their electrochemical performances are also discussed by virtue of the effects of morphology, structure, and composition of Ni compound-based materials. Finally, the main challenges as well as opportunities that researchers are facing now in the field of Ni compound-based nanomaterials for electrochemical energy storage are summarized. We hope that this review will shed light on the development of Ni compound-based nanostructured electrodes and exploitation of their energy storage mechanisms towards high-performance electrochemical capacitor applications. [ABSTRACT FROM AUTHOR]

Pseudocapacitors, which can store more energy at high charge/discharge rates, have attracted considerable attention. The performance of a pseudocapacitive material mainly depends on the interaction between electrode materials and the electrolyte ions. However, the understanding of the interaction is still limited. Here, the performance of Ti2CT2 (T = O, F, and OH) nanosheets as pseudocapacitor electrode materials has been investigated through a novel first-principles approach. The results suggest that O-terminated Ti2C nanosheets are shown to be pseudocapacitive cathode materials. The pseudocapacitance is attributed to the large intrinsic capacitance of Ti2CO2 nanosheets and contact adsorbed cations. The former mainly decides the capacity of charges and the latter reduces the change of potential in the electrode. The integral capacitance and Na-ion capacity of Ti2CO2 nanosheets are simulated to be 291.5 F g−1 with a broad potential window range from 0 to 2.80 V (versus Na/Na+). Low diffusion energy barriers on Ti2CO2 and Ti2CF2 nanosheets indicate fast transportation and high charge and discharge rate for Na-ions. Our results provide insight into the origin of pseudocapacitance on stacked two-dimensional materials. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition-metal (TM) compound nanomaterials, due to their high-surface-area and large potential charge capability of TM atoms, have been widely investigated as electrochemical capacitors. However, the understanding of charge-storage mechanisms of 2D transition-metal compounds as electrode materials is still limited. In this study, using density functional theory computations, we systematically investigate the electrochemical properties of monolayer VS2 and V2C. Their electronic structures show a significant electron storage capability of around 0.25 V, referenced to the standard hydrogen electrode, and indicate redox pseudocapacitance characteristics as cathodes. The different charge densities visually confirm that excess electrons tend to localize in the vanadium atoms nearby contact-adsorbed Li ions, corresponding to the redox of vanadium atoms. In contrast, only the electric double layer acts as a charge-storage mechanism in the V2C monolayer. However, the O saturation would induce redox pseudocapacitance in the V2C monolayer. Furthermore, the calculated metallic behavior and low Li ion diffusion barriers substantiate that V2C and VS2 monolayers would manifest low resistance in the charging process. Our findings provide insights for the different charge-storage mechanism of VS2 and V2C monolayers. [ABSTRACT FROM AUTHOR]

NiCo2S4 porous nanotubes are synthesised by a sacrificial template method based on the Kirkendall effect. The as-prepared NiCo2S4 nanotube electrode shows a specific capacitance of 1093 F g−1 at a current density of 0.2 A g−1 (933 F g−1 at 1 A g−1). [ABSTRACT FROM AUTHOR]