Graphene, the most fascinating 2D form of carbon with closely packed carbon atoms arranged in a layer, needs more attention in various fields. For its unique electrical, mechanical, and chemical properties with a large surface area, graphene has been in the limelight since its first report. Graphene has extraordinary properties, making it the most promising electrode component for applications in supercapacitors. However, the persistent re-stacking of carbon layers in graphene, caused by firm interlayer van der Waals attractions, significantly impairs the performance of supercapacitors. As a result, many strategies have been used to get around the aforementioned problems. The utilization of graphene-based nanomaterials has been implemented to surmount the aforementioned constraints and considerably enhance the performance of supercapacitors. This review highlights recent progress in graphene-based nanomaterials with metal oxide, sulfides, phosphides, nitrides, carbides, and conducting polymers, focusing on their synthetic approach, configurations, and electrochemical properties for supercapacitors. It discusses new possibilities that could increase the performance of next-generation supercapacitors.
All-solid-state batteries with solid ionic conductors packed between solid electrode films can release the dead space between them, enabling a greater number of cells to stack, generating higher voltage to the pack. This Review is focused on using high-voltage cathode materials, in which the redox peak of the components is extended beyond 4.7â V. Li-Ni-Mn-O systems are currently under investigation for use as the cathode in high-voltage cells. Solid electrolytes compatible with the cathode, including halide- and sulfide-based electrolytes, are also reviewed. Discussion extends to the compatibility between electrodes and electrolytes at such extended potentials. Moreover, control over the thickness of the anode is essential to reduce solid-electrolyte interphase formation and growth of dendrites. The Review discusses routes toward optimization of the cell components to minimize electrode-electrolyte impedance and facilitate ion transportation during the battery cycle.
Solar-driven water electrolysis to produce hydrogen is one of the clean energy options for the current energy-related challenges. Si as a photocathode exhibits a large overpotential due to the slow hydrogen evolution reaction (HER) kinetics and hence needs to be modified with a cocatalyst layer. MoS2 is a poor HER cocatalyst due to its inert basal plane. Activation of the MoS2 basal plane will facilitate HER kinetics. In this study, we have incorporated SnS2 into MoS2 ultrathin sheets to induce defect formation and phase transformation. MoS2/SnS2 composite ultrathin sheets with a Sn2+ state create a large number of S vacancies on the basal sites. The optimized defect-rich MoS2/SnS2 ultrathin sheets decorated on surface-modified Si micro pyramids as photocathodes show a current density of -23.8 mA/cm2 at 0 V with an onset potential of 0.23 V under acidic conditions, which is higher than that of the pristine MoS2. The incorporation of SnS2 into 2H-MoS2 ultrathin sheets not only induces a phase but also can alter the local atomic arrangement, which in turn is verified by their magnetic response. The diamagnetic SnS2 phase causes a decrease in symmetry and an increase in magnetic anisotropy of the Mo3+ ions.
Metal-CO2 rechargeable batteries are of great importance due to their higher energy density and carbon capture capability. In particular, Na-CO2 batteries are potential energy-storage devices that can replace Li-based batteries due to their lower cost and abundance. However, because of the slow electrochemical processes owing to the carbonated discharge products, the cell shows a high overpotential. The charge overpotential of the Na-CO2 battery increases because of the cathode catalyst's inability to break down the insulating discharge product Na2CO3, thereby resulting in poor cycle performance. Herein, we develop an ultrathin nanosheet MoS2/SnS2 cathode composite catalyst for Na-CO2 battery application. Insertion of SnS2 reduces the overpotential and improves the cyclic stability compared to pristine MoS2. As shown by a cycle test with a restricted capacity of 500 mAh/g at 50 mA/g, the battery is stable up to 100 discharge-charge cycles as the prepared catalyst successfully decomposes Na2CO3. Furthermore, the battery with the MoS2/SnS2 cathode catalyst has a discharge capacity of 35â¯889 mAh/g. The reasons for improvements in the cycle performance and overpotential of the MoS2/SnS2 composite cathode catalyst are examined by a combination of Raman, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure analysis, which reveals an underneath phase transformation and changes in the local atomic environment to be responsible. SnS2 incorporation induces S-vacancies in the basal plane and 1T character in 2H MoS2. This combined impact of SnS2 incorporation results in undercoordinated Mo atoms. Such a change in the electronic structure and the phase of the MoS2/SnS2 composite cathode catalyst results in higher catalytic activity and reduces the cell overpotential.
Alkali metal-carbon dioxide (Li/Na-CO2) batteries have generated widespread interest in the past few years owing to the attractive strategy of utilizing CO2 while still delivering high specific energy densities. Among these systems, Na-CO2 batteries are more cost effective than Li-CO2 batteries because the former uses cheaper and abundant Na. Herein, a Ru/carbon nanotube (CNT) as a cathode material was used to compare the mechanisms, stabilities, overpotentials, and energy densities of Li-CO2 and Na-CO2 batteries. The potential of Na-CO2 batteries as a viable energy storage technology was demonstrated.
Hydrogen energy is a promising alternative for fossil fuels because of its high energy density and carbon-free emission. Si is an ideal light absorber used in solar water splitting to produce H2 gas because of its small band gap, appropriate conduction band position, and high theoretical photocurrent. However, the overpotential required to drive the photoelectrochemical (PEC) hydrogen evolution reaction (HER) on bare Si electrodes is severely high owing to its sluggish kinetics. Herein, a molybdenum tungsten disulfide (MoS2-WS2) composite decorated on a Si photoabsorber is used as a cocatalyst to accelerate HER kinetics and enhance PEC performance. This MoS2-WS2 hybrid showed superior catalytic activity compared with pristine MoS2 or WS2. The optimal MoS2-WS2/Si electrode delivered a photocurrent of -25.9 mA/cm2 at 0 V (vs reversible hydrogen electrode). X-ray absorption spectroscopy demonstrated that MoS2-WS2 possessed a high hole concentration of unoccupied electronic states in the MoS2 component, which could promote to accept large amounts of carriers from the Si photoabsorber. Moreover, a large number of sulfur vacancies are generated in the MoS2 constituent of this hybrid cocatalyst. These sulfur defects served as HER active sites to boost the catalytic efficiency. Besides, the TiO2-protective MoS2-WS2/Si photocathode maintained a current density of -15.0 mA/cm2 after 16 h of the photocatalytic stability measurement.
The marriage between a Li metal anode and the solid-state electrolyte is expected to limit the safety risk of secondary batteries. However, dendrites and interfacial stability hinder the combination of Li metal anode and solid-state electrolyte. Herein, a plastic crystal electrolyte (PCE) and three-dimensional (3D) host structure played the role of a matchmaker in combining the solid-state electrolyte and Li metal anode. Succinonitrile cooperated with Li salt and Li6.4La3Zr1.4Ta0.6O12 nanosize powder and built a PCE interphase, which enhanced the interfacial stability between Li1.5Al0.5Ge1.5(PO4)3 and Li metal anode. To protect the soft PCE from the dendrite penetration, commercially sold Super P, carbon nanotube, KS6, and Ketjen black were co-heated with the melted Li metal. However, only KS6 built a 3D host in Li metal successfully because of its high graphitization and layered structure. Benefitting from the matchmakers, the solid-state batteries exhibited enhanced cycling stability.
Li-air batteries are limited to lab-scale research owing to the uninterrupted formation of discharge products. In the case of Li-CO2 batteries, the increase in overpotential caused by Li2 CO3 formation results in cell death. In this study, Cu2 O crystals having three different types of shapes (i.e., cubic, octahedral, and rhombic) were synthesized to compare their catalytic activity toward CO2 reactions. The full-cycle and long-term stability test revealed that rhombohedral Cu2 O facilitates Li2 CO3 decomposition more efficiently than that of cubic and octahedral Cu2 O. The cycle was extended to investigate the photocatalytic activity of the rhombic Cu2 O by illuminating the cell. The repeated cycles to 1â h showed a maximum overpotential of 1.5â V, which is 0.5â V lower than that of the cell without illumination. A postmortem analysis of the cell after dividing the cycles into segments demonstrated interesting results concerning the role of light and Cu2 O during the cell cycle.
Li-CO2 batteries are of great interest among researchers due to their high energy density and utilization of the greenhouse gas CO2 to produce energy. However, several shortcomings have been encountered in the practical applications of Li-CO2 batteries, among which their poor cyclability and high charge overpotential necessary to decompose the highly insulating discharge product (Li2CO3) are the most important. Herein, the spinel zinc cobalt oxide porous nanorods with carbon nanotubes (ZnCo2O4@CNTs) composite is employed as a cathode material in Li-CO2 batteries to improve the latter's cycling performance. The ZnCo2O4@CNT cathode-based Li-CO2 battery exhibited a full discharge capacity of 4275 mAh g-1 and excellent cycling performance over 200 cycles with a charge overpotential below 4.3 V when operated at a current density of 100 mA g-1 and fixed capacity of 500 mAh g-1. The superior performance of the ZnCo2O4@CNT cathode composite was attributed to the synergistic effects of ZnCo2O4 and CNT. The highly porous ZnCo2O4 nanorod structures in the ZnCo2O4@CNT catalyst showed enhanced catalytic activity/stability, which effectively promoted CO2 diffusion during the discharging process and accelerated Li2CO3 decomposition at a low charge overpotential.
In the present report, vertically-aligned graphene nanowalls are grown on Ni foam (VA-G/NF) using plasma-enhanced chemical vapor deposition method at room temperature. Optimization of the growth conditions provides graphene sheets with controlled defect sites. The unique architecture of the vertically-aligned graphene sheets allows sufficient space for the ionic movement within the sheets and hence enhancing the catalytic activity. Further modification with ruthenium nanoparticles (Ru NPs) drop-casted on VA-G/NF improves the charge overpotential for lithium-oxygen (Li-O2) battery cycles. Such reduction we believe is due to the easier passage of ions between the perpendicularly standing graphene sheets thereby providing ionic channels.
