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Electrochemical ultracapacitors exhibiting high energy output and an ultra-long cycle life, utilizing green and sustainable materials, are of paramount importance for next-generation applications. Developing an ultracapacitor that has high output energy under high power conditions in a high-voltage non-aqueous electrolyte and maintaining a long cycle life is an ongoing challenge. Herein, we utilize watermelon seeds, a bio-waste from watermelons, for use in high-voltage, high-energy, and high-power ultracapacitors in a sodium ion-based non-aqueous electrolyte. The as-synthesized hierarchically porous, high surface area carbon is surface-engineered with a large quantity of nitrogen and sulfur heteroatoms to give a high specific capacitance of ∼252 F g−1 at 0.5 A g−1 and 90 F g−1 at 30 A g−1. An ultra-high stability of ∼90% even after 150 000 cycles (10 A g−1) with 100% coulombic efficiency is achieved at room temperature (25 °C), equivalent to an ultra-low energy loss of ∼0.0667% per 1000 cycles. Furthermore, the porous carbon demonstrates remarkable stability even at high temperature (55 °C) for 100 000 cycles (10 A g−1), ensuring the safety of the device and enabling it to outperform graphene-based materials. A maximum energy of ∼79 W h kg−1 and a maximum power of 22.5 kW kg−1 with an energy retention of ∼28.2 W h kg−1 was attained. The results provide new insights that will be of use in the development of high-performance, green ultracapacitors for advanced energy storage systems.

Sodium hybrid capacitors (NHCs) have tremendous potential to meet the simultaneous high energy–high power requirement of next-generation storage applications. But NHCs still face some obstacles due to poor sodium ion kinetics, low power, and poor cyclability while working with several inorganic sodium ion hosts. Additionally, developing high-performance NHCs that are sustainable and versatile is more crucial from the perspective of energy storage devices. Here, we report a conceptually new and high performance organic sodium hybrid capacitor (ONHC) system, developed by substituting a conventional toxic-metal-containing inorganic battery electrode of an NHC with a nano-structured, metal free, and renewable organic molecule – disodium rhodizonate – to host sodium ions. The sustainability of the ONHC is greatly enhanced by the simultaneous utilization of high surface area cardamom shell (as biomass)-derived porous carbon as a high-power capacitor electrode. The new system exhibits an outstanding performance, delivering a high energy density of ∼87 W h kg−1 along with a high specific power of 10 kW kg−1 (based on the mass in both electrodes), outperforming inorganic sodium hosts. High durability over 10 000 cycles (∼85% retention) with an ultra-low energy loss of ∼0.15% per 100 cycles is also demonstrated, indicating its emergence as a rival to conventional metal containing lithium and sodium hybrid capacitors. The current study provides new opportunities for developing greener and sustainable devices beyond conventional systems for next-generation storage applications.

Developing sodium based energy storage systems that retain high energy density at high power along with stable cycling is of paramount importance to meet the energy demands of next generation applications. This requires the development of electrodes beyond the conventional intercalation-based chemistry to overcome the sluggish diffusion-limited reaction kinetics and limited cycle life. Herein, we report a rationally designed hollow graphene nanosphere (HGS) cathode, which utilizes non-destructive, ultra-fast surface redox reactions at oxygen functional groups and delivers a discharge capacity of ∼155 mA h g−1 (0.1 A g−1) corresponding to a high energy of ∼415 W h kg−1 and retains ∼88 W h kg−1 of energy at a remarkable specific power of ∼84 kW kg−1 (40 A g−1), which are beyond the capabilities of intercalation-based electrodes. Moreover, the achieved cycling performance (86% capacity retention after 50 000 cycles at 10 A g−1) is the most stable cathode performance reported so far. The rationally designed sodium metal battery full cells with a sodium metal deposited aluminium current collector anode and the HGS cathode showed a similar sodium ion storage performance with high capacity, good rate capability, and stability. We certainly believe that the current research could direct the future research development towards transition metal-free, ultra-high power and super stable cathodes for sodium energy storage devices.

The growth of silicon nanoparticles on a graphene surface without forming the unwanted silicon carbide (SiC) phase has been challenging. Herein, the critical issues surrounding silicon anode materials for lithium-ion batteries, such as electrode pulverization, unstable solid electrolyte interphase and low electrical conductivity, have been addressed by growing silicon nanoparticles smaller than 10 nm, covalently bonded to a reduced graphene oxide (rGO) surface. The successful growth of SiC-free silicon nanoparticles covalently attached to the rGO surface was confirmed by using various spectroscopic and microscopic analyses. The rGO–Si delivered an initial discharge capacity of 1338.1 mA h g−1 with capacity retention of 87.1% after the 100th cycle at a current rate of 2100 mA g−1, and exhibited good rate capability. Such enhanced electrochemical performance is attributed to the synergistic effects of combining ultra-small silicon nanoparticles and rGO nanosheets. Here, rGO provides a continuous electron conducting network, whereas, ultra-small silicon particles reduce ionic diffusion path length and accommodate higher stress during volume expansion upon lithiation.

Mesoporous carbon on nitrogen and sulfur co-doped graphene nanosheets (NSGC) was synthesized and its bi-functional catalytic activity toward oxygen reduction reaction and oxygen evolution reaction was investigated. The NSGC material showed high bi-functional catalytic activity due to the synergistic effect of co-doping of sulfur and nitrogen, as well as the presence of a hierarchical porous structure. The enhanced bi-functional catalytic activity of NSGC facilitated the efficient formation and decomposition of Li2O2 on the air cathode. The lithium–oxygen cell assembled with the NSGC-based air cathode delivered a high initial discharge capacity of 11 431 mA h g−1 and exhibited good cycling stability. The hybrid structure consisting of mesoporous carbon with co-doped graphene nanosheets can be an effective strategy to improve the round-trip efficiency and cycle life of lithium–oxygen batteries.

A highly efficient nitrogen and sulfur co-doped graphene (NSG) nanosheet for dye-sensitized solar cells (DSSCs) was synthesized using a simple hydrothermal method, and its electrocatalytic activity towards the I3−/I− redox reaction was investigated. The NSG materials showed a uniform distribution of nitrogen and sulfur heteroatoms throughout the graphene nanosheet. The doped nitrogen was present in the form of pyridinic, pyrrolic and graphitic states, and the doped sulfur was present in the C–S–C configuration. The DSSC with the NSG counter electrode exhibited a high conversion efficiency (7.42%), similar to that of the Pt counter electrode (7.56%) and much higher than that of the only N- or S-doped graphene electrodes. The high catalytic activity of the NSG electrode is attributed to the synergistic effect of the high charge polarization arising from the difference in electronegativity between nitrogen and carbon as well as the structural distortion caused by the bigger atomic size of the sulfur atom. To the best of our knowledge, the synergistic effect of co-doping of graphene on the counter electrode performance in DSSCs is demonstrated for the first time, and co-doping is proposed as a promising approach to enhance the photovoltaic performance of DSSCs.

The application of lithium ion batteries in high power applications such as hybrid electric vehicles and electric grid systems critically requires drastic improvement in the electronic conductivity using effective materials design and strategies. Here, we demonstrate that the growth of a multi-component structure of composition LiTi2(PO4)3 [LTP] on a reduced graphene oxide (rGO) surface via a facile synthetic strategy could achieve an ultrahigh rate capability with the total carbon content as low as 1.79 wt%. The rGO–LTP hybrid material has been prepared using a two-step approach, where the growth of TiO2 nanoparticles on the graphene oxide surface is followed by the high temperature growth of LTP on graphene sheets and simultaneous thermal reduction of graphene oxide. The LTP particles are densely packed within the ripples of rGO and form a compact, well-connected graphene network requiring no additional conductive carbon to facilitate fast electron transport from active materials to the current collector. Here, graphene not only acts as a stable conductive substrate but also helps to control the size of the formed particles. The rGO–LTP hybrid as a cathode in lithium ion batteries achieves an ultrahigh specific power of 10000 W kg−1 at a specific energy of 210 W h kg−1, which corresponds to a charge and discharge time of 36 s and also retains 92% of the initial capacity after 100 cycles at a 10 C charge–discharge rate. Such an excellent performance is attributed to the multifunctional roles performed by rGO such as controlling the particle size, enhancing the electronic conductivity through a highly conductive network and rendering stability during cycling. This provides an effective design strategy for growing complex hybrid materials on graphene and engineering graphene nanosheets for advanced energy storage applications.