Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO4), basic (NaOH), and neutral (Na2SO4) electrolytes of similar cations but different anions as well as similar anions but different cations (Na2SO4 and Li2SO4) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO42– and Li+ showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO42– > HSO4– > OH– and Li+ > Na+, respectively. Consequently, the CNT//Li2SO4//CNT cell displayed outstanding charge storage indicators (operating voltage ∼0–2 V, specific capacitance ∼122 F·g–1, specific energy ∼67 W h·kg–1, and specific power ∼541 W·kg–1 at 0.5 A·g–1) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li2SO4//CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5–2.5 A·g–1) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density

Insights into the charge storage mechanism of binder-free electrochemical capacitors in ionic liquid electrolytes

Electrochemical capacitors (synonymously supercapacitors) working under an electrochemical double-layer charge storage mechanism (EDLC) are widely investigated because of their excellent power density and cycle life; however, their energy density is lower than those of lithium-ion batteries. Ionic liquids (ILs) are of great interest as electrolytes for EDLCs due to their wide operational voltage window. Here, we provide a systematic investigation on the influence of anions of ILs on the charge storage mechanism and electrochemical stability of EDLC electrodes. Two IL electrolytes, viz., [1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)], having similar cations but different anions and carbon nanotube (CNT) electrodes are chosen for this study. The CNT//BF4:TFSI//CNT-based device showed superior electrochemical performance (∼69 F•g-1 gravimetric specific capacitance, ∼949 W•kg-1 power density, and ∼139 Wh•kg-1 energy density at 0.5 A•g-1) to CNT//EMIMBF4//CNT and CNT//EMIMTFSI//CNT devices. The device using a mixture of BF4:TFSI (1:0.5) electrolytes has an operating voltage of 0-3.8 V and specific capacitance retention of ∼45% at 0.5 A•g-1 after 500 cycles. In the case of the IL mixture (BF4:TFSI), the combined anion structure and their properties play very crucial part in the improvement of the electrochemical performance of the CNT//BF4:TFSI//CNT device. The assembled Teflon Swagelok-type cell could light up green (3.3 V) and red (2.1 V) light-emitting diodes for more than 5 min

Simple Bottom-Up Synthesis of Bismuthene Nanostructures with a Suitable Morphology for Competitive Performance in the Electrocatalytic Nitrogen Reduction Reaction

Nitrogen reduction to ammonia under ambient conditions has received important attention, in which high-performing catalysts are sought. A new, facile, and seedless solvothermal method based on a high-temperature reduction route has been developed in this work for the production of bismuthene nanostructures with excellent performance in the electrocatalytic nitrogen reduction reaction (NRR). Different reaction conditions were tested, such as the type of solvent, surfactant, reducing agent, reaction temperature, and time, as well as bismuth precursor source, resulting in distinct particle morphologies. Two-dimensional sheet-like structures and small particles displayed very high electrocatalytic activity, attributed to the abundance of tips, edges, and high surface area. NRR experiments resulted in an ammonia yield of 571 ± 0.1 μg h-1 cm-2 with a respective Faradaic efficiency of 7.94 ± 0.2% vs Ag/AgCl. The easy implementation of the synthetic reaction to produce Bi nanostructures facilitates its potential scale up to larger production yields

Electrochemistry of layered metal diborides

In the last decade, layered materials and their 2D counterparts, such as graphene, transition metal chalcogenides or black phosphorus, have been tested for their electrocatalytic properties. The main application is in the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR), in which layered materials could possibly replace platinum. Although platinum offers both the lowest overpotential and the highest current density, it is also expensive and rare and has little tolerance to passivation. On the other hand, metal diborides are well known for their high chemical stability and low cost. Surprisingly, until now, only a few electrochemical properties of diborides have been studied. In this paper, we present the study of the electrocatalytic properties of metal borides (namely AlB2, CrB2, HfB2, MgB2, NbB2, TaB2, TiB2, VB2 and ZrB2) towards the HER and ORR together with their structural and chemical characterization.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore

Layered ZnIn2S4 single crystals for ultrasensitive and wearable photodetectors

Zinc indium sulfide belongs to the family of layered ternary chalcogenides. Although ZnIn2S4 has a suitable bandgap in the visible range, its optoelectronic properties are not fully investigated. Most photodetectors based on layered semiconductors suffer from large dark currents, which hamper their performance and energy efficiency. In this work, high quality ZnIn2S4 single crystals are synthesized via chemical vapor transport. The free-standing crystals are ≈20 μm thick and up to 2 cm2 in area and produce large photocurrents upon UV–vis illumination, while also maintaining extremely low currents in the dark. This allows to fabricate a simple photodetector with ohmic contacts, exhibiting extremely small dark currents down to 10–12 A. The ON/OFF (light/dark) switching ratio reaches value of 106, the highest reported for a layered semiconductor. Furthermore, the photodetector exhibits remarkable responsivity of 173 A W–1 and excellent detectivity of 1.7 × 1012 Jones. To demonstrate sensitivity and flexibility of the ZnIn2S4 crystals, a wearable device is also fabricated. The wearable is able to record human heart rate and compare it with signal measured by a commercial smartwatch. The results suggest a substantial research potential in further explorations of ZnIn2S4 and other ternary chalcogenides for optoelectronic applications.This work was supported by project LTAUSA19034 from Ministry of Education, Youth and Sports (MEYS) and by the specific university research IGA grant -A2_FCHT_2021_006. The TEM measurements were performed in the Laboratorio de Microscopias Avanzadas (LMA) at the Universidad de Zaragoza (Spain). R.A. acknowledges funding from the Spanish MICINN (project grant PID2019-104739GB-100/AEI/10.13039/501100011033), Government of Aragon (project DGA E13-20R), and European Union H2020 programs “ESTEEM3” (823717) and Graphene Flagship (881603).Peer reviewe

Hydrogenation of fluorographite and fluorographene : an easy way to produce highly hydrogenated graphene

Fluorographene is an excellent precursor for the synthesis of graphene derivatives. Relative to pure graphene, fluorographene possesses higher reactivity and, in comparison with graphene oxide, is also homogenous in composition, which enables the preparation of well-defined materials. Recently, it has been shown that several graphene derivatives can be synthesized from fluorographene, thus yielding various products such as graphene acid or alkylated graphene. This study focuses on the hydrogenation of fluorographene by using various hydrogenation reactions, including the use complex hydrides and solvated electrons in different media. In addition, a comparison of these reactions shows that fluorinated graphite has significantly lower reactivity than fluorographene. The conversion rates of these reactions are higher when fluorographene is used relative to fluorographite. These reactions can be used to tune the hydrogen/fluorine composition on a graphene backbone

2D layered bimetallic phosphorous trisulfides MI MIII P2 S6 (MI = Cu, Ag; MIII = Sc, V, Cr, In) for electrochemical energy conversion

Considerable improvements in the electrocatalytic activity of 2D metal phosphorous trichalcogenides (M2 P2 X6 ) have been achieved for water electrolysis, mostly with MII 2 [P2 X6 ]4- as catalysts for hydrogen evolution reaction (HER). Herein, MI MIII P2 S6 (MI  = Cu, Ag; MIII  = Sc, V, Cr, In) are synthesized and tested for the first time as electrocatalysts in alkaline media, towards oxygen reduction reaction (ORR) and HER. AgScP2 S6 follows a 4 e- pathway for the ORR at 0.74 V versus reversible hydrogen electrode; CuScP2 S6 is active for HER, exhibiting an overpotential of 407 mV and a Tafel slope of 90 mV dec-1 . Density functional theory models reveal that bulk AgScP2 S6 and CuScP2 S6 are both semiconductors with computed bandgaps of 2.42 and 2.23 eV, respectively and overall similar electronic properties. Besides composition, the largest difference in both materials is in their molecular structure, as Ag atoms sit at the midpoint of each layer alongside Sc atoms, while Cu atoms are raised to a similar height to S atoms, in the external segment of the 2D layers. This structural difference probably plays a fundamental role in the different catalytic performances of these materials. These findings show that MI (Cu, Ag) together with Sc(MIII ) leads to promising achievements in MI MIII P2 S6 materials as electrocatalysts.publishe