Electrochemical investigation of ionic liquid-derived porous carbon materials for supercapacitors: pseudocapacitance versus electrical double layer

Deep Eutectic Solvent for Facile Synthesis of Mn3O4@N-Doped Carbon for Aqueous Multivalent-Based Supercapacitors: New Concept for Increasing Capacitance and Operating Voltage

The capacitance and operating voltage of supercapacitors as well as their energy density have been increased by development of different materials and electrolytes. In this paper, two strategies, for the first time, were used to improve energy density: Mn₃O₄- and N-dual doped carbon electrode and aqueous mixture of multivalent ions as electrolyte. Mn₃O₄- and N-dual doped carbon was prepared by a novel and cost-effective procedure using deep eutectic solvent. XRD, XPS, and FTIR confirmed presence of Mn₃O₄ and nitrogen, while SEM and EDS elemental mapping showed micrometer-sized nanosheets with uniform distribution of C, O, N, and Mn atoms. Charge storage behavior of carbon was tested in aqueous multivalent-based electrolytes and their mixture (Ca²⁺-Al³⁺). Regarding both specific capacitance and workable voltage, the Ca²⁺-Al³⁺ mixed electrolyte was found as the best optimal solution. The calcium addition to the Al-electrolyte allows the higher operating voltage than in the case of individual Al(NO₃)₃ electrolyte while the addition of Al³⁺ ion in the Ca(NO₃)₂ electrolyte improves the multivalent-ion charge storage ability of carbon. As a result, the specific energy density of two-electrode Mn₃O₄@N-doped carbon//Al(NO₃)₂+Ca(NO₃)₂//Mn₃O₄@N-doped carbon supercapacitor (34 Wh kg^-1 at 0.1 A g^-1) overpasses the reported values obtained for Mn-based carbon supercapacitors using conventional aqueous electrolytes.

Electrocatalytic Behavior of an Amide Functionalized Mn(II) Coordination Polymer on ORR, OER and HER

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec-1 with mixed kinetics of 2e- and 4e- pathways indicated by the Koutecky-Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH.

Enzymatic Hydrolysis Lignin-Derived Porous Carbons through Ammonia Activation: Activation Mechanism and Charge Storage Mechanism

The low energy density and low cost performance of electrochemical capacitors (ECs) are the principal factors that limit the wide applications of ECs. In this work, we used enzymatic hydrolysis lignin as the carbon source and an ammonia activation methodology to prepare nitrogen-doped lignin-derived porous carbon (NLPC) electrode materials with high specific surface areas. We elucidated the free radical mechanism of ammonia activation and the relationship between nitrogen doping configurations, doping levels, and preparation temperatures. Furthermore, we assembled NLPC∥NLPC symmetric ECs and NLPC∥Zn asymmetric ECs using aqueous sulfate electrolytes. Compared with the ECs using KOH aqueous electrolyte, the energy densities of NLPC∥NLPC and NLPC∥Zn ECs were significantly improved. The divergence of charge storage characteristics in KOH, Na₂SO₄, and ZnSO₄ electrolytes were compared by analyzing their area surface capacitance. This work provides a strategy for the sustainable preparation of lignin-derived porous carbons toward ECs with high energy densities.

Heteroatom-doped porous carbon microspheres derived from ionic liquid-lignin solution for high performance supercapacitors

In this work, nitrogen and phosphorus dual-doped alkali lignin-based carbon microspheres (MLCM) were prepared by pre-oxidation and carbonization of ionic liquid ([Mmim]DMP) -lignin solution and used as green-based supercapacitor electrode materials. Compared with the directly carbonized alkali lignin carbon (LC), MLCM had a spherical structure with higher specific surface area (938.1 m²/g) and pore volume (0.64 cm³/g). Moreover, MLCM materials showed superior electrochemical performance. In the 1 mol/L H₂SO₄ electrolyte system, MLCM presented the highest specific capacitance of 338.2F/g at a current density of 0.8 A/g. Furthermore, MLCM was used as a positive and negative electrode material to assemble a symmetrical supercapacitor. The resultant device maintained excellent cycle stability after 5000 times of charging and discharging process at 2 A/g. Overall, the facile, green and sustainable synthesis strategy of heteroatom-doped porous carbon microspheres developed in this work opens a new avenue for the fabrication of high-performance carbon electrode materials, especially based on abundant and renewable lignin.

Mixed chloridometallate(ii) ionic liquids with tunable color and optical response for potential ammonia sensors

Eight d-metal-containing N-butylpyridinium-based ionic liquids (ILs) were synthesized, characterized, and investigated for their optical properties.

Sustainable Preparation of Nanoporous Carbons via Dry Ball Milling: Electrochemical Studies Using Nanocarbon Composite Electrodes and a Deep Eutectic Solvent as Electrolyte

The urgent need to reduce the consumption of fossil fuels drives the demand for renewable energy and has been attracting the interest of the scientific community to develop materials with improved energy storage properties. We propose a sustainable route to produce nanoporous carbon materials with a high-surface area from commercial graphite using a dry ball-milling procedure through a systematic study of the effects of dry ball-milling conditions on the properties of the modified carbons. The microstructure and morphology of the dry ball-milled graphite/carbon composites are characterized by BET (Brunauer-Emmett-Teller) analysis, SEM (scanning electron microscopy), ATR-FTIR (attenuated total reflectance-Fourier transform infrared spectroscopy) and Raman spectroscopy. As both the electrode and electrolyte play a significant role in any electrochemical energy storage device, the gravimetric capacitance was measured for ball-milled material/glassy carbon (GC) composite electrodes in contact with a deep eutectic solvent (DES) containing choline chloride and ethylene glycol as hydrogen bond donor (HBD) in a 1:2 molar ratio. Electrochemical stability was tracked by measuring charge/discharge curves. Carbons with different specific surface areas were tested and the relationship between the calculated capacitance and the surface treatment method was established. A five-fold increase in gravimetric capacitance, 25.27 F·g-1 (G40) against 5.45 F·g-1, was found for commercial graphene in contact with DES. Optimal milling time to achieve a higher surface area was also established.

Lignocellulose-based free-standing hybrid electrode with natural vessels-retained, hierarchically pores-constructed and active materials-loaded for high-performance hybrid oxide supercapacitor

Lignocellulose including cellulose, lignin, and hemicellulose could be extracted from wood, and has been used to prepare carbon electrode. However, complicated extraction greatly increases preparation cost. To achieve maximum utilization of lignocellulose and avoid complicated extraction, wood with porous structure and good mechanical strength is used as carbon precursor. Additionally, chemical activation is commonly used to create micropores to provide high capacitance, but it brings in natural structure destruction, and generation of wastewater during pickling. Moreover, to achieve desirable energy density, multi-step strategy with long duration is required for loading active materials on carbonized lignocellulose (CL). Herein, a one-step method is developed to prepare a free-standing hybrid CL electrode (CLE) by using Lewis acid in three aspects: (1) as structure protection agent, (2) as activating agent, (3) as active materials donor, which bypasses pickling and further avoids the generation of wastewater. Additionally, natural vessels in wood can not only provide large space for active materials loading, but also act as rapid ions diffusion way, simultaneously confining active materials detachment. Benefiting from the synergistic effect of porous structure and Lewis acid, this work not only makes full utilization of lignocellulose, but also makes CLE exhibit excellent performance in hybrid oxide supercapacitor.

Ionic Liquid-Derived Carbon-Supported Metal Electrocatalysts as Anodes in Direct Borohydride-Peroxide Fuel Cells

Three different carbon-supported metal (gold, platinum, nickel) nanoparticle (M/c-IL) electrocatalysts are prepared by template-free carbonization of the corresponding ionic liquids, namely [Hmim][AuCl4], [Hmim]2[PtCl4], and [C16mim]2[NiCl4], as confirmed by X-ray diffraction analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and Raman spectroscopy. The electrochemical investigation of borohydride oxidation reaction (BOR) at the three electrocatalysts by cyclic voltammetry reveals different behavior for each material. BOR is found to be a first-order reaction at the three electrocatalysts, with an apparent activation energy of 10.6 and 13.8 kJ mol−1 for Pt/c-IL and Au/c-IL electrocatalysts, respectively. A number of exchanged electrons of 5.0, 2.4, and 2.0 is obtained for BOR at Pt/c-IL, Au/c-IL, and Ni/c-IL electrodes, respectively. Direct borohydride-peroxide fuel cell (DBPFC) tests done at temperatures in the 25–65 °C range show ca. four times higher power density when using a Pt/c-IL anode than with an Au/c-IL anode. Peak power densities of 40.6 and 120.5 mW cm−2 are achieved at 25 and 65 °C, respectively, for DBPFC with a Pt/c-IL anode electrocatalyst.

Sustainable biomass-based hierarchical porous carbon for energy storage: A novel route to maintain electrochemically attractive natural structure of precursor

The development of sustainable and renewable energy storage devices with low cost and environment friendly features is an extremely urgent issue that needs to be solved. Herein, low-cost and sustainable biomass chitin, possessing natural fibrous, O/N-enriched and porous structure, was employed as a porous carbon (PC) precursor. However, a huge challenge in PC preparation is to maintain the natural electrochemically attractive structure of chitin while obtaining highly porous structure. In this study, by utilizing the molten protecting effect and micropore-creating ability of CuCl₂ 2H₂O, the obtained PCs maintain the natural structure, achieve high yield (46%), and simultaneously develop hierarchical pores with a specific surface area range of 1635-2381 m² g^-1, a tunable micropore volume ratio range of 63.5-96.8%, and high surface O/N contents (N: 3.1-9.0 wt% and O: 10.5-12.8 wt%). Benefiting from these excellent properties, optimized PC achieves a high specific capacitance of 286 F g^-1 at 0.5 A g^-1 and a remarkably high rate capability of 88% at 10 A g^-1; moreover, it even exhibits a rate capability of 80% at an ultrahigh current density of 50 A g^-1. The optimized PC-based supercapacitor assembled in Na₂SO₄ electrolyte shows a high energy density of 15.41 W h kg^-1 at 0.19 kW kg^-1 and achieves 76% energy density retention when the power density increased tenfold. Thus, this study presents a new way to fully utilize biomass, especially with electrochemically attractive natural structure, for developing advanced energy storage devices.

Ex-situ nitrogen-doped porous carbons as electrode materials for high performance supercapacitor

Nitrogen (N) doping of porous carbon materials is an effective strategy for enhancing the electrochemical performance of electrode materials. Herein, we report on ex-situ (post) nitrogen-doped porous carbons prepared using a biomass waste, peanut shell (PS) as a carbon source and melamine as the nitrogen source. The synthesis method involved a two-step mechanism, initial chemical activation of the PS using KOH and post N-doping of the activated carbon. The effect of the activating agent/precursor ratio and the ex-situ N-doping on the structural, textural, electrochemical properties of the porous carbons was studied. The ex-situ N-doped porous carbon with an optimum amount of KOH to PS exhibited the best capacitance performance with a specific surface area (SSA) of 1442 m² g^-1 and an enriched nitrogen content (3.2 at %). The fabricated symmetric device exhibited a 251.2 F g^-1 specific capacitance per electrode at a gravimetric current of 1 A g^-1 in aqueous electrolyte (2.5 M KNO₃) at a wide cell voltage of 2.0 V. A specific energy of 35 Wh kg^-1 with a corresponding specific power of 1 kW kg^-1 at 1 A g^-1 was delivered with the device still retaining up to 22 Wh kg^-1 and a 20 kW kg^-1 specific power even at 20 A g^-1. Moreover, long term device stability was exhibited with an 83.2% capacity retention over 20 000 charge/discharge cycles and also a good rate capability after 180 h of floating at 5 A g^-1. This great performance of the symmetric supercapacitor can be correlated to the surface porosity and post nitrogen-doping effect which increased the electrochemically-active sites resulting in a remarkable charge storage capability.