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Pectin, natural polysaccharide biopolymer, was chelated with cations (Mg2+/Ca2+) to form an interwoven framework. Herein, the graphene hydrogel electrodes were self-assembled by the synergistic effects of pectin-cations. The optimum combination proportion was determined, the Mg2+/Ca2+-pectin matrix cross-linked graphene hydrogel (Mg2+/Ca2+-PGH) electrodes exhibited a large specific capacitance of about 839.2 F g−1 with high coulombic efficiency of 191.8% at a current density of 1 A g−1. The assembled flexible supercapacitor displayed excellent stability (capacitance retention of 98.5% after 2000 charge/discharge cycles) and flexibility (the specific capacitance remained 98.4% of its original value after 500 folding/unfolding cycles). Such flexible and high-performance Mg2+/Ca2+-PGH electrodes are attractive in the field of lightweight, miniature and wearable energy storage devices.

An automated multi-filtration cleanup (Auto m-FC) method with nitrogen-enriched activated carbon material based on modified QuEChERS (quick, easy, cheap, effective, rugged, and safe) extracts was developed. It was applied to pesticide multi-residue analysis in six representative crop commodities. The automatic device was aimed to improve the cleanup efficiency and reduce manual operation workload in cleanup step. By controlling extracts volume, flow rate and Auto m-FC cycles, the device could finish cleanup process accurately. In this work, nitrogen-enriched activated carbon mixed with alternative sorbents and anhydrous magnesium sulfate (MgSO4) was packed in a column for Auto m-FC and followed by liquid chromatography with tandem mass spectrometric (LC-MS/MS) detection. This newly developed carbon material showed excellent cleanup performance. It was validated by analyzing 23 pesticides in six representative matrices spiked at two concentration levels of 10 and 100μg/kg. Water addition volume, salts, sorbents, Auto m-FC procedure including the flow rate and the Auto m-FC cycles for each matrix were optimized. Then, three general Auto m-FC methods were introduced to high water content, high oil and starch content, difficult commodities. Spike recoveries were within 82 and 106% and 1-14% RSD for all analytes in the tested matrices. Matrix-matched calibrations were performed with the coefficients of determination over 0.997 between concentration levels of 10 and 1000μg/kg. The developed method was successfully applied to the determination of pesticide residues in market samples.

In this work, a lignosulfonate has been used for synthesizing the graphene quantum dots (GQDs) with many functional groups by a two-step method. Then a graphene-based composite hydrogel (GH-GQD) was obtained through the modification of the functionalized GQDs. Results showed that the composite hydrogel had a highly porous structure and numerous active sites. Accordingly, the assembled GH-GQD electrode exhibited an impressive specific capacitance of 451.7 F g−1 at a current density of 0.5 A g−1, a good cycle stability (89.0% capacitance retention after 10000 charge/discharge cycles) and mechanical flexibility (93.3% capacitance retention bent to 180°). The present findings indicate a great potential of the graphene hydrogels modified with the functionalized GQDs in fabricating flexible supercapacitor electrodes.

In this work, the Pd@ Cd(2-methylimidazole)2 (Pd@CdIF-8) was synthesized by two facile methods, in which the original cubic Pd nanoparticles anchored on the surface/interface of CdIF-8 and supported on the acidified activated carbon (AAC). Interestingly, CdIF-8 exhibited sphericity under hydrothermal condition and presented hexahedron under mild liquid. A(B)-PdxCdyIF-8@AAC hybrids are researched by FESEM, TEM, contact angle measurement, XPS, XRD and BET analytical techniques. A(B)-PdxCdyIF-8@AAC hybrids are employed as nonenzymatic sensors for electrochemical detection of H2O2, and both exhibited excellent electrocatalytic activity, especially for A(B)–Pd0.2Cd1.5IF-8@AAC/GCE sensors. A(B)–Pd0.2Cd1.5IF-8@AAC/GCE sensors demonstrated a linear response to H2O2 in the range of from 10 μM to 10.2 mM and 2 μM to 17.5 mM, the sensitivity of 0.195 μAμM−1 and 0.266 μAμM−1, respectively. In addition, asymmetric supercapacitors have been fabricated using A(B)-PdxCdyIF-8@AAC hybrids as positive electrode and exhibited exceptional electrochemical properties with high specific capacitance (452 F g−1 at 0.05 A g−1 and 372 F g−1 at 1 A g−1 of A-Pd0.2Cd1.5IF-8@AAC, and 502 F g−1 at 0.05 A g−1 and 395 F g−1 at 1 A g−1 of B–Pd0.2Cd1.5IF-8@AAC), excellent stability (83.579% maintenance of specific capacitance at 5 A g−1 of A-Pd0.2Cd1.5IF-8@AAC and 78.313% maintenance of B–Pd0.2Cd1.5IF-8@AAC), respectively. Keywords: Acidified activated carbon, A(B)-PdxCdyIF-8@AAC, Electrochemical sensor, Asymmetric supercapacitor

In this work, flexible graphene supercapacitor electrodes were prepared from biomass-derived carbon dots/graphene composite film (CDGF) and hydrogel (CDGH) with/without an electrodeposited NiCo2S4 layer. Results showed that the dispersive CDs with abundant heteroatoms effectively inhibited the aggregation of graphene sheets and increased the number of active sites. By combining CDs modification and NiCo2S4 electrodeposition, the electrochemical performance of the prepared ternary composite electrodes is significantly enhanced. In particular, the CDGF-NiCo2S4 electrode has a specific capacitance of up to 1348 F g−1 at a current density of 0.5 A g−1. The resultant CDGF-NiCo2S4 flexible symmetric supercapacitor using 1.0 M H2SO4 as aqueous electrolyte shows a large specific capacitance (313 F g−1 at 0.5 A g−1) and a high energy density of 85.1 Wh kg−1 at a power density of 353 W kg−1. By contrast, the CDGH-NiCo2S4 flexible solid-state supercapacitor using polyvinyl alcohol (PVA)/H2SO4 as gel electrolyte has improved cycling stability (83.1% capacitance retention after 10000 charging/discharging cycles) and a better mechanical flexibility (96.8% capacitance retention after 1000 bending cycles). This work demonstrates the importance of CDs and metal sulfides in modifying graphene-based composite electrodes for flexible supercapacitor.

Due to the urgent demand for portable electronics, enormous research efforts are devoted for the development of flexible, wearable and lightweight supercapacitors. Herein, free-standing, binder-free and high-performance metal @ carbon sphere/graphene film electrodes (Fe@C/G and Co@C/G), consisting of β-cyclodextrins (β-CDs) as carbon sources, iron and cobalt as metal materials, and graphene sheets as “binders”, have been synthesized and characterized. The resultant Fe@C/G and Co@C/G film electrodes exhibit maximum areal specific capacitances of 2.19 F cm−2 and 0.80 F cm−2 at 0.5 mA cm−2 (438 F g−1 and 160 F g−1), respectively. Furthermore, a flexible asymmetric all-solid-state supercapacitor (Fe@C/G//Co@C/G) is assembled and it displays a series of intriguing features including high volumetric specific capacitance of about 13.3 F cm−3 at 0.5 mA cm−2, excellent volumetric energy density of up to 5.99 mWh cm−3 at a volumetric power density of 0.013 W cm−3, and good cycle stability of 87.3% capacitance retention after 10,000 charge/discharge cycles. Even after 500 bending/unbending cycles, still about 80.5% of its initial capacitance is retained. Such results well demonstrate its strong potential for the practical applications of flexible energy storage devices.