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Flexible supercapacitors with considerable energy storage performance from green/sustainable materials have attracted significant attention in many fields, such as portable and wearable electronics. In this work, flexible cellulose nanofibers/reduced graphene oxide/polypyrrole (CNFs/rGO/PPy) aerogel electrodes with well-defined three-dimensional porous structures are prepared using citric acid-Fe3+ (CA-Fe3+) complexes as oxidant precursors to command the deposition of PPy. The in situ gradual release of Fe3+ leads to the formation of thin and uniform polypyrrole in the composites. A flexible all-solid-state supercapacitor is then prepared by the CNFs/rGO/PPy aerogel film electrode and poly­(vinyl alcohol) (PVA)/H2SO4 gel electrolyte and separator. Due to the porous structure, high electrical conductivity, and remarkable wettability of the electrodes, the assembled supercapacitors show excellent electrochemical properties with maximum areal capacitance of 720 mF cm–2 (405 F g–1 for single electrode) at 0.25 mA cm–2 and good cycle stability (95% retention after 2000 cycles). The device with maximum energy density of 60.4 μW h cm–2 also exhibits nearly constant capacitance under different bending conditions, suggesting their great potential for applications in flexible electronics.

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BackgroundCofactor engineering is involved in the modification of enzymes related to nicotinamide adenine dinucleotides (NADH and NAD+) metabolism, which results in a significantly altered spectrum of metabolic products. Cofactor engineering plays an important role in metabolic engineering but is rarely reported in the sterols biotransformation process owing to its use of multi-catabolic enzymes, which promote multiple consecutive reactions. Androst-4-ene-3, 17-dione (AD) and androst-1, 4-diene-3, 17-dione (ADD) are important steroid medicine intermediates that are obtained via the nucleus oxidation and the side chain degradation of phytosterols by Mycobacterium. Given that the biotransformation from phytosterols to AD (D) is supposed to be a NAD+-dependent process, this work utilized cofactor engineering in Mycobacterium neoaurum and investigated the effect on cofactor and phytosterols metabolism.ResultsThrough the addition of the coenzyme precursor of nicotinic acid in the phytosterols fermentation system, the intracellular NAD+/NADH ratio and the AD (D) production of M. neoaurum TCCC 11978 (MNR M3) were higher than in the control. Moreover, the NADH: flavin oxidoreductase was identified and was supposed to exert a positive effect on cofactor regulation and phytosterols metabolism pathways via comparative proteomic profiling of MNR cultured with and without phytosterols. In addition, the NADH: flavin oxidoreductase and a water-forming NADH oxidase from Lactobacillus brevis, were successfully overexpressed and heterologously expressed in MNR M3 to improve the intracellular ratio of NAD+/NADH. After 96 h of cultivation, the expression of these two enzymes in MNR M3 resulted in the decrease in intracellular NADH level (by 51 and 67%, respectively) and the increase in NAD+/NADH ratio (by 113 and 192%, respectively). Phytosterols bioconversion revealed that the conversion ratio of engineered stains was ultimately improved by 58 and 147%, respectively. The highest AD (D) conversion ratio by MNR M3N2 was 94% in the conversion system with soybean oil as reaction media to promote the solubility of phytosterols.ConclusionsThe ratio of NAD+/NADH is an important factor for the transformation of phytosterols. Expression of NADH: flavin oxidoreductase and water-forming NADH oxidase in MNR improved AD (D) production. Besides the manipulation of key enzyme activities, which included in phytosterols degradation pathways, maintenance the balance of redox also played an important role in promoting steroid biotransformation. The recombinant MNR strain may be useful in industrial production.