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We report here a study on the Li ion storage performance of binary phased SnO 2 /rGO and ternary phased SnO 2 -Fe 2 O 3 /rGO composite nanostructures. The SnO 2 /rGO and SnO 2 -Fe 2 O 3 /rGO were prepared by a facile wet-chemical approach. The Li storage performances of these samples were closely related to the weight ratio of SnO 2 : rGO or SnO 2 : Fe 2 O 3 : rGO. It was found that ternary SnO 2 -Fe 2 O 3 /rGO composite nanostructures (e.g. with a weight ratio of SnO 2 : Fe 2 O 3 : rGO ¼ 11 : 1 : 13) showed significant enhancement of the specific capacities and cyclabilities as compared to that of SnO 2 /rGO samples. For example, the SnO 2 -Fe 2 O 3 /rGO electrode depicted a specific capacity of 958 mA h g À1 at a current density of 395 mA g À1 (0.5 C) during the 100 th cycle. Such Li storage performances of the SnO 2 -Fe 2 O 3 /rGO electrodes, especially at high current densities (e.g. 530 mA h g À1 at 5 C rate), were also much better than those reported for either SnO 2 -based or Fe 2 O 3 -based electrodes. Such a synergetic effect in the SnO 2 /Fe 2 O 3 /rGO composite nanostructures is promising for the development of advanced electrode materials for rechargeable Li-ion batteries.

h i g h l i g h t sHigh surface area activated carbon was synthesized from orange peel (OP-AC). OP-AC showed high porous structure and rich oxygen-containing functional groups. OP-AC coated electrode showed less contact resistance and good catalytic activity. OP-AC coated electrode enhanced voltage and energy efficiency in VRB static cell. a b s t r a c t Activated carbon (AC) with high surface area (1901 m 2 g À1 ) is synthesized from low cost bio-waste orange (Citrus sinensis) peel for vanadium redox flow battery (VRB). The composition, structure and electrochemical properties of orange peel derived AC (OP-AC) are characterized by elemental analyzer, field emission-scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. CV results show that OP-AC coated bipolar plate demonstrates improved electro-catalytic activity in both positive and negative side redox couples than the pristine bipolar plate electrode and this is ascribed to the high surface area of OP-AC which provides effective electrode area and better contact between the porous electrode and bipolar plate. Consequently, the performance of VRB in a static cell shows higher energy efficiency for OP-AC electrode than the pristine electrode at all current densities tested. The results suggest the OP-AC to be a promising electrode for VRB applications and can be incorporated into making conducting plastics electrode to lower the VRB cell stack weight and cost.

Vanadium redox flow batteries (VRFBs) are becoming an integral component of renewable energy solutions, microgrids, and backup storage systems. The reduction in cell resistance due to advancements in the electrode and overall component design allows flow cells to operate at higher current/power densities. During discharging of an experimental flow cell at higher current densities, under certain conditions, an unexpected voltage drop and subsequent recovery were observed. Similar effects of voltage drop and recovery have been observed in NiCd and primary lithium–thionyl chloride (LTC) batteries but have not yet been reported for the VRFB in peer-reviewed publications. This study aims to understand the causes behind the voltage drop effect in a VRFB under high power density operation.

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