Graphite felt anchored with fused nanowires from bismuth metal-organic framework as negative electrode for all-vanadium redox flow batteries

Abstract

The performance of all-vanadium redox flow batteries (VRFBs) is hindered by limited kinetics of vanadium redox couples, particularly V3+/V2+, at graphite felt (GF) electrodes. This study is focused on bismuth-based metal-organic frameworks (Bi-MOF) with nanotube morphology and MOF-derived Bi2O3 fused nanowires as novel and efficient electrocatalysts for V3+/V2+ redox reactions. In-situ growth of Bi2O3 nanowires from Bi-MOFs provided improved hydrophilicity and higher defect sites for efficient inter-particle electron transfer, thus enhancing its electrochemical activity towards V3+/V2+ redox reactions. The VRFB single cells utilizing Bi-MOF-GF and Bi2O3-GF as anode materials achieved energy efficiency of 81 and 82 %, respectively, outperforming cell employing pristine GF (P-GF) and thermally treated (HT-GF), which only achieved 64 % and 78 %, respectively at current density of 100 mA cm−2. Moreover, even at a higher current density of 300 mA cm−2, Bi-MOF-GF and Bi2O3-GF cells delivered average discharge capacities of 8 and 12 Ah L−1 respectively, while cells with P-GF and HT-GF electrodes failed to perform beyond 100 and 200 mA cm−2, respectively. The cycling stability tests over 100 charge/discharge cycles of Bi2O3-GF and Bi-MOF-GF cells at 150 mA cm−2 demonstrated capacity retentions of 86 and 69 %, respectively, with negligible decay in energy efficiency throughout the cycling duration.