A graphite felt decorated with bismuth nanoparticles was studied as negative electrode in a vanadium redox flow battery (VRFB). The results confirm the excellent electrochemical performance of the bismuth modified electrode in terms of the reversibility of the V(3+) /V(2+) redox reactions and its long-term cycling performance. Moreover a mechanism that explains the role that Bi nanoparticles play in the redox reactions in this negative half-cell is proposed. Bi nanoparticles favor the formation of BiHx , an intermediate that reduces V(3+) to V(2+) and, therefore, inhibits the competitive irreversible reaction of hydrogen formation (responsible for the commonly observed loss of Coulombic efficiency of VRFBs). Thus, the total charge consumed during the cathodic sweep in this electrode is used to reduce V(3+) to V(2+) , resulting in a highly reversible and efficient process.
Graphite felt modified with nanodispersed bismuth was studied as electrode in the positive half-cell of a vanadium redox flow battery (VRFB). The felt was easily modified by immersion in a Bi 2 O 3 solution followed by thermal reduction at 450°C in air. Despite the low metal content (1 at. %), the Bi-modified felt showed an excellent electrochemical performance (at 1 mVs -1 ) in terms of anodic and cathodic peak current densities (21 and 17 mA/cm 2 , respectively), reversibility (ΔE p = 0.050 V) and overpotential for the V(IV)/V(V) redox reactions. Furhtermore, repetitive cyclic voltammetry measurements, at various scan rates, evidenced the long term stability of this material. These results demonstrate that bismuth nanoparticles on the carbon surface act as stable active sites to promote these reactions, and represent a significant step forward towards the development of outstanding electrode materials for VRFB.
Two graphene-like materials, obtained by thermal exfoliation and reduction of a graphite oxide at 700 and 1000ºC, were studied as active electrodes in the positive half-cell of a Vanadium Redox Flow Battery (VRFB). In particular, that obtained at 1000ºC exhibited an outstanding electrochemical performance in terms of peak current densities (30.54 and 30.05 mAcm -2 for the anodic and cathodic peaks at 1 mVs -1 , respectively) and reversibility (ΔE p = 0.07 V). This excellent bahaviour is attributed to the restoration of sp 2 domains after thermal treatment, which implies the production of a graphene-like material with a high electrical conductivity and accessible surface area. Moreover, the residual functional groups, -OH, act as active sites towards the vanadium redox reactions. This represents a significant step forward in the development of highly effective VRFB electrode materials.
The specific capacitance of the MWCNTs was improved by the addition of an electrochemically active compound (Indigo carmine) to an electrolyte generally used in electric double layer capacitors. The pseudocapacitive contribution of the IC trebled the specific capacitance values of the MWCNTs at low current densities (from 17 Fg-1 to 50 Fg-1). The good resistance obtained for the MWCNT-based capacitor was not modified with the use of this novel redox-active electrolyte. A reversible process associated to the redox reaction of IC was found to be responsible for the capacitance increase observed. Therefore, a combined effect of double layer formation and pseudocapacitive phenomena is presented. Long-term cycling experiments performed showed good stability with a reduction of the initial capacitance values of 30 % after 10,000 galvanostatic cycles at 360 mAg-1. The efficiency of the cell was close to 100 % throughout the experiment.
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