Redox flow batteries (RFBs) have emerged as prime candidates for energy storage on the medium and large scales, particularly at the grid scale. The demand for versatile energy storage continues to increase as more electrical energy is generated from intermittent renewable sources. A major barrier in the way of broad deployment and deep market penetration is the use of expensive metals as the active species in the electrolytes. The use of organic redox couples in aqueous or nonaqueous electrolytes is a promising approach to reducing the overall cost in long-term, since these materials are low-cost and abundant. The performance of such redox couples can be tuned by modifying their chemical structure. In recent years, significant developments in organic redox flow batteries has taken place, with the introduction of new groups of highly soluble organic molecules, capable of providing a cell voltage and charge capacity comparable to conventional metal-based systems. This review summarises the fundamental developments and characterization of organic redox flow batteries from both the chemistry and materials perspectives. The latest advances, future challenges and opportunities for further development are discussed.
Metal deposition or anode in one half-celli.e. Lithium/ organic hybrid FB Metal 'This review'
The nanostructured BiVO 4 photoanodes were prepared by electrospinning and were further characterized by XRD, SEM, and XPS, confirming the bulk and surface modification of the electrodes attained by W addition. The role of surface states (SS) during water oxidation for the asprepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (N SS) with respect to the charge donor density (N d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest N d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V 5+ /V 4+ in semiconductor bulk to form water oxidation intermediates through the electron trapping process. Otherwise, the irreversible surface reductive reaction of VO 2 + to VO 2+ though the electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of the BiVO 4 electrode. It can tune the electron trapping process to obtain a high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.
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