Abstract
Facile and efficient methods to prepare active electrodes for redox reactions of electrolyte ions are required to produce efficient and low-cost redox flow batteries (RFBs). Carbon-fiber electrodes are widely used in various types of RFBs and surface oxidation is commonly performed to enhance the redox reactions, although it is not necessarily efficient. Quite recently, a technique for nanoscale and uniform surface etching of the carbon fiber surface was developed and a significant enhancement of the negative electrode reaction of vanadium redox flow batteries was attained, although the enhancement was limited to the positive electrode reaction. In this study, we attempted to obtain an additional enhancement effect of metal-oxide nanoparticles without the need for further processing steps. A coating with carbonaceous thin films was obtained coating by sublimation, deposition, and pyrolysis of tin(II) phthalocyanine (SnPc) on a carbon fiber surface in a single heat-treatment step. The subsequent thermal oxidation concurrently achieved nanoscale surface etching and loading with SnO2 nanoparticles. The nanoscale-etched and SnO2-loaded surface was characterized by field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The activity for the vanadium ion redox reactions was evaluated by cyclic voltammetry (CV) to demonstrate the enhancement of both the positive and negative electrode reactions. A full cell test of the vanadium redox flow battery (VRFB) showed a significant decrease of the overpotential and a stable cycling performance. A facile and efficient technique based on the nanoscale processing of the carbon fiber surface was presented to substantially enhance the activity for the redox reactions in redox flow batteries.
Highlights
Redox flow batteries (RFBs) are energy conversion and storage devices that involve the reduction and oxidation of electroactive species in electrolyte solutions and have attracted much attention due to their scalability and safety
The reactions shown below are involved in the vanadium redox flow batteries (VRFBs), which are in the most advanced stage of research and development: trode reactions of a vanadium redox flow battery (VRFB) were clearly enhanced at the finely etched and SnO2-loaded carbon-fiber electrode and a stable performance was demonstrated by full cell cycle tests
The cyclic voltammetry (CV) in the potential ranges corresponding to the positive and negative electrode reactions in an acidic electrolyte containing vanadium ions are shown in Figure 5 for TGP and TGPCSnPc-TAir
Summary
Redox flow batteries (RFBs) are energy conversion and storage devices that involve the reduction and oxidation of electroactive species in electrolyte solutions and have attracted much attention due to their scalability and safety. Carbon-fiber electrodes are conventionally used in RFBs and surface oxidation is often performed to enhance the redox reactions [7,8,9,10,11,12,13,14], a sufficient activity has not yet been obtained.
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