Kinetics Test And Performance Optimization On Carbon-based Electrode For The Vanadium Redox Flow Batteries

In the context of global "carbon-neutral",energy storage industry faces a significant opportunity.Vanadium flow battery(VFB),as a kind of electrochemical energy storage technology,is expected to become the preferred technology for large-scale and long-term energy storage owing to its merits such as high safety,long cycle-life and so on.At present,the full commercialization of VFB industry is still limited by its high cost.Improving battery performance including power/energy density is considered as an effective way to reduce storage cost.Carbon-based electrode is one of the critical components in VFB,which provides a place for the redox reactions and thus plays a decisive role for the performance.Nevertheless,its unsatisfactory catalytic activity and poor wettability greatly limit the improvement of VFB.Moreover,it is difficult to accurately acquire electrode kinetics parameters and provide further theoretical guidance for electrode optimization by existing ex-situ methods due to the fact that the electrode in VFB is in a complex environment such as compression and convection.Therefore,in-situ kinetics analysis and performance optimization of the electrode are very important for the development of VFB.Herein,we design a symmetrical cell configuration to conduct in-situ polarization analysis.Subsequently,various electrode optimization strategies are proposed from different perspectives and scales to improve the drawbacks of carbon-based materials.Detailed content are as follows.Firstly,we propose an in-situ analysis method for porous electrode by designing symmetrical cell to measure kinetics parameters under practical battery condition.The total polarization,ohmic polarization and concentration polarization of the symmetrical cell are determined by steady-state polarization curve,electrochemical impedance spectroscopy and limit current measurement,respectively.Then the activation polarizations of V2+/V3+ and VO2+/VO2+ reactions are calculated.The reaction rate constants and transfer coefficients of the porous electrode are obtained based on Tafel fitting of the activation polarization curve.The accuracy and applicability of the proposed in-situ polarization analysis method are highlighted by comparing the measured polarization curves with the finite element fitting results.Secondly,to unveil the regulation essence of electrode compression,the impacts of the electrode compression on various polarizations are quantified by using the methods of ex-situ material characterization and in-situ cell testing.Meanwhile,we conduct the charge-discharge tests to further understand the essential relationship between cycling performance and battery polarizations under different electrode compression states.The results indicate that the electrode compression decreases the ohmic polarization while increaseing the activation polarizations.Consequently,the positive and negative effects of the compression result in an optimal compression ratio.Based on these,reasonable electrode compression state can effectively reduce the total polarizations in the cell,and thus widen the effective potential window to improve the efficiency and capacity of a VFB.Next,to figure out the sluggish anode redox chemistry in VFB,we propose a novel room-temperature modification approach to tailor the oxygen functional groups on carbon felt surface based on ozone oxidation.The content and composition of oxygen functional groups are further accurately tailored by varying the treatment time,which significantly enhances the anode V2+/V3+ kinetics.Theoretical calculations show that the introduction of oxygen functional groups alters the charge distribution,and facilitates the adsorption process of vanadium-ion on the electrode interface.However,the oxygen functional groups can synergistically suppress electron transfer process to a certain extent.Therefore,the modulation essence of oxygen functional groups is to balance their adsorption enhancement effect and electron transfer inhibition effect.Finally,we fabricate highly active Nitrogen-doped carbon felts via in-situ interfacial co-polymerization of dopamine and polyethylenimine followed by a pyrolysis process.Through the regulation of co-polymerization reaction conditions,a hierarchical electrode interface is constructed,providing multiscale channels for mass transfer and redox reactions.Formed microvillus-liked Nitrogen-doped carbon layer with a higher content of pyridinic N is able to offer more effective active sites for redox reactions.As a result,a VFB assembled with the prepared felts achieves a maximum output power density of 750.6 mW cm-2.Theoretical calculations show that the essence of Nitrogen doping is that pyridinic N enhances the adsorption process of vanadium-ion and reduces the activation energy barrier for the positive reaction.