Modification And Characterizations Of High-performance Bipolar Plates For Redox Flow Batteries

The wide application of renewable energy in electricity generation facilitates the development of energy storage batteries like vanadium redox flow battery（VRFB）.Bipolar plate,as one of the vital components of VRFB,is responsible for collecting conductive current,connecting two adjacent cells and supporting the electrode.However,it may be difficult for traditional bipolar plates to possess high electrical conductivity and high flexural strength simultaneously.To solve this,the dominant factors and mechanisms that influence the flexural strength and electrical conductivity are disclosed,and a low-carbon-content thermosetting composite bipolar plate and a low-carbon-content thermoplastic composite bipolar plate are designed.In order to eliminate the resin-rich layer on the surfaces of composite bipolar plate,surface modification with cactus-like carbon nanofibers is carried out.After surface treatment,durability and performance of the composite bipolar plate are significantly enhanced.Key results are presented in the following.（1）Taking advantage of bridging effect of graphene and matrix of epoxy resin,a low-carbon-content thermosetting composite bipolar plate with both high electrical conductivity and sufficient flexural strength is designed.Electrical conductivity,flexural strength,corrosion resistance,vanadium permeability and single cell performance of the bipolar plate are measured and evaluated.Results show that the bipolar plate with extremely low carbon content of 18.7wt.%possesses low area specific resistance,high flexural strength,and high energy efficiency of 85.7%at 100m A cm^-2.（2）In order to simplify the production process and improve material utilization,a low-carbon-content thermoplastic composite bipolar plate is further promoted.The bipolar plate uses polyethylene as binder,graphene and expanded graphite powder as conductive agent,and carbon fibers as toughening agent.Morphology,in-plane and through-plane conductivity,flexural strength,vanadium permeability,corrosion resistance,and its performance in single cell are investigated.Results show that the bending strength of the thermoplastic composite bipolar plate can be dramatically enhanced by adding carbon fibers.Energy efficiency of the single cell with this type of bipolar plate reaches as high as 85.9%at 100 m A cm^-2.（3）To remove the resin-rich layer on the surface of composite bipolar plates,cactus-like carbon nanofibers are designed and applied in the surface modification of the bipolar plates.Morphology,electrical conductivity,electrochemical characteristics,and single cell performance of the bipolar plates before and after surface treatment are characterized and compared.The surface-treated bipolar plate exhibits improved electrical conductivity and larger electrochemical surface area.As a result,single cell with the surface-treated bipolar plates presents high energy efficiency of 86.28%at100 m A cm^-2.It is worth noting that the main performance of the above-mentioned bipolar plates meets the standards established by the Department of Energy of United States.They all have great potential to be applied in flow batteries or proton exchange membrane fuel cells.