A review of bipolar plate materials and flow field designs in the all-vanadium redox flow battery

A bipolar plate (BP) is an essential and multifunctional component of the all-vanadium redox flow battery (VRFB). BP facilitates several functions in the VRFB such as it connects each cell electrically, separates each cell chemically, provides support to the stack, and provides electrolyte distribution in the porous electrode through the flow field on it, which are quite similar to the functions of fuel cell BP. However, unlike fuel cells, a BP in the VRFB encounters a highly acidic environment (⁓2.5 M H2SO4) and varying voltage conditions, which seriously restrict the choice of materials selection. Metals become the most unsuitable candidate as BP in the VRFB due to the highly corrosive media. Furthermore, since the conventional VRFB uses a 3D porous electrode, the flow fields on BP play a very crucial role in electrolyte distribution on the electrode surface. In this review, the BP materials and the flow field of various designs are discussed in a holistic manner. As a main part of the review, various types of BP materials (metal-based, graphite-based, and carbon/polymer composite-based) and their processing methods are described in detail. The novel properties of BP materials are characterized by both physical and electrochemical processes. The challenges associated with BP materials such as corrosion, interfacial contact resistance, swelling, and electrolyte leakage are included. Further, the effect of different flow field designs on pressure drop, pumping losses, power-based efficiency, and overpotential of the VRFB at different flow rates and compression ratios are discussed and compared with the properties of the VRFB without flow field. To date, there is no review paper available on BP and flow field design in the VRFB system. Hence, this paper reviews and discusses the progress and challenges in BP and flow field designs for the VRFB system.