**Abstract**

Graphite filled thermoplastic based composites are an adequate material for bipolar plates in redox flow battery applications. Unlike metals, composite plates can provide excellent resistance to the highly aggressive chemical environment at elevated temperatures in combination with an electrochemical potential in battery operation. The chapter therefore gives an overview of the most important requirements for the graphite-plastic composite material and thus also for the bipolar plates, as well as the different characterization methods of the bipolar plates. In the following, both the modern composite materials based on polypropylene (PP) and polyvinylidene fluoride (PVDF) and their general properties are described with a focus on improved long-term stability. Furthermore, recycling is also considered. One section is dedicated to seals, which - as so often - are an underestimated component of redox flow batteries. In this gasket part of the chapter, the most common materials and interactions between gaskets and other stack components are presented, as well as the material properties, characterization and processing methods of the gaskets.

**Keywords:** bipolar plate (BPP), gasket, graphite compound, composite, graphite plate, polypropylene (PP), polyvinylidene fluoride (PVDF), ethylene-propylene-dien-monomer (EPDM), fluoroelastomer (FKM)

## **1. Introduction**

Redox flow batteries (RFB) are electrochemical reactors suitable for storing electrical energy by chemical reactions [1]. Depending on the technology used, this reaction can take place at elevated temperatures and/or in aggressive media, with an electrochemical potential superimposed. In recent years, the technical requirements on materials and components of the reactor of the Redox flow battery have therefore become more and more demanding. The battery unit consists of many stacked cells which are connected in series to a Flow battery stack. Each cell in turn consists of various components such as the proton exchange membrane, seals, frames and

the conductive bipolar plate which provides the connection from cell to cell up to the end of the stack where the generated current is collected.

RFBs, in particular vanadium redox flow batteries (VRFBs), have now reached a considerable degree of technical maturity and the systems are available on the market through many suppliers. However, due to a high remaining cost structure partly due to a lack of economies of scale - the profitable market introduction of flow batteries still suffers from a high market acceptance.

On the one hand the membrane is considered the heart of a redox flow battery. On the other hand, the bipolar plate is one of the key components of an RFB. However, the Bipolar plate is important, since the plate has an impact on the complete systems, as far as total dimensions, total weight, thermal and electrical properties of the stack and thus of the system is determined by the bipolar plate technology [2].

As already mentioned, the chemical conditions for the materials used in redox flow batteries are challenging [3]. Most systems are operated between 40°C and 60°C in a liquid of dissolved vanadium salts in sulfuric acid. Besides the Vanadiumtechnology, there also some other technologies (metallobased or organic RFBs), which will not be further considered.

Due to these harsh conditions, superimposed by an electrochemical potential, graphite-based bipolar plates with polymeric binders are used in almost all applications in these battery stacks. The graphite composite plates are an unbeatable material in terms of stability under the above-mentioned corrosive conditions, and the cost-intensive coated metal plates have no chance.

They have been operated very adequately several times over the years. However, due to an intrinsic fragility caused by a high filling load with graphite, graphite composite plates require a greater thickness than metal plates, resulting in more weight and volume of the stack. From a cost point of view, the membrane is certainly considered the dominant part of the redox flow battery stack. However, the bipolar plates tend to be underestimated both in terms of their technical requirements and, in particular, their contribution to the cost structure.

Graphite composite based bipolar plates are manufactured using highly filled compounds [2]. They contain fillers like graphite and/or other electrically conductive carbons incorporated in polymers performing as a gluing binding matrix. The key challenge is the competing interaction between electrical conductivity - achieved by the carbon component - and mechanical stability as well as liquid tightness which is provided by the binding polymer.

The compounding process is the first step to produce highly filled, electrically and thermally conductive pellets for the subsequently following step of forming bipolar plates.

Both compounding and molding processes, which can be injection molding, compression molding or continuously extrusion, are very sensitive to process parameters and need to be carefully controlled. The objective is to manufacture bipolar plates in large volumes and high quality more or less like standard plastic parts. Only by using price cost attractive materials and the consequent focus on process automation by higher volume, the bipolar plate can contribute significantly to a better market acceptance of RFB.

Besides the bipolar plate, the gasket is a very important component of the battery stack and tends to be heavily underestimated. It plays a key role in the mechanical properties of the stack. Inappropriately selected gasket materials may cause cracks in the bipolar plates or may affect the membrane-structure negatively. Despite the fact that the gasket has to seal the stack, the cooperation with other stack components and their cumulative tolerance effects have to be on focus for the stack design and for the operation of its.

*Key Components in the Redox-Flow Battery: Bipolar Plates and Gaskets – Different Materials… DOI: http://dx.doi.org/10.5772/intechopen.94863*

The same which is evident for each component is also obviously for the gasket; they have to be cost attractive. Therefore, in some research projects, it is the objective is to suspend gaskets completely and use welding or bonding processes instead.

Technically, the bipolar plate of a RFB stack has to accomplish the following functions [3, 4]:


However, the functions of the gasket are completely different. The main functions of gaskets in a RFB stack are [5, 6]:

