Bipolar plates for redox flow batteries

What is a bipolar plate?

Bipolar plates (BPP) are one of the key components of redox flow batteries (RFB). They serve multiple functions: Not only do they connect adjacent cells electrically and separate them physically, but they also provide structural support to the stack and, in special designs, even prevent leakage.

As the name implies, a “bipolar” plate provides a series electrical connection between adjacent cells. It conducts current in both directions between the cells and can transmit positive and negative charges.

Important characteristics of BPPs include high electrical conductivity, electrochemical stability in the applied media – even if these are highly aggressive or corrosive –, mechanical stability, and electrolyte impermeability through the BPP structure. Hence, careful selection of a suitable BPP material is important for optimum battery performance. BPPs available on the market are either metal- or carbon-based.

BPPs are not only suitable for RFBs, but also for many other applications such as fuel cells and electrolysers. Highly conductive and resistant BPPs are also required for various other electrochemical processes, such as water treatment processes (coagulation, desalination, disinfection, precipitation, purification, demineralization, and water softening) as well as some electrochemical metal separation and extraction technologies. Due to the harsh environment and applied potentials in RFBs, BPPs are usually composed of carbon or graphite/polymer composite materials, as these are superior to metals in terms of corrosion resistance.

Carbon or graphite-based composite BPPs consist of a polymer matrix (thermoplastic or thermoset) and a carbon or graphite compound. The latter makes up the largest share and ensures the required electrical conductivity of the plates. The main carbon/graphite components used in commercial products are synthetic graphite powder, natural graphite, carbon black, and expanded graphite.

SIGRACELL BPPs are made from expanded graphite, a product SGL Carbon has been producing for more than 50 years. Since then, SGL Carbon has been one of the market leaders in the field of expanded graphite. Our products are well known for best quality, high reliability, and long service life.

Above all, expanded natural graphite is characterized by its resistance against media and in corrosive conditions as well as its good electrical conductivity. These are exactly the main properties demanded of a RFB BPP. In addition to the stack efficiency, the BPP influences the batteries weight and dimensions, too.

Area specific resistivity: 70 mΩ x cm²
Permeability: < 1 x 10^-3 mg/(s x m²)
Impurities: < 150 ppm
Sealability (no extra gasket/seal needed)

Expanded graphite – also known as flexible graphite -, the graphite material SGL Carbon uses to manufacture its SIGRACELL bipolar plates, is produced from highly purified natural graphite flakes only, i. e. it is binder and additive free. In the production process, the unique properties of natural graphite are contained e.g. its excellent conductivity, and even more are added: The material becomes flexible, soft, and easy-to-process.

These great properties still remain during BPP production when expanded graphite and a high-quality polymer are combined to form a graphite/polymer composite with distinct properties.

SIGRACELL BPPs are lightweight – especially when compared to metal BPPs –, conductive, and withstand corrosive conditions. The polymer itself provides the demanded thermal stability, and mechanical strength plus is chemically resistant, too. Both components complement each other perfectly, forming a high performance BPP.

But that’s not all, SIGRACELL BPPs due to their non-resin impregnated, soft surface distinguish by high adaptability, surface tolerance, compressibility, sealability, and a favorable processability. All those properties being inherent to flexible graphite, too.

In 2001, SGL Carbon created the first expanded graphite bipolar plates at its competence center in Valencia, California, USA. Since then, our SIGRACELL BPPs have been widely known for their excellent performance, quality, and reliability.

The very low area specific resistance of SIGRACELL expanded graphite BPPs enables a significant increase in stack current density. Therefore, less bipolar plates are needed to achieve the desired performance output of the RFB.

Typical area specific electrical resistance (perpendicular to their surface) vs. surface compression of two SIGRACELL BPP grades

Especially due to their great resistance to corrosion, thin, high-density SIGRACELL BPPs are ideally suited for the demanding cell chemistry in various RFB systems as well as for other electrochemical processes.

Typical properties	Units	TF6	PV15
Polymer type		Fluoropolymer	Fluoropolymer
Polymer content	%	6	15
Density	g/cm³ (lb/ft³)	1.75 (110)	1.75 (110)
Tensile strength (ASTM F152D)	MPa (psi)	20 (2900)	25 (3600)
Thickness	mm (in)	0.6 (0.024)	0.6 (0.024)