Polybenzoxazine based composite bipolar plate: selective distribution of carbon black to construct a conductive network, with a total filler content of 60 wt% meeting the standard

Research background and material design: Polymer based ion-exchange membrane fuel cells (PEMFC) composite bipolar plates typically require the addition of more than 80 wt% of conductive fillers to meet conductivity requirements, which puts pressure on the lightweighting of the fuel cell stack. A study published in the Journal of Applied Polymer Science proposes that optimizing conductive pathways with multi-scale conductive fillers is an effective strategy to improve filler efficiency. A study was conducted on the preparation of polybenzoxazine (PBA) – based composite bipolar plates using graphite sheets (GF) as the main conductive filler and carbon black (CB) as the secondary filler through a dry dispersion process. The effects of graphite sheet size and content, GF/CB ratio on conductivity and various service properties were systematically investigated.

Key data and performance

The results indicate that smaller sized graphite sheets contribute to better electrical performance.

. Due to the selective distribution of carbon black within the PBA matrix phase, the conductivity of the PBA/GF/CB composite bipolar plate was significantly improved. Under the same total filler content, the binary PBA/GF system containing only graphite sheets failed to meet the DOE target of 100 S/cm, while the addition of 4 wt% carbon black resulted in a conductivity of 116.36 S/cm, meeting DOE requirements. Among them, the composite bipolar plate containing 52 wt% graphite flakes and 8 wt% carbon black showed the best performance, with an in-plane conductivity of 153.12 S/cm and a maximum power density of 673.13 mW/cm ². In addition, PBA/GF/CB composite bipolar plates exhibit satisfactory bending strength, thermal conductivity, corrosion resistance, and hydrophobicity.

Implications for the polymer additive industry

The core idea of this study is to use the synergistic distribution of fillers at different scales to reduce the overall amount of conductive fillers used.

. The selective positioning of carbon black in the matrix phase essentially plays a role similar to “bridging the conductive network”, which is similar to the idea of constructing a filler network through reinforcing fillers and carbon black in rubber formulations. In the field of tires and rubber, the combination of white carbon black and carbon black has become a common means of reducing rolling resistance, balancing wear resistance and grip performance. The dispersion and distribution of fillers in the matrix also directly determine the final performance. From the perspective of formula design, this study suggests that the efficiency of fillers does not only depend on the amount added, but also on the spatial distribution and interconnection of fillers in the polymer phase. For rubber mixing and vulcanization systems, the dispersion uniformity of fillers, the interface interaction between fillers and rubber matrix, and the influence of processing technology on the formation of filler networks are all key variables that determine the conductivity, thermal conductivity, and mechanical properties of the product. Although this study focuses on fuel cell bipolar plates, its methodology for multi-scale filler composition and selective distribution can provide reference for the optimization of rubber reinforced filler systems. It should be pointed out that this study used a dry dispersion process to prepare polybenzoxazine based composite materials, and the processing path is significantly different from rubber mixing. The relevant conclusions cannot be directly applied to tire formulations. In addition, the study did not involve the effects of rubber additives such as anti-aging agents and antioxidants on the long-term aging performance of the system, and the applicability of the selective distribution mechanism of carbon black in the rubber matrix also needs further verification.

Information source:Conductive Network Enhancement in Polybenzoxazine‐Based Composite Bipolar Plates via Selective Localization of Carbon Black， Organized by Herun Chemical.