## 1 Introduction  
Researchers are still making efforts to achieve better electrical-mechanical performance of conductive polymer composite materials (CPCs) for various engineering applications [1; 2; 3]. Other properties of these composites that are also of interest are their thermal conductivity, thermal stability, corrosion resistance, and the lightweight nature of the composites [4; 5; 6]. Improvement of the electrical properties of CPCs can be achieved by using conductive fillers such as graphite (G), carbon black (CB), carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and carbon fibers (CF) [7; 8; 9; 10; 11]. Different factors play crucial roles in determining the final properties of CPCs: the filler content, size, shape, and distribution in the matrix have been shown to be the most important [12; 13; 14; 11]. If a conductive filler with a specified particle shape and size is used, then optimizing the processing technique for the best distribution of the filler particles in the matrix is crucial. Successfully achieving uniform distribution of the filler in the matrix results in better mechanical and electrical properties and lower filler loading, which in turn reduces the weight and cost of the product. Researchers have typically used high conductive filler loadings of up to 90 wt.% to achieve high electrical conductivity in engineering applications, such as for the bipolar plate in polymer electrolyte membrane fuel cells (PEMFCs), for which the US Department of Energy (DOE) set a target electrical conductivity of greater than 100 S/cm [15]. Many studies have been conducted in response to achieve the DOE target [16; 17]. However, a degradation of the mechanical properties was obvious in the composite material with a high filler content, and its fabrication process was also more difficult [18]. However, the diversity in material types and their processing techniques and the operator's skills still open the door to running increasingly fundamental studies to understand the overlap between the structure-property-processing relationship and the performance of CPCs. To date, improved electrical-mechanical properties at lower filler content have only been achieved by using multifiller conductive composites with micro- and nano-sized particles and fibers. Among the various conductive fillers used in CPCs, graphite is still of interest because it has high electrical conductivity and various shapes and sizes, and it is easily obtained using conventional processes [19; 20]. The synergistic effect of compositing small particle size and large particles in the polymer matrix has been extensively investigated by many researchers [21; 22]. The use of large particles as a main filling component in the matrix or vice versa was also studied. Researchers found that combining a larger primary conductive filler with a smaller secondary conductive filler could increase the electrical conductivity value and improve the mechanical properties of the conductive composite. [23; 24; 25]. [PERSON] et al. [26] studied the influence of the incorporation of different sizes of conductive filler materials on the electrical conductivity of composite materials for the applications of bipolar plates in fuel cells. They found that the electrical conductivity of the produced composite material was also influenced by the incorporation of multiple conductive filler materials, and thus, the appropriate particle size of the conductive filler increased the density of the composite material and reduced the electrical resistivity. [PERSON] et al. [16] used different graphite particle sizes in their study and found that the electrical conductivity increased with increasing graphite particle size. The electrical properties of composites containing expanded graphite (EG) of different sizes (30, 50, 150, and 300 \(\upmu\)m) and phenolic resin as a matrix were investigated by [PERSON] et al.