## 3 Results and Discussion
### In-Plane Electrical Conductivity Results
A clear improvement in the electrical conductivity was observed by the addition of different G sizes. An increase of approximately 35% in the electrical conductivity compared to that of the master batch epoxy/80 wt.% G150 sample was achieved by the combination of 70 wt.% of the largest size G150 with 10 wt.% of a smaller size G75. The combination of G150 with a smaller 10 wt.% G44 size resulted in an approximately 16% improvement in the electrical conductivity. [PERSON] et al. [26] studied the effect of combining different sizes of conductive fillers on the electrical conductivity values for bipolar plate composite materials. They foundthat the electrical conductivity increased by 37% when combining 10 wt.% of the small particles (<45 \(\upmu\)m) with the larger particles (>90 \(\upmu\)m) at an overall G loading of 60 wt.% and attributed that to the reduction in the pore volume due to the accumulation of the large G size and the increase in the particle-to-particle contacts. The conductivity decreased when the proportion of small G size increased. [PERSON] et al. [52] also reported an increase in the electrical conductivity of epoxy/G composites when incorporating up to 15 wt.% synthetic graphite (SG) with a particle size <1 \(\upmu\)m into the natural graphite (NG) of particle size (75-150 \(\upmu\)m) and attributed that to the ability of the small particles to fill the voids between NG flakes. The SEM images of the surfaces and fracture surfaces of epoxy/G150/G44 and epoxy/G150/G75 composites in Figure 4 showed a similar microstructure as the epoxy/G150 composites. The smaller G particle sizes (G44) might have a higher tendency to agglomerate and unevenly distribute and did not sufficiently fill the micro-voids that existed in the composite, which in turn did not contribute to increasing the electrical conductivity compared to that of the larger G particles (G75). [PERSON] et al. [16] found that the increase of the particle size in epoxy/G composites decreased the number of particles and the contact resistance, which consequently improved the conductivity. [PERSON] et al. [26] investigated the particle size gradation of different conductive filler particle sizes in the polymer composite and reported that the optimal small particle size (d), which can enter into the accumulation of large particle sizes (D), is d = 0.154D in the closest accumulation mode and d = 0.414D in the loosest accumulation mode. Based on these ratios, the optimal small particle size when using large G particle sizes of G150 is d = -23 \(\upmu\)m for the closest accumulation and d = -62 \(\upmu\)m for the loosest accumulation, which deviated from the smaller particle sizes used in this study (i.e., G44 and G75). The curing conditions were found to have a crucial role in determining the in-plane electrical conductivity, as shown in Figure 5. At a specified curing time of 60 min, the conductivity of the epoxy/G150/G75 composite was increased sharply from \(\sim\)32 to 43 S/cm by increasing the curing temperature from 110 to 150 \({}^{\circ}\)C (Figure 5a). [PERSON] et al. [16] studied the effect of curing temperature on the electrical conductivity of novolac epoxy/G composite and reported an increase of about 10% when raising the temperature. [PERSON] et al. [13] in a previous work found the same trend when producing epoxy/40 wt.% G/5 wt.% CNTs composites and achieved an increase of about 40% (from 14.5 to 21 S/cm) in the electrical conductivity by increasing the curing temperature from 80 to 120 \({}^{\circ}\)C. They attributed the increase in the electrical conductivity to the matrix viscosity, which decreased at higher  
Figure 2: In-plane electrical conductivity values of the epoxy/G composites with different G particle sizes and contents: (**a**) epoxy/G150 and (**b**) epoxy/G150/G75 and epoxy/G150/G44 prepared at 110 \({}^{\circ}\)C for 60 min. curing temperatures, allowing the G filler to be more evenly dispersed, and thus a conductive network can be formed more easily in the matrix.