## 3 Results and Discussion
### Formic Acid Oxidation Reaction on Carbon Black Embedded in Carbon Nanofibers  
Figure 56 shows the effect of the CB content in the CNF on the FAOR activity during an anodic scan. Pd/CNF exhibited a relatively low FAOR activity due to the excessive development of micropores, which is similar to that reported by other researchers (around 105-410 mA mg\({}_{\text{Pd}}\)\({}^{-1}\)) [13; 30; 31; 32; 33]. On the other hand, the FAOR activity increased after embedding the CB into the CNFs, and there was a significant difference in the FAOR activity depending on the CB/PAN ratio. The CB/PAN ratio of 0.25 for both Pd/VENF and Pd/KECNF showed the highest FAOR activities of 1477 mA mg\({}_{\text{Pd}}\)\({}^{-1}\) and 2022 mA mg\({}_{\text{Pd}}\)\({}^{-1}\), respectively. These values are higher than those obtained for the unembedded CBs that were carbonized and steam activated: Pd/C\({}_{\text{v}}\)-all and Pd/C\({}_{\text{k}}\)-all. Essentially, the FAOR activity significantly increased in the case of Pd/KECNF. The maximum catalytic activity obtained in Figure S6 and ECSA for Pd/VECNF and Pd/KECNF are summarized in Table 4. ECSA for Pd/VECNF at the various CB/PAN ratios only have a small deviation despite the mass activity. Hence, the mass activity is not only influenced by the ECSA but may be caused by the metal-support interaction. The metal-support interaction and chemical state influence were investigated by XPS measurements in the next section. Even though the mass activity for the Pd/VECNF and Pd/C\({}_{\text{v}}\) only have a small difference, the ECSA value of Pd/VECNF being higher than that of Pd/C\({}_{\text{v}}\) may due to the change in the surface area of the embedded CNF. ECSA for Pd/KECNF was enhanced with the mass activity of the catalyst, reaching the highest ECSA value at 56.9 m\({}^{2}\) g\({}_{\text{Pd}}\)\({}^{-1}\) for Pd/KECNF[0.25] which is comparable to  
Figure 3: Effect of CB/PAN ratio of VECNF and KECNF on pore distribution for (**a**) Vulcan and (**b**) Ketjen. the Pd/Graphene, which obtained 56.0 m\({}^{2}\)\(g_{\text{Pd}}\)\({}^{-1}\) as reported by [PERSON] et al. [27]. Generally, Pd/KECNF shows a higher ECSA value compared to Pd/C\({}_{\text{k}}\) whether heat-treated nor untreated. Note that this was not due to the difference in the Pd particle size, because the Pd crystallite size was much larger in Pd/VECNF and Pd/KECNF than in Pd/CB, with a range 13.4-17.0 nm, as shown in Table 1. The larger Pd particles naturally reduce the electrochemical surface area, which causes a lower FAOR activity. The larger particle size of Pd may affect the FAOR activity; however, this is not a decisive factor for a high electro-catalytic activity especially in this study. The effect of particle size on FAOR in this study is less important because the particle size is larger than 10 nm for Pd/KECNF and Pd/VECNF [29]. Moreover, there was a higher ECSA value in Pd/KECNF and Pd/VECNF compared to Pd/CB despite the smaller crystallite size of Pd/CB, which was caused by the Pd particles in Pd/CB being hidden in the macropore of Pd/CB, which was attributed to low Pd utilization. Pd/KECNF and Pd/VECNF have a higher amount of smaller pores, especially micropore and mesopore along the fiber structure, which provide higher utilization of Pd, hence the enhanced ECSA value. The high FAOR activities of Pd/VECNF and Pd/KECNF must be due to the positive effect of the heat-treated CB, as a result, for example, of the appearance of hydrogen spillover as already mentioned; this does not involve the splitting of the agglomerates or aggregates of the CB due to the protection by the CNF. Since the pore structure of Ketjen was more influenced by heat treatment than that of Vulcan due to its hollow structure (as previously mentioned), a significant increase in the FAOR activity was observed in the case of Pd/KECNF due to the positive effect of the heat treatment of Ketjen [0.25]. Regarding the CB/PAN ratio, there was an optimum weight ratio for both types of CB.