## 3 Results and Discussion
### Effect of Heat Treatment of Carbon Blacks on Formic Acid Oxidation Reaction Activity  
Figure S3 shows the effect of the heat treatment of CB on the FAOR activity of the catalysts. The figure shows only the anodic scan of the CV for Pd/C, which corresponds to the electro-oxidation of formic acid. The peak current density occurs between 0.44 and 0.84 V vs. normal hydrogen electrode (NHE). In the case of Vulcan, the heat-treated CB, especially Pd/C\({}_{\rm v}\)-carb and Pd/C\({}_{\rm v}\)-act, showed significantly higher FAOR activities than Pd/C\({}_{\rm v}\)-no, as shown in Figure S3a. It has been reported that heat treatment increases the BET surface area and the electrical conductivity of CB [9; 24]. The presence of defects on the catalyst support surface is understood to enhance the hydrogen spillover. Moreover, it has also been reported that the heat-treated CB improved the hydrogen spillover due to the increasing of the surface area and pore volume of the support [21; 25]. The hydrogen spillover occurs when hydrogen atoms tend to diffuse and migrate from the external surface into the inter-planar spacing through defect sites on the surface of the heat-treated catalyst support. The acceleration of the dehydrogenation during the FAOR on a Pd surface is affected by the hydrogen spillover [25]. The higher FAOR activity of the heat-treated Pd/C\({}_{\rm v}\) observed here is attributed to these findings. On the other hand, in the case of Pd/C\({}_{\rm v}\)-all, the FAOR activity was lower than those of Pd/C\({}_{\rm v}\)-carb and Pd/C\({}_{\rm v}\)-act, even though the BET surface area was higher. This was due to the excessive heat treatment. Micropores were significantly developed by the heat treatment. [PERSON] et al. reported that Pt particles  
were observed in the interior of the CB in which the microstructure developed, resulting in a decreased effective surface area of the Pt [26]. As a result of the excessive heat treatment, the micropore structure was excessively developed, resulting in a reduction in the effective surface area of Pd in the case of Pd/C\({}_{\mathrm{v}}\)-all. Moreover, there is the possibility that the splitting of the aggregates or agglomerates affects the FAOR activity since the electrical conductivity would be changed due to less contact between the primary carbon particles. These results suggest that there is an appropriate intensity of heat treatment to maximize the FAOR activity for the CB supports. The effect of the heat treatment of Pd/C\({}_{\mathrm{v}}\) on the ECSA of each catalyst is explained in the latter part of the paper. In the case of Ketjen, the heat treatment did not improve the FAOR activity, despite an increase in the BET surface area. This is due to the pore structure of Ketjen, which has numerous pores and an initially high BET surface area compared to the Vulcan even without heat treatment. Similar to the case of Pd/C\({}_{\mathrm{v}}\)-all, the excessive development of a structure with small pores during heat treatment caused a decrease in the FAOR activity. Therefore, it was found that the heat treatment of the porous carbon, such as Ketjen, was not effective at enhancing the FAOR activity even though the BET surface area increased. Since both heat treatments for carbonization and steam activation result in a degraded Pd/C\({}_{\mathrm{k}}\) and Pd/C\({}_{\mathrm{v}}\) catalytic activity of the FAOR, it can be concluded that excessive heat treatment of the catalyst-supported CB caused deterioration of the CB structure. Hence, both heat treatments are only appropriate for the CB-embedded CNF. Table 2 summarizes the maximum mass activity of the FAOR and ECSA for Pd/C with various heat treatments. The calculated ECSA value for Pd/C\({}_{\mathrm{k}}\) was generally lower than that of Pd/C\({}_{\mathrm{v}}\) except for Pd/C\({}_{\mathrm{k}}\)-no.