## 2 Experimental  
### Catalyst Preparation  
Commercial Vulcan XC-72R (Cabot Corp., MA, USA) and Ketjen black (Lion Speciality Chemicals Co. Ltd., Tokyo, Japan) were used for the Pd support and were embedded into the CNFs. Vulcan embedded into the CNFs (VECNF) and Ketjen embedded into the CNFs (KECNF) were prepared using the following procedure. Polyacrylonitrile (PAN, Sigma-Aldrich Co., Ltd., MO, USA) was used as the carbon source for the nanofibers. Solutions containing PAN and CB were individually prepared by dissolving PAN and CB into dimethylformamide (DMF, Wako Pure Chemicals Ind., Ltd., Osaka, Japan) using a planetary centrifugal mixer (ARE-310, THINKY Corp., CA, USA) for 10 min for each solution. The CB/DMF solution was added to the PAN/DMF solution and mixed for 5 min in a planetary centrifugal mixer. The weight ratio of CB to PAN was varied between 0 and 0.5. The mixture of CB and PAN was transferred to a glass syringe equipped with a stainless steel needle for electrospinning at 0.05 mL min\({}^{-1}\) in an electric field through a syringe pump (AS One Co., Ltd., Osaka, Japan) to control the solution feed rate of the ink. Electrospinning was conducted by applying an 18 kV potential difference between the needle and the rotating collector using a high-voltage power supply (Element Co., Ltd., Tokyo, Japan). The collector was covered with aluminum foil and placed at a distance of 20 cm from the needle. After the polymer nanofibers were obtained from the electrospinning process, they were stabilized in a tube furnace at 250 \({}^{\circ}\)C in an air atmosphere for 10 h. After stabilization, the nanofibers were calcined at 1000 \({}^{\circ}\)C in a nitrogen atmosphere for one hour (carbonization process). Finally, steam activation was conducted at 850 \({}^{\circ}\)C for one hour in a humidified nitrogen flow with a 70 \({}^{\circ}\)C dew point. To check the effect of the heat treatment on the Vulcan (C\({}_{\rm v}\)) and Ketjen (C\({}_{\rm k}\)), several CB samples were also prepared with different heat treatments. After stabilization, one set of CB samples was carbonized at 1000 \({}^{\circ}\)C in flowing nitrogen, and the samples were denoted as C\({}_{\rm v}\)-carb or C\({}_{\rm k}\)-carb. Another set of CB samples was subjected to steam activation at 850 \({}^{\circ}\)C in flowing humidified nitrogen with a 70 \({}^{\circ}\)C dew point, and these samples were denoted as C\({}_{\rm v}\)-act or C\({}_{\rm k}\)-act. A final set of CB samples was heat-treated in a similar manner to the VECNF and KECNF samples and was denoted as C\({}_{\rm v}\)-all or C\({}_{\rm k}\)-all. As a reference, CBs without heat treatment were also used as catalyst supports and these samples were denoted as C\({}_{\rm v}\)-no and C\({}_{\rm k}\)-no.  
A 20 wt.% quantity of Pd was deposited on each of the supports by chemical reduction using NaBH\({}_{4}\) (Wako Pure Chemicals Ind., Ltd.) and PdCl\({}_{2}\) (Wako Pure Chemicals Ind., Ltd.) as the precursors. After the heat treatment process, the supports were ultrasonically dispersed in distilled water for 30 min. The precursor was added to this solution while stirring, then heated to 80 \({}^{\circ}\)C for 1 h. The pH of the solution was maintained at 8-9 by adding NaOH. A 0.2 M NaBH\({}_{4}\) solution was then added dropwise. The solution was stirred for a further 1 h, then cooled before drying and washing with distilled water. The catalyst was finally dried for 3 h at 120 \({}^{\circ}\)C. The catalysts were denoted as Pd/C\({}_{\rm v}\), Pd/C\({}_{\rm k}\), Pd/VECNF, and Pd/KECNF, respectively. For VECNF and KECNF, the CB/PAN ratio was also indicated after the sample name; for example, Pd/VECNF [0.5] denotes the 0.5 CB/PAN ratio and [0.0] denotes CNFs without CB (Pd/CNF).