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Nanomaterials 2021, 11, 1017 3 of 12 be improved by employing metal oxides, which enhanced the stability and prevented degradation of the composites. In this study, Pd nanoparticles loaded onto Co3O4 supported by multi-walled carbon nanotubes (Pd/Co3O4@MWCNT) were synthesized as a promising cathode catalyst for enhancing the electrocatalytic activity and ORR of cathodes in DUFCs. Co3O4@MWCNT and Pd/Co3O4@MWCNT were synthesized using a simple two-step hydrothermal process. The uniform dispersion of Pd nanoparticles on the Co3O4 surface was confirmed using FESEM and TEM. A Pd/MWCNT catalyst was also prepared and evaluated to investi- gate the effects of Co3O4 on the electrocatalytic performance of the Pd/Co3O4@MWCNT catalyst. Cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE) techniques are used to examine the ORR activity and electrochemical properties of the catalyst. This study reports the effect of Co3O4 on the composite of Pd-coated MWCNTs. 2. Materials and Methods 2.1. Materials Cobalt(II) acetate tetrahydrate, palladium(II) chloride (PdCl2, 99.9%), ethylene glycol (anhydrous, 99.8%), and MWCNTs (with diameters of 50–90 nm, ≥95% carbon basis) were obtained from Sigma Aldrich (St. Louis, MO, USA). Commercial Pt/C (20 wt.%; Sigma Aldrich) was used for comparative purposes. All chemicals were used as received, without further purification. 2.2. Preparation of Co3O4/MWCNT Nanocomposite First, the raw MWCNTs were pretreated with 6.0 M nitric acid and then sonicated for 2 h at room temperature, followed by magnetic stirring at 80 ◦C for the duration of acid treatment of the MWCNTs. Finally, the mixture was washed with DI water a few times to remove the residues, followed by centrifugation at 8000 rpm for 5 min. Subsequently, the mixture was suspended in 50 mL of ethanol under continuous magnetic stirring for 8 h at room temperature. Co(CH3COO)2 was used at different weight ratios. Note that the calculated weight ratio of the MWCNTs vs. Co(CH3COO)2 was 1:3 and 1:5, respectively. Co(CH3COO)2 (0.5 g) was added to the MWCNT suspension. Thereafter, NH3 (25%) aqueous solution was slowly added dropwise under continuous magnetic stirring at 45 ◦C. After 1 h, the mixture was placed into a Teflon-lined vessel, sealed, and maintained at 150 ◦C for 4 h. Finally, the sample was dried at 70 ◦C overnight and collected for further synthesis [21–23]. 2.3. Preparation of Pd/Co3O4@MWCNT Catalyst A modification of the experimental procedure described by Carrión-Satorre et al. was implemented herein [4]. First, chloropalladic acid was obtained by the reaction between palladium(II) chloride (0.2 M) and hydrochloric acid (0.06 M) under continuous magnetic stirring, followed by the addition of 30 mL of surfactant (ethylene glycol). After magnetic stirring for 30 min, the mixture was adjusted to pH 8 by dropwise addition of 0.5 M KOH. Subsequently, the mixture was added to 40 mg of the Co3O4@MWCNT suspension and sonicated for 2 h. Thereafter, the mixture was magnetically stirred continuously for another 2 h at 80 ◦C. The mixture was then placed into a Teflon-lined vessel, sealed, and maintained at 150 ◦C for 4 h. The mixture was centrifuged and washed with DI water and ethanol. Finally, the obtained composite was dried overnight in a vacuum oven at 70 ◦C. Pd@MWCNT composite was also synthesized for comparison with the same preparation of Pd/Co3O4@MWCNT catalyst, except for using the equal amount of MWCNTs instead of the Co3O4@MWCNT. 2.4. Characterization of the Materials The morphologies of the prepared catalysts were examined using transmission elec- tron microscopy (TEM, G2 F30, Tecnai, OR, USA) and scanning electron microscopy (SEM,PDF Image | Effect of Co3O4 Nanoparticles on Improving Catalytic Behavior
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