Pd-Co Alloy Nanoparticles Confined In Nitrogen-doped Hollow Carbon Spheres As A Highly Efficient Catalyst For Direct Ethanol Fuel Cell

With the aggravation of energy crisis and increasing prominent of environment issues, direct ethanol fuel cell(DEFC) have attracted more and more attention as clean and efficient energy conversion devices. However, the oxygen reduction reaction(ORR) and ethanol oxidation reaction(EOR) occuring on cathodes and anodes of DEFC, respectively, are both sluggish. State-of-the-art electrocatalysts consist of Pt can be of great help, however, the high cost, scarcity, electrochemical instability and facile poisoning of Pt directly limited the commercial success of DEFC. Great efforts have been focused on efficient electrocatalysts with little or no Pt for both ORR and EOR. Herein, we present a facile yet effective method for the synthesis of PdCo alloy confined in nitrogen doped hollow carbon spheres(PdCo@NHCSs) using SiO2 nanospheres as the sacrificed templates. Biomimetic molecule dopamine can self-assemble to form conformal polydopamine(PDA) coatings on the surface of SiO2 nanospheres while metal salts can be easily absorbed onto PDA nanostructures by the facile method of impregnation. After optimizing the preparation process, such as atomic ratios of Pd to Co, temperature of carbonization and so on, the TEM images of final PdCo@NHCSs clearly show that the PdCo nanoparticles are well alloyed and dispersed.Next, we investigate the electrocatalytic properties of PdCo@NHCSs for ORR and EOR, Pd@NHCSs, Co@NHCSs and NHCSs were synthesized for comparative purposes. We find that the electrocatalytic activity and kinetics of PdCo@NHCSs for ORR are comparable to that of commercial Pt/C, and noble metal contents in PdCo@NHCSs(2.4 wt%) is much lower than that in commercial Pt/C(10 wt%). Moreover, the long-term durability and tolerance to methanol crossover effect are even better than commercial Pt/C. Besides, PdCo@NHCSs also exhibits significantly improved electrocatalytic activity and durability for EOR compared with Pd@NHCSs and even commercial Pd/C catalysts, and PdCo@NHCSs has the highest oxidation peak current of 1308mA/mg, almost 14 times higher than that of commercial Pd/C. We conclude that the as-prepared PdCo@NHCSs exhibits enhanced electrocatalytic activity mainly due to following factors, firstly, the synergistic effect between reactive metal Pd and Co could efficiently enhance the catalytic activity, and Co could effectively change the lattice parameters and electronic state of Pd in favor of breaking the O-O bonds. Also, the XPS analysis reveals that the addition of Co can be of great help to pyridinic N content increasing as a result of the bonding interaction between Co and N, probably. As is known, carbon atoms with lewis basicity next to pyridinic N can be the active sites for ORR, thus directly improving the activity of catalysts. The hollow spheres are favorable to the transfer of mass and electrons. Most importantly, because of the special fabrication strategy, PdCo alloy can be well confined in the carbon shell, enabling its better stability.In conclusion, PdCo@NHCSs synthesized herein is a promising catalyst for both ORR and EOR of DEFC. Finally, the synthesis methodology was also broadly applicable to many other products. PdFe@NHCSs, Fe@NHCSs, PdNi@NHCSs and Ni@NHCSs were successfully prepared. Accordingly, we believe that the synthetic methodology described here is a general method for the production of various metal or alloy confined in nitrogen doped carbon.