| Direct borohydride-hydrogen peroxide fuel cell（DBHFC）, which is consisted of alkaline aqueous solution of Na BH4 as fuel and acid H2O2 as oxidant has achieved extensively attention for many advantages, such as its high theoretical open cell voltage（3.01 V） and high energy density(9.3 Wh g-1), easily to storage and transport, and no catalytic poisoning. In theory, the oxidation of BH4— at anode can tranfer eight electrons. However, the utilization of BH4—decreases and some security risks are produced in fact, due to the borohydride hydrolysis with hydrogen generation. Therefore, the key factor for the study of DBHFC is to develop anode electrocatalysts with high selectivity and high catalytic acitivity.The catalysis of noble metal Au is relative low to the hydrolysis of BH4—, and the transfer electron is close to 8. However, Au could not be the anode catalyst alone, due to the low reaction kinetics. In this paper, bimetal catalysts consisting of Au are prepared by the inverse microemulsion method with two-step reduction method. The prepared catalysts are investigated by physical and electrochemical characterization.First, the Co@Au/C core-shell nanoparticles with different atomic ratios are prepared by the successive reduction. The morphology and structure are investigated by transmission electron microscopy（TEM） and X-ray diffraction（XRD）. Electrochemical characterizations are performed by cyclic voltammetry（CV）, linear scan voltammetry with rotating disc electrode（LSV RDE）, electrochemical impedance spectroscopy（EIS）, chronopotentiometry（CP）, chronoamperometry（CA）, and fuel cell test. The results of physical characterization show that the Co@Au/C nanoparticles are core-shell catalysts, and the average size is approximately 13.4 nm. The results of electrochemical tests indicate that the reaction mechanism of Co@Au/C and Au/C are similar. Among all the catalysts, the performance of Co4@Au1/C catalyst is better than the others, such as, the Tafel slope on Co4@Au1/C electrode is 0.25, and the charge transfer resistance is 2.310 Ω cm2. Furthermore, DBHFC fabricated using Co4@Au1/C as anode catalyst attains a maximum power density of 102.4 m W cm-2, which is 2.14 times higher than Au/C in the same conditions.Second, the Cu@Au/C core-shell and Cu-Au/C alloying nanoparticles are prepared by the inverse microemulsion method with two-step and one-step reduction, respectively. The average size of Cu@Au/C and Cu-Au/C is 15 to 17 nm. Electrochemical results are shown as follows: Cu3@Au1/C and Cu4-Au1/C catalysts have the best performance in the corresponding bimetallic catalysts, and the electrocatalytic oxidation for BH4— is better on Cu4-Au1/C electrode. In addition, DBHFC fabricated using Cu3@Au1/C and Cu4-Au1/C as anode catalysts attains maximum power density of 71.4 m W cm-2 and 77.1 m W cm-2, respectively. It can be obviously seen that the electrocatalytic activity of Cu4-Au1/C is better than Cu3@Au1/C.In the paper, the electrocatalytic performance of bimetallic core-shell and alloying catalysts is better than that on monometallic catalyst, which is due to the electronic effect. With regard to the core-shell structure, the core metal may have a suitable induced effect to d-orbit electrons of the shell metal when have an appropriate shell thickness, thus, makes BH4— has a more suitable adsorption capacity on metal atoms of shell, then makes the process of BH4— adsorption and desorption occurs more easily. For Cu-Au with alloying structure, the doping of transition metal Cu changes the electronic state of Au, and Au atom could get electron from Cu atom because Au has high electronegativity. Thus, the synergistic effect is produced between Cu and Au, which increases the catalytic activity. The experimental results show that the performance of alloying catalysts is better than core-shell catalysts with the same metals, indicating that the synergistic effect of alloy is superior to increasing d-orbit vacancy of core-shell catalysts. |
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