Correlation Of Surface Ag Contents Of Core-shell Structured Au@AgPd Nanoparticles With Their Electrocatalytic Performances

Compared with Pt-based catalysts, Pd-based catalysts exhibit better electrocatalytic performance for alcohol oxidation in alkaline media. Thus, the synthesis of Pd-based catalysts is also one of the most attractive research themes in development of highly efficient fuel cells. It has been demonstrated that the catalytic performance of Pd-based catalysts is strongly related to their composition, architecture, particle size, and morphology. For instance, the presence of Au in Pd-based catalysts can efficiently remove intermediates by oxidation and thus significantly improve the catalyst resistance to poisoning for better durability. Bimetallic particles with a unique core-shell (CS) structure can minimize the utilization of precious metal precursors, since the majority of Pd atoms are distributed at the electrochemical reaction interface.At present, the durability of Pd-based CS NPs still needs to be improved, as the most parts of the Pd shells are still directly exposed to CO and then poisoned. The introduction of Ag in Pd shells can improve the anti-CO poison ability in addition to the catalytic activity enhancement. However, the total Ag contents in the Pd-based catalysts with best catalytic performance are different in the literatures. Moreover, in our recent work, it is also found that although the whole Ag-to-Pd molar ratios in the AgPd shells of two types of NPs are nearly same, their electrocatalytic performances on ethanol oxidation are different. Thus, the Ag content on the NP surfaces rather than the total Ag content in the NP bulk may be the determinant for improvement of Pd-based catalysts. On the other hand, Pd-based CS NPs with thinner shells would have improved the utilization efficiency of Pd.Based on the current issues mentioned above, herein, we firstly synthesized well-dispersed, ultrasmall CSm Au@AgPd-s-Agn Pd1-n NPs with different surface Ag contents via the galvanic reaction of Ag shells of CSm Au@Ag NPs with Pd2+ions in water at room temperature. Next, we studied the correlation of electrocatalytic performance on ethanol oxidation, methanol oxidation and oxygen reduction of these CSm Au@AgPd-s-Agn Pd1-n NPs with surface Ag contents in their AgPd shells. Then, we further studied the correlation of electrocatalytic performance on ethanol oxidation, methanol oxidation, and oxygen reduction of these CSm Au@AgPd-s-Agn Pd1-n NPs with both surface Ag contents in the AgPd shells and KOH concentration. Lastly, empirical rules for the design and preparation of CS Au@AgPd-s-Agn Pd1-n NPs and other composite metal particles for better electrocatalytic performance are summarized.In Chapter 2, we synthesized high yield, well-dispersed, ultrasmall CSm Au@AgPd-s-Agn Pd1-n NPs with different surface Ag contents via the galvanic reaction of Ag shells of CSm Au@Ag NPs with Pd2+ ions in water at room temperature. We studied the correlation of the electrocatalytic performance on ethanol oxidation, methanol oxidation, oxygen reduction of these CSmAu@AgPd-s-AgnPd1-n NPs with surface Ag contents in the AgPd shells.(i) Electrocatalytic activity on ethanol oxidation. The surface Ag content in AgPd shells of CS0.60 Au@AgPd-s-Ago.294 Pd0.706 NPs is about 29.4%. This core-shell structural feature and alloyed composition render the resulting CS0.60 Au@AgPd-s-Ago.294 Pdo.706 NPs with significantly increased electrochemically active surface area (up to 77.5 m2 g-1) and long-term stability on ethanol oxidation in alkaline media. In comparison with commercial Pd/C catalyst (0.20 A mgpd-1), the CS0.60 Au@AgPd-s-Ago.294 Pdo.706 NPs have superior catalytic activity (1.16 A mgpd-1) towards ethanol oxidation (0.50 M) in alkaline media (0.30 M KOH).(ii) Electrocatalytic activity on methanol oxidation. The surface Ag content in AgPd shells of CS0.90 Au@AgPd-s-Ag0.588 Pdo.412 NPs is about 58.8%. CS0.90 Au@AgPd-s-Ag0.588 Pd0.412 NPs show optimal electrocatalytic activity on methanol oxidation and increased electrochemically active surface area (up to 101.1 m2 g-1). In comparison with commercial Pd/C catalyst (0.23 A mgp-1), the CS0.90 Au@AgPd-s-Ag0.588 Pd0.412 NPs have superior catalytic activity (1.05 A mgp-1) towards methanol oxidation (1.0 M) in alkaline media (1.0 M KOH).(iii) Electrocatalytic activity on oxygen reduction. The surface Ag content in AgPd shells of CS0.30 Au@AgPd-s-Ag0.388 Pd0.612NPs is about 38.8%. In comparison with commercial Pd/C catalyst (1.20 A mgpd-1), the CS0.30Au@AgPd-s-Ag0.388 Pd0.612NPs have superior catalytic activity (2.64 A mgpa-1) towards ethanol oxidation (0.50 M) in alkaline media (0.30 M KOH). This core-shell structural feature and alloyed composition render the resulting CS0.30 Au@AgPd-s-Ago.388 Pdo.612 NPs with significantly increased electrochemically active surface area (up to 94.2 m2 g-1) and long-term stability on ethanol oxidation in alkaline media.Due to the formation of AgPd alloy and the incorporation of proper Ag in the Pd shells, these CSm Au@AgPd-s-AgnPd1-n NPs bear better electrocatalytic performance on ethanol oxidation, methanol oxidation, and oxygen reduction. Moreover, the Ag content on the NP surface rather than the total Ag content in the NP bulk may be the determinant for improvement of Pd-based catalysts. Furthermore, the proper Ag in the AgPd shells for ethanol oxidation, methanol oxidation, oxygen reduction is different. The surface Ag content of corresponding CSmAu@AgPd-s-AgnPd1-n NPs for ethanol oxidation, methanol oxidation and oxygen reduction is about 30%, about 60% and about 40%, respectively. This is mainly due to the different adsorption density of ethanol, methanol, and oxygen molecules on Pd-Ag surface.In Chapter 3, CS Au@AgPd-s-Agn Pd1-n NPs is chosen as the model to study the correlation of both KOH concentration and surface Ag contents in the AgPd shells with the electrocatalytic performance on ethanol oxidation, methanol oxidation and oxygen reduction of these CS Au@AgPd-s-Agn Pd1-n NPs. We also studied the correlation of ethanol and methanol concentration with their electrocatalytic performance of these CS Au@AgPd-s-AgnPd1-n NPs under the KOH concentration usually used in the literature. (i) The influence of KOH concentration on ECSA of CS Au@AgPd-s-Agn Pd1-n NPs with different surface Ag contents in the AgPd shells is negligible; (ii) The appropriate concentration of KOH is important for their electrocatalytic performance of CS Au@AgPd-s-AgnPd1-n NPs on ethanol oxidation; (iii) Enough concentration of KOH is important for their electrocatalytic performance of CS Au@AgPd-s-Agn Pd1-n NPs on methanol oxidation; (iv) At the same concentration of KOH, the electrocatalytic performance of these CS Au@AgPd-s-Agn Pd1-n NPs on ethanol oxidation and methanol oxidation increase gradually with the increase of concentration of ethanol and methanol, respectively; (v) Electrocatalytic performance of these CS Au@AgPd-s-AgnPd1-n NPs on oxygen reduction is better under low concentration of KOH.In summary, the results obtained in the thesis can provide empirical rules for the design and preparation of CS Au@AgPd-s-Agn Pd1-n NPs and other composite metal particles to bear better electrocatalytic performance on ethanol oxidation, methanol oxidation and oxygen reduction.