Effect of heat treatment temperature on the Pt3Co binary metal catalysts for oxygen reduced reaction and DFT calculations
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摘要: 制备低成本、高活性、高稳定性的铂(Pt)基氧还原反应(ORR)催化剂是质子交换燃料电池(PEMFC)大规模商业化应用的关键。以钴(Co)等非贵金属与Pt掺杂制备二元合金PtM催化剂不仅可以减少Pt用量，还可以获得高于Pt金属催化剂的ORR催化活性和稳定性。本研究使用浸渍还原法制备碳载铂钴ORR催化剂，通过控制热处理还原温度来控制纳米颗粒的结构、晶相、尺寸等，从而改善催化剂的ORR性能。XRD、TEM和电化学分析结果综合表明，热处理温度对纳米颗粒合金度和平均粒径有显著的影响，平均粒径和合金度随着热处理温度升高而增大。通过控制热处理温度可以获得粒径与合金度之间的最优值从而提高催化剂氧还原活性，实验表明，800 ℃是低粒径和高合金度的平衡点，在所有制备的催化剂中有最高的质量活性(0.41 A/mgPt)和稳定性。进一步的密度泛函理论(DFT)计算表明高合金度的Pt3Co结构表面可以降低速控步反应势垒，提高ORR活性。Abstract: Synthesis of low-cost, high-activity and high-stability Pt-based catalysts is of great importance to the large commercialization of proton exchange membrane fuel cell (PEMFC). Doping non-precious metals such as cobalt (Co) with Pt is attractive due to the reduced depletion of Pt and, more importantly, the enhanced activity on the oxygen reduction reaction (ORR) compared with pure Pt. In this work, carbon-supported platinum-cobalt nanoparticles (NPs) were prepared by the impregnation reduction method for the ORR catalyst. By changing the heat treatment temperature, the structure, the crystal phase and the size of the Pt3Co nanoparticles could be controlled. TEM and XRD characterizations show that larger size NPs with higher alloying degree are obtained at higher temperature. The electrochemical results demonstrate that the Pt3Co NPs at 800 ℃ have the highest mass activity (0.41 A/mgPt) and the best stability among all the samples due to their lower particle size and higher alloying degree. Further Density functional theory (DFT) calculation shows that the surface of the Pt3Co structure with high alloying degree can reduce the rate-determining step barrier and improve the ORR activity.
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图 1 使用浸渍还原法制备的不同温度下Pt3Co催化剂的TEM照片及相应的粒径尺寸分布
Figure 1 TEM images and the corresponding size distribution of the Pt3Co catalysts prepared with the impregnation reduction method at different temperature of (a) 700 ℃ (Pt3Co-700), (b) 800 ℃ (Pt3Co-800) and (c) 900 ℃ (Pt3Co -900)
图 2 Pt3Co-700、Pt3Co-800和Pt3Co-900的高分辨TEM照片 （a）、（d）和（g），元素线性扫描图（b）、（e）和（h）（红色为 Pt 分布曲线，绿色为 Co 分布曲线）和元素分布图（c）、（f）和（i）
Figure 2 Characterization of Pt3Co-700, Pt3Co-800 and Pt3Co-900 (a), (d) and (g): high-resolution TEM image; (b), (e) and (h): linear scan of a single particle, red line is the Pt atom distribution curve, green is the Co atom distribution curve; (c), (f) and (i): the nanoparticle element distribution diagram
图 3 （a）Pt3Co-700、Pt3Co-800和Pt3Co-900的XRD谱图，蓝色线为Pt3Co的标准卡片（ICSD #102624）衍射峰，（b）为图（a）的XRD谱图在36°−43°附近的局部放大图，（c）为Pt3Co-700、Pt3Co-800和Pt3Co-900的Pt 4f XPS谱图，（d）为Co 2p XPS谱图
Figure 3 (a) XRD patterns of Pt3Co-700, Pt3Co-800 and Pt3Co-900, the blue line is the diffraction peak of Pt3Co standard card (ICSD #102624), (b) is the partial enlarged view XRD curve of Figure (a) around 37°− 44°, (c) is Pt 4f XPS spectra of Pt3Co-700, Pt3Co-800 and Pt3Co-900, (d) is Co 2p XPS spectra
图 4 Pt3Co-700、Pt3Co-800和Pt3Co-900 ADT前后的（a）CV曲线，（b）LSV曲线，内嵌有Pt3Co-700、Pt3Co-800和Pt3Co-900在不同转速下LSV曲线，（c）ECSA，（d）0.9 V vs. RHE下质量活性，（e）0.9 V vs. RHE下本征活性，（f）极化曲线，实线为电压随电流密度变化的极化曲线，虚线为功率随电流密度变化的功率曲线
Figure 4 For Pt3Co-700, Pt3Co-800 and Pt3Co-900 (a) CV curve, (b) LSV curve, embedded with Pt3Co-700, Pt3Co-800 and Pt3Co-900 LSV curve at different rotation rates, (c) ECSA, (d) 0.9 V vs. RHE mass activity and (e) specific surface area activity before and after the ADT (f) Polarization curve, which the solid line is the polarization curve of voltage changing with current density, and the dashed line is the power curve of power changing with current density
图 5 （a）Pt3Co表面O、OH、OO和OOH吸附结构示意图；（b）Pt3CoCorich表面O、OH、OO和OOH吸附结构示意图其中银色为Pt原子，蓝色为Co原子，红色为O原子，白色为H原子; （c）为Pt3Co和Pt3CoCorich在U = 1.23 V下associative机理反应自由能对比；(d)为dissociative机理反应自由能对比
Figure 5 (a) Schematic diagram of the adsorption structure of O, OH, OO and OOH on the surface of Pt3Co, (b) Schematic diagram of the adsorption structure of O, OH, OO and OOH on the surface of Pt3CoCorich. Silver ball corresponds to Pt atom, blue ball corresponds to Co atom, red ball corresponds to O atom , and white ball corresponds to H atom, (c) is the comparison of the associative mechanism reaction free energy diagram of Pt3Co and Pt3CoCorich at U = 1.23 V, (d) is the comparison of the dissociative mechanism reaction free energy diagram
表 1 ADT前后LSV曲线半波电位
Table 1 Half wave potential before and after ADT test.
Pt3Co-700 before ADT Pt3Co-700 after ADT Pt3Co-800 before ADT Pt3Co-800 after ADT Pt3Co-900 before ADT Pt3Co-800 after ADT Half wave potential
(V vs. RHE)0.89 0.86 0.89 0.86 0.81 0.76 
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表 2 ORR反应机理步骤
Table 2 Reaction steps of ORR mechanisms
ORR mechanism Reaction steps Associative O2 (g) + * →*O2 *O2 + H+ + e− →*OOH *OOH + H+ + e− → H2O(l) + *O *O + H+ + e−→ *OH *OH + H+ + e−→ H2O(l) + * Dissociative 1/2O2 (g) + * →*O *O + H+ + e−→*OH *OH + H+ + e−→ H2O(l) + *
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