摘要
金属酞菁化合物具有高共轭结构和化学稳定性,将其负载于碳材料上作为氧还原(ORR)催化剂时表现出良好的电催化活性,目前成为催化剂研究领域的一个新方向。该类催化剂的制备包含四个基本要素:过渡金属、氮源、碳载体和热处理温度。金属酞菁的中心离子通常为Fe、Co、Ni、Mn等,其中Fe和Co的酞菁化合物对氧还原具有较高的催化活性。
研究建立了固相加热一步合成碳载钴酞菁(CoPc/C)复合催化剂的制备方法。制备的钴酞菁复合催化剂形貌均匀,分散性好,催化材料颗粒呈球形,粒径约为10-30nm。
研究了碳载体的预处理对复合材料催化性能的影响。碳粉经酸处理后其催化性能显著提高,在空气中-0.2V(vs.Hg/HgO)电位下,氧还原的电流密度达到了60 mA·cm-2。和未处理碳相比,以酸处理碳作为载体时制备的钴酞菁复合催化材料性能有了显著提高,电极在同样条件下电流密度增加了70 mA·cm-2。这说明在氧电极中,碳材料不仅作为惰性载体而存在,它的孔隙结构和表面性质会影响催化剂的活性和选择性。
对不同温度下制备的催化剂的性能进行了比较分析,600℃的催化剂具有最佳催化效果。XRD结果显示,600℃时催化剂中存在大量的CoPc分子,继续升高温度到900℃后得到的是钴的氧化物。据此分析,催化剂的活性位点应该主要是600℃时CoPc分子与碳载体形成的Co-N-C结构。另外,催化剂含量对电极活性有着重要影响。实验表明,当催化剂中金属含量5 wt%(Co/C)时电极性能最好。在最佳条件下制备的氧电极0 V即可发生显著氧还原反应,-0.1V时电流密度达到100 mA·cm-2。
对电化学测试结果进行了数据拟合和相关分析。结果显示,含催化剂电极的交换电流密度i0b=2.59×10-10 A·cm-2,比纯碳电极氧还原的交换电流密度i0a=2.61×10-12 A·cm-2提高了两个数量级,动力学过程明显加快。研究结果还显示,催化剂的加入增加了氧还原时电极的双电层电容,反应的有效活性位增加。
Metal-phthalocyanine compounds show good electrocatalytic activity for oxygen reduction due to its high conjugate structure and chemical stability. Thus, it has becomed a new direction in catalyst research field. There are four basic factors in the preparation of Metal-phthalocyanine compounds:Transition metals, Nitrogen sources, carbon support and pyrolysis temperatures. The central transition metals are usually Fe, Co, Ni and Mn. Metal-phthalocyanine which containing Fe and Co as central ions are reasearched more due to its higher catalytic activity.
In this paper, carbon supported Cobalt-phthalocyanine (CoPc/C) catalyst was synthesized in one step through calcinations. With this method, a catalyst with high activity was obtained.The result showed that the product was CoPc/C with the average particle size of 30 nm and had a well distribution and morphology.
We studied the influence of pretreated carbon carrier for ORR catalytic activity. When carbon was treated by nitric acid, the current density increased obviously to 60 mA·cm-2 at potential of-0.2 V versus an Hg/HgO reference electrode. Compared to cabon material without any treatment, the Co-phthalocyanine compounds CoPc/C catalysts with acid-treated carbon as support showed a higher catalytic activity and the current density increased 70 mA·cm-2 at the same condition. This supported an opinion that carbon material not only acted as an carrier, but also affected the activity and selectivity for ORR due to its hole fabric and apparent status.
After comparision and analysis to the CoPc/C catalysts prepared at different temperatures, the optimal pyrolyzed temperature was observed at 600℃. XRD patterns of catalysts prepared under these temperatures showed that Co-phthalocyanine polymer is stable up to 600℃and pyrolized to oxides containing Co at higher temperature as to 900℃. Based on this facts, we speculated that the active site for ORR should be Co-N-C structure formed at 600℃when CoPc connected to carbon suface.Various Co/C weight ratios were examined and the maximum activity was found to be 5% Co/C weight ratio. With the sample prepared under the optimal condition as catalyst for ORR, the onset potential of ORR is 0 V and the current density can reach 100 mA·cm-2 at-0.1V.
The electrochemical testing results were data-fitted and analysed. The fitting value in polarization curves indicates the exchange current density of ORR on the carbon-supported catalyst electrode was i0b=2.59×10-10 A·cm-2, raised by two orders of magnitude compared to naked carbon electrode. This means the catalyst could facilitate kinetic process of ORR obviously. The increased double layers capacitance had a main contribution to the activity of catalyst, the effectively active site has been increased in this way.
引文
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