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>The evolution of hierarchical porosity in self-templated nitrogen-doped carbons and its effect on oxygen reduction electrocatalysis
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The evolution of hierarchical porosity in self-templated nitrogen-doped carbons and its effect on oxygen reduction electrocatalysis
Pyrolitic self-templating synthesis is an effective method for creating hierarchically porous N-doped carbons. We study the evolution of microstructure in self-templated carbons derived from magnesium nitrilotriacetate, in the 600-1000 degrees C temperature range. The materials are characterised using N-2 adsorption, Hg intrusion, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, elemental analysis, scanning electron microscopy and transmission electron microscopy. The carbons display high specific surface areas (up to 1830 m(2) g(-1)), and high pore volumes (up to 3.1 mL g(-1)). Interestingly, each porosity type -micro, meso, and macro -evolves along its own route. Micropore growth is most significant between 600 and 700 degrees C, yet it slows down and stops around 800 degrees C; this indicates that micropores form by removal of tarry matter from the interstices between graphitic sheets, rather than by physical/chemical etching of these sheets. Mesopores, templated by spontaneously forming MgO nanoparticles, become dominant at 800 degrees C; further agglomeration of these particles leads to macropore templating at 900 degrees C. The porosity evolution is explained by the growth of MgO particles, as monitored by XRD broadening. Furthermore, the degree of disorder decreases with the pyrolysis temperature, most significantly between 700 and 800 degrees C, with the Raman I-D/I-G ratio dropping from 1.36 to 1.17. Correspondingly, the in-plane length of graphitic crystallites increases along the series, from 14 to 17 nm. Although the nitrogen content decreases with pyrolysis temperature, from 6.9 to 4.1 at%, the ratio between graphitic and pyridinic nitrogens remains constant. We then measure the performance of these carbons as electrocatalysts in the oxygen reduction reaction (ORR) at pH 13 using rotating disk electrode voltammetry and electrochemical impedance spectroscopy. Remarkably, the ORR activity trend is independent of nitrogen concentration or degree of disorder. Instead, it is governed by the microstructural parameters, most importantly surface area and microporosity.
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机译：热解自模板合成是创建分层多孔的N掺杂碳的有效方法。我们研究了在600-1000摄氏度的温度范围内自次氮基三乙酸镁衍生的自模板碳的微观结构演变。使用N-2吸附，Hg侵入，X射线衍射，X射线光电子能谱，拉曼光谱，元素分析，扫描电子显微镜和透射电子显微镜对材料进行表征。碳显示出高的比表面积（高达1830 m（2）g（-1））和高的孔体积（高达3.1 mL g（-1））。有趣的是，每种孔隙度类型（微观，中观和宏观）都沿其自身的路径发展。在600至700摄氏度之间，微孔的生长最为明显，但在800摄氏度左右时，微孔的生长速度会减慢并停止。这表明微孔是通过从石墨薄片之间的空隙中除去焦油物质而不是通过对这些薄片的物理/化学蚀刻而形成的。通过自发形成MgO纳米颗粒而模板化的中孔在800摄氏度时变得占优势。这些颗粒的进一步团聚导致在900摄氏度下形成大孔模板。孔隙度的演变可以通过XRD展宽监测到的MgO颗粒的生长来解释。此外，无序度随热解温度而降低，最显着的是在700至800摄氏度之间，拉曼I-D / I-G比从1.36降至1.17。相应地，石墨微晶的平面内长度沿该系列增加，从14nm至17nm。尽管氮含量随热解温度而降低，从6.9at％降至4.1at％，但石墨氮和吡啶二氮之间的比率保持恒定。然后，我们使用转盘电极伏安法和电化学阻抗谱法在pH值为13的氧还原反应（ORR）中测量这些碳作为电催化剂的性能。显着地，ORR活性趋势与氮浓度或无序度无关。相反，它受微观结构参数控制，最重要的是表面积和微孔率。
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