Preparation of CoP/Co@NPC@rGO nanocomposites with an efficient bifunctional electrocatalyst for hydrogen evolution and oxygen evolution reaction
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摘要: 简单的热处理和热处理磷化ZIF-67/氧化石墨烯(GO)前驱体得到具有典型的多孔碳结构特征的CoP/Co@NPC@rGO纳米复合材料电催化剂。通过扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)、拉曼光谱(Raman)和N2等温吸脱附曲线等对其形貌、成分和结构进行分析和表征。采用线性扫描伏安法、电化学阻抗谱和计时电位法探讨了CoP/Co@NPC@rGO纳米复合电催化剂对氢气析出反应（HER）和氧气析出反应（OER）的电催化活性和稳定性。结果表明，CoP/Co@NPC@rGO‒350在1.0 mol·L–1 KOH溶液中达到10 mA·cm‒2电流密度的析氢过电位仅127 mV；同时，在1.0 mol·L–1 KOH溶液中显示出优于贵金属RuO2的析氧性能，达到10 mA·cm‒2电流密度的过电位为276 mV，塔菲尔斜率仅为42 mV·dec‒1。这种高析氢和析氧电催化活性主要归因于高度石墨化的N掺杂多孔碳与N掺杂石墨烯之间的协同效应。CoP/Co@NPC@rGO是电催化全解水电催化剂的候选材料，且为基于金属有机骨架(MOFs)/氧化石墨烯复合材料的高效电催化剂的设计开辟了一条新的途径。Abstract: The construction of the highly active transition-metal phosphide/carbon-based electrocatalyst from metal-organic frameworks (MOFs) precursors is considered as an efficient approach. In this work, ZIF-67/GO precursors were firstly obtained by the in situ controllable growth of ZIF-67 nanocrystals on both surfaces of GO sheets. Then, a highly efficient bifunctional electrocatalyst CoP/Co@NPC@rGO nanocomposite was derived by the thermal pyrolysis of ZIF-67/GO precursors under N2 atmosphere and a subsequent phosphatization process. The structure and elemental composition of the ZIF-67/GO, Co@NPC@rGO, and CoP/Co@NPC@rGO nanocomposites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and N2 ad-/desorption isotherms analysis. The Co@NPC@rGO‒800 nanocomposite exhibits a high Brunauer-Emmett-Teller (BET) surface area of 186.27 m2·g‒1, indicating that both micropores and mesopores existed. Subsequently, the electrocatalytic properties of the CoP/Co@NPC@rGO nanocomposites for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were investigated by electrochemical measurements. The results suggest that the obtained CoP/Co@NPC@rGO‒350 nanocomposite only requires an overpotential of 127 mV to reach a current density of 10 mA·cm‒2 for HER in 1.0 mol·L–1 KOH solution. For OER, CoP/Co@NPC@rGO‒350 nanocomposite can reach a current density of 10 mA·cm‒2 at an overpotential of 276 mV, with a Tafel slope of 42 mV·dec−1, in the same alkaline aqueous solution, which is superior to RuO2. In addition, for both HER and OER, CoP/Co@NPC@rGO‒350 nanocomposite also shows impressive strong durability in alkaline aqueous solution. The outstanding performance can be attributed to the synergistic effect of coupled highly graphitized N-doped porous carbon and N-doped graphene. The as-prepared CoP/Co@NPC@rGO‒350 electrocatalyst is a promising candidate for overall water splitting in the alkaline solution. This development offers an attractive catalyst material based on MOF/GO composites. It is expected that the presented strategy can be extended to the fabrication of other composites electrode materials for more efficient water splitting.
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图 1 ZIF-67/氧化石墨烯的扫描电子显微镜图片（a）和X射线衍射谱图（b）
Figure 1. SEM image (a) and XRD pattern of ZIF-67/GO
图 2 Co@NPC@GO‒800的表征。（a）扫描电子显微镜图片；（b）透射电子显微镜图片；（c）X射线衍射谱图；（d）拉曼光谱；（e）氮气等温吸脱附曲线；（f）孔径分布
Figure 2. Characterization of Co@NPC@GO‒800: (a) SEM; (b) TEM; (c) XRD; (d) Raman spectrum; (e) N2 ad-/desorption isotherms; (f) pore size distribution
图 3 CoP/Co@NPC@rGO‒X的X射线衍射谱图
Figure 3. XRD patterns of CoP/Co@NPC are braided once clockwise. $\gamma_2$ crosses the branch cut of the votex hosting $\gamma_3$ and gains a minus sign while $\gamma_3$ crosses the branch cut of the votex hosting $\gamma_2$ and gains a minus sign. Hence the result is given by $\gamma_2\to-\gamma_3,\gamma_3\to\gamma_2$. (b) Worldlines in a space-time (x; t) diagram, describing four MZMs. @rGO‒X
图 4 CoP/Co@NPC@GO‒350的结构表征。（a）扫描电子显微镜图片；（b）透射电子显微镜图片
Figure 4. Characterization of CoP/Co@NPC@rGO‒350: (a) SEM image; (b) TEM image
图 5 CoP/Co@NPC@rGO‒350的X射线光电子能谱图。（a）全谱图；（b）N 1s；（c）Co 2p；（d）P 2p
Figure 5. XPS spectrum of CoP/Co@NPC@rGO‒350: (a) full spectrum; (b) N 1s; (c) Co 2p; (d) P 2p
图 6 Co@NPC@rGO‒800、CoP/Co@NPC@rGO‒X和RuO2的析氧性能。（a）线性扫描伏安曲线；（b）塔菲尔斜率；（c）电化学阻抗；（d）计时电位曲线
Figure 6. Oxygen evolution performance of Co@NPC@rGO‒800, are braided once clockwise. $\gamma_2$ crosses the branch cut of the votex hosting $\gamma_3$ and gains a minus sign while $\gamma_3$ crosses the branch cut of the votex hosting $\gamma_2$ and gains a minus sign. Hence the result is given by $\gamma_2\to-\gamma_3,\gamma_3\to\gamma_2$. (b) Worldlines in a space-time (x; t) diagram, describing four MZMs. CoP/Co@NPC@rGO‒X, and RuO2: (a) Lsv curves; (b) Tafel slope; (c) EIS; (d) chronoamperometry
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