物理化学学报 >> 2019, Vol. 35 >> Issue (5): 523-530.doi: 10.3866/PKU.WHXB201806191

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盐封后碳化钼电催化剂的制备及其氢析出性能

林舟,申琳璠,瞿希铭,张俊明,姜艳霞*(),孙世刚   

  • 收稿日期:2018-04-11 发布日期:2018-10-19
  • 通讯作者: 姜艳霞 E-mail:yxjiang@xmu.edu.cn
  • 基金资助:
    国家重点研究与发展项目(2017YFA0206500);国家自然科学基金(21773198);国家自然科学基金(U1705253)

Molybdenum Carbide Prepared by a Salt Sealing Approach as an Electrocatalyst for Enhanced Hydrogen Evolution Reaction

Zhou LIN,Linfan SHEN,Ximing QU,Junming ZHANG,Yanxia JIANG*(),Shigang SUN   

  • Received:2018-04-11 Published:2018-10-19
  • Contact: Yanxia JIANG E-mail:yxjiang@xmu.edu.cn
  • Supported by:
    the National Key Research and Development Program of China(2017YFA0206500);the National Natural Science Foundation of China(21773198);the National Natural Science Foundation of China(U1705253)

摘要:

碳化钼具有低廉的价格、优越的催化性能以及良好的稳定性而被人们认为是极好的可以替代Pt等贵金属的氢析出反应(HER)催化剂。本工作采用钼酸钠和2, 6-二氨基吡啶为反应原料,之后不断进行盐封的过程直到前驱体被紧紧包覆在NaCl的晶格中,最后置于惰性气氛下煅烧即可制得盐封后碳化钼。本工作采用了扫描电子显微镜(SEM)、透射电子显微镜(TEM)、能量色散x射线光谱(EDS)、X射线衍射(XRD)以及X射线光电子能谱(XPS)等技术对盐封后碳化钼的形貌、组成以及晶体结构进行了表征。结果表明盐封后的产物形貌并不均一,其中包括纳米颗粒及纳米棒。比较Mo2C/SS与Mo2C的TEM图,可以发现盐封后Mo2C的尺寸变小,表明盐封的方法可以有效地避免颗粒的团聚。根据产物的氮气吸脱附等温线得到的催化剂的Brunauer-Emmett-Teller (BET)比表面积,盐封后Mo2C的BET表面积由2.55提高至8.14 m2·g−1,可以证明盐封过程中孔的生成。EDS、XRD及XPS分析的结果表明盐封后的产物是斜方晶系的Mo2C,并且表面由于被氧气氧化而带有氧化钼。结合XPS和周转率(TOF)数计算的结果,可以说明盐封过程中孔的形成有助于暴露更多活性位点,然而同时也扩大了与氧气的接触面积,催化剂表面形成的氧化钼的含量也增多。因此,催化剂表面活性中心即碳化钼所占的比例降低。另一方面,氧化钼的法拉第反应产生的赝电容会与碳化钼催化剂的双电层电容叠加,导致得到的比容量数值偏大。而氧化钼的赝电容效应对Mo2C/SS催化剂的影响是更显著的,因此盐封后的TOF数降低。同Mo2C相比,Mo2C/SS展现出更高的HER质量活性的原因可归结如下:(1)盐封过程中大量孔的形成有助于提高产物的BET表面积并暴露出更多的活性位点;(2)盐封后的多孔结构和较大的表面积有利于传质传荷;(3)盐封后碳化钼的Tafel斜率由145降至88 mV·dec−1。总的来说,盐封后碳化钼的HER活性有了明显的提高,当电流密度达到10 mA·cm−2时,过电位为175 mV左右。盐封后碳化钼的Tafel斜率为88 mV·dec−1,证明催化剂表面发生的氢析出反应遵循Volmer-Heyrovsky机理并以电化学脱附步骤为反应的速控步骤。

关键词: 碳化钼, 氢析出反应, 盐封, 活性位点, 电催化

Abstract:

Molybdenum carbide is regarded as an excellent substitute for Pt-based catalysts in the hydrogen evolution reaction (HER), owing to its low cost, superior catalytic performance, and long-term stability. In this work, salt-sealed molybdenum carbide was prepared using sodium molybdate and 2, 6-diaminopyridine as the reactive raw materials, followed by continuous salt sealing and calcination of the precursor under an inert atmosphere. The morphology, composition and structure of salt-sealed molybdenum carbide were determined by scanning electron microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results indicate that salt-sealed molybdenum carbide has irregular morphology and includes nanoparticles and nanorods. A comparison of the TEM images of Mo2C with salt sealing (Mo2C/SS) and Mo2C without salt sealing (Mo2C) indicates that Mo2C/SS exhibits a smaller particle size. This suggests that salt sealing can efficiently avoid particle aggregation. The Brunauer-Emmett- Teller (BET) specific surface area of the catalysts was obtained from nitrogen adsorption/desorption isotherms. The increase in BET surface area from 2.55 to 8.14 m2·g−1 after salt sealing provides evidence for the formation of pores in the product. The results of XRD, EDS and XPS analyses show that Mo2C/SS has an orthorhombic crystal structure with molybdenum oxides on the surface, which may originate from surface oxidation. Considering the results of XPS and the turnover frequency (TOF) calculation, we can conclude that the formation of pores via salt sealing contributes to the exposure of more active sites, while simultaneously enlarging the contact area with oxygen. Therefore, higher molybdenum oxide content is generated on the surface, resulting in a lower proportion of active centers (molybdenum carbides) on the catalyst surface. Furthermore, the pseudocapacitance generated by the faradaic reaction of molybdenum oxides is superimposed on the double-layer capacitance of Mo2C catalysts, which increases the double layer capacitance. Since the effect of pseudo-capacitance on Mo2C/SS is more significant, the TOF number declines after salt sealing. Compared with Mo2C, Mo2C/SS exhibits three features that promote HER mass activity: (1) the generation of large quantities of pores via salt sealing leads to an increase in the BET surface area and exposure of more active sites, which is beneficial for improving HER performance; (2) the porous structure and enlarged surface area pave the way for effective mass and charge transfer; (3) the decrease of the Tafel slope from 145 to 88 mV·dec−1. In summary, salt-sealed Mo2C exhibited enhanced HER activity with an overpotential of 175 mV to achieve a current density of 10 mA·cm−2. The Tafel slope for HER on salt-sealed Mo2C is 88 mV·dec−1. This can be considered as the proof of the Volmer-Heyrovsky mechanism with electrochemical desorption as the rate-determining step.

Key words: Molybdenum carbide, Hydrogen evolution reaction, Salt sealing, Active site, Electrocatalysis

MSC2000: 

  • O646