Electronic Regulation on Mo2c Nanowires Toward Efficient Hydrogen Evolution Reaction
Authors: Gao Qingsheng , Lin Huanlei 
Published: February 2020

Advanced catalysts with element-abundance, economic-cost and stability are desired for electrochemical hydrogen evolution reaction (HER) to scale-up the promising clean energy of hydrogen.[1] Recently, early-transition metal carbides have been developed as noble-metal free electrocatalysts for HER.[2] Among them, Mo2C have received special attentions because of its varied electronic feature and catalytic performance related to the tunable phases and composition.[3] However, the negative hydrogen-binding energy (∆GH*) on Mo2C indicates a strong Mo-H, resulting in the restricted Hadsdesorption (i.e., Heyrovsky or Tafel step) and thus low HER efficiency. An optimized electron density in Mo center is highly desired.

In our recent work, the effective electronic regulation on Mo2C nanowires was accomplished via constructing MoC-Mo2C heteronanowires and cobalt doping, leading to the significantly improved HER activity in both acidic and basic electrolytes.[4~6] On one hand, MoC-Mo2C heteronanowires composed of well-defined nanoparticles were accomplished via the controlled carbonization of tailored MoOx-amine precursors, which showed the reduced electronic density around Mo active centers on MoC-Mo2C interfaces. This would weaken the Mo-H for promoted Hads desorption. As expected, the optimal one consisting 31.4 wt% MoC displayed a low overpotential (η10 = 126 and 120 mV for reaching a current density of -10 mA×cm−2), a small Tafel slope (43 and 42 mV×dec−1) and a low onset overpotential (38 and 33 mV) in 0.5 M H2SO4 and 1.0 M KOH, respectively. On the other hand, an effective cobalt-doping based on Co-modified MoOx-amine precursors was developed, in which the consequently increasing electron density around E
F facilitated HER kinetics. With an optimal Co/Mo ratio of 0.020, the Co-Mo2C presented a low overpotential (η
10 = 140 and 118 mV for reaching a current density of -10 mA×cm−2; η
100 = 200 and 195 mV for reaching a current density of -100 mA×cm−2), a small Tafel slope (39 and 44 mV×dec−1) and a low onset overpotential (40 and 25 mV) in 0.5 M H2SO4and 1.0 M KOH, respectively.

Noticeably, the sufficient variation of electron density around E
F, either the increase or decrease, can be used to optimize the hydrogen binding on metal carbides.[7] The former results in the electrons transferred into anti-bonding of Mo-H, while, the latter leads to the reduced electrons in bonding orbitals, both of which will be responsible for the weakened Mo-H and the in turn favored Hads desorption. The obviously improved HER on our MoC-Mo2C and Co-Mo2C strongly demonstrated the efficiency of above two mechanisms to optimize HER kinetics. More importantly, this work advanced the understanding of electrocatalysis on carbide surface, and opened up new opportunities to develop high-performance catalysts via rational engineering on compositions and nanostructures.

Reference:

[1] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577.

[2] C. G. Morales-Guio, L. Stern and X. L. Hu, Chem. Soc. Rev., 2014, 43, 6555.

[3] L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M. D. Scanlon, X. Hu, Y. Tang, B. Liu, H. H. Girault, Energy Environ. Sci., 2014, 7, 387.

[4] H. L. Lin, Z. P. Shi, S. N. He, X. Yu, S. N. Wang, Q. S. Gao, Y. Tang, Chem. Sci. 2016, DOI: 10.1039/c6sc00077k.

[5] H. L. Lin, N. Liu, Z. P. Shi, Y. L. Guo, Y. Tang, Q. S. Gao, Adv. Funct. Mater., 2016, DOI: 10.1002/adfm.201600915.

[6] Z. P. Shi, Y. X. Wang, H. L. Lin, H. B. Zhang, M. K. Shen, S. H. Xie, Y. H. Zhang, Q. S. Gao, Y. Tang, J. Mater. Chem. A 2016, DOI: 10.1039/c6ta01900e.

[7] Q. S. Gao, N. Liu, S. N. Wang, Y. Tang, Nanoscale, 2014, 6, 14106.

Fig. 1 (a) j
0 (obtained by extrapolating Tafel curves to η= 0 mV) and j
150 (current density at η= 150 mV) of the MoC-Mo2C heteronanowires, which are associated with the ratio of surface Mo3+/Mo2+ determined by XPS analysis. (b) Schematic illustration for the HER activity relying on the electron density of Mo in a series of MoCx electrocatalysts.

Figure 1