Two-Dimensional Lamellar Mo₂C for Electrochemical Hydrogen Production: Insights into the Origin of Hydrogen Evolution Reaction Activity in Acidic and Alkaline Electrolytes

Abstract

Developing high surface area Mo2C with certain crystal plane exposed is an efficient strategy but is an urgent challenge to optimize the hydrogen evolution reaction (HER) catalytic performances. In addition, the effects of certain crystal faces on catalytic performance have been limitedly understood. Toward this end, the (1 0 0) plane oriented two-dimensional lamellar Mo2C transformed from carbon fibers is synthesized successfully in a molten salt system. Subsequently, the electrocatalytic properties toward HER show that (1 0 0) plane oriented Mo2C functions well in both acidic and basic media. The density functional theory calculations show that the most stable Mo/C termination of the (1 0 0) plane contains multiple catalytically active centers. These close-to-zero ΔG H* values verify its better HER performance. Besides, the correlation between hydrogen adsorption behavior and the water dissociation process as well as their corresponding roles in the overall acid and alkaline HER rates have been discussed in depth. A simple mechanistic analysis is put forward to explain the favorable HER performance of the lamellar structure β-Mo2C in alkaline other than acid electrolytes. The molten salt method may provide a new way for developing electrocatalysts with oriented crystal faces.

Synthesis of Mo2C and W2C Nanoparticle Electrocatalysts for the Efficient Hydrogen Evolution Reaction in Alkali and Acid Electrolytes

“…The extracted slope values from the Tafel plots were 83 and 86 mV dec −1 for the acidic and 65 and 100 mV.dec −1 for the alkali for reduced Mo 2 C and W 2 C, respectively. These values indicate that the Volmer-Heyrovsky reaction step is obeyed for the whole HER process (Bockris and Potter, 1952; Yuan et al, 2018). Another important parameter, the exchange current density (j 0 ), was extracted to evaluate HER activity by extrapolating the linear region of the Tafel plot.…”

Recent advances in transition metal-based electrocatalysts for alkaline hydrogen evolution

“…Most of the research articles and reviews reported that the inferior performance of electrocatalysts in alkaline solution to the acidic one is due to the extra energy required in the Volmer step. 4,20 However, several emerging catalysts (e.g., Ru-MoO 2 , 21 MoO 2 /α-Mo 2 C, 22 Mo 2 C, 23,24 Mo 2 C/G-NCS, 25 C-CWC, 26 DR-MoP, 27 NiCo 2 P 2 -ACNT, 28 NCP, 29 NiCo 2 P x , 30 Ni-Cu-P 31 ) have been developed which are active both in acid and alkaline media and demonstrate better catalytic capability in alkaline electrolytes than in acidic media. Nevertheless, there is a lack of understanding of their better performance in alkaline solutions.…”

“…234 The d orbitals of M atoms will be broadened with the help of sand p-orbitals of C atoms, thus forming a similar Pt d-band state. 235 Recently, TMCs like Mo x C y , 23,235,236 W x C y , 237,238 39,239 and Ni x C y 240 have been applied in alkaline HER, especially Mo 2 C. An informative summary can be found in the latest review. 235 Despite these advantages, the relatively poor conductivity of TMCs requires a highly conductive substrate.…”

Synthesis of Ultrathin and Grid‐Structural Carbon Nanosheets Coupled with Mo2C for Electrocatalytic Hydrogen Production

“…According to the corresponding Tafel slope (Figure 5b), the mechanism of the HER was studied [18] . The three reaction equations in alkaline medium as an example are shown as follow, [19] …”