Date Published: June 28, 2016
Publisher: John Wiley and Sons Inc.
Author(s): Chongyi Ling, Li Shi, Yixin Ouyang, Qian Chen, Jinlan Wang.
Developing alternatives to precious Pt for hydrogen production from water splitting is central to the area of renewable energy. This work predicts extremely high catalytic activity of transition metal (Fe, Co, and Ni) promoted two‐dimensional MXenes, fully oxidized vanadium carbides (V2CO2), for hydrogen evolution reaction (HER). The first‐principle calculations show that the introduction of transition metal can greatly weaken the strong binding between hydrogen and oxygen and engineer the hydrogen adsorption free energy to the optimal value ≈0 eV by choosing the suitable type and coverage of the promoters as well as the active sites. Strain engineering on the performance of transition metal promoted V2CO2 further reveals that the excellent HER activities can maintain well while those poor ones can be modulated to be highly active. This study provides new possibilities for cost‐effective alternatives to Pt in HER and for the application of 2D MXenes.
Hydrogen has been considered to be one of the most important candidates for the energy source of the next generation,1, 2 owing to the high energy density and environmentally friendly combustion product (H2O). Hydrogen evolution from electrocatalytic water splitting is one of the most efficient ways, where an ideal catalyst would be the key factor to the production of hydrogen. Precious metal platinum (Pt) is the most popular electrocatalyst for hydrogen evolution reaction (HER).3 However, the high cost and the insufficiency of Pt greatly hamper their practical utilization. To assure a sustainable hydrogen generation, tremendous efforts have been made to develop the earth abundant and cost‐effective alternatives to Pt in the past few decades, including non‐precious metal alloys, metal chalcogenides, metal carbides, metal nitrides, metal phosphides, and so on.3, 4, 5, 6, 7, 8 Among these alternatives, 2D layered materials (such as MoS2) have gained broad interest recently because of their extremely large surface areas, low cost, and excellent catalytic activity.9, 10, 11
Due to the high surface activity, all the MXenes produced to date are terminated by functional groups, such as OH, O, and F.26 The terminated groups (T) in MXenes have two energetically favorable orientations, resulting in three distinct structures:25 (i) the T groups are positioned above the top of X atoms, formed three T—M bonds with neighboring M atoms on both surfaces; (ii) the T groups are located above the hollow sites of M3X3 on both sides; (iii) T groups are above the top of X atoms on one surface and above the hollow sites of M3X3 on the other surface. Different MXenes and T groups have different ground state structures.25, 26 We considered all the possible absorption sites and found that the O atom favorably absorbs above the hollow site of C3V3. The stability of partial and fully oxidized V2C was also evaluated by computing the formation energies and the fully oxidized V2C is always thermodynamically most favorable when the chemical potential exceeds −7 eV, which corresponds to the ultralow oxygen partial pressure (see Figure S2a,b in the Supporting Information for more details). The stability of fully oxidized vanadium carbides is further evaluated by ab initio molecular dynamics simulation. As shown in Figure S2c (Supporting Information), the structure remains well even at a temperature of 1000 K, indicative of the high thermodynamic stability of V2CO2. Therefore, fully oxidized vanadium carbides (V2CO2) is selected as the study prototype in this work.
In summary, we study the HER performance of fully oxidized vanadium carbides V2CO2 with and without the promotion of transition metals within the framework of first‐principle calculations. Our calculations show that pure V2CO2 is not an ideal catalyst for HER, while it can be engineered to be an excellent HER catalyst by introducing the TM atoms onto the surface. The influences of the TM promoter type, coverage, and the active site on the HER performance of V2CO2 are further explored in details and ≈16.7% ML Fe‐promoted, 16.7%–25% ML Co‐promoted, ≈25% ML Ni‐promoted systems are found to be the best catalysts for HER activity with the optimal ΔGH of ≈0 eV. Moreover, these TM‐promoted catalysts show good catalytic stability and can be further modulated by applying external strain as well. It is worth pointing out that assembling various TM onto the surfaces of materials can be easily realized in experiment, while the size and coverage can also be controlled by adjusting the ratio of reactants, react time, type, and amount of surfactant.33, 34 Therefore, these TM promoted V2CO2 are expected to be a kind of easy‐synthesized and highly active catalyst for HER. In short, the findings unveiled here would open a new window for the application of 2D MXenes and for the development of cost‐effective alternatives to Pt in HER.
All first‐principle calculations were performed by using projector augmented wave method35 as implemented in the Vienna ab initio simulation package.36, 37 The generalized gradient approximation in the Perdew–Burke–Ernzerhof form38, 39 and a cut‐off energy of 600 eV for plane‐wave basis set were adopted. The convergence threshold was 10−5 eV for energy and 0.02 eV Å−1 for force, respectively. To avoid the interaction between two periodic units, a vacuum space at least 20 Å was used. Both non‐polarized and spin‐polarized calculations were employed to determine the ground state structures. Supercells consisting of 4 × 2 × 1, 3 × 2 × 1, and 2 × 2 × 1 unit cells of V2CO2 ML were used for 12.5% ML, 16.7% ML, and 25% ML hydrogen adsorbed or promoter covered systems, respectively, as shown in Figure S1 in the Supporting Information. The corresponding Brillouin zone was sampled by Monkhorst–Pack k‐point mesh of 4 × 8 × 1, 5 × 9 × 1, and 9 × 9 × 1, respectively.