Transition Metal‐Promoted V2CO2 (MXenes): A New and Highly Active Catalyst for Hydrogen Evolution Reaction

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.


Introduction
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 (H 2 O). 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 atoms on one surface and above the hollow sites of M 3 X 3 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 C 3 V 3 . The stability of partial and fully oxidized V 2 C was also evaluated by computing the formation energies and the fully oxidized V 2 C 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 V 2 CO 2 . Therefore, fully oxidized vanadium carbides (V 2 CO 2 ) is selected as the study prototype in this work.
We first study the HER catalytic activity of pure V 2 CO 2 by computing the reaction free energy of hydrogen adsorption at different hydrogen coverage (12.5%, 16.7%, and 25% monolayer (ML) H coverage on one surface), where the H atom prefers to adsorb on the top site of surface O atoms. The calculated ΔG H is −0.45, −0.42, and −0.37 eV for 12.5%, 16.7%, and 25% ML H coverage (see Figure S1, Supporting Information), respectively, indicative of the strong interaction between H and surface O. Thus, the pure V 2 CO 2 is not a good catalyst for HER activity.
Since the high H binding strength causes the poor performance of V 2 CO 2 in HER, it is natural to seek the way to weaken the interaction between H and O to improve the HER activity. In fact, when H adsorbs on surface O atom, the combination of H 1s orbital and O 2p z orbital will form a fully filled bonding orbital (σ) and a partially filled anti-bonding orbital (σ*). According to molecular orbital theory, the H bonding strength is determined by the occupancy of the partially filled antibonding orbital, that is, the higher σ* occupancy, the weaker binding strength. So if we can introduce an electron donor (such as metal atoms) onto the surface, the O atom will receive extra electron from the donor, leading to more filled p-states of O atom. As a result, σ* occupancy will increase when forming the H O bond on the surface of V 2 CO 2 . Meanwhile, the extra electron that surface O gained will also lead to less charge transfer from H to O. Therefore, the interaction between H and O will be weakened and the HER performance will be improved greatly (Figure 1). To verify this point, we study the HER performance of transition metal (TM) absorbed V 2 CO 2 .
For TM-promoted V 2 CO 2 , the TM atoms prefer to locate above the hollow sites of O 3 V 3 and form three TM O and three TM V bonds with neighboring O and V atoms, respectively. The calculated binding energies (E b ) between different TM atoms and V 2 CO 2 are illustrated in Table S1 (Supporting Information), and they are all larger than 1.0 eV, indicative of the strong binding strength between TM atoms and V 2 CO 2 . The stability of TM-promoted V 2 CO 2 is further evaluated by ab initio molecular dynamics simulation. As shown in Figure S3 (Supporting Information), no structure reconstruction is found to occur in all of the cases under the temperature of 353 K. Even the temperature increases to 500 K, which far exceeds the typical experimental reaction temperature, the structures of the TM-V 2 CO 2 still remain in good shape, indicative of the high stability of TM-V 2 CO 2 . Moreover, the interaction between the promoter atoms and surface O atoms follows the type of electron transfer, due to the relatively large electron transfer (larger than 0.7 e) from TM atoms to V 2 CO 2 and the high formation energy of an oxygen vacancy (larger than 3.0 eV) on both the pure and TM-promoted V 2 CO 2 surface, which is similar to that of Pt-CeO 2 catalyst. [27] On the surface of TM-promoted V 2 CO 2 , H prefers to adsorb on the top of the O atom which is not bonded with the TMs, as shown in Figure 2a-c. Four different active sites can be obtained, labeled as T 0 , T 1 , T 2 , and T 3 sites, respectively, in which "T" means the "top site of surface O atom" and the subscript is the number of surrounding V atoms that connect to both the active O atom and the promoter atom. Clearly, the larger the subscript number is, the more TM atoms near the active site. For V 2 CO 2 with 12.5% ML coverage of TM promoter, each configuration contains five possible active sites, i.e., three T 0 , one T 1 , and one T 2 site (Figure 2a). Similarly, the systems with 16.7% ML promoter coverage also have three kinds of active sites: T 0 , T 1 , and T 2 sites (Figure 2b). For the case of 25% ML promoter covered V 2 CO 2 , its surface is much simple with only a T 3 site (Figure 2c). We also examine other possible adsorption sites: (i) H adsorbs on the top site of TMs, where the calculated ΔE H is positive, indicating it is not an active site for HER; (ii) H adsorbs on the hollow site of O 3 , however, H will diffuse to the top site of neighboring O spontaneously after full relaxation. The corresponding configurations of H adsorbed TM-promoted V 2 CO 2 are illustrated in Figure S4 (Supporting Information).
We select iron (Fe), cobalt (Co), and nickel (Ni) to investigate the promotional effect of different TMs on the HER performance of V 2 CO 2 and the results are summarized in Table 1 and Figure 2d. As clearly shown in the table and figure, the calculated ΔG H of the TM-promoted V 2 CO 2 is always larger than that of pure V 2 CO 2 , suggesting that introducing the TM onto the surface of V 2 CO 2 can indeed weaken the H O binding strength and thereby improve the HER performance. Most importantly, the ΔG H at T 3 site of 25% ML Ni-V 2 CO 2 (denoted as "T 3 (25% ML-Ni)" for the sake of concision), T 0 (16.7% ML-Fe), T 1 (16.7% ML-Co), T 1 (16.7% ML-Fe), and T 2 (12.5% ML-Co) site is only −0.01, −0.04, −0.05, 0.03, and −0.03 eV, respectively. The |ΔG H | at these sites are even smaller than the ΔG H of Pt(111) surface (≈0.09 eV), [28] indicating the extremely high catalytic activity for HER. Other sites, such as T 3 (25% ML-Co), T 0 (16.7% ML-Co), T 2 (16.7% ML-Co), T 2 (16.7% ML-Ni), T 2 (12.5% ML-Fe), and T 2 (12.5% ML-Ni) have similar |ΔG H | (≈0.1 eV) to that of Pt (111) surface. Therefore, these TM-promoted V 2 CO 2 are expected to have HER performances that are comparable to or even better than the Pt surface.
Moreover, the HER catalytic activities of TM-V 2 CO 2 show strong dependence on the types and coverage of the promoters as well as the active sites. First, at the same level of promoter coverage and the same type of active site, the calculated ΔG H always decreases from Fe-to Co-to Ni-V 2 CO 2 . For example, at the T 3 site of V 2 CO 2 with 25% ML promoter coverage, the calculated ΔG H is 0.31 eV for Fe-V 2 CO 2 , 0.10 eV for Co-V 2 CO 2 , and −0.01 eV for Ni-V 2 CO 2 , respectively. Second, for a given active site and a given promoter type, the calculated ΔG H always increases with the increase of the promoter coverage. This can  be seen vividly from Figure 2d that the yellow plane which presents the case of 12.5% ML promoter coverage is always under the transparent plane which describes the situation of 16.7% ML promoter coverage. Third, the HER activity is also sensitive to the active site. As displayed in Figure 2d, the calculated ΔG H for 12.5% ML Ni-promoted V 2 CO 2 is −0.32 eV at T 0 site, and it increases to −0.27 and −0.12 eV at T 1 and T 2 site, while the T 3 site has the largest ΔG H , respectively. For the case of Fe-and Co-promoted systems, similar tendency is also observed. That is, ΔG H of T 0 site is the lowest, followed by that of T 1 , T 2 , and T 3 sites. Therefore, we can conclude, for a certain promoter, the H binding strength will decrease monotonously from T 0 to T 1 to T 2 and to T 3 site, leading to the increase of ΔG H gradually. These promoter type, coverage, and active site dependent behaviors offer us flexibility for modulating the HER performance of the systems and the optimal coverage for Fe-, Co-, and Ni-V 2 CO 2 is ≈16.7% ML, 16.7%-25% ML, and ≈25% ML, respectively. Moreover, at these optimal coverages, all the active sites present high HER activity, which is actually important, since it is hard to control the reaction occurred at certain sites in real experiments.
The promotional effect of TM on HER performance of V 2 CO 2 can be ascribed to the significant charge transfer between promoter atom and surface O atoms. Through Bader charge analysis, for pure V 2 CO 2 , each surface O atom gains about 0.895 e from surrounding V atoms (see Table 1). For the TM promoted V 2 CO 2 , the electrons that surface O atoms received (N e ) range from 0.899 to 0.967 e (Table 1 and Figure 3a), larger than those in pure V 2 CO 2 . As discussed above, these extra electrons will lead to more p-states of O atom and increase the σ* occupancy in TM promoted V 2 CO 2 . As a result, the H O bond of TM-V 2 CO 2 is weakened, as compared with that in pure V 2 CO 2 and eventually increases ΔG H . Moreover, different TM promoters cause different charge transfer, which is determined by the intrinsic electronegativity of the TM. As the electronegativity order of these three TMs follows: Fe (1.83) < Co (1.88) < Ni (1.92), Fe will provide the largest charge transfer to O atom, followed by Co and Ni. As a result, at 12.5% ML TM coverage, the charge near the active sites of Fe-promoted V 2 CO 2 is the densest, followed by Co-and Ni-promoted systems (Figure 3b-d). For other TM coverage, similar tendency is observed, as shown in Table 1 and Figure 3a. Correspondingly, the H O bond in Fe-promoted V 2 CO 2 is always weaker than that in Co-and Nipromoted systems. Therefore, the ΔG H in Ni-V 2 CO 2 is always the smallest, followed by Co-V 2 CO 2 and Fe-V 2 CO 2 . As more TM atoms surround the active site (from T 0 to T 1 to T 2 to T 3 ), the number of received electrons of O atom increases for all TMpromoted systems. As a result, the H O binding strength is attenuated and ΔG H is thus enhanced. With the increase of Ni coverage from 12.5% ML (Figure 3d) to 16.7% ML (Figure 3e), the charge density at the same type of active site always increases. For other promoters, the received charge of a certain site at a lower TM coverage is also less than that at a higher TM coverage (Table 1 and Figure 3a). Therefore, the H O bond is weakened and ΔG H is augmented with the increase of TM coverage. These results are in perfect accord with the promoter type, coverage and active site dependent behaviors discussed above, which shed light on the nature of the promotional effects of transition metals on the catalytic activity.
A complete HER is a multistep reaction. The first step is the hydrogen adsorption, named Volmer reaction, in which a proton gains an electron from the surface of catalyst or electrode to form adsorbed hydrogen. The subsequent step has two possibilities: one is the homolytic Tafel reaction, 2H ad → H 2 ; the other is the heterolytic Heyrovsky reaction, H ad + H + + e − → H 2 . The pathway of the second step is strongly dependent on the inherent properties of the catalyst or electrode. Above discussion belongs to Volmer reaction. To give a comprehensive understanding of the HER process of TM-V 2 CO 2 , we further explore possibilities of Tafel mechanism or Heyrovsky mechanism. In fact, all the active sites that present high activity for Volmer reaction will maintain their catalytic activity for Heyrovsky reaction. Therefore, the second step of HER on the surface of TM-promoted V 2 CO 2 can follow the Heyrovsky mechanism. To verify whether the reaction can also follow the Tafel mechanism, we select 16.7% Fe-and Co-promoted V 2 CO 2 as prototypes and calculate the ΔG H for the adsorption of the second hydrogen. As shown in Figure S5 (Supporting Information), with the adsorption of the second hydrogen, the ΔG H greatly increases from −0.04 and −0.10 eV to 0.42 and 0.37 eV for Fe-and Co-V 2 CO 2 , respectively, indicating that the second HER step of TM-promoted V 2 CO 2 may not follow the Tafel mechanism. To further verify this, the activation barriers following Tafel mechanism are calculated. As shown in Figure S6 (Supporting Information), relative high energy barriers need to be overcome, i.e., 2.13 and 2.22 eV for Fe-and Co-V 2 CO 2 , respectively. Therefore, we can conclude that the HER on the surface of Fe-and Co-promoted V 2 CO 2 follows the Heyrovsky mechanism rather than the Tafel mechanism. Strain engineering has been proved to be an efficient way to tune the physical and chemical properties of 2D materials, including MXenes, [29][30][31] which may have influence on the HER performance as well. Moreover, the real HER experiments are generally very complicated. Materials may suffer deformations and form curved surfaces, in which the concave and convex can be regarded as suffering compressive and tensile strain, respectively. We select 12.5% ML TM covered V 2 CO 2 which contains T 0 , T 1 , and T 2 sites and the 25% ML TM covered V 2 CO 2 which contains T 3 site as representatives to study the strain influence on the HER performance of the TM-promoted V 2 CO 2 . Here, the biaxial strain is considered and defined as ε = Δa/a 0 , where the a 0 and Δa +a 0 are the lattice constants of the unstrained and strained supercells, respectively. Thus, the positive or negative values of ε correspond to the tensile or compressive strain, respectively. Figure 4 presents the reaction free energy for hydrogen adsorption at T 0 , T 1 , and T 2 sites of V 2 CO 2 with the promoter coverage of 12.5% ML and at T 3 sites of V 2 CO 2 with the coverage of 25% ML as a function of strain. A general tendency is observed, that is, the ΔG H monotonously decreases with the increase of ε. Consequently, for strong H binding strength systems that have relatively negative ΔG H , the compressive strain can improve their HER performance. For example, the ΔG H of T 0 (12.5% ML-Co) and T 1 (12.5% ML-Fe) sites is −0.24 and −0.16 eV at unstrained state and reduces to −0.09 and −0.03 eV under −2.5% strain, respectively. On the contrary, the tensile strain is an efficient way to improve the HER performance of weak H binding strength systems, such as T 3 (25% ML-Co) site, whose ΔG H will decrease from 0.10 to 0.03 eV when applying a 2.5% tensile strain on it. Moreover, the HER catalytic performance of V 2 CO 2 can even be tuned to reach the optimal ΔG H of 0 eV by applying a biaxial strain, i.e., T 2 (12.5% ML-Co) under −0.5% strain and T 3 (25% ML-Ni) under −0.27% strain. Most interestingly, for highly active sites such as T 2 (12.5% ML-Co) and T 3 (25% ML-Ni) sites, the calculated |ΔG H | is always smaller than 0.1 eV within a relatively wide range of strain, indicating their highly catalytic stability when used in real condition.  The strain engineering HER performance of TM-promoted V 2 CO 2 can be profoundly understood in light of the partial density of states of surface O under different strain. As clearly illustrated in Figures S7-S18 (Supporting Information), the p-orbital DOS of surface O atom in TM-promoted V 2 CO 2 shows an upward shift with the increase of ε. These upward shifts will lead to fewer filled p-states, [32] and less σ* occupancy with the increase of ε; as a consequence, the binding strength of H O bond will be enhanced and the ΔG H is accordingly decreased.

Conclusion
In summary, we study the HER performance of fully oxidized vanadium carbides V 2 CO 2 with and without the promotion of transition metals within the framework of first-principle calculations. Our calculations show that pure V 2 CO 2 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 V 2 CO 2 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 ΔG H 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 V 2 CO 2 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.