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Ultra-high electrochemical catalytic activity of MXenes

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ABSTRACT

Cheap and abundant electrocatalysts for hydrogen evolution reactions (HER) have been widely pursued for their practical application in hydrogen-energy technologies. In this work, I present systematical study of the hydrogen evolution reactions on MXenes (Mo2X and W2X, X = C and N) based on density-functional-theory calculations. I find that their HER performances strongly depend on the composition, hydrogen adsorption configurations, and surface functionalization. I show that W2C monolayer has the best HER activity with near-zero overpotential at high hydrogen density among all of considered pure MXenes, and hydrogenation can efficiently enhance its catalytic performance in a wide range of hydrogen density further, while oxidization makes its activity reduced significantly. I further show that near-zero overpotential for HER on Mo2X monolayers can be achieved by oxygen functionalization. My calculations predict that surface treatment, such as hydrogenation and oxidization, is critical to enhance the catalytic performance of MXenes. I expect that MXenes with HER activity comparable to Pt in a wide range of hydrogen density can be realized by tuning composition and functionalizing, and promotes their applications into hydrogen-energy technologies.

No MeSH data available.


Calculated overpotentials as a function of H-coverage on oxidized Mo2X monolayers: (a) differential Gibbs free energy for H atoms adsorbed on HC sites, (b) average Gibbs free energy for H atoms adsorbed on HC sites, (c) differential Gibbs free energy for H atoms adsorbed on TX sites, (b) average Gibbs free energy for H atoms adsorbed on TX sites.
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f6: Calculated overpotentials as a function of H-coverage on oxidized Mo2X monolayers: (a) differential Gibbs free energy for H atoms adsorbed on HC sites, (b) average Gibbs free energy for H atoms adsorbed on HC sites, (c) differential Gibbs free energy for H atoms adsorbed on TX sites, (b) average Gibbs free energy for H atoms adsorbed on TX sites.

Mentions: Similarly, a supercell with 2 × 2 × 1 unit cells is constructed based on the unit of M2XO2-mH-Ad (m = 1 and 2, Ad = HC and TX). By removing H atom one by one from a H-covered surface, the H-coverage dependent HER activity can be evaluated. The DFT-calculated Gibbs free energies show that Mo2X monolayers with oxidization at HC sizes are better than those at TX sites in HER activities (Fig. 6). Mo2XO2-1H-HC (X = C and N) shows the best HER performance at n = 2 (d-ΔGH = −0.03~0.03 eV), and the HER activities of Mo2XO2-2H-HC are better if 1 < n < 2 where the d-ΔGH cross the reference line (0 eV) in individual processes (Fig. 6a). Importantly, the a-ΔGH of Mo2XO2-1H-HC (X = C and N) at full H-coverage (n = 4) are near-zero (0.01~0.04 eV, Fig. 6b), indicating the collective processes can take place. It is expected that the collective HER process on Mo2XO2-1H-HC is dominant at high H-density, and the individual processes may take over the role when H-density is low. Similarly, the collective processes of Mo2XO2-2H-HC can take place within the H-coverage from 2 to 3 (Fig. 6b). For Mo2X monolayers with oxidization at TX, only Mo2CO2-2H-TX shows high HER activity at low H-coverage (n = 1, ΔGH = 0.02 eV) (Fig. 6c,d). Although the oxidization results in the improvement of the HER performances of W2CO2 monolayers with O atoms at HC sites at low H-coverage, but their HER activities at high H-coverage are greatly reduced (Supporting data, S-2a,b). W2CO2 monolayers with O atoms at TX sites shows poor HER performances (S-2c,d). Our calculations also show that W2NO2 monolayers with partial H-coverage (0 < n < 4) are unstable and their Gibbs free energies are not calculated.


Ultra-high electrochemical catalytic activity of MXenes
Calculated overpotentials as a function of H-coverage on oxidized Mo2X monolayers: (a) differential Gibbs free energy for H atoms adsorbed on HC sites, (b) average Gibbs free energy for H atoms adsorbed on HC sites, (c) differential Gibbs free energy for H atoms adsorbed on TX sites, (b) average Gibbs free energy for H atoms adsorbed on TX sites.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5015052&req=5

f6: Calculated overpotentials as a function of H-coverage on oxidized Mo2X monolayers: (a) differential Gibbs free energy for H atoms adsorbed on HC sites, (b) average Gibbs free energy for H atoms adsorbed on HC sites, (c) differential Gibbs free energy for H atoms adsorbed on TX sites, (b) average Gibbs free energy for H atoms adsorbed on TX sites.
Mentions: Similarly, a supercell with 2 × 2 × 1 unit cells is constructed based on the unit of M2XO2-mH-Ad (m = 1 and 2, Ad = HC and TX). By removing H atom one by one from a H-covered surface, the H-coverage dependent HER activity can be evaluated. The DFT-calculated Gibbs free energies show that Mo2X monolayers with oxidization at HC sizes are better than those at TX sites in HER activities (Fig. 6). Mo2XO2-1H-HC (X = C and N) shows the best HER performance at n = 2 (d-ΔGH = −0.03~0.03 eV), and the HER activities of Mo2XO2-2H-HC are better if 1 < n < 2 where the d-ΔGH cross the reference line (0 eV) in individual processes (Fig. 6a). Importantly, the a-ΔGH of Mo2XO2-1H-HC (X = C and N) at full H-coverage (n = 4) are near-zero (0.01~0.04 eV, Fig. 6b), indicating the collective processes can take place. It is expected that the collective HER process on Mo2XO2-1H-HC is dominant at high H-density, and the individual processes may take over the role when H-density is low. Similarly, the collective processes of Mo2XO2-2H-HC can take place within the H-coverage from 2 to 3 (Fig. 6b). For Mo2X monolayers with oxidization at TX, only Mo2CO2-2H-TX shows high HER activity at low H-coverage (n = 1, ΔGH = 0.02 eV) (Fig. 6c,d). Although the oxidization results in the improvement of the HER performances of W2CO2 monolayers with O atoms at HC sites at low H-coverage, but their HER activities at high H-coverage are greatly reduced (Supporting data, S-2a,b). W2CO2 monolayers with O atoms at TX sites shows poor HER performances (S-2c,d). Our calculations also show that W2NO2 monolayers with partial H-coverage (0 < n < 4) are unstable and their Gibbs free energies are not calculated.

View Article: PubMed Central - PubMed

ABSTRACT

Cheap and abundant electrocatalysts for hydrogen evolution reactions (HER) have been widely pursued for their practical application in hydrogen-energy technologies. In this work, I present systematical study of the hydrogen evolution reactions on MXenes (Mo2X and W2X, X&thinsp;=&thinsp;C and N) based on density-functional-theory calculations. I find that their HER performances strongly depend on the composition, hydrogen adsorption configurations, and surface functionalization. I show that W2C monolayer has the best HER activity with near-zero overpotential at high hydrogen density among all of considered pure MXenes, and hydrogenation can efficiently enhance its catalytic performance in a wide range of hydrogen density further, while oxidization makes its activity reduced significantly. I further show that near-zero overpotential for HER on Mo2X monolayers can be achieved by oxygen functionalization. My calculations predict that surface treatment, such as hydrogenation and oxidization, is critical to enhance the catalytic performance of MXenes. I expect that MXenes with HER activity comparable to Pt in a wide range of hydrogen density can be realized by tuning composition and functionalizing, and promotes their applications into hydrogen-energy technologies.

No MeSH data available.