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Ultra-high electrochemical catalytic activity of MXenes

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 = 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.

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Calculated differential Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers (M2XH-Ad) with H atoms adsorbed on: (a) HC, (c) TX, and (e) TM; Calculated average Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers with H atoms adsorbed on: (b) HC, (d) TX, and (f) TM.
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f4: Calculated differential Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers (M2XH-Ad) with H atoms adsorbed on: (a) HC, (c) TX, and (e) TM; Calculated average Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers with H atoms adsorbed on: (b) HC, (d) TX, and (f) TM.

Mentions: It had been reported that MXene can be easily functionalized by H, OH, and F, which affect their performance in energy storage404143. To study the effect of hydrogenation on HER activity, a supercell with 2 × 2 × 1 unit cells is constructed based on the unit of MXene with both surfaces fully covered by hydrogen atoms at different adsorption sites (M2XH2-Ad). By removing H atom one by one from one of its surfaces, the effect of hydrogenation on the H-coverage dependent HER activity can be evaluated. To calculate the Gibbs free energies, it only needs to replace M2X in Eqs (3) and (4) by M2XH. Therefore, E(M2XH + nH) is the energy of monolayer with one side covered with variable hydrogen atoms (n) and another side fully covered by hydrogen atoms (M2XH-Ad-nH), and E(M2XH) is the energy of M2X monolayer with one side fully covered by hydrogen atoms (M2XH-Ad). The calculated overpotentials show that hydrogenation can efficiently improve the HER activities of M2X monolayers (Fig. 4). For M2X monolayer with hydrogenation at HC sites on one of its surfaces (M2XH-HC), the HER activities are improved because of the reduced overpotentials in individual processes (Fig. 4a). In particular, the d-ΔGH of W2C monolayer is about 0.05 eV at full H-coverage (n = 4), which just satisfies the basic requirement for an efficient electrocatalyst with ΔGH = 0 eV. Although the average Gibbs free energies (a-ΔGH < 0) for M2XH-HC in collective processes are increased (Fig. 4b), they are still less than −0.2 eV, indicating that the collective processes are difficult to take place.


Ultra-high electrochemical catalytic activity of MXenes
Calculated differential Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers (M2XH-Ad) with H atoms adsorbed on: (a) HC, (c) TX, and (e) TM; Calculated average Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers with H atoms adsorbed on: (b) HC, (d) TX, and (f) TM.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Calculated differential Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers (M2XH-Ad) with H atoms adsorbed on: (a) HC, (c) TX, and (e) TM; Calculated average Gibbs free energy as a function of H-coverage on one-side hydrogenated M2X monolayers with H atoms adsorbed on: (b) HC, (d) TX, and (f) TM.
Mentions: It had been reported that MXene can be easily functionalized by H, OH, and F, which affect their performance in energy storage404143. To study the effect of hydrogenation on HER activity, a supercell with 2 × 2 × 1 unit cells is constructed based on the unit of MXene with both surfaces fully covered by hydrogen atoms at different adsorption sites (M2XH2-Ad). By removing H atom one by one from one of its surfaces, the effect of hydrogenation on the H-coverage dependent HER activity can be evaluated. To calculate the Gibbs free energies, it only needs to replace M2X in Eqs (3) and (4) by M2XH. Therefore, E(M2XH + nH) is the energy of monolayer with one side covered with variable hydrogen atoms (n) and another side fully covered by hydrogen atoms (M2XH-Ad-nH), and E(M2XH) is the energy of M2X monolayer with one side fully covered by hydrogen atoms (M2XH-Ad). The calculated overpotentials show that hydrogenation can efficiently improve the HER activities of M2X monolayers (Fig. 4). For M2X monolayer with hydrogenation at HC sites on one of its surfaces (M2XH-HC), the HER activities are improved because of the reduced overpotentials in individual processes (Fig. 4a). In particular, the d-ΔGH of W2C monolayer is about 0.05 eV at full H-coverage (n = 4), which just satisfies the basic requirement for an efficient electrocatalyst with ΔGH = 0 eV. Although the average Gibbs free energies (a-ΔGH < 0) for M2XH-HC in collective processes are increased (Fig. 4b), they are still less than −0.2 eV, indicating that the collective processes are difficult to take place.

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.