Heterostructures of MXenes and N-doped graphene as highly active bifunctional electrocatalysts (1805.04213v1)

Abstract: MXenes with versatile chemistry and superior electrical conductivity are prevalent candidate materials for energy storage and catalysts. Inspired by recent experiments of hybridizing MXenes with carbon materials, here we theoretically design a series of heterostructures of N-doped graphene supported by MXene monolayers as bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Our first-principles calculations show that the graphitic sheet on V2C and Mo2C MXenes are highly active with an ORR overpotential down to 0.36 V and reaction free energies for the HER approaching zero, both with low kinetic barriers. Such outstanding catalytic activities originate from the electronic coupling between the graphitic sheet and the MXene, and can be correlated with the pz band center of surface carbon atoms and the work function of the heterostructures. Our findings screen a novel form of highly active electrocatalysts by taking advantage of the fast charge transfer kinetics and strong interfacial coupling of MXenes, and illuminate a universal mechanism for modulating the catalytic properties of two-dimensional hybrid materials.

Design and Catalytic Performance of MXene and N-Doped Graphene Heterostructures

This paper explores the catalytic potentials of hybrid materials formed by integrating N-doped graphene with MXene monolayers, primarily targeting the enhancement of electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). The paper leverages density functional theory (DFT) calculations to delve into the electronic and catalytic characteristics of these heterostructures, thereby proposing new avenues for efficient energy conversion processes.

MXenes, with their promising electrical conductivity and diverse chemistry, serve as attractive candidates for material innovations in energy storage and catalytic applications. The paper articulates the design framework by juxtaposing MXenes—such as Ti2C, V2C, Nb2C, and Mo2C—with N-doped graphene, aiming to form bifunctional electrocatalysts with superior performance in ORR and HER.

Key Findings and Results

• Electronic Coupling and Charge Transfer: The heterostructures exhibit significant electron transfer from MXene to graphene, enhancing the reactivity of carbon atoms. The electron transfer dynamics were intricately tied to the graphene's band structure and catalytic sites' electronic environment.
• ORR Performance: Among the MXenes evaluated, V2C and Mo2C exhibited noteworthy catalytic activities, marked by low ORR overpotentials of 0.36 V and 0.39 V, respectively. The high activity is attributed to the adept binding abilities of graphene/MXene hybrids, ensuring efficient adsorption and reduction of oxygen intermediates.
• HER Performance: The hydrogen evolution reaction benefited from moderate binding strengths within these heterostructures. Notably, the heterostructures demonstrated near-equilibrium hydrogen adsorption free energies, endorsing them as competitive alternatives to conventional platinum catalysts.
• Synergic Catalytic Mechanism: The synergic interactions between the MXene substrates and graphene's surface facilitated robust catalytic activities, mainly by tuning the electronic band centers and work function of the hybrids. This approach aligns catalytic activity with theoretically favorable band energetics and adsorption potentials.

Implications and Future Directions

The insights offered in the paper pave the way for advancing the development of MXene-based hybrids as multifaceted electrocatalysts. By exploiting the synergies in electronic structures and charge distribution, researchers can strategically select MXene compositions and functionalize carbon materials to optimize catalytic performance across various electrochemical applications.

The paper also suggests that further exploration into other 2D compositions, such as metal nitrides, could diversify the catalytic capabilities of these hybrids. The paper's findings underscore the significance of electronic structural modulation as a design criterion, advocating more nuanced efforts to experimentally validate and refine these theoretical models.

In summary, the research delineated in this paper marks a constructive step toward enhancing the efficiency and versatility of composite materials in electrocatalytic processes. By embracing the dynamics of MXene/graphene heterostructures, it lays foundational concepts for the rational design of next-generation catalysts tailored for flexible and sustainable energy technologies.