MXene-supported single-atom or cluster catalysts have emerged as a promising class of materials for electrochemical nitrogen (N2) reduction to ammonia (NH3). The unique properties of MXenes, particularly Ti3C2Tx, make them ideal substrates for stabilizing highly active catalytic sites. Their high electrical conductivity, tunable surface chemistry, and abundant anchoring sites enable efficient N2 adsorption and activation, leading to improved Faradaic efficiency and NH3 yield compared to conventional transition metal sulfide catalysts.
### Synthesis and Surface Engineering of Ti3C2Tx MXenes
The synthesis of Ti3C2Tx MXenes typically involves selective etching of the aluminum (Al) layer from the parent MAX phase (Ti3AlC2) using hydrofluoric acid (HF). The process begins with immersing Ti3AlC2 powder in concentrated HF (e.g., 49% HF) for several hours at room temperature. The Al layers are selectively removed, resulting in a multilayered Ti3C2Tx structure with surface terminations (-Tx) such as -O, -F, and -OH. The etching time and HF concentration influence the degree of delamination and the distribution of surface functional groups.
After etching, the multilayered MXene is often subjected to intercalation and sonication to produce few-layer or monolayer Ti3C2Tx nanosheets. The surface terminations play a critical role in catalytic performance. Oxygen-rich terminations (-O) enhance N2 adsorption due to their electron-donating properties, while fluorine (-F) terminations can modulate the electronic structure of anchored metal sites. Post-synthesis treatments, such as thermal annealing in inert or reducing atmospheres, can further tailor the surface chemistry to optimize catalytic activity.
### Single-Atom and Cluster Catalysts on MXene Supports
Single-atom catalysts (SACs) and metal clusters anchored on Ti3C2Tx MXenes exhibit exceptional activity for electrochemical N2 reduction. The high surface area and defective sites on MXenes provide strong metal-support interactions, preventing aggregation of active sites. Transition metals such as Fe, Mo, and Ru are commonly dispersed as single atoms or small clusters on MXene surfaces.
The anchoring mechanism involves coordination between metal atoms and the oxygen or fluorine terminations of MXenes. For example, Fe single atoms may bind to -O sites, forming Fe-O-MXene configurations that facilitate electron transfer during N2 activation. Density functional theory (DFT) calculations suggest that the negatively charged MXene surface stabilizes metal centers in low oxidation states, promoting back-donation of electrons to the N2 antibonding orbitals.
### N2 Adsorption and Activation Mechanisms
The electrochemical N2 reduction reaction (NRR) on MXene-supported catalysts proceeds through associative or dissociative pathways. In the associative mechanism, N2 molecules adsorb end-on or side-on onto the metal sites, followed by sequential protonation to form NH3. The dissociative mechanism involves cleavage of the N≡N triple bond before hydrogenation.
MXene-supported SACs favor the associative pathway due to the moderate binding strength of N2. The presence of -O terminations enhances N2 adsorption by polarizing the N2 molecule, weakening the N≡N bond. In contrast, -F terminations may alter the charge distribution around metal centers, influencing the energetics of N2 dissociation. Operando spectroscopic studies have confirmed that the rate-determining step is often the first protonation of adsorbed N2 (*N2 → *NNH), where MXene-supported catalysts exhibit lower overpotentials compared to bulk metal electrodes.
### Performance Comparison with Transition Metal Sulfides
Transition metal sulfides (e.g., MoS2, FeS2) have been widely studied for NRR due to their sulfur-rich surfaces that mimic nitrogenase enzymes. However, MXene-supported catalysts demonstrate superior performance in terms of NH3 yield and selectivity.
For instance, Fe single atoms on Ti3C2Tx MXene have achieved NH3 production rates exceeding 30 μg h⁻¹ mgcat⁻¹ at -0.3 V vs. RHE, with Faradaic efficiencies above 20%. In contrast, MoS2-based catalysts typically show NH3 yields below 10 μg h⁻¹ mgcat⁻¹ under similar conditions. The higher activity of MXene-supported catalysts is attributed to their superior electrical conductivity, which minimizes charge transfer resistance, and the tailored metal-support interactions that optimize N2 binding energy.
A comparison of key performance metrics is summarized below:
Catalyst System | NH3 Yield (μg h⁻¹ mgcat⁻¹) | Faradaic Efficiency (%) | Overpotential (V vs. RHE)
Ti3C2Tx-Fe SAC | 30-35 | 20-25 | -0.3
MoS2 nanosheets | 8-10 | 10-15 | -0.4
The lower overpotential and higher selectivity of MXene-based systems highlight their potential for energy-efficient NH3 synthesis.
### Challenges and Future Directions
Despite the promising performance, challenges remain in scaling up MXene-supported catalysts for industrial applications. The stability of MXenes under prolonged electrochemical cycling needs improvement, as oxidation or restacking can degrade performance. Strategies such as covalent functionalization or encapsulation in protective matrices may enhance durability.
Future research should focus on optimizing the metal loading and termination balance (-O vs. -F) to further boost NRR activity. Advanced characterization techniques, such as in situ X-ray absorption spectroscopy, can provide deeper insights into the dynamic changes in catalyst structure during N2 reduction.
In summary, MXene-supported single-atom and cluster catalysts represent a significant advancement in electrochemical N2 reduction. Their tunable surface chemistry and strong metal-support interactions enable high NH3 yields and selectivity, outperforming traditional transition metal sulfides. Continued development of these systems could pave the way for sustainable ammonia production.