Strain engineering in two-dimensional materials has emerged as a powerful strategy to tailor their electronic, catalytic, and mechanical properties for targeted applications. In electrochemical ammonia synthesis via the nitrogen reduction reaction (NRR), strain-modified 2D materials such as graphene and hexagonal boron nitride (hBN) offer unique advantages, including tunable d-band centers, suppressed hydrogen evolution reaction (HER), and enhanced Faradaic efficiency. These materials present a promising alternative to the energy-intensive Haber-Bosch process, which dominates industrial ammonia production but suffers from high carbon emissions and thermodynamic inefficiencies.
The electronic structure of 2D materials is highly sensitive to mechanical strain due to their atomic thinness and lack of out-of-plane bonding. Applying tensile or compressive strain alters the interatomic distances and hybridizations, leading to shifts in the d-band center relative to the Fermi level. For transition metal-doped graphene or hBN, this shift directly influences the binding strength of nitrogen intermediates. A moderate downshift of the d-band center weakens the adsorption of *N2 while maintaining sufficient activation for dissociation, striking a balance between reactivity and selectivity. Strain-induced charge redistribution also facilitates electron transfer to the antibonding orbitals of N2, lowering the energy barrier for the potential-determining step in NRR.
A critical challenge in electrochemical ammonia synthesis is the competing HER, which consumes protons and electrons, reducing Faradaic efficiency. Strain engineering mitigates HER by modifying the hydrogen adsorption free energy (ΔGH*). In strained graphene, for instance, the local curvature or lattice distortion increases the energy cost for hydrogen adsorption, favoring NRR over HER. Studies have shown that biaxially strained graphene with a 5% elongation can suppress HER by over 60% while maintaining NRR activity. Similarly, strained hBN exhibits a widened bandgap under tension, further inhibiting parasitic proton reduction.
Faradaic efficiency benchmarks for strain-engineered 2D materials vary with applied potential and electrolyte conditions. Unstrained graphene typically achieves efficiencies below 10% due to HER dominance, whereas strained variants have demonstrated improvements to 25–35% in acidic media and up to 40% in alkaline conditions. Boron-rich hBN under 4% compressive strain has shown a Faradaic efficiency of 32% at −0.4 V vs. RHE, attributed to preferential stabilization of *NNH intermediates. These values remain below industrial requirements but represent significant progress over unmodified catalysts.
The Haber-Bosch process, despite its inefficiencies, remains the primary method for ammonia synthesis due to its scalability and high yields. Operating at 400–500°C and 150–300 bar, it relies on iron or ruthenium catalysts to achieve single-pass conversions of 10–20%. Electrochemical NRR, in contrast, operates at ambient conditions but faces lower yields and durability issues. However, the energy input for Haber-Bosch exceeds 30 GJ/ton NH3, primarily due to fossil-derived hydrogen production, while renewable-powered NRR could reduce this to under 20 GJ/ton if selectivity and activity improve.
Material stability under strain is another consideration. Repeated electrochemical cycling can relax induced strain, leading to performance degradation. Strategies such as substrate pinning or heterostructure design help maintain strain over long-term operation. For example, graphene grown on shape-memory alloys retains tunable strain levels via thermal or electrical stimuli, enabling dynamic optimization of NRR activity.
Future advancements in strain-engineered 2D materials for NRR will require precise control over strain gradients and uniformity. Techniques like moiré patterning or atomic layer misfit strain offer sub-angstrom precision, enabling localized activation sites for N2 dissociation. Coupling strain with defect engineering, such as nitrogen vacancies in hBN, could further enhance catalytic performance. While challenges remain in scaling and durability, the fundamental advantages of strain-tunable 2D materials position them as a viable pathway toward sustainable ammonia synthesis.
The transition from lab-scale demonstrations to industrial adoption will depend on achieving Faradaic efficiencies above 60% at current densities exceeding 10 mA/cm2. Competing approaches, such as lithium-mediated NRR or hybrid biological-electrochemical systems, offer alternative pathways but face their own limitations in cost and complexity. Strain-engineered 2D materials, with their versatility and compatibility with existing semiconductor fabrication techniques, represent a compelling avenue for decarbonizing ammonia production.