Mechanochemical graphene exfoliation with succinic acid to confine Ni nanoclusters for electrochemical nitrate reduction

Developing a sustainable synthesis method for functionalized graphene remains in a great demand. In this work, we developed a mechanochemical method for graphene production from graphite with succinic acid as the functionalization agent. Succinic acid effectively functionalized the surface of graphite with oxygen containing groups with the mechanical energy generated during ball milling, resulting in smooth exfoliation of graphene sheets. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations calculations revealed that the intact carboxyl –COOH group of the succinic acid interacts most favorably with the graphene surface, exhibiting a significantly lower adsorption energy barrier compared to the epoxyl >O and hydroxyl –OH groups. This result helps understand the thermodynamically predominant role of –COOH group in the functionalization and exfoliation process. On the other hand, the enriched oxygen groups provide abundant nucleation sites, obtaining smaller Ni nanoclusters (8–10 nm) on the graphene. Using the exfoliated graphene doped with highly dispersed Ni nanoclusters for electrochemical nitrate reduction yielded an outstanding ammonia production rate of 12.9 mg·cm⁻²·h⁻¹ at −1.0 V versus reversible hydrogen electrode (RHE), with remarkable Faradaic efficiency of 90% and selectivity of 78%. Overall, this study establishes a sustainable and scalable method to synthesize functionalized graphene for efficient electrochemical conversion from nitrate to ammonia.