Thermodynamic Stability, Mechanistic Evaluation, and Electronic Properties of Polyoxoniobate-Based SACs (TM@PONb): A Promising Multifunctional Electrocatalyst for HER/OER/ORR

Polyoxometalate-based single-atom catalysts (POM-SACs) have garnered significant interest owing to their ability to effectively catalyze transition metals (TMs) through tailored coordination environments provided by the polyanionic surfaces of POMs. With the accelerating advancement of renewable energy technologies, there is a growing demand for highly efficient and economically viable multifunctional electrocatalysts that can simultaneously drive the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). This study thoroughly investigated the HER, OER, and ORR performance of a series of TM anchored hexaniobate anion clusters [(Nb6O19)]8–, also known as Lindquist-type polyoxoniobate (PONb) clusters, as SACs employing spin-polarized density functional theory calculations via Grimme’s D3 dispersion correction. According to recent experimental studies, introducing foreign dopant atoms onto polyoxometalate (POM) surfaces has been shown to significantly enhance the electronic properties and catalytic activity. We first identify the most favorable dopant-site on the PONb surface and confirm its thermal stability through ab initio molecular dynamics (AIMD) simulations. The structural, thermodynamic, and electrochemical stability are evaluated by determining the binding energy (Eb), cohesive energy (Ec), formation energy (Efor), and dissolution potential (Udiss). In the HER mechanistic analysis, we found that Sc@PONb (ΔGH* = −0.01 eV) is the best catalyst for the Volmer–Tafel mechanism, whereas Os@PONb (ΔGH* = −0.01 eV) is a superb catalyst via the Volmer–Heyrovsky step. In our work, Cr@PONb (ΔGH* = 0.01 eV) exhibited superior HER activity at the Volmer step. The best OER/ORR activity was demonstrated by Pd@PONb and Fe@PONb catalysts. We notice that Pd@PONb exhibits excellent catalytic activity, making it a trifunctional electrocatalyst for the HER via the Volmer–Tafel mechanism (ΔGH* = 0.07 eV), as well as OER (ηOER = 0.24 V) and ORR (ηORR = 0.43 V). However, the Fe@PONb serves as a bifunctional electrocatalyst for the OER (ηOER = 0.41 V) and ORR (ηORR = 0.53 V). Based on the solvation model findings on our optimal OER/ORR catalyst, we attain significantly reduced overpotential for the Pd@PONb catalyst (ηOER/ORR = 0.10/0.25 V). This study lays a foundation for the rational design of multifunctional polyoxometalate-based single-atom catalysts (POMs-SACs), offering promising prospects for advanced energy conversion and production technologies.