Transport-Guided Proton Storage in Sulfur-Deficient Few-Layer MoS2 Using Concentrated Phosphoric Acid

Aqueous proton batteries based on strong mineral acids can provide high conductivity but may accelerate electrode dissolution and hardware degradation. Here, few-layer, sulfur-deficient MoS2 nanosheets are paired with concentrated phosphoric acid (H3PO4), and multinuclear NMR, self-diffusion, and conductivity measurements are used to identify a composition that balances carrier availability and mobility. The highest conductivity (192 mS·cm–1) is found at 0.18 mole fraction (8.3 M) H3PO4, where a Nernst–Einstein partition assigns ≈17% of the conductivity to vehicular H3O+, ≤17% to H2PO4, and ≈65% to structural (Grotthuss-type) transport. The binder-free MoS2-derived electrode delivers 243.0 mA·h·g–1 at 1.0 A·g–1 and retains 88% after 1000 cycles, compared with 79% in a 0.5 M H2SO4 benchmark. Density functional theory (DFT) shows that proton storage in 2H-MoS2 cannot account for this charge in the galvanostatic window (≈0.22–0.87 V vs RHE): hydrogen binding is strongly unfavorable on the basal plane (ΔGH = +2.02 eV) and in the van der Waals gallery (+1.59 eV), and sulfur vacancies (–0.17 eV) are occupied only below ≈0.17 V vs RHE. Because MoS2 becomes unstable toward oxidation above 0.37–0.47 V vs RHE and MoO3 stores protons at ≈0.1–0.35 V vs RHE, we propose that a partially oxidized MoOx surface layer stores part of the charge, with background currents contributing the remainder. Within the gallery, DFT identifies a low-barrier sulfur-mediated proton relay (S–H···S transfer, 0.06 eV) that is unoccupied at the operating potentials, and phosphate binds sulfur vacancies slightly more strongly than bisulfate, consistent with a passivating role.