Microcalorimetric quantification of hydrogen adsorption thermodynamics in water-solvated systems on Pt/C.

Abstract

Adsorption of simple gas phase molecules (e.g., H₂) on metal nanoparticles in the aqueous phase link thermo- and electrocatalysis communities through common elementary steps. Yet, a key facet of this linkage remains incomplete: the effects of water solvation on coverage-dependent adsorption thermodynamics, the migration and speciation of adsorbates across surfaces, and related electrical polarization, eludes current understanding but represents a necessary benchmark to relate computational and experimental investigations of such systems. Here, we describe an experimental approach to quantify adsorption thermodynamics of hydrogen, a species relevant for both thermo- and electrocatalysis in the condensed phase, utilizing volumetric adsorption uptakes, microcalorimetric assessments of adsorption enthalpies, and in situ measurements of catalyst open circuit potentials (E_cat) for water-wetted Pt nanoparticles dispersed on carbon supports as a model system. Precise control of H₂O thermodynamic activity and hydrogen fractional coverages reveals nearly constant molar enthalpies of adsorption (ΔH_ads = -32 vs. -27 kJ per mol-H) coupled with greater entropy losses (ΔS_ads = -100 vs. -62 J per mol-H per K) upon introducing H₂O. Additionally, hydrogen uptakes increase drastically in the presence of coadsorbed water and exceeds 20 mol-H per mol-Pt_surf, which indicates chemical species migrate from Pt nanoparticles to the carbon support. Analysis of adsorption free energies and E_cat measurements indicate that these migrated species remain bound as hydronium-electron pairs dispersed across the carbon support following the equilibrium of Tafel (H₂ + 2* ⇌ 2H*) and Volmer (H* + H₂O ⇌ H₃O⁺ + e^- + *) elementary steps commonly invoked in hydrogen evolution electrocatalysis alongside an electrostatic capacitive interaction step (H₃O⁺ + C^- ⇌ H₃O⁺⋯C^-). Dissociative adsorption of H₂ proceeds more rapidly in the presence of co-adsorbed water as a consequence of hydronium shuttling enabled by the Volmer step. This case study illustrates a generalizable methodology to directly measure thermodynamic quantities for molecular and dissociative adsorption at solid-liquid interfaces at controlled thermodynamic activities of all species. We anticipate this form of measurement will serve as a foundation for connections between theory and experiment in pursuit of increasingly complex descriptions of chemical reactions in these environments.