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DOE OSTI2025

Universal Reversible Hydrogen Potential for Electrocatalytic Ammonia Splitting Reactions in Nonaqueous Solvents from Unified pH Measurements

In this work, we introduce a new approach of using differential potentiometric measurements in four nonaqueous solvents─MeCN, THF, DMF, and PC─to determine the universal pH abs H 2 O values aligned to the aqueous pH scale for dilute NH 4 + /NH 3 solutions. Knowledge of the pH abs H 2 O values allows simple determination of the reversible hydrogen potential in any given solvent relative to the aqueous standard hydrogen electrode (SHE) and, most importantly, ensures comparability across different solvents. As an independent method, Open Circuit Potenial measurements were carried out in the same solvents titrated with NH 4 + /NH 3 to obtain alternative values for the reversible hydrogen potential in these solvents. The close agreement of these two methods, as well as calculated potentials from literature values when available, substantiates the new, simpler, and more robust approach to determine the reversible hydrogen potential introduced here. We further use the reversible hydrogen potential values established here to report, for the first time, the overpotential for ammonia oxidation as a function of solvent, with a recently discovered ruthenium catalyst.

ammonia↗
DOE OSTI2026

Multi-Spectroscopic Determination of Exchange Coupling, Zero-Field Splitting, and g-Matrices in Radical-Bridged Dinuclear Fe(III) Complexes

When the energy gap, Δ, between the lowest-lying spin manifolds within a spin-exchange coupled molecule approaches Δ/k B ≈ 300 K, the traditional temperature-dependence (T < 400 K) of the molar magnetic susceptibility is not always a reliable way to obtain a good estimate of intramolecular exchange couplings. We develop a spectroscopic approach capable of accurately parametrizing complex magnetic Hamiltonians by exploiting the separation of the anisotropy and exchange energy scales in strongly coupled magnetic molecules. Specifically, we combine inelastic neutron scattering, high-frequency electron paramagnetic resonance, far-infrared magneto-spectroscopy and magnetometry, and obtain detailed information about the magnetic properties of a series of diiron complexes derived from [[Fe(cth)] 2 (dxbq)] 3+ (H 2 dxbq: 2,5-dihydroxy-1,4-benzoquinone (x = h) or 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone (x = c), cth: 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Well-isolated S = 9/2 ground states emerge due to strong direct antiferromagnetic exchange between the Fe 3+ centers (S = 5/2) and the radical bridging benzoquinone ligand (S = 1/2). The specific sensitivities and transition selection rules of the applied methods allow us to determine the parameters of the microscopic Hamiltonian including exchange coupling, fourth-order Stevens operators and g-factors. Our methodology is directly portable to other strongly coupled molecular compounds.

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