Interfacial Engineering for proton transfer pathways in electrochemical Low-voltage H2 storage

Abstract

The global pursuit of carbon neutrality has intensified the transition toward hydrogen as a clean and high-energy-density carrier for sustainable energy systems. This dissertation systematically explores the interfacial engineering principles governing hydrogen generation, storage, and conversion across electrochemical systems. The discussion begins with the fundamentals of conventional water electrolysis, highlighting their operational mechanisms, material designs, and limitations imposed by the sluggish oxygen evolution reaction (OER) and high cell voltages (> 1.8 V). To mitigate these kinetic and energetic barriers, low-energy electrochemical strategies are introduced, where anodic OER is replaced by the oxidation of small organic and inorganic molecules such as formic acid, ethanol, hydrazine, ammonia, and hydrogen peroxide. These reactions, characterized by lower redox potentials (−0.33 to 0.69 VRHE), enable hydrogen production at substantially reduced voltages. Building upon efficient hydrogen generation, the work extends to hydrogen storage coupling with electrochemical systems. Traditional thermochemical hydrogenation of liquid organic hydrogen carriers (LOHCs) such as the toluene/methylcyclohexane pair requires high-temperature and high- pressure operation. In contrast, the electrochemical palladium membrane reactor (ePMR) offers low- voltage alternative by directly coupling water electrolysis with in-situ hydrogen absorption, diffusion, and spillover across a palladium membrane. This electrochemical approach enables controllable hydrogenation under ambient conditions, bridging hydrogen production and storage toward sustainable, distributed LOHC-based hydrogen systems. In the conversion domain, ammonia emerges as a key carbon-free energy carrier. The sesction compares three promising electrochemical routes such as lithium-mediated nitrogen reduction (Li- NRR), direct N₂ reduction (NRR), and nitrate electroreduction (NO₃RR). Among them, NO₃RR offers a particularly attractive pathway due to its lower activation barriers and high selectivity for ammonia formation under mild conditions. The mechanistic distinction between the electron-transfer route and the hydrogen-atom-mediated route highlights how surface H+/H* relay through co-adsorbed oxoanions governs the selectivity toward NH₃ formation. Finally, the dissertation integrates these concepts through interfacial engineering. The strategic control of molecular orientation and proton dynamics at solid–liquid interfaces. On the anodic side, orientation control through ligand and steric effects tunes adsorption geometry and reactivity of bulky molecular substrates. On the cathodic side, cooperative electronic and electrolyte effects modulate interfacial H⁺/H* behavior, governing proton-coupled electron transfer (PCET) and hydrogen spillover processes. Collectively, these principles establish a unified framework for low-voltage hydrogen production, liquid-phase storage, and selective nitrogen conversion, paving the way for next-generation electrocatalytic energy systems that integrate.