Space–surface charge synergy for enhanced ionic thermoelectric power in nanofluidic channels

Space charge and its synergistic interaction with surface charge have recently been shown to enhance ion transport in regions of weak charge density within the electrical double layer (EDL) in nanofluidic systems for osmotic energy conversion. However, the influence of space charge on ionic thermoelectric power generation has not yet been explored. To address this gap, this study employs numerical simulations based on the coupled non-isothermal Poisson–Nernst–Planck, Stokes–Brinkman, and energy equations to investigate the role of space charge and its synergy with surface charge in governing the ionic thermoelectric response in nanochannels containing space-charge materials. Three charge models are considered: surface charge, space charge, and a combined all-charge model. The results demonstrate that the introduction of space charge significantly enhances both the ionic Seebeck coefficient and the thermoelectric current, primarily due to the strengthened EDL effect and enhanced temperature-dependent ion electrophoretic mobility (TDIEM) and thermoosmotic flow (TOF) effects. Furthermore, the coexistence of surface and space charges gives rise to a pronounced synergistic effect, which depends on nanochannel radius, length, charge density, electrolyte concentration, and temperature difference, leading to a substantial enhancement in ionic thermoelectric power output. A normalized maximum power density exceeding 100 mW/m 2 ∙K 2 can be achieved. Notably, the maximum synergistic effect is achieved under partial-to-weak EDL overlap conditions, corresponding to a Debye–Hückel parameter ( r n / λ D ) of approximately 1.3–3.7, and is further enhanced by increasing temperature gradient and charge density. The underlying mechanisms of space charge and charge synergy are systematically elucidated, providing critical insights for the design and optimization of ionic thermoelectric energy conversion devices in nanofluidic systems.