Giant and bidirectionally tunable ionic thermopower in Cu-based liquid flow thermocells enabled by solvation entropy for low-grade heat harvesting

Abstract

Ionic thermocells (iTCs) are promising candidates for efficiently harvesting low-grade waste heat. Their integration inevitably requires different p/n-type redox units in series to output sufficient voltage. However, the possible redox pairs are fairly limited, especially n-type, and furthermore the precious metals used as electrodes are commonly adopted to improve the power density, which significantly increases the complexity and cost of the integrated device. Herein, we demonstrated a cost-effective and facile high-entropy Cu-based liquid-flow thermocell (LiTC) with giant and bidirectional ionic thermopower (S_i) through tunable solvation entropy. The Cu-based LiTCs consisted of a Y-shaped channel with Cu electrodes as the walls and CuSO₄/H₂SO₄ injected separately from Y-shaped inlets. The significant change in solvation entropy induced by the interaction of Cu²⁺ and H⁺/SO₄²⁻ can achieve n–p conversion and present a tunable S_i range of −33 ∼ +2.8 mV K⁻¹ for different CuSO₄ concentrations, which are 4.7–55 times higher than those of pristine iTCs. The P_max/ΔT² can reach the highest value of 1.58 mW m⁻² K⁻² with separate flows of 0.01 mol L⁻¹ CuSO₄ and 0.1 mol L⁻¹ H₂SO₄ by using foam Cu electrodes, higher than the previous P_max/ΔT² records of Cu-based iTCs. The Cu-based LiTCs with a low cost-performance metric of ∼1.15 $ W⁻¹ and electrolyte forced convection have great potential to realize efficient heat-to-electricity conversion and thermoregulation for high-power devices.