Experimental insights into the trade-off in thermally regenerative electrochemical batteries

Abstract

Notwithstanding the prevailing global energy crisis and the concomitant environmental pollution, the world's demand for electricity and cooling is continuing to grow at a rapid rate. The thermally regenerative electrochemical battery (TREB) has emerged as a promising solution that can provide switchable power generation and cooling capabilities both environmentally and efficiently. Nevertheless, despite ongoing research, a fundamental trade-off between specific heat capacity and entropy change, the latter being governed by temperature coefficient and internal resistance, remains largely unexplored experimentally, limiting the optimization and practical deployment of TREBs. Hence, this study proposes a dimensionless parameter, Θ, which consolidates the thermal properties from the perspective of electrolyte concentration, thereby offering an effective means to elucidate the key trade-off. Results show that increasing Θ leads to a decrease in temperature coefficient and specific heat capacity, but the internal resistance exhibits a non-monotonic trend due to the combined effect of osmotic pressure. For power generation, the relative Carnot efficiency initially rises with Θ and then declines, reaching a maximum of 33.52 % when $\Theta =0.30$, accompanied by a peak power density of 12.85 mW/g. It also highlights the significance of heat recovery to mitigate the negative impact of high specific heat capacity under low electrolyte concentrations. Under cooling conditions, the Coefficient of Performance relative to the Carnot limit across each Θ remains outstanding, ranging from 86.17 % to 68.39 %. This work removes a key barrier to TREB optimization by systematically exploring the trade-off mechanism and offering a concentration-based strategy for performance improvement.