A flow field with superior rib-under mass transfer: Dual-boost for thermally regenerative flow battery output and thermoelectric conversion efficiency

The Serpentine Flow Channel (SFC) has been proven to have great potential in enhancing the performance of Thermally Regenerative Ammonia-based Flow Battery (TRAFB). However, its existing design is limited by the uniform short rib structure, making it difficult to achieve stronger under-rib mass transfer, which has become a key bottleneck restricting the further performance improvement of TRAFB. To mitigate this limitation, this study designed a novel Alternating-path Serpentine Flow Channel (ASFC) for the TRAFB by systematically reconstructing the flow path arrangement of the traditional SFC. The flow field plate of the ASFC incorporates several long rib structures. The results show that the ASFC enhances convective mass transfer in the under-rib electrode region and suppresses the formation of mass transfer dead zones and reactant-starved zones. As the channel-to-rib width ratio or transverse-channel number of the ASFC increases, the output performance, energy storage capacity, and low-grade waste heat recovery efficiency of the TRAFB first increase and then decrease slightly, with a channel-to-rib width ratio of 1:1 and a transverse-channel number of 15 being the best parameter combination. On this basis, a total of 16 ASFC configurations have been derived by adjusting the number and sequence of sub-paths. Among these, the best configuration enables the TRAFB to achieve a peak net power density of 243.15 W m−2 (approximately 19.60 % higher over traditional SFC) and a Carnot relative efficiency of 48.72 % (surpassing the traditional SFC by about 10.74 %). Increasing the sub-path number does not always improve TRAFB performance; a positive effect can only be achieved when the parity (odd or even nature) of the sub-paths remains consistent. Furthermore, three necessary conditions for achieving high-performance ASFCs are revealed: the sub-path number must be odd; Sub-path 1 and Sub-path 2 should be arranged at the start and the end respectively; and the sub-path containing the flow channel outlet should be arranged in the middle. These provide design heuristics for practical engineering scale-up.