kW-grade unitized regenerative fuel cell stack design for high round-trip efficiencies.

• The concept of bifunctional channel is proposed to balance water–gas management. • New bifunctional flow channel design index is proposed. • A kW-grade URFC stack is revealed with the RTE over 45%. • Good bifunctional performance is due to the improvement of mass transfer. Unitized regenerative fuel cell (URFC) is a promising electrochemical device, which can function either in fuel cell (FC) mode or water electrolysis (WE) mode. However, few applications of kW-grade URFC stack are available due to their low round-trip efficiency. In this study, a new approach to URFC stack design is provided to break the limitations of round-trip efficiency (RTE) and output power, by introducing the concept of bifunctional flow channels for the first time. Hence, a two-phase flow model based on the volume of fluid (VOF) method for bifunctional flow channels is implemented first. It reveals that URFC suffers from the issues of water adhesion on the channel wall in FC mode and the problem of air film coverage on the GDL surface in WE mode. To address this, water volume fraction in FC mode and gas coverage fraction of GDL in WE mode are proposed as new bifunctional flow channel design index, and the optimized configuration of the bifunctional flow channel is provided to balance the drainage and exhaust capabilities. Eventually, a high power-density URFC stack is demonstrated to achieve a high RTE of 45 %, with an output power of 6.01 kW in FC mode and a power requirement of 18.16 kW in WE mode. Moreover, validation experiments show that the good bifunctional performance is since the little water/gas transport problem of the URFC at high current densities. Further sensitivity experiments of the operating conditions also show that the mass transfer capacity of URFC is well adapted to various operating conditions, which implies that the bifunctional flow channel successfully balances good drainage and exhaust capabilities. [ABSTRACT FROM AUTHOR]