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Highlights • Design and fabrication of BPP is further introduced based on our previous study. • Round corner and clearance of moulds play important role in maximum channel height. • Channel dimensions are selected by formability model and reaction efficiency. • Channel height, channel thickness and effect of blank holder are discussed. • Performance of single cell and short stack is tested to evaluate method reliability. Abstract This study offers an efficient design method of flow channels of metallic bipolar plates (BPPs) to improve manufacturing technique of BPPs and maximize power density in proton exchange membrane (PEM) fuel cells. Stamped thin metallic BPPs with anticorrosive and conductive coating are promising candidates for replacing conventional carbon-based BPPs. Nevertheless, unlike carbon-based BPPs, the flow channel design of metallic BPPs should take into account not only the reaction efficiency, but also formability due to the possible rupture of the metallic channel during the micro-forming process. In our previous study, a forming limit model was first proposed to predict the maximum allowable channel height by the forming process. This study is conducted to further propose the method of the design and fabrication of metallic BPPs based on the numerical model. In order to determine channel geometry design from formability perspective, response surface method is utilized to build a formability model. Combining the formability model and reaction efficiency, flow field design for metallic BPPs (channel width of 0.9 mm, rib width of 0.9 mm, channel depth of 0.4 mm and radius of 0.15 mm) is proposed. Experiments on BPP fabrication and assembled 20-cell fuel cell testing are conducted to observe forming quality of micro channel and output performance on the real fuel cell. It is shown that the stamping force grows with increasing channel depth in a nonlinear manner and a blank holder is needed to eliminate the sheet wrinkle in the forming process. The uniformity of the voltage distribution in the 1000 W-class stack further proves the reliability of metallic BPPs designed by our method. The methodology developed is beneficial to the fabrication management of metallic BPPs and effective supplement to the channel design principle for PEM fuel cells. [ABSTRACT FROM AUTHOR]

This study offers an efficient method to improve manufacturing technique of metallic bipolar plates (BPPs) so as to simultaneously reduce contact resistance (CR) and maximize power density in proton exchange membrane fuel cell (PEMFC). CR plays an important role in the energy conversion in the cell and can be normally reduced by coating on the BPP surface. Nevertheless, the effect of fabrication process on the CR has not been revealed, especially for the welding and forming characteristics. In this paper, a comprehensive three-dimensional finite element model of BPP/gas diffusion layer (GDL) assembly was established to investigate the influences of coating, weld and dimensional error on the CR, which are produced during the fabrication process of metallic BPPs. Experiments were carried out to validate the accuracy of the model. The results indicate that direction and distribution of the current in the cell change significantly with altering the weld path of metallic BPPs, which are different from graphite BPPs. 47% CR reduction is observed for the case of dense weld arrangement. For the coating process, it is found that the necessity of coating on both sides of single BPP is quite low if channel number is less than 20. Statistic simulation was conducted to investigate the effect of dimensional error on CR. Specially, 14.5% increment in CR is found when the dimensional error exceeds 30 μm. The methodology developed is beneficial to the fabrication management of metallic BPPs and the efficiency improvement of PEMFC. [ABSTRACT FROM AUTHOR]

• 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]

• Method combining computational fluid dynamics model and empirical model is proposed. • Voltage distribution of unit cells shows the similar trend as the flow distribution in manifold. • U-type configuration stack promotes more uniform voltage among unit cells than Z-type. • The voltage unevenness of unit cells climbs dramatically as the cell number increased. This study offers an efficient method for commercial-size proton exchange membrane fuel cell (PEMFC) stack performance evaluation to improve designing of fuel cell and maximize power density in PEMFC stack. The flow distribution in the manifold of the stack is critical to the energy conversion of the assembled unit cells in series due to the typical short-board effect. Most existing works studying on the flow distribution focus on small fuel cell stack and does not establish a clear relationship to performance, which are not able to thoroughly guide commercial-size PEMFC stack design and development. In the present study, an effective method combining computational fluid dynamics (CFD) model and empirical model is proposed to evaluate the performance of commercial-size stack considering air flow distribution in the manifold. Firstly, the air flow distribution in the manifold is predicted by a CFD model. A performance evaluation empirical model is developed by a series of experiments to evaluate the effects of flow maldistribution on the performance of PEMFC stack. Then, the predicted flow distribution and performance are respectively validated by a novel experimental setup. Finally, the effects of stack configuration, cell number, and current density on flow distribution and performance of PEMFC stacks are discussed. The results show that U-type configuration promotes more uniform voltage among unit cells than Z-type. The voltage unevenness of unit cells caused by flow maldistribution climbs dramatically as the cell number and current density increase. The methodology developed is beneficial to the energy management and the efficiency improvement of commercial-size PEMFC stack. [ABSTRACT FROM AUTHOR]