## 1 Introduction  
A proton exchange membrane (PEM) fuel cell is a power generation device which directly converts the chemical energy stored in fuel and oxidants into electric energy via electrochemical reactions. Its advantages include high energy conversion efficiency, no emissions of nitrogen oxides and sulfur compounds, low operating noise, and high reliability. As one of the important components of a PEM fuel cell, flow field accounts for about 60-80% of the total weight of the fuel cell and 30-50% of the total cost [1; 2; 3], which are key factors that can influence the output performance and cost of the fuel cell. Thus, this area is one of the focuses of current scientific research. At the macro level, the structure of the flow field has two main functions. On the one hand, it provides fuel and oxidants for the electrochemical reaction of the whole cell and discharges unreacted gas and reaction products (mainly water); on the other hand, the flow field collects the current generated by the electrochemical reaction. In brief, the structure of flow field has a great effect on the output performance of PEM fuel cells, and thus affects the commercialization. Although many types of flow fields have been studied and reported in the literature, such as parallel flow field [4; 5; 6; 7; 8], serpentine flow field [4; 9; 10], interdigitated flow field [11; 12], and so on, every type has advantages and disadvantages. However, there are still many challenges to be overcome to enhance the performance of flow fields, therefore, it is necessary for researchers to study and optimize the flow field structure. In an experimental simulation of PEM fuel cell flow field, [PERSON] et al. [13] designed novel flow field \"speed bumps\" to generate pressure fluctuation in the flow passage, and then numerically simulated its impact on PEM fuel cell performance. The results showed that \"speed bumps\" improved the polarization curve performance of proton exchange membrane fuel cells (PEMFCs). [PERSON] et al. [14] and [PERSON] et al. [15] proved with their experiments and simulations that the protruding part of the flow field significantly improved the drainage capacity and the efficiency of oxygen transfer to the catalyst layer, so as to improve the performance of PEM fuel cells at medium and low potentials. [PERSON] et al. [16] simulated the flow path and protrusion of the flow field and found that the oxygen transfer capacity and drainage capacity were significantly improved. [PERSON] et al. [17] studied the different performances of PEM fuel cells in parallel flow fields under different blocking conditions. Their simulation results demonstrated that the performance of parallel flow fields with alternating blockage was not different from that of a serpentine flow field. [PERSON] et al. [7] and [PERSON] et al. [18] simulated the influence of changing flow field lengths and cross-sections on PEM fuel cell performance. The results showed that these two factors played important roles in the design of PEM fuel cell flow fields. [PERSON] et al. [19; 20] proposed a new shunt design for a parallel cathode flow field, and studied the polarization curve, pressure drop, cathode oxygen distribution, and water management of the fuel cell via experiments and simulations, and then optimized and analyzed the shunt design. [PERSON] et al. [21] studied the water transfer and discharge in a flow field by using a hydrophilic field plate. The results indicated that the hydrophilic field plate was useful in the discharge of water, which is of great significance for PEM fuel cell water management. By means of numerical simulation, [PERSON] et al. [22] studied the effect of temperature on a high-temperature proton exchange membrane (HT-PEM) fuel cell output power, with results showing that the output power of the HT-PEM fuel cell increased with increasing temperature.