Tailoring Heat and Mass Transfer for Next Generation of PEM Fuel Cells
Authors: Penga Željko , Xing Lei , Penga Jure , Vidović Tino , Vukoja Tomislav , Radica Gojmir , Qu Xian , Hacker Viktor 
Published: December 2024

The distribution of operating parameters inside PEM fuel cells under dynamic operating conditions is non-uniform. Designing operating conditions using constant Pt loading and temperature seems to result in a high number of problems for operation due to non-optimized heat and mass transfer. During operation using humidified reactants, large quantities of liquid water are accumulated inside the cell in porous layers and for this reason, it is very difficult to achieve high current densities without purging the cell which results in large waste of hydrogen fuel. If the reactants are not fully humidified, during operation at constant operating temperature, the ionomer in membrane and catalyst layers is prone to dehydration and starvation. By using a spatially variable temperature profile, i.e. variable temperature flow field, it is possible to achieve a delicate balance between the rate of produced water and water vapor partial pressure in such a manner as to achieve fully humidified conditions along the entire active area of the cell without the necessity for external humidification. This concept was studied using a combination of computational fluid dynamics and experimental research and it was shown that the application of a variable temperature flow field can be achieved using liquid coolant with a sufficiently low flow rate which is gradually heated up during the flow through the cell by utilizing the heat generated by the cell for establishing and maintaining the desired temperature profile. This concept was further improved by using a variable temperature profile in conjunction with a graded catalyst design. An experimental and numerical study was conducted to determine the optimal combination of variable temperature and graded Pt loading in the cathode catalyst which will show the highest performance. By comparing the optimal design with variable temperature and graded catalyst vs. isothermal operation with uniform catalyst loading, it was found that the performance can be dramatically increased. The operating current density at 0.6 V for the optimal case was 260% higher while Pt utilization was 19% lower when compared to the baseline isothermal constant catalyst loading case with notably enhanced current density distribution. To further enhance the performance we have designed and optimized novel flow fields with secondary channels for water removal near the cathode outlet using 3D metal printing and the results have shown that it is possible to achieve higher current densities with this simple flow field modification. Our investigation of the novel flow fields also included biomimetic design where we based our flow field on the design of shark skin, which was manufactured using a high-end milling machine with vibration compensation. This novel design utilizes inertial effects in the diffusion layers due to high flow velocities which is a novelty for PEM fuel cells since the flow velocities are commonly in a laminar regime. Utilization of these different new concepts has shown that it is possible to significantly enhance the performance of PEM fuel cells with combined numerical and experimental efforts and elucidates the direction of the research toward the development of tailored anisotropic heat and mass transfer for the next generation of fuel cells with improved performance and durability.
Funding: This work has been supported by the Croatian Science Foundation under the project IP.2020-02-6249.

Figure 1