Cathode catalyst layer (CL) design is important for improved performance, durability and stability of proton exchange membrane fuel cells (PEMFCs). An effective cathode CL must serve multiple functions simultaneously: electron and H⁺ conduction, O₂ or H₂ supply, and effective water management. The reaction in the CL requires three-phase boundaries (or interfaces) among ionomer (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as voids (reactant diffusion); an optimized cathode CL structure should have micropores and mesopores to balance water egress and O₂ diffusion.

Our previous research showed that increasing microporosity in the cathode CL increases the power production of the PEMFCs [1]. Modeling indicates that high CL microporosity enhances water evaporation, which reduces mass transport resistances to enable higher power production [2]. Meanwhile, low microporosity encourages liquid water retention.

It is known that catalyst support type has a profound effect on electrode porosity, influencing both micro- and mesoporosity [3, 4]. By constructing electrodes with layers of different carbon supported catalyst types, we demonstrate that we can create porosity gradients in the CLs that vary the electrode water management properties and the power of the PEMFC. Two different types of commercial platinum/carbon electrocatalysts are used in dual cathode CLs – 40 wt. % Pt on Vulcan carbon (Pt/VC) having low micropores and low porosity and 40 wt. % Pt on Ketjen black (Pt/KB) having large micropores and high porosity. CLs are created with both homogeneous porosity in the Z direction (Figure 1a), and compared to ones with non-uniform porosity across the Z-direction (Figure 1b).

The microstructure of these cathode CLs is characterized using N₂-sorption porosimetry. The CLs are assembled into PEMFCs and probed with polarization curves (at 100, 50 and 25% RH), cyclic voltammetry, O₂ gain, electrochemical impedance spectroscopy, and limiting current measurements to quantify the sources of mass transport resistance.

The results indicate that the regions with low microporosity encourage liquid water retention, while high microporosity encourages water evaporation. These countering actions are balanced for high CL hydration and gas diffusion, and better PEMFC performance.

References

[1] Y. Garsany, R.W. Atkinson, M.B. Sassin, R.M.E. Hjelm, B.D. Gould, K.E. Swider-Lyons, J. Electrochem. Soc., 165 (2018) F381-F391.

[2] M. Eikerling, J. Electrochem. Soc., 153 (2006) E58-E70.

[3] T. Soboleva, K. Malek, Z. Xie, T. Navessin, S. Holdcroft, ACS Appl. Mater. Interfaces, 3 (2011) 1827-1837.

[4] Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, Journal of Power Sources, 315 (2016) 179-191.