Grooved Electrodes to Enhance Mass Transport in Thick Platinum Group Metal-Free Fuel Cell Cathodes

Abstract

Producing “green” hydrogen at a low cost is one of the main strategies adopted by several countries to reduce their greenhouse gas emissions.1 To enable the hydrogen utilization for transportation and stationary power generation, developing low-cost and efficient proton exchange membrane fuel cells (PEMFC) is essential.2 To date, platinum (Pt) is used as the preferred catalyst in PEMFCs due to its high catalytic activity for both hydrogen oxidation and oxygen reduction reaction (ORR).3 Due to high cost and scarcity of Pt, research has extensively focused on developing low-cost platinum group metal (PGM)-free electrocatalysts for ORR, with impressive performance improvements recently achieved.4,5 However, the lower mass activity of PGM-free catalysts compared to Pt requires much higher loadings in the cathode, resulting in ca. 10 times thicker catalyst layers (i.e., ~100 vs. ~10 µm), with consequent negative impact on mass transport.6 In the fuel cell group at Los Alamos National Laboratory, a series of structured electrodes with differentiated and ordered segments have been developed, enabling a faster transport of ORR reactants (O2, H+) and products (H2O).7 To enhance the O2 gas transport towards the active sites and improve the catalyst utilization, we developed an electrode structure with alternated catalyst and void (or grooves) domains, aiming to improve the electrodes operational robustness. The grooved structure provides a more direct and less tortuous path for O2 diffusion within the catalyst layer, enabling at the same time a faster removal of the liquid H2O. Using a commercial Fe-N-C PGM-free catalyst, we fabricated a series of grooved electrodes and corresponding flat electrodes with the same catalyst loading and ink composition. From tests under different relative humidity (RH) conditions, we found that the grooved electrodes outperform the flat baseline electrodes at high RH and in oversaturated conditions, where the mass transport is more hindered by the massive presence of liquid water and ionomer swelling.8,9 The grooves improved the O2 mass transport by facilitating a fast removal of the liquid water, without negatively affecting the H+ conductivity. In addition, we showed that filling the grooves with a hydrophobic carbon-based material further improved the water removal from the catalyst layer, increasing the performance at high RH. References A. M. Oliveira, R. R. Beswick, and Y. Yan, Curr. Opin. Chem. Eng., 33, 100701 (2021).D. A. Cullen et al., Nat. Energy (2021).D. Banham et al., Sci. Adv., 4, 1–7 (2018).H. Zhang et al., Energy Environ. Sci., 12, 2548–2558 (2019).A. Uddin et al., ACS Appl. Mater. Interfaces, 12, 2216–2224 (2020).L. Osmieri and Q. Meyer, Curr. Opin. Electrochem., 31, 100847 (2021).J. S. Spendelow, DOE Annual Merit Review - Accessible PGM-free Catalysts and Electrodes (2021).A. Kusoglu, T. J. Dursch, and A. Z. Weber, Adv. Funct. Mater., 26, 4961–4975 (2016).G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, J. Electrochem. Soc., 167, 044519 (2020).