Development of Novel Nanoporous Carbon Scaffolds for PEMFC Applications
Creators: name: Atwa, Marwa H
Publisher: name: Science
Year: 2021
Subjects: subject: polymer electrolyte membrane fuel cell
subject: oxygen reduction reaction, atomic layer deposition, platinum nanoparticles, nanoporous carbon scaffold, tunable mesopores
subject: Physical Sciences
subject: Health And Environmental Sciences

description: The microstructure of conventional polymer electrolyte fuel cell (PEMFC) catalyst layers (CLs) consists of a mixture of high surface area carbon powder, Pt or Pt alloy nanoparticles (NPs), and an ionomer, which acts as a binder and a proton-conducting electrolyte. These CLs contain complex pathways and a poorly controlled distribution of Pt NPs and ionomer, which can cause kinetic and transport limitations for the oxygen reduction reaction (ORR). Therefore, we have developed a self-supported, robust, and binderless carbon membrane with an ordered, tunable pore structure, namely the nanoporous carbon scaffold (NCS). The NCS films have a high surface area (200-610 m2/g), ultra-low tortuosity (close to 1), high porosity (ca. 90%) and electrical conductivity (? 2 S/cm), and excellent 3-dimensional scalability (1-150+ cm2 demonstrated area, 1-1000 µm thickness). Herein, the effect of the microstructure of various NCS materials, loaded with both catalytic Pt NPs and ionomer, on the kinetics of the oxygen reduction reaction (ORR), which is the rate limiting reaction in PEMFCs, was explored. Two different methods of Pt NP loading (wet impregnation and atomic layer deposition (ALD)) were examined and two types of NCS films with two different structures (monodisperse and bimodal pore size distributions) were prepared and then integrated into a membrane electrode assembly (MEA) as a PEMFC cathode. For the Pt-loaded NCS films with monodisperse pore sizes (uniform pore size distribution with a standard deviation of ± 10-15%) of either 22, 50, or 85 nm, it was found that ALD Pt loading results in a much narrower NP size distribution than wet impregnation, and that the ALD-Pt/NCS85 catalysts out-perform the ALD-Pt/NCS22 materials, likely as Nafion can better penetrate the larger pores to provide proton conductivity. The much smaller neck diameter may be the real problem, where it was shown that larger neck diameters, keeping all other factors constant, can significantly lower the mass transport resistance. The bimodal NCS12 films possess a Craspedia (flower)-like structure, with the spheres (~900 nm) fully saturated with close-packed 12 nm primary pores and connected to neighboring spheres through non-porous carbon fibers, leaving ~ 0.25 µm secondary pores between the spheres. It was found that the 12 nm pores contain the vast majority of the highly dispersed ca. 2-3 nm Pt NPs, and yet contain no Nafion, thus indicating that water must be conducting protons in these pores during the ORR. These Pt NPs also show signs of surface strain, perhaps contributing to their high ORR activity. These factors, including the absence of blocking or poisoning of the Pt NPs by Nafion, have resulted in an extraordinary ORR mass activity (0.58 ± 0.1 A/mgPt), retained after 10k ADT cycles, and a very high limiting current of close to 2.7 A/cm2 in MEA cathode studies.